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Process equipment Frequently asked questions


1. What is particle size reduction and how does British Rema achieve this?

Particle size reduction is a mechanism by which particles are physically reduced in size. It is also commonly known as milling, grinding and micronising.

In nearly every case this is achieved by mechanical impact and attrition. In British Rema’s air jet mills this is achieved by particle-particle interaction. In our mechanical mills, depending on the specific type of mill, it is achieved via collision of the particle into another object having a different velocity. This ‘other object’ can be the internal wall of the mill, a metal stator, a target, or, in the case of a hammer mill, a beater.

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2. Can you provide a machine to mill my material to a certain particle size?

The broad answer to is yes, we can. Our range of milling equipment includes opposed air jet mills, classifier mills and larger scale equipment such as ball mills.

Typically, a minimum average particle size of 2 microns would be the practical minimum. Our equipment is not designed to produce output at the nanoscale.

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3. What type of machine would be best suited to milling my material to a certain particle size?

It is difficult to generalise but the smaller the output size you require the more likely it is that an air jet mill will be the most appropriate solution. For a slightly larger product or a coarser feedstock a rotary impact mill, with or without a classifier, could be more appropriate.

We are happy to discuss your requirements to identify the optimum equipment or system for your application.

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4. What is a jet mill?

Jet mills are also referred to as micronisers, spiral jet mills, or opposed jet mills.

All mills achieve particle size reduction through mechanical impact. The energy provided for this impact is invariably kinetic; supplied by a blade, wheel, or compressed air. In a jet mill the energy is supplied through compressed air, accelerated to subsonic or even supersonic velocities.

In an opposed jet mill the jets are equally distributed around the circumference of the mill and inject air into a bed of material at sonic velocities via Laval nozzles forming a fluidised bed of material being milled. Particles caught within the jets of air are accelerated and collide with other particles either stationary within the bed or particles accelerated in the opposing direction by the opposing jets. A dynamic classifier wheel is mounted above the milling chamber controlling release of the particles which have been milled down to the desired particle size.

In a spiral flow jet mill these jets are set tangentially to provide energy in a rotary direction around the perimeter of the mill. Larger particles requiring the most milling energy are then concentrate towards the outside of the chamber where the jets enter, ensuring impact with both other particles and the periphery of the mill’s surface. This tangential inlet sets up a vortex action where finer particles migrate to the centre for release from the mill. Spiral jet mills are often used to de-agglomerate in industries such as pharmaceuticals as well as genuinely milling the product.

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5. When should I choose an opposed jet mill rather than a rotary classifier mill?

Rotary classifier mills can achieve smaller particle sizes in comparison to many other milling systems by virtue of the integrated classifier which returns coarse material to the milling chamber. However, as particle hardness increases, wear rates of the milling rotor and stator components also increase.

In an opposed jet mill, the milling process involves particle-particle collisions in a compressed air stream, rather than particle-rotor or particle-stator collisions. These jet mills also have an integral classifier, allowing narrower particle size distributions to be achieved. Opposed jet mills are useful either for very hard, abrasive materials or for temperature-sensitive materials as the product is continually cooled by a compressed air stream.

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6. I am interested in a continuous ball mill; which type of mill should I choose: grate discharge or screen discharge?

The choice between these types of mill generally comes down to the desired particle size.

A screen discharge mill lends itself well to coarse grinding, with minimum particle size typically above 300 microns. The final particle size is controlled by the discharge screen aperture. In contrast, the particle size achieved in a grate discharge mill is, to some extent, influenced by the design of the mill (length to diameter ratio and media charge). However, grate discharge mills lend themselves well to closed-circuit milling processes in which oversized particles are returned to the mill for further grinding, typically via a classifier system. In contrast, the powder exiting a screen discharge mill tends to be the final product, exiting into an integral discharge hopper.

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7. What size machine would I need to mill my material to a certain particle size at a given production rate?

This is one of the most common questions we are asked but it is very difficult to answer directly as the answer depends on many factors and may require an element of compromise. At British Rema we appreciate that a customer requires the most exacting of products, produced at the most cost-effective rate, and we are happy to work with you to determine an appropriate strategy.

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8. What is particle size control and how can it be achieved?

British Rema uses the term particle size control to mean the method by which the desired fraction of a material is obtained. Particle size control can be achieved using a classifier or a sieve. In these cases, particle size control is commonly referred to classification or sieving.

Without any form of particle size control, the product achieved would invariably consist of a range of particle sizes, from the feedstock size down to the smallest nanoparticles. The aim of particle size control is to obtain a predetermined particle size range.

