NO143495B - ALUMINUM OXYDE AGGLOMERATES WITH GOOD MECHANICAL STRENGTH AND ADJUSTABLE PARTICLE SIZE DISTRIBUTION AND PROCEDURE FOR THE PREPARATION OF SUCH AGGLOMERATES - Google Patents

Description

The present invention relates to aluminum oxide agglomerates with good mechanical strength and adjustable particle size distribution produced by thermal treatment of hydrated aluminum sulphate and with a sulfur content expressed as SO^,

of less than 1 percent by weight, and with a specific BET surface area of 2-120 m ~ 2/g, and the peculiarity of the alumina agglomerates according to the invention is that they are freni-set by compression of an intermediate product obtained by incomplete thermal cleavage of hydrated aluminum sulfate and which still contains 1-15, but preferably 3-10 percent by weight of S expressed as SO^, and with subsequent granulation and calcination of the compacted intermediate product.

The invention also relates to a method for producing the aforementioned aluminum oxide agglomerates, and the distinctive feature of the method according to the invention is that an intermediate product obtained by incomplete thermal decomposition of hydrated aluminum sulfate with the formula A1203 " xSO3 • yU^ O where x has the values 0, 5-5 and y have values 0-18, is compressed by a pressure of at least 200 kp/cm 2 or by being compressed continuously between two cylinders which between them exert a compression force of at least 0.5 tonnes per linear cm across the width of the cylinders, after which it 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 the patent claims.

Due to their good mechanical strength and adjustable particle size, the aluminum oxide agglomerates are easy to adapt to the user's technical requirements.

The aluminum oxide agglomerates can be produced in various forms obtained by forming under pressure followed by the aforementioned calcination.

The industry which specializes in the production of aluminum oxide and the conversion of this into aluminum with melt electrolysis has long been exposed to difficulties which efforts have been made to resolve.

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

Another difficulty consisted in the recovery of some of the elements that were included in the gaseous outflow from the cells for the melt electrolysis.

A technique which has hitherto been used for this purpose consists in maintaining intimate contact between the gaseous outflow and the aluminum oxide which is used as supply to the cells.

It has been confirmed that the aluminum oxide that is brought into contact in this way must have a specific surface area adapted to this practice if the elements are to be satisfactorily absorbed.

A further difficulty which proved to be of considerable importance consisted in the variations found in the particle size of the alumina. The person skilled in the art 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 adversely affect these variations.

Due to these many difficulties and disadvantages, the person skilled in the art has considered the idea of transferring alumina to agglomerates in a form that is particularly suitable for melt electrolysis, so that a product is obtained in which the desired properties would be reproducible, i.e. permanent over the time.

In order to find a solution for these difficulties, many methods for the agglomeration of aluminum oxide have been proposed, and these are described in the specialist literature.

A first type of proposed methods consisted in mechanical agglomeration of a paste obtained by mixing a Bayer alumina and a suitable binder which could be a solution of an acid, of aluminum salts, e.g. aluminum nitrate or aluminum stearate.

After agglomeration by extrusion, compression or other mechanical measures, 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 whatever

was left of this after the calcination.

Another method which represented a marked improvement

is described in French patent document no. 2,267,982 and consisted of producing an agglomerated active alumina using the aluminum hydrate obtained by the Bayer process itself as raw material.

The raw material, which could only contain a small amount of impurities and more particularly sodium impurities, was first dried to remove the impregnation water. It was then compressed, without the addition of any binder, by passing it continuously between 2 cylinders with the desired pressure established between them. The continuous strip thus produced was then fragmented to the desired dimensions and the fragments subjected to conventional activating heat treatment.

The various processes which have hitherto been proposed relate to the agglomeration of a hydrated alumina, mainly obtained from impact on bauxite by means of the Bayer process. Apart from this basic process, there is an acid process, which includes digestion of the starting ore with H2SO^. This constitutes an important intermediate step in the production of pure aluminum oxide, by converting the aluminum oxide in the ore into a hydrated aluminum sulfate with the general formula A^O^, xSO^, yH20, where x and y can vary within broad limits corresponding to the known sulphates, which belong to the group comprising acidic basic and neutral hydrated aluminum sulphates.

x can generally have a value from 0.5 to 5 while y can generally have a value from 0 to 18.

By thermal decomposition of the aforementioned hydrated sulfate

by the equation A1203# xS03, yH20 -i-> A1203 + xS03 + yH20

it proved possible to control the cleavage by varying the time and temperature so that an incompletely cleaved sulphated and hydrated intermediate was obtained.

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

It was therefore desirable to aim at agglomerating the aluminum oxide obtained by thermally splitting hydrated aluminum sulphates.

The realization that underlies the present invention is that it was found that it was possible to produce aluminum oxide granules with good mechanical strength and adjustable particle size from a hydrated aluminum sulfate.

The intermediate product that is compressed is obtained by incomplete thermal decomposition of hydrated aluminum sulfate produced, e.g. by the action of acid on aluminosilicate-containing ores, in such a way that the content of S, expressed as SO^, is still from 1 to 15, but preferably 3 to 12 percent by weight.

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

The intermediate product defined in this way is then postponed

for the agglomeration process and Figure 1 shows an exemplary and preferred embodiment of this.

