Stable Phase Diagram of the Quaternary Water-Salt System Li+, Na+, Rb+//SO42-•H2O at 298.2 and 323.2 K (P=94.5 kPa)

The stable phase equilibrium of the quaternary system Li+, Na+, Rb+// SO42- - H2O at T = 298.2 and 323.2 K (P = 94.5 kPa) was studied by an isothermal dissolution equilibrium method. The solubilities, density, and refractive index of equilibrated solution were determined. It is found that the projected phase diagrams of the quaternary system at 298.2 and 323.2 K (P = 94.5 kPa) both contain four invariant points, nine univariant curves, and six crystallization regions. By comparing the phase diagrams at 298.2 and 323.2 K, it is found that at 298.2 K, the region of single salt Rb2SO4 is the largest one and that of Li2SO4 center dot H2O is the smallest one. At T = 323.2 K, the single salt Na2SO4 center dot 10H(2)O transforms into Na2SO4 and the lithium sodium double salt is transformed from 3Na(2)SO(4)center dot Li2SO4 center dot 12H(2)O to Li2SO4 center dot Na2SO4. Meanwhile, the maximum phase region shifts from Rb2SO4 to Li2SO4 center dot Na2SO4. This indicates that with the rise of temperature, the solubilities of Rb2SO4 and Li2SO4 center dot Na2SO4 changed significantly and the lithium sodium double salt will be crystallized out from the system in the form of Li2SO4 center dot Na2SO4, with a higher lithium sodium ratio. In other words, it is unfavorable to extract rubidium sulfate from sulfate brine, while it has obvious promoting effects on extracting lithium sulfate.