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

Qaidam Basin is rich with salt lakes brine in which not only abundant with common ions such as Na,K and Mg,but also with rare ions such as Li,Rb and Cs.In view of the characteristics of brine in Qaidam Basin and the extracting difficulties of rubidium resources,the stable phase equilibrium of the quinary systems Na+,K+,Rb+,Mg2+//SO42--H2O and its Partial Subsystems(Rb+,Mg2+//SO42--H2O,Na+,Rb+//SO42--H2O,Na+,Rb+,Mg2+//SO42--H2O,Na+,K+,Rb+//SO42--H2O)were studied at 298.2 K and 323.2 K in this paper.The data of solubility and physicochemical properties(density and refractive index)of the equilibrium solution of the systems mentioned above were determined,and the space phase diagram,planar projection diagram,multi-temperature phase diagram,water-phase diagram,density vs composition diagram and refractive index vs composition diagram were constructed.The ternary system Rb+,Mg2+//SO42--H2O is of a complex type system at 298.2 K or at 323.2 K,with double salt Rb2SO4·MgSO4·6H2O formed.Its stable phase diagram is composed of two invariant points,three univariate curves and three crystallization fields.Along with the change of temperature,the crystalline form of MgS04 changes from MgSO4·7H2O at 298.2 K to MgSO4·6H2O at 323.2 K.The order of the three crystalline region of salts is Rb2SO4·MgSO4·6H2O>MgSO4·7H2O(MgSO4·6H2O)>Rb2SO4.The ternary system Na+,Rb+//SO42--H2O is of a simple system at 298.2 K or at 323.2 K,without double salt or solid solution formed.Its stable phase diagram is composed of one invariant point,two univariate curves and two crystallization fields.The precipitation form of Na2SO4 is Na2SO4·10H2O at 298.2 K,while that is Na2SO4 at 323.2 K.Salt Rb2SO4 has a strong salting out effect on salt Na2SO4,and the salting out effect is stronger at 298.2 K than 323.2 K.Compared with the phase diagrams of the system at 298.2 K and 323.2 K,it was found that the size of the crystallized zone has changed,with the increase of temperature,the crystallization region of Rb2SO4 decreases means that it is posssiple to separate Rb2SO4 by crystallization method.The quaternary system Na+,Rb+,Mg2+//SO42--H2O is of a complex system at 298.2 K or at 323.2 K,with double salts Rb2SO4·MgSO4·6H2O and Na2SO4·MgSO4·4H2O formed.Its stable phase diagram consists of three invariant points,seven univariate curves and five crystalline regions at these two temperature.The invariant points of the quaternary system are of commensurate invariant points.With the temperature increases in the range from 298.2 K to 323.2 K,the crystalline from of MgSO4 changes from MgSO4·7H2O at 298.2 K to MgSO4·6H2O at 323.2 K,while the crystalline form of Na2SO4 changes from Na2SO4·10H2O(298.2 K)to Na2SO4(323.2 K).With the temperature increases in the range from 298.2 K to 323.2 K,the crystalline region of Rb2SO4 and Na2SO4·MgSO4·4H2O increases,while that of Rb2SO4·MgSO4·6H2O decreases.The quaternary system Na+,K+,Rb+//SO42--H2O is of a complex system at 298.2 K or at 323.2 K,with double salt Na2SO4·3K2SO4 and solid solution[(K,Rb)2SO4]formed.Its stable phase diagram consists of three invariant points,seven univariate curves and five crystalline regions at these two temperatures.Along with the temperature changes,the crystalline form of Na2SO4 changes from Na2SO4·10H2O at 298.2 K to Na2SO4 at 323.2 K.With the temperature increases in the range from 298.2 K to 323.2 K,the crystalline region of Na2SO4·3K2SO4 and[(K,Rb)2SO4]increases,while that of K2SO4 and Rb2SO4 decreases.The stable phase diagram of the quinary system Na+,K+,Rb+,Mg2+//SO42--H2O consists of six invariant points,nineteen univariate curves and nine crystalline regions at 298.2 K or at 323.2 K,and the crystalline form of the salts change with the rise of temperature.At 298.2 K,the crystalline regions are corresponding to K2SO4,Rb2SO4,Na2SO4·10H2O,MgSO4·7H2O,Na2SO4·3K2SO4,Na2SO4 MgSO4·4H2O,Rb2SO4·MgSO4·6H2O,K2SO4·MgSO4·6H2O,[(K,Rb)2SO4];while which are corresponding to K2SO4,Rb2SO4,Na2SO4,MgSO4·6H2O,Na2SO4·3K2SO4,Na2SO4·MgSO4·4H2O,Rb2SO4·MgSO4·6H2O,K2SO4·MgSO4·4H2O,[(K,Rb)2SO4]at 323.2 K.At 298.2 K,the crystalline area of Rb2SO4·MgSO4·6H2O is the largest and Na2SO4·MgSO4·4H2O is the smallest;whereas the crystalline region of Na2SO4·3K2SO4 is the largest and Rb2SO4 is the smallest at 323.2 K.With the temperature increases,the crystalline region of Na2SO4·MgSO4·4H2O,Na2SO4·3K2SO4 and[(K,Rb)2SO4]increases,while that of Rb2SO4·MgSO4·6H2O,K2SO4 and Rb2SO4 decreases.Therefore,in the sulfate system with sodium,potassium,rubidium and magnesium coexisted,the single salts K2SO4 and Rb2SO4 can be separated and extracted by crystallization method.