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

The salt lake in Qaidam Basin is abundant with valuable components such as rubidium,cesium and magnesium and has huge reserves and high exploiting value.In order to exploit and utilize the beneficial elements from salt lake and enrich the thermodynamic data,it is necessary to study the phase equilibria aimed at the component of Qaidam Basin salt lake brine.In this paper,the stable phase equilibrium of some systems were studied at 298.2 K or 323.2 K.The systems include the quinary system Na⁺,Rb⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O,the quaternary system Rb⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O at T=298.2 K,323.2 K,Na⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O at T=298.2 K,Na⁺,Rb⁺,Cs⁺//SO₄^2--H₂O at T=323.2 K and the ternary systems Rb₂SO₄+Cs₂SO₄+H₂O,Cs₂SO₄+MgSO₄+H₂O at T=298.2K,323.2 K.Based on the experimental data,a searies diagrams were constructed.The diagrams are including phase diagram,the space phase diagram,density vs composition diagram,refractive index vs composition diagram.There is a double salt Cs₂SO₄·MgSO₄·6H₂O formed in the ternary system Cs₂SO₄+MgSO₄+H₂O at 298.2 K and 323.2 K,which indicates the system is a complex saturated type.At the two different temperatures,the number of invariant points?four?,univariate curves?three?,and crystalline regions?three?,and the crystalline?Cs₂SO₄·MgSO₄·6H₂O and Cs₂SO₄?have no changes,whereas the crystallization form of salt magnesium sulfate changes from MgSO₄·7H₂O to MgSO₄·6H₂O.The ternary system Rb₂SO₄+Cs₂SO₄+H₂O is of a complex type with solid solution[?Rb,Cs?₂SO₄]formed at 298.2 K and 323.2 K.Its stable phase diagram is composed of 2 invariant points,3 univariate curves,3 single salts crystallization fields and 1 solid solution crystallization fields.With the temperature increases in the range from 298.2 K to 323.2 K,the type of salts crystalline form?Rb₂SO₄,Cs₂SO₄ and[?Rb,Cs?₂SO₄]?has no changes,while the area of crystalline regions decrease.The crystalline area of solid solution[?Rb,Cs?₂SO₄]occupies the majority of the system,which indicates that[?Rb,Cs?₂SO₄]is easily formed between rubidium and cesium sulfate.The quaternary system Rb⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O is of a complex type system no matter at 298.2 K and at 323.2 K,with double salt Rb₂SO₄·MgSO₄·6H₂O,Cs₂SO₄·MgSO₄·6H₂O and the solid solution[?Rb,Cs?₂SO₄]formed.Their stable phase diagrams consist of 4 invariant points,6 crystalline regions and 9 univariate curves.Compared the stable phase diagrams at different temperature?298.2 K,323.2K?,along with the temperature rises,the crystallization form of magnesium sulfate changes from MgSO₄·7H₂O?298.2 K?into MgSO₄·6H₂O?323.2 K?,and the crystallization area of Cs₂SO₄,Rb₂SO₄,[?Rb,Cs?₂SO₄]and Cs₂SO₄·MgSO₄·6H₂O increases,which indicates that higher temperature is beneficial to the formation of Cs₂SO₄,Rb₂SO₄,[?Rb,Cs?₂SO₄]and Cs₂SO₄·MgSO₄·6H₂O.The phase diagram of the quaternary system Na⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O at 298.2K is of a complex type,with double salts Na₂SO₄·MgSO₄·4H₂O and Cs₂SO₄·MgSO₄·6H₂O formed.Comparisons between the stable phase diagrams of this quaternary system at 298.2 K and 323.2 K show that the same double salts are formed,but the crystalline?Na₂SO₄·10H₂O and MgSO₄·7H₂O?and the number of invariant points,univariate curves,and crystalline regions decreased with the increase of temperature.The hydrated salts Na₂SO₄·10H₂O and MgSO₄·7H₂O are only formed at 273.2 K.The quaternary system Na⁺,Rb⁺,Cs⁺//SO₄^2--H₂O is of a complex type system at323.2 K,with the solid solution[?Rb,Cs?₂SO₄]formed.The crystallization field size of salt Cs₂SO₄ is the smallest,while that of[?Rb,Cs?₂SO₄]is the biggest and it occupies about half of the entire crystallization field.Comparing the stable phase diagram of the quaternary system at 298.2 K and at 323.2 K,it is found that the number of invariant points,crystallization regions and univariate curves decreases at323.2 K.Sodium sulfate crystallizes as Na₂SO₄ and Na₂SO₄·10H₂O at 298.2 K,whereas only crystallizes as Na₂SO₄ at 323.2 K.At these two temperature points,the crystallization area of Rb₂SO₄,Cs₂SO₄ and[?Rb,Cs?₂SO₄]has no significant change,means that the change of temperature has no distinct effect on the salt separating.The phase diagram of the quinary system Na⁺,Rb⁺,Cs⁺,Mg²⁺//SO₄^2--H₂O at298.2 K consists of 7 saturation points,21 univariate curves and 9 crystalline regions,while at 323.2 K consists of 5 saturation points,16 univariate curves and 8 crystalline regions.The three double salts Rb₂SO₄·MgSO₄·6H₂O,Cs₂SO₄·MgSO₄·6H₂O and Na₂SO₄·MgSO₄·4H₂O,and one solid solution[?Rb,Cs?₂SO₄]are all formed at 298.2K and at 323.2 K;the change of temperature has no effect on the crystalline form of Rb₂SO₄·MgSO₄·6H₂O,Cs₂SO₄·MgSO₄·6H₂O,Na₂SO₄·MgSO₄·4H₂O and[?Rb,Cs?₂SO₄].Along with the temperature changes,the crystalline region of Cs₂SO₄·MgSO₄·6H₂O,Na₂SO₄·MgSO₄·4H₂O,Rb₂SO₄,Cs₂SO₄,and[?Rb,Cs?₂SO₄]increases.It is indicated that higher temperature is beneficial to single salt Rb₂SO₄,Cs₂SO_4,double salt Rb₂SO₄·MgSO₄·6H₂O,Cs₂SO₄·MgSO₄·6H₂O,and solid solution[?Rb,Cs?₂SO₄]form in the system coexisted of Na⁺?Rb⁺?Cs⁺?Mg²⁺and SO₄^2-.