This can be achieved in two ways: sieving following milling can be effective but is often problematic and there are lower limits to the particle sizes that can successfully be differentiated through sieves. In general, air classification is a far more effective technique.

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9. What is air classification?

Air classification is the technology by which particles are separated predominantly according to size in a continuous process. When a classifier is attached to a mill, the classifier ensures material stays in the mill until it reaches the desired size. Air classification works by means of a rapidly rotating slotted wheel that allows finer particles to pass through, whilst rejecting coarser particles. The faster the wheel rotates the smaller a particle needs to be to pass through. A useful, but ultimately incorrect, analogy is that the faster the wheel rotates, the smaller the gaps effectively are.

The actual particle size segregated depends upon mass and density, in addition to diameter: a concept is known as ‘aerodynamic diameter’.


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10. What can classification of my powders help me achieve?

Air classification is an excellent method for separating powders into coarse and fine streams to,

  • achieve a consistent material
  • guarantee product purity
  • ensure product performance
  • separate a waste stream

Some examples of applications which benefit from classification are pigments, silica mixed with rubber for the print industry, segregating proteins and starches for food supplements, avoiding nozzle-blocking problems in spray coating, metal powders for 3D printing, and the separation of waste streams in recycling operations.

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11. What is the difference between a dynamic air classifier and a static classifier?

All classifiers are used to separate coarse and fine particles from a feedstock.

A dynamic air classifier, as supplied by British Rema, has an in-built slotted classifier rotor, which rotates at high speed. Particles are transported in an air stream. Particles which are sufficiently fine (or of sufficiently low density) will pass through the classifier wheel and exit via the fines discharge. Coarser particles cannot pass through the classifier wheel and so they exit via the coarse discharge. The classification point (or cut-point) can be controlled by variation of the classifier wheel speed and airflow. In the case of a static classifier, particles are carried within an air stream, but this type of equipment tends to have static vanes and does not have a rotating classifier wheel.  In general, finer particle sizes are achieved a dynamic air classifier.

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12. Is there a difference between mixing and blending?

The terms ‘mixing’ and ‘blending’ tend to be used interchangeably and this is quite normal, often depending on the industry.

Technically, mixing involves the process of combining different materials to produce a homogeneous product; whereas blending tends to be a gentler process, which can also involve combining dissimilar materials, occurring over relatively short distances within the bulk material. In fact, the distinction between the two processes is not always clear and, therefore, British Rema refers to ‘mixers and blenders’ or ‘mixing and blending’ rather than differentiating between the two.

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13. What types of blender does British Rema offer and which one is right for my application?

British Rema offers blenders in various geometries including single and double cone, V-cone and ribbon, with volumes from only a few litres to thousands of litres.

A blender’s size or capacity refers to its physical size and not the volume it will blend in a single batch. Determining which is the right blender for your application depends on the specific nature of the materials to be blended, the maximum bulk density, the volumes involved and the desired result.

There are some other key things to consider in determining the right blending solution, for example: How much room/floor space is available? Is gentle blending required? Is complete discharge of the product needed? Is the product heat-sensitive? What is the volume of the batch to be blended?

Our Mixing and Blending Test Centre is available for customers to take advantage of our expertise and conduct trials on a variety of mixers and blenders. We can help with identifying the right blender type and also process parameters, including optimum fill ratios, methods of loading, blend times and rotational velocity – all of which impact on achieving optimum blending and the optimum amount of additive components.

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14. Can I use British Rema’s Mixing and Blending Test Centre?

Yes, our in-house Mixing and Blending Test Centre is available for customers to take advantage of our expertise and conduct small- to medium- scale trials on a variety of mixers and blenders. We can help with identifying the right blender type and also process parameters, including optimum fill ratios, methods of loading, blend times and rotational velocity – all of which impact on achieving optimum blending and the optimum amount of additive components.

Our specialist facility is headed up by Dr Jose Perdomo who has years of experience in the field of mixing and blending. We can handle a wide variety of powered materials for test blending including chemicals, minerals, metal powders, plastics and polymers, plus food and pharmaceuticals. We will ask for MSDS(s) before accepting materials in our processing facility.

Whether you are looking for a series of trials or one-off we will be happy to work with you to help you ensure the right solution. If you think you can benefit from using our facilities please contact us.

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15. When should a V-cone blender be chosen rather than a double cone blender?

It depends upon the difficulty associated with achieving homogeneity within the material. Traditionally, double cone blenders are used for ‘easy’ to combine materials, like sugar and salt, with equivalent particle sizes. V-cone blenders tend to be used when it is a little more difficult to achieve a desirable result, such as when a free-flowing powder is to be combined with a less free-flowing or sticky material. In a V-cone blender, the material is continually divided into two, then re-combined, with mixing taking place as material flows across the walls of the ‘V’ as the cone rotates.

