Abstract: An improved mixing apparatus involves a vessel having an interior surface in the shape of at least two intersecting conical frustums with axes parallel and substantially vertical. At least two interengaging helical screw elements are rotatably mounted within the vessel such that when co-rotated they interengage along their length and also conform to the interior surface of the vessel to effect a complete cleaning of each other and of the interior surface of the vessel. The bottom portion of the screw elements form a pressure generating zone and the upper portion of the screw elements form a mixing zone having a hollow center described by the co-rotating screw elements. In the mixing zone each screw element includes a continuous transition from a multilobal cross-section, e.g., trilobal cross-section (bounded by three equal arcs), to a circular cross-section. The circular cross-sections of the screw elements in the upper part of the mixing zone, i.e.