GB1565408A - Alumina and its production - Google Patents

Description

(54) ALUMINA AND ITS PRODUCTION (71) We, ALUMINIUM PECHINEY, a French body corporate, of 28, rue de Bonnel, 69003 Lyon, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The invention concerns alumina agglomerates with high mechanical strength and with an adjustable particle size adaptable to the technical requirements of the user; it also concerns methods of obtaining such agglomerates.

The industry that specialises in obtaining alumina and converting it into aluminium through igneous electrolysis, e.g. electrolysis of a molten material, has long been encountering serious difficulties and disadvantages which they have tried to overcome.

A first disadvantage was loss of alumina through flying dust; this was experienced when handling the alumina and when using it in tanks for igneous electrolysis. It was consequently found necessary to design expensive recovery and dusting installations.

Another disadvantage encountered has to do with the recovery of some of the elements included in the gaseous effluent emerging from tanks for igneous electrolysis. A technique commonly used nowadays for this purpose comprises creating intimate contact beetween the gaseous effluent and the alumina used for feeding the tanks. To obtain satisfactory absorption of these elements, experts have confirmed that the alumina thus put into contact must have a BET specific surface area adapted to this practice.

Finally, a serious disadvantage has to do with the variations found in the particle sizes of the alumina. Experts would like to have a reproducible particle size, so that the operation of the tanks for igneous electrolysis would not be troubled by such variations.

Because of these many difficulties and drawbacks, experts have been wondering about the desirability of putting alumina into agglomerate form, particularly appropriate for igneous electrolysis, so as to provide a product where the desired properties would be reproducible, i.e. permanent in time.

Many methods of agglomerating alumina have been proposed and widely described in the specialised literature with a view to finding a way of overcoming these disadvantages.

A first type of process proposed comprised mechanically agglomerating a paste obtained by mixing a Bayer' alumina and an appropriate binder, which could be a solution of an acid or of an aluminium salt such as aluminium nitrate or aluminium stearate. After being agglomerated by extrusion, compacting or any other mechanical means, the granules obtained were calcined. Such processes were expensive and gave granular products polluted not only with small quantities of Na2O from the Bayer process itself but also with the binder or what was left of it after the heat treatment.

Another process, which constituted an important improvement, was subsequently proposed. Described in French Patent No. 2,267,982, it comprised producing an agglomerated active alumina by using as the raw material the aluminium hydrate obtained by the Bayer process.

The raw material, which could only contain a small quantity of impurities and more particularly sodium impurities, was first subjected to drying to eliminate the water of impregnation. It was then compacted, without the addition of any binder, by passing it continuously between two cylinders between which the desired pressure was set up. The continuous strip thus produced was fragmented according to the dimensions desired, and the fragments were subjected to a conventional activating heat treatment.

In our French Patent No. 1,419,879 is described a method of converting and purifying aluminium chloride hexahydrate. This method forms an important intermediate state in obtaining pure alumina from silica-aluminous ores which cannot be treated by the alkaline Bayer process and for which acid attacking means are used to convert the alumina in the ore into a hydrated chloride.

However, hexahydrated aluminium chloride obtained by precipitation is known to be in the form of rods which commonly vary in length between 50 u and 1500 ,u, depending on the crystallising conditions chosen.

When the hydrated aluminium chloride is thermally decomposed in accordance with the equation: 2A1C13 . 6H2O A12O3 + 6HCl + 9H2O at a temperature e.g. from 600 to 900"C in a state of dynamic bed, the alumina collected also has the appearance of rods, their volume being equal to about 35 Ó of that of the initial rods of hexahydrated aluminium chloride. The alumina rods are fragile and can ill tolerate the attrition which takes place both during heat treatment and during mechanical, pneumatic or other types of transferring operations. For these reasons the alumina thus obtained is generally in the form of very fine particles which are liable to raise a dust and which simultaneously have several of the disadvantages previously described.

It was thus desirable to envisage agglomerating the alumina obtained by thermal decomposition of hexahydrated aluminium chloride.

The present invention is based on the interesting discovery that it is possible to produce alumina granules of high mechanical strength and predeterminable particle size from hexahydrated aluminium chloride.

According to the invention, alumina agglomerates with high mechanical strength and predeterminable particle size are obtained by compacting an intermediate product, preferably in the dry state, that results from incomplete decomposition of aluminium chloride hexahydrate and contains 0.8 to 15% by weight of chlorine, and then granulating the compacted product, selecting particles of the desired size from the granulated product and calcining those particles of the granulated product to ensure that alumina is obtained.

