Cork separation (natural corks vs. synthetic corks)

  • #1
TL;DR Summary
How to separate natural corks from synthetic corks?
Hi all,

At my workplace we have a waste stream that includes both natural and synthetic corks. The synthetic corks we can further process, so the natural corks have to be selected out. We have two Polish workers who are able to do this just by sight, 8 hours a day. However, we tried to put other people at the machine and even I tried it, but we just don’t see it.

Now as an Engineer I would of course like to optimise and if possible automise this process. So I asked myself if there are no different properties based on which we could determine if it concerns a synthetic or natural cork. It turns out that the density of the natural corks is between 180-200 g/cm^3 and the synthetic corks are 230-260 g/cm^3. Now as this is way lower than the denisty of water or ethanol or any other liquid I know of, just floating and sinking is not an option.

So, I have been searching online for an other density separation method. Unfortunately, I have not found a method yet that looks convincing. The best solution I have come up with is putting them on a machine that would consecutevily weigh each cork and then push it under wather, so the density can be calculated automatically and that way they could be separated. However, I doubt there is a setup that would make this (economically) interesting as we have to separate 15T on a weekly basis which means roughly 12 million corks.

Is there any other method which I have not come across that could suit for density separation?

Another solution would be if there is a chemical that reacts with natural cork, which results in a physical/visual change and is inert with respect to synthetic corks (TPE). However I think this would be a question more for a chemistry forum.
 

Answers and Replies

  • #2
Summary:: How to separate natural corks from synthetic corks?

180-200 g/cm^3
Do you mean milligrams?
 
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  • #3
Could you use an airstream that would push the less dense ones further?
 
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  • #4
How about a liquid bath carefully selected for its density. The real corks might float while the artificial ones don't (or visa versa).

Perhaps you can add an additive that continuously changes the density of the liquid until it fits your needs.
 
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  • #5
If the liquid is flowing and the depth is shallow enough to ground the synthetics the corks will flow with the liquid.
 
  • #6
How about a liquid bath carefully selected for its density
The OP mentioned the density is 30% that of water. So finding an homogeneous liquid may by tough. Liquid H has a density 7% that of water but the temperature may by a bit difficult.
## ^3He## even less dense but cost might be prohibitive in addition to Temperature thing.
More seriously, perhaps using aerated water somehow.[
Or perhaps some sort of vibratory bath of lightweight (styrofoam?) beads to act as the intermediate density fluid? A kind of "quicksand" in air
A centrifuge works for ## ^{235}U##. How about spinning the mixture while vibrating.
Are the corks regular enough in size to just send them over an air curtain on a mesh belt and "tune" the air speed like @Dale pointed out?
 
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  • #7
Just check out my suggestion. The synthetic I had is shorter than the cork and seems to have nearly the same draft, but the synthetic seems a bit thicker and had more height above the water so you might block the synthetics with a low "bridge".
 
  • #8
Look at the corks in the dark, with an IR capable camera, while illuminating them with IR light. I would expect the synthetic corks to remain cool, while the natural corks reflect more IR and so appear brighter. If it is not that simple, then do some more involved spectroscopy to identify the polymer presence in the synthetics.

Corks could be separated by density in air with a cyclone separator.
Use the input like a vacuum cleaner to pick up the corks from the waste stream.
https://en.wikipedia.org/wiki/Cyclonic_separation

A preliminary sort could be done in air on a shaker table, if size and the Brazil nut effect is not a problem.
https://en.wikipedia.org/wiki/Granular_convection

A stable, low density foam can be used to sort material in flotation tanks or on tables.
Other foam techniques use hydrophobic / hydrophilic differences.
https://en.wikipedia.org/wiki/Froth_flotation
 
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  • #9
Are these corks cylindrical like from wine bottles? If so, here is a kitchen countertop experiment I tried. I attached turkey lacing pins (thin nails would work just as well) to the bottom of one synthetic and one natural cork (figure below, left.) Then I floated them in a glass of water. The pins ensured that the corks would float more or less upright (figure below, right). One can use the diameter of each cork as a measuring stick to estimate the fraction of submerged length to diameter.

