History
Magnesium derives its name from magnesite, a magnesium carbonate mineral, and this mineral in
turn is said to owe its name to magnesite deposits found in Magnesia, a district in the ancient
Greek region of Thessaly. The British chemist Humphry Davy is said to have produced an amalgam
of magnesium in 1808 by electrolyzing moist magnesium sulfate, using mercury as a cathode. The
first metallic magnesium, however, was produced in 1828 by the French scientist A.-A.-B. Bussy.
His work involved the reduction of molten magnesium chloride by metallic potassium. In 1833 the
English scientist Michael Faraday was the first to produce magnesium by the electrolysis of
molten magnesium chloride. His experiments were repeated by the German chemist Robert
Bunsen.
The first successful industrial production was begun in Germany in 1886 by Aluminium und
Magnesiumfabrik Hemelingen, based on the electrolysis of molten carnallite. Hemelingen later
became part of the industrial complex IG Farbenindustrie, which, during the 1920s and '30s,
developed a process for producing large quantities of molten and essentially water-free magnesium
chloride (now known as the IG Farben process) as well as the technology for electrolyzing this
product to magnesium metal and chlorine. Other contributions by IG Farben were the development
of numerous cast and malleable alloys, refining and protective fluxes, wrought magnesium
products, and a vast number of aircraft and automobile applications. During World War II the Dow
Chemical Company of the United States and Magnesium Elektron Limited of the United Kingdom
began the electrolytic reduction of magnesium from seawater pumped from Galveston Bay, Texas,
and the North Sea at Hartlepool, Eng. At the same time in Ontario, Can., L.M. Pidgeon's process
of thermally reducing magnesium oxide with silicon in externally fired retorts was introduced.
Following the war, military applications lost prominence. Dow Chemical broadened civilian markets
by developing wrought products, photoengraving technology, and surface treatment systems.
Extraction remained based on electrolysis and thermal reduction. To these processes were made
such refinements as the internal heating of retorts (the Magnetherm process, introduced in France
in 1961), extraction from dehydrated magnesium chloride prills (introduced by the Norwegian
company Norsk Hydro in 1974), and improvements in electrolytic cell technology from about 1970.
Ores and raw materials
The eighth most abundant element in nature, magnesium constitutes 2.4 percent of the Earth's
crust. Owing to its strong reactivity, it does not occur in the native state, but rather it is found in a
wide variety of compounds in seawater, brines, and rocks.
Among the ore minerals, the most common are the carbonates dolomite (a compound of
magnesium and calcium carbonates, MgCO3 CaCO3) and magnesite (magnesium carbonate,
MgCO3). Less common is the hydroxide mineral brucite (Mg[OH]2) and the halide mineral
carnallite (a compound of magnesium and potassium chlorides and water, MgCl2 KCl 6H2O).
Magnesium chloride is recoverable from naturally occurring brines such as the Great Salt Lake
(typically containing 1.1 percent by weight magnesium) and the Dead Sea (3.4 percent), but by far
the largest source is the oceans of the world. Although seawater is only approximately 0.13
percent magnesium, it represents an almost inexhaustible source.
Mining and concentrating
Both dolomite and magnesite are mined and concentrated by conventional methods. Carnallite is
dug as ore or separated from other salt compounds that are brought to the surface by solution
mining. Naturally occurring magnesium-containing brines are concentrated in large ponds by solar
evaporation.
Extraction and refining
A strong chemical reagent, magnesium forms stable compounds and reacts with oxygen and
chlorine in both the liquid and gaseous state. This means that extraction of the metal from raw
materials is an energy-intensive process requiring well-tuned technologies. Commercial production
follows two completely different methods: electrolysis of magnesium chloride or thermal reduction
of magnesium oxide. Where power costs are low, electrolysis is the cheaper method--and, indeed,
it accounts for approximately 75 percent of world magnesium production.
Electrolysis
Electrolytic processes consist of two steps: the preparation of a feedstock containing magnesium
chloride and the dissociation of this compound into magnesium metal and chlorine gas in
electrolytic cells.
In industrial processes, cell feeds consist of various molten salts containing anhydrous (essentially
water-free) magnesium chloride, partly dehydrated magnesium chloride, or anhydrous carnallite. In
order to avoid impurities present in carnallite ores, dehydrated artificial carnallite is produced by
controlled crystallization from heated magnesium- and potassium-containing solutions. Partly
dehydrated magnesium chloride can be obtained by the Dow process, in which seawater is mixed
in a flocculator with lightly burned reactive dolomite. An insoluble magnesium hydroxide
precipitates to the bottom of a settling tank, whence it is pumped as a slurry, filtered, converted to
magnesium chloride by reaction with hydrochloric acid, and dried in a series of evaporation steps to
25 percent water content. Final dehydration takes place during smelting.
