In view of ionic theory, a subregular solution model for cations was applied to evaluate the thermodynamic properties of complex slags. The slags studied are Fe_tO-CaO, Fe_tO-SiO₂, Fe_tO-CaO- SiO₂ and Fe_tO-MgO-SiO₂ systems. The influence of slag composition on the activity of iron oxide and the ferric to ferrous iron ratio have been discussed.
It has been confirmed that the subregular solution model is satisfied over the wide composition range in above slag systems. The iso-activity contours of iron oxide in Fe_tO-CaO-SiO₂ and Fe_tO-MgO-SiO₂ slaps in equilibrium with liquid iron at 1600°C have been calculated using the subregular solution model.