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.