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The gradient solar pond technique is an economic separating process employed in the inor-ganic chemical industrial production of salt lake chemical engineering processes. In this paper,a novel isothermal evaporation experimental method was employed to simulate the evaporation phase equilibri-um for the reciprocal quatary system (LiCl + NaCl + Li 2 SO4 + Na2 SO4 + H2 O)at 348. 15 K to serve as a useful guide for lithium salt production via the depth solar ponds. The isothermal evaporation equi-librium solubilities and physicochemical properties,including the densities and pH values,were experi-mentally investigated. The dry-salt phase diagram,water-phase diagram,and the diagram of the physi-cochemical properties versus composition were plotted with respect to the experimental data. The dry-salt phase diagram consists of three invariant points,seven univariant solubility curves,and five crystal-lization regions,specifically halite (NaCl,Ha),thenardite (Na 2 SO4 ,Th),double salt (Li 2 SO4· Na2 SO4 ,Db 2 ),lithium sulfate monohydrate (Li 2 SO4·H 2 O,Ls),and lithium chloride monohydrate (LiCl·H 2 O,Lc). Based on Pitzer and its extended HMW model,the Pitzer single salt parameters, mixing ion interaction parameters,and thermodynamic stable equilibrium constants for the quatary system at 348. 15 K were obtained. The calculated phase diagram and experimental isothermal phase diagram at 348. 15 K exhibited a great difference. Based on these results,the isothermal evaporation phase diagram can truly reflect the salt sedimentary in salt lakes and can be applied as a useful guide for the solar pond process.