HU191492B - Process for the recovery of aluminium hydroxide from aluminate alkal of alumina factory - Google Patents

Abstract

The prod. is obtd. from the aluminate liquors of the alumina process by using hydrothermal boehmite as seed for the pptn. of the aluminate liquor and sepg. the solid ppte. from the liq. phase in the known way. - Pptn. is performed pref. as a cyclic process, using as seed at least partly the material pptd. in the previous cycle.

Description

(54) PROCEDURE FOR THE RECOVERY OF ALUMINUM HYDROXIDE FROM ALUMINUM LUBRICANTS OF TIME MANUFACTURE (57)

BACKGROUND OF THE INVENTION The present invention relates to a novel process for the recovery of aluminum hydroxide from alumina alumina liquor by mixing the alumina liquor in the presence of a vaccine by using hydrothermal boehmite as the vaccine and separating the precipitated solid product from the liquid phase in a known manner. The agitation is preferably performed cyclically and the product selected at least in part from the previous cycle is used as the vaccine.

The process of the present invention enables the production of any particle size and particle size distribution of alumina hydrate under the usual conditions of agitation of aluminate.

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The present invention relates to a novel process for the recovery of aluminum hydroxide from alumina alumina. The process is mainly used in Bayer alumina production to mix alumina liquor.

As is known, aluminum hydroxide (alumina hydrate) obtained after the alkaline digestion of the bauxite and separation of the digested slurry is obtained by mixing the aluminate lye in the presence of seed crystals and lowering its temperature. The precipitated crystalline aluminum hydroxide is separated from the mother liquor by filtration or other suitable means and converted into alumina by calcination or otherwise processed.

The extraction of aluminum hydroxide, known as blending in alumina production, is accomplished by adding alumina hydrate as an inoculum in an amount depending on the desired properties of the product, and the system is continuously stirred. Meanwhile, a

NaA10 ₂ + H ₂ O 4 Al 1 (OH) j + NaOH is a reversible reaction, this time towards the right side of the reaction equation.

The properties of the alumina to be produced, in particular its particle size and granulometric distribution, are decisively influenced by the conditions under which the above operation is carried out: the caustic Na ₂ O and Al ₂ O ₃ concentration, the temperature, the seeding ratio, the mixing time, the solution. purity and other characteristics of the operation. The particle size does not change significantly during calcination. The more active the vaccine is, the greater the effect of the vaccine on the particle size of the alumina.

Aluminum metallurgy has become increasingly demanding of coarse-grained so-called alumina, while the demand for conventional fine-grained, flour-like alumina has been decreasing. The alumina factories are therefore primarily interested in producing coarse alumina. However, this is hindered by the fact that the lower Na ₂ O concentration is favorable for the production of the required large particle alumina hydrate, so that the solution efficiency of the mixing is significantly lower than that of the fine particle alumina hydrate. (Solution efficiency is defined as the amount of alumina hydrate, expressed as alumina, which can be obtained from a unit volume of alumina by mixing.) Thus, the production of coarse alumina hydrate poses a significant efficiency or capacity problem and requires a radical modification of the Bayer process.

It is an object of the present invention to provide a process for the recovery of aluminum hydroxide from aluminate alkali which allows the adjustment of the particle size and particle size distribution of alumina hydrate without any significant reduction in solution efficiency.

It has been found in our experiments that the above objective can be achieved by mixing the aluminate alkali in the presence of a vaccine containing hydrothermal bohemite.

Hydrothermal bohemite is called bohemite (A1OOH) prepared from aluminum hydroxide by hydrothermal treatment at temperatures above 180 ° C. Details of hydrothermal treatment will be described later. This treatment is, moreover, generally known in the literature.

The process of the present invention is based on the unexpected and surprising discovery that, contrary to prior art, foreign material other than rhodium hydrate may be used. In the past, it was believed that the quality of alumina would be impaired by the use of foreign materials as vaccines. The use of hydrothermal bohemite, on the other hand, presents no disadvantage in terms of alumina quality, since it has been found that during the calcination process, the alumina hydrate, together with the hydrothermal bohemite contained therein, is converted to a uniform crystalline aluminum hydroxide.

According to the invention, hydrothermal bohemite is added to the alumina lye as a vaccine, either alone or in combination with alumina hydrate. Due to its high activity, hydrothermal bohemian provides the desired result even if it is only part of the vaccine. Advantageously, the mixing is performed cyclically (this is advantageous in most cases anyway due to operational conditions) and the first cycle uses hydrothermal bohemite as the vaccine, while the other cycles are supplemented with the alumina hydrate obtained in the previous cycle containing hydrothermal bohemite. liquor. If the hydrothermal bohemite content of the vaccine drops below a certain value, the cycle is restarted. However, it is also possible in a cyclic system to adjust the hydrothermal bohemite content of the vaccine from the previous cycle to a desired value by adding fresh hydrothermal bohemite. Then, in theory, the cycle can be sustained for as long as possible.

The stirring is otherwise carried out in the usual manner. The particle size distribution of the product is primarily controlled by the amount of vaccine, of course, in combination with other parameters (caustic Na ₂ O concentration, temperature, cooling profile, etc.), and the ability to control other parameters as needed. In order to carry out the process according to the invention, no special parameters need to be provided. The process can be performed on conventional equipment.

