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In order to develop a new processfor concentrating and separatingsodium sulfate (Na2SO4) and magnesium sulfate(MgSO4) of the bloedite based on tetrahydrofuran (THF)hydrate method, the equilibrium of the ternary systems THF–Na2SO4–H2O and THF–MgSO4–H2O and the quaternary system THF–Na2SO4–MgSO4–H2O were measured at 5 °C, and the phase digrams of these threesystems were investigated. It showed that the mass percentage of Na2SO4(MgSO4) in equilibrium liquid washigher than in feed and we could obtain Na2SO4·10H2O and MgSO4·7H2Oin turn. It proved that THF hydrate could be used to the concentrationand separation of Na2SO4and MgSO4. The extended Pitzer model of the electrolyte–electrolyte–nonelectrolyte–watersystem was derived and applied into THF–Na2SO4–MgSO4–H2O system andits subsystems. The necessary thermodynamic parameters had been derivedfrom a least-squares optimization program. The model calculation valuewas in good agreement with experimental solubility for ternary andquaternary mixtures, which indicated that the Pitzer model could besuccessfully used to predict the phase equilibrium of the electrolyte–electrolyte–nonelectrolyte–watersystems containning THF hydrate. [ABSTRACT FROM AUTHOR]