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Occurrence and Production of Magnesium
From the ground
Although magnesium is found in over 60 minerals, but only dolomite (CaMg(CO3)2), magnesite (MgCO3), brucite (Mg(OH)2), carnallite (KMgCl3.6(H2O)), talc (Mg3Si4O10(OH)2), and olivine ((Mg,Fe)2SiO4) are of commercial importance.
In the United States this metal is principally obtained by electrolysis of fused magnesium chloride from brines, wells, and sea water:
cathode: Mg2+ + 2e- Mg
anode: 2Cl- Cl2(gas) + 2e-
The United States has traditionally been the major world supplier of this metal, supplying 45% of world production even as recently as 1995. Today, the US market share is at 7%, with a single domestic producer left, US Magnesium, a company born from now-defunct Magcorp. As of 2005 China has taken over as the dominant supplier, pegged at 60% world market share, which increased from 4% in 1995. Unlike the above described electrolytic process, China is almost completely reliant on a different method of obtaining the metal from its ores, the silicothermic Pidgeon process (the reduction of the oxide at high temperatures with silicon).
From the Sea
Sea water contains magnesium salts, such as magnesium chloride. To extract the magnesium, calcium carbonate is added to sea water to form magnesium carbonate.
MgCl2 + CaCO3 MgCO3 + CaCl2
Magnesium carbonate is insoluble in water so it can be filtered out, and reacted with hydrochloric acid to obtain concentrated magnesium chloride.
MgCO3 + 2HCl MgCl2 + CO2 + H2O
From magnesium chloride, electrolysis produces magnesium.