JPS5817128B2 - Method for producing alumina lumps with high mechanical strength - Google Patents

Description

Detailed Description of the Invention The present invention involves compressing an intermediate product obtained by incomplete decomposition of aluminum chloride hexahydrate, then granulating the compressed product, and then heat-treating the granulated product. The present invention also relates to a method for producing alumina agglomerates having high mechanical strength and adjustable particle size, which can be adapted to the technical requirements of the user.

The present invention also relates to a method for manufacturing alumina lumps having high mechanical strength in various forms obtained by compression molding followed by heat treatment.

The industry that specializes in obtaining alumina and converting it to aluminum by fusion electrolysis has for many years encountered significant difficulties and drawbacks that researchers have attempted to overcome.

The first drawback is the loss of alumina due to spatter, which was experienced when handling the alumina and when using it in melting electrolysis tanks.

Therefore, it was necessary to design an expensive dust collection and treatment device.

Another drawback encountered was that some of the elements contained in the gaseous effluent leaving the melting electrolysis tank had to be recovered.

The method for doing this commonly used today consists of creating intimate contact between the gaseous effluent and the alumina used to charge the tank.

Those skilled in the art have determined that in order to obtain satisfactory absorption of these elements, the alumina contacted must contain "alpha" alumina and have a suitable BET surface area to perform this. .

Finally, among all other deficiencies, it was the variation observed in the alumina particle size that proved to be a significant one that had to be addressed.

A person skilled in the art would prefer to have a particle size that is always constant so that the operation of the melting electrolysis tank is not disturbed by such variations.

These many difficulties and drawbacks have made it difficult for those skilled in the art to bring alumina into the form of a mass that is particularly suitable for melt electrolysis in order to produce a product whose desired properties are reproducible, i.e., time-invariant. I continued to doubt the possibility of

Many methods of alumina agglomeration have been proposed, and ideas for finding ways to overcome these drawbacks have been widely described in the professional literature.

The first type of proposed method is the "Bayer" method, in which the paste obtained by mixing the alumina with a suitable binder, which can be an acid or a solution of an aluminum salt like aluminum nitrate, aluminum stearate, etc. It was composed of agglomerates.

After being agglomerated by extrusion, compression or any other mechanical means, the resulting granules were calcined.

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

Other methods were subsequently proposed which constituted significant improvements.

The process disclosed in French Patent No. 2,267,982 consists of producing agglomerated activated alumina using aluminum hittard obtained by the Bayer process as its raw material.

Impurities, More Details The feedstock containing only small amounts of impurities was first dried to remove its permeate water.

It was then compressed without adding any binder by passing it continuously between two cylinders where the desired pressure was set.

The continuous strip thus produced was crushed according to the desired dimensions and the pieces were subjected to a conventional activation heat treatment.

Now, our French Patent No. 1,419,879 discloses a process for the conversion and purification of aluminum chloride hexahydrate.

This process forms an important intermediate step in obtaining pure alumina from aluminum silicate ore, which cannot be treated with the alkaline Bayer process, by treating the ore with acid to hydrate the alumina in the ore. convert into things.

However, the aluminum chloride hydrate obtained by precipitation is usually 50μ to 1500μ depending on the crystallization conditions chosen.
It is known to have a rod-like shape whose length varies within the range of .

When the aluminum chloride hydrate is pyrolyzed in a fixed or moving bed at a temperature of, for example, 600 to 900°C according to the formula 2, the alumina collected also has a rod-like appearance and its volume is equal to approximately 35% of the original rod volume of hydrated aluminum chloride.

The alumina rods are brittle and during heat treatment and mechanical
Inability to withstand excessive friction during pneumatic or other types of transport operations.

For these reasons, the alumina thus obtained generally has a very fine grain form, is prone to dusting, and at the same time has some of the disadvantages mentioned above.

It was thus desirable to attempt to agglomerate the alumina obtained by thermal decomposition of aluminum chloride hexahydrate.

Continuing research in this field, the inventors were interested to discover that it is possible to produce alumina particles of high mechanical strength and adjustable particle size from aluminum chloride hexahydrate.

