DE2732983B2 - Alumina agglomerates - Google Patents

Description

50

The invention relates to mechanically high-strength alumina agglomerates in a grain size distribution that can be controlled according to the technical needs of the user and processes for their manufacture. These agglomerates are compacted or Compaction of an intermediate product obtained, in turn, from the incomplete decomposition of Aluminum chloride hexahydrate is derived; the compacted product is granulated and the granulated product finally heat treated. ω

Furthermore, the invention relates to alumina agglomerates with high mechanical strength, which in different shapes have been obtained by means of compression molding and subsequent heat treatment are. t'5

The one on the production of alumina or aluminum oxide and its conversion to aluminum Industries specialized in fused flux electrolysis have been struggling with difficulties for a long time Disadvantages and endeavors to remedy them.

A first problem was the alumina residue caused by airborne dust, especially when handling and Bringing the alumina into the tubs for the melt flow electrolysis was noticeable. To the solution to address this problem, expensive dust collection and dedusting systems have had to be developed.

Another problem arose from the fact that certain exhaust gases from the wells of the Melt flow electrolysis elements or substances present must be intercepted. One for this developed, currently used common working method is to bring these exhaust gases into intimate contact with the clay to be fed into the vats. It has been shown that with The alumina brought into contact with the gases has a correspondingly adapted content of Λ-ΑΙ2Ο3 and a appropriately adapted specific surface BET must have, so that a satisfactory absorption these substances or elements is achieved.

A final and very serious problem is the observed fluctuations in the grain size distribution the alumina, while on the other hand a practically time-independent constant grain size distribution or setting is desirable so that the operation of the electrolysis tanks due to the differences in the grain size is not disturbed.

Because of these numerous difficulties, a move has been made to agglomerate the clay deform, which are particularly suitable for the melt flow electrolysis in order to have a product, whose desired properties are reproducible, i.e. do not change over time. In this In connection with this, numerous agglomeration processes have been developed for alumina and are in the specialist literature described.

According to an initially developed working method, a paste is mechanically agglomerated, which by Mixing Bayer clay (trihydrate) with a suitable binder, for example an acid solution or an aluminum salt solution such as aluminum nitrate or aluminum stearate solution. After agglomeration by extrusion, compression (compacting) or any other mechanical means Way, the granules obtained are burned. These first procedures proved costly and supplied granulated products that not only have a low content of Na2O from the Bayer process itself, but also contaminated by the binder or its residue left over after firing was.

A significant improvement in this procedure was achieved with the process of French Pat. No. 22 67 982. Thereafter, an agglomerated active clay is made from the according to the Bayer process obtained alumina hydrate. This starting material, which has a low content of accompanying substances or should contain impurities, especially accompanying substances containing sodium, is first dried, to remove the impregnation water; then it is compacted by adding it without adding any binder is passed continuously between two cylinders or rollers, between which the desired Pressure has been set. The continuous belt-shaped material obtained in this way becomes then crushed according to the desired dimensions and finally a conventional thermal

Subject to activation treatment.

In FR-PS 14 19 879 a process for converting and purifying aluminum chloride hexahydrate is used described; this process is an important intermediate stage in the production of pure alumina or pure aluminum oxide AI2O3, from aluminum silicate ores or minerals that are not by the alkaline Bayer process, but must be digested by means of acid, with the in the ore contained alumina is converted into an aluminum chloride hydrate. It is also known that the means Precipitation obtained aluminum chloride hexahydrate in the form of rods, the length of which is usually 50 to 150 μπι is, depending on the chosen Crystallization conditions.

In the thermal decomposition of aluminum chloride hydrate according to the equation

2 (AlCl ₃ , 6 H ₂ O) -Al ₂ O ₃ + 6 HCl + 9 H ₂ O

At a temperature, for example, in the range from 600 to 900 ^° C., in a static or dynamic bed, the alumina obtained is also obtained in the form of rods, the volume of which makes up about 35% of the volume of the rods of the aluminum chloride hexahydrate used as the starting material. These clay sticks are fragile and prone to the abrasion phenomena that occur in the course of heat treatments and during mechanical, pneumatic or other transport.

The alumina thus obtained is therefore generally in the form of very fine particles. the easily fly away and at the same time raise several of the problems described earlier.

The invention was therefore based on the object of a possibility of agglomerating by means of thermal Provide decomposition of aluminum chloride hexahydrate obtained alumina.

According to the invention, this object is achieved by starting from aluminum chloride hexahydrate Alumina granules with high mechanical strength and adjustable grain size distribution are produced can be.

According to the invention, the new alumina agglomerate is characterized in that a "Intermediate product" is preferably compacted in the dry state, which consists of the incomplete Decomposition is derived from aluminum chloride hexahydrate and contains 0.8 to 15 wt% Cl; that condensed The product is granulated and the granulate is finally thermally treated.

