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 1573 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.