Impurities Removal From Metallurgical Grade Silicon Using Gas Blowing Refining Techniques

Abstract

The removal of impurities from metallurgical grade silicon using the O2 and H2O-O2 gas blowing techniques was firstly studied by thermodynamics. The relationships between the boron content in refined silicon and the equilibrium partial pressures of gaseous boride species were established, which shows a theoretical limitation for boron removal from metallurgical grade silicon using the H2O-O2 gas blowing technique. The data also showed that the impurity boron in silicon was mainly volatilized in the form of B3H3O6, BHO2 and BO and the volatilization of boric hydrate species was much more than that of the oxide species. The impurities removal from metallurgical grade silicon including Fe, Al, Ca, Ti, B, P and C was studied using an O2 gas blowing in a ladle and in succession a mixed Ar-H2O-O2 gas blowing was operated in a DC arc furnace for boron removal. It showed a removal efficiency higher than 90 % for Al, Ca and 50 % for B using the O2 gas blowing technique in the ladle. Impurity boron was reduced from 35 ppmw to 18 ppmw in the ladle and it was once again reduced to 0.6 ppmw using an Ar-H2O-O2 gas blowing technique in the DC arc furnace for a systematic pressure of 5 Pa when the ratio of H2O to O2 and the refining times are 2:1 and 12 min, respectively.