Page 222 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 193

FIGURE 7.23 Vertical hammermill (Pfeffer, J.T., Solid Waste Management Engineering, Prentice Hall,
Englewood Cliffs, NJ, 1992). Reproduced with kind permission of Pearson Education, Inc., Upper Saddle
River, NJ

If the input material is difficult to break, for example a chunk of steel, the continued impact of
the hammers will impart a centrifugal motion that will eventually direct the difficult item to an ejec-
tion portal. Such items will normally be ejected rather than remain in the mill and damage the ham-
mers (Pfeffer, 1992).
The primary factors affecting particle size in the vertical-shaft hammermill include retention
time in the mill and the number of impacts by the hammer. The clearance between hammers and
wall of the housing in both the upper and lower portions of the machine regulates passage through
the mill. By changing the number and position of the hammers, the particle size is changed.
An advantage of the vertical shaft hammermill is the ability to achieve a high degree of size
reduction. Energy consumption per ton of waste processed is less for the vertical shaft hammermill
than for the horizontal-shaft model. Primary disadvantages include high-energy costs and high
maintenance. Operational problems encountered with the vertical shaft unit may include internal
jamming and explosions.
A common waste processing problem when using a hammermill is contamination of organic mate-
rials with inorganics. For example, paper wastes are impregnated with shards of glass when the high-
speed hammer shatters glass containers. The best course to pursue, therefore, is separation of potential
contaminants before they enter the mill. Screening and hand-sorting are appropriate in this case.

7.4.3.8 Flail Mill

The flail mill (Figure 7.20c) is similar to the hammermill in that a rotary shaft is secured with a
number of rotating appendages; however this provides only coarse shredding since the hammers are