GB2188915A - Tablets - Google Patents

Abstract

Tablets, pellets, blocks formed of particles of common salt or other crystallisable material can be produced without a tendency to disintegrate, even when an anti-caking agent is included in the material being tabletted, by moistening the tablet and then drying it, or by moistening the particles either just before or during the tabletting and then drying.

Description

SPECIFICATION Tablets This invention relates to tablets, pellets, blocks and the like (hereinafter referred to as "tablets") of salt or other crystalline material which may be of organic or inorganic nature. More particularly, though not exclusively, the present invention relates to tablets consisting of or comprising common salt (sodium chloride).

Common salt tablets are currently used for medicinal and culinary purposes. They are also used in water softening and, in the form of blocks, are used for animal licks. The function of such tablets is to enable controlled release or dissolving of the salt without premature disintegration of the tablet. For example, in the water softening field, it is undesirable for the salt tablet to disintegrate because the fine salt particles resulting from disintegration of the tablet can cause undue wear and sticking of automatic valves in the water softening system and does not lead to effective utilization of the salt.

It is common practice to include an anti-caking agent in salt in orderto maintain it in afree-running condition. In this way, handling, storage and transportof salt isfacilitated. Withoutthe provision of an anti-caking agent, the saltforms into solid lumps, which is highly undesirable, However, it has heretofore been impossible to form tablets having the required coherence (i.e. lackofatendencyto disintegrate) using salt containing an anti-caking agent. Thus, it has been the practice to form tablets from salt which does not contain an anti-caking agent, with all the attendant difficulties in handling, transport and storage.

Most surprisingly, we have now found thattablets having the required coherence can be produced from salt containing anti-caking agent if, afterthe tablet has been formed, it is moistened and then dried.

Thus, according to one aspect of the present invention, there is provided a method of manufacturing tablets (as herein defined) of particles of salt or other crystallisable material, comprising the steps of compacting such particles to form tablets, moistening the tablets, and then removing the moisture therefrom.

In an alternative embodiment, the salt is moistened slightly either just before or during the tabletting operation and is then dried. The moistened salt may be in the form of commercially available undried salt typically having a water content of about 2.5% Although the mechanism is not yet fully understood, it is believed that moistening ofthe tablet serves to dissolve a small proportion of the material at the surface of the particles and that, after removal of the moisture, bridges of salt or other crystalline material are established which extend between the particles in order to hold such particles in a coherent state, thus overcoming the effect of the anti-caking agent.

Thus, according to another aspect of the present invention,there is provided a tablet (as herein defined) of particles of salt or other crystallisable material, wherein the particles are held together in a coherent state within the tablet by bridges of said material which extend between the particles.

Whilstthe present invention is particularly concerned with tablets formed of salt or other crystallisable material which also contain an anti-caking agent, it will be appreqiated that the concept of the present invention can also be applied to tablets formed ofsaltorothercrystallisable material which do not contain an anti-caking agent if it is desired to improve furtherthe coherence of a tablet containing no anti-caking agent.

Typical anti-caking agents for common salt are aluminosilicatesofsodium or potassium, magnesium carbonate and sodium or potassium salts of carboxylic acid such as citric acid, oleic acid, stearic acid and palmitic acid, and the present invention is applicable to tablets containing such caking agents. In particular, however, the present invention is applicable to tablets formed of salt containing the hexacyanoferrate II salt of sodium or potassium, which is the most common anti-caking agent for common salt. The anti-caking agent is usally provided in the salt at a concentration in the range of 5 to 15 ppm by weight to provide the necesssaryanti-caking properties.

During the moistening step, the tablets can be moistened with water, either in the form of fine spray or droplets, or by briefly immersing thetablets in water. Alternatively,thetablets may be moistened with a solution of salt or other crystallisable material, e.g brine. Any concentration of solution up to a saturated solution may be used. The amount of liquid required is not critical, butwe have found that quite a light moistening of the surface ofthetablet appears to be sufficient. If it is desired to introduce other materials, such as trace elements, into the tablet, then these can be introduced conveniently by incorporating them into the solution used to moisten the tablets. For example, trace elements may be advantageously incorporated into the tablets when such areintheform of animal licks.

After moistening, the tablets are dried essentially to remove the added moisture. Drying can be effected by raising the temperature in an oven to a temperature typically between 1 000C and 200"C for a period of time to effect drying. Alternatively, the moistened tablets can be dried in a microwave oven.

In a particular example of the present invention, tablets are formed in a tabletting machine, (e.g a COLTON 31 STATION DOUBLE ROTATORY TABLET PRESS) using salt having a particle size distribution such that 2.5% of the particles are retained on an 850 micron sieve, 70% are retained on a 300 micron sieve and 99.9% are retained on a 75 micron sieve. The salt contains 5to 5 ppm by weight of sodium hexacyanoferrate II as an anti-caking agent and is better than 99.9% by weight pure sodium chloride.

Thetabletting machine produces tablets having a diameter of 5to 30 millimetres (in this example 25 mm) and athickness of 5to 20 mm (in this example 12 mm) but the tablet dimensions are not critical to the invention. After moistening the tablets with a fine water spray, the tablets are then dried in an oven at 1 70 C for 2 minutes. The resultanttablets are found to be coherent in use in that they do not disintegrate when immersed in water and this is believed to be due to bridges of salt which are formed between the salt particles within the tablet.

Claims (8)

1. A method of manufacturing tablets (as herein defined) of particles of salt or other crystallisable material, comprising the steps of compacting such particles to form tablets, moistening the tablets, and then removing the moisture therefrom.

2. A modification of the method as claimed in claim 1, wherein the salt is moistened slightly either just before or during the tabletting opeation and is then dried.

3. A method as claimed in claim 2, wherein the moistened salt is in the form of undried salttypically having a water content of about 2.5%.

4. A method as claimed in any preceding claim, wherein said tablets also contain an anti-caking agent.

5. Atablet (as herein defined) of particles of salt or other crystallisable material, wherein the particles are held together in a coherent state within the tablet by bridges of said material which extend between the particles.

6. Atablet as claimed in claim 5, which also contains an anti-caking agent.

7. A method as claimed in claim 1, substantially as hereinbefore described.

8. Atabletwhen produced bya method as claimed in anyone of claims 1 to 4 or claim 7.