The "intermediate product" which is to be compressed is obtained from the incomplete thermal decomposition of aluminum chloride hexahydrate, which, for example, in the acid digestion νυΐ; aluminum silicate ores is obtained such that the chlorine content is 0.8 to 15%, preferably 2 to 10%. The content of AbO ₃ as well as the constitutional water of the intermediate product depends of course on the chlorine content; the impregnation water is evaporated before the decomposition of the aluminum chloride hexahydrate

This incomplete decomposition of the aluminum chloride hexahydrate is carried out in a manner known per se performed.

As already stated above · the "intermediate product" is usually dry compressed or compacted. It has been found, however, that no more than 20% by weight of the product can be moistened with a little water before compacting without the properties of the final alumina agglomerate product being impaired.
The "intermediate product" is agglomerated, as shown schematically in the drawing.

The "intermediate product PI" stored in A is fed via line 1 into mixer B , where it is mixed with a granulated product fed via line 6, which is undesirably small in particle size.

This mixture reaches press C via line 2, in which the compression takes place. This press C is equipped with, for example, a press roller of the usual type combined with a means for pre-compression or pre-compression. The pressure in the roller is at least 3 t per (linear) cm of the roller length (width of the cylinder) such that the apparent density of the compacted product is preferably 1 to 1.5 g / cm ³ . The compacted product has the form of a continuous belt which, when exiting the compression zone, breaks up into coarse pieces and which is fed via line 3 into a comminution device D , in which the comminution to the desired dimensions with the help of devices of conventional design such as a pin mill ( Spiked roller), jaw crusher, hammer mill and ii. m. takes place.

The granulate arrives from D via line 4 in a selection or classification area and is divided there into at least three fractions a, b and c of different grain sizes.

jo Fraction a includes the granules, their dimensions lie in the range desired by the later user. This fraction is about 7 in an oven F known design and there heat-treated in the necessary manner so that the alumina

j5 receives the desired properties, for example in Temperature range from 600 to 1500 ° C.

Fraction b consists of the grain that is too small and is returned to the process via line 6 to mixer B.

Fraction c consists of the grain that is too coarse and is fed via line 5 to device D and here again subjected to a comminution process and then returned to classifying or selection zone F via line 4.

After the heat treatment in F, fraction a is deducted from G with a view to later use.

According to a variant of the method, the device C for continuous compression or pressing consists of a pellet press with a pressing pressure in the range from 2000 to 10,000 kp / cm ² .

The pelletized product is then fed to stage D and continues through the treatment cycle described above.

After the heat treatment has been completed, they have no addition or involvement of any Alumina agglomerates obtained binder are particularly important or interesting physical properties on, above all a regular crown size distribution, which is according to the wishes of the User adjustable.

In general, the chlorine content is very low and, depending on the conditions of the heat treatment, is in Range from 0.005 to 0.5%.

b5 In addition, the BET specific surface area, determined by nitrogen absorption in accordance with the French standard AFNOR XI1-621, is 2 to 120 m ² / g and the content of «-AI2O3 is also dependent

from the conditions of the heat treatment 95 to 0%.

It has been shown, surprisingly, that with identical Conditions of the heat treatment the alumina agglomerates produced according to the invention have a chlorine content and have a larger BET surface area than alumina, which is based on aluminum chloride hexahydrate, but had been produced without the agglomeration cycle provided according to the invention.

The alumina agglomerates produced according to the invention are characterized by very good abrasion resistance which have a very good resistance to the disintegration or crumbling of the grain or the Granules can be seen in the event of repeated thermal or mechanical shocks or impacts.

According to the invention, agglomerates with well-defined or specific shapes can also be produced, with the help of the working methods known and customary in this field, for example compression molding, Extruding, etc. In this way, pellets of different sizes, solid cylinders or Hollow cylinders, plates, grooving rolls, etc. m., being for the Heat treatment after the shaping, a heat cycle is selected and determined, which is based on the intended use of the molded body.

The following examples serve to explain the invention in more detail.

example 1

In B , 15 kg of fraction b consisting of sifted fine grains <1 mm, which came from a previous cycle, were mixed with 15 kg of "intermediate" which had been obtained by incomplete decomposition of aluminum chloride hexahydrate; the mixture still contained 4.55% by weight of Cl. It was pre-compressed in the conical space of a filling funnel with an Archimedean screw rotating in it. The base of this pre-compressor flows «- directly onto or into the press; this consisted of; two rollers with a diameter of 600 mm, which were equipped with perforated belts, the belts being 1.5 mm apart prior to the introduction of the product.

ίο During the entire process, a constant pressure of 6 t / cm belt width on right. The speed of rotation was 4rpm.