Edit: A little dye in the bath will show the submerged volume better when the corks are pulled out.

Floating Corks.png
 
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  • #10
One could make use of the static charge accumulation on the cork, being of either natural or synthetic. Natural cork is supposedly neutral; the synthetic being some sort of plastic may collect electrons( or is it give up electrons ). The cork synthetic should lie somewhere on the triboelectrostatic series.


One such unit, I bet a bit large for your fancy, but you get the idea.
Just look up triboelectrostatic separator for more information.
And make a table top version to test it out before going all in.
https://steqtech.com/wp-content/uploads/2016/03/STET-Physical-Separation-2015.pdf

Desirable features are:
No fess, no muss.
Bin separation is automatic and continuous
High level of separation.
 
  • #11
Decidedly NOT environment friendly, but...
An acid bath would dissolve the natural cork, leaving the synthetics untouched.

Come to think of it, an alkali bath would also work.

Does that mean you could use both bath types and mix them to neutralize each other before disposal?

I'm sure the Chemistry gurus will have plenty to say about this post!
 
  • #13
Cool. So, as we used to say laughingly in the R@D lab, "in principle the problem is solved"
 
  • #14
Do you mean milligrams?
That’s indeed a small mistake, thanks for pointing out.
 
  • #15
Could you use an airstream that would push the less dense ones further?
That’s something I have been thinking about as well. However, as they are cilinder shaped I think this might give some difficulties.
 
  • #16
How about a liquid bath carefully selected for its density. The real corks might float while the artificial ones don't (or visa versa).

Perhaps you can add an additive that continuously changes the density of the liquid until it fits your needs.
That would work, but I don’t know about any additve that could make the density of water or any other liquid as low as 200 mg/cm^3.
 
  • #17
If the liquid is flowing and the depth is shallow enough to ground the synthetics the corks will flow with the liquid.
I was also thinking about this, but in ethanol the synthetic corks would float with about 30% of their volume beneath the surface and the natural corks 25%. As the dimensions are not that stable, I doubt this would be failliet. Nevertheless, I could try.
 
  • #18
The OP mentioned the density is 30% that of water. So finding an homogeneous liquid may by tough. Liquid H has a density 7% that of water but the temperature may by a bit difficult.
## ^3He## even less dense but cost might be prohibitive in addition to Temperature thing.
More seriously, perhaps using aerated water somehow.[
Or perhaps some sort of vibratory bath of lightweight (styrofoam?) beads to act as the intermediate density fluid? A kind of "quicksand" in air
A centrifuge works for ## ^{235}U##. How about spinning the mixture while vibrating.
Are the corks regular enough in size to just send them over an air curtain on a mesh belt and "tune" the air speed like @Dale pointed out?
Hi, thanks for your response. Indeed liquid H and He wouldn’t be a cost effective solution. The dimensions can vary, so I should try the air curtain to see if it’s failproof. How would the quicksand in air setup look like? Have you got an example of that?
 
  • #19
Have you got an example of that?
Sorry I don't. These were just my own ruminations. I do like @Baluncore examples very much.
 
  • #20
Summary:: How to separate natural corks from synthetic corks?

We have two Polish workers who are able to do this just by sight, 8 hours a day.
If the Polish workers can do it by sight then there must be some optical indicator. Perhaps they can explain what it is that they see, or perhaps you can use them to produce a training set for AI.
 
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  • #21
I too like @Baluncore's suggestions. But I also like the quicksand idea.

Rather than a liquid, it might be better to have a fluidized bed of plastic pellets with a source of air from below. Adjustment of the air flow should vary the buoyancy of the bed. That sounds like air separation with a twist.
 
  • #22
Synthetic cork is an insulator, whereas natural cork is a bad conductor. It takes a second per cork to separate them in this way.
 
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  • #23
We have two Polish workers who are able to do this just by sight
So there must be some visual difference. Have you tried image recognition?

