An impact mill equipped with transmission machinery (10) to rotate two coaxial rotors (16, 18) of a rotor apparatus (12) by one single driving apparatus (14). The transmission device includes, according to one alternative, a secondary shaft (32; 36b) equipped with a gear pair (28, 30) or a conic gear (29). The first gear (28) rotates a gear (24) at the shaft (20) of the first rotor (16), and the second gear (30) rotates a gear (26) at the shaft (22) of the second rotor (18), but in the opposite direction or at a different speed. The conic gear (29) rotates the gear (24a) at the shaft (20) of the first rotor (16) in one direction, and the gear (26a) at the shaft (22) of the second rotor (18) in the opposite direction.