'Manufacturing alumina\r'

'The production of atomic number 13 begins with the mining and ore edgeing of bauxite. At the mine (usually of the excavate type), bauxite ore is removed to a crusher. The crushed ore is then screened and stockpiled, ready for deliin truth to an aluminium oxide plant. At the alumina plant, the bauxite ore is further crushed or ground to the correct particle size for expeditious extraction of the alumina through digestion by bouncy sodium hydroxide liquor. After removal of â€Å" departure flub” (the insoluble part of the bauxite) and fine solids from the process liquor, aluminium trihydrate crystals atomic number 18 precipitated and calcined in rotary kilns or fluidized bed calciners to urinate alumina (Al2O3). (Bounicore & Wayne 1992)\r\nsome(a) alumina processes include a liquor nuance step. pristine atomic number 13 is invoked by the electrolytic simplification of the alumina. The alumina is dissolved in a melt bath of fluoride compounds (the electrol yte), and an electric current is passed through the bath, do the alumina to dissociate to ashes liquid atomic number 13 and oxygen.\r\nThe oxygen moves with blow in the electrode to produce carbon dioxide and carbon monoxide. Molten atomic number 13 collects in the git of the individual cells or pots and is removed under make clean into tapping crucibles. . Depending on the desired application, additional refining may be necessary. For demagging (removal of magnesium from the melt), dotty substances such as chlorine and hexachloroethane argon often utilize, which may produce dioxins and dibenzofurans. (Bounicore & Wayne 1992)\r\nIn remainsrial forms of atomic number 13 include commercially pure metal and alloys with early(a) metals such as chromium, copper, bid, 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 authority fit hazard. In sheet or block forms, aluminum will non normally propagate or sustain combustion. ( admixtures & Alloys, 1976)\r\nHazards and Risks Entail in process\r\nAt the bauxite production facilities, disperse is emitted to the atmosphere from dryers and materials- discussion equipment, through vehicular movement, and from blasting. The trunk is not hazardous; it sack up be a nuisance if containment systems are not in place, especially on the dryers and manipulation equipment. Other air emissions could include nitrogen oxides (NOx), southward dioxide (SO2), and other products of combustion from the bauxite dryers. (genus Paris Com, 1992)\r\nOre washing and beneficiation may yield process wastewaters containing suspended solids. Runoff from rashness may in like manner contain suspended solids. At the alumina plant, air emissions can include bauxite sprinkle from handling and processing; unslaked burnt unslaked quicklimestone sparge from limestone handling, burnt lime dust from conveyors and bins, alumina dust from materials handling, red mud dust and sodium salts from red mud lade 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 emanation effluents, heavy rainfalls can result in surface runoff that exceeds what plant can use in process. (Brady & Humiston, 1982)\r\nHydrogen Generating Reactions\r\naluminum is a very reactive metal, and the greatest industrial hazards associated with aluminum are chemical reactions. aluminium is an excellent reduction agent, and should react with water readily to liberate hydrogen. However, the protective(p) aluminum oxide coating protects it from reaction with moisture or oxygen. If the protective coating is broken, for example, by scratching or b y amalgamation (the process of coating with a conduct of mercury in which the metallic aluminum dissolves; the aluminum oxide coating does not adhere to the amalgamated surface), quick reaction with moisture and/or oxygen can occur.\r\nThe significance of this reaction is dependent upon the quantity of aluminum available to react. Aluminum is also oxidized by stir up at a temperature dependent rate. (Ogle, Beddow, Chen, Butler, 1982) Aluminum metal is amphoteric (exhibits some(prenominal) acidic and base characteristics). Therefore, aluminum will react with acids or bases; both reactions liberate hydrogen, a flammable gas. However, aluminum does not react with slenderized nitric acid be defecate the oxidizing potential of the acid contributes to the formation of the protective aluminum oxide coating. (Martin, 1976)\r\nThermite Reactions\r\nAluminum readily extracts oxygen from other metal oxides to form