Improving Purity and Process Volume During Direct Electrolytic Reduction of Solid SiO2 in Molten CaCl2 for the Production of Solar-Grade Silicon

Abstract

The direct electrolytic reduction of solid SiO2 is investigated in molten CaCl2 at 1123 K to produce solar-grade silicon. The target concentrations of impurities for the primary Si are calculated from the acceptable concentrations of impurities in solar-grade silicon (SOG-Si) and the segregation coefficients for the impurity elements. The concentrations of most metal impurities are significantly decreased below their target concentrations by using a quartz vessel and new types of SiO2-contacting electrodes. The electrolytic reduction rate is increased by improving an electron pathway from the lead material to the SiO2, which demonstrates that the characteristics of the electric contact are important factors affecting the reduction rate. Pellet- and basket-type electrodes are tested to improve the process volume for powdery and granular SiO2. Based on the purity of the Si product after melting, refining, and solidifying, the potential of the technology is discussed.