Modeling the dependence of alumina solubility on temperature and melt composition in cryolite-based melts

The solubility of alumina in NaF-AlF₃ melts was calculated and modeled thermodynamically for the temperature range of 1240 to 1300 K (967 °C to 1027 °C). The solute complexes of alumina in the cryolite melts were identified to be Na₂Al₂OF₆ (acidic solute), Na₂Al₂O₂F₄ (neutral solute), and Na₄Al₂O₂F₆ (basic solute). The assumption that the oxygen-free solute species in solution were Na₃AlF₆ and NaAlF₄ was supported by the modeling results. The equilibrium constants for the formation reactions of the solutes were calculated and the corresponding ΔG _f ⁰ values were evaluated as a function of temperature. The interaction derivatives (∂ ln a _NaF/∂x _add, ∂ ln a _NaF/∂x _add, and ∂ ln a _AlF3/∂x _add) for small additions of LiF, CaF₂, and MgF₂ to the NaF-AlF₃-Al₂O₃ ternary system were also estimated as a function of temperature and melt composition.