Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author. CHARACTERISATION AND AMELIORATION OF LOW pH CONDITIONS IN PYRITIC MINE PITW ALL MATERIALS, MARTHA MINE, W AIHI, NEW ZEALAND A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science Massey University Shivaraj Gurung 1998 A b s t r a c t The objective of this thesis was to research the processes associated with the generation of low pH conditions in pitwall rock material at Martha Mine, Waihi, and evaluate the ameliorating effectiveness of some selected acid neutralising materials with an aim to create suitable plant growth media. Approximately 25% of the current pitwall area is affected by pyrite oxidation, resulting in the formation of acid mine drainage (AMD) which limits long-term establishment of vegetation. The results of this study showed that slope gradient, variable cover material distribution and persistent rill and sheet erosion on the pitwall are some of the physical characteristics restricting plant establishment. Weathered cover materials varied in depth from 5 mm on the upper slopes to > 300 mm in the lower colluvial section of the pitwall. The uneven distribution of pyrite mineralisation has resulted in microenvironments of "acid pockets" in oxidised parts of the pitwall. The fresh pyritic rock had a near neutral pH while the strongly weathered materials generally had pH < 3.0. Based on the total sulphide S content (2.51 %), the fresh rock had a net acid producing potential (N APP) of 51 kg CaC03 ( I . Weathered material still contained significant amounts of sulphide S but because of negative neutralisation potential (NP), it had a higher NAPP of 82 kg CaC03 (I. Kinetic net acid generation (NAG) test revealed that the fresh rock, when exposed, had a lag-period of 22 weeks for the onset of biochemical oxidation. However, the degree of pyrite liberation from the host rock materials is likely to effect the lag-period. The effect of progressive weathering and oxidation was to cause major losses in base cations except for K, which showed an anomalous enrichment, due to incorporation into clays and jarosite-type minerals. Weathering also caused relative enrichment in Ba and As contents of the pitwall materials. Run-off water collected from the bottom of the pitwall had the characteristic AMD composition of low pH and high dissolved metal concentrations. The spatial variation of pH of the weathered pitwall rock in the study area was in the range 2.0-4.6 while EC varied from 1.9 to 4.3 dS m- I . The study area generally contained high 11 concentrations of soluble Fe (2506-5758 mg kg-I) , Mn (203-635 mg kg-I) , exchangeable­ Al (4.8- 10 .8 cmole kg- I), sol (1650-3400 mg ktl) and acidity ( 1 2 1 -668 kg CaC03 f\ Overall, NAPP distribution varied from 35 to 1 43 kg CaC03 (I. A buffer curve lime requirement (LRBuffer) to raise the pH of the weathered pitwall rock material to 6 (29 kg CaC03 fl) amounted only to 35% of the acid base accounting (AEA) value of 82 kg CaC03 fl. This suggested that the LRButfer only accounted for the acid generated from dissolutjon of hydroxide precipitates of Fe and AI . It was found that in order to account for the NAPP of the pitwall material, it was important that the lime required to neutralise the potential acidity (LRNAPP) be added to the LRBuffer to give the total lime requirement (LRTotal) for long-term control of acid generation. A 90 days incubation assessment of selected neutralising materials (limestone, LST; dolomite, DOL; reactive phosphate rock, RPR; fluidised bed boiler ash, FBA) indicated that LST, DOL and FBA were similar in attaining the target pH of 6 at a carbonate content equivalent rate (CER) of 30 kg CaC03 t-l. The RPR did not raise the pH > 4.5 even at CER of 50 kg CaC03 (1 but it was equally effective in overall reduction of EC, sol-, acidity, Fe, Mn and Al in the incubated pitwal1 rock material . The coarser the grain size, the less reactive the neutralising material was, mainly due to an armouring effect from the Fe and Al hydroxide coatings. While fine-grained material provided quick neutralisation of acid, long-term buffering of the pH may not be possible due to continued generation of acid as more pyrite grains are liberated for oxidation. On the other hand, materials like RPR and coarse LST may provide slow release neutralisation from repetitive dissolution of hydroxide coatings when reacidification occurs. Results of the column experiments on the assessment of ameliorative effectiveness of neutralising materials on leachate quality and subsurface acidity indicated that although application of amendments significantly raised the pH at 0-60 mm column depth, the leachate pH remained below 2.5 throughout the 1 2 weeks leaching cycle. The concentrations of EC, SO/ , acidity, Fe, Mn and Al were however, significantly reduced both in the leachate and subsurface column sections. At depth > 60 mm, the leached columns remained acidic irrespective of treatments . Broadcasted and incorporated 111 methods of application of neutralising material amendments showed similar trends in effectiveness of amelioration. However, the overall ameliorative effectiveness was significantly better with incorporated method of amendments. Surface application of a shallow depth of topsoil (TS) and incorporation of bactericide ProMac (PM) were found effective in the amelioration of low pH conditions of the pitwall rock material by raising pH and significantly reducing sub-surface concentrations of S042-, acidity, Fe, Mn and Al. The amended columns however, still produced effluent pH of <2.5. The overall results from the study indicated that with detailed on-site characterisation and using laboratory studies to formulate appropriate combinations of neutralising materials, the pyritic pitwall rock materials could be suitably modified for plant growth. In practice, the placement of the amendments on the pitwall remains an engineering challenge. IV Acknowledgments I would like to thank my supervisory panel, Dr. R.B. Stewart (Chief), Professor P.E.H. Gregg, Dr. N.S. Bolan and Dr. C.W. Ross for their constructive advice, interest in the research, encouragement and patience towards completion of this thesis. My special thanks to Professor Paul Gregg for initiating this research. My gratitude and appreciation for assistance also extends to the following technical staff of the Earth & Soils Science Depmtment:- Messrs Lance D. Currie (Senior Technical Manager), Mike Bretherton, Bob Toes, lan Furkert, Alton McDonald, Joe Whitten, Ann West, Glenys Wallace and Ross Wallace. I would like to thank Doug Hopcroft at Horticulture and Food Research Institute for assisting with Scanning Microscopy. To the academic staff and to Denise and Marian at the office I say "thank you" for all the help. Funding for this research was kindly provided by Waihi Gold Mining Company. I wish to thank Keith Brodie (Environmental Manager), Tim Gosling (Mine Manager) and Stewart Miller (EGi.Ltd) for facilitating field studies and for their continued support and keen interest in this mine reclamation research. Kathy Mason (Environmental Officer) , thank you for your help in the field and all the documents you provided so readily. I also acknowledge the fundings from Fertiliser & Lime Research Centre, Helen E. Akers postgraduate scholarship and a grant for XRF analysis at Spectra Chem Analytical, Wellington. Thanks to all my friends and colleagues in the department who have helped keep my sanity intact. My special thanks to John Morrell , Jim Moir, Shane Cronin & Iris Vogeler, Bret Robinson and Andrew Hammond for inspirational chats and putting up with my antics . Your friendship and companionship will be remembered through the yem·s. To my family Shanthini, Kesherie, Ishka and Aasha for their love, support and understanding through the student years. Dr & Mrs S. Nissanga and Thilani, thank you for your love and support. Finally, I would like to dedicate this thesis to my parents Ratna Bahadur & Dil Maya Gurung for aspirations fulfilled, obligations neglected. v TABLE OF CONTENTS Abstract Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV Table of Contents . . . . . . . . . . ............................... ................................................. ............ V List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . Xl List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........ ........................... ...... .... ..... ..... .... ......... ........... Xlll List of Plates . . . . . . . . . ...... .... ......................................................... .................................. XVI Chapter 1 Introduction 1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....... ................................ 1 1.2 Objectives of this Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ....................................... 5 1.3 Outline of this Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Chapter 2 Literature Review: Mined Land Reclamation 2.1 Introduction . . . . . . . . ................................................................................................. 7 2.2 Acid Mine Drainage (AMD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 Sources of AMD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ................................................ 8 2.4 Biochemical Aspects of AMD Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.4.1 The chemistry of AMD generation from pyrite oxidation . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4.2 Accretion and migration of AMD . . . . . . . . . ............... ..................................... 13 2.4.3 By-products associated with AMD.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.4.3.1 Metal hydroxides . . . . . . . . ... . . . . . . . . .. . . . . . . .. . . .. . . . . . . . . . .. . . ... . . . .. . . . . . . . . . . . . . . . . .. . . . . 16 2.4.3.2 Sulphate salts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.4.3.3 Acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 2.4.3.4 Aluminium . . . . . . . . . . .. . . .. .. . ... . . . . . . . . .. . . . . . . .. . . .. .. . .. . .... . . . ... . . .. . . . . . ..... . . . . . . . . . . . . 18 2.4.4 In-situ neutral isation of acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........................ 19 2.5 Predictive Techniques for AMD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.5.1 Static tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.5.2 Kinetic tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.5.3 Evaluation of the predictive techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.6 Prevention and Control of AMD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.6.1 Preventive coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . ... . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . 30 2.6.2 Selective handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 2.6.3 Bactericides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 VI 2.6.4 Oxidant infiltration barriers .. .... ... . . . . . . . . . . .. ... . . .... . .. . . . . ........ .. . .. . . .. . .... .. . ... ...... . 3 1 2 .7 Treatment of AMD .. . . . . .......... ...... . . . . . ......... .... . . . . . . ... . . .. . . ... . .. . . . . .. . .. ... ......... ..... . ...... 3 5 2 .7 . 1 Active treatment systems .. . . . . .. . ... ... . . .. . . . ... . . ... . . . .. ..... .. .. .. .. .. .. . . . .. . . . . . .. . . .. .. . .. . . . 35 2.7 .2 Passive treatment systems .. . . . . .... . .. ...... .. . . . . . . . .. ... . . . ......... . . . . ... . .. .. . . . . . . .. .. . . . .... 37 2.8 Mined Land Reclamation Methods ......... . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 8 2 .8 . 1 Lime requirement of sulphidic mine wastes . .. . . ... . ... .. .. . ...... ....... ... . . . . .. . .. .... 39 2 .8 .2 Alkaline amendments in reclamation ..... . . .. . .. .... .. . . . . . . . . .. . . ....... . .. . . ... .. . . .. . . . ... 39 2 . 8 . 3 Organic amendments . .. . . . . .. . . . . . .. .. .. .... . .. .. .. .. . . . . ..... ... .. . .. . . . ... ...... . . .. . ... .. . ... . . .. .. 43 2 .8 .4 Bactericides . . . . ... . . . . . . . . .. . . . . . . . . ........ .. . .. . . . . .... .. . . . . . . . ... . . . . .. . . .... . . . ... .. . . . .. . ... .. . . . .. . . . 45 2 .8 .5 Reclamation by revegetation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 2 .9 Revegetative Reclamation in New Zealand ....... . . . .. . . .. .. . . ... ... ..... . . .... .. .. . . .. .. . .. . . . . . . 47 2. 1 0 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Chapter 3 Characterisation of Pyritic Pitwall Rock at Martha Mine, Waihi 3 . 1 Introduction...... ..... .... . ..... ... ............. .. . . . . . . . . . . . . . . . .. ........ . . . . . . . . . ... ...... .. . . . . .. . . ... . . . . . .. .. .. 5 1 3 .2 Materials and methods ... . . . . . . . .. . . . . . . . .. . . ..... . .. ... . . . . .. . .. . . . . . . . . . ..... .. . . . .. ....... . ... . . .. . .. . . .. . . . 52 3 .2 . 1 S ite selection and sample collection . . . .. . .. . . . . .. .. . . . ... .. . ... .. . .... ......... .. . . . . .... .. .. . 52 3 .2 .2 Grid sampling .. .. . . . . . . . . . . . . . . .. .. .... . .. ...... ............ .. . . . . ... . .... . . .. .. ...... ... .... ...... . .. . . .. 52 3 .2 .3 Spatial data analysis . .. .. ... .... .. . . . . . .... . . . . . ... .. . . .. .. ... . . . . . . . .... .. . . .. . . .. .. ..... .. .. .. . ...... . 52 3 .2.4 Mineralogical and geochemical studies ... . . . . ... . . ... .. ... . . ... .. .. . ... .. . .. . .... .. . . . ... .. 55 3 .2 .5 Analytical methods ... . . . .. ... . . .. ... . . . . . .. . ... . . . . . . . . .. . .. .. . ... . . . . . .. .... . .. . . . . . . . . . .. . .. . . . .. . .. . 56 3 .2 .6 Net acid generating (NAG) static test . .. .. ........ . . . . . .. ...... .. .. .. .. . . .... ... ... ...... .. . 59 3 .2 .7 Acid neutralising capacity (ANe) . .. . . . .. .. .. ... . . . . . .. . .. ..... ... . . .. ......... . ... . ... ..... .. . 59 3 .2 .8 Acid base accounting (AB A) . . . . . . . . ... .. . . . .. .. . .. ..... .. . . . . .. .. . ..... .. . .. . . .... . .. . .. . . . ... . . 59 3 .2 .9 Net acid generating (NAG) kinetic test ... . . . ...... . .. . . . . .. . . . ..... .. . . . . ... . . ..... .. . . . ... . 59 3 .2 . 1 0 Column test .... . .. . . .. . . . . ... .. . .. . .. . ...... . .. . . ... . . .. . .. . . . .. .. ... ... .. .. .. .. .. ........ . . .. . ... .. .. . . . . 60 3 . 3 Results and Discussion . .... ................ . . .. . . ... .. .. . . . . . . .. ......... ... . . . ..... . . . . . . .. . . . . . .. . . ... ...... . 62 3 .3 . 1 Physical and geochemical characteristics of the pitwall rock . . . . .. . . . .. . . . .. . .. . 62 3.3.1.1 Physical characteristics . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 3.3.1.2 Mineralogical properties................................................................ 63 3.3.1.3 Chemical properties .. . . . . . .. .. . . . .. .. . . . .. .. .. . . .... ........ .. . . . . . ... ....... .. . . ...... ... 77 3.3.1.4 Depth-profile chemistry of the weathered pitwall rock .... .. . .. ... . ... . 78 3.3.1.5 Geochemical Properties . .. . . . . . .. . .. .. .. .. . ... .... .... . ...... . . . . . .. . . .. .. . .. . .. . .. .. .. . 79 3.3.1.6 Effect o.fweathering on geochemical properties .. .. . . .. .. ........ . . . .... . . 8 1 3 .3 .2 Acid generation properties of the pitwall rock... . .. . . . .... .. . . .. . . . . .. .. .. . . . .. . .... . ... 85 3.3.2.1 Static net acid generation (NAG) test ..... .... ...... . .. .. . . ... . ... . . ....... . . ... . 85 3.3.2.2 Acid base accounting (ABA)........................................................... 85 3.3.2.3 Kinetic NAG test - Lag period . ... . .. ... . . .... .. . . . ..... .... .. . .... ..... . . . . . . .. . . ... 88 V11 3.3.2.4 Column test - rate of acid generation ............. . . . . . . .... . . . . ... .... . . . . ...... 90 3.3.3 Characterisation of pitwall run-off drainage . . . . . ..... ....... . . ... . .. . ........ . . . . . . . .... 94 3.3.4 Spatial characterisation of the weathered pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.3.4.1 Geostatistics . . .. . ... .... ...... ................ . . . . . . . . . . . . . . . ..... . . ............ . . . . . . . . . . ..... 98 3.3.4.2 Descriptive statistics ...................................................................... 99 3.3.4.3 Distribution of cover material and moisture content .................... 102 3.3.4.4 pH and EC....... . .. .. ... . . . . . . . . . ... . .................................. . ......... . . . ... ..... . . . 104 3.3.4.5 Exchangeable Al, soluble Fe and Mn. . . . . ....... ...... ... . . . . . . . . .. . . . . . . ....... 106 3.3.4.6 sol' and Acidity............................... . . .. . . ..... . . ...... . . . . . . ... . . .. . .. .. . ....... 109 3.3.4. 7 Total Sand NAPP ...... .. .. . . ........................ .... . . . . .................. ............ 111 3.3.4.8 Distribution of pyrite ... . . . . ........ .. . ..... . . . .. . . .. ........ . .. .... ....... . . . . .. . . . . ..... 113 3.3.4.9 Base cations (Ca, Mg, K and Na}. . ... . ..... ........... . . . . . . . . . . .. . . . . ............ 115 3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Chapter 4 An Assessment of the Effectiveness of Neutralising Materials in Ameliorating Acidic Pitwall Rock 4.1 Introduction ........ ............................ ....................................................... .............. 119 4.1.1 Neutralisation of acid in pyritic mine waste rocks . . . . . . . .. . . ...... . . . . . . .. . ...... . .. . 119 4.2 Materials and Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120 4.2.1 Pitwall rock bulk sample ........ . . .. . ............... ..................... . ......... ..... . . .. . . . . . . . 120 4.2.2 Neutralising materials ..... . . .... ....... . ......... ... . ......... . . ........ ..... . . . . ..... . . . ........... . 120 4.2.3 Neutralising potential (NP) of the neutralising materials . . . . . . . . . . . . . . . . . . . . . . . . . . 121 4.2.4 Lime requirement of the pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.2.5 Neutral ising material requirements: an incubation study . . . . . . . . . . . . . . . . . . . . . . . . . . 122 4.2.6 Determination of limestone particle size effect on pitwall rock . . . . . . . . . . . . . . . 122 4.2.7 Characterisation of hydroxide coating on l imestone particles . . . . . . ......... ... 123 4.2.8 Analytical methods .... . . . . ........ ............... . .. . . . . . . . . . . ............... ............. ... .. . . . . . . . 123 4.3 Results and Discussion ..... . . . . . . . . .. . ..... ... . . . . . .... . . . . .. . . . . . . . .... . . . . ....... . . . . . . . . . . . . . ......... .. . . 123 4.3.1 Effectiveness of alkaline materials in acid neutralisation . . . . . . . . . . . ... ........... 123 4.3.1.1 Lime requirement of the pitwall rock ..... ....... .... ... . .... ...... . ..... ........ 123 4.3.1.2 Neutralising material requirements of the pitwall rock... . . ........... 126 4.3.1.3 Neutralisation of acidity ................................................................ 129 4.3.2 Effect of neutralising materials on chemical properties of the pitwall rock 131 4.3.2.1 pH...... ..... .............. . . . .. .... . . . . . . . . .. . . . .. . . ..... . ..... ..... . . . . . . . .. . . . . ..... . . . . . ......... 131 4.3.2.2 Electrical conductivity (EC) . . . ...... . . . . . . . . . . ... ... . . . . . . . . . . . . . . . . . . ..... . .. ... . ... 131 4.3.2.3 Exchangeable Al (Alexc) . . . . . . . . . . . . . . . . .. . . .. . . ... ... . . ...... . .. . .. . . ...... . . ... ... . . . . . 132 4.3.2.4 Sulphate (S042-). . ............... . . . ........... . . .. . . . . . .... . . . .•.•.••.••••.•.•••••. . .•••.••.• 134 Vlll 4.3.2.5 Fe and Mn....................................................................................... 1 34 4.3.2.6 Overall neutralising effectiveness of alkaline materials . . . . . . . . . . . . . . . 1 35 4 .3 .3 Neutralising effect of limestone particles on pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . 1 37 4.3.3.1 Lime requirement based on limestone particle size . . . . . . . . . . . . . . . . . . . . . . . 1 37 4.3.3.2 Limestone particle size effect on chemical properties . . . . . . . . . . . . . . . . . . . 1 37 4.3.3.3 Characterisation of the hydroxide coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 4 1 4.3.3.4 Quantification of elements in the hydroxide coating . . . . . . . . . . . . . . . . . . . . . 1 44 4 .3 .4 Effect of incubation time on pitwall rock chemical properties . . . . . . . . . . . . . . . . . . 1 45 4.3 .5 Effect of incubation on physical properties of the pitwall . . . . . . . . . . . . . . . . . . . . . . . . . 1 47 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 48 ChapterS Effectiveness of Surface Broadcasted Neutralising Materials in Ameliorating Low pH Conditions in Pyritic Pitwall Rock 5 . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 53 5 .2 Material and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 56 5 .2 . 1 Pitwall rock bulk sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 56 5 .2 .2 Neutralising materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 56 5 .2 .3 Column set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 56 5 .2 .4 Column leaching protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 58 5 .2 .5 Leachate analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 5 8 5 .2.6 Column section analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 59 5 . 3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 59 5 .3 . 1 Characterisation of the leachate quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 60 5 .3 . 1 . 1 Leachate pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 60 5 .3 . 1 .2 Leachate EC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 6 1 5 .3 . 1 .3 Leachate S042- . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 63 5 .3 . 1 .4 Leachate acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 64 5 .3 . 1 .5 Leachate Fe and Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 65 5 .3 . 1 .6 Leachate Al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 1 67 5 .3 .5 . 1 Leachate Ca-Mg-K-Na . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 68 5 . 3 .2 Column section chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 70 5 .3 .2. 1 Column section pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 70 5 .3 .2.2 Column section EC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 72 5 .3 .2 .3 Column section S042- ..• . . . . . . . ...... .... . . ...... . . .. ..... .................. . . ... . ........ 1 73 5 .3 .2 .4 Column section acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 74 5.3 .2.5 Neutralisation of acidity in the pitwall rock column . . . . . . . . . . . . . . . . . . . . . 1 77 5 .3 .2.6 Column section Fe and Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 80 5 .3.2.7 Column section exchangeable Al (A1exc) . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 80 5.3 .2.8 Column section Ca-Mg-K-Na. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 8 1 5 .3 . 3 Mineralogical composition of leached columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 83 lX 5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . 185 Chapter 6 Effectiveness of Depth Incorporated Neutralising Materials in Ameliorating Low pH Conditions in Pyritic Pitwall Rock 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 6.2 Materials and Methods. . . . . . . . . . . . . . . . . . . . ................................... ........... ...... ................ 188 6.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.2.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 6.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......................................... 188 6.3.1 Effect on chemical properties of the pitwall rock by incorporated neutralising materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 6.3.1.1 pH and EC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 6.3.1.2 sol' and acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 6.3.1.3 Fe, Mn and Al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .............. 195 6.3.1.4 Base cations (Ca, Mg, K and Na) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196 6.3.2 Comparison between broadcasted and incorporated methods of application of neutralising materials . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . ................... ........... ......... 199 6.3.2.1 Leachate chemistry ' . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 6.3.2.2 Column section chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.3.2.3 Overall effectiveness of amendment methods' . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208 6.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..................... 209 Chapter 7 An Assessment of AMD Inhibitors (Topsoil and ProMac) in Ameliorating Low pH Conditions in Pyritic Pitwall Rock 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 7.2 Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 7.2.1 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 214 7.2.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 7.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 7.3.1 Characterisation of the leachate . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 7.3.1.1 Leachate pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 7.3.1.2 Leachate EC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 7.3.1.3 Leachate S042. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 7.3.1.4 Leachate acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 221 7.3.1.5 Leachate Fe and Mn' . . . . . . . . ........ ..................................... ................. 222 7.3.1.6 Leachate AI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 7.3.2 Column section Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 x 7 .3 .2. 1 Column section pH. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 7 .3 .2.2 Column section EC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 7 .3 .2.3 Column section SO/. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 225 7 .3 .2.4 Column section acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 7 .3 .2.5 Column section Fe and Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . 227 7 .3 .2.6 Column section Al . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228 7 .3 .2.7 Effect of topsoil placement on total Al pool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 7 .3 .3 Effects of combination amendments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1 7 .3 .3 .1 Effect of combination treatments on pH and EC. . . . . . . . . . . . . . . . . . . . . . . . . . . 23 1 7 .3 . 3 .2 Effect of combination treatments on leachate sol', Fe, Mn and Al 234 7 .3 .3 .3 Effect of combination treatments on column chemistry . . . . . . . . . . . . . . . 235 7.4 Conclusions. . . . . . . . . . . . . . ........ .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . .. . . . . . . . . . . . . . 236 Chapter 8 Summary and Conclusions 8 . 1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239 8 .2 Literature Review ... . . . . . .. . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 239 8.3 Characterisation of the Pyritic Pitwall Rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . 24 1 8 .4 Lime Requirements of the Pitwall Rock . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 8 .5 Ameliorating Effectiveness of Selected Amendments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245 8 .6 Future Directions . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . 247 References . . . . . . . . . . . . . . . . . . . . . . . . . .. ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 LIST OF TABLES Table 2.1 A B A procedures ............................. . .......................... ... . .... ......... ............. . Table 2.2 Screening criteria in ABA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 2.3 Comparisons of ABA procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 2.4 Reclamation of AMD contaminated mine sites in New Zealand . . . . . . . . . . . . Table 3 .1 Description of the pyritic pitwall rock samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3.2 XRD analysis of major minerals (wt.%) in the pitwall rock . . . . . . . . . . . . . . . . . . . . Table 3.3 Chemical properties of the pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3 .4 Average composition of the pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3.5 NAG test results for the pitwall rock samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3 .6 Acid base accounting (ABA) analysis of pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3 .7 Chemical characteristics of the runoff drainage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3 .8 Descriptive statistics of spatial characteristics of the pitwall rock (n=47) Table 3 .9 Correlation coefficients of the selected parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 3 .10 A comparison of pyrite estimation from various methods . . . . . . . . . . . . . . . . . . . . Table 4.1 Selected properties of the bulk pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 4.2 Properties of the selected neutralising materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 4.3 Non-linear regression coefficients for pH-CER curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 4.4 Neutralising material required to raise pH>6 in pitwall rock . . . . . . . . . . . . . . . . . . Table 4.5 Graded limestone requirement of the pitwall rock to raise pH>6 . . . . . . . . . . . Table 4.6 Metal and sulphate analysis of the coated limestone grain . . . . . . . . . . . . . . . . . . . . . . Table 4.7 Munsells colours after 90 days incubation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Table 5 .1 Correlation coefficients (r) of the measured parameters in the leachate . Table 5.2 Average release rates of the concentrations of measured parameters leachate from broadcasted pitwall rock column . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Table 5 .3 Correlation coefficients (r) of measured parameters in the column section Xl 22 22 23 48 55 64 78 82 85 86 96 101 101 114 121 121 128 129 137 144 147 163 in the 165 173 Table 5.4 Overall comparison of the mean concentrations of chemical properties of the leached pitwall rock columns broadcasted with neutralising materials 173 Table 5.5 Depth-wise base saturation (BS%) in the broadcasted columns . . . . . . . . . . . . . 183 Table 6.1 Average release rates of the concentrations of measured parameters in the leachate from incorporated columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Table 6.2 Overall comparison of the mean concentrations of chemical properties of the leached pitwall rock columns incorporated with neutralising materials 192 Table 6.3 Overall means comparison of the broadcasted (BC) and incorporated (lC) methods of amendments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Table 7.1 Selected properties of Waihi topsoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214 Table 7.2 Amendments and treatment design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216 Table 7.3 Average release rates of the concentrations of measured parameters in the leachate from columns treated with various combinations of amendments 221 Xli Table 7.4 Comparison of the mean distribution of selected chemical parameters in leached columns treated with various amendments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Table 7.5 Average release rates of the concentrations of measured parameters in the leachate from columns treated with various combinations of amendments 234 Table 7 .6 Comparison of the mean distribution of selected chemical parameters leached columns treated with various amendments . . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . 235 LIST OF FIGURES Figure 1 . 1 Location map, North Island, New Zealand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 2. 1 A simplified cycle in the generation of AMD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Figure 3 . 1 Grid sample locations (PWGS- l, GSC- l . .) of the study area in Contour values represent depth of cover material in mm . . . . . . . . . . . . . . . . . . . . . Figure 3.2 Colurrm test set up (adapted from Miller and Jeffrey, 1 995) . . . . . . . . . . . . . . . . . . X1ll 3 1 3 Plate3 . l . 53 6 1 Figure 3 . 3 An EDS spectrum o f semi-quantitative elemental analysis of the pyrite grain in Plate 3.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Figure 3 .4 An EDS spectrum and semi-quantitative elemental analysis of pyrite grain shown in Plate 3.6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Figure 3 .5 Depth variations of selected chemical properties of the pitwall rock. Horizontal bars represent LSD(5%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Figure 3.6 Chemical gain-loss diagrams relative to the fresh rock (PWR- 1 ) , for the pitwall rock undergoing progressive weathering and oxidation . . . . . . . . . . . . . . . . . . . . . . . 84 Figure 3 .7 Pyrite inclusions and oxidation scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Figure 3 .8 NAG test kinetics for (a) fresh PWR- l , (b) freshly weathered PWR-2, (c) moderately weathered PWR-4 and Cd) strongly weathered PWR-5 . . . . . 89 Figure 3 .9 Leachate characteristics of the pitwall rock under laboratory controlled colurrm test. Vertical bars represent LS D(5%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1 Figure 3 . 1 0 Pyritic pitwall showing runoff drainage flow direction and sampling locations C 1 -8). Contour values represent kriged % pyrite CFeS2) in the weathered material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Figure 3 . 1 1 Chemical characteristics of the runoff drainage (a) pH and EC (b) cumulative loading of Fe, S042-, AI and Mn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Figure 3 . 1 2 Distribution of (a) weathered pitwall rock cover depth (CD) and (b) moisture content (MC) of the cover material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 03 Figure 3 . 1 3 Spatial variations in Ca) pH and Cb) EC levels in the weathered pitwall cover material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 05 Figure 3 . 1 4 Distribution of (a) exchangeable AI, (b) soluble Fe and (c) soluble Mn in the weathered pitwall cover material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 08 Figure 3 . 1 5 Distribution of Ca) SO/- and Cb) acidity in the weathered pitwall cover material. ........................................ ....................................................................... 1 1 0 Figure 3 . 1 6 Spatial distribution of (a) total S content and Cb) NAPP of the weathered pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 2 Figure 3 . 1 7 Distribution of pyrite in the weathered pitwall rock material. . . . . . . . . . . . . . . 1 1 4 Figure 3 . 1 8 Spatial distribution of base cations Ca) Ca, Cb) Mg, Cc) K and (d) Na in the weathered pitwall rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 6 Figure 4. 1 (a) NaOH-pH and CaC03-pH buffer curves and (b) acidimetric titration curves for solutions containing Fe, AI and pitwall rock sample extract . . . . . . . . . . 1 25 Figure 4.2 Response to pH with increasing carbonate content equivalent rate (CER) of neutralising materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 28 XlV Figure 4.3 Reduction in acidity in 90 days incubated pitwall rock (a) with varying CER and (b) overall comparison between neutralising materials . . . . . . . .. . . . . . . . . . . . .. . . . 1 30 Figure 4.4 Effect on selected chemical properties of the pitwal1 rock after 90 days incubation with varying CER of neutralising materials. Vertical bars represent LSD(5%). .. ....... . .... . . . ..... . . . ... ... . . . . . ..... ..... . . .. . ... .... . .. . .... . . . .. . .. .... .. . . . .... . . ... . .. 1 33 Figure 4.5 Overall effect of neutralising materials on selected chemical properties of the pitwaU rock after 90 days incubation. LST, l imestone; DOL. dolomite; FBA, fluidised bed boiler ash; RPR, reactive phosphate rock. Vel1ical bars represent LSD(5%). .. ... . . . ...... . . . . . . .. . . .. . ........ . . . .... .... . ... ... . .... . . . .. . ... .. .. . . . ............... ..... . 1 36 Figure 4.6 Effect on selected chemical properties of the pitwall rock after 90 days incubation with varying CER of different particle size limestone. LST VF, very fine limestone; LST F, fine l imestone; LST Co coarse limestone; LST AR, as received limestone. Vertical bars represent LSD(5%) .. .. . ... . .......... .. .. . .. 1 39 Figure 4.7 Reduction in acidity in 90 days incubated pitwall rock. (a) With varying CER of graded limestone and (b) overall comparison between the graded limestone. 1 40 Figure 4.8 EDS spectra with accompanying tables of elemental concentrations of hydroxide coated limestone grain incubated for 90 days. Core (limestone), Middle (hydroxide coating) and Outer (pitwall rock front) ...... .... .. .. . .. . 143 Figure 4.9 Effect of incubation time on selected chemical properties of the pitwall rock treated w ith neutralising materials at CER = 30 kg CaC03 f I. Vertical bars represent LSD(5%) . . ..... .. . .. ... . ... . . ... . . . . . ..... . ... . .. . ...... ... . .. . .. . . . . . .... .. . . . ...... . . . 1 46 Figure 5 . 1 Reconstructed column set up for leaching pitwall rock under glasshouse condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 57 Figure 5 .2 Ca) pH and Cb) EC of the leachate from columns broadcasted with neutralising materials . Vertical bars represent LSD(5%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 62 Figure 5 .3 Concentrations of (a) solo, (b) acidity, (c) Fe, (d) Mn and (e) Al in the leachate from columns broadcasted with neutralising materials . Vertical bars represent LSD(5%) ................................................................................................ 1 66 Figure 5 .4 Concentrations of (a) Ca, (b) Mg, (c) K and (d) Na in the leachate from columns broadcasted with neutralising materials . Vertical bars represent LSD(5%) .... . . .. . .. .... . . . .. . ..... .. ... ... . .. .. . .. .. . .. .. .. . . . ... . . ..... . ... . ... . ........ ...... ... 169 Figure 5 .5 (a) pH and (b) EC of the sectioned samples from leached columns. Horizontal bars represent LSD(5% ) .. . . .. .... . ..... .. . . . . . ... ... . . ............. ... ... . ...... . .. . . . . . . . . . . ... 1 7 1 Figure 5 .6 Distribution of (a) solo, (b) acidity, (c) Fe, (d) Mn and (e) A1 exc in the sectioned samples from leached columns. Horizontal bars represent LSD(5%) 176 Figure 5.7 A regression plot of overall mean acidity and S042- values in leachate from control and amended columns . Data points are means of four replicates. 1 79 Figure 5 .8 Distribution of (a) Ca, (b) Mg, (c) K and (d) Na in sectioned samples from leached columns broadcasted with neutralising materials . Horizontal bars represent LSD(5%) ...... ........ ... .. . . . . ............... . .. . . . . ... .. ... ... . . . ........ .. ...... . . . .. 1 82 Figure 5 .9 Distribution of (a) pyrite, Cb) gypsum, (c) silica and (d) clay minerals in the leached columns broadcasted with neutralising materials . . . . . . ........ . . . . . . 1 84 xv Figure 6. 1 (a) pH and (b) EC of the leachate from columns incorporated with neutralising materials. Vertical bars represent LSD(S%) . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 190 Figure 6.2 (a) pH and (b) EC of the sectioned samples from leached columns. Horizontal bars represent interaction LSD(S% ) . . . . ... ... . .. . . .. . . . . . .. . .. . . . . . . . . . . . . . . . . . . . . . . . . ... 190 Figure 6.3 Distribution of (a) Ca, (b) Mg, (c) K and (d) Na in the sectioned samples from leached columns incorporated with neutralising materials. Horizontal bars represent LSD(5 %) . . . .. . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . .. . ... . . .. . . . . . . . . . . . . . . . . . . . . . . 198 Figure 6.4 Treatment-wise comparison of leachate chemical properties of the broadcasted and incorporated columns. Vertical bars represent method x treatment interaction LSD(S%) . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . ...... . . ........ ... . . . . . . . . . . . . . .... . . . . . . . 201 Figure 6.S Week -wise variations in the leachate chemical properties of the broadcasted and incorporated pitwall rock columns. Vertical bars represent LSD(S%) 202 Figure 6.6 Treatment-wise comparisons of chemical properties of the broadcasted and incorporated columns. Vertical bars represent LSD(S%) . . . . . . . . . . .. . . . . . . . . . 204 Figure 6.7 Comparisons of (a) treatment-wise and (b) depth-wise distributions of exchangeable AI (Alexc) in the broadcasted and incorporated columns 20S Figure 6.8 Distribution of chemical parameters in broadcasted and incorporated columns. Horizontal bars represent (LSDS%)..... . . . ... . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . 207 Figure 7.1 (a) pH and (b) EC of the leachate from columns amended with TS and PM. Vertical bars represent LSD(S%) .. . . . . . . . . . . . . . . . . . . ... .... ... . .... . .. . ... . . . . . . . ... . . . . . . 218 Figure 7.2 Concentrations of (a) SO/-, Cb) acidity, Cc) Fe, (d) Mn and Al in the leachate from columns amended with TS , PM and TS+PM. Vertical bars represent LSD(SO/o).. . . . . . . . . . . . . . ... . . ...... . . . ... . . . . . ... . . . . . . . . . . . . . .... . . ... . .. . . .. . . . . . ... . . . . ... . . . . .. . . .... 220 Figure 7.3 (a) pH and (b) EC of the sectioned samples from leached columns. Horizontal bars represent LSD(S%) . . . . . . . . . . . . . . . ... . ... . . . . . ... . . . . . . . ... . . . . . . . . . ... . . . . . ... . . . .. . . . .... 224 Figure 7.4 Distribution of (a) SO/-, (b) acidity, (c) Fe, (d) Mn and (e) Alexc in the sectioned samples from leached columns. Horizontal bars represent LSD(S%). . . ............ . . . . ....... . .... ............ .......... . . .. . . .. ................. . . . . . .. . . .. . . .... . ......... . . . .. .. 226 Figure 7.5 Distribution of different forms of Al in topsoil amended columns . . . . . . . . 230 Figure 7.6 Overall effect on (a) leachate pH and (b) column pH by various combinations of treatments . . .... .. . . ...... .. . . . . . . . . . . . . . . . . . . . ..... . . .... . . . . . . . . . . .. . . '" . . . . . . . . . . . . . . . . . . . . . . '" 232 Figure 7.7 Overall effect on (a) leachate EC and (b) column EC by various combinations of treatments ... . ... . . . . ..... . . . . . . . .. . . . ..... . . .. . . .. . . . . . . . . . . . . . . . . ... . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . 233 Figure 7.8 A conceptual scenario of micro-bench amendment on pyritic pitwall at Martha rrune. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 XVI LIST OF PLATES Plate 1 . 1 Martha mine and tailings disposal site ( 1 994), Waihi . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 3 Plate 3 . 1 Martha mine (Waihi) shoeing the study area on exposed north face pitwall ( 1 994) 53 Plate 3 .2 A section of the pitwall showing (a) Fe-hydroxide coatings and rill erosion pattern and (b) fresh rock in weathered matrix . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 65 Plate 3 .3 Back-scattered SEM scans of (a) disseminated pyrite crystals and (b) fine grained pyrite lens . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . .. . . . . . 67 Plate 3 .4 SEM scan of euhedral pyrite grain from fresh rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Plate 3 .5 Forms of pyrite grains (a) sub-rounded pyrite crystals in weathered pitwall rock and (b) flaky calcite grains in fresh rock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . . .. . . . . 7 1 Plate 3 .6 SEM scan of silica coated pyrite grains in weathered pitwall rock . . . . . . . . . . . 73 Plate 3 .7 SEM scan of pyrite grain with etch holes (a) Magnification x2000 (b) Magnification x20000 . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 75 Plate 4. 1 SEM scan of sectioned 90 days incubated limestone grain. Refer Figure 4.8 for EDS spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 1 4 1 Plate 4.2 Physical effect on pitwall rock incubated with nil (Control), low ( 10 kg CaC03 f I), medium (25 kg CaC03 fl) and high (50 kg CaC03 (I) rates of neutralising materials. Labels on pitwall rock indicate Munsells colour notations . . 149 1 C h a p t e r 1 Introduction 1 . 1 Background Waihi Gold Company (WGC) operates the Martha Mine located within the northern perimeter of Waihi township on the southern end of the Coromandel Peninsula (Figure 1 . 1 & Plate 1.1). The Martha Mine is one of the previously worked epithermal gold-silver deposits in the Hauraki goldfield and was reopened in 1988 as a result of resurgence in gold prices and technological advances in ore processing methods. The ore is extracted by open pit mining and is processed by the carbon-in-pulp process. The current annual production rate is 0.85 Mt of ore and 3.5 Mt of waste rock with an average ore grade of 3 .1 g Cl gold and 24 g C l silver (Brodie et al., 1996). The total production of precious metals during the period 1 988- 1 996 was 645 ,000 oz gold and over 3 mill ions oz of silver respectively (Gregg et al . , 1998). An estimated 11 million tonnes of gold-silver bearing ore and 25 million tonnes of overburden is expected to be mined during the lifetime of the current mining operation. The waste rock is disposed at a site about 2.3 km away from the mine, where it is used as an embankment to store tai l ings from the processing plant. Since the mine is located in close proximity to the Waihi township and the disposal site is on pastoral land, the mining licence required that stringent environmental controls were implemented from the onset of mining. Under the current l icensing agreement, the final land use of the disposal site is to be pastoral farming and the post-mining option for the pit is a recreational lake. The WGC is also required to revegetate the pitwall above the projected final lake level, primarily for aesthetic reasons as well as to prevent erosion (WGC, 1 985; Gregg and Stewart, 1 996). A preliminary study undertaken by Widdowson et al. (1984) indicated that although the oxidised waste rock showed potential as a plant growth medium, the unoxidised rock, which contained significant amounts of pyrite mineralisation, was highl y unsuitable due to its poor texture, formation of excessive amounts of dissolved aluminium and salt content when oxidised. Onsite baseline studies and environmental impact assessments of the mine site (WGC, 1985 ; Miller, 1 986) showed that pyrite exposed in some areas of the 2 pit had the potential to generate acid when oxidised, resulting m acid mine drainage (AMD). Open pit mining of base metal deposits generally exposes large volume of sulphidic rocks to weathering and oxidation processes and is often regarded as an unsightly visual impact on the local landscape. With an annual average rainfall of 2300 mm and dry spells in summer, the local climatic conditions at Waihi provide a high erosion index, resulting in acidic nmoff from pitwalls and surface precipitation of metal salts respectively. Such characteristics pose serious limitations to revegetation of the pyritic pitwalls. Pre-mining and on-going geochemical characterisation of the waste rock generated from open pit mining have indicated that approximately 40% of the waste rock is classified as a potentially acid generating type (WOe, 1 985; Brodie et aI . , 1 996). In tem1S of pitwall area, about 25% of the cun-ent pit slopes contain pyritic host rock (andesite) undergoing varying degrees of oxidation and weathering processes. Acid generation from oxidation of pyrite exposed on parts of the pitwall has created a highly acidic pitwall rock material with serious limitations to plant growth. Past revegetation efforts on the pitwall have had mixed success. Whilst the non-pyritic sections of the pitwall have been successfully revegetated by hydroseeding with grass seed mixture, the pyritic areas of the pitwall have failed to sustain any form of vegetation (Orcgg and Stewart, 1 996). Large surface area exposed on steep pit slope (43°), low pH conditions created by accelerated pyrite oxidation and mobilisation of oxidation by­ products are considered primary factors restricting plant growth on the pitwalL Mining is a disruptive activity, especially so when it involves open-pit mining methods. The environmental impact of mining is now a global issue and the struggle to remediate AMD contamination is a challenging problem. Acid mine drainage poses a significant threat to the environment and a liability for sulphide mine operations, if not properly managed. Many countries in the world are bringing out newer legislation with stricter guidelines for environmentally responsible mining (so much so that in New Zealand, in an effort to project the country as a "clean green image", there is even stricter Resource Management Act ( 199 1 ) to control the mining agencies activities) . x OOllOtI'lINDEL x x x x X x. XX X )It. t If Figure 1 . 1 Location map, North Island, New Zealand. Plate 1 . 1 Martha Mine and tailings disposal site (1994), Waihi. 3 4 5 Most of the research on mine environments focuses on the development and testing of better tools for prediction and treatment of AMD problems with particular emphasis put on the generation, migration, and ultimate fate of mining-related contaminants in the environment. The development of a "safety-proof' treatment technology and remediation program however, represents the "holy grail" of much of the current research on mine waste. A "tug of war" exists between the regulators, reclamation experts, the environmentalists and mine operators . Regulators are faced with the overall problem of contamination of the environment from mining activity, mine operators envision the liability and expenditure of scarce resources, the "reclaimers" want expensive undertakings to rehabilitate the disturbed site, while the environmentalists want to close down the mine. Amidst these tensions lies the underlying fact that something must be done to reclaim the disturbed land to avoid the legacy of an abandoned mine, of which the Tui Mine at Te Aroha is a classic example (Morrel l, 1 997). The process of erosion and chemical evolution that develop into AMD conditions can have serious impacts on mine development and mine site rehabilitation. Pyritic pitwalls are dynamic environments where steep slope gradients facilitate rapid weathering and erosion, resulting in continued exposure and oxidation of pyrite and migration of AMD. Many of the current reclamation and remediation techniques have been used with a varying degree of success to rehabilitate mine wastes, coal mine spoils and tailings disposal sites. There is, however, an apparent lack of attention directed towards reclamation of mine pitwalls and therefore this area warrants further research in terms of how low pH conditions are generated and the applicability of amendments for vegetative establishment. 1.2 Objectives of this Study Past attempts to revegetate the pyritic pitwall at Martha mine have indicated that long­ term sustenance of vegetative growth is limited by the low pH conditions created by AMD. The general obj ective of this study was to highlight the importance of site knowledge for the reclamation of acid generating mine pitwalls with an aim to study the generation of low pH conditions limiting plant growth and to investigate the ameliorative effectiveness of some selected amendments. 6 The specific objectives were directed towards: • Detailed geochemical characterisation of the pyritic pitwall rock material • Spatial distribution of the geochemical properties on the pitwall . • Assessment of the l ime requirements to neutralise of pyritic pitwall rock. • Kinetic evaluation of the ameliorative effectiveness of selected amendments . 1 .3 Outline of this Research This thesis comprises eight chapters. Following introduction of this study in Chapter 1 , a literature review of acid generation processes and the remediation of materials resulting from weathering and oxidation of sulphides in hard rock mining activities is presented in Chapter 2. The results of the characterisation of the pitwall rock materials are presented in Chapter 3 and include mineralogical studies, geochemical analysis, static and kinetic predictive tests and spatial distribution of the geochemical properties limiting revegetation on the pitwall . An incubation study to evaluate the liming requirements of the pitwall rock material and effectiveness of selected neutralising materials to ameliorate low pH is presented in Chapter 4. This is followed by kinetic evaluation of the comparative effects of these neutralising materials on leachate and subsurface chemistry of the pitwal l rock under broadcasted and incorporated methods of amendments, in Chapters 5 and 6. An investigation of the effects of topsoil as cover material and application of a commercial bactericide to reduce acid generation are presented in Chapter 7. A general summary and implications of this research are presented in Chapter 8 . 7 C h a p t e r 2 Literature Review: Mined Land Reclamation 2.1 introduction This chapter outlines an overVIew of the problems associated with reclamation of sulphidic mine waste materials applicable to pitwall revegetation. The significance of the processes leading to acid mine drainage (AMD) and its impact on the substrate environment is highlighted. The critical role of predictive techniques in optimising the prevention, control and treatment of AMD is examined. Establ ished, innovative and pre­ commerc ial amendment methods are reviewed with particular reference to alkaline materials used in AMD mitigation and their effective performance. Reclamation practices pertinent to revegetation of pitwall are reviewed and conclusions are drawn with implications for the direction of this research. 2.2 Acid Mine Drainage (AMD) Acid mine drainage (AMD l ) from both active and abandoned mme sites IS a major environmental issue for the mining industry in environmentally concerned regions of the world (Caruccio, 1975; Ritcey, 1989 ; Gray and Sul livan, 1995 ; Taylor, 1997) . The term AMD is used to describe any seepage, leachate or drainage affected by the oxidation products of sulphide minerals when exposed to air and water . Both chemical reactions and biological acti vities are recognised as responsible for generating AMD ( Kleinmann and Crerar, 1979; Nordstrom, 1982). The AMD is typical ly characterised by low pH and high levels of dissolved metal salts, as well as high concentrations of acidity, sulphate, iron and other metals (Caruccio, 1975). Once the AMD process begins it is difficult to control, often accelerates and is likely to persist for decades or centuries. In the absence of neutral ising materials (carbonate minerals such as calcite or dolomite), the AMD may contain toxic levels of metals such as Fe, AI, Mn, Cu, Pb, Zn, Cd etc. which can cause serious environmental problems in soil and water systems (Sengupta, 1994) 1 In Australia and Pacific regions AMD is the standard term used to refer to problems associated with the development of acidic drainage at sulphide-rich rock surfaces exposed by mining and other industrial activities and involves a complex combination of chemical and biological reactions. The same phenomenon is described as acid rock drainage (ARD) in North America (Murray et aI., 1 995). 