Measurements and predictions of phase equilibria in the quaternary system KBr + MgBr2 + SrBr2 + H2O at 273 K and 308 K

The underground brines in Sichuan Basin of China are rich in potassium, magnesium, strontium, bromine. Consequently, the studies of phase equilibria for the underground brines are of great significance. The phase equilibria of the quaternary system KBr + MgBr2 + SrBr2 + H2O at 273 K and 308 K are determined by the isothermal dissolution equilibrium method in this work. Firstly, the equilibrium liquid phase compositions at two temperatures were measured by chemical analysis. Secondly, the corresponding solid phases were identified by X-ray powder diffraction method. As a result, the corresponding phase diagrams were drawn based on the obtained data. The equilibrium phase diagrams all include four crystallization fields (SrBr2·6H2O, MgBr2·6H2O, KBr·MgBr2·6H2O and KBr), five isothermal solubility curves and two invariant points, which present the same trend at the two temperatures. Furthermore, the mixed ion interaction parameters (θMg,Sr, φK,Sr,Br, φMg,Sr,Br) of Pitzer model, which have not been reported in the literatures, were obtained by using the solubility data of the ternary system and multiple linear regression. And Pitzer equation and particle swarm optimization algorithm were used to simulate and predict the solubility of salts in the quaternary system. By comparing the calculated phase diagrams at 273 K and 308 K, some obvious deviation in crystalline phase zone of MgBr2·6H2O and KBr·MgBr2·6H2O at 273 K have been found. And both of them are within the reasonable deviation range. The results show that the fitted parameters and the calculated model of the mixed ion interaction are reliable, and the calculated data agree well with the experimental data.