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