Energy and Resource Saving of Steelmaking Process: Utilization of Innovative Multi-phase Flux During Dephosphorization Process

An increase in the utilization efficiency of CaO, one of the major fluxing agents used in various steelmaking processes, is required to reduce the amount of discharged slag and energy consumption of the process. The authors have intensively focused on the development of innovative dephosphorization process by using so called “multi-phase flux” composed of solid and liquid phases. This article summarizes the research on the above topic done by the authors, in which the formation mechanisms of P₂O₅-containing phase during CaO or 2CaO·SiO₂ dissolution into molten slag, the phase relationship between solid and liquid phases at equilibrium, and thermodynamic properties of P₂O₅-containing phase have been clarified. The reactions between solid CaO or 2CaO·SiO₂ and molten CaO-FeO _x -SiO₂-P₂O₅ slag were observed by dipping solid specimen in the synthesized slag at 1573 K or 1673 K. The formation of the CaO-FeO layer and dual-phase layer of solid 2CaO·SiO₂ and FeO _x -rich liquid phase was observed around the interface from the solid CaO side toward the bulk slag phase side. Condensation of P₂O₅ into 2CaO·SiO₂ phase as 2CaO·SiO₂-3CaO·P₂O₅ solid solution was observed in both cases of CaO and 2CaO·SiO₂ as solid specimens. Measurement of the phase relationship for the CaO-FeO _x -SiO₂-P₂O₅ system confirmed the condensation of P₂O₅ in solid phase at low oxygen partial pressure. The thermodynamics of 2CaO·SiO₂-3CaO·P₂O₅ solid solution are to be clarified to quantitatively simulate the dephosphorization process, and the current results are also introduced. Based on the above results, the reduction of CaO consumption, the discharged slag curtailment, and energy-saving effects have been discussed.