Study On Metastable Phase Equilibria Of The Quinary Reciprocal System Li～+, K～+//Cl～-, CO₃～(2-), B₄O₇～(2-)-H₂O At T=298K

In accordance with the composition of the brine of Zabuye Saline Lake, Tibet,metastable phase equilibria of the quinary reciprocal system Li⁺, K⁺//Cl^-, CO₃^2-,B₄O₇^2-H₂O were studied at 298 K with isothermal vaporization method and Pitzer'stheory, respectively, which have not been reported in the literature. Solubility of eachsalt and physico-chemical properties such as density, viscosity, refractive index, andpH value of the metastable equilibrium solutions in the system were determined.Based on the determined values, the solubility projected-diagram (saturated withLi₂CO₃) of the quinary system, isothermal solubility diagrams of the ternary andquaternary sub-systems, and the physico-chemical property vs composition diagramsof the metastable equilibrium solutions were plotted.For the systems mentioned above, it is found that they are simple-eutectic-typeones with no complex salt and solid solution. Potassium carbonate (K₂CO₃) showsstrong salting-out effect on other salts in the systems containing K₂CO₃. Lithiumchloride shows strong salting-out effect on other salts in the systems containing LiCl.The isothermal solubility diagram of the quinary system Li⁺, K⁺//Cl^-, CO₃^2-,B₄O₇^2--H₂O consists of seven univariant curves, three invariant points, and fivecrystallization regions that correspond to lithium chloride hydrate (LiCl·H₂O),potassium chloride, potashite (K₂CO₃·3/2H₂O), lithium borate quihydrate(Li₂B₄O₇·5H₂O, and potassium borate quadrithydrate (K₂B₄O₇·4H₂O).In the metastable equilibrium phase diagram of the quinary system (saturatedwith Li₂CO₃), the crystallization region of lithium borate mineral is the largest one,the crystallization regions of potassium borate and potassium chloride are larger thanthose of potashite (K₂CO₃·3/2H₂O) and lithium chloride hydrate (LiCl·H₂O). It isfound in the metastable equilibrium phase diagram that:â‘ the crystallizationspeciations of potashite, lithium chloride, potassium borate, and potassium chlorideare the same as those in the stable equilibrium one;â‘¡lithium borate terhydrate andits crystallization region that were found in the stable equilibrium phase diagram arereplaced by lithium borate quihydrate;â‘¢potassium borate crystallization field isdistinctly enlarged, potassium chloride field is also enlarged, lithium borate field isobsviously shrinked, and the areas of potashite and lithium chloride fields are almostthe same as those in the stable equilibrium one. This feature is very important to extract lithium and potassium and borate from the salt lake brine.Using the metastable solubility data of binary and ternary systems at 298 K,ion-interaction parameters of individual salts and mixing ion-interaction parameters inPitzer's equations were fitted by multiple linear regression method. Metastabledissolving equilibrium constants of salts at 298 K were evaluated with Pitzer's model.The solubilities of quaternary and quinary systems were calculated by Pitzer'sequations. Concretely to say, the first step is to establish a simultaneous equationthrough the Pitzer's theory, then solving it. The variable in the equation is molality ofthe ion. The improvement simplex method in the non-restraint optimizationcomputational method was used. The calculation process was programmed withMATLAB (FORTRAN). The prediction results show agreement with experimentalones.