Carbothermal reduction of silica was investigated in a fixed bed reactor at 1300-1650 °C in nitrogen at 1-11 atm pressure and in hydrogen-nitrogen mixtures at atmospheric pressure. Samples were prepared from silica-graphite mixtures in the form of pellets. CO evolution in the reduction process was monitored using an infrared sensor; oxygen, nitrogen and carbon contents in reduced samples were determined by LECO analyses. Phases formed in the reduction process were analysed by XRD. Silica was reduced to silicon nitride and silicon carbide; their ratio was dependent on reduction time, temperature and nitrogen pressure. Reduction products also included SiO which was removed from the pellet with the flowing gas.
In the temperature-programmed experiments, reduction of silica started below 1300 °C; the reduction rate increased with increasing temperature. Silicon carbide was the major product at the early stage of reduction; a fraction of silicon nitride increased with increasing reaction time. Maximum silicon nitride to carbide ratio (68.0/9.8 specifically for 720 min) in the reduction of silica in nitrogen at atmospheric pressure was observed at 1450 °C. Further increase in temperature decreased Si3N4/SiC ratio. When nitrogen pressure was 11 atm, maximum Si3N4/SiC ratio of 71.4/13.3 was observed at 1550-1600 °C. Increasing nitrogen pressure increased reduction and nitridation rates and suppressed SiO loss under otherwise the same conditions.