Mechanism of electrolytic reduction of SiO₂ at liquid Zn cathode in molten CaCl₂

The reaction mechanism of electrolytic reduction of SiO₂ at a liquid Zn cathode in molten CaCl₂ was investigated with the aim of establishing a new production process of solar-grade Si. Three types of Zn/SiO₂ contacting electrodes were prepared depending on the objectives. Cyclic voltammetry suggested two reduction mechanisms of SiO₂ at a Zn electrode. One is a direct electrolytic reduction that proceeds at potentials more negative than 1.55 V vs. Ca²⁺/Ca. The other is an indirect reduction by liquid Ca-Zn alloy at potentials more negative than 0.85 V. The both reduction mechanisms were confirmed to proceed at 0.60 V by electrolysis and immersion experiments. Impurity analysis by ICP-AES was conducted for the Si prepared by potentiostatic electrolysis at 0.60 V, and confirmed that the concentrations of the metal elements and P were lower than the target levels for primary Si before directional solidification process.