The intermediate product to be compacted is obtained on incomplete thermal decomposition of aluminium chloride hexahydrate obtained e.g. by the action of acid on silico-aluminous ores, such that the Cl content is from 0.8 to 15 wt.%, preferably from 2 to 10%. From then onwards the content of Awl203 and the content of water constititionally present in the intermediate product can be deduced naturally from the Cl content, the water of impregnation having been evaporated before decomposition of the aluminium chloride hexahydrate.

The incomplete decomposition of the aluminium chloride hexahydrate is carried out by known methods.

As already mentioned, the intermediate product' is normally compacted dry.

However, it has been found that if a certain amount of water is added to the product to be compacted, not exceeding 20% by weight of the said product, this will not substantially affect the final properties of the alumina agglomerates.

The intermediate product thus defined is then subjected to the agglomerating process, a non-restrictive industrial example of which is given in the single Figure of the accompanying drawings.

In this process the intermediate product (P.I.) stored at A is fed through line 1 into a mixer B, which also receives a portion through 6, consisting of granulated products with smaller than the desired dimensions. It is then passed through line 2 into a unit C where continuous compacting takes place. The unit C comprises a pressing means which may e.g. be a cylinder compacter of the conventional type with an associated precompacting means. The compacting pressure is at least 3 tonnes per linear centimetre across the width of the cylinders, so that the apparent density of the product is preferably from 1 to 1.5 g/cm3. From then onwards the compacted product is in the form of a continuous strip which is broken up roughly on leaving the compacting stage and then taken through line 3 to a granulator D where it is fragmented to the desired dimensions. Fragmentation is carried out by a known type of apparatus, such as spiked rollers, jaw-type crushers or hammer mills.

The granules discharged from fragmenting station D are directed through line 4 to a selecting zone E, where they are divided into at least three grades I, II and III of different dimensions.

I grade covers granules with dimensions which come within the range of measurements desired by the subsequent user. This grade is thereafter passed through line 7 into a known type of furnace F where heat treatment is applied at the appropriate temperature to give alumina with the desired properties, e.g. from 600 to 15000C.

II grade consists of granules having dimensions which are too small. This is conveyed through line 6 into the mixer B for recycling into the process.

III grade consists of particles of excessively large dimensions. This is conveyed through line 5 into the granulator D, where it is refragmented then reintroduced through 4 into the selection zone E.

After heat treatment (calcination) at F, I grade is collected at G ready for use.

In an alternative form of the process the continuous compacting unit C may comprise a pelletising press with a compacting pressure chosen between 2000 kgF/cm2 and 10000 kgF/cm2.

The pelletised product is then fed into the granulator, after which it follows the cycle of treatment previously described.

As a result of the heat treatment, the alumina agglomerates, which are obtained without the aid of any binder, have particularly interesting physical properties, apart from that of retaining a regular, adjustable particle size according to the wishes of the user.

Generally speaking the chlorine content is very low and varies from 0.005 to 0.5% according to the conditions of heat treatment.

The BET specific surface area, measured by nitrogen absorption in accordance with AFNOR standard XII--621, is from 2 m2/g to 120 m2/g, and the alpha-alumina content is up to 95% according to the conditions of heat treatment.

It has been surprising to find that, given identical heat treatment conditions, the alumina agglomerates have a lower Cl content and a larger BET surface area than alumina obtained from hexahydrated chloride which has not followed the agglomerating cycle according to the invention.

Finally, the alumina agglomerates according to the invention offer very good resistance to attrition, which is manifested in their good resistance to particle crumbling when subjected to repeated thermal and mechanical shocks.

In accordance with the invention agglomerates can also be made in well defined forms by known techniques, e.g. moulding under pressure and extrusion. It thus becomes possible to produce e.g. balls of varying dimensions, solid or hollow cylinders, small plates, grooved pulleys and reversing dual wheels. For these products the heat treatment subsequent to shaping follows a selected heating cycle determined by the uses for which the shaped articles are intended.

Other features and advantages of the invention will be understood better than the illustrative examples of how the process is carried out.

Example 1.

15 kg of grade II, comprising fines screened to 1 millimetre and obtained from previous operations carried out under the same industrial conditions, is mixed at B with 15 kg of 'intermediate product' resulting from incomplete decomposition of hexahydrated aluminium chloride, the mix still containing 4.55% by weight of chlorine. The mix is precompacted in a conical vessel with a spiral conveyor turning within it, the spiral conveyor fitting the conical shape of the hopper.

The base of the precompacter discharges directly into the compacter, which comprises two cylinders 600 mm in diameter fitted with pitted hoops, the hoops being 1.5 mm away from one another before the product is introduced.

The squeezing pressure, which is kept constant throughout the operation, is 6 tonnes per centimetre across the width of the hoop.

The speed of rotation is 4 revolutions per minute.