The novel alumina mass according to the invention has the following characteristics, namely that it is an "intermediate" - its "
"Intermediate product" is an incomplete decomposition product of aluminum chloride hexahydrate and containing 0.8 to 15% by weight of chlorine - preferably dry compressed, and then the compressed product is granulated. The granulated product is further characterized in that it is produced by applying heat treatment to the granulated product.

The "intermediate product" to be compressed is obtained, for example, by incomplete pyrolysis of aluminum chloride hexahydrate obtained by the action of an acid on aluminum silicate ore (1 content 0.8 to 1
5%, but preferably something like 2 to 10%.

Proceeding from that, A1 present in the "intermediate product"
The content of 1203 and the structural water content can be naturally estimated from the 1 content, since the impregnated water was evaporated before the decomposition of chloride-hexahydrate.

Incomplete decomposition of aluminum chloride hexahydrate is carried out by methods known in the art.

As already mentioned, the "intermediate product" is usually dry compressed.

However, even if an amount of water not exceeding 20% of the product is added to the product to be compressed,
It has been found that this does not substantially affect the final properties of the alumina mass.

A non-limiting industrial example of an "intermediate product" thus defined which was then subjected to an agglomeration process is shown in FIG.

In this method, intermediate product 1.P is stored in A.
, J are fed through 1 to mixer B, which likewise receives through 6 a portion of the granular product smaller than the desired size.

It then passes through 2 to unit C where it is continuously compressed.

Unit C is constituted by compression means, which may be, for example, a cylinder compressor of the usual type with attached pre-compression means.

The compression pressure is at least 3 tons per crrl over the width of the cylinder so that the apparent density of the product is preferably 1 to 1.5 cm/cd.

The continuous elongate compacted product then proceeds to be coarsely crushed as it leaves the compaction stage and is passed through 3 to a granulator to crush it to the desired size.

Crushing is carried out in known types of equipment such as spike trawlers, mills, crushers, hammer mills, etc.

The particles coming out of the crushing field are led to the sorting zone E by 4, where they are separated into at least three classes of "α", "β" and "γ" having different sizes.

The "α" class includes particles of a size that falls within the size range desired by the subsequent user.

This grade is then passed through a furnace of known type through 700 to give alumina with the desired properties, e.g.
Heat treatment is performed at a suitable temperature of 0 to 1500°C.

The "β" class is made up of particles with undersized dimensions.

This is transported through 6 to mixer B for recirculation to the process.

The "gamma" class consists of particles of extremely large size.

This is sent to the granulator through 5, where it is crushed again and then 4
The sample is introduced into the sorting zone E again through the filter.

After heat treatment in F, the "α" class is collected in G for use.

In another form of the process, the continuous compression unit C is 2000
It can also consist of a press for pellet production with a compression pressure selected between 10,000 kf/cat and 10,000 kf/cat.

The pelletized product is then fed to a granulator and then subjected to the processing cycle described above.

The alumina agglomerates obtained without the aid of any binder by applying a heat treatment have particularly interesting physical properties besides the property of retaining a regular adjustable particle size according to the wishes of the user.

Generally speaking, the chlorine content is very low and varies between 0.005 and 0.5% depending on the heat treatment conditions.

BET specific surface area measured by nitrogen adsorption according to AFNOR standard ■-621 from 2 m / g to 120
m/9N and the “α” (alpha) alumina content is between 95% and 0% depending on the heat treatment conditions.

The present inventors under the same heat treatment conditions (1)
and the alumina agglomerates are more C4 than alumina obtained from chloride-hexahydrate without following the agglomeration cycle of the present invention.
I was surprised to find that the content was low and the BET surface area was large.

Finally, the alumina agglomerates according to the invention exhibit very good resistance to friction, which is manifested by a good resistance to powdering of the particles when subjected to repeated thermal and mechanical shocks.

According to the invention, the mass can also be made into defined shapes by techniques known in the art, such as compression molding, extrusion, etc.

In this way, for example, balls of various dimensions, solid or hollow cylinders, small flat plates, grooved pulleys, reversible double wheels, (diat)olo
It becomes possible to produce products such as sde renvoi).

For these products, the heat treatment following molding follows a selected heating cycle determined by the use for which the molded article is intended.

