As is known, the phase diagram is the basis and guidance for separating and purifying salt from the complex liquid minerals such as salt lake brine and underground brine [1]. Therefore, it should be no surprise that phase equilibria research of binary, ternary, quaternary, and multi-component system has attracted much attention. Reviews of existing research found that phase equilibria research mainly concentrated on two types, one is salt – water system, and the other is aqueous two – phase system (ATPS).

Salt – water system phase equilibria research began with Van’t Hoff [2] in 1912 for the seawater system. Since then, salt – water phase diagram became the basis and guide for the utilization of liquid mineral resources. Over the past century, a lot of phase equilibria research have been carried out focused on different type’s brine: (1) Seawater system, Na+, K+, Ca2+, Mg2+//Cl−, SO42− – H2O from 263 K to 373 K [3], [4], [5], [6], [7], [8], [9]; (2) Carbonate-Borate system, Li+, Na+, K+ // Cl−, CO32–(HCO3–), SO42−, borate – H2O from 273 K to 298 K [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]; (3) Nitrate system, Na+, K+(Mg2+)//Cl−, SO42−, NO3− – H2O at 298 K [21], [22], [23]; (4) Chloride type of underground brine containing with Li+, K+, Rb+, Mg2+, and Cs+ [24], [25], [26], [27], [28], [29], [30], as the parameters and size of the positive ion lattice in KCl, RbCl, and CsCl salts are similar to each other and their lattice type of components is the same, when separated by crystallization from the saturated solution, a solid solution likely [(K, Rb)Cl], [(K, Cs)Cl], and [(Rb, Cs)Cl] are easily formed, the complex salt interaction has brought more difficulties to the separation and purification of salt.

The aqueous two phase system (ATPS) was found in 1896 by Martinus Willem Beijerinck [31], and its real application was discovered by Albertsson in 1950 s [32]. During the last decades, ATPS was used for separation and purification of inorganic salts [33], [34], [35], [36], [37]. ATPS formed via adding two polymers or one polymer and one inorganic salt are mixed with proportional concentration in an aqueous solution. In aqueous two phase system, there are two immiscible phases, the top phase is rich in one polymer, and the bottom phase is rich in the other polymer or the salt. Generally, a nontoxic hydrophilic polymer polyethylene glycol (PEG) was often used in ATPS’ studies. And PEG-salt based ATPSs are prepared using Na2SO4, (NH4)2SO4, MgSO4, KH2PO4, and Na2CO3, etc. The phase equilibrium data for PEG1000/4000/6000 + CsCl system are scarce. Phase diagram data are required for the design of aqueous two-phase extraction process. Therefore, in this work, the phase equilibria of systems containing CsCl and PEG1000/4000/6000 at T = 298.15 K were done. Meanwhile, experimental data were modelled using Othmer – Tobias and Bancroft models.