DE2732983A1 - CLAY AGGLOMERATES - Google Patents

Description

DIi. ING. F. WIJKSTIIOFF DIt. K. ν. PKClI MANX

DK. INC. I). IiKIIKKNS DIPL .. ING. R. GOKTZ

PATENT LAWYERS

8OOO MÜNOHKN OO SCIlVVKUiEKSTHASSE τκι.KFON <089) ee2U.11 TKI.KX 3 24 070

TKLRdVAM MKt

1A-49 497

P a tent application

Applicant: ALUMINUM PECHINEY

28, rue de Bonnel, 69003 Lyon, France

Title: Alumina Agglomerates

709885/0775

DH. ING. F. WUKSTHOKK DH. Kt. PKC H MAN N DH. IN". U. KKlIHKNS DIPL. JN (J. H. (JOKTZ PATENTAN U ^- ALTF.

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1-49 497

Applicant: Aluminum Pechiney

description

The invention relates to mechanically high-strength alumina agglomerates in a manner corresponding to the technical needs of the user controllable grain size distribution and process for their production. These agglomerates are contacted or Compaction of an intermediate product obtained, in turn, from the incomplete decomposition of aluminum chloride hexahydrate comes from; the compacted product is granulated and the granulated product is finally heat-treated.

Furthermore, the invention relates to alumina agglomerates with high mechanical strength, which in different Forming have been obtained by means of compression molding and subsequent heat treatment.

Those on the production of alumina or aluminum oxide and Its conversion to aluminum by means of fused flux electrolysis has been struggling with difficulties for a long time and disadvantages and endeavors to remedy them.

A first problem was the loss of alumina due to airborne dust, which was particularly noticeable when handling and placing the alumina in the tanks for the fused-salt electrolysis. To solve this problem, expensive dust collection and dedusting systems had to be developed.

709885/0775

Another problem arose from the fact that certain gases were present in the flue gases from the fusible electrolysis pans Elements or substances must be intercepted. A customary working method developed for this purpose that is currently used is to bring these exhaust gases into intimate contact with the clay to be fed into the tubs. It has been shown that the alumina brought into contact with the gases has a correspondingly adapted content of ed-AlpO, and a appropriately adapted specific surface BET must have, so that a satisfactory absorption of these substances or elements is achieved.

A final and very serious problem is the observed fluctuations in the grain size distribution of the alumina, while on the other hand a practically time-independent constant grain size distribution is desirable so that the Operation of the electrolysis tank is not disturbed by the differences in the grain size properties.

Because of these numerous difficulties, a move has been made to deform the alumina into agglomerates, which in particular are suitable for fused metal 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 for alumina have been developed and described in the specialist literature.

According to an initially developed way of working, a paste mechanically agglomerated 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 has been obtained 1st. After agglomeration by extrusion, compression

- 3 -709885/0775

The granules obtained are fired (compacting) or in any other mechanical manner. These first processes turned out to be costly and provided granulated products that _{were contaminated not only by low levels of Na 2} O from the Bayer process itself, but also by the binder or its residue after firing.

A significant improvement in this mode of operation was achieved with the process of FR-PS 2,267,982. Thereafter, an agglomerated active alumina is produced starting from the alumina hydrate obtained by the Bayer process. This starting material, which contains a low content of accompanying substances or impurities, especially sodium-containing accompanying substances should, is first dried in order to remove the impregnation water; then it is compacted without the addition of binder by being passed continuously between two cylinders or rollers between which the desired pressure has been set. The continuous, strip-shaped material is then comminuted according to the desired dimensions and finally subjected to a conventional thermal activation treatment.

FR-PS 1 419 879 describes a process for converting and purifying aluminum chloride hexahydrate; this process represents an important intermediate stage in the production of pure alumina or pure aluminum oxide AlpO ,, from aluminosilicate ores or minerals that cannot pass through the alkaline Bayer process, but must be digested by means of acid, whereby the clay contained in the ore is converted into an aluminum chloride hydrate. It is also known that the aluminum chloride hexahydrate obtained by means of precipitation is obtained in the form of rods, the length of which is usually 50 to 150 μm, depending on the crystallization conditions chosen.

