Phase equilibria in the ternary systems LiCl–LiBr–H2O and LiCl–LiBr–CH3OH at 298 K

Abstract

Oil gas field brine is a dominant liquid mineral resource in Sichuan Basin, and its rational development and utilization is of great significance to the national economy. In order to explore the physicochemical properties of brine and provide theoretical guidance for the comprehensive utilization of brine, the isothermal solubilities of the ternary systems LiCl − LiBr − H2O and LiCl − LiBr − CH3OH at 298 K were determined by employing the isothermal dissolution equilibrium method. Based on the measured composition of the liquid phase, wet solid phase and equilibrium solid phase, isothermal phase diagrams of the two systems at 298 K were drawn. The results show that according to the phase diagram of the ternary system LiCl − LiBr − H2O at 298 K, there is no single salt and double salt formation, but the crystalline region precipitated by the solid solution occupies all the crystalline space. The phase region is divided by a solubility boundary line into two parts, namely the liquid phase unsaturated region located below the isotherm and the completely mutually soluble solid solution crystallization region above it. Therefore, the two single salts LiCl and LiBr could not be successfully separated with the guidance of this phase diagram. The ternary system LiCl − LiBr − CH3OH at 298 K has single salt and solid solution formed, but no double salt determined. The phase diagram of the ternary system LiCl − LiBr − CH3OH consists of an invariant point, two isothermal solubility lines, and two crystallization regions precipitated with Li(Cl, Br) and LiCl, respectively. Using the phase diagram as a guiding tool for phase separation, LiCl can be acquired by crystallization and filtration operations, but it cannot be achieved for LiBr. In this work, a combination of wet residue method and XRD is used to identify the equilibrium solid phase. Selecting the particle swarm optimization algorithm, the solubility predictions of the ternary systems LiCl − LiBr − H2O and LiCl − LiBr − CH3OH at 298 K were achieved through the well-established Pitzer model. The solubility simulation results satisfactorily matched with the experimental values, indicating that the parameters selected for the simulation have strong applicability.