An apparatus (1) for mechanical processing of materials (7) in the dry state is of the type comprising a casing (2), within which are set a plurality of rollers (3) and at least one diffuser and/or contrast element (4) for said plurality of rollers. The casing likewise has at least one input section (5) and at least one output section (6a, 6b) for the materials. The apparatus is characterized in that the rollers (3a, 3b) are arranged around the diffuser and/or contrast element so as to define, for each roller, a gap (d) between the diffuser and/or contrast element and the side surface of the roller, and in that it comprises means for setting in rotation the diffuser and/or contrast element with respect to the plurality of rollers and/or for setting in rotation at least one roller, or the entire plurality of rollers, with respect to the aforesaid diffuser and/or contrast element.