At present two basic processes are used to produce metallic magnesium. These are: 1) the electrolysis of fused anhydrous magnesium chloride (MgCl2) derived from magnesite, brine, or seawater, which accounts for about 80% of the output, and 2) thermal reduction of magnesium oxide (MgO) by ferrosilicon derived from carbonate ores. A third process has been recently developed which uses electrolysis of fused anhydrous MgCl2 derived from serpentine ores.
The extraction of magnesium by electrolysis involves two steps: (1) preparation of the magnesium chloride cell feed and (2) electrolysis. All extraction processes are followed by refining and casting.
Electrolysis, or the dissolving of magnesium oxide in fused fluorides, has also been considered. However, the solubility of magnesium oxide in the electrolyte is low, and practical solutions for metal collection have not been found. Composite MgO-C anodes have been tested in fluoride, fluoride-chloride, and chloride electrolytes on a laboratory scale.
Electrolysis of magnesium chloride in aqueous solution liberates hydrogen, not magnesium, at the cathode. Attempts to electrolyze magnesium salts in organic solvents have not been successful.
Carbothermic reduction of magnesium oxide is not used industrially.
The main problems are: high reaction temperature (1800-2000 °C) and rapid cooling of reaction gases to suppress magnesium oxide formation. Recent experimental studies have been based on rapid adiabatic expansion of the gases or cooling in liquid metal. Attempts to enhance selectivity of the reduction step in the carbothermic reduction of magnesium oxide in molten CaO • Al2O3 • MgO slag have been reported.