Separation of boron and phosphorus from Cu-alloyed metallurgical grade silicon by CaO-SiO2-CaCl2 slag treatment

A purification method using copper (Cu) alloying and CaO-SiO2-CaCl2 slag treatment had been studied in connection with the purification of metallurgical grade silicon (MG-Si). The principle of this method was that low-purity MG-Si alloyed with Cu to form Si-Cu alloy phase, and then the Si-Cu alloy was treated using CaO-SiO2-CaCl2 slag to remove impurities. Finally, selective acid leaching was applied to remove the Si-Cu alloy phase, whereupon high-purity silicon could be recovered from the Si-Cu alloy. Effects of slag composition and Cu content on the removal of boron (B) and phosphorus (P) were also investigated. When the Si-50 wt% Cu alloy treated with 45 wt% CaO-45 wt% SiO2-10 wt% CaCl2 slag for 30 min at 1550 °C, the final concentrations of B and P in the purified Si decreased from 3.12 to 0.35 ppmw and 17.14 to 7.27 ppmw, respectively. Furthermore, the corresponding removal mechanism was investigated. It was found that the reaction kinetics followed a first-order rate law with respect to B and P, and were controlled by mass transfer with mass transfer coefficients of 6.25 × 10−4 cm s−1 and 2.55 × 10−4 cm s−1, respectively.