The production of aluminum begins with the mining and beneficiation of bauxite. At the mine (usually of the surface type), bauxite ore is removed to a crusher. The crushed ore is then screened and stockpiled, ready for delivery to an alumina plant. At the alumina plant, the bauxite ore is further crushed or ground to the correct particle size for efficient extraction of the alumina through digestion by hot sodium hydroxide liquor. After removal of “red mud” (the insoluble part of the bauxite) and fine solids from the process liquor, aluminum trihydrate crystals are precipitated and calcined in rotary kilns or fluidized bed calciners to produce alumina (Al2O3). (Bounicore & Wayne 1992)
Some alumina processes include a liquor purification step. Primary aluminum is produced by the electrolytic reduction of the alumina. The alumina is dissolved in a molten bath of fluoride compounds (the electrolyte), and an electric current is passed through the bath, causing the alumina to dissociate to form liquid aluminum and oxygen.
The oxygen reacts with carbon in the electrode to produce carbon dioxide and carbon monoxide. Molten aluminum collects in the bottom of the individual cells or pots and is removed under vacuum into tapping crucibles. . Depending on the desired application, additional refining may be necessary. For demagging (removal of magnesium from the melt), hazardous substances such as chlorine and hexachloroethane are often used, which may produce dioxins and dibenzofurans. (Bounicore & Wayne 1992)
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Industrial forms of aluminum include commercially pure metal and alloys with other metals such as chromium, copper, iron, magnesium, manganese, nickel, titanium and zinc. Aluminum alloys may contain as much as fifteen percent of the alloying metals. In powder form, aluminum and its alloys are combustible in air and present a potential explosion hazard. In sheet or block forms, aluminum will not normally propagate or sustain combustion. (Metals & Alloys, 1976)
Hazards and Risks Entail in Processing
At the bauxite production facilities, dust is emitted to the atmosphere from dryers and materials- handling equipment, through vehicular movement, and from blasting. The dust is not hazardous; it can be a nuisance if containment systems are not in place, especially on the dryers and handling equipment. Other air emissions could include nitrogen oxides (NOx), sulfur dioxide (SO2), and other products of combustion from the bauxite dryers. (Paris Com, 1992)
Ore washing and beneficiation may yield process wastewaters containing suspended solids. Runoff from precipitation may also contain suspended solids. At the alumina plant, air emissions can include bauxite dust from handling and processing; limestone dust from limestone handling, burnt lime dust from conveyors and bins, alumina dust from materials handling, red mud dust and sodium salts from red mud stacks impoundments), caustic aerosols from cooling towers, and products of combustion such as sulfur dioxide and nitrogen oxides from boilers, calciners, mobile equipment, and kilns. The calciners may also emit alumina dust and the kilns, burnt lime dust. Although alumina plants do not normally discharge effluents, heavy rainfalls can result in surface runoff that exceeds what plant can use in process. (Brady & Humiston, 1982)
Hydrogen Generating Reactions
Aluminum is a very reactive metal, and the greatest industrial hazards associated with aluminum are chemical reactions. Aluminum is an excellent reducing agent, and should react with water readily to liberate hydrogen. However, the protective aluminum oxide coating protects it from reaction with moisture or oxygen. If the protective coating is broken, for example, by scratching or by amalgamation (the process of coating with a film of mercury in which the metallic aluminum dissolves; the aluminum oxide coating does not adhere to the amalgamated surface), rapid reaction with moisture and/or oxygen can occur.
The significance of this reaction is dependent upon the quantity of aluminum available to react. Aluminum is also oxidized by heat at a temperature dependent rate. (Ogle, Beddow, Chen, Butler, 1982) Aluminum metal is amphoteric (exhibits both acidic and basic characteristics). Therefore, aluminum will react with acids or bases; both reactions liberate hydrogen, a flammable gas. However, aluminum does not react with concentrated nitric acid because the oxidizing potential of the acid contributes to the formation of the protective aluminum oxide coating. (Martin, 1976)
Aluminum readily extracts oxygen from other metal oxides to form aluminum oxide with the simultaneous release of large amounts of heat (enough heat to melt the products of the reaction). For example, the reaction of aluminum with ferric oxide to produce liquid aluminum oxide and liquid iron produces temperatures approaching 3000°C (5400°F). This reaction, referred to as the "thermite reaction," has been used to weld large masses of iron and steel; when enclosed in a metal cylinder and ignited by a ribbon of magnesium has been used in incendiary bombs; and, with ammonium perchlorate added as an oxidizer, has provided the thrust for the space shuttle booster rockets. (May & Berard, 1987)
A dust explosion is a complex phenomenon involving simultaneous momentum, energy, and mass transport in a reactive multi-phase system. Aluminum particles, when in dust, powder, or flake forms from operations such as manufacturing powder, grinding, finishing, and processing, may be suspended as a dust cloud in air and consequently may ignite and cause serious damage.
