JPS5855085B2 - alumina lump - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention describes
It is obtained by compression molding an intermediate product, then granulating the compression molded product, and then heat treating the granulated product, which has good mechanical strength and is adapted to the technical requirements of the user. The present invention relates to an alumina agglomerate having a particle size that can be adjusted to .

The invention also relates to a method for obtaining the above-mentioned mass.

The present invention likewise relates to alumina lumps having good mechanical strength in various forms obtained by molding under pressure and subsequent heat treatment.

The industry that specializes in producing alumina and converting it to aluminum via fire electrolysis has long encountered and attempted to overcome various serious difficulties.

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

This is what is experienced when handling raw materials and when using raw materials in electric arc electrolyzers.

For this reason, it has been proven that expensive dust collectors and dust removers need to be included.

Another difficulty that arises is the recovery of some components in the gaseous effluent from the electric arc cell.

Today, the technique commonly employed for this purpose is to bring the gaseous effluent into intimate contact with the alumina used to feed the electrolyzer.

Those skilled in the art will appreciate that in order to sufficiently absorb the above components, the alumina that undergoes such contact is suitable for this implementation.
It has been confirmed that the material must have a specific surface area.

The last difficulty, which is important among all other difficulties, concerns the variation in alumina particle size.

Those skilled in the art desire that the particle size remain essentially constant at all times so that the operation of the electric arc cell is not adversely affected by such fluctuations.

These many difficulties and disadvantages have led those skilled in the art to use alumina in a form that is particularly suitable for electric arc electrolysis and which gives a product in which the desired properties are reproducible, i.e. permanent. I had doubts about the importance of

Many methods of alumina agglomeration have been proposed and widely described in the specialized literature in order to find solutions to these drawbacks.

The first type of proposed method is to machine a paste obtained by mixing Bayer process alumina with a suitable binder (which can be an acid solution of an aluminum salt such as aluminum nitrate or aluminum stearate). It consists of clumping together.

That is, after agglomeration by extrusion, compression molding or any other mechanical means, the resulting granules are calcined.

Such a process is expensive and produces a granular product contaminated not only with small amounts of Na20 from the Bayer process itself, but also with the binder or its residue after heat treatment.

Another, significantly improved method was subsequently proposed.

The process described in French Patent No. 2,267,982 consisted of producing bulk activated alumina using aluminum hydrate obtained by the Bayer process as raw material.

That is, the raw material, which may contain only a small amount of impurities, more specifically sodium impurities, is first dried to remove the moisture content, and then placed between two cylinders applying the desired pressure. Compression molding was carried out by continuous passage without the addition of binders, the continuous strip thus obtained was cut into pieces of the desired dimensions, and the pieces were then subjected to conventional heat treatment activation.

Various methods proposed so far include the Bayer method;
Concerning the agglomeration of irrigated alumina obtained substantially from its action on bauxite.

In addition to this basic method, raw ore (fni tial or
There is an acid process consisting of reacting e) with H2SO4.

This method converts alumina in ore into a compound with the general formula Al2O3,X
SO3゜yH20, where X and y can vary within a wide range, but correspond to the sulfates known in the art, which belong to the group consisting of acidic, basic and neutral aluminum sulfates. .

Generally, X has a value of 0.5 to 5, while y has a value of 0 to 18.

) forms an intermediate step in the production of pure aluminum by converting it into hydrated aluminum sulfate.

Equation A6203. xs03. yH20-)A 120
For the pyrolysis of the water-containing aluminum sulfate by 3+xSO3+yH20, the inventors have developed the above method to obtain incompletely decomposed sulfate and the hydrated "intermediate" by varying the time and temperature. It seemed possible to tune the decomposition.

When the aqueous aluminum sulfate is completely decomposed, the resulting alumina is generally in very fine particle form, easy to fly, and suffers from some of the drawbacks mentioned above.

Therefore, it was desirable to take into account the agglomeration of alumina obtained by pyrolysis of water-containing aluminum sulfate.

