Measurements and Thermodynamic Model on the Solid–Liquid Phase Equilibria of the Ternary System LiBr–Li2SO4–H2O at Multitemperatures

The solid–liquid phase equilibria of the ternary system LiBr–Li₂SO₄–H₂O at 273.15, 288.15, and 308.15 K were investigated by the isothermal dissolution equilibrium method. The equilibrium solid phases were identified by X-ray powder crystal diffraction. The isothermal phase diagrams of the ternary system at different temperatures were drawn in detail. The equilibrium phase diagrams of the ternary system at 273.15, 288.15, and 308.15 K have one invariant point, two univariate curves, and two crystallization fields (corresponding to LiBr·2H₂O and Li₂SO₄·H₂O), respectively. The results show that LiBr has a strong salting-out effect on Li₂SO₄, and the solubilities of LiBr will be greatly improved with an increase in temperature. Furthermore, multiple linear regression and programming analysis are used to fit the single salt parameters (β⁽⁰⁾, β⁽¹⁾, and C^φ) and the mixed ion interaction parameters (ψ_{Li⁺,Br^–,SO4^2–}) of lithium salt at 288.15 and 308.15 K. The Pitzer model and particle swarm optimization (PSO) were used to predict the solubilities in the ternary system at multitemperatures, and the phase diagrams of calculation and experiment are drawn accordingly. The experimental results and model predictions are in good agreement, indicating that fitted ion interaction parameters in this work of the Pitzer model have good applicability.