Reaction Mechanism and Kinetics of Boron Removal from Molten Silicon by CaO–SiO2–CaCl2 Slag Treatment

Abstract

The mechanism by which B is removed from Si to a molten CaO–SiO2–CaCl2 slag, and its subsequent evaporation to a gas phase at 1723 K, has been evaluated with a view to optimize the production of solar-grade Si. By assuming a diffusion model for the transfer of B from Si to molten slag, and its evaporation to a gas phase, the diffusion coefficient of B in slag was investigated using the tube-molten pool method. The mass transfer coefficients of B in slag were also measured and calculated according to Fick’s law. Through this, it was found that the rate-limiting step of B removal in the slag refining process is controlled by B transfer from both the slag interface and surface. Of these, however, it is the former that has the greater influence, as the slag boundary layers in the interface are thicker than those at the surface.