GB2595046A

GB2595046A - Desiccant granules and their method of production

Info

Publication number: GB2595046A
Application number: GB2104913.5A
Authority: GB
Inventors: Raymond Meyer Christopher; Somerville-Roberts Nigel
Original assignee: Birch Chemicals Ltd
Current assignee: Birch Chemicals Ltd
Priority date: 2020-04-08
Filing date: 2021-04-07
Publication date: 2021-11-17
Anticipated expiration: 2041-04-07
Also published as: GB202005222D0; GB2595046B; GB202104913D0

Abstract

Desiccant granules comprise: (a) calcium oxide; (b) inert oil; and (c) optionally, other ingredients, wherein the ratio of (a) to (b) is from 3:1 to 6:1, wherein (a) +(b) is greater than 90wt% of the granule, and wherein the calcium oxide has an oil absorbency of greater than 28% to less than 40 wt%. The granules may have a particle size distribution of 1.0-6.0 mm of at least 90wt%. The inert oil may be a hydrocarbon oil or a silicon-based oil. The granules may have a minimum compaction strength of greater than 100g/cm3. A process for the production of the granules comprises: obtaining calcium oxide with an oil absorbency of greater than 28% to less than 40 wt%; contacting the CaO powder with an inert oil under conditions of high agitation; contacting the granules formed in the previous step with obtaining calcium oxide with an oil absorbency of greater than 28% to less than 40 wt%.under conditions of low agitation. The calcium oxide particles may have a d50 particle size of less than 4.0µm.

Description

Desiccant granules and their method of production

Field of the Invention

The present Invention relates to desiccant granules and their method of production. The granules comprise calcium oxide and inert oil. The granules are free-flowing and have sufficient robustness and integrity to be easily packed and shipped, accurately dosed and are readily dispersible in use.

Background of the invention

Granules comprising calcium oxide are used as desiccants with a particular focus on the processing of plastics and rubbers. Calcium oxide is able to absorb and react with water that would otherwise boil and cause defects in the plastic or rubber during processing. Such materials are commonly referred to as "defoamers" or "antifoamers" in the context of plastics and rubber processing.

The processing of plastics and rubber is highly complex and has multiple process issues that need to be addressed. One issue is the evaporation of moisture during processing. Processing, such as extrusion or vulcanisation, typically happens at high temperatures. During extrusion, the flash evaporation of even tiny amounts of water immediately after extrusion, due to the high temperature and drop in pressure, can cause surface defects as the moisture boils off This is referred to as "foaming". To deal with this water, desiccants are added to the reaction mixes. These desiccants need to be able to react with and hold onto the water at high temperatures. A very common material for this application is calcium oxide, or quicklime, which is able to react with water to form calcium hydroxide. This is stable at the temperatures of use, so the water is removed from the process. This reaction depends on surface contact, hence the greater the surface area of the calcium oxide, the better the efficiency of the reaction I. It is very important to ensure that any calcium oxide powder that is added to a reaction mix is well dispersed. This is to maximise the surface area of the calcium oxide in contact with the surrounding material. This is also necessary to ensure that there are no larger clumps or aggregates of fine calcium oxide particles which could be visible in the plastic or rubber and cause defects. One problem is that differences in the surface properties between calcium oxide and an organic material such as a plastic or rubber increase the tendency of calcium oxide particles to clump into larger aggregates when mixed into a plastic or rubber mix rather than being dispersed as individual particles. In addition, calcium oxide is highly alkaline and dangerous for health if inhaled or if in contact with the skin or eye. When fine powder is used, this is also highly dusty. This presents a significant risk to health when handling calcium oxide powder, such as when it is added to a plastic/rubber reaction mix.

To overcome these health risks and processing issues when using calcium oxide as a desiccant for plastic/rubber processing, calcium oxide powder is either blended with other materials and/or processed further, e.g. into extrudates, and/or packaged into speciality packages. One such material is an oil. Mixing the calcium oxide with a more hydrophobic material such as an inert oil makes the surface of the calcium oxide particles more compatible with the surrounding plastic medium and hence easier to disperse. The oil is inert, in that it does not react chemically with the calcium oxide. However, commercially available granules (which are always extrudates) made with high levels of oil are typically soft, which results in handling and storage problems. The high level of oil and property of the calcium oxide that makes such mixtures extrudable also makes the resulting extrudates soft.

