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

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.