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Powder processing Frequently asked questions

1. Which materials can you process?

The most common question and the most difficult to answer as it can depend not only on the material but also the input size and shape, as well as the required output size distribution.

For micronising, as a rule, the more brittle, friable or crystalline a material, the more likely it is to process easily.

Most polymers, minerals, dried grains and chemicals can be milled, although a moisture, fat or oil content above 10–15% can mean that a material may either not mill easily or tend to (re)-agglomerate afterwards. The same characteristics can also affect the efficiency of classification (i.e. the sharpness of cut at the desired cut-point and/or the rate of throughput).

As well as the physical characteristics, for toll/contract processing at our facility we need to consider health and safety aspects. Generally, we can handle “st2” explosive materials (i.e. kst value up to 300 bar.m/s) and many toxic and hazardous materials.

Our opposed jet mills can process materials of any hardness, including those at the very top of the Mohs scale such as corundum and diamond, whilst our classifier mills can provide efficient and cost-effective particle size reduction to intermediate levels of hardness and on larger sized feedstock.

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2. What types of mills do you have at your contract processing facility?

British Rema’s processing facility operates a suite of commercial mills in addition to smaller-scale pilot plant that can be used for testing and small-scale processing of materials.

Our range of mills includes jet mills, impact mills and classifier mills which, between them, provide a comprehensive range of options.

We have equipment manufactured from stainless steel (which is suitable for pharmaceutical and food products, for example) and most of our plants are completely enclosed to eliminate the risk of cross-contamination and allow for the safe processing of hazardous materials.

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3. I am interested in using contract processing services, what information do you need from me?

Along with the details of the raw material and the level of processing you require, we require a safety data sheet (SDS or MSDS) so that we can conduct a risk assessment prior to materials being allowed on site.

We appreciate that many formulae are proprietary but without full disclosure we are unable to process a material for safety reasons. However, we are used to handling commercially sensitive materials and information and are willing to sign appropriate confidentiality agreements.

Occasionally, depending upon the nature of the material and the level of information already available, we may also require testing to be conducted to determine the exact explosive nature of a compound or component. If it’s necessary, we can arrange this for you.

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4. How fine or coarse can you process my material?

This depends on the type of mill being used and the physical and chemical characteristics of your feed material.

An air jet mill can typically be used to mill product to an output size from about 250 microns down to an average of only a few microns, with a relatively tight size distribution. A mechanical mill can provide either a broad or narrow distribution, depending on whether a classifier is used.

We will always conduct trials to ensure that we use the most effective processing method for your material.

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5. How do you test for particle size?

Testing for particle size is carried out in our on-site laboratory and is generally conducted using a highly accurate laser diffraction system, usually a Malvern Mastersizer. This is a measurement system commonly used in the powder industry which enables us to compare results with other quoted size distributions on a consistent basis.

The grading of particles can also be done using sieves. This is a more traditional technique and can be effective even in the most demanding of applications. Some of our customers prefer results from sieving, however, we find that the results can be unreliable below about 325 mesh (approximately 44 microns).

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6. Can you match a sample?

We are happy to take a sample and determine its particle size to determine its suitability for our processes. We can then advise whether or to what degree we can match it.

We can also work to a series of data points (such as d10, d50, d90) or a third-party particle size distribution, but working to a physical sample is our preferred option.

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7. Can you conduct trials?

Following a material assessment, we are happy to trial a product for you, whether it’s a single trial or a series of iterations or an extended development. Please note that the amount of work required for the set up and clean down of a machine, even for a small trial, can be quite considerable so such work would normally attract a charge.

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8. How much material do you need?

That’s dependent upon the type of work being conducted. For a pilot plant trial, we typically require about 1kg of material or 1 litre for less dense materials. For a production machine trial, the minimum amount required can be anything from 50kg upwards.

Whilst we appreciate that large trial volumes can be problematic for reasons of value or availability, customers often request information related to production rate (throughput), and larger trials on full scale production equipment will give a far better indication than smaller ones.

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9. What are your lead times?

