The activity coefficients of MnO and FeO in an FeO-MnO-MgO- P 2O5-SiO2 (-CaO) slag system were measured on the basis of the equilibrium between Ag and molten slag at 1 673 K under a controlled atmosphere. On the basis of experimental results, the activity coefficients of MnO and FeO in this multicomponent slag were evaluated using a regular solution (RS) model, FactSage, and an empirical formula. In the case of the RS model, the interaction energies and conversion factors to fit the calculated values to experimental results were reassessed. By the used of the empirical formula, FactSage and the regular solution model of present work, the activity coefficient ratio of MnS and FeS in a Fe-Mn-O-S matte that was equilibrated with a FeO-MnO-MgO-P2O5-SiO2 slag system was evaluated. When the RS model was used to calculate the γ MnO/γ FeO ratio, the γ MnS/γ FeS ratio decreased slightly with an increase in the XMnS/XFeS ratio. In contrast, when the empirical formula and FactSage were used, the γ MnS/γ FeS ratio was almost constant when the XMnS/XFeS ratio was increased.
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