GB1578806A

GB1578806A - Alumina and its production

Info

Publication number: GB1578806A
Application number: GB30922/77A
Authority: GB
Original assignee: Aluminium Pechiney SA
Current assignee: Rio Tinto France SAS
Priority date: 1976-07-23
Filing date: 1977-07-22
Publication date: 1980-11-12
Also published as: NL7708197A; NL182876B; NO143495C; SE7708434L; DE2733055C3; NO772588L; PL199725A1; FR2359095B1; JPS5855085B2; NO143495B; NL182876C; DE2733055A1; IL52526A; FR2359095A1; PT66834B; IL52526A0; DE2733055B2; SE427454B; TR19325A; PL111185B1

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 cvlinders between which the desired pressure was set up. The continuous strip thus produced was fragmented according to the dimen sions desired, and the fragments were subiected to a conventional activating treatment.

The various processes hitherto proposed concerned the agglomeration of a hydrated alumina obtained essentially from action on bauxite by the Bayer process. Apart from this basic process there is an acid process, which comprises reacting the initial ore with H2SO,. This forms an important intermediate stage in the preparation of a pure alumina, by converting the alumina in the ore to a hydrated aluminium sulphate of the general formula Al?O . xS0 . yH.O where x is from 0.5 to 5 and y from 0 to 18, which values correspond to known acid, basic or neutral hydrated aluminium sulphates.

On thermal decomposition of the said hydrated sulphate by the equation Awl203 . xSO, . YH2O AlO. A1203 t xSO3 + 3'H2O, it appeared possible to control the decomposition by varying the times and temperatures so as to obtain an incompletely decomposed sulphated and hydrated intermediate product.

then hydrated aluminium sulphates are totally decomposed, the alumina obtained is generally in the form of very fine particles which are liable to fly about and which also suffer from several of the above-mentioned disadvantages.

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

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

According to the invention, alumina agglomerates with high mechanical strength and predeterminable particle size are obtained by compacting an intermediate product containing 1 to 15% by weight of S expressed as SO3, and resulting from incomplete decomposition of at least one hydrated aluminium sulphate of the formula Awl203 . xSO3 . yH2O, in which x and y are as defined above, 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 decompoosition of hydrated aluminium sulphate prepared e.g. by the action of acid on silica-aluminous ores, in such a way that the content of S expressed as SO, is from 1 to 15%, but preferably from 3 to 12%.

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% 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 0.5 tonne 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 the temperature, preferably not exceeding 15000 C, required to obtain alumina with the desired properties.

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 of at least 200 kg/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 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 sulphur content expressed as S03 is less than 1%.

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 aluminum 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 cylinders, small plates, grooves 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.

Intermediate products containing 3.8, 5.4 and 11.9% by weight of sulphur expressed as SO,3, and prepared by incomplete decomposition of Al2(SO4), . 18H2O, are pelletised under various pressures.

Compacting is effected with a hydraulic press where the pressure is varied from 400 to 3000 kgF/cm2.

The pellets have a diameter of approximately 24 mm and a thickness which varies from 2 to 6 mm, according to the quantity of intermediate product introduced.

The pellets thus obtained are calcined at 10500C in a muffle furnace which is heated gradually with the temperature rising at SC degrees per minute.

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

Apparent Attrition test Intermediate product BET surface average Height of Content of S expressed Pelletising area in m2/g density after drop of ball as SO, pressure after heat heat in cm % by weight kgF /cm2 treatment treatment causing break 11.9 3000 107 0.83 8 to 10 3.8 3000 104 1.17 8 to 10 5.4 800 95 0.88 8 to 10 5.4 ~ 5 4 400 100 0.7 5 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 24.80 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 fall of the ball causes the pellet to break.

Example 2.

To demonstrate that the presence of water does not adversely affect the mechanical properties of the pellets made, the intermediate product is wetted with water weighing 5%, 7% and 15% of its mass.

When the sample has been homogenised it is compacted by means of the same hydraulic press as in Example 1.

The pellets have a diameter of approximately 24 mm and a thickness of 3 to 5 mm, depending on the quantity of intermediate product introduced.

After being dried at 1 100C the pellets are calcined at various temperatures in a muffle furnace, which is heated gradually with the temperature rising 5 C per minute.

