Change of phosphorus-concentrated phase in low basicity steelmaking slag

Recently, interest in the art of dephosphorization in steelmaking has turned toward relatively lower basicity slag saturated with a [2CaO·SiO₂-3CaO·P₂O₅] solid solution rather than CaO-saturated slag. In this work, laboratory experiments were carried out in order to investigate the change of the phosphorus-concentrated phase in low basicity slag. Phosphorus-containing slag was added onto 10 kg of hot metal, and its basicity was lowered through the desiliconization reaction of the hot metal at 1 573 K. EPMA observation of the slag after the experiment showed different mineral phases corresponding to the slag basicity (mass%CaO)/(mass%SiO₂). When basicity was larger than 0.8, the phosphorus-concentrated phase (P-phase) was observed but was different from the solid solution phase in the initial slag. When basicity was lower than 0.8, the P-phase was not observed, and phosphorus was distributed through the slag at a low concentration. These results would reflect decomposition of the P-phase and formation of homogeneous liquid slag. Based on a thermochemical consideration with the phase diagram of the [CaO–SiO₂–FeO] system, the dissolution of the P-phase observed in the present experiment would be due to the slag composition approaching the SiO₂ saturated region and a resulting decrease in CaO activity. From the viewpoint of the phase diagram of the [CaO–SiO₂–P₂O₅] system, lowering the slag basicity to CaO·SiO₂ saturation would come to coexistence of a higher P-phase such as 5CaO·SiO₂·P₂O₅ or even 3CaO·P₂O₅, which would lead to an increase in the driving force for phosphorus transfer from slag to metal (so-called rephosphorization) at lower slag basicity.