Reaction Mechanism and Kinetics of Boron Removal from Metallurgical-Grade Silicon Based on Li2O-SiO2 Slags

Boron removal from metallurgical-grade silicon by Li2O-SiO2 slag refining under an air atmosphere was experimentally investigated to explore the potential of this method for achieving high boron removal. The boron concentration in the refined silicon was studied under different conditions of holding time, slag composition, and mass ratio of slag to silicon. The boron concentration in metallurgical-grade silicon was successfully reduced from 8.6 ppmw to 0.4 ppmw after slag refining for 0.5 h at 1973 K when the mass ratio of the 60 wt.% Li2O-40 wt.% SiO2 slag to metallurgical-grade silicon was 3. It was proposed that the addition of CaF2 decreased the activity of silica, which brings about a negative effect on the boron removal capacity of Li2O-SiO2 slags. Moreover, the reaction mechanism and kinetics of boron removal were further studied. Analysis of the boron concentration in the resulting slag after refining revealed that the primary boron removal approach was that a large amount of boron was oxidized and then volatilized to the atmosphere in the form of gaseous borates. Based on the two-film theory, the total mass transfer coefficient of boron was determined to be 2.3 x 10(-2) mu m/s.