Thermodynamic Assessment of the P₂O₅-Na₂O and P₂O₅-MgO Systems

Knowledge about the thermodynamic equilibria of the P₂O₅-Na₂O and P₂O₅-MgO systems is very important for controlling the phosphorus content of steel materials in the process of steelmaking dephosphorization. The phase equilibrium and thermodynamic data of the P₂O₅-Na₂O and P₂O₅-MgO systems were critically evaluated and re-assessed by the CALPHAD (CAlculation of PHAse Diagram) approach. The liquid phase was described by the ionic two-sublattice model for the first time with the formulas (Na⁺¹)_P(O^-2, PO₃^-1, PO₄^-3, PO_5/2)_Q and (Mg⁺²)_P(O^-2, PO₃^-1, PO₄^-3, PO_5/2)_Q, respectively, and the selection of the species constituting the liquid phase was based on the structure of the phosphate melts. A new and improved self-consistent set of thermodynamic parameters for the P₂O₅-Na₂O and P₂O₅-MgO systems was finally obtained, and the calculated phase diagram and thermodynamic properties exhibited excellent agreement with the experimental data. The difference in the phase composition of invariant reactions from the experimentally determined values reported in the literature is less than 0.9 mol.%. The present thermodynamic modeling contributes to constructing a multicomponent oxide thermodynamic database in the process of steelmaking dephosphorization.