Significant reduction in the dehydration temperature of magnesium chloride hexahydrate through the addition of AlCl3 and NH4Cl

The electrolytic production of magnesium metal requires anhydrous MgCl 2 with a high-purity, which is usually prepared through the dehydration of MgCl 2 ·6H 2 O at a relatively high temperature. The Mg 2+ cations in MgCl 2 ·6H 2 O have a high affinity for water and the complete dehydration of MgCl 2 ·6H 2 O is difficult due to the hydrolysis of the hydrated Mg 2+ cations. The hydrolysis usually results in the formation of unwanted oxycompounds of magnesium. In this study, the effects of the addition of AlCl 3 and NH 4 Cl on the dehydration of MgCl 2 ·6H 2 O were investigated by comparing the thermal decomposition processes of MgCl 2 ·6H 2 O, MgCl 2 ·6H 2 O/AlCl 3 , and MgCl 2 ·6H 2 O/NH 4 Cl, using Raman, FT-IR, TG-DSC, XRD techniques and theoretical calculations. It was found that the addition of AlCl 3 or NH 4 Cl significantly reduces the dehydration energy barrier of MgCl 2 ⋅6H 2 O, decreases the dehydration temperature, and suppresses the hydrolysis of MgCl 2 ·6H 2 O. Anhydrous AlCl 3 can extract water from MgCl 2 ·6H 2 O at room temperature. The interactions between Al 3+ or NH 4 + and H 2 O, together with the substitution of Cl - for H 2 O, result in the easier production of anhydrous MgCl 2 . During the dehydration of MgCl 2 ·6H 2 O, several impurities including MgO are generated, indicating that the environment remains to be an important controlling factor for the dehydration of MgCl 2 ⋅6H 2 O. The results obtained would be helpful for understanding the dehydration processes and mechanisms of MgCl 2 ·6H 2 O under the assistance of AlCl 3 and NH 4 Cl, and for exploring new techniques for MgCl 2 ·6H 2 O dehydration.