8 Regulations in many countries have developed ways to address the issue of AMD at the permitting stage rather than as an afterthought. There are signs that some operators, working in partnership with regulatory authorities and other stakeholders, are developing proactive methodologies based in part upon improvements in predictive techniques (Taylor et al . , 1 997). Many of the currently practised preventive measures, however, still require extensive field validation before they can be prescribed as standard techniques in the environmental management of AMD. Since it is now a recognised fact that AMD generation is a site-specific phenomenon, effective control and treatment measures have been directed towards fulfilling the problem on site rather than to providing universal solutions. 2.3 Sources of AMD There are five major sources of AMD, namely drainage from underground workings; run-off and discharges from open pits; waste rock dumps; tailings and ore stockpiles. The sources may be locally significant, for example spent heap-leach piles, stockpiles of segregated sulphides and natural seeps and springs in areas of sulphidic mineral isation. In general terms, potential sources of AMD in mining activities are well understood and widely documented (Bloomfield, 1 972; Dent . 1 986; Doyle, 1 990; B rodie et al . , 1 99 1 ; Broughton and Robertson, 1 992; Bhole, 1 994 ; Durkin and Herrmann, 1 996; Robertson, 1 996) . Abandoned underground workings can result in the release of high concentrations of metal salts into the aqueous environment as the water table rebounds and the workings flood. These metal salts accumulate when the mine is pumped "dry" and in-place sulphides are exposed to oxygen and moisture. AMD generation may continue even after flooding if there is a persistent source of dissolved oxygen. Open pit mining can expose very large areas of sulphide-bearing rock to air and water. Failure to control water flow into open pit slopes can result in large volumes of AMD. As oxidation of the sulphides proceeds, fresh sulphides may be exposed by spalling of the rock face , resulting in the constant renewal of the AMD source (Kuyucak et al . , 1 99 1 ; Morin and Hutt, 1 995) . 9 Waste rock has become a more significant threat as open pit nunmg has replaced underground mining, particularly in developed countries, and the volumes produced have increased (Morin, 1990) . The highly permeable coarse nature of the waste rock facilitates rapid oxidation of sulphides after disposal . During extended dry periods, dumps may build-up "stored" acid products and salts through evaporation and supersaturation processes which are then released in the form of highly contaminated AMD during the next significant precipitation event (USEPA, 1994) . Tailings often have a high sulphide content (mainly in the form of rejected pyrite, marcasite and pyrrhotite) and are much finer than waste rock. Although tailings have a much higher specific surface area than waste rock, the uniform and fine particle size leads to a much lower permeability than that seen in waste rock piles. Therefore, the increased surface area available for oxidation and leaching reactions is balanced by reduced contact with oxygen due to saturation by a relatively static water body. Consequently, tailings often generate AMD more slowly than coarser, but more permeable, waste rock (Filipek et aI , 1996) . 2.4 Biochemical Aspects of AMD Formation Numerous workers have studied the reaction kinetics of pyrite oxidation (S inger and Stumm, 1970; Walsh and Mitchell, 1972; Kleinmann and Crerar l 979; Nordstrom, 1982 ; Williams et al . , 1982; Dent, 1986; Caruccio et aI . , 1988; Ritcey, 1989; Evangelou and Zhang, 1995). In simpl icity, acid generation involves a complex combination of organic and inorganic processes and reactions. This happens when reactive sulphide rock (eg. pyrite) is initially exposed to air and water. The extent and duration of acid generation depend on the intrinsic geochemical characteristics of the sulphidic rocks . The generation of acid in the oxidation of pyrite is considered to involve abiotic and biotic processes under aerobic environments. The former i s catalysed by ferric iron (Fe3+) while the latter is catalysed by acidophilic bacteria, chiefly Thiobacillus ferrooxidans. The role of bacteria in AMD generation from oxidation of pyrite has been recognised and studied by many workers (Kleinmann and Crerar; 1979; Williams et al . , 1982; Bruynesteyn & H ack!, 1984) . Both abiotic and biotic processes are considered pH and temperature dependent. The net generation of acidity in any sulphidic substrate will 1 0 ultimately depend o n the host rock mineralogy, bio-geochemical and environmental factors. The pyrite crystal morphology and degree of liberation of grains are important factors affecting oxidation rates (Mills , 1 996). Generation and migration of AMD are complex subjects and intimately linked with the nature of the ore body, host rock mineralogy and local and regional hydrology and hydrogeology. The occurrence of AMD does not necessarily lead to its migration as there are a number of chemical processes that prevent the movement of AMD away from its source (Doyle, 1 990; Sengupta, 1 994; Gray and Sullivan , 1 995; Sherlock et aI. , 1 995; B owell e t aI, 1 996). The main points of AMD forming processes are summarised , briefly below. 2.4.1 The chemistry of acid generation from pyrite oxidation The dominant sulphide mineral involved in the development of AMD in sulphidic mine waste rocks are commonly pyrite (FeS2) , marcasite (FeS2) and pyrrhotite (FeS) . Other sulphide minerals such as chalcopyrite (CuFeS2), Chalcocite (CU2S) , covellite (CuS) , pentlandite [ (Fe,Ni)8S9] , arsenopyrite (FeAsS), stibnite (Sb2S3) , molybdenite (MoS2), sphalerite (ZnS) and galena (PbS) are also likely to make a secondary contribution to sulphide oxidation and contribute towards acid generation (Jambor and B lowes, 1 994; MVTI, 1 994 ; Mills , 1 997), although PbS, ZnS and bornite (CuSFeS4) are considered non-acid generating (Bmynesteyn and Hackl, 1 984; Kwong, 1 995) . These non-pyritic sulphide minerals are however, likely to be subjected to direct chemical attack by FeJ+ generated from oxidation of associated pyrite minerals or indirect oxidation by thiobacilli resulting in the generation of significant amount of acid (MVTI, 1 994; Kwong, 1 995). The biochemistry of pyrite oxidation and its products have been studied in detail and described by many workers (Harnsen, 1 954; Le Roux et aI . , 1 974; Nordstrom, 1 982; Backes et aI . , 1 986 ; Chander and Briceno, 1 988 ; Camccio et aI . , 1 990; Arora et aI . , 1 992; Evangelou and Zhang, 1 995; Ciminelli and Osseoasare, 1 995; Taylor and Thornber, 1 995 ; B ronswijk and Groenenberg, 1 996) . The basic chemistry of AMD generation from oxidation of pyrite is summarised in the following equations. 2 FeS2 + 7 O2 + 2 H20 ---7 2 Fe2++ 4 S042- + 4 H+ [1] 2 Fe2+ + 112 O2 + 2 H+ ---7 2 Fe3+ + H20 [2] FeS2 + 14 Fe3+ + 8 H20 ---7 15 Fe2+ + 2 S042- + 16 H+ [3 ] Fe3+ + 3 H20 ---7 Fe(OH)3 + 3 H+ [4] FeS2 + 15/4 O2 + 712 H20 ---7 Fe(OH)3 + 2 H2S 04 [5 ] 1 1 Ferrous iron (Fe2+) is initially released by the oxidation of FeS2 (Eqn.1) . After the sequence has been initiated, a cycle is established in which Fe2+ is oxidised by oxygen to Fe3+(Eqn.2). FeS2 is subsequently oxidised by Fe3+, generating additional Fe2+ and acid (Eqn.3) . Certain conditions can cause rapid oxidation and hydrolysis of Fe3+, resulting in generation of further acidity (Eqn.4). The forward reaction in Eqn.4 depends on the rate of production of Fe3+ in Eqn. 2 and the reaction in Eqn . 2 is therefore the rate l imiting step in the generation of acid in sulphidic ores (Nordstrom, 1982). The principal products of the overall pyrite oxidation are therefore, ferric oxyhydrox ide and sulphuric acid (Eqn.5) . The rate controlling factor in pyrite oxidation is thus the availability of Fe3+ ion or the products of its hydrolysis and secondary reactions, Fe(OHh and Fe(S04h The hydrolysis of the soluble hydrous metal sulphates formed during the oxidation reactions and formation of ferric oxyhydroxides will result in the net production of acidity (Backes et aI . , 1986; Caruccio et aI . , 1988). Pyrite oxidation in the absence of Fe3+ is relatively slow (Eqn. l ) . The rate of abiotic oxidation of Fe2+ to Fe3+ (Eqn.2) is also slow in acidic environ ments. However, the presence of certain catalysing bacteria (eg: Thiobacillus ferrooxidans) can increase the rate of Fe2+ oxidation by as much as a factor of 106 (Singer and Stumm, 1970; KJei nmann and Crerar, 1979; Evangelou and Zhang, 1995 ), pushing Eqn. 2 to the right. The bacteria Thiobacillus ferrooxidans is considered to be the most important organism involved in the biochemical oxidation of sulphide minerals (LeRoux et aI . , 1974; Will iams et aI., 1982; Bruynesteyn and Hackl, 1984). B iologically catalysed oxidation of pyrite leads to the production of ferrous iron, sulphate and hydrogen ions. Most of the H+ generated from the oxidation of pyrite is spent on the subsequent oxidation of Fe2+ to Fe3+. Catalytic oxidation of pyrite predominantly occurs at pH < 4 because Fe3+ is only soluble and Thiobaccillus ferrooxidans activity is optimum under 1 2 these acidic conditions (Dent, 1 986). Both pH and temperature control the biological oxidation rate: the optimum pH range being 1 . 5-3 .5 and temperature range of 30-35°C (Roman and Benner, 1 973 ; Caruccio et aI. , 1 988 ; Ritcey, 1 989) . The pH also has a significant effect on the oxidation rate of ferrous ion. For example, Hutchison and Ellison ( 1 989) indicate that at pH 7, the oxidation of Fe2+ to Fe3+ happens within minutes while it takes about 300 days at pH 4.5 and about 1 000 days at pH 3 .5 (Roman and Benner, 1 973) . Acid formation occurs due to hydrolysis of ferric sulphate (Nordstrom, 1 982) . Ferric ions produced by the oxidation of pyrite usually either precipitate as ferric oxyhydroxides if the pH is sufficiently high, or serve to oxidise other metal ions and become reduced again to supply additional substrate for microbial growth. The rate of pyrite oxidation by bacteria depends on the amount and form of pyrite, pyrite activity, O2 and CO2 availability, pH, temperature and presence of inhibiting compounds (Le Roux et aI . , 1 974; Nordstrom, 1 982; Ritcey, 1 989) . The Fe3+ generated is a very strong oxidant, and will readily attack pyrite (generating acidity and Fe2+) and oxidise other metal sulphides (releasing the metals into the acid aqueous phase) while itself being regenerated by biotic and abiotic reactions. The dissolution of non-pyritic minerals can be accelerated by galvanic interactions causing preferential dissolution in acid solutions (Doyle, 1 990; Kwong, 1 995) , possibly enhanced by the presence of certain bacteria which continuously oxidise the layer of elemental sulphur formed on the pyrite surface that would otherwise prevent galvanic action (Gray and Sullivan, 1 995). Sulphate concentrations can often also rise to high levels . Because of these reinforcing "feedback" loops, AMD generation is considered autocatalytic . Once the process has started, it can be very difficult to halt (Doyle, 1 990) indicating the importance of taking preventative measures. An overall generalised acid production cycle for pyritic mine waste i s shown in Figure 2 . 1 . AMD generation depends primarily on the abundance or ratio of acid producing sulphide minerals to acid neutralising alkaline mineral s . The presence or absence of alkaline material is general ly regarded as the most critical element in determining AMD characteristics (Caruccio et aI . , 1 988) . The degree of acidity and concentration and 1 3 speclatlOn of dissolved contaminants vanes according to a number of site-specific factors, but typical contaminants include Fe, Mn, AI , Cu, Pb, Zn, Cd, As, sol- and Cl (Herr et aI . , 1 996) . Less common dissolved components may also be present, depending on localised and regional mineralogy. Dissolved concentrations can range from below the limits of detection up to thousands or tens-of-thousands of mil l igrams per l itre while pH can vary from near neutrality down to one and below. FeSl + O2 + H2O Abiotic �� B iotic I Acid + SO/ + Fe2+ I � O2 + H2O I FeSl + H20 I I Acid + Fe(OHh I ... '" Fe2(S04h = D AMD Figure 2 . 1 A simpli fied cycle in the generation of AMD 2.4.2 Accretion and migration of AMD The processes that occur in the accretion and migration of AMD are controlled by both physical and chemical factors. Local environmental conditions no doubt play an important role in the accretion and migration of AMD. The degree of l iberation of pyrite grains, the amount of rainfal l infiltration, sulphide rock permeability, availabi li ty of pore water pressure and migration mechanism of AMD (surface flow, capil lary flow, discreet seepage) are important physical characteristics. 1 4 The migration and fate o f many o f the dissolved metals and metallo ids present i n AMD is significantly affected by adsorption by, or co-precipitation with, the various iron compounds that may form under favourable conditions . In general , the iron species that precipitates is determined by the iron concentration, redox potential , pH, concentration of complexing anions such as carbonate, sulphate and sulphide and partial pressure of CO2 and O2 (Schwertmann and Taylor, 1 989) . For example, Fe 3+ is rapidly hydrolysed, even at relatively low pH, to form iron oxyhydroxides (ochre) . These ochres can contain significant concentrations of metals through co-precipitation and adsorption (Bloomfield, 1 972 ; Bowell et aI , 1 996; ) . S imilarly, aluminium hydroxide can play an important role in the adsorption and precipitation of other metals . In relatively dry environments, various iron sulphates may crystall ise; if Fe2+ is oxidised to Fe3+, basic sulphates and oxyhydroxides may form. In low pH, high sulphate environments, jarosite (KFe3(S04)2(OH)6) may form while at higher pH and lower sulphate concentrations, iron oxide species such as goethite and hydroxides may precipitate (Herbert, 1 995) . These minerals have a large capacity to adsorb non-ferrous metals and metalloids, and will also remove metals from solution by co-precipitation processes. Such reactions are considered capable of preventing the dispersion of dissolved contaminants (Sull ivan and Yelton, 1 988) . The other mam chemical control on AMD accretion IS the neutralising effect of minerals such as carbonates and silicates . However, even if the host rock contains significant quantities of acid-neutralising minerals, the rate of acid generation may exceed that of neutralisation, leading to AMD accretion. The rate of neutral isation is influenced by pH, the partial pressure of CO2, temperature, mineral composition and structure, redox conditions and the presence of impurity ions in the neutral ising mineral 's lattice (Sherlock et aI, 1 995) . The most important factor in determining the extent of AMD is not the pH, but the total acidity (Erickson and Hedin, 1 988 ) . Total acidity is a measure of the excess amount of H+ over other ions present in the solution. This takes into account the neutralisation of the acid by other materials present. Usually, however, a high acidity is accompanied by a low pH in the AMD. Consequently, a low pH has detrimental effects on the bicarbonate buffering system. The concentrations and mobility of metal ions in AMD are important aspects in the assessment of the environmental impact of mining base metal sulphide deposits. Metal 1 5 concentrations mobilised in soil and water exert a strong influence on vegetation since plant uptake of heavy metals is considered a function of element speciation (Dunn, 1 989). The distinction between "total" metal and "available metal is critical to the understanding of potential contamination/pollution problems. Although total metal concentrations in various forms can have l ittle impact on the environment, bioavailability of dissolved metals in phytotoxic levels can be a limiting factor to plant growth (Lindsay and Norvell , 1 978; Luo and Rimmer, 1 995) . A clear understanding of the accretion of AMD, its migration and metal loading can be a powerful tool in planning preventative or control measures. Natural processes such as attenuation and neutralisation by alkaline minerals present can go some way toward mitigating the impacts of AMD. Long-term preventative measures and effective treatment prescriptions adapted to site-specific criteria accordingly may be viable options for mitigating low pH environment created by AMD. 2.4.3 By-products associated with AMD Pyrite oxidation and its oxidation by-products are as much environmental problems as AMD itself. Sulphide oxidation and migration of its by-products have been studied mainly on coal overburden using complex geochemical and numerical models (Vanberk and Wisotzky, 1 995; Wunderly, et aI . , 1 996). However, the overall products of pyrite oxidation would be similar for hard rock mine waste materials as well . The primary reaction products resulting from biochemical oxidation of pyrite are H+, Fe2+, Fe3+, SO/- and Fe(OH)3 . The Fe(OH)3 remains in solution as long as the pH of the AMD is <4. At thi pH, most of the metals ions (Fe, Mn, Al, Cu, Zn, Pb, As etc . ) remain mobilised in AMD (Nordstrom, 1 982; Wunderly et aI . , 1 996). If the pH of the AMD is increased by contact with carbonate minerals ( if present) or entry into water system of higher pH, a number of secondary products (metal hydroxides and sulphate salts) can form under surficial conditions resulting in considerable build up of reserve acidity (Wunderly et aI . , 1 995) . 2.4.3. 1 Metal hydroxides When the pH AMD increases to >4, metallic ions such as Fe3+, AI3+, Cu2+, Zn2+, Pb2+ and As3+ will react with hydroxyl ions to eventually form hydroxides as precipitates by the general reaction:- Mn+ + nOH' H M(OH)n where M = metal ion, OH' = hydroxyl 1 6 ion and M(OH)n = metal hydroxides, n = oxidation state . Fe-oxyhydroxides [ (FeO.OH) and Fe(OHhJ precipitate directly by oxidation of dissolved Fe2+ (Backes et al . , 1 986; Dent, 1 986) . These highly insoluble amorphous Fe-oxydroxide gels often precipitate as rusty-brown or yellowish surface expression in areas of intense sulphide mineral oxidation or as "ochre" in pore fillings (Tucker et al . , 1 987 ; Taylor and Thornber, 1 995). The formation of ironstone (ferricrete) from cementing of unconsolidated surficial material by oxyhydroxides in fracture zones and slope surfaces of weathering iron sulphides is generally indicative of natural AMD (Taylor and Thornber, 1 995) . 2.4.3.2 Sulphate salts Associated with the acid environment are elevated levels of soluble salts which solubilise during wet periods and severely affect plant growth on the reclaimed mine waste. In the presence of carbonate minerals such as calcite (CaC03) and dolomite (CaMgC03) , the sulphate salt most commonly formed i s gypsum (CaS04.2H20), which may precipitate copiously on surface during dry periods . Unlike iron, which remains immobil ised in the waste rock substrate, most of the soluble sulphate salt is leached with the AMD and only a fraction of sulphate may be retained as gypsum, jarosite [(KFe3(S04h(OH)6] , alunite [(KAh(S04h(OH)6] and mallardite (MnS04.7H20) (Kerth and Wiggering, 1 990; Herbert, 1 995; Wiggering, 1 977) . Under very low pH (pH<3 .7) and strongly oxidising environments (Eh>400 m V), characteristic pale yellow deposits of minerals in the iron and aluminium jarosi te­ natrojarosite range [ (K,Na)(Fe,Al)3(S04)2(OH)6] commonly precipitate as pore fillings and coatings on exposed surfaces (K wong and Huang, 1 979; Dent, 1 986). J arosite is unstable at higher pH where it is hydrolysed to iron oxide (goethite) , releasing further acidity (Bloomfield, 1 972). Thus neutralisation with alkaline amendments is l ikely to effect dissolution of jarosite and formation of Fe-hydroxide. Since the precipitation of iron hydroxide leads to formation of free acidity, jarosite acts as a storage sink for the acidity formed by pyrite oxidation and weathering (van Breeman, 1 973 ; Kerth and Wiggering, 1 990). The formation of efflorescence minerals such as jarosi te (KFe(S04h.2Fe(OH):;) and melanterite (FeS04.7H20), which usually occur as surface encrustation during dry periods, will through hydrolysis result in the release of acidity to the ambient AMD (Eqns. 6 and 7) . This is considered to be the reason why in the 1 7 presence o f jarosite, mine waste dumps remain acidic long after the cessation o f pyrite oxidation (Dent, 1 986; Kwong and Ferguson, 1 997). 3Fe2(S04)3 + 2KOH + l OH20 � 2K[Fe(S04h.2Fe(OH)3] + 5H2S04 [6] 2FeS2 + 702 + 1 6H20 � 2FeS04.7H20 + 2S0/- + H2S04 [7] Accumulation of acid in pyritic waste materials also facilitates rapid chemical degradation, leading to progressive decomposition of clay minerals and extreme acidification of the spoil can have a detrimental impact on the wider environment through salinisation and contamination of ground waters. 2.4.3.3 Acidity Acidity is the base-neutralising capacity of the solution, which results from H+, Ae+, Fe3+, Fe2+, Mn2+, and other hydrolyzable cations in a sample (Hedin et aI . , 1 994; Cravotta, 1 995 ; Hedin and Erickson, 1 998) . In polymetallic base metal deposits, Cu2+, Zn2+ and Pb2+ can also contribute significantly to total acidity of the material (Skousen, 1 995) . Acidity data is therefore useful in defining AMD, which commonly contains substantial quantities of Fe2+, Mn2+ and A13+ (and other hydrolyzable cations), the oxidation and hydrolysis of which can have significant affect on pH of the mine drainage. Oxidation and hydrolysis of Mn2+ is known to occur at circumneutral pH 6-7 and therefore can contribute significant acidity even in AMD with neutral pH (Hem, 1 963) . In fact, Fe(OH)3 can be directly involved in the heterogeneous adsorption and oxidation of Mn2+ to Mn02. Hedin et al .( l 994) have indicated that in the absence of acid neutralising materials, a theoretical total acidity of the mine drainage containing significant amount of Fe2+, Mn2+ and A13+ can be calculated from the equation: Acidity (mg CaC03 L·I) = [50 * (2*Fe/56) + (2 * Mn/55) + (3 * A1/27) + 103 * ( 10exp-pH)] (Metal concentrations are in mg L· I , converted to cmole kg· l ) Thus, the emerging AMD containing abundant Fe2+ and Mn2+ can have pH -6, but it may also have significant acidity. Alternatively, if the AMD contains low concentrations of dissolved metals (hence low acidity) , a very low pH does not necessarily mean high acidity. In other words, just knowing the pH of the AMD does not really tel l you much about the inherent acidity of the solution. This has important 1 8 implications for treatment and amendment of low pH conditions i n oxidising pyritic materials . For example, a material with a pH of 2, but very low metal concentrations maybe neutralised more readily than a material with a pH of 5 and high metal concentrations . Acidity determination IS, therefore, very important for assessing the buffering capacity of the material. 2.4.3.4 Aluminium Dissolution of aluminium (AI) in acid soils is one of the most pronounced soil chemical effects of AMD. Because of its toxicity to plants, the chemical behaviour of Al has been a major research topic for decades of agronomic and environmental studies (Mulder et al . , 1 989). Low pH conditions promote solubility of Al and result i n the concentration of phytotoxic levels of Al in the soil solution (Conyers, 1 990; Aitkin, 1 992; Helyar et aI . , 1 993) .The distribution of aluminium in acid soil materials is dependent on the forms of inorganic and organic anionic ligands present and the relative competition between Al and other cations for ligands. There is increasing evidence that the phytotoxicity of aluminium is reduced in the presence of inorganic and organic complexing anions (Bloom and McBride, 1 979; Kwong and Huang, 1 979; Young and Bache, 1 985 ; Hue et aI. , 1 986; Gurung et aI . , 1 996). Possible inorganic ligands with tendencies to form polymeric complexes with solution Al are OH, S04, F, Si (OH)4 and H2P04 (Ritchie, 1 989) and these ligands have been found to detoxify Al (Helyar, 1 978 ; Blarney et aI . , 1 983, Alva et aI . , 1 986, Cameron et aI . , 1 986). Soluble AI-hydroxy phosphate polymers were found at a pH region of 4. 1 -4.8 and at P concentrations <25 /-lM (Blarney et aI . , 1 983) . At pH values <5.5, silicate ions are found to form stable complexes with AI-hydroxy polymers (Luciuk and Huang, 1 974). The AI-hydroxy silicates are considered a major cause of the Al and pH buffering in acid soils (Bloom and McBride, 1 979). The effect of Al adsorption on pH depends on the type of clay mineral present and the OH:AI ratio in the solution. Thus removal of hydrolysed Al species from solution would induce further hydrolysis of Al and therefore a lowering of pH is expected (Bache, 1 974). Manganese (Mn) in soil is considered to behave similarly to Al in that i t may be adsorbed onto the surface of hydrous oxides, clay particles and organic matter, or exist as discrete Mn compounds. However, it differs from Al because i t exists in more than one oxidation state under conditions naturally found in soils . In very acid soils, 1 9 however, Mn 2+ may be the dominant specIes and competes with other cations for exchange sites (Walker and Barber, 1 960; Salcedo et aI . , 1 979). 2.4.4 In-situ neutralisation of acidity In sulphidic deposits, acid generating minerals such as pyrite often occur m close association with acid neutralising minerals such as calcite (CaC03) and dolomite [CaMg(C03)2] that normally occur as late stage mineralisation. The acid produced from pyrite oxidation is neutralised, in-situ, by dissolution of these basic minerals if in contact with the migrating AMD, resulting in precipitation of sulphate salts (Eqns. 8 & 9) . These chemical reactions must also be examined to fully understand the processes occurring and to be able to predict the chemistry of solutions resulting from the combination of oxidation and neutralisation processes (Blowes et aI . , 1 994; Morin et aI . , 1 995) CaC03 + H2S04 � CaS04 + H20 + CO2 [ 8] CaMg(C03h + 2H2S04 � CaS04 + MgS04 + 2H20 + CO2 [9] Sil icate minerals such as plagioclase fe1dspars also have the potential to neutralise acid under specific pH conditions (Ritchie, 1 994; Mills, 1 997), by the dissolution reactions listed below (Eqns. I O- 1 4) . In acidic conditions, the silicate minerals rapidly decompose clay products as well as releasing significant amount of A13+ and K+ in the soil solution. The dissolution of Fe(OH)3 is also considered acid consuming reaction (Eqn. 1 5) . Muscovite dissolution KAh[AlSi30IO] (OHh(s) + H+ + 3/2H20 � K+ + 3/2AhSi20s(OH)4 Biotite dissolution [ 1 0] KMgl .sFe 1 .5AIShOIO(OHh + 7H+ + 1 I2H20 � � + 1 .5Mg 2+ + 1 .5Fe2+ + H4Si04 + 1 I2A}zSi20s(OH)4 Albite dissolution NaAlSi308 + H+ + 912H20 -> Na+ + 2H4Si04 + 1 I2AI2Si20s(OH)4 Anorthite dissolution CaA}zSi208 + 2H+ + H20 � Ca2+ + A}zSi20S(OH)4 [ 1 1 ] [ 1 2] [ 1 3 ] 20 K-feldspar dissolution KAlSi30s + H+ + 9/2H20 ---7 K+ + 2H4Si04 + 1 I2AhSi20S(OH)4 Iron oxyhydroxide dissolution Fe(OH)3 + 3H+ ---7 Fe3+ + 3H20 [ 1 4] [ 1 5] The in-situ neutralisation processes described above highlights the fact that mineralogy of the pyritic rocks is a key factor in defining the composition of the AMD and emphasises the importance of mineralogical characterisation. The amelioration of AMD in pyritic materials is primarily based on the neutralisation of the acid as it occurs naturally in-situ conditions. 2.5 Predictive Techniques for AMD The ability to predict acid generation from mine waste materials is an important step in the process of preventing AMD. This allows the appropriate control measures to be taken to prevent/mitigate the formation of AMD. Predictive tests specifically designed for coal mine waste have been used for decades and significant advances in predictive techniques applied to hard rock metal mine waste have been made in the last 5 to 1 0 years (Sobek et al . , 1 987 ; Skousen et al . , 1 990; Miller e t aI, 1 994). The prediction of acid producing potential (APP) of materials begins with an understanding of the geology and geochemical properties of the rock type encountered during the mining process (Miller and Murray, 1 988) . Several laboratory and field test procedures to assess or predict the generations of AMD are in use or have been proposed. These include: geochemical static tests, geochemical kinetic tests, mineralogical and petrologic studies (J ambor and Blowes, 1 994; K wong and Ferguson, 1 997) , mathematical and geochemical modelling (Ritcey, 1 989 ; Sengupta, 1 994; White et al . , 1 994) and remote sensing (Paterson and S tanton-Gray, 1 995) . Geochemical static and kinetic tests form the most commonly used techniques currently used for prediction of acid generation from mine waste rock materials . To be useful , the techniques must firstly predict whether a particular mine waste will generate acid at some time and, if so, the rate at which it will occur and secondly , it should predict the characteristics of the drainage leaving the mine waste, from both controlled and 2 1 uncontrolled sites. The current static and kinetic testing procedures used are described below (sections 2.5 . 1 & 2 .5 .2 respectively) and their merits evaluated in section 2 .5 .3 . 2.5.1 Static tests The static test is generally the first step in the analysis of acid generation potential. It is based on an acid-base accounting (ABA) procedure whereby the acid neutral ising capacity (ANC) and acid producing potential (APP) of the samples are determined, and the difference, net acid producing potential (NAPP) is calculated. It serves as a screening process to categorise materials into potentially acid generating, potentially non-acid generating and uncertain groups. There are several different static test procedures, all serving the basic requirements of ABA (Table 2 . 1 ) AEA remains the most widely used screening test procedure for AMD prediction. It is based on the total sulphide S content and acid neutralising capacity (AN C) of a sample. The APP of the material is estimated stoichiometrically by assuming ideal oxidation of pyrite (Eqn.5) and the equivalent CaC03 required to neutralise the potential acidity produced from the oxidation of 1 mole of pyrite (Eqn . 1 6) . [ 1 6] According to the equation 5 , one mole of pyrite contains 64 g of pyritic S which will theoretically produce 4 moles of acidity (H+), equivalent to 2 moles of CaC03 or 200 g CaC03 equivalent acidity i .e. 1 g S will potentially produce acidity equivalent to 3 . 1 25 g CaC03. A sample containing 1 % pyritic S (sulphide-S), therefore, will have an APP of 3 1 .25 kg CaC03 fl . The ABA of a sample is calculated according to the following relationship: APP (kg CaC03 ( 1 ) = Total sulphide-S (%) x 3 1 .25 . The ANC « kg CaC03 fl) of the pyritic material is determined by chemically digesting a pulverised sample with dilute acid (HCI) and the solution back-titrated to a predetermined endpoint (pH 7 or 8 .3 ) with a standard base (NaOH) to determine the amount of acid consumed by the material. The ANC value is subtracted from the APP to derive NAPP where: NAPP (kg CaC03 ( 1 ) = APP - ANC. Lime requirement for sulphidic mine waste is generally based on this NAPP value. 22 Table 2 . 1 ABA procedures PROCEDURE Reference Sobek Neutralisation Potential Method Sobek et al. ( 1978); Skousen et al. (1 997). and Peroxide Siderite Correction for Sobek Method BCRI Initial Test Duncan & Bruynesteyn ( 1 979). BCRI Confirmation Test Bruynesteyn & Hackl (1 984). Coastech Research Modified B iological Lawrence & Sadeghnobari (1 986). Oxidation Test Lawrence et al. ( 1 989). Lapakko Neutralisation Potential Lapakko (1994). Modified Sobek Neutralising Potential Coastech Research ( 1991); Lawrence & Method Wang (1 997). Static Net Acid Generation (NAG) Lawrence (1990); Miller ( 1 996); Miller et al. ( 1997) . .... Paste pH Sobek et al. (1978); Price ( 1 997). Alternatively, the NAG test procedure based on the accelerated oxidation of a sample by H202, and measuring the resultant pH, predicts the acid generating nature of the material if the pH of the solution (NAGpH) is < 4 (Miller & Jeffrey, 1 995) . However, the ratio ANC:APP is now more often used to determine the potential for AMD generation, with incorporation of a factor-of-safety into the ratio to reduce risks arising from unknown parameters . For example, Placer Dome Inc. uses the following ANC:APP ratios (Table2.2) to screen samples (Robertson and Ferguson, 1 995) . Table 2 .2 Screening criteria in ABA ANC/APP < 1 Likely to generate AMD unless sulphides are unreactive ANC/APP 1 -2 Possible AMD generation if neutralising minerals are preferential ly depleted, coated or otherwise unreactive ANC/APP >2 AMD generation not expected A still higher ANC:APP ratio may be warranted to cover the situation where the dissolution rate of acid neutralising minerals is generally low compared with the rate of pyrite oxidation. If an apparently "safe" ANC:APP ratio overestimates the neutralisation 23 capacity or rate, then there can be severe environmental and ultimately cost implications in terms of both site operation and closure (Sherlock et aI, 1 995). In the Australasia and Pacific regIOns, both NAPP and NAG test procedures are commonly used as initial screening and monitoring tools for predicting acid generation from mine waste. Currently both the North American and the Australian terminology and units expressing the results of ABA testing are in use. Both procedures give the desired end results, as compared in Table 2.3 below (Murray et aI . , 1 995). Table 2.3 Comparisons of ABA procedures Australia Acid Producing Potential (APP) Acid Neutralising Capacity (ANC) Net Acid Producing Potential (NAPP) (NAPP = APP - ANC) Result Interpretation: Potential AMD, NAPP = positive (+ve) Standard units kg H2S04 ( I Note: I k g H 2S04 ( I = 0.98 k g CaC03 C l 2.5.2 Kinetic tests North America Acid Potential (AP) Neutralisation Potential (NP) Net Neutralisation Potential (NNP) (NNP = NP - AP) Potential ARD, NNP = negative (-ve) Standard units, kg (t) CaC03 ( I For materials where the potential for acid generation i s uncertain, kinetic test work is performed to attempt to define acid generation characteristics. In kinetic test procedures, the acid generation (and metal mobilisation and transport) characteristics of a sample are measured with respect to time. Kinetic test procedures include humidity cells (Sobek et aI, 1 978 ; Sullivan and Sobek, 1 982; ASTM, 1 996; Price, 1 997; Soregaroli and Lawrenc, 1 997), column tests (Sturey et aI . , 1 982; Price, 1 997), soxhlet reactors (Sullivan and Sobek, 1 982), Shake Flasks (Bruyensteyn and Hackl, 1 984), field lysimeters and test plots (Eger and Lapakko, 1 98 1 ) and barrel tests (Kalin et al . 1 995). Miller and Jeffrey ( 1995) suggested that the static net acid generation test (NAG) procedure could also be considered as a kinetic test as the data generated provide information on the kinetics of the reaction from the short-term simulation of the weathering process. A development stage procedure, "Minewall" is also being tested for 24 determining the kinetic AMD characteristics of in-situ rock such as pitwal ls and the rock surfaces of adits, stopes and other underground workings (MEND, 1 995). Currently there has been l ittle standardisation of kinetic testwork procedures with regard to sample preparation, cell or column design and operation or data reporting. Most kinetics tests involve weathering of samples under laboratory controlled conditions to simulate time dependent chemical changes in the mine waste and determine the potential to generate net acidity, the rates of sulphide oxidation and neutralisation and the quality of the leachate/drainage. The most commonly used laboratory scale kinetics test procedures are humidity cells and columns. Humidity cells are typical ly laboratory units m which samples are subjected to accelerated weathering by cyclic permeation of dry and humid air fol lowed by flushing with water (Caruccio et aI . , 1 990; ASTM, 1 996) . The test usually determines if a given sample will generate acidity but not when the material will produce acidity since the cells undergo accelerated oxidation of sulphide minerals (Price, 1 997). The accelerated rate of oxidation processes will thus result in an accelerated rate of generation of oxidation products as dissolved metals and/or precipitated metal compounds. The humidity cell tests are not designed to provide leachates that are similar to the actual leachate produced in field conditions and therefore are not intended to simulate site­ specific leaching conditions (ASTM, 1 996; Guard, 1 997) . The humidity cell tests were ori ginally developed for coal overburden in which the lag period is shorter and then run for only 1 0 weeks. For hard rock materials, the lag periods are longer and a 1 0 week period is not adequate to give accurate results (Miller and Jeffrey, 1 995). Column testwork is generally conducted to simulate the leaching effects of precipitation infi ltration into, and drainage from, material exposed to the atmosphere. The aim is to monitor leachate quality with time by cyclic flushing with water to simulate seasonal variations at site. Unlike humidity cells, there is no standard testwork procedure and the operation can be highly site or material specific with regard to material particle size range, sample mass, water infiltration rate and degree of oxygenation (Sturey et aI . , 1 982; Mills, 1 995) . The column is operated without aggressive flushing so that oxidation product may accumulate at particle surfaces in addition to being removed in 2S leachate . This behaviour paral lels field conditions and, as a result, leachate analyses from column testworks are a better indicator of expected water quality than leachate analyses from humidity cells-particularly if column infiltration rate is varied to simulate site conditions (Mill , 1 995; 1 996). There is no standardisation of column testwork procedures, allowing considerable flexibility, which permits column operation to be highly site or material specific with regard to material particle size range, sample mass, water infiltration rate and degree of oxygenation. Column tests also allow treatments to be tested and compared which is major advantage over humidity cells (Mil ler and Jeffrey, 1 995) . Column tests are intended to simulate natural conditions and are simple to construct, operate and monitor. Various environmental factors can be assessed, as can the influence of various control measures such as cover systems. However, the kinetics of reaction may not be distinguishable from rate l imiting transport phenomena and bacterial populations may differ from those found under field conditions. Column tests, on the other hand can become water saturated and interpretations of the results become difficult (Lawrence, 1 99 1 ; Mil ler and Jeffrey, 1 995 ; Perkins et aI . , 1 995) . Kinetic tests generally only attempt to predict what wil l happen in the early stages of acid generation processes (Miller and Murray, 1 988; Sherlock et ai , 1 995) and the data interpretation and modelling is complex irrespective of the nature of the tests carried out. Irrespective of the different types of kinetic tests, the overall objective of the tests is to provide data on the rate of acid generation and acid neutralisation under laboratory controlled conditions. In most kinetic tests water is commonly added to a sample, the mixture is allowed to incubate for a certain period to allow acid-ba e reactions, and samples of leachate or extracts are collected and analysed. The tests are required to run for a long time to generate overall acid generation information. The major parameters measured in kinetic tests are trends in pH, sulphate, acidity or alkalinity and metals . The pH identifies the stage of the acid generating process, sulphate production relates to rates of sulphide oxidation, acidity/alkalinity give an indication of the rate of acid generation/neutralisation and metal levels evaluate metal solubility and leaching behaviour. 26 2.5.3 Evaluation of the predictive techniques Although data generated from predictive techniques give a base for planning preventive, corrective and remedial measures, so far they have been found to have restricted application as universal predictive tool. Sherlock et al ( 1 995 ) considered that predictive techniques must be applied on a site-specific basis and take into account the mineralogy of the waste material. For example, minerals present, percent sulphides and their distribution within the rock mass and along joints and other discontinuities and the likely durability of the waste rock are likely to effect the predictive test results (MEND, 1 99 1 ; Orava and Swider, 1 996). Moreover, predictive techniques alone do not account for the build-up of metal salts that may occur after disposal but prior to final reclamation. The tests also fail to incorporate an assessment of the coating of sulphide phases with unreactive coatings that may occur naturally (Pratt et aI , 1 996) . Most of these tests also force oxidation or neutralisation reactions that may never occur in the real situation. Acid generation (and neutral isation) are time dependent phenomenon, and until someone develops a test that takes into account time dependency, there will never be an "exact" predictor. For example a test that indicates the presence of neutralisers, inherent or added does not mean that the net result wil l be no acid. That depends on the reactivity and kinetics . If the acid is generated faster than it can be neutrali sed, the net result will be an acid effluent regardless of how much neutraliser is available. Short-term static tests, which are conducted to determine the acid generating or acid consuming potential, will usually provide only an indirect assessment of the net acid generating potential. A longer-term kinetic test, which allows reactions to occur, will provide a more comprehensive assessment. However, even a kinetic test may not predict the net acid generating potential accurately because the ongoing sulphide oxidation will continue to produce acid and acid consumption by the alkalinity present in the system may be dominant only in the beginning. Conversely, the alkalinity in the system could overcome and exhaust the supply of sulphide bearing rock present. It is therefore c lear that both the static and kinetic tests must be designed to suit the mineralogy of the waste rock being tested. Both acid generating and acid consuming reaction rates in the 27 oxidation of pyrite and reactions with carbonate and sil icate minerals must be considered for predictive acid generation tests. The issue of appropriate NP/AP ratios is a key area of debate among the regulatory agencies and the mining industry. To date there is no comprehensive compilation of case histories of mine sites with significant ABA data, NP/AP ratios and AMD problems. The NP/ AP ratio is the most significant of the variables which regulatory agencies are attempting to use as a prescriptive measure. Ferguson and Morin ( 1 99 1 ) and Cravotta et al . ( 1 990) suggested that NP/ AP criterion separating potentially acid generating and non-acid generating samples could be about 2: 1 . However, in the data base presented by them, no sample with NP/AP > 1 produced acidic leachate in 1 66 laboratory leaching tests. There is also no field evidence of NP/AP> 1 producing AMD. The NP/ AP ratio may be considered as a safety factor with a higher safety factor probably required for mines in wet climates where carbonate minerals may be preferentially leached from the mine waste relative to the oxidation of the contained sulphide minerals . The first interpretative use published for ABA was an estimate of NNP>S kg CaC03 fl producing alkaline conditions (Sobek et aI . , 1 978) . This screening criteria was selected based on soil quality and plant growth media considerations, not mine drainage prediction. ABA later began to be applied to coal mine drainage prediction, beginning in the late 1 970's . At this time it became apparent that ABA interpretation depended on whether the end use was mine drainage prediction or mine spoil/growth media suitability. ABA is still used for both purposes today and two sets of interpretative frameworks have developed. Several researchers have suggested that the standard ABA procedures may in fact substantially underestimate the neutrali ing material requirement of the potential ly acid generating materials (Cravotta et aI . , 1990; Brady and Cravotta, 1 992; Brady et aI . , 1 994). It has been suggested that the currently used value of 3 . 1 25 g CaC03 equivalent to neutralise acidity from oxidation of 1 g S should in fact be 6.25 g CaC03 to assure a neutral AMD (Cravotta et aI . , 1 990; Brady et aI . , 1 994). Field studies conducted by Brady et al . ( 1 994) found that the alkaline material requirement calculated from ABA 28 analysis using sulphide-S to CaC03 ratio of 3 . 1 25 was inadequate for controlling AMD. Only when the ratio was doubled to 6.25, was there an overall net alkal inity of water at 1 1 of 1 2 coal mine sites studied. Perry and Brady ( 1 995) showed that material with an NP>2 1 generall y produced alkaline drainage whereas NP< l O produced acidic drainage. The APP on the other hand, is considered adequate in predicting AMD only in materials that contained insignificant amount of carbonates « 1 % CaC03), in which case a relationship between total sulphide-S and acidity could be defined (Perry and Brady, 1 995) . Other studies have shown that factors other than mine waste characteristics may be involved in the generation of AMD (DiPretoro and Rauch, 1 988 ; Erikson and Hedin, 1 988) . They found that there was poor correlation among APP, NP and NNP from ABA and net alkalinity from drainage water. O 'Hagan and Caruccio ( 1 986) found that addition of CaC03 at 5% by weight to a coal refuge containing 1 % S produced alkaline drainage whereas Lapakko ( 1 988 ) indicated that CaC03 > 3% was needed to neutralise an overburden material with 1 . 1 7% S . The discussions above indicate that there are still discrepancies in the use o f ABA as screening tool in predicting AMD. It is also evident from their results that lime requirement assessed from ABA analysis did not always produce neutral drainage from waste rock dumpsites . There was a need for further studies on the rate, application and placement of alkaline materials in mine waste and mine sites with potential to generate AMD. There appears to be no universal set of threshold numbers for defining cut-offs on ABA. Instead, the data tend to group themselves in ranges (Brady et al . , 1 990; Cravotta et al . , 1 990; DiPretoro and Rauch, 1 988) . However, ABA testing procedures are finnly entrenched in the mining industry and it is likely to stay in use because it is familiar to industry, consultants and regulators, and i t is low cost and has rapid turnaround time. As with the static tests, kinetic tests are also subject to queries about their accuracy in predicting real situation AMD conditions . Kinetic tests (humidity cells , columns and soxhlets) not only produce a unique leachate but also modify the sample and a significant variation in the accuracy of the results was observed (Bradham and Caruccio, 1 990) . No data is yet available from weathering tests which were run long enough to see the sulphate generation rate begin to taper off. Most weathering test 29 results are assessed as "acid or not acid" producing. In other words, there is not anything particularly kinetic about the data analysis . While static and kinetic tests and the associated models are far from perfect in their capacity to predict the generation or migration of AMD, they do allow for a more systematic approach to understanding the potential problems. 2.6 Prevention and Control of AMD Various physical, chemical and biological control measures have been used to prevent, minimise and treat AMD. Basically there are two types of prevention and control, the first relates to the generation of AMD, while the second relates to its in-situ mobilisation and subsequent migration. Both are inter-related in as much as certain approaches to preventing and controlling generation can reduce migration and vice versa (Mills, 1 996) . Methods proposed for the prevention and control of AMD generation include treatment of sulphide surfaces via the formation of inert surface "coatings" , the use of bactericides, the segregation of the principal AMD generating waste fraction and control of oxygen and/or water infiltration of the sulphide-bearing material. Currently, waste segregation and prevention or control of water/oxygen infiltration dominate, with the other methods having only limited application at full-scale . Certain approaches such as inert surface coatings are considered unproven at present but worthy of further research. In addition to the use of engineered covers, the most common approaches to preventing or controlling the migration of AMD are the re-routing of water away from the source or the use of sub-surface seals and barriers to impede the movement of contaminated ground water (Filipek et aI , 1 996). The greater the control achieved, the smaller the volume of AMD that is likely to require treatment. Just as waste minimisation is typical ly more cost-effective than waste management, prevention or minimisation of AMD is generally considered a cheaper option than long-term treatment (Filipek et aI, 1 996) . The principle involved in the prevention and control of AMD from mining activities has been centred on removal and abatement of one or more of the essential components in the acid generation process. These components are the sulphide mineral, bacteria, air and water. The prevention and control procedures currently practised to abate acid generation are described in the following sections. 30 2.6.1 Preventive coatings Several techniques have been experimented within recent years, which inhibit pyrite and pyrrhotite oxidation using inert coatings. Most of the studies centre around coating of isolated sulphide mineral grains with coating agents such as iron phosphate (Evangelou and Huang, 1 992, Fytas et aI . 1 994, Evangelou, 1 994, Georgopolou et al . 1 995) , acetyl acetone , humic acid, oxalic acid, sodium silicate and l ignin (Maki et al . 1 995 ; Mitchell and Atkinson, 1 995) . Cathodic protection of the weathering sulphide ore body has also been attempted (Shelp et aI . , 1 995). Preventive coating experiments have been done mainly on separated grains of pyrite crystals and their use in the field is impractical, because of different forms and types of pyrite present in the waste rock. Attempts were also made by Ahmed ( 1 994) to form pyrrhotite hard pans on tailings surfaces to prevent atmospheric oxidation of pyrrhotite. The method involved electrochemical treatment of FeS rich tailings with ferrous solution to form an oxyhydrate (goethite) matrix . A recent advance has seen research on the use of fatty acid amines, which suppress bacterial activity and chemical oxidation through a process of hydrophobic coatings (Nyavor et aI . , 1 996). Treatment with the amine makes the pyrite highly hydrophobic and the pyrite surface consequently repels oxidising ions (i .e . : Fe3+). It is, however, unclear whether the effect of the hydrophobicity prevents the bacteria from contacting the pyrite surface. While some of these techniques have prevented further oxidation of pyrite minerals, their applicability to the field situation is not proven as yet. Further research is necessary to detail the economic costs and technical constraints of larger-scale applications, but this approach does look promising, possibly as a means of treating segregated high-sulphide wastes prior to disposal. 2.6.2 Selective handling Partitioning of wastes into sulphide-rich and sulphide-depleted fractions offers the chance to expand waste management options in the control and prevention of AMD. In theory, the low-volume, sulphide-rich fraction can be disposed of at a highly engineered disposal site or at least isolated as buried "cells" within the bulk waste while the high volume sulphide-depleted fraction can be disposed of as an inert waste. B lending of 31 acid-generating and non-acid generating wastes can also be used to prevent or control AMD. However, as the latter waste is often exempt from permitting due to its inert nature, blending can sometimes result in the permitting of a much larger volume. This approach may, therefore, be environmental ly attractive, but constrained by operator' s reticence t o extend or further complicate the permitting process. Several mineral processmg techniques are available for separating acid-generating sulphides. Methods include gravity separation (e.g. centrifugal concentrator, shaking table, spiral concentrator), flotation, magnetic separation and cyclone classification. The segregated pyrite can be substituted for elemental sulphur in the production of sulphuric acid. Research also shows that pyrite may have some use in the removal of dissolved arsenic species by adsorption (Humber, 1 995). 2.6.3 Bactericides Bactericides normally contain amomc surfactants that destroy the greasy outer cell membrane coating of the bacteria Thiobacillus ferrooxidans. The greasy film coating normally protects the bacteria from the surrounding acid environment and once it is destroyed, the bacteria can not survive in the acid conditions (MVTI, 1 994; Sanda, 1 989). ProMac System, comprising commercial short-term and long-term slow release bactericides have been widely used in the treatment of AMD (Rastogi et aI . , 1 986; MVTI, 1 994; Sengupta, 1 994). Alternative bactericides such as sodium dodecylbenzene sulfonate (SDS), sodium laurel sulfate (SLS) and thiol-blocking agents have also been suggested for the inhibition of Thiobacillus bacteria (Sengupta, 1 994; Stichbury et aI, 1 995) . 