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 L_P 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–L_P and IMCT– models, have been developed by coupling with the developed thermodynamic model for calculating the mass action concentrations N_i of structural units or ion couples in the slags, i.e. the IMCT–N_i model, based on the IMCT. The developed IMCT–L_P and IMCT– models have been verified with the experimental results from the literature and the predicted results by the summarised five L_P models and three models. Besides the total dephosphorisation ability and potential of the slags as L_P and , the respective phosphorus distribution ratio L_P,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–L_P and IMCT– models. 3CaO·P₂O₅ accounts for 95.00% of dephosphorisation products com- pared with 5% for 4CaO·P₂O₅. The comprehensive effect of CaO + Fe_tO, which can be described by the mass action concentration ratio , controls dephosphorisation ability and potential of the slags. An exponentially growing relationship of lg L_P 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.

Keywords:

• Phosphorus distribution ratio
• Phosphate capacity
• Dephosphorisation mechanism
• Thermodynamic model
• CaO–FeO–Fe₂O₃–Al₂O₃–P₂O₅ slags
• Ion and molecule coexistence theory
• Secondary refining of molten steel
• Mass action concentration
• Structural units
• Ion couples
• Slag oxidation ability
• Fe_tO activity

Acknowledgement

This work is supported by the National Natural Science Foundation of China (NSFC) by grant no. 51174186.