GB1565315A - 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 between 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.

However, the various processes so far proposed concerned the agglomeration of a hydrated alumina produced essentially by action on bauxite by the Bayer process. Apart from this basic process there is an acid process which comprises reacting the initial ore with HNO3. This method is an important intermediate stage in obtaining a pure alumina, by converting the alumina in the ore to a hydrate aluminium nitrate of the formula Al(NO2)3.nH2O, where n is generally 9 but may be equal to 8 or 6.

When the nitrates are thermally decomposed by the following equations 2Al(NO3)3.9H2OAl2O3 + 3N205 + 181130 2AI(NO3)3.8H20~AI203 + 3N205 + 161120 2Al(No3)3.6H2o~Al2o3 + 3N205 + 121120 N205 may be decomposed into various other oxides of nitrogen, depending on the temperature.

It appeared possible to control the decomposition by varying the times and temperatures, so that an incompletely decomposed nitrated and hydrated intermediate product could be obtained.

When hydrated nitrates of aluminium are totally decomposed, the alumina obtained is generally in the form of very fine particles which are liable to fly away and which also have several of the above-mentioned disadvantages.

It was therefore desirable to envisage agglomerating the alumina obtained by thermal decomposition of hydrated aluminium nitrate.

The present invention is based on the interesting discovery that it is possible to produce alumina granules with good mechanical strength and predeterminable particle size from the said hydrated aluminium nitrate.

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, containing 0.5 to 15% by weight of nitrogen oxide expressed as N205 and resulting from incomplete decomposition of hydrated aluminium nitrate, 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 is prepared, on incomplete thermal decomposition of hydrated aluminium nitrate obtained e.g. by the action of acid on silicoaluminous ores, so that the nitrogen oxide content, expressed as N205, is from 0.5 to 15% but preferably from 2 to 8% by weight. Consequently the content of A1203 and that of water constitutionally present in the intermediate product can be deduced naturally from the content of nitrogen oxide, since the water of impregnation has been evaporated before the hydrated nitrate is decomposed.

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

However, it has been found that the addition of a certain amount of water to the product to be compacted, not exceeding 15 Ó by weight thereof, does 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 over the width of the cylinders. 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 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 a maximum temperature of 1500"C.

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.

Ill 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 of at least 1500 kgF/cm2 and preferably from 3000 kgF/cm2 to 5000 kgF/cm2.

The pelletised product is then fed into the granulater, 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 use of any binder, have particularly interesting physical properties, apart from that of keeping a regular particle size which can be adjusted according to the wishes of the user.

Generally speaking the nitrogen oxide content, expressed as N205, is from 0% to 0.5% depending on the conditions of heat treatment.

The BET specific surface area, measured by nitrogen absorption in accordance with AFNOR Standard XII--621, is from 2 to 150 m2/g according to the conditions of heat treatment.

Finally, the alumina agglomerates according to the invention offer good resistance to attrition, which takes the form of good resistance to crumbling of the grains when repeated thermal and mechanical shocks are applied.

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 cyclinders, 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 from the illustrative examples of how the process is carried out.

Example 1.

An intermediate product containing 2.3% by weight of nitrogen oxide, expressed as N205, is pelletised at various pressures.

Compacting is effected with a hydraulic press where the pressure is varied from 2000 to 5000 kgF/cm2.

The pellets have a diameter of approximately 24 mm and a thickness which varies from 5 to 7 mm, according to the quantity of intermediate product introduced.

The pellets thus obtained are then placed in a muffle furnace which has previously been brought to the selected calcination temperature, and are kept at that temperature for 2 hours.

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

Apparent average Pelletising Calcining BET specific density after Attrition test pressure temperature surface area heat treatment Height of drop kgF/cm2 C m2/g kg/dm3 of ball cm 2000 650 1.20 8-10 3000 650 1.24 10-20 4000 650 127 1.26 20 5000 650 1.33 10-20 2000 750 1.34 8-10 3000 750 1.36 10 4000 750 127 1.43 20 5000 750 1.44 10-20 2000 850 1.28 8-10 3000 850 1.39 10-20 4000 850 124 1.36 10-20 5000 850 1.41 10-20 2000 950 1.47 8-10 3000 950 1.54 8-10 4000 950 106 1.59 20 5000 950 1.55 8-20 The BET surface area is measured by nitrogen absorption in accordance with AFNOR Standard XII-621.

The pellet breaking test is carried out by dropping a steel ball 18.25 mm in diameter and weighing 24.60 g, which is guided in a glass tube 20 mm in diameter.

The ball drops 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 2.

To demonstrate that the pressure of water does not adversely affect the mechanical properties of the pellets made, the intermediate product, containing 2.6% of nitrogen oxide expressed as N2O5, is wetted with water, the amount of water being 5% by weight of its mass.

When the sample has been homogenised it is compacted with the same hydraulic press as in Example 1 with the pressure set to 4000 kgF/cm2.

The pellets have a diameter of approximately 24 mm and a thickness of 5 to 7 mm, according to the quantity of intermediate product introduced.

After drying at 1 l0 C. the pellets are placed in a muffle furnace which has previously been brought to the chosen calcining temperature, and are kept at that temperature for 2 hours.