2.6.4 Oxidant infiltration barriers Seals, grouting, cover layers, interception trenches and sub-surface barriers have been used to prevent infiltration of oxidants (air and water) to control AMD generation (Nicholson et aI . , 1 989; Schueck et aI . , 1 994; Tremblay, 1 994; Stogran and Wiseman, 1 995 ; Scheetz et aI . , 1 995; Foote, 1 996; McCloskey, 1 996; Mueller et aI . , 1 996). Seal ing mine openings, tunnels and adits can prevent the infiltration of water into, and the migration of AMD out of underground workings. Preventing the movement of water through such workings can minimise sulphide oxidation even if the workings are flooded, as static water wil l quickly become anoxic as oxygen is consumed by chemical 32 and biological reactions. Cementitious grouts can also be applied to mme adit and pitwalls to prevent the infiltration of oxygenated water (Scheetz et aI, 1 995) . Interception trenches for directing AMD to passive treatment systems have also been successfully used to prevent the migration of AMD (Mueller et aI, 1 996) . Although waste rock dump and pitwall geometry can be important in defining surface area exposure and air infi ltration rates (USEP A, 1 994; Rastogi et aI , 1 995) , engineered covers are effective at controlling oxygen and water infiltration and can be classified as oxygen barriers, oxygen consumers or reaction inhibitors (MEND, 1 994a) . PitwalIs generally have steep gradients and therefore placement of amendments becomes impractical . Often heavy engineering of the acid generating pitwall area is required to implement stabilised placement of suitable cover system (Watson, 1 995) . Engineered cover systems often include layers that promote lateral rather than vertical movement of water, as well as providing a substrate for vegetation and protective layers between the geofabric and the waste to reduce the risk of physical damage to plant root systems. Synthetic geofabrics such as polyvinyl chloride (PVC) and high density polyethylene (HDPE), have been used to prevent and control water and oxygen infiltration into AMD generating wastes (Nicholson et aI . , 1 989 ; Ritcey, 1 989; Sengupta, 1 994; Jones and Wong, 1 994) . Geofabrics are expensive but if applied properly (i .e . to avoid punctures and rips) they are l ikely to have useful working lives in excess of 1 00 years (Filipek et aI , 1 996). Clays and clay mixtures such as kaolin amorphous derivatives (KAD) and sand-bentonite mixture have often been used as oxidant infiltration barriers because of their minimal permeability when compacted and relatively low cost (Yanful and Shikatani , 1 995 ; Mackinnon et al . , 1 997) . However, there is a danger that if the clay cover dries, deep cracks can occur, allowing the rapid ingress of water and oxygen. Moreover, compacted clays provide poor growing media for plants. Organic covers consisting of sewage sludge, papermill sludge, topsoil and wood bark have also been found to be effective in reducing oxygen infiltration in sulphidic mine wastes and tailings ponds (Pierce et ai . , 1 994; Tremblay, 1 994; Stogran and Wiseman, 1 995) . Surface application of organic materials not only provides a physical barrier to oxygen and moisture but also provides leachate rich in soluble organic compounds that 33 encourage the activity of sulphate reducing bacteria (Pierce et aI . , 1 994) . The placement of a biologically active organic layer on top of tailings has been suggested as a means of reducing oxygen diffusion and to confine metal contaminants (Ritchie, 1 997). Oxygen infiltration into the tail ings is controlled by its consumption in the organic layer (e.g. via conversion to carbon dioxide and water), while metal diffusion into the water can be further controlled by inoculation of the organic layer with sulphate-reducing bacteria capable of precipitating the metals as sulphides (Nicholson et aI . , 1 989). Organic l igands leached from decomposing organic matter are found to be effective in reducing phytotoxic levels of Al by readily complexing with the exchangeable Al (Hargrove and Thomas, 1 98 1 ; Young and Bache, 1 985 ; Hue et aI . , 1 986; Hem et aI . , 1 988 ; Shuk-Ching and McColl, 1 990; Gurung et aI . , 1 996). One potential drawback to this approach has been highlighted by recent research which has shown that ferric oxyhydroxides present in weathered tailings dissolve when in contact with organic acids originating from carbon-rich oxygen consuming covers (Ribet et aI, 1 995). The dissolution of the oxyhydroxide phase can result In the release of adsorbed or co­ precipitated non-ferrous metals into the aqueous phase. Although reclamation of mine waste materials by municipal sludge application has been shown to be beneficial to plant growth by providing a growth medium and nutrient reserves (Dinauer et aI . , 1 977 ; Gemmell, 1 98 1 ; Sopper, 1 992), there are serious environmental problems associated with such organic-based amendments (Sopper, 1 992; LRNL, 1 993) . Composted organic sludge materials commonly contain high concentrations of heavy metals such as Cd, Cu, Mo, Mn, Pb and Zn (Forstner, 1 99 1 ) which can be hazardous contaminants in soils, water and plants. Surface applications of sludge materials can mobilise heavy metals through reductive dissolution as well as mobilising nitrate into the ground water system (Voos and Sabey, 1 987; Sopper, 1 992; Ribet et aI . , 1 995) . Field and laboratory studies have shown that surface applied sewage sludge to promote revegetation at reclaimed mines resulted in increased down gradient concentrations of sulphate and acidity, indicating that surficially applied sludge was not an effective barrier to O2 entering into underlying zones (Cravotta, 1 997) . Laboratory leaching tests also indicated that sewage sludge additions produced significant increases 34 in microbial pyrite oxidation and that only when CaC03 was added was there a reduction in bacterial oxidation of pyrite (Cravotta and Trahan, 1 996; Cravotta, 1 997) . Composite cover system consisting of layers of non-acid generating geologic materials has been developed with an aim to establish vegetation as well as isolating sulphidic waste materials (Harris and Richie, 1 988 ; Aachib et al . , 1 994; Bell et al. , 1 995) . Composite soil covers are often up to 1 m thick and they can be applied only on low gradient rehabilitation sites. Such a cover system has been used in the rehabilitation of Rum Jungle mine site in Austral ia (Ryan, 1 987; Harries and Richie, 1 988; Bennett et al . , 1 989; Sengupta, 1 994). On-going monitoring of the effectiveness of the composite cover system at Rum Jungle has shown significant reduction in metal loading in the AMD from the rehabilitated dump sites (Bennett et al . , 1 987; Sengupta. , 1 994) . Relevant research and modelling work has been published on soil covers which limit oxygen influx to tailings and their subsequent effect on oxidation of pyrite! pyrrhotite. For example, Nicholson et al . ( 1 989) demonstrated that, in addition to a fine particle size of the cover material , maintenance of high moisture content is essential to minimise diffusion of oxygen. Furthermore, although erosion and stability are important considerations with respect to slope, one must also consider that the slopes are influx zones for oxygenated air entering the pitwall (Murray, 1 997) . Buoyant air, heated by pyrite oxidation tends to rise, drawing fresh air into the slopes. Accordingly, convective airflow may develop within the sloughed pitwall rock, maintaining oxidation despite burial of the acid-forming material. In order to inhibit continued oxidation, the slopes need low-penneability or oxygen-consumptive covers (Guo and Cravotta, 1 996). Sub-aqueous disposal of reactive waste rocks by flooding of open pits or dumping into natural lakes or impoundments has been practised by many mining operations, although such practices require a thorough understanding of local and regional hydrology and hydrogeology (Fraser and Robertson, 1 994; Dave and Vivyyurka, 1 994; S t-Arnaud, 1 994; Filipek et aI, 1 996). In Canada, the recommended method of preventing AMD is to dispose of sulphide waste rocks and tailings deposits sub-aqueously (MEND, 1 99 1 ; Pederson et al . , 1 994). While this is feasible in areas of high precipitation, low temperature and abundance of lakes, i t cannot be considered in areas of low 35 precipitation and lack of fresh water lakes. Disposal of sulphidic waste into sea water has also been practice by mining operators in Norway (Sengupta, 1 994) and in Papua New Guinea, river disposal of tailings has been practised for some time at the Pogera Gold and Ok Tedi copper mines (Harries, 1 997) . Sub-aqueous disposal controls acid generation from sulphidic wastes by limiting the diffusion rate of oxygen through the water cover (Fil ipek et al . , 1 996). However, research over a three-year period at the Noranda Technology Centre has shown that although water covers can reduce the rate of acid generation by 99.7% (Payant et aI , 1 995), the concentration of metals in the surface water can still exceed regulatory limits (Aube et aI , 1 995) . The efficacy of subaqueous disposal and the environmental impact on the aquatic life are yet to be fully assessed. Other barrier methods such as deliberate construction of hardpan (using electrochemical methods), which consist of a cementitious iron oxyhydrate matrix, may also help control infiltration by water and thus reduce or prevent the generation of AMD (Ahmed, 1 995) . 2.7 Treatment of AMD There are number of overlapping approaches to the treatment of AMD, which are nominally categorised here as active, passive and active-passive hybrid systems. In the recent past, research into the use of ion exchange resins and natural and synthetic zeolites to treat AMD have also shown promising results (Zarnzow et al . , 1 989; Schultze et al . , 1 994; Mondale et al . , 1 995 ; Riveros, 1 995) . The most common methods employed in the treatment of AMD are through chemical and biological processes. The chemical treatment processes include complexation, oxidation and reduction principles (Cohen, 1 996). Although all of these methods show promising inhibitive properties in chemical sense, they are not all found to be practical under field conditions. 2.7.1 Active treatment systems Active treatment systems basically involve neutralisation of AMD with l ime and precipitation of metals as hydroxides. There are numerous studies carried out on the variations of this technique but the ultimate process involves the addition of base to neutral ise acid (Bell et al . 1 982; Skousen et al . , 1 990; Boling and Kobylinski , 1 992; 36 Bhole, 1 994; Kuyucak et aI . , 1 995; Murdock et aI . , 1 995 ; Taylor et aI . , 1 997). Liming materials commonly used in neutralisation techniques are hydrated l ime [Ca(OH)2] , sodium bicarbonate (NaHC03) and sodium hydroxide (NaOH) (Bell et aI . , 1 982; Skousen et aI . , 1 990) . Agricultural limestone and phosphate rocks have also been used to treat AMD (Hil l and Willmouth, 1 970; Renton et aI . , 1 988 ; Hart et aL , 1 990; Spotts and Dollhopf, 1 990; Fyson et aI. , 1 994; Kalin et aI . , 1 997) . In conventional l ime treatment, there are five basic steps following collection of the AMD:- 0) equalisation to ITLiniITLise variations in water quality; (d) aeration to oxidise Fe2+ to the less soluble Fe3+, (iii ) neutralisation to increase pH to precipitate metals as hydroxides, (iv) sedimentation to separate water and solids and (v) sludge disposal (Bell et aI. , 1 982; Bhole, 1 994; Murdock et aI. , 1 995 ; Taylor et aI . , 1 997). Lime is normally introduced into the system as a 5-20% (by weight) water-based slurry, although i t is sometimes applied as a dry powder when the water volume to be treated is low. The principal reactions are summarised in Equations 1 7 to 1 9 where M represents dissolved metals (Fe, Zn, Cu, Zn, Mn, Al etc .) . Detailed analysis of the reactions between lime and acidic metal contaminated AMD is explained in Marchant ( 1987). Ca(OHh + H2S04 � Ca2+ + S042- + 2 H20 Ca(OH)z + MS04 � M(OH)2 + Ca 2+ + sol- 3 Ca(OHh + M2(S04h � 2 M(OHh + 3 CaS04 [ 17 ] [ 1 8] [ 1 9 ] Major drawbacks of the l ime treatment system are the large volume of potential ly toxic sludge produced (which must be physicall y removed) and a relatively high pH required to remove metals such as Mn. Some metal hydroxides such as Fe(OHh Al(OH)3 may re-dissolve in the highly alkaline solutions required to complete metal precipitation, necessitating a multi-stage treatment to reduce all metals to acceptable concentrations. Sludges derived from the liming of AMD are chemicall y unstable and will partially redissolve if exposed to a sufficiently acidic environment (Kuyucak et aI, 1 995 ; Murdock et aI . , 1 995) . To address some of these drawbacks, a number of refinements to standard liming treatments have been developed in recent years. Two such examples include the High 37 Density S ludge (HDS) process and the patented Noranda Technology Centre (NTC) process which are being applied increasingly within the mining industry. The HDS process produces a more compact and higher density sludge (Murdock et aI , 1 995) and is carried out in aerated reactors. Part of the settled sludge is recycled to the beginning of the process, where it is mixed with the lime slurry. The NTC process uses pH controlled reactors in which sludge density and settling rates are improved relative to the HDS process (Kuyucak, 1 995; Kuyucak et aI, 1 995) . However, the benefit of such lime treatment is a proven technology with well documented and understood mechanisms of metal removal and acid neutralisation. Despite the commercial availability of processes such as HDS and NTC, increasingly stringent legislation is likely to drive mine operators to look beyond the use of lime to avoid incurring growing disposal costs, and to avoid the possibility of future liability and litigation. One alternative to which serious consideration has been given is the use of passive treatment systems. 2.7.2 Passive treatment systems Passive treatment systems encompass a number of discrete neutralisation processes and they are described by numerous researchers (Faulkner and Skousen, 1 984; Holm and Elmore, 1 986; Howard et aI . , 1 989; Kleinmann and Hedin, 1 993; Hedin et al . , 1 994; Gusek and Wildeman, 1 995; Cambridge, 1 995; Cohen, 1 996; Gazea et aI . , 1 996; Robbins et aI . , 1 996). These include anoxic ponds (AP), l imestone ponds (LSP), anoxic limestone drains (ALD), alkalinity producing system (APS) , Reverse alkalinity producing system (RAPS), open limestone channels (OLC), aerobic and anaerobic wetlands (Faulkner and Skousen, 1 984). Porous reactive alkaline walls have also been proposed as a means of treating sub-surface AMD in-situ (Blowes et aI, 1 995; Waybrant et aI, 1 995). This approach has only been attempted on a limited scale, although the preliminary results appear promising in terms of both economic and technical performance (Blowes et aI, 1 995). Passive treatment systems have demonstrated substantial mitigation in AMD quality in some cases, while in other situations, less dramatic results has been obtained (Brodie and Hedin, 1 994; Falkner and Skousen, 1 996; Skousen, 1 995) . High concentration of Fe and Al in the AMD often precipitate as hydroxide when in contact with the alkaline treatment systems, causing an "arrnouring 38 effect" on the limestone, thereby reducing the generation of further alkalinity and impeding flow through the drain (Filipek et aI, 1 996; Robbins et aI . , 1 996) . The use of sulphate-reducing bacteria in open pits or underground workings has been suggested as a means of treating AMD in-situ (Kuyucak and St.-Germain, 1 994) . This approach might be suitable for Iow load scenarios, where a suitable organic substrate for bacterial growth is locally and cheaply available. However, efficiency might be compromised by a single addition if the substrate is so deep that mass-transfer is detrimentally affected. The operational temperature at depth is also an issue as this will influence bacterial activity. Constructed or engineered aerobic and anaerobic wetIands are passive treatment systems which attempt to duplicate natural systems and use chemical and biological processes to reduce dissolved metal concentrations and neutralise acidity (Holm and Elmore, 1 986). Compared with conventional active chemical treatment by liming, passive methods generally require more land area, but use cheaper materials to support the chemical and biological processes, and require less operational attention and maintenance (Cambridge, 1 995). However, wetlands are not "walk-away" solutions but, rather, low maintenance, low energy systems designed to treat effluent AMD and are not suitable treatment options for in-situ neutralisation of AMD. 2.8 Mined Land Reclamation Methods Reclamation and revegetation of sulphidic mine waste rock materials have become a challenging practical problem for the mining industry worldwide. The acid produced in the mine waste substrate solubilises toxic quantities of heavy metals and reaches elevated levels of sal inity, making reclamation difficult, if not impossible, without physiochemical modification of the material. Amendment of the sulphidic waste with alkaline materials have been commonly practised to ameliorate acidic conditions for establishment of vegetation (Pulford, 1 99 1 ; Dollhopf, 1 992). Alkaline addition to acid producing sites during surface mining and reclamation has shown variable success in ameliorating low pH conditions (Brady et aI . , 1 990; Ziemkiewicz and Skousen, 1 995) . Several studies have been conducted on the efficacy of alkaline amendments to ameliorate acidi ty problems in reclaimed coal mine sites (Costigan et aI. , 1 984; 39 Bhumbla et aI . , 1 990; Brady et al . , 1 990; Cravotta et aI . , 1 990; Evans and Rose, 1 995). The type of alkaline materials and the method of applications have been important factors both in terms of economy and effectiveness. 2.8.1 Lime requirement of sulphidic mine wastes For many years lime requirement determinations of acid agricultural soi ls have been made by quick-test buffer methods involving buffer mixtures (Shoemaker et aI . , 1 96 1 ; McLean et aI . , 1 966; McLean, 1982) . Recently, methods have been developed which predict lime requirements from measurements of soil pH, organic matter, extractable Al and clay content of the soil (Oates and Kamprath, 1 983; Bailey et aI . , 1 989; Aitkin, 1 992). While these methods appear to accommodate acid agricultural soils, they have been found to be inappropriate for mine wastes that generate continuous acidity from pyrite oxidation. In the mining industry, lime requirements for acidic overburden and mine waste dumps are commonly determined from standard ABA procedures (Sobek et al . , 1 978) . The liming rate assessed from ABA analysis, however, is found to vary considerably and does not always bring the expected results in long-term neutralisation of acidity and prevention of mobilisation of heavy metals (Costigan et al . , 1 982 ; Brady et al . , 1 990; Cravotta et aI . , 1 990; Skousen et aI . , 1 997). S ite specific variations in the sulphide content of the mine wastes make it difficult to specify a standard rate of l ime requirement (Sorensen et aI . , 1 980). Large amounts of liming materials are therefore, commonly required for long-term amelioration of acidic conditions in sulphidic waste rock materials. Even then, reacidification can occur when the added neutralising materials are exhausted or "armoured" by oxyhydroxides (Hill and Willmouth, 1 970; Costigan et aI . , 1 982). 2.8.2 Alkaline amendments in reclamation Large quantities of alkaline materials such as limestone or l ime products (CaO, Ca(OHh, fly ash, lime kiln dust) dolomite and alkaline shale are often used in mine reclamation to control acidity. There have been several studies carried out on the effectiveness of alkaline addition to sulphidic mine waste rock in ameliorating low pH conditions and creating suitable growing media for plant growth (Sorensen et al , 1 980; Costigan et aI . , 1 982; Hoving and Hood, 1 984; Stiller et aI . , 1 984; Joost et aI . , 1 987; 40 Sullivan and Yelton, 1 988 ; Brady et aI . , 1 990; Meek, 1 994; Skousen and Larew; 1 994) . Although alkaline materials are effective in short-term neutralisation of acid and aid in the initial establishment of plant growth, long-term sustenance of the ameliorative effectiveness has not been successful. Several factors such as local climatic conditions and site-specific nature of the waste rock can affect the dissolution of various types of alkaline materials in highly acidic materials (Sorenson et aI. , 1 980; Costgan et aI . , 1 98 1 ) . The effect of limestone grain size on its solubility and neutralisation of substrate acidity in acidic agricultural soils has been studied in detail (Albrecht, 1 946; Meyer & Garth, 1 952 ; Elphick, 1 955 ; Gemmell, 1 98 1 ; Costigan et aI . , 1 9 84) . All of these authors have shown that limestone solubility in acidic soils is affected by its effective grain size, carbonate content and the amount of dissolved metals (especially Fe and AI) in pore solutions. In highly acidic mine waste materials, the hming agents were found to react vigorously and often the rate applied was insufficient to counterbalance the acid released from oxidation of pyrite . Alternatively, the armouring effects by precipitated metal hydroxides on the coarser limestone particles rendered it ineffective in neutralising acid (Gemmell, 1 98 1 ; Costigan et aI . , 1 982 , 1 984) . When large quantities of limestone are applied to acidic colliery spoils, Costigan et a1 . ( 1 984) have observed that coarser grained l imestone reacts more slowly than fine grades although it has no significantly different effect on the plant growth compared with that of fine grade limestone . Their study also indicated that depth incorporation did not raise subsurface pH significantly from that of equal rates of surface application and that the long-term neutralisation effect was only evident from particle size < 3 mm. Leaching experiments usmg lime kiln flue dust amendments on pyritic shale, have shown that it significantly reduce acidity, S04, Fe, Mn, Al and Mg by up to 85% compared to a control despite still producing acidic effluent from the test cells (Evan and Rose, 1 995). Their study also showed that alkaline amendments produced leachate pH that remained <3 even at the highest rate of application ( 1 7 1 % of that required to neutralise APP of 3 1 . 25 kg CaC03 ( I at 1 % S) . They found that the amount of lime needed to prevent AMD could not be predicted from the calculated rates based on ABA 41 analysis because of site specific nature of the sulphidic waste rock and the environmental conditions. Agronomic effectiveness and environmental impact of fly ashes used to reclaim pyritic acid mine spoils have been investigated in the laboratory and field ( Bhumbla et aI . , 1 990; Bhumbla, 1 992). Application o f fly ash decreased bulk density, increased moisture retention capacity of spoils and reduced toxic levels of Fe, Mn and Al in soils by buffering soil pH at 6 .5 , and retarding pyrite oxidation. Fluidised bed boiler bed ash (FBA) generated from fossil fuel-fired boilers has been much investigated for use as a liming material in acid agricultural soils because of its high alkalinity (Stenhouwer et al . , 1 995; Wang et aI . , 1 994). Because FBA contains both lime and gypsum, it has been shown to ameliorate both surface and subsurface acidity (Sumner et aI . , 1 986; Wang et aI . , 1 994). Its use as AMD ameliorant in reclamation of coal mine spoil has been investigated by several workers (S iddle et aI . , 1 979; Dick et aI . , 1 994; Stehouwer et aI . , 1 995) . Similar other by-products such as fluidised bed combustion ash (FBC), fluidised bed waste (FBW) and flu gas desulphurisation sludge (FGD) have been investigated as possible ameliorants for AMD in coal mine waste (Bhumbla, 1 992). S ince FBA is a waste product, its use in large quantities in reclamation of coal refuse serves as a viable alternative for its disposal. Greenhouse column leaching of pyritic mine spoil has shown that FBA applied at the rate of 30 to 60 g kg- 1 mine spoil, was effective in neutralising acidity, increasing leachate pH, Ca, Mg and reducing concentrations of Fe, Mn and AI (Stehouwer et aI . , 1 995) . However, their results also indicated that EC and SO/- increased w ith increasing rates of FBA applications . Other workers (Balla and Edwards, 1 995; Reichert and Norton, 1 996) have explored the possible use of FBA to improve the physical properties of acid agricultural soils (increase water infiltration, reduce surface sealing and erosion control. A major drawback in using FBA as amendment is that it is composition ally similar to portland cement . When applied at a high rate, it can form a cementitious crust (which can impede water infiltration and plant root growth) from the reaction of CaO with aluminosilicate minerals (Korcak, 1 988; Dick et al. 1 994; Dawson et al, 1 995; S tehouwer et al . 1 995). 42 Several studies have been done on the use of phosphate rock as a potential AMD ameliorant (Sti ller et aI . , 1 986; Renton et aI . , 1 988 ; Sencindiver and Bhumla, 1 989; Spotts and Dollhopf, 1 990; Hart and Stiller, 1 99 1 ; Evangelou et aI . , 1 992; Fyson et aI . , 1 994; Kalin et aI . , 1 997) . Laboratory studies have shown that precipitation of iron phosphate on pyrite surfaces has an armouring effect that prevents further oxidation (Evangelou, 1 994, Huang and Evangelou, 1 995) . Other studies have reported that pyritic waste amended with natural phosphate rock can induce significant reductions in acidity, Fe, Mn and Al (Kahn et aI., 1 997; Fyson et aI. , 1 994). Natural phosphate rocks such as North Carolina reactive phosphate rock (RPR) which contains about 1 1 .7% free CaC03 and 1 3 . 1 % P has been shown in the laboratory and field studies to be a potential ameliorative material in acid agricultural soils (Bolan et al. 1 988 ; Loganathan et aI . , 1 995) . The theoretical liming value from complete dissolution of RPR is equivalent to 600 kg CaC03. For every tonne of RPR dissolved, the estimated liming value is 4 1 9 kg CaC03 from dissolution of P plus 1 1 7 kg CaC03 from free CaC03 to give a total 536 kg CaC03 (Bolan, 1 995) . In pyritic mine waste devoid of Ca and P, phosphate rock is considered an effective liming material in reducing substrate acidity, providing Ca and P as well as inhibiting further chemical oxidation of pyrite by coating with Fe-P04 compound (Pulford and Duncan, 1 975 ; B ackes et aI . , 1 987) . The P04 formed from dissolution of RPR combines with Fe to form Fe-P04 which has been shown in laboratory studies to armour the pyrite surface from further attack by oxidising agents (Evangelou and Huang, 1 992 ; Georgoupoulou et aI . , 1 995) . Phosphate rock not only reduces soil acidity but also reduces phytotoxic Al concentrations and leads to a significant increase in the effective cation exchange capacity of the soils (Loganathan et aI . , 1 995) . The use of natural phosphate rock to reduce AMD from pyritic waste rock and colliery spoil has also been studied by several workers (Backes et aI . , 1 987; Sencindiver and Bhumla, 1 989; Hart et aI . , 1 990; Hart and Stiller, 1 99 1 ; Kalin et aI. , 1 995) . Laboratory experiments by Hart et aI . , ( 1 990) and Hart and Stiller ( 1 99 1 ) have shown that application of natural phosphate to acid generating coal waste was an effective 43 ameliorant. In a drum test trial of pyritic mine waste rock using a natural phosphate rock amendment, Kalin et al. ( 1 995) have shown that phosphate rock applied at 1 1 5 kg ( 1 waste rock significantly reduced cumulative acidity (74%) over 989 days of monitoring. They found that mixing phosphate rock with the waste rock was more effective in reducing acidity than placement on the surface. A long term field experiment on the effect of alkal ine amendments on mine waste rock pi les has shown that over an 1 1 year period limestone addition had no effect on the rate of sulphate production from pyrite oxidation although leachate pH remained above neutral (Ziernkiewicz and Meek, 1 994). A similar effect on sulphate generation was indicated for phosphate amended pi les but there was a significant drop in leachate pH from near neutral to pH<4 over an 1 1 year period. In both limestone and phosphate amendments, the high rates of application (2.4% limestone and 0.3% phosphate by mass) generally resulted in acidic leachate, indicating possible armouring effects on the limestone and phosphate grains from metal precipitates. In the same field experiment, they also showed that application of bactericide (sodium lauryl sulphate) had no effect on the rate of pyrite oxidation and the pH and sulphate fluxes in the leachate were similar to the lowest rates (0.5% limestone and 0. 1 5% phosphate by mass) of alkal ine addition. The concluding results from their 1 1 year monitoring of the leachate characteristics from alkaline and bactericide amended test piles were that pyrite oxidation was not affected by these amendments and independent of leachate pH. This review on alkaline amendments indicates that various alkaline materials can be used to mitigate acidic conditions in sulphidic mine wastes, albeit with varying degree of success. Given favourable conditions and with appropriate rates of application, alkaline materials are an important chemical additive in modifying mine wastes to create suitable plant growth media in reclamation practices (John son et al . , 1 977). This ameliorative approach is one of the frequently used methods to establish vegetation on mine wastes. 2.8.3 Organic amendments The potential use of organic and inorganic cover materials to inhibit the oxidation of pyrite and thereby mitigate acid mine drainage (AMD) have been investigated by 44 several workers (Nicholson et aL, 1 989; Daniel and Koerner, 1 993; Guo et aI . , 1 994; Bell et aI . , 1 995 ; S togran and Wiseman, 1 995 ; Swanson et aI . , 1 995 ; Guo and Cravotta, 1 996; Miller, 1 997). As most mine waste materials are devoid of organic matter, many researchers have tested several organic matter sources (chiefly municipal sludge, peat, wood-wastes, papermill sludge and topsoil materials) as amendments in land reclamation (Pulford, 1 99 1 ; Gregg et aI . , 1 998) . An extensive review of the organic materials as amendments in land reclamation can be found in LRNL ( 1 993) and comparisons of various amendments in several research papers (Haghiri and Sutton, 1 982; Hoitink et aL 1 982; Bennett et aI . , 1 984; ; Voos and Sabey, 1 987; Pichtel et aI. , 1 994) . Organic wastes such as municipal sludge (Halderson and Zenz; 1 978 ; loost, et aI . , 1 987 ; Sopper, 1 992), organic compost (Pierce et aI . , 1 994) and papermill sludge (Hoitink et aI . , 1 982) are some of the waste materials commonly used in the reclamation of lands disturbed by mining. The use of municipal sludge in reclamation and revegetation of disturbed land has been extensively investigated by the above authors, especial ly Halderson and Zenz ( 1 978) . The results to date are encouraging and show that stabilised municipal sludge, if applied properly according to guidelines used, can be used to revegetate mined l ands in an environmentally safe manner with no major adverse effects on the vegetation, soi l , or groundwater quality. Field and laboratory studies conducted by Cravotta ( 1 997) using surface applied sewage sludge to promote revegetation at two reclaimed mmes m western Pennsylvania showed that concentrations of sulphate and acidity increased down gradient and that surficially appl ied sludge was not an effective barrier to O2 entering into underlying zones. Laboratory leaching tests also indicated that sewage sludge additions produced a small increase in microbial pyrite oxidation and that highest oxidation rates resulted when sewage sludge was added. The lowest oxidation rates resulted when CaC03 was added, indicating promotion of bacterial oxidation of sulphides under sewage treated conditions (Cravotta, 1 997) . Pichtel et al. ( 1 994) indicated that sewage sludge, papermill sludge, and topsoil amendments were superior to fly ash in maintaining forage crops on pyritic mine spoils . Their conclusion was that both sewage sludge and papermill sludge amendments were equivalent to l imed topsoil for successful reclamation of toxic mine spoils . 45 In open-cast strip mining, large volumes of topsoil are available for use in reclamation. The reclamation practice in New Zealand is to replace topsoil to at least the original depth where applicable (Gregg et aI . , 1 998). In acidic mine waste materials with poor chemical and physical properties suitable for plant growth, use of topsoil or surrogate soil material becomes a necessity to provide an uncompacted growing medium. What depth of topsoil to use is limited by the configuration of the reclamation site, topsoil availability and the local climatic conditions. Current reclamation trial work at Martha mine site (see Plate 1 , Chapter 1 ), has demonstrated that depth of topsoil wa not a critical factor in the land being returned to pasture under Waihi' s local annual average rainfall regime of 2 100 mm (Gregg and Stewart, 1 990) . 2.8.4 Bactericides Several studies have investigated the use of bactericides in the control of acid mine drainage and in land reclamation (Rastogi et aI . , 1 986; Watzlaf, 1 988 ; MVTI, 1 994; Parisi et aI . , 1 994; Splittorf and Rastogi, 1 995). Bactericide agents such as sodium lauryl sulphate (SLS) have been shown to effectively kill Thiobaccilus ferrooxidans (Kleinmann and Erickson, 1 983). Field trial results after the application of 5 1 4 L ha- I of 30% SLS solution has been reported to reduce acid production by 60% to 90% over a period of 4 to 5 months (Erickson et aI . , 1 985) . Although bactericide agents such as ProMac have been used for controll ing acid generation from bacterial oxidation of sulphidic mine wastes, their application in mine reclamation and revegetation have been trialled only recently (Watzlaf, 1 988a; Parisi et aI . , 1 994; MVTI, 1 995). Column leaching studies on ProMac-amended mine wastes have been shown to effectively reduce acidity and sulphates in the column leachate by 85% and 73% respectively (Parisi et aI . , 1 994). In some reclamation trials, use of ProMac-based slow-release capsules have been shown to reduce acidity and aluminium by up to 98% and 95% respectively (MVTI, 1 989). However, in sulphidic materials that are partially oxidised and already colonised by ThiobaciLli, Watzlaf ( 1 988) indicated that bactericides might not be effective in suppressing bacterial activity. Although bactericides can temporari ly disrupt the activity of acidophilic bacteria, recolonisation of the local environment is likely once the bactericides become depleted (Fil ipek et ai, 1 996) . Most bactericides are surface-applied as spray or in powder form. 46 On steep mine pitwall slopes, the effectiveness of bactericides by such methods of application may be affected by high losses due to erosion and runoff leaching. Slow­ release formulations are commercially available that reduce acid generation up to 1 0 years (Sanda, 1 989) . Ideally, this i s long enough to establish stable soil and vegetative covers that prevent further acidification. 2.8.5 Reclamation by revegetation The primary purpose of establishment of vegetation on barren mine sites has been to control against erosion from wind and water and stabilise slope surfaces as well as providing an aesthetic appearance to the site (Sengupta, 1 994). A more obligatory . purpose is that many countries with active mining operations have promulgated stringent, specific, and detailed rules and regulations concerning mined land reclamation. Therefore, the issue of environmentally responsible mining is not whether a mine site will be reclaimed, but rather, the manner in which reclamation is accomplished (Todd and Struhsacker, 1 997) . In its most basic form, mined land reclamation is a "green is good" and "grow grass" proposition. Revegetation of metall iferous mine sites and coal spoils have had varying degrees of success mainly because the ameliorative approaches used to establish plant growth have not been perfected as yet . Several approaches to revegetation have been practised in mine sites contaminated by AMD. Most mine waste dump sites and abandoned pits have steep slopes which require heavy engineering modification prior to placement of stabilised amendments (Jeffrey et al . , 1 974; Johnson et al. , 1 977 ; Sencindiver and Bhumbla 1 989, Sengupta, 1 994; Dick et al . , 1 994). Alkaline materials such as limestone, dolomite and fluidised bed boiler ash are the most commonly used amendments for pH modification to suit establishment of vegetation (Joost et al . , 1 987 ; Pulford, 1 99 1 ) . A variety of organic waste materials and industrial by-products have found increasing use in reclamation of mined land (Hoitink et al . , 1 982 ; Joost et al . , 1 987 ; Pulford, 1 99 1 , Sopper, 1 992; LRNL, 1 993 ; Shelp et al . , 1 994) . In mine waste material devoid of plant growth media, an engineered soil cover not only aids in vegetation establishment but also serves as oxidant infiltration barrier for 47 sulphide oxidation (Cairney, 1 987 ; Bell et aI . , 1 995 ; Swanson et aI . , 1 995). Design and performance of soil cover in terms of thickness and stabi lity has been site specific so far (Hoving and Hood, 1 984; Barton-Bridge and Robertson, 1 989). In plant growth trials conducted at Martha mine site on oxidi ed mine waste, Gregg et al. ( 1 990) indicated that topsoil depth was not a critical factor in the reclamation of land being returned to pasture. While this may be true for pasture grass that requires a minimum rooting depth for propagation, establishment of higher order plant species (shrubs and native plants) may not be possible on slopes that restrict placement of amendments. Bioengineering (synonymous with biotechnical erosion control , biostabilisation or soil­ bioengineering) is an alternative approach to revegetation on steep slopes areas prone to intense erosion. B ioengineering is a method of construction using l ive plants alone or combined with dead or inorganic materials, to produce living functional systems to provide erosion protection for hillslopes, stream banks and lakeshores (Gray and Laiser, 1 982; Franti, 1 997). There are a number of bioengineering techniques available for erosion control and these include : - (i) contour wattling, (ii) brush layering, (iii) trench packing (iv) brush matting (v) prevegetated mats (vi) interplanting rip rap and (vii) staking. Often a combination of these methods are used depending upon site-specific requirements (Franti, 1 997) . These methods are, however, designed for plant establishment on non-sulphidic material where low pH conditions are not a limitation for plant establishment. Methods such as brush layering (construction of micro-trenches on slopes and using live branches and soil cover to create a series of reinforced benches) and prevegetated mats ( live plants grown on a movable mat of organic material) may be suitable for application to pyritic pitwalls, provided AMD is controlled with suitable amendments. 2.9 Revegetative Reclamation in New Zealand In New Zealand, the 1 99 1 Resource management Act (RMA) imposes resource consent that require a mining company to "promote the sustainable management of natural and physical resources, which underpins the environmental requirement to reclaim land disturbed by mining operations " (Gregg et al . , 1 998). Compared to USA, UK, Canada and Austral ia, revegetative rec lamation practices for mined land in New Zealand is only a few decades old. This is mainly due to the fact that compared to many other countries, 48 New Zealand has been l ittle impacted by mining activities. Since the advent of active mining in New Zealand in the early 1 850s, only < 0.02% of the country ' s land surface is estimated to be disturbed by some form of mining activities (Gregg et aI . , 1 998) . While there are several active and abandoned mine sites in New Zealand, lack of mine survey data make it difficult to estimate the number affected by AMD. This author believes that many coal mines in New Zealand would experience some degree of AMD from dumpsites because of the fact that coal seams often contain pyrite mineralisation. The prominent mines sites in New Zealand that are affected by AMD and need some form of remediation measures for successful reclamation are listed in Table 2.4. Reviews of land reclamation by revegetation on alluvial gold mine, aggregate mine and coal mines occur in Morrell ( 1 997) and Gregg et al . ( 1 998) . Many mines in New Zealand are located on pasture lands, and therefore, restoration to pasture is the norm in the reclamation practices. Table 2 .4 Reclamation of AMD contaminated mine sites in New Zealand. Mine sites Reclamation Martha Mine, Waihi . Oxidised waste and tailings revegetated with pasture Open pit mining of gold- grass. Lime, organic compost and ferti lisers used as silver ore. Dominant sulphide amendments (Gregg & Stewart, 1 990, Gregg et aI . , mineral- FeS2 . 1 995 ; Gregg et aL , 1 998; Mason et aI . , 1 995) . Pitwall hydro seeded with grass and legume mixture. Successful on areas containing weathered oxidised rock materiaL Plant growth failed to establish on pyritic areas of the pitwall (Mason, 1 996; Gregg et aI . , 1 998) . Golden Cross Mine, Waihi . Small scale revegetation with exotic grasses and Open pit/underground mining native plants (Bokich, 1 995) of gold-silver ore. Dominant sulphide- FeS2 Tui Mine, Te Aroha. None. Over 20 years of abandonment , natural Underground base-metal vegetation colonisation on the acidic tailings dam has mining. Dominant sulphides- not occurred. Glasshouse plant growth trials have PbS , ZnS , CuS and FeS2 indicated that the tailings material has the potential for revegetation with suitable metal tolerant plant species and lime and organic based amendments (Morrell et aL , 1 995) . Macraes Flat Mine, Eastern Significant areas revegetated with pastoral grass. Otago. Open pit gold mining. Arid conditions help minimise AMD from sulphide Dominant sulphides- FeS2 . oxidation (Gregg, 1 997). 49 2.10 Summary and Conclusions Acid mine drainage from mines containing sulphidic ores and its preventive and control measures are a major environmental issue in the mining industry. The understanding of AMD, its prediction and treatment are the subject of a substantial research effort by government, the mining industry, universities and research establishments with input from the general public and environmental groups . Increasing worldwide awareness of the environmental impacts of mining activities has directed government agencie to lay down strict guidelines for reclamation and rehabilitation of mined lands. So much so that in New Zealand, in an effort to project the country as having a "clean green image", there are even stricter Resource Management Act criteria that control the mining agencies activities. Acid generation from pyrite oxidation is the principle cause of AMD. The understanding of the biochemistry of pyrite oxidation and the resulting by-products has lead to significant advances in AMD mitigation technology. The nature of pyrite oxidation itself is site-specific and dependent upon a number of factors including host rock mineralogy, weathering conditions, forms and distribution of pyrite and local climatic conditions. AMD generation depends primarily on the abundance or ratio of acid producing sulphide minerals to acid neutralising alkaline minerals. The presence or absence of acid- neutralising material is general ly regarded as the most critical factor in determining the intensity and migration of AMD in the mine waste rocks. Predictive techniques employed in the assessment of AMD do not have standardised applications as yet mainly because acid generation in mine sites is site-specific. Appropriate predictive testing along with waste characterisation and scientific interpretation of the data are essential if proper AMD prevention and management practices are to be developed, disseminated and sustained. Despite the uncertainties in AEA screening criteria, the ABA method of evaluating acid generating potential of sulphidic mine waste materials has become mandatory requirement of regulatory guidelines in mining industry. Liming, organic waste application and less commonly bactericides, have so far been the common amendment materials in the vegetative reclamation of mine sites. However, 50 these amendments have had varying successes in alleviating low pH conditions created by AMD mainly because of difficulties in assessing the l ime requirement of sulphidic waste rocks. Because acid generation from pyrite oxidation is a continual process, the ABA method of e timating the l iming requirement of sulphidic waste rock has been inadequate for providing long-term pH modification suitable for sustaining plant growth. There is an apparent lack of kinetic data on comparative effectiveness of alkaline materials (l imestone, dolomite, reactive phosphate rock, fluidised bed boiler ash etc . ) in in-situ amelioration of AMD conditions in weathered pyritic waste rock materials. The site specific characteristics of the AMD necessitates investigative evaluations of the ameliorating effectiveness of amendments in creating suitable plant growth media. It is evident from this review that the major emphasis has been placed on investigations regarding mechanisms and probable technologies (e.g. phosphate coatings, cover materials, collection and treatment with lime additions and bactericides) for prevention and control of AMD primari ly arising from coal refuse, mill tai l ings and waste rock piles . Many of the procedures and techniques discussed in the review are site specific in nature and often expensive to implement in field scale. Very l ittle research has been undertaken to investigate the in-situ characterisation and mitigation of AMD conditions on active mine pitwalls like that of Martha mine at Waihi , although many of the waste rock characterisation procedures and preventive measures can equal ly be applied to pitwall rock materials. 51 C h a p t e r 3 Characterisation of Pyritic Pitwall Rock at Martha Mine, Waihi 3.1 Introduction Pyritic pitwall rock surfaces are significant sources of AMD and metal contaminants. The primary step in the prediction and control of AMD has been the identification of source material and characterisation of sites affected by AMD. The sites of concern have generally been the sulphidic waste rock dumps from hard rock and coal mining and the control-treatment efforts are general ly centred on the resultant AMD prior to discharge into a natural drainage system. Much of the effort to mitigate AMD in the past has been directed at both management of waste rock and effluent streams but little has been directed towards mitigating AMD problems at source (mine pit). A literature review of AMD revealed that virtually no attention has been directed towards AMD generation and mitigation in open pit mines and pitwalls. Low pH conditions resulting from oxidation of pyrite in parts of the pitwall rock exposed on the north face pitwalI of Martha mine has been a major obstacle to revegetation. Amelioration of such low pH conditions for plant growth will involve combinations of control , prevention and treatment of AMD. The primary step in the mitigation of AMD effects at any mine site must, therefore, involve detailed characterisation of the mineralogical and geochemical properties of the pitwall rock, and identifying the AMD products that are likely to cause limitation to plant growth. The objectives of this part of the study were thus to examine the site specific geochemical characteristics of part of the pyritic rock exposed on the pitwall at Martha mine and map spatial distributions of selected chemical properties. Such a characterisation is an important primary investigative step in defining the factors preventing establishment of plant growth and providing prescriptive design for control and remediation of sites affected by AMD. 52 3.2 Materials and Methods 3.2.1 Site selection and sample collection The study area is located on the north face of the Martha Mine pitwall and covers a slope area of 1 200 m2 (Plate 3 . 1 ) . This area of the pitwall contained pyritic andesite undergoing progressive weathering and a varying degree of pyrite oxidation, resulting in low pH conditions. The area had been hydroseeded with grass in an earlier unsuccessful attempt to revegetate the pitwall . Representative samples of the pitwall rock used to study the geochemical characteristics of the pitwall are described in Table 3 . 1 . About 1 0 kg of each representative samples were collected from the north facing pitwall in November 1 995. Duplicate samples were oven dried and ground to � 2 mm and < 1 50 !im fractions for chemical and elemental analysis respectively. The samples were stored in sealed containers to prevent further oxidation. 3.2.2 Grid sampling The grid samples for spatial characterisations of weathered material were collected from the north face of the pitwall (Plate 3 . 1 and Figure 3 . 1 ) . A 40 m x 30 m surface on the pit slope was measured and sampled at a major grid interval of 5 m. Grid sampling was also done at 2.5 m intervals along the down-slope profiles. At the time of sampling (November 1 995) the cover depth of weathered materials on the study area (Figure 3 . 1 ) varied from 0 mm on hard rock surfaces to 600 mm in post-blast cavities and in colluvia1 materials deposited along the bottom of the pitwal l . The pitwall surface had a closely spaced, parallel rill erosion pattern from surface runoff and patches of reddish-brown coatings of Fe-hydroxides (Plate 3 .2) . Pit slope angle was 43° with a total slope length of 30 m. Where possible core samples a t each grid site were taken down to the depth of the hard rock surface. About two kilogram samples (47 samples) were collected from each of the grid points. Sampling locations and the grid plan is shown in Figure 3 . 1 . The samples were air-dried and ground to � 2 mm and � 1 50 !im fractions for chemical and total elemental analysis respectively. 3.2.3 Spatial data analysis Spatial distribution maps of the selected chemical properties of the pitwall rock within a 40 m x 30 slope area were obtained by the geostatistical method of kriging. Kriging is a geostatistical interpolation technique based on the theory of rationalised variables with a Plate 3 . 1 Martha mine showing the study area on exposed north face pitwall ( 1994). 3 2 2 E - .c -C) c 1 � 8. 0 en 0,--- • 5 1 0 • 1 5 20 25 Slope width (m) 30 Top wal l GSC-1 • 35 40 Bottom wall Figure 3. 1 Grid sample locations (PWGS- l , GSC- l . .) of the study area in Plate 3 . l . Contour values represent depth of cover material in mm. 53 54 55 spatial distribution of values (Webster and Burges, 1 984; Bemdtsson et al . , 1 993). Surfer for Windows software (Keckler, 1 994) was used to generate contour and 3D surface maps. Data kriging was performed using a linear semi-variogram model. 3.2.4 Mineralogical and geochemical studies Petrographic examination of the samples described m Table 3 . 1 were conducted by transmitted and reflected light microscopy on polished thin sections and grain mounts respectively. Scanning electron microscopy (SEM) examination and energy dispersion spectroscopy (EDS) analysis of the representative samples were made with a Cambridge Stereoscan 250 Mark 3 Scanning Electron Microscope, with an attached Link E543 1 EDS detector. Table 3 . 1 Description of the pyritic pitwall rock samples. Sample No. Description PWR- I Light bluish-grey, hard, massive fresh pyritic andesite PWR-2 PWR-3 PWR-4 PWR-5 Freshly weathered, bluish-grey, pyritic andesite containing disseminated pyrite crystals and lenses of pyrite Freshly weathered dark blue pyritic rock with disseminated pyrite mineralisation Light bluish-grey, moderately weathered pyritic andesite containing disseminated pyrite crystals and lenses of pyrite Light grey, strongly weathered pyritic rock forming loose cover material on the pitwall surface Samples were collected from north face pitwall during November 1 995. Determination of major oxides and trace elements was carried out usmg a Siemens SR303AS wavelength dispersive fluorescence spectrometer (XRF). Samples for the major oxides were prepared by fusion with lithium tetraborate/metaborate flux and cast into solid solution glass discs. Trace elements were determined on 40 mm diameter pressed powder samples with 1 0% wax added as a binder. Relative masses of the main minerals were also determined semi-quantitatively by X-ray diffraction analysis (XRD) using a Philips PW 1 7 1 0 instrument. 56 3.2.5 Analytical methods The analytical procedures outlined below apply throughout the thesis and will be referred to frequently. pH in water The pH was measured in a suspension containing sample to deionised water ratio of 1 :2 .5 ( 1 0 g air-dried ::; 2 mm waste rock to 25 ml deionised water) . The stirred sample mixture was left to stand overnight and stirred again 30 minutes prior to pH measurement. A Radiometer PHM83 Autocal pH meter fitted with a Radiometer GK240 1 C combined glass/reference electrode was used to measure pH. The method for measuring pH is outlined in Blakemore et al. ( 1 987). Leachate pH was measured in about 25 m} solution. Electrical conductivity (EC) The methods for extraction of soluble salts and EC measurement is described in detail by Metson ( 1 956) and is outlined in Blakemore et al . ( 1 987). The EC was measured in a solution extracted from 1 : 5 sample to deionised water ratio. A 1 0 9 air-dried (::; 2 mm) sample was mixed with 50 ml deionised water in a 1 00 m! beaker, stirred well , allowed to stand 1 hour and agitated for 5 minutes prior to filtering through Whatman filter paper No. I . A Portable Conductivity Meter HI8633 (HANNA Instruments) was used to measure EC (dS mo l ) at 25°c solution temperature. A reference 0.0 1 M KCI solution was used to standardise the EC reading at 35°C. The EC ratings in dS mo l ) are: >2.0 := very high, 0.8-2.0 := high, 0.4-0.8 := medium, 0. 1 5- 0.4 := low and <0. 1 5 := very low. Total soluble salts (%) = EC (dS m-I) x 0.35. (Blakemore et aI . , 1 987). Leachate EC was measured in 30 ml solution. Base cations (Ca, Mg, K and Na) Exchangeable bases were determined by the Silver Thiourea (AgTU) method of Pleysier and Juo ( 1980) as outlined in Blakemore et al . ( 1 987). Standard CEC methods using 1 M ammonium acetate (pH-7.0) and high electrolyte concentrations can result in hydrolysis or salt adsorption and is considered to overestimate CEC for variable-charge soils (Gill man , 1 979). The single extraction AgTU method for measuring exchangeable cations and effective cation exchange capacity (ECEC) uses an unbuffered 0. 1 M solution. The AgTU complex is considered an efficient exchanger of cations on clay surfaces (Gillman, 57 1 979) and the method has b�en used in a wide range of New Zealand soils (Searle, 1 986). A 0.8 g air-dried (2 � mm) sample was mixed with 40 ml 0.0 1 M AgTU in a 50 ml polypropylene centrifuge tube and shaken overnight (- 1 6 hrs) on a end-over-end shaker and centrifuged @2000 rpm for 1 0 min and filtered through Whatman filter paper No.4 1 . The concentration of bases in the extract was measured by atomic absorption/emission spectrometry (AAS/AES). The ECEC (cmolc kg-I) = L bases + exchangeable Al (Alexc) and the base saturation (BS%) = L bases / ECEC. KCI-extractable exchangeable Al (Alexc) Exchangeable Al was extracted with 1 M KCI solution according to method outlined in Blakemore et al . , ( 1 987) with a minor modification in the extraction procedure. 5.0 g air­ dried (� 2mm) sample was mixed with 25 ml 1 M KCI in a 50 ml polypropylene centrifuge tube and shaken overnight (- 1 6 hrs) on a end-over-end shaker and centrifuged @2500 rpm for 5 min and filtered through Whatman filter No.42. The Alexc in the extract was measured by AAS using a N20/acetylene flame. Dissolved Al concentrations in the leachate and runoff drainage water were measured by AAS in 0. 1 M HCI matrix. Total Sulphur (S) Total S was determined by using the sodium bicarbonate/silver oxide (NaHC03/AgO) fusion method of Steinbergs et al . ( 1 962). The method involves ashing of 0.25 g of sample « 1 50 Ilm) with 0.5 g of NaHC03/AgO mixture at 550°C for 3 hours. The ashed sample is solubilised in 5 M HCI and an appropriate dilution carried out before determining the concentration of total S by the turbidimetric method using an auto­ analyser as described in Blakemore et al. ( 1 987). Sulphate sulphur (SOl) Adsorbed sulphate concentrations in the sample was determined as per method of Searle ( 1 979) outlined in Blakemore et al . ( 1987) . The method basically involves displacement of sol- by HPol- ion and measuring the displaced solo. 5.0 g of air-dried (� 2 mm) sample was extracted with 25 ml 0.0 1 M calcium di-hydrogen phosphate [(Ca(H2P04h.H20] solution in a 50 ml polypropylene centrifuge tube. The mixture was shaken for 0.5 hour on an end-over-end shaker, centrifuged @9000 rpm for 5 min and filtered through Whatman filter No.4 1 . The sol in the extract was measured turbidimetrically in an auto-analyser. Leachate and runoff drainage water sol- was 58 measured in a 5 ml aliquot with appropriate dilutions. Sulphide S in the sample was obtained by subtracting sol- from the total S (organic S is assumed negligible in the pitwall rock). Determination ofF e and Mn Concentrated HN03 and dilute HC} are conunonly used in extracting total and soluble plant available metals in soils (Viets and Boawn, 1 965 ; Smith and Bradshaw, 1 979). In a comparison of chelating agents for extracting for metals from diverse soil materials, Norvell ( 1 984) observed that 0. 