Page 228 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 199

FIGURE 7.29 Overhead belt magnet in operation.

FIGURE 7.30 The height of a magnet above
the waste stream affects the efficiency of fer-
rous recovery. Reproduced with kind permis-
sion of B.L. Parker.

• Depth of burden on conveyor. The deeper the waste on the belt, the lower recovery of ferrous.
• Material density. Dense wastes such as steel containers will sink below other wastes on
a conveyor. Such settling is increased with increased time on the conveyor belt (Vesilind
et al., 2002).

7.4.5 EDDY CURRENT DEVICE
The eddy current unit operation separates aluminum products from other nonmetals. An aluminum
separator employs either a permanent magnetic or electromagnetic field to generate an electrical
current (eddy), which causes aluminum cans (nonferrous) to be ejected and separated from other
materials. Eddy current separation is based on the use of a magnetic rotor with alternating polarity,
spinning rapidly inside a nonmetallic drum driven by a conveyor belt. Eddy current separation is
based on Faraday’s law of electromagnetic induction:
−dB/dt V/A (7.10)

where B is the magnitude of magnetic flux density (T), V the voltage, and A the cross-sectional area
2
normal to magnetic field (m ).