AU753380B2 - Process for the manufacture of chemical absorbents, and novel chemical absorbent formulations - Google Patents

Description

Title: Process for the Manufacture of Chemical Absorbents, and Novel Chemical Absorbent Formulations This invention relates to a process for the manufacture of chemical absorbents, and to novel chemical absorbent formulations.

Chemical absorbents are used to remove one or more molecular species, for example to remove specific molecular species from mixtures of gases. Soda lime is one such chemical absorbent and is widely used to absorb carbon dioxide, for instance in anaesthetic breathing systems and other applications involving air which is to be breathed.

In order to make chemically active soda lime, calcium hydroxide is mixed with sodium hydroxide and/or potassium hydroxide to produce a caustic lime mixture containing water. The ratio of components in the caustic lime mixture is 96-98% calcium hydroxide with 2-4% sodium and/or potassium hydroxide. The finished product contains typically 81-87% caustic lime mixture and 13-19% water. In addition, a pH sensitive indicator dye is normally present to give a visual indication of the extent of usage and exhaustion.

00 This indicator dye is present in the caustic lime mixture at very low levels, typically 0.01-0.1%.

15 To varying degrees, all known soda lime formulations contain dust and fine particles generated as a result o0% of the friability and breakdown of the finished product. The presence of such dust is undesirable, S° particularly when the product is intended for use in anaesthetic (or other) breathing systems since the fine particles may be inhaled.

There has now been devised a progress for the manufacture of chemical absorbents which overcomes or substantially mitigates the above problem.

According to the invention, a process for the manufacture of a chemical absorbent in solid, granular form includes the steps of 9 a) mixing the components of the chemical absorbent with water to form a paste; b) passing the paste between a pair of counter-rotating, contacting rollers, the rollers having formed therein corresponding substantially hemispherical depressions; and c) directing high velocity and energetic air at the surface of the rollers, thereby dislodging the granules of chemical adsorbent.

The process according to the invention is advantageous primarily in that the chemical absorbent is formed into substantially spherical granules. Consequently, the granules have no projecting edges or corners which can break off and create dust. The granules are considerably less friable than conventional granules and maintain the integrity of their shape throughout normal use.

The surface and rotation of the counter-rotating rollers are such that, at the point at which the rollers touch, depressions in the rollers form substantially spherical moulds. Most preferably, the hemispherical depressions are arranged in a hexagonal close-packed array.

Preferably, the direction of rotation of the rollers and the direction in which the paste is fed between them are such that the paste travels downwards, ie movement of the paste between the rollers is assisted by S.gravity. As the rollers rotate, the spherical moulds re-open, exposing the moulded paste spheres. It is 15 preferred that the air knife be positioned adjacent each roller to generate a tangential blade of high velocity and energetic air.

Spherical solid granules of a chemical absorbent formulation are believed to be novel, and represent a further aspect of the present invention.

The process according to the invention may be utilised to produce spherical granules of a wide range of chemical absorbent formulations. However, it has been found to be particularly useful in relation to soda lime formulations. Furthermore, novel soda lime formulations have been developed which demonstrate improved mechanical strength and are especially well suited to manufacture by the process of the invention.

Thus, according to another aspect of the invention there is provided a soda lime formulation comprising a major proportion of calcium hydroxide in admixture with a minor proportion of sodium hydroxide and/or potassium hydroxide and water, the formulation further comprising a zeolite.

Various forms of zeolite, eg zeolites containing sodium, calcium, barium, strontium or potassium, may be utilised. The presently preferred zeolite is sodium aluminium silicate.

The zeolite is preferably present in the formulation to a level of between 0.1 and 10% w/w, more preferably 2% to 6% w/w.

The invention will now be described in greater detail, by way of illustration only, with reference to the manufacture of spheres of the following, presently most preferred, soda lime formulation: r Calcium hydroxide Sodium hydroxide Sodium aluminium silicate Water pH sensitive indicator dye 77 parts by weight 3 parts by weight 4 parts by weight 16 parts by weight 0.03 parts by weight.

The required quantities of calcium hydroxide, sodium aluminium silicate and indicator dye (all in the form of fine powders) are mixed to form a homogeneous powder mix.

The required amount of sodium hydroxide and water are mixed to form a homogeneous caustic solution.

The caustic solution is added to the powder mix. Mixing then takes place to form a stiff paste.

Before the above paste is processed it has been found to be beneficial to allow a dwell time of to 60 minutes, during which the paste hardens to a stiffer consistency.

The paste is then loaded into a processor which is shown schematically in Figure 1.

The processor comprises two counter rotating and touching rollers 1,2. The direction of rotation of the rollers 1,2 at the point where they touch is such that the surface of both roller I, has a downward motion.

Each roller 1,2 is profiled with hemispherical cavities, each cavity being of the same diameter.

Depending on the product to be made, this my be 3, 3.5 or 4mm. The hemispherical cavities are an-anged in a hexagonal close packed arrangement.