1 M HC} was severe enough to extract most metals particularly Fe, Mn and AI. Dilute HCl is commonly used for extracting plant available heavy metals. The original method of Viets and Bowen ( 1 965) was used to extract soluble fonns of Fe and Mn in the sample. 5 .0 g sample was mixed with 25 ml 0. 1 M HCI in a 50 ml centrifuge tube and shaken overnight in an end-over-end shaker. The mixture was centrifuged @9000 rpm for 5 min and metals in the extract were measured by AAS . Dissolved Fe and Mn in the leachate and drainage water were measured by AAS in 0. 1 M HC} matrix . Total Fe and Al was extracted with 25% HN03 according to the method outlined in Smith and Bradshaw, ( 1 979) . 0.5 g air-dried « 1 50 Ilm) sample was digested with 25 ml 25% HN03 in a 200 ml digestion tube @ 1 00°e. The digest was centrifuged @9000 rpm for 5 min, filtered and diluted before measuring the concentration of Fe by AAS. Pyritic Fe in the sample was obtained by subtracting 0. 1 M HCl-extractable Fe from total Fe. Acidity Acidity was measured in 1 : 5 pitwall rock to deionised water extract. 1 0 g sample was mixed thoroughly in 50 ml deionised water and left to stand overnight (- 1 6 hr) . The solution mixture was filtered through Whatman filter paper No.4 l . A 1 0 ml aliquot was diluted with 20 ml deionised water and titrated with 0. 1 M NaOH to pH 8 . 3 using a Mettler DL2 1 Autotitrator. Acidity in the leachate was measured with the same dilution ratio. The amount of NaOH consumed was converted to kg CaC03 equivalent. 59 3.2.6 Net acid generation (NAG) static test Static NAG tests were carried out according to the method outlined by Miller and Jeffrey ( 1 995), where 2 .5 g of pulverised (S; 1 50 �m) sample was treated with 250 rnl 1 5 % H202 (pH stabilised to 5 .5 with 0. 1 M NaOH) in a 500 rnl beaker under a fume-hood. The beaker was kept covered with a watch glass until vigorous reaction stopped. The sample was heated on a hot plate for 2 hours and cooled to room temperature before measuring the pH (NAGpH) in the residual solution. The sample was then made up to 250 rnl with deionised water and titrated with standardised 0.5 M NaOH to pH 7. The net acid generation (NAG) by the sample was calculated as : I 50 x [volume NaOH, ml] x [molarity of NaOH ] NAG (kg CaC03 f ) = ---------------'----'---- weightof the sample (g ) 3.2.7 Acid neutralising capacity (ANe) The neutralisation potential (NP) method from Sobek et al . ( 1 978) was adapted for determination of ANC. It involved treating 2 g of sample with 20 rnl of 0. 1 M HCI in 250 rnl flasks, heating nearly to boiling with periodical swirling until no gas evolution was observed. The sample was made up to 1 25 rnl with deionised water, boiled for 1 minute, then cooled to room temperature. The treated samples were back-titrated with standard 0.5 M NaOH to pH 7. The ANC was calculated as the amount of HCI consumed by the sample and converted to equivalent CaC03. I 50 x [volume HCl consumed, mll x [molarity of HCll ANC (kg CaC03 f ) = ------------------- weightofth esample (g) 3.2.8 Acid base accounting (ABA) The net acid producing potential (NAPP) of the sample was derived from total sulphide-S analysis as per standard acid base accounting analysis (ABA) procedures described in section 2 .5 . 1 (Chapter 2) . 3.2.9 Net acid generation (NAG) kinetic test The NAG kinetic test was carried out according to the procedure described by Miller ( 1 996). Although this method is still in its development stage, it has been routinely used 60 by Environmental Geochemistry International Pty. Ltd. to determine the real time kinetics of sulphide oxidation and the onset of acid generation, termed a ' lag-period ' . The NAG test kinetic resul t is compared with the column test data to estimate the approximate lag­ period. The NAG test reaction time (X) to reach pH 4 is used to extrapolate the Jag-period (Y) from the linear regression equation of the form: Y = 2.4*X + 0.60 (R2 = 0.88) obtained by Miller ( 1 996) from static and kinetic data of 1 8 different waste rock samples c lassified as potentially acid forming. In this test, 2 .5 g of pulverised pitwall rock sample « 1 50 /-lm) was oxidised with 1 5% H20z, following the procedure for the NAG static test. Temperature and pH of the test samples were monitored at 2 minute intervals until the reaction was complete. The temperature and pH were plotted against time for extrapolation of approximate time taken for complete oxidation of sulphide mineral . 3.2.10 Column test Column test for kinetic evaluation of the acid generation by fresh rock (PWR- l ) and freshly weathered/partially-oxidised rock (PWR-3) was done according to the modified method of Miller and Jeffrey ( 1 995). Although the test is still in its development stages, such a real time kinetic test can give an approximate indication of the time (lag-period) involved in the oxidation of pyrite present in a sample. Samples PWR- l and PWR-3 were used in the column test in order to compare the rate of release of sulphate and acid produced between fresh and partial ly oxidised samples of pitwall rock. The column test set up for evaluating acid generation and leachate chemistry is shown in Figure 3 .2 . One kilogram of representative sample (nominal size :::; 4 mm) was packed in the column and initially leached with 200 ml of deionised water over a two-day period. The leachate collected was labelled week- I . Subsequent leaching was done with 100 ml of water on a 7 days cycle. During the 7 days non-leaching period the column surface was kept at a temperature of 35°C ± 2°C with an overhead heat lamp to simulate a dry period and to provide an optimum temperature for Thiobacillus ferrooxidans activity. Bacterial activity is considered to be most active at this temperature range (Carrucio et aI . , 1 988) . The leaching was carried out in duplicate for 1 0 weeks. Leachate pH, EC, S042- and acidity were monitored weekly. 60 Ilm nylon mesh 40 W heat lamp adjusted to provide - 35 °C surface temperature Leachate 1 00 mm Shelf Bench Figure 3 .2 Column test set up (adapted from Mil ler and Jeffrey, 1 995) 61 62 3.3 Results and Discussion 3.3.1 Physical and geochemical characteristics of the pitwall rock 3.3. 1. 1 Physical characteristics At the time of field sampling (December 1 994 and November 1 995) , the pitwall consisted of waste rock materials undergoing various degrees of physiochemical weathering. The weathered rock material generally appeared l ight-grey in colour with numerous patches of reddish-brown Fe-hydroxide coatings (Plate 3 .2a). Freshly weathered wall rock generally appeared light bluish-grey whereas the fresh pyritic rock appeared dark blue. Cobble sized fresh pyritic rock commonly occurs embedded within the finer grained, light-grey weathered material (Plate 3 .2b). The pitwall rock as a whole appeared highly fractured and the fractures were commonly filled with limonitic clay material . Disseminated grains and lenses of pyrite and occasional fine-grained quartz and calcite veins were observed in some hard, fresh rock. Weathering was found to be more intense in areas where pyrite vein mineralisation was most abundant. Due to the steep gradient of the pit slope (43°) , most of the weathered, loose material had gravity-sloughed towards the bottom of the pitwall to form a colluvial deposit. Rill and sheet erosion of the weathered and oxidised loose rock materials facilitated rapid down-slope migration of the weathered materials and pyrite oxidation products. Grain-size analysis of the weathered rock materials indicated that it contained 2% clay (S:; 0.06 mm), 35% fines (0.06 - 2 mm) and 63% coarse materials (2 - 1 00 mm). Bulk density varied from 1 .23 to 1 .6 Mg m-3 with a mean of 1 .4 Mg m-3 . The field moisture content of the weathered pitwall rock varied from 4.3 to 1 6.3% by weight (mean 9 .3%) . The water content (field capacity) of the weathered pitwall rock was 1 4.2% by weight at - 1 0 KPa. Pore water content of the pitwall rock samples (S:; 4 mm fractions), determined by saturating a 1 kg column of sample with distilled water and oven drying at 1 05 °C, was 47% by weight. 63 3.3. 1.2 Mineralogical properties Petrographic examination of fresh pyritic rock samples showed that pyrite commonly occurred as fine-grained, disseminated euhedral crystals and as lenses. Colloform bands of very fine-grained pyrite were also observed. Some pyrite grains showed scoured surface striations and circular cavities which may have been remnants of fluid inclusions or etch marks caused by bacterial activity. No framboidal forms of pyrite were observed in the thin sections. An average of 1 1 liberated or partially l iberated grains up to 0.5 mm size were identified in 1 0 g sample (� 2 mm) of the fresh rock (PWR- l ) under reflected light. In the strongly weathered material (PWR-5), the average number of liberated pyrite grains was 5 in 1 0 g sample (� 2 mm). Most pyrite grains appeared euhedral, except in some weathered samples where some of the pyrite grains showed rounded edges which were due to the effect of weathering and oxidation. SEM and EDS examinations of the fresh rock samples highlighted the disseminated and lensoid forms of pyrite mineralisation observed under the petrographic microscope (Plate 3 .3a & b). The pyrite grains in the lenses (Plate 3 .3b) were very fine-grained « 40 Ilm). Observation of fresh pyrite crystals under SEM scans showed that they have a perfect euhedral form (Plate 3 .4). An EDS analysis of the grains indicated that the crystals were stoichiometrically pure pyrite (Fe : S ratio of 1 : 2) as indicated by the EDS spectrum and elemental analysis (Figure 3 .3 ). Exposed pyrite crystals in weathered rocks showed moderately rounded crystals (Plate 3 .Sa). Fresh rock also contained rhombohedral crystals of calcite as vein fillings with perfect 0 1 0 cleavage surfaces, giving them a flaky appearance (Plate 3 .Sb). No framboidal forms of pyrite were observed in the samples, consistent with their abiotic origin. In strongly weathered samples, clusters of pyrite crystals were entirely coated with a thin film of highly siliceous material (Plate 3 .6) . An EDS analysis of the coated pyrite grains (Figure 3 .4) indicated that the presence of a significant amount of Si ( 16.4%), Al ( 1 .0%) and K ( 1 .8%). It is possible that under very acidic conditions, decomposition of silica and clay minerals may form precipitates that selectively coat the pyrite grain. Possible 64 evidence of bacterial oxidation of pyrite was observed in SEM scans of some pyrite grains where characteristic etch pits were observed (Plate 3 .7), which are considered to result from bacterial activity (Mustin et aI. , 1 992). Similar etch pits were also reported in pyrite from Tui mine tailings by Morrell et al . ( 1 996). XRD analysis of the rock samples (Table 3 .2) indicated that the fresh rocks (PWR- l ) contained up to 1 2% by weight CaC03. Pyrite content was greater in the rocks containing both vein as well as disseminated pyrite mineralisation. A trend of increasing quartz content with increasing degree of weathering was observed. Feldspars remained resistant to weathering except in strongly weathered rocks (PWR-5) . Clay content (mostly chlorite, illite and verrniculite) generally increased with progressive weathering and was highest in the strongly weathered sample. Table 3 .2 XRD analysis of meUor minerals (wt. %) in the pitwall rock PWR- I PWR-2 PWR-3 PWR-4 PWR-5 Quartz (Si02) 50 60 60 67 70 Pyrite (FeS2) 1 0 1 6 6.8 1 4 4 .3 Calcite (CaC03) 1 2 1 .3 0 0 0 Gypsum (CaS04) 0 0 0 0 2 .5 Feldspars 25 20 20 30 0 Clays * 1 1 I S 1 3 1 5 1 7 * About 60% kaolinite, m inor i l l i te and chlorite. Strongly weathered samples contained <5% pyrite compared with > 1 0% pyrite in fresh to moderately weathered rocks. With progressive weathering, the reduction in rock grain­ size resulted in more liberation of the entrapped pyrite and hence a lower content of pyrite was expected in more weathered rocks, due to loss from oxidation and dissolution. The strongly weathered samples also contained 2 .5% gypsum, indicating that salt precipitation is a common phenomenon during weathering of pyritic pitwall rock under dry environmental conditions. 65 Plate 3 .2 A section of the pitwall showing (a) Fe-hydroxide coatings and rill erosion pattern and (b) fresh (blue) rock in weathered matrix. 66 67 Plate 3.3 Back-scattered SEM scan of (a) disseminated pyrite crystals and (b) fine grained pyrite lens. 68 .' �. F':'-'..1- r'1E Plate 3 .4 SEM scan of euhedral pyrite grain from fresh rock F 0 • '_J 5 . 90 0 k e:l.) 1 K c h 30 5= ELMT. S Fe ATOM.% 65.9 34. 1 1 77 1 . 0 > e t,. :; 69 Figure 3.3 EDS spectrum of semi-quantitative elemental analysis of the pyrite grain In Plate 3 .4 . 1 1 1 1 1 1 1 1 1 70 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 I \1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ \ \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 1 \ 71 Plate 3 .5 Forms of pyrite grains Ca) sub-rounded pyrite crystals in weathered pitwall rock and Cb) flaky calcite grains in fresh rock. 72 Plate 3 .6 Si lica coated pyrite grains in weathered pitwall rock. � �' � I rr K m . ", F !I :l : � : ! , i := 4 1 k j' :i� :i j ; ::;� r !�r,:i: Cod . K i!�: L l i J :i � ' lJ::: � ! !� ' ; : I::;: fll!: f:: ' . . -iV:!' : i - .. · , I; ' , . " \ .< • E, FS= 1 K r'1Et'1 1 : PL·JF� 1 I::" 720 I.' -. I t _J . I r. t:. ... I:: h 296= ELMT. AI S i S K Fe % ELMT. I . ! 1 6.4 42.9 1 . 8 1 7 .9 1 0 . ::: > 1 6lf c t. s Figure 3 .4 EDS spectrum and quantitative elemental analysis of pyrite grain shown in Plate 3 .6. 73 74 75 Plate 3 .7 Pyrite grains with etch holes (a) Magnification x2000 (b) Magnification x20000. 76 77 3.3. 1.3 Chemical properties Chemical characteristics of the pitwall rock showed that, with progressive weathering and oxidation, there were gross chemical changes in the composition of the rocks (Table 3 .3) . Mean chemical properties of the pitwall rock samples listed in Table 3 .3 showed that the fresh sample (PWR- 1 ) and the freshly weathered sample (PWR-2) had a near neutral pH of 7 .3 and slightly acid pH of 5 .9 respectively. With increasing degrees of weathering there was a corresponding decrease in pH, the strongly weathered rock sample (PWR-5) having a pH as low as 1 . 8 with a mean of 2 . 1 ± 0.4. Fresh and freshly weathered rocks had an EC < 2 dS m- I whereas moderately to strongly weathered samples in colluvial deposits had an EC > 2 dS m- I because of increase in sol- levels. There was a general increase in solo, soluble Fe, Mn and exchangeable Al (Alexc) from fresh to weathered pitwall rock. The increase in A1exc from fresh to weathered rock samples is mainly due to contribution from decomposition of aluminosilicate minerals and increasing mobilisation with decreasing pH. Soluble Fe and sol- in the samples ranged from as low as 477 mg kg- I and 460 mg kg- I to 4 1 00 mg kg- I and 7440 mg kg- I from fresh to strongly weathered pitwall rock, respectively. Total S was highest in the samples containing pyrite lenses as well as disseminated pyrite mineralisation (PWR-2, PWR-3 and PWR-4). The total Fe was highest (4.6%) in PWR- l , remaining constant in all other samples . Generally, the pitwall rock had very low concentrations of base cations. The effective cation exchange capacity (ECEC) was in the medium range for all the samples although there was nearly a two-fold increase in ECEC from fresh to strongly weathered sample mostly due to the effect of increase in Alexc. The base saturation (BS%) however, was very high in the fresh rock and decreased gradually with increased weathering, as a result of leaching of base cations. An apparent increase in exchangeable Ca in PWR-5 and Mg in PWR-2 and PWR-4 was possibly due to release of these elements from decomposition of feldspars and ferromagnesian minerals in acidic conditions. Exchangeable K+ and Na+ remained constantly low in all the samples in spite of the differing degrees of weathering and oxidation. 78 Table 3 . 3 Chemical properties of the pitwall rock. Parameters Units PWR- l PWR-2 PWR-3 PWR-4 PWR-5 pH (H2O) 7 .3 5.9 4.7 4.2 2. 1 EC dS m- ' 0. 1 1 .3 0.3 2.8 3 .8 S042- mg kg- ' 460 680 960 3200 7440 Total S % 2.6 3.5 2.5 4.0 3 .4 Soluble Fe mg kg- ' 477 398 2970 3050 4 1 00 Total Fe % 4.6 3 .2 3.6 4. 1 3 .8 Soluble Mn mg kg- ' 2 1 5 27 1 295 357 328 Exchangeable AI cmole kg" ' 2 . 1 6. 1 5 .4 10.6 14.9 Ca cmolc kg- ' 7 .7 4.7 2.4 l .9 5 .3 M a 0 cmole kg- I 2 .5 2.4 3 .3 5 .4 1 .3 K cmole kg- ' 0.3 0.6 0.9 0.4 0.7 Na cmole kg- ' 0.4 0.3 0.4 0.2 0.2 ECEC cmole kg- ' 1 3 .0 14. 1 1 2 .4 1 8 . 5 22.4 Base saturation % 84 57 56 43 33 3.3. 1.4 Depth-profile chemistry of the weathered pitwall rock Selected chemical properties (pH, EC, sol, Al and Mn) of the strongly weathered 60 cm of sloughed pitwall rock material (grid site PWGS-5, Figure 3 .2) were measured at 10 cm depth intervals in order to determine whether stratification of pyrite oxidation products occurred. Results of the analyses of the selected chemical properties are presented in Figure 3 .5 . Vertical profiling of pH variation showed that there was a significant change in pH from the surface to a depth of 40 cm (Figure 3.5a). The mean profile pH was 2.5 ± 0.2. The EC varied from 9.2 dS m- I at surface to 5 .7 dS m- I at 60 cm depth with a profile mean of 7 .2 ± 1 .4 dS m- i . A significantly higher EC at the surface than at depth (Figure 3 .5b), indicated evidence of sulphate salt formation on the surface during the dry period existing at the time of sampling. Formation of sulphate salts such as epsomite (MgS04.7H20), alunite [(KAh(S04h(OH)6] and minor gypsum (CaS04.2H20) on the surface of the pitwall rock were observed both in field and laboratory (section 3 .3 .2.4) studies. The mean SO/- in the 60 cm depth profile was 4950 ± 649 mg kg- ' and varied significantly within the depth profile (Figure 3 .5c). The high and low SO/- levels within the depth profile are likely to be due to distribution of oxidisable liberated grains of pyrite and subsequent formation of sulphate salts with the cations released from weathering and 79 oxidation processes. The maxima in the depth profile could also be due to increased activity of T. ferrooxidans under optimum conditions. Alexc remained at 7. 1 ± 0.8 cmolc kg- ' and showed a significant increase at depth except at a median sampling depth of 35 cm (Figure 3 .5d). Soluble Mn showed an initial increase at median depth of 1 5 cm but otherwise remained more or less constant throughout the profile depth at an average of 467 ± 20 mg kg- ' (F.igure 3 .5e) . Acidity showed a very similar trend to that of sol- and varied significantly within the depth profile (Figure 3 .5f). The high and low acidity values within the depth profile were consistent with the trends in sol- levels in the column sections. Step-wise multiple regression of the measured chemical properties of the depth profile showed that as expected, there was a positive correlation (R2 = 0.93) between acid production and sol­ release. However, S042- versus EC showed a negative correlation (R2 = 0.82) in the depth profile, contrary to the expected positive trend. This could be due to periodic flushing of the salt by infiltrating runoff water and gypsum formation under dry conditions prevailing at the time of sampling. Because of the coarse texture and high porosity of the weathered rock material , moisture and oxygen readily diffuse through to greater depth to facilitate biochemical and abiotic pyrite oxidation and hence acid generation. This characteristic of the weathered pitwall rock has an important implication to remediation measures for revegetation. Any amendments for amelioration of low pH conditions in the pitwall rock must take into consideration the subsurface distribution of pyrite oxidation products (acidity, EC and SOlO) especially in relation to amendment application and selection of suitable plant species to be grown under site specific conditions. 3.3.1.5 Geochemical properties Table 3 .4 is a summary of the average major oxide and trace element concentrations present in the pitwall rock samples characterised in this study. The pyritic rocks showed chemical compositions typical of the Coromandel Group andesitic rocks (Brathwaite and Christie, 1 996) . Weathered pitwall rock generally had higher Si02 contents, with highest 80 (a) pH (b) EC (dS m-1 ) a·o 4 6 8 1 0 I I I I 1 0 I----l - 20 E � .s:: 30 -a. Q) o 40 50 60 (c) (d) AI (cmolckg-1 ) WOO 4 6 8 1 0 I I I I 1 0 I---i - 20 E � .s:: 30 -a. Q) o 40 50 60 (e) Mn (mg kg-1 ) (f) Acidity (t CaC03Mt-1) 800 300 400 500 600 I I I I 1 0 - 20 E 0 - .s:: 30 -0-Q) o 40 50 60 Figure 3.5 Depth variations of selected chemical properties of the pitwall rock. Horizontal bars represent LSD(%) . 81 SiOz (70%) being in PWR-3, indicating a possible residual effect of weathering. FeZ03 was highest (6%) in the fresh rock but was generally 1 % less in the weathered rocks. Both CaO and MgO showed decreasing trends with increasing degree of weathering while NazO was mostly enriched in PWR-4. Water content (L.O. I . ) was highest in the strongly weathered PWR-5. An anomalously high KzO content in PWR-3 and PWR-4 may have been initial residual enrichment of K-bearing feldspar minerals resistant to dissolution under conditions of light weathering. Except for slight decreases in PWR-3, both Ah03 and TiOz remained immobile to weathering. The trace element compositions of the pitwall rock indicated a nearly two-fold increase in Ba in weathered samples (53 1 mg kg- I ) from that of the fresh pyritic rock (PWR- l ), indicating that it is strongly immobilised by S042- to form BaS04. Over most of the Eh­ pH field of water, Ba is immobile as BaS04 and is fixed in soils and rocks materials (Sullivan and Yelton, 1 988; Larocque and Rasmussen, 1 998). A strong linear regression coefficient of RZ = 0.98 between SO/- and Ba supports the above observation. Trace amounts of heavy metals (Cu, Zn, Pb, Cr and Co) were mobilised into AMD solution and removed as indicated by the decreasing levels of metals with increasing degree of weathering. Arsenic (As) increased with progres ive weathering from 2 mg kg- ' in PWR- 1 to 1 0 mg kg- ' in PWR-5 as a result of oxidation of arsenopyrite (FeAsS) and subsequent accumulation in the weathered rocks as insoluble As(S04h complex. Strontium (Sr) was unusually high (2 10 mg kg- I ) in PWR- l , but the level dropped to < 1 00 mg kg- ' in rocks undergoing progressive weathering, indicating that it is highly mobile in the acidic environment. The higher Sr in the fresh PWR- I may have resulted from substitution of Sr for Ca. Rubidium (Rb) concentrations were very high (>200 mg kg- I ) in moderately weathered rocks whereas fresh and strongly weathered rocks contained 10 mg kg- ' and 98 mg kg- ' respectively. The Rb/Sr ratios for the pitwall rocks were thus 1 .0 (PWR- l ) : 0.005 (PWR-2): 0.0 14 (PWR-3) : 0.0 1 9 (PWR-4) : 0.009 (PWR-5) from fresh to strongly weathered rocks. 3.3. 1.6 Effect of weathering on geochemical properties The major changes in mineralogical compositions of the rocks under surficial weathering are described by either changes in volume or mass of the rock with respect to constant mass/volume of resistant oxides such as A}z03 and TiOz. The most commonly used 82 Table 3 .4 Average composition of the pitwall rocks Oxides, % PWR- l PWR-3 PWR-4 PWR-5 Si02 57.95 62.4 1 69.46 67.2 1 65.34 Ti02 0.65 0.69 0.58 0.60 0.78 Ah03 1 5 .35 1 6 .52 1 3 .70 1 4. 1 3 1 5.S7 Fe203 5 .9 1 4.S0 4.09 4.48 4 . 1 2 MnO 0. 1 3 0.09 0.02 0. 1 4 0.02 MgO 4.84 3 . 54 0.75 1 .29 2.90 CaO 7.3 1 2 . 1 1 0.89 1 . 07 0.99 Na20 1 .80 0. 1 6 0.22 1 .67 0. 1 3 K20 0.26 2 .01 6.26 5 .32 1 .53 PzOs 0. 1 9 0. 1 6 0. 1 3 0. l 7 0. 1 4 L.O.I. 5.42 5 .25 3 .S7 4.0S 8. 1 7 SUM 99.79 97.70 99.94 1 00. 1 3 99.97 Trace elements, mg kg-l As 2 4 8 7 1 0 Ba 265 368 588 534 470 Cc 25 23 23 1 9 1 4 Co 22 24 j'J -"- 1 2 1 0 Cr 3 1 7 1 56 1 1 5 1 93 289 Cu 28 29 3 1 26 1 3 Ga 1 5 1 7 1 6 1 5 l 7 La 1 7 1 7 2 1 1 4 7 Nb 6 6 6 7 6 Ni 86 36 25 39 28 Pb 4 5 6 8 1 Rb 1 0 226 3 1 3 227 98 Se 1 0 20 1 8 1 7 22 Sf 2 1 0 23 90 94 1 8 Th 6 1 0 1 1 8 5 U 3 3 5 4 4 V 1 39 1 44 1 06 1 08 148 Y 1 8 I S 1 1 1 3 9 Zn 74 61 67 59 34 Zr 1 00 1 1 3 1 0 1 99 1 1 6 83 method has been the calculation of gains-losses from chemical analysis either in weight (%) or volume (%) basis. Gresens ( 1 967) originally considered compositional-volume relations in metasomatised rocks to determine compositional changes during alteration. Grant ( 1 986) later modified the same principle and he termed it the isocon method. Other workers (Mountain and Williams-Johns, 1 995; Huston, 1 993) used the isocon methods to represent mass transfer of elements in metasomatic and hydrothermal alteration processes. In this study, simple gain-loss diagrams were used to represent compositional changes in the mass of the oxides (%) in the pitwall rock during progressive weathering, relative to constant mass of Ah03. Based on the constant mass of Ah03, the change in mass (.6.M) of the oxides can be represented in simplified Gresens 's equation: Ch · f · Wt. % AI203 (F) Wt. % Oxide (W) ange m mass 0 oXIdes (.6.M) = x ------ Wt. % AI203 (W) Wt. % Oxide (F) Where F, fresh rock composition; W, weathered rock composition. Major gams and losses of oxides calculated from Table 3 .3 and presented diagrammatically in Figure 3.6, showed that the progressive weathering of the pitwall was accompanied by a consistent addition of Si and K, and general depletions in Ca, Mg, Na, P, Fe and Mn. The apparent gain in K in PWR-3 and PWR-4 could be the result of acid leaching of Ca and Na and residual incorporation of K in the formation of the clay mineral i l lite [KA}z(AISi3)O IO(OHh] from weathering of potassium feldspars (KAISi30g) under acidic environments. Gains in K may also reflect a pre-weathering enrichment of K-rich minerals such as sericite and potassic feldspars. The re-precipitation of minerals such as alunite [(KAI3(Si04)z(OH)6J and jarosite [(KFe3(S04)2(OH)6J during dry periods could also elevate the K content temporarily as would in the transformation of feldspars to a potassic phase. Most of the Fe2+ oxidised to Fe 3 + would initially be precipitated as ferric hydroxide [(Fe(OH)3J and release acid in the process. Similar gains in Si and K and losses in Ca, Na and Fe were also suggested by Huston ( 1993) as typical in alteration of pyritic volcanic rocks. The losses in base cations Ca, Mg and Na showed the expected trend of preferential leaching during progressive weathering of the pitwall rock. Losses in base cations 84 30 � 25 <1 Cl) � 20 >< o -0 Cl) Cl) m E c: Q) Cl c: m � 0 1 5 1 0 5 0 30 � 25 < o -0 Cl) Cl) m E c: Q) Cl c: m � 0 1 5 1 0 5 0 N 0 u; N Q Cl) (a) PWR-2 N C') C') 0 0 0 c f= N N :2: � Cl> u.. (c) PWR-4 N C') C') 0 0 0 c f= N N :2: � Cl> u.. en Cl) N :2: <.) Cl) Z en Cl) N :2: <.) Cl) Z � N I (b) PWR-3 N N M C") o 0 0 0 u; f= � N (£ (d) PWR-5 Figure 3 .6 Chemical gain-loss diagrams relative to the fresh rock (PWR- 1 ), for the pitwall rock undergoing progressive weathering and oxidation. 85 generall y increased with increasing intensity of weathering. Greatest amounts of depletion were in PWR-3 and PWR-5. Most of the Mn was also depleted with increasing degree of weathering except for PWR-4, which showed a gain compared to the fresh rock. With progressive weathering and partial oxidation, the overall trend in the distribution of major oxides showed general depletion in major cations except for K, which showed an anomalous enrichment possibly in clays and as a substituted ion in jarosite type minerals. 3.3.2 Acid generation properties of the pitwall rock 3.3.2.1 Static net acid generation (NAG) test When the pitwall rock samples were treated with unstabilised 1 5% H202 solution, the most vigorous reaction was observed in the freshly weathered (PWR-2) and moderately weathered (PWR-4) pitwall rocks. These samples contained pyrite lenses as well as disseminated crystals, which contributed to a higher amount of liberated pyrite grains and resulted in greater reactivity with the oxidant. The average time taken for the reaction to be completed until no effervescence was observed was 44 minutes. Results of the NAG test on the pitwall rock samples showed that they are potential ly acid generating types, as indicated by their NAGpH < 3 (Table 3 .5) . Sulphidic materials with NAGpH < 4 are considered to be potential ly of acid forming type (Miller and Jeffrey, 1 995). The resultant acidity obtained from titration of the test solution to pH 7 showed that samples containing pyrite lenses as well as disseminated crystals, that are undergoing a moderate degree of weathering had potential to produce higher acidity than samples with disseminated pyrite crystals alone. Lower net acid generation in PWR- I reflected the presence of some carbonate minerals in the sample (Table 3 .2, 3 .3 and 3.4). Table 3.5 NAG test results for the pitwall rock samples. NAGpH NAG, kg CaC03 ( I PWR- 1 PWR-2 PWR-3 2.98 ± 0.33 42 ± 3 2 . 1 1 ± 0.24 52 ± 8 1 .95 ± 0.23 44 ± 3 3.3.2.2 Acid base accounting (ABA) PWR-4 2.03 ± 0. 1 4 68 ± 7 PWR-5 2.08 ± 0.22 52 ± 4 Analysis of the sulphur forms present in the pitwall rock indicated that samples with pyrite vein mineralisation generall y contained higher amounts of sulphide S than the samples that only contained disseminated pyrite crystals. The acid base analysis (ABA) of the pitwall rock samples is given in Table 3 .6. The result of the ABA analysis indicated 86 that all the samples, irrespective of weathering, showed positive net acid producing potentials (NAPP), indicating that all of the pyritic rocks in the pitwall are potentially acid generating types. NAPP > 0 is considered potentially acid forming material in ABA analysis. Table 3 .6 Acid base accounting (ABA) analysis of pitwall rock Sulphur forms, % ABA, kg CaC03 f! Sample Paste pH Total Sulphate Sulphide APP ANC NAPP PWR- l 7 .3 2.56 0.046 2.5 1 78 27 5 1 PWR-2 6.9 3 .54 0.068 3 .47 1 08 - 1 1 08 PWR-3 4.7 2.54 0.096 2.44 76 -5.8 76 PWR-4 4.2 4.02 0.320 3 .70 1 1 6 -3 1 1 6 PWR-5 2. 1 3 .36 0.744 2.62 82 -8 82 Organic S is assumed to be negligible. Negative acid neutralisation capacity CANC) values i ndicate n i l ANC, in which case NAPP = APP where APP is acid producing potential C= total ulphide-S * 3 1 .25 ) . The APP values obtained from ABA analysis were general ly higher than the NAG test values but there was a positive correlation between the NAG and APP data from the five samples of pitwall rocks (NAG = 2.27 * APP + 32. 1 4, R2 = 0.8 1 ) . The NAPP values in Table 3 .6 indicated that the fresh sample containing an ANC of 27 kg CaC03 ( 1 resulted in much lower NAPP than the progressively weathered samples. It is l ikely that the higher NAPP in the weathered samples is due to residual concentration of pyrite. Although pyrite grains and veins that are directly exposed undergo rapid oxidation to produce a very low pH in weathered pitwall rock, a significant amount of disseminated pyrite grains will not be immediately oxidised in the host rock and will form an unoxidised pyrite concentrate as weathering progresses. This is reflected in the significant pyrite content in the weathered samples (Table 3 .2) and high total sulphide-S content (Table 3 .6), indicating that moderately to strongly weathered pyritic rock will still have the potential to generate acidity more readily than the fresh rock. It is important to consider the effect pyrite grain liberation during weathering has on the NAPP under field conditions. Even with an advanced degree of weathering and oxidation, fine-grained pyrite crystals may remain encapsulated and unoxidised in coarse grained fractions of the pitwall rock. Thus, only the pyrite grains that are directly or partially 87 exposed by weathering are likely to be involved in acid generation under field conditions (Figure 3 .7 ) . Pyrite grain Pitwall rock (a) (b) (c) (d ) Figure 3 .7 Pyrite inclusions and oxidation scenario. Grains (b) and (d) are available for oxidation while grains (a) and (c) will remain inert until exposed by rock weathering. Thus the amount of acid generated and the intensity of AMD in the substrate will depend on the textural relationships between the pyrite mineral and the host rocks under given conditions of atmospheric exposure and weathering intensity. Where pyrite presence is dominated by vein mineralisation, one may find anomalously high AMD compositions. Even if the rock appears to be strongly weathered/oxidised it may contain 'entombed' pyrite grains which may eventually produce acid. The above scenario can have important implications for the prediction of AMD and in kinetic testwork. The degree to which individual pyrite grains are susceptible to oxidation depends largely on the distribution of the mineral grains, nature of mineralisation and, therefore, the size of the sample involved. The ABA test assumes that the small sample used in the test is representative of an uniformly mineralised strata. However, this is not always the case and unless the ABA test is performed on large bulk sample, the NAPP properties of the sample will not be properly assessed. Field observation indicated that pyrite mineralisation in the pitwall at Martha mine is not uniformly distributed. This necessitated the detailed spatial delineation of the AMD properties of the pitwall rock discussed later in section 3 .3 . 88 3.3.2.3 Kinetic NAG test - Lag-period Although standard ABA techniques have been widely used as screening tools to identify potentially acid producing sulphidic waste rocks, there is still an uncertainty in the prediction of AMD generation. In order to establish the real time pH-temperature dependent kinetics involved in pyrite oxidation and acid generation, a kinetic NAG test was performed on fresh and progressively weathered samples. The method of Miller ( 1 996) was adopted for this test. The results of the NAG test kinetics (Figure 3 .8) showed that fresh samples took approximately 80 minutes before vigorous reaction started while partially oxidised/weathered samples (PWR-2, PWR-4, PWR-5) took comparatively much less time; 30, 20 and 20 minutes respectively. Studies by Miller and Jeffrey ( 1 995) and Miller ( 1 996) have attempted to correlate NAG test kinetic data with that of conventional column test results of potentially acid forming mine wastes materials. They indicated that the time taken (minutes) for the pH of the NAG test solution to decrease by 1 unit from the starting value (X) is directly related to the time taken (weeks) to reach pH 4 in the column weathering test (Y) and obtained a linear regression equation of the form: Y = 2.40 * X + 0.60 (R2 = 0.88) for 1 8 samples classified as potential ly acid forming by the NAPP and static NAG test results. Based on this equation, the fresh PWR- l gave a lag-period of 22 weeks (X = 9 minutes) while progressively weathered PWR-2, PWR-4 and PWR-5 had much shorter lag-periods of 1 3 weeks (X = 5 minutes), 1 3 weeks (X = 5 minutes) and 7 weeks (X = 3 minutes) respectively. The result for PWR-3 was similar to PWR-2 and therefore not included in this discussion. The lag-period of 22 weeks for fresh rock to generate acid is above the upper range of field lag-period ( 1 5-20 weeks) indicated by Miller ( 1 986) and Brodie et al. ( 1 996) in their on-site prediction of AMD from fresh unoxidised waste rock from borehole cores. Miller ( 1 986) also indicated that about 20 to 30 weeks time period was taken for bacterially catalysed oxidation to take place in the unoxidised waste rock. Under the local environmental conditions of Waihi (dry periods in summer and wet winters, and an annual average rainfall of 2500 mm), the pitwal l affected by pyrite mineralisation will remain continuously acidic due to varying lag-periods of the pitwall rocks. The nature of pyrite mineralisation and amount of pyrite grains liberated during progressive weathering of the pitwall rock directly control acid generation in the pitwall. Although the fresh rock showed longer lag-period (22 weeks) , once weathering 6 6 5 4 2 1 (a) PWR-1 (b) PWR-2 o o 50 1 00 1 50 200 250 300 0 50 1 00 1 50 200 250 300 Reaction time (minutes) (c) PWR-4 Reaction time ( m i n utes) • pH .. Temperature (d) PWR-S o o (I) .... ::J -ca .... (I) a. E (I) .... Or-����������� o 50 1 00 1 50 200 250 300 0 50 1 00 1 50 200 250 300 Reaction time ( m inutes) Reaction time ( m i n utes) Figure 3.8 NAG test kinetics for (a) fresh PWR- l , (b) freshly weathered PWR-2, (c) moderately weathered PWR-4 and (d) s trongly weathered PWR-S . 89 90 commences the lag-period is significantly reduced and the rate of AMD generation will be rapidly increased. 3.3.2.4 Column test - rate of acid generation Leaching studies are concerned with the kinetics of mineral dissolution. Thus, a basic understanding of the leaching of constituents from the pitwall rock is required in order to quantify the rate of release of the pyrite oxidation products. The objective of the kinetic test was to provide real time data on the rate of acid generation/neutralisation under laboratory controlled conditions so that quantitative information on leachate characteri tics could give some indication of l ikely AMD contaminants under field conditions. Such information not only complements the static test results but also provides an assessment of treatment options for remedial measures. Kinetic column tests involve repetitive oxidation during leaching cycles on a sample and are frequently used for predicting acid generation and neutral isation. The kinetic NAG test (section 3 .3 .2 .3) indicated a widely varying lag-period of 22, 1 3 and 7 weeks for fresh to strongly weathered pitwall rocks respectively; they gave no indications of the amount of acid generated and the characteristics of the AMD. In this study, a column test was conducted on the fresh (PWR - 1 ) and freshly weathered (PWR -3 ) rock samples for a 1 0 week period. These two samples were selected so that a comparison could be made of leachate quality and rate of acid generation between fresh and freshly weathered rocks. Results of 10 weeks of column leaching showed that in the freshly weathered sample (PWR-3) there was a steady decrease in pH up to week 7 (Figure 3 .9a). The lowest pH reached was 2.3 at week 7 after which it remained constant. The largest drop in pH occurred in weeks 4 to 7. There was no significant drop in pH of the leach ate in fresh rock (PWR- I ) columns, even after 1 0 weeks of leaching. It is possible that in the fresh rock, only chemical oxidation of pyrite may have been taking place, which is insignificant compared with the rate of oxidation due to bacteria. Leachate EC in the PWR-3 remained in the mean range of 2 to 3 dS m- I , except for a sudden increase to about 4 dS m- I at week 7 (Figure 3 .9b). This was in response to the sudden drop in pH and increase in sol- released as a result of possible bacterial oxidation r-� tn (a) 8 6 =a 4 2 (c) 800 600 .§. 400 N ..,. o en 200 2 I . ...... . . . 4 6 No_ of weeks No. of weeks I 8 1 0 I I • .. (b) 5 4 '7E 3 Cl) � 0 2 w 1 2 4 6 No. of weeks Fresh pyritic rock (PWR-1 ) Freshly weathered pyritic rock (PWR-3) r- (d) 800 :.... 600 M o U n:s c; 400 E - :2 200 o « I 8 1 0 I • • • 91 • • • • • • • Or---.---.---.---.--. o 2 4 6 No. of weeks 8 1 0 Figure 3 .9 Leachate characteristics of the pitwall rock under laboratory controlled kinetic column test. Vertical bars represent LSD(5%). 92 of pyrite. In contrast, the slight increase in the EC levels in the leachate fresh sample from week-6 onwards was insignificant. Sulphate-S (SO/) level in the column leachate from freshly weathered rock (PWR-3) reached a maximum (634 mg L-1 ) at week 7, after which there was a sudden drop to a level below 400 mg L- 1 at week S (Figure 3 .9c). The increase in sol production from week 5 to week 7 indicated that bacterial oxidation of pyrite began to be effective only when the pH decreased to < 3.5 (Figure 3 .8a) in spite of the optimum surface temperature of 35 0c. The sudden drop in leachate SO/- after week-S also indicated that the amount of free pyrite (grains b & c in Figure 3 .7) available for oxidation by T. ferrooxidans was only sufficient to support bacterial activity for about 3 weeks. In comparison, there was no indication of pyrite oxidation in the fresh samples, although some amount of chemical oxidation may have been taking place. Total acidity in the leachate from freshly weathered samples increased to a peak of 704 mg CaC03 LI at week 7 (Figure 3 .8d) after which it decreased to levels < 500 mg CaC03 L-1 . Acid generation in the weathered samples appeared to be directly proportional to SO/- production during week-3 to week-7 but the overall con-elation was poor (R2 = 0.55). This may be due to neutralisation of some of the acid by dissolution of silicate and carbonate minerals present. Although there were some variations in S042 production and acid generation, there were steady increases in the cumulative levels of EC, SO/- and acidity over the l O week period of leaching. The linear trends in cumulative amounts of the measured parameters in the PWR-3 leachate over the l O weeks were highly significant (Eqns. 3 , 4 & 5) . S ince fresh rock remained inert throughout the leaching period, only trends in the cumulative amounts of the EC, SO/- and acidity in the PWR-3 leachate are considered. EC = 2.73 * week - 0.S7 (R2 = 0.99) [3] S042- = 43 1 * week - 309 (R2 = 0.98) [4] Acidity = 497 * week - 406 (R2 = 0.99) [5] 93 The rate of acid generation is commonly measured in terms of rate of SO/- production and in simple terms, the amount of S042- in the column leachate is a measure of the amount of acid generated. Over the 1 0 weeks period, the cumulative amount of SO/­ produced was 3977 mg L- I and the rate of SO/- release was 398 mg L- I week- I . The corresponding release of acid in the leachate was 463 mg CaC03 L- I week- I . The column test results indicated that the onset of acid generation started in week 5, as reflected in the significant increases in S042. and acidity. This corresponded to a lag-period of 5 weeks for freshly weathered sample compared to a lag-period of 1 3 weeks predicted from the NAG test kinetics. This indicated that under leaching conditions, assumed to be simulative of field conditions, acid generation in the pyritic pitwall rock at Martha mine is predicted to take a much shorter period to start once weathering commences. The PWR-3 sample had a pyrite content of 4.6% (based on the total sulphide-S content of 2.4%) . This meant that only 1 .6 % of the total pyrite was being oxidised per week in the column. Assuming the same average rate of SO/- release (398 mg L· I ), it would take approximately 63 weeks for the PWR-3 to completely oxidise, all factors (grain liberation, environmental conditions etc.) being favourable. The ideal oxidation of pyrite produces 2 moles of acidity (equivalent to 1 00 mg CaC03 ; given that 1 mole H+ is equivalent to 0.5 mole CaC03) for every mole of sol· produced (96 mg SO/") according to Eqn. 6. [6] This molar ratio of 2: 1 (acidity to sol weight ratio 1 00/96) should theoretically give a weight equivalent acidity versus S042. slope ratio of 1 .04. A regression plot of acidity versus SO/- for the column test on PWR-3 gave a linear relationship of the form: Acidity = 0. 79 * X + 150 (K = 0.55) with a slope of 0.79. This slope is clearly less than the ideal slope of 1 .04 expected for stoichiometric oxidation of pyrite, indicating that about 25% 0 .04 - 0.79 / 1 .04 * 1 00) of the acid produced is unaccounted for. It is possible that the acid produced is being neutralised by dissolution of silicate minerals or transformation of K-feldspars to kaolinite (Eqn. 7) . 94 The lower slope ratio of 0.79 may also be due to incomplete rinsing of the interstitial SO/-, thus giving lower concentration in the leachate. The molar ratio of 2 : 1 for acidity to sol is based on the ideal conditions of pyrite oxidation and thus may also not be applicable to samples that have variable pyrite content and liberated pyrite grains. Both sol and acidity levels in the column test leachate should be accounted for in order to give a reliable value for the rate of pyrite oxidation and hence acid generation. In sample PWR-3 columns, clusters of mushroom-shaped salt crystals were observed on the surface a day after the leaching. These crystals continued to form throughout the test period, although fewer crystals were observed towards the end. An XRD analysis of the crystals indicated that they consisted mainly of epsomite (MgS04.7H20), alunite [(KAh(S04)2(OH)6] and minor gypsum (CaS04.2H20). This indicated that in pyritic materials that have low neutralisation potential, preferential precipitation of Mg, AI , Fe and possibly Mn sulphate salts are likely to occur during dry conditions. These observations suggest that under field conditions, a dry spell after a rain event may result in high concentrations of salt in pore waters, leading to salt precipitation, which may be detrimental to plant growth. Column tests on fresh rock (PWR- l ) and freshly weathered rock (PWR-3) indicated that only PWR-3 was susceptible to biochemical oxidation and acid generation over a 1 0 week period. Sample PWR- l remained inert over this period, indicating that under column test leaching conditions (section 3 .2 .2 .2 .2) fresh rock may remain unoxidised over a long period (lag-period > 23 weeks) until exposed to oxidants and bacterial activity. Even though the column test was IUn for twice the length of time suggested in the standard method of Miller and Jeffery ( 1 995), the fresh rock continued to produce neutral AMD and may continue to do so until lag-period of over 22 weeks is completed. 3.3.3 Characterisation of pitwall runoff drainage In order to assess the pitwall runoff drainage characteristics, drainage water from the bottom of the pitwall was collected after one brief rainfall event in November 1 995. Drain water samples were collected at 8 locations ( 10 m intervals) along the base length of the 95 pitwall up to the manhole point (Figure 3.9). Drainage water pH was measured in the field with a portable pH meter as well as in the laboratory in collected samples. There were no differences between field and laboratory pH measurements. EC was measured in filtered, undiluted solution. The drainage water samples were analysed for sol, Fe, Mn and Al and the results presented in Table 3.7 and Figure 3 . 1 0. PYRITIC PITW ALL Gradient = 43° Drainage flow PIT BENCH Figure 3 . 1 0 Pyritic pitwall showing runoff drainage flow direction and sampling locations ( 1 - 8) . Contour values represent kriged % pyrite (FeS2) in the weathered material. The results of the analysis showed that the runoff drainage from the exposed pyritic pitwall was highly acidic and contained elevated levels of soluble salts, as reflected in high EC (Figure 3 . 1 l a) . The runoff drainage water was found to be extremely acid all along the 70 m flow. Away from location 1 the pH increased slightly and reached a maximum of 3 . 1 2 at location 8 as a result of progressive dilution downstream. Runoff drainage EC ranged in value from a high of 3 .7 dS m- I at sampling location 1 to < 2.0 at downstream location 8. Soluble metal (Fe, Mn and AI) loading and SO/- were generally higher at location 1 and decreased in concentration with increasing distance towards downstream location 8 . Total soluble Fe and S042- ranged in value from 1 7040 mg L- I and 8300 mg L- I at location 1 to 1 750 mg Cl and 89 1 mg L· I respectively towards downstream location 8 . A similar trend was also shown by the soluble Al, which ranged from 8220 mg L- I to 264 mg Cl from 96 location 1 to downstream location 8. Soluble Mn loading was also highest at source and decreased gradual ly downstream. Table 3 .7 Chemical characteristics of the runoff drainage# Loc. Distance pH EC sol Fe, Mn Al - \ 0 2.44 3 .68 8300 1 7040 495 7940 2 10 2 .83 2.77 6800 1 3400 423 5400 3 20 2 .85 2.74 4680 8200 383 3940 4 30 2 .80 2 .27 4 1 90 7 1 00 38 1 3550 5 40 2.86 2 . 1 0 2900 4200 364 2980 6 50 2 .88 2 . 1 2 3200 3700 323 3 1 50 7 60 2.92 2 . l4 2660 2300 248 1 770 8 70 3 . 1 2 1 .8 1 2 1 00 1 600 1 40 1 690 # Sample collected during short rainfall in November 1 995 . The gradual decrease in the concentrations of sol, Fe, Al and Mn indicated that either there was mass dilution or mass adsorption on sediments of these species downstream. However, the drainage also contained a high sediment loading and as a result selective adsorption/precipitation of metal ions are likely to effect the downstream concentration. The cumulative loading of Fe, Mn, Al and sol in the runoff drainage water are shown in Figure 3 . 1 Ob. The maximum cumulative loading of SO/-, Fe, Mn and Al at 70 m distance were 34830 mg L- 1 , 57540 mg L l , 2757 mg Ll and 34020 mg L- 1 respectively. Although the cumulative loading of these elements indicated toxic levels, the extent of exponential reduction due to dilution with increasing distant and with increasing volume of flow is going to be major factor in affecting the final drainage water quality. Overal l , the cumulative concentrations of SO/-, Fe, Al and Mn followed a logarithmic trend with distance (x) away from source of the drainage as explained by the following equations. S042- = 1 2884 * Log(x) + 6878, R2 = 0.99 Fe = 20047 * Log(x) + 1 704 1 , R2 = 0.99 Al = 1 1 1 06 * Log(x) + 6442, R2 = 0.98 Mn = 1 143 * Log(x) + 259, R2 = 0.96 [8 ] [9 ] [ 10] [ 1 1 ] (a) 4.0 3.5 - ,.... 'E 3.0 en "0 - U w ::i 2.5 c.. 2.0 1 .5 0 (b) 60000 - ,.... 50000 � Cl E s::::: 40000 0 -m .... -s::::: Q) 30000 0 s::::: 0 0 Q) > ; m :::l E :::l U o 1 0 20 1 0 20 30 40 Distance (m) 30 40 Distance (m) 50 50 • pH • EC 60 70 -a- Fe � SO/· ----- AI ----*-- Mn 60 70 Figure 3 . 1 1 Chemical characteristics of the run-off drainage. (a) pH and EC (b) cumulative loading of Fe, SOl-, Al and Mn. 97 98 The rainfall event had occurred for a brief period ( 1 .5 hours) after a spell of dry weather. The runoff water thus had the effect of flushing accumulated salts. The rapid dissolution of salts, formed during dry periods by surface evaporation is the likely cause of elevated levels of metal ions at source. Salt formation during dry periods [(mainly alunite: KAb(S04h(OHk epsomite: MgS04.7H20 and jarosite : KFe3(S04h(OH)6)] was evident in column tests. Although metal and salt loadings in the drainage water are likely to decrease with distance and flow volume, the intensity and frequency of rainfall are going to be important factors affecting loading capacity of the drainage. The pitwall is prone to drying out under low rainfall conditions, resulting in the precipitation of metal salts on the surface. During rainfall events, periodic flushes of AMD pollutants can be expected and may temporarily exceed toxic levels for some metals. The runoff drainage also contained a significant sediment loading. Although not reflected in the water analysis, large amounts of metal ions will be transported in the fine sediments as adsorbed species. With eventual increase in pH, these adsorbed metal species will desorb and co-precipitate with Fe as hydroxide sludge on drainage beds. 3.3.4 Spatial characterisation of the weathered pitwall rock 3.3.4. 1 Geostatistics Spatial properties of the parameters listed in Table 3 .8 were investigated by use of geostatistical procedures. Estimates for the mean spatial values of the pitwall rock properties within a small rectangular block have been obtained by kriging using a l inear semi-variogram model. Kriging is a geostatistical procedure based on the concept of regionalised variables and used to study spatial characteristics (Webster and Burgess, 1 984). The method has been traditionally used for geochemical distribution of elements at a regional scale but in recent years it has found use in characterising spatial variability of soil chemical properties and crop yields (Bahri et al. , 1 993; Berndtsson et aI . , 1 993 ; Persicani, 1 995). Webster and Burgess ( 1984) have shown that data kriging can also be applied to small areas « 1 0000 m2) with regular grid spacing and that minimum variance was observed when the sampling interval = block length / -wz (Where n = sample size). In this study, the pitwall block size was 40 m x 30 m and therefore a sampling grid interval of S; 5 m and sample size of 47 was sufficient to provide values with standard errors ± 1 0% of the mean. 99 The mean spatial value, z(A), of the pitwall rock property, z, in a small rectangular shaped area A, is estimated from the observed values, Z(Xi) measured at grid points Xi where i = 1 to n. Thus, 11 z(A) = I wiz(xi) , Wi = 1 to n are weights. i=1 Kriging is the procedure for finding the weights Wi and using them in the estimation of regionalised variables using a semi-variogram model. The semi-variogram, y(h), is a measure of the variance of the estimation of the sample value z at a location (x+h) using the sample value measured at location (x) and is defined as (Bemdtsson et al . , 1 993 , Webster and Burgess, 1 984) : y(h) = V2 E { [Z(x) - Z(x+h)f l where E is the expected value (mean) of y(h). In the simplest case, the senu-varlOgram is linear and isotropic, at least over small distances (Webster and Burgess, 1 984). Regression fits for )'(h) for all the variables over a 40 m distance gave a linear relationship with regression coefficients (R2) greater than 0.60 for most of the vari ables described in the spatial distributions. For a small area ( 1 200 m2), sampled at grid interval of < 5 m, the variogram fits for 47 samples along the 40 m profile were sufficient to perform data kriging within a search radius of 1 8 m and standard error of < 1 0%. 3.3.4.2 Descriptive statistics Descriptive statistics for the selected parameters of the pitwall used in the geostatistical analysis are shown in Table 3 .8 . The data showed that, in most cases, there was a wide variation in measured parameters. Weathered pitwall rock cover depth varied from a minimum of 5 mm on the top end of the pitwall to up to 3 30 mm towards the bottom end of the pitwall where colluvial deposition was highest. In some areas of the pitwall not covered by the grid, up to 600 mm of cover depth was observed. Moisture content ranged from 9.3 in upper slopes to 1 6.3% in lower parts of the pitwall where cover material was thickest. The pH was generally lowest in strongly weathered parts of the pitwall and ranged from 2.9 to a maximum of 4 . 3 . The highest deviations from the mean (n=47) were 1 00 in cover depth, SO/, soluble Fe, soluble Mn, acidity and NAPP. The variations in Ca and Mg were relative to dissolution of carbonates and enrichment from dissolution of alkali feldspars. The distribution of pyrite also showed a wide variation, pyrite content ranging from 2 . 1 to 8. 1 %. While variability in cover depth is controlled by the physical aspects of the slope and local climatic conditions, variability in chemical parameters were most likely to be controlled by the reactive elements in the cover depth and mobilisation of these elements by AMD. The degree of spatial interdependence of measured variables within the cover depth of the pitwall rock is reflected in the correlation coefficients shown in Table 3 .9. Correlation coefficients, r>0.40 were statistically significant at the probability level of p < 0.005 . As expected, the moisture content in the cover depth is highly dependent on the depth of weathered cover material . Both cover depth and moisture content were significantly correlated (p = 0.00 1 ) with EC, AI, SO/, Fe, Mn, acidity and K. At the same probability level, pH showed negative correlation with EC, Al and acidity indicating that the low pH conditions in the cover depth are buffered by mainly solubilised AI , and SO/- salts of the metal ions. Acidity in the cover depth was found to be directly controlled by cover depth, moisture content, pH, EC, AI, S042-, Fe and Mn. It is interesting to note that K had a strong correlation with cover depth, moisture content, EC, AI, SO/, Fe, Mn and acidity. A large fraction of K released during weathering is probably retained in the cover depth as metal salt complexes with AI , Fe and Mn. The high linear correlation between Fe, Al and Mn is indicative that these elements coexist as hydroxide complexes as well S042- salts. Magnesium content in the cover depth is proportional to the buffer acidity. Most of Mg would be derived from breakdown of ferromagnesian minerals with increasing acidity of the substrate. The strong correlation (r=0.7 1 ) between FeS2 and NAPP is expected since both are dependent on the total Fe and S content of the pitwall rock material . 