Several sources indicate that MgSO4.7H2O melts at a temperature range of 50-52.5°C during dehydration (see for example Refs [2, 7]). Melting is a problem, since it reduces the bed porosity of the material, and consequently the vapor transport through the bed, which limits the ability of the material to take up water again. The TGA-DSC curve for heating rate at 1 K/min (Figure 1) shows that for MgSO4.7H2O is completely converted to MgSO4.6H2O at 50°C and therefore no melting occurs.

However, if the heating rate is increased, the dehydration peak is shifted to higher temperatures due to kinetics. Figure 3 illustrates the effect of the heating rate on the dehydration peak:

It means that for higher heating rates (> 1 K/min), MgSO4.7H2O may still be present at 50-52.5°C and starts to melt. The melting of MgSO4.7H2O was studied by a combined TGA-DSC measurement. This can be distinguished from dehydration since melting causes a heat uptake of the material without a change in mass as illustrated in Figure 4.

The results of the combined TGA-DSC experiments indicated that melting of MgSO4.7H2O typically occurred for high heating rates, large sample masses and/or large particles. The experiments also indicated that only melting of MgSO4.7H2O occurs during dehydration of the material.