Solid–Liquid Phase Equilibria of Quaternary System Na+, K+, Rb+//SO42––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 Na₂SO₄·3K₂SO₄, 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.