Abstract: Apparatus for mixing materials, including a fluid, in a vessel, which apparatus includes a hollow elongated shaft mounted so as to be driven at one end for rotation about its axis of elongation within the vessel, the other end of the shaft in the vessel being open. A planar baffle plate extends across the vessel intermediate the ends of and transversely to the shaft so as to divide the vessel's interior into two zones joined by the fluid pervious clearance around the edge of the plate. The open end of the shaft is located in one of the zones; the walls of that portion of the shaft located in the second of the zones are perforated to permit fluid flow between the second zone and the shaft interior. A turbine impeller is also mounted on the shaft adjacent the open end of the latter so as to be rotatable with the shaft for creating a relatively low pressure in the volume of materials and fluid disposed immediately adjacent the open end of the shaft.