Pin, air classifying and turbo mills

A pin mill comprises two discs, one rotating and one stationary which are fitted with intermeshing pins set in a concentric pattern. The charge is fed into the centre of the discs and is broken down as it moves outwards through the pins which are moving at very high speed - up to 20,000 rpm The air classifying mill is similar in construction to the pin mill but incorporates a built-in classifier. This type of mill produces a significant airflow through the machine to aid with keeping temperature as low as possible. Oversize grains which pass through the mill have to be recycled. Turbo mills use a similar concept but the rotating disc is fitted with paddles or bars rather than pins. This rotating disc sits within a cage which is fitted with grids, screens or breaker plates. The mill is configured in such a way as to produce the desired particle size.

Pin mills are capable of very fine grinding without the need for screens and provide a uniform product size. Air classifying mills are used where the product is temperature sensitive. They are widely used in the pharmaceutical and fine chemical industries. They are suitable for relatively soft materials (below Moh 3) and for small quantities of material. Wear on the pins is significant if used continuously.