Thermochemical Aspects of Boron and Phosphorus Distribution Between Silicon and BaO-SiO2 and CaO-BaO-SiO2 slags

Abstract

In the production of solar grade silicon by metallurgical route the distribution of B and P between slags and liquid silicon is the most important key issue. The equilibrium and thermochemistry of reactions between liquid silicon and BaO-SiO2 slags and up to 10% BaO-containing CaO-BaO-SiO2 slags is studied through experimental work and using thermodynamic calculations. It is shown that the distribution coefficient of B (LB) is higher for the CaO-BaO-SiO2 slags than that for BaO-SiO2 slags and it is not significantly affected by temperature and composition changes of the slags. In contrast, the distribution coefficient of P (LP) is higher for BaO-SiO2 slags than that for the CaO-BaO-SiO2 slags, and it is higher at lower temperatures. The chemical activities of the dilute solutions of Ba in liquid silicon, and the dilute solutions of B2O3, P2O5 and BaO in the slags are calculated. Moreover, the reaction mechanisms for B, P, Ba and Ca transport between liquid silicon and the slags are explained.