Abstract: According to the invention a truncated conical disc is provided with inward and outward flanges carrying projections and a plurality of curved bars or ribs located so that the projection thereof onto a plane perpendicular to the axis of spinning is a plurality of converging spirals. The height of each rib at a section through the normal to the truncated conical surface of the disc and the plate is between 0.2 and 0.5 the height of the projections. As reviewed in section, the edge of each rib, meets head-on the flow of the liquid clarified, and makes an angle approximately 90 deg. with the truncated conical surface of the disc.