The physical properties of the pellets after heat treatment are set out in the table which follows:

Pressure Apparent average Attrition test exerted Calcining density after heat Height of drop kgF/cm2 temperature 0C treatment kg/dm3 # of ball cm 4000 650 1.27 20-30 4000 750 1.36 10-20 4000 850 1.42 20-30 4000 950 1.59 10-20 Example 3.

An intermediate product containing 5.9% by weight of nitrogen oxide expressed as N2O5, is pelletised at various pressures.

Compacting is carried out under the same conditions as in Example 1.

The pellets obtained are placed in a muffle furnace which has previously been brought to the chosen calcining temperature, and are kept at that temperature for 2 hours.

The physical properties of the pellets after heat treatment are set out below:

Attrition test Pressure BET specific Apparent density Height of exerted Calcining surface area after heat treat- drop of ball kgF/cm2 temperature C m2/g ment kg/dm3 drop cm 2000 7500 1.33 10 - 20 3000 7500 1.39 30 4000 7500 107 1.35 20 5000 7500 1.43 8 - 10 2000 8500 1.40 8 - 10 3000 8500 1.46 8 - 10 4000 8500 100 1.51 8 - 10 5000 8500 1.50 10 --20 WHAT WE CLAIM IS: l. Alumina agglomerates with high mechanical strength and predeterminable particle size obtained by compacting an intermediate product containing 0.5 to 15% by weight of nitrogen oxide expressed as N206 and resulting from incomplete decomposition of hydrated aluminium nitrate, 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 hydrated aluminium nitrate contains 2 to 8% by weight of the NO5.

3. Agglomerates as claimed in Claim 1, in which the nitrogen oxide content, expressed as N2O5, after compacting and heat treatment is from 0% to 0.5% by weight.

4. Agglomerates as claimed in any preceding claim, in which the product to be compacted is moistened with a quantity of water not exceeding 15% by weight of the product.

5. Agglomerates as claimed in any preceding claim having a BET specific surface area from 2 to 150 m2/g.

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

7. A method of obtaining agglomerates as claimed in any one of Claims 1 to 5, in which the said intermediate product is pelletised at a pressure of at least 1500 kgF/cm2.

8. A method of obtaining agglomerates as claimed in any one of Claims 1 to 5, in which the said intermediate product is continuously compacted between two cylinders which exert between them a compressive force of at least 3 tonnes per linear centimetre over the width of said cylinders.

9. A method as claimed in Claim 7 or 8, in which the compacted intermediate product is then granulated by fragmentation and particles of the desired size are then selected.

10. A method as claimed in any one of Claims 7 to 9, in which the maximum temperature for the calcination is 1500 C.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   Example 3.

   An intermediate product containing 5.9% by weight of nitrogen oxide expressed as N2O5, is pelletised at various pressures.

   Compacting is carried out under the same conditions as in Example 1.

   The pellets obtained are placed in a muffle furnace which has previously been brought to the chosen calcining temperature, and are kept at that temperature for 2 hours.

   The physical properties of the pellets after heat treatment are set out below:

   Attrition test Pressure BET specific Apparent density Height of exerted Calcining surface area after heat treat- drop of ball kgF/cm2 temperature C m2/g ment kg/dm3 drop cm 2000 7500 1.33 10 - 20 3000 7500 1.39 30 4000 7500 107 1.35 20 5000 7500 1.43 8 - 10 2000 8500 1.40 8 - 10 3000 8500 1.46 8 - 10 4000 8500 100 1.51 8 - 10 5000 8500 1.50 10 --20 WHAT WE CLAIM IS: l. Alumina agglomerates with high mechanical strength and predeterminable particle size obtained by compacting an intermediate product containing 0.5 to 15% by weight of nitrogen oxide expressed as N206 and resulting from incomplete decomposition of hydrated aluminium nitrate, 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. 2. Agglomerates as claimed in Claim 1, in which the hydrated aluminium nitrate contains 2 to 8% by weight of the NO5.
3. 3. Agglomerates as claimed in Claim 1, in which the nitrogen oxide content, expressed as N2O5, after compacting and heat treatment is from 0% to 0.5% by weight.
4. 4. Agglomerates as claimed in any preceding claim, in which the product to be compacted is moistened with a quantity of water not exceeding 15% by weight of the product.
5. 5. Agglomerates as claimed in any preceding claim having a BET specific surface area from 2 to 150 m2/g.
6. 6. Agglomerates as claimed in any preceding claim of defined shapes obtained by moulding under pressure or by extrusion.
7. 7. A method of obtaining agglomerates as claimed in any one of Claims 1 to 5, in which the said intermediate product is pelletised at a pressure of at least 1500 kgF/cm2.
8. 8. A method of obtaining agglomerates as claimed in any one of Claims 1 to 5, in which the said intermediate product is continuously compacted between two cylinders which exert between them a compressive force of at least 3 tonnes per linear centimetre over the width of said cylinders.
9. 9. A method as claimed in Claim 7 or 8, in which the compacted intermediate product is then granulated by fragmentation and particles of the desired size are then selected.
10. 10. A method as claimed in any one of Claims 7 to 9, in which the maximum temperature for the calcination is 1500 C.
11. 11. A method as claimed in Claim 7 or 8, substantially as hereinbefore

    described in any one of the Examples.
12. 12. Agglomerates as claimed in Claim 1, when prepared by a method as claimed in any one of Claims 7 to 11.