NO143494B - ALUMINUM OXYDE AGGLOMERATES WITH HIGH MECHANICAL STRENGTH AND ADJUSTABLE PARTICLE SIZE DISTRIBUTION AND PROCEDURE FOR THE PREPARATION OF SUCH AGGLOMERATES - Google Patents

Description

The present invention relates to aluminum agglomerates

with high mechanical strength and adjustable particle size distribution, with a nitrogen oxide content, calculated as N2°5'

of 0-0.5% by weight, and a specific BET surface area of 2-150 m /g, and the peculiarity of alumina agglomerate

the atene according to the invention is that they are produced by compression of an intermediate product obtained by incomplete

thermal decomposition of hydrated aluminum nitrate containing 0.5-15% and preferably 2-8% by weight of nitrogen oxide, calculated as ^Oj-, with subsequent granulation of the compressed product and calcination of the granulated product.

The invention also relates to a method for production

of the aforementioned agglomerates, and the peculiarity of progress-

the method according to the invention is that an intermediate product obtained by incomplete thermal decomposition of aluminium-

nitrate containing 0.5-15 and preferably 2-8 percent by weight of nitrogen oxide, calculated as ^0,-, is continuously compressed between two cylinders which between them exert a compression

power of at least 3 tonnes per linear cm across the width of the cylinders, or that it is compacted by pelletizing under a pressure of at least 1500 kp/cm 2 , after which the compressed intermediate

product is granulated by fragmentation, classified according to the desired particle size and calcined at a maximum of 1500°C.

These and other features of the invention appear in patent

the requirements.

A significant advantage of the invention is that the particle size

can be adapted to the user's technical requirements.

The agglomerates can be obtained in varying forms obtained by

forming under pressure followed by the aforementioned calcination.

The industry which has specialized in the manufacture of aluminum oxide and its conversion to aluminum by smelting electrolysis has long encountered serious difficulties which efforts have been made to solve.

The first difficulty was the loss of alumina by dust formation and this difficulty occurred when the material was handled and when it was used in the cells for fusion electrolysis. Consequently, it was found necessary to construct expensive facilities for recycling dust.

Another difficulty that arose related to recycling

of certain elements contained in the gaseous outflow from the cells used for the melt electrolysis.

A method commonly used for this purpose consisted of

to establish intimate contact between the gaseous outflow and the aluminum oxide which was used as supply to the cells.

It is now recognized that alumina which is brought into contact in this way must have a specific BET surface area adapted to this practice if the elements are to be satisfactorily absorbed.

A final difficulty which proved serious concerned the variations found in the particle sizes of the alumina. Experts wanted to have a particle size that would be essentially constant over time, so that the operation of the cells for the melt electrolysis would not be adversely affected by these variations.

Because of these many difficulties and disadvantages, experts have considered the idea of transferring aluminum oxide into an agglomerate form particularly suitable for melt electrolysis, so that a product is obtained in which the desired properties are reproducible, that is to say permanent over time. In order to find a solution to these difficulties, many methods have been proposed for the agglomeration of aluminum oxide and these are described in the specialist literature.

A proposed first type of method comprised mechanical agglomeration of a paste obtained by mixing a Bayer alumina and a suitable binder, which could be a solution of acid, of salts of aluminum such as aluminum nitrate, aluminum stearate etc.

After agglomeration by extrusion, compression or other mechanical means, the obtained granules were calcined. These methods were expensive and produced granulated products that were not only contaminated with small amounts of Na 2 O from the Bayer process itself but also with the binder or what was left of it after the heat treatment.

Another process, which represented a great improvement,

is described in French patent document 2,267,982 and consisted of producing an agglomerated active aluminum oxide using the aluminum hydrate obtained by the Bayer process as raw material.

The raw material, which could contain only a small amount of impurities and more particularly sodium impurities, was first dried to remove the impregnation water and then compacted, without the addition of any binder, by

passing it continuously between two cylinders between which the desired pressure had been established. The continuous strip thus produced was fragmented to the desired dimensions and the fragments subjected to conventional activating heat treatment.