If the dust cloud is unconfined, the effect is simply one of flash fire. If, however, the ignited dust cloud is at least partially confined, the heat of combustion may result in rapidly increasing pressure and produce explosion effects such as rupturing of the confining structure. Aluminum dust is not always easily ignitable, and, therefore, the hazard of dust explosions is often ignored. Minimum explosive concentrations of aluminum dust have been reported upwards from about 40 grams per cubic meter (0.04 ounces per cubic foot) of air. (May & Berard, 1987)
Effects on Health
Aluminum particles deposited in the eye may cause local tissue destruction. Aluminum salts may cause eczema, conjunctivitis, dermatoses, and irritation of the upper respiratory system via hydrolysis-liberated acid. Aluminum is not generally regarded as an industrial poison, although inhalation of finely divided aluminum powder has been reported as a cause of pneumoconiosis. In most investigative cases, however, it was found that exposure was not solely to aluminum, but to a mixture of aluminum, silica, iron dusts, and other materials.
Aluminum in aerosols has been referenced in studies involving Alzheimer's disease. Most exposures to aluminum occur in smelting and refining processes. Because aluminum may be alloyed with various metals, each metal (e.g., copper, zinc, magnesium, manganese, nickel, chromium, lead, etc.) may possibly present its own health hazards. (Buonicore & Davis, 1992)
Aluminum dust is strongly fibrogenic. Metallic aluminum dust may cause nodular lung fibrosis, interstitial lung fibrosis, and emphysema as indicated in animal experimentation, and effects appear to be correlated to particle size of the dust30; however, when exposure to aluminum dusts have been studied in man, most exposures have been found to be to other chemicals as well as aluminum. (Buonicore & Davis, 1992)
Safety Measures: Prevention and Control
The American Council of Governmental Industrial Hygienists (ACGIH) recommends the need for five separate Threshold Limit Values (TLVs) for aluminum, depending on its form (aluminum metal dust, aluminum pyro powders, aluminum welding fumes, aluminum soluble salts, and aluminum alkyls). The Occupational Safety and Health Administration (OSHA) has also established Permissible Exposure Limits (PELs) for aluminum. (May & Berard, 1987)
Pollution prevention is always preferred to the use of end-of-pipe pollution control facilities. Therefore every attempt should be made to incorporate cleaner production processes and facilities to limit, at source, the quantity of pollutants generated. In the bauxite mine, where beneficiation and ore washing are practiced, tailings slurry of 7– 9% solids is produced for disposal.
The preferred technology is to concentrate these tailings and dispose of them in the mined-out area. A concentration of 25–30% can be achieved through gravity settling in a tailings pond. The tailings can be further concentrated, using a thickener, to 30–50%, yielding a substantially volume reduced slurry. The alumina plant discharges red mud in slurry of 25–30% solids, and this also presents an opportunity to reduce disposal volumes. (May & Berard, 1987)
Today’s technology, in the form of high-efficiency deep thickeners, and large-diameter conventional thickeners, can produce a mud of 50–60% solids concentration. The lime used in the process forms insoluble solids that leave the plant along with the red mud. Recycling the lime used as a filtering aid to digestion to displace the fresh lime that is normally added at this point can minimize these lime-based solids. Finally, effluent volume from the alumina plant can be minimized or eliminated by good design and operating practices: reducing the water added to the process, segregating condensates and recycling to the process, and using rainwater in the process. (Ogle, Beddow, Chen, Butler, 1982)
- Brady, James E. and Humiston, Gerard E. (1982), General Chemistry: Principles and Structure, Third Edition, John Wiley and Sons, New York.
- Bounicore, Anthony J., and Wayne T. Davis, eds. (1992), Air Pollution Engineering Manual.New York: Van Nostrand Reinhold.
- Martin, R. (1975), "Dust-Explosion Risk with Metal Powders and Dusts," P/M Group AnnualMeeting 1975: Handling Metal Powders, Session I: Health and Safety in PowderHandling," Powder Metallurgy, No. 2.
- May, David C., and Berard, David L. (1987), "Fires and Explosions Associated with Aluminum Dust from Finishing Operations," Journal of Hazardous Materials, 17.
- "Metals and Alloys," (1976), Loss Prevention Data 7-85, Factory Mutual Engineering Corporation.
- Paris Commission. (1992), “Industrial Sectors: Best Available Technology—Primary Aluminium Industry.”
- Ogle, R. A., Beddow, J. K., Chen, L. D., and Butler, P. B. (1988), "An Investigation ofAluminum Dust Explosions," Combust. Sci. and Tech.
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