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

Abstract

1. Introduction In order to replace the present manufacturing process of solar-grade silicon (SOG-Si), the Siemens process, the establishment of low-cost SOG–Si production process is required. As a new approach, we demonstrated that solid SiO2can be directly reduced to solid Si by electrolysis in molten salts at 1123 K [1]. SiO2 + 4 e− (through conductor) = Si + 2 O2−(1) We also proposed that the electrolytic reduction of purified SiO2 is a potential method to realize the low-cost production of SOG-Si [2]. In recent years, we have been studying the electrochemical reduction of powdery SiO2 to establish a continuous process [3]. However, washing treatment using water or acid is necessary to recover the produced powdery Si from molten CaCl2 and the residual SiO2. To effectively recover the product, we recently proposed an electrolytic process utilizing a liquid Si–Zn alloy cathode as shown in Fig. 1 [4, 5]. In this process, solid SiO2is reduced to form a Si–Zn liquid alloy on the cathode, and then metallic Si is recovered by lowering the temperature to decrease the solubility of Si in the alloy. The Zn-rich liquid alloy after the separation step is recycled for the cathode. The produced Si is further purified to SOG-Si by directional solidification process. In this study, we investigated (i) the electrolytic reduction of SiO2 granules on a liquid Zn cathode in molten CaCl2 and (ii) the impurity segregation during the precipitation of Si from liquid Si-Zn alloy. 2. Experimental High-purity CaCl2 was charged in an alumina crucible and dried under vacuum at 773 K for 24 hours. All the electrochemical experiments were carried out in a dry Ar atmosphere at 1123 K. Electrochemical behavior was investigated by cyclic voltammetry and potentiostatic electrolysis using a liquid Zn electrode. The counter electrode was a carbon rod. An Ag+/Ag electrode was used as a reference electrode. 3. Results and discussion The potentiostatic electrolysis was conducted using a liquid Zn electrode with SiO2 granules (diameter ca. 0.1 mm) at 0.9 V vs. Ca2+/Ca for 1 h. By cross-sectional SEM measurement, several particles were observed in the Zn sample after the electrolysis. EDX analysis confirmed that the SiO2 powder had been reduced to Si. According to the Si–Zn phase diagram, solubility of Si in liquid Si–Zn alloy is 6 at% at 1123 K and that at room temperature is very low. Thus, it is concluded that SiO2 was reduced to form liquid Si–Zn alloy and then Si particles were precipitated during the cooling process. In the presentation, the mechanism of SiO2reduction and the impurity segregation behavior during the cooling process will be discussed. Acknowledgement This study was partly supported by the Core Research for Evolutionary Science and Technology (CREST) of the Japan Science and Technology Agency (JST), and Grant-in-Aid for Scientific Research (A) from Japan Society for the Promotion of Science (JSPS). Reference [1] T. Nohira, K. Yasuda, and Y. Ito, Nat. Mater., 2, 397 (2003). [2] K. Yasuda, T. Nohira, R. Hagiwara, and Y. H. Ogata., Electrochim. Acta, 53, 106 (2007). [3] T. Toba, K. Yasuda, T. Nohira, X. Yang, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Electrochemistry, 81, 559 (2013). [4] T. Shimao, X. Yang, K. Yasuda, T. Nohira, R. Hagiwara, K. Ichitsubo, K. Masuda, and T. Homma, Technical Digest for The 6th World Conference on Photovoltaic Energy Conversion, 4WePo.7.52, (2014). [5] K. Yasuda, Molten Salts (Yoyuen Oyobi Koon Kagaku), 58, 20 (2015). Figure 1