Na2SO4–NaCl–H2O system with a binary homogeneous critical point: Phase equilibria at 475–520°C and to 130 MPa

Phase equilibria were studied at temperatures 475–520°C and a pressure to 130 MPa in the ternary system Na 2 SO 4 –NaCl–H 2 O with boundary binary subsystems of two types. In type 1 subsystems (the NaCl–H 2 O subsystem in this work), there are no critical phenomena in saturated solutions. Type 2 subsystems (the Na 2 SO 4 –H 2 O subsystem in this work) have terminal critical points p (G = L – S N a 2 S O 4 ) and Q (L 1 = L 2 – S N a 2 S O 4 ). It was shown that the ternary system contains two regions of three-phase equilibria ((G–L–S) and (L 1 –L 2 –S)), divided by a two-phase fluid region (F – S N a 2 S O 4 ), and two types of monovariant critical curves ((G = L – S N a 2 S O 4 ) and (L 1 = L 2 – S N a 2 S O 4 )). With increasing temperature, these three-phase regions approach each other until the two-phase fluid equilibrium vanishes and the monovariant critical curves meet at a binary homogeneous critical point (G = L–S ⇔ L 1 = L 2 – S N a 2 S O 4 ) at a maximal temperature of ~495°C and a pressure of ~75 MPa.