Abstract
Oxidative removal of boron from molten silicon has been investigated at 1773 K (1500 °C) by CaO-based flux treatment with oxygen gas injection. Since oxygen gas is injected into the silicon melt after adding CaO- or CaCO3-CaF2 flux onto the melt, high oxygen partial pressure is maintained at the flux-O2-Si interface and the removal of boron proceeds under nonequilibrium conditions. The experimental results clarified that the behavior of boron removal from molten silicon depends on the competition between the oxidation reactions of boron and silicon. On the basis of the results obtained, optimum operating conditions for boron removal by the flux treatment were examined from the viewpoints of initial flux composition, reaction time, oxygen gas flow rate, and orifice size of gas injection nozzle. By repeating the batch operation for 120 seconds three times under the optimum conditions determined in the present study, boron concentration in metallurgical-grade silicon could be reduced from 14 to 7.6 mass ppm efficiently.
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