The platelets collected at the exit of C.

ι Ι were fed in D into a hammer mill, which was equipped with a grid or grate with a mesh size of 6.5 mm. The granules obtained in this way were classified in E, namely in a device which enabled the separation into four fractioners or fractions:

The fraction <5 mm, which made up 2% by weight of the total granulated mass and fed back ^{into D!} became;
the fraction desired by the user 2 to 5 mm made up 43% by weight of the granulated mass;

the also desired fraction 1 to 2 mm, the If % By weight of the granulated mass and

the fraction below 1 mm, which made up 37% by weight of the granulated mass and fed back into B.

jo was.

The two fractions 2 to 5 mm and 1 to 2 mrr were fired separately from one another in G at different temperatures and then had the properties summarized in the table below as a function of these temperatures.

Factions Brenn Brenn Cl Surface Apparent Abrasion test average 0 average 0 temperature loss BET density before test after test generated μπι Fine 3050 2900 ■ c O / o o / n m ² / g kg / dm3 % <160 μΐπ 2 to 5 mm not ge 4.55 - 0.78 0.02 burned 2800 2650 granules 2850 2600 850 10.9 0.3 72 0.74 0.16 3050 2850 950 12.35 0.26 39 0.74 0.16 1080 1020 1050 13.4 0.07 20th 0.73 0.04 1150 1020 1 to 2 mm 850 11.0 0.3 69 0.68 2.5 900 12.0 0.28 47 0.65 1.0

The abrasion test was carried out with a mixer consisting of a cylindrical glass container with a volume of 250 ml. Inside diameter 55 mm and inside length 105 mm. This container was subjected to a complex movement going in the three directions of space, the thrusts and rotations and rhythmic rocking movements worked.

The container was filled with 50 g of alumina agglomerate, namely the residue on the 160 μπι sieve.

The sample was moved continuously for 2 hours and then again passed through the same sieve 160 μm. on this way became the percentage of that through the abrasion test determined the fine grain produced.

Finally, before and after the abrasion was made mean diameter determined with the aid of the methods familiar to the person skilled in the art.

With the help of this example, the essential properties of the alumina agglomerates are determined for « Features shown for compression and for a chlorine content of the »intermediate product« of 4.55%.

Example 2

In this example the determining influence is de: Chlorine content of the "intermediate product" on the essential properties of the herge according to the invention presented agglomerates.

There are b in B 15 kg group consisting of the

h5 sieved fine fraction <1 mm as in example 1 with 15 k | "Intermediate product" mixed; the mixture still contains 2.5% by weight of C1.

This mixture was described as in Example 1!

compressed with the modification that the speed of rotation of the cylinders from 4 rpm to 2 rpm was slowed down.

After sieving or classifying in E , the following fractions were obtained:

The fraction <5mm, which is 1% of the granulate mass made out; the desired fraction 2 to 5 mm, which made up 28% of the granulate mass;

the also desired fraction 1 to 2 mm, which made up 17% of the granulate mass and

the fraction <1 mm, which made up 54% of the granulate mass and was fed back into B.

For comparison, a mixture of "fine grain" and "intermediate product" was used under the same conditions. compressed, the chlorine content of which was 0.9% by weight before compression.

After sieving and classifying were here

receive the following fractions:

a fraction> 5 mm corresponding to 1% of the granulate mass, a desired fraction 2 to 5 mn corresponding to 24% of the granulate mass,

a fraction 1 to 2 mn, which is also aimed at, corresponding to 11% of the granulate mass and

a fraction <1 mm corresponding to 64% of the granulate mass.

In comparison with the granules made from the mixture with 2.5% chlorine, the granules proved themselves made from the mixture 0.9% chlorine as relath friable.

The two fractions 2 to 5 mm were fired at different temperatures and then had depending on the firing temperature, different mechanical strength in the abrasion test; the results are in dei summarized in the following table.

% Cl im
Pl mixture

Firing temperature

Apparent Density Abrasion Test

kg / dm ^ produced fines
% <160 μm

average 0 before test μιη

average 0 after test μιη

not burned

850

950
1050

not burned

850

950
1050

0.96
0.87
0.86
0.88

0.83
0.78
0.78
0.79

3100
3000
2900
3000

2650
2650
2800
2750

2600 2750 2750 2800

2500 2500 2600 2600

The granules made from the intermediate with lower chlorine content were found to be less abrasion resistant.

Examples 3 to 5

There were three samples "intermediate product" with different chlorine content of 1.2 wt .-% and 4.4 Wt% and 12.7 wt% pelletized.

To demonstrate that the presence of water affects the mechanical properties of the manufactured Pellets not impaired, the respective intermediate product was added with 10% by weight or 20% by weight based on the mass of the intermediate product water.