The profiled surface and relative movement of the two touching rollers 1,2 are synchronised such that at the point at which the surfaces of the two rollers 1,2 meet, a row of completely spherical moulds are created from the two facing rows of hemispheres. The speed of rotation of the two rollers 1,2 is synchronised and can be varied between 0 and 30 revolutions per minute.

Paste 3 is fed in lumnps into a feed hopper 4 mounted on top of the processor such that the paste 3 is resting on the downwardly rotating surfaces of the rollers 1,2. In an alternative anrangemet the paste is formed into a sheet which is fed between the rollers.

The motion of the rollers draws paste down between them at the point where they touch. Soda *lime paste is thus squeezed and forced to fill the spheri cal moulds as they form.

i~A the roiier rotate, the spherical cavities re-open exposing a row of moulded paste spheres These spheres 5 generally do riot drop out of the processor under their own weight, instead they stick inside one or the other of the two hemispherical cavities from which they were moulded.

There is no factor influencing which roller cavity the moulded spheres 5 stick to and so the result is that both rollers as they rotate from thle under side have half of their profiled pits empty while the other half are full of moulded sphere.

The profiled cavities on the surface of the rollers must be emptied of the moulded paste sphes in order that they are available to take up more paste the next time around. Therefore, an ejection mechanism is necessary to remove the moulded spheres 5 from the cavities.

Air knives 6,7 are mounted adjacent each roller 1,2 such thar a blade of high velocity and energetic air'is directed at a tangent to the surface of the rollers 1,2 down their entire length.

As the rollers 1,2 rotate, the rows of cavities filled with moulded spheres 5 come into line with the blade of air which hits the side of the moulded spheres 5 thus ejecting them :from the cavities.

Aconveyor belt 8 starting a sufficient distance behind the processor passes under it, collecting spheres 5 as they drop. The moulded spheres 5 travel along the conveyor and into a continuous belt oven 9.

mre fst drying stage is a gentle drying at around 100 0 C during which the majorit of the water 0. is removed from the product. This low temperature stage has two beneficial effects. Firstly, it is believed that it prevents rapid drying which could create stress within the stucture, reducing *the subsequent physical strength of the product Secondly, it is believed that the low temperature prevents migration of sodium hydroxide to the surface of the spheres 5 while it is still in solution.

This would create an outer layer of high alkalinity but an interior of low alkalinity and po or *activity.

The second drying stage is at an elevated temrperature of around 140T. In this stage the product is completely dried to less thani 1% water. This level of dryness is important as it allows for.

bonding to take place between microscopic lime particles within the soda lime spheres 5 (it is 00 believed that as the dissolved ions come out of solution they form bonds between the lime particles).

On exiting the oven 9, the dried spheres 5 are tipped from the conveyor belt 8 and are collected in a suitable receptacle During the formation of soda lime spheres 5 by the invented process, some of the spheres are found to possess a slight "moulding seam" around the circumference. Although the dusting of this finished product is already low this "moulding seam"i may present a region of increased fliability on the soda lime sphere. It has been found to be beneficial to include a de-dusting stage in the production process.

The de-duster comprises a rotating cylinder made from perforated steel sheet. The size and pitch of the perforations are such that only fine particles and dust are allowed to fall through while the spheres remain in the cylinder. The speed of rotation of the de-dusting cylinder is between and 60 revolutions per minute.

The completely dried soda lime spheres are introduced into the de-duster. As the spheres begin to tumible they settle into a stable flowing cyclic motion. As they do so, the spheres roll against one another creating an abrasive affiect which causes the pieces of "flash" or moulding "seami" to be broken off. After a sufficient time the spheres are smoothed while the generated dust falls through the perforations and is thus separated from the product. The resulting product possess *a significantly reduced friability since any irregularities have been removed.

In order for soda lime to be chemically active (able to absorb GO 2 the final product must contain a level of water between 13 and 19% (16% in the above formulation).

During the drying process it is necessary to completely dry the product in order to create the physical strength. Therefore, there is insufficient moisture present for C0 2 activity and the necessary water must be added back into the product to achieve this.

To achieve the required end product moisture content of 16% by weight, the necessary quantity of water to "wet back" the de-dusted product is calculated. This wate r is then added to the dry product and the mixture is mechanically agitated for a sufficient time to disperse the water. The soda lime is then sealed in an air tight container until complete equilibrium of the moisture has taken place.

The product may contain partially formed spheres and/or fine particles created during the wetting back process. In order to remove these, the soda lime is sieved over a suitable screen before packing.

Claims (3)

1. 2. A process as claimed in Claim I, wherein the hemispherical depressions are arranged in a hexagonal close-packed array.
2. 3. A process as claimed in Claim 1 or Claim 2, wherein the direction of rotation of the rollers and the direction in which the paste is fed between them are such that the paste travels downwards. .:o.oi
3. 4. A process as claimed in any one of Claims 1 to 3, wherein the high velocity and energetic air os provided by an air knife positioned adjacent each roller. o: 5. A process for the manufacture of a chemical absorbent in solid, granular form substantially as hereinbefore described with reference to the example. .Dated this 12' day of August 2002 INTERSURGICAL LIMITED S"By their Patent Attorneys AHEARN FOX S St *o oo S