The shipping and use of such materials, especially blends or particles of calcium oxide with high and commercially preferred levels of added oil, can be difficult due to their softness and deformability. Mixtures of calcium oxide and oil may be made into extrudates but, after shipping and the compaction that can happen during transport, such extrudates will often have compacted together and be hard to use. This softness and deformability of many products limits the size of containers that can be used. Some materials are supplied as pastes or putty. Such mixes are hard to use especially when partial amounts of a package are needed, and material needs to be scooped out of a container.

There is therefore a need for free-flowing and easily handled calcium oxide/oil granules, preferably which have the high and commercially preferred levels of oil, that can be easily processed and that do not require the use of speciality processing aids, such as high levels of solidification agents or other non-preferred materials. Such granules need to remain free-flowing over extended periods of time, even under pressure, and be easy to dose in variable quantities from the package if required.

The inventive granules are robust enough to be easy to handle and be weighed. However, they are not so hard as to be difficult to disperse well in rubber and plastic mixes during processing They comprise inert oil to ensure ease of dispersion in plastics and rubber and protection from moisture uptake during storage.

US2007/078210 relates to a desiccant material that comprises calcium oxide. However, the oil absorbency of the calcium oxide is not disclosed.

Summary of the invention

The present invention relates to desiccant granules comprising: (a) calcium oxide; and (b) inert oil, wherein the ratio of (a) to (b) is from 3:1 to 6:1, wherein (a) +(b) is greater than 90wtll/O of the granule, and wherein the calcium oxide has an oil absorbency of from greater than 28wt% to less than 40wt%.

The present invention also relates to a process for the production of these desiccant granules, wherein the process comprises the steps of: (a) obtaining calcium oxide powder having an oil absorption of from greater than 2Swt% to less than 40wt%; (b) contacting the calcium oxide powder with an inert mineral oil under conditions of high agitation to form intermediate granules, (c) contacting the intermediate granules from step (b) with calcium oxide powder having an oil absorption value of from greater than 28wt% to less than 40wt% under conditions of low agitation to form granules according to any preceding claim; and (d) optionally, sieving the granules obtained in step (c) so as to remove particles greater than 6 Omm in diameter.

Detailed Description of the Invention

Desiccant granules. The desiccant granules comprise: (a) calcium oxide; (b) inert oil; and (c) optionally, other ingredients. The ratio of (a) to (b) is from 3:1 to 6:1. (a) +(b) is greater than 90wt% of the granule.

Preferably, the ratio of (a) to (b) is from 3:1 to 6:1. An especially preferred ratio is from 3.5:1 to 4.5:1, or even 4:1.

The granules can comprise from 5wt% to 25wt% inert oil. The granules can comprise from 75wtl, to 95wt% calcium oxide. Preferably, (a) +(b) is greater than 95wt%, or even 99wtl, of the granule. Preferably the calcium oxide content is greater than 75wt% or even greater than 85w-t%.

The granules preferably have a particle size distribution such that at least 90wt%, or at least 95wt%, or even 99wt%, of the granules have a particle size range between 1.0mm and 6.0mm, or even between 2.0mm and 6.0mm. A preferred size distribution is such that at least 90wt%, or even at least 95wt%, of the granules have a particle size in the range of between 1.0mm and 5.0mm, or even between 2.0mm and 5.0mm. The size distribution of these granules can be controlled by the use of suitable sieves.

The granules are preferably spherical. Typically, the granules are free-flowing.

The robustness of the granules can be defined by their minimum compaction pressure (MCP). The granules preferably have a minimum compaction pressure of greater than 100 g/cm2 or preferably greater than 150g/cm2, or even greater than 200 g/cm2.

