Apparatus (1) for breaking up and mixing blocks of fibrous material comprises a container (2). A mixing rotor comprising a shaft (32) and mixing blades (37) carried on carrier members (36) extending radially from the shaft (32) is rotatable in the container (2) for mixing the fibrous material. Four stationary blades (44) cooperating with the mixing rotor (30) for breaking up the blocks of fibrous material extend from a side wall (15) of the container (2) transversely of and above the mixing rotor (30). As the mixing rotor (30) rotates in the direction of the arrow A, the mixing blades (37) sequentially engage portions of the block of fibrous material, which portions are in turn separated into smaller portions by the mixing blades (44) as the mixing blades (37) urge the fibrous material against the mixing blades (44). Further rotation of the mixing rotor (30) rips the portions of fibrous material from the block of fibrous material where it is supported by the stationary blades (44).