Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 225

8.12.2 Magnesium
Magnesium is the third most abundant metal after aluminium and iron in the
Earth's crust. Deposits of magnesium, especially magnesite MgCO 3 and dolomite
MgCO 3 CaCO 3 are widely distributed in large quantities and are of adequate
quality for the production of the metal. Carnallite, with an 8% magnesium content,
is the third most frequently exploited metal ore, after the previous two. Other de-
posits occur in magnesium salts, especially chloride, double chlorides and sulphates,
which are constituents of many water sources including saline lakes and the ocean.
Indeed each cubic kilometre of seawater contains 1.3 million tonnes of magnesium,
at an average magnesium concentration of 1.28 ppm (Quinby-Hunt and Turekian,
1983).
Magnesium as a metal is obtained either by a thermal reduction process or
electrolysis. The metallothermic option consists of reducing dolomite, which has
been previously calcined in a rotary furnace, with ferrosilicon in a retort vessel
where aluminium is sometimes also added. The process takes place under a vacuum
or in an argon-inert atmosphere at 1700 o C. The furnace is electrically heated and the
liquid slag opposes resistance between the electrode and the carbonaceous bottom
lining. It produces magnesium vapour which afterwards condenses in a condenser
cooled by water. The crucible is then conveyed to the foundry for the casting of
magnesium ingots (IPPC, 2009).
Most magnesium metal production (around 80%) is obtained through the elec-
trolysis of magnesium chloride. The chloride is produced from roasting dolomite
in kilns where magnesium and calcium carbonates are converted into their oxides
(dolime). Thereafter, the dolime is quenched with seawater precipitating the mag-
nesium hydroxide and thus separating the magnesium from its calcium compounds.
The magnesium hydroxide can then be subjected to hydrochloric acid in order to
produce magnesium chloride. The latter must be further dehydrated in fluidised
beds employing hot air at 150-180 o C and then later hydrogen chloride gas at the
higher temperature of 300-400 o C.
But there can be another route whereby magnesium hydroxide is instead reverted
into its oxide by reducing its water content and calcining the feed in a rotary kiln.
The product is subsequently pelletised with charcoal and magnesium chloride brine
and is processed in a chlorinator furnace where chlorine gas converts the oxide into
chloride. This process produces molten magnesium chloride for the electrolysis stage
(IPPC, 2009; Friedrich and Mordike, 2006). As electrolysis of magnesium chloride
in aqueous solution releases hydrogen instead of magnesium, the cell used must be
composed of a molten mixture of magnesium chloride with alkali chlorides at 700-
800 o C. The fused MgCl2 at 300-400 kA, allows for the co-production of a highly
concentrated chlorine gas and liquid magnesium metal 22 . The reaction generates
magnesium thanks to the density differences that exist within the molten bath. The
22 See http : ==www:magnesium:com=. Accessed Nov. 2011.