Preparation of nanocrystalline titanium dioxide particles from New Zealand ilmenite : a thesis submitted in fulfillment of the requirements for the degree of Master of Engineering in Chemical Engineering and Nanotechnology at Massey University, New Zealand, May 2011
Titanium dioxide being one of the most important composite precursors has wide range of application due to the unique properties that it exhibits. TiO2 with varying amount of anatase and rutile phases were prepared by controlled hydrolysis of dissolved liquor (Ti—Fe—Cl solution) from dissolution of New Zealand ilmenite followed by calcination of the hydrate sample at different temperatures. The kinetics of ilmenite digestion is examined based on the factors affecting the ilmenite dissolution rate such as acid/ilmenite ratio, additive (iron powder) and optimum dissolution temperature. In hydrolysis, the use of structure determining agents (SDA) that alters the morphology of TiO2 fine particles is analyzed. Samples without SDA have resulted in rutile phase formation at 110°C, while samples with SDA (phosphoric acid/tri-sodium citrate/citric acid) resulted in either anatase phase or mixed phase (both anatase and rutile) at 110°C. The phosphate and citrate ions (0.35% P2O5 and 0.4% citrate) helps in promoting an anatase phase of TiO2 particles. Along with SDA, parameter such as hydrolysis temperature and percentage seed also affects the intermediate product.
The influence of calcination temperature ranging from 925°C—1000°C on anatase-rutile phase transformation and variation in crystallite size was studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize the resultant TiO2 phase, crystallite size and particle size and shape. The degree of conversion to rutile was higher at higher calcination temperature. Introducing potassium additive (0—2 mass% K2O) in the hydrate sample enhanced the anatase-rutile phase transformation at higher calcination temperature. However, the potassium content in the hydrate sample has a negligible effect on the crystallite
size of anatase and rutile after calcination. The XRD pattern shows an increase in the rutile peak intensity and a decrease in the anatase peak intensity with higher calcination temperature. SEM images show that the particle size of the calcined product at 975°C with 1% K2O ranges from 230nm—300nm.