On leaving C the small plates collected are fed at D into a hammer mill, fitted with a screen with a mesh cavity of 6.5 millimetres. The granules thus obtained are screened at E in an apparatus which separates them into four grades: a grade larger than 5 millimetres, which is directed to granulator D and which constitutes 2% by weight of the mass of granules a 2-5 millimetre grade, desired by the user, which constitutes 43% by weight of the mass of granules a 1--2 millimetre grade, also desired, constituting 18% by weight of the mass of granules a grade smaller than 1 millimetre, constituting 37% by weight of the mass of granules, which is recycled to B.

The 2-5 millimetre and 1-2 millimetre grades are calcined separately at G at different temperatures and, as a function of these temperatures, show the specific properties set out in the table below:

attrition test Percent fines average average calining weight apparent formed diameter diameter temperature loss on density % before in afterwards grades heating % Cl BET kg/cm3 160 in 2-5 initially - 4.55 - 0.78 0.02 3050 2900 millimetres untreated, not calcined 850 C 10.9 0.3 72 0.74 0.16 2800 2650 950 C 12.35 0.26 39 0.74 0.16 2850 2600 1050 C 13.4 0.07 20 0.73 0.04 3050 2850 1-2 850 C 11.0 0.3 69 0.68 2.5 1080 1020 millimetres 900 C 12.0 0.28 47 0.65 1.0 1150 1020 The attrition test is carried out with a mixer comprising a cylindrical glass receptacle with a capacity of 250 cc, an internal diameter of 55 mm and an internal length of 105 mm. A complex three-dimensional movement is imparted to the receptacle, producing effects of shaking, rotation and rhythmical rocking.

50 g of alumina agglomerates left behind on previous sieving at 160 is placed in the cylindrical receptacle. Then the sample is subjected to continuous agitation for two hours, after which time it is resieved at 160 . In this way the percentage of fines' created during the attrition test is determined.

Finally, the average diameter is measured before and after attrition, by known methods.

This first example thus demonstrates the essential properties of the alumina agglomerates for specific compacting properties and for a chlorine content of 4.55% in the intermediate product.

Example 2.

The purpose of this Example is to show that the chlorine content of the intermediate product is a factor which determines the fundamental properties of the agglomerates according to the invention.

15 kg of grade II comprising 'fines' screened at 1 mm is mixed with 15 kg of intermediate product at B, as in Example 1; the mix still contains 2.5% by weight of chlorine.

The mix is compacted under the conditions described in Example 1, except that the speed of rotation of the cylinders is reduced from 4 revolutions per minute to 2 revolutions per minute.

After screening at E the following grades are collected: a grade larger than 5 millimetres, representing 1% of the mass of granules the required 2-5 millimetre grade, representing 28% of the mass of granules the 1--2 millimetre grade, also required, representing 17% of the mass of granules a grade smaller than I millimetre, representing 54% of the mass of granules, which is recycled to B.

As a comparison, a mixture of 'fines' and intermediate product in which the chlorine content before compacting is 0.9% by weight is also compacted under identical operating conditions.

After screening the following grades are collected: a grade larger than 5 millimetres: 1% of the mass of granules a desired 2-5 millimetre grade: 24% of the mass of granules a 1--2 millimetre grade, also desired: 11% of the mass of granules a grade smaller than 1 millimetre: 64% of the mass of granules.

Compared with granules containing 2.5% of chlorine, granules containing 0.9% are found to be relatively friable.

The 2-5 millimetre grades are calcined at various temperatures, revealing the differences in mechanical strength on application of the attrition test as a function of these temperatures. These differences are set out in the table as follows:

Attrition test % Cl in average average inter- calcining apparent fines diameter diameter mediate temperature density formed before afterwards product in OC kg/cm3 % -160, in tL in IL not 0.96 19.7 3100 2600 calcined 0.9% 8500 0.87 10.5 3000 750 9500 0.86 9.7 2900 2750 10500 0.88 7.4 3000 2800 not 0.83 10.8 2650 2500 calcined 2.5% 850 0.78 9.5 2650 2500 9500C 0.78 6.7 2800 2600 10500 0.79 2.5 2750 600 Granules obtained from intermediate products with a lower chlorine content are less strong.

Examples 3 to 5.

3 samples of intermediate product, respectively containing 1.2%, 4.4% and 12.7% by weight of chlorine, are pelletised.

To prove that the presence of water does not adversely affect the mechanical properties of the pellets formed, the intermediate products are wetted with a quantity of water representing 10% or 20% by weight of the mass of intermediate products.

When the samples have been homogenised compacting is carried out using a hydraulic press where the pressure is varied from 3100 to 7700 kgF/cm2.

The pellets have a diameter of approximately 20.3 millimetres and a thickness which varies from 7 to 11 millimetres depending on the quantity of intermediate product introduced.

