Phase diagrams of the Li2SO4–LiCl–H2O system at 490–520°C

Abstract

Phase equilibria were studied in the Li2SO4–LiCl–H2O system with boundary subsystems of various types: type 1 subsystem LiCl–H2O and type 2 subsystem Li2SO4–H2O. The ternary critical curves (G=L–S) and (L1=L2–S) originating at the terminal critical points p and Q of the type 2 binary system do not combine to form a single curve with the binary homogeneous critical point (G=L–S) ⇔ (L1=L2–S) but end at the invariant points (G=L1–L2–S) and (G–L1=L2–S) connected by the monovariant four-phase curve (G–L1–L2–S) at a temperature below 490°C. It was found that, at 490–520°C, there are no critical phenomena in the solutions, and the solubility of each of the salts varies continuously from the solubility in the binary aqueous system to the solubility in an anhydrous melt. Above the temperature of the four-phase equilibrium (G–L1–L2–S), a region (G–L1–L2) of phase separation of unsaturated solutions in the presence of gas is observed, which is bounded by the monovariant critical curves (G=L1–L2) and (G–L1=L2). With increasing temperature, this region shrinks and collapses at the tricritical point (G=L1=L2) at ~520°C.