Thermodynamic models for predicting dephosphorisation ability and potential of CaO–FeO–Fe2O3–Al2O3–P2O5 slags during secondary refining process of molten steel based on ion and molecule coexistence theory

Abstract

Thermodynamic models for predicting phosphorus distribution ratio LP and phosphate capacity of CaO–FeO–Fe2O3–Al2O3–P2O5 slags during secondary refining process of molten steel, according to the ion and molecule coexistence theory (IMCT), i.e. IMCT–LP and IMCT– models, have been developed by coupling with the developed thermodynamic model for calculating the mass action concentrations Ni of structural units or ion couples in the slags, i.e. the IMCT–Ni model, based on the IMCT. The developed IMCT–LP and IMCT– models have been verified with the experimental results from the literature and the predicted results by the summarised five LP models and three models. Besides the total dephosphorisation ability and potential of the slags as LP and , the respective phosphorus distribution ratio LP,i and the respective phosphate capacity of six dephosphorisation products as P2O5, 3FeO·P2O5, 4FeO·P2O5, 2CaO·P2O5, 3CaO·P2O5 and 4CaO·P2O5 can also reliably be predicted by the developed IMCT–LP and IMCT– models. 3CaO·P2O5 accounts for 95.00% of dephosphorisation products com- pared with 5% for 4CaO·P2O5. The comprehensive effect of CaO + FetO, which can be described by the mass action concentration ratio , controls dephosphorisation ability and potential of the slags. An exponentially growing relationship of lg LP against slag oxidisation ability and slag basicity as the simplified complex basicity (%CaO)/[(%P2O5) þ (%Al2O3)] or optical basicity can be established for the slags in a temperature range from 1811 to 1927 K. However, a linearly increasing relationship of lg against slag oxidisation ability and the aforementioned two kinds of slag basicity can be correlated for the slags in the lower temperature range from 1811 to 1828 K and the middle temperature range from 1870 to 1876 K; an inversely asymmetrical U type relationship of lg against the aforementioned parameters can be observed for the slags in the higher temperature range from 1918 to 1927 K. The influences of the aforementioned parameters on dephosphorisation ability are significantly different from those on dephosphorisation potential of the slags.