Boron removal in purifying metallurgical grade silicon by CaO-SiO2 slag refining

Boron removal from metallurgical grade silicon (MG-Si) using a calcium silicate slag was studied. The results show that it is impossible basically to remove boron using a pure SiO2 refining. The oxidizing ability of CaO-SiO2 slag for boron removal was characterized by establishing the thermodynamic relationship between the distribution coefficient of boron (LB) and the activities of SiO2 and CaO. The experimental results show that the distribution coefficient and the removal efficiency of boron are greatly improved with the increase of CaO proportion in the slag. The maximal value of LB reaches 1.57 with a slag composition of 60%CaO-40%SiO2 (mass fraction). The boron content in the refined silicon is reduced from 18×10−6 to 1.8×10−6 using slag refining at 1600 °C for 3 h with a CaO-SiO2/MG-Si ratio of 2.5, and the removal efficiency of boron reaches 90%.