A flow cytometry-based sorting method of many different types of plastic waste
Image credit: J. Nikola
jnikolaJan 08, 2022
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Summary
"Flow plasticometry" device that sorts up to 13 types of plastics by:
aerodynamic focusing of the particles in one-by-one formation
detecting the type of plastics by the hyperspectral camera and dedicated software
sending particles to separate containers ready for further processing and market.
Introduction
Let's say we want to recycle all the plastic waste and return it to the industry as pure as possible. Now imagine we somehow collected it all. The cigarette buds, coffee cups, straws, bottles, mobile phone masks, packaging, everything is here. Now we shred it on pieces smaller than 0.5 cm in diameter by a huge multi-level shredder. Now we have tons of plastic shreds that are of different shapes, colors, materials, and sizes but don't have the technology that can sort them out.
Recently, these guys used hyperspectral imaging with wavelengths from 955 to 1700 nm on thirteen different plastics and successfully differentiated them. The technology is not new, but they were first to use it in a way to detect which particle is of which plastic material (between 13 of them!) and that's great.
What they didn't find a solution for is how to sort and collect them in a fast and efficient way. The existing technologies of pneumatic ejection units or robotic arms are reliable and fast for separating two types of plastic or other waste. When it comes to simultaneous sorting of more materials, pneumatic ejection units are time-consuming, since the same material needs to be run multiple times, while robotic arms are just not precise enough. That's where I jump in.
How it works?
"Flow plasticometry" scanning and sorting device
The machine should, just like the flow cytometer, have a bottleneck where plastic pieces of shredded size enter one by one.
To ensure regular one-by-one flow of plastics, air could be used (airdynamic focusing). Along with the plastics, air would be blow in the machine (just like the shealth fluid in flow cytometer) and ensure the plastics is propelled inside in a single row while helping the removal of any leftover pieces from the machine.
Once inside the machine in one-by-one order, plastics would pass through a set of hyperspectral cameras that would determine the material (the recent, above-mentioned invention).
Based on the material, plastic particles would then be further propelled by the air and sorted in one of the thirteen containers (one for each type of plastic material). The sorting would be done by the set of valves which would be regulated by the scan reads.
Why is this needed?
simulatenous recycling of all the plastic material, not just the most popular ones
recycling of the sea waste that is often deteriorated and pale (unable to use existing color sorting methods)
high purity of the material --> important because the industry requires recycled resources to be clean enough to ensure consistency and uniformity of the product performance and quality, especially for the food-containing packaging
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General comments
Povilas S2 years ago
Isn't flow cytometry too slow to be used for such a purpose? Since particles have to pass through a "bottleneck" one by one, I believe counting many individual cells may take hours for the machine to do the task. Wouldn't we end up with very little plastic sorted out in a long time using the proposed method?
Also, I'm not sure if it would be beneficial for the recycling industry to have all types of plastic in shredded form. For some it definitely would, some types of plastic melt easily (e.g. HDPE) and can be reshaped into new products, packages, etc., but not all, I'm wondering couldn't that be one of the main reasons why only some types of plastic are now being recycled.
Having finely shredded (it would basically be pulverized, as I understand, what particle sizes are we talking here actually?) plastic you'd have to fully melt it and reshape into something new, on the contrary, having larger pieces, you might "weld" something new from parts, which require less heating and melting. But even leaving the "welding" aside, it might be simply easier to recycle some types of plastic when it's shredded into larger bits, not super tiny ones. But I'm not an expert on this, just thinking here.
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jnikola2 years ago
Povilas S Well, that's a great question. The speed would depend on the size of the particles. Depending on the shredding techniques, I guess the first thing would be to determine the size range of the particles entering the flow cytometer. I proposed a size around 0.5 cm in diameter. Now, the flow speed of the flow cytometer is around 50 000 - 100 000 cells per minute. If the device is proportionally larger and can achieve the same speed, I guess that would be a speed of at least 200 shredded bottles in a minute (250 shredded pieces per bottle).
These are great thoughts, Povilas S. Depending on the industry requirements, some types of plastic would definitely have to be larger than the others. That all should be taken into consideration when designing the device.
If the industry requirements are not so high that a new device is needed, I would still build it and use it to separate microplastics from the water systems. For this purpose, even the standard flow cytometer with small changes could work :)
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Povilas S2 years ago
J. Nikola 0.5 cm is a huge size compared to the size of an average cell which is at the range of μm. Since the technique would use spectrometry to identify different types of materials, it might be important that the size of particles would be closer to the wavelength of the light used. 0,5 cm might be way too big.
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Darko Savic2 years ago
Is this conceptually the same?
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Darryl Koh Yuan Jie2 years ago
I am not fully aware of the mechanics behind such sorting machines but I would imagine one of the main problems would be the shredding of different kind of plastic wastes. There are many kind of plastic waste, ranging from different densities and material properties. I am not sure if you can shred a plastic bag the same way you can shred a plastic container. You mentioned you would prefer if the final result was plastic that is as pure as possible to be returned to the industry but how would the process be? Say we sort out and grind down all the different plastic, it would still require I imagine some chemical process of sorts in order to do so?
Personally, I think it may not be necessary to simply reform everything into one kind of plastic only. Perhaps, we could look at the different kinds of plastic needed on the market and ways to form them from unwanted reused plastic.
jnikola Jan 08, 2022
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