Interestingly, by continuing research in this field, the inventors have discovered that alumina particles with good mechanical strength and adjustable particle size can be produced from aqueous aluminum sulfate.

According to the invention, the novel alumina agglomerate has the formula: Al
2O3,
The "intermediate product" obtained from the incomplete decomposition of hydrated aluminum sulfate seeds is compression molded, the compression molded product is then granulated, and the granulated product is then heat treated. Features.

The "intermediate" to be compression molded may be, for example, silicone.
It is obtained by incompletely pyrolyzing aluminum sulfate produced by the action of acid on alumina ore so that the content of S, expressed as SO3, is from 1 to 15 φ, preferably from 3 to 12%.

As already mentioned, the "intermediate" is usually compression molded in a dry state.

However, it has been found that the addition of some amount of water to the product to be compression molded, i.e. not exceeding 15 weight types of the product, does not substantially affect the final properties of the alumina mass.

The "intermediate" as defined above is then subjected to an agglomeration operation.

A non-limiting example of this operation is shown in FIG.

In this operation, the intermediate product (1, P
, ) J is introduced into mixer B via (1).

B also receives via (6) a portion of the particulate product that is smaller than the desired size.

The "intermediate product" is then introduced into unit C via step (2), where compression molding is continuously performed.

Unit C comprises pressure means, which may for example be a cylindrical compression molding machine of the conventional type coupled with precompression molding means.

The compression pressure is at least 0.5 metric tons per linear cIrL relative to the width of the cylinder.

The compression molded product then exits the compression molding process in the form of a coarsely segmented continuous strip;
(3) and is taken into a granulator, where it is crushed into desired dimensions.

Crushing is carried out in known types of equipment such as claw cylinders, show crushers, hammer mills, etc.

The granules coming out of the crushing process pass through (4) to the sorting zone E.
Then, the granules have different dimensions α.

It is divided into at least three grades: β and γ.

The alpha class includes particulates having dimensions that fall within the size range desired by the subsequent user.

This grade then passes through (7) into a furnace F of known type where it is heat treated at the temperature necessary to obtain the alumina with the desired properties.

The beta class consisting of undersized granules is transported through (6) to mixer B and recycled to this operation.

The gamma class consisting of oversized granules is transported through (5) to the granulator where it is re-crushed and then returned to the sorting zone E through (4).

The α class is collected in G after heat treatment in F and prepared for use.

In an improved form of this method, a continuous compression molding unit C
can be replaced with a pelletizing press with a compression pressure of at least 200 kg/ffl.

The pelletized product is then fed to a granulator and subsequently subjected to the above-mentioned recycling process.

Alumina lumps obtained without using the binder at full heat are
Since it is heat treated, it exhibits particularly interesting physical properties, in addition to maintaining a constant particle size that can be adjusted according to the user's wishes.

Generally speaking, the content of sulfur, expressed as SO3, is less than 1.

The BET specific surface area measured by nitrogen adsorption using AFNOR standard type X[l-6,21 is from 2 to 150 m/g depending on the heat treatment conditions.

As a result, the alumina agglomerates of the present invention exhibit good abrasion resistance and assume a shape with good resistance to particle disintegration when subjected to repeated thermal and mechanical shocks.

According to the invention, it is also possible to produce a mass of well-defined shape by methods known in the art, such as, for example, molding under pressure or extrusion.

This includes, for example, spheroid bodies of various sizes, solid,
or hollow cylinders, platelets, groove wheels, diab
This makes it possible to produce products such as olos de renvoi).

The heat treatment is carried out after molding according to a selected heating cycle determined by the intended use of the molded article.

Other features and advantages of the invention are more fully explained by examples of implementation of the method.

Example 1 **
Contains 3.8% by weight of sulfur, 5.4% by weight φ and 11.9% by weight of sulfur expressed as SO3, (SO4)3A12
The "intermediate" produced by incomplete decomposition of 18H20 was pelletized under various pressures.

Compression molding is performed using a hydraulic press, with a pressure of 400 to 30
It was varied over 00 kf/-.

