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The different sorting methods for plastic scraps
 
Technology in this field did big steps ahead lately.
Of course we're talking about automatic sorting systems.
NIR detectors can sort all kind of different plastics, together with colors etc.
Easy, isn't it ?  OK, lets go to see the back face of the medal.
Let's divide also this page into different chapters:
 
Plastics coming from garbage selection:
For this purpose we are keen to approve this way of doing because the NIR detectors can handle a pretty good quantity of scrap material and because separation is, in practice, something like 92-95% the machine pays for itself in not a long time.
Please note we said 95% top as separation efficiency. (actually like manufacturers do)
So, if after this you go into a washing and separation process, you' will remove the rest of contamination pretty easily with some exceptions.
The HDPE bottles stream will have some (little) film (LDPE), some PET and PVC bottles and practically nothing else, separation in any washing line will easy and the product at the only HDPE and the 2% LDPE that, may be remains inside, doesn't bother at all.
Almost the same applies to PET bottles that require further sorting anyway. (we'll talk about this later)
As far as LDPE film concern here the true:
Together with film there will be HDPE bottles, and this not big deal, some PET bottles as, and this can be handle quite easily as well, some PS trays not easy to remove by sink-float tanks but, the most difficult and messy materials will be sponges, all kind of foamed materials and last but not least, cleaning pads, a piece of cloth, some cotton fabric and craps like these.
OK, you'll say, it will be matter to put on more NIR device to sort out the 95% of the 5% that was left before.
Wrong.
If for some reasons the machine didn't recognize in the first separation step, there is a reason and, more than likely, it will not recognize it in the second step, the third and so on.
So, efficiency of a second machine will be way less than 95 % and up to you to state if it is convenient to go this way or do it in some different way.
What we suggest in this case is to improve the efficiency of separation into the washing line to get rid of most of contamination (all contamination separation will never happen) without pretending to get a pellet good for film or even close to this; pellets will be good for injection molding, very thick sheet and applications like these.
For more information go to Mix Plastics Washing Line.
 
As far as HDPE bottles, not much to say, simply because also if there is some contamination left, a good washing line will take care of all of it and the product at the end will be absolutely pure.
 
Now, let's go to the PET bottles sorting:
If what's coming is supposed to be only PET bottles, here what you'll find:
HDPE bottles
Aluminum cans
Little glass (normally broken already)
Little PS in form of trays, cups and sticks.
PVC bottles (it is in the last place for a reason)
If you find something else, just start complaining with your supplier, doesn't matter what the price of bottles is.
HDPE bottles don't bother at all, unless it is 50% of the total amount, but this never happens.
As far as Aluminum cans, they can be sorted out by three different methods:
First is electrostatic separation and, because it is a costly device, it worth having it if the amount of cans is pretty high and what you get from the aluminum itself pays off for the machine.
Second is what everybody does, that is manual sorting; Aluminum is easy to recognize even for a sleepy person in the middle of the night and shouldn't be a big problem anyway.
Third is to wait till it gets to the metal detector and remove it when the conveyor belt stops; this, of course, if the amount of Aluminum is very little.
There are also continuous detectors scanning Aluminum flakes and sorting them out but this should be put at the end of the line only for security purposes and, if it is discarding too muck, because it removes also quite many PET flakes together with  the Aluminum one, you better find a better way to sort it out at the beginning of the line.
Glass, in pieces, will