Removal of Boron from Molten Silicon Using CaO-SiO2 Based Slags

The global shortage of solar grade silicon for the production of photovoltaic cells has motivated many researches on the refining of silicon, especially the refining of metallurgical-grade silicon. In order to obtain better control of the removal process of boron from molten silicon using silicate slags, precise thermodynamic data are required. In the present study the partition ratio of boron (L-B) between the slag and silicon phases and the activity coefficient of boron oxide in CaO-SiO2, CaO-SiO2-25%CaF2, and CaO-SiO2- 40%CaF2 slags were evaluated at 1823 K. The activity coefficient of 130(1.5) was investigated for slags of CaO/SiO2 ratio varying from 0.3-70; further, the borate capacity of the slag is defined in this study as: CBO33- = (mass%BO33-)/a((B) under bar)P(O2)(3/4) = K(1)a(O2-)(3/2)/f(BO33-) The addition of Na2O to CaO-SiO2 slags was found to increase the partition ratio of boron between the slag and the silicon phase.