CHEMICAL-EQUILIBRIA BETWEEN SILICON AND SLAG MELTS

The equilibria between silicon and slags of the systems CaO-SiO2, Na2O -SiO2, and CaO-SiO2-Y with Y being Al2O3, MgO, TiO(x), B2O3, and Na2O have been investigated in silica crucibles. The calcium content under silica-saturated CaO-SiO2 slag is 262 parts per million (ppm) at 1500- degrees-C. The aluminum and magnesium contents increase with increasin g alumina or magnesium oxide contents, respectively, reaching about 18 00 ppm Al at silica/mullite or about 390 ppm Mg at silica/protoenstati te saturation. Boron has a distribution ratio [B]/(B2O3) of 0.18. The sodium content under silica-saturated Na2O-SiO2 slag is 25 ppm at 1500 -degrees-C. In contrast, the titanium content of the silicon, if Y is TiO(x) and (Ti) is in the percent range, is high and varies with the t itanium content of the slag according to [wt pct Ti] = 2.7 square-root (wt pct Ti). In other experiments. it is shown that metallurgical gra de (MG) silicon can be purified from aluminum, magnesium, and calcium by treatment with suitable silicate slags.