JPS5817129B2 - Alumina lumps with good mechanical strength and adjustable particle size and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention involves compressing an "intermediate product" formed by incomplete decomposition of aluminum nitrate hydrate, then granulating the compressed product, and then heat-treating the granulated product. The present invention relates to alumina agglomerates with good mechanical strength and adjustable particle size to suit the technical requirements of users.

The invention also relates to a method for obtaining such a mass.

The present invention also relates to alumina masses with good mechanical strength in various shapes obtained by processing subsequent to compression molding.

Industries specializing in obtaining alumina and then converting it to aluminum via molten electrolysis have long encountered the significant difficulties they were intended to solve.

The first difficulty is the loss of alumina due to scattering as dust.

This difficulty was experienced when handling the material and when using it in a tank for melt electrolysis.

As a result, a need was found to design an expensive dust treatment and recovery system.

Another difficulty that has arisen has been related to the recovery of certain elements contained in the effluent gas from the tanks used for melt electrolysis.

The currently commonly applied method for this purpose consists in bringing the gaseous effluent into intimate contact with the alumina used to feed the tank.

Experts have now confirmed that the alumina contacted in this way must have a BET specific surface area suitable for this implementation if the element is to be satisfactorily absorbed. A final difficulty that has proven to be significant is related to the variations found in the alumina grain size.

The experts desired to have a particle size that would be essentially constant over time so that the operation of the tank for melt electrolysis would not be adversely affected by such fluctuations.

These many difficulties and disadvantages have led experts to make alumina in the form of a mass specially suited for melting electrolysis, and in this way to produce a product whose desired properties are reproducible, i.e. permanent in time. With a view to finding a solution to these shortcomings, which have always been questioned as to their significance, a number of methods for alumina agglomeration have been proposed and widely described in the specialized literature.

The first type proposed was "Bayer process" alumina,
and a suitable binder, which may be a solution of an acid or a salt of aluminum such as aluminum nitrate, aluminum stearate, etc., by mechanical agglomeration of a paste obtained.

After agglomeration by extrusion, compression or any other mechanical means, the resulting particles are calcined.

Such a process is expensive and gives a granular product contaminated not only with small amounts of Na2O from the Bayer process itself, but also with binder or what remains of it after heat treatment.

Later, other methods were proposed that were significantly improved.

It is disclosed in French Patent No. 2,267,982 and consists in producing agglomerated activated alumina using aluminum hydrate obtained by the Bayer process as raw material.

The raw material, which contains only a small amount of impurities, more specifically sodium impurities, is first dried to remove the impregnation water and then, without adding any binder, is placed at the desired pressure in two cylinders. It is compressed by continuously passing through the space.

The continuous strip thus produced is crushed to the desired dimensions and the fragments are subjected to a conventional activation heat treatment.

However, the various methods proposed essentially concern the agglomeration of hydrated alumina produced by the action of bauxite by the Bayer process.

Apart from this basic method, there is an acid method which involves reacting raw ore with HNO3.

This method converts alumina in ore into the formula (NOs) s
The conversion to aluminum nitrate hydrate of Al t n N20, where n is generally 9 but can also be 8 or 6, is an important intermediate step in obtaining pure alumina.

The following formula, 2(N03)3Al・9H20→Al2O3+3N20
5+・18H20 2(NO3)3Al・8H20-4”Al 203 +
3N20. +6H20 2(NO3)3A'・6H20→A 1203+3N2
05 +2H20 when pyrolyzed by N20. will be decomposed into various other nitrogen oxides depending on the temperature.

It appears to the inventors that by varying the time and temperature it is possible to limit the decomposition to incomplete nitrification and water utilization [intermediate products]. It was done.

If the aluminum water nitrates are completely decomposed, the resulting alumina is generally in the form of very fine particles, easily dispersed, and has some of the disadvantages mentioned above.

□Therefore, it was desirable to intend to lump the alumina obtained by thermal decomposition of aluminum nitrate hydrate.

Continuing our research in this field, we found it interesting that it is possible to produce alumina particles with good mechanical strength and adjustable particle size from the aforementioned aluminum nitrate hydrate. remembered.

