Removal of boron from Silicon-tin solvent by slag treatment

To eliminate B effectively from Si for its use in a solar cell, a novel process involving the slag refining of molten Si with Sn addition was investigated. The partition ratio of B between CaO-SiO-24 mol pct CaF slag and Si-Sn alloy at 1673 K (1400 C) was determined by the chemical equilibrium technique. It was found that the partition ratio of B was remarkably increased with the increase in Sn content of alloy, which attributes to the increase in activity coefficient of B as well as the oxygen partial pressure. The partition function was accounted as much as 200 when the alloy composition was Si-82.4 mol pct Sn, which was much higher than the reported values in the range of 1 to 3. The required amounts of slag used for B removal from Si-30, 50, and 70 mol pct Sn melts were only 15.6 pct, 6.5 pct, and 1.2 pct of that used for the removal of B directly from MG-Si without Sn addition in a single slag treatment

Removal of Boron and Phosphorus from Silicon Using CaO-SiO2-Na2O-Al2O3 Flux

A combination of solvent refining and flux treatment was employed to remove 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 1500 °C. The distribution 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. With silica affecting both parameters in these slags, a critical P_(O_2 ) could be identified that yields the highest impurity pick–up. 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 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 present study was partially supported by NSERC