Plate Heat Exchanger For Alumina

Alumina

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1. Process of Alumina Production

The basic principles of the Bayer process for alumina production are as follows:

(1) Use NaOH solution to dissolve the bauxite. The alumina in the resulting sodium aluminate solution is separated as aluminium hydroxide under adding seeds or stirring continuously, that is, the seed precipitation process.

(2) After decomposition, the mother liquor can be used to dissolve out new bauxite at high temperatures after evaporation and concentration, that is, the dissolution process.

Using these two processes alternately, you can get a batch of aluminium hydroxide by processing each batch of ore, which constitutes the so-called Bayer process cycle.

The reaction equation is as follows:

Al2O3(1or3)H2O+2NaOH+aq2NaAl(OH)4+aq

2. Process Introduction

Bauxite is a complex aluminosilicate ore with alumina hydrate as the main constituent. The main chemical compositions are Al2O3, SiO2, Fe2O3, TiO2, a small amount of Cao and MgO sulfide, trace amount of gallium compounds, vanadium, phosphorus, and chromium other elements. The main compositions of industrial sodium aluminate are NaAl (OH) 4, NaOH, Na2CO3, Na2SiO4, etc. Generally, Na2O in NaAl (OH) 4 is called compound alkali, which in NaOH is called free alkali and that in Na2CO3 is called carbonate alkali. The carbonate alkali and caustic alkali are collectively called the total alkali.
(1) Use NaOH solution to dissolve out the bauxite, and the alumina in the resulting sodium aluminate solution is separated out as aluminium hydroxide under the conditions of adding seeds or stirring continuously, that is, seed precipitation process.
(3) After decomposition, the mother solution can dissolve new bauxite at high temperatures after evaporation and concentration, that is, the dissolution process.
Stir the raw pulp at 90 ℃ for 6~10h before high-pressure digestion and add sodium hydrate alumino-silicate to convert silicon minerals into silica slag as much as possible. The process is called pre-desalinization.
Seed precipitation decomposes the sodium aluminate solution in the cooling, seeding, stirring conditions to obtain aluminium hydroxide products having certain performance and mother liquor with the high molecular weight used as the circulating mother liquor of bauxite digestion.
The main purposes of evaporation are as follows:
(1) To eliminate excess water in the process of maintaining the balance of the circulatory system.
(2) To evaporate and condense the mother liquor till its concentration meets the requirements concentration of the bauxite digestion (the mother liquor) or the preparation of raw pulp (carbonate mother liquor);
(3) To eliminate accumulated impurities in the production process.

3. Heat Exchanger Type

Bayer process of alumina production: to use NaOH solution to dissolve the bauxite and the alumina in the resulting sodium aluminate solution is separated as aluminium hydroxide under adding seeds or stirring continuously. The heat exchange equipment used in this process is as follows:

Pre-heater: to heat up the raw pulp to 90 ℃, removing the silicon from the pulp.

Condenser: to cool down the raw pulp after high-pressure dissolution, for pulp dilution.

Plate heat exchanger: to heat the refined sodium aluminate solution, separated to obtain more mother liquor.

Evaporator: to evaporate and condense the mother liquor generated in the whole process, which can be used to dissolve out new bauxite at high temperature and achieve the cycle collection of mother liquor.

Due to the sodium aluminate, sodium hydroxide, sodium carbonate, sodium silicate, and other substances in the pulp (such as nickel). At the same time, the recommended pre-heater and condenser should adopt the tube type to prevent the particles in the pulp and the adhesive silicon aluminium from blocking the machine.

The mother liquor may contain sodium, aluminium ions, silicon ions, calcium ions, magnesium ions, iron ions, titanium ions, sulfur ions, etc.