Phosphorus Removal from Si-Fe Alloy Using SiO₂-Al₂O₃-CaO Slag

Phosphorus removal from silicon using a combination of solvent and slag refining, with lower carbon footprint and lower energy requirement than the current industrial process, is investigated by evaluating the distribution of phosphorus between Si-Fe alloy and SiO₂-Al₂O₃-CaO slag. An alloy with the composition of silicon-20 wt pct iron was treated with a slag of CaO-Al₂O₃-SiO₂ at 1600 °C. The effects of slag basicity and oxygen potential on the distribution of phosphorus were studied via changing the slag composition. The critical oxygen potential, at which the maximum removal of phosphorus is obtained, was calculated as 6.3 × 10⁻¹⁸ atm. Normalized distribution and phosphate capacity values were calculated for each slag composition with the purpose of isolating the effect of basicity. In a separate experiment, calcium was also added to the alloy with the purpose of improving the removal efficiency of phosphorus. Results show that the addition of calcium can almost double the partition ratio of phosphorus. The removal rate of phosphorus was quantified via kinetics calculations, and the total mass-transfer rate of phosphorus was estimated to be 8.58 × 10⁻⁷ cm/s.