Effect of Si Addition on the Electrochemical Reduction Rate of SiO₂ Granules in Molten CaCl₂

The effect of adding Si powder/granules on the electrochemical reduction rate of SiO₂ granules in molten CaCl₂ was investigated. Various starting materials were prepared by mixing different amounts (up to 80 mass pct) of Si powder/granules (four different sizes: < 5, 20 to 100, 100 to 250 µm, 0.5 to 1.0 mm) with SiO₂ granules, with these materials then being electrochemically reduced for 20 minutes in molten CaCl₂ at 0.6 V vs Ca²⁺/Ca and a temperature of 1123 K (850 °C). The effects of the size and amount of Si added on the reaction rates were then studied. When the size of the Si powder added is less than 5 µm, the reaction rate decreases as more Si is added. When Si with a size of 20 to 100 or 100 to 250 µm is used, the reaction rate does not change substantially with the amount added. When 0.5 to 1.0 mm Si is used, the reduction rate is the largest when 40 mass pct Si is added. The reduction rate is influenced by the reaction surface and the diffusion of O²⁻ ions in the mixture.