Kinetics of carbothermic reduction of natural aluminosilicates
Creators: name: Wong, Siu Chung
Publisher: name: University of British Columbia
Year: 2009

description: Thermogravimetric analysis was used to investigate the kinetics of the carbothermic reduction of aluminosilicates between 1450 and 1550°C. The majority of the experiments involved the reduction of meta-kaolin, mullite or silica with lamp black within an argon atmosphere This dissertation focused on the mechanism and the rate limiting step involved in the reduction process. Also investigated were various aluminosilicates, reductants and reactor atmosphere to identify less expensive sources of raw materials. The carbothermic reduction of aluminosilicates is believed to be simply the reduction of mullite and silica after their formation at the reaction temperatures. Thermodynamics show that only the silica phase will be reduced within the temperature range of this study. The reduction mechanism of aluminosilicates is therefore based on the reduction mechanism of silica. This mechanism can be described as follows: SiO₂ + C = SiO[sub (g)] + CO Step (1) SiO[sub (g)] + 2C = SiC + CO Step (2) From examination of the mechanism, the two possibilities for the rate limiting step are the SiO formation step and the SiC formation step. Experiments involving temperature effect, particle size effect, Si and SiC seeding effect, reactor atmosphere effect (CO[sub (g)] and N₂[sub (g)]) and various combinations of precursor powders were carried out. The collected weight loss data are presented as conversion, X, versus time, t, and conversion rate, dX/dt, versus time, t. The activation energies for the different reaction mixtures have been determined and compared with values from literature. Product powders were characterized by XRD (X-ray diffraction), particle size analysis and SEM (scanning electron microscopy). The two step mechanism is supported by experimental evidence. However, the results of this study are not conclusive in defining the rate limiting step. The rate data are fitted to various rate models, the Contracting Volume Model (phase boundary control) and Jander's equation (diffusion control through a product layer) provide the best overall fit. The activation energies have been determined to be between 260 and 290 kJ/mole for aluminosilicates (meta-kaolin and mullite) reduction and approximately 360 to 420 kJ/mole for silica reduction. The activation energy values for silica reduction agree with published data. The activation energies for mullite and meta-kaolin reduction are approximately 100 kJ/mole lower than those for silica reduction. The alumina phase of the aluminosilicates may play an important role in this difference. A study of alkali aluminosilicates reduction has been carried out to examine the possibility of using these materials as precursors for low technology applications. The reduction rates of bentonite, illite, mica and syenite are similar to meta-kaolin. The study of various carbon reductants shows that graphite and coke may be used in place of lamp black. The use of nitrogen as a purge gas has been examined and the results show that although SiC is retained in the product powders, its yield is low.