Metastable Stable Phase Equilibrium Of Quinary System Na⁺//Cl^-，CO₃^2-，SO₄^-2-，B₄O₇^2--H₂O At273K

Zhabuye Salt Lake, located in Tibet, was abundant with lithium, sodium,potassium, borate, rubidium. In order to comprehensive utilize the resource in theZhabuye Slat Lake, the metastable equilibria of subsystems of the quinary system Na⁺//Cl-, CO₃^2-, SO₄^2-, B₄O₇^2--H₂O was studied at273K using an isothermal evaporationmethod, which has not reported in the literature. The solubilities, densities and pHvalues of the equilibrated solutions were determined. Based on the experimental data,the stereo-diagram, the projected phase diagram, corresponding water contentdiagram, density vs composition diagram and the pH value vs composition diagramwere constructed.Results show that the double salt2Na2SO4·Na2CO3is formed in the quinarysystem Na⁺//Cl^-, CO₃²-, SO₄^2-, B₄O₇^2--H₂O. The stereo–diagram of the quinarysystem consists of two invariant points, seven univariant curves and fivecrystallization fields. Five crystallization fields correspond to single salts sodiumchloride （NaCl）, sodium sulfate decahydrate （Na₂SO₄·10H₂O）, sodium carbonatedecahydrate （Na₂CO₃·10H₂O）, sodium tetraborate decahydrate （Na₂B₄O₇·10H₂O） andsodium carbonate-sulfate double salt （2Na₂SO₄·Na₂CO₃）. The crystallization fields ofsodium carbonate septahydrate （Na₂CO₃·7H₂O） and trona （Na₂CO₃·NaHCO₃·2H₂O）.The projected diagram of this system （saturated with NaCl） consists of fourcrystallization fileds, five univariant curves, and two invariant points.Invariant point H1is an commensurate point, which is saturated with four saltsNa₂CO₃·10H₂O+Na₂B₄O₇·10H₂O+2Na₂SO₄·Na₂CO₃+NaCl and an equilibratedsolution, the composition of the corresponding liquid phase is w （Na⁺）=11.63%，w （Cl-）=13.37%，w (CO₃^2-)=2.90%，w (SO₄^2-)=0.95%，w (B₄O₇^2-)=0.80%，w （H₂O）= 70.34%.Invariant point H2is an incommensurate point, which is saturated with four saltsNa₂SO₄·10H₂O+Na₂B₄O₇·10H₂O+2Na₂SO₄·Na₂CO₃+NaCl and an equilibratedsolution, the composition of the corresponding liquid phase is w （Na⁺）=11.92%，w（Cl-）=13.37%， w (CO3^2-)=3.02%， w (SO4^2-)=1.12%， w (B₄O₇^2-)=1.19%， w（H₂O）=69.38%.The J necke index of water and the physicochemical properties of equilibratedsolution change with the J necke index of sodium. The calculated densities usingempirical equations have a maximum relative error of <0.02.