According to the present invention, the novel alumina agglomerates are obtained by compressing an "intermediate product" resulting from the incomplete decomposition of aluminum nitrate hydrate and containing 0.5 to 15% by weight of nitrogen oxides expressed in N20°. The compressed product is granulated,
It is characterized in that it is obtained by subsequently applying heat treatment to the granulated product.

The "intermediate product" is, for example, an aluminum silicate ore with a content of nitrogen oxides expressed as N2O5 of 0.
．． It is produced by incomplete thermal decomposition of aluminum nitrate hydrate, which is obtained by working to a concentration of 5 to 15%, preferably 2 to 8%.

Since the impregnating water is evaporated before decomposing the nitrate hydrate, the content of AA'203 and the content of structurally present water can naturally be estimated from the nitrogen oxide content. Can be done.

As already mentioned, ``Intermediate product J is usually dry compressed.

However, it has been found that adding an amount of water, not exceeding 15% by weight thereof, to the product to be compacted does not substantially affect the final properties of the alumina mass.

A non-limiting example of the "intermediate product" thus defined is then subjected to an agglomeration step is shown in FIG.

The intermediate product J (I, P,) stored in A is introduced into mixer B through 1.

A portion of the granulated product, which is smaller than the desired size, is also admitted to B through 6.

It then enters unit C where compression is carried out continuously.

Unit C has compression means, which may be, for example, a cylindrical compressor of the usual type with precompression means.

The compression pressure is 1 crf for the entire width of the cylinder.
At least 3 tons per liter.

The compacted product then proceeds to form a continuous band, which is coarsely ground upon exiting the compaction stage and then passed through 3 into a granulator where it is ground to the desired size.

Crushing is done using a spiked cylinder, a JIYO crusher,
It is carried out in known types of equipment such as hammer mills and the like.

The particles from the crushing stage are led through 4 to a sorting zone E where they are separated into at least three classes of different sizes "α", "β" and "γ".

The "α" class covers particles that fall within the size range desired for their subsequent use.

This grade is then passed through 1 into a furnace of known type where a heat treatment is applied at temperatures of up to 1500°C.

The "β" class, consisting of particles of small size, is transported to B through 6 for recycling the process.

The "γ" class, which is made up of larger sizes, is sent through 5 to the granulator, where it is re-crushed and then returned through 4 to the sorting zone E.

When heat treated with F, the "α" class is simply collected in G and used.

In a variation of that method, the continuous compression unit C is compressed at a minimum compression pressure of 1500 kf/CyL! , and preferably 3000
It may be replaced with a pellet compressor having kfF/cIrL2 to 5000 kfF/F2.

The pelletized product is then fed into a granulator,
After that, it is subjected to the processing cycle described above.

The alumina agglomerates obtained without the use of any binder due to the application of heat treatment have physical properties of special interest besides retaining a regular particle size adjusted according to the wishes of the user.

Generally speaking, the nitrogen oxide content, expressed as N2O5, is between 0% and 0.5%, depending on the heat treatment conditions.

The BET specific surface area, determined by nitrogen adsorption according to AFNOR standard X11-621, is between 2 and 150 m2/7, depending on the conditions of the heat treatment.

Finally, the alumina agglomerates of the invention offer good resistance to friction, which takes the form of good resistance to particle crushing when subjected to repeated thermal and mechanical shocks.

According to the invention, methods known in the art, such as compression molding,
It is also possible to produce a well-defined mass by extrusion or the like.

This applies, for example, to balls of various dimensions, solid or hollow cylinders, small plates, grooved pulleys, reversible double wheels (diabolos de renbois).
voi) etc.

The heat treatment following molding follows a selected heating cycle determined by the intended use of the molded article.

Other aspects and advantages of the invention will be better understood from the examples of how the method is carried out.

EXAMPLE 1 An "intermediate product" containing 2.3% by weight of nitrogen oxides expressed as N2O5 was pelletized under various pressures.

Compression was carried out using a hydraulic press whose pressure could be varied from 2000 to 5000 Kf/cIri2.

The pellets have a diameter of approximately 2411g11 and the thickness varies from 5 to 7M depending on the amount of "intermediate product" placed inside.

