Research of boron removal from polysilicon using CaO–Al2O3–SiO2–CaF2 slags

•The whole melting process was carried out under atmosphere condition.•Silicon and slags were separated before solidification.•Ion structure theory was used to explain the B removal.•Results is more instructive for industrial production.

Purification process of MG-Si was limited by the difficulties involved in reducing boron content. The possibility of removing impurity boron (B) in MG-Si using CaO–Al2O3–SiO2–CaF2 slags was investigated in the present study. Different from traditional research methods, the whole melting process was carried out under atmosphere condition, which was closer to the actual production conditions. The thermodynamic and kinetic mechanisms of B removal were analyzed. Based on the ionic structure theory, the relation between ionic structure of slags at high temperature and optical basicity can be explained firstly. The parameters, including the slag optical basicity, melting time and CaF2 content were discussed. The boron content was reduced from original 25 ppmw to 4.4 ppmw through 120 min melting with the optical basicity of 0.551 and 5 wt% CaF2 additions.