Paul Kellett
Automated Imaging Assn.
Ann Arbor, Mich.
Recycling was a somewhat
onerous process when first introduced in North America and
Europe. Participants presorted
glass, paper, plastic, and metal
into separate, color-coded containers and placed them curbside. Special trucks with compartmentalized bins hauled off
the segregated recyclables to a
material-recovery facility (MRF)
where they were screened and
hand sorted.
Recycling is still done this
way in many places. But socalled single-stream recycling
is catching on. Here, all recyclables go in a single container
and off to the MRF by ordinary
refuse trucks. The practice not
only encourages wider participation in nonmandatory recycling programs, waste-management firms benefit as well. The
elimination of presorting lets
drivers make more stops per
day and cover a wider area, so
companies can reduce the number of trucks and save fuel.
Of course, single-stream recycling does not eliminate sorting. The sorting process simply
moves downstream from the
curb to the MRF. Mixed recyclables go by conveyor belts to processing stations where
they are screened, identified,
sorted by type, and removed
from the material stream by
magnets, air jets and other mechanical means. It is here that
machine vision plays a key role.
Machine vision targets wanted
recyclables and guides the process of separating them from
the remaining material at rates
to 10,000 kg/hr.
Plastics: In the case of plastics, a conveyor belt transports
the material under an optical
sensor where it is illuminated by
near infrared light. Software analyzes captured images and flags
plastic objects for removal by
air jets. Certain optical scanning
systems can also identify and
remove a specific type of plastic, say, PET (polyethylene terephthalate) from other types such
as PVC (polyvinyl chloride), PP
(polypropylene), PE (polyethylene) or PS (polystyrene).
One setup from Mogensen in
Sweden uses a vibrating sorter/
feeder that forms a single layer
of material. The material conveys to a chute where it is
scanned by a color-line (scan)
camera at the chute edge. An
industrial parallel computer
evaluates the images and signals one or more compressed
air jets to eject the unwanted
components from the stream as
they free-fall from the chute.
Paper: Optical sorting of paper typically involves a ‘‘negative-sort'‘ technique. That is,
all nonpaper objects are removed from the materials flow,
leaving behind the paper. An
installation from Bollegraaf
Recycling Machinery, Netherlands, provides an example of how de-inking cardboard is optically sorted from household
wastepaper: A shovel puts the
collected waste into a bunker
(drum) with a drum feeder at its
end. A conveyor moves the material to a screen that separates
large pieces of cardboard. The
remaining paper flow goes to a
sorting drum via a fine screen
that removes the smallest cardboard particles from the wastepaper. From there, the paper
flow goes to two or three paper
spikes, machines that remove
small pieces of cardboard.
Cardboard stuck on the V-beltmounted spikes goes to a conveyor belt where it joins with
large pieces that come from the
cardboard screen.
The process produces a flow
of de-inking cardboard containing about 4 to 5% contaminants.
A subsequent optical sorting
step directs an air-jet system
that halves contamination levels to about 2%. The sorted,
de-inking cardboard automatically loads into trailers for
bailing (800 to 900-kg bales).
Such systems can process 30 to
40 tons of domestic waste paper per hour.
Glass: An MRF in Australia
uses optical sorting for glass recycling. High-speed color cameras identify glass fragments
by color as they move down a
high-speed conveyor. Air jets
shoot identified fragments into
separate material streams. The
cameras used in this process
identify up to 1 million glass
fragments/min and can detect
up to 16 million colors. Each
1,000 kg of glass recycled in this
manner saves 1,100 kg of raw
material needed to make glass from scratch.
Aluminum: On average, 85%
of the aluminum in cars is recycled. Most of the recovered
aluminum goes into automotive
castings instead of higher-value
wrought products. Once aluminum casting is separated from
wrought, optical sorting can
segregate the wrought into alloy groups. Laser-induced optical-emission spectroscopy can
also be used for this purpose.
Other recyclables: Optical
scanning may have an increasingly important role in the sorting of construction and demolition waste, which is extremely
challenging given the complex
and variable composition of
the refuse. End-of-life electronics sorting is also extremely
challenging, because components tend to be interconnected. Currently, manual disassembly and magnetic sorting
of shredded materials is principally used in the recovery of
electronic materials.
In general, optically sorted
recyclables contain fewer contaminants than those that are
hand sorted and therefore are
more valuable. Today, there are
about 2,000 large MRFs worldwide, of which some 1,500 reside
in Europe, with the remainder in
the U.S., according to a study by
Tomra, a maker of waste recognition and sorting equipment in
Norway.
MAKE CONTACT
Automated Imaging Assn., www.machinevisiononline.org
Bollegraaf Recycling Machinery,
www.bollegraaf.com