Alumina production technology (1)

1. alumina production technology and production worldwide in 1995, 68 in the production of alumina plant, metallurgical grade alumina output of 47.4 million tons, 3.69 million tons of specialty alumina output, a total of 51.09 million tons. In 1997, there was an alumina plant in production with a production of 50.58 million tons.
In addition to the use of multiple production methods in countries such as China and Russia, Bayer is used in other countries. The determination of the production method depends mainly on the nature of the raw materials to be treated.
(1) Foreign alumina production technology and production.
(2) China's alumina production technology and production situation There are 6 alumina plants in China. In 1999, alumina production was 3.32 million tons.
1 Shandong Aluminum Company Alumina Plant This plant is China's first alumina plant, using soda lime sintering method, with an annual output of 500,000 tons of alumina.
2 Zhengzhou Aluminum Plant In 1999, the annual output was 800,000 tons of alumina, and in 1998, it produced 730,000 tons of alumina.
The hybrid method is China's original production method. It was first practiced in Zhengzhou Aluminum Plant. It has the advantages of tandem method and flexibility in production organization. It is suitable for the treatment of insoluble low- iron bauxite in Henan Province.
Zhengzhou Aluminum Factory uses a unique technology in its production – Bayer Pipeline Dissolution Technology.
Process: atmospheric desiliconization → high pressure diaphragm pump → first casing slurry preheating → 2-9 secondary steam casing preheating → casing molten salt heating → staying pipe section → 8 grade self-evaporator. Lime milk is added to the staying pipe section.
The main equipments are: 12 cylinder horizontal single-acting diaphragm pumps, 4-tube tubular preheater, sluice tube section Ф355mm, length 915m.
Main technical indicators: raw pulp processing capacity 300m 3 /h; mother liquor concentration 180g / L Na 2 O; dissolution temperature 270-280 ° C; dissolution time 40-45min; alumina dissolution rate > 92%; equipment operating rate > 80%; The heating method is molten salt heating, molten salt furnace fuel; alumina production is 165,000 tons/year.
3 Guizhou Aluminum Plant The annual production capacity is 400,000 tons of alumina, using the combined method (mixed method) production process; in 1998, it produced 420,000 tons of alumina. The production technology and equipment used mainly include circulating fluidized bed alumina roaster, large six-effect plate falling film evaporator group, high-pressure circulating fluidized bed boiler, indirect heating dissolution tank dissolution and high-efficiency sedimentation tank.
4 Shanxi Aluminum Plant Shanxi Aluminum Plant is the largest alumina plant in China, with an annual output of 1.2 million tons of alumina. It handles the medium-grade bauxite mine in Xiaoyi, Shanxi Province, using a completely separate Bayer-sintering combined process.
5 Zhongzhou Aluminum Plant The original design of Zhongzhou Aluminum Plant is to produce 1.2 million tons of alumina per year, using the combined method (mixed method).
6 Pingguo Aluminum Company Alumina Plant Pingguo Alumina Plant is the most technologically advanced alumina plant in China. It adopts the new Bayer process. The first phase of the project is designed to produce 300,000 tons of alumina per year. [next]
2. Mechanism, process flow and characteristics of each production method Alumina is an amphoteric oxide which can be dissolved in an acid and dissolved in a caustic solution; accordingly, the method for extracting alumina from ore is divided into an acid method. And alkali method.
Because acid is corrosive, acid-resistant equipment is difficult to solve, so acid production has not been applied in large industries. The method currently used in the industry is alkaline production.
The alumina production methods are: 1 Bayer process, high quality bauxite, A1 2 O 3 /SiO 2 ≥ 8 (mass ratio), SiO 2 <9%; 2 sintering method, treatment of low grade aluminum ore, A1 2 O 3 /SiO 2 =3.5-5.0; 3 combined method, processing medium grade bauxite, A1 2 O 3 /SiO 2 =5.0-8.0, combined method is divided into parallel method, series method and hybrid method.
(1) Bayer Bayer method is suitable for processing high-grade bauxite, which is a production method for heating and dissolving alumina in bauxite with caustic solution, which has the advantages of simple process, high product purity and good economic benefit. The process is shown in Figure 1.


1 Method principle The basic principle of the Bayer method is divided into dissolution and decomposition. Dissolution is the dissolution of alumina from bauxite with caustic soda. The reaction is:
A1 2 O 3 ·H 2 O+2NaOH====2NaA1O 2 +2H 2 O
A1 2 O 2 ·3H 2 O+2NaOH====2NaA1O 2 +4H 2 O
The gibbsite or gibbsite dissolves to form sodium aluminate into the lye, while other impurities do not enter the solution and are present in the solid phase, which is called red mud.
The dissolution temperature of gibbsite (A1 2 O 3 ·3H 2 O) is 105 ° C, diaspore (α-A1 2 O 3 ·H 2 O) is 220 ° C, boehmite (γ- A1 2 O 3 ·H 2 O) is 190 °C.
The decomposition is carried out by using a NaAlO 2 solution to precipitate solid phase Al(OH) 3 under reduced temperature, seed addition and agitation. The decomposition reaction is:
NaAlO 2 +2H 2 O====Al(OH) 3 ↓+NaOH
Seed is the Al (OH) 3, added in an amount (calculated as the amount of A1 2 O 3) of the above solution and A1 2 O 3 content is doubled; temperature control is reduced from 75 ℃ 55 ℃; stirring time is about 60h. The obtained Al(OH)3 is then dehydrated and dehydrated to become Al 2 O 3 , and the Al 2 O 3 crystal form is transformed to meet the requirements of aluminum electrolysis. The calcination reaction is:

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