709885/0776 - 4 -

During the thermal decomposition of aluminum chloride hydrate according to equation 2 (AlCl ₃ , 6H ₂ O) ^ Al ₂ O ₃ + 6HCl + 9H ₂ O

at a temperature, for example in the range from 600 to 90O ⁰ C, in a static or dynamic bed, the obtained alumina 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 susceptible to the abrasion phenomena that occur in the course of heat treatments and during mechanical, pneumatic or other transport.

The clay obtained in this way is therefore generally in the form of very fine particles which easily fly off 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 decomposition of aluminum chloride hexahydrate to provide obtained alumina.

This object is achieved according to the invention in that, starting from aluminum chloride hexahydrate, alumina granules with a high mechanical strength and controllable grain size distribution can be produced.

According to the invention, the new alumina agglomerate is characterized in that that for its production an "intermediate product" is preferably compacted in the dry state, which consists of the incomplete decomposition of aluminum chloride hexahydrate originates and contains 0.8 to 15 parts by weight of Cl; the compacted product is granulated and the granulate is finally thermally treated.

The "intermediate product" which is to be compacted is obtained from the incomplete thermal decomposition of aluminum chloride hexahydrate, which, for example, in the case of acid

709885/0776

- 5-

is obtained on the decomposition of aluminosilicate ores in such a way that the chlorine content is 0.8 to 15 %, preferably ^ to 10 % . The content of Al ₂ O and of constitutional water of the "intermediate product" depends of course on the chlorine content; the impregnation water is evaporated before the aluminum chloride hexahydrate decomposes.

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

As already indicated, the "intermediate" is usually used dry compacted or compacted. However, it has been shown that with little water, not more than 20% by weight of the product can be moistened prior to compaction without adversely affecting the properties of the final alumina agglomerate product will.

The "intermediate product" is agglomerated as shown schematically in the accompanying drawing.

The "intermediate P.I." stored in A is over the line 1 led into the mixer β and there with one over the line 6 supplied granulated product mixed undesirably small grain size. This mixture reaches the line 2 in the Press C, in which the compaction takes place. This press C is combined with, for example, a press roll of the usual type equipped 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. The compacted product is in the form of a continuous strip, which breaks up into coarse pieces when it exits the compression zone and which passes through line 3 in a shredding device D is performed, in which the shredding to the desired dimensions takes place with the help of devices of conventional design such as pin mill (spiked roller), bakery crusher, hammer mill, etc.

709886/0776

The granules come from D via line 4 to a selection or Classification area E and is divided there into at least three fractions a, b and c of different grain sizes.

Fraction a comprises the granules, the dimensions of which are in the range desired by the subsequent user. This fraction is passed through 7 into a furnace F of known design and there is heat-treated in the required manner so that the alumina has the desired properties, for example in the temperature range from 600 to 1500 ^° C.

The fraction fc consists of the grain which is too small and is over the line 6 returned to the mixer B in the process.

Fraction c consists of the grain that is too coarse and is over the line 5 led to the device D and here again subjected to a 'crushing process and then via the line 4 returned to the classification or selection zone E.

After the heat treatment in F, fraction a becomes in G with respect to deducted for later use.

According to a variant of the method, the device C consists for continuous compression or pressing from a pellet

Press with a pressing pressure in the range from 2000 to 10 000 kp / cm ♦

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 alumina agglomerates obtained without the addition or involvement of any binder particularly important or interesting physical properties, especially a regular grain size distribution, which is adjustable according to the wishes of the user.

709885/0775

AO

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

In addition, the BET specific surface area determined by nitrogen absorption in accordance with the French standard APNOR XII-621 is 2 to 120 m ² / g and the content of ^ L-Al ₂ O is 95 to 0% , also depending on the conditions of the heat treatment .

It has been shown, surprisingly, that the heat treatment produced according to the invention under identical conditions of the heat treatment Alumina agglomerates had a lower chlorine content and 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 clay agglomerates produced according to the invention are characterized by a very good abrasion resistance, which is based on a very good resistance to the disintegration or crumbling of the grain or granules in the event of repeated thermal shocks or mechanical shocks or bumps can be detected.

According to the invention, well-defined agglomerates can also be used or produce certain shapes, using the methods known and customary in this field, for example Compression molds, extrusions, etc. In this way, pellets of different sizes, solid cylinders or hollow cylinders, Platelets, grooving rolls, etc., whereby a heat cycle is selected and specified for the heat treatment after shaping, which depends on the intended use of the molded body.

