An Experimental Study On Metastable Phase Equilibrium Of Subsystems Of The Potassium Hexahydric System On Zhabuyesaltlake At273K

Zhabuye Salt Lake, located in Tibet, famous for its abundance resources, such as lithium, potassium, borate, rubidium, and cesium. In order to comprehensive utilize the resource in the Zhabuye Salt Lake, the metastable equilibria of subsystems of the hexahydric system Li⁺, K⁺//Cl^-, CO₃^2-,SO₄^2-, B₄O₇^2- H₂O were studied at273K using an isothermal evaporation method, respectively, which have not been reported in the literature. They are four quinary systems Li⁺//Cl^-, CO₃^2-, SO₄^2-, B₄O₇^2- H₂O, K+//Cl-, CO₃^2-,SO₄^2-, B₄O₇^2- H₂O,Li⁺, K⁺//Cl^-, CO₃^2-, SO₄^2- H₂O and Li⁺, K⁺//Cl-, SO₄^2-, B₄O₇^2- H₂O, four quaternary systems K⁺//Cl^-, SO₄^2-, B₄O₇^2- H₂O, Li⁺, K⁺//Cl^-,SO₄^2- H₂O, Li⁺, K⁺//Cl^-, B₄O₇^2- H₂O,and K⁺, Na⁺//Cl^-, SO₄^2- H₂O.The solubilities, densities and pH values of the equilibrated solutions were determined. The metastable equilibrium phase diagrams, the water-content diagrams, the density vs composition diagrams and the pH values vs composition diagrams of the quaternary systems were plotted the stereo-diagram and projected phase diagrams （saturated with Li₂CO₃or Li2B4O7）, corresponding water content diagrams, density vs composition diagrams, the pH value vs composition diagrams and lithium content diagrams of the quinary systems were constructed.There is no double salt or solid solution formed in the simple quinary system Li⁺//Cl^-, CO₃^2-,SO₄^2-, B₄O₇^2- H₂O. The metastable phase diagram of the system consists of one invariant point, four univariant curves and four crystallization fields. Because of the structure similarity between LiCl and H₂O, salt LiCl has strong salting-out effect and the role of making other hydrate dehydration, with strong salting-out effect onsalt Li2SO₄in this system.The quinary system K⁺//Cl^-, CO₃^2-, SO₄^2-,B₄O₇^2- H₂O and the quaternary system K⁺//Cl^-, SO₄^2-, B₄O₇^2- H₂O are of simple type. No double salt or solid solution formed. The metastable phase diagram of the quinary system consists of one invariant point, four univariant curves and four crystallization fields. Salt K2CO3has strong salting-out effect on the other salts. Due to the large solubility, salt K2CO3has strong effect on the physicochemical properties in the solution of this equilibrium system, which presents alkalescency. Potassium borate is metastabled obviously.Results show that the double salt KLiSO+4is formed in the quinary system Li⁺, K⁺//Cl^-, CO₃^2-,SO₄^2- H₂O and the subsystem Li⁺, K⁺//Cl^-, SO₄^2- H₂O. The stereo-diagram of the quinary system consists of four invariant point, thirteen univariant curves and seven crystallization fields. The projected diagram of this system （saturated with Li₂CO₃） consists of sixcrystallization fields, nine univariant curves and all the four invariant points. The crystallization fields of Li2SO₄H₂O and K2SO₄are larger, while the salt LiCl H₂O and K2CO33/2H₂O have smaller crystallization fields. A lower temperature is necessary for separating pure potassium sulfate and higher temperature is helpful to obtain lithium carbonate.The quinary system Li⁺, K⁺//Cl^-, SO₄^2-, B₄O₇^2- H₂O is of complex type with the double salt KLiSO₄formed. The stereo^-diagram of the quinary system consists of four invariant point, thirteen univariant curves and seven crystallization fields. The projected diagram of this system （saturated with Li2B4O7） consists of six crystallization fields, nine univariant curves and all the four invariant points. Invariant point E1is the commensurate invariant point, while points E2, E3, E4are the incommensurate invariant points. The crystallization field of K₂SO₄is the largest, but the salt LiCl H₂O has smallest crystallization fields.The crystallizated forms of lithium borate and potassium borate are K₂[B₄O₅（OH）₄]₂H₂O and Li[B（OH）₄]₆H₂O, respectively. Borate salt shows obviously metastablity and has complicated dissolution behaviour in aqueous solution. The borate crystallization fields in metastable equilibrium diagram are smaller than that in stability diagram. For the systems mentioned above, it is found that physicochenmical properties of the equilibrium solutions saturated with the LiBO₂·8H₂O and K₂B₄O₇·4H₂O show irregular changes with the solution composition.In the metastable equilibria of subsystems of the hexahydric system Li⁺, K⁺//Cl^-, CO₃^2-, SO₄^2-,B₄O₇^2- H₂O at273K, salts Li₂CO₃and K₂SO₄precipitate easily due to the larger crystallization fields. The crystallization area of Li₂SO₄H₂O has the positive correlation with the concentration of LiCl in the equilibrated solution. Borate only formed single salts in equilibrated solution, which can be separated by isothermal evaporation method. Salts LiCl H₂O and K2CO33/2H₂O have the smaller crystallization zone, thus it is difficult to obtain them only by evaporation method. This feature is very important to extract lithium, potassium and borate from the salt lake brine.