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16. When should a ribbon blender be chosen in preference to a tumbling action blender?

A ribbon blender imparts far more shear into the powder during the blending process. This type of blender can be more effective than a gentler, tumbling action blender when two or more materials which are ‘difficult’ to combine are to be processed. Ribbon blenders can also be used when a small quantity of liquid is to be combined with a powder.

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17. How does British Rema measure particle size distribution? (What is it?)

Particle size distribution (PSD) is measured in several ways. The PSD of coarser materials tends to be measured by physical means such as through sieves. The PSD of smaller particle sizes tends to be determined by other techniques such as laser diffraction, which uses highly defined light sources from lasers to ‘measure’ the particles.

British Rema’s own laboratory has a variety of equipment to measure PSD including a full range of sieves and two laser diffraction machines: a Malvern 2000 and a Coulter LS230.

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18. What do you need to know from me when I make an enquiry about processing equipment?

We welcome the opportunity to discuss your requirements and will always try to help.

To progress your enquiry, we will need to know about your material, its chemical and physical properties. When a trial may be involved, we will ask you for the material safety data sheet (MSDS or SDS). We appreciate this information can be commercially sensitive, and we are happy to enter into a non-disclosure agreements (NDA).

We will also need to know about the feedstock, including its physical size, moisture content and any incompatibilities; your requirements and expectations for the product, for example, particle size distribution, production rate and any physical constraints applicable to the machinery installation, for example, lack of physical space or available headroom. We have an application check sheet which we ask you to fill out, which ensures we capture as much relevant information as possible.

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19. How do I know the machine you are proposing will be suitable for my application?

We have a wealth of experience in this field and have been supplying mills and classifiers for over 90 years. Our powder processing division, operates more than a dozen machines, working on a contract/toll processing basis with an array of materials for exacting applications such as aerospace and medical. We can offer equipment trials to demonstrate the suitability of the technique and equipment being proposed.

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20. Do you offer equipment 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.

We work in conjunction with our powder processing division, to provide trials at our contract processing facilities based in Chesterfield, UK. We operate more than a dozen machines of various formats and sizes and this equipment is available for trials to establish how easily a material will process and which settings are required to meet the required specification.

The smallest trial requires about a kilo of material, and can provide the most basic of information such as: Will it mill? How easily? What settings are required to match a sample?

At this scale we cannot determine how any tonnes an hour a machine 5 or 10 times the size will produce so larger scale trials may be required which require greater masses of material – typically, from tens or hundreds of kilos.

Before any trial can go ahead, a full health and safety assessment will be conducted. To protect IP we also adhere to strict rules regarding attendance at trials.

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21. Can you meet a particle size distribution specification?

Yes, we can. A customer’s specification may be as simple as ‘everything less than 50 microns’ or ‘a d50 10 microns’, and, in such cases, we have no doubts we can meet your requirements. On occasions when a customer requires an exact match to a third-party particle size distribution, we will ask for a sample of material to be provided for us to match against. We prefer to match an existing sample since just changing the feedstock size or moisture content can affect the produced particle size from a mill supplied by any company.

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22. Can you manufacture equipment for use in the food and pharmaceutical industries?

Yes, we can. We tailor the design of our equipment and systems to meet the specific requirements of any industry.

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23. Can British Rema help me if I need a turnkey solution?

Yes, we can. We offer provide partial and complete solutions.

British Rema’s mills and classifiers are stand-alone pieces of equipment which require ancillary equipment to function effectively. As most of our mills and classifier require an air stream pull through to transport the powder particles through the equipment, it is likely you will need to consider fans, filters, valves, feeders and control systems.

We can integrate a machine into an existing system or physical space, or we can provide a complete bespoke solution, including the project managing of all the other components from concept to commissioning.

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24. How can I improve the wear resistance of my processing equipment?

Materials of construction are very much dependent on a customer’s requirements.

Our standard equipment is manufactured from mild steel and the classifier blades from tool steel but various grades of stainless or high-wear steels can be used.

For example, you may be milling a particularly hard material, or you may require a construction material that offers reduced potential contamination such as stainless steel. Often applications require special surface treatments or mills partially- or fully-lined with ceramics, rubber or other protective materials to prolong wear resistance and help to avoid product contamination.

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25. What is ATEX and does it apply to me?

ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres, its name derived from the French: ATmospheres EXplosives. For example, Directive 99/92/EC (also known as ‘ATEX 137’ or the ‘ATEX Workplace Directive’) outlines the minimum requirements for improving the health and safety protection of workers potentially at risk from explosive atmospheres. The other: directive 2014/34/EU (previously 94/9/EC also known as ‘ATEX 95’ or ‘the ATEX Equipment Directive’) provides for approximation of the laws of Members States concerning equipment and protective systems intended for use in potentially explosive atmospheres.

In Great Britain the requirements of Directive 99/92/EC were put into effect through regulations 7 and 11 of DSEAR (Dangerous Substances and Explosive Atmospheres Regulations 2002). The requirements in DSEAR apply to most workplaces where a potentially explosive atmosphere may occur, as is the case with many finely divided powders. Some industry sectors and work activities are exempted because there is other legislation which fulfils the necessary requirements.

The onus for establishing the potential explosion risk and specifying ATEX zoning requirements of the equipment to operate safely with the material are part of ATEX137 is the legal responsibility of the end user of the equipment. Guidance on methodology of establishing appropriate zoning is set out in standard EN 60079-10-2:2015 “Explosive atmospheres. Classification of areas. Explosive dust atmospheres”.

Once the zone requirements are specified, it is the legal responsibility of the equipment manufacturer to supply equipment which is certified for use in the appropriate area. For non electrical ATEX category 1 equipment (zone 20) an independent notified body must be involved with the certification process. For category 2 (zone 21) the manufacturer’s technical file must be lodged with a notified body. For category 3 (zone 22) this may be self certified by the manufacturer.

Our team can advise when ATEX certification is likely to be required and what impact this will have on any of the relevant aspects of your equipment build.

If you are handling a potentially explosive material, then you will require ATEX approved equipment. It should be noted that the particle size distribution and concentration of particles forming a dust cloud has a great influence on how potentially explosive the atmosphere is. High concentrations of very fine particle size are much more explosive than coarser particle sizes, as such it is normal to have explosion indices testing carried out by an accredited laboratory on the specific powder formulation in question prior to equipment build.

Usually our milling and classification equipment, when used to mill or classify a potentially explosive product, will contain a potentially explosive atmosphere continuously internally within the machine (zone 20). If the mill or classifier is integrated into a full production line where the product entering and leaving the system is enclosed, then externally an explosive atmosphere is not likely to occur in normal operation, but if it does occur, will persist for a short period only (zone 22). In some cases where a more manual product discharge or filling scheme is used, which is not fully enclosed, then externally an explosive atmosphere is likely to occur in normal operation occasionally (zone 21).

For large bespoke plants, these may sometimes be considered ‘installations’ rather than ‘equipment’, in these cases it is outside the scope of Directive 2014/34/E. In these cases the end user takes responsibility for the risk assessment and certification for the plant installation.

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26. Are your machines CE marked?

CE marking is a certification mark that indicates conformity with health, safety, and environmental protection standards for products sold within the European Economic Area (EEA).

CE marking of machinery indicates that the machine complies with the requirements of the Machinery Directive. This is mandatory for machines sold within the European Economic Area (EEA). This lists various safety requirements through the whole life of the equipment from transporting, installation, operation, maintenance and decommissioning. As a manufacturer of equipment we need to carry out a comprehensive risk assessment and conformity assessment to demonstrate how the various risks identified with the use of the equipment in relation to the essential H&S requirements mandated have been addressed. Ideally via elimination through design or if not feasible to design out to give adequate instruction on the accompanying equipment or PPE, measures which must be adopted before putting the equipment into service. 

With process equipment of the type British Rema supplies, we often effectively only supply part of the overall machine. For example the control system may be by another party or it may discharge directly into another piece of machinery supplied by another party. In these cases we do not have control over all elements influencing the risk profile and compliance with the essential health and safety (EHSR) requirements of the directive. In such cases we will supply with a declaration of incorporation, setting out which EHSR’s are fulfilled and it is then the end user or system integrators role to ensure that the partly completed machinery must not be put into service until the final machinery into which it is to be incorporated has been declared in conformity with the provisions of the directive. So for example ESHR 1.2.6 “Failure of Power Supply – Has machine been designed so that an interruption, the re-establishment after an interruption or the fluctuation of power supply to the machinery does not lead to dangerous situations?”; if British Rema are not supplying the control system, we could not ensure that this had been designed such that the machine could not restart after power failure without a conscious operator intervention through for example pressing a start button.

For large bespoke plants, often with a large element of in-situ manufacture, these may sometimes be considered ‘installations’ rather than ‘machinery’. These will normally be risk assessed and CE marked in-situ by the end user or system integrator. It is important in these cases that exact responsibilities are established and agreed in the contract prior to manufacture, as establishing compliance with EHSR’s after the event may be much more difficult and costly.

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