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> Metal and plastic are super easy to separate by density or by electrical conductivity Yeah, what I meant by better segmentation is not to separate them from

> Metal and plastic are super easy to separate by density or by electrical conductivity Yeah, what I meant by better segmentation is not to separate them from each other (we do that) but rather by type. Some plastics are easier to recycle than others and I don't know if metals can be easily and automatically sorted from each other.

MayeulC 6y ago
Well, some are magnetic, some are not, that's just one way to differentiate them. Here are a couple videos I know (in French) from consignesdetri.fr: https://youtu.be/KgW1TP909NU?t=36 https://youtu.be/KgW1TP909NU?t=36 (you can see the magnetic belt at that timestamp) https://www.youtube.com/watch?v=vMJ-pDG_afk https://www.youtube.com/watch?v=vMJ-pDG_afk For an aluminium can. They mention doing the same with eddy currents [1] [2] I also recall seeing a big, spiky magnetic roller in some video instead of a magnetic belt, so implementation details can vary, but this seems to be pretty standard stuff. Less standard is the plastic-sorting equipment, though that can be achieved with hyperspectral cameras and the like (read: expensive). In some places, there are separate bins for colored and transparent glass. Some facilities use detectors instead. [1] https://en.wikipedia.org/wiki/Eddy_current_separator https://en.wikipedia.org/wiki/Eddy_current_separator [2] https://en.wikipedia.org/wiki/Eddy_current#Repulsive_effects_and_levitation https://en.wikipedia.org/wiki/Eddy_current#Repulsive_effects...
0_____0 6y ago
Metals can be sorted fairly efficiently. Most types of steel can be pulled out with magnets. Nonmagnetic metals can be sorted from plastic by having a conveyor belt chuck the incoming stream over an gap with a magnetic field applied to it. The magnetic field induces a current in the flying metal, causing the metal itself to become magnetic. This causes the metal to slow down (eddy current braking), and now you have physically separate streams of plastic etc. (fast) and metal (slow).
neltnerb 6y ago
Plastic separation is tricky, they tend to have similar physical properties so you end up relying on small density differences or optical properties or computer vision which is obviously not so great. I think most places it's still more cost effective to do it by hand. Metals are easy to separate from one another, they tend to have substantially different densities and electrical properties. Aluminum for instance is not magnetic while steel cans are, but aluminum is conductive so if it passes through a magnetic field it will still get slowed down. Those are the two big ones in any case. Then you could get to things like nickel (magnetic but intermediate conductivity) or tin (light but non magnetic) or maybe zinc/magnesium, but basically the densities are more different and the properties are more different so it's easier to tell them apart. And for some things that it can't figure out it makes sense to just landfill it. Plastics though, absolutely vastly harder. You can use density but it only varies by a few percent. You can use diffraction gratings and cameras to see crystals and maybe sort a bit by that. But really it's just very hard, especially since the same polymer has different molecular weights and crystallinity. Your milk jug is milky while the same molecule processed differently is clear. But even that's a mess -- you can't generally "depolymerize" these things very easily, you are really melting them back together and reusing the melt. When you start from monomers you can control the molecular weight (chain length) which controls things like crystallinity and stiffness, so the quality is higher. If you melt a variety of different molecular weights back together even if they're the same exact molecule the physical properties will just by worse. It's just the nature of the beast, and it sucks. Even if you recycle by perfectly melting all the PET with perfect sorting and no food residue you'll be guaranteed to increase the variability in the physical properties of the output. It's just chemistry, unless you can convert it back to monomers it'll never be as good even in the best case. For metals at least you can count on a fairly full recovery if you invest in it, it's not like polymers where it's just not possible. Even with alloys like steel you can at least in principle go back to the raw materials if you wanted, even if they are quite tricky.
- SahAssar 6y ago
  So what you're saying is that separating glass, cardboard, paper, compost, metal (which we can separate from one another if it is just metal by the methods you described) is valuable, but the all plastic can go in the "general" type, right? Glass, cardboard, paper and compost are either recyclable or reusable, metal is recyclable when sorted correctly, and plastics are pretty much a no-go as the parent article suggests, right?
  - neltnerb 6y ago
    Except by burning it as fuel, yes, that's basically what I'm saying. I don't think this would be news to a material scientist trained since the 90s, but marketing is strong.
    - SahAssar 6y ago
      I think some people take the "plastic recycling is not viable" to mean that all recycling is not a thing, so I wanted to make sure I didn't misunderstand you about things like glass, cardboard, paper, metal and so on. I think it's even harder to separate those concepts if you throw all recycling in the same bin.
      - neltnerb 6y ago
        I think the point of the NPR piece is that the recycling symbols started intentionally and have continued to be for marketing. I don't think it's viable to recycle paper products meaningfully, but they're also made from farmed trees and burn so I don't object overmuch to them. Worst case they sequester carbon... Glass is tricky and probably cheaper to make fresh right now but it's not that big a difference to recycle it. You'd only need a marginal incentive, or for sand to somehow become more expensive. Metal is really worth recycling economically, energetically, and technically. Glass is worth keeping separate since it can be recycled pretty easily. Paper products frankly just burn em as biofuels. Plastics... well... I just kind of wish they didn't exist except for special applications where their unique properties were necessary. They are space age, high performance, specialty materials and we use them like they're paper cups. Use PTFE for specialty tubing, great. Use rubber tires for cars, great. Use plastics where the properties matter. Don't use it for cups... it's just so sad...