The pellets thus obtained are placed in a Matsufuru furnace which has been brought to a preselected calcination temperature and maintained at that temperature for 2 hours.

The physical properties of the pellets after heat treatment can be seen in the table below summarizing them.

BET surface area is measured by nitrogen adsorption according to AFNOR standard X11-621.

The pellet destruction test was conducted with a diameter of 18.2571 m and a weight of 24.
A steel control ball of 80.9 is played by dropping a glass tube with a diameter of 2071g11 as a guide tube.

Drop the ball into the center of the pellet.

Glass tubes of increasing height are used until a single drop of the ball destroys the pellet.

Example 2 In order to demonstrate that the presence of moisture does not adversely affect the physical properties of the pellets produced, an "intermediate product" containing 2.6% nitrogen oxides, expressed as N2O5, was
Moisten with an amount of water that is % by weight.

Once the sample was homogeneous, the pressure was increased to 400
Compress using the same hydraulic press as in Example 1 set at 0 Kf/C112.

The diameter of the pellet is approximately 247 tons! , and the thickness is from 5 to 7M depending on the amount of "intermediate product" placed therein.

After drying at 110° C., the pellets are placed in a Matsufuru furnace that has been brought to a preselected calcination temperature and held at that temperature for 2 hours.

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

EXAMPLE 3 An "intermediate product" containing 5.9 weight fractions of nitrogen oxides, expressed as N205, is pelletized under various pressures.

Compression is carried out under the same conditions as in Example 1.

The resulting pellets are placed in a Matsufuru furnace that has been set to a preselected calcination temperature and held at that temperature for 2 hours.

The physical properties of the pellets after heat treatment are shown in the table below.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet of the agglomeration process of the present invention. A: Raw material "intermediate product" storage tank, B: Mixer, C: Compression unit, D: Granulation equipment (crushing equipment), E: Sorting (sieving) equipment, F: Heat treatment furnace, G: Product (use) immediately before), 1.
2, 3...7 represent transfer routes.

Claims (1)

[Claims] 1 N produced by incomplete decomposition of aluminum nitrate hydrate
obtained by compressing a solid intermediate product containing 0.5 to 15% by weight of nitrogen oxides expressed as 2O5, then granulating the compressed product, and applying a heat treatment to the granulated product. A method for producing alumina agglomerates with large mechanical strength and adjustable particle size, characterized by: 2 The intermediate product is N20. Nitrogen oxide expressed as 2
The method for producing an alumina lump according to the above item 1, characterized in that the alumina lump contains 8 to 8 parts by weight. 3. The method for producing an alumina lump according to item 1 above, wherein the nitrogen oxide content expressed as N2O5 after compression and heat treatment is 0% by weight to 0.5% by weight. 4, characterized in that the product to be compressed is moistened with water in an amount not exceeding 15 parts by weight of the product,
The method for producing an alumina lump according to item 2 or 3. 5 Its BET specific surface area is 2 to 150m2/! q. The method for producing an alumina lump according to the above item 1, 2, 3, or 4. 6. Items 1, 2, 3 and 4 above in the prescribed shapes, such as spheres, solid or hollow cylinders, small flat plates, etc., characterized in that they are obtained by compression molding or extrusion. 5. A method for producing an alumina lump according to item 5. 7 If the intermediate product is pelletized, at least 1
Items 1, 2 and 3 above, characterized in that the intermediate product is compressed under a pressure of 500 k f/CIn2.
5. The method for producing an alumina lump according to item 4, item 5, or item 5. 8. Items 1 and 2 above, characterized in that the intermediate product is continuously compressed between two cylinders with a compression pressure of at least 3 tons per ICIrL over the entire width of the cylinders. , the method for producing an alumina lump according to item 3, item 4, or item 5. 9 The compressed intermediate product is then crushed and granulated,
9. The method for producing an alumina lump according to item 1 or 8 above, wherein the alumina lump is then sorted according to a desired particle size. 10. The method for producing a lump according to item 7, 8 or 9 above, wherein the maximum temperature for the heat treatment is 1500°C.