Immiscibility of Liquid Phases and Critical Phenomena in the Na₂SO₄–Na₂B₄O₇–H₂O System at 350–460°C and a Pressure of up to 100 MPa

Abstract

The phase equilibria and critical phenomena in the Na₂SO₄–Na₂B₄O₇–H₂O system with the boundary subsystems Na₂B₄O₇–H₂O (type 1 with the phase separation of the solutions) and Na₂SO₄–H₂O (type 2) at 350–460°C and a pressure of up to 100 MPa are studied. It is shown that there exist two three-phase regions L₁–L₂–S, which are adjacent to two water–salt subsystems and divided by a fluid region, at temperatures ranging from 440°C (the critical point Q (L₁ = L₂–S) of the type 2 binary system) to ~455°C (at which these regions unite). The ternary system has two critical curves in saturated solutions. The curves originate from different critical points of the type 2 binary subsystem, point p (G = L–S) and point Q (G = L₁–L₂–S), respectively, and end at the end ternary critical points pR (G = L₁–L₂–S) and QN (G–L₁ = L₂–S), respectively. Between the last two points, there is a four-phase equilibrium (G–L₁–L₂–S), which, according to the visual observations in the sealed quartz ampules, exists in the temperature range 350–380°C.