A preferred feature is that the granules have a non-uniform distribution of oil throughout the particle with a higher level of oil to calcium oxide in the centre of the granules compared to the outside. Preferably, the ratio of (a) to (b) on the surface of the granules is greater than the ratio of (a) to (b) in the middle of the granules. This is a preferred feature as it reduces the surface stickiness. The simplest way to tell if a granule has the desired structure is by visual examination of a cross-section of the granule. The colour of the particle is darker with increasing oil level. A granule with the required structure therefore has a lighter-coloured outer edge under visual examination compared to the core.

It is beneficial for the granules to have a minimum residual oil absorption (ROA) value, that is they retain a residual ability to absorb further oil. The greater the ROA capacity of the granules, the more robust the granules will be. If a granule has a low, or zero, ROA value, there will be some oil able to act as a lubricant, which will make the granules easier to deform and cake under pressure. The granules preferably have a residual oil absorption capacity of greater than 3.0wt%, or greater than 4.0wt.%, or preferably greater than 5.0wt%.

The granules may optionally comprise other ingredients. If present, these other ingredients may be present at a level of up to lOwt%, or up to 5wt% of the granule. One suitable optional ingredient is polyethylene glycol. Polyethylene glycol can be included in the granule to enhance the performance of the granule. A suitable polyethylene glycol has a molecular weight of from 600 to 8000. Another suitable ingredient is a coupling agent. Suitable coupling agents are silanes.

Preferably, the granule is an agglomerate.

Calcium oxide. The calcium oxide has an oil absorbency of from greater than 28wt% to less than 40wt%. It is critical to use a suitable calcium oxide that has an oil absorption value above this minimum value and below this maximum value. Different calcium oxide materials will have different oil adsorption values based on the complex interaction of particle morphology, porosity and size. By using a calcium oxide material with a suitable oil absorption value, such mixtures can be granulated by a high energy granulation process, such as by the use of Eiricr) granulator, to form robust desiccant granules, and especially spherical granules. If the oil absorption value is too low, the process mixture will typically over-granulate in the high energy granulation process and the resulting desiccant granules can be too soft and deformable due to oil being present on surface granules and being able to act as a lubricant. In many products, the oil level cannot be adjusted to compensate for differences in calcium oxide properties due to end-user requirements who often have processes and compositions optimised for a specific amount of oil and calcium oxide. A very common commercially used level of inert oil is from 5wt% to 25wt%, or even from 15wt% to 25wV,"0, or even 20wVio.

An appropriate selection of calcium oxide allows the oil absorption value to be controlled. The oil absorption value of the calcium oxide can be controlled by the particle size as well as the source of the calcium oxide. Preferably, the calcium oxide has a dso particle size of less than 4.0pm, preferably less than 3.0m. Calcium oxide that is too coarse will typically have a relatively low oil absorption value. Calcium oxide that is derived from soft chalk is especially suitable.

An especially preferred calcium oxide is sold as CIO by Singleton BirchOn" Limited, Barnetby, UK.

Inert oil. Suitable inert oil is selected from hydrocarbon-based oils and/or silicon-based oils. The inert oil may be a plant-based oil. The principal requirement is that the oil is a material that does not react chemically with the calcium oxide and that it is hydrophobic so as to be more compatible with any plastic matrix than the calcium oxide surface. The insert oil is typically non-volatile so as to minimise risks associated with processing.

Suitable hydrocarbon-based oils are hydrotreated heavy paraffinic distillates having the CAS number 64742-54-7. Other suitable hydrocarbon-based oils include saturated hydrocarbons. A suitable hydrocarbon-based oil is sold by Safic Alcan(ne as PW 1 083C.

A suitable inert oil is a vegetable oil. Suitable vegetable oils include non-drying and semidrying oils, typically having an iodine value of less than 150, as measured according to ASTM D5768 -02(2018).

Suitable inert oils include sunflower oil, olive oil, avocado oil, rapeseed oil and soybean oil. Soybean oil is preferred This may be due to its greater thermal stability as indicated by its higher smoke point value compared to many other vegetable oils.

Suitable granules comprise an inert oil that is a hydrocarbon-based oil; and wherein the ratio of (a) to (b) is from 6:1 to 3:1, or even from 5:1 to 3:1.

Suitable silicon-based oils are polysiloxanes having the CAS number 63148-62-9. A suitable material is sold as SeraSense SF 1K by KCC Beauty"1), of Abingdon, UK.