101 Table 3 . 8 Descriptive statistics o f spatial characteristics o f the pitwall rock ( n = 47). Parameters Unit Mean Median SD Min. Max. CD Mm 1 37 1 50 1 02 5 330 MC % 9.3 9.2 3.0 4.3 1 6.3 pH 2.9 2.8 0.6 2.0 4.6 EC dS m- ' 3 .2 3 .3 0.6 1 .9 4.3 Exch. Al cmolc kg- ' 7.4 7.3 1 .5 4.8 1 0.8 Total Al % 1 . 8 1 .8 0.5 l . 1 3 .4 Total S % 2.7 2.5 0.8 1 .4 4.8 S 042- mg kg- 1 2467 2500 453 1 650 3400 Total Fe % 2.7 2.7 0.6 l . 5 4. 1 Soluble Fe mg kg" 4084 4 1 38 88 1 2506 5758 Soluble Mn mg kg- ' 342 323 1 02 203 635 Acidity kg CaC03 ( ' 402 420 1 1 6 1 2 1 668 FeS2 % 5.0 5. 1 1 .4 2. 1 8. 1 NP kg CaC03 t- 1 - 1 0.5 -9.9 3.2 - 1 9.4 -5.4 NAPP kg CaC03 ( ' 74.4 68.0 24.4 35.4 143.0 Ca cmolc kg- ' 4.4 4. 1 1 . 8 1 . 8 9.5 Mg cmolc kg" 1 .8 1 .6 0.6 0.8 3.2 K cmolc kg- 1 0.6 0.6 0.2 0.3 1 .0 Na cmolc kg- ' 0.2 0.2 0. 1 0. 1 0.5 SD, standard deviation; CD, cover depth; MC, moi sture content; EC, electrical conductivity, NP, neutralisation potential; NAPP net acid producing potential. Table 3 .9 Correlation coefficients of the selected parameters CD MC pH EC AI S042. Fe Mn Acidity Ca Mg K FeS2 MC 0.83 pH -0.38 -0.36 EC 0.74 0.72 -0.44 AI 0.69 0.65 -0.44 0.57 S042. 0.43 0.36 -0.33 0.34 0.2 1 Fe 0.76 0.68 -0. 3 8 0.72 0.6 1 0.34 M n 0.65 0.66 -0.28 0.5 1 0.57 0.4 1 0.44 Acidity 0.6 1 0.53 -0.5 1 0.5 1 0.55 0.50 0.50 0.44 Ca -0.07 -0.0 1 0.0 1 -0.03 0.0 1 0.20 0.08 -0. 1 4 0.06 M g 0.37 0.22 -0.09 0.22 0.27 0. 1 6 0.29 0.27 0.4 1 0.03 K 0.72 0.62 -0.22 0.56 0.56 0.4 1 0.49 0.60 0.43 -.26 0.22 FeS2 -0.26 -0.3 1 0. 1 9 -0. 1 8 -0.20 0.05 -0.22 -0. 1 1 -0.03 0.43 -. 1 3 -0.35 NAPP -0.27 -0.24 0.07 -0.02 -0.2 1 0. 1 6 -0. 1 2 -0.09 0.04 0.49 0.05 -0.42 0.7 1 Underlined coefficients are statistically significant at 5 % significance level ( n=47 ). CD, cover depth, MC, moisture content 102 3.3.4.3 Distribution of cover material and moisture content The distribution of weathered pitwall rock cover material and moisture content within the grid block are shown in Figures 3 . 1 2a and 3 . 1 2b respectively. Most of the cover material had accumulated at the bottom half of the pitwall as a 400 mm thick band of colluvial material deposited from erosion and gravity sliding. The cut and fill volume calculation approximated by trapezoidal rule (Golden Software Inc . , 1 994) gave a positive volume of 1 60 m3 of weathered cover material at the time of sampling. Taking l A Mg m- 3 as a mean bulk density of the material, the 1 200 m2 pitwall , contained 224 Mt of weathered cover material at the time of sampling. The volume calculated from a mean cover depth of 1 37 mm (Table 3 .8) was 1 64 m 3 (40 m x 30 m x 0. 1 37 m), which gave 230 Mt of pitwalI cover material. The kriged cover depth variability in the pitwall is thus well within the 1 0% standard error margin. The moisture content of the cover material was directly related to the thickness of the cover material . This direct relationship is reflected in the high correlation coefficient of 0.83 between the moisture content and cover depth (Table 3 .9). The pitwall cover material had a very low mean moisture content of 9 .3% (with values ranging from 4 .3 to 1 6.3 %) and was much lower than the field capacity value of about 1 4% for the bulk sample from maximum depth of colluvial deposition (600 mm). The high macroporosity of the cover material and the time of sampling (during a dry period) would have resulted in such low moisture contents. Towards the top half of the pitwall, where mean depth of weathered cover material was < 1 00 mm, moisture content was generally < 5% whereas lower half of the pitwall had a mean moisture content of 1 5% (Figure 3 . 1 2) . Although, the lower half of the pitwall contained sufficient depth of weathered pitwall material to support plant root systems to a depth of about 300 mm, high macroporosity, very low moisture content and surface encrustation with salts during dry periods are likely to be the major physical constraints to plant growth. Figure 3 . 1 2 Distribution of (a) weathered p itwall rock cover depth (CD) and (b) moisture content (MC) of the cover material . 1 03 1 04 3.3.4.4 pH and EC The spatial representation of the pH in the weathered pitwall rock is shown in Figure 3 . 1 3a. Generally, the lower half and the top left quadrant of the pitwall had pH < 3 .0. The top right quadrant of the pitwall and certain areas in the lower half had peak pH of >3 .5 . Overall, the entire grid surface had a mean pH of 3 . 3 with a variance of 2 .0 to 4 .6 . The high and low zones of pH were a result of varying degrees of weathering, dissolution of calcite and oxidation of l iberated pyrite. The residence time of the weathered materials on the pitwall slope surface may influence the development of low pH micro-environments since static conditions give rise to a favourable environment for oxidants to oxidise liberated pyrite grains and for bacterial activity. Thus, the lower part of the pitwall with thickest colluvial deposit provided a stable environment for AMD generation, as indicated by generally low pH of < 3 .0. Electrical conductivity (EC) showed a very uneven spatial distribution tluoughout the gridded pitwall surface (Figure 3 . 1 3b). Except for the top left quadrant which had EC < 2.5 dS m- I , the rest of the pitwall area had EC > 3dS m- I . There were distinct high and low areas of EC levels indicating that salt levels are primarily determined by the distribution of SO/- and buffer acidity. The mean EC was 3 .2 dS m- I , equivalent to 1 . 1 % total soluble salt (total soluble salt = EC x 0.35) over an area of 1 200 m2. Salt levels in the weathered cover material are likely to vary seasonally. Since the grid samples were collected during a dry period, the distribution of soluble salt is l ikely to be less during a wet period when most of the precipitated salts will be transported down slope by the runoff water and enter the runoff drainage along the base of the pitwall . (b) 1 05 Figure 3 . 1 3 Spatial variations in Ca) pH and Cb) EC in the weathered pitwall cover material. 106 3.3.4.5 Exchangeable AI, soluble Fe and Mn The level of exchangeable Al (Alexc) in the weathered pitwall rock generally increased gradually down the slope of the pitwall (Figure 3 . l 4a). Higher A1exc levels were observed in lower parts of the pitwall where high acidity and accumulation of clay minerals facilitated greater mobilisation of Al species in pore water solution. The mean Alexc content in the pitwall rock was 7 .4 cmole ki l and ranged in value from 4.8 to 1 0.8 cmole kg- I in an area of 1 200 m2. At the time of grid sampling, the 1 200 m2 area of pitwall contained 1 49 kg of Alexc. The A1exc has been identified as one of the main factors restricting the plant growth potential of unoxidised pitwall rock (Widdowson et aI . , 1 984; Mason, 1 996) as well as oxidised waste rock (Gregg and Stewart, 1 990; Gurung et aI . , 1 996) from Waihi mine. Aluminium phytotoxicity under low pH conditions has been well documented by several workers, notably Foy ( 1 97 1 ), Hargrove and Thomas ( 1 98 1 ) and Barcello et al ( 1 996). Exchangeable AI > 1 00 mg kg- l ( 1 . 1 cmole kg- I ) is considered a critical toxicity level for plant growth (Cameron et aI . , 1 986; Percival et aI. , 1 994; Barcelo et al . , 1 996) . Although Al phytotoxicity in acid soils can be alleviated by the standard practice of l iming (Shoemaker et aI . , 1 96 1 ; Oats and Kamprath, 1 983 ; Hem et aI . , 1 988; Conyers, 1 990; Slattery et aI. , 1 995), phosphate addition (Alva et aI . , 1 988 ; Loganathan et al . , 1 995) and organic matter complexation (Hargrove and Thomas, 1 98 1 ; Hue et aI . , 1 986, Young and Bache, 1 985 ; Tan and Binger, 1 986), the long-term amelioration of Al in the pitwall rock will no doubt depend on the control of AMD and alleviation of low pH environment. Soluble Al along with Fe and Mn are considered the major cations involved in keeping waste rock buffered at low pH (Luciuk and Huang, 1 974; Huang, 1 988; Sue et aI. , 1 995) . At pH < 3 .5 , the depletion of solution Al in pitwall rock by formation of salts like aIunite with SO/-, AI-hydroxy compounds and by leaching is going to be minimal in comparison with Al on exchange sites in the pitwall material. The overall mean distribution of soluble Fe was 4084 mg kg- l and ranged in value from 2506 to 5758 mg kg - 1 . Soluble Fe distribution showed a broad band across the middle of the pitwall (Figure 3 . l 4b). A large fraction of the soluble Fe in the weathered pitwall rock would be precipitated as hydroxide coatings (Plate 3 .2) , jarosite and copiapite during dry periods while dissolution of these precipitates during wet periods will release further 1 07 acidity in the system, resulting in very acidic AMD. Under acidic conditions, jarosite is probably precipitated in inter-granular spaces. Like Al , soluble Fe is also largely responsible for keeping the pitwall rock buffered at low pH. Although, from plant the growth point of view, soluble Fe is not a maj or toxicity concern, its involvement in the formation of j arosite at low pH can lead to storage of acid which is released during subsequent rainfall event . The distribution of soluble Mn in the pitwall rock was general ly higher towards the down­ slope on the pitwall (Figure 3 . 1 4c). The mean distribution was 342 mg kg" and range was from 203 to 635 mg kg- I . Because Mn is highly mobile, the distribution of Mn in the pitwall rock will largely depend on the distribution of amount of cover material and its moi ture content. The dependence of Mn mobility on moisture content of soil has been suggested by Christensen et al . ( 1 950). Because Mn exists in up to six oxidation states, its chemistry and mobility in acid soils is considered to be controlled by mainly the Eh-pH environment (Watzlaf, 1 988b; Ritchie, 1 989; Willet et al . , 1 992 ) . Soluble Mn is known to be toxic to both plant and aquatic biota (Helyar, 1 978). The major concern about high levels of mobilised Mn in the pitwall rock AMD will be its entry into the drainage system. Liming has been shown to reduce the availability of Mn to plants (Sanchez and Kamprath, 1 959; White, 1 970). Its affinity to adsorption by soil aggregates has been shown to result in longer residence times (Ritchie, 1 989). 1 08 (a) 1{) � , � a ....... 11 0 � '- � Cl � '" < Figure 3 . 1 4 Distribution of (a) exchangeable Al (AIexc), (b) Fe and (c) M n i n the weathered pitwall cover material . 1 09 3.3.4.6 SO/- and Acidity The distribution of sol- in the weathered pitwall cover material was spatially dependent and remained high throughout the pitwall area under study (Figure 3 . 1 5a). The mean distribution of sol- content was 2467 mg kg- I although the top 5 m band width of the pitwall had a S042- content < 2000 mg kg- I . As with EC, there were areas of high and low sot content and at any one time the sol- level in the pitwall cover material was dependent on the level of dissolved metals and acidity. However, at probability level p < 0.003, only Mn, K, acidity and cover depth were significantly correlated with the sol- content. This may indicate that both Fe and Al were either highly mobilised under very acidic conditions or they may have formed interstitial and surface precipitates of hydroxides (Fe(OH)3, AI(OH)3) and stable salt complexes such as j arosite and alunite. The high correlation of Mn and K with sot suggest that these elements exist mainly as sulphate complexes in the pitwall cover material . Acidity levels in the pitwall cover material were > 300 kg CaC03 ( I throughout the grid area with the maximum being towards the lower areas of the pitwall (Figure 3 . 1 4b). The mean acidity level in the pitwall rock was 402 kg CaC03 ( I and ranged from 1 2 1 to 668 kg CaC03 ( I . This corresponds to an equivalent of 90 t CaC03 equivalent acidity in the 1 200 m2 of pitwall having a mean cover depth of 1 37 mm at any one time. In terms of acid neutralisation, carbonate equivalent of 90 t of liming material will be required for an area of 1 200 m2 of pitwall . The level of acidity in the pit cover material was positively correlated at probability level p = 0.002 with cover depth, MC, EC, Al, sol-, Fe and Mn (Table 3 .7b), and is consistent with low pH level of the pitwall surface (negative correlation) . 1 1 0 � � � '..;:. � t;:)" Cl) (a) 3%0 3000 3), lower EC, sol-, acidity, Fe, Mn and exchangeable AI. This attribute of the pitwall rock may have been due to the less weathered materials at the upper pit slope area. While lower pit slope area contained thicker deposition of cover material for suitable amendments to be applied, the upper surface generally had lesser degree of AMD effects. Provided suitable depth of cover material can be retained on the slope surface by engineering modifications, the pitwall rock can be amended with acid neutral ising materials, which are evaluated in Chapters 4, 5 and 6. Any attempts to revegetate the pitwall must, therefore, overcome toxicity from H+ and A13+ species plus possible heavy metal and soluble salt effects. Both chemical and biological oxidation of pyrite must be controlled long enough for the establishment of suitable plant species and pH must be raised to provide a less hostile environment. Because mine pitwall slopes are often too steep for stabilised application of amendments, revegetation techniques have to be a combination of engineering, chemical and biological modifications along with the selection of plant material adapted to cope with the adverse physical and chemical conditions. 1 1 9 C h a p t e r 4 An Assessment of the Effectiveness of Neutralising Materials in Ameliorating Acidic Pitwall Rock 4.1 Introduction 4. 1 .1 Neutralisation of acid in pyritic mine waste rocks Acid mine drainage (AMD) from sulphidic mine waste rock dumps and open pits is a major concern in the rehabil itation of both active and abandoned mines associated with sulphide bearing ores. Numerous methods of acid neutralisation have been used with varying degree of success to abate AMD in pyritic coal refuge and mine waste rock dumpsites. Large amounts of limestone are commonly used for presumed long-term control of acid generation and reclamation of sulphidic mine wastes (Costigan et al . , 1 982, 1 984; Pulford et aI . , 1 984; Pulford, 1 99 1 ; Dollhopf, 1 992). Although liming initially helps establish plants, the rapid depletion of the neutral ising material in a continually acid producing media may result in plant die-back. The relatively slow reactivity of coarse-grained limestone and the armouring effect from Fe-hydroxides may also result in reacidification (Gemmell, 1 98 1 ; Costigan et aI . , 1 984). While problems of reacidification may arise from underestimating the l ime requirement, overliming with high rates of limestone can have a negative impact on plant growth due to Ca induced deficiency in plant available nutrients such as P, Mg and Cu (Costigan et aI . , 1 982; Brady e t aI . , 1 990; Pulford, 1 99 1 ) . Characterisation of the weathered pyritic pitwall rock material in Chapter 3 showed that low pH conditions resulting from oxidation of pyrite and associated high levels of S 042, AI, acidity and soluble salts are some of the chemical characteristics of the pitwall rock limiting plant growth. Geochemical characterisation showed that acid generation IS active on the pitwall and the spatial distribution of low pH micro-environments IS primarily controlled by the degree of pyrite oxidation and distribution of the weathered cover material. Metastable salts such as epsomite (MgS04.7H20), alunite ( KA13(S04h and jarosite [ (KFe3(S 04h(OHh] have been identified as common minerals likely to be formed at the surface during dry periods. The dissolution of salts like j arosite and 1 20 hydrolysis of Fe and Al during wet period may often result in the fonnation of free acidity, and thus keeping the pitwall rock buffered at a l ow pH at all times. Neutralisation of strongly buffered acidic pyritic rocks therefore, requires consideration of both active and potential aciditidy to prevent reacidification as a result of continued oxidation of pyrite. An assessment of the effectiveness of various neutralising materials in reducing pitwall rock acidity and toxic elemental concentration is thus a primary step in ameliorating the low pH condition on the pitwal l . The primary objective of the incubation study reported in this section was to assess the ameliorating properties of selected neutralising materials added to partially oxidised pyritic pitwal1 rock. A second obj ective was to investigate the acid neutralising effectiveness of limestone particle size in partially oxidised pyritic rock material . 4.2 Materials and Methods 4.2.1 Pitwall rock bulk sample Approximately 500 kg bulk sample of weathered pyritic pitwall rock was collected from a single location on the north face pitwall at Martha mine (Grid location PWGS-5, Figure 3 . 1 ) . The bulk sample was air-dried and crushed to nominal 4 mm size prior to making into duplicate subsampJes by coning and quartering. In this incubation study, the sample was further crushed to ::; 2 mm fractions. The chemical properties of the pitwall rock bulk sample used in the incubation study are given in Table 4 . 1 . 4.2.2 Neutralising materials Limestone (LST) used in this study was obtained from a Te Kuiti limestone quarry (MacDonalds). The as-received limestone (LST AR) had approximately 55% fines (::; 2 mm). Slaked, fluidised bed boiler ash (FBA) was obtained from the Te Awamutu Milk Powder factory operated by the NZ Dairy Company. Reactive phosphate rock (RPR) used was North Carolina phosphate rock and dolomite (DOL) was commercial dolomite dust. Pure calcium carbonate used was of laboratory analytical grade. The physical and chemical characteristics of the neutralising materials are given in Table 4.2 . Table 4. 1 Selected properties of the bulk pitwall rock Pro�ies _ ______ ______ _ _ pH ( 1 : 2 .5 w/v soil to water) Electrical conducti vity (EC) Total Fe (25% HN03-extractable) Soluble Fe (0. 1 M HCl-extractable) Soluble Mn, (0. 1 M HCl-extractable) Total S Sulphate-S (SO/-) Acid producing potential CAPP) Acid neutralising capacity (ANC) Net acid producing potential (NAPP) Exchangeable Al (Alexc) Ca2+ M 0-2+ b K+ Na+ Base saturation (BS) Olsen-P Moisture content Bulk densi ty Units Amount ---- -- -------- dS m- ' % mg kg- ' mg kg- ' % mg kg- ' kg CaC03 f ' kg CaC03 f ' kg CaC03 f ' cmolc kg- ' cmolc kg- ' cmolc kg- ' cmolc kg- ' cmolc kg- ' % mg kg- ' % Mg m- 3 2. 1 3 .8 3 .7 4 1 00 328 3 .36 7440 82 -8 .0 82 14.9 5 .3 1 .3 0.7 0.2 33 8.6 1 5 1 .4 Table 4.2 Properties of selected neutralising materials. Particle size Equivalent, Neutralisation Neutralising materials (mm) CaC03 (%) potential (NP) 1 2 1 _____ _ ____ . __ . __ . __ ___ . ___ .. _____ .. ____ . ____ . __ . ___ . _____ _ (�g CaC03 !�I_) __ . Limestone ( LST) � 0.5 95 1 0 1 0 Dolomite (DOL) � 0.5 1 14 1 1 35 Fluidised bed boiler ash CFBA) � 2.0 42 438 Reactive phosphate rock (RPR) � 2.0 27 689 For RPR, ANC includes 4 1 9 t CaC03 from dissolution of RPR 4.2.3 Neutralisation potential (NP) of the neutralising materials The NP of the neutralising materials was determined by treating a 0.5 g sample with 50 ml of 0.5 M HCl, heating nearly to boiling and swirling periodically until no gas (C02) evolution was observed. The samples were made up to 1 25 ml with distilled water, boiled for one minute, and cooled to room temperature. The treated sample was then back titrated with standard 0.5 M NaOH to pH 7 using a Mettler DL2 1 Autotitrator. The NP was calculated as the amount of HCI consumed by the sample and converted to units of kg CaC03 f ' material . 1 22 I 50 x [volume Hel consumed, ml] x [molarity of Hel] NP (kg CaC03 r ) = ------------------- weightofthe sample (g) 4.2.4 Lime requirement of the pitwall rock The l ime requirement of the pitwall rock bulk sample used in this study was determined by equilibrating the pitwall rock with both 1 M NaOH and pure CaC03 and obtaining their respective buffer curves. For the NaOH-pH buffer curve, 20 g of pitwall rock was incubated with 0 to 20 ml of I M NaOH (with step-wise increments of 2 ml) for two weeks . The pure CaC03-pH buffer curve was obtained by incubating 200 g of pitwall rock with 1 0 rates of 0 to 1 0 g of CaC03 (equivalent to 0 to 50 kg CaC03 f l ) for 60 days @ 20 °C and 80% moisture content. Acidimetric titration was done in 1 0 9 : 50 ml (pitwall rock to deionised water ratio) equilibrated overnight and titrated with 0.5 M NaOH to pH 8 .2 using a Mettler DL2 1 Autotitrator. Titration curves for Al and Fe were obtained by titrating solutions containing 1 000 mg L- 1 Al(N03h and 400 mg L- 1 Fe(N03h respectively. 4.2.5 Neutralising material requirements: an incubation study Air dried pitwall rock bulk samples (200 g) were weighed into the plastic bags and mixed with varying CaC03 equivalent rates (CER) of the four neutralising materials used and incubated at 20 (lC for 90 days . Moisture levels at 80% field capacity were maintained throughout the incubation period. The CER of neutralising materials added were in the range 0 to 50 kg CaC03 r l with increment of 5 kg CaC03 r l . The incubation experiment was a complete factorial design with 1 0 different CER of neutralising materials replicated 4 times. SAS for Windows software was used to perform relevant statistical analyses to test significant differences between treatment effect and CER effect. 4.2.6 Determination of limestone particle size effect on pitwalJ rock A second set of incubation experiments was conducted to study the effective neutralisation properties of different particle s izes of l imestone. Very fine limestone (LSTvF, < 0.5 mm), fine limestone (LSTF, 0.5-2 mm), coarse limestone (LSTc, 2-4 mm) 1 23 and as received limestone from quarry (LST AR, 55% :::; 2 mm) were incubated with bulk pitwall rock samples as per the method described above. 4.2.7 Characterisation of hydroxide coating on limestone particles Various sizes (2 mm, 4 mm and 8 mm) of limestone grains were also incubated with pitwall rock for 90 days in order to quantify the amount of Fe, Al and SO/- in their associated coatings. The coated grains were dissolved in 1 M Hel, fi ltered and the Fe, Al and S in the solution analysed by atomic absorption spectrometry (AAS) and sulphate Auto-Analyser respectively. An 8 mm limestone grain incubated for 90 days was impregnated with epoxy resin, cut in half, polished and the surface carbon coated for scanning electron microscopy (SEM )/energy dispersion spectronomy (EDS ) probe analysis for semi-quantitative elemental analysis. 4.2.8 Analytical methods Chemical parameters were measured on air-dried samples after 45 and 90 days incubation. pH, electrical conductivity (EC), exchangeable Al (Alexc), SO/-, Fe, Mn and acidity were determined according to methods outlined in section 3 .2.4 (Chapter 3) . 4.3 Results and Discussion 4.3.1 Effectiveness of neutralising materials in acid neutralisation 4.3. 1.1 Lime requirement of the pitwall rock The NaOH and CaC03 equilibrated buffer curves for the pitwall rock used in this study indicated that a large amount of base was required to neutralise acidity because of the substantial buffer exerted by Fe and Al at pH range 2.0 to 4.5 (Figure 4. 1 a) . The pitwall rock used in this study contained 4 1 00 mg kg- I soluble Fe, and exchangeable Al up to 1 5 cmole kg- I ( 1 350 mg kg- I ) . At the pH range of the pitwall rock (2 .0-4.5) , a large amount of the base added was buffered by the precipitation of Fe and Al hydroxides. This is shown by the acidimetric titration of pitwall rock extract and solutions containing approximately equivalent concentrations of Fe and Al (Figure 4. 1 b) . The horizontal section of the curves indicates the formation of metal hydroxides. 1 24 The increase in pH with incremental CER fol lowed a sigmoidal logistic equation of the form shown below with accompanying table of respective regression coefficients. a Y = Yo + ---- l + (�)h Xo CaCOrpH NaOH-pH yo 2 .7 2 .3 xo 27 557 a 5 . 1 8 .9 - 1 2 .0 0.99 -8.2 0.99 Where y :::: target pH, Yo :::: i nit ial pH, Xo :::: minimu CER to overcome buffer requirement, x :::: CER requ ired to reach target pH, a and b are regression coefficients. The lime requirement predicted from the NaOH-pH and CaC03-pH buffer curves described above indicated that buffer lime requirements (LRBuffer) of 29 kg CaC03 C l pitwall rock was required to raise the pH > 6.0 (Figure 4. 1 a) . This LRBuffer however, only takes into account the neutralisation of active acidity and does not therefore assess the lime required to neutralise the acid generated from further oxidation of pyrite disseminated in larger fractions of the pitwall rock. The LRBuffer therefore, grossly underestimated the l ime requirement based on theoretical NAPP value of 82 kg CaC03 (1 pitwall rock determined by ABA. In order to account for the potential acid generation from pyrite oxidation, it is important that the lime required to neutralise the potential acidity (LRNAPP) be included in the LRBuffer to give "total lime requirement" (LRTolal ) ' This LRTotal would thus be the true lime requirement for long-term control of acid generation in pitwall rock undergoing progressive weathering and oxidation. The LRTotal of the pitwall rock that takes into account the neutralisation of both the active and potential acidity from further oxidation of unoxidised pyrite is thus : LRTotal = LRBuffer + LRNAPP = 29 kg CaC03 (1 + 82 kg CaC03 C l = 1 1 1 kg CaC03 r I pitwall rock The direct establishment of vegetation on the pyritic pitwall is entirely dependent upon acid neutralisation for both pH control and for reducing toxic levels of soluble metals . The assurance of long-term revegetation success demands that amelioration of low pH conditions be maintained for several years . S ince it i s impractical for subsurface l: a. l: a. (a) 1 2 - e- 1 M NaOH 1 0 -.- Pure CaC03 8 6 4 2:���������--���� o 200 400 600 800 1 000 mole OH- kg-1 I I i I i I i I i I i I i I i I i I i I o 5 1 0 1 5 20 25 30 35 40 45 50 Rate (kg CaC03 t-1 ) (b) 1 0 -B- Pit wall rock extract -B- Fe (400 mg L-') 8 --f:r- AI (1000 mg L-') O ��-'--'-�--��--r-�-'--'-�--' o 2 4 6 8 0.5 M NaOH, rnl 1 0 1 2 Figure 4 . 1 (a) NaOH-pH and CaC03-pH buffer curves and (b) acidimetric titration curves for solutions containing Fe, Al and pitwall rock sample extract. 1 25 1 26 incorporation of liming materials into the reclaimed area on a regular basis, the LRTotal has to be met in a single treatment before the establishment of vegetation. However, application of large amounts of agricultural l imestone may cause excess alkalinity, adversely atlect the availability of phosphorus and trace elements and increase salinity of the pitwall rock by mass precipitation of metal sulphate salts . On the other hand, armouring effects of oxyhydroxides on the liming material may suppress its reactivity and thus provide a long-term slow release carbonate reserve to maintain pH at levels favourable to plant growth conditions. Acidimetric titrations (Figure 4. 1 b) showed that a large part of the buffer l ime requirement was induced by the co-precipitation of Fe and Al hydroxides and that only a small fraction of the l ime requirement will be generated by the H+ in the solution. This study showed that acidimetric titration of the p itwall rock not only determined the Fe-AI induced acidity but also the active acidity due to H+ protonation. Such a titration curve may therefore be used to predict the neutrali sation requirement of the pitwall rock active acidity. The pH range in which the pitwall rock is buffered is closely s imilar to the NaOH and CaC03 buffer range. From the acidimetric titration curve (Figure 4. 1 b), the amount of OH- consumed to pH 6 was approximately 300 mmol OH- kg- I pitwall rock, which is equivalent to 30 kg CaC03 r l . This value is similar to that obtained from NaOH and CaC03 buffer curves . 4.3.1.2 Neutralising material requirements of the pitwall rock The rise in pH of the 90 days-incubated pitwall rock with increasing CER of neutralising materials followed a sigmoidal type log function (Figure 4.2) s imilar to that for buffer curves in Figure 4. 1 a. The respective regression coefficients for the equation are given in Table 4.3 . The comparative neutral ising material requirements to raise pH >6 in 90 days incubated pitwall rock predicted from the above equation are presented in Table 4.4. Both LST and DOL were applied as fine materials (::::; 0.5 mm) and may have therefore been completely dissolved to provide a quick pH response. Larger fractions (2:: 2 mm) of these materials may not be as effective in neutral ising pitwall rock acidity, as indicated by the l imestone particle size effect (section 4 .3 .3 .3 ) . On the other hand, incorporation 1 27 of large amount of FBA may create highly alkaline conditions in the pitwall rock and this is likely to affect plant growth, causing an imbalance in the CalMg ratio and phytotoxic salt concentrations. The mean pH buffer curves presented in Figure 4.2 after 90 days of incubation showed that LST, DOL and FB A were comparable to pure CaC03 at same CER level of application in raising the pH to > 6. At a lower CER, however, DOL was significantly (p=0.05) more effective in acid neutral isation due to its higher neutralisation value but the trend was reversed at CER > 30 kg CaC03 fl . There w as no further significant increase in pH at CER > 30 kg CaC03 f l for LST, DOL and FBA. On the other hand, although RPR significantly (p = 0.05) raised the pH to 4.2 from a control pH value of 2 . 3 , it was unable to raise the pH to 6.0, even at the highest CER of 50 kg CaC03 f l . The incubation experiment showed that although RPR had a higher theoretical NP value than FBA, it was ineffective in neutralising pitwall rock acidity. The theoretical hming value from complete dissolution of North Carolina RPR ( 1 kg of P dissolved from RPR has an estimated liming value of 3.2 kg CaC03) is estimated to be 536 kg CaC03 C l based on 1 3 . 1 % P and 1 l .7 % CaC03 (Bolan, 1 995) . It is likely that only the carbonate fraction of the RPR was responsible for neutralising the pitwall rock acidity. Acid neutralisation through dissolution of RPR and adsorption of H2P04- to Fe and Al hydroxides through ligand exchange processes and subsequent release of OH" may thus be negligible. However, formation of Fe and Al phosphate compounds may reduce the activities of these ions in the acid solution. Incorporation of RPR to neutralise pitwall rock acidity may have the beneficial effect of providing a slow release, long-term acid neutralisation effect as well as reducing the oxidant (Fe3+) level and raising the P content of the pitwall rock. Furthermore, laboratory studies have shown that FeP04 coating of pyrite grains can effectively control further oxidation of pyrite (Evangelou, 1 994 & 1 995 ; Fytas et aI . , 1 994; Georgopoulou et aI., 1 995 & 1 996) . However, application of RPR at a very high rate (LRTolal of 630 t ha- I ) on the pitwall is unlikely to be cost effective viable option (at a current price of NZ$200 f l ) . 1 28 Table 4.3 Non-linear regression coefficients for pH-CER curves Yo Xo a b Ri LST 2 .43 27 5 . 2 6 -5 . 1 2 0 .99 DOL 2 .69 20 4.33 -6.03 0 .99 FBA 2.4 1 26 5 . 2 1 -7. 1 6 0.99 RPR 2 .44 40 3 . 1 1 -2 .26 0 .99 Where y :::: target pH, Yo = i n it ial pH. XC) = minimu CER to overcome buffer requirement, x == CER required to reach target pH, a and b are regression coefficients. 8 �. Limestone (LST) � Dolomite (DOL) -X- Fluidised bed boiler ash (FBA) 7 � Reactive phosphate rock (RPR) 6 :I: 5 0. 4 3 I LSD(5%) 2�--�--�--�--�---r--�---'----r---�� o 1 0 20 30 CER (kg CaC03 r1 ) 40 50 Figure 4.2 Response to pH with increasing carbonate content equivalent rate (CER) of neutralising materials. 1 29 Table 4.4 Neutralising material required to raise pH>6 in pitwall rock. Neutralising LRBuffer (CER) CaC03 equivalent Actual rate LRTotal materials kg CaC03 f l (%) kg f l kg f l -�.-... � .. ,---.-• . .. -.---------... -.. ---.--�---.---------.----.-.------ CaC03 29 1 00 29 1 1 1 LST 32 95 34 1 20 DOL 25 1 1 4 22 94 FBA 30 42 7 1 266 RPR* 88 27 326 630 * Extrapolated rate (pH = 0.04 * CER + 2.22, Ri = 0.97). Actual rate is based on the CaC03 (%) equivalent of the neutral ising materials (Table 4 .2) . LRTo,al = LRsuffer + LRNAPP (equi valent). 4.3.1.3 Neutralisation of acidity Overall, there was a significant (p=0.05) reduction in acidity in the pitwall rock material treated with neutralising materials compared with that of the control with nil treatment. The effectiveness of neutralising materials in reducing acidity in 90 days-incubated pitwall rock is shown in Figure 4.3a. Acidity in the pitwall rock was reduced to � 50 kg CaC03 f ' at CER =30 kg CaC03 f ' for all the treatments except for RPR treated pitwall rock in which it remained above 75 kg CaC03 f ' at the same CER . However, at CER �40 kg CaC03 r ' all the amendments gave similar values. At a CER of 30 kg CaC03 f ' , the overall reduction in acidity in the 90 days incubated pitwall rock was 85 % . The rate of decrease in acidity of the pitwall rock was more rapid with DOL and FBA than with either LST or RPR at low rates (CER<20 kg CaC03 r ' ) of neutralising materials. In the case of RPR, some of the phosphate released during dissolution may be used up in the formation of Fe-P04 and Al-P04 compounds and therefore the total neutralisation potential would be much less than the theoretical value of 689 kg CaC03 f l . The net neutralisation of acid will depend on the rate of dissolution of the carbonate fractions of the neutralising materials (LST, DOL, FBA and RPR) . Gypsum and portlandite in FBA are far more soluble and faster reacting than either LST or RPR and hence the greater reduction in acidity. The overall comparison of the effectiveness of neutralising materials in the reduction of pitwall rock acidity showed that at CER > 30 kg CaC03 f ' , all the neutralising materials were equally effective in reducing the acidity of the 90 days incubated pitwall rock . (Figure 4.3b). 1 30 (a) 400 - '7 .... C') 8 300 et! (.) en ..:.c: - >- 200 ..... :0 '0 - 200 ."t:: :"2 (J « 1 00 1 0 Control 20 30 CER (kg CaC03 t-1 ) I LSD(5%) LST DOL 40 50 FBA RPR Figure 4 .3 Reduction in acidity in 90 days incubated pitwall rock (a) with varying CER and (b) overall comparison between neutralsiing materials . 1 3 1 4.3.2 Effect of neutralising materials on chemical properties of the pitwall rock 4.3.2. 1 pH The response of pH with increasing rates of appl ication of neutralising materials was similar for LST and FBA and both required CER of about 20 kg CaC03 ( I to satisfy the buffering exerted by co-precipitation of Fe and Al at pH < 3 . 5 (Figure 4.2) . In contrast, DOL required only half as much CER to satisfy the buffer. The rate of increase in pH was significantly (p = 0.05) greater for DOL compared with other treatments. At CER > 30 kg CaC03 ( I , the neutralising effects of LST, DOL and FBA was restricted and there was no further increase in pH. Even with highly alkaline FBA, the pH did not rise above 7 .3 at higher rates of appl ication. This may be due to excessive precipitation of Fe hydroxide, which could reduce the reactivity of these neutralising materials. The pH in RPR treated pitwal l rock increased l inearly (pH = 0.04 * CER + 2.22, R2 = 0.97) and the maximum pH reached was 4.3 at the highest rate of application (50 kg CaC03 ( I ) . The regression coefficients in Table 4 .3 suggest that appl ication of RPR at CER > 50 kg CaC03 ( I , may not result in a significant increase in pH. 4.3.2.2 Electrical conductivity (EC) The pitwal l rock initial ly maintained a very high level of EC ( 3 .6 dS m- I ) , equivalent to a total dissolved salts content of 1 .3 % . Although 90 days incubation of the pitwall rock with neutral ising materials significantly reduced the EC to < 2.5 dS m- I at CER 30 kg CaC03 ( I , it was not possible to reduce the EC levels to < 2 dS m- I , even at the highest CER of 50 kg CaC03 ( I (Figure 4.4a). The EC in FBA treated pitwall rock was significantly (p=0.05) higher than the LST treated pitwall rock at CER >30 kg CaC03 ( I . There was no significant difference in the reduction of EC among LST, DOL and RPR. Since FBA contains both gypsum (CaS04.2H20) and portlandite (Ca(OHh), the solubil ised gypsum would have contributed to elevated levels of EC in the pitwall rock for this treatment. There was a l inear decrease in EC with increasing rates as indicated in the regression equations 1 - 4. The lower regression correlations for DOL and FBA were due to h igher solubilities of these neutral ising materials and precipitation of Mg2+ and Ca2+ with SO/- to form MgS04 and CaS04 respectively . 1 32 LST, EC = -0.03 * CER + 3 . 35 , R2 = 0.95 DOL, EC = -0.02 * CER + 3 . 3 1 , R2 = 0.88 FBA, EC = -0.02 * CER + 3 .52, R2 = 0 .89 RPR, EC = -0.02 * CER + 3 .38 , R2 = 0.95 [ 1 ] [2] [ 3 ] [4] In pitwall rock which contains elevated levels of SO/-, Fe and AI , a large part of the applied neutralising materials will be consumed in satisfying the large buffering demand by the formation of Fe and Al hydroxides. Formation of gypsum with the increasing amount of Ca2+ will initially consume acid but it may have the undesirable effect of raising the salt concentration, as indicated by the slow decrease in EC levels (Figure 4 .4a) . 4.3.2.3 Exchangeable Al (Alexc) Incubation with neutralising materials caused a sharp decrease in A1exc content of the pitwall rock and there was no significant difference between the treatments (Figure 4.4b) . There was however, a significant (p=0.05) reduction in Alexc from a control value of 1 4 cmole kg- I to less than 2 cmole kg- I in the treated pitwall rock after 90 days of incubation period at a CER of 30 kg CaC03 r I . The maximum reduction in A1exc was in the CER range of 10 to 20 kg CaC03 r l . At this rate, DOL and FBA were the most effective whereas the reduction of Alexc in LST and RPR treated pitwall rock were significantly less (p=0.05) . Overal l , the decrease in Alexc showed an exponential (function) trend with increasing rates . Since the concentration of Al in soi l solution is highly pH dependent, the observed reduction of Alexc at a buffer lime requirement (LRBuffer) rate of 30 kg CaC03 f l is sufficient to ameliorate Al toxicity in the weathered pitwall rock material . Highly acidic conditions in pitwall rock resulting from oxidation of FeS2 increases both soluble and AIexc concentration whereas introduction of neutralising materials to increase pitwall rock pH has the opposite effect . The results of this incubation study showed that, irrespective of type of neutralising materials used, A1exc in the pitwal l rock can be effectively reduced to levels � 2 cmole kg- 1 at CER of 30 kg CaC03 r l . ,.... 'E (a) 4.0 3.5 � 3.0 (.) w 2.5 (c) 1 2000 1 000 - '70) 8000 � 0) E 6000 - N � o 4000 en 2000 (e) 250 22 � 200 '0) � 0) 1 75 .s c: :2 1 50 1 25 I I I 1 00�.-.-���,-���� o 1 0 20 30 40 50 CER (kg CaC03 el ) -,.... (b) 20 1 6 '0) � 1 2 o "0 E (.) - (d) 3500 � 3000 '0) � 0) E - .f. 2500 1 33 I 2000 �.---.---.---.---.---,-,,-::;�� o 1 0 20 30 40 50 CER (kg CaC03 el) -a- Limestone (LST) � Dolomite (DOL) -)E-- Fluidised bed boiler ash (FBA) -fr- Reactive phosphate rock (RPR) Figure 4.4 Effect on selected chemical properties of the pitwall rock after 90 days incubation with varying CER of neutralising materials . Vertical bars represent interaction LSD(5%). 1 34 The importance of Al in soil acidity and its physiochemical effects on plant growth is well recognised (Foy, 1 97 1 ; Wright, 1 989) . Liming acid soils has been shown to significantly reduce Alcxc (Reeve and Sumner, 1 972; MacLean et al . , 1 992) . Neutralisation of acid pitwal l rock to a pH value of 6.0 can require very large amounts of neutralising materials because of high buffer capacity exerted by Fe and Al hydroxide precipitates. Although AIexc is a measure of reserve acidity and has been found to be related to plant root growth and yield, it does not always relate well in soil materials containing high concentrations of metal ions that readily form oxyhydroxide gels under alkaline environments. High concentration of Alexc (> 1 cmole kg" I ) are known to be detrimental to plant root growth and interferes with P and Ca uptake by plants (Fox et al . , 1 964; Alva et al . 1 986; Sloan et al . 1 995) . Although RPR was less effective in raising the pitwall rock pH, it significantly reduced Alexc (Figure 4. 1 c). This reduction is l ikely to be due to formation of AI-P04 polymers and thereby reducing the solubility and exchangeability of Al in the pitwall rock. 4.3.2.4 Sulphate (50/") Levels of S042" in the 90 days-incubated pitwall rock was significantly reduced to half the amount ( 1 0000 mg kg" I to 5000 mg kg" l ) in the control by all the treatments at the low CER of 1 0 kg CaC03 fI (Figure 4.4c) . At CER > 1 0 kg CaC03 ( l there was no significant decrease in the SOl" levels. FBA incubated pitwalI rock generally had higher levels of SOl" due to the contribution from gypsum. The initial decrease in solo levels is due to formation of salt precipitates with Fe, Al and possibly with Mg and K. The constant level of SO/ at CER > 20 kg CaC03 f l indicates that no further generation of acid from oxidation of pyrite is taking place. 4.3.2.5 Fe and Mn S01uble Fe was significantly reduced from the control value of 3300 mg kg" l to less than 2500 mg kg" 1 at CER of 1 0 kg CaC03 (I but it remained constant at around 2200 mg kg" l with higher rates of application (Figure 4.4d) . There was no significant difference in the Fe levels (p=0.05) between the various treatments. In the case of RPR the initial reduction in Fe level is mostly due to formation of Fe-P04 compound because at pH < 4, very little Fe(OHh is likely to be formed. 1 35 The soluble Mn level was reduced from 228 mg kg- l in the control to about 1 75 mg kg- l by all the treatments at low rate application of 1 0 kg CaC03 f l . At the CER of 30 kg CaC03 f l , the Mn level was reduced to less than 1 50 mg kg- l by LST, DOL and RPR but it remained above 1 50 mg kg- l in the FBA treated pitwall rock. At CER > 30 kg CaC03 f l , the Mn levels in FBA incubated pitwall rock were significantly less than for LST, DOL and RPR. 4.3.2. 6 Overall effectiveness of neutralising materials Analysis of v ariance of the overall mean difference in the neutralising values between the neutralising materials indicated that LST, DOL and FBA were similar in their effectiveness in ameliorating low pH conditions in the pitwall rock. They significantly increased the pitwall rock pH to above 5.0 from the control (nil treatment) value of $2 .5 (Figure 4 . 5 a) . Although EC levels in FBA incubated pitwall rock were slightly higher, the overall mean differences between the neutralising materials effects on EC, solo, Fe, Mn and Al were insignificant at p=0.05 . There was a significant reduction in the levels of these measured parameters in the neutral ising material incubated pitwall rock from that of control levels of nil treated pitwall rock (Figure 4.5b, c, d, e and f). Addition of RPR caused a significant reduction in the acidity of the pitwall rock, but it was not effective in raising the pH to optimum level for plant growth (pH 6.0) even at the highest levels of CER . 1 36 (a) 6 (b) 4.0 ld) 1 2000 I 1 00���������� Control LST DOL FBA RPR Figure 4.5 Overall effect of neutral ising materials on selected chemical properties of the pitwall rock after 90 days incubation. LST, l imestone; DOL, dolomite; FBA, fluidised bed boiler ash; RPR, reactive phosphate rock. Vertical bars represent LSD(5%). 4.3.3 Neutralising effect of limestone particles on pitwall rock 4.3.3. 1 Lime requirement based on limestone particle size 1 37 Lime requirements of the pitwall rock obtained from rate versus pH curves (Figure 4.6a) based on the varying sizes of limestone particles are given in Table 4.5 . The Te Kuiti limestone which is currently used at Waihi mine to control AMD in wa te rock has a CaC03 value of 95% and fineness (::; 2 mm) of about 55%. The 90 days incubation study showed that the effectiveness of the graded limestone in raising the pitwall rock pH was directly related to the particle size. There was no significant effect on pH of the pitwall rock by limestone > 2 mm. Only very fine-grained limestone (LST VF, < 0.5 mm) was effective in raising the pH to > 5 at a CER of 30 kg CaC03 ( I . Based on the LRNAPP of 82 kg CaC03 ( I (86 kg LSTAR ( I , 95% CaC03), under the current practice of Iiming at Waihi, it would require LRTotal of 1 94 kg LSTAR ( I (272 t LSTAR ha· I I 0 cm) of Te Kuiti limestone to effectively control AMD from pyritic waste rock materials. Table 4.5 Graded li mestone requirement of the pitwall rock to raise pH to > 6. Limestone LRBuffer (CER) CaC03 equi valent Actual rate kg CaC03 ( I ( % ) kg ( I - -- - . ---_ .. "._----. ---- 32# - ------_ .. -_. __ ._---_ ..... _-- --_ ... _------'-- ._--_._-_ .. _".- LSTvF 95 34 LSTF 62' 95 65 LSTc 4 1 4' 95 436 LSTAR 1 03 ' 95 1 08 LRTotal k -I g t - "- -_.- .-. 1 20 1 5 1 522 1 94 # Same as LST * Extrapolated rates from l i near equations: LST F, pH = 0.07 * CER + 1 .93, R2 = 0.96; LST c, pH = 0.0 1 * CER + 2.3 1 , R2 = 0.86; LST AR, pH = 0.04 * CER + 2.03, R = 0.93 4.3.3.2 Limestone particle size effect on chemical properties The effective final in pH at the highest CER (50 kg CaC03 ( I ) for LST VF, LST F and LSTc were 7 . 3 , 5 .0 and 4.0 (Figure 4.6a) . The LSTAR, although it had 55% fi nes, did not raise the pH above 2.5 at the highest CER. Coarse grained limestone (LST c) was ineffective in raising the pitwall rock pH to desired level of 6.0, even at the highest CER (kg CaC03 ( I ) due to an armouring effect of metal hydroxide and sulphate salt coating on the limestone grains. Very fine grained limestone (LST VF) provided efficient neutralisation and raised the pH to the desired level of 6.0 at CER of 34 kg CaC03 (I (48 t LVF ha- I 1 0 cm). The rate of increase in pH with lime addition for LST F , LST c and 1 3 8 LST AR followed a l inear function and the predicted CER shown in Table 4.5 indicated that it would require 6 1 0 t LSTc ha- i (LRTotal of 73 1 t LSTc ha- i ) of pitw all surface to effectively raise the pitwall rock pH to 6.0 and above. The pit slope angle and local environmental conditions will restrict application of such a large amount of LST c on the pitwall surface. Although larger particle l imestone was not effective in raising the pH of tbe pitwall rock, the trend in reduction in the levels of EC, SO/- , Fe, Mn and Al in the pitwall rock (Figure 4 .6b, c , d, e and f) were significant at p=0.05 and were very similar to the reduction by the neutral ising materials. The reduction in EC, Mn and Al by LST c was significantly less than that for LST VF, LST F and LST AR · The LST VF, LST F and LST AR all reduced acidity to levels < 1 00 g CaC03 at CER of 30 kg CaC03 C l (Figure 4 .7a). The reduction in acidity by LST c was significantly lower than that of LST VF, LST F and LST AR . Generally, the trend in the reduction of acidity by LSTc fol lowed a linear function (Acidity = -8.4 * CER + 495, R2 = 0.99) and only at the highest CER (50 kg CaC03 C i ) , was the reduction < 1 00 g CaC03. The overall comparison of acid reduction by v arying particle size limestone showed (Figure 4.7b) that LST VF, LST F and LST AR all reduced acidity by about 80% of the control value at the CER of 30 kg CaC03 f I whereas LST c only reduced acidity b y about 50%. �a) 7 6 � 5 0.. 4 3 2 (C) 20 1 ... 'Cl .:.:. 0 '0 E � « (e) 3500 :; 3000 Cl .:.:. 0') E - � 2500 I 2000 r-.-.-.-.-.-.-.-.-�� o 1 0 20 30 40 50 4lf) ... 'E � 3.0 U w 2.5 2.0 1 200 �d) 1 000 �O') 8000 .:.:. 0') §. 6000 , N O� 4000 Cl) 2000 0 (f) 250 22 :::- 200 '0') .:.:. 0') 1 75 §. � 1 50 1 25 1 39 ---- LSTVF I � LSTF -f:r- LSTc --- LSTAR I I 1 00 �.-.-.-.-.-.-.-.-.-. o 1 0 20 30 40 50 Figire 4.6 Effect on selected chemical properties of the pitwall rock after 90 days incubation with varying CER of different particle size limstone. LST VF, very fine limestone ; LST p, fine limestone ; LST c, coarse limeston e ; LSTAR, a s received limestone. Vertical bars represent LSD( 5 % ) . 1 40 0 0 10 20 30 40 50 CER ( kg CaC03 t-1 ) (b) 500 400 I LSD(5%) - .... � M 0 300 <.> (G <.> Cl .:t:. - >- 200 ::: "0 'u �"' ... """"--¥.;,., ! ���c;.;. : · ·�+. iIt . • , • • -• ..g� • . . ' , ,' �. . . EDS Scan path __________ _ • _______ _ . ___ ._w._._ ... :.. __ � __ _<::. ____ • _________________ ______________________ • '�� .-.... ."r Core _ Mi. 88 e t. s %ELMT ATOM% 1 .4 1 .6 72.9 76.8 1 .2 1 .0 1 7 .3 1 2.8 3 . 1 3.8 2.3 I .7 1 .8 2.3 1 0 . :3 > , 1 1 2 e t, ::: ' 1 43 Figure 4.8 EDS spectra with accompanying tables of elemental concentrations of hydroxide coated limestone grain incubated for 90 days. Core (limestone), Middle (hydroxide coating) and Outer (pitwall rock front) . 1 44 4.3.3.4 Quantification of elements in the hydroxide coating In order to quantify the amount of Fe, Al and SO/- in the hydroxide coatings, and whether duration of incubation had any effect on the amount of metals and SO/­ precipitated in the coatings, l imestone grains that had been incubated for 45 and 90 days were dissolved in 1 M Hel. The amount of Fe, Al and S dissolved from coatings on the 2 and 4 mm grains was proportionate to the diameter of the grains but there was no difference in the amount between the 4 and 8 mm particles (Table 4.6) . Since the reactivity and relative coating by the Fe hydroxide are directly proportional to the surface area of the limestone grain, smaller grains which have larger surface areas are expected to adsorb higher concentrations of metal precipitates. On a specific surface area basis, S042- in the 45 days incubated grains followed the expected trend with adsorption of 5 .2 , 2 .7 and 0.7 j.lg mm2 for 2, 4 and 8 mm grains respectively. A similar trend was observed for AI . Whereas Fe adsorption in the 8 mm grain was twice (2.6 j.lg ') . mm-) the amount adsorbed on 2 and 4 mm grams. Table 4.6 Metal and sulphate analysis of the coated l imestone grain 45 days 90 days mm Fe, j.lg AI, j.lg S04 , Fe, j.lg AI , j.lg 2 1 6 ± 2 6 ± 1 67 1 5 ± 2 4 ± 0.5 4 52 ± 8 1 1 ± 1 1 37 53 ± 5 1 3 ± 1 8 76 ± 6 1 2 ± 2 1 32 68 ± 3 9 ± 1 S, j.lg 44± 5 98 ± 7 1 1 2 ± 8 The results showed that there was in fact a reduction in the amount of coating of the relative quantity of Fe, Al and SO/- in the 90 days incubated grains compared with the 45 days incubated grains. This may indicate that the amount of precipitate is directly dependent upon the reactivity of the limestone and the degree of acidity of the pitwall rock . Once a certain thickness of coating is developed, no more Fe, Al or sol will precipitate. When a new front of acid lowers the pH of the contact zone, some of the coating may start to dissolve and may finally expose the limestone once again to provide further neutralisation. While fine grained l imestone provide immediate neutralisation of the active acidity, the possibility of providing long-term slow release 1 45 neutralisation of sulphide acidity by coarse grained limestone In pitwall rock seem feasible. 4.3.4 Effect of incubation time on pitwall rock chemical properties Comparison of the 45 days and 90 days incubated pitwall rock showed that there were no significant differences in the changes in the chemical properties due to the addition of neutralising materials (Figure 4.9a,b, c, d, e, f). The mean pH of the 90 days incubated pitwall rock were generally slightly higher whereas the levels of EC and Mn were lower than that of 45 days incubated pitwall rock. However, the differences were not statistically significant at p=0.05 . Interesting to note though was that levels of EC, Fe and Mn in the control (nil treated pitwall rock) were lower in 90 days incubated pitwall rock whereas there was an increase in the exchangeable AI. This may indicate possible reduction of Fe and Mn, making them less soluble, and release of Al from the silicate matrix. 1 46 (a) 7 6 5 4 ::I: Q. 3 2 1 0 (C) 20 __ 1 5 ..... , 0') � u "0 1 0 E 0 - LL 1 000 o (b) 5 4 - ":"E 3 (/) "0 --- 0 2 w 1 0 (d) 1 2000 -. "-0, 8000 � 0') E - N 0"'" 4000 (/) - ,... '0') � (1) 400 300 0') 200 E --- s::: :lE 1 00 o EZZ221 45 days - 90 days Figure 4.9 Effect of incubation time on selected chemical properties of the pitwall rock treated with neutralising materials at CER= 30 kg CaC03 C l . Vertical bars represent LSD(5%). 1 47 4.3.5 Effect of incubation on physical properties of the pitwall rock Application of higher rates of neutralising materials were not only effective in raising the pitwall rock pH and reducing levels of EC, sol-, Fe, Mn and Al but also changed pitwall rock consistence from clayey (control) to structurally better friable forms. The LST c and LST AR incubated pitwall rock were the least structurally developed. There was a marked colour change from earthy bluish grey (SY61 l , control) to reddish brown with increasing rates of application of LST and DOL, as indicated by changes in Munsells colours (Table 4.7 and Plate 4.2) . At the highest CER (SO kg CaC03 ( I ) the colour change was most pronounced in the pure CaC03 ( l OYR7/6), DOL ( l OYR6/8) and LST ( l OYR7/8) incubated pitwall rock. These changes in colour are indicative of increasing precipitation of ferric-hydroxide with increase in pitwall rock alkal inity. However, very little colour change was observed in the pitwall rock incubated with LST c and RPR. Hydroxide coating on limestone particles were observed all through low CER ( 10 kg CaC03 ( I ) to high CER (SO kg CaC03 ( I ) in both LSTc and LSTAR. The FBA treated pitwall rock had the best structure development but at high rates of application (SO kg CaC03 ( I ) , cementitious matrix formation was observed. Table 4.7 Munsells colours after 90 days incubation . ������U���g_��te�(�l� . Control Nil . .. .. ... . ........ -.. Low rate .. _ _ M�ct.il!J:!1J�!�. _ ____ ! lg.h._ ra� _ ........ __ ... _._-. _ .. ' ---.