Recovery of silicon from silica fume

Feasibility of producing high purity silicon from amorphous silica fume, using a low temperature magnesiothermic reduction was demonstrated. Commercial silica fume containing 97.5% amorphous silica was first purified by acid leaching and roasting to remove large quantities of transition metals and carbon. The product was then reduced using magnesium as the reductant. The effect of the amount of reductant, initial temperature, and dwell time were investigated on the quantity and type of the reaction products. The optimum reduction conditions were decided based on the maximum yield of the Si metal. These corresponded to Mg/SiO2 molar ratio of 2.0, preheating temperature of 750 °C, and holding time of 2 h. High purity silicon (> 99 wt.%) containing < 3 ppmw B and 12 ppmw P was obtained after leaching purification of the reduction products, showing that the material is superior to metallurgical grade silicon, for use as solar-grade silicon feedstock.