The pellets had a diameter of approximately 24 mm and a thickness varying from 2 to 6 mm depending on the amount of "intermediate" introduced.

The pellets thus obtained were heated to 1050 °C in a Matsufuru furnace that was gradually heated at a temperature increase of 5 °C per minute.
Calcined at a temperature of °C.

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

BET surface area uses Afnor standard type W-621,
Measured by nitrogen adsorption.

The pellet destruction test was conducted by dropping a steel ball with a diameter of 18.25 mm and a weight of 24.80.9 mm guided into a glass tube with a diameter of 20 mm.

This steel ball was dropped into the center of the pellet.

The height of the glass tube was increased until the pellet was destroyed by a single drop of a steel ball.

Example 2 In order to demonstrate that the presence of water does not have an adverse effect on the mechanical properties of the pellets produced, the "intermediate" was injected with 5 parts, 7 parts and 15 parts of the weight of water. It was damp.

When the sample was homogenized, it was compression molded in a hydraulic press identical to that described in Example 1 above.

The pellets had a diameter of approximately 24 mm and a thickness of 3 to 5 mm, depending on the amount of "intermediate" introduced.

After drying the pellets at a temperature of 110° C., they were calcined in a Matsufuru furnace at various temperatures.

The Matsufuru furnace was gradually heated at a rate of 5° C. per minute.

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

Example 3 Pellets previously obtained by pelletizing an "intermediate" containing 85.4%, expressed as SO3, at a pressure of 3'OOOOkf/i,
Calcined at 1300° C. in a Matsufuru furnace heated gradually with a temperature increase of 5° C. per minute and then held at that temperature for 1 hour.

From that time on, the BET surface area was 3 m/g.

The height to which the steel ball had to be dropped to break the pellets was approximately 5 CIn.

[Brief explanation of the drawing]

The accompanying drawings are process diagrams illustrating embodiments of the present invention. In the figure, A: storage tank, B: mixer, C: unit,
D: Granulator, E: Sorting zone, F: Heating furnace.

Claims (1)

[Claims] 1. From the incomplete decomposition of hydrated aluminum sulfate having the formula AA203, xs03, yH20 and containing 1 to 15% by weight, preferably 3 to 10% by weight of S, expressed as SO3. Good mechanical strength, characterized in that it is obtained by compression molding the resulting solid state intermediate product, then granulating the compression molded product, and then heat treating the granulated product; Alumina agglomerates with adjustable particle size. 2. X can have a value of 0.5 to 5, y can have a value of 0 to 18, and the sulfate belongs to the group consisting of acidic, basic and neutral hydrated aluminum sulfates. The alumina lumps described in section. 3. After compression molding and heat treatment, the content of S expressed as SO3 is 1φ or less.
The alumina lump according to any one of the following items. 4. Alumina mass according to any one of the preceding clauses 1 to 3, in which the product to be compression molded is wetted with water in an amount not exceeding 15% by weight of the product. 5. The alumina lump according to any one of the above items 1 to 4, which has a BET specific surface area of 2 to 150 m/.9. 6. According to any one of the above items 1 to 5, having a defined shape such as a sphere, a solid cylinder, a hollow cylinder, a small plate, etc., which is obtained by molding or extrusion under pressure. Any one of the above items 1 to 5, characterized in that when the intermediate product is pelletized, the intermediate product is compression molded at a pressure of at least 200 kf//criL. The alumina block form described in Section 8 consists of continuous compression molding of the intermediate product between two cylinders, in which case there is a gap of 1 cm in a straight line with respect to the width of the cylinder between the two cylinders. At least 0.0 per meter.
8. The alumina lump according to items 1 to 7 above, wherein a compressive force of 5 metric tons is applied. 9. The alumina lump according to any one of the above items 7 and 8, wherein the compression-molded intermediate product is granulated by crushing and then sorted according to a desired particle size. 10. The alumina lump according to any one of items 7 to 9, wherein the heat treatment is performed at a maximum temperature of 1500°C.