In this process, the intermediate product Pi, stored at (A), is introduced through (1) into a mixing device (B), which through (6) also receives a part comprising the granulated products which are smaller than the desired size. The mixture is then fed through (2) into the unit (C) where compression is carried out continuously.: The unit (C) has a pressing device such as can be a conventional compression device of the cylinder type with an associated pre-compression device. The compression pressure is at least 0.5 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 after it has left the compression step and is passed through (3) into the granulation device.

(B) where it is fragmented to the desired dimensions. Fragmentation is carried out using a known type of apparatus

like for example. spike cylinders, jaw crushers, impact mills etc.

The granules that emerge from the fragmentation step (D)

are passed through (4) to a selection zone (E) where they are divided into at least 3 qualities a, 3 and y, with different dimensions.

Quality a covers 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 the necessary temperature to produce aluminum oxide with the desired properties.

Quality 3 consists of too small granules that are passed through

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

The quality y consisting of too large granules is passed through

(5) to a granulation device (D) where it is fragmented again and then returned through (4) to the selection zone (E).

After the calcination at (F), the quality a wood is collected

(G) ready for use.

In the case of a modified form of the process, it can be continuous

compaction unit C is replaced by a pelletizing press with a compaction pressure of at least 200 kp/cm 2.

The pelletized product is then fed into the granulation device, after which it follows the treatment cycle as previously described.

After calcination, the alumina agglomerates obtained without the use of any binder have particularly interesting physical properties, apart from the fact that they adhere to a regular particle size which can be set according to the user's desire.

In general, the sulfur content expressed as SO^ is less than 1%.

Specific BET surface area, measured by nitrogen absorption

in accordance with the standardized method AFNOR Standard XII-6 21

is from 2 to 150 m 2 /g according to the conditions of the calcination.

Finally, the alumina agglomerates according to

to the invention good resistance to abrasion, in form. of good resistance to collapsing of the grains when exposed to repeated thermal and mechanical shocks.

It is also possible to produce agglomerates with well-defined shapes using known methods, 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 be apparent from

the subsequent execution examples.

Example 1

Intermediate products containing 3.8, 5.4 and 11.9 percent by weight of sulfur expressed as SO^, and produced by incomplete cleavage of A12(S04)3 • 18H20, are pelletized under different pressures.

Compaction is carried out with a hydraulic press at varying press pressures from 400 to 3000 kgF/cm 2.

The pellets have a diameter of 24 mm and a thickness that varies from 2 to 6 mm, according to the amount of intermediate product introduced.

The pellets thus obtained are calcined at 1050°C in a muffle furnace which is heated gradually with a temperature that rises by 5°C per minute.

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

BET surface area is measured by nitrogen absorption in accordance with AFNOR Standard 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 grams, which is guided in a qlass tube with a diameter of 20 mm down onto the center of the pellet. Glass tubes of increasing 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 adversely affect the mechanical properties of the manufactured pellets, the intermediate product is moistened with water weighing 5%, 7% and 15% of the intermediate product.

When the sample is homogenized, it is compressed using the same hydraulic press as in example 1.

The pellets have a diameter of 24 mm and a thickness of 3 to

5 mm, depending on the amount of introduced intermediate product.

After drying at 110°C, the pellets are calcined at different pressures in a muffle furnace which is heated gradually with a temperature increase of 5°C per minute.

The physical properties of the pellets after calcination

is summarized in the following table.

Example 3

Pellets obtained in advance by pelletizing an intermediate product containing 5.4% S expressed as S0-. at a pressure of 3000 kgF/cm 2 is calcined at 1300 C in a muffle furnace which is heated gradually with a temperature increase of 5°C per minute, and kept at this temperature for one hour. The BET surface area is then 3 m 2 /g. The height from which the ball must fall to crush the pellets is approximately 5 cm.

Claims (4)

1. Aluminum oxide agglomerates with good mechanical strength and adjustable particle size distribution produced by thermal treatment of hydrated aluminum sulphate and with a sulfur content expressed as SO^ of less than 1 percent by weight, and with a specific BET surface area of 2 - 120 m /g, characterized in that they are produced by compression of an intermediate product obtained by incomplete thermal cleavage of hydrated aluminum sulfate and which still contains 1-15, but preferably 3-10 weight percent S expressed as SO^, and with subsequent granulation and calcination of the compressed intermediate. 2. Aluminum oxide agglomerates as specified in claim 1, characterized in that they are obtained from a hydrated aluminum sulfate belonging to the group of acidic, basic and neutral hydrated aluminum sulfates, with the general formula Al^ O^ ' xSO^' yH20 nvori- x has values 0.5-5 and y has values 0-18. 3. Aluminum oxide agglomerates as specified in claim 1 or 2, characterized in that they are obtained from an intermediate<p>roduct which, before compaction, is moistened with an amount of water that does not exceed 15% by weight of the intermediate product. 4. Process for producing the aluminum oxide agglomerates specified in claim 1, characterized in that an intermediate product obtained by incomplete thermal cleavage of hydrated aluminum sulfate with the formula Al2°3 "xSO^<*> yH20 in which x has values 0.5- 5 and y have values 0-18, is compressed by a pressure of at least 200 kp/cm 2 or by being compressed continuously between two cylinders which between them exert a compression force of at least 0.5 tonnes per linear cm across the width of the cylinders, after which it is granulated by fragmentation, classified according to the desired particle size, and calcined at a maximum of 1500°C.