Enhanced Phosphorus Removal from Metallurgical Grade Silicon by the Combined Process of Si-Cu Solvent Refining and CaO-CaF2-CaCl2 Slag Treatment

To develop a high-efficiency process for removing phosphorus (P) from metallurgical grade silicon, a novel method of combining Si-Cu solvent refining and CaO-CaF2-CaCl2 slag treatment was investigated through simultaneously re-constructing P-containing phases of CaCu2Si2 in the Si-Cu alloy and Ca3P2 in the slag. After acid leaching, P-containing phases can be eliminated, whereupon high-purity silicon could be recovered from the Si-Cu alloy. The effect of slag components and alloy composition on the P removal efficiency was studied systematically. When the Si-40 wt.% Cu alloy is treated with 20 wt.% CaO-32 wt.% CaF2-48 wt.% CaCl2 slag for 60 min at 1400 °C, the P removal efficiency reaches 90.1%. Furthermore, the mechanism of enhanced P removal was also discussed. It was indicated that a silicothermal reduction reaction occurred between CaO and Si, which caused Ca to migrate into the alloy and precipitate the P-containing CaCu2Si2 in the Si-Cu alloy. Simultaneously, P in silicon is reduced to P3− at the slag–alloy interface, forming Ca3P2 in the slag, thereby establishing a dual-path purification mechanism. Hence, this study provides new insight into silicon high-efficiency purification from economical and practical considerations.