On both sides of the conveyor belt (10) there are e.g. triangular cloth walls (16). Each has one edge (17) fastened to the conveyor belt frame and an adjacent edge (18) connected to the mixing container (1). The free edge (19) has eyelets (20) through which an elastic tensioning cord (21) is run. In a slight variant, the cloth shape is trapezoidal. It has an approximately central, diagonal support, i.e. a lever swinging out at an angle on a pivot near the container base. The lever is retained in this position by a locking device.
