Extraction of phosphorus from metallurgical grade silicon using a combined process of Si-Al-Ca solvent refining and CaO-CaF2 slag treatment

A combined purification process of Si-Al-Ca solvent refining and CaO-CaF2 slag treatment was investigated with a focus on removing phosphorus (P) from metallurgical-grade Si (MG-Si). The primary precipitates in the slag-treated Si-Al-Ca alloy are CaAl2Si2 and CaSi2, and these precipitates can be eliminated via leaching with HCl + CH3COOH and HCl + HF in sequence. Compared to MG-Si, a higher extraction efficiency of P was achieved with Si-Al-Ca alloy after CaO-CaF2 slag treatment. Influences of alloy composition, slag components, and operation parameter on the P removal efficiency were studied systematically. The optimal dephosphorization efficiency reached 98.6% when the 70%Si-Al-Ca alloy was treated with 20%CaO–80%CaF2 slag twice. Furthermore, the mechanism of P removal using the combined process was studied. The results indicate that the Si-Al-Ca solvent refining causes the segregation coefficient of P to be lower, thus facilitating its mass transfer in slag treatment. A large amount of P was reduced and then diffused to the slag phase in the slag experiment. Residual P in the slag-treated alloy was trapped by the CaAl2Si2 phase and was then removed via acid leaching.