Magnesium is extracted in one of three ways. The first is the electrolytic process, which uses magnesium chloride produced from magnesite or seawater. The second is the silicothermic process, which mixes calcined dolomite or magnesite with ferrosilicon alloy. The third involves dissolving magnesite in hydrochloric acid to make magnesium chloride.
In the electrolytic process, which is primarily used in the United States, seawater is treated with calcium oxide, causing magnesium hydroxide to precipitate out. The magnesium hydroxide is collected and converted into magnesium chloride by treating it with hydrochloric acid and heating it. At that point, the compound is electrolyzed, creating magnesium and chlorine gas.
In the silicothermic process, which is primarily used in China, ferrosilicon alloy is mixed with magnesite. Magnesium is highly reactive in nature, so the closest thing to it that can be mined from the ground is magnesite. Once the magnesite and ferrosilicon alloy are mixed, they are heated until they vaporize, and the resultant magnesium vapor is collected in cooling vessels to create magnesium.
In the third process, which is primarily used in Australia, magnesite ore is dissolved in hydrochloric acid, creating magnesium chloride. The magnesium chloride is purified and dehydrated to a dry feed before being electrolyzed in an Alcan cell. The molten magnesium is removed and the leftover chlorine gas is recycled and combined with hydrogen to make more hydrochloric acid.