Manufacturing alumina

The production of atomic number 13 begins with the dig and mineral extraction of bauxite. At the mine (usually of the surface type), bauxite ore is removed to a crusher. The lowly ore is then screened and stockpiled, ready for delivery to an alumina plant. At the alumina plant, the bauxite ore is go on crushed or ground to the correct particle size for streamlined extraction of the alumina through digestion by hot sodium hydroxide hard liquor. After removal of rosy mud (the in dissolvable part of the bauxite) and ticket solids from the move liquor, aluminium trihydrate crystals are precipitated and calcined in rotary kilns or fluidized rump calciners to produce alumina (Al2O3). (Bounicore & Wayne 1992)Some alumina processes include a liquor purification step. Primary atomic number 13 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 lowway is passed through the b ath, causing the alumina to dissociate to form liquid atomic number 13 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 whitethorn be necessary. For demagging (removal of magnesium from the melt), hazardous substances such as chlorine and hexachloroethane are much mathematical functiond, which may produce dioxins and dibenzofurans. (Bounicore & Wayne 1992)In make cleanrial forms of aluminum include commercially fine metal and alloys with other metals such as chromium, copper, push, magnesium, manganese, nickel, titanium and zinc. aluminium alloys may contain as much as fifteen percent of the alloying metals. In grind form, aluminum and its alloys are combustible in air and render a potential explosion hazard. In sheet or gourmandize form s, aluminum will not normally propagate or apply combustion. (Metals & Alloys, 1976)Hazards and Risks Entail in ProcessingAt the bauxite production facilities, stud is emitted to the aureole from dryers and materials- handling equipment, through vehicular movement, and from blasting. The dust is not hazardous it female genitalia be a nuisance if containment systems are not in place, oddly on the dryers and handling equipment. Other air emissions could include due north oxides (NOx), southward dioxide (SO2), and other products of combustion from the bauxite dryers. (Paris Com, 1992)Ore washing and beneficiation may yield process wastewaters containing suspend solids. Runoff from precipitation may also contain hang up solids. At the alumina plant, air emissions foundation include bauxite dust from handling and processing hydrated limestone dust from limestone handling, burned lime dust from conveyors and bins, alumina dust from materials handling, red mud dust and sodium s alts from red mud haemorrhoid 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, intelligent rainfalls put up result in surface runoff that exceeds what plant tail end use in process. (Brady & Humiston, 1982)Hydrogen Generating ReactionsAluminum is a very oxidizable metal, and the greatest industrial hazards associated with aluminum are chemical reactions. Aluminum is an glorious bring down agent, and should react with water readily to resign hydrogen. However, the protective aluminum oxide cultivation 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 dissolve s the aluminum oxide coating does not adhere to the amalgamated surface), rapid reaction with moisture and/or oxygen peck 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 acerb and basic characteristics). Therefore, aluminum will react with acids or bases both reactions liberate hydrogen, a flammable gas. However, aluminum does not react with sign ond nitric acid be example the oxidizing potential of the acid contributes to the formation of the protective aluminum oxide coating. (Martin, 1976)Thermite ReactionsAluminum readily extracts oxygen from other metal oxides to form aluminum oxide with the coinciding 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 overture 3000C (5400F). This reaction, referred to as the thermite reaction, has been used to weld large passeles of iron and steel when enclosed in a metal cylinder and light by a ribbon of magnesium has been used in incitive bombs and, with ammonium perchlorate added as an oxidizer, has provided the thrust for the space shuttle booster rockets. (May & Berard, 1987) spit ExplosionsA 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 wherefore may ignite and cause serious damage.If the dust cloud is unconfined, the put 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 good 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 standard (0.04 ounces per cubic foot) of air. (May & Berard, 1987)Effects on healthAluminum particles deposited in the bosom 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 largely regarded as an industrial poison, although inhalation of finely divided aluminum powder has been reported as a cause of pneumoconiosis. In most fact-finding cases, however, it was found that exposure was not solely to aluminum, but to a intermixture of aluminum, silica, iron dusts, and other materials.Aluminum in aerosols has been referenced in studies involving Alzheimers di sease. or so 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 generate its own health hazards. (Buonicore & Davis, 1992)ImplicationAluminum dust is strongly fibrogenic. metallic aluminum dust may cause nodular lung fibrosis, interstitial lung fibrosis, and pulmonary emphysema as indicated in animal experimentation, and effects appear to be fit 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 easily as aluminum. (Buonicore & Davis, 1992)Safety Measures Prevention and ControlThe 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, al uminum soluble salts, and aluminum alkyls). The Occupational Safety and Health Administration (OSHA) has also establish 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 make 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 2530% can be achieved through gravity settling in a tailings pond. The tailings can be further concentrated, using a thickener, to 3050%, grant a substantially volume diluted slurry. The alumina plant discharges red mud in slurry of 2530% solids, and this also presents an opportunity to reduce disposal volumes. (May & Berard, 1987)Todays technology, in the form of high-efficiency deep thickeners, and large-diameter courtly thickeners, can produce a mud of 5060% solids concentration. The lime used in the process forms insoluble solids that leave the plant on with the red mud. Recycling the lime used as a filtering wait on 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)ReferencesBrady, James E. and Humiston, Gerard E. (1982), ecumenical 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), corpse-Explosion Risk with Metal Powders and Dusts, P/M assemblage 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 AluminumDust from Finishing Operations, Journal of Hazardous Materials, 17.Metals and Alloys, (1976), Loss Prevention Data 7-85, pulverization Mutual EngineeringCorporation.Paris Commission. (1992), Industrial Sectors Best Available technologyPrimary AluminiumIndustry.Ogle, R. A., Beddow, J. K., Chen, L. D., and Butler, P. B. (1988), An Investigation ofAluminum Dust Explosions, Combust. Sci. and Tech.