Solid–Liquid Phase Equilibria of Quaternary System Na⁺, K⁺, Rb⁺//SO₄^2––H₂O at <i>T</i> = 298.2 and 323.2 K

To explore the crystallization behavior of rubidium under conditions of multi-ion coexistence in the sulfate system and the influence of temperature changes, the solid–liquid equilibria of the quaternary system Na+, K+, and Rb+//SO42––H2O at 298.2 and 323.2 K was studied using the isothermal dissolution equilibrium method. The results are as follows: the quaternary system Na+, K+, Rb+//SO42––H2O is a complex system at both 298.2 and 323.2 K, with the formation of the solid solution [(K, Rb)2SO4] and the double salt Na2SO4·3K2SO4, where the crystal phase region of the double salt Na2SO4·3K2SO4 is consistently the largest. Comparing these two phase diagrams at 298.2 and 323.2 K, it was found that the precipitation form of Na2SO4 changes from Na2SO4·10H2O at 298.2 K to Na2SO4 at 323.2 K. As the temperature increases, the phase regions of the double salt Na2SO4·3K2SO4 and the solid solution [(K, Rb)2SO4] expand, while the precipitation phase regions of the single salts K2SO4 and Rb2SO4 decrease relatively. Therefore, this change can be utilized to separate rubidium and potassium sulfates through cooling crystallization in a sulfate system containing sodium, potassium, and magnesium.