EFFECT OF CARBONACEOUS MATERIALS ON KINETICS OF CARBOTHERMIC REDUCTION OF SIO2

In relation to the preparation of SiC, the mechanism of reduction of SiO2 by carbon black and activated carbon has been investigated. Using powdered mixtures with a molar ratio C/SiO2 of 3, the reduction rate has been determined by thermogravimetry and chemical analysis at temperatures from 1873 K to 2073 K under an atmosphere of argon. The reduction of SiO2 with carbon black is slow. A small quantity of SiO2 evaporates as SiO and a high yield of SiC can be obtained. W.Jander's rate equation is applicable to the rate of reduction. This result suggests that the dense particles of carbon black are continuously surrounded by SiC at the early stage of reduction. The activation energy is 544 kJ/mol. The rate-determining step is the diffusion of carbon in SiC. Activated carbon reduces SiO2 rapidly because of its high porosity and reactivity. As the evolution of SiO is serious, a yield of SiC is low. The initial rate of reduction is controlled by gas diffusion through the porous layer of carbon particles. The activation energy is 244 kJ/mol. At a late stage of reaction, gas diffusion becomes difficult as a consequence of a decrease in the population and size of pores.