When the top size of a product is the crucial quality required then a Jet Mill with a dynamic classifier will generally be much more energy efficient. A dynamic classifier is a variable speed vaned wheel through which the air and product must pass on exiting a mill. The higher the  tip  speed  of

this wheel the finer the product passing through it will be. It is therefore driven by a variable speed motor controlled by a frequency inverter to allow fingertip control of the product particle size distribution (PSD). The physics of this process is governed by Stokes Law equation which relates the peripheral speed of the dynamic classifier (the resistance) to the radial approach

velocity of a given particle towards the centre of the classifier wheel. Particle density and air density are also parameters.

The air flow through the mill will decide the Radial Approach Velocity of the particle to the classifier. Increasing air flow increases Radial Approach Velocity (RAV), which will increase the particle size cut point of the product. Decreasing RAV makes the product finer. It follows that when a dynamic classifier is at the limit of its performance to get as fine a product as possible, a final trimming can be achieved by reducing the air flow and, as a result, the RAV.

The peripheral velocity of the classifier wheel increases or decreases the resistance to product flow through the mill in line with its rotational speed. Resistance from this speed-controlled vortex either rejects or allows through particles of given mass. If the mass is too high the particle is thrown out tangentially by the classifier, back to the milling zone.

As Mass is directly related to the diameter/dimensions of the particle it follows that, with any given dynamic classifier, the higher the density of the material, the finer the potential product from this classifier. Eg. limestone with a density of 2.7 g/cc may be produced to a fineness limit of d97 = 10 µm on a given dynamic classifier. On the same classifier, Silicon Metal, with a density of 6.3 g/cc, may be produced to a fineness of say d97 = 6 µm. The density used in this relationship is the Actual Density of the material and should not be confused with Bulk Density. Classification of needle or laminar crystals such  as

Wollastonite or talc is most efficient with a special form of classifier such as that developed by PMT Jetmill GmbH.

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