Studies On The Multi-temperature Phase Equilibria Of Systems Containing SrCl₂ And SrBr₂ In The Underground Brine Of Sichuan Basin

The underground brine resources in Sichuan Basin are very advantageous liquid mineral resources,which are rich in potassium,sodium,chlorine,bromine,strontium,lithium,magnesium and other useful components.In order to comprehensively develop the brine resources,reveal the ore-forming process of various salts in the underground brine and the geochemical equilibrium rules of brine migration and transformation,it is necessary to study the multi-temperature solid-liquid phase equilibria of the brine systems.The results of experimental and theoretical phase equilibrium studies can not only guide the comprehensive development and utilization of the brine,provide scientific guidance for the formulation of processing route,but also predict the dynamic equilibrium process of the brine.In addition,there are few reports on the phase equilibria of the coexistence of magnesium and strontium,so it is necessary to study the phase equilibria of the coexistence of magnesium and strontium under multi-temperature conditions.In this paper,in order to understand the thermodynamic behavior of the underground brine system rich in strontium,magnesium and potassium in Sichuan Basin,the stable phase equilibria of the quinary systems Na Cl+KCl+Mg Cl2+Sr Cl2+H2O and Na Br+KBr+Mg Br2+Sr Br2+H2O and related subsystems under multi-temperature conditions have been studied.The equilibrium liquid phase of each system was measured and the crystal substances were identified using chemical analysis and X-ray powder crystal diffraction method,the isothermal phase diagrams were drawn.At the same time,the Pitzer electrolyte solution theory model was used to establish the theoretical model for calculating the solubilities of ternary and quaternary subsystems,and the calculated results were compared with the experimental data,which would lay a foundation for the subsequent thermodynamic phase equilibrium simulation calculations of related systems.No solid solution or double salts were found in the ternary systems Mg Cl2+Sr Cl2+H2O（273 K,288 K）,Na Cl+Sr Cl2+H2O（273 K）and KCl+Sr Cl2+H2O（273 K）under the corresponding temperature conditions,but there was isothermal solubility curve saturated with Sr Cl2·6H2O in three ternary systems,so the phase diagrams of three ternary systems belong to hydrate type I.Through the comparisons of the ternary system Mg Cl2+Sr Cl2+H2O under multi-temperature conditions,it was found that with the increase of temperature,the hydrate of strontium chloride will dehydrate with the addition of magnesium chloride,and the salting out effect of magnesium chloride on strontium chloride is obvious.The ternary systems Na Cl+Sr Cl2+H2O and KCl+Sr Cl2+H2O have the same configurations in the phase diagrams under multi-temperature conditions.The compositions of the liquid phase change regularly with the changes of temperatures.The quaternary system Na Cl+KCl+Sr Cl2+H2O is the same as its subsystems at 273 K and 308 K,it belongs to hydrate type I.By comparing the multi-temperature phase equilibria,it was found that Sr Cl2·6H2O will dehydrate to be Sr Cl2·2H2O with the increase of temperature,except that the compositions of saturated liquid phase change regularly with temperature.In quaternary systems Na Cl+Mg Cl2+Sr Cl2+H2O and KCl+Mg Cl2+Sr Cl2+H2O,Mg Cl2 has strong salting out effect on Na Cl,KCl and Sr Cl2 at 273 K,288 K and 308 K.Due to the influence of temperature and coexisting ion Mg²⁺at 308 K,the crystallization region of Sr Cl2·2H2O appears in the phase diagrams of two quaternary systems at 308 K.At 373 K,the forms of strontium chloride hydrate are Sr Cl2·2H2O and Sr Cl2·H2O.The phase diagram of the quaternary system KCl+Mg Cl2+Sr Cl2+H2O shows the formation of double salts KCl·Mg Cl2·6H2O.Double salts KCl·Mg Cl2·6H2O are