Mixing Machines

MIXING MACHINES

Trough, Ribbon, and Paddle Mixers

A simple trough mixer consists of a semicircular trough in which an impeller, such as a number of paddles mounted at diverse angles on a shaft running the length of the trough, rotates, lifting and distributing the material in an irregular manner. Convective and shear mixing occur. Some fine-scale diffusive mixing occurs when the impeller lifts material clear of the main charge.

The ribbon mixer employs a ribbon-like conveying scroll. The helix, which may be continuous or interrupted, is rotated in a semicircular trough, and mixing again occurs through convection and shear, giving rapid coarse-scale dispersion. Two ribbons set to convey material in opposite directions are fre-quently fitted to the shaft. Although little axial mixing in the vicinity of the shaft occurs, mixtures with high homogeneity can be produced by prolonged mixing even when components differ in particle size, shape, or density or there is some tendency to aggregate.

Tumbler Mixers

Tumbler mixers operate by a mainly diffusive mechanism, and their use is confined to free-flowing and granular materials. The mild forces employed, which preclude the mixing of materials that aggregate strongly, allows friable materials to be handled satisfactorily. The more elaborate geometrical forms are most commonly used because movement of material in all planes, which is necessary for rapid overall mixing, is induced. Internal baffles and lifter blades may also be incorporated. For example, axial movement of material along the length of a simple drum mixer is slow and can be enhanced by these methods.

Mullers and Impact Millers

The function of machines of this type in mixing was discussed earlier in the chapter.