GB1570261A

GB1570261A - Process for the production of basic aluminium-sodium carbonate

Info

Publication number: GB1570261A
Application number: GB3945/78A
Authority: GB
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1977-02-01
Filing date: 1978-02-01
Publication date: 1980-06-25
Also published as: DE2708861C3; FR2378718A1; FR2378718B1; JPS5396998A; DE2708861A1; DE2708861B2

Description

(54) PROCESS FOR THE PRODUCTION OF BASIC ALUMINIUM-SODIUM CARBONATE (71) We, SwIss ALUMINIUM LTD., a company organised under the laws of Switzerland, of Chippis (Canton of Valais), Switzerland, 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 a process for the production of basic aluminium-sodium carbonate, also known as dawsonite.

The composition of dawsonite can be represented by the following equivalent formulae: Al20n;.NaOO.2CO2.2H00 or AlNaO(OH)HCO3 or as A1Nas(C0,),.2A 1(OH), All three formulae are found frequently in technical literature.

A number of applications has been found for dawsonite e.g. medicaments to counter hyperacidity of the stomach, filler material for light-sensitive papers, additive for lacquers and in the paint manufacture industry.

Recently dawsonite was proposed as a filler material with reinforcing and/or flame resisting properties for polymeric synthetic resins such as polystyrene. polypropylene or polyethvlene. As far as its flame resisting properties are concerned, it was already admitted that a corresponding substance. in which sodium was replaced by ammonia, would seem to be more suitable for this purpose.

Both natural and synthetic dawsonite occurs in the form of fine needles.

The known processes for producing aluminium-sodium carbonate can be divided into the following three groups: a) Reacting an aluminate solution with a cndillm bicarbonate solution.

hl Reacting an alumina gel with a sodium hicarhonate solution.

c) Reacting an aluminium salt with a suitable bicarbonate or carbonate solution, in accordance with the desired molar ratio of Al/CO2.

Unfortunately the product obtained is impure to a greater or lesser extent. It contains in particular various amounts of aluminum hydroxide.

It has been suggested, for example, that an initial aluminate solution with a high molar ratio of Na20/Al203 e.g. > 6 should be used as a means of improving the purity of the dawsonite. In this case however the resultant mother liquors are rich in sodium carbonate and bicarbonate which are difficult to enrich.

If, on the other hand, a "Bayer" aluminate solution is used as is usually the case, the dawsonite obtained is not only impure but also mostly coloured a strong yellow or brown. This can represent a significant disadvantage, depending on the application.

If an aluminum hydroxide is employed, it must be particularly active if it is to react more or less completely under the conditions used up to now. In other words the starting point must be an alumina gel, with all the well known disadvantages which are associated with this process.

The object of the present invention is therefore to provide a simple and successful process which allows the above mentioned disadvantages to be avoided. In doing so the starting material is a pure material which can be manufactured very reproducibly and employing an inexpensive substance as source of aluminium viz., aluminium trihydroxide.

The process of the invention which fulfils this object is such that the reaction of aluminum hydroxide with a sodium bicarbonate solution is carried out at a temperature of 160 to 240"C and at a pressure of 5 to 50 atm., while the aluminium hydroxide is in the form of a suspension in the solution, the suspension being agitated by stiring, the temperature being selected in accordance with the desired length of crystal fibres.

It was found that the NaHCOs content of the suspension is usefully 40 to 250 g/l (0.475-3.0 M). It was also found that the best results are obtained if the molar ratio Na2O/AI203 of the reagents is from 2 to 6.

The reaction is carried out under pressure in a container such as, for example, an autoclave. At the end of the reaction the suspension is cooled to 1000C or lower, and the resultant dawsonite is separated from the mother liquor, and then washed and dried.

If desired, after the separation, the mother liquor is enriched with sodium bicarbonate and is used as the aqueous sodium bicarbonate solution in a repetition of the process.

The reaction time, i.e. the time for which the suspension is held at the reacting temperature, is in fact not critical. However, a reaction time of 20 to 180 min. is recommended; in a reaction time of less than 20 min. the conversion is not complete and after 180 min. the reaction is almost ended.

A reaction time of 120 min is preferred.

Generally there is no purpose in exceeding 240 min.

The results have shown that the process of the invention allows a simple way of obtaining a product which is not only very pure and white, but which also crystallises out in fine fibrous-like needles, the length of which depends on the temperature and duration of the reaction. As such, it suggests that interesting possibilities are conceivable.

It has also been found that prior addition of dawsonite seed crystals to the reacting suspensions of advantage in that amongst other things it allows the length of the fibrous-shaped crystals to be influenced.

These dawsonite seed crystals can be prepared in the course of a previous run of the process. If seeding is desired, then the quantity of seed crystals depends principally on the length of fibres desired and can usually vary from 5 to 50% of the weight of the final product.

Obviously the rate of the reaction is greater the finer the gain size of the aluminium hydroxide. This means of course that gel-like aluminium hydroxide can be added as a starting material, but this is not indispensible; crystalline aluminium hydroxide (Al2O.3H2O) can be employed, in particular a well known aluminum hydroxide (alumina hydrate) which is obtained from the "Bayer" process. In this case, which for obvious reasons is in fact the most common, it is sufficient if the aluminum hydroxide is well washed; it can also be used while damp.

The hydroxide should be as fine as pos site, in order to increase the rate of reaction. In general this can be achieved by mechanical grinding of the hydroxide. In the case of the "Bayer" hydroxide however, the starting material can be obtained in a very fine form by employing precipitation conditions which are generally known to produce a fine or very fine hydroxide.

The molar ratio (Na2OJAl2O3 or NaHCO3 /AI(OH)3) can also have a certain influence on the resultant product. If in the case of the dawsonite this molar ratio is small, i.e.

less than 2, then Boehmite, an oxihydroxide of aluminum (Al2O3H2O) is formed, even at temperature below 150"C. If on the other hand the ratio is more than 6, then the concentration of the suspension must be increased (e.g. by 150 g/l NaHCO3) to achieve a prescribed production volume; in this case the suspension can become too thick and too viscous.

The concentration of the bicarbonate influences the rate of reaction considerably.

This rate is low at concentrations below 40 to 50 g/l NaHCOa; above 200 to 250 g/l, depending on the molar ratio adopted, the reaction mixture can become very thick and cause difficulties in handling.

It has already been said that the resultant product is exceptionally pure and almost free of aluminum hydroxide, in contrast to the same kind of product obtained by conventional means. The process presented here produces crystals which grow in the form of needles of greater or shorter length and which are always well formed. They can therefore be easily filtered and washed.

Chemical analysis shows that the composition is very close to that predicted by theory.

The yield obtained in the conversion to dawsonite, under the reaction conditions of the invention presented here, is exceptionally high and is of the order of 98.599.5% on the basis of the initial, dry aluminum hydroxide.

The process of the invention can also be carried out in a continuous manner.

Other features and advantages of the invention will now be presented by way of description of a number of examples.

Example 1 1.5 1 of a sodium bicarbonate solution which contains 140 g/l NaHCO3 were placed in an autoclave which was 2 1 in capacity, provided with a stirring device and electrically heated. Next 40 g (weight: Al(OH)3) of fine grained "Bayer" aluminum tri-hydroxide (1-20 çam particle size) was added, and stirred in to form a suspension.

The autoclave was then closed and heated to 200"C within 30 minutes. The autoclave was maintained at this temperature for 120 minutes with the stirrer in operation the whole of this time; The pressure was within the range 5 to 50 atm. After this the autoclave was then cooled to about 100"C and opened. The reaction product from the autoclave was filtered, washed three times with hot water and then dried for 8 hours at llO"C. The weight of the dried product was 73.5 g. An x-ray analysis showed that the product was a dawsonite as is defined in the ASTM spectrographic index. No evidence could be found for the presence of any other compound whatsoever.The chemical analysis of the dried product was as follows: measured (%) theoretical (%) CO2 29.8 30.5 Awl203 34.8 35.5 Na2O 21.9 21.6 H2O (difference) 13.5 12.4 100.0 100.0 Examination under the optical microscope showed that the product consisted of long, thin fibrous-shaped crystals which were 10--50 pm long and we were 0.5-2 1Lm in diameter.

Example 2 The mother liquor from the previous example was used again after it had been brought to the same bicarbonate content (140 g/l NaHCO2) as in the first example.

In the same auto clave as was used in the first example a suspension containing 40 g aluminum hydroxide was allowed to react for 60 minutes at 200"C. The product of the reaction was then treated as in the previous example. Its weight after drying was 73.2 g and its CO2 content 29.9%. The X-ray analysis and the microscope investigation confirmed the result from example 1.

Example 3 The conditions for this trial were the same as in example 1 with the exception of the temperature which in this case was 120 min. at 2250C. The reaction product was then treated in the same manner as in example 1. The weight of the dried product was 73.6 g. Microscopic examination showed that the fibrous crystals were 50-100 iam long.

Example 4 The reaction suspension was made up of 30 g "Bayer" aluminum hydroxide (grain size 1-20 pm), 20.0 g of the dawsonite obtained in example 3, 158.0 g NaHCO2 and 1.5 1 water. The suspension was heated to 2200C in the autoclave and held at this temperature for 120 min. The reaction pro duct was then treated as in the previous examples. The weight of the dried product was 75.0 g, including the 20.0 g of dawsonite seeding crystals. Microscopic examination showed that the product consisted of daw sonite crystals which were 100--150 um long.

Example 5 The previous examples have already shown that the length of crystalline fibres obtained depends on the treatment temperature. This will be made clearer by the present example in which the same reaction conditions were employed, with the exception of the temperature. The following results were obtained: Reaction Temperature Approx. length of fibres (OC) Glum) 190 10-30 200 20-50 225 50-100 WHAT WE CLAIM IS: 1.A process for the production of basic aluminium-sodium carbonate by reacting aluminum hydroxide with an aquesous sodium bicarbonate soultion, in which this reaction is carried out at a temperature of 160 to 240"C and under a pressure of 5 to 50 atm., while the aluminium hydroxide is in the form of a suspension in the solution, the suspension being agitated by stirring, the temperature being selected in accordance with the desired length of crystal fibres.

2. A process according to claim 1, in which the aluminium hydroxide obtained from the "Bayer" process is employed.

3. A process according to claim 1 or claim 2, in which a sodium bicarbonate solution containing 40 to 250 g/l NaHCO2 is employed.

4. A process according to any of claims 1 to 3, in which a suspension containing a molar ratio of reagents (NaO/AI203) of from 2 to 6 is employed.

5. A process according to any of claims 1 to 4, in which the duration of the reaction is 20 to 240 minutes.

6. A process according to claim 5, in which the duration of the reaction is 20 to 180 minutes.

7. A process according to any of claims 1 to 6, in which, before the start of the reaction, dawsonite seed crystals are added to the aqueous suspension.

8. A process according to claim 7, in which the quantity of seed crystals is 5 to 50 wt. % of the resultant product.

9. A process according to any of claims 1 to 8, in which at the conclusion the basic aluminium-sodium carbonate is separated from mother liquor, and then the mother liquor is enriched with sodium bicarbonate solution in a repetition of the process.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. opened. The reaction product from the autoclave was filtered, washed three times with hot water and then dried for 8 hours at llO"C. The weight of the dried product was 73.5 g. An x-ray analysis showed that the product was a dawsonite as is defined in the ASTM spectrographic index. No evidence could be found for the presence of any other compound whatsoever.The chemical analysis of the dried product was as follows: measured (%) theoretical (%) CO2 29.8 30.5 Awl203 34.8 35.5 Na2O 21.9 21.6 H2O (difference) 13.5 12.4 100.0 100.0 Examination under the optical microscope showed that the product consisted of long, thin fibrous-shaped crystals which were 10--50 pm long and we were 0.5-2 1Lm in diameter. Example 2 The mother liquor from the previous example was used again after it had been brought to the same bicarbonate content (140 g/l NaHCO2) as in the first example. In the same auto clave as was used in the first example a suspension containing 40 g aluminum hydroxide was allowed to react for 60 minutes at 200"C. The product of the reaction was then treated as in the previous example. Its weight after drying was 73.2 g and its CO2 content 29.9%. The X-ray analysis and the microscope investigation confirmed the result from example 1. Example 3 The conditions for this trial were the same as in example 1 with the exception of the temperature which in this case was 120 min. at 2250C. The reaction product was then treated in the same manner as in example 1. The weight of the dried product was 73.6 g. Microscopic examination showed that the fibrous crystals were 50-100 iam long. Example 4 The reaction suspension was made up of 30 g "Bayer" aluminum hydroxide (grain size 1-20 pm), 20.0 g of the dawsonite obtained in example 3, 158.0 g NaHCO2 and

1.5 1 water. The suspension was heated to 2200C in the autoclave and held at this temperature for 120 min. The reaction pro duct was then treated as in the previous examples. The weight of the dried product was 75.0 g, including the 20.0 g of dawsonite seeding crystals. Microscopic examination showed that the product consisted of daw sonite crystals which were 100--150 um long.