Abstract: A mixer comprising a conical mixing vat, tapering downward toward a material egress, and a motor-driven mixing worm which rotates on its axis and revolves within the mixing vat, the mixing worm having an upper region with a small screw pitch and, a lower region with a large pitch and a neutral conveyance zone between the upper and lower regions. In order to achieve a stable pattern of the junction formed by the neutral conveyance zone of the mixing worm between the upper and lower regions, which will be independent of the degree to which the mixer is filled, a mixing worm rotates around another mixing worm which is placed centrally in the upper region of the mixing vat along its longitudinal axis, this worm being of cylindrical shape along its length and extending into the conveyance neutral zone and with its exterior diameter nearly touching the other mixing worm.