Environmental Engineering FOURTH EDITION

Reuse, Recycling, and Resource Recovery 275

however, the material must be in separate and discrete pieces, a condition clearly not
met by most components of mixed refuse. An ordinary “tin can” contains steel in its
body, zinc on the seam, a paper wrapper on the outside, and perhaps an aluminum
top. Other common items in refuse provide equally or more challenging problems in
separation.
The separation process can be facilitated by decreasing the particle size of refuse,
thus increasing the number of particles and achieving a greater number of “clean” par-
ticles. The size reduction step, although not strictly materials separation, is commonly
a first step in a solid waste processing facility.

Size Reduction

Size reduction, or shredding, is brute force breaking of particles of refuse by swinging
hammers in an enclosure. ’pwo types of shredders are used in solid waste processing:
the vertical and horizontal hammermills, as shown in Fig. 14-1. In vertical ham-
mermills, the refuse enters the top and must work its way past the rapidly swinging
hammers, clearing the space between the hammer tips and the enclosure. Particle
size is controlled by adjusting this clearance. In the horizontal hammermill, the ham-
mers swing over a grate that my be changed, depending on the size of product
required.
The solid waste processing facility in Fig. 14-2 has a conveyor belt leading up
to a vertical shredder, with a control room above and to the left. The hammers inside
the shredder are shown in Fig. 14-3. As the hammers reduce the size of the refuse
components, they are themselves worn down. mically, a set of hammers such as
those shown can process 20,000 to 30,000 tons of refuse before having to be replaced
(Vesilind 1980).

Feed \ chute convevor Feed

Liners - #

*-

Main frame -3, pocket

% Discharge

Discharge

chute

conveyor

Figure 14-1. Vertical and horizontal hammermills.