Metastable Equilibria Of The Quaternary System(Li⁺, Mg²⁺//Cl^-, SO₄^2--H₂O) At75℃

Salt lakes in China are characterized by huge amounts, large areas, rich resourcesand complete types, and it is the important treasury of inorganic salts. It is well knownthat the metastable equilibria and phase diagram of salts lake brine systems playimportant roles in the exploring of salts lakes resources.In the thesis, the metastable equilibria of the quaternary system (Li⁺, Mg²⁺//Cl-,SO₄^2--H₂O) at75℃and the ternary systems （LiCl-Li₂SO₄-H₂O） and （MgCl₂-MgSO₄-H₂O） at75℃have been studied using the isothermal evaporation method. Solubilitiesand physicochemical properties including density and pH in those mentioned systemswere determined experimentally. According to the experimental data, the phasediagrams of the ternary systems and the dry-salt diagram, water-phase diagram of thequaternary system were drawn, respectively. The diagrams of physicochemicalproperties versus compositions in the metastable ternary and quaternary systems wereall plotted.In the metastable phase diagram of the ternary system （LiCl-Li₂SO₄-H₂O） at75℃,there is one invariant point, two uninvariant curves, and two crystallizationzones corresponding to lithium sulfate monohydrate （Li₂SO₄·H₂O） and lithiumchloride monohydrate （LiCl·H₂O）. It is belonging to a hydrate type I, and neitherdouble salts nor solid solutions were formed. The density value and pH value of themetastable equilibrium solution change regularly with the increasing composition oflithium chloride in the solution, the former achieving one maximum value at theinvariant point and the latter achieving one minimum value at the invariant point.Compared with the stable and metastable diagram at the same temperature of75℃, itwas found that the metastable phenomenon of the ternary system is not obvious.In the metastable phase diagram of the ternary system （MgCl₂-MgSO₄-H₂O） at75℃,there is three invariant points, four uninvariant curves, and four crystallization zones corresponding to hexahydrite （MgSO₄·6H₂O）, tetrahydrite （MgSO₄·4H₂O）,kieserite （MgSO₄·H₂O） and bischofite （MgCl₂·6H₂O）. It is belonging to a hydrate typeI, and neither double salts nor solid solutions were formed. The density value and pHvalue of the metastable equilibrium solution change regularly with the increasingcomposition of magnesium sulfate in the solution. In the diagram of pH value versuscomposition, it achieves the maximum value in the invariant point. Compared withthe stable and metastable diagram at the same temperature of75℃, it indicates thatthe metastable phenomenon of magnesium sulfate in the ternary system is obvious. Inthe metastable phase diagram, obviously, the area of bischofite is enlarged, and thearea of kieserite is decreased, while the two small crystallization zones of hexahydriteand tetrahydrite are presented.In the metastable dry-salt diagram of the quaternary system (Li⁺, Mg²⁺//Cl-,SO₄^2--H₂O) at75℃, thereis five invariant points, eleven univariant curves, and sevencrystallization zones corresponding to hexahydrite, tetrahydrite, kieserite, bischofite,lithium sulfate monohydrate （Li₂SO₄·H₂O）, lithium chloride monohydrate （LiCl·H₂O）and lithium carnallite （LiCl·MgCl₂·7H₂O）. It was found that there is no solid solutionexcept one double salt of lithium carnallite existed in the reciprocal quaternary system.Meanwhile, the physicochemical properties （density value and pH） in the metastableequilibrium solution are in regular changes with the increasing Janecke index of Li⁺.In the diagram of pH value versus composition, one of the invariant points （E1）achieves the maximum value. Compared with the stable and metastable diagram at thesame temperature of75℃, it indicates that the metastable phenomenon ofmagnesiumsulfate in the quaternary system is obvious tremendously. In the metastable phasediagram, the area of lithium sulfate monohydrate is enlarged obviously, and the areaof kieserite is decreased while the two small crystallization zones of hexahydrite andtetrahydrite are presented.On the basis of Pitzer model and its extended HW model, the predictivesolubilities of the metastable equilibrium quaternary system (Li⁺, Mg²⁺//Cl-,SO₄^2--H₂O) at50℃were done. A comparison of the calculted and experimental datashows that they agree well. Meanwhile, a comparison of the experimental metastablediagram at75℃and the caculated metastable diagram of thequaternary system (Li⁺,Mg²⁺//Cl-, SO₄^2--H₂O) were also done.