Stable Phase Equilibria Of The Quinary System Li⁺,Na⁺,K⁺,Cs⁺//SO₄^2--H₂O And Its Lithium Containing Subsystems At 298.2 K And 323.2 K

The Qaidam Basin on the Qinghai-Tibet Plateau,which is the main distribution area of magnesium sulfate and chloride salt lake brines in China.Because of its rich mineral resources,it has been given the reputation of"the world of salt".Among the natural brine mineral resources in the Qaidam Basin,besides the common elements sodium（Na）,potassium（K）,magnesium（Mg）,there are also a lot of valuable elements,such as lithium（Li）,cesium（Cs）,rubidium（Rb）,etc.Alkali metal elements are easy to form double salts or solid solutions among themthelves,which causes many difficulties in the extraction and separation of these elements.In order to extract and separate the alkali metal elements from the brines effectively,the stable phase equilibria of the quinary system Li⁺,Na⁺,K⁺,Cs⁺//SO₄^2--H₂O,quaternary system Li⁺,Na⁺,Cs⁺//SO₄^2--H₂O and Li⁺,K⁺,Cs⁺//SO₄^2--H₂O were studied at 298.2 K and 323.2 K using isothermal dissolution method.The solid phase composition was determined by X-ray powder diffraction method.In addition,the solid solution was also verified by Polarizing Microscope,Scanning Electron Microscope,and Energy Dispersive Spectroscopy.The space diagram,projection diagram,physicochemical properties（densityρand refractive index n_D）-composition diagram and water content-composition diagram are drawn base on the measured data.The quaternary system Li⁺,Na⁺,Cs⁺//SO₄^2--H₂O is both of a complex system at298.2 K and 323.2 K,whereas the number of invariant points of these two systems（298.2 K:5;323.2 K:4）,the number of univariate curves（298.2 K:11;323.2 K:9）,crystalline regions（298.2 K:7;323.2 K:6）are different,means the temperature effects the dissolution behaviours of the coexisted salts.At these two temperatures,although the crystalline forms of the single-salt Li₂SO₄·H₂O,Na₂SO₄,Cs₂SO₄ and double-salt Li₂SO₄·Cs₂SO₄,3Li₂SO₄·Cs₂SO₄·2H₂O are the same,the crystalline form of the lithium-sodium double salt has changed from 3Na₂SO₄·Li₂SO₄·12H₂O（298.2K）to Li₂SO₄·Na₂SO₄（323.2 K）,and the single-salt sodium sulfate converted from Na₂SO₄·10H₂O（298.2 K）to Na₂SO₄（323.2 K）.At 298.2 K and 323.2 K,there are two kinds of lithium and cesium sulfate double salts formed,those are Li₂SO₄·Cs₂SO₄and 3Li₂SO₄·Cs₂SO₄·2H₂O,and also the crystalline phase region of the double salts is larger than the crystalline phase region of the single salt,which means that it is difficult to separate Li₂SO₄·H₂O and Cs₂SO₄ directly from the system only by evaporation and crystallization methods,the way to separate lithium and cesium is first precipitated in the form of lithium-cesium double salt and then separated from each other.Temperature also effects the precipitation of double salt,decreasing the temperature will facilitate the precipitation of the double-salt Li₂SO₄·Cs₂SO₄,while increasing the temperature will facilitate the precipitation of the double-salt3Li₂SO₄·Cs₂SO₄·2H₂O.The quaternary system Li⁺,K⁺,Cs⁺//SO₄^2--H₂O is all of a complex system at298.2 K and 323.2 K.The two stable phase diagrams both consist of 5 invariant points,11 univariate curves and 7 crystalline regions.Except for the slightly different size of the crystal phase region of each salt,the crystal precipitation form of each salt is the same.The area order of the crystal phase region is[K,Cs]₂SO₄>K₂SO₄>Li₂SO₄·Cs₂SO₄>Li₂SO₄·K₂SO₄>3Li₂SO₄·Cs₂SO₄·2H₂O>Cs₂SO₄>Li₂SO₄·H₂O.The solid solution[K,Cs]₂SO₄ has the largest crystalline phase area and is the easiest to precipitate,while Li₂SO₄·H₂O and Cs₂SO₄ have the smallest crystalline phase area and the most difficult to precipitate.The temperature rise causes the crystalline phase regions of the double-salts 3Li₂SO₄·Cs₂SO₄·2H₂O,Li₂SO₄·Cs₂SO₄,and the solid solution[K,Cs]₂SO₄ to become larger.Therefore,increasing the system temperature is more conducive to the precipitation of lithium and cesium through the double-salts and solid solution.The quinary systems Li⁺,Na⁺,K⁺,Cs⁺//SO₄^2--H₂O are complex systems containing single-salts,double-salts and solid solution at 298.2 K and 323.2 K.The system has 9 invariant points,28 univariate curves and 12 crystal phase regions at298.2 K,and 8 invariant points,25 univariate curves and 11 crystal phase regions at323.2 K.The temperature has a greater effect on the stable phase diagram.The area of the crystalline phase region of each salt changes significantly,and the type of lithium-sodium double-salt changes.It changes from 3Na₂SO₄·Li₂SO₄·12H₂O（298.2 K）to Li₂SO₄·Na₂SO₄（323.2 K）,Na₂SO₄·10H₂O only exists at 298.2 K.2Li₂SO₄·Na₂SO₄·K₂SO₄,3Li₂SO₄·Cs₂SO₄·2H₂O,Li₂SO₄·Cs₂SO₄,Li₂SO₄·K₂SO₄,Na₂SO₄·3K₂SO₄,Li₂SO₄·H₂O,K₂SO₄ and Cs₂SO₄ are relatively stable,and their crystalline forms do not change with increasing temperature.solid solution[K,Cs]₂SO₄ has the largest crystalline phase region and Cs₂SO₄ has the smallest crystalline phase region.The temperature of the quinary system rose from 298.2 K to323.2 K,which resulted in the solid solution[K,Cs]₂SO₄,the two lithium cesium sulfate double salts 3Li₂SO₄·Cs₂SO₄·2H₂O and Li₂SO₄·Cs₂SO₄ crystal phase area becoming larger and the solubility decreased.