Processing of Alumina

The process of producing pure alumina from bauxite ore (Bayer process) has changed very little since its incephon in 1893, and can be divided into three stages of extraction, precipitahon, and calcination. 

The aluminum hydroxide (gibbsite, y-Al(OH)3) and oxyhydroxide (boehmite, y-AlOOH diaspore a-AlOOH) minerals in bauxite ore are selectively extracted from the insoluble components (mostly quartz, clay minerals, and iron and tita- 

1) Minor amounts of transitional alumina phases such as T -alumina (Tilley and Eggleton, 1996) or X-alumina (Singh and Gilkes, 1995) may also be present in natural bauxite ore. 

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The overall manufacturing process of / "-alumina tubes can be subdivided into the production stages powder preparation,... 

Aluminum oxide (alumina), AI2O3, has lugh technological value. Sol-gel processing of alumina has created novel applications and improved some of its properties. Products such as catalyst earners, abrasives, fibers, films for electronic applications, aerogels, and membranes for molecular filtration have been developed based on sol-gel processing. See also Alumina Adsorption (Process) and Bauxite Aluminum and Aluminum Alloys and Engineered Materials. 

Tung and Mclninch 129) describe still another rehydroxylation process of alumina that leads to the formation of passive protons. It is assumed that a water molecule initially coordinated to a cation in an anion vacancy, dissociates, the OH group being held in the vacancy while the proton is trapped in a cation vacancy ... 

Singh, B.P, Bhattacharjee, S., and Besra, L., Influence of surface charge on maximizing solids loading in colloidal processing of alumina, Mater Lett.. 56, 475. 2002. Bhattacharjee, S., Singh, B.P, and Besra, L., Effect of additives on electrokinetic properties of colloidal alumina suspensions, J. Colloid Interf. Sci., 254, 95, 2002. Boily, J.-F. and Fein, J.B., Experimental study of cadmium-citrate co-adsorption... 

In this review, the processing of alumina is discussed next, and then its properties are tabulated and described. In a summary future uses of alumina are considered. 

Garrido L B, Aglietti E F. Reaction-sintered mullite-zirconia composites by colloidal processing of alumina-zircon-Ce02 mixtures. Mater Sci Eng A, 2004, 369 (12) 250-257. 

Sikora M, Schilling CH, Tomasik P, Li CP (2002) Dextrin plasticizers for aqueous colloidal processing of alumina. J Eur Ceram Soc 22 625-628... 

N. P. Bansal and S. R. Choi, Processing of alumina-toughened zirconia composites, NASA/TM—2003-212451, National Aeronautics and Space Administration, Glenn Research Center, Cleveland, OH (2003). 

Subsec uently, the electrolysis processes of alumina underwent major technological changes (with the automation of machines, recycling fumes, overcoming the electrolyte and metal stirring problems due to magnetic field, which made a considerable current increase possible). However the principle of the process remained the same. 

Table III. Property change during the processing of alumina fibers containing 4% silica.

Thick-fihn processing of alumina ceramics has traditionaUy been performed in oxidizing atmospheres at moderate temperatures (800-1000°C). Precious metals are used, such as gold, silver, platinum, palladium, and their combinations. A glassy phase is usuaUy incorporated in the thick-fUm paste. 

Fu G., Nazar L.F., Bain A.D. Aging processes of alumina sol-gel characterization of new aluminum polyoxycations by Al NMR spectroscopy. Chem. Mater. 1991 3 602-610 Furusaki T., Kodaira K. Preparation and properties of IniOs thin films by the sol-gel method. In High Performance Ceramic Film and Coatings. Elsevier Sdaice Pub, 1991, pp. 241-247 Furusaki T., Takahashi J., Takaha H., Kodaira K. Formation process ofSn02 thin films by sol-gel method. J. Ceram. Soc. Jpn. 1993 101 451 55 Furusaki T., Takahashi J., Kodaira K. Preparation ofTTO thin films by sol-gel method. J. CCTam. Soc. Jpn. 1994 102 200-205... 

Chatelier had patented a reduction process of alumina with sodium carbonate, a plant was built in Salindres, close to Ales in the Gard country, not far from the bauxite supply and the salt fields of La Camargue. The production in Salindres has varied between 505 kg, when the plant was started in 1860, and 2959 kg in 1880, when this process was discontinued. 

LeChatelier s principle favors the generation of the product in a non-aqueous environment. In hydrothermal processing, the solvent is a product and the level of a-AI2O3 permitted in the reaction is inhibited. Hydrothermal processing produces alumina at 405°C and 34.5 MPa in a reaction time of days to weeks. In contrast, glycothermal processing of alumina proceeds at a minimum temperature of 270°C, 4.1 MPa and 12 hours. 

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