Carbothermal reduction of bauxite

The commercial technologies for aluminium production include production of alumina from bauxite by the Bayer process and smelting of alumina to produce aluminium by Hall-Heroult process. The current technology is energy intensive, a major source of greenhouse gas emissions and harmful fluoride emissions. These issues have stimulated the interests in search for alternative technologies of aluminium production. Carbothermal reduction of alumina is considered a promising alternative technology for aluminium production. However, the carbothermal process in investigation still needs pure alumina which does not solve the problems related to Bayer process, such as generation of harmful red mud, requires very high temperatures, and is overall still an energy intensive process.This thesis is concerned with the development of an environmentally sustainable technology for aluminium production that will avoid the generation of environmentally negative red mud sludge. It is to investigate the feasibility of stepwise carbothermal reduction of bauxite at different temperatures with emphasis on the examination of the mechanisms and kinetics of reduction of different metal oxides in bauxite, and also on the deportment of impurities among the various phases formed. It is expected that, if successful, this technology will significantly decrease energy consumption and CO2 emissions compared to conventional carbothermal reduction for aluminium production without generation of red mud waste.Understanding the kinetics and mechanisms of reduction of the different metal oxides in bauxite and the effects of operational parameters is essential for the achievement of the optimal conditions for the production of metallic aluminium and by-products such as ferroalloys and possibly titanium and silicon carbides.This project investigated the carbothermal reduction of Western Australian and Queensland bauxites in argon, carbon monoxide and hydrogen atmospheres. Experiments were performed in a high temperature vertical tube furnace and the off-gas composition was monitored using an infra-red gas analyser. The phase composition of reduced samples was characterized by X-ray diffraction (XRD). Oxygen and carbon contents in reduced samples were determined by LECO analysers. The morphology of the surface and intersections was observed by Scanning Electron Microscopy (SEM). The chemical compositions of the phases in the reduced samples were also detected by Energy-dispersive X-ray spectroscopy (EDS).The results of this study have proved the concept of stepwise reduction of bauxite ores in solid state by appropriate controlling reduction temperature. The products in reduced bauxites by temperature programmed reduction to 1600oC include ferroalloy of silicon and aluminium, carbides of titanium, silicon and aluminium, and unreacted alumina. It also showed that temperatures and the gas environment affected the extent of reduction. These results are of importance to an explanation of the stepwise carbothermal reaction of bauxites, as well as providing aluminium industry with a better understanding of alternative ways to produce aluminium