Sodium aluminate solution was concentrated to give concentrated high temperature alkali leaching for producing alumina from bauxite Bayer process, aluminum hydroxide is precipitated from it a very important step. The Al in the Bayer solution is in the form of [Al(OH) 4 ] - complex ions, which is unstable, and the precipitate of aluminum hydroxide is resolved by water. The reaction is as follows:
(1)
The precipitated aluminum hydroxide may be crystalline or colloidal, depending on the conditions of the precipitation, including the composition of the mother liquor, the temperature and the presence or absence of seed crystals. A typical Bayer solution contains about 80 kg ∕m 3 of Al 2 O 3 , and the Na 2 O ∕ Al 2 O 3 ratio (referred to as the molar ratio, the same below) is between 1.5 and 2.5. The simple dilution or cooling can only obtain colloidal hydrogen. Alumina, difficult to separate and wash. In practice, seeding is added to help crystallization separation, which is customarily called “speciesâ€. The practice is to use the newly formed 5-150 μm aluminum hydroxide crystal in the previous cycle as a seed crystal, and greatly introduce it into a new crystallization cycle and cool down. The mixture was stirred slowly for about 4 d to obtain coarse-grained aluminum hydroxide crystals. In the initial stage of precipitation, the rate of crystallization is proportional to the surface area of ​​the seed crystal. Effective agitation is necessary, otherwise fine seed crystals tend to coalesce and reduce the rate of crystallization. About 70% of aluminum was crystallized by stirring at 25 to 35 ° C for 36 hours. Certain components such as dissolved iron , vanadium and calcium salts have a negative effect on crystallization and are therefore often referred to as inhibitors or poisoning agents. These inhibitors should be limited to the specified low levels to ensure the necessary crystallization rate. The precipitated aluminum hydroxide is settled to the bottom of the tank, filtered, washed and calcined to form an alumina product. The mother liquor was concentrated by evaporation to a density of 1.45 kg·m -3 and returned to leaching.
Another method of crystallizing aluminum hydroxide from a concentrated solution of sodium aluminate is to pass carbon dioxide to neutralize excess alkali, which is conventionally referred to as "carbon", generally at 70 ° C. The relevant neutralization reaction is as follows:
(2)
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