---. -_ ...... . Pure CaC03 LST F (O.S - 2 mm) LSTc (2 - 4 mm) LST AR (SS% < 2 mm) DOL « O.S mm) RPR « 2 mm) FBA « 2 mm) ISY61 l 1 0YR7/6 l OYR61 l 2.SY6/0 2.SY7/0 l OYR8/6 2.SY6/0 l OYR61 l Low rate = CER 1 0, Medium rate= CER 25, H igh rate = CER 50 l OYR6/8 7.SYRS/8 l OYR8/8 l OYR7/8 2.SY6/0 2.SY6/0 2.SY8/4 2.SY7/4 l OYR7/8 10YR6/8 2.SY6/0 2.SY6/0 10YR7/4 l OYR7/6 148 4.4 Conclusions In a highly acidic pyritic pitwall rock containing high levels of Fe, AI, sol- and soluble salts, l ime requirement assessed from standard buffer methods grossly underestimates the actual l ime requirement of the pitwall rock. Both the buffer exerted by Fe-AI­ hydroxides and potential acidity of the pitwall rock contribute significantly to the neutralisation requirement of the pitwall rock. The type of neutralising material required is directly dependent upon their relative reactivity of the carbonate fraction. The l ime requirement predicted from the NaOH-pH and CaCOrpH buffer curves indicated that a buffer l ime requirements (LRBuffer) of 29 kg CaC03 C l was required to raise the pH of the pitwall to 6.0 . The lime requirement to neutralise active acidity of the pitwall rock, determined by the incubation method, is substantially less than that estimated from ABA. The total l ime requirement (LRTotal) of the pitwall rock ( 1 1 1 kg CaC03 ( I ) for complete neutralisation of acidity in the pitwall rock material, is nearly four times the LRBuffer' LST, DOL and FBA were equally effective in overcoming the l arge buffer exerted by Fe and Al hydroxides to raise pH to 6.0 at the same CER of 30 kg CaC03 r l . At lower CER, DOL was sign ificantly more effective than LST and FBA in overcoming the Fe-AI buffer. RPR did not raise the pH above 4 .2 even at the highest CER of 50 kg CaC03 r I , despite i t having a higher theoretical neutralisation potential than FBA. Under the present conditions of the pit slope, the weathered pitwall rock would require a LRTotal of 1 70, 1 3 1 , 392 and 576 t ha- l . lO cm of LST, DOL, FBA and RPR respectively Overall , there was a significant reduction in acidity in the pitwall rock material amended with neutralising materials compared with that of the control with nil treatment. Acidity in the pitwall rock was reduced to � 50 kg CaC03 r l at CER � 30 kg CaC03 ( I for LST, DOL and FBA treatments whereas acidity in the RPR treated pitwall rock remained above 7 5 kg CaC03 ( 1 at the same CER. At a CER ==30 kg CaC03 r i , the overall reduction in acidity in the 90 days incubated pitwall rock was 85%. At CER > 30 kg CaC03 r i , all the neutralising materials were equally effective in reducing the acidity of the 90 days incubated pitwall rock. • 11 \ � " f, • \ I \ RS \ • " ';:: , , 1 1 " , • 1 I. , t � ..:; , H 149 , < Plate 4.2 Physical effect on pitwall rock incubated with nil (Control), low (10 t CaC03 Mf1), medium (25 t CaC03 Mfl) and high (50 t CaC03 Mfl) rates of neutralising materials. Labels on pitwall rock indicate Munsells colour notations. 1 50 1 5 1 The incubation experiment showed that although RPR had a higher theoretical NP value than FBA, it was ineffective in neutralising pitwall rock acidity. Amendment with RPR however, may have the beneficial effect of providing a slow release, long-term acid neutralisation effect as well as reducing the oxidant (Fe3+) level and raising the P content of the pitwall rock. The overall mean differences between the neutralising materials' effects on EC, S04 2-, Fe, Mn and Al were insignificant at p=0.05. However, incubation with neutralising materials caused a significant reduction in the levels of these measured parameters from that of control levels of nil treated pitwall rock. Incubation with neutralising materials caused a sharp decrease in exchangeable Al in the pitwall rock and all the treatments were equally effective in reducing exchangeable Al. Since the concentration of Al in soil solution is highly pH dependent, the observed reduction of exchangeable Al at buffer lime requirement (LRBuffer) rate of 29 kg CaC03 ( I is sufficient to ameliorate Al toxicity in the weathered pitwall rock material . Limestone particle size had a marked effect on reactivity and neutralisation of acidity. Coarse grained limestone (LST c) was ineffective in raising the pitwall rock pH to desired level of 6.0, even at the highest CER (kg CaC03 ( I ) due to an armouring effect of metal hydroxide and sulphate salt coating on the limestone grains. Although the neutralisation effect of coarse grained l imestone (> 2 mm) was affected by hydroxide coatings, the initial neutralisation provided by the larger particle limestone was sufficient to significantly lower levels of EC, solo, acidity, Fe, Mn and Al in the acidic pitwall rock. While fine grained limestone provide immediate neutralisation of the active acidity, the possibility of providing long-term slow release neutralisation of sulphide acidity by coarse grained l imestone in pitwall rock seems feasible. When the pH of pitwall rock is increased, as would happen with contact with the incremental addition of neutralising materials, metal ions such as Fe and Al readily form hydroxide precipitates and possible sulphate salts, which is shown to have an armouring 1 52 effect on larger particle size l imestone. SEMlEDS examination showed that the coating on limestone particles consisted mainly of Fe and Al hydroxides and sulphate salts in a matrix of amorphous silica. Significant amounts of Ca had moved into the pitwall rock l- and may have complexed with S04- to form CaS04·2H20 as reflected in the relatively high levels of EC. Amendments with neutralising materials were not only effective in raising the pitwall rock pH and reducing levels of EC, sol-, Fe, Mn and Al but also changed pitwall rock consistency from clayey (control) to structurally better friable forms . The LST c and LSTAR incubated pitwall rock were the least structurally developed. Hydroxide coating on limestone particles were observed all through low CER ( 1 0 kg CaC03 C l ) to high CER (50 kg CaC03 r l ) in both LSTc and LSTAR . Best structure development was observed in FBA incubated pitwall rock. 1 53 C h a p t e r 5 Effectiveness of Surface Broadcasted Neutralising Materials in Ameliorating Low pH conditions in Pyritic Mine Pitwall Rock 5.1 Introduction Acid mine drainage (AMD) from oxidation of pyritic waste rock generally produces low pH drainage capable of mobilising heavy metals. The resulting low pH conditions not only facilitate mobilisation of toxic metals but also accelerate geochemical weathering, growth of acidophilic bacteria and the rate of sulphide oxidation. The release of metals potentially toxic to plants such as Fe, Mn, Al and sol- and dissolved salts from pyritic waste rock depends on several chemical and physical processes. Geochemical processes under given environmental conditions largely control the evolution of low pH from oxidation of pyrite in the presence of oxygen and moisture. The quality of drainage emerging from pitwalls as surface runoff depends in large part on the reactions with minerals capable of neutralising the AMD as it migrates downslope. The reactions and relative effectiveness of most processes, which neutralise AMD under leaching conditions, have not been fully assessed. Alkaline addition to pyritic waste rock has had mixed success in preventing AMD. For example, of eight alkaline addition sites studied by Brady et al ( 1 990), six sites still produced acid after reclamation, though generally at reduced concentrations. In a field experiment to investigate the process of AMD generation in pyritic shale treated with alkaline material, Evans and Rose ( 1 995) found that although lime treated cells showed significantly reduced acidity, sol-, AI, Fe and Mn, the effluent pH remained below 3, i rrespective of treatments. At the same time higher rates of lime did not further decrease acidity as expected, leading to the conclusion that there was no clear cut relationship between the lime requirement and the AMD released. The rate of acid neutralisation is difficult to measure because of several factors affecting the neutralisation processes. Ferguson and Morin ( 1 99 1 ) and Morin and Hutt ( 1994) suggested that the dissolution of carbonate minerals and acid neutralisation can simply be represented by the ratio of the molar concentr tions of dominant cation (Ca and Mg) and sol- (Ca/S04 or Ca+MglS04). Ferguson and Morin ( 1 99 1 ) defined a number of curves 1 54 for sulphate production from 28 1 kinetic tests and found that most test cases produced logarithmic growth curves for cumulative sulphate, indicating gradual decrease through time in the rate of acid generation. The effectiveness of various materials in neutralising acid will largely depend on the mineralogical characteristics of the mine waste rock and the environmental conditions of the mine location. Each treatment measure for acid neutralisation is therefore site specific. The acid consuming reactions involved during neutralisation processes using neutralising materials selected can be summarised in the following equations. LST, CaC03 + 2H+ -7 ci+ + CO2 + H20 [ 1 ] DOL, CaMg(C03)2 + 2H+ -7 Ca 2+ + Mg2+ + CO2 + H20 [2] FBA, [CaS04.2H20, Ca(OHh] + 2H+ -7 Ca 2+ + sol- + H20 [3] RPR, Cal O(P04)6F2 + 1 2H+ -7 l OCa 2+ + 6H2P04- + 2F [4] In the presence of pyrite, the carbonate fraction of the neutralising materials will react according to equation 5 and the SO/ released from oxidation of pyrite will further react with CaC03 to form gypsum (Eqns.6 and 7) . 4FeS2 + 8CaC03 + 1 502 + 6H20 -7 4Fe(OH)3 + 8S0/- + 8ci+ + 8C02 [5] CaC03 + SO/ + 2H+ + H20 -7 CaS04 .2H20 + CO2 [6] Ca(OH)2 + + sol- + 2H+ -7 CaS04.2H20 [7] Beside carbonates, the decomposition of clay minerals and alkali feldspars (even under moderately acidic conditions) can also contribute towards acid neutralisation processes. Under acidic environments aluminosilicates such as mica and clay minerals are capable of removing H+ ions by ion exchange reactions (see section 2 .4.4, Chapter 2) . The effectiveness of alkaline materials in neutralising acid conditions in oxidising pyritic waste rock is also dependent on the depth of weathered cover material available for neutralisation. In the presence of elevated levels of SO/, heavy precipitation of CaS04 and metal hydroxides at the surface may deter the downward movement of the alkaline front. The effectiveness of limestone in ameliorating subsoil acidity is generally 1 55 considered poor due to the slow rate of downward movement of the alkaline front (Costigan et aI. , 1 98 1 ; Dollhopf, 1 992). Therefore, an alternate choice of amendments and the method of application are vital in creating a suitable depth of growing media. The methods of application of neutralising materials (surface broadcast versus incorporation) are going to be the detenninant factor in their effectiveness in alleviating subsurface acidity in the pitwall rock. Chemical characterisation of pitwall bulk samples and the AMD from runoff showed that high levels of dissolved Fe, Al, Mn, S042- and soluble salts are the major components of the AMD besides its characteristic low pH (Chapter 3) . Neutralisation of acid in the pitwall rock material by pH modification will undoubtedly result in the precipitation of the metals as sulphate and hydroxide complexes, which will affect the nature, and chemistry of the leachate. Since addition of neutralising materials to ameliorate AMD conditions in mine waste materials is currently a standard practice, it is important to evaluate the characteristic effectiveness of some selected neutralising materials. The objective of this study was to detennine the effect that surface broadcasted neutralising materials has on 1eachate quality and subsurface amelioration of low pH condition in weathered pyritic pitwall rock. Column leaching cycles under glasshouse conditions were intended to provide conditions that closely approximate field conditions. The main quantitative aspect of this study inv lved an assessment of the effectiveness of selected neutralising materials in ameliorating low pH conditions in weathered pitwall rock under accelerated leaching conditions. The specific objectives of this experiment were: 1 . Characterisation of leachate from pitwall rock columns broadcasted with selected neutralising materials 2. Assessing the extent of subsurface amel ioration of low pH condition by surface broadcasted neutralising materials 1 56 5.2 Materials and Methods 5.2.1 Pitwall rock bulk sample The pitwall rock bulk sample used in the column study w as from the north face of the pit at Martha mine, Waihi. The sampling location and chemical characteristics of the bulk sample are described in section 4.2. 1 and Table 4. 1 respectively (Chapter 4). The bulk sample was air-dried and crushed to nominal 4 mm fractions prior to packing into the columns. The samples contained approximately 35% fines « 0.5 mm) and had a composite pyrite content of 4.5%. 5.2.2 Neutralising materials The neutralising materials used were fine limestone (LST), dolomite (DOL), reactive phosphate rock (RPR) and fluidised bed boiler ash (FB A). These materials were described in Chapter 4 under section 4.2.2 and Table 4.2. 5.2.3 Column set up Controlled leaching columns as shown in Figure 5 . 1 were used to evaluate the effectiveness of surface broadcasted neutralising materials in ameliorating acidity on weathered partially oxidised pitwall rock. The columns were 1 6 cm long with an internal diameter of 8 cm. One kilogram of pitwall rock bulk sample was packed to a column depth of 1 4 cm (approximate average depth of weathered pitwall rock on the pitwall) . Neutralising materials (LST, DOL, RPR and FBA) at a carbonate content equivalent rate (CER) of 0 (control) and 30 kg CaC03 ( 1 were uniformly spread on the surface of the pitwall rock column. The column set up was then left to equilibrate undisturbed for one week at a moisture content of 80% field capacity. At the end of the week, the columns were weighed and leached as per the protocol outlined in section 5.2 .4 below. This experiment was designed to investigate the effects of four different neutralising materials at two rates of applications (CER = 0 and 30 kg CaC03 f l ) on the AMD compositions of repacked and leached pyritic pitwall rock columns. The experimental design was a randomised block design (consisting of 4 treatments x 2 rates x 4 replicates) and was c arried out under glasshouse conditions. SAS for Windows software was used to perform relevant statistical analyses to test significant differences in the interaction between time x treatment, treatment x depth and time x treatment x depth effects from neutralising materials. ·SUO!l!PUO:J ;:)snoqssuI� l;:)pUn ){:JOl IfBM1!d �U!q:JB;:)I 10J dn l;:)S UUInl0:J P;:)l:JrulSUO:J;:)"M 1 S ;:)l�!d unu OtT unu oz LS L IduunJ U!BI;:):J10d ;:)lod unu I < �' i .. . ".".".!.:. 1 qs;:)w uOIAU wri 09 If.-ci-!i?i?i?i?i4� (unu t 5) ){:JOl I fBM1!d �){ T A A �, A I I I I I I I I ( \ _){ddM {1U O�Z) l;:)lBM P;i1lE1S!Q 08 .p. !) uwnr0:J :JJ\d ,) foJeJ �)f O£ = 1I3J :SlU;:)WPU;:)UlB d:JBJlnS 1 58 5.2.4 Column leaching protocol Preliminary tests using pitwall rock :::; 2 mm fractions showed that water flow through the column was impeded due to clogging of the pore space with Fe-AI hydroxide coatings formed during reaction with the neutralising materials . Columns packed with pitwall rock of nominal grain size of 4 mm were found satisfactory in eluting ?: 70 % of the saturated pore volume. In the case of FBA application, surface encrustation from reaction with Ca(OHh to form cementitious gel (ettringite) was observed during non-leached periods. The cement had to be broken regularly during leaching to facilitate water infiltration. After one week of equilibration, the columns were wetted to saturation by gradually adding 3S0 ml of distilled water over 7 days (SO ml per day) . The effluent was allowed to drain into collection flasks and the leachate collected was labelled as week-2 leachate. When leaching stopped, the wet weight of the column was recorded and compared with the dry weight for estimation of the saturated pore volume. The wetted columns were then allowed to react for one week, ensuring that the saturation water content was kept constant by daily weighing the columns. The subsequent leaching during alternate weeks (week 2, 4, 6, 8 , 1 0 and 1 2) were done with 2S0 m! of distilled water (36 ml per day). The leaching volume of 2S0 ml over a one week period was approximately equivalent to mean weekly rainfall amounts at Waihi (-SO mm, -2S0 ml) . The overall cycle of alternate dry reaction period and wet leaching period were repeated every 7 days for 1 2 weeks. The leached and non-leached weeks were thus simulative of wet and dry period field conditions at the Martha Mine site. The leaching was discontinued after 1 2 weeks when the measured parameters in the leachate were deemed to have stabilised. 5.2.5 Leachate analysis For each leaching cycle, leachate pH and EC were measured immediately after collection. For total dissolved elemental analysis, about 50 ml of the leachate samples were acidified with 2 ml of 1 0 M HCI and filtered through Whatman #42 in order to bring into solution any metals adsorbed in the suspended materials. Total dissolved Fe, Mn and Al in the leachate were measured by atomic absorption spectroscopy (AAS) using water analysis methods outlined by Rayment and Higginson ( 1 994) . Leachate sulphate (SOl-) was 1 59 measured by turbidimetric method in an Auto Analyser (Blakemore et al . 1 987). Major cation concentrations in the leachate were analysed by atomic absorption/emission spectrophotometry. 5.2.6 Column section analysis At the end of the 1 2 weeks leaching period, the pitwall rock columns were sliced into 20 mm sections (0-20, 20-40, 40-60, 60-80, 80·· 1 00, 1 00- 1 20, 1 20- 140 mm). The sliced sections were crushed and air-dried prior grinding to � 2 mm fractions for chemical analysis. Column section pH, EC, S042-, acidity, soluble Fe, Mn, and exchangeable AI (AIexc) were determined according to methods outlined in section 3 .2. 1 .3 (Chapter 3) . Compositional volume percentage (semi-quantitative) of minerals present in the sectioned samples were analysed by X-ray diffraction (XRD). 5.3 Results and Discussion The average volume of the leachate col lected was 140 ml week- I and the rate of elution during leaching was about 2 ml per hour. The glasshouse temperature during the leaching period was 22 ± SO C and at this temperature the rate of evaporation of column surface water was approximately O.S ml m-2 da/ (O. l ml dai l from a column surface area of O. 1 3 m2) . Since the downward leaching of solute in the column depended upon the pore volume, solubility of the reaction products, hydraulic conductivity of the material and surface evaporation, the volumes of leachate collected were highly variable. In general , there was a rapid release of dissolved metals (Fe, Mn and AI), sol- and EC over the first 8 weeks of leaching period fol lowed by steady decrease until a constant level was released. The initial high concentrat ions in the leach ate are considered to be flushing of the accumulated oxidation products but the rapid decline from week-6 onwards was considered to be due to the effect of neutralising materials. In describing the leachate and column characteristics of the pitwall rock, the following assumptions were made: • Sample characteristics were uniform in the column. 1 60 • Downward migration of leachant was uniform and unimpeded by oxyhydroxide formations. • All the reaction products were flushed by 250 ml de ionised water used. 5.3.1 Characterisation of the leachate quality 5.3. 1 . 1 Leachate pH Surface amendment of pitwall rock columns with neutralising materials had no statistical significant effect on the leachate pH throughout the 1 2 weeks period of leaching. The leachate pH remained below 2.5 throughout the 1 2 weeks period of leaching, irrespective of surface amendments (Figure 5 .2a). A stable pH plateau between 2.0-2.5 existed after week 6, indicating no net neutralisation of the acid in the leachate by surface amendment of neutralising materials. The equivalent rate of neutralising materials (CER = 30 kg CaC03 f l ) although being sufficient to raise the pitwall rock pH to 6 during incubation to 90 days, had no significant effect on the leachate pH. The consistent low leachate pH from the neutralising material broadcasted pitwall rock may suggest that bacterial ly catalysed oxidation of pyrite produced acid more rapidly than the ability of the neutralising materials to neutralise microenvironments of acid fonnation. Since the downward movement of the alkaline front was limited to the upper 60 mm of the 1 40 mm pitwall rock column (see section 5 .3 .2 . 1 ), it is possible that the acid produced in the lower half of the column will not be atfected by the surface broadcasted neutralising materials. Low pH (pH<2.5) leachate from columns broadcasted with neutral ising materials was unusual but consistent with observations made by other workers in results of alkaline additions to pyritic materials. (Evans and Rose, 1 995; Parisi et aI, 1 994; Brady et al, 1 990). B loomfield ( 1 972) also noted that, although hming had considerably raised the subsurface pH of the leached pyritic soil cores, it had no significant effect on the pH of the effluent. Hoving and Hood ( 1984) and Doepker ( 1988) also observed a similar effect in their studies on the treatment of pyritic material with l ime and phosphate respectively. The pH of the leachate is primarily controlled by the oxidation of pyrite and subsequent dissolution of buffering minerals in the pitwall rock material and the neutralising effect of the amendments. After complete dissolution of a buffering mineral, the pH decreases until equilibrium is attained with respect to the next buffering mineral. These pH-buffering 1 6 1 reactions are indicated by near-constant pH-plateaus after week-5 o f leaching. Such low pH leachate, despite surface amendment with neutralising materials, showed that AMD from pitwall surface runoff can be considerably more acid than the bulk of the surface material partially neutralised by surface application of neutralising materials. Correlation coefficients (Table 5 . 1 ) showed that the pH of the leachate, although remaining unchanged over the leaching peliod, had a highly significant (p=O.OO l ) negative linear relationship with EC, S042-, ac idity, Fe, Mn and AI . However, there was no significant correlation with the base cations (Ca, Mg, K and Na). This may suggest mass precipitation of sulphate salts in the presence of elevated levels of S042. in the upper section of the column affected by the neutralising materials. At the same time, the displacement of H+ and Al 3 + from exchange sit s by the base cations will lower the pH of the leach ant as it moves down the column. This is possibly one of the reasons for continued low pH leachate from surface broadcasted columns. 5.3. 1.2 Leachate EC Leachate EC in the untreated column remained high (EC> 6 dSm·l ) through to 1 2 weeks of leaching although it was significantly lower than the initial EC level of about 1 1 dSm· 1 (Figure 5 .2b). The EC from the untreated column was higher than from surface broadcasted columns throughout the leaching period. This indicated that most of the S042. accumulated as surface precipitations of gypsum (CaS04.2H20), epsomite (MgS04.7H20) and alunite [KAl3(S04h(OH)6]' which are solubilised during leaching and thus produce a highly salt laden leachate. Jarosite [KFe3(S04h(OH)6] accumulated under low pH conditions during non-leached periods (when oxidative reaction takes place) will also be solubilised by the leachate water and contribute towards the increase in EC levels in the leachate and corresponding decrease in pH. The apparent reduction in EC level in the leachate was due to precipitation of gypsum and metal hydroxide at higher pH effected by the neutralising materials . The leachate volume of 250 rnl was thus sufficient only to partially solubilise a fraction of the salt in the leachate. The initial (week 2 and 4) EC levels in the �urface broadcasted column were also high, in the range 7 to 10 dS m· l , but there was rapid decrease after week-6 until it stabi lised in the range 2-4 dS m· 1 from week-8 onward. Irrespective of neutralising 1 62 (a) 2.6 :a 2.2 2.0 1 .8 0 (b) 1 2 1 0 8 -... 'E Cl) 6 "'C - U w 4 2 0 0 2 I 2 4 6 8 1 0 1 2 --- Control -a- Limestone (LST) � Dolomite (DOL) -t::r- Reactive phosphate rock (RPR) --)E-- Fluidised bed boiler ash (FBA) 4 6 8 1 0 1 2 Time (weeks) Figure 5 .2 (a) pH and (b) EC of the leachate from columns broadcasted with neutralising materials . Vertical bars represent LSD(5 % ) . 1 63 material type, the EC level did not fall below 2 dSm- ' during the leaching period. There was no significant difference in the EC of the leachate treated with the four types of neutralising materials . Initially, the FBA broadcasted column produced slightly higher EC than the other three neutralising materials . This was to be expected as FBA already had CaS04.2H20, which would contribute total salt level in the leachate. The soluble salt concentrations in the leachate, as indicated by the EC, depend on the composition and solubility of the amendments applied at the surface and the extent of their reactivity within the column. The exchangeable ions displaced from the soil exchange sites and their cumulative concentrations in the leachate also strongly contribute towards the total electrolyte concentrations. Strong positive correlation of the EC with SO/" , acidity, Fe, Mn, AI, Mg and K and (Table 5 . 1 ) showed that these are the major components significantly (p=O.OO I ) affecting the salt concentration in the leachate. Table 5 . 1 Correlation coefficients (r) of the measured parameters in the leachate. pH EC S04�- acidity Fe Mn Al Ca Mg K EC -0.83 S042- -0.76 0.93 acidity -0.89 0.92 0.90 Fe -0.86 0.93 0.90 0.97 Mn -0.8 1 0.92 0.89 0.95 0.95 Al -0.88 0.93 0.92 0.96 0.97 0.96 Ca 0.2 1 0. 1 6 0.2 1 0.05 0.04 0.2 1 0.08 Mg -0.27 0.57 0.50 0.53 0.52 0.67 0.53 0.73 K -0. 1 1 0.47 0.48 0.37 0.38 0.50 0.38 0.85 0.85 Na 0. 1 3 0.26 0.26 0.07 0.07 0.20 0. 1 0 0.88 0.68 0.87 Coefficients r � 0.46 and r � 0.36 are significant at p=O.OO I and p=0.OO5 level of significance respectively (n=30). 5.3. 1.3 Leachate SO/- Sulphate flux in the leachate is an indication of rates of pyrite oxidation, assurrung optimum conditions for both chemical and bacterial reactions. Generally, it is reasonable to assume that sulphate concentration in the leachate is proportional to the sulphate flux and hence the extent of acid generation from pyrite oxidation (Morin and Hutt, 1 994; Miller et al . , 1 994). From the correlation matrix (Table 5 . 1 ) it is evident that sulphate flux in the leachate is directly proportional to relea e of acidity, Fe, Mn, AI, Mg and K. Significant positive correlation with pH and EC also indicated the generation of acid and corresponding precipitation of salts were probably occurring simultaneously. 1 64 Leachate sol from the untreated column remained higher than from the treated columns throughout the 1 2 weeks of leaching period. The sol- flux from the untreated and broadcasted columns rapidly decreased over the 1 2 week period (Figure 5 . 3a). The untreated column initially released 5588 SO/- mg L- 1 in the lcachate which gradually stabilised at around 3500 mg sol L- I after week 8 onwards. Leachate from the amended columns generally produced lower concentrations of sol. Compared to the untreated column, LST and DOL induced the most significant reduction in S042- level. The cumulative rate of release of sol in the leachate is compared in Table 5 .2. LST and DOL caused the largest reductions in sol- and they were significantly more effective than FBA and RPR by about 2 1 %. The overall effectiveness of reductions in the concentration of sol- in the leachate compared with that of control with nil treatment were in the order: DOL>LST» RPR>FBA. 5.3.1.4 Leachate acidity Acidity release from the untreated column was 3 1 2 1 mg CaC03 Cl in week-2 to about 1 477 mg CaC03 L-1 from week 8 onward, after which the level remained constant. Generally, LST, DOL and FBA broadcasted columns showed significant reduction in acidity compared with the untreated column (Figure 5 .3b) . The RPR broadcasted column produced significantly higher leachate acidity than LST, DOL and FBA but from week 10 onwards, there was no difference between the amendments. With time, the surface broadcasted columns generally produced lower acidity compared with the untreated column with nil amendment. At the end of 1 2 weeks, the comparative average acidity levels and relative reduction by the neutralising materials were as shown in Table 5 .2. The relative effectiveness of neutralising materials in reducing leachate acidity were in the order : FBA>LST>DOL» RPR. The results of the leaching experiment showed that although leachate pH remained low, surface applied neutralising materials had significantly lowered sol and acidity in the leachate towards the end of the leaching cycle as a result of oxyhydroxide formation and precipitation of sulphate minerals. 1 65 Table 5 .2 Average release rates of the concentrations of measured parameters in the leachate from broadcasted pitwall rock column. Overall means comparison LSD % reduction (-) / increase (+) Control LST DOL RPR FBA a=0.05 LST DOL RPR FBA S042- 42 1 3 2327 1 977 3000 3 1 1 3 224 -45 -53 -29 -26 Acidity 2 1 96 1066 1 1 32 1485 992 24 -5 1 -48 -32 -55 Fe 3253 1 575 1492 1 773 1 1 53 1 53 -52 -54 -45 -65 Mn 1 34 79 82 9 1 63 7 -4 1 -38 -32 -53 Al 377 2 1 1 l 79 245 1 60 2 1 -44 -52 -35 -58 Ca 234 368 35 1 303 4 1 2 1 3 +57 +50 +29 +76 Mg 82 106 1 38 90 1 1 1 7 +29 +68 + 1 0 +35 K 1 8 22 24 1 7 28 2 +24 +34 -4 +54 Na 24 36 38 29 5 1 2 +46 +54 +20 + 1 1 0 . . Except for aC idity (mg CaCO) L I week \ the umts are 1 0 mg L I week I As a measure of acid neutralisation by neutralising material addition in the pitwall rock column, cumulative acidity was calculated for 1 2 weeks duration of leaching. Cumulative acidity, given in equivalent mg CaC03 C' represents the total acidity discharged since the beginning of the experiment. The mean cumulative rate of acidity released from the untreated control column was 2 196 mg CaC03 L- ' week- I . In the untreated column, the cumulative increase in acidity followed a linear trend with time (y = 394 * X + 4 1 0.3 1 , R2 = 0.99) indicating continued generation of acid in the column. The initial high levels of acid release from all the columns represented the initial flushing of stored SO/- and dissolution of jarosite type minerals. The corresponding cumulative release of acidity from the amended columns fol lowed trends : - RPR: y = 7 1 3 * Log(x) + 557, R2 = 0.99; LST and DOL: y = 494 * Log(x) + 537, R2 = 0.98 ; FBA: y = 4 1 0 * Log(x) + 464, R2 = 0.99] . A strong positive correlation (Table 5 . 1 ) of the acidity with the major components of the leachate indicated that the solubility of Fe, Mn, AI, salt and cations in the leachate will be directly controlled by the high acidity and low pH of the emerging leachate . 5.3. 1.5 Leachate Fe and Mn A significant reduction in the concentrations of soluble Fe and Mn were observed in leachate irrespective of treatments. The concentrations in the leachate from amended columns were significantly lower than that of control column throughout the leaching period (Figure 5 .3c & d). The mean release rate of Fe and Mn concentrations from the 1 66 (a) 6000 5000 I ..... � Cl ..s N � 0 Cl) 4000 3000 2000 1 000 0 (c) 5000 4000 I I I � 3000 � Cl I E ' - I Co> 2000 !-U. � 1 000 � (e) 500 400 :. 300 Cl E ...: 200 « 1 00 I O�-r--���-r--�� o 2 4 6 8 1 0 1 2 Time (weeks) (b) 4000 - � 3000 I 0 0 ('0 0 Cl 2000 ..s >-;!:! :s? () « 1 000 0 (d) 250 200 I T"" � 1 50 Cl E c: 1 00 :a 50 Or-�--�--r-�--�� o 2 4 6 8 1 0 1 2 Time (weeks) -e- Control -a- Limestone (LST) ----+- Dolomite (DOL) ---t:s:- Reactive phosphate rock (RPR) � Fluidised bed boiler ash (FBA) Figure 5 .3 Concentrations of (a) S 042-, (b) acidity, (c) Fe, (d) Mn and (e) Al in the leachate from columns broadcasted with neutral ising materials . Vertical bars represent LSD(5%). 1 67 control columns were 3253 mg Fe L- I week- I and 1 34 mg Mn L- I week- I respectively (Table 5 .2) . All the treatments caused rapid reduction in the concentration of Fe and Mn after the second cycle of leaching. The relative reduction of both Fe and Mn contents was generally higher in the leachate from the FBA broadcasted columns. Both Fe and Mn showed strong positive correlation with EC, acidity, SO/-, Al, Mg and K but showed insignificant (at p=0.005) relationship with Ca and Na in the leachate. Highly significant (p=O.OO I ) negative correlation of Fe and Mn with leachate pH and corresponding positive significance with acidity (Table 5 . 1 ) confirm their solubilised state in very low pH conditions. In general , the reduction in the concentrations of Fe and Mn in the leachate from the control columns fol lowed the trends:- Fe = 3079 * X + 1 8 1 3 , R2 = 0.99 and Mn = 336 * Ln(X) + 1 82 , R2 = 0.99. In the presence of elevated levels of sulphate, raising the pH with neutralising materials has resulted in the precipitation of these metals as sulphate (FeS04, MnS04) and hydroxides (Fe(OH)3, Mn(OH)3) along with gypsum (CaS04.2H20). The FeS04 can further react with the CaC03 to form more Fe(OH)3 and CaS04.2H20 resulting in immobilisation of Fe and Mn in the column. 5.3. 1 .6 Leachate Al Surface application of l ime in acid agricultural soils is commonly intended to raise the pH and neutralise or precipitate plant-toxic levels of Al. In the absence of mobile anions such as SO/- the downward movement of lime has been found to be extremely slow especially in variable charge soi ls. Under severely acid conditions (pH<3), the low pH condition is largely buffered by acid hydrolysis of aluminosilicates and is characterised by high AI contents in both leachate and column sections of untreated columns (Figure 5.3e and 5.5e). As the pH of the column surface is raised by the alkaline amendments, Al is likely to precipitate as hydroxide [Al(OH)3J or basic sulphate (van Breemen, 1 973) releasing more soluble acid that can be leached from the system. The solubil ised Al content in the leachate from surface broadcasted columns compared with the untreated column showed a rapid decrease with time (Figure 5 .3e). The average release of Al in the leachate from the untreated control column was 377 mg Cl week- I 1 68 whereas the concentration in the leachate from broadcasted columns was significantly lower (Table 5 .2 ) . The Al in the leachate was shown to be strongly correlated with pH, EC, SOl", acidity, Fe, Mn, Mg and K (Table 5. 1 ) indicating that in low pH conditions all of these dissolved constituents are likely to influence the release of Al in the leachate. The rel ative reduction in Al (Table 5 .2) in the leachate from amended columns were in the order FBA ( 5 8 % ) > DOL (53%) > LST (44%) » RPR (35%). 5.3. 1 . 7 Leachate Ca - Mg - K · Na Amendment with neutralising materials caused significant increase in the leachate Ca concentration compared to that from control column. The FBA treated column produced leachate with highest Ca concentration whereas Ca level in the leachate from RPR was not significantly different from that of control column (Figure 5 .4 ) . This may suggest that in the presence of high amounts of SOl", Fe, Mn, Al and dissolved salts, the contribution of Ca from the carbonate fraction of the neutralising material may be minimal. Under alkaline conditions created at the surface by the neutralising materials, most of the Ca is likely to be paired with SO/ and precipitated as gypsum, as reflected in the significant (p=O.OO 1 ) linear correlation coefficients (Table 5 . 1 ) . It is possible that some Ca in the leachate may have dissolved from decomposition of clay and Ca-feldspar minerals in very acid pore solutions. The relative increases of 57%, 50%, 29% and 76% in Ca concentrations of the leachate from LST, DOL, RPR, and FBA amended columns respectively were statistically significant. There was also significant reduction in the Mg concentrations except for the DOL amended column, which showed an 68% relative increase in the Mg concentration of the leachate compared with that of control column (Table 5 . 2 ) . Both FBA and DOL broadcasted columns produced leachate with increased concentrations of K in the leachate. There was a large increase ( 1 1 0%) in Na concentration in the leachate from the FBA amended columns. The FBA used in this experiment had a Na content of 7000 mg Na kg" ' (0.7 % , Wang et aI, 1 994) and because Na is highly mobile, the high concentration reflected in leachate is to be expected. Both Mg and K showed strong positive correlation with EC, sol, acidity, Fe, Mn and Al (Table 5 . 1 ) indicating that their mobility in the acid environment is strongly affected by acidity generated from pyrite oxidation. (a) 500 400 - I (b) 200 1 50 I 1 69 -;- 300 ..J ..... � C'I .s � 100 - «S 200 0 100 (c) 40 30 I 1 0 O�--r---.---.---.---.-� o 2 4 6 8 1 0 1 2 Time (weeks) 50 (d) 70 60 50 � 40 C'I E -; 30 z 20 10 I ___ Control � Umestone (LST) � Dolomite (DOL) --f:r- Reactive phsophate rock (RPR) ---*- Fluid ised bed boiler ash (FBA) 2 4 6 8 1 0 1 2 Time (weeks) Figure 5 .4 Concentrations of ( a) Ca (b) Mg (c) K and (d) Na in the leachate from columns broadcasted with neutral ising materials . Vertical bars represent LSD (5%) . 1 70 5.3.2 Column section chemistry 5.3.2. 1 Column section pH Surface application of neutralising materials significantly increased the pH in top 50 nun of the column (Figure 5.5a). The degree of effectiveness of raising the pH was in the order FBA > LST, DOL > RPR. A large fraction of the surface applied FBA remained undissolved at the surface, resulting in very high pH (>8.0) in the 0-30 mm of the column. FBA amendment also caused formation of a cementitious crust on the surface of the column during the non-leached period, which impeded the infiltration of the leachate water. The FBA used in this study contained a large fraction of burnt lime (CaO) and reactive CaS04 as well as significant amounts of amorphous aluminosilicates (Wang et al . , 1 994). When FBA is mixed with water it quickly forms hydrated calcium sulphate (CaS04.2H20) or calcium aluminium sulphates (ettringite, Ca3AhOs.3CaS04.32H20), resulting in the formation of a cementitious constituent of portland cement called the C-S­ H gel (Morin et aI, 1 995) . Both LST and DOL raised the pH to about 7 in the upper 0-30 mm section of the column. RPR raised the pH to about 5 only in the top 20 mm of the column, and at column depth > 20 mm, it remained below 4. FBA, LST and DOL were all able to raise the pH to >4 up to a column depth of 60 mm. Although the neutralising materials had significantly raised the surface pH, there was little effect at depths greater than 60 mm. The results suggested that in acidic materials with high levels of S042- and metal concentrations, the downward mobility of the alkaline front created by the neutralising materials is severely restricted by mass precipitation of salts and hydroxide compounds. These compounds can keep the material buffered against further rise in pH, at lower section of the pitwal l rock column. The pH of the untreated column remained unchanged « 2.5) throughout the 1 40 mm column depth. As with the leachate analysis, the pH of the column section was negatively correlated with Ee, sol-, Fe, Mn and AI. However, significant amounts of Ca, Mg, K and Na accumulated in the columns affected by the neutralisation processes as ion pairs with sol. This is reflected in the strongly positive correlation (p=O.OO l ) of pH with these cations (Table 5 .3 ) . (a) 0 0 20 - 40 E .s or. 60 -a. Q) "0 r:::: 80 E :::I 0 u 1 00 1 20 1 40 (b) 0 0 20 - 40 E .s or. 60 -a. Q) "0 r:::: 80 E :::I "0 u 1 00 1 20 1 40 2 4 1 pH 6 8 � Control -a- Limestone (LST) -+- Dolomite (DOL) 1 0 -fr- Reactive phosphate rock (RPR) -)E- Fluidised bed boiler ash (FBA) 3 4 Figure 5 .5 Ca) pH and Cb) EC of the sectioned samples from leached columns. Horizontal bars represent LSD(5%). 171 1 72 The initial dissolution of c arbonate and hydroxide fractions of the neutralising materials is rel atively fast, resulting in the precipitation of Fe(OHh and Al(OH)), thereby increasing the buffering capacity of the pitwall rock. However, the downward movement of the alkaline front was not sufficient to significantly raise the pH at column depth > 60 mm due to possible armouring effect of the carbonate grains with Fe(OH)3 and dissolution of j arosite [ ( KFe3(S04)2 (OH)6]. Thus in upper sections of the column the pH of the leachate will be controlled by the l arge buffer exerted by Fe(OH)3 and AI(OHh whereas at depth the rate of acid generation and dissolution of j arosite may keep the leac hate at a low pH level at all times. The acid neutralisation processes are, therefore primarily c ontrolled by the dissolution reactions of the carbonate, hydroxide and aluminosilicate minerals, as suggested by B l owes et al . ( 1 994) . 5.3.2.2 Column section EC The EC i n the control column remained h i g h and constant ( 3 . 1 ± 0 . 2 dSm- 1 ) throughout the column depth. Application of amendments resulted in significant reductions in the EC levels i n the upper 0-8 0 mm depth of the columns (Figure 5 . 5b). The FBA had signifi c antly higher EC than LST, DOL or RPR in the OAO mm section but at depths >40 . mm, there were no differences in the EC levels between the amended columns. The higher EC level in the upper section of the FBA amended column is attributed to the gypsum contained in the FB A . At depths of 0-80 mm the EC in the LST, DOL and RPR wa<; < 2 d S m- 1 but increased at greater depths as a result of accumulated s alt. The low level of EC in the column was expected since it was sectioned after leaching of the soluble components. Therefore, any salt left in the column is either from incomplete flushing or from S 042- generated thereafter. S ignificant positive correlation of EC with solo, acidity, Fe, Mn and exchangeable Al ( A1exc) suggested that metal sulphate s alt formation under a highly acid leaching environment is a common occurrence. The l aboratory ( section 3 . 3 . 2.4, Chapter 3) and field evidence of copious amounts of salt encrustation on the surface support this above postulation. The relative reductions in the amount of EC were in the order: RPR>LST>DOL>FB A (Table 5 .4). 1 73 Table 5 .3 Correlation coefficients (r) of measured parameters in the column section. pH EC sol acidity Fe Mn Alexc Ca Mg K EC -0.43 sol -0.66 0.78 acidity -0.88 0.58 0.70 Fe -0.88 0.68 0.86 0.83 Mn -0.86 0.64 0.73 0.78 0.85 A1exc -0.78 0.84 0.89 0.82 0.89 0.89 Ca 0.89 -0.3 1 -0.52 -0.7 1 -0.77 -0.74 -0.67 Mg 0.53 -0.32 -0.42 -0.5 1 -0.49 -0.29 -0.46 0.60 K 0.62 0. 1 5 -0.25 -0.3 1 -0.49 -0.43 -0.29 0.68 0.30 Na 0.58 -0.04 -0. 1 8 -0.33 -0.45 -0.54 -0.39 0.72 0.30 0.75 Coefficients r � 0.46 and r � 0.36 are significant at p=O.OO 1 and p=0.OO5 level of signi ficance respecti vely ( n=30). Table 5 .4 Overall comparison of the mean concentrations of chemical properties of the leached pitwall rock columns broadcasted with neutralising materials. Overal l means comparison LSD % reduction (-) / increase (+) Control LST DOL RPR FBA a=0.05 LST DOL RPR FBA pH 2.5 4.6 4.3 3.3 5.2 0. 1 2 +86 +75 +34 + 1 1 3 EC 3 . 1 l .8 2.0 l .7 2.5 0. 14 -4 1 -37 -45 -2 1 SO/- 3263 2 1 97 2072 1 953 2555 1 4 1 -33 -37 -40 -22 Acidity 276 1 1 0 1 4 1 058 2 1 39 1 1 94 1 6 -63 -62 -23 -57 Fe 2538 1 585 1 638 1 745 1 588 1 08 -38 -35 -3 1 -37 Mn 260 145 1 95 1 55 1 35 1 1 -44 -25 -40 -48 Alexc 806 339 358 300 377 29 -58 -56 -63 -53 Ca 1 5 1 3 10255 9 1 1 1 5 1 27 1 3645 60 1 +578 502 +239 +802 Ma eo 382 42 1 1 24 1 500 63 1 4 1 + 10 225 +3 1 +65 K 236 1 98 1 98 207 298 1 5 - 1 6 - 1 6 - 1 2 +26 Na 57 8 1 1 08 1 56 249 7 +4 1 +88 + 1 72 +335 . . Except for pH, EC(dS m I ) and aCidity ( mg CaC03 kg" I ), the UnIts are In mg kg" l . 5.3.2.3 Column section SO/- Average soluble SO/- concentrations in the untreated column ranged from 3 1 50 mg kg- I at the surface to 3720 mg kg- I in the bottom section of the column, indicating that not all the sulphate was leached. The mean SO/- concentration in the 140 mm column was 3263 ± 376 mg kg- 1 compared to the original concentration of 7440 mg kg- 1 in the bulk sample (Table 4. 1 , Chapter 4). This indicated that intermittent leaching of the column without amendment alone was responsible for reducing the SO/- level in the column by about 75%. 1 74 Soluble sol- concentrations In the pitwall rock column sections after 1 2 weeks of leaching cycle showed that the LST, DOL and RPR broadcasted columns had significantly reduced concentrations up to 0-60 mm depth (Figure 5 .6a). The FBA amended column had an elevated level of sot in the top 0-40 mm section as a result of solubilised CaS04 from FBA as well as the ma,>s precipitation of CaS04.2H20. However, in lower sections of the column (>60 mm depth), the sol- levels of the broadcasted columns were not significantly different from that of the untreated column. Although surface broadcasted columns had significantly lower sulphate levels at 0-40 mm depths, it is probable that a large fraction of the sulphate diffused to low pH section of the column and remained trapped as insoluble salts in the pore spaces. A significant amount of soluble SO/- in the column section samples indicated that either not all SO/- was flushed or pyrite oxidation is still active despite amendment at the surface by highly alkaline neutralising materials . The reduced level of S042- in the 0-40 mm section of the column broadcasted with LST, DOL and RPR indicated that these materials facilitated precipitation of sparingly soluble sulphate salts. This possibility is reflected in the significant correlation of SO/- with Ca, Mg (Table 5 .3 ). Column leaching affected by gypsum precipitation and hydroxide coatings can be due to insufficient rinse water and incomplete rinsing of the reaction products due to channelling. At high sulphate production, gypsum and metal sulphate precipitate at the surface when rinse water is added (increase in pH) resulting in apparent low sulphate levels in the leachate. When sulphate production is decreased due to reduction in pyrite oxidation (exhaustion of exposed pyrite grains or non-reactive coating by hydroxides), the rinse water can flush the accumulated gypsum and thus providing an apparent increase in sulphate levels in the leachate. 5.3.2.4 Column section acidity Acidity in the untreated column remained high at around 2000 mg CaC03 kg- 1 throughout the column depth (Figure 5 .6b). There was an apparent elevated level of acidity at median column depths of 50-70 mm depth of the control column indicating possible acid generation by bacterial activity. There was significant reduction in acidity in the upper 60 mm section of the columns broadcasted with LST, DOL and FBA. Acidity reduction by 1 75 RPR was approximately 60% less effective than either LST, DOL or FBA. Compared to the untreated column, there was significant reduction in acidity up to a depth of 80 mm in the surface broadcasted columns. Column section acidity showed strong negative correlation with Ca, Mg and K, indicating dissolution of these ions from silicate matrix and rapid removal from the system (Table 5 .2) . As expected, acidity showed a strong negative correlation with pH and a strong positive correlation with EC, sol·, Fe, Mn and AI. Although there was a gradual reduction of the leachate acidity in the untreated column with each successive leaching (Figure 5 .4b), acidity levels in the leached columns remained high through the column. On the other hand, the surface broadcasted columns showed a significant decrease in acidity in both leachate and in column sections (Figures 5 .3b and 5 .6b) . This characteristic corresponds to the raised pH level in the upper 60 mm of the column depth by the surface broadcasted neutralising materials . The result showed that under the leaching conditions of this experiment, there was significant amelioration of subsurface acidity by neutralising materials despite generation of very low pH leachate. Free acid may be readily leached from the column, however, in the presence of heavily adsorbed Fe and Al , acidity represented by these species may remain in the column as indicated by high acidity content throughout the column. The amount of acid generated depends on the amount of oxidisable pyrite present and solubilisation of the reaction products. In order to assess the extent of acid neutralisation by surface applied alkaline amendments under oxidative leaching conditions, total acidity and the rate of acid production and leaching need to be monitored over a length of time. Most of the acidity generated from oxidation of the pyrite in the column should ideally be consumed in the subsequent oxidation of Fe2+ to Fe3+, but the resulting net precipitation of Fe-hydroxide would generate further acidity. Under very low pH conditions, jarosite may precipitate in pores and micro-channels as yellowish-brown deposits of Na, K and H20 polymorphs. In severely acid and oxidising conditions of pH < 4, j arosite is considered to be the dominant precipitate over goethite resulting in temporary storage of acidity (van Breemen, 1 973) . 1 76 (a) o°r-��������� 20 E E 40 --- E. 60 Q) "t:I s:::: 80 E 2 1 00 o U 1 20 1 40 (c) o°r-��������� 20 E E 40 .....,. ..s:: Q. 60 Q) "t:I s:::: 80 E 2 1 00 o U 1 20 1 40 - E .s ..s:: -Cl.. Q) "t:I (b) Acidity (mg CaC03) (d) o � L � r­ I-r � r r I r I t_ • D � 6. )( 1 000 2000 3000 I I I Mn (mg kg-1 ) 1 00 200 300 400 I I I I H Control Limestone (LST) Dolomite (DOL) Reactive phosphate rock ( RPR) Fluidised bed boiler ash (FBA) Figure 5 .6 Distribution of (a) sOi-, (b) acidity , (c ) Fe, Cd) M n and (e) A1exc i n the sectioned samples from leached columns. Horizontal bars represent LSD(5%), 1 77 However, a likely scenario would be that as exposed pyrite grains are exhausted or coated by Fe-hydroxide and Fe-phosphate coatings, the pH of the pitwall rock column would increase and at pH > 4, jarosite is metastable and will hydrolyse to goethite (FeO.OH) and Fe(OH)3, resulting in further release of acidity in the leachate. 5.3.2.5 Neutralisation of acidity in the pitwall rock column The continued release of acidity in the leachate from surface broadcasted columns indicated that dissolution of the neutralising materials was not fast enough to counterbalance the rate of acid generation in the pitwall rock column. Since partially oxidised pitwall rock was used, there was already a large amount of pyrite oxidation products f�rmed in the column. In the presence of elevated levels of Fe, Mn, Al and S042- , the alkalinity generated by the surface placement of the neutralising materials was therefore rapidly consumed in the precipitation of metal hydroxide complexes and salt precipitates in the upper section of the column. Initially, the rate of amendment application used was only sufficient to neutralise the active acidity present in the pitwall rock up to a column depth of 60 mm from the surface. A large part of the hydroxyl ions generated by the broadcasted neutralising materials were consumed by the formation of Fe and AI hydroxide precipitates which kept the system buffered against increases in pH. Since grain size and reactivity affect the dissolution of alkaline fraction in a very acidic environment, the reaction with finer carbonate fractions will be relatively fast and effect the immediate precipitation of metal hydroxides. In contrast, larger grains of the neutralising materials may be armoured by oxyhydroxide coatings by initial reactions in the neutralisation process. This is evident in SEMlEDS scans of the limestone grains in contact with acidic solutions containing high concentrations of Fe and AI (Chapter 4). Therefore, neutralisation of acidity by addition of neutralising materials has some drawbacks because solutes are precipitated as insoluble precipitates such as floes of Fe and AI oxides and hydroxides which can severely affect the micro-environment by clogging the free pore spaces. The neutralisation of acid by the carbonate fraction of the neutralising materials (Egn.5, 6 & 7)) and for decomposition of silicate minerals is a function of the total acidity produced 1 78 in the system and formation of secondary minerals such as gypsum (Eqns. 6 & 7), Fe-AI sulphates and jarosite. According to an ideal oxidation of pyrite, 2 moles of acidity are produced for every one of S042- and the neutralisation of acid follows at the rate of 2 moles of H+ consumed for each mole of CaC03 equivalent (Eqns. 1 & 8, Chapter 2). Therefore, any deviation from the ratio of acidity to sol- ( l OO mg CaC03 L- II 96 mg sol Cl =1 .04) is an indication of the acid generation/neutralisation. Regression plots showed close linear relationships between acidity and sulphate by all the treatments (Figure 5 .7). The slope of the regression equation for the untreated column leachate was 0.66 whereas the combined average slope of the LST, DOL, RPR and FBA amended column leachate was about 0.49. These slopes were unusually low in comparison to the slope of 1 .04 for acid generation. Therefore, if the acid released in the leachate was proportional to the acid generation (S042- flux) in the column, these slopes indicated acid neutralisations of 37% (0.661 1 .04* 100 - 1 ) and 53% in the untreated and amended columns respectively. However, acidity measurements of the column section showed that only LST, DOL and FBA had significantly reduced the column acidity (Figure 5.6b) and that the untreated column, still had a similar level of acidity as in the leachate. This observation suggested that sol, and acidity levels in the leachate from pyritic waste rock may not reflect the acid generation and neutralisation processes in the columns as suggested by Evans and Rose ( 1 995) in their study on the effect of alkaline additions to coal mine spoil . The ideal molar ratio of 1 .04 for acidity to sulphate is never achieved under field conditions and, therefore, the measurement of acidity does not indicate the rate of release of acidity in the leachate because some of the acidity is neutralised by the dissolution of alkali minerals. Neither can sulphate release be indicative of acid production because some of the sulphate will be involved in the formation of gypsum and metal sulphate precipitates. Gypsum precipitation and solubility under low pH has been shown to affect the sulphide oxidation and acid generation resulting in apparent lower rates of sulphide oxidation (Ferguson and Morin, 1 99 1 ; Morin et al, 1995). The measurement of sol- in the leachate therefore, is not indicative of acid generation and hence pyrite oxidation in the pitwall rock sample. The weekly rinse water equivalent to the weekly average rainfall 1 79 amount at Waihi (50 mm) or the equivalent pore volume often used to flush the reactants are in reality insufficient for the removal of all of the leaching cycle' s reaction products. 3500 3000 2500 � :.... '" 8 2000 as o C'I E ;: 1 500 - :0 'u et 1 000 500 Leachate (Control): Y = 0.66 · X- 569, � = 0.86 Leachate (amended): Y = 0.49 • X - 1 06, � = 0.75 • • • • • O+-----�------�------�----�------�----� o 1 000 2000 3000 SO/- (mg L-1) 4000 5000 6000 Figure 5 .7 A regression plot of overall mean acidity and sol- values in leachate from control and amended columns. Data points are means of four replicates. The comparIson of sulphate releases between the untreated column and the surface broadcasted columns showed that although sulphate release rates were limited by gypsum precipitation and metal sulphate complexing, there was a general trend of decreasing S042- release with time. Although there was a strong linear relationship between sulphate and acidity levels in the leachate, the relative slope of the curve did not relate to the molar ratio slope for ideal oxidation of pyrite. 1 80 5.3.2.6 Column section Fe and Mn Most of the soluble Fe mobilised by oxidation of pyrite remained in the untreated column and for surface broadcasted columns, increased pH caused rapid precipitation of Fe as goethite or amorphous Fe-CaC03 compounds. The precipitation of Fe and Mn as hydroxides and sulphate metal salts is reflected in the correlation matrix which shows significant (p=O.OO l ) positive relationship with EC, sol and acidity and corresponding negative correlation with base cations (Table 5 .3). Strong correlation (p=O.OO I ) was also found between Fe and Mn in both leachate and column sections, indicating that these metals may be co-precipitating under alkaline conditions created by surface broadcasted neutralising materials. There were no significant differences between the effect of different types of neutralising materials on the Fe content of the column sections. Soluble Fe in the treated columns was reduced to < lOO mg kg· ! at 0-40 mm depth and remained significantly less than the soluble Fe content of the untreated column (Figure 5 .6c) . On the other hand, FBA and LST amended columns showed significant reduction in Mn in the upper 60 mm column depth (Figure 5.6d). The reductions in soluble Fe and Mn were less than 50 mg kg- ! (Table 5.4), indicating that these metals remained in the pitwall rock as insoluble metal hydroxide complexes. 