Occurrence and uses
Aluminium is the most abundant metal in the earth’s crust, where it is found in combination with oxygen, fluorine, silica, etc., but never in the metallic state. Bauxite is the principal source of aluminium. It consists of a mixture of minerals formed by the weathering of aluminium-bearing rocks. Bauxites are the richest form of these weathered ores, containing up to 55% alumina. Some lateritic ores (containing higher percentages of iron) contain up to 35% Al2O3· Commercial deposits of bauxite are mainly gibbsite (Al2O3·3H2O) and boehmite (Al2O3·H2O) and are found in Australia, Guyana, France, Brazil, Ghana, Guinea, Hungary, Jamaica and Suriname. World production of bauxite in 1995 was 111,064 million tonnes. Gibbsite is more readily soluble in sodium hydroxide solutions than boehmite and is therefore preferred for aluminium oxide production.
Aluminium is used widely throughout industry and in larger quantities than any other non-ferrous metal; worldwide primary metal production in 1995 was estimated at 20,402 million tonnes. It is alloyed with a variety of other material including copper, zinc, silicon, magnesium, manganese and nickel and may contain small amounts of chromium, lead, bismuth, titanium, zirconium and vanadium for special purposes. Aluminium and aluminium alloy ingots can be extruded or processed in rolling mills, wire-works, forges or foundries. The finished products are used in shipbuilding for internal fittings and superstructures; the electrical industry for wires and cables; the building industry for house and window frames, roofs and cladding; aircraft industry for airframes and aircraft skin and other components; automobile industry for bodywork, engine blocks and pistons; light engineering for domestic appliances and office equipment and in the jewellery industry. A major application of sheet is in beverage or food containers, while aluminium foil is used for packaging; a fine particulate form of aluminium is employed as a pigment in paints and in the pyrotechnics industry. Articles manufactured from aluminium are frequently given a protective and decorative surface finish by anodization.
Aluminium chloride is used in petroleum cracking and in the rubber industry. It fumes in air to form hydrochloric acid and combines explosively with water; consequently, containers should be kept tightly closed and protected from moisture.
Alkyl aluminium compounds. These are growing in importance as catalysts for the production of low-pressure polyethylene. They present a toxic, burn and fire hazard. They are extremely reactive with air, moisture and compounds containing active hydrogen and therefore must be kept under a blanket of inert gas.
For the production of aluminium alloys, refined aluminium is melted in oil or gas-fired furnaces. A regulated amount of hardener containing aluminium blocks with a percentage of manganese, silicon, zinc, magnesium, etc. is added. The melt is then mixed and is passed into a holding furnace for degassing by passing either argon-chlorine or nitrogen-chlorine through the metal. The resultant gas emission (hydrochloric acid, hydrogen and chlorine) has been associated with occupational illnesses and great care should be taken to see that appropriate engineering controls capture the emissions and also prevent it from reaching the external environment, where it can also cause damage. Dross is skimmed off the surface of the melt and placed in containers to minimize exposure to air during cooling. A flux containing fluoride and/or chloride salts is added to the furnace to assist in separation of pure aluminium from the dross. Aluminium oxide and fluoride fumes may be given off so that this aspect of production must also be carefully controlled. Personal protective equipment (PPE) may be required. The aluminium smelting process is described in the chapter Metal processing and metal working industry. In the casting shops, exposure to sulphur dioxide may also occur.
A wide range of different crystalline forms of aluminium oxide is used as smelter feed stock, abrasives, refractories and catalysts. A series of reports published in 1947 to 1949 described a progressive, non-nodular interstitial fibrosis in the aluminium abrasives industry in which aluminium oxide and silicon were processed. This condition, known as Shaver’s disease, was rapidly progressive and often fatal. The exposure of the victims (workers producing alundum) was to a dense fume comprising aluminium oxide, crystalline free-silica and iron. The particulates were of a size range that made them highly respirable. It is likely that the preponderence of disease is attributable to the highly damaging lung effects of the finely divided crystalline free-silica, rather than to the inhaled aluminium oxide, although the exact aetiology of the disease is not understood. Shaver’s disease is primarily of historical interest now, since no reports have been made in the second half of the 20th century.
Recent studies of the health effects of high level exposures (100 mg/m3) to the oxides of aluminium amongst workers engaged in the Bayer process (described in the chapter Metal processing and metal working industry) have demonstrated that workers with more than twenty years of exposure can develop pulmonary alterations. These changes are clinically characterized by minor, predominantly asymptomatic degrees of restrictive pulmonary function changes. The chest x-ray examinations revealed small, scanty, irregular opacities, particularly at the lung bases. These clinical responses have been attributed to deposition of dust in the lung paraenchyma, which was the result of very high occupational exposures. These signs and symptoms cannot be compared to the extreme response of Shaver’s disease. It should be noted that other epidemiological studies in the United Kingdom regarding widespread alumina exposures in the pottery industry have produced no evidence that the inhalation of alumina dust produces chemical or radiographic signs of pulmonary disease or dysfunction.
The toxicological effects of aluminium oxides remain of interest because of its commerical importance. The results of animal experiments are controversial. An especially fine (0.02 μm to 0.04 μm), catalytically active aluminium oxide, uncommonly used commercially, can cause lung changes in animals dosed by injection directly into the lung airways. Lower dose effects have not been observed.
It should also be noted that so-called “potroom asthma” which has frequently been observed among workers in aluminium processing operations, is probably attributable to the exposures to fluoride fluxes, rather than to the aluminium dust itself.
The production of aluminium has been classified as a Group 1, known human carcinogenic exposure situation, by the International Agency for Research on Cancer (IARC). As with the other diseases described above, the carcinogenicity is most likely attributable to the other substances present (e.g., polycyclic aromatic hydrocarbons (PAHs) and silica dust), although the exact role of the alumina dusts are simply not understood.
Some data on the absorption of high levels of aluminium and nervous tissue damage are found among individuals requiring kidney dialysis. These high levels of aluminium have resulted in severe, even fatal brain damage. This response, however, has also been observed in other patients undergoing dialysis but who did not have similar elevated brain aluminium level. Animal experiments have been unsuccessful in replicating this brain response, or Alzheimer’s disease, which has also been postulated in the literature. Epidemiological and clinical follow-up studies on these issues have not been definitive and no evidence of such effects has been observed in the several large-scale epidemiological studies of aluminium workers.