Page 211 - Materials Chemistry, Second Edition
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                       182                       Waste Management Practices: Municipal, Hazardous, and Industrial
                       might consist primarily of old corrugated cardboard (OCC) and newspaper, depending on the com-
                       position of the incoming waste stream. Screening is carried out either wet or dry, although dry sep-
                       aration is most common. Screens are classified as primary, secondary, or tertiary, depending on
                       where they are situated in the sequence of separation steps. The primary applications of screening
                       during MSW processing include:

                           • Removal of oversized materials
                           • Removal of undersized materials
                           • Recovery of paper and plastics for recycling or as refuse derived fuel (RDF)
                           • Separation of soil, glass, and grit from combustible materials

                          Screens have a long history in various industries for particle separation by size. Rotary (trom-
                       mel) screens have been used in the mineral industry for many years for coarse screening of ores,
                       gravel, and rock.
                          As discussed in Chapter 4, different components of MSW possess characteristic size ranges. If
                       screen size is properly selected, it is possible to create a fairly enriched stream of a particular waste
                       component. Of course, due to the great variability of sizes of a single waste component, only partial
                       separation is possible; additional processing is still required for further purification of the product.
                          There are three major types of screening used in materials recovery: trommel screening, disk
                       screening, and vibrating shaker screening.

                       7.4.2.1 Trommel Screens
                       Of the major forms of MSW screening the trommel is by far the most popular. Trommel screening
                       is primary screening, designated as such because it is usually placed before all other separation units
                       in an MRF.
                          The trommel is a rotating perforated cylinder with a diameter ranging between 0.6 and 3 m (2
                       and 10 ft) with a screening surface consisting of a perforated plate or wire mesh (Figure 7.11). Some
                       are equipped with spikes, usually within the first third of the drum, to break open plastic trash bags.
                       The drum is inclined at a slight angle. A motor is attached at one end which rotates the drum at a
                       rate of about 10 to 15 r/min. The waste is introduced at the elevated end via a conveyor belt. As the
                       drum rotates, waste particles are carried up the side until they reach a certain height and then fall to
                       the bottom. The waste which falls through the openings is collected by a conveyor or a hopper and
                       the fraction retained within the trommel is collected on a separate belt.
                          A typical trommel screen is depicted in Figure 7.12. The length and diameter of the drum have
                       a direct relationship with the efficiency of the trommel. The longer the drum, the longer the MSW
                       will remain in contact with the screen; and, the greater the diameter, the more effective the trommel
                       will be in breaking up large objects such as trash bags. Large trommels (2.5 to 3 m, or 8 to 10 ft in
                       diameter, up to 15 m long) have been used to separate large OCC and newsprint from mixed paper
                       or commingled containers (particularly from glass containers). Small trommels (0.3 to 0.6 m in
                       diameter by 0.6 to 1.2 m long) have been used to separate labels and caps from crushed glass. These
                       small units have been used in conjunction with an air stream to aid in separation (U.S. EPA, 1991).
                          Two-stage or compound trommels have been used in waste processing. In two-stage trommels
                       the first section is set with small openings (e.g., 2 to 3 cm [approx 1 in.] diameter) which permit
                       soil, broken glass, and other small fragments to fall through and be collected. This material is
                       largely nonrecyclable and will probably be landfilled. The second stage is provided with larger
                       apertures (e.g., 12 to 15 cm diameter), which allows glass, aluminum, and plastic containers to be
                       removed from the waste stream.
                          Trommel screens separate waste materials based on size and do not identify the material by
                       any other property. As a result, trommels are used as a classification step before true “separation” of
                       materials. For example, smaller particles such as grit and broken glass can be removed early in the
                       processing scheme to produce better quality (i.e., higher quality, greater purity) recyclables such as
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