After drying at 1 100C, the pellets are calcined in a muffle furnace, which is heated gradually from 200 to 9000C with the temperature rising 3.40C per minute.

The physical properties of the pellets after heat treatment can be seen from the summarising table which follows:

% Cl in pelletising apparent average height of ball intermediate % H2O pressure density after drop resulting product wetting kgF/cm2 heat treatment in breaking cm 3100 1.03 20 1.2 10 4600 1.23 25 6200 1.31 25 3100 1.12 75 4600 1.24 100 10 6200 1.34 100 7700 1.38 100 4.4 3100 1.13 75 4600 1.32 150 20 6200 1.36 100 7700 1.42 75 3100 1.12 20 4600 1.16 10 10 6200 1.22 10 7700 1.20 10 12.7 3100 1.30 10 4600 1.33 10 20 6200 1.36 10 7700 1.35 10 The pellet breaking test is carried out by dropping a steel ball, 18.25 mm in diameter and 24.80 g in weight, which is guided in a glass tube 20 mm in diameter.

The ball falls on the centre of the pellet. Glass tubes of increasing height are used until a single drop of the ball causes the pellet to break.

Example 6.

The purpose is to measure the comparative effect of the size of the agglomerates according to the invention, and of an alumina of the same origin of various sizes which have not undergone the agglomeration cycle according to the invention, on chlorine content and BET surface area for specific temperatures and periods of calcination.

To this end three samples are used, made up as follows: a sample 'A' comprising alumina granules with dimensions between 0.5 and l millimetre a sample 'B' comprising alumina granules with dimensions between 0.25 and 0.5 millimetre a sample 'C' resulting from incomplete decomposition of hexahydrated aluminium chloride with a fine particle size, the average diameter of the intermediate product being approximately 50 ,u.

These various samples are calcined in a fluidised bed furnace, the fluidising gas being dry air.

The characteristics of the products thus treated can be seen from the table below:

Characteristics of products Characteristics of treatment after treatment calcining calcining speed of BET temperature time fluidising surface area C minutes gas cm/sec cm2/g % Cl 90 52 0.13 1000 120 32 51 0.06 A 60 37 0.036 1050 90 34 < 0.01 90 49 0.16 1000 120 15 51 0.07 B 15 60 32 0.05 1050 90 31 {0.01 90 30 0.21 1000 120 30 0.15 C 17 60 22 0.09 1050 90 19 0.05 These results make it possible to measure the simultaneous change in the percentage of chlorine and in the BET specific surface areas, depending on whether or not the intermediate products treated have been agglomerated according to the invention.

Claims (13)

WHAT WE CLAIM IS:

1. Alumina agglomerates with high mechanical strength and a predeterminable particle size obtained by compacting an intermediate product containing 0.8% to 15% by weight of chlorine and resulting from incomplete decomposition of aluminium chloride hexahydrate, granulating the compacted product, selecting particles of the desired size from the granulated product and calcining those particles of the granulated product to ensure that alumina is obtained.

2. Agglomerates as claimed in Claim 1, in which the aluminium chloride hexahydrate contains 2 to 10% by weight of chlorine.

3. Agglomerates as claimed in Claim 1 or 2, in which the apparent density after compacting but before heat treatment is from l to 1.5 g/cm3.

4. Agglomerates as claimed in any preceding claim, in which the Cl content after compacting and heat treatment is from 0.005% to 0.5% by weight.

5. Agglomerates as claimed in any preceding claim, in which the product to be compacted is moistened with a quantity of water not exceeding 20% by weight of the said product.

6. Agglomerates as claimed in any preceding claim, in which the BET specific surface area is from 2 to 120 m2/g and the content of alumina of the dimensions desired is up to 95%.

7. Agglomerates as claimed in any preceding claim of defined shapes obtained by moulding under pressure or extrusion.

8. A method of obtaining agglomerates as claimed in any one of Claims 1 to 6, in which the said intermediate product is pelletised at a pressure of 2000 to 10000 kgF/cm2.

9. A method of obtaining agglomerates as claimed in any one of Claims 1 to 6, in which the intermediate product is continuously compacted between two cylinders which exert between them a compressive force of at least 3 tonnes per linear centimetre of the width of said cylinders.

10. A method as claimed in Claim 8 or 9, in which the compacted intermediate product is granulated by fragmentation and particles of the desired size are then selected.

11. A method as claimed in Claim 8, 9 or 10, in which the calcination is carried out at a temperature from 600 to 15000 C.

12. A method as claimed in Claim 8 substantially as hereinbefore described in any one of the Examples.

13. Agglomerates as claimed in Claim 1, when prepared by a method as claimed in any one of Claims 8 to 12.