Removal of boron and phosphorus from silicon using CaO-SiO2-Na2O-Al2O3 flux

摘要

In the present study a combination of solvent refining and flux treatment of silicon was carried out, aiming to remove boron and phosphorus to acceptable levels for solar applications. Metallurgical grade silicon (MG-Si) was alloyed with pure copper, and the alloy was subjected to a refining by liquid CaO-SiO2-Na2O-Al2O3 slags at 1500 °C. The distributions of B and P between the slags and the alloy was examined 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 distributions of B and P, and the basicity and the oxygen potential competed with each other to control the distributions. When the oxygen partial pressure (........2) was higher than a critical oxygen partial pressure, distributions increased with basicity, and in the low ........2 range the distributions increased with oxygen potential. The addition of Na2O to the slag system was found to increase the distributions of boron and phosphorus. 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 distribution coefficients are 47 and 0.7, respectively. Using these optimum slags to reduce boron and phosphorus in MG-Si to solar grade level, a slag mass about 0.28 times and 27 times mass of alloy would be required, respectively.