The hydrothermal bohemite is prepared from aluminum hydroxide by heat treatment in a liquid phase at temperatures above 180 ° C. Preferably, the aqueous suspension of aluminum hydroxide is charged to an autoclave which is heated indirectly by heating to a temperature greater than 180 ° C and heating is continued until the formation of hydrothermal boehmite is complete. Conversion can be readily detected by examining the glow loss. The approximately 34% annealing loss of aluminum hydroxide decreases below 17% during conversion. The transformation time depends on the temperature. The relationship is shown in the table below.

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Table 1

tem- ture 'C Loss on ignition (% by weight) 1 2 hour hydro 3 thermal ke 4 : after gelling 5 150 34.3 34.2 34.1 34.0 33.8 185 34.3 34.0 33.0 19.1 16.9 200 34.3 32.4 16.8 - - 215 34.3 16.7 - - 250 16.8 - - - - 300 1/2 hour 16.6 - - - -

According to scanning electron microscopy scans and physico-chemical examinations, hydrothermal bohemite has substantially different physical and physico-chemical properties than the parent alumina hydrate. It has a highly articulated structure and a specific surface area that is one order of magnitude larger than alumina hydrate. (Approximately 2.2-2.4 m ² / g of alumina hydrate as opposed to 0.1-0.2 m ² / g.) Its properties are also different from those of mineral bohemite as shown by X-ray diffraction comparison of elemental cell parameters.

The use of hydrothermal bohemite as an extinguishing agent can significantly increase the efficiency of the mixing operation. However, there are a number of other benefits of mixing according to the invention. The most important of these are listed below.

1. Improvement in agitation efficiency is somewhat at the expense of particle size growth. In this way, the alumina hydrate required for the production of sandy alumina can also be prepared with sufficient efficiency.

2. The control of the mixing system can be significantly simplified, since by varying the amount of the particularly active vaccine, it is possible to influence the structure of the particle and the particle size very effectively, and the undesirable effects due to the fluctuation of other technological parameters.

3. European plants using Bayer technology (characterized by high caustic Na ₂ O concentrations (120-150 g / dm ³ )) can be easily converted without the need for capacity reduction to produce the coarse-grained alumina required by modem aluminum smelters.

4. The concentration of organic matter in the alkaline cycle is significantly reduced. In fact, when using hydrothermal bohemite as an extinguishing agent, about three times more organic material is removed from the cycle.

. The invention is illustrated by the following examples.

Example 1

Alcoholic liquor prepared by the European Bayer process with a caustic Na ₂ O concentration of 130.0 g / dm ³ and an Al ₂ O ₃ concentration of 142.6 g / dm ³ , and having a caustic molar ratio of 1.5, is mixed under the following conditions:

initial temperature: 75 'C final temperature: 63' C vaccine:

hydrothermal bohemian at first mixing, product of previous mixing at other mixing extinguishing ratio: 0.3 mixing time: 40 hours

The table below shows the evolution of mean particle size, particle size distribution, and loss on ignition over each cycle. The product of Cycle 1 is also the vaccine of Cycle 2, and the product of the other cycles is the same as the product of the next cycle.

Table 2

hydro- produce. 1. 2. 3. 4. 5. 6th bőh- what product of mixing cycle Average particle size gm 45.1 47.1 60.9 71.6 87.4 109.3 113.6 Granules above 80 gm 11.5 4.6 25.4 62.0 88.9 89.5 87.8 % by weight 40-80 µm 29.3 55.6 60.6 29.5 7.4 5.2 6.1 crowd% Granules by weight below 40 gm 59.2 39.8 14.0 8.5 3.7 5.3 6.2 Loss on ignition by weight 18.1 28.3 31.8 33.0 34.1 34.3 '34.3

Stirring is continued until the desired particle size and particle size distribution are achieved.

Example 2

Aluminum alumina liquor containing 130.0 g / dm ^{3 of} caustic Na ₂ O and 142.6 g / dm ^{3 of} Al ₂ O ₃ at 1.5 caustic molar ratio is mixed under the following conditions:

initial temperature: 66 'C final temperature: 55' C inoculum: hydrothermal bohemite 3 weight percent recycled alumina hydrate 97 weight percent inoculum ratio: 1.0 mixing time: 40 hours

The following table shows the average particle size and particle size distribution of cyclically repeated mixing products.

Table 3

First Second kikeveré Third si cycles 4th products 5th Average particle size gm 73.15 73.40 72.30 72.60 72.90 % By weight of particles above 80 gm 55.5 56.0 52.3 47.5 49.5 40-80 gm of granules by weight 34.7 35.8 38.9 43.1 42.1 Granules by weight below 40 gm 9.8 8.2 8.8 9.4 8.4

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As shown in the table, the use of an unchanged vaccine produces a product having substantially the same particle size and particle size distribution in each mixing cycle.

Claims (2)

Claims 1. A process for recovering aluminum hydroxide Timföldgyári aluminátlúgból by mixing with the liquor in the presence of seed crystals, characterized in that the liquor is stirred vaccine containing hydrothermal boehmite in the presence of ₅ off, and the precipitated solid product is separated in known manner from the liquid 2. A process according to claim 1, wherein the mixing is carried out cyclically and at least in part the timfold hydrate selected in the previous cycle is used as the vaccine. Without drawing Published by the National Inventory Office responsible for publishing: Zoltán Himer Head of Department Szedte Printing House of the Printing Industry (878347/09) 89-0010 - Dabasi Printing House, Budapest - Dabas Responsible Manager: Csaba Bálint Director