Boron and Phosphorus Removal from Si-Cu Alloy Using CaO-SiO 2-Na2O-Al2O3 Slag

A combination of solvent refining and slag treatment was employed to investigate the possibility of removing boron and phosphorus from crude silicon to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper and the alloy was subjected to refining by liquid CaO-SiO2-Na2O-Al2O3 slags at 1773K (1500°C). The distribution of B and P between the slags and the alloy was measured under a range of slag compositions, varying in CaO:SiO2 and SiO2:Al2O3 ratios and the amount of Na2O. The results showed that both basicity and oxygen potential have a strong influence on the distribution of B and P. With silica affecting both parameters in these slags, a critical PO2 could be identified that yields the highest impurity pick-up. The addition of Na2O to the slag was found to increase the distributions of boron and phosphorus into the slag. A thermodynamic evaluation of the system showed that alloying copper with MG-Si leads to substantial increase of boron distribution coefficient. The highest boron and phosphorus distributions were 47 and 1.1, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.3 times and 17 times mass of alloy would be required, respectively. The loss of Na2O and B2O3U from CaO-SiO2-Al2O 3 slags containing a high content of Al2O3 and Na2O was studied in the temperature range of 1573-1773K (1300-1500°C) by thermogravimetric analysis (TGA) under isothermal conditions. Higher basicity and larger B2O3 and Na2O concentrations were also found to increase the evaporation rate. The rate appears to be controlled by chemical reaction at the surface in the beginning, followed by a mixed chemical reaction-mass transfer regime, and finally a liquid-phase mass transport step. The apparent activation energies for the evaporation reaction were calculated for different slag compositions.