Solid–Liquid Phase Equilibria in the Quaternary System Na ⁺ , Cs ⁺ //Cl ^– , CO ₃ ^2– –H ₂ O at 308.15 K

The solid–liquid phase equilibria of the quaternary system Na+, Cs+//Cl–, CO32––H2O and its subsystem Na2CO3–Cs2CO3–H2O at 308.15 K were studied by the isothermal dissolution method. The phase diagram of the system Na2CO3–Cs2CO3–H2O is composed of two single-salt crystallization fields of Na2CO3·H2O and Cs2CO3·H2O, two solubility univariant curves, and one invariant point. The dry-salt phase diagram of the quaternary system consists of three invariant points, seven univariant curves, and five crystallization regions corresponding to Na2CO3·H2O, NaCl, solid solution [Cs1–x(Na·H2O)x]Cl, Cs2CO3·H2O, and double salt CsCl·2NaCl·2H2O. The areas of crystallization zones decrease according to the above order. The Jan̈ecke index of H2O, pH, and refractive index of the liquid phase in the quaternary system change regularly with the Jan̈ecke index of Cs+. These results on the phase diagram of the quaternary system can supply theoretical references for cesium salt separation from carbonate-type brine.