It turns out that the density of the natural corks is between 180-200 g/cm^3 and the synthetic corks are 230-260 g/cm^3.
These densities are very close and so I suspect that any attempt at centrifuging or air separation is going to be thwarted by shape and size variation (Brazil nut effect).

How about some kind of hopper feed to create a single line of corks and then measure and weigh them individually? 12 million corks a week processed 24/7 is about 20 per second or 1 m/s line speed.
 
  • #24
Or can you poke them with something and detect a difference in hardness, elasticity or speed of restitution?

Surely there are some frequencies of light that one will absorb much better than the other?
 
  • #25
Synthetic cork is an insulator, whereas natural cork is a bad conductor.
Being soaked in a sweet wine might make some synthetics more conductive, unless all corks were washed before testing.

I would expect synthetic corks to bounce better than natural corks, but I don't drink enough wine to be sure. Maybe you could take the corks to a golf driving range and as an experiment, sort them by distance.

A cyclone separator is used to separate particles based on density by adjusting the diagonal flow to the sink rate of particles. Low density particles rise with the air, while more dense particles sink.
The aim would be to remove the top and bottom quarters in the first pass, the marginal product can go around again inside the cyclone, or pass through the process again. That way the purity of the sort can be high. If high purity was not needed then why sort the stock? Consider inventing a recycling process that can handle the mix of both synthetic and natural as a feed stock.
I remember one 8 inch industrial system being used to vacuum out semitrailer loads of threshed crop, and then sorting it into different process lines. I built a “blockage” alarm with a “slow it down” bell for the vacuum cleaner guy who would stand in the load and could feed it too fast. The portable alarm bell came back a few times for repairs after getting sucked up, luckily it wasn't wireless. In the end the bell was tied to the operators belt, so I lost contact with that part of the project. In an emergency, after the power failed at the peak of harvest, I was shanghaied into a gang of four, shovelling a couple of 75 m³ trailers out over the side by hand. The crop was bulky, so it weighed little, like cork. Vacuum handling, with a cyclone separator, was certainly the clever way to go there.
 
  • #26
I would expect synthetic corks to bounce better than natural corks
I tried that first thing, couldn't see much difference (I had some corks propping up my laptop).

Now, brainstorming, the less dense ones would fly farther, neglecting air resistance.
Toy_popgun.jpg

[Jerrid322]
 
  • #27
If the Polish workers can do it by sight then there must be some optical indicator. Perhaps they can explain what it is that they see, or perhaps you can use them to produce a training set for AI.
Why would they help in automating their job?
 
  • #29
Now as an Engineer I would of course like to optimise and if possible automise this process..
What happens when you compress/crush the two types of cork? I don't have test samples here. But if for example the natural cork breaks into small pieces easier, while synthetic cork stays in one piece under the same deformation, that would make it much easier to separate the material with an air blower.
 
  • #30
When I rub corks on a plastic surface, the synthetics have more friction (some plastics much more than others).

bounce
They might have a different sound when whacked.

Also:

Making Agglomerated Corks

Cork waste is processed by a machine that breaks it down into small bits. The pieces are cleaned and dried before being sent through two grinders in succession to reduce particle size further. These particles are filtered for uniform size after further washing and drying process.

Pure agglomerated cork is created by compressing the cork particles into a mold and firmly sealing it. The mold is filled with superheated steam (about 600°F or 315°C).

Alternatively, the mold can be baked for four to six hours at 500°F (260°C). By activating the inherent resins in the cork particles, either method bonds them into a solid block.

Compound agglomerated cork, also known as composition cork, is created by evenly coating cork granules with a light layer of an extra adhesive agent. The coated grains are placed in a mold and gradually heated.

After being extracted from the mold and allowed to cool, the blocks are stacked to facilitate air circulation and seasoning. The agglomerated cork is then trimmed to meet the size for its intended purpose.
https://advancedmixology.com/blogs/art-of-mixology/do-you-know-your-wine-corks
 
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