Page 229 - Materials Chemistry, Second Edition

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200 Waste Management Practices: Municipal, Hazardous, and Industrial

FIGURE 7.31 An eddy current separator in operation.

As nonferrous metals pass over the drum, the alternating magnetic field creates eddy currents
in the particles, repelling the material away from the conveyor. While other materials drop off at the
end of the conveyor, the nonferrous metals are propelled forward over a splitter for separation
(Figure 7.31) (Walker Magnets, no date).
A time-varying field can be created either by rapidly reversing the voltage on an electromagnet
(i.e., using alternating current ) or by using strips of permanent magnets with alternating polarities.

7.4.6 AIR CLASSIFIER
Air classification is a unit operation designed to separate light waste components such as paper and
plastic from heavier materials based on their differential behaviors when subjected to a stream of
air. When a waste mixture is fed into an air stream of sufficient velocity, the lighter materials will
be carried away with the air stream while the heavier components fall.
Air classification has been employed by industry for many years for the separation of various
components in mixtures. Air classifiers are used in waste-to-energy processing lines to segregate the
MSW stream into two fractions. One fraction consists of light materials (paper, plastic, wood, and
dust), and the other is composed of heavy materials (metals, glass, and stones). In most MSW, the
light fraction constitutes 60 to 75% of the total (see Chapter 4). Air classification concentrates the
combustibles into the light fraction as a fuel product. Also, the metals and glass can be separated
from the heavy fraction and sold in secondary markets. Often in the processing scheme an air clas-
sifier is situated after the magnetic separator and before the secondary shredder.
Separation is optimized through the proper design of the separation chamber, airflow rate, and
material feed rate (Tchobanoglous et al., 1993). Specific variables of the input waste feed will affect
material separation through air classification. Variables include:
• Particle density
• Particle size
• Particle surface area

Air classifiers may be configured in several designs of varying capacity and efficiency of sepa-
ration. A schematic diagram of a typical air classifier is provided in Figure 7.32. The vertical,
straight type is one of the most common and basic configurations of air classifiers. In the vertical
unit, shredded MSW is dropped downward into the chute. An upward stream of air, fed by blowers,
lifts lightweight materials upward for subsequent capture in a cyclone or other receptacle. There is
little breakage of aggregated particles. Airflow direction is fairly uniform and the airflow rate is held
constant. Variations to the simple vertical design include the installation of baffles along the length.