Fail-Stop Signature.

A drum mill intended for autogenous or semi-autogenous wet-grinding processes has a screening wall, which is arranged adjacent an end wall provided with a hollow material-discharge trunnion and which separates said end wall from the grinding space, whereat said screening wall has at least one first group of relatively small grate openings through which fine material ground in the mill constantly leaves the grinding space and departs through the discharge trunnion, and at least one second group of relatively large openings through which mixed material, comprising both fine and coarse material, can leave the grinding space. This second group of openings communicates with an individual material outlet via a switching means by which the material mixture passing through said second group of openings can be prevented from leaving the mill. The mill is also provided with classifying means for dividing said material mixture upstream of the outlet therefore into at least one coarse fraction and at least one finer fraction having a maximum particle size which is equal to or greater than the mazimum particle size of said fine material. To avoid disturbances in the grinding process, means are also provided for returning at least the major part of said finer fraction to the grinding space.