Effect of slag basicity and oxygen potential on the distribution of boron and phosphorus between slag and silicon

The distribution of boron and phosphorus between Al2O3–CaO–MgO–SiO2 and Al2O3–BaO–SiO2 slags and silicon metal was examined for the purposes of establishing the possibility of refining metallurgical-grade silicon to solar grade silicon. Distribution coefficients of B and P were normalized with the estimated oxygen partial pressure of the system to isolate the influence of basicity as the composition of the slag was varied in terms of the basicity (as CaO:SiO2 ratio) and oxygen potential (as Al2O3:SiO2 ratio) of the former and of the basicity (as BaO:SiO2 ratio) of the latter slag systems. For both of these impurity elements, the normalized distribution showed a different dependence on corrected optical basicity in each slag system studied. Values were highest in the Al2O3–CaO–MgO–SiO2 slag with varying SiO2:Al2O3, where the impurities are thought to associate with Mg, and lowest in the Al2O3–BaO–SiO2 slag, where the impurities are thought to associate with Ba. The different dependence on basicity in each system is attributed to the differing manner in which the activity coefficient of the actual impurity compound in the slag varies with basicity. From thermodynamic calculations it was concluded that the difference in normalized distributions between the slag systems is attributed to the magnitude of the ratio of equilibrium constant to activity coefficient of the impurity oxides. It was found that where basicity is equal and the form of the impurity oxide in the slag is equivalent, phosphorus is expected to have a much smaller activity coefficient in the slag than boron.