MayeulC 6y ago

Well, some are magnetic, some are not, that's just one way to differentiate them. Here are a couple videos I know (in French) from consignesdetri.fr: https://youtu.be/KgW1TP909NU?t=36 https://youtu.be/KgW1TP909NU?t=36 (you can see the magnetic belt at that timestamp) https://www.youtube.com/watch?v=vMJ-pDG_afk https://www.youtube.com/watch?v=vMJ-pDG_afk For an aluminium can. They mention doing the same with eddy currents [1] [2] I also recall seeing a big, spiky magnetic roller in some video instead of a magnetic belt, so implementation details can vary, but this seems to be pretty standard stuff. Less standard is the plastic-sorting equipment, though that can be achieved with hyperspectral cameras and the like (read: expensive). In some places, there are separate bins for colored and transparent glass. Some facilities use detectors instead. [1] https://en.wikipedia.org/wiki/Eddy_current_separator https://en.wikipedia.org/wiki/Eddy_current_separator [2] https://en.wikipedia.org/wiki/Eddy_current#Repulsive_effects_and_levitation https://en.wikipedia.org/wiki/Eddy_current#Repulsive_effects...

0_____0 6y ago

Metals can be sorted fairly efficiently. Most types of steel can be pulled out with magnets. Nonmagnetic metals can be sorted from plastic by having a conveyor belt chuck the incoming stream over an gap with a magnetic field applied to it. The magnetic field induces a current in the flying metal, causing the metal itself to become magnetic. This causes the metal to slow down (eddy current braking), and now you have physically separate streams of plastic etc. (fast) and metal (slow).

neltnerb 6y ago

Plastic separation is tricky, they tend to have similar physical properties so you end up relying on small density differences or optical properties or computer vision which is obviously not so great. I think most places it's still more cost effective to do it by hand. Metals are easy to separate from one another, they tend to have substantially different densities and electrical properties. Aluminum for instance is not magnetic while steel cans are, but aluminum is conductive so if it passes through a magnetic field it will still get slowed down. Those are the two big ones in any case. Then you could get to things like nickel (magnetic but intermediate conductivity) or tin (light but non magnetic) or maybe zinc/magnesium, but basically the densities are more different and the properties are more different so it's easier to tell them apart. And for some things that it can't figure out it makes sense to just landfill it. Plastics though, absolutely vastly harder. You can use density but it only varies by a few percent. You can use diffraction gratings and cameras to see crystals and maybe sort a bit by that. But really it's just very hard, especially since the same polymer has different molecular weights and crystallinity. Your milk jug is milky while the same molecule processed differently is clear. But even that's a mess -- you can't generally "depolymerize" these things very easily, you are really melting them back together and reusing the melt. When you start from monomers you can control the molecular weight (chain length) which controls things like crystallinity and stiffness, so the quality is higher. If you melt a variety of different molecular weights back together even if they're the same exact molecule the physical properties will just by worse. It's just the nature of the beast, and it sucks. Even if you recycle by perfectly melting all the PET with perfect sorting and no food residue you'll be guaranteed to increase the variability in the physical properties of the output. It's just chemistry, unless you can convert it back to monomers it'll never be as good even in the best case. For metals at least you can count on a fairly full recovery if you invest in it, it's not like polymers where it's just not possible. Even with alloys like steel you can at least in principle go back to the raw materials if you wanted, even if they are quite tricky.

SahAssar 6y ago
So what you're saying is that separating glass, cardboard, paper, compost, metal (which we can separate from one another if it is just metal by the methods you described) is valuable, but the all plastic can go in the "general" type, right? Glass, cardboard, paper and compost are either recyclable or reusable, metal is recyclable when sorted correctly, and plastics are pretty much a no-go as the parent article suggests, right?
- neltnerb 6y ago
  Except by burning it as fuel, yes, that's basically what I'm saying. I don't think this would be news to a material scientist trained since the 90s, but marketing is strong.
  - SahAssar 6y ago
    I think some people take the "plastic recycling is not viable" to mean that all recycling is not a thing, so I wanted to make sure I didn't misunderstand you about things like glass, cardboard, paper, metal and so on. I think it's even harder to separate those concepts if you throw all recycling in the same bin.
    - neltnerb 6y ago
      I think the point of the NPR piece is that the recycling symbols started intentionally and have continued to be for marketing. I don't think it's viable to recycle paper products meaningfully, but they're also made from farmed trees and burn so I don't object overmuch to them. Worst case they sequester carbon... Glass is tricky and probably cheaper to make fresh right now but it's not that big a difference to recycle it. You'd only need a marginal incentive, or for sand to somehow become more expensive. Metal is really worth recycling economically, energetically, and technically. Glass is worth keeping separate since it can be recycled pretty easily. Paper products frankly just burn em as biofuels. Plastics... well... I just kind of wish they didn't exist except for special applications where their unique properties were necessary. They are space age, high performance, specialty materials and we use them like they're paper cups. Use PTFE for specialty tubing, great. Use rubber tires for cars, great. Use plastics where the properties matter. Don't use it for cups... it's just so sad...