Multiple Slag Operation on Boron Removal from Metallurgical-Grade Silicon Using Na2O-SiO2 Slags

The removal of boron from metallurgical-grade silicon by Na2O-SiO2 slag refining was investigated. The final content of boron in refined silicon was examined under different conditions of temperature, slag composition, holding time, mass ratio of slag to silicon, and slag refining times. The content of boron in silicon decreased from 10.6 to 0.65 ppmw by slag treatment in one time, and increasing slag refining times was beneficial for removing boron at the conditions of the small mass ratio of slag to silicon and short holding time. Moreover, the removal mechanism of boron was also discussed. The primary removal mechanism of boron was that a large amount of boron was oxidized and then volatilized to the atmosphere in the form of boron oxides. The mass transfer coefficient of boron from silicon to slag was connected with the radius of silicon droplet in the slag refining process.