Boron removal in molten silicon by a steam-added plasma melting method

Abstract

Oxidation and removal of boron from molten silicon by a steam-added plasma melting method was investigated as an important part of a sequential metallurgical process for producing high-purity solar grade silicon (SOG-Si) from commercially available metallurgical grade silicon (MG-Si). Experiments were carried out with the mass of silicon per charge varied in the range from 0.6 to 300kg, corresponding to the laboratory scale to industrial scale. Boron was removed to [B] < 0.1 mass ppm, which is the permissible boron content for SOG-Si. The deboronization rate was proportional to the steam content, 3.2th power of the hydrogen content of the plasma gas, boron content of the molten silicon, and area of the dimple formed by the plasma gas jet, and was inversely proportional to the mass of the molten silicon. A thermodynamic study showed that preferential oxidation of boron in molten silicon is positively related to higher temperatures, supporting the conclusion that this plasma method, which causes a local increase in the temperature of the reaction surface, is in principle advantageous.