The Kinetics of Boron Transfer in Slag Refining of Silicon

Abstract

Slag refining is one of the few metallurgical methods for efficient removal of boron from silicon. In industrial slag refining for production of silicon for photovoltaic applications, the relationship between the slag composition and the mass transfer rate of boron from metal to slag is of great importance. The kinetics of boron removal from liquid silicon during slag refining has been investigated in the present work by means of several small-scale experimental series at temperatures between 1,600°C and 1,650°C. Slag and metal, in batch weights of 30 g, were heated together in a graphite crucible placed in a resistance-heated tube furnace. The slags were produced from powdered SiO2, CaO, and MgO. The oxides applied were of high purity. Experiments were carried out at slag-to-metal ratios of 1 or 2, where the silicon initially contained approximately 250 ppm boron. Metal–slag mass transfer coefficients were calculated based on chemical analyses of the silicon and ranged from 1.7 μm/s for slag consisting of equal amounts of SiO2 and CaO by weight, to 4.3 μm/s for a 40%–40%–20% SiO2-CaO-MgO slag.