Thermodynamic Propertiesand Phase Equilibria of theLiCl–KCl–CH3OH–H2O MixedSolvent System at 308.2 K

Targeting the characteristic of brines from the oil-gas field being rich in lithium and potassium, this study investigates the thermodynamic activity coefficients and phase equilibria of the LiCl–KCl–CH3OH–H2O mixed solvent system at 308.2 K using the cell potential method and isothermal dissolution equilibrium method, respectively. Based on the measured cell potentials of the mixed solvent, the mean activity coefficients of KCl under different ionic strengths (I = 0.05–2.00 mol·kg–1) and LiCl ionic strength fractions (yb = 0, 0.2, 0.4, 0.6, 0.8) are calculated via the Nernst equation. The Pitzer single-salt parameters (β(0), β(1), CΦ) of LiCl and mixed ion interaction parameters (θK,Li and ψK,Li,Cl) in the mixed solvent are fitted by multiple linear regression. Using the fitted Pitzer model parameters, the mean activity coefficient of LiCl, water activity, excess Gibbs free energy, and osmotic coefficient in the mixed solvent are calculated. Additionally, the phase equilibria of the LiCl–KCl–CH3OH–H2O mixed solvent system are measured. The phase diagram contains one invariant point, two univariant solubility curves, and two crystalline regions (KCl and LiCl·H2O). The solubility modeling results based on the Pitzer model are in good agreement with the experimental values. The research findings provide theoretical support for the separation of lithium and potassium resources from brines using mixed solvent phase equilibrium.