Boron Removal From Molten Metallurgical Grade Silicon via Flux-Powder Injection

This work assessed boron removal from molten metallurgical grade silicon (MG-Si) using highly basic CaO-CaF2 flux-powder together with the injection of gaseous oxygen. The dynamics of both the oxygen bubbles and the injected flux particles were examined in detail. An elevated oxygen partial pressure was achieved at the flux-O-2-Si interface and boron removal occurred in a non-equilibrium state. This technique was found to reduce the boron concentration in MG-Si to a level on the order of 1 mass ppm. The effects of the flux particle dynamics at the flux-O-2-Si interface close to the exit of the injection nozzle on boron removal were also evaluated. The rate of boron removal was determined based on the flux particle kinetic energy, which in turn depended on the oxygen gas flow rate and the particle size in the injected flux-powder. Increasing the oxygen flow rate increased the kinetic energy of the flux, allowing it to penetrate the O-2-Si interface at the exit of the nozzle. This effect produced a new reaction field for further removal of boron from molten MG-Si. In contrast, lower flow rates did not allow penetration through the interface. In this case, reaction field formation was inhibited and the boron removal rate was independent of the flux injection rate. Eight consecutive runs for 60 seconds using optimized parameters decreased the boron concentration in MG-Si from 14 to 1.5 mass ppm.