In order to develop a new process for concentrating and separating sodium sulfate (Na2SO4) and magnesium sulfate (MgSO4) of the bloedite by gas hydrate, the equilibrium of the ternary system H2O-Na2SO4-C3H8, H2O-MgSO4-C3H8 and the quaternary system H2O-Na2SO4-MgSO4-C3H8 with tetrahydrofuran (THF) additive were studied at 0 degrees C. It showed that the mass percentage of Na2SO4 (MgSO4) in equilibrium liquid was higher than in feed. It proved that gas hydrate could be used to the concentration and separation of Na2SO4 and MgSO4. The solubility of the propane was calculated based on the Krichevsky-Kasarnovsky equation. The extended Pitzer model of electrolyte-electrolyte-nonelectrolyte-water system was derived and applied into H2O-Na2SO4-MgSO4-C3H8 system and its sub-systems. The necessary thermodynamic parameters had been derived from a least-squares optimization program. The model calculation value was in good agreement with experimental solubility for ternary and quaternary mixtures, which indicated that the Pitzer model could be successfully used to predict the phase equilibrium of the electrolyte-electrolyte-nonelectrolyte-water systems containing gas hydrate. (C) 2013 Elsevier B.V. All rights reserved.