Anhydrous magnesium chloride is produced by two principal methods: dehydration of magnesium
chloride brines or chlorination of magnesium oxide. In the latter method, exemplified by the IG
Farben process, lightly burned dolomite is mixed with seawater in a flocculator, where magnesium
hydroxide is precipitated out, filtered, and calcined to magnesium oxide. This is mixed with
charcoal, formed into globules with the addition of magnesium chloride solution, and dried. The
globules are charged into a chlorinator, a brick-lined shaft furnace where they are heated by carbon
electrodes to approximately 1,000-1,200 C (1,800-2,200 F). Chlorine gas introduced through
portholes in the furnace reacts with the magnesium oxide to produce molten magnesium chloride,
which is tapped at intervals and sent to the electrolytic cells.
Dehydration of magnesium brines is conducted in stages. In the Norsk Hydro process, impurities
are first removed by precipitation and filtering. The purified brine, which contains approximately 8.5
percent magnesium, is concentrated by evaporation to 14 percent and converted to particulates in
a prilling tower. This product is further dried to water-free particles and conveyed to the electrolytic
cells.
Electrolytic cells are essentially brick-lined vessels equipped with multiple steel cathodes and
graphite anodes. These are mounted vertically through the cell hood and partially submerged in a
molten salt electrolyte composed of alkaline chlorides to which the magnesium chloride produced
in the processes described above is added in concentrations of 6 to 18 percent. The basic reaction
is:
Operating temperatures vary from 680 to 750 C (1,260 to 1,380 F). Power consumption is 12 to 18
kilowatt-hours per kilogram of magnesium produced. Chlorine and other gases are generated at the
graphite anodes, and molten magnesium metal floats to the top of the salt bath, where it is
collected. The chlorine can be reused in the dehydration process.
Thermal reduction
In thermal production, dolomite is calcined to magnesium oxide (MgO) and lime (CaO), and these
are reduced by silicon (Si), yielding magnesium gas and a slag of dicalcium silicate. The basic
reaction,
is endothermic--that is, heat must be applied to initiate and sustain it. With magnesium reaching a
vapour pressure of 100 kilopascals (1 atmosphere) at 1,800 C (3,270 F), heat requirements can be
quite high. In order to lower reaction temperatures, industrial processes operate under vacuum.
There are three principal methods, differing by their means of supplying heat. In the Pidgeon
process, ground and calcined dolomite is mixed with finely ground ferrosilicon, briquetted, and
charged into cylindrical nickel-chromium-steel retorts. A number of retorts are installed horizontally
in an oil- or gas-fired furnace, with their lids and attached condenser systems extending out of the
furnace. After a reaction cycle at a temperature of 1,200 C (2,200 F) and under a reduced pressure
of 13 pascals, magnesium crystals (called crowns) are removed from the condensers, slag is
evacuated as a solid, and the retort is recharged. In the Bolzano process, dolomite-ferrosilicon
briquettes are stacked on a special charge support system through which internal electric heating
is conducted to the charge. A complete reaction takes 20 to 24 hours at 1,200 C below 400
pascals.
The dicalcium silicate slag produced by the above processes has a melting point of about 2,000 C
(3,600 F) and is therefore present as a solid, but, by adding alumina (aluminum oxide, Al2O3) to
the charge, the melting point can be reduced to 1,550-1,600 C (2,825-2,900 F). This technique,
utilized in the Magnetherm process, has the advantage that the liquid slag can be heated directly
by electric current through a water-cooled copper electrode. The reduction reaction occurs at 1,600
C and 400-670 pascals pressure. Vaporized magnesium is condensed in a separate system
attached to the reactor, and molten slag and ferrosilicon are tapped at intervals.
Refining
After extraction by the processes described above, crude magnesium metal is transported to cast
shops for removal of impurities, addition of alloying elements, and transformation into ingots, billets,
and slabs. During melting and handling, molten magnesium metal and alloys are protected from
burning by a layer of flux or of a gas such as sulfur hexafluoride or sulfur dioxide. For shipping and
handling under severe climatic conditions, suitable ventilated plastic or paper wrappings are
required to prevent corrosion.
The metal and its alloys
Primary magnesium is available in grades of 99.90, 99.95, and 99.98 percent, but, in practice,
grades 99.95 and 99.98 have only limited use in the uranium and nuclear industries. For bulk use,
grades 99.90 and 99.80 are supplied.
