A Thermodynamic Model of Sulfur Distribution Ratio between CaO–SiO₂–MgO–FeO–MnO–Al₂O₃ Slags and Molten Steel during LF Refining Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the sulfur distribution ratio between ladle furnace (LF) refining slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating the mass action concentrations of structural units in LF refining slags, i.e., CaO–SiO₂–MgO–FeO–MnO–Al₂O₃ hexabasic slags, based on the ion and molecule coexistence theory (IMCT). The calculated mass action concentrations of structural units in CaO–SiO₂–MgO–FeO–Al₂O₃–MnO slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than mass percentage of components, in the slags can represent their reaction abilities. The calculated total sulfur distribution ratio shows a reliable agreement with the measured or the calculated sulfur distribution ratio between the slags and molten steel by other models under the condition of choosing oxygen activity based on (FeO)–[O] equilibrium. Meanwhile, the developed thermodynamic model for calculating sulfur distribution ratio can quantitatively determine the respective contribution of free CaO, MgO, FeO, and MnO in the LF refining slags. A significant difference of desulfurization ability among free component as CaO, MgO, FeO, and MnO has been found with approximately 87–93 pct, 11.43–5.85 pct, 0.81–0.60 pct and 0.30–0.27 pct at both middle and final stages during LF refining process, respectively. A large difference of oxygen activity is found in molten steel at the slag–metal interface and in bulk molten steel. The oxygen activity in molten steel at the slag–metal interface is controlled by (FeO)–[O] equilibrium, whereas the oxygen activity in bulk molten steel is controlled by [Al]–[O] equilibrium. Decreasing the high-oxygen-activity boundary layer beneath the slag–metal interface can promote the desulfurization reaction rate effectively or shorten the refining period during the LF refining process.