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

709885/0776

example 1

In B, 15 kg of fraction b consisting of sifted fine grain <C1 mm, which came from a previous cycle, were included 15 kg of "intermediate" mixed by incomplete Decomposition of aluminum chloride hexahydrate had been obtained; the mixture still contained 4.55% by weight of Cl. It was * in the conically tapered space of a feed hopper with a rotating * screw in it. The basis of this pre-compressor led directly to or in the press; this consisted of two rollers with a diameter of 600 mm, those with perforated belts with the tapes 1.5 mm apart before the product was introduced.

A constant pressure of 6 t / cm belt width was maintained during the entire operation. The speed of rotation was 4 rpm.

The platelets collected at the exit of C were fed into a hammer mill in D, 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 in a device that made it possible to separate them into four fractions or proportions:

The fraction i »x 5 mm, which made up 2 % by weight of the total granulated mass and was fed back into D *;

the fraction desired by the user 2 to 5 mm, the 43 wt. Ji made of granulated mass;

the also desired fraction 1 to 2 mm, which made up 18 wt. -96 of the granulated mass and

the fraction below 1 mm, the 37 wt -.% of the granulated mass was accounted for and fed back into B.

♦ Archimedean

709885/0775

-r-

The two fractions 2 to 5 mm and 1 to 2 mm 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 on.

Factions

Firing temperature

Burn loss

Cl

Surface, sham abrasion test

BET bare produc- Mean mean .2 / _ Density te 0 before 0 after

kg / dm ^{3 fine test test}

m ² / g

% Ct 60 / um / um

Around

mm

2 to 5 unbaked granules

850 950 1050

4.55

10.9 0.3 72

12.35 0.26 39

13.4 0.07 20

0.78 0.02 3050 2900

0.74
0.74
0.73

0.16
0.16
0.04

2800 2850 3050

2650 2600 2850

1 to 2,850 mm 900

11.0 0.3 69 0.68 2.5 1080 1020 12.0 0.28 47 0.65 1.0 1150 1020

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 became one going in the three directions of space subject to complex movements that acted like jolts, rotations and rhythmic rocking movements.

The container was filled with 50 g of alumina agglomerate, namely the residue on the 160 / µm sieve. The sample became continuous Stirred for 2 hours and then passed through the same 160 μm sieve again. In this way, the percentage determined on the fine grain produced by the abrasion test.

Finally, before and after the abrasion test, the mean diameter was determined using methods familiar to the person skilled in the art certainly.

709885/0776

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With the help of this example, the essential properties of the alumina agglomerates are shown for certain characteristics during compression and for a chlorine content of the "intermediate product" of 4.55 % .
Example 2

In this example, the determining influence of the chlorine content of the "intermediate product" shown on the essential properties of the agglomerates produced according to the invention.

There were 15 kg of fraction b in B consisting of the sieved fines <: 1 mm as in example 1 with 15 kg "intermediate product" mixed; the mixture still contained 2.5% by weight of Cl.

This mixture was compressed as described in Example 1 with the modification that the rotational speed of the cylinder was slowed down from 4 rpm to 2 rpm.

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

The fraction;> - 5 mm, which made up 1 % of the mass of granules;

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 3.

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

After sieving and classifying, the following fractions were obtained here:

709885/0776 - 11 -

γ \ -

a fraction> 5 mm corresponding to 1 % of the granulate mass,

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

a fraction 1 to 2 mm, 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 produced from the mixture with 2.5 % chlorine, the granules produced from the mixture 0.9% chlorine proved to be relatively friable.

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

% Cl im Burned apparent generated
Fine Abrasion test average 0
after test P.I.Ge temperature Density. % <Λ60 / um middle 0
before test ,around mix ⁰ C kg / dm ^ 19.7 /Around 2600 - not ge
burns 0.96 10.5 3100 2750 850 O, O7 9.7 3000 2750 0.9 950 0.06 7.4 2900 2800 1050 0.88 10.8 3000 2500 not ge
burns 0.83 9.5 2650 2500 850 0.78 6.7 2650 2600 2.5 950 0.78 2.5 2800 2600 1050 0.79 2750

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

709885/0776

- 12 -

Examples 3 to 5 **

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

To prove that the presence of water does not affect the mechanical properties of the pellets produced, the respective intermediate product was 10 96% by weight or 20 96% by weight based on the mass of the intermediate product Water moistened.

