Electrochemical Purification of Molten Chloride/Fluoride Salt for Silicon Electrorefining

With the ever-increasing demand for solar modules nowadays, there is a growing interest in developing an alternative, eco-friendly and low-cost manufacturing process for solar-grade silicon. Silicon electrorefining in a molten salt is a promising technique for producing solar-grade silicon directly from metallurgical-grade silicon with low carbon emission and low energy consumption. Due to the stringent requirement in the purity of the silicon for solar applications (at least 99.9999% or 6N), a high-purity molten salt electrolyte is necessary for the solar-grade silicon electrorefining process. In this report, an electrochemical purification process for the eutectic CaCl2-CaF2 (CaCl2:CaF2 = 86.2:13.8 by weight) molten salt at 900℃ has been studied. The impurity level in the salt was in-situ examined by cyclic voltammetry (CV). After the electrochemical purification process, a decrease in peak current of the CV is observed, indicating the removal of the impurities. According to energy-dispersive x-ray spectroscopic results, Fe, Mn, Mg and Al were successfully extracted from the salt. This electrochemical purification process and in-situ salt purity examination technique are useful for purity control of the molten salt in the solar-grade silicon electrorefining process.