Magnesium chloride, calculated by weight as anhydrous material, is composed of 74.54% chlorine and 25.48% magnesium. However, according to the phase diagram analysis of the MgCl2-H2O system, in addition to anhydrous substances, there are also hydrates containing 1, 2, 4, 6, 8, and 12 crystal water molecules. When dehydrated in air, only about 4 molecules of water can be removed, without causing serious side reactions. If the remaining two molecules of water are removed, magnesium chloride will significantly hydrolyze and rise to hydroxymagnesium chloride, magnesium oxide, and hydrogen chloride. At different stages of temperature, the reaction is:

1. 96-117 ℃:

MgCl2 · 6H2O ≈ MgCl2 · 4H2O+2H2O

2. 135~180 ℃:

MgCl2 · 4H2O → MgCl2 · 2H2O+2H2O

3. 185~230 ℃:

MgCl2 · 2H2O → MgCl2 · H2O+H2O

MgCl2 · 2H2O → MgOHCl+HCl+H2O

4. Above 230 ℃:

MgCl2 · H2O → MgCl2+H2O

MgCl2 · H2O → MgOHCl+HCl

5. 304~554 ℃:

MgCl2+H2O ≈ MgOHCl+HCl

6. Above 527 ℃:

MgOHCl ≈ MgO+HCl

From the above reactions, it can be seen that magnesium chloride hydrates cannot become anhydrous magnesium chloride when dehydrated in air. The dehydration methods used to produce anhydrous magnesium chloride now include: (1) hydrogen chloride dehydration method; (2) Generating double salt dehydration method; ⑶ Generating complex dehydration method.

Anhydrous magnesium chloride is a white glossy hexagonal crystal with a melting point of 714 ℃ and a boiling point of 1412 ℃. It has extremely strong deliquescence and is higher than that of 6-water magnesium chloride and 2-water magnesium chloride. The physical and chemical properties are basically the same as those of 6-water magnesium chloride. Mainly used for the production of magnesium metal and sectors such as chemistry, light industry, coal, and construction.