Magnesium (Mg) was first recognised as an element in 1755 by Black however, it was not isolated until 1808 by Davy who evaporated the mercury from a magnesium amalgam made by electrolysing a mixture of moist magnesia and mercuric oxide, and was not prepared in its coherent form till 1831 by Bussy.
Magnesium is the eighth most abundant element in the Earth’s crust (approximately 2.5% by weight) it is one of the alkaline-earth metals and does not occur uncombined. It is found in mineral deposits such as magnesite (the oxide), dolomite, brucite, serpentine, chrysolite, meerschaum, talc, Epsom salts (the sulphate) magnesia alba (the carbonate) and most kinds of asbestos. Seawater contains about 0.13% magnesium, mostly as the dissolved chloride, which imparts the characteristic bitter taste.
Magnesium is commercially produced by electrolysis of molten magnesium chloride (MgCl2), processed mainly from brines, wells and seawater and by the direct reduction of its compounds with suitable reducing agents (as from calcined dolomite with ferrosilicon).
Key engineering properties of magnesium include:
- It is the lightest engineering metal (excluding exotic materials such as beryllium and lithium) with a density of 1.73g.cm-3
- Has good toughness
- Has a high co-efficient of thermal expansion at 25.5x10-6m/m/°C
- It has a reasonably high thermal conductivity of approximately 150 W/m.K
- It has a reasonably high electrical conductivity at around 40% that of copper
- High damping capacity
- Is excellent machinablity
Magnesium develops a corrosion-inhibiting film upon exposure to clean atmospheres and freshwater. However, the film breaks down in the presence of chlorides, sulphates and other media. It is rapidly attacked by mineral acids, except for chromic and hydrofluoric acids. It is however, resistant to dilute alkalies, aliphatic and aromatic hydrocarbons, particular alcohols, and dry bromine, chlorine and fluorine gases. Anodising magnesium improves its corrosion resistance.
Magnesium has a good affinity for oxygen and if finely divided will readily ignite upon heating in air and burns with a dazzling bright white flame. For this reason magnesium should be handled carefully for risk of serious fires and water should not be used on burning magnesium or on magnesium fires.
Magnesium in the metal form is used in:
- An alloying element in aluminium, zinc, lead and other non-ferrous alloys.
- Cathodic protection of other metals against corrosion.
- Flashlight photography, photoengraving plates, flares and pyrotechnics (which includes incendiary bombs).
- Airplane alloys and missiles construction.
- As a reduction agent for the production of pure uranium from its salt.
- Building construction as a flashing material.
Its alloys are implemented in lightweight applications, auto parts, aerospace equipment, power tools, sporting goods, fixtures and materials-handling equipment.
Magnesium and its compounds are used in:
- Magnesium nitrate [(Mg(NO3)2.6H2O], is used in dry colours, pyrotechnics and to produce magnesia.
- Magnesium methoxide [Mg(OCH3)2], used to dry alcohols to produce absolute fuels.
- Magnesium fluoride (MgF), or sellaite, having a very low refractive index is used for lens and instrument windows to eliminate reflection.
- Its hydroxide, chloride, sulphate and citrate are used in medicines.
- Magnesium carbonate (MgCO3), is used also in insulating coverings for steam pipes and furnaces, making of oxychloride cement, in boiler compounds, and as a filler for rubbers and paper.
- Magnesium sulphate (MgSO4.7H2O) is used also in leather tanning, as a mordant in dyeing and printing textiles, as filler for cotton cloth, for sizing paper, in water-resistant and fireproof magnesia cements and as a laxative.