Kunio Saegusa

Process for Solar Grade Silicon Production by Molten Salt Electrolysis Using Aluminum-Silicon Liquid Alloy

A new process for solar grade silicon production by molten salt electrolysis has been proposed and its feasibility discussed. This process consists of an electrolysis step of SiO 2 dissolved in a fluoride melt using an aluminum-silicon liquid alloy cathode and a precipitation step of silicon from the liquid alloy. According to the calculation based on literature data, the proposed process has the possibility to produce solar grade silicon in combination with directional solidification, even if the electrolysis step has no purification effect. A preliminary electrolysis using aluminum cathode and carbon anode in a NaF-AlF 3 -SiO 2 melt at 1273 K revealed that silicon was formed by the electrolysis and that sub-reactions, such as the reduction of SiO 2 by aluminum and metal fog formation, also took place. Especially, SiO 2 reduction by aluminum was found to be fast compared to the reduction by the electrolysis, which made it difficult to evaluate the current efficiency. Thus, these sub-reactions were further investigated in order to measure the reaction rates, and their potential dependence. Based on the obtained data, the cathodic current efficiency during the preliminary electrolysis was evaluated to be greater than 46%.

Reduction Behavior of SiCl4 During its Aluminothermic Reduction

With a long term aim of utilization of SiCl4 as a resource for solar grade silicon, we have investigated the aluminothermic reduction of SiCl4. The rate-controlling step for reduction of SiCl4 with liquid aluminum with the gas flow at the melt surface was estimated to be mass transfer in the liquid aluminum. When the Ar-SiCl4 gas was either blown onto or injected into the liquid aluminum, the maximum silicon contents obtained were below the saturated silicon content, and the formation of solid silicon precipitates was not observed. This was believed to be because solid silicon film forms at the gas/liquid interface and acts as a protective layer, stopping further reaction. To overcome this, the injection of Ar-SiCl4 gas into liquid aluminum kept under a temperature gradient was conducted, and the precipitation of solid silicon during the reaction was achieved.