Suitable granules comprise an inert oil that is a silicon-based oil.

Process for the production of desiccant granules. The desiccant granules can be prepared by processes including granulation, roller compaction, briquetting and tabletting as the particle-forming steps. However, granulation and especially granulation using Eirich(Rmumixer type granulators, is preferred due to the ability to create rounded, spherical granules which can have improved flowability (compared to granules made by other processes) due to their shape and the ease of applying an external coating of calcium oxide powder.

Preferably, the process does not comprise an extrusion step. Preferably, the granules are nonextrudates. Preferably, the process comprises one or more, preferably two or more, agglomeration steps. Preferably, the granule is an agglomerate. Preferably, the process does not comprise the step of extruding the calcium oxide together with the inert oil.

In the Eirich(RT"-type granulators, the granules are built up by a layering process. An alternative process that can also form rounded granules is pan granulation. A preferred process route is to use a mixer such as an Liner) mixer for both the granule forming and layering/coating stages. An alternative process is to use another higher shear mixer such as a CB-type mixer from Loedige Brothersnr) to form initial granules which are then fed to a pan granulator so as to further increase the size and sphericity of the granules. Granules having a particle size between 1.0mm and 6.0mm, or even between 2.0mm and 6.0mm are preferred for robustness and flowability.

The process of the present invention comprises the steps of: (a) obtaining calcium oxide powder having an oil absorption of from greater than 28wt% to less than 40wt%; (b) contacting the calcium oxide powder with an inert mineral oil under conditions of high agitation to form intermediate granules; (c) contacting the intermediate granules from step (b) with calcium oxide powder having an oil absorption value of from greater than 28web to less than 40wt% under conditions of low agitation to form granules according to any preceding claim; and (d) optionally, sieving the granules obtained in step (c) so as to remove particles greater than 6.0mm in diameter.

Step (a) obtaining calcium oxide powder. Step (a) obtains calcium oxide powder having an oil absorption of from greater than 28wt9/0 to less than 40wt%.

Step (b) forming intermediate granules. Step (b) contacts the calcium oxide powder with an inert mineral oil under conditions of high agitation to form intermediate granules. High agitation refers to the situation in which a tool, such as the blade on an impellor, moves at a speed of greater than 3.0m/s or even greater than 5.0m/s or even greater than 10.0m/s through the powder mass. The high level of agitation disperses the oil to form a more uniform mixture. The use of a high level of agitation during the addition of the mineral oil helps ensure uniformity of composition in the granules as well as a tighter and narrower particle size.

Step (c) forming the desiccant granules. Step (c) contacts the intermediate granules from step (b) with calcium oxide powder having an oil absorption value of from greater than 28wt% to less than 40w-fliIi under conditions of low agitation to form the desiccant granules. The intermediate granules are then coated with additional calcium oxide powder when the granules are of a suitable size. A low level of agitation is beneficial here. By "low level" it is meant that no surface or edge that the granules come into contact with is moving at a speed of greater than 3.0m/s, or greater than 2.0m/s. The higher the level of agitation that the intermediate granules plus additional calcium oxide powder is subjected to, the less effective the additional calcium oxide coating layer is as the agitation and work done to the granules forces more oil to the surface due to the continual compression and compaction of the intermediate granule core.

Optional step (d) sieving. If desired, there can be an optional step (d) that sieves the granules obtained in step (c) so as to remove particles greater than 6.0mm in diameter or which can be used to sieve the particles to within a desired size range, such as between 2.0mm diameter and 5.0mm diameter. Suitable equipment for doing this are mechanical sieves, especially a vibratory sieve, such as a Mogensen(nu Vibratory Screen Deck having at least two mesh screens to remove the oversize and undersized fractions.

Other process features. A preferred feature is that the granules have a non-uniform distribution of oil throughout the particle with a higher level of oil to calcium oxide in the centre of the granules compared to the outside. This can be achieved by performing an initial granulation with a higher ratio of oil to calcium oxide to help form the granules. Then additional calcium oxide powder can be added to form a non-homogenous layer on the surface. This assists with flowability and ability to avoid caking under compaction.

