A new example of alumina production by mineral processing Bayer process

The so-called "Bayer" based Austrian chemist K · J · Bayer alumina in a method of processing high-quality bauxite 1887 manufactured by the invention taken. Bayer method is to treat bauxite with circulating mother liquor containing a large amount of free caustic, dissolve the alumina in it to obtain sodium aluminate solution, add aluminum hydroxide seed crystal to sodium aluminate solution, and precipitate after a certain period of stirring and decomposition. The aluminum hydroxide, the decomposition mother liquid is evaporated to be used to dissolve the next batch of bauxite.

Bayer process production often uses concepts such as caustic ratio, silicon content index, cycle efficiency, and seed coefficient. The Bayer process is the dissolution of alumina from bauxite by alkali. Industrially, Na 2 O in the form of NaAlO 2 and NaOH in solution is called caustic (denoted as Na 2 O k ), and Na 2 O in the form of Na 2 CO 3 is called carbonate (referred to as Na 2 O c ). Na 2 O in the form of Na 2 CO 4 is called sulfate base (referred to as Na 2 O), and the sum of all forms of base is referred to as total base (referred to as Na 2 O t ).

The caustic ratio is the molar ratio of Na 2 O k to Al 2 O 3 in the sodium aluminate solution, which is denoted as α ko . The United States is accustomed to the mass ratio of Al 2 O 3 to Na 2 O k in a sodium aluminate solution, symbol A/N.

The silicon content index refers to the ratio of Al 2 O 3 to SiO 2 content in the sodium aluminate solution, symbol A/S.

The cycle efficiency refers to the amount (t) of Al 2 O 3 produced by 1 t Na 2 O in a sodium aluminate solution in a single Bayer cycle, denoted by E. It indicates the level of utilization of the base.

Coefficient seed (seed ratio) means the addition of seed crystals of aluminum hydroxide in the amount of Al 2 O 3 and liquid decomposition of Al 2 O 3 ratio of the number.

It refers to the dissociation of the decomposition of aluminum hydroxide Al 2 O 3 accounts for the number 2 O 3 ratio of the number of Al contained in the semen. The calculation formula is:

η=(1-α a /α m )×100%

Where α a , α m - represent the caustic ratio of the decomposed semen and the decomposed mother liquor, respectively.

The Bayer process consists of four processes: (1) dissolving the alumina in the bauxite with a decomposition mother liquor of α k = 3.4, so that the α k of the eluate is 1.6 to 1.5; (2) diluting the eluate, washing and separating the refined Aluminate solution (semen); (3) semen and seed crystal decomposition; (4) decomposition of mother liquor evaporation to concentration of caustic to reach dissolution requirements (230 ~ 280g / L). The process of producing alumina by Bayer process is shown in Figure 1.

Figure 1 Process flow chart of alumina production by Bayer process

In these four processes, the dissolution of bauxite is a key process in the Bayer process. The gibbsite in the bauxite dissolves quickly into the caustic solution at 140 ° C. The boehmite dissolves quickly into the caustic solution at 180 ° C, while the diaspore is at 240 ° C. Above, the caustic soda is dissolved faster, and the corundum is insoluble at all. In order to bring the temperature of the caustic solution to the temperature required for dissolution, the caustic lye is heated by an autoclave. The slurry of the dissolved alumina is high in temperature and pressure, and it is necessary to take out the energy contained therein, and the secondary steam is produced by the self-steaming method to preheat the slurry before entering the autoclave, and at the same time, the temperature and pressure of the eluate are lowered. The autoclave exits the open vessel to wash and separate the sodium aluminate semen. Modern production involves continuous operation of a series of preheaters, autoclaves and auto-evaporators in series as a dissolver. The slurry is pumped into the jackpot by a high pressure pump, and the pressure of the diaspore is required to reach 3.33 MPa/cm 2 or more. Therefore, the high-pressure dissolution workshop is likened to the heart of the alumina production process.

The high pressure dissolution is heated by high pressure steam and is divided into direct heating and indirect heating. Direct heating and high pressure dissolution is to press the prepared raw slurry (pre-desiliconized at 95 °C) into a two-way preheater, heat to 160 ° C, flow to the dissolver, and directly into the first two or three autoclaves. The steam is heated to the dissolution temperature and then sequentially passed to the subsequent dissolver to dissolve. The steam is directly heated and dissolved without mechanical agitation, and the structure is simple. However, there are also many disadvantages. As soon as the slurry enters the autoclave, it is diluted, requiring a high concentration of caustic alkali in the circulating mother liquor, thereby increasing the load of evaporation of the mother liquor. Indirect heating uses 10 times of self-evaporating secondary steam to preheat the slurry to 200 ° C. In the autoclave, it is indirectly heated to the dissolution temperature with new steam. The steam condensate does not enter the slurry. The autoclave needs mechanical agitation, and the slurry flows out through the ten. The first stage self-evaporation recovers heat and has high heat utilization rate. However, the indirect heating of the high pressure dissolver group is more complicated than the direct heater and the investment is large.

When the Bayer process is dissolved, in order to reduce nodular nodules, desiliconization is usually carried out. In the original slurry tank, low temperature desiliconization below 90 °C and medium temperature desiliconization at 150 °C are used. Well, the silicon content index of the eluate is also high. The changes in the concentration of Al 2 O 3 and SiO 2 in the Bayer process solution are shown in Fig. 2.

Fig. 2 Mass concentration of Al 2 O 3 and SiO 2 in solution in the production of alumina by Bayer process

The Bayer method requires that the Al 2 O 3 content in the bauxite be more than 55%, and the larger the A/S, the better. A/S dropped from 11 to 7, and production costs will increase by 14.5%. Most of China's bauxite mines are A/S=5-7 (59.3% of all mines), and Bayer's method cannot be economically processed. This part of the bauxite is treated with soda lime sintering.

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