The phytoextraction of gold and palladium from mine tailings : this thesis is presented in fulfilment of the requirements for the degree of Master of Philosophy

Abstract

The extraction of gold and palladium from a South African mine tailing (Klipfontein) and artificial substrate was examined. A variety of solutions were tested and extractants observed to dissolve large quantities of metal were subsequently used in trials investigating plant uptake of gold and palladium. Extraction by thiocyanate amended with an oxidising agent dissolved large amounts of gold and palladium from the test substrates. Various combinations of thiocyanate/Fe(III) and thiocyanate/H 2 O 2 were examined. Metal extraction in the thiocyanate/Fe(III) showed dependence on redox potential and acidity of the solution; this dependence was not observed in the thiocyanate/H 2 O 2 system where production of cyanide may be an important factor. The addition of iodide to thiocyanate/Fe(III) did not affect dissolution of metals. Thiourea was also tested. This chemical was shown to be a relatively poor extractant of gold and palladium, with and without an oxidant. Two plant species, Berkheya coddii and Brassica juncea, were investigated in plant trials. Initial experiments showed uptake of metals to be independent of plant species. Greatest metal uptake was achieved using cyanide as a chemical amendment, with nearly 500 ppm gold accumulation in B. juncea planted in artificial substrate and treated with 1 gL -1 KCN every day over 6 days. Nearly 13 ppm palladium had accumulated in these plants - the highest average concentration observed with any treatment. KCN also induced the largest metal uptake from Klipfontein substrate – nearly 1600 ppb gold and 7700 ppb palladium accumulation in B. coddii. As an exercise it was shown that the value of gold and palladium that would be recovered from a phytomining operation on Klipfontein substrate would be greater than the cost of cyanide added in such an operation. Plant uptake of gold and palladium from the mine tailing after treatment with thiocyanate plus an oxidant was poor. Gold and palladium uptake by B. coddii from artificial substrate after treatment with thiocyanate + H 2 O 2 was improved, with levels of accumulation similar to that of cyanide. Metal uptake by thiocyanate + Fe(III), however, remained poor. The conclusion of this thesis is that phytomining of gold and palladium offers large potential in both practical and research terms. The relative importance of the species thiocyanate, H 2 O 2 , and cyanide remain unknown in the thiocyanate/H 2 O 2 system and further research is needed to elucidate this behaviour.