Reaction Mechanism and Kinetics of Boron Removal from Metallurgical-Grade Silicon Based on Li2o-SiO2 slags

Abstract

Slag refining is generally considered as one of the most optimal options for boron removal from metallurgical-grade silicon. Therefore, finding an effective refining agent to eliminate boron from metallurgical-grade silicon is of great importance. In this work, a new Li2O-SiO2 slag system was chosen as a promising agent to remove boron impurity from metallurgical-grade silicon at the conditions of holding time, slag composition and mass ratio of slag to silicon. To lower the production cost and achieve industrial application, slag treatments were carried out under an air atmosphere rather than under protective gas atmosphere. 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 60wt.% Li2O-40wt.% SiO2 slag to metallurgical-grade silicon was 3. This figure is significantly close to the required boron purity of SOG-Si (below 0.3 ppmw). 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 investigated. 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 2.3×10-2 μm s-1.