Minimum compaction pressure. The robustness of the granules can be defined by their minimum compaction pressure (MCP). The minimum compaction pressure is the minimum pressure needed to compact material together into a cake or solid mass that does not immediately break up with even gentle handling.

The minimum compaction pressure can be tested as follows. 500g of granules, preferably having a size between 1.0mm and 6.0mm, are placed in a flexible container such as a bag having dimensions of 10cm by 10cm and the top surface levelled. The granules are then compressed uniformly by placing a 10kg weight on the granules and leaving the weight in place for 24 hours. This subjects the granules to a pressure of 100g/cm2. This is equivalent to a 1000 kg weight resting on 1.0m2 which approximates to the situation when 2 big bags of material are "double-stacked" with one on top of the other. Double-stacking is widely used in industry for space reasons so having a material that can be double-stacked, that is it has a minimum compaction pressure of greater than 100 g/cm2, is highly preferred.

Oil absorption. The oil absorption can be measured according to a modified version of ASTM D-281-12. The test sample size is 20g whether for calcium oxide powder or granules, and the linseed oil specified in D-281-12 is replaced by a heavy paraffinic distillate having a CAS number 64742-54-7. Linseed oil can react chemically with calcium oxide, thus giving misleading results. 20g of the material to be tested is mixed dropwise with the paraffinic distillate and mixed with a spatula until a smooth paste is formed that can be spread uniformly over a flat surface. The oil absorption value is the wt% of distillate in the final distallate:powder mix. For example, if 5g of distillate are added to 20g of a powder sample to get to the end point, then the oil absorption value of the powder is (5/(5+20))wt% = 20wt%.

Residual oil absorption. The residual oil absorption (ROA) value of the granules is simply the oil absorption test value obtained when using the granules rather than the calcium oxide powder.

Method of measuring particle size of calcium oxide. The particle size distribution of the calcium oxide is typically measured by laser diffraction. A suitable standard for size analysis by laser diffraction is given in ISO 13320:2009. Suitable size analysers are the Mastersizeia) 2000 and 3000 instruments by Malvern Instruments(RT". It is preferred to disperse the samples by compressed air (usually with a Sciroccoun" 2000 unit) where the material is tested as a powder stream, rather than the wet method where the test material is dispersed in a fluid first. However, it is possible to disperse and test these ceramic mixtures in non-aqueous liquids The measurement is typically done as per the manufacturer's instruction manual and test procedures. The results are typically expressed in accordance with ISO 92762, Method of measuring particle size of the granules. The particle size of the granules is typically measured by sieve analysis. A suitable method is to sieve 100g of the granules using shaking for 1 minute through a set of sieves consisting of a 6mm mesh size, 5mm mesh size, 2mm mesh size and the pan. The material sieves easily due to the larger size of the granules. The wt% of the granules between 2mm and 6mm can be calculated by adding the weights of the granules on 5mm and 2mm sieves. The wt% fraction between 2mm and 5 mm can be calculated by weighing the fraction on the 2mm screen after sieving.

Examples

Example 1:

Three samples of calcium oxide were tested for oil absorption values and their ability to be formed into granules.

Comparative example la: Ultrafine Calcium Oxide ReagentPlus Grade 208159 from Sigma-Aldrichum" was tested for oil adsorption as per the method described above. The oil adsorption value was 42wt%.

50g of this calcium oxide material was then placed in a Kenwoor) kitchen mixer. Oil (Safi c Alcar) Alconplast(n1) PW1083C) was then added in a step-wise manner to granulate the material. However, even when 20wt% oil had been added, the powder mix was still very dusty and fine. Adding extra oil ultimately caused the powder mix to very rapidly over-agglomerate in an uncontrolled manner. No viable granule could be formed.

Comparative example lb: Calcium oxide powder was made in the lab by calcining available calcium carbonate quarry fines. This CaO powder was tested for oil adsorption as per the method described above. The oil adsorption value was 25wt%.

