Studies On Phase Equilibria In The Quinary System Li⁺,Na⁺,K⁺,Sr²⁺//Br^-–H₂O And Partial Subsystems At 308 K

Sichuan Basin is rich in underground brine resources,which contains many components with high utilization value,such as Li⁺,K⁺,Sr²⁺and Br^-,etc.In view of this characteristic,the studies of stable phase equilibria at 308 K were carried out in the quinary system Li⁺,Na⁺,K⁺,Sr²⁺//Br^-–H₂O and partial ternary and quaternary systems containing lithium,strontium and bromine in the underground brines of Sichuan Basin.The research systems are as follows:the ternary systems LiBr-KBr-H₂O,Na Br-SrBr₂-H₂O and KBr-SrBr₂-H₂O;the quaternary systems Na Br-KBr-SrBr₂-H₂O,LiBr-Na Br-KBr-H₂O and LiBr-KBr-SrBr₂-H₂O;the quinary system LiBr-Na Br-KBr-SrBr₂-H₂O?KBr saturated?.Moreover,the comparison experiment of the ternary system LiBr-KBr-H₂O at 273 and 308 K was performed in this work.The isothermal dissolution equilibrium method was used in the above experiments,and ion components in the equilibrium liquid phase were determined by chemical analysis method and flame atomic absorption spectrometry method.Wet solid phase method and X-ray powder diffraction method were investigated for the analysis of the equilibrium solid phases at the invariant points of the ternary systems,while the saturated solid phases at the invariant points of the quaternary systems and the quinary system were analyzed by X-ray powder diffraction method.Thus,the solubility data of each salt and the density data of the solution were obtained when the above research systems reached equilibrium.On the basis of these data,the corresponding stable equilibrium phase diagrams,the density composition diagrams as well as water content diagrams of the quaternary systems and the quinary system were constructed,and the X-ray powder diffraction diagrams of the equilibrium solid phases at the invariant points were also drawn in this paper.In addition,the comparative discussions of the related ternary and quaternary systems at different temperatures as literature reported were conducted,and the phase diagrams of the comparative solubility were plotted in this paper.The experimental results show that the ternary system LiBr-KBr-H₂O at 273 and308 K belongs to the hydrate type I.There is no double salt or solid solution in the saturated solid phases.The two phase diagrams are both composed of one invariant point,two solubility curves and two crystalline regions?where the solids are LiBr·2H₂O and KBr,respectively?.LiBr has a strong salting-out effect on KBr at these two temperatures,that is,with the gradual addition of LiBr,the mass fraction of KBr in the solution gradually decreases until equilibrium.At the same time,the ternary system was compared with the reported system at 323 K,and it is found that with the increase of temperature,LiBr·2H₂O is dehydrated and converted to LiBr·H₂O.The equilibrium phase diagrams of the ternary systems Na Br-SrBr₂-H₂O and KBr-SrBr₂-H₂O at 308 K also belong to the hydrate type?,including one invariant point,two univariate curves as well as two solid phase crystallization regions,which are either a single salt or a single salt hydrate,without complex salt or solid solution.Among them,the ternary system Na Br-SrBr₂-H₂O has two solid phase crystallization regions where the solids are Na Br·2H₂O and SrBr₂·6H₂O,respectively.The solid phase crystallization regions of the ternary system KBr–SrBr₂–H₂O correspond to KBr and SrBr₂·6H₂O.In this paper,the invariant point data of two ternary systems at different temperatures were compared,which indicates the solubility of strontium bromide gradually increases with temperature in the ternary system Na Br-SrBr₂-H₂O,and sodium bromide has the decreasing trend.With the increase of temperature,equilibrium solid phase Na Br·2H₂O dehydrates into anhydrous Na Br.The solubility of strontium bromide raises with temperature in the ternary system KBr–SrBr₂–H₂O,while the solubility data of potassium bromide changs little.The phase diagram of the quaternary system Na Br-KBr-SrBr₂-H₂O belongs to the hydrate type I,the saturated surface of hydrate replaces the saturated surface of corresponding anhydrous salt,and three solubility curves intersect at one invariant point in dry salt phase diagram.Then the phase diagram is divided into three regions,which are the solid-phase crystallization areas of Na Br·2H₂O,KBr and SrBr₂·6H₂O,the crystalline region of KBr is the largest,indicating that its solubility is the smallest.A comparative analysis was conducted in this quaternary system at different temperatures,and it manifests that as the temperature rises,equilibrium solid phase Na Br·2H₂O is dehydrate and changes to anhydrous Na Br.The quaternary systems LiBr-Na Br-KBr-H₂O and LiBr-KBr-SrBr₂-H₂O are two simple systems,which consist of two invariant points,five univariate curves,and four equilibrium solid phase crystallization regions,neither of the two quaternary systems has double salt or solid solution.The quaternary system LiBr-Na Br-KBr-H₂O contains four equilibrium solid phase crystallization zones corresponding to LiBr·2H₂O,Na Br·2H₂O,Na Br and KBr.For the quaternary system LiBr-KBr-SrBr₂-H₂O,the corresponding crystallization fields are LiBr·2H₂O,SrBr₂·6H₂O,SrBr₂·2H₂O and KBr.The area of the KBr crystalline regions in the phase diagrams of these two simple quaternary systems are the largest,indicating that KBr is easily crystallized from a saturated solution to achieve separation.The phase diagram of the quinary system LiBr-Na Br-KBr-SrBr₂-H₂O?KBr saturated?has three invariant points,seven univariate curves,and five equilibrium solid phase crystallization regions:LiBr·2H₂O,Na Br·2H₂O,Na Br,SrBr₂·2H₂O and SrBr₂·6H₂O,there is neither a solid solution nor a double salt in the system.Among them,the crystallization area of LiBr·2H₂O is the smallest,which indicates that its solubility is the largest and it is the most difficult to crystallize and separate from the mixed solution.