Page 223 - Materials Chemistry, Second Edition

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194 Waste Management Practices: Municipal, Hazardous, and Industrial
spaced farther apart. In some units, chains or knives replace the hammers. Flail mills tend to be sin-
gle-pass devices whereas in the hammermill, wastes may be retained until they are small enough to
pass through the gratings at the bottom of the unit. Flail mills tend to have a low power requirement
(Pfeffer, 1992).
As the waste passes through the mill, the hammers strike the MSW and smash it against the
anvil plate. If the particle is sufficiently small it will pass through the mill without size reduction.
Typically the mill will impact larger containers, bags, boxes, and so on.
Advantages of the flail mill include low power requirements and low maintenance. The major
disadvantage is the limited capability for size reduction.

7.4.3.9 Rotary Shear

The rotary shear or shear shredder contains two parallel counterrotating shafts with a series of disks
mounted perpendicularly that act as cutters, working in a scissor-like fashion (Figure 7.20b). Rotary
shears are low-speed devices (60 to 190 rpm) compared with hammermills. The input to be shred-
ded is directed to the center of the rotating shafts. The size of the input is reduced by the shearing
or tearing action of the cutter disks. The spacing and orientation of the shafts and the spacing
between the shafts control the final particle size of the product (Pfeffer, 1993). Particle sizes can
range from as low as 2.5 to 25 cm (approximately 1 to 10 in.). The shredded materials fall through
or are pulled through the disks. Shutdown is not typical because even large bulky objects like rail-
road ties can be processed. Most are driven by hydraulic motors that can be reversed automatically
in the event of an obstruction (Tchobanoglous et al., 1993).
Advantages of the rotary shear include slower speeds and the consequent lack of brute, destruc-
tive force. This will help in preventing contamination of organic wastes by broken glass — any glass
containers or other small input can simply fall between the shears, given adequate spacing of the
shafts. Additionally, lower speeds imply lower energy costs and also less maintenance of moving
parts. As was the case for other shredders, large steel and other durable objects pose a problem for
the rotary shear and should be removed prior to entering the shredder.

7.4.3.10 Hammer Wear

Due to the abrasive nature of high-speed shredding of highly heterogeneous wastes, hammers,
grates, and housing walls are all subject to excessive wear. Both high- and low-speed shredders are
maintenance-intensive due to their violent type of work.
Hammer damage and wear is the major maintenance issue related to MSW shredders (Figure
7.24). As the hammers wear, their effectiveness in shredding the waste decreases due to blunting the
hammer tips and increased clearance between hammers and the housing or grates. Hammers are
usually double-sided, and hammer wear occurs primarily on the outer edge, since this is the area of
impact as the material is crushed against the grate. The wear is mostly due to abrasion although
severe impact with very hard objects also contributes to wear. Maintenance involves periodically
turning the hammers or knives. Once both sides are worn down, however, they must either be
replaced, retipped, or resurfaced. Resurfacing involves rewelding a work surface on the hammers
followed by resharpening to a cutting edge (Stessel, 1996). After a number of retippings the entire
hammer must be replaced.
Hammer wear is reduced by employing special hardened facings to the hammers with abrasion-
resistant alloys and by slowing the speed of shredding (Stessel, 1996; Vesilind et al., 2002). Shredding
after removal of metals, concrete, glass, and ceramics greatly extends the lifetime of the hammers.

7.4.3.11 Safety Issues Related to Size Reduction

A number of hazards are possible during the operation of high-speed shredders. First, because waste
materials are so heavily pulverized, dust concentrations can become sufficiently high, both in the