Anhydrous magnesium chloride, the dehydration product from magnesium chloride hydrate(MgCl2·nH2O), is a conventional raw material to prepare electrolytic magnesium.Because the dehydration of magnesium chloride hydrate can be accompanied by hydrolysis leading to the production of undesirable hydroxy chloride and oxide compounds of magnesium, it should be carried out in hydrogen chloride gas atmosphere.