Improving Purity and Process Volume During Direct Electrolytic Reduction of Solid SiO₂ in Molten CaCl₂ for the Production of Solar‐Grade Silicon

Abstract

The direct electrolytic reduction of solid SiO2 is investigated in molten CaCl2 at 1123 K to produce solar‐grade silicon. The target concentrations of impurities for the primary Si are calculated from the acceptable concentrations of impurities in solar‐grade silicon (SOG‐Si) and the segregation coefficients for the impurity elements. The concentrations of most metal impurities are significantly decreased below their target concentrations by using a quartz vessel and new types of SiO2‐contacting electrodes. The electrolytic reduction rate is increased by improving an electron pathway from the lead material to the SiO2, which demonstrates that the characteristics of the electric contact are important factors affecting the reduction rate. Pellet‐ and basket‐type electrodes are tested to improve the process volume for powdery and granular SiO2. Based on the purity of the Si product after melting, refining, and solidifying, the potential of the technology is discussed.

High purity silicon materials prepared through wet-chemical and electrochemical approaches

“…Si(s) + 2O 2À Therefore, if the electrode material is Si, contamination free reduction of silica to Si can be achieved in principle [18,19]. However, there was a problem of low productivity for such earlystage study because batch type electrolysis was used.…”

Electrolytic Reduction of SiO2 Granules in Molten CaCl2

“…4448 In our previous studies, in which SiO 2 plates and powder-molded SiO 2 pellets were used, 3039 the amount of impurity elements were mostly controlled not to exceed the target levels, except for boron (B) and carbon (C). 39 However, the process for the fabrication of SiO 2 plates or pellets from high purity SiO 2 powder or granules increases the cost and the possibility of undesired contamination. Therefore, the direct reduction of SiO 2 powder or granules would be more practical.…”

Kinetic Characteristics of Electrochemical Reduction of SiO2Granules in Molten CaCl2

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Meanwhile, the authors [26][27][28][29][30][31][32][33] and other researchers [34][35][36][37][38][39][40][41][42][43] have demonstrated that solid silica (SiO 2 ) can be directly reduced to solid Si via electrolysis in molten salts. 44 Recently, the authors proposed an improved electrochemical process in which SiO 2 granules are directly used as the raw material. The produced O 2− ions are transferred to the carbon anode through the melt, and then reacted to form CO or CO 2 .…”