Numerical study into gravity separation of phosphorus from BOS slag during solidification

Phosphorus is known to partition to a dicalcium silicate – tricalcium phosphate solid solution (C S–C P) during the solidification of basic oxygen steelmaking (BOS) slag. Typically, C S–C P solidifies first and has a lower density than the remaining liquid slag, suggesting that gravity separation may be possible. This study simulated the cooling behaviour of BOS slag, and predicted the potential for spherical C S–C P particles to float. A lumped parameter heat transfer model based on ordinary differential equations was developed to predict the temporal variations of slag temperature in a 5 mm diameter Pt crucible. Hydrodynamic calculations were also carried out to study the floating behaviour of the spherical particles. The results showed reasonable agreement between predictions and experimental measurements for the slag’s cooling rate. In the separation experiments, coarse C S–C P crystals were observed in the upper section of the crucible, while a glassy slag was observed in the lower section. This is consistent with the hydrodynamic calculations that showed the single particles floating up to the interface. Preliminary approximations were also performed for industrial slag pots which showed the higher possibility of separation for a shallow and insulated slag pot. Further studies are required to confirm the nucleation and growth behaviour in the experiments. 2 3 2 3 2 3 2 3