Electrowinning of magnesium is also a major industrial process, and is the most commonly used industrial process for magnesium production although other methods such as chemical reduction of magnesium compounds at high temperatures with carbon, calcium carbide, or ferrosilicon have been used. The raw material for magnesium production is well brines or more commonly seawater, which is about 0.13% magnesium by mass.
The Mg2+ ion is precipitated from seawater by addition of CaO (lime), produced by calcining (strong heating) of CaCO3 (limestone or oyster shells). The insoluble Mg(OH)2 is removed by filtration. Acidification of a slurry of this solid and an aqueous MgCl2 solution with HCl converts the Mg(OH)2 to soluble MgCl2, which is recovered as solid MgCl2 by evaporation. This is dried to the hydrate MgCl2.1.5H2O, which is used as the feedstock to an electrolytic cell.
The cell electrolyte is a molten mixture containing about 25% MgCl2 - 15% CaCl2 - 60% NaCl, and the cell is operated between 700oC and 750oC. The cell electrolytic reaction is:
MgCl2 --> Mg(l) + Cl2(g)
The magnesium produced is about 99.9% pure as it comes from the cell, since neither sodium nor calcium is reducible more easily than is magnesium. The chlorine produced in the cell is converted to HCl for use in the acidification process by reaction with natural gas and steam or in a hydrogen/chlorine burner.