Solid–Liquid Equilibria of the K+,Al3+//SO42–-H2O and Rb+,Al3+//SO42–-H2O Systems at 298.15 and 323.15 K

Abstract Rubidium is currently extracted primarily from rubidium ores, and its low-grade and complex associated components lead to difficult separation and high production costs, hindering large-scale utilization. Alum salt precipitation is a low-cost and industrializable method for K–Rb separation, while its phase equilibrium theory is underdeveloped. This work studies the phase equilibrium of multicomponent sulfate systems to support rubidium purification. The isothermal dissolution equilibrium method was utilized to explore the existing forms of potassium salts, rubidium salts, and aluminum salts in the sulfate system. The solubility and density of the K+,Al3+//SO42–-H2O and Rb+,Al3+//SO42–-H2O systems were determined at 298.15 and 323.15 K. The experimental data were utilized to generate the phase diagrams and density-composition diagrams for the two systems at distinct temperatures. For the K+,Al3+//SO42–-H2O and Rb+,Al3+//SO42–-H2O systems, the crystallization regions decrease with increasing temperature, while the crystallization form remains unchanged. The crystallization regions of KAl(SO4)2 · 12H2O and RbAl(SO4)2 · 12H2O occupy the largest area at both temperatures. The K+,Al3+//SO42–-H2O and Rb+,Al3+//SO42–-H2O subsystems advance the phase diagram research of the quaternary K+,Rb+,Al3+//SO42–-H2O system, complement missing phase equilibrium data, and provide basic thermodynamic parameters for K and Rb separation.