Trash in, cash out

Aug. 9, 2007
Machine vision spots treasure in trash and makes possible single-stream recycling.

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.

Automated Imaging Assn.,
Bollegraaf Recycling Machinery,

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