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

Abstract

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. Ca 2+ /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.

Mechanism of Electrolytic Reduction of SiO2at Liquid Zn Cathode in Molten CaCl2

“…In our previous study, the suppression of evaporation of Zn metal by covering it with molten CaCl 2 was confirmed at 1123 K. 36,37 Moreover, the alloying rate between solid Si and liquid Zn was measured, and the formation of liquid Si-Zn alloy at a liquid Zn cathode was demonstrated. 36,37 After potentiostatic electrolysis at 0.9 V, Si particles of sizes in the range 2-30 μm were precipitated in the solidified Zn matrix. The formation of liquid Ca-Zn alloy (Reaction 4) was also suggested by cyclic voltammetry.…”

“…Recently, we proposed an electrolytic reduction process of SiO 2 using a liquid Zn cathode in molten CaCl 2 . Since the electrolysis product is liquid Si-Zn alloy, its separation from unreacted SiO 2 and molten salt is expected to be easier than that entailed in the use of the conventional solid cathode 36,37 . The choice of Zn as an alloying element stems from the very factors that render the Du Pont process advantageous, i.e., the use of Zn ensures the formation of no intermetallic compounds with Si, and facilitates easy removal of both Zn and ZnCl 2 .…”

“…The choice of Zn as an alloying element stems from the very factors that render the Du Pont process advantageous, i.e., the use of Zn ensures the formation of no intermetallic compounds with Si, and facilitates easy removal of both Zn and ZnCl 2 . Here, the most important point is that the existence of molten salt over liquid Zn effectively suppresses the evaporation of Zn even at high temperatures such as 1123 K. Figure 1 schematically illustrates the proposed process 36,37 . The overall process consists of three major steps:…”

“…Since the electrolysis product is liquid Si-Zn alloy, its separation from unreacted SiO 2 and molten salt is expected to be easier than that entailed in the use of the conventional solid cathode. 36,37 The choice of Zn as an alloying element stems from the very factors that render the Du Pont process advantageous, i.e., the use of Zn ensures the formation of no intermetallic compounds with Si, and facilitates easy removal of both Zn and ZnCl 2 . Here, the most important point is that the existence of molten salt over liquid Zn effectively suppresses the evaporation of Zn even at high temperatures such as 1123 K. Figure 1 schematically illustrates the proposed process.…”

“…Here, the most important point is that the existence of molten salt over liquid Zn effectively suppresses the evaporation of Zn even at high temperatures such as 1123 K. Figure 1 schematically illustrates the proposed process. 36,37 The overall process consists of three major steps: electrolysis, precipitation, and refining. In the electrolysis step, solid SiO 2 is reduced to form liquid Si-Zn alloy at a liquid Zn cathode.…”