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7.4.7 EFFICIENCY OF SEPARATION IN AN AIR CLASSIFIER
As may be obvious from discussions of the various unit operations in an MRF, complete separation
of one material from all others is not possible. In the case of air classification, the recovery of organ-
ics is complicated by two factors (Vesilind et al., 1988):
• Not all organics are aerodynamically light, and many inorganics (e.g., aluminum foil) are
aerodynamically heavy.
• Perfect separation of heavy and light materials is difficult because of the stochastic (i.e.,
any phenomenon obeying the laws of probability) nature of material movement within
the classifier.

In Figure 7.37, terminal settling velocity (i.e., air velocity at which the particle will just begin
to rise with the air stream) is plotted against the fraction of particles of various materials.
Regardless of the air velocity chosen there will never be a complete separation of the lighter organ-
ics (paper and plastic) from the heavier inorganics (steel). Figure 7.38 shows the efficiency of sep-
aration of fractions vs. the feed rate to the air classifier. With a greater loading of solids, an
increasing proportion of light particles will fall into the underflow stream rather than be separated
as originally planned.
The effectiveness of air classification can be estimated using published data (Figure 7.37). As the
air velocity in an air classifier is increased from zero, the first material to float upwards and collect
in the extract is paper. At an air velocity of about 500 cm/sec (1000 ft/min) all of the paper occurs in
the extract. However, at this velocity about 50% of the plastic is also entrained. If we increase the air
velocity beyond 500 cm/sec, some of the aluminum will start to become entrained with the extract.
At an airflow of 1010 cm/sec (2000 ft/min), all of the paper and plastic will be collected in the
extract; however, this will be contaminated by 50% of the aluminum. The steel component would not
become entrained until the air velocity exceeded 1010 cm/sec (2000 ft/min). At 1525 cm/sec (3000
ft/min), virtually all of the input feed would become entrained. From the data in Figure 7.37, then, if
the goal of an MRF is to produce high-quality feedstock for RDF, the air velocity is best maintained
under 500 cm/sec (1000 ft/min).

FIGURE 7.37 Terminal settling velocities of MSW components. (Vesilind, P.A. et al Environmental
Engineering, 2nd ed., Butterworths, Boston, MA, Reproduced with kind permission of Elsevier, Oxford, U.K.)