THE PHASE DIAGRAM OF THE SYSTEM Na₃AlF₆–Na₂SO₄ AND THE DISSOCIATION OF THE CRYOLITE ANION IN MOLTEN SODIUM SULPHATE

Abstract

The binary system Na 3 AlF 6 –Na 2 SO 4 has been investigated. A eutectic point was found at 91 mole % Na 2 SO 4 . On the Na 3 AlF 6 side of the diagram extensive solid solution was found. No solid solution was observed on the Na 2 SO 4 side, either by thermal analysis or by Debye–Scherrer diagrams. The freezing point depression of Na 2 SO 4 caused by additions of Na 3 AlF 6 has been used to evaluate the dissociation scheme of the cryolite anion in a sodium sulphate melt. The calculations favor the following scheme:AlF 6 3− = AlF 4 − + 2F − .