Solid–Liquid Equilibria (SLE) of the Sulfate Aqueous System Containing Potassium, Rubidium, and Magnesium at 298.2 and 323.2 K

Abstract

In order to obtain the form of salt crystals in the aqueous system K⁺, Rb⁺, Mg²⁺//\({\text{SO}}_{4}^{{2 - }}\)–H₂O at 298.2 and 323.2 K, the phase equilibria of the quaternary system were investigated by isothermal dissolution method. Results show that both the phase diagrams of the quaternary system K⁺, Rb⁺, Mg²⁺//\({\text{SO}}_{4}^{{2 - }}\)–H₂O at 298.2 and 323.2 K has four invariant points, nine univariate curves and six crystallization regions which corresponding to salts K₂SO₄, Rb₂SO₄, K₂SO₄⋅MgSO₄⋅6H₂O, Rb₂SO₄⋅MgSO₄⋅6H₂O, MgSO₄⋅7H₂O, and [(K, Rb)₂SO₄]. According to the comparison of these phase diagrams, it has found that the crystalline forms of these salts K₂SO₄, Rb₂SO₄, Rb₂SO₄⋅MgSO₄⋅6H₂O, and [(K, Rb)₂SO₄] did not change with temperature variations, while the crystalline forms of potassium-magnesium double salts and magnesium sulfate changed with the increase in temperature: 298.2 K (K₂SO₄⋅MgSO₄⋅6H₂O, MgSO₄⋅7H₂O); 323.2 K (K₂SO₄⋅MgSO₄⋅4H₂O, MgSO₄⋅6H₂O). With the temperature increasing, the crystallization areas of Rb₂SO₄⋅MgSO₄⋅6H₂O and K₂SO₄ decrease, of which Rb₂SO₄ and [(K,Rb)₂SO₄] increase.