Boron Removal from Silicon by CaO-Na2O-SiO2 Ternary Slag

Abstract

Boron removal from silicon is an important issue for solar-grade silicon feedstock production. In the present study, the removal of B from liquid silicon by a CaO-Na2O-SiO2 slag is studied and it is shown that B can be rapidly removed from silicon within short refining times. Based on mass balance and chemical analysis of the reacted silicon and slag, it is indicated that the kinetics of B removal is dependent on the Na2O in the slag, the main reactive agent for B removal from the system. The transported B into the slag is gasified at the slag-gas interface through sodium metaborate evaporation, which is a rate controlling reaction for B transport to the gas phase. It is indicated that B removal rate by CaO-Na2O-SiO2 slag is considerably higher than that by CaO-SiO2 slags. It is proposed that boron oxide (B2O3) is better embedded in the structure of CaO-SiO2 slags than Na2O-containing slags.