Study On Phase Equilibria Of Underground Brine In The Sichuan Basin For The Relevant Subsystem Li⁺,K⁺,Ca²⁺,Sr²⁺//Cl^-,Br^-—H₂O

As an important liquid mineral resource in China,underground brine in the Sichuan Basin has abundant resource reserves and is widely distributed.In addition to a large amount of sodium chloride,brine also includes useful mineral elements such as potassium,lithium,rubidium,cesium,strontium,bromine,iodine,and boron.And its content exceeds the industrial comprehensive utilization standard,and has high comprehensive development and utilization value.It can not only be used as an important raw material water for chemical production,but also provide essential raw materials for the new energy field.In order to make rational use of the underground brine resources in the Sichuan Basin,it is necessary to conduct a profound investigation on the characteristic of dissolution and crystallization of ore-forming elements in the brine.The underground brine in the Sichuan Basin is rich in mineral elements such as lithium,potassium,calcium,and strontium.Based on this feature,this paper uses the research method of isothermal solution equilibrium method to conduct experiments to determine the multi-temperature solid-liquid equilibrium of the main quinary,quaternary,and ternary sub-systems subordinate to the multi-component system Li⁺,K⁺,Ca²⁺,Sr²⁺//Cl^-,Br^--H₂O under the conditions of 273,298 and 323 K.The simulation calculation based on the Pitzer model is performed and the solid-liquid composition relationship of the ternary solid solution system is established.In the experiment,the solid-liquid composition of the phase equilibrium of a series of research systems was determined.The Schreinemakers'method was used to determine the equilibrium solid phase.And combined with X-ray powder crystal diffraction,scanning electron microscopy and X-ray energy spectrum analysis,the equilibrium solid phase at the invariant point was further identified.In this study,the phase diagrams were drawn for the experimental and calculated results of phase equilibrium,and analysis and discussion were carried out.The ternary systems studied in this paper include:LiCl-LiBr-H₂O,KCl-KBr-H₂O,CaCl₂-CaBr₂-H₂O,SrCl₂-SrBr₂-H₂O,LiCl-SrCl₂-H₂O,LiBr-SrBr₂-H₂O,LiBr-CaBr₂-H₂O and CaBr₂-SrBr₂-H₂O;the quaternary systems are:Li⁺,Sr²⁺//Cl^-,Br^--H₂O,Li⁺,Ca²⁺//Cl^-,Br^--H₂O,Ca²⁺,Sr²⁺//Cl^-,Br^--H₂O,Li⁺,Ca²⁺,Sr²⁺//Cl^--H₂O,Li⁺,Ca²⁺,Sr²⁺//Br^--H₂O,Li⁺,K⁺,Sr²⁺//Br^--H₂O,Li⁺,K⁺,Ca²⁺//Br^--H₂O and K⁺,Ca²⁺,Sr²⁺//Br^--H₂O.The quinary system is:Li⁺,K⁺,Ca²⁺,Sr²⁺//Br^--H₂O.The isothermal phase diagrams of the ternary systems LiCl-LiBr-H₂O(273 and323 K),KCl-KBr-H₂O(273 K),CaCl₂-CaBr₂-H₂O(273 and 323 K),and SrCl₂-SrBr₂-H₂O(273,298 and 323 K)are classified as phase diagrams with complete solid solution formation.That is,there is continuous solid solution formation in the equilibrium solid phase,but no single salt or double salt formation is found.Its phase diagram has only one isothermal dissolution line,and the invariant point is absent.This isotherm divides the phase diagram region into an unsaturated liquid phase region and a solid solution crystallization region.The isothermal phase diagrams of the ternary systems LiCl-SrCl₂-H₂O(323 K),LiBr-SrBr₂-H₂O(298 K),LiBr-CaBr₂-H₂O(273,298 and 323 K),and CaBr₂-SrBr₂-H₂O(273,298 and 323 K)belong to simple hydrate type.That is,no double salt and solid solution are formed in the equilibrium solid phase,only its crystal hydrate.Due to the partial dehydration of hydrates,the isothermal phase diagrams of the ternary systems LiCl-SrCl₂-H₂O(323 K)and LiBr-CaBr₂-H₂O(298K)both contain two invariant points and are divided into three hydrated salt crystallization areas by three isothermal dissolution lines.The quaternary system Li⁺,Sr²⁺//Cl^-,Br^--H₂O has no invariant point in the isothermal phase diagram at 323 K,and contains two saturation lines which separate the phase region into three crystallization regions.The crystalline solid phase corresponds to three different crystalline solid solutions of Li(Cl,Br)·H₂O,Sr(Cl,Br)₂·2H₂O,and Sr(Cl,Br)₂·6H₂O.There is no invariant point in the isothermal phase diagram of the quaternary system Li⁺,Ca²⁺//Cl^-,Br^--H₂O at 323 K.It contains two solution boundary lines and two solid phase crystallization regions saturated with solid solution Li(Cl,Br)·H₂O and Ca(Cl,Br)₂·2H₂O respectively.The isothermal phase diagram of the quaternary system Ca²⁺,Sr²⁺//Cl^-,Br^--H₂O at 323 K contains an invariant point.Its three solid phase crystallization regions are saturated by Ca(Cl,Br)₂·2H₂O,Sr(Cl,Br)₂·2H₂O and Sr(Cl,Br)₂·6H₂O respectively,and are divided by three isothermal.The isothermal phase diagram of the quaternary system