Studies On The Phase Equilibria Of The Quinary System K⁺,Rb⁺,Mg²⁺//Cl^-,Borate-H₂O And Its Subsystems At348K

The Pingluo underground brine, located in the west of Sichuan Basin, belongs tothe marine sedimentary deep brine, abundant with minerals as potassium, rubidium,boron etc. It is well-known that the phase diagram provide a very important basis forthe comprehensive utilization of brine. According to the composition of Pingluounderground brine, the metastable phase equilibria in the ternary system RbCl+MgCl2+H2O and quaternary system K+, Rb+, Mg2+//Cl--H2O at348K wereinvestigated using an isothermal evaporation method. The stable phase equilibria inthe ternary system MgCl2+MgB4O7+H2O, quaternary systems Rb+, Mg2+//Cl-,borate-H2O and K+, Rb+, Mg2+//Cl--H2O and quinary system K+, Rb+, Mg2+//Cl-,borate-H2O were investigated at348K using an isothermal dissolution method.The solubility values and physicochemical properties (densities and refractiveindices) of the equilibrated solution were determined experimentally. On the basis ofthe measured data, the metastable (stable) phase diagrams of the ternary systems, thespace phase diagrams, the planar projection diagrams, and the water content diagramsof the quaternary systems and quinary system were plotted. The diagrams of densityversus composition and refractive index versus composition for the above systemswere also constructed.The ternary system MgCl2+MgB4O7+H2O is of a simple eutectic type, thecorresponding stable phase diagram contains one invariant point, two univariantcurves, and two crystallization fields at348K. The crystallization region ofMgCl2·6H2O is smaller than MgB4O7·9H2O, and the salt MgCl2·6H2O has a strongsalting in effect on salt MgB4O7·9H2O.The ternary system RbCl+MgCl2+H2O is of a complex type, with a double saltrubidium carnallite RbCl·MgCl2·6H2O formed at348K. The metastable phasediagram of this ternary system consists of two invariant points, three isothermalevaporation curves, and three crystallization fields corresponding to single salts RbCl,MgCl2·6H2O and double salt RbCl·MgCl2·6H2O. The salt RbCl·MgCl2·6H2O has thelargest crystallization field in the phase diagram, salt MgCl2·6H2O has the smallestcrystallization filed, which indicates that in this ternary system, RbCl·MgCl2·6H2Ohas the lowest solubility, and MgCl2·6H2O has the largest solubility.The metastable and stable phase diagrams of the quaternary system K+, Rb+, Mg2+//Cl--H2O at348K are all consist of four invariant points, nine univariant curves,and six crystallization fields. And there are two carnallite type double saltsKCl·MgCl2·6H2O, RbCl·MgCl2·6H2O and a solid solution (K, Rb)Cl formed in themetastable and stable phase diagrams of this quaternary system at348K. Comparisons with the metastable phase diagram and stable phase diagram at348Kindicate that the crystallization forms of the salts have not changed, while in themetastable phase diagram, the crystallization zones of RbCl and (K, Rb)Cl decreased,the crystallization zone of KCl enlarged, and the others in this quaternary system haveslightly changed. Comparisons between the metastable phase diagrams at348K andat323K show that the crystallization forms have not changed, while thecrystallization fields have slightly changed. The crystallization zone of (K, Rb)Cldecreased at348K, while the crystallization zones of double salts RbCl·MgCl2·6H2O,KCl·MgCl2·6H2O and single salts RbCl, MgCl2·6H2O, KCl enlarged at348K. Thesetwo comparison results indicate that the increase of temperature is benefit for theseparating of salt KCl from brine in the evaporation process.The quaternary system Rb+, Mg2+//Cl-, borate-H2O belongs to a complex typealong with the double salt rubidium carnallite (RbCl·MgCl2·6H2O) formed at348K.The stable phase diagram consists of three invariant points, seven univariant curvesand five crystallization zones corresponding to four single salts RbB5O8·4H2O,MgB4O7·9H2O, RbCl, MgCl2·6H2O and a double salt RbCl·MgCl2·6H2O. With aview to the crystallization zones, the salt MgCl2·6H2O has the smallest crystallizationfield and the salt RbB5O8·4H2O has the largest crystallization in the phase diagram,which presents the salt MgCl2·6H2O has the largest solubility and the satlRbB5O8·4H2O has the smallest solubility among the coexisting salts.The quinary system K+, Rb+, Mg2+//Cl-, borate-H2O is of a complex type, thereare two carnallite type double salts: potassium carnallite KCl·MgCl2·6H2O, rubidiumcarnallite RbCl·MgCl2·6H2O and solid solution (K, Rb)Cl formed at348K. Thecrystalloid forms of potassium borate, rubidium borate and magnesium borate of thisquinary system at348K is K2B4O7·4H2O, MgB4O7·9H2O, RbB5O8·4H2O,respectively. The planar projected diagram (saturated with salt KCl) consists of twoinvariant points, five univariant curves and four crystallization zones corresponding toK2B4O7·4H2O, MgB4O7·9H2O, KCl·MgCl2·6H2O and (K, Rb)Cl. The solid solution(K, Rb)Cl has the largest crystallization field and the double salt KCl·MgCl2·6H2Ohas the smallest crystallization field. The planar projected diagram (saturated with saltRb2B4O7) consists of four invariant points, nine univariant curves and sixcrystallization fields. The crystallization field is the largest for MgB4O7·9H2O, thesecond for (K, Rb)Cl and the smallest for MgCl2·6H2O.