Reductive Removal of Phosphorus in Silicon Using CaO-CaF2 Slag

In order to verify an alternative metallurgical process of phosphorus removal for solar grade silicon (SOG-Si), slag treatment of metallurgical grade silicon (MG-Si) was conducted followed by acid leaching in the present study. MG-Si containing certain amount of phosphorus and calcium was equilibrated at 1723 and 1773 K with several compositions of the CaO-CaF2 slags and phosphorus in molten silicon was confirmed to be removed into slag phase also by reducing reaction as a form of phosphide ion, P3-, in addition to the phosphate ion, PO43-. These contents were separately determined by a wet chemical analysis method developed by ourselves. Although the distribution ratio of phosphorus could not exceed the highest reported values of 3, subsequent leaching brought about considerably high fraction of P removal. The removal fraction of 95.6% was attained when 5 g of silicon was treated with 10 g of the slag at 1773 K followed by the acid leaching, which would be much higher than that expected by the ordinary oxidizing slag treatment. Although the possibility of reducing dephosphorization by slag treatment was clarified, more effective condition should be pursued by changing slag composition, calcium content of silicon, temperature, etc.