aluminum oxide with the simultaneous release of large amounts of pepperin ess (enough heat to melt the products of the reaction). For example, the reaction of aluminum with ferrous oxide to produce liquid aluminum oxide and liquid iron produces temperatures approaching 3000°C (5400°F). This reaction, referred to as the â€Å"thermite reaction,” has been used to dyers mignonette large masses of iron and steel; when wrap in a metal cylinder and flash by a ribbon of magnesium has been used in incendiary bombs; and, with ammonium perchlorate added as an oxidizer, has provided the fox for the space shuttle booster rockets. (May & Berard, 1987)\r\n ashes Explosions\r\nA dust explosion is a tangled phenomenon involving simultaneous momentum, energy, and mass transport in a reactive multi-phase system. Aluminum particles, when in dust, powder, or bit forms from operations such as manufacturing powder, grinding, finishing, and processing, may be suspended as a dust asperse in air and consequently may ignite and cause serious damage.\r\nIf t he dust cloud is unconfined, the answer is simply one of flash fire. If, however, the ignited dust cloud is at least partially confined, the heat of combustion may result in speedily increasing pressure and produce explosion make 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)\r\n set up on Health\r\nAluminum particles deposited in the heart and soul 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 investigativ e cases, however, it was found that picture was not solely to aluminum, but to a categorization of aluminum, silica, iron dusts, and other materials.\r\nAluminum in aerosols has been write in studies involving Alzheimers disease. Most exposures to aluminum occur in smelting and refining processes. Because aluminum may be alloy with various metals, each metal (e.g., copper, zinc, magnesium, manganese, nickel, chromium, lead, etc.) may mayhap present its own health hazards. (Buonicore & Davis, 1992)\r\n price reduction\r\nAluminum 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)\r\n safe Measures: barroom and Control\r\nT he American Council of Governmental industrial Hygienists (ACGIH) recommends the need for five separate Threshold gear up 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)\r\n contaminant prevention is always preferred to the use of end-of-pipe contaminant 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.\r\nThe preferred technology is to concentrate these tailings and dispose of them in the mined-out area. A concentration of 25â€30% can be achieved through s olemnity settling in a tailings pond. The tailings can be further concentrated, using a thickener, to 30â€50%, yielding a substantially volume bring down slurry. The alumina plant discharges red mud in slurry of 25â€30% solids, and this also presents an opportunity to issue disposal volumes. (May & Berard, 1987)\r\nToday’s technology, in the form of high-efficiency deep thickeners, and large-diameter conventional thickeners, can produce a mud of 50â€60% solids concentration. The lime used in the process forms insoluble solids that relegate the plant along with the red mud. Recycling the lime used as a filtering aid to digestion to make a motion the fresh lime that is normally added at this intimate 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 ra inwater in the process. (Ogle, Beddow, Chen, Butler, 1982)\r\nReferences\r\nBrady, James E. and Humiston, Gerard E. (1982), General Chemistry: Principles and Structure,\r\nThird Edition, fundament Wiley and Sons, virgin York.\r\nBounicore, Anthony J., and Wayne T. Davis, eds. (1992), Air Pollution Engineering Manual.\r\nNew York: Van Nostrand Reinhold.\r\nMartin, R. (1975), â€Å" distribute-Explosion Risk with Metal Powders and Dusts,” P/M Group Annual\r\nMeeting 1975: Handling Metal Powders, Session I: Health and Safety in Powder\r\nHandling,” Powder Metallurgy, No. 2.\r\nMay, David C., and Berard, David L. (1987), â€Å"Fires and Explosions Associated with Aluminum\r\nDust from Finishing Operations,” Journal of Hazardous Materials, 17.\r\nâ€Å"Metals and Alloys,” (1976), Loss Prevention Data 7-85, Factory Mutual Engineering\r\nCorporation.\r\nParis Commission. (1992), â€Å"Industrial Sectors: Best Available Technologyâ€Primary Aluminium\r\nIndus try.”\r\nOgle, R. A., Beddow, J. K., Chen, L. D., and Butler, P. B. (1988), â€Å"An Investigation of\r\nAluminum Dust Explosions,” Combust. Sci. and Tech.\r\n \r\n'