Studies On The Metastable Phase Equilibria Of The Quinary System Li⁺,K⁺,Rb⁺,Cs⁺// Cl^--H₂O And It Subsystems

Pingluo brine is one of the most abundant areas of underground brine,and its reserves are very enormous.It not only contains the salt NaCl,but also contains different concentrations of other useful components,such as K+,Br-,I-,B3+,Li⁺,Br-,Rb+ and so on,which have broad prospects.Therefore,the rational use and development of brines can increase the quantity of lithium,potassium,rubidium and cesium meantime provides a series of inorganic chemical products.In accordance with the composition characteristics of Pingluo underground brine,the metastable phase equilibria of the quinary system Li⁺,K⁺,Rb⁺,Cs⁺// Cl^--H₂O at 298 K,and its quaternary subsystem K⁺,Rb⁺,Cs⁺// Cl^--H₂O at 298 K and 323 K,Li⁺,K⁺,Rb⁺,Cs⁺// Cl^--H₂O at 298 K,323 K and 348 K were investigated using an isothernal evaporation method.According to the measured data of solubility and physicochemical properties,the phase diagrams,and the diagrams of water content and physicochemical properties vs composition of the system mentioned above were constructed.The quaternary system Li⁺,K⁺,Rb⁺,Cs⁺// Cl^--H₂O belongs to a complex system at 298 K,323 K and 348 K,and its phase diagram is composed of two invariant points,five univariate curves and four crystal phase regions.The four crystallization fields correspond to the single salt LiCl�H₂O,KCl,CsCl and the solid solution [?K,Cs?Cl].In general,the temperature and coexisting ions effect the presence of solid solution.In this quaternary system,the solid solution [?K,Cs?Cl] is extremely sensitive to temperature.Along with the temperature increases,the solubility of single salt decreases,while the solubility of [?K,Cs?Cl] solid solution increases,shown in the phase diagram with the LiCl�H₂O,KCl and CsCl crystallization area increase.Among them,the crystallization area of KCl is obviously increased,and the crystal area of the solid solution [?K,Cs?Cl] is greatly reduced.It shows that the increase of temperature is conducive to the precipitation of single salt.The quaternary system K⁺,Rb⁺,Cs⁺// Cl^--H₂O belongs to a complex system at 298 K and 323 K,and its phase daigram is made up of two invariant points,five univariate curves,and six crystal phase regions,containing the solid solution [?K,Rb?Cl],[?K,Cs?Cl] and [?Rb,Cs?Cl].Comparisons between the metastable phase diagrams of the system 298 K and 323 K show that the crystalline forms of salts are the same,whereas the size of the crystalline regions is different.The system belongs to temperature-sensitive type,especially the solubility of solid solution is greatly affected by temperature.With the increase of temperature,the crystallization area of [?K,Cs?Cl],[?Rb,Cs?Cl] and increase obviously,whereas the solubility of [?K,Rb?Cl] increases obviously,leading to a significant decrease in the crystalline area on the phase diagram.Therefore,the changes of temperature has greatly affect on the solubility and the precipitation of solid solution,and even affect the trend of isothermal evaporation curve.The metastable phase diagram of this complex quinary system Li⁺,K⁺,Rb⁺,Cs⁺// Cl^--H₂O at 298 K consists of five invariant points,fourteen univariant curves and seven crystallization zones.Seven crystalline forms of salts correspond to single salts potassium chloride?KCl?,caesium chloride?CsCl?,rubidium chloride?RbCl?,lithium chloride monohydrate?LiCl�H₂O?,and solid solution [?K,Rb?Cl],[?K,Cs?Cl] and [?Rb,Cs?Cl].In this system,K,Rb and Cs belong to the alkali metal group,has the similar ionic radius and easy to form solid solution among them chloride,which lead to form solid solution [?K,Rb?Cl],[?K,Cs?Cl] and [?Rb,Cs?Cl],respectively.In the projected phase diagram saturated with salt LiCl,the crystallization areas of salts is in the order KCl>[?K,Rb?Cl]>RbCl>CsCl.In the projected phase diagram saturated with salt RbCl,the crystal areas of salt is in the order [?K,Rb?Cl]>[?Rb,Cs?Cl] >CsCl>LiCl�H₂O.