Abstract: A mixing device for the treatment of viscous substances is described, which mixing device comprises a heatable container 1 with product inlet 5 on the one side of the container and product outlet 6 on the other side of the container, and where appropriate a vapor connection 7, which mixing device comprises a plurality of shafts 2,2' rotating in the same direction. Annular discs 4 are fitted to a shaft 2 by means of spokes 3; in this case, the annular discs 4 are corrugated in the axial direction, and annular discs 4 and spokes 3, together with the shaft 2 carrying them, kinematically clean the annular discs 4', spokes 3' and the adjacent shaft 2' carrying the latter. At all times, the housing 1 is kinematically cleaned by the annular discs 4,4' and spokes 3,3' in the course of rotation of the shafts 2,2'.