Gunnar Nordberg
Magnesium (Mg) is the lightest structural metal known. It is 40% lighter than aluminium. Metallic magnesium can be rolled and drawn when heated between 300 and 475 ºC, but is brittle below this temperature and is apt to burn if heated much above it. It is soluble in, and forms compounds with, a number of acids, but is not affected by hydrofluoric or chromic acids. Unlike aluminium, it is resistant to alkali corrosion.
Occurrence and Uses
Magnesium does not exist in a pure state in nature, but is generally found in one of the following forms: dolomite (CaCO3·MgCO3), magnesite (MgCO3), brucite (Mg(OH)2), periclase (MgO), carnallite (KClMgCl2·6H2O) or kieserite (MgSO4·H2O). In addition, it is found as a silicate in asbestos and talc. Magnesium is so widely distributed over the earth that facilities for processing and transporting the ore are often the determining factors in selecting a site for mining.
Magnesium is used, mainly in alloy form, for components of aircraft, ships, automobiles, machinery and hand tools for which both lightness and strength are required. It is used in the manufacture of precision instruments and optical mirrors, and in the recovery of titanium. Magnesium is also extensively used in military equipment. Because it burns with such intense light, magnesium is widely used in pyrotechnics, signal flares, incendiary and tracer bullets, and in flash bulbs.
Magnesium oxide has a high melting point (2,500 ºC) and is often incorporated into the linings of refractories. It is also a component of animal feeds, fertilizers, insulation, wallboard, petroleum additives and electrical heating rods. Magnesium oxide is useful in the pulp and paper industry. In addition, it serves as an accelerator in the rubber industry and as a reflector in optical instruments.
Other important compounds include magnesium chloride, magnesium hydroxide, magnesium nitrate and magnesium sulphate. Magnesium chloride is a component of fire extinguishers and ceramics. It is also an agent in fireproofing wood and textile and paper manufacture. Magnesium chloride is a chemical intermediate for magnesium oxychloride, which is used for cement. A mixture of magnesium oxide and magnesium chloride forms a paste which is useful for floors. Magnesium hydroxide is useful for the neutralization of acids in the chemical industry. It is also used in uranium processing and in sugar refining. Magnesium hydroxide serves as a residual fuel-oil additive and an ingredient in toothpaste and antacid stomach powder. Magnesium nitrate is used in pyrotechnics and as a catalyst in the manufacture of petrochemicals. Magnesium sulphate has numerous functions in the textile industry, including weighting cotton and silk, fireproofing fabrics, and dyeing and printing calicos. It also finds use in fertilizers, explosives, matches, mineral water, ceramics and cosmetic lotions, and in the manufacture of mother-of-pearl and frosted papers. Magnesium sulphate increases the bleaching action of chlorinated lime and acts as a water-correcting agent in the brewing industry and a cathartic and analgesic in medicine.
Alloys. When magnesium is alloyed with other metals, such as manganese, aluminium and zinc, it improves their toughness and resistance to strain. In combination with lithium, cerium, thorium and zirconium, alloys are produced which have an enhanced strength-to-weight ratio, along with considerable heat-resisting properties. This renders them invaluable in the aircraft and aerospace industries for the construction of jet engines, rocket launchers and space vehicles. A large number of alloys, all containing over 85% magnesium, are known under the general name of Dow metal.
Hazards
Biological roles. As an essential ingredient of chlorophyll, the magnesium requirements of the human body are largely supplied by the consumption of green vegetables. The average human body contains about 25 g of magnesium. It is the fourth most abundant cation in the body, after calcium, sodium and potassium. The oxidation of foods releases energy, which is stored in the high-energy phosphate bonds. It is believed that this process of oxidative phosphorylation is carried out in the mitochondria of the cells and that magnesium is necessary for this reaction.
Experimentally produced magnesium deficiency in rats leads to a dilation of the peripheral blood vessels and later to hyperexcitability and convulsions. Tetany similar to that associated with hypocalcaemia occurred in calves fed only milk. Older animals with magnesium deficiency developed “grass staggers”, a condition which appears to be associated with malabsorption rather than with a lack of magnesium in the fodder.
Cases of magnesium tetany resembling those caused by calcium deficiency have been described in humans. In the reported cases, however, a “conditioning factor”, such as an excessive vomiting or fluid loss, has been present, in addition to inadequate dietary intake. Since this tetany clinically resembles that caused by calcium deficiency, a diagnosis can be made only by determining the blood levels of calcium and magnesium. Normal blood levels range from 1.8 to 3 mg per 100 cm3, and it has been found that persons tend to become comatose when the blood concentration approaches 17 mg per cent. “Aeroform tumours” due to the evolution of hydrogen have been produced in animals by introducing finely divided magnesium into the tissues.
Toxicity. Magnesium and alloys containing 85% of the metal may be considered together in their toxicological properties. In industry, their toxicity is regarded as low. The most frequently used compounds, magnesite and dolomite, may irritate the respiratory tract. However, the fumes of magnesium oxide, as those of certain other metals, can cause metal fume fever. Some investigators have reported a higher incidence of digestive disorders in magnesium plant workers and suggest that a relationship may exist between magnesium absorption and gastroduodenal ulcers. In foundry-casting magnesium or high-magnesium alloys, fluoride fluxes and sulphur-containing inhibitors are used in order to separate the molten metal from the air with a layer of sulphur dioxide. This prevents burning during the casting operations, but the fumes of fluorides or of sulphur dioxide could present a greater hazard.
The greatest danger in handling magnesium is that of fire. Small fragments of the metal, such as would result from grinding, polishing or machining, can readily be ignited by a chance spark or flame, and as they burn at a temperature of 1,250ºC, these fragments can cause deep destructive lesions of the skin. Accidents of this type have occurred when a tool was sharpened on a wheel which was previously used to grind magnesium alloy castings. In addition, magnesium reacts with water and acids, forming combustible hydrogen gas.
Slivers of magnesium penetrating the skin or entering deep wounds could cause “aeroform tumours” of the type already mentioned. This would be rather exceptional; however, wounds contaminated with magnesium are very slow to heal. Fine dust from the buffing of magnesium could be irritating to the eyes and respiratory passages, but it is not specifically toxic.
Safety and Health Measures
As with any potentially hazardous industrial process, constant care is needed in handling and working magnesium. Those engaged in casting the metal should wear aprons and hand protection made of leather or some other suitable material to protect them against the “spatter” of small particles. Transparent face shields should also be worn as face protection, especially for the eyes. Where workers are exposed to magnesium dust, contact lenses should not be worn and eyewash facilities should be immediately available. Workers machining or buffing the metal should wear overalls to which small fragments of the metal will not adhere. Sufficient local exhaust ventilation is also essential in areas where magnesium oxide fumes may develop, in addition to good general ventilation. Cutting tools should be sharp, as blunt ones may heat the metal to the point of ignition.
Buildings in which magnesium is cast or machined should be constructed, if possible, of non-flammable materials and without ledges or protuberances on which magnesium dust might accumulate. The accumulation of shavings and “swarf” should be prevented, preferably by wet sweeping. Until final disposal, the scrapings should be collected in small containers and placed apart at safe intervals. The safest method for disposal of magnesium waste is probably wetting and burying.
Since the accidental ignition of magnesium presents a serious fire hazard, fire training and adequate firefighting facilities are essential. Workers should be trained never to use water in fighting such a blaze, because this merely scatters the burning fragments, and may spread the fire. Among the materials which have been suggested for the control of such fires are carbon and sand. Commercially prepared firefighting dusts are also available, one of which consists of powdered polyethylene and sodium borate.