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

Apparent Attrition Intermediate BET average test product surface density Height of Content of S Content of Pelletising area in after drop of expressed H2O expressed pressure m2/g after heat ball in as SO, as in heat treatment cm causing % by weight % by weight kgF/cm2 treatment break 11.9 5 5000 105 1.45 8 to 10 3.8 7 1000 100 1.00 5 3.8 7 2000 98 1.32 8 to 10 3.8 7 3000 102 1.39 8 to 10 3.8 15 2000 95 1.33 5 Example 3.

Pellets previously obtained by pelletising an intermediate product, containing 5.4% of S expressed as SO at a pressure of 3000 kgF/cm2, are calcined at 1300"C in a muffle furnace, which is heated gradually with the temnerature rising 50C per minute, and are kept at that temperature for 1 hour. The BET surface area is then 3 m2/g. The height from whirh the ball has to drop to break the pellets is approximately 5 cm.

WHAT WE CLAIM IS: 1. Alumina agglomerates with good mechanical strength and a predeterminable particle size obtained by compacting an intermediate product containing 1 to 15% by weight of S expressed as SO,, and resulting from incomplete decomposition of an acid, basic or neutral hydrated aluminium sulphate of the formula Al2O . xSO3 . yH2O, in which x is in the range from 0.5 to 5 and y is in the range from 0 to 18, granu lating the compacted product, selecting particles of the desired size from the granulated product and calcining these particles of the granulated product to ensure that alumina is obtained.

2. Agglomerates as claimed in claim 1, in which the dehydrated aluminium sulphate contains 3 to 10% by weight of S expressed as SO,.

3. Agglomerate as claimed in claim 1 or 2, in which the content of S expressed as SO after compacting and heating treatment is less than 1%.

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 in the range from 2 to 150 m2/g.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Example 2. To demonstrate that the presence of water does not adversely affect the mechanical properties of the pellets made, the intermediate product is wetted with water weighing 5%, 7% and 15% of its mass. When the sample has been homogenised it is compacted by means of the same hydraulic press as in Example 1. The pellets have a diameter of approximately 24 mm and a thickness of 3 to 5 mm, depending on the quantity of intermediate product introduced. After being dried at 1 100C the pellets are calcined at various temperatures in a muffle furnace, which is heated gradually with the temperature rising 5 C per minute. The physical properties of the pellets after heat treatment are summarised in the table which follows: Apparent Attrition Intermediate BET average test product surface density Height of Content of S Content of Pelletising area in after drop of expressed H2O expressed pressure m2/g after heat ball in as SO, as in heat treatment cm causing % by weight % by weight kgF/cm2 treatment break 11.9 5 5000 105 1.45 8 to 10 3.8 7 1000 100 1.00 5 3.8 7 2000 98 1.32 8 to 10 3.8 7 3000 102 1.39 8 to 10 3.8 15 2000 95 1.33 5 Example 3. Pellets previously obtained by pelletising an intermediate product, containing 5.4% of S expressed as SO at a pressure of 3000 kgF/cm2, are calcined at 1300"C in a muffle furnace, which is heated gradually with the temnerature rising 50C per minute, and are kept at that temperature for 1 hour. The BET surface area is then 3 m2/g. The height from whirh the ball has to drop to break the pellets is approximately 5 cm. WHAT WE CLAIM IS:

1. Alumina agglomerates with good mechanical strength and a predeterminable particle size obtained by compacting an intermediate product containing 1 to 15% by weight of S expressed as SO,, and resulting from incomplete decomposition of an acid, basic or neutral hydrated aluminium sulphate of the formula Al2O . xSO3 . yH2O, in which x is in the range from 0.5 to 5 and y is in the range from 0 to 18, granu lating the compacted product, selecting particles of the desired size from the granulated product and calcining these particles of the granulated product to ensure that alumina is obtained.

6. Agglomerates as claimed in any preceding claim of defined shapes obtained

by moulding under pressure or 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 200 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 compacted continuously between two cylinders which exert between them a compressive force of at least 0.5 tonne per linear centimetre over the width of the 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 selected.

10. A method as claimed in any one of claims 7 to 9, in which the calcination is carried out at a maximum of 15000C.

11. A method of obtaining agglomerates as claimed in claim 1 substantially as hereinbefore described in any one of the Examples.

12. Agglomerates as claimed in claim 1 when prepared by a method as claimed in any one of claims 7 to 11.