Solid-Liquid Phase Equilibria of Quaternary System Na+, K+, Rb+//SO4 2--H2O at T=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+//SO4 2--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+//SO4 2--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<middle dot>3K2SO4, where the crystal phase region of the double salt Na2SO4<middle dot>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<middle dot>10H2O at 298.2 K to Na2SO4 at 323.2 K. As the temperature increases, the phase regions of the double salt Na2SO4<middle dot>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.