Prediction model of sulphur distribution ratio between CaO–FeO–Fe 2 O 3 –Al 2 O 3 –P 2 O 5 slags and liquid iron over large variation range of oxygen potential during secondary refining process of molten steel based on ion and molecule coexistence theory

Abstract

A thermodynamic model for predicting sulphur distribution ratio L _S between CaO–FeO–Fe₂O₃–Al₂O₃–P₂O₅ slags and liquid iron, i.e. the ion and molecule coexistence theory (IMCT)–L _S model, has been developed for slags in a large variation range of slag oxidisation ability based on the IMCT. The developed IMCT–L _S model has been verified with measured data of oxygen, phosphorus and sulphur distribution equilibria between the slags and liquid iron from the literature. The results indicate that the desulphurisation reaction is mainly controlled by reaction (CaO)+[S] = (CaS)+[O] in the reducing zone with the optimised standard molar Gibbs free energy change of ; the desulphurisation reaction is dominated by reaction (FeO)+[S] = (FeS)+[O] in the oxidising zone with the reported standard molar Gibbs free energy change of . The influence of CaO on the desulphurisation ability of the slags can be counteracted by that of Fe _t O because higher CaO content corresponds to lower Fe _t O content in the slags. An asymmetric V type relationship between sulphur distribution ratio L _S and the mass action concentration ratio or or or , or the mass percentage ratio (%FeO)/(%CaO) or (%Fe₂O₃)/(%CaO) or (%Fe _t O)/(%CaO) or the simplified complex basicity or optical basicity can be established for the slags equilibrated with liquid iron in a temperature range from 1811 to 1927 K (1538 to 1654°C). The abovementioned mass action concentration ratios or mass percentage ratios of various iron oxides to basic oxide CaO can be recommended to represent the comprehensive effect of iron oxides Fe _t O and basic oxide CaO on desulphurisation ability of the slags.