Studies On The Stable Phase Equilibria Of The Quinary System Na⁺,K⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O At 298.2K And 323.2K

The Qaidam Basin is extremely rich in salt lakes abundant with valuable components,such as Li⁺,Na⁺,K⁺,Cs⁺,Mg²⁺etc,and has a high scientific research value and economic exploiting value.The thermodynamic behaviors of the brine is complicated,with double salt or solid solutions formed,which makes the extracting elements from brine difficult.In order to utilize the saline lake brine comprehensively,it is necessary to study the phase equilibria of Qaidam Basin brine.The component solubilities,densities and refractive indices of the equilibrated solution in the quinary system Na⁺,K⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O were measured with the isothermal dissolution equilibrium method at 298.2 K and 323.2 K.Based on the experimental data,the space phase diagram,the plane projection phase diagram,density vs composition diagram,and refractive index vs composition diagram were constructed.There is no double salts or solid solution formed in the ternary system Na⁺,Cs⁺//SO₄^2--H₂O at 298.2 K and 323.2 K.Because the crystalline form of sodium sulfate is greatly affected by temperature,there are significant differences in the stable phase diagrams of the ternary system Na⁺,Cs⁺//SO₄^2--H₂O at 298.2 K and 323.2 K.The ternary system Na⁺,Cs⁺//SO₄^2--H₂O is of the hydrate type II,with the dissolution curves of Na₂SO₄·10H₂O and Na₂SO₄ at 298.2 K.While it is belong to simple system with only Na₂SO₄ formed at 323.2 K,its phase diagrams is comprised of one invariant points,two univariant curves and two crystallographic zones correspond with salt Na₂SO₄ and Cs₂SO₄.The ternary system K⁺,Cs⁺//SO₄^2--H₂O is of complex saturated type with solid solution[?K,Cs?₂SO₄]formed at 298.2 K and 323.2 K.The stable phase diagram is similar,contained two cosaturated points,three monovariant curves,and three crystallization fields.The crystallization area of[?K,Cs?₂SO₄]is the largest phase area at 298.2 K and 323.2 K,which indicatd that the solid solution[?K,Cs?₂SO₄]has formed easily in the sulphate-type brine,furthermore,the crystallization areas of[?K,Cs?₂SO₄]increased with the increase of temperature.The quaternary system Na⁺,K⁺,Cs⁺//SO₄^2--H₂O is of complex system with solid solution[?K,Cs?₂SO₄]and double salts Na₂SO₄·3K₂SO₄ formed at 298.2 K and323.2 K.The crystalline form of sodium sulfate has changed from Na₂SO₄·10H₂O to Na₂SO₄ with the rise of temperature.Comparisons between the stable phase diagrams at 298.2 K and 323.2 K,the crystallization form of[?K,Cs?₂SO₄]and Na₂SO₄·3K₂SO₄ are remaind the same,whereas the invariant points,univariate curves and crystalline regions have decreased.The quaternary system Na⁺,K⁺,Mg²⁺//SO₄^2--H₂O is of complex system with fthree types of double salts?Na₂SO₄·MgSO₄·4H₂O,K₂SO₄·MgSO₄·6H₂O and Na₂SO₄·3K₂SO₄?formed.The phase diagram are composed of four quaternary saturated points,nine univariate curves and six crystallographic zones at 298.2 K and323.2 K.With the temperature increaseing,the crystalline form of the hydrated salt changed.The hydrated salts K₂SO₄·MgSO₄·6H₂O,Na₂SO₄·10H₂O and MgSO₄·7H₂O losed crystal water and turn into K₂SO₄·MgSO₄·4H₂O,Na₂SO₄ and MgSO₄·6H₂O affected by the temperature respectively,whereas the crystallization form of crystals Na₂SO₄·MgSO₄·4H₂O and Na₂SO₄·3K₂SO₄ has no changes with the change of temperature.Moreover,the crystalline regions of the double salts Na₂SO₄·MgSO₄·4H₂O and Na₂SO₄·3K₂SO₄ increased with increasing temperature,which indicated that the elevated temperature is favorable for its formation.The quaternary system Na⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O is of complex type system,with two double salts Na₂SO₄·MgSO₄·4H₂O and Cs₂SO₄·MgSO₄·6H₂O formed at323.2 K.Its stable phase diagram contains three invariant points,seven isothermal dissolution curves and five crystallization regions.The single salt Cs₂SO₄ has the highest solubility,and the phase region is the smallest.The Cs₂SO₄·MgSO₄·6H₂O phase region has the largest area,and it is the easiest to reach the saturated crystallization from the mixed solution.Furthermore,comparisons the stable phase diagrams at 298.2 K and 323.2 K show the hydrate form of Na₂SO₄ has changed and the number of invariant points,univariant curves and zones has decreased with the rise of temperature.The quinary system Na⁺,K⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O is of complex type system,with four double salts and one solid solution formed.Comparisons between those phase diagrams at 298.2 K and 323.2 K show that the crystal forms of Na₂SO₄·MgSO₄·4H₂O,Na₂SO₄·3K₂SO₄,Cs₂SO₄·MgSO₄·6H₂O and[?K,Cs?₂SO₄]have no changes with the increase of temperature,except for K₂SO₄·MgSO₄·6H₂O.In the systems contained with salt K₂SO₄ and Cs₂SO₄,whether it is ternary system,quaternary system or quinary system,there is[?K,Cs?₂SO₄]always formed.The crystallization area of[?K,Cs?₂SO₄]is large,which suggested that the solid solution[?K,Cs?₂SO₄]formed easily in sulfate type water salt system containing potassium and cesium.