GB1577801A

GB1577801A - Alumina and its production

Info

Publication number: GB1577801A
Application number: GB30923/77A
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: 1980-10-29
Also published as: DE2733004A1; NL7708199A; NO143496C; JPS5314699A; NO143496B; SE427176B; NO772589L; PT66835A; IL52527A; DE2733004B2; NL182556C; DE2733004C3; JPS5537488B2; PL111053B1; PL199726A1; SE7708435L; PT66835B; FR2359096A1; TR19324A; IL52527A0

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 ststement:-- 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 apporpriate for igneous electrolysis, so as to provide a product where the desired properties would be repro ducible, i.e. permanent in time.

Many methods of agglomerating alumina have been proposed and widely described in the specialised literature with a view to find ing 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 appro priate binder, which could be a solution of an acid or of an aluminium salt such as alumi nium stearate. After being agglomerated by extrusion, compacting or any other mechanical means, the granules obtained were calcined.

Such processes were expensive and gave granu lar 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 ari 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.

The various processes hitherto proposed concerned the agglomeration of a hydrated alumina resulting essentially from action on bauxite by the Bayer process. Apart from this basic process there is an acid process which consists of reacting the original previously calcined ore with sulphurous acid at a relatively high pressure of 5 to 10 bars and a temperature below 100"C. This process is an important intermediate stage in the preparation of an alumina by conversion of the alumina in the ore to a hydrated aluminium sulphite of the general formula A120, . xSO, .yH2O where x is from 0.2 to 3 and y has a maximum value of 5, which values correspond to known hydrated basic sulphites and a neutral sulphite of aluminium.

On thermal decomposition of the said hydrated sulphite in accordance with the equation of Al2O, . xSO2 . yH2OAl2Oa + XSO2 + yH2O, it appeared possible to control the decomposition by varying the times and temperatures, so as to obtain an incompletely decomposed hydrated intermediate product still containing a small quantity of sulphur oxides.

When there is total decomposition of the hydrated aluminium sulphites, the alumina obtained is generally in the form of 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 aluminium sulphites.

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 an aluminium sulphite.

According to the invention alumina agglomerates with high mechanical strength and predeterminable particle size are obtained by compacting an intermediate product containing 3 to 15% by weight of sulphur expressed as SO2, and resulting from incomplete decomposition of an aluminium sulphite of the formula Alto,. xSO2 . yH2O, where x and y are as defined above, granulating the com- pacted product, selecting particles of the de sired 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 hydrated aluminium sulphite, prepared e.g. by the action of acid on previously calcined silico-aluminous ores, in such a way that the sulphur content expressed as SO2 is from 3 to 15% but preferably from 5 to 10% by weight.

As already mentioned, the intermediate product is normally compacted dry. However, it has been found that the addition of a cer tain amount of water to the product to be compacted, preferably not exceeding 10% by weight of the product, 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 accom panying drawings.

In this process the intermeriate 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 1 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 whose heat treatment is applied at a maximum temperature of 15000 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.

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 600 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 SO, is very low: less than 0.6%.

The BET specific surface area, measured by nitrogen absorption in accordance with AFNOR Standard XII-621, is from 2 to 130 m2/g, according to the conditions of heat treatment.

Finally, the alumina agglomerates accord ing 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, grooved pulleys and reversing dua! wheels.

For these products the heat treatment subsequent to shaping follows a selecred 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 example of how the process is carried out.

Example.

An intermediate product containing 6.4% by weight of sulphur expressed as SO, and resulting from incomplete decomposition of Al2Oa .2SO2 . 5H20 is pelletised at various pressures.

Compacting is effected by means of a hydraulic press at a pressure of 3000 kgF/ cm2.

The pellets have a diameter of approximately 24 mm and a thickness which varies from 4 to 7 mm, according to the quantity of intermediate product introduced.

The pellets thus obtained are then calcined at 10500C 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 can be seen from the summarising table below.

Intermediate product Pelletising Apparent average Attrition test Sulphur content pressure density after Height of ball expressed as SO2 kg/F/cm2 heat treatment drop 6.4 3000 0.96 8 to 10 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.

WHAT WE CLAIM IS:- 1. Alumina agglomerates with good mechanical strength and predeterminable particle size obtained by compacting an intermediate product containing 3 to 15% by weight of sulphur expressed as SO,, and resulting from incomplete decomposition of a basic or neutral hydrated aluminium sulphite of the formula Al2O,. xSO, . yH2O, in which x has a value in the range from 0.2 to 3 and y is not greater than 5, 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 sulphur content of the hydrated aluminium sulphite is 5 to 10% by weight.

3. Agglomerates as claimed in claim 1 or 2, in which the sulphur content, expressed as SO,, after compacting and heat treatment is less than 0.6% by weight.

4. Agglomerates as claimed in any preceding claim, in which the product to be compacted is moistened with water.

5. Agglomerates as claimed in claim 4, in which the quantity of water does not exceed 10% by weight of the product.

6. Agglomerates as claimed in any preceding claim having a BET spceific surface area in the range 2 to 130 m2/g.

7. Agglomerates as claimed in any preceding claim, of defined forms 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 at least 600 kgF/cm2.

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

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

11. A method as claimed in any one of Claims 8 to 10, in which the heat treatment is carried out at a maximum temperature of 15000C.

12. A method of obtaining agglomerates as claimed in Claim 1, substantially as hereinbefore described in the Example.

13. Agglomerates as claimed in Claim 1 when prepared by a method as claimed in any one of Claims 9 to 13.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 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, grooved pulleys and reversing dua! wheels. For these products the heat treatment subsequent to shaping follows a selecred 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 example of how the process is carried out. Example. An intermediate product containing 6.4% by weight of sulphur expressed as SO, and resulting from incomplete decomposition of Al2Oa .2SO2 . 5H20 is pelletised at various pressures. Compacting is effected by means of a hydraulic press at a pressure of 3000 kgF/ cm2. The pellets have a diameter of approximately 24 mm and a thickness which varies from 4 to 7 mm, according to the quantity of intermediate product introduced. The pellets thus obtained are then calcined at 10500C 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 can be seen from the summarising table below. Intermediate product Pelletising Apparent average Attrition test Sulphur content pressure density after Height of ball expressed as SO2 kg/F/cm2 heat treatment drop 6.4 3000 0.96 8 to 10 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. WHAT WE CLAIM IS:-

1. Alumina agglomerates with good mechanical strength and predeterminable particle size obtained by compacting an intermediate product containing 3 to 15% by weight of sulphur expressed as SO,, and resulting from incomplete decomposition of a basic or neutral hydrated aluminium sulphite of the formula Al2O,. xSO, . yH2O, in which x has a value in the range from 0.2 to 3 and y is not greater than 5, 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.