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 had a diameter of about 20.3 mm and a thickness in the range of 7 to 11 mm, depending on the one introduced "Intermediate product" quantity.

After drying at 110 ⁰ C, the pellets were fired in a muffle furnace which was heated at a rate of temperature rise of 3.4 ° C / min gradually from 200 to 900 ° C.

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

Tabel;

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% Cl
in PI
mixture % H ₂ O
added Press pressure
kp / cm middle
apparent
density
g / cm ³ Ball drop
height at
fracture
cm 1.2 10 3100
A600
6200 1.03
1.23
1.31 - 20th
25th
25th A, A 10 3100
A600
6200
7700 1.12
1.24
1.34
1.38 75
100
100
100 12.7 20th 3100
A600
6200
7700 1.13
1.32
1 »36
1.42 75
150
100
75 10 3100
A600
6200
7700 1.12
1.16
1.22
1.20 20 '
10
10
10 20 · 3100
A600
6200
7700 1.30
1, 33
1.36
1.35 10
10
10
10

The breaking strength of the pellets was tested in the ball drop test; a steel ball with a diameter of 18.25 mm and weight was used for this 24.80 g in a guide tube made of glass with a diameter of 20 mm are each dropped onto the center of the pellet. There were Glass tubes of increasing length were used until a single drop of a ball caused the pellet to break.

709885/0776

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Example 6

In this example, the influence on the chlorine content and the BET surface area is investigated in comparison with one another of the size of the agglomerates according to the invention and one Alumina of the same origin but different dimensions, which was not subjected to the agglomeration cycle according to the invention, for precisely defined firing temperatures and firing times be evoked.

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 Intermediate product about 50 / um.

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

Tabel:

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709885/0776

treatment Burn time
min Carrier gas *
swiftly
^speed cm / s Properties of the behan
delten products % Cl Burned
temperature 90 surface
- BET ₂
cm / g 0.13 A. 120
60 32 52 0.06
0.036 1000 90 51
37 <0.01 1050 90 34 0.16 B. 120 15th 49 0.07 1000 60 51 0.05 90 32 <0.01 1050 90 31 0.21 C. 120 17th 30th 0.15 1000 60 30th 0.09 90 22nd 0.05 1050 19th

These results enable the measurement or determination of the simultaneous development of the percentage chlorine content and the BET specific surface area as a function of whether the treated intermediate products agglomerate according to the invention were or not.

709885/0776

Claims (8)

Claims High-strength alumina agglomerates with controllable particle size distribution, which contain 0.005 to 0.5% by weight of chlorine, have a BET specific surface area of 2 to 120 m / g and a content of 0 ^ -Al ₂ O ₅ of 95 to 0 % and are compacted an intermediate product obtained in the incomplete decomposition of aluminum chloride hexahydrate with 0.8 to 15 percent by weight, preferably 2 to 1.0 percent 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 shape such as spheres, hollow and solid cylinders or platelets, which have been obtained by means of compression molding or extrusion. 3 · Process for the production of the alumina agglomerates according to claim 1, characterized in that one intermediate product obtained from incomplete decomposition of aluminum chloride hexahydrate with a Cl content of 0.8 to 15, preferably 2 to 10 wt .- # compacted, the compacted product granulated and the Desired grain size distribution thermally treated * 4. The method according to claim 3 _t characterized in that Cl-containing intermediate product is compressed in a pellet press under a pressure of 2000 to 10 000 kp / cm. 709885/0775 ORlQJNAL IN8PECTSD 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 one of claims 3 to 5, characterized characterized in that it is compressed to a product with an apparent density of 1 to 1.5 g / cm ^. 7. The method according to any one of claims 3 to 6, characterized in that the Cl-containing intermediate product is mixed with a maximum of 20 wt .- # water, based on the dry weight of the intermediate product, moistened. 8. A method according to any one of claims 3 to 7, characterized in that treating the granulated product at a temperature in the range from 600 to 1500 ⁰ C. 709885/0775