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Milling and separating powders

Milling of powder has been around for centuries and is a process integral to the manufacture of a number of foods and food ingredients.

The addition of a Dynamic Air classifier in a milling circuit has now made out-of-date milling processes into state-of-the-art systems.

A classifier utilises the high g forces in a rotating wheel to separate the coarse particles from the fine.

It also allows the manufacture of powders with narrow particle distributions as well as finer powders which demand a higher added value.

These powders are in the 10µm to 600µm size range.

This need has occurred due to the new nanotechnologies of powders, which enhance performance.

It all starts with high performance milling (size reduction) being more demanding.

Finer powders or narrow particle distributions are now demanded.

New Zealand company CAS Enterprises has designed and built Air Swept Mills and a high-performance Dynamic Classifier to meet these requirements.

It ensures consistent quality, a more practical system and value for money.

The first principle is the CasMill which has a static circular housing using a specialised rotational beater plate.

  • The rotating beater creates airflow to pneumatically convey the product into the milling chamber.
  • The beater then imparts a force on the product to fracture the particles into smaller particles in the milling (or grinding) zone.

The second principle is a Dynamic Air Classifier, which is less known for powder processing.

This is used to separate the coarse particles from fine particles.

It separates only, there is no milling involved.

  • The airflow induced in the machine sweeps the milling chamber with product further breaking down the particles and reducing their size.
  • Once the product particles have reached the end of the milling chamber they are pneumatically conveyed into a dust collec- tor as finished product or passed through an optional dynamic air classifier to sepa- rate the particles for further milling.
  • The Dynamic Air Classifier is driven by an independent drive motor but forms an integral part of the complete mill.
  • Specialised beater plates are available in sizes 250mm, 500mm, and 750mm diameter.

While the primary function of an air swept mill is to reduce particle size, additional functions can be incorporated.

Applications

The CasMill is suitable for fine or ultra-fine milling of semi-hard, crystalline, brittle, fibrous and oily materials with a hardness of up to 5 on Mohs Hardness Scale.

Typical maximum feed stock is 20mm.

Minimum finished particle size is 10µm depending on the product.

Materials suitable include casein, cereals and pulses, wheat, dried products, freeze-dried products, lactose, milk powders and sugar.

Benefits

If a mill/pulveriser is in an existing circuit a CAS Dynamic Classifier can be added and will ensure the material is not overground or that unwanted fine powders are not produced.

It also reduces the power requirements of the mill.

If a narrow band of particle size is required the ‘cut point’ of the separation can be set and adjusted by simply changing the speed of the classifier.

High throughputs are achievable.

The classifier produces ultra fine particles down to 3µ subject to type and specific gravity of the product.

There is no messy or slack wire mesh to contaminate the powders.

An opposing pneumatic conveying air stream is utilised, which separates particles from the centrifugal force in the wheel.

The rejected or coarse particles are discharged and will not be allowed to get through the classifying wheel, which results in two size fractions.

The ‘cut point’ at which the particles are divided can be easily changed by altering speed or air flow.

This means that both the oversized and undersized particles can be removed if two passes are run.

Air flow is balanced with the opposing forces making the particles ‘float’ through the wheel.

This minimises the effect of wear or attrition on the wheel.

Complete systems include a powder feeder, classifier, dust collector and control system, allowing the production of powders to order.

www.cas.co.nz

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