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

Example 1 15 kg of "β" grade consisting of fine particles obtained in a previous operation carried out under the same industrial conditions and passed through a sieve of 17 ILm were obtained by incomplete decomposition of aluminum chloride-hexahydrate. Mix 15 kg of "intermediate product" in B,
The mixture still contains 4.55% by weight of chlorine.

The mixture is precompressed in a conical container with a spiral conveyor rotating therein.

The spiral conveyor is made to match the conical shape of the hopper.

The bottom of the precompressor leads directly to the compressor, which is a 60 mm diameter steel strip with a recessed steel strip.
It consists of two cylinders of 0 mm, the strips of which are separated from each other by 1.5 mm before entering the product.

The squeezing pressure, which is constant during work, is 1 cI across the width of the steel strip.
6 tons per rL.

The rotation speed is 4 revolutions per minute.

The platelets collected from C are fed into a hammer mill D equipped with a screen having 6.5 inch mesh holes.

Sorting device E that sorts the particles obtained in this way into four classes
And sieve.

The grade of 5 or higher destined for the granulator constitutes 2% by weight of the amount of particles, the grade of 2-5 am as desired by the user constitutes 43% by weight of the amount of particles; The 1-2 mm grade desired for B constitutes 18% by weight of the amount of particles, and the smaller than 1 mm grade recycled to B accounts for 37% of the amount of particles.
% by weight.

2-5 mrtt grades 1-2 mm grades were calcined in G separately at different temperatures and their individual properties as a function of these temperatures are disclosed in the following table.

The friction test was conducted using an inner volume of 2 with an inner diameter of 55 mm and an inner length of 105 mm.
It is carried out using a mixer consisting of a 50 ml glass cylindrical vessel.

Complex three-dimensional movements are applied to the container, producing the effects of shaking, rotation, and rhythmic rocking.

50 g of aluminum lumps remaining from the previous sieving at 160μ are placed in a cylindrical container.

After stirring the sample continuously for 2 hours, it is sieved again at 160μ.

The percentage of "fine particles" generated during the friction test is thus determined.

Finally, the average diameter before and after rubbing is measured using methods known in the art.

This Example 1 thus demonstrates the particular compaction properties of the "intermediate product" and the essential properties of the alumina mass for a chlorine content of 4.55%.

Example 2 The purpose of this example is to demonstrate that the chlorine content of the "intermediate" is a determining factor in the fundamental properties of the agglomerates of the invention.

15 kg of "β" class consisting of "fine particles" when sieved at 1 mm are mixed with 1 intermediate product J15ky in B as in Example 1.

The mixture still contains 2.5% by weight of chlorine.

The mixture was compressed under the conditions described in Example 1 except that the cylinder rotation speed was reduced from 4 revolutions per minute to 2 revolutions per minute.

After sifting through E, I collected the next grade.

Grades larger than 5 mm represent 1% of the amount of particles.

2-5 mm class, representing 28% of the amount of particles.

1-2mm class, also required, 17 of the amount of particles
represents %.

The smaller than 1 mm class, those recycled to B, represent 54% of the amount of particles.

As a comparative example, a mixture of "fine particles" and "intermediate product" with a chlorine content of 0.9% before compression is similarly compressed under the same working conditions.

Collect the next can rim after sifting.

Grade larger than 5 mm, 1% of the amount of particles, desired 2-5 mm grade, 24% of the amount of particles, -2m
7ft class, also desired, 11% of the amount of particles, 1m7I
Grade smaller than L, 64% of the amount of particles.

It has been found that particles with a chlorine content of 0.9% are relatively more fragile compared to particles with a chlorine content of 2.5%.

Calcining 2-5 mm grade materials at various temperatures,
Friction tests show differences in mechanical strength as a function of these temperatures.

These differences are disclosed in the following table.

Particles obtained from intermediate products with lower chlorine content have lower strength.

Example 3-5 Chlorine was added at 1.2% by weight, 4.4% by weight, and 12% by weight, respectively.
Three samples of "intermediate product" containing 7% by weight were pelleted.

