Thermodynamic models for predicting phosphorus distribution ratio P and phosphate capacity C PO 32 4 of CaO-FeO-Fe 2 O 3 -Al 2 O 3 -P 2 O 5 slags during secondary refining process of molten steel, according to the ion and molecule coexistence theory (IMCT), i.e. IMCT2 P and IMCT2C PO 32 4 4 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.

A thermodynamic model for predicting sulphur distribution ratio L S between CaO-FeO-Fe 2 O 3 -Al O -P O slags and liquid iron, i.e. the ion and molecule coexistence theory (IMCT)-2 3 2 5 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, pho-The results indicate that the desulphurisation reaction is mainly controlled by reactionthe reducing zone with the optimised standard molar Gibbs free energy change of D r G H m;CaS ¼ 147 023:63 2 27:94T ðJ mol 21 Þ; the desulphurisation reaction is dominated by reactionthe oxidising zone with the reported standard molar Gibbs free energy change of D r G H m;FeS ¼ 115 526 2 33:35T ðJ mol 21 Þ. The influence of CaO on the desulphurisation ability of the slags can be counteracted by that of Fe O because higher CaO content corresponds to lower Fe O content in the slags. An asymmetric V type relationship between sulphur distribution ratio L S and the mass action concentration ratio N FeO =N CaO or N Fe 2 O 3 =N CaO or N FeO•Fe 2 O 3 =N CaO or N Fe t O =N CaO , or the mass percentage ratio (%FeO)/(%CaO) or (%Fe 2 O 3 )/(%CaO) or (%Fe O)/(%CaO) or the simplified complex basicity ð%CaOÞ=½ð%Al 2 O 3 Þ þ ð%P 2 O 5 Þ or optical basicity can be established for the slags equilibrated with liquid iron in a temperature range from 1811 to 1927 K (1538 to 16548C). 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 O and basic oxide CaO on desulphurisation ability of the slags.

t O of iron oxides based on the ion and molecule coexistence theory (IMCT). The proposed coupling relationships between dephosphorisation and desulphurisation abilities or potentials of the slags have been verified to be valid using reported data of oxygen, phosphorus and sulphur distribution equilibria between the slags and liquid iron over a temperature range from 1811 to 1927 K (1538-1654 °C) and predicted results of dephosphorisation and desulphurisation abilities or potentials. Not only the proposed coupling relationships between dephosphorisation and desulphurisation potentials but also the correlated coupling relationships between dephosphorisation and desulphurisation abilities for the slags in reducing and oxidising zones have been verified to be indeed independent of slag oxidisation ability.