5.3.2. 7 Column section exchangeable Al (Alexc) Surface amendments with neutralising materials resulted in significant reduction of A1exc at 0-80 mm column depths (Figure 5 .6e). At 0-60 mm depths, Alexc was less than 1 80 mg kg- ! in the broadcasted columns. The increase in pH at the upper section of the column was primarily responsible for precipitating Alexc as insoluble complexes. Although RPR did not raise pH to > 3.5 in the 0-60 mm depth, it was effective in reducing AIexc possibly as AI-P04 complexes. Strong correlation of AIexc with pH, EC, solo, acidity, Fe and Mn suggested that as pH increased, most of the AI in the upper section of the column precipitated either soluble salt (alunite) or hydroxide precipitates (gibbsite). Strong negative correlation with cations also indicated that AI3+ is preferentially displaced from the exchange sites along with H+. The relative reduction in the overall means of Alexc in the amended columns were in the order: RPR>LST>DOL>FBA (Table 5 .4) 1 81 5.3.2.8 Column section Ca - Mg - K - Na The distribution of the base cations in the sectioned columns is shown in Figure 5.8 . As expected, FBA caused the greatest increases in Ca, K and Na concentrations of the column section. Downward migration of Mg, K and Na was evident in FBA and DOL broadcasted columns but Ca was raised only at the 0-40 mm section of the column. High Mg concentrations were observed only in the DOL broadcasted column and at the surface section (0-20 mm) of the FBA broadcasted column. The cation concentrations in the untreated column remained low and at depth greater than 20 mm, both LST and RPR broadcasted columns contained similar levels of cation concentrations to the untreated column. The distribution of cations in the column is largely controlled by the degree of alkalinity created by the neutralising materials. Thus FBA, which created a highly alkaline condition at 0-40 mm depths, would in fact contribute high amounts of Ca as reflected in Figure 5.8a and in the strong positive correlation of the cations with the pH and corresponding negative trend with the acidity (Table 5 .3). Significant negative correlation of the cations with Fe, Mn and Alexc suggested preferential displacement and precipitation of the metal ions. Although surface broadcasted columns produced low pH leachate and significant amounts of acidity were resident in the columns, there were significant increases in the base saturation (BS%) of the pitwall rock as a result of Ca and Mg input from the dissolution of neutralising materials. Generally there was a 2-fold increase in BS% at the surface in all the amended columns (Table 5.5). This may have important implications for the availability of nutrient cations (Ca, Mg, K and Na) for plant growth. There were no significant differences (P>0.05) in the BS% distributions between the amendments, especially at 0-40 mm depth. As expected, there were gradual decreases in the BS% down the column profile. It is interesting to note that RPR was equally effective in providing base cations as the other three amendments. The BS% of the untreated column remained low « 65%) and relatively uniform throughout the column depth. 1 82 (a) 0 0 20 E 40 g .s::. 60 -Cl. Cl) "C c:: 80 E :J 8 1 00 1 20 140 (c) 0 0 20 - 40 E g .s::. 60 -Cl. Cl) "C c:: 80 E :J 8 1 00 1 20 140 --*- Control -a-- Limestone (LST) � Dolomite (DOL) (b) o --b.-- Reactive phosphate rock (RPR) � Fluidised bed boiler ash (FBA) K (mg kg-1 ) (d) Na (mg kg-1 ) o Figure 5.8 Distribution of (a) Ca, (b) Mg, (c) K and (d) Na in sectioned samples from leached columns broadcasted with neutralising materials. Horizontal bars represent LSD(5%). 1 83 Table 5 . 5 Depth-wise base saturation (BS%) in the broadcasted columns. Control LST DOL RPR FBA Depth, mm ------------------------------- ---B S 0/0-------------------------------- 0-20 50 1 00 1 00 1 00 1 00 20-40 6 1 97 98 98 99 40-60 62 94 92 89 94 60-80 62 90 89 86 87 80- 1 00 56 86 83 78 83 1 00- 1 40 53 83 8 1 7 1 70 %BS = [Ibase (cmolc kg" ) / ECEC (cmolc kg" )]* 1 00 where ECEC = [Ibase + Alexc (cmolc kg" )] 5.3.3 Mineralogical composition of leached columns An X-ray diffraction (XRD) analysis of the sectioned samples indicated anomalously high amounts of pyrite and gypsum at a column depth of 50 mm (Figure 5 .9). B oth untreated and FBA broadcasted columns showed unusually high amounts of pyrite at a column depth of 60 mm. This may have been due to selective accumulation of hydroxide coated pyrite being analysed by the XRD, since there was no anomalous high acidity or sol­ generation at this depth. Another possible explanation for the anomaly could be that during column packing, this section of the column depth might have contained a greater amounts of unliberated pyrite. The weight percent pyrite analysed by XRD ranged from 0 to 9% in the untreated and FBA treated columns whereas it remained at 2 to 4 % in the LST, DOL and RPR broadcasted columns. The high pyrite content throughout the column indicated that only a fraction of the pyrite in the column oxidised, either due to the armouring effect of oxyhydroxide precipitates and Fe-P04 coatings, or because of incomplete oxidation due to "entombment" in the pitwall rock material. High gypsum contents at 50 mm depth for LST and DOL amendments (Figure 5.9b) could be either due to residual precipitation of the mineral in macropores resulting from aggregation of large particles sample during column packing or selective coating of the precipitated gypsum crystals by the carbonate from LST and DOL, as suggested by Keren and Kauschansky ( 1 98 1 ). In the presence of elevated levels of Ca and sol' ions in the pore solution, dissolution of gypsum is also found to decrease due to the common-ion effect between Ca and sol- (Kemper et al, 1 975) . 1 84 E .s � -Co Cl) "t:l s:::: E :J (a) 0 0 20 40 60 80 Pyrite (wt.%) 4 6 (b) o 8 1 00 - E E - � -Co Cl) "t:l s:::: E :J 120 1 40 (c) �O 20 40 60 80 ____ Control -a- Limestone (LST) -+- Dolomite (DOL) ---tr- Reactive phosphate rock (RPR) � Fluidised bed boiler ash (FBA) (d) o 8 1 00 120 1 40 Figure 5.9 Distribution of (a) pyrite, (b) gypsum, (c) silica and (d) clay minerals in the leached columns broadcasted with neutral ising materials. 1 85 The significant reduction of silica in the surface section of the FBA broadcasted column (Figure 5 .9c) confinns the suggestion of cement fonnation at the surface of the column. A large fraction of silica at the surface may have been incorporated in the formation of calc­ silicate hardpan at the surface. There were no significant changes in the clay content of the column section as a result of surface broadcasted neutralising materials (Figure 5.9d). 5.4 Conclusions Surface amendment of pitwall rock with neutralising materials (LST, DOL, RPR and FBA) had no significant effect on the leachate pH, which remained low (pH<2.5) throughout the 12 week period of leaching. This indicated that acid generation from oxidation of pyrite at lower sections of the columns continued irrespective of the presence of neutralising materials at the surface. Observation of sectioned columns showed that there was marked evidence of flow impediment due to blockage of pores by precipitate formation during the non-leaching period. This raises the question that although alkaline amendments may initially create higher pH environments at the surface and an increase in CEC, rapid reaction of the neutralising materials with highly acid pore solutions causes mass precipitation of oxyhydroxides and sulphate salts which may create a reducing environment. In the case of FBA, surface amendment resulted in highly alkaline pitwall rock material at 0-40 mm depths as well as forming cementitious crust that impeded surface infiltration. Products of pyrite oxidation (SO/-, Fe2+, H+) stored in the oxidising pitwall rock were initially leached from the untreated column, whereas in the amended columns these products quickly reacted with the carbonate phases of the neutralising materials and resulted in mass precipitation of Fe and Al oxyhydroxides and gypsum. The rate of sulphide oxidation, as indicated by sol production in the control column ranged from 5588 mg SO/- kg- I week- I in the initial week of leaching to 3270 mg sol kg- I week- I in the final week of leaching indicating an overall reduction in SO/­ production of 4 1 %. The corresponding reduction in sol production by the surface broadcasted neutralising material over 1 2 week period were in the order DOL (90%) > LST (83%) > RPR (73%) > FBA (65%). 1 86 Although sol- and acidity were linearly correlated, the slope of acidity versus sol­ indicated only about 5 1 % and 20% neutralisation of the leachate and column section acidity respectively. There were however, significant reductions in acidity of the leachate and column section, indicating that despite low pH leachate, subsurface amelioration of exchangeable acidity was achieved by amendments with neutralising materials. High alkalinity created by surface broadcasted neutralising materials resulted In significant increase in the base saturation properties of the pitwall rock as well as oxyhydroxide precipitates which, may have kept the material buffered against further rise in pH. The amount of liberated pyrite in any given sample of the pitwall rock is responsible for the level of acid generation and efficiency of neutralisation. Surface broadcasting of neutralising materials, although it did not improve leachate quality, significant pH modification and acidity reduction were obtained in the subsurface. This indicated that plant materials with minimum rooting depth could be grown on surrogate soils placed over the surface modified pitwall rock materials. 1 87 C h a p t e r 6 Effectiveness of Depth Incorporated Neutralising Materials in Ameliorating Low pH conditions in Pyritic Mine Pitwall Rock 6.1 Introduction Surface broadcasting of neutralising materials on pyritic pitwall rock columns resulted in significant reductions in the levels of EC, S O/- and soluble metals (Fe, Mn, AI), in the leachate and column sections (Chapter 5) . However, the leachate pH remained low despite a significant rise in pH in the column section of the pitwall. Soil solution chemistry of the column sections showed that the downward movement of the alkaline front effected by the broadcasted neutralising materials was l imited to the upper 40 mm section of the pitwall rock column. The reaction of neutralising materials with acid solution has resulted in mass precipitation of Fe and Al hydroxides on the surface as well as elevated levels of sulphate salts. Formation of metastable precipitates such as Fe-AI hydroxides and dissolution of j arosite may also have been responsible for keeping the lower section of the pitwall rock column buffered at low pH at all times. Other than improving the leachate quality, one of the maj or objectives of applying various amendments was to ameliorate subsurface low pH conditions in pitwall rock so that a suitable plant growth medium could be created. The choice of amendment methods will become important, especially on pitwalls where the slope factor may facilitate accelerated weathering and thus limit the effectiveness of amendments to ameliorate low pH conditions. Thus, the choice of neutralising material and the method of amendment are important in providing an effective and economically viable option for treating pit slope areas affected by low pH conditions created by acid generation from pyrite oxidation. Pit slopes and mine waste dump sites are commonly engineered to the most stable slope angle and it is likely that any amendments applied by surface broadcast will result in high losses due to erosion. This will result in incomplete neutralisation of acid due to insufficient residence time of the neutralising materials on the pit slope. In steep pitwall , the methods o f application of neutralising materials (broadcasted versus incorporated) are going to be the determinant factor in their effectiveness in alleviating subsurface acidity in the pitwall rock. Provided the pitwall contains sufficient cover materials , 1 88 incorporation of neutralising materials by mixing may help prevent losses from runoff erosion and thus provide longer residence, time and more effective neutralisation of acid. The obj ective of this study was to examine the effect that depth incorporated neutralising materials have on leachate and subsurface chemistry of pyritic pitwall rock columns. The results are compared with the surface broadcasted amendments (Chapter 5) in order to assess their relative effectiveness. 6.2 Materials and Methods 6.2.1 Materials The pitwall rock bulk samples and the neutralising materials used in this study were the same as that for the surface broadcast experiment and are described in Tables 4. 1 and 4.2 respectively (Chapter 4). The pitwall rock was crushed to a � 4 mm nominal size fraction and one kilogram of it was packed into each column (Figure 5. 1 , Chapter 5). 6.2.2 Methods Except for incorporation of neutralising materials ((limestone, LST; dolomite, DOL; reactive phosphate rock, RPR; fluidised bed boiler ash, FBA) in the top 50 mm of the pitwall rock column, the experimental design, column specifications, leaching protocol, analytical and statistical methods were the same as that for the surface broadcasted amendments. Once the column was packed, the top 50 mm of the pitwall rock was taken out and mixed thoroughly with neutralising materials (�2 mm) at a carbonate content equivalent rate (CER) of 30 g CaC03 kg- I . The mixture was replaced in the column and left standing for one week prior to the commencement of leaching. 6.3 Results and Discussion The average volume of the leachate collected from the control column was 1 40 ml week- I whereas the columns incorporated with neutral ising materials produced significantly lower volumes of leachate. The corresponding mean leach ate volumes were 1 1 9, 1 1 5, 1 23 and 1 1 5 rn1 week- I for LST, DOL, RPR and FBA respectively. The treated column leachate colour appeared dark reddish-brown because of high concentrations of particulate Fe-oxyhydroxides. Due to formation of excessive Fe and Al hydroxides, the downward movement of leachant was slow and in some columns, there was a significant decrease in the flow rate of the leachate. 1 89 The trends in the distribution of measured chemical parameters in the leachate and column sections were very similar to that of broadcasted method of amendments. Instead of describing the results in detail , this chapter briefly highlights the significant outcome of the experiment and then focuses on the overall comparison of the results with that of the broadcasted method of amendments discussed in Chapter 5. In doing so, a qualitative assessment of the ameliorating effectiveness of neutralising materials under broadcasted and incorporated conditions can be made in terms of their suitability for alleviating low pH conditions in the pit wall rock. Table 6. 1 and 6.2 summarises the overall effect of incorporated neutralising materials on some selected chemical properties of the pitwall rock. 6.3. 1 Effect on chemical properties of the pitwall rock by incorporated neutralising materials. 6.3. 1. 1 pH and EC Incorporation of neutralising materials in 0-50 mm depth of the pitwall rock column resulted in no significant effect on the leachate pH throughout the 1 2 weeks period of leaching. All the columns produced very low pH leachate irrespective of amendments (Figure 6. 1 a). Although the 1 2 weeks mean pH remained in the range 1 .9 to 2.4, the • gradual increase in pH from week 2 to week 8 was statistically significant. Low pH leachate from columns incorporated with neutralising materials was consistent with the observations made by several other workers (Bloomfield, 1 972; Hoving and Hood, 1 984; Doepker, 1 988; Brady et al . , 1 990; Parisi et al . , 1 994; Evans and Rose, 1 995). Despite low pH leachate, there was a significant rise of the column section pH by depth incorporated amendments (Figure 6.2a). As expected, FBA incorporation resulted in highly alkaline conditions within the depth of incorporation (50 mm) but indicated no subsurface effect on the pH. Both LST and DOL raised pH above 7 at the near surface sections but pH gradually decreased towards the unamended interface. On the whole, RPR was more consistent in keeping the pH uniform at around 4 within the depth of incorporation. All the amendments were ineffective in raising the pitwall rock pH below 70 mm column depth. The overall relative effectiveness of the neutralising material on pH modification m the pitwall rock columns were m the order: FBA» LST=DOL>RPR. 1 90 (a) 2.6 2.4 :a. 2.2 2.0 2 4 6 8 10 1 2 Time (weeks) (b) 14 1 2 10 .... . E en 8 � 0 w 6 4 2 2 4 6 8 1 0 1 2 Time (weeks) Figure 6. 1 (a) pH and (b) EC of the leach ate from columns incorporated with neutralising materials. Vertical bars represent interaction LSD(5%). 20 E 40 S .; 60 Cl. Q) "0 s: 80 E :::l 8 1 00 1 20 140 pH (b) o ___ Control -a- Limestone (LST) � Dolomite (DOL) ----.- Reactive phosph e rock (RPR) -)E- Fluidised bed boil� ash (FBA) Figure 6.2 ( a) pH and (b) EC of the sectioned samples from leached columns. Horizontal bars represent interaction LSD(5%). 191 All the columns, irrespective o f treatments, produced leachate with very high E C (9- 1 3 dS m- I ) during first two weeks of leaching after which it decreased significantly (p<0.05) to levels 4-6 dS m- I (Figure 6. 1 b). The decrease in EC for all the treatments over 1 2 weeks period was significant (P<0.05) . The EC levels did not fall below 4 dS m- I within the 1 2 week leaching period. In the presence of high levels of sol, the decrease in EC level in the leachate was probably due to precipitation of gypsum and metal hydroxide at higher the pH effected by the neutralising materials . The neutralising materials incorporated columns consistently had E C significantly lower than the control column (Figure 6.2b). The FBA incorporated column had significantly higher EC at 0-40 mm depth than LST, DOL or RPR incorporated column as a result of contribution of S042- from dissolution of gypsum. Successive leaching of the columns significantly reduced levels of EC 10 the neutral ising material incorporated column and this is considered to be due to preferential leaching of the soluble electrolyte components. H igh concentration of sol­ in the pitwall rock should ideally facilitate downward movement of base cations (Ca and Mg) through the ion pairing effect. However, in the presence of excessive base cations, a significant amount of Ae+ and H+ will also be displaced from the exchange sites, resulting in a build up of reserve acidity. Comparisons of the overall means showed significant leaching of soluble salts from the columns during the 1 2 weeks leaching cycle (Table 6. 1 ). The higher EC level in the FBA treated column is likely to be due to residual gypsum from incomplete dissolution of FB A . All the treatments were equally effective in reducing the pitwall rock EC. The overall reduction in column section EC was in the order RPR ( 5 1 %) > LST (48%) > DOL (4 1 %) > FBA (3 1 %) and the means differences in the relative effectiveness of the neutralising materials were significant at P>O.05 (Table 6.2) . 6.3. 1.2 SO/- and acidity Leachate sol- concentrations significantly decreased with each successive leaching cycle for all the treatments. Generally, all the treatments produced leachate with significantly (P>0.05) lower S O/- concentration than the control column with nil incorporation of neutralising materials. The LST and DOL incorporated columns 1 92 showed release of similar amounts of S042- whereas S042- releases from RPR and FBA incorporated columns were significantly higher- By the end of 1 2 weeks of leaching period, the sol- flux in the leachate from incorporated columns decreased from 4500 mg L- 1 to <2000 mg L- 1 while the corresponding decrease in the control column was from 5500 to 3200 mg L- 1 • Comparison of the overall means of the sol release over 1 2 weeks leaching period (Table 6. 1 ) relative to the control value (4 1 55 mg sol- kg­ I week- I ) showed that although there was significant lowering of the sol- concentration in the leachate, the apparent reduction was <50% for all the treatments. The order of reduction in leachate sol- were LST (42%) > DOL (39%) > RPR (29%) > FBA (20%). Table 6. 1 Average release rates of the concentrations of measured parameters in the leachate from incorporated pitwall rock column. Overall means comparison LSD % reduction (-) / increase (+) Control LST DOL RPR FBA a=0.05 LST DOL RPR FBA sol- 4 1 55 24 1 3 253 1 2958 3305 1 92 -42 -39 -29 -20 Acidity 1 437 70 1 675 842 679 1 1 9 -5 1 -53 -4 1 -53 Fe 2862 1 908 1 726 1 500 2027 1 72 -33 -40 -48 -29 Mn 1 22 9 1 97 1 0 1 78 8 -26 -20 - 1 7 -36 Al 389 1 63 1 5 3 1 83 1 3 1 1 2 -58 -6 1 -53 -66 Ca 247 376 355 294 457 1 6 +52 +44 + 1 9 +85 Mg 1 22 1 58 23 1 1 34 1 69 7 +29 +90 + 1 0 +39 K 20 23 24 22 3 3 2 + 1 1 + 1 9 +5 +59 Na 23 28 35 34 58 3 +25 +56 +5 1 + 1 60 . . .\ - \ . \ Except for aCidity (mg CaC03 L week ), the umts are In mg L week - I Table 6.2 Overall comparison of the mean concentrations of chemical properties of the leached pitwall rock columns incorporated with neutralising materials. Overall means comparison LSD % reduction (-) / increase (+) Control LST DOL RPR FBA a=0.05 LST DOL RPR FBA pH 2.5 4.9 4.9 4.2 6. 1 0. 1 +95 +92 +67 + 1 4 1 EC 3. 1 1 .6 1 .8 1 .5 2. 1 0. 1 -48 -4 1 -5 1 -3 1 S042- 3353 1 85 3 1 952 1 6 1 8 2265 105 -45 -42 -52 -32 Acidity 25 1 9 655 692 1 256 63 1 94 -74 -73 -50 -75 Fe 2097 977 1 072 9 1 2 995 67 -53 -49 -57 -53 Mn 1 49 4 1 52 36 4 1 5 -73 -65 -76 -73 Al 245 1 7 45 27 59 4 -93 -82 -89 -76 A1exc 8 1 2 2 1 4 203 209 2 1 1 32 -74 -75 -74 -74 Ca 1 682 1 1 605 1 0624 6880 1 6357 559 +590 +5 32 +309 +873 Mg 299 424 1 460 455 523 52 +42 +388 +52 +75 K 207 1 05 82 90 1 90 1 0 -50 -6 1 -57 -8 Na 54 86 1 1 4 1 39 3 1 0 1 3 +60 + 1 1 2 + 1 59 +478 -\ -\ .\ Except for pH, EC (dS m ), Alexc (cmole kg ) and aCidity ( mg CaC03 kg ), the umts are In mg kg . 1 93 Incorporation of neutralising materials to 0-50 mm depth had significantly reduced the distribution of S 042- concentration in the upper 80 mm section of the pitwall rock column. There was no change in the S042- distribution of the control column indicating continual oxidation of pyrite. The sol- level in the control column remained at around 3 3 5 3 mg kg- 1 throughout the column section (Table 6.2) compared to the original concentration of 7440 mg kg- l in the bulk sample (Table 4. 1 , Chapter 4). Although a significant amount of sol- was leached (4 1 55 mg sol- kg- I ) from the control column, it appeared that about 45 % of the sol (335317440 x 1 00) remained in the column as insoluble precipitates and complex sulphate salts. The FBA incorporated column generally showed higher sol- content whereas RPR incorporation resulted in the most significant lowering of sol- in the pitwall rock column. Both LST and DOL showed similar distribution of sol- throughout the column depth. There was significant overall reduction in sol- concentrations in the pitwall rock columns amended with incorporated neutralisation materials (Table 6.2). The RPR incorporated column showed the highest reduction in S 042-, possibly due to inhibition of pyrite oxidation by Fe-P04 coatings. Several workers (Evangelou, 1 994; Fytas et aI . , 1 994 ; Georgopoulou et al . , 1 995) have demonstrated micro-encapsulation of pyrite grains by Fe-P04 coating in laboratory scale experiments. The relative reduction in sol- distribution in the neutralising material incorporated columns were RPR (52%) > LST (45%) > DOL (42% ) > FBA (32%). There was a rapid decrease In acidity levels in the leachate from all the amended columns. The overall mean acidity release rate in the leachate from control column over 1 2 weeks period was 1437 mg CaC03 Cl week- I (Table 6. 1 ) . The reduction in acidity in the neutralising materials incorporated columns compared to the acidity level from the control column were 5 3 % , 5 3 % , 5 1 % and 4 1 % DOL, FBA, LST and RPR respectively. Significant reduction in acidity was observed in the neutralising materials incorporated columns. The LST, DOL and FB A incorporated columns showed similar reduction in acidity whereas the acidity in the RPR incorporated column was generally higher. The untreated control column contained consistently higher acidity of 25 1 9 mg CaC03 kg- I throughout the column depth. Although the leachate pH remained low (pH -2.5) in the neutralising material incorporated columns, amelioration of subsurface acidity in the pitwall rock column was significant (P>0.05) throughout the column depth ( 1 40 mm) . 1 94 The relative reduction in acidity i n the column section by incorporated neutralising materials were in the order: FBA (75 % ) > LST (74%) > DOL (73%) » RPR (50% ). The continued release of acid in the leachate from incorporated columns indicated that the liming rate required to raise the pH of the pitwall rock to 6 (30 kg CaC03 (I ) is sufficient to provide only a short-term elevation of pH in the pitwall rock. Armouring effects on the neutralising materials from hydroxide coatings may result in re­ acidification of the entire column section. The rapid precipitation of gypsum from dissolution of neutralising materials is likely to result in the reduction of the neutralisation capacity through an armouring effect and possible cement formation. Thus in the presence of gypsum or elevated levels of sulphate salts, the S042- concentration in the leachate will not reflect the acid generation due to pyrite oxidation. Regression fits between S042- and acidity concentrations of the leachate and column section gave slopes of 0.36 and 0.76 respectively. Compared to the ideal slope of 1 .04 for complete neutralisation of acid (see section 5 . 3 .2 . 5 , Chapter 5), these slopes indicated corresponding neutralisation of 65% and 27 % of total acidity in leachate and column section respectively. These values were comparatively less than the reductions indicated by the overall effect of incorporation of neutralising materials on leachate and column section acidity (Tables 6. 1 & 6.2). The observations above indicated that although there was a significant linear relationship between the S042- and acidity in the leachate and column section, the relationship may not reflect the acid neutralisation processes under leaching conditions (or field conditions). S ince a large fraction of the S042- produced from oxidation of pyrite will be precipitated as sulphate salts of Fe, AI, Ca and Mg, the sol- fluxes in the leachate may vary considerably depending on leaching conditions and the volume of leachant used. This observation suggested that sol- and acidity fluxes in the leach ate from pyritic waste rock may not reflect the actual acid generation and neutralisation processes in the column. The results indicated that although leachate pH from neutralising materials amended columns remained low during the leaching period, significant reductions in the acidity levels were observed. This suggested that in subsurface, most of the acid is "locked" in 195 Al and Fe hydroxide and sulphate complexes as a result of downward intrusion of alkaline front. Formation of j arosite in lower sections of columns would also consume acidity and as long as the system is kept buffered by Al and Fe hydroxide gel formation, acidity will remain at reduced level despite low pH. 6.3. 1.3 Fe, Mn and Al Leachate Fe and Mn concentrations decreased rapidly with each successive leaching cycle for all the treatments . Most of the decrease in Fe and Mn in the leachate occurred during 2 to 6 weeks of leaching, after which, they remained constant during further leaching cycles. The Fe and Mn concentrations decreased from initial values of about 5000 mg Cl and 1 50 mg L- I in the week 1 leachate to < 1 000 mg L- I and 80 mg L- I respectively in the week 6 leachate. Soluble Fe and Mn concentrations in the leachate from the control columns were significantly higher (P>0.05) than the leachate from amendments incorporated columns. Leachate from FBA incorporated column contained consistently higher Fe whereas the RPR incorporated column produced leachate with the lowest Fe concentration. Overall mean distributions of Fe and Mn in the leachate from untreated columns were 2862 mg L- I week- I and 1 22 mg L- I week- I respectively (Table 6. 1 ) . The relative reduction of Fe in the leach ate were RPR (48%» DOL (40%» LST ( 3 3 % » FBA (29%) and the corresponding reduction i n Mn were FBA (36%» LST (26%» DOL (20%» RPR ( 1 7%). The alkalinity generated by incorporated neutralising materials had a contrasting effect on the distribution of Fe and Mn in the leachate. While RPR may have fixed Fe in the column by formation of an insoluble Fe­ P04 compound, the higher release of Mn in the leachate from RPR incorporated column indicated that it had little effect on the mobilisation of Mn. It appears that the high alkalinity created by the FBA was more effective in solubilising Mn in the leachate. The precipitation of sulphate salts of Fe and Mn (FeS04, MnS04) and oxyhydroxides (Fe(OH)3, Mn(OH)3) may also have affected the downward movement of the metals in the alkaline pore solutions formed by the neutralising materials. Both soluble Fe and Mn distribution in the untreated control column remained significantly (P>0.05) high at 2097 mg kg- I and 1 49 mg kg- I respectively throughout the column depth. All the neutralising materials showed similar effects in reducing the Fe and Mn concentrations in the column sections. There were no significant differences 1 96 among the different types of neutralising materials on the Fe and Mn content of the column section. In the neutralising material incorporated columns, soluble Fe was reduced to < 1 000 mg kg- I at 0-60 mm depth and remained significantly less than the Fe content of the untreated column in the 1 40 mm column depth . Soluble Mn was lowered to < 75 mg kg- I throughout the column depth by the amendment effects. The overall reduction in Fe and Mn contents of the amended columns was approximately 50% and 72% respectively (Table 6.2). The distribution of Al in the leachate was strongly affected by the incorporated neutralising materials. There was a significant (P DOL ( 6 1 %) > LST (58%» > RPR (53%). Exchangeable Al (Alexc) concentration in the control column sections showed no change throughout the 1 40 mm column depth of pitwall rock. The mean depth profile distribution of Alexc was 8 1 2 mg kg- I . The pitwall rock material initially had AIexc content of 1 34 1 mg kg- ! ( 1 4.9 cmole kg- I , Table 4. 1 ) . Assuming the leached pitwall rock column had similar chemical properties and pyrite content to the bulk sample, a 1 2 week leaching cycle resulted i n about 39% displacement of Alexc from the exchange sites. This indicated that a large fraction (6 1 %) of the Alexc remained mobilised in the column section, possibly as sulphate and hydroxide precipitates in the upper section of the column where the pH was significantly raised. Most of the reduction in Alexc was observed in the upper 80 mm section of the column. The overall reduction of Alexc in the column section was about 74% for all the neutralising materials. 6.3. 1.4 Base cations (Ca, Mg, K and Na) The distribution of base cation concentrations in the leachate decreased gradually over the 1 2 week cycle of leaching. The FBA incorporated columns consistently produced leachate with significantly (P>0.05) higher concentrations of Ca, K and Na while DOL 1 97 incorporated released the highest amount of Mg. The higher levels of Ca and Mg in the leachate from FBA and DOL amended columns can be due to downward movement of Ca and Mg, paired with S O/-. This is reflected in the relatively high concentration of Ca (derived from decomposition of alkali feldspars) leached from the control column as well . Higher cumulative K and Na released in the leachate from FBA incorporated column may have resulted from the significant amount of these cations contained in the FB A used as well as preferential displacement of these ions by Ca. The LST, DOL and RPR released similar amounts of Ca, K and Na in the leachate. Amendment incorporation significantly raised the concentration of Ca in the upper 80 mm section of the column depth (Figure 6.3a). This may suggest that although there was a significant amount of Ca leached from columns irrespective of amendments, a large fraction of it has remained unreactive possibly due to the armouring effect from oxyhydroxide precipitates and gypsum coating on the carbonate fractions. The FBA treated column had the greatest amount of Ca immobilised possibly in the formation of cementitious material. The distribution of Ca in the LST and DOL amended columns were similar at all depths. As expected there was a significant (P>O.O l ) increase in Mg content of the DOL incorporated column in the upper 0-80 mm depth (Figure 6.3b). Except for the FBA incorporated column, the distribution of K was significantly lower than that of the control value, throughout the column depth (Figure 6.3c). The K content of the FB A treated column was higher than the control value at the 50 mm depth of incorporation possibly due to dissolution of potassic minerals in highly alkaline solution. The high anomalous amount of Na in the upper 60 mm section of the FBA incorporated column may have been due to dispersive effect on Na by alkaline conditions (Figure 6.3d). A significant amount of Na could be contributed from initial high concentration of this ion in the FBA (Wang et aI . , 1 994) . There were significant increases in the overall distribution of Ca, Mg and N a in the amendments incorporated columns. A general depletion in K was observed in columns incorporated with LST, DOL and RPR (Figure 6.3c)_ Except for K, the base cation concentrations in the untreated column remained low at all depths. The high K content in the control column may have been due to decomposition of potassic feldspars by acidic solution in the pitwall rock and subsequent formation of j arosite K[Fe(S04h.2Fe(OHhD under low pH ( -2.5) conditions. 198 (a) Mg (mg kg-') o°r--'r-��-.������ (b) o I 500 1 000 1 500 2000 2500 I I i I I i I 20 E 40 .s a 60 Cl) 't:J c: 80 E ::J 8 1 00 1 40 (c) 00 20 E 40 .s .r:. 60 -a. Cl) 't:J c: 80 E ::J 8 1 00 120 140 --- Control � Limestone (LST) -+- Dolomite (DOL) -.- Reactive phosphate rock (RPR) -)(- Fluidised bed boiler ash (FBA) K (mg kg-') (d) Figure 6.3 Distribution of (a) Ca, (b) Mg, (c) K and (d) Na in the sectioned samples from leached columns incorporated with neutralising materials. Horizontal bars represent interaction LSD(%). 199 6.3.2 Comparison between broadcasted and incorporated methods of application of neutralising materials 6.3.2. 1 Leachate chemistry Treatment-wise comparison of the leachate chemical properties of the broadcasted and incorporated columns showed that although there were close similarities in the general trends in the characteristics of the chemical properties of the leachate and columns, significant differences were observed in the overall distribution of the chemical properties. Despite leachate pH remaining low for both methods of amendment, incorporated columns generally produced leachate with lower pH (Figure 6.4a) and significantly higher EC (Figure 6.4b) particularly for LST, DOL and FB A amended columns. There were no differences in the overall treatment-wise sol- release rates in the leachate from broadcasted and incorporated columns but acidity of the leachate from incorporated columns was consistently lower (Figure 6.4c and 6.4d). Except for significant increase in Fe in the leachate from the FBA incorporated column, both Fe and Mn also remained similar in both the methods of amendments (Figure 6.4e and 6.4f). There was also no difference in the leachate Al from both methods of amendments. The distribution of base cations (Ca, K and Na) in the leachate was similar in both the methods of amendments. The Mg distribution, however, was significantly higher (P>O.O l ) in the leachate from incorporated columns . The Mg level in the leachate from incorporated columns was generally two-fold higher than in the leachate from broadcasted columns as a result of high solubility of MgS04. Week-wise companson of the chemical parameters in the leachate showed that incorporated columns produced significantly (P>O.OS) lower pH than the broadcasted columns in the first 1 0 week period of leaching cycle (Figure 6.Sa). Low pH inducement with high rates of l imestone application in acidic mine waste was also observed by Costigan et al . ( 1 982). The trend was reversed for EC, which remained consistently higher in the leachate from incorporated columns (Figure 6.Sb). This suggested that 200 hydrolysis and precipitation of metal salts maybe inducing low pH conditions at subsurface depths. There was no difference in the week-wise leachate sol- release from both methods of amendment applications throughout the 1 2 week leaching c ycle (Figure 6.Sc). Amendment incorporated columns however, produced leachate containing significantly lower acidity than the leachate from broadcasted columns (Figure 6.Sd). The greater depth of neutralisation effect by the incorporated amendments may have caused dissolution of metastable salts such as j arosite, thereby releasing stored acid in the leachate. Soluble Fe was initially higher in the leachate from incorporated columns but this maybe j ust a flushing effect because there was no difference observed in the leachate Fe concentration in the weeks there after (Figure 6.Se). High alkalinity created by the incorporated amendments in the column section may have initially precipitated Mn as Mn(OHh as reflected in the lower M n content in the leachate from incorporated columns (Figure 6.Sf). The week-wise distribution of leachate Al and the base cations were simi lar for both the methods of application of amendments. Generally, the incorporated columns produced leachate with lower concentrations of Al and higher concentrations of base cations but the overall differences between the methods of amendments were not significant at P>O.OS except for Mg, which showed consistently higher amounts in the leachate from incorporated columns irrespective of treatment types. 2.Ja) 2.4 J: c.. 2.2 2.0 4000(C) -... 3000 :... Cl E - N � g 2000 1 000 (e) 2500 � 2000 � Cl E - Cl.) LL 1 500 1 000 � Broadcasted I - Incorporated I 1 �b) 8 "': E Cl) 6 � 0 w 4 2 200Jd) ... :"'M 1 500 0 0 ftI 0 Cl 1 000 .§. >-- :0 ·u 500 et: (f) 1 20 1 00 -.,... :... Cl 80 E - c: :lE 60 40 Figure 6 .4 Treatment-wise comaparison of leachate chemical properties of the broadcasted and incorporated columns. Vertical bars represent method x teratment interaction LSD(5%). 201 202 (a) 2.6 2.4 a 2.2 2.0 1 .8 (c) 5000 4000 .... � en .s 3000 N � 0 en 2000 1 000 (e) 6000 � 4000 � en E - Cl) LL 2000 I O r--,---.---.--.---,-� o 2 4 6 8 10 1 2 Week No. (b) 1 2 10 -.... 'E 8 Cl) � 0 6 w 4 2 (d) -B-- Broadcasted 2500 ---- Incorporated � 2000 M 0 0 1 500 ca 0 en .s 1 000 >-:: '0 ·u 500 < 0 (f) 200 1 50 I -.... � E 1 00 - c: :lE 50 O �-,---.---.--.---.-� o 2 4 6 8 1 0 1 2 Week No. Figure 6.5 Week-wise variations in leachate chemical properties of broadcasted and incorporated pitwall rock columns. Verical bars represent interaction LSD(5%). 203 6.3.2.2 Column section chemistry Treatment-wise comparisons of the column section chemical properties of broadcasted and incorporated amendments showed that there was some significant improvement in amelioration of subsurface low pH conditions by the incorporated neutralising materials. Except for LST, the pH in the depth-incorporated column was significantly higher in the column broadcasted with DOL, RPR and FBA (Figure 6.6a). There was no difference in the distribution of EC except for FBA incorporated columns, which had significantly, lower EC than broadcasted columns. Generally, the incorporated columns had lower EC for all the amendments (Figure 6.6b). Amendment incorporation also resulted in generally lower levels of SO/- than the broadcasted columns but the difference was not significant at P>0.05 (Figure 6.6c). Acidity, however, was significantly lower for all the treatments in the incorporated columns (Figure 6.6d). This was to be expected since incorporation of neutralising materials affected a greater column depth (0-80 mm) of acid neutralisation compared to broadcasted columns in which the maximum depth of effective neutralisation was only in the top 50 mm section of the pitwall rock column. Incorporation of fine-grained neutralising materials may also provide greater reactivity with the acid solution and hence resulting in more effective neutralisation. However, this might not be the case if the grain sizes of the neutralising materials were larger, in which case, armouring effect from hydroxide coatings may result in incomplete dissolution of the amendment materials The reductions of soluble Fe and Mn in the incorporated columns were significantly (P0.05) higher amounts throughout the column depth for incorporated amendments, there were no differences in the distribution of Ca, Mg and Na in the columns from both methods of amendment application. The higher K content in the amendment-incorporated column may have been mainly due to the contribution from dissolution of FBA, which initially had a high K content. (a) 2 20 E E 40 .r:; - 60 Co Cl) "0 C E � 1 0 E E .r:; -Co Cl) "0 E E .r:; 1 2 1 (C) (e) o 0. 60 Cl) "0 pH 0 Broadcasted Incorporated 207 (b) EC, dS m-1 1 .0 1 .5 2.0 2.5 3.0 I I I I I I------l (d) Acid ity, mg CaC03 0 1 000 2000 3000 I I I I H (t) o Figure 6.8 Overall depth-wise distribution of chemical parameters in broadcasted and incorporated columns. Horizontal bars represent interaction LSD(5%). 208 6.3.2.3 Overall effectiveness of amendment methods The individual treatment-wise and depth-wise statistical treatment of the data showed varying differences in the chemical parameters measured in the leachate and column sections amended with broadcasted and incorporated neutralising materials. When the same data were statistically tested to show the overall differences (method x treatment x time) between the method of application of amendments, the incorporated columns consistently showed significantly lower pH, acidity and Al and higher EC, sol·, Fe, Mn, Ca and Mg in the leachate (Table 6.3). The corresponding overall differences in means for the columns showed significantly (P>0.05) higher pH and Ca and lower distribution of EC, sol·, acidity, Fe, Mn, AI, Mg, K and Na in the incorporated columns. The incorporated method of amendment, although indicating a more effective amelioration of the low pH pitwall rock, requires that at a reasonable depth of weathered material remain stable on the pit slope (-43°) in order for application to be effective. B ecause the pitwall contained only a thin layer of weathered cover materials which is prone to rill and sheet erosion, incorporation of neutralisation materials may not have the desired effect of pH modification. On the other hand, surface broadcasted neutralising materials was just as effective in significantly improving the leachate quality (except for pH) and subsurface amelioration up to 0-40 mm depth of pitwall rock. However, in steep slope areas, the residence time of the broadcasted neutralising materials may be short due to loss from erosion and re-acidification of the pitwall rock. Revegetation of the steep pitwall no doubt requires engineered slope modification for stabi lised placement of amendments and plant growth media on which vegetation can be established long enough for natural succession . Provided this criterion is met, the results of this study indicated that amelioration of low pH conditions in the pyritic pitwall can be achieved by either methods of application of suitable neutral ising materials. 209 Table 6.3 Overall comparison of means between the broadcasted (BC) and incorporated (IC) methods of amendment. Leachate Column section BC IC LSD(5%) BC IC LSD(5 %) pH 2.30 2.22 0.02 3 .98 4.53 0.05 EC 6.00 8.03 0.22 2.23 2.04 0.06 sol- 2926 3072 93 2382 22 1 1 55 acidity 1 374 867 58 1 634 1 1 68 40 Fe 1 880 2004 73 1 8 1 9 1 2 1 0 4 1 Mn 90 98 3 1 78 64 4 Al 234 204 8 436* 322* 1 3 Ca 334 346 7 7930 9430 258 Mg 1 05 1 63 3 635 632 20 K 23 24 1 228 1 35 6 Na 36 37 1 1 30 1 4 1 5 -) . . - I Except for pH, E C (dS m ) and aCIdIty (eqUIvalent mg CaC03), the umts are In mg L for leachate and mg kg· ) for the column section. * = exchangeable Al (Alexc) 6.4 Conclusions Results of this study showed that depth incorporation of neutralising materials had no significant effect on the leachate pH, which remained low (pH<2.4) throughout the 1 2 weeks period of leaching. The oxyhydroxide precipitates of Fe and Al formed due to a sudden rise in pH within the column may have rendered a large fraction of the incorporated neutralising material ineffective due to possible armouring effect. These precipitates may also have buffered the leachate pH against further increase. This may have been the cause for only 50% reduction in the release of acidity over the 1 2 weeks period despite > 70% reduction in the column section acidity. The rapid precipitation of gypsum and metal sulphate salts during neutralisation made it difficult to estimate the acidity from sol- production. The predicted v alue of neutralisation of acidity, as estimated from the rate of sol- production, were 66% and 27% for the leachate and column section respectively. The actual reduction in acidity in the leachate (-50%) and in the column section (-70%) by all the treatments with varying neutralising potentials indicated that neutralisation of acid in the column may occur only during the initial contact but will stop immediately after sulphate salts begin to form. 210 The effect o f incorporation o f the neutralising material was t o significantly increase base cation concentrations in the leachate as well as in the column section The FBA treatment caused the largest increase of Ca in both leachate and column section while DOL incorporation resulted in 90% and 388% increases in the Mg concentration of the leachate and column section respectively. An anomalous high K and Na contents waere observed in leachate ( 1 60%) as well as in column section (478%) in FBA amended pitwall rock and this was thought to be dissolving directly from the FBA. Among the treatments, LST and DOL gave the most consistent ameliorative effects on low pH. The FBA amendment created a very highly alkaline material (pH>8) in the 50 mm depth of incorporation . The RPR incorporated column generally produced larger volumes of leachate and that a significant amount of RPR remained unreacted in the upper section of the column. Physical effects on the pitwall rock were evident in the LST and FBA incorporated columns. The depth of incorporation (50 mm) contained mottled grains of Fe­ hydroxide-coated limestone and orange brown precipitates in the LST and DOL incorporated columns. H ard concretions of cement were common in FBA incorporated columns. Both these decreased the rate of downward infiltration of leachate during leaching cycles. In a pitwall rock column of 1 40 mm depth, only the top 60 mm of the column was significantly ameliorated by depth incorporation of neutralising materials. Very high pH, Ca and Mg concentrations in the top 50 mm section of the column indicated that there was very little downward movement of the alkaline front. It is possible that the high Fe and SO/- content of the pitwall rock will have caused rapid precipitation of gypsum, Fe and Al hydroxides (and sulphates) in contact with the highly alkaline neutralising material . These precipitates not only buffer against an increase in pH but also severely reduce the neutralising capacity of the amendment materials by the armouring effect of the hydroxide coatings. Although RPR was the least effective material in improving the pitwall rock pH, it had significant effect on the overall reduction of sol-, acidity, Fe, Mn and AI, possibly due to ready formation of insoluble phosphate complexes which may have coated pyrite grains and thus partially inhibited the production of S 042- from biochemical oxidation of pyrite. 21 1 Amelioration of low pH conditions in pitwall rock materials with large amounts of neutralising materials may result in the discharge of leachate saturated with sulphate salts. This may eventually lead to leachate waters containing high levels of dissolved metal sulphate salts. Since the pitwall rock contains very high levels of SO/-, precipitation of metal sulphate salts is most likely when large amounts of highly alkaline neutralising materials are used to ameliorate the acidity problem. Thus precipitation of sulphate salts like epsomite (MgS04.7H20), alunite (KAb(S04h(OH)6) and j arosite (KFe3(S04h(OH)6) is likely to strongly influence the leachate chemistry. It is most likely that the discharge of low pH leachate despite amendments is largely due to release of acid from dissolution of Al and Fe hydroxides as well as j arosite precipitated at lower depths. S ince metastable precipitates such as Al and Fe­ oxides/hydroxides and j arosite strongly buffer the system to a low pH at lower depth, amendment with neutralising materials may have the desired effect of reducing acidity provided moisture at depth is kept at a minimum. With appropriate rates of application of neutralising material amendments, the pitwall rock material can be ameliorated for plant growth as long as water infiltration is controlled to prevent leaching and re­ acidification. Incorporated amendments were found to be more effective than broadcasted method in the overall amelioration of low pH conditions in the pitwall rock, provided that the neutralising materials are fine-grained (�2 mm). Large particle size neutralising materials may be rendered less effective by the armouring effect of oxyhydroxide coatings. 2 1 3 C h a p t e r 7 An Assessment of AMD Inhibitors (Topsoil and ProMac) in Ameliorating Low pH conditions in Pyritic Mine Pitwall Rock 7.1 Introduction Soil covers are often used as barriers to prevent influx of oxygen and water to limit acid generating processes in the waste rock (Nicholson et aI . , 1 989). The cover materials also provide organic ligands that readil y complex with mobilised metals such as Fe, Mn and Al (Miller and Ohlrogge, 1 95 8 ; H argrove and Thomas, 1 98 1 ; Davis, 1 984). Organic cover materials, such as topsoil and compost, have been shown to provide an effective means of reclaiming acidic soil material and have been shown to provide a protective layer which inhibits sulphide mineral oxidation (Pierce et aI. , 1 994; Stogran and Wiseman, 1 995). Several workers have demonstrated the role of soil organic matter in ameliorating Al toxicity in acidic soils and mine waste materials (Hargrove and Thomas, 1 98 1 ; Young and B ache, 1 98 5 ; Hue et aI. , 1 986; Shuk-Ching and McColl, 1 990; Gurung et aI. , 1 996; Smith et al. , 1 997). The use of topsoil as cover material serves not only to reduce acid generation processes but also to assist in achieving the aesthetic and ecological benefits of revegetation by acting as a substrate for plant growth (Gregg et aI . , 1 998) . The effectiveness of soil cover is however, strongly dependent on the local climatic conditions, organic matter content, nutrient levels (N, P, K) and the depth of soil cover placement although Gregg et al . ( 1 998) indicated that in oxidised mine waste materials topsoil depth was not a major factor for vegetative establishment. On steep pitwalls however, the slope factor is going to be the major limitation to effective placement of soil cover. Except for limited research by Hoving and Hood ( 1 984) and Tisch and Winterhalder ( 1 995), very little research has been conducted on the effect of topsoil cover material on the leachate and subsurface chemistry of pyritic mine waste materials. In recent times, commercial grade bactericide has been used increasingly in the reclamation of mined lands with varying degree of success (Pari si et aI. , 1 994; Splittorf and Rastogi, 1 995). Since mine waste rock materials are deemed site specific in their geochemical properties, the effectiveness of the bactericide in controlling AMD may not 2 1 4 be universally applicable. Attempts have also been made to formulate slow-release forms of surfactants (Erickson et al . , 1 985) and some of the products such as ProMac slow release pellets are now commercially available (Shellhom et aI . , 1 985). To date, no bactericide products have been used to control or inhibit AMD conditions in sulphidic _ mine wastes in New Zealand mines. The obj ectives of this study were to investigate the influence of topsoil cover and a commercial bactericide (ProMac) on leachate and subsurface column chemistry of partially oxidised pyritic pitwall rock columns under accelerated leaching cycles in glasshouse conditions. 7.2 Materials and Methods 7.2.1 Materials The columns used for the leaching study were the same as shown in Figure 5 . 1 (Chapter 5) . The physical and chemical properties of the pitwall rock bulk samples and neutralising materials used in this study are described in Chapter 4 (Table 4. 1 ) . The topsoil (TS) was from the Waihi mine site area (Baxter Road location) and its selected chemical properties are given in Table 7 . 1 . The ProMac (PM) product used in this study was in powder form (ProMac 2000SB) . It had a solution pH of 6.8 ( 1 :2.5 ProMac to water ratio) and electrical conductivity (EC) of 22 dS m- i . The recommended dosage for column a test according to M VTechnologies guidelines was 0.5 kg Mg- ' material (0.5 g kg- I ) dissolved in water to make 5 % solution (MVTI, 1 995). Table 7 . 1 Selected properties of Waihi topsoil Organic pH EC S 042- Alcu Alexe Alca CEC matter ( % ) ( d S m- i ) --------- mg kg- ' --------- cmole kg- ' 3 1 % 5.2 1 .3 257 756 1 70 7 34 Alcu = 0.5 M CuCIz-extractable organic (Alorg) + exchangeable (Aim) + soluble (AlcaJ) aluminium. 2 1 5 7.2.2 Methods About 50 mm of topsoil was placed on the pitwall rock column. Only 700 g of pitwall rock (about 90 cm column depth) was used in order to create space for topsoil placement. Where neutralising material was applied ( at CER of 30 kg CaC03 ( I , required to raise pitwall rock pH to 6), it was incorporated in the top 50 mm of the pitwall rock column prior to placement of topsoil. In this study, ProMac powder at the rate of 0.5 g kg- I pitwall rock (Vij ay Rastogi, pers comm., 1 998) w as directly incorporated in the top 50 mm section of the pitwall rock column since surface application created excessive foam which slowed the downward movement of the leachate considerably. The leaching protocol, analytical methods and statistical data analyses were carried out according to methods described in Chapter 3 (section 3.2 . 1 .3) and Chapter 5 (section 5.2.4). The experiment was initially set up as a factorial design comparing various combinations of five amendments (Table 7 .2). The results discussed in this chapter relate mostly to selected four treatments that include control (nil treatment), topsoil (TS) , ProMac (PM) and TS+PM. The remediation of low pH conditions in pitwall rock by pH modification using acid neutralising materials or topsoil placement to inhibit oxidation and provide an organic ameliorant to immobilise toxic metal concentrations (especially AI), or bacterial inhibition of pyrite oxidation, may all become compli mentary in their relative effectiveness. Often a combination of treatments may be necessary to achieve the desired results. Thus various combinations of treatments using the amendments TS and PM with three other amendments [neutralising materials : limestone (LST), fluidised bed boiler ash (FBA) and reactive phosphate rock (RPR)] were used (Table 7 .2) and discussed briefly in section 7 . 