Stable-Phase Diagram of the Quaternary Water–Salt System K+, Rb+, Cs+//SO42––H2O at T = 323.2 K

Abstract

The stable-phase equilibria of the aqueous quaternary system K+, Rb+, Cs+//SO42––H2O was investigated at T = 323.2 K using the isothermal dissolution equilibrium method. The solubility, density, and refractive index of equilibrium solutions were determined using the chemical/instrument analysis method, the specific gravity bottle method, and the WYA Abbe refractometer, respectively. The solid phases were identified by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results reveal that besides three single salts (K2SO4, Rb2SO4, and Cs2SO4), three complete solid solutions are formed in this system at 323.2 K, namely, [(Kx, Rb1–x)2SO4], [(Kx, Cs1–x)2SO4], and [(Rbx, Cs1–x)2SO4], increasing the difficulty of separation potassium from the solution composed of rubidium, cesium, and sulfate. The stable-phase diagram of the quaternary system K+, Rb+, Cs+//SO42––H2O at T = 323.2 K consists of four invariant points, nine univariant curves, and six crystallization regions. The sequence of the size of the salt crystal region is [(Kx, Rb1–x)2SO4] > [(Kx, Cs1–x)2SO4] > K2SO4 > [(Rbx, Cs1–x)2SO4] > Rb2SO4 > Cs2SO4. The phase diagrams at different temperatures (T = 298.2 K and T = 323.2 K) show that temperature is an important factor affecting salt crystallization, especially for the solid solution. As the temperature increases, the crystallization region of [(Kx, Cs1–x)2SO4] conspicuously decreases, whereas the crystallization regions of [(Kx, Rb1–x)2SO4] and [(Rbx, Cs1–x)2SO4] increase slightly.