Thermodynamic study on the formation of Mg(Al_1-x,Fe_x)₂O₄ between single-crystal MgAl₂O₄ and Fe_tO-containing slag at 1550°C

Herein, the effects of Fe_tO content and basicity (CaO/Al₂O₃) on MgO solubility in calcium aluminate slag were evaluated. The experimental results revealed that the dependence of MgO solubility on basicity reversed across the equilibrium region as the phase changed from MgO to spinel. Meanwhile, a dense layer of Mg(Al_1-x,Fe_x)₂O₄ solid solution formed at the interface between the single-crystal MgAl₂O₄ and Fe_tO-containing MgO-saturated slag. Thermodynamic evaluation of the spinel solid solution formation indicated that the solid solubility of ferric ions (Fe³⁺) in the reaction layer was dominated by the redox equilibria of ferrous (Fe²⁺) and ferric ions in the slag. The standard Gibbs free energy change (ΔG°) of the formation of Mg(Al_1-x,Fe_x)₂O₄ solid solution was derived considering both the energy change in mixing (ΔG°_Mix) from the configurational change and the cation distribution owing to the inversion in the spinel, as well as a chemical reaction (ΔG°_Rxn) with the Fe_tO-containing slag. The dissolution mechanism via the substitution of cations, as inferred from these calculations, is discussed in detail.