However, the various processes which have been proposed so far have concerned the agglomeration of a hydrated alumina produced mainly by impact on bauxite by the Bayer process. Apart from this basic process, there is an acidic process which includes conversion of the starting ore with HNO^. This method is an important intermediate step in the production of a pure aluminum oxide, by converting the aluminum oxide in the ore into a hydrated aluminum nitrate with the formula AICNO^J^ " nl^O, in which n is usually 9 but can also be equal to 8 or 6.

When the nitrates are thermally decomposed by the following reactions

N2°5 can split into other different nitrogen oxides, depending on the temperature.

It proved possible to control the cleavage by varying the time and temperatures, so that an incompletely cleaved, nitrated and hydrated intermediate could be obtained.

When hydrated nitrates of aluminum are completely decomposed, the alumina obtained is usually in the form of very fine particles which tend to spread and which also have several of the above-mentioned disadvantages.

It was therefore desirable to seek to agglomerate the aluminum oxide obtained by thermal decomposition of hydrated aluminum nitrate.

The realization that underlies the invention is

that it was found that it was possible to produce aluminum oxide granules with good mechanical strength and adjustable particle size from the aforementioned hydrated aluminum nitrate.

The aforementioned intermediate product is produced by incomplete thermal cleavage of hydrated aluminum nitrate obtained e.g. by the action of acid on aluminosilicate ores, so that the nitrogen oxide content, expressed as ^0^., is from 0.5-15% but preferably from 2-8%. Consequently, the content of A^O-j and the content of water constitutionally present in the intermediate product can be derived naturally from the content of nitrogen oxide, as the impregnation water has been evaporated before the hydrated nitrate is decomposed.

The intermediate is usually compressed dry. However, it has been found that the addition of a certain amount of water to the intermediate product to be compressed, not exceeding 15 percent by weight thereof, does not significantly affect the final properties of the alumina agglomerates.

The intermediate product defined in this way is then subjected to an agglomeration process and an example of such is given in fig. 1.

In this process, the intermediate product PI, stored at (A), is introduced through 1 into the mixer (B), which also through 6 receives a part comprising the granulated products which are smaller than the desired size. The mixture is then passed through 2

into a unit (C) where compression is carried out continuously. The unit (C) has pressing devices which can be e.g. a compression device of the conventional cylinder type with an associated pre-compression device. The compression pressure is at least 3 tonnes per linear cm across the width of the cylinders. The compressed product is then in the form of a continuous strip which is broken up into coarse pieces when it leaves the compression step and is taken through 3 into a granulation device (D) where it is fragmented to the desired dimensions. Fragmentation is carried out with the help of a known type of device such as e.g. spike cylinders, impact mills etc. The granules

which comes out of the fragmentation step (D) is passed through 4 to a selection zone (E) where they are divided into at least 3 qualities et, 8 and y with different dimensions.

Quality a denotes granules with dimensions that fall within the size range desired by the user. This quality is then fed through 7 into a known type of furnace (F) where calcination is carried out at a maximum temperature of 1500°C.

Quality &, consisting of too small granules, is passed through

6 into the mixing device (B) for recycling in the process.

Quality y, which consists of oversized granules, is passed through 5 to a granulation device (D) in which it is fragmented again and then returned through 4 to the selection zone (E).

After the calcination at (F), quality a is collected at (G) ready for use.

In a modified form of the process, the continuous compression unit (C) can be replaced with a pelletizing press with a compression pressure of at least 1500 kp/cm 2 and preferably from 3000 kp/cm<2> to 5000 kp/cm<2>.

The pelletized product is then fed into the granulation device, after which the processing cycle described above follows.

After the heat treatment has been carried out, alumina agglomerates obtained without the use of any binder have particularly interesting physical properties, except that they maintain a regular particle size which can be set according to the user's wishes.

Generally speaking, the nitrogen oxide content, expressed as

N2°5' ^ra 0-0.5% depending on the conditions of the heat treatment.

