Influence of pellet composition and structure on carbothermic reduction of silica

The melting zone in a cupola has temperatures greater than 1773 K and a reducing atmosphere. This condition is suitable for the carbothermic reduction of silica. The key to the applicability of carbothermic reduction of silica for ferroalloy production is rapid in situ production of SiC and its subsequent dissolution in the hot metal. The main objective of this investigation was to study the kinetics of the carbothermic reduction process and determine the optimum parameters for rapid and complete in situ conversion of silica to SiC. At temperatures above 1773 K, the key reactions in the carbothermic reduction process are (1) SiO(2) (s) + CO (g) = SiO (g) + CO(2) (g), (2) SiO (g) + 2C (s) = SiC (s) + CO (g), (3) C (s) + CO(2) (g) = 2CO (g). To meet the objective of this study, conditions must be such that the surface reactions occurring at the carbon and silica surfaces are rate limiting and the entire silica is converted to SiC. Pellet composition and structure in terms of carbon to silica ratio, their particle sizes, and compaction pressure that ensure surface reaction is rate controlling were determined. The gas-solid reaction kinetics was mathematically modeled in terms of the process parameters. The reaction kinetics improved by reducing both carbon and silica particle sizes, However, below a certain critical particle size, there was no significant improvement in the reaction kinetics. For complete conversion of SiO(2) (s) to SiC (s), excess carbon and critical porosity are necessary to ensure that the entire SiO (g) generated by Reaction [1] is consumed via Reaction [2] within the pellet.