Li⁺,Ca²⁺,Sr²⁺//Cl^--H₂O at 323 K includes an invariant point,four isothermal dissolution lines,and four hydrate crystallization regions.The equilibrium hydrates in the crystallization regions are SrCl₂·6H₂O,SrCl₂·2H₂O,LiCl·H₂O and CaCl₂·2H₂O.The isothermal phase diagram of the quaternary system Li⁺,Ca²⁺,Sr²⁺//Br^--H₂O at 323 K consists of two invariant points,five saturation solution lines,and four hydrate crystalization regions.The equilibrium solid phase forms of the crystalization regions are SrBr₂·6H₂O,SrBr₂·2H₂O,LiBr·H₂O and CaBr₂·4H₂O,respectively.The three isothermal dissolution lines in the isothermal phase diagram of the quaternary system Li⁺,K⁺,Sr²⁺//Br^--H₂O at 273 and 298 K intersect at an invariant point,and the phase regions are divided into three crystallization regions,corresponding to the anhydrous salt KBr and the two hydrated salts SrBr₂·6H₂O and LiBr·2H₂O.The isothermal phase diagrams of the quaternary system Li⁺,K⁺,Ca²⁺//Br^--H₂O at 273 and 323 K all have three isotherms intersecting at an invariant point.While,there are five isotherms and two invariant points at 298 K.The three equilibrium solid phase crystallization regions at 273 K are LiBr·2H₂O,KBr and CaBr₂·6H₂O,and they are LiBr·H₂O,KBr and CaBr₂·4H₂O at323 K.Under the condition of 298 K,due to the partial dehydration of CaBr₂·6H₂O into CaBr₂·4H₂O,four equilibrium solid phase crystallization regions appear,namely:LiBr·H₂O,KBr,CaBr₂·4H₂O and CaBr₂·6H₂O.The isothermal phase diagrams of the quaternary system K⁺,Ca²⁺,Sr²⁺//Br^--H₂O at the three temperature conditions of 273,298 and 323 K all contain only one invariant point,three isothermal dissolution lines and three equilibrium solid phase crystallization regions.The crystalline substances are KBr,CaBr₂·6H₂O and SrBr₂·6H₂O at 273 and 298 K,and those are KBr,CaBr₂·4H₂O and SrBr₂·6H₂O at 323 K.The three-dimensional dry basis phase diagram of the quinary system Li⁺,K⁺,Ca²⁺,Sr²⁺//Br^--H₂O at 273 K has four isotherm dissolution lines,and they intersect at an invariant point,which corresponds to the four crystalline solid phase LiBr·2H₂O,KBr,CaBr₂·6H₂O and SrBr₂·6H₂O.The three-dimensional dry basis phase diagram of the quinary system at 298 K contains two invariant points.The four equilibrium solid phases corresponding to the first invariant point are:LiBr·H₂O,KBr,CaBr₂·4H₂O and SrBr₂·6H₂O.The solid phases corresponding to another invariant point are KBr,CaBr₂·6H₂O,CaBr₂·6H₂O and SrBr₂·6H₂O.The three-dimensional dry basis phase diagram of the quinary system at 298 K consist of two invariant points.The four equilibrium solid phases corresponding to the first invariant point are:KBr,LiBr·H₂O,CaBr₂·4H₂O and SrBr₂·6H₂O.The solid phases corresponding to another invariant point are KBr,CaBr₂·6H₂O,CaBr₂·4H₂O and SrBr₂·6H₂O.The solubility data was used to fit the interaction parameters of mixed ions and the dissolution equilibrium constants of salts that were not reported in the literature.Based on Pitzer model,the solubility in the ternary and quaternary sub-systems under the studied quinary system Li⁺,K⁺,Ca²⁺,Sr²⁺//Br^--H₂O and the related ternary system containing chlorine-bromine solid solution was theoretically predicted.The calculation results show that the deviation between predicted and the experimental results in the quaternary system is larger than that in the ternary system.The calculation results further verify the applicability of the model parameters and the possibility of the Pitzer model in predicting the solubility of high-concentration solutions containing lithium and calcium.The stable phase equilibrium experiments at 273,298,and 323 K were carried out on the sub-systems of the Li⁺,K⁺,Ca²⁺,Sr²⁺//Br^--H₂O quinary system and the related chlorine and bromine coexisting solid solution systems,which enriched the solubility data of the bromine-containing brine system.It reveals the law of dissolution and crystallization of coexisting salts in the research system,and provides important data support for the enrichment of underground brine resources in the Sichuan Basin and the subsequent comprehensive utilization of resources.By regressing the solid-liquid composition relationship of the studied chlorine and bromine coexisting solid solution system,its dissolution characteristics are further described.Using multiple linear regression method,part of the mixed Pitzer parameters and salt dissolution equilibrium constants under the quinary system were fitted,and the parameter database of the Pitzer model was expanded.The accuracy of the fitting parameters and the applicability of the Pitzer model to calculate high-concentration bromine-containing systems are verified,which provides a reference for the solubility calculation of related systems.