The mentioned specific BET surface area, measured by nitrogen absorption in accordance with the standardized method AFNOR•Standard XII-621, is from 2-150 m 2 /g in accordance with the conditions for the calcination.

Finally, the alumina agglomerates exhibit according

to the invention good resistance not abrasion, with good resistance to the collapsing of the grains when these are subjected to repeated thermal and mechanical shocks.

It is also possible to produce agglomerates with well-defined shapes by methods known per se, e.g. forming under pressure, extrusion, etc. This makes it possible to produce e.g. spheres of different dimensions, compact or hollow cylinders, small plates, etc. The calcination after shaping follows a selected heating cycle determined by the use for which the shaped objects are intended.

Other features and advantages of the invention will become more apparent

of the following examples of preferred embodiments.

Example 1

An intermediate product containing 2.3% by weight of nitrogen oxide, expressed as ^0,., is pelletized at various pressures.

The compression is carried out with a hydraulic press where the pressure is varied from 2000-5000 kp/cm 2.

The pellets have a diameter of approximately 24 mm and a thickness that varies from 5 to 7 mm, in accordance with the amount of intermediate product introduced.

The pellets obtained in this way are then placed in a muffle furnace which has previously been brought to the selected calcination temperature and kept at this temperature for two hours.

The physical properties of the pellets after heat treatment can be seen from the summary in the following table.

BET surface area is measured using nitrogen absorption in accordance with standardized method AFNOR Standar XII-621.

The pellet crushing test is carried out by dropping a steel ball with a diameter of 18.25 mm and a weight of 24.80 g, which is guided in a glass wheel with a diameter of 20 mm down onto the center of the pellet. Glass windows with covering height are used until a single drop of the ball causes the pellet to shatter.

Example 2

To show that the presence of water does not affect the mechanical properties of the manufactured pellets in a harmful direction, the intermediate product containing 2.6% nitrogen oxide expressed as NO is moistened with water, the amount of water being 5% by weight.

When the mixture has been homogenized, it is compressed with the same hydraulic press as in ex. 1 with the pressure set to 4000 kp/cm 2.

The pellets have a diameter of approximately 24 mm and a thickness of 5 to

7 mm, in accordance with the amount of inserted intermediate product.

After drying at 110°c, the pellets are placed in a muffle furnace which has previously been brought to the chosen calcination temperature, and kept at this temperature for two hours.

The physical properties of the pellets after the heat treatment are indicated in the following table

Example 3

An intermediate containing 5.9% by weight of nitric oxide expressed as ^0^. are pelletized at different pressures.

Compression is carried out under the same conditions as in ex. 1. The obtained pellets are placed in a muffle furnace which has previously been brought to the selected calcination temperature and kept at this temperature for two hours.

The physical properties of the pellets after heat treatment are stated below;

Claims (3)

1. Aluminum oxide agglomerates with high mechanical strength and adjustable particle size distribution, with a nitrogen oxide content, calculated as ^Oj-, of 0-0.5 percent by weight, and a specific BET surface area of 2-150 m<2>/g, characterized in that they are produced by compressing an intermediate product obtained by incomplete thermal decomposition of hydrated aluminum nitrate containing 0.5-15% and preferably 2-8% by weight of nitrogen oxide, calculated as ^0,., with subsequent granulation of the compressed product and calcination of the granulated product. 2. Aluminum oxide agglomerates as stated in claim 1, characterized in that they are obtained from an intermediate product which, before compaction, has been moistened with an amount of water that does not exceed 15 percent by weight of the said product. 3. Procedure for producing the agglomerates specified in claim 1, characterized in that an intermediate product obtained by incomplete thermal decomposition of aluminum nitrate containing 0.5-15 and preferably 2-8 percent nitrogen oxide by weight, calculated as N^O^, is continuously compressed between two cylinders which between them exert a compression force of at least 3 tonnes per linear cm across the width of the cylinders, or that it is compressed by pelletizing under a pressure of at least 15 00 kp/cm 2, after which the compressed intermediate product is granulated by fragmentation, classified according to the desired particle size and calcined at a maximum of 1500°C.