Abstract: At least two levels of mixing, one adjacent the top of the vessel and one adjacent the bottom of the vessel are provided and individually driven at appropriate speeds to produce a single flow pattern in the fluid in the vessel. The bottom mixing apparatus is preferably a single large diameter impeller whereas the top mixing apparatus includes a plurality of smaller diameter impellers positioned equally distant from and symmetrical about the lower impeller's vertical axis of rotation. Each of the impellers have a converging flow field exiting the impeller.

Abstract: Mixing systems for liquids having fibers suspended therein wherein the fibers tenaciously adhere to and accumulate on the leading edge of the impeller, thereby increasing the drag of the liquid on the impeller as it rotates and the consequent increase in the power required to rotate the impeller. Axial flow of the liquid in a tank is produced by the impeller within a draft tube. The leading edge of the blades of the impeller are inclined with respect to radial lines extending from the axis of rotation of the impeller beyond the angle of repose of the fibers on the leading edge. Also, the coefficient of friction of the portion of the impeller, extending from the leading edge toward the trailing edge thereof, to which the fibers adhere is reduced by providing the surface thereof with a low coefficient of friction material.