Studies On The Metastable Phase Equilibria Of The Quinary System Li⁺,K⁺,Rb⁺,Mg²⁺//Cl^--H₂O₂at98K、348K

The underground brine in Pingluo, located in the western of Sichuan province,belongs to one of the three big deep underground brines in China and is abundant withpotash and rubidium salts, whose mineral content is considered the first in Asia andthe second in the word. The rational exploitation and comprehensive utilization of theunderground brine are helpful to provide good quality of potassium, rubidium andlithium chemical products for our country.The metastable phase equilibria of the quinary system Li+, K+, Rb+, Mg2+//Cl--H2O at298K and348K, and one subsystem Li+, K+, Mg2+//Cl--H2O at348K wereinvestigated using an isothermal evaporation method．The densities and refractiveindices of the equilibrated solution were also determined experimentally. On the basisof the obtained experimental data, the space diagrams of the quinary system at298Kand at348K, and the planar projection diagrams (saturated with KCl or LiCl),potassium content diagrams, and lithium content diagrams, and the metastable phasediagram of the quaternary system, corresponding water content diagram were poltted.And the diagrams of the density and refractive index against composition were alsoconstructed.The quaternary system Li+, K+, Mg2+//Cl--H2O at348K is of a complex type,with two carnallite type double salts LiCl·MgCl2·7H2O and KCl·MgCl2·6H2O formed.There are five salts, seven unvariant curves, and three invariant points included in thismetastable phase diagram. The five salts are K-carnallite (KCl·MgCl2·6H2O), Li-carnallite (LiCl·MgCl2·7H2O), lithium chloride monohydrate (LiCl·H2O), andpotassium chloride (KCl), bischofite (MgCl2·6H2O). Comparisons among the stablephase diagram at298K, the metastable phase diagrams at298K and at348K of thisquaternary system show that all the crystalline forms of salts in this system are thesame, whereas the crystallization areas of salts are changed. With the temperatureincreasing, the crystallization area of KCl is enlarged obviously, which shows that theincrease of temperature is benefit for the seperating of potassium chloride.The quinary system Li+, K+, Rb+, Mg2+//Cl--H2O at298K and348K are ofcomplex type, with three carnallite type double salts K-carnallite (KCl·MgCl2·6H2O),Li-carnallite (LiCl·MgCl2·7H2O), Rb-carnallite (RbCl·MgCl2·6H2O) and one solidsolution (K, Rb)Cl formed. The metastable phase diagrams of this system at298K and348K are consisted of six invariant point, eighteen univariant curves and eightcrystallization fields. Eight crystalline forms of salts correspond to four single saltslithium chloride monohydrate (LiCl·H2O), and potassium chloride (KCl), bischofite(MgCl2·6H2O), rubidium chloride (RbCl), three carnallite type double salts Li-carnallite (LiCl·MgCl2·7H2O), Rb-carnallite (RbCl·MgCl2·6H2O), and K-carnallite(KCl·MgCl2·6H2O), and one solid solution (K, Rb)Cl, respectively. Compared themetastable phase diagrams of the quinary system Li+, K+, Rb+, Mg2+//Cl--H2O at298K,323K and348K, results show that all of the crystalline forms of salts are notchanged, meanwhile the crystallization areas of salts are changed unobviously. Thecrystallization area of (K, Rb)Cl salt is the largest among the eight salts at differenttemperatures, which shows that it is difficult to separate potassium and rubidiumchloride only by evaporating-crystallization method.