Diffusion and Mass Transfer of Impurity P in the Purification of Metallurgical-Grade Silicon Using Calcium Silicate Slag

The study of diffusive mass transfer of impurity phosphorus (P) in silicon melts was a crucial aspect of achieving efficient deep P removal. To elucidate the mass transfer mechanism of P in silicon melts, this paper presents a kinetic analysis of impurity P removal from silicon melts. To minimize the effects of convection on diffusion, a capillary diffusion device was employed to conduct experimental investigations of P diffusion in silicon melts at different temperatures. According to slag–silicon refining experiments, the mass transfer process of P between the silicon melt and the slag phase was characterized and the mass transfer coefficient of P in the silicon melt was calculated. The experimental results indicated that the diffusion coefficients of P in silicon melt at 1450 °C, 1500 °C, and 1550 °C were 2.81 × 10 −8 , 3.24 × 10 −8 , and 3.91 × 10 −8 m 2 ·s −1 , respectively. Through the calculation of the activation energy for P diffusion, the relationship between the diffusion coefficient and temperature was determined as \(D=1.23\times {10}^{-5}\cdot {\text{e}}^{-\frac{\text{87,260}}{RT}}\) . At a refining temperature of 1550 °C, the mass transfer coefficient of P in the silicon melt was found to be 2.11 × 10 −5 m·s −1 when calcium silicate slag was used for slag refining. These findings provided essential kinetic data, including diffusion and mass transfer coefficients, to support the advanced removal of impurity P from metallurgical-grade silicon (MG-Si).