Page 220 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 191

FIGURE 7.20 Three major types of shredders: (a) hammermill; (b) rotary shear (Pfeffer, 1992); and (c) flail mill
(Pfeffer, J.T., Soild Waste Management Engineering. Prentice Hall, Englewood Cliffs, NJ, 1992). Reproduced
with kind permission of Pearson Education, Inc., Upper Saddle River, NJ.

exit the bottom through a grate. Most horizontal hammermills have a grate placed across the outlet
under the swinging hammers. The grate, possessing specific sized openings, may be changed
depending on the desired size of the final product. This allows for more accurate control over final
particle size. The hammers pound the material until it is small enough to pass through the grate
openings. The size of the output material in horizontal type hammermills is therefore ultimately
controlled by the size of the openings in the grate.
A disadvantage of the horizontal shaft unit is that if a durable waste such as a small engine block
enters, it may remain there until it is broken to smaller sizes. This will result in considerable wear
on the hammers; furthermore, excessive heat and sparks create a fire or explosion risk. Rejection
portals in horizontal units are utilized in such cases.

7.4.3.7 Vertical-Shaft Hammermills

Vertical-shaft hammermills (Figure 7.23) are designed with the shaft mounted vertically. The verti-
cal unit was originally designed in the United Kingdom for MSW processing. Large steel hammers