The feeder is mounted on the top of the hammer mill to obtain a uniform feed rate. It also functions as a gravity separator and iron separator. The aspiration air volume is controlled by means of a gate. For coarse control, the inlet gate can be opened continously, with precise proportioning being effected via a frequency-controlled corrugated roll. The mill offers excellent access for screen and hammer changes through two large air-purged doors. For safety reasons, the doors are interlocked with the rotor is still turning, and the mill cannot be started as long as the doors are open.