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Bayer Process Chemistry

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Bayer Process Chemistry BBaayyeerr PPrroocceessss CChheemmiissttrryy By Kenneth R Seecharran Process Engineer Alumina Plant, Guymine, Linden The process of producing pure alumina from bauxite - the Bayer Process - has changed very little since its invention by Austrian chemist Karl Josef Bayer while working in Saint Petersburg, Russia. He develop a method for supplying alumina to the textile industry (it was used as a mordant in dyeing cotton), in 1888. He found that the aluminium hydroxide that precipitated from alkaline solution was crystalline and could be easily filtered and washed, while that precipitated from acid medium by neutralization was gelatinous and difficult to wash. Figure 1: Karl Josef Bayer The alumina produced today, can then be used for various industrial purposes or smelted to provide aluminum. The first commercial plant was commissioned in 1893. The Bayer process can be considered in three stages: 1 Figure 2: Aerial view of a Bayer processing plant 2 Extraction The aluminium-bearing minerals in bauxite - Gibbsite, Böhmite and Diaspore - are selectively extracted from the insoluble components (mostly oxides) by dissolving them in a solution of sodium hydroxide (caustic soda): The first step in the process is the mixing of ground bauxite into a solution of sodium hydroxide. By applying steam and pressure in tanks containing the mixture, the bauxite slowly dissolves. The alumina released reacts with the sodium hydroxide to form sodium aluminate Gibbsite: + - - + Al(OH)3 + Na + OH Î Al(OH)4 + Na Böhmite and Diaspore: + - - + AlO(OH) + Na + OH + H2O Î Al(OH)4 + Na Depending on the quality of the ore it may be washed to beneficiate it prior to processing. The ore is crushed and milled to reduce the particle size and make the minerals more amenable to extraction. It is then combined with the process liquor and sent in a slurry form to a heated pressure digester. Conditions within the digester (concentration, temperature and pressure) are set according to the properties of the bauxite ore. Ores with a high Gibbsite content o -1 can be processed at 140 C with about 110gl of Na2O. Processing of Böhmite on the other hand requires harsher conditions, between 200 and 240°C and -1 140-170 gl of Na2O. The pressure is not important for the process, as such but is defined by the steam pressure during the actual process conditions. At 240°C the pressure is approximately 35 atmospheres (atm). Although higher temperatures are often theoretically advantageous there are several disadvantages including corrosion problems and the possibility of oxides other than alumina dissolving into the caustic liquor, leading to impurities problems in the final product, and scaling of pipes, pumps and tanks. After digestion about 30% of the bauxite mass remains in suspension as a thin red mud slurry of silicates, and oxides of iron and titanium. The mud-laden liquor leaving the digestion vessel is flash-cooled to atmospheric boiling point by flowing through a series of flash vessels which operate at successively lower pressures. The flash steam generated is used to preheat incoming caustic liquor in tubular heat exchangers located parallel to the flash tank line. Condensate from the heat exchangers is used for boiler feed water and washing waste mud. 3 After the extraction stage the insoluble bauxite residue must be separated from the Aluminium-containing liquor by a process known as settling. The mixture of solid impurities is called red mud, and presents a disposal problem. The liquor is purified as much as possible through filters before being transferred to the precipitators. Slaked lime is added to dilute caustic liquor in the washing process to remove carbonate (Na2CO3) which forms by reaction with compounds in bauxite and also from the atmosphere and which reduces the effectiveness of liquor to dissolve alumina. Lime regenerates caustic soda, allowing the insoluble calcium carbonate to be removed with the waste mud. The insoluble red mud from the first settling stage is thickened and washed to recover the caustic soda, which is then recycled back into the main process. The mud is washed with fresh water in counter-current washing trains to recover the soda and alumina content in the mud before being pumped to large disposal dams. Precipitation Crystalline aluminium trihydroxide (Gibbsite), conveniently named "hydrate", is then precipitated from the digestion liquor: - + + - Al(OH)4 + Na Î Al(OH)3 + Na + OH This is basically the reverse of the extraction process, except that the product's nature is carefully controlled by plant conditions, including seeding or selective nucleation, precipitation temperature and cooling rate. The cooled pregnant liquor flows to rows of precipitation tanks which are seeded with crystalline tri hydrate alumina, usually of an intermediate or fine particle size to promote crystal growth. Each precipitation tank is agitated, with a holding time of about three hours. During the 25-30 hours pass through precipitation, alumina of various crystal sizes is produced. The entry temperature and the temperature gradient across the row, seed rate and caustic concentration are control variables used to achieve the required particle size distribution in the product. The finished mix of crystal sizes is settled from the liquor stream and separated into three size ranges in three stages "gravity" classification tanks. The primary classifiers collect the coarse fraction which becomes the product “hydrate”. The intermediate and fine crystals from the secondary and tertiary classifiers are washed and returned to the precipitation tanks as coarse and fine seed. The process engineer decides on the ratio of coarse and fine seed to be added, based on screen analyses. 4 The slurry of coarse “hydrate” (Al2O3.3H2O) from the primary thickeners is pumped to hydrate storage tanks and is filtered and washed on horizontal-table vacuum filters to remove and recycle process liquor, prior to calcination in fluid bed or rotary calciners. Heating the slurry before filtration, with waste steam, gives a cleaner, drier filter cake and leads to less soda loss. The heated cake entering the calciner also helps fuel efficiency. Calcination "Hydrate", is calcined to form alumina for the aluminium smelting process. In the calcination process water is driven off to form alumina, this takes place at 1050oC: 2Al(OH)3 Î Al2O3 + 3H2O The calcination process must be carefully controlled since it dictates the properties of the final product. Sandy alumina particles are 90%+ 45 µm (microns) in size, this is the product sought after by most smelters. A large amount of the alumina so produced is then subsequently smelted in the Hall-Héroult process, named after inventors Charles Hall and Paul-Louis Héroult, in order to produce aluminium. Smelting The purified Al2O3 is dissolved in molten cryolite, Na3AlF6, which has a melting point of 1012oC and is an effective conductor of electric current. In the following schematic diagram of the electrolysis cell graphite rods are employed as anodes and are consumed in the electrolysis process. The cell electrolytic reaction is: 2Al2O3 + 3C Î 4Al(l) + 3CO2(g) 5 Figure 3: Aluminium smelting The production of one ton of aluminum requires about 65-70 GJ (18-20 MWh) and about half a ton of carbon. Figure 4: Schematic diagram of aluminium production 6 Figure 5: 15-year aluminium price chart Written in 1979 and updated in 2010 7.
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