Silicon Refining by Solidification from Liquid Si–Zn Alloy and Floating Zone Method

This study evaluated the refining ability of a proposed production process for solar-grade silicon utilizing the electrolytic reduction of SiO₂ on a liquid zinc electrode in molten salt. The distribution behaviors of impurity elements during the precipitation of solid silicon from a liquid Si–Zn alloy were studied by thermodynamic calculations at 923 K. In the precipitation experiment, silicon granules were recovered from a liquid Si–Zn alloy, which was prepared from metallurgical-grade silicon. The impurity removal ratios exceeded 99% for C, Al, and Ca, and 90% for Fe. High removal ratios were attained for B and O as well. As the post-processing, a silicon ingot was produced from the precipitated silicon granules by the floating zone method. The Zn residue in the precipitated silicon was completely evaporated during the floating zone refining. The total content of metallic elements (Al, Ca, Fe, Ti, and Zn) was lower than 0.2 ppmw, even though metallurgical-grade silicon was used as the starting material

Electrolytic Production of Silicon Using Liquid Zn Alloy Cathode in Molten CaCl₂

2019 Liquid Metal Processing & Casting Conference (LMPC2019), Birmingham, UK, 8-11 September, 2019.A new electrolytic production process for solar-grade Si has been proposed utilizing a liquid Si–Zn alloy cathode in molten CaCl₂. The process consists of three major processes: electrolysis, precipitation, and refining. One of the advantages of the process is the attainability of high purity by application of a solidification refining from the liquid Si–Zn alloy. The residual zinc is easily removed afterwards due to its high volatility. To establish this process, the behavior of liquid Zn metal in molten CaCl₂ at 1123 K was investigated. Evaporation of Zn metal was largely suppressed by immersion into the molten salt, which enabled the use of a Zn electrode despite its high vapor pressure. Based on the results of cyclic voltammetry, the reduction of SiO₂ on a liquid Zn cathode was conducted by potentiostatic electrolysis at 0.9 V vs. Ca²⁺/Ca. Precipitated Si particles were recovered in the solidified Zn matrix

A New Electrolytic Production Process of Silicon Using Liquid Zn Alloy Cathode in Molten Salt

PRiME 2016/230th ECS Meeting, October 2, 2016 - October 7, 2016, Honolulu, HI.Electrolytic production process for solar-grade Si utilizing liquid Si–Zn alloy cathode in molten CaCl₂ has been proposed. Toward the establishment of the process, the behavior of liquid Zn metal was investigated in molten CaCl₂ at 1123 K. The evaporation of Zn metal was largely suppressed by an immersion into molten salt, which enables the use of Zn electrode in spite of the high vapor pressure of Zn. The cyclic voltammetry suggested the reduction of SiO₂ at 1.45 V vs. Ca²⁺/Ca on a Zn cathode. After the potentiostatic electrolysis at 0.9 V, Si particles with diameters of 2–30 µm were precipitated in the solidified Zn matrix by the slow cooling process of the produced liquid Si–Zn alloy. The alloying rate between solid Si and liquid Zn was measured as 4.56 μm s⁻¹, and it linearly decreased with the Si content in liquid Zn phase

Electrolytic Production of Silicon Using Liquid Zinc Alloy in Molten CaCl2

A new electrolytic production process for solar-grade Si has been proposed utilizing liquid Si–Zn alloy cathode in molten CaCl2. To establish this process, the behavior of liquid Zn metal in molten CaCl2 at 1123 K was investigated. Evaporation of Zn metal was largely suppressed by immersion in the molten salt, which enabled the use of a Zn electrode despite its high vapor pressure. Cyclic voltammetry results suggested that the reduction of SiO2 on a Zn cathode proceeded at a more negative than 1.45 V vs. Ca2+/Ca. After potentiostatic electrolysis at 0.9 V, Si particles with sizes of 2–30 μm were precipitated in the solidified Zn matrix by a slow cooling process. The rate-determining step for electrochemical reduction of SiO2 on the Zn cathode was discussed on the basis of a measurement of the alloying rate between solid Si and liquid Zn