Behavior and State of Boron in CaO-SiO2 Slags during Refining of Solar Grade Silicon

The thermodynamics of boron removal during the refining of solar grade silicon by slag treatment was investigated using CaO-SiO2 slags. The partition ratio of boron between slag and silicon phases and the state of boron in slag were investigated for a composition range of CaO/SiO2 between 0.55-1.21, and for boron concentrations varying from 50 ppm to 0.25 wt% at 1823 K. The best boron partition ratios were attained for CaO/SiO2 values of 1.21 and 0.55 (5.5 and 4.3, respectively), and it had a minimum value of 1.9 for a CaO/SiO2 value of around 0.8. The dependence of the boron distribution with content between silicon and slag phase showed that boron exists as a monomer for all the slag compositions studied in this work. A thermodynamic evaluation of the system was carried out by calculating the activity coefficient of BO1.5, revealing a decrease in the activity coefficient with an increase in the silica content when the CaO/SiO2 ratio was lower than 0.8. The decrease in the activity coefficient was attributed to the incorporation of boron oxide in the silicate network present in slags having high silica content. This assumption was confirmed by solid state B-11 NMR analysis and supported by quantum chemical calculations.