The samples were homogenized and then compacted with the aid of a hydraulic press with a pressure of 3100 to 7700 kp / cm ^2.

The pellets were about 20.3 mm in diameter and ranging from 7 to 11 mm in thickness "Intermediate product" depending on the amount introduced.

After drying at 11O ⁰ C, the pellets were fired in a muffle furnace which was heated at a rate of temperature rise of 3.4 ° C / min allmählieh from 200 to 900 ° C.

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

% Cl im
Pl mixture

Vo H ₂ O
added

Press pressure

kp / cm ² mean

apparent

density

g / cm ³

Ball height in the event of breakage

1.2

4.4

3100 1.03 20th 4600 1.23 25th 6200 1.31 25th 3100 1.12 75 4600 1.24 100 6200 1.34 100 7700 1.38 100 3100 1.13 75 4600 1.32 150 6200 1.36 100 7700 1.42 75

9 continuation % H ₂ O
added 27 32 983 Medium
apparent
density 10 Spherical
height at
fracture % Cl im
PI mixture g / cm ι cm 10 Press pressure 1.12
1.16
1.22
1.20 20th
10
10
10 12.7 20th kp / cm- 1.30
1.33
1.36
1.35 10
10
10
10 3100
4600
6200
7700 3100
4600
6200
7700

The breaking strength of the pellets was tested in the ball drop test; a steel ball with a diameter was used for this 18.25 mm and weight 24.80 g in a guide tube made of glass with a diameter of 20 mm in each case on the center of the Pellets dropped. Glass tubes of increasing length were used up to a single ball fall Rupture of the pellet resulted.

Example 6

In this example, the influence on the chlorine content and the surface BET is investigated, which im Comparison with one another of the dimensions of the agglomerates according to the invention and an alumina the same Origin, but different dimensions, which are not un-

has been subjected to, for precisely defined firing temperatures and firing times.
The following three samples were used for this:

Sample A consists of alumina granules with a grain size between 0.5 and 1 mm,

Sample B consists of alumina granules with a grain size of 0.25 to 0.5 mm,

Sample C from the incomplete decomposition of fine grain aluminum chloride hexahydrate; mean diameter of the intermediate product about 50 μίτι.

The various samples were fired in a fluidized bed furnace using dry air as Carrier gas. The properties of the treated products are summarized in the following table:

treatment Burn time Carrier gas properties the treated % C1 speed delten products Brenn min cm / s surface temperature 90 32 BET 0.13 ⁰ C 120 cm ² / g 0.06 A. 60 52 0.036 1000 90 51 <0.01 90 15th 37 0.16 1050 120 34 0.07 B. 60 49 0.05 1000 90 51 <0.01 90 17th 32 0.21 1050 120 31 0.15 C. 60 30th 0.09 1000 90 30th 0.05 22nd 1050 19th

These results make it possible to measure or determine the simultaneous development of the percentage Chlorine content and the specific surface BET depending on whether the treated intermediates were agglomerated according to the invention or not.

1 sheet of drawings

Claims (8)

Patent claims: 1. High-strength alumina agglomerates with controllable particle size distribution, which contain 0.005 to 0.5% by weight of chlorine, a BET specific surface area of 2 to 120m ² / g and a content of «-Al ₂ O ₃ of 95 to 0% and have Compaction of an intermediate product obtained in the incomplete decomposition of aluminum chloride hexahydrate with 0.8 to 15% by weight, preferably 2 to 10% by weight, of Cl, granulation of the compacted product and thermal treatment of the granulated product have been obtained. 2. Alumina agglomerates according to claim 1, in a defined form such as spheres, hollow and solid cylinders or platelets obtained by compression molding or extrusion. 3. Veifahren for producing the alumina agglomerates according to claim 1, characterized in that an intermediate product obtained in the incomplete decomposition of aluminum chloride hexahydrate with a Cl content of 0.8 to 15, preferably 2 to 10% by weight, is compressed compacted product granulated and the desired particle size distribution thermally treated. 4. The method according to claim 3, characterized in that the Cl-containing intermediate product is compressed ^{in a pellet press under a pressure of 2000 to 10,000 kp / cm 2.} m 5. The method according to claim 3, characterized in that the Cl-containing intermediate compressed between two cylinders with a pressing force of at least 3 t / cm cylinder width. 6. The method according to any one of claims 3 to 5, characterized in that it is compressed ^{to a product with an apparent density of 1 to 1.5 g / cm 3.} 7. The method according to any one of claims 3 to 6, characterized in that the Cl-containing Intermediate product before compaction with a maximum of 20% by weight of water, based on the dry weight of the intermediate product, moistened. 8. The method according to any one of claims 3 to 7, characterized in that the granulated product is treated at a temperature in the range from 600 v> to 1500 ° C.