Screw Mixers

Screw mixers use a rotating screw that progresses around the periphery of a conical hopper. The screw lifts solids from the bottom of the hopper to the top, where the mixture flows by gravity back into the screw. Mixing occurs around the open screw, where the solids transported by the screw exit at various levels and are replaced by other solids at that level. The screw’s shearing action also intimately mixes the various components. Gross mixing action also occurs within the mixture by the velocity profile created in the conical hopper as it feeds the screw. This gross mixing action is most effective when the solids move along the conical hopper walls.
This requires a hopper index (HI)* less than the conical hopper angle measured from the vertical. If there is no flow at the hopper walls, a complete progression around the cone will be required to incorporate all the solids in the mixing action. Consequently, a screw mixer hopper without flow along the walls tends to mix much more slowly than mixer hoppers with flow along the walls.

Demixing possibilities include sifting and angle-of-repose demixing. Sifting produces a high concentration of coarse particles in the last discharge. A screw’s high rpm may propel superfines into the air, allowing them to resettle on the screw drive and on the top of the mixer. Additionally, using an extremely fluidizable component as the major component can cause a concentration of fines to discharge last. Angle-of-repose demixing will concentrate a low angle-of-repose solid away from the screw. Solids that do not have an HI greater than the typical cone angle of 17 degrees from the vertical will cause the lower angle-of-repose, easier-flowing solids to discharge from the mixer first.

Solids appropriate for a screw mixer include those with moderately cohesive fines, those that are not prone to angle-of-repose segregation and those that require a moderate amount of liquid addition.