Boron removal from metallurgical grade silicon using a CaO–Li2O–SiO2 molten slag refining technique

The distribution of boron between CaO–SiO2 slag phase and silicon phase was examined in order to establish the effects of slag composition and basicity on boron removal from metallurgical grade silicon (MG-Si). Though the addition of Li2O to CaO–SiO2 slag showed a little obvious advantage to boron removal, the boron concentration in MG-Si can be reduced from 18 ppmw to 1.3 ppmw with the ratio of CaO–Li2O–SiO2 slag to MG-Si for 4:1 (mass). The reactive mechanism between boron in MG-Si and CaO–Li2O–SiO2 slag is described using an ionization theory of molten slag and an electrochemical model was established, which depicted the oxidation and transfer of boron between silicon and slag phases. The experiment of slag refining for rich-B Si-B alloy showed that the generated (BO3)3− combines into Li2O·2B2O3 with slag instead of CaO·B2O3 on account of the more negative standard Gibbs free energy of formation for Li2O·2B2O3 than that of CaO·B2O3.