Kinetics and Mechanism of an Electrically Enhanced Silicon Refining

Abstract The current study investigates the kinetics and mechanism of boron (B) removal from silicon through electrically enhanced slag treatment using a 45CaO–45SiO 2 –10Al 2 O 3 wt pct slag system. Experiments were conducted at 1500 °C to 1600 °C under an argon atmosphere, with varying reaction times (0 to 120 minutes) and applied currents of 0 and 1.25 A, resulting in cell potential difference of approximately 3 to 6 V. Improved approach in analyzing the kinetics was developed by considering the diffusivity of B determined through molecular dynamics simulations. The results indicate that the overall boron removal process appeared to be governed by mass transfer in the slag phase with k s (mass transfer coefficient in slag) values between 1.61 and 2.34 × 10 −6 m s −1 . These values are 20 pct slower than the observed mass transfer in the silicon which was between 1.95 and 2.31 × 10 −6 m s −1 . The applied electrical current showed an enhancement in boron removal (approximately by 10 to 20 pct) indicated by an increase in distribution coefficient L B (wt pct B in slag/wt pct B in Si) between 10 and 50 pct. Open-circuit experiments demonstrated transient electrochemical activity, with no sustained potential difference beyond 1000 seconds. Microstructural analysis identified silicon formation on the graphite cathode, attributed to SiO 2 reduction. The electrode introduction into the melt in the current experimental setup reduced the slag–silicon contact area, partially offsetting the benefits of electrical enhancement. The proposed mechanism involves slag melting, ionic transfer, interfacial chemical reactions, and mass transport. During electric current application, Si 4+ ion reduction to Si is enhanced by supplying electrons from the cathode. On the other hand, O 2− ion reaction with boron in the interface produces (BO 3 ) 3− and three electrons. This reaction is enhanced by consuming electrons in the anode. Although electrical current enhanced boron oxidation and SiO 2 reduction, its effect was limited by slow slag phase diffusion—the rate-determining step, as seen in similar studies. The findings suggest that while electrical currents can improve boron removal by 10 to 20 pct, optimizing slag properties may offer more practical pathways for silicon refining purposes. It is proposed that improving the cell configuration and applying stirring can further enhance efficiency.