Although the precipitation of aluminium trihydroxide as gibbsite, via the Bayer process has been widely studied, the mechanism of crystal growth is poorly understood. This work focus on the morphology of gibbsite and the relative growth rates of individual crystal faces.Initial work was carried out to characterize aluminium trihydroxide and it was found that bayerite, another polymorph, precipitated at temperatures below 50 [degrees] C and its morphology depended on the method of precipitation. Gibbsite however, precipitated above this temperature and its morphology depended on the type of alkali aluminate solutions used. The method of precipitation does not affect the morphology, only the size of the precipitate formed. The morphology of gibbsite can be altered by the addition of organic compounds which are known to inhibit gibbsite precipitation. Some of these compounds were found to selectively inhibit the growth of individual crystal faces, thus altering the overall morphology of gibbsite. Boehmite, a polymorph of aluminium hydroxide, can be produced by partial dehydration of gibbsite at 300T. The morphology of boehmite consisted of diamond shaped crystals.The influence of cation incorporation on the morphology of gibbsite was studied experimentally and computationally (molecular modelling). These studies showed that there is a linear relationship between the amount of cation incorporated and atomic radii and between the amount of cation incorporated and the defect energy calculated. The equilibrium morphology of gibbsite predicted in the absence of media matched the morphology of gibbsite grown slowly from sodium aluminate, implying that the amount of sodium incorporation is low in these crystals.The growth rates of individual crystal faces were measured in situ, and found to be a function of supersaturation squared for the prismatic faces, possibly indicating ++ / that E growth occurs by spiral growth mechanism. The growth of the basal face was found to follow the spiral growth mechanism below a relative supersaturation of 0.815 and the birth and spread mechanism above this level. The activation energies and kinetic coefficients for the individual prismatic faces were also determined.Growth rate dispersion was observed in these microscopic studies, but the question of size dependency remains unanswered.The overall growth rates of gibbsite crystal, determined using rapid dynamic light scattering, was found to be an exponential function of supersaturation indicative of a birth and spread growth mechanism.

Low temperature (165°C) Wet Oxidation (WO) and Catalytic Wet Oxidation (CWO) of 12 organic compounds has been studied in highly alkaline, high ionic strength solution (simulating that encountered in the Bayer process used to refine alumina) for the first time. Most (11 out of 12) of the 12 organic compounds studied (formic, acetic, propionic, butyric, oxalic, malonic, succinic, glutaric, citric, lactic, malic and tartaric acids) have been identified in various worldwide Bayer liquors. The various aspects of WO and CWO studied for each of the above-mentioned compounds were as follows; -Extent of complete oxidation to carbonate (i.e. extent of removal of organic compound) -Extent of overall oxidation (i.e. extent of complete oxidation and partial oxidation to stable products) -The product(s) formed from partial (incomplete) oxidation -The reaction mechanism occurring -Why certain compounds undergo low temperature WO and/or CWO in highly alkaline, high ionic strength solution -The ability of various transition metal oxides to catalyse the WO of the selected organic compounds Of the 12 organic compounds studied only six (formic, malonic, citric, lactic, malic and tartaric acids) underwent appreciable (>2% overall oxidation) WO in isolation under the reaction conditions used (4.4 -7.0 M NaOH, 165°C, 500 kPa Po₂, 2 hours). Each of these six compounds underwent some complete oxidation and therefore can be partly removed from highly alkaline, high ionic strength solution using low temperature WO. The order of extent of complete oxidation determined was as follows tartaric> citric> malonic> formic> lactic> malic. All of these compounds also underwent some partial oxidation under the reaction conditions used, excluding formic acid, which only underwent complete oxidation. Oxalic acid was a major product of partial oxidation of all of the above-mentioned compounds (excluding formic acid), while acetic acid was a major product of partial oxidation of citric, lactic, malic and tartaric acids. The WO of formic, malonic, citric, lactic, malic and tartaric acids varied considerably with NaOH concentration over the NaOH concentration range studied (4.4 - 7.0 M). The extent of overall oxidation undergone by each of these compounds increased significantly with increasing NaOH concentration. All of the compounds that underwent appreciable WO under the reaction conditions studied contained hydrogen(s) significantly more acidic then the compounds that did not undergo appreciable WO, thus indicating that only organic compounds that contain acidic (albeit weakly acidic) hydrogens undergo low temperature (165°C) WO in highly alkaline, high ionic strength solution. Two different reaction mechanisms were identified to occur during low temperature WO in highly alkaline, high ionic strength solution. Malonic and formic acids underwent WO predominantly via a free radical based reaction mechanism, while citric, lactic, malic and tartaric acids underwent WO predominantly via an ionic based reaction mechanism. The six organic compounds that did not undergo appreciable WO in isolation (acetic, propionic, butyric, oxalic, succinic and glutaric acids) all underwent appreciable WO when in the presence of malonic acid undergoing low temperature WO. Hence, low temperature WO of all of the above-mentioned compounds can be initiated by free radical intermediates produced by malonic acid undergoing WO in highly alkaline, high ionic strength solution. The ability of several transition metal oxides to catalyse the WO of the chosen 12 organic compounds was investigated. Of the transition metal oxides studied CuO was clearly the most active. Five of the organic compounds studied (malonic, citric, lactic, malic and tartaric acids) were catalytically wet oxidised by CuO in highly alkaline, high ionic strength solution in isolation. The order of catalytic activity observed was malonic > tartaric> lactic> malic> citric. Two different catalytic reaction mechanisms were identified for CuO catalysed WO in highly alkaline solution for the organic compounds studied. CuO catalysed the WO of malonic acid predominantly by catalysing the formation of free radical intermediates. CuO catalysed the WO of citric, lactic, malic and tartaric acids predominantly via a complexation-based reaction mechanism.