Lead times depend on the levels of processing we are experiencing. For a pilot plant trial, we would generally aim to turnaround less complex processing within a few days. Major trials on a commercial plant need to fit in with pre-scheduled production so lead times can be up to a few weeks.

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10. What types of milling do you offer?

We have a variety of mills on-site which allow us to offer air jet milling and mechanical milling techniques.

Our suite of mills includes opposed jet mills, hammer mills, rotary impact mills and classifier mills.

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11. Which milling technique is the most appropriate for my needs?

That very much depends upon your material and your requirements.

Highly amorphous materials tend to mill better with mechanical milling whereas semi-crystalline and crystalline materials lend themselves better to air jet milling.

Sometimes the most appropriate way to process a material could be through two machines, such as rotary impact milling followed by air jet milling. For more information on this please see this article on Mill selection.

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12. What about contamination and FOD?

With major customers in markets such as aerospace and medical implants, quality generally, and control of FOD specifically, is a very high area of focus. We have standard operating procedures relating to FOD (compliant, for example, with NAS 412 for the aerospace industry), use plant in isolated bays, and test internal air quality regularly.

In terms of contamination from plant (e.g. metal pick-up from wear) air jet milling inherently has the lowest risk of FOD, since it functions through particle to particle collision, rather than through attrition with a mill part.

The mills themselves are sealed systems, and all air used within them is pre-treated. Handling of materials is kept to a minimum using pneumatic feed and discharge systems.

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13. What is classification?

Classification means the separation of particles according to particle size in a continuous process. This can, of course, be achieved by sieving (screening), particularly for particle sizes that are not too small, but the term is normally used to refer to systems using ‘air classification’ technology. To separate an existing input material into its constituent parts based on size, a stand-alone air classifier unit would be used, but the technology can also be integrated into a mill such that material stays in the mill until it reaches the desired size.

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14. How does air classification work?

Air classification works by drawing the solid particles, entrained in an air stream, through a rapidly rotating slotted wheel. The various forces acting on an individual particle in this environment allows lighter particles to pass through, whilst rejecting heavier particles. In a homogeneous, single product powder, particle mass is proportional to particle size, so the effect of the classifier is to separate by size. All else equal, the faster the wheel rotates the smaller a particle needs to be to pass through.

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15. How fast can you produce the product?

The rate at which a material can be processed depends on a range of criteria including the nature of the material, the machine, the specification of the output and the feed size. Throughput can range between 1kg and 1,000kg per hour on a commercial scale machine.

Approximate production rates can be assessed through trials on small machines or by short runs, but a true indication is best obtained by processing a commercial scale run on the appropriate machine.

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16. What about packaging?

We can pack off into a range of container types and sizes, from steel drums and paper sacks to FIBCs. Often our customers supply their own packaging, but we are more than happy to source packaging if required.

Fine powders are susceptible to static which can be particularly problematic for subsequent handling and which sometimes increases explosion risk. We are familiar with sourcing specialist packaging suitable for these applications, as well as sourcing and arranging UN certification for the transportation of hazardous materials.

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17. Can you mill mixed components at the same time?

Yes, however, if there is a large disparity in the materials size or density, some localised enrichment can take place, which may be beneficial or detrimental depending upon your requirements.

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18. Can you blend milled materials?

Yes. We can mill and blend some or all your components and are able to advise on the best way to achieve the blend required.

Determination of the homogeneity of a blend is usually determined by our customers, using their own preferred techniques.

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19. Can you blend materials with different sizes and/or densities?

Yes, although the greater the disparity the more unpredictable the results can be. We would strongly recommend a trial.

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20. Do you offer blending trials?

Yes. Subject to a health and safety assessment of the materials, we can conduct trials using the machines we have on site. We have a 30-litre double cone, a 30-litre V-cone, a 50-litre ribbon and a 500-litre double cone blender.

We can mix and blend dry powders or products with a very low liquid content.

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21. What kinds of certification can you offer?

We can offer certificates of analysis, certificates of conformity and can complete customers’ supplied documentation and traceability records as required.

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22. Do you hold any accreditations?

We operate a quality system accredited to ISO 9001:2015.

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