3 . 1 . 8 . Thiobacillus ferrooxidans is considered t o b e most active a t optimum pH and temperature ranges of 1 .5-3.5 and 30-350C respectively (Walsh and Mitchell, 1 97 2 ; Roman and Benner, 1 973; Ahonen and Touvinen, 1 99 1 ; B rown e t aI . , 1 993). Therefore, the columns amended with ProMac were heated with overhead high wattage lamps to provide an approximate constant surface temperature of about 35°C . The lamps weft� frequently shifted around to provide even distribution of temperature on column surface. 2 1 6 Table 7 .2 Amendments and treatment design. Pitwall rock (g) Treatments Treatment rate 1 000 Control Nil 700 Topsoil (TS) 50 mm placed on top 1 000 ProMac (PM) 0.5 g PM 700 TS+PM 50 mm TS + 0.35 g PM 700 TS+LST 50 mm TS + 22 g LST 1 000 PM+LST 0.5 g PM + 22 g LST 700 TS+FBA 50 mm TS + 49 g FBA 1 000 PM+FB A 0.5 g PM + 70 g FBA 700 TS+RPR 50 mm TS + 39 g RPR 1 000 PM+RPR 0.5 g PM + 56 g RPR 700 TS+PM+LST 50 mm TS + 0.35 g PM + 22 g LST 700 TS+PM+FBA 50 mm TS + 0.35 g PM + 49 g FBA 700 TS+PM+RPR 50 mm TS + 0.35 g PM + 39 g RPR PM and neutralising materials (LST, FBA and RPR) were incorporated in the top 50 mm depth of pitwall rock column. The pH, electrical conductivity (EC), sol·, acidity, Fe, Mn and Al were measured in the leachate from each leaching cycle. At the end of the 1 2 weeks leaching period, the TS amended columns were sliced into sections (0-20, 20-40, 40-50, 50-70, 70-90, 90- 1 1 0, 1 1 0- 1 40 mm), crushed and air-dried prior to grinding to ::; 2 mm for chemical analysis. The PM and neutralising material incorporated columns were sliced into 20 mm sections. Soluble labile Al ( Alca) and organic+polymeric+exchangeable Al (Alcu) in the sectioned pitwall rock samples were extracted with 0.02 M CaCh (Hoyt and Nyborg, 1 972), and 0.5 M CuCh (Juo and Kamprath, 1 979) respectively and their concentrations were measured by atomic absorption spectrometry (AAS). 7.3 Results and Discussion Columns with topsoil (TS ) placements consistently produced leachate volumes of about 80% of the input volume applied (250 ml). On the other hand, leachate volumes from ProMac (PM) treated columns were considerably lower (46% of input volume) as a 2 1 7 result of impediment of infiltration from blockage of pore spaces with clay particles dispersed during foaming and as well as loss from evaporation under heated lamps. The average volume of leachate collected from the control column was 1 9 1 ml week- I . In most cases, the leachate colour from all the treatments was dark orange brown which changed to a clear solution when acidified with 1 0 N HN03, indicating the coloration was mainly due to dissolution of particulate Fe3+ in the solution. In the discussion of results for the distribution of measured parameters in the TS amended pitwall rock column sections, the equivalent column depth 50- 1 30 mm is compared with the 0-80 mm section of the control and PM treated columns. 7.3.1 Characterisation of the leachate 7.3. 1.1 Leachate pH Topsoil (TS) and ProMac (PM) treated columns produced leachate with very low pH « 2. 6) throughout the 1 2 week leaching cycle (Figure 7 . 1 a). The overall mean pH of the leachate from control and TS amended columns remained below 2.2 whereas leachate pH from PM treated columns was higher by about 0.2 pH units. The production of very low pH leachate from the TS and PM amended columns is comparable to results obtained from the neutralising material amended columns (Chapter 5 and 6). In a column test (9 weeks duration) on partially oxidised silver mine waste (pH 4.8), Parisi et al . ( 1 994) indicated an effluent pH improvement of about 1 6% (pH 3.3 to 4.3) in PM treated columns (7 g PM kg- I ) compared to control columns. However, no significant improvement in leachate pH from PM treated columns was observed in this study. This may indicate that in partially oxidised pyritic pitwall rock materials, ProMac may not have inhibited bacterial oxidation as suggested by Watzlaf ( l 988a). ProMac solution had near neutral pH (6.8) and therefore, the slight increase in pH of the leachate from ProMac treated columns may well be due to pH dilution effect. 2 1 8 (a) 2.8 2.6 2.4 2.2 2.0 I 1 .8�----�----�-----.----�------.-----. - � 'E Cl) � 0 w o 2 4 6 8 1 0 1 2 • Control (b) 1 6 * Tosoil (T8) )( ProMac (PM) E8 T8 + PM 1 2 I 8 4 Or-----.-----�----_.----_.------._--� o 2 4 6 8 TIme (weeks) 1 0 1 2 Figure 7. 1 (a) p H and (b) EC of the leachate fro m columns amended with TS and PM. Vertical bars represent LSD(5%). 2 1 9 7.3. 1.2 Leachate EC The distribution of EC in the leach ate over 1 2 weeks period showed a gradual decrease for all the treatments (Figure 7 . 1 b). During week 2 to week 6, the PM treated column produced leachate with the highest EC, after which it remained similar in value to that of the leachate from the control column . At the end of the 1 2 week leaching period, the EC in leachate from the control, TS and PM treated columns remained above 5 dS m· l . The TS treated column consistently produced leachate with significantly lower EC than either control or PM treated columns during week 2 to week 8. During week 2 to week 6, both TS and TS+PM treated leachate EC were similar but thereafter, the TS+PM treated column produced leachate with EC significantly lower (EC< 3.5 dS m- I ) than all the other treatments as a result of continued flushing of the columns during leaching cycles. 7.3. 1.3 Leachate SO/- The sol- concentration in the leachate from the control columns varied from 4650 mg L- 1 in week 2 to 2350 mg L- I in the final week of leaching (Figure 7.2a). The TS and ProMac amendments significantly lowered the S042- levels in the leachate. The TS and TS+PM amended columns produced the lowest levels of sol- while PM treated column released twice as much sol- in the leachate as that from either TS or TS+PM treated columns. The overall mean release of S042- in the control column leachate (Table 7 . 3 ) was 3480 mg sol- kg- 1 week- 1 and the corresponding reduction of sol­ relative to the control value were 74%, 47 % and 74% for TS, PM and TS+PM respecti vel y. While reductions in leachate sol- levels from PM treated columns reflected an apparent inhibition of acid generation by Thiobacillus ferrooxidans activity, the significantly lower release of sol- from TS amended columns could be either due to restricted of oxygen diffusion or immobilisation of sol- in the column as metal salt complexes. Preliminary results of a 9 week column leaching experiment on ProMac amended (7 g kg- I ) silver mine waste rock containing total S of about 0.5% was shown to improve sol- reduction by about 72% (Parisi et al., 1 994) . In situ reclamation trials carried out by M VTI ( 1 989) using ProMac spray have been shown to reduce sol­ concentration in lysimeter samples by as much as 82% . 220 (a) (b) 5000 3000 4000 I ... .:... CO) I ... o 2000 .:... 3000 0 ca 0) 0 E 0) - N� 2000 E - 0 l!' 1000 tJ) :s 1000 'u « 0 0 (c) (d) 5000 350 4000 I 300 I � 3000 ;:-- 250 .:... 0) � 200 E ;- 2000 - c: LL � 150 1000 100 0 50 0 2 4 6 8 1 0 Time (weeks) (e) 500 400 I - � 300 • Control 0) E • Topsoil (TS) ::::- 200 « )( ProMac (PM) 100 EH TS + PM 0 0 2 4 6 8 1 0 1 2 Time (weeks) Figure 7.2 Concentrations of ( a) SOl-, (b) acidity, ( c) Fe, ( d) Mn and (e) Al in the leachate from columns amended with TS , PM and TS+PM. Vertical bars represen t LSD(5%) . 1 2 22 1 Result of this study however indicated sol reduction of only about 47% in leachate from the PM treated columns. The site-specific nature of the mine waste rock type and pyrite content may have affected the performance of ProMac in alleviating sol- levels in the leachate. ProMac is a sulphonic salt (EC, 22 dS m- I ) and therefore it would be contributing some sol- to the system rather than reducing sol- in the leachate. Table 7.3 Average release rates of the concentrations of measured parameters in the leachate from columns treated with various combinations of amendments. S042- acidity Fe Mn Al Control 3480 2025 3078 2 1 7 366 TS 902 1075 968 178 109 PM 1 842 382 1423 144 222 TS+PM 9 1 5 282 773 1 37 90 LSD(5%) 247 209 234 67 27 Relative reductions ( - ) i n the measured parameters TS -74 -47 -69 - 1 8 -70 PM -47 -8 1 -54 -34 -39 TS+PM -74 -86 -75 -37 -75 Except for acidity (mg CaC03 L-\ the units are in mg L-i_ 7.3.1.4 Leachate acidity Acidity in the leachate from amended columns decreased significantly in the initial week of leaching cycle (Figure 7 .2b). Leachate acidity from the control column decreased from 2372 mg CaC03 kg- I in week 2 to 1 690 mg CaC03 kg- I in week 1 2 whereas the corresponding range in the TS and PM amended column leachate were 1 50 1 -830 mg CaC03 kg- I and 765- 1 35 mg CaC03 kg- I respectively. The lowest release of acidity was obtained in the TS+PM amended column leachate (389-270 mg CaC03 kg- I ) . Comparisons of the overall differences in means (Table 7.3) indicated that the PM amendment caused an 8 1 % reduction in the level of leach ate acidity relative t� the control mean of 2025 mg CaC03 kg- I . This reduction in acidity due to PM amendment is comparable to 93% reported by Parisi et al. ( 1 994) and mine water quality improvement of 87-98% under ProMac treatment systems (MVTI, 1 994) . The corresponding reduction in the leachate acidity by TS amendment was 47% whereas the combination treatment of TS+PM resulted in 86% reduction. 222 The ratio of acidity to sol- is an indication of the amount of acid generated or neutralised/inhibited. According to the ideal oxidation of pyrite (section 5 .3 .2.5, Chapter 5) the acidity to sol- ratio is 1 .04. The level of sol- and acidity in the leachate showed a highly significant (p=O.OO I ) linear relationship (R2 > 0.95) for all the treatments. The slope of the regression fit for the distribution of acidity and sol- in the leachate from the control column was 0.58, indicating that despite nil treatment, about 42% of the acid was being neutralised by possible dissolution of silicate minerals and precipitation of jarosite type minerals. The corresponding slopes of the regression fits for TS and PM amended columns were 1 . 19 and 0.2 1 respectively. This ratios indicated neutralisation of 79% and 1 9% acidity in the leachate from PM and TS treated column respectively. Overall mean reduction of acidity compared to nil treatment were 8 1 % and 47% in the leachate from PM and TS amended columns respectively (Table 7.3) . 7.3. 1.5 Leachate Fe and Mn Both Fe and Mn concentrations In the leachate decreased significantly over the 1 2 weeks leaching period (Figure 7 .2c & d). Incorporation of PM powder and TS placement resulted in significant reduction in the level of Fe in the leachate. The Fe concentration in the control column leachate decreased from 4270 to 1 690 mg L- t (60%) over 1 2 weeks period. The TS and PM amended columns produced leachates that were significantly lower in Fe and Mn concentrations than the values for the control column. The cumulative concentration of Fe followed a logarithmic trend with R2 >98 for all the treatments. The TS+PM amended columns produced the lowest level of Fe in the leachate while the PM treated column produced leachate with a significantly higher Fe content. The overall mean release of Fe by the control column was 3078 mg L- tweek- I (Table 7 .3) . The reduction in Fe concentration by the TS, PM and TS+PM amendments were 60%, 54% and 75% respectively. This reduction by PM in the present study was much lower than the reduction of 95% (Parisi et aI. , 1 994) and 88% (MVTI, 1995) in ProMac affected leachate from silver mine waste rocks. The soluble Mn levels decreased rapidly with each successive leaching for all the treatments (Figure 7 .2b). There was a gradual decrease in the concentration of Mn in the leachate from all the treatments. Generally, the Mn in the control column remained high throughout the 1 2 week leaching cycle. The PM treated column produced significantly 223 lower Mn levels in the leachate than the TS amended columns. The overall mean distribution of Mn in the leachate (Table 7.3) indicated a relative reduction of 1 8%, 34% and 37% respectively for TS, PM and TS+PM amendments. The reduction in Mn in leachate from PM treated columns could be due to a slight increase in soluble Mn in the column sections. 7.3. 1. 6 Leachate Al The leachate Al gradually decreased with each successive leaching cycle (Figure 7 .2e) . The TS amended columns produced a significantly lower level of Al in the leachate compared to that of PM and nil treatment control columns. It is possible that a high fraction of the Al may have been retained in the column complexed with water-soluble organic ligands leaching from the topsoil . The order of overall reduction in the leachate Al were TS+PM (75%) > TS (70%) > PM (39%). Soluble AI reduction in leachate from PM amended waste rock columns reported by Parisi et al ( 1 994) and MVTI ( 1 994) was about 95%. The lower reduction in leachate Al in this study may have been due to site specific characteristic of the waste rock and the leaching conditions. 7.3.2 Column section chemistry 7.3.2. 1 Column section pH The column section pH remained constantly low (2.5) throughout the column depth (80 mm) for both the control and TS amended columns (Figure 7.3a) . The pH in the PM treated columns was significantly raised to pH > 4.0 at 0 to 40 mm depth and remained above 3 .5 at lower depths. The distribution of pH in the TS+PM treated pitwall rock column was similar to that of PM alone treatment. The rise in pH effected by the bactericide ProMac was significantly higher (P>0.05) than either control or TS treated columns. S ince at pH > 3 .5 , Thiobacillus ferrooxidans is considered inactive (Walsh and Mitchell, 1 972; Kleinmann and Crerar, 1 979; Brown et aI . , 1 993), the continual discharge of low pH leachate from the PM treated column indicated that abiotic oxidation of pyrite alone maybe creating low pH leachate. 7.3.2.2 Column section EC Column section EC levels were significantly lower in PM (EC < 1 dS m- I ) and TS (EC < 2 dS m- I ) treated columns throughout the depth profile compared to the control columns (Figure 7 .3b). The distribution of the EC in the column depth profile of the TS 224 E E - � -c.. 20 -8 40 c: E ::l o o - E E - � -c.. 60 80 (b) 8.0 20 Cl) 40 "t:J c: E ::l 0 0 60 80 pH I--t • Control ... Topsoil (TS) )( ProMac (PM) EH TS+PM Figure 7 .3 (a) pH and (b) EC of the sectioned samples from leached columns. Horizontal bars represent LSD(5%). 225 and PM treated columns were similar to that of the neutralising material incorporated columns (Chapter 6). The control column EC was slightly higher at the surface, possibly due to metal salt precipitation under drying conditions. The lower EC down the profile may reflect the removal of solute during successive leaching of the columns. The overall mean EC was 2.8 dS m- I for the control column which was significantly reduced to 1 .8, 0.6 and 0.4 dS m- I respectively by TS, PM and TS+PM treatments . The relative reduction of the EC by the amendments were 86%, 79% and 36% for the TS+PM, PM and TS treatments respectively. 7.3.2.3 Column section SO/- The TS, PM and TS+PM treatments caused a significant reductions (P>O.05) in the distribution of sol in the leached pitwall rock column compared to the control columns with nil treatment (Figure 7.4a). The PM and TS+PM amended columns showed the largest reduction in the S042- levels throughout the column depth of 80 mm. Although the PM treatment resulted in only a 47% reduction in leachate SO/-, the column section SO/- level was reduced by 88% (Table 7 .4). Combination of TS+PM caused an overall reduction of sol by 97%. The elevation of pH to >3 .5 and the subsequent significant reduction in the S042- indicated that the PM treatment may have inhibited bacterial activity and this reduced the acid generation process in the column section. Although the TS amendment significantly reduced (P>0.05) the SO/- in the 80 mm section of the column by 35% (Table 7.4), the mean sol distribution remained > 2000 mg kg- I . The reduction in sol- by TS amendment may be due to reduced pyrite oxidation under restricted diffusion of oxygen because of the 50 mm thick topsoil placement. 7.3.2.4 Column section acidity The mean distribution of acidity in the column sections closely followed the trend shown by S042- distribution. All the treatments caused significant reductions in column section acidity throughout the 80 mm column depth (Figure 7 .4b) . This was to be expected as the level of acidity retained in the column section would in fact be directly proportional to the production of SO/-. Overall mean acidity level in the control column was 2432 mg CaC03 kg- I , and was reduced by 59% in the PM incorporated column whereas the TS amendment resulted in 35% reduction in acidity (Table 7 .4). The TS+PM treatment caused 7 1 % reduction in the column section acidity. 226 (a) o°r-�����'=�'-� - E 20 E - .s= -a. .g 40 c: E ::J (5 60 u 80 (c) o°r-��-.������ .s= -a. .g 40 c: E ::J (5 60 U 80 H (b) o (d) o • * )( m Control Topsoil (TS) ProMac (PM) TS + PM Figure 7.4 Distribution of (a) S042-, (b) acidity, (c) Fe, (d) Mn and (e) Alexc in the sectioned samples from leached columns.Horizontal bars represent LSD(5%) , 227 Table 7.4 Comparison of the mean distribution of selected chemical parameters in leached column treated with various amendments. S04�- acidity Fe Mn Al Alexc Control 3 143 2432 1 8 10 1 85 353 894 TS 2067 1 583 488 263 1 92 779 PM 389 1 00 1 543 22 7 654 TS+PM 106 7 1 8 89 32 68 7 1 8 LSD(5%) 220 188 156 26 28 108 Relative reductions (-) in the measured parameters TS -34 -35 -73 42 -46 - 1 3 PM -88 -59 -70 -88 -98 -27 TS+PM -97 -7 1 -95 -82 -8 1 -20 Except for acidity (mg CaCO) kg-\ the units are in mg kg-i. The available literature (Parisi et aI . , 1 994; MVTI, 1 994) on column leaching studies using bactericide and soil cover materials characterises only the improvements on leachate water quality and to date no data are available to compare the subsurface amelioration of subsurface low pH conditions. Field and laboratory studies conducted by Cravotta ( 1 996) on the biogeochemical interactions and transport processes affecting the oxidation of pyrite and the formation of AMD in surface coal mines showed that despite low concentrations of dissolved O2 « 1 mg L- 1 ) in ground water beneath sludge­ treated spoil , concentrations of sol- and acidity increased down gradient. The present study showed that both subsurface sol- and acidity are significantly reduced by TS, PM and TS+PM amendments. The depth of amelioration of low pH conditions in the column section (--80 mm) was comparable to that of neutralising material amended columns (Chapter 5 & 6). The acidity versus SO/- distribution in the column section was linearly regressed (y = 0.52 * x + 733, R2 = 0.88) with a slope of 0.52, indicating overall reduction of acidity of about 50%. The corresponding acidity to SO/- ratio for the TS, PM and TS+PM treated columns were 0.65 , 0.4 1 and 0.29 respectively. These reduction in slopes (compared to 1 .04 for ideal oxidation of pyrite) indicated relative acid neutralisation of 38%, 6 1 % and 72% by TS, PM and TS+PM amendments respecti vel y. 7.3.2.5 Column section Fe and Mn Soluble Fe in the columns treated with various amendments was significantly reduced (P>0.05) when compared to that of the control column (Figure 7.4c). The columns with the nil treatment showed a uniform distribution of Fe throughout the 80 mm depth 228 section. The TS amended columns reduced Fe to < 200 mg kg- I in the top 0-20 mm section but the Fe distribution appeared to increase down depth. This reduction in Fe is reflective of the possible inhibitory effect by TS and PM amendments on the oxidation of pyrite, as indicated in the suppression of S042- and acidity production in the amended columns. The overall comparison of means showed Fe reduction in the column to be 73%, 70% and 9S% by TS, PM and TS+PM respectively (Table 7.4). The distribution of Mn in the column section was significantly reduced by the incorporation of PM in the top SO mm section of the pitwall rock column. At depth 30- 80 mm, there was an apparent increase in the distribution of Mn in the TS amended column sections, and this increase was significantly greater (P>O.OS) than in the column with nil treatment (Figure 7.4d). This apparent increase of Mn in the TS amended column section may be due to mobilisation of Mn by dissolved organic matter in the leachate. The overall reductions in column section Mn were 42%, 88% and 82% respectively for TS, PM and TS+PM treatments. Ferric-sulphate and Fe-Mn oxide minerals, which may have formed under oxidising conditions before the application of topsoil, were likely to dissolve close to the surface locations producing SO/- and Fe3+ ions and precipitates salts. The increased EC, SO/-, Fe, Mn and Al in the topsoil at the interface indicated that there was upward migration of salts and metals, which may affect the plant root system. 7.32. 6 Column section Al Soluble Al (Alw) in the column section was most affected by PM treatment possibly due to an increase in the column section pH and hence reduction in the solubility of monomeric Al (Table 7.4). At pH>3.S, Alw may go into polymeric forms as well as precipitating as hydroxy gels and sulphate salts such as alunite in the presence of elevated levels of SO/-. In acidic conditions ' of pH<2.S, as is present in the control column, Alw was highly mobile and remained high at 300 mg kg- I . As reflected in the reduced release of Alw in the leachate, a large fraction (- 1 92 mg kg- I ) of Alw was retained and distributed evenly in the 80 mm depth section of pitwall rock (Figure 7.8a). The overall reduction in the Alw was in the order PM (98%) > TS+PM (8 1 %) > TS (46%). 229 The levels of exchangeable Al (Alexc) in the amended columns were >500 mg kg- I (>6 cmole kg- I ) throughout the 80 mm column depth and was similar for all the treatments (Figure 7.4e)_ The mean Alexe in the control columns remained constant at 894 mg kg- I (9.9 cmole kg- I ) At depth 0 to 50 mm, the amended column had significantly lower Alexc than the nil treated control columns. PM amendment appeared to have significantly lowered Alexc at all depths possibly either due to the pH effect caused by ProMac solution (pH 6.8) or the dispersive effect on clay by PM may have released Alexe in the effluent solution. The relative reduction in the distribution of Alexc were 1 3%, 27% and 20% respectively for TS, PM and TS+PM. 7.3.2. 7 Effect of topsoil placement on total Al pool Topsoil (TS) covers not only provide an oxygen diffusion barrier (and plant growth media where vegetative reclamation is a necessity) but also act as a source of soluble organic ligands that can bind readily with the labile Al in the pore water solution. · An earlier investigation by Gurung et al. ( 1 996) has shown that organic matter leachate from topsoil cover on oxidised mine waste rock was responsible for providing organic ligands which complexed with labile exchangeable Al (Alexc) and thereby reduced the phytotoxic levels of AI. The reactive Al pool in very acid soils would ideally include soluble monomeric and total exchangeable AI, Al in octahedral clays and Al-hydroxide-organic matter complexes (Juo and Kamprath, 1 979). Aluminium extracted with 0.5 M CuCh solution ( Alcu) is mainly attributed to total Al pool containing organic-AI + hydroxy-AI complexes + exchangeable-AI + labile-Al. In the absence of organic ligand sources, the largest fraction of the total Al pool extracted by 0.5 M CuCh solution should ideally be hydroxy-bound Al + exchangeable Al + soluble AI. S ignificantly, higher levels of Alexc followed by Alcu dominate the Al distribution in the pitwall rock (Figure 7.5) . The distribution of Al in the TS amended pitwall section of columns was not significantly different from that of the control column (nil amendment) which had mean column section Al concentrations of 60 1 ± 1 03 mg kg- I (Alcu), 9 1 2 ± 59 mg kg- I (AleJ and 29 1 ± 1 5 1 mg kg- I (Alca) respectively. 230 This suggested that the 0.5 M CuClz solution only extracted hydroxy-bound Al and very little exchangeable AI. Results of this experiment showed that there was no evidence of AI-organic matter complexation in TS amended columns, which were leached for 1 2 weeks although there was an apparent increase in the Alcu j ust above the topsoil-pitwall rock interface (Figure 7.5). The TS sections (0-50 mm) contained mostly Alcu and very little Alexc, indicating that the organic ligands in the topsoil were probably already bound with labile Al and that the residual organic ligands moving down the column during leaching cycles was not sufficient to cause significant changes in the Al pool of the pitwall rock below the depth of placement. It is also probable that in the pyritic rock, most of the labile Al would be already hydroxy-bound or in the presence of elevated S042- level, sulphate­ bound. E E - or; -c.. Cl) '"0 2 Pitwall rock AICa Figure 7 .5 Distribution of different forms o f AI in topsoil amended co lu mns . 23 1 7.3.3 Effect of combination amendments Research has shown that no single treatment system alone is effective in the control and treatment of AMD. S ince the geochemical environment in which pyrite oxidation and hence acid generation occur is so complex, a combination of treatments may be more advantageous than a single ameliorant in mitigating low pH conditions in acidic mine waste rocks. While neutralising materials may provide short -term amelioration of low pH conditions, their effectiveness was commonly affected by rapid exhaustion of neutralisation capacity by the armouring effect of oxyhydroxide precipitates (Chapter 4). Fluidised bed boiler ash (FB A ) caused excessive alkalinity and cement formation while reactive phosphate rock (RPR) was ineffective in raising pH to 6 achieved at carbonate equivalent rates of application. Mine site rehabilitation by revegetation methods requires control of acid generation and subsurface amelioration of the growing media long enough for vegetation to establish permanently. Topsoils, bactericides and neutralising materials are therefore some of the important materials in the amelioration of low pH conditions in pyritic pitwall rock and in the creation of a suitable plant growth medium. The overall effects of various combination treatments on leachate quality and column chemistry are briefly discussed below and summarised in Table 7.5 and 7.6 respectively. 7.3.3.1 Effects of combination treatments on pH and EC Combined treatments did not raise the leach ate pH > 2.3 although the increase in pH effected by combination of TS with either PM, LST and FBA where significantly higher (Figure 7 . 6a) . Except for the RPR amended column, there were significant increases in pH of the pitfall rock columns amended with combination treatments of TS , PM, LST and FBA (Figure 7 .6b). The PM amended columns produced high leachate EC level similar to that of the control column and the improvement on leachate EC by combination treatments were not significantly different to that of TS or PM amendment alone (Figure 7 .7a). Amendment combinations of either TS or PM with LST and FBA caused significant increases in the EC content of the pitwall rock column (Figure 7 .7b) . This increase in EC of the pitwall rock could be due to contribution of sol- from FB A and sulphonic salt from PM. 232 (a) 2 .4 2.3 2.2 2.1 (b) 8 [ LSD(5%) '0 en :E ... � 0.. 'E 0 (.) 6 I LSD(5%) ::r:: Q.. 4 '0 ... 'E 0 (.) a: � 0.. en a: ..J + + en :E � 0.. � en ..J + :E Q. et a: CD 0.. u.. a: + + :E :E 0.. 0.. TS= Topsoil PM= ProMac � et a: en CD 0.. ..J u.. a: + + + :E :E :E 0.. 0.. 0.. + + + en en en � � � � et a: en CD 0.. ..J u.. a: + :E + + :E :E Q. Q. 0.. + + + en en en � � � Figuer 7 .6 Overall effect on (a) leachate pH and (b) column pH by various combinations of treatmen ts. .,.... E en '0 - (.) w (a) 1 0 4 (b) 8 7 6 -.,.... 'e 00 5 '0 - (.) w 4 3 2 [ LSO(5%) '0 en I-::E ::E et � l- Q. Q. en ID - + ...J "-c 0 en + + () I- en en I- I- I LSO(5%) '0 � C 0 () a: I- et a: Q. en ID Q. a: ...J "- a: + + + + en ::E ::E :E l- Q. Q. Q. TS= Topsoil PM= ProMac I- et a: en ID Q. ...J "- a: + + + ::E :E :E Q. Q. Q. + + + en en en I- I- I- I- et en ID ...J "-+ + ::E ::E Q. Q. + + en Cl) I- I- Figure 7 .7 Overaall e ffect on ( a) leachate EC and (b) c olumn EC b y various combination s of treatments . 233 234 7.3.3.2 Effects of combination treatments on SO/·, acidity, Fe, Mn and Al Combination of TS and PM treatments with neutralising materials-amended pitwall rock columns resulted in higher improvement in leachate quality than the TS or PM alone treatment (Table 7.5) . The overall reductions in the release levels of solo, acidity, Fe, Mn and Al in the leachate were generally higher for pitwall rock columns amended with combinations of TS and PM with either LST or FBA. The PM+LST amendment caused significantly higher release of sol- than other combinations. All the combination treatments had similar effect on the release of acidity, Fe and Al in the leachate and were significantly lower than the levels in the leachate from control column with nil treatment. Table 7.5 Average release rates of the concentrations of measured parameters in the leachate from columns treated with various combinations of amendments. sol acidity Fe Mn Al Control 3480 2025 3078 2 1 7 366 TS+LST 73 1 200 677 1 05 59 PM+LST 1 45 8 220 1 0 1 0 1 1 6 65 TS+FB A 409 763 906 66 26 PM+FBA 692 343 1 090 1 35 43 TS+RPR 500 1 7 1 895 1 63 33 PM+RPR 7 1 3 289 967 1 5 1 29 TS+PM+LST 459 27 1 549 85 4 1 TS+PM+FB A 476 243 1 053 78 28 TS+PM+RPR 397 1 64 890 1 46 25 LSD(5%) 247 209 234 67 2 7 Relative % reductions (-) in the measured parameters TS+LST -79 -90 -78 -5 1 -84 PM+LST -58 -89 -67 -46 -82 TS+FBA -88 -63 -7 1 -70 -93 PM+FBA -80 -83 -65 -38 -88 PM+RPR -80 -86 -69 -30 -92 TS+RPR -86 -92 -7 1 -25 -9 1 TS+PM+LST -87 -87 -82 -6 1 -89 TS+PM+FBA -86 -88 -66 -64 -92 TS+PM+RPR -89 -92 -7 1 -32 -93 Except for acidity (mg CaC03 L-\ the units are in mg L- ' . 235 7.3.3.3 Effects of combination treatments on column section chemistry The effect of using neutralising materials (LST, FBA and FB A) in combination with either TS or PM caused a general decrease in the efficiency in reduction of S042- compared to reductions with TS or PM alone (Table 7 . 6 and compare with Table 7.4). There was however, a general increase in the reduction efficiency (compared to individual treatments with TS , PM, LST, FBA or RPR) of acidity, Fe, Mn and Al in the columns amended with combination treatments. The result demonstrated the fact that under real field situations, amendment of pyritic materials may require combinations of treatment measures for effective amelioration of low pH conditions created by pyrite oxidation. Table 7.6 Comparison of the mean distribution of selected chemical parameters in leached column treated with various amendments. sol acidity Fe Mn Control 3 1 43 2432 1 8 1 0 1 85 TS+LST 2306 292 420 1 1 8 PM+LST 877 57 370 5 TS+FBA 1 469 373 2 1 1 80 PM+FBA 1 327 25 1 335 1 2 TS+RPR 1 352 723 234 1 26 PM+RPR 1 033 366 2 1 2 34 TS+PM+LST 65 364 22 6 TS+PM+FBA 7 1 5 536 1 1 6 TS+PM+RPR 204 735 1 8 23 LSD(5%) 221 188 156 76 Relative % reductions (-) in the measured parameters TS+LST -27 -88 -77 PM+LST -72 -98 -80 TS+FB A -53 -85 -88 PM+FB A -58 -90 -8 1 TS+RPR -57 -70 -87 PM+RPR -67 -85 -88 TS+PM+LST -98 -85 -99 TS+PM+FBA -77 -78 -99 TS+PM+RPR -94 -70 -99 Except for acidity ( rng CaC03 kg"I), the units are in mg kg" l . -36 -97 -57 -93 -32 -82 -97 -97 -87 Al Alexc 353 894 84 98 1 8 99 75 298 32 77 86 35 1 44 1 1 0 3 1 02 4 205 9 1 26 18 108 -76 -89 -95 -89 -79 -67 -9 1 -9 1 -76 -6 1 -88 -88 -99 -89 -99 -77 -97 -86 236 7.4 Conclusions Topsoil and bactericide ProMac were found effective in the amelioration of low pH conditions in pyritic pitwall rock from Martha mine. Placement of 50 mm thick topsoil layer on a 80 mm depth pitwall rock column effectively reduced leachate S O/- (74%), acidity (47%), Fe (69%) and Al (70%). Significant reduction in S O/- (47% ) and acidity (8 1 %) in leachate from ProMac treated columns indicated evidence of possible inhibition of acid generation from biochemical oxidation by Thiobacillus ferrooxidans had occurred. Use of topsoil in combination with ProMac further improved the leachate quality. Irrespective of amendments, the leachate pH remained very low « 2.5) throughout the 1 2 weeks period of the leaching cycle. Such a low pH leachate is responsible for mobilising soluble Fe, Mn, and different forms of Al in the pore solution. Incorporation of neutralising materials (LST, FB A and RPR) in combination with topsoil or ProMac resulted in an overall efficiency but did not result in significant rise in the leachate pH Topsoil cover had no significant effect on the pH of the pitwall rock material but the bactericide ProMac significantly raised the pH > 3.5, resulting in the possible inhibition of bacterial oxidation of pyrite. Incorporation of neutralising materials in combination with topsoil and ProMac significantly raised the column section pH. Limestone incorporation in the pitwall rock material with either topsoil or ProMac created the most favourable increase in pH (5.2-7 .3) while combination with RPR did not raise the pH above 4.5. The ProMac + FBA combination raised the pH above 8, thereby creating a highly alkaline pitwall rock material, whereas ProMac + topsoil resulted in more favourable pH of about 6.8. Conclusions from Chapters 4, 5 and 6 showed that with appropriate rates of application of neutralising materials, the pitwall rock can be modified to create a pH environment suitable for plant growth. Results of this chapter indicated that topsoil and ProMac could also be used as suitable amendments. For plant growth to establish, suitable growing media must be placed on top of the modified pitwall rock material. Partially 237 oxidised pyritic pitwall on steep slopes is difficult to revegetate mainly because amendments have low residence time and lack of sufficient cover material depth for plant establishment. Provided the pitwall slope gradient is engineered for stable placement of minimum depth of topsoil, revegetation of the pitwall may be achieved with suitable amendments. The results of this study must be however, be tested with plant growth trials in glasshouse and under in situ field conditions. Developments of site-specific combinations of treatment measures are required for successful revegetation of the pyritic pitwall materials on engineered pit slopes. In the case of pitwalls, engineered slope configuration modification is a requirement for stabilised placement of materials. A section of the micro-bench depicting a likely scenario for amendment of the pitwall rock cover material is shown in Figure 7 . 8 . Suitable amendment materials can however, be placed on current pit bench for establishment of wide canopy native plants that can serve the purpose of aesthetic req�irement of revegetating the pitwall in general. Plant growing media i i i i i1 Amended cover material Figure 7 . 8 A conceptual scenario of micro-bench amendment on pyritic pitwalls at Martha mine. 239 C h a p t e r 8 Summary and Conclusions 8.1 Background Mining is a disruptive activity, especially so when it involves open-pit mining methods. Pyritic pitwalls are dynamic environments where steep slope gradients facilitate rapid weathering and erosion, resulting in continued exposure and oxidation of pyrite and migration of acid mine drainage (AMD). Approximately 25% of the current pitwall area at Martha mine (Waihi) contains pyritic host rock undergoing varying degrees of oxidation and weathering processes. Acid generation from oxidation of pyrite exposed on parts of the pitwall has created a highly acidic pitwall rock material with serious limitations to plant growth. Under the current mining licence, revegetation of the upper pitwalls is a requirement primarily for aesthetic reason as well as to prevent erosion. The primary objective of this thesis was to characterise the factors limiting plant growth on pyritic pitwalls and to investigate the ameliorating effectiveness of some selected amendments. 8.2 Literature Review Acid mine drainage (AMD) from abandoned and active nune sites is a maj or environmental problem facing the mining industry. AMD occurs when sulphide minerals such as pyrite are exposed and undergoes biochemical oxidation, resulting in the generation of acidic effluent typically characterised by low pH and high levels of dissolved heavy metal . Pyrite oxidation and its oxidation by-products are as much environmental problem as AMD itself. The ability to predict acid generation from mine waste materials is an important step in preventing AMD. Predictive tests specifically designed for coal mine waste have been used for decades and significant advances in predictive techniques applied to hard rock metal mine waste have been made in the last 5 to 1 0 years. Geochemical static and kinetic tests form the most commonly used techniques currently used for prediction of acid generation from mine waste rock materials. The obj ective of the geochemical tests is to provide data on the acid generating potential of sulphidic mine waste materials so 240 that control and treatment of AMD could be incorporated in environmentally safe management and reclamation of mine sites . Current predictive techniques are however, still in developing stages and subject to queries about their accuracy in predicting real situation AMD conditions. Because of variability in the ore types, these tests are currently accepted only on site specific basis. Even then, with increasing worldwide focus on environmental issues, the geochemical static and kinetic tests have become a mandatory requirement of regulatory guidelines in mining industry. Lime requirements for acidic mme waste materials are commonly determined from standard acid base accounting (ABA) procedures rather than from conventional pH buffer methods used for agricultural soils . The l iming rate assessed from ABA analysis, however, is found to vary considerably and does not always bring the expected results in long-term neutralisation of acidity and prevention of mobilisation of heavy metals . . S ite specific variations in the mine waste materials and continued generation of acid from sulphide mineral oxidation have made it difficult to standardise lime requirement to sustain long-term neutralisation. Large amounts of liming materials are therefore, commonly used to treat AMD and in the reclamation of mine waste. The acid produced from pyrite oxidation is neutralised, in-situ, by dissolution of basic minerals if in contact with the migrating AMD. This neutralisation process is the basis for developing many of the current treatment strategies to mitigate AMD problems in mine wastes and reclamation sites. Alkaline addition to acid producing sites during surface mining and reclamation has shown variable success in ameliorating low pH conditions. Several studies have been conducted on the efficacy of alkaline amendments to ameliorate acidity problems in reclaimed coal mine sites. The effectiveness of alkaline materials in ameliorating low pH conditions is however, a topical issue amongst researchers investigating cost-effective means of treating AMD problems. Various physical, chemical and biological control measures have been used to prevent, minimise and treat AMD. Methods attempted for the prevention and control of AMD generation include treatment of sulphide surfaces via the formation of inert surface "coatings" , soil cover, subaqueous burial, bactericide treatment and segregation of sulphidic waste fraction. Neutralisation with l i ming agents and precipitation of metals as 24 1 hydroxides is currently an effective method for treating AMD generated from coal refuse, waste rock heaps and tailings ponds. Reclamation and revegetation of sulphidic mine waste rock materials contaminated by AMD conditions have become a challenging practical problem for mining industries worldwide. Several studies on the prevention and treatment of AMD have been mainly directed towards improving the effluent drainage quality of the mine wastes. Long-term preventative measures and effective treatment prescriptions adapted to site-specific criteria have been recognised as viable options for mitigating low pH environment created by AMD. Many of the currentl y practised remediation measures are therefore, learning curves for the mine operators and "reclaimers" alike. Open pit mining can expose very large areas of sulphide-bearing rock to air. and water. Although mine pitwalls are recognised as one of the maj or sources of AMD, no detailed studies on the characterisation and reclamation of pitwall rock in situ materials were cited. 8.3 Characterisation of the Pyritic Pitwall Rock Pyritic pitwall rock at the M artha mine is a significant source of AMD and metal contaminants. Acid generation from oxidation of pyrite has created hot spots of low pH / high acidity microenvironments with elevated levels of Fe, sol-, Mn, Al and soluble salts. Low pH conditions in parts of the pitwall rock exposed on the north face pitwall of Martha mine has been recognised as a maj or limitation to revegetation. Steep slope, thin cover material distribution and a high erosion index are some of physical characteristics restricting plant establishment. Progressive weathering of the pyritic pitwall rock has accelerated the acid generation process by rapidly liberating pyrite grains from the host rock. Petrography and mineralogy of the pitwall rock indicated that the weathered cover materials contained localised concentrates of pyrite as a result of a "panning" effect from weathering. Completely weathered samples contained <5% pyrite compared with > 1 0% pyrite in fresh to moderately weathered rocks. With progressive weathering, the reduction in rock grain-size resulted in only moderate liberation of the pyrite grains. 242 Completely weathered samples contained about 2.5% gypsum, indicating that salt precipitation is a common phenomenon during weathering of pyritic pitwall rock under dry environmental conditions. Weathering and biological oxidation has resulted in the deformation of pyrite crystals as reflected in the range of grain shapes and etches pits . SEMlEDS examinations showed that the sulphide mineral in the fresh rock is dominantly pyrite with minor calcite vein mineral. Liberated pyrite grains showed moderate degree of roundness due to weathering. Possible evidence of bacterial activity was indicated by characteristic etch pit on some pyrite grains. XRD analysis of the rock samples indicated that the fresh rocks contained up to 1 2% by weight CaC03. Pyrite content was greater in the rocks containing both vein as well as disseminated pyrite mineralisation. Acid base accounting (AEA) analysis indicated that, irrespective of the degree of weathering, the pitwall rock showed a positive net acid producing potential (NAPP) and that the NAPP values were proportional to the amount of pyrite contained in the representative samples. Although pyrite grains and veins that are directly exposed undergo rapid oxidation to produce a very low pH in weathered pitwall rock, a significant amount of disseminated pyrite grains will not be immediately oxidised in the host rock and will form an unoxidised pyrite concentrate as weathering progresses. Even with an advanced degree of weathering and oxidation, fine-grained pyrite crystals may remain 'entombed' in coarse-grained fractions of the pitwall rock and may thus eventually produce acid. A completely weathered pyritic rock will therefore, still have the potential to generate acidity. Kinetic NAG test indicated that the pitwall rock materials are potentially acid generating types, as indicated by their NAGpH < 3 . S amples containing pyrite lenses as well as disseminated crystals that are undergoing a moderate degree of weathering had potential to produce higher acidity than samples with disseminated pyrite crystals alone. Based on kinetic test results, the fresh pyritic pitwall rock had a "lag-period" of approximately 22 weeks before the onset of acid generation from pyrite oxidation. However, this lag period is likely to vary under site-specific conditions. 243 Chemical characteristics of the pitwall rock showed that, with progressive weathering and oxidation, there were gross chemical changes in the composition of the rocks. While the freshly weathered pitwall rock had a near neutral pH of 7, the completely weathered rock sample had a mean pH of 2 . 3 and EC > 2 dS m- I as a result of accumulated salts. There w as a general increase in solo, soluble Fe, Mn and Al from fresh to weathered pitwall rock. Soluble Fe and sol- in the samples ranged from as low as 477 mg kg- I and 460 mg kg- I to 4055 mg kg- I and 7440 mg kg- I from fresh to weathered pitwall rock, respectively. Both exchangeable Al and soluble AI increased from fresh to weathered rock samples due to increasing solubility with higher acidity. A 60 cm depth profile of the weathered pitwall rock material indicated no variation with depth in pH, EC, solo, Al and Mn. In terms of plant growth potential of the weathered pitwall rocks, it is evident that high acidity, soluble salts and Al remained at toxic levels even up to 60 cm depth and may persist to greater depths. Coarse texture and high porosity of the weathered rock material facilitate diffusion of moisture and oxygen through to greater depth to facilitate biochemical and abiotic pyrite oxidation and hence acid generation. This characteristic of the weathered pitwall rock has important implications to remediation measures for revegetation. Any amendments for amelioration of low pH conditions in the pitwall rock must take into consideration the additional acid generated at depth, especially in the selection of suitable plant species to be grown under such acidic conditions. Geochemical analysis of the pitwall rock showed that weathered materials generally had higher SiOz contents whereas Fez03 was highest (6% ) in the fresh rock. An anomalously high KzO content in moderately weathered pitwall rock is considered to be residual incorporation of K in the formation of clay minerals and possible re­ precipitation of alunite and j arosite under low pH and dry conditions. The trace element compositions of the pitwall rock indicated a nearly two-fold increase in Ba in weathered samples indicating possible immobilisation by sol- to form B aS 04. Arsenic increased with progressive weathering from 2 mg kg- I in fresh rock to 1 0 mg kg- I in completely weathered rock indicating that it has probably accumulated in the weathered rocks as insoluble metal complexes under low pH high Eh condition. 244 Spatial characterisation indicated that the pitwall is a dynamic environment undergoing progressive weathering and oxidation of pyrite. There w as a strong spatial relationship in the distribution of pyrite oxidation products in the weathered pitwall rock over an area of 1 200 m2. Cover material and moisture content were both associated with the spatial distribution of pyrite oxidation products in the pitwall. The distribution of the weathered pitwall rock material primarily controlled the spatial variations in pH, EC, solo, acidity, Fe, Mn, AI, Ca, Mg, K and Na in pitwall area in this study. Variations in the distributions of metals, salts and cations indicated that the pitwall contained highly variable micro-environments, largely controlled by the distribution of pyrite and degree of liberation of pyrite grains during weathering of the pitwall rock. Physical constraints such as steep slopes, insufficient weathered covered material, high macroporosity, low moisture content and l ack of textural development are going to be maj or physical limitations to plant growth potential of the pitwall rock. Both H+ and Ae+ species as well as a significant amount of reserve acidity must be chemically modified with suitable amendments prior to emplacement of growing media on the pitwall surface. 8.4 Lime Requirements of the Pitwall Rock In a highly acidic pyritic pitwall rock materials containing high levels of Fe, AI, S 04 and soluble salts, the lime requirement assessed from standard buffer methods grossly underestimates the actual lime requirement of the pitwall rock. Both the buffer exerted by Fe-AI-hydroxides and potential acidity of the pitwall rock contribute significantly to the neutralisation requirement of the pitwall rock. The total lime requirement of the pitwall rock must take into account both active acidity and the potential acidity from complete oxidation of pyrite present in the pitwall rock. Neutralisation of acid pitwall rock to the often recommended pH value of 6.0 will require very large amounts of alkaline materials because of the high buffer capacity exerted by Fe and Al hydroxide precipitates. Neutralising material particle size had a marked effect on reactivity and neutralisation of acidity. Coarse grained neutralising material was ineffective in significantly raising the pitwall rock pH due to an armouring effect of metal hydroxide and sulphate salt coatings. Wheras, fine grained neutralising 245 materials provided an immediate neutralisation of the active. There is a possibility though that once reacidification occurs, the hydroxide coating may redissolve to neutralisation source once again. The possibility of providing long-term slow release neutralisation of acidity by coarse grained neutralising materials in pitwall rock seem feasible. An incubation assessment of the neutralising effects of selected alkaline materials indicated that LST, DOL and FB A were equally effective in overcoming the large buffer exerted by Fe and Al hydroxides to raise pH to 6.0. RPR did not raise the pH above 4.2 even at the highest CER of 50 kg CaC03 r l . 8.5 Ameliorating Effectiveness of Selected Amendments Both broadcasted and incorporated methods of application of neutralising material amendments had no significant effect on the leachate pH, which remained below 2.5 throughout the 12 weeks period of leaching. The consistently low leachate pH from the neutralising material amended pitwall rock suggested that the alkalinity released from dissolution of neutralising materials was ineffective in ameliorating subsurface pH. At subsurface depths, both bacterially catalysed generation of acid and the acid stored as hydronium sulphate salts such as j arosite are likely to influence the effluent characteristics irrespective of amendment at the surface . Despite low pH effluent, there was significant reduction in acidity in the upper 60 mm section of the columns amended with LST, DOL and FBA. This indicated that under the leaching conditions of this experiment, there was significant amelioration of subsurface acidity by neutralising materials despite generation of very low pH leachate. Continued release of acidity in the leachate from neutralising material amended columns indicated that the dissolution and downward migration of alkaline front was restricted by the large buffer exerted by the hydrolysis of Fe, Mn and AI. There was however, a significant increase in the base saturation of the pitwall rock amended with neutralising materials . Downward migration of Mg and K and Na was evident in FBA and DOL broadcasted columns but Ca remained immobilised in the 0- 40 mm section of the column. High Mg concentrations were observed only in the DOL amended columns. 246 Incorporation of neutralising materials to 0-50 mm depth had significantly reduced the distribution of EC, S O/-, acidity, Fe, Mn and Al concentrations in the upper 80 mm section of the pitwall rock column. There were no differences in SO/- release rates for the broadcasted and incorporated columns but acidity of the leachate from incorporated columns was consistently lower. The Mg level in the leachate from incorporated columns was generally two-fold higher than in the leachate from broadcasted columns. Of the four types of neutralising materials used, LST provided the best circumneutral pH range suitable for plant growth. RPR was ineffective in raising the pH of the pitwall rock material above 4.5 while FBA created highly alkaline conditions as well as . forming cementitious material, both of which make it an unsuitable candidate for ameliorating low pH pyritic rock. On the other hand, the cement forming properties of the FBA can be utilised to stabilise the cover materials on the pitwalls as well as to provide infiltration barriers against runoff water. Placement of topsoil cover significantly lowered metal levels in the leachate and immobilised most of the soluble aluminium in the partially pitwall rock. The bactericide ProMac applied at a rate of 0.5 kg ( 1 resulted in the leachate pH increase of 1 unit from control value of 2 . 5 . A significant reduction in S04, Fe, Mn and Al was associated with this change. ProMac seemed to buffer pH at slightly above 3.5 but whether this increase is as a result of inhibition of bacterial oxidation of pyrite, remains questionable. It is evident from this study that slope factor and high acidity condition are the two main constraints limiting plant growth potential of the pitwall rock. However, pH modification with suitable amendments seems a viable option for creating growth a medium conducive to plant growth provided the slope is engineered to gradients suitable for stabilised placement of amendment materials. The effectiveness of amendment materials in ameliorating low pH conditions on the pitwall will largely depend on the prescriptive combination rather than their acid-neutralising capacity. The success of effective neutralisation of acid will of course, ultimately depend on the assessment of the amendment materials under field conditions, although in active mine pitwalls this may be l imited by ever extending mine w al l . In such a case, one of the best 247 options may be to leave the pitwall materials to fully oxidise naturally prior to implementation of reclamation measures. 8.6 Future Directions This thesis discussed the physical and chemical characteristics of the pitwall rock and its intrinsic hostile nature to plant growth. Whilst laboratory and glasshouse experiments explored the possibility of ameliorating the low pH conditions in the pitwall, field testing of the findings is imperative in validating the results under in situ conditions. Whilst revegetation of the pyritic pitwall area is limited by the steep slope and rapid acid generation, several options are open for future research directions. These include: • Bioengineering of part or all of the pitwall for slope stabilisation and erosion control. • Field plant growth trial with a combination of amendments on the lower section of the pitwall containing maximum thickness of weathered materials. • Use of FB A slurry to stabil ise the cover materials. • Grouting the pitwall with alkaline rods to provide continual source of in-situ neutralisation. • Revegetation of pit bench with large canopy native species to provide screening effect to the exposed pitwall . • Placement of moveable "biomats" on which vegetation could be established. • Use of acid tolerant creeper plants and grass species. • Retardation of acid generation by injecting slow release bactericide pellets • Engineered micro-benches or micro-trenches to hold plant growing media • It i s evident from the characterisation part of this research that acid generation on the pitwall materials is rapid under accelerated weathering conditions. 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