Solubility and crystallization in the system MgCl2-MgSO4-H2O at 50 and 75°C

The system MgCl2-MgSO4-H2O has been investigated experimentally and modeled thermodynamically according to the Pitzer method at 50 and 75 degreesC. It was found that, even when seemingly all requirements for reaching the stable thermodynamic equilibrium are fulfilled, the crystallization of higher hydrates as metastable phases is possible, and cannot be avoided in each crystallization field of a stable lower hydrate of magnesium sulfate. Crystallization of MgSO4.x H2O (x = 1, 4, 6) and MgCl2.6 H2O at 50 degreesC and of MgSO4.H2O and MgCl2.6 H2O at 75 degreesC as stable phases has been observed. Three metastable crystallization fields of MgSO4.x H2O (x = 4,6, 7) have been detected at 50 degreesC and two of MgSO4.x H2O (x = 4, 6) at 75 degreesC. The results obtained and the contradictions existing in the literature with respect to the solubility and the crystallizing solid phases are discussed in terms of the crystal structures.