In order to demonstrate that the presence of water does not have an adverse effect on the mechanical strength of the pellets formed, the "intermediate product" was added in an amount corresponding to 10% or 20% by weight, respectively, of the amount of "intermediate product". Moisten with water.

3100 to 77 depending on the water pressure after the sample is homogenized.
Apply pressure varying between 00 kf/d.

The pellets have a diameter of approximately 20.3 mm and a thickness that varies between 7 and 11 mm depending on the amount of "intermediate product" placed therein.

After drying at 110°C, the pellet was heated at 3.4°C per minute.
It is calcined in a Matsufuru furnace whose temperature is gradually raised from 200°C to 900°C.

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

Pellet destruction test has a diameter of 18.25 mm and a weight of 24.8
(The test was carried out by dropping a steel ball of Bi' using a glass tube with a diameter of 20 mrIL as a guide.

The ball is dropped into the center of the pellet.

Use progressively longer glass tubes until a single drop destroys the pellet.

Example 6 The objective was to determine the dimensions of the agglomerates according to the invention and the chlorine content and BET surface area for a particular calcination temperature and time of alumina of the same origin that had not undergone the agglomeration cycle of the invention of various dimensions. The objective is to measure the comparative effectiveness of

Three samples are used for this purpose, prepared as follows.

Sample rAJ Sample rBJ consisting of alumina particles with a size of 0.5 to 111L7IL Sample "C" consisting of alumina particles with a size of 0.25 to 0.5 mm Fine particle size whose intermediate product has an average diameter of about 50μ , obtained by incomplete decomposition of aluminum chloride-hexahydrate.

These various samples are calcined in a fluidized bed furnace where air is the fluidizing gas.

The properties of the products treated in this way are shown in the table below.

These results depend on whether the treated intermediate product was agglomerated according to the present invention or not. % and BET surface area simultaneously.

[Brief explanation of drawings]

FIG. 1 is a flow sheet of the method of the present invention. A; storage container for "intermediate products";B;mixer;C; unit for compression; D; granulator; E; screening zone (sieving); F; calcination furnace; G; product; 12.3 ,4,5,6
．． 7 indicates the route.

Claims (1)

[Claims] 16 0 produced by incomplete decomposition of water-use aluminum chloride
, an intermediate product containing 8 to 15% by weight of chlorine is selected, and this intermediate product is compression molded into pellets at a pressure of 2000 to 10000 kg/crrt.
A method for obtaining alumina lumps, characterized by heating to a temperature of 500°C. 2 After compression, but before heat treatment, the apparent density is between 1 and 1.
．． 5 g/cril, the method for producing an alumina lump according to item 1 above. 3 C4 content after compression and heat treatment is 0. The first above, characterized in that OO' is 5% by weight to 0.5% by weight.
2. A method for producing an alumina lump according to item 1 or 2. 4. A process for producing alumina agglomerates according to item 1, 2 or 3 above, characterized in that the product to be compressed is moistened with an amount of water not exceeding 20% by weight of the product. 5. Item 1, 2, 3 or 4 above, wherein the product has a BET specific surface area of 2 to 120 m7g and an alpha alumina content of between 95% and 0%.
A method for producing an alumina lump as described in section. 6. The first above, characterized in that they are given a predetermined shape, such as a sphere, a solid or hollow cylinder, a small plate, etc., making them into compressed products by compression molding or extrusion.
A method for producing an alumina lump as described in Item 2, Item 3, Item 4, or Item 5. 1. When the product is pelletized, the intermediate product is compressed under a pressure of 2000 to 10000 kf/i,
The method for producing an alumina lump according to item 4 or 5. 8 The intermediate product is compressed continuously between two cylinders, with 1 cf of the width of the cylinders being compressed between the cylinders.
The method for producing an alumina lump according to the above item 1, 2, 3, 4 or 5, characterized in that a compression pressure of at least 3000 kg per rL is applied. 9. The method for producing an alumina lump according to item 7 or 8 above, characterized in that the compressed intermediate product is granulated by crushing and then sorted according to a desired particle size. 10 The alumina lump according to item 7, 8 or 9 above, characterized in that the heat treatment is set at a temperature of 600 to 1500°C in order to impart desired physical properties to the alumina. Production method.