produced in the quinary system Na Cl+KCl+Mg Cl2+Sr Cl2+H2O at 273 K,288 K and 308 K,and the crystallization area of KCl is the largest under the condition of Na Cl saturation,indicating that it is the easiest to crystallize from the saturated solution.When the temperature rises to 308 K,Sr Cl2·6H2O dehydrates and exists in the forms of Sr Cl2·6H2O and Sr Cl2·2H2O.In the quinary system at 273 K,the crystallization area of Na Cl is the largest under the condition of KCl saturation,and the crystallization area of anhydrous single salt KCl is the largest in the simplified dry base phase diagram when Sr Cl2 is saturated.The ternary systems Mg Br2+Sr Br2+H2O（273 K,288 K and 308 K）,Na Br+Sr Br2+H2O（273 K）and KBr+Sr Br2+H2O（273 K）in the corresponding research conditions all contain one invariant point and the isothermal solubility curve saturated with Sr Br2·6H2O,so three phase diagrams of the ternary systems all belong to hydrate typeΙ.By comparing the multi-temperature phase equilibria,it was found that the shapes of the phase diagrams of three systems are similar at different temperatures,and the compositions of the saturated liquid phase change regularly with the increase of temperature.Mg Br2 has strong salting out effect on Sr Br2,while Sr Br2 also shows strong salting out effect on Na Br and KBr.At 348 K,solid phase Na Br·2H2O in the system Na Br+Sr Br2+H2O disappears and Na Br is generated.The ternary system Na Br+Mg Br2+H2O has one invariant point at 288 K,Na Br·2H2O dehydrates water and becomes Na Br at 308 K with the continuous addition of magnesium bromide.Therefore,the phase diagram of ternary system Na Br+Mg Br2+H2O at 308 K belongs to hydrate type II.When the temperature rises to 348 K,Na Br·2H2O disappears and only Na Br exists.The quaternary system Na Br+KBr+Sr Br2+H2O has one invariant point at 273K and 288 K.With the increase of temperature,the forms of saturated solid phases are consistent with those of corresponding subsystems,Na Br·2H2O dehydrates and becomes Na Br at 348 K.The phase diagram configurations of three systems Na Br+KBr+Mg Br2+H2O,Na Br+Mg Br2+Sr Br2+H2O and KBr+Mg Br2+Sr Br2+H2O are similar at 273,288 and 308 K,respectively.The area of KBr and Na Br·2H2O in the crystallization region accounts for the largest proportion in the whole phase diagram,which is due to the strong salting out effect of Mg Br2 on KBr and Na Br.The differences are that due to the influences of temperature and coexisting ion Mg²⁺,the quaternary systems Na Br+KBr+Mg Br2+H2O and Na Br+Mg Br2+Sr Br2+H2O have two crystal forms of Na Br·2H2O and Na Br at 308 K,while the quaternary systems Na Br+KBr+Mg Br2+H2O and KBr+Mg Br2+Sr Br2+H2O both have double salts KBr·Mg Br2·6H2O at three temperature conditions.Double salts KBr·Mg Br2·6H2O,hydrated salts Mg Br2·6H2O and Sr Br2·6H2O exist in the quinary system Na Br+KBr+Mg Br2+Sr Br2+H2O at 273 K,288 K and 308 K.When KBr is saturated,the crystallization area of Na Br·2H2O is the largest,and that of KBr·Mg Br2·6H2O is the smallest.At 273 K,under the condition of Na Br or Sr Br2saturation,the phase region of KBr in the dry phase diagram is the largest,indicating that KBr can be separated first.On the basis of the Pitzer electrolyte solution theory model,the unreported mixing ion-interaction parameters（θMg,Sr、ΨMg,Sr,Cl、ΨNa,Sr,Br、ΨK,Sr,Br和ΨMg,Sr,Br）containing strontium chloride at 273 K,288 K and 308 K were fitted by multiple linear regressions,and the correlation equation between the mixing ion-interaction parameters and temperature containing strontium chloride and magnesium chloride was established.The Pitzer equation and particle swarm optimization algorithm were used to simulate and predict the solubilities of the subsystems containing strontium,chlorine and bromine.The experimental and calculated values were compared and discussed.The predicted values were consistent with the experimental values.