Solid-Liquid Phase Equilibria of the Aqueous Quaternary System K+, Mg2+ // Cl-, So42- - H2o at T = 363.2 K

To prepare for the exploitation of deep-buried polyhalite resources, the phase equilibria of the ternary system K+ // Cl-, SO42- - H2O, Mg2+ // Cl-, SO42- - H2O and the quaternary system K+, Mg2+ // Cl-, SO42- - H2O at 363.2 K were investigated by using the isothermal dissolution method. The solubility and density of the systems were measured experimentally, and the related phase diagrams and density vs composition diagrams are plotted. The ternary system K+ // Cl-, SO42- - H2O and Mg2+ // Cl-, SO42- - H2O are simple systems at 363.2 K. Both systems have one invariant point corresponding to the solid phases KCl + K2SO4 and MgCl2·6H2O + MgSO4·H2O, respectively. The quaternary system K+, Mg2+ // Cl-, SO42- - H2O generates four double salts corresponding to KCl·MgCl2·6H2O, KMg(SO4)Cl·3H2O, K2SO4·2MgSO4, K2SO4·MgSO4·4H2O, and the phase diagram consists of seven quaternary invariant points, fourteen isothermal dissolution curves and eight crystallization regions at 363.2 K. By comparing the phase diagram of the quaternary system at 348.2 K, 363.2 K and 373.2 K, it can be seen that the crystallization zones of K2SO4·MgSO4 grow significantly with the increase of temperature, while the crystallization zones of MgSO4·H2O, KCl and K2SO4 shrink, and the crystallization zone of K2SO4·MgSO4·4H2O on the verge of disappearing at 363.2 K. Based on Pitzer - Simonson - Clegg (PSC) model, the solubility data of the ternary subsystems and the quaternary system were calculated. The results showed that the calculated values were close to the experimental values.