Phosphorus Removal in Metallurgical-Grade Silicon via a Combined Approach of Si-Fe Solvent Refining and SiO2-TiO2-CaO-CaF2 Slag Refining

As a critical impurity in the production of solar-grade silicon, the concentration of phosphorus (P) significantly affects photoelectric conversion efficiency. To address the challenge of P removal in solar-grade silicon production, this study proposes a combined process of Si-Fe solvent refining and SiO2-TiO2-CaO-CaF2 slag treatment. Under conditions utilizing collaborative refining with an alloy composition of Si-10 wt. %Fe and a slag composition of 32 wt. %SiO2-48 wt. %CaO-10 wt. %TiO2-10 wt. %CaF2, the removal rate of P in silicon can reach up to 96.8%. This paper investigates the effectiveness of combining solvent refining with slag making under fixed conditions of a Si-10 wt. %Fe alloy paired with various slag systems (no slag addition, binary slag SiO2-TiO2, ternary slag SiO2-CaO-TiO2, and quaternary slag SiO2-TiO2-CaO-CaF2). Based on the experimental results, the optimal TiO2 content in the slag system for maximizing P removal was analyzed and determined. Finally, leveraging both theoretical analysis and experimental findings, the mechanism of P removal was elucidated as a dual process: P is oxidized into Ca3(PO4)2 within the slag phase, and residual P is captured by the Fe-Si-Ti ternary phase.