New technique automates sorting of plastics for recycling
By measuring a plastic item's fluorescence half-life, it's possible to tell what kind of plastic it's made from (Photo: Shutterstock)
If you've ever had to separate different types of plastic for recycling, then you'll know how much it slows down the recycling process. Now, imagine how much harder it is for staff receiving huge amounts of unsorted plastic at municipal recycling plants. New technology developed at Ludwig Maximilians Universitat in Munich, however, identifies plastic types automatically.
In the new system, plastic items are exposed to a brief flash of light which causes them to fluoresce. Photoelectric sensors then measure how long it takes for that fluorescence to fade. Because different types of plastic polymers have different fluorescence lifetimes, a measurement of that lifetime can reliably identify the plastic in question.
Even in its present form, the technology can sort up to 1.5 tons (1.4 tonnes) of plastic per hour. According to the researchers, this figure meets the specifications required for its application on an industrial scale.
"With this process, errors in measurement are practically ruled out; for any given material, one will always obtain the same value for the fluorescence half-life, just as in the case of radioactive decay," said project leader, Prof. Heinz Langhals.
A paper on the research was recently published in the journal Green and Sustainable Chemistry.
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An experienced freelance writer, videographer and television producer, Ben's interest in all forms of innovation is particularly fanatical when it comes to human-powered transportation, film-making gear, environmentally-friendly technologies and anything that's designed to go underwater. He lives in Edmonton, Alberta, where he spends a lot of time going over the handlebars of his mountain bike, hanging out in off-leash parks, and wishing the Pacific Ocean wasn't so far away.
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It still looks inferior to any of the Any plastic to fuel systems.
It still leaves the thorny problem of screw caps, which are not recyclable with their parent bottle, often being left on.
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This sensor technology can easily be paired with sorting technology that is common in the food industy. That would provide relatively pure streams, with perhaps a mixed one. The question would then be how to set up the logistics of moving that plastic about (to recyclers or one of those fuel converter systems) in an economical way.
Bruce H. Anderson
Will the technique work on pigmented plastics?
I've seen a fair of amount of Twitter traffic over this, but readers should know: a) Optical Sorting has been around a LONG time, so this is hardly revolutionary when b) identifying the plastics isn't the biggest barrier to recycling. Mechanical/Pneumatic sorting is required. And consistent market (commodity) values! So three things are required: the efficient processing of clean, post-consumer materials, manufacturer/commodity demand and public/consumer demand. It doesn't matter if a new reflective fluorescence technology helps recognize the "good" stuff; recognizing materials has only been part of them challenge. Sorting out the rest, if it's oddly shaped, an awkward weight, or a hybridized combination of materials, quickly makes material recovery technically/economically infeasible
There are many ways to sort plastics automatically. First, uniformly shred it then throw it all into a water bath. It needs cleaned anyway.
Skim what floats off the top then drain the tank and take out what sank.
A whirlpool separator could be used to produce a stronger force gradient than normal gravity, enabling pickup pipes to suction up plastic of different densities.
There's a technique to use precisely sized air bubbles to sort various denser than water plastics. Different sized bubbles stick to different types of plastics and cause small pieces to float.
Optically sorting pieces of plastic, glass or other materials could be done by spreading the chips out on a wide conveyor then running it under cameras. Angled air jets under the belt would then puff pieces back up the belt a ways and left or right to different color zones. After passing several such stages the plastic stream on the belt would be neatly arranged like a rainbow.
Yet another possibility is optical recognition of the type marks on many plastic containers. Do that prior to shredding.
What will really help is when someone invents a machine to automatically sort out containers with the cap on then remove the caps without getting chunks off cap or container to contaminate each other.
Then there's separating thermoplastic and thermoset plastic. Thermosets can be burned, chemically converted to other products, ground up and used as fillers in other products, but they can't be melted and re-used for their original application or other uses of the same type like thermoplastics can.
Another thing that drastically complicates recycling is most beverage bottles and some other containers (like STAX potato chip cans) are not homogenous. They're a multi-layer sandwich of different types/grades of plastics and often contain a center layer that's made all or partially of recycled plastic.
A soda bottle may be 3 to 5 layers, mainly PETE. The inside and outside will be a layer of freshly made "virgin" PETE. The next layer under on the inside is often a special gas barrier designed to keep the CO2 in longer. Some drinks need a UV light barrier to help preserve color and slow down chemical changes due to light. The bulk of the very thin sandwich will be recycled PETE.
Recycled plastics often have properties significantly below unrecycled. Testing has shown that adding a low percentage of virgin plastic can get the properties to 90% or better of unrecycled - but for health and safety reasons recycled plastic is never used for any surface that will have food contact.
Doesn't matter that no known pathogen could survive the temperatures and pressures of plastic injection or blow molding, there's the "ick factor".
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