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Best Practices in Wood Waste Recycling Click here for printable PDF version Automatic Detection of Metal
Material: Wood Waste
Issue: One of the most common contaminants
of recovered wood waste supplies is
ferrous metal. One can find ferrous metal within crates, pallets,
construction and demolition debris
as nails, screws, staples and other
hardware.
Larger pieces such as scrap
metal can also be found within the
wood waste supply. Other sources of metal contaminant include damaged parts from the
processing equipment (for example,
sections of chain, bolts, and tools). Metal contaminants create problems for processing
equipment.
For example, chippers are sensitive
to even the smallest pieces of metal.
Even the sturdiest hog can receive
serious damage from a piece of metal. Most importantly, to safeguard against damage to the equipment,
detect ferrous metals at the in-feed
of
certain critical pieces of
process equipment, like size-reduction equipment.
Best
Practice: This Best Practice recommends
wood waste processors install a metal
detection system upstream of sensitive
or critical process equipment.
Generally, metal detection equipment is installed in
a conveyor system.
The basic equipment consists
of a section of metal free conveyor
trough, usually fiberglass, that contains
a metal detecting coil.
The detector is activated by
the presence of magnetic or non-magnetic
metal in the material flow.
Any metal passing over the
coil produces an electronic signal
that activates an alarm system. The
alarm usually sets off a warning light
and stops the conveyor. Once the metal is removed, the operator can
reset the detector and restart the
conveyor to resume operation. The detection system can be adjusted to allow
small pieces of metal to pass through
and only activate for large pieces
of metal, if appropriate.
Also available are more sensitive systems that incorporate
coils that completely surround the
conveyor. These systems contain more sophisticated electronics
which create several advantages compared
to the flat coil systems. One advantage is better resolution in distinguishing
between small and large pieces of
metal.
When the burden depth on the
conveyor goes above ten inches, the
‘surround’ type detector is better
able to distinguish between a small
piece of metal on the bottom of the
conveyor and a large piece of metal
riding on the top of the material
flow.
The surround type coil is also
able to incorporate a discriminate
circuit that can be set to reject
certain non-ferrous metal like aluminum
or copper wire.
Implementation:
Metal
detection systems can be installed
in new or existing facilities. The metal detection system should be installed
at an adequate distance from the chipper
in-feed spout to allow belt travel
after the detector stops the drive
motor.
Benefits: Metal detection systems provide protection for sensitive
or critical process equipment.
The systems are an excellent
safeguard against excessive equipment
maintenance and catastrophic equipment
damage.
Application Site: Processing Facility and Manufacturing Plant.
Contact: For more information about this Best Practice, contact CWC (206) 443‑7746, e-mail info@cwc.org.
References:
1.
Perdew,
Frank.
Metal Detectors, Inc. Eugene, OR.
2.
Willis,
Ken.
Rens Manufacturing Company. Creswell, OR.
Issue Date / Update: March 1997 |
