Page 225 - Materials Chemistry, Second Edition

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196 Waste Management Practices: Municipal, Hazardous, and Industrial
Blockage of materials due to material jamming between the rotor and the housing within the
shredder is a frequent cause of operational problems. Should a high-speed shredder encounter an
object that it cannot cut through, there is no reverse or overload setting. Either the problem material
or the revolving shaft will have to give way. Preferably, the machine will simply jam up, although
shafts have been known to break. In the event of a simple jam it is necessary to open the machine
and remove the object. There may be substantial down time in order to remove jammed articles and
to repair hammers. A hazard exists for workers entering the hammermill as there may be a buildup
of pressure on the rotor and hammers caused by the obstruction.
Problems with flying objects and noise are also encountered in MSW shredding. Since raw
MSW is contaminated, any dust produced contains a broad range of microorganisms and the atmos-
phere in the vicinity of the shredder may become a potential health hazard to facility personnel.

7.4.4 MAGNETIC SEPARATION

Magnetic separation is a relatively simple unit process designed to recover magnetic material, pri-
marily ferrous metals, from mixed MSW. Two important reasons for removing the ferrous metals
in a MRF are to increase the heat content of RDF and to recover a saleable product. On average,
there is approximately 5.3% ferrous metal in the incoming MSW (U.S. EPA, 2001). Furthermore,
metal removal reduces wear on subsequent processing and handling equipment and also reduces the
amount of ash generated if the waste is to be incinerated. Magnetic recovery systems have also been
used at landfill sites to recover product for recycling.
Magnetic separators are available in three primary configurations, i.e., the drum, the magnetic
head pulley, and the magnetic belt pulley (Figures 7.25–7.27). Either permanent or electromagnets
are employed. Magnets may be composed of exotic (e.g., rare earth) metals and are expensive. The
principle of a single drum-type magnetic separator is shown in Figure 7.25. The drum is positioned
under the lead pulley of a conveyor belt carrying mixed, shredded MSW. A stationary magnet is
located inside the revolving drum. The ferrous metal in the MSW is attracted to the magnet against
the force of gravity and is conveyed around the drum circumference until it exits the magnetic field

FIGURE 7.25 Drum magnet.
(Vesilind, P.A., et al., Solid Waste
Engineering, Brooks/Cole, Pacific
Cole, CA, 2002. Reproduced with
kind permission of Brooks/Cole, a
division of Thomson Learning.)