The preparation of magnesium chloride is primarily categorized into large-scale industrial production and small-scale laboratory preparation; the core approach in both cases involves reacting magnesium compounds with hydrochloric acid or extracting the substance from seawater bittern.
1. It is obtained from the bittern (a byproduct of seawater salt production) by concentrating it into a carnallite (KCl·MgCl₂·6H₂O) solution, cooling to remove potassium chloride, and then subjecting the remaining solution to further concentration, filtration, cooling, and crystallization. It can also be produced by dissolving magnesium oxide or magnesium carbonate in hydrochloric acid. Anhydrous magnesium chloride is obtained by heating ammonium magnesium chloride to drive off ammonia.
2. Anhydrous magnesium chloride can be prepared by dehydrating a mixture of ammonium chloride and magnesium chloride hexahydrate-or a double salt formed from them-in a stream of hydrogen chloride gas.