Bayer process alumina refining

Small amounts of sodium aluminate are prepared in the lab by fusion of equimolar quantities of sodium carbonate [497-19-8] and aluminum acetate [139-12-8], A1(C2H202)3, at 800°C (4). Other methods involve reaction of sodium hycboxide with amorphous alumina or aluminum [7429-90-5] metal. Commercial quantities of sodium aluminate are made from hydrated alumina, in the form of aluminum hydroxy oxide [24623-77-6], AIO(OH), or aluminum hycboxide [21645-51 -2], Al(OH)3, a product of the Bayer process (5,6) which is used to refine bauxite [1318-16-7], the principal aluminum ore. 

FIGURE 3.1 Bayer process for alumina refining. (From U.S. EPA, Profile of the Nonferrous Metals Industry, publication EPA/310-R-95-010, U.S. EPA, Washington, DC, September 1995.)... 

The Bayer Process, patented by Karl Josef Bayer in 1888 (Pearson (1)) is still widely adopted in refining alumina trihydrate from bauxite. Despite its long history, the unit operations involved in the process have remained basically unchanged with the reversible reaction below as its backbone. 

Iron(III) oxide or alumina is refined from bauxite. Approximately 175 million tons of bauxite are mined annually worldwide, with virtually all of this processed into alumina. Alumina is a white crystalline substance that resembles salt. Approximately 90% of all alumina is used for making aluminum, with the remainder used for abrasives and ceramics. Alumina is produced from bauxite using the Bayer process patented in 1887 by Austrian Karl Josef Bayer (1847-1904). The Bayer process begins by grinding the bauxite and mixing it with sodium hydroxide in a digester. The sodium hydroxide dissolves aluminum oxide components to produce aluminum hydroxide compounds. For gibbsite, the reaction is Al(OH)3 + NaOH —> Al(OH)4 + Na+. Insoluble impurities such as silicates, titanium oxides, and iron oxides are removed from the solution while sodium hydroxide is recovered and recycled. Reaction conditions are then... 

The principal method used in producing aluminum metal involves three major steps refining of bauxite by the Bayer process to produce alumina, electrolytic reduction of alumina by the Hall-Heroult process to produce aluminum, and casting of aluminum into ingots (Browning 1969 Dinman 1983 IARC 1984). 

The recovery of aluminum metal is divided into two steps, i. e., the production of pure alumina (Bayer Process) and its molten salt electrolysis. Raw aluminum obtained by reduction electrolysis already has a high purity (99.5-99.7%). Refining methods for raw aluminum to obtain higher purities include the segregation process (99.94-99.99% Al) and three-layer electrolysis (99.99-99.998% Al) [142, 236]. Besides these, processes are available whereby the aluminum is anodically dissolved in an organic electrolyte and then cathodically deposited [37, 118, 217, 221]. The dissolution as well as the deposition process contribute to the electrolytic refining of aluminum. 

Nearly all A1 is obtained from bauxite, which contains 40- 60% alumina. Approximately 90% is refined to alumina by the Bayer process, after which the alumina is reduced to A1 metal by electrolysis in the Hall-Heroult process. In the year 2000, worldwide A1 production was -24x10 metric tons, while in 2001 the US production was -2.6x10 metric tons (USGS... 

Western-world bauxite production in 1988 totaled about 90 x 10 t, approximately 90% of which was refined to aluminum hydroxide by the Bayer process. Most of the hydroxide was then calcined to alumina and consumed in making aluminum metal. The balance, which constituted about 2.3 x 10 t in 1988 (Table 2), was consumed in production of abrasives (qv) adhesives (qv) calcium aluminate cement used in binding ceramics (qv) and refractories (qv) catalysts used in petrochemical processes and automobile catalytic converter systems (see Petroleum Exhaust control, automotive) ceramics that insulate electronic components such as semiconductors and spark plugs chemicals such as alum, aluminum halides, and zeoHte countertop materials for kitchens and baths cultured marble fire-retardant filler for acryhc and plastic materials used in automobile seats, carpet backing, and insulation wrap for wire and cable (see Flame retardants) paper (qv) cosmetics (qv) toothpaste manufacture refractory linings for furnaces and kilns and separation systems that remove impurities from Hquids and gases. 

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