150g of this calcium oxide material was then placed in a Kenwood(nu kitchen mixer. Oil (Safi c Alcan(RT" Alconplastun" PW1083C) was then added in a step-wise manner to granulate the material. However, the powder mix started over-granulating in a hard-to-control manner even after the addition of low levels of oil. By the time 25g of oil had been added, much of the mix was smeared as make-up along the side and bottom of the mixer bowl. No viable granule could be formed.

Inventive example le: Calcium oxide powder (C10 supplied by Singleton Birch(n) Ltd) was tested for oil adsorption as per the method described above The oil adsorption value was 31wt%.

125g of this calcium oxide material was then placed in a Kenwoodvr" kitchen mixer. Oil (Safic Alcanvr" Alconplast(') PW1083C) was then added in a step-wise manner to granulate the material. By the time 35g of oil were added the material had formed damp granules. 18g of further calcium oxide (C10 supplied by Singleton Birch(Rml) Ltd) was added to give free-flowing granules.

Example 2:

43.75 kg of calcium oxide derived from a soft chalk and having an oil absorption value of 30wt% was added to an Eirich(RTNII R07 mixer. The mixer was operated at a medium drum speed with the impellor set to maximum. 12.5 kg of an inert mineral oil, Safic AlcanGn" Alconplast(') PW1083C, was added over three minutes followed by a further 3 minutes at the same speeds. The impellor speed was then reduced to 40% of maximum and 6.25 kg of additional calcium oxide powder as added with agitation for 30 seconds.

This formed spherical granules with a yield of 95wt% with a particle size between 2mm and 6mm.

The resulting granules could be dosed successfully out of automatic dosing equipment such as a Hayssen Sandiacre(') TG320-LD.

The minimum compaction pressure of the granules was tested as described. After testing, the granules were still free-flowing with no evidence of any caking. Hence their MCP was > 100 g/cm2.

In comparison, when the MCP test was performed on a commercially available extrudate of similar composition, Kezadolvr" GR, the extrudates formed on solid mass after the test.

The inventive granules had a ROA value of 7wt% when tested.

The commercially available granules had a ROA of less than 3wt% and formed a paste with the addition of minor amounts of oil When tested for performance in a production environment, the inventive granules worked as effectively as other granules or compositions with the same level of oil.

When the above process was repeated but using a calcium oxide material with an oil absorption value of 25wt%, the mixture over-granulated, despite changes in the process conditions, and formed large, soft granules and an excessive amount of make-up.

Claims

Claims Desiccant granules comprising: (a) calcium oxide, and (b) inert oil; wherein the ratio of (a) to (b) is from 3:1 to 6:1, wherein (a) +(b) is greater than 90wt% of the granule, and wherein the calcium oxide has an oil absorbency of from greater than 28wt% to less than 40wt% Granules according to claim 1, wherein the granules have a particle size distribution such that at least 90wt(14) of the granules have a particle size range between 1.0mm and 6.0mm.Granules according to any preceding claim, wherein the granules have a minimum compaction strength of greater than 100 g/cm2.Granules according to any preceding claim, wherein the ratio of (a) to (b) on the surface of the granules is greater than the ratio of (a) to (b) in the middle of the granules.Granules according to any preceding claim, wherein the granules have a residual oil absorption capacity of greater than 5.0wt%.Granules according to any preceding claim, wherein the granules are spherical Granules according to any preceding claim, wherein the inert oil is a hydrocarbon oil.Granules according to any of claims 1-6, wherein the inert oil is a silicone.9. Granules according to any preceding claim, wherein the granules are non-extrudates.10. Granules according to any preceding claim, wherein the granules are agglomerates.11 A process for the production of granules according to any preceding claim, wherein the process comprises the steps of: (a) obtaining calcium oxide powder having an oil absorption of from greater than 28wt% to less than 40wt%; (b) contacting the calcium oxide powder with an inert mineral oil under conditions of high agitation to form intermediate granules; (c) contacting the intermediate granules from step (b) with calcium oxide powder having an oil absorption value of from greater than 28wt% to less than 40wt% under conditions of low agitation to form granules according to any preceding claim; and (d) optionally, sieving the granules obtained in step (c) so as to remove particles greater than 6 Omm in diameter.12. A process according to claim 11, wherein the calcium oxide particles have a d5() particle size of less than 4.0p.m.