GB2027682A - Formed Bodies of Alkali Metal Hydroxides - Google Patents

Abstract

Sodium or potassium hydroxide monohydrate in compacted form is obtained in a process in which crystallised alkali metal hydroxide monohydrate is separated off from mother liquors, washed and compacted to give solid formed bodies. Sodium hydroxide monohydrate crystals suitable for compacting are produced in a process wherein a cold, saturated and a hot solution of sodium hydroxide are brought together, the amount, concentration and/or temperature of the two solutions being so coordinated with one another that the temperature of the mixture is one at which a supersaturated solution is obtained containing sodium hydroxide monohydrate crystals, which are separated off.

Description

SPECIFICATION Formed Bodies of Alkali Metal Hydroxides The present invention is concerned with formed bodies of sodium or potassium hydroxide monohydrate and with the production thereof.

The commercially available forms of sodium and potassium hydroxide are normally anhydrous products in the form of pellets, pills, flakes or blocks, most of these products being solidified melts. Thus, for example, the most frequently used pellets are solidified droplets of a melt.

German Patent Specification No. 1,567,927 describes non-porous formed bodies made of alkali metal hydroxides which are pressed from anhydrous moulding masses of the corresponding alkali metal hydroxides, with the application of pressure and with simultaneous shaping.

However, these formed bodies suffer from certain disadvantages. Due to the conventional production process, namely, evaporation of aqueous lyes without separation of a mother liquor, they contain a number of impurities.

However, the products so produced frequently no longer satisfy the requirements demanded today for many purposes and especially in the case of the increasing demands for the purity of starting materials. Furthermore, the evaporation process involves a high consumption of energy.

Attempts have already been made to produce these alkali metal hydroxides in purer form via the readily crystallising hydrates. In this case, crude lyes are first highly concentrated and then cooled to the temperature at which the desired hydrate crystallises, whereafter this hydrate is separated off. In particular, sodium hydroxide monohydrate has been used for this purpose. However, the sodium hydroxide monohydrate obtainable in this manner is difficult to handle, it usually being present in the form of very caked or caking crystals. Therefore, these are normally further worked up either by dissolving in water or melting and subsequently evaporating to give anhydrous forms of a conventional nature.However, this circuitous way via the alkali metal hydroxide monohydrate results in a considerable increase in the cost of the purer alkali metal hydroxides and has, therefore, not been used to any great extent.

Thus, it is an object of the present invention to provide sodium or potassium hydroxide monohydrate in a purer form which is easy to handle and to improve the known production processes.

We have now found that, surprisingly, alkali metal hydroxide monohydrate in a form which is easy to handle is obtained when the crystallisate separated off from the mother liquor is compacted to give formed bodies. We have also found that a very pure sodium hydroxide monohydrate can be obtained surprisingly simply and with a saving of energy by bringing together a cold, saturated and a hot solution of sodium hydroxide, the large consumption of energy and heat for the removal of the water in the case of the conventional evaporation processes thereby being avoided.

It was not to have been foreseen that the already so widely elaborated processes for the production of pure alkali metal hydroxides could be so decisively improved in this simple manner.

Although the alkali metal hydroxides in solid form are used to a very wide extent and a need has long existed to have available such widely usable products in greater purity and, nevertheless, in a form which is easy to handle, this problem has hitherto not been solved in such a convincingly simple manner. Improved processes for producing such cheap mass products often prove to be unsatisfactory because the expense necessary for carrying out these processes is too high. The solution of the problem provided by the present invention, which simultaneously includes an energy-saving process and gives pure and compact alkali metal hydroxides in a form which is ready for use is all the more surprising.

Thus, the present invention provides formed bodies of alkali metal hydroxides, consisting of sodium or potassium hydroxide monohydrate in compacted form.

Furthermore, the present invention provides a process for the production of these formed bodies, wherein the crystallised alkali metal hydroxide monohydrate obtained in the production is separated off from the mother liquors, washed and compacted to give solid formed bodies.

The present invention also provides a process for the production of formed bodies of sodium hydroxide monohydrate, wherein a cold, saturated and a hot solution of sodium hydroxide are brought together, the amount, concentration and/or temperature of the two solutions being so coordinated with one another that the temperature of the mixture is one at which a supersaturated solution is obtained containing sodium hydroxide monohydrate crystals, which are separated off, washed and compacted to give solid formed bodies.

In comparison with the previously conventional formed bodies and processes, the formed bodies and the processes according to the present invention have a number of advantages. Thus, on the one hand, for the first time, there is provided a pure alkali metal hydroxide monohydrate in the form of compacted formed bodies which are easy to handle. On the other hand, the process according to the present invention is very economical. The previously necessary high consumption of energy for the concentration of sodium hydroxide lyes to give anhydrous sodium hydroxide is avoided, the total heat of the system during the production process being economically taken into account. Thus, a pure, crystalline sodium hydroxide monohydrate is obtained with a low consumption of energy.

The formed bodies of sodium or potassium hydroxide monohydrate according to the present invention can, depending upon the compacting device employed, have a large variety of shapes.

As a rule, square individual pellets are obtained with a weight of 0.1 to 2 g. and preferably of about 0.55 to 0.6 g. These formed bodies have a height of 3 to 1 5 mm., preferably of 5 to 15 mm.

and especially of about 10 mm., a breadth of 3 to 1 5mm., preferably of 5 to 1 5 mm. and especially of about 10 mm., and a thickness of 2 to 10 mm., preferably of 5 to 10 mm. and especially of about 6 mm.

According to the process of the present invention, for the production of sodium hydroxide monohydrate, a cold, saturated and a hot solution of sodium hydroxide are mixed together. The amount, concentration and temperature of these two starting solutions can vary within a wide range but they must be so coordinated that, in the mixture obtained, the solubility and the melting point of the monohydrate is gone below. For the cold, saturated solution, the individual pairs of values for the temperature and concentration can be taken from the solubility diagram of the sodium hydroxide-water system (see Pickering, J.

Chem. Soc., 63, 890/1893). Thus, the temperature of a cold, saturated 50% by weight aqueous solution of sodium hydroxide is about 1 OOC. The temperature of the hot solution can vary in wide ranges above the melting point of the monohydrate, which is about 64"C. These values are also to be takem from the solubility diagram.

Thus, for example a 74% by weight saturated aqueous solution of sodium hydroxide has a temperature of about 80"C. In principle, substantially higher temperatures can also be used for this hot solution. However, such a further concentrating and increase of temperature of the hot solution of sodium hydroxide involves a high expenditure for energy.

The temperature and concentration of the mixture are given by the particular amounts, concentrations and temperatures of the two starting solutions according to the mixing procedures and from phase diagram of the sodium hydroxide-water system. The parameters of the starting solutions are preferably selected in such a manner that the temperature of the mixture obtained is from 40 to 640 C. (melting point of the monohydrate).

The two starting solutions are mixed in known manner. The two starting solutions can be brought together, for example with stirring, or they can be allowed to flow tangentially against one another.

Stirring generally promotes the uniform distribution of the heat within the solution and, during crystallisation, prevents the formation of crystai agglomerates.

Separation of the crystallised out monohydrate from the mother liquor obtained in the production also takes place in known manner. Mechanical separation of the solid-liquid mixture can take place not only by sedimentation but also by filtration. As a rule, separation of the crystallisate is carried out with a sieve centrifuge. However, other conventional devices can also be employed.

After separation, the crystallisate is washed in order to remove adhering mother liquor and crystallised out impurities. Various washing liquids can be used to this purpose but, as a rule, the crystals are washed with water.

The purity of the product obtained depends, in known manner, upon the ratio of crystallisate mother liquor in the case of given purity of the two starting solutions.

It is especially advantageous to carry out the process according to the present invention continuously.

The mother liquors, which usually still contain crystal nuclei, can be recycled or, depending upon the desired purity of the product, can be removed from the process. Recycling of the mother liquor is an important factor for the optimum carrying out of the process according to the present invention.

The system can be operated in an especially energy-saving manner when its total heat is economically utilised. In this case, mother liquor is expediently fed at a relatively high mixing temperature to the evaporation stage; in the case of a lower mixing temperature, recycling into the cooling stage is frequently more economical or the mother liquor is, in each case, recycled in appropriate amounts. The whole plant can be operated continuously with a closed heat system comprising, on the one hand, an evaporation and cooling unit and, on the other hand, a mixing and crystallising section, with subsequent solid/liquid separation and compacting. In this way, for the first time, there is provided an especially energy saving process, with the help of which the alkali metal hydroxides, which are required in large amounts, can be produced in a pure form which is easy to handle.

The sodium hydroxide monohydrates crystals obtained are subsequently compacted in known manner. Potassium hydroxide monohydrate which is usually obtained in known manner by cooling crystallisation, can be compacted in the same way as the sodium hydroxide monohydrate.

For carrying out the process according to the present invention, use can be made of all known devices for compacting alkali metal hydroxide monohydrates. However, it is preferable to use a roller compacting machine, in combination with appropriate moulds, for producing the desired formed bodies of alkali metal hydroxide monohydrate. Such a device comprises two rollers which move against one another and are provided with milled-in moulds. The rollers are so adjusted that they run against one another precisely.

The crystals are continuously fed in via a supply hopper and pass into the gap between the rollers provided with the milled-in half moulds.

The roller speed can be varied according to the rate of feed in of the crystals. The crystals must always be available in such an amount that the moulds on the rollers are sufficiently filled. After passing through the pressing zone between the two rollers, the formed bodies produced fall on to a sieve and, after the removal of small particles, are packed. In addition, it is also possible to subject the crystals to a precompacting even in the supply hopper.

The alkali metal hydroxide monohydrates in compact form can be employed everywhere and in the same manner as the previously usual forms of sodium and potassium hydroxide. The water content does not usually have a disturbing influence because aqueous alkali metal hydroxide solutions are preponderantly produced from the solid forms.

The following Examples are given for the purpose of illustrating the present invention Example 1 kg. of a 50% by weight sodium hydroxidecontaining aqueous solution with a temperature of 100C. is mixed, while stirring, with 1 kg. of a 74% by weight sodium hydroxide-containing aqueous solution with a temperature of 800C., the resulting mixture having a temperature of 560C. The crystals thus obtained are separated off with a sieve centrifuge, washed with water and subsequently compacted on a commercially available compacting machine to give formed bodies with a weight of about 0.55 to 0.6 g. The compacting machine employed has rollers with a diameter of 20 cm. and a breadth of 5 cm., which corresponds, in the case of this roller breadth of 5 cm., to a pressing of about 4000 kg./cm. The yield of sodium hydroxide monohydrate is 285 g.Since they still have a high heat content, the mother liquors can be recycled to the evaporation stage.

The formed bodies obtained have a height and a breadth of 10 mm. and a thickness of about 6 mm.

Example 2 1.5 kg. of a 50% by weight sodium hydroxidecontaining aqueous solution with a temperature of 1 00C. is mixed with 1 kg. of a 74% by weight sodium hydroxide-containing aqueous solution with a temperature of 680C., in a manner analogous to that described in Example 1. The temperature of the mixture obtained is 430C. The crystals thus obtained are separated off in a manner analogous to that described in Example 1, washed and compacted on a compacting machine as in Example 1 to give formed bodies with a weight of about 0.55 to 0.6 g. The yield of sodium hydroxide monohydrate is 220 g. Because of their low heat content, the mother liquors are recycled to the cooling stage.

Example 3 Potassium hydroxide monohydrate formed bodies are produced in the compacting machine described in Example 1, potassium hydroxide monohydrate obtained by cooling crystallisation being used as starting material. In a manner analogous to that described in Example 1, there are obtained formed bodies with a weight of about 0.55 to 0.6 g., a height and a breadth of 10 mm. and a thickness of 6 mm.

Claims (7)

Claims

1. Formed bodies of alkali metal hydroxides, consisting of sodium or potassium hydroxide monohydrate in compacted form.

2. Formed bodies according to claim 1, characterised in that they possess a height and breadth of, 'in each case, 3 to 1 5 mm. and a thickness of 2 to 10 mm.

3. Formed bodies according to claim 1, substantially as hereinbefore described and exemplified.

4. Process for the production of the formed.

bodies according to claim 1, wherein the crystallised alkali metal hydroxide monohydrate obtained in the production thereof is separated off from the mother liquors, washed and compacted to give solid formed bodies.

5. Process for the production of formed bodies of sodium hydroxide monohydrate, wherein a cold, saturated and a hot solution of sodium hydroxide are brought together, the amount, concentration and/or temperature of the two solutions being so coordinated with one another that the temperature of the mixture is one at which a supersaturated solution is obtained containing sodium hydroxide monohydrate crystals, which are separated off, washed and compacted to give solid formed bodies.

6. Process for the production of formed bodies according to claim 1, substantially as hereinbefore described and exemplified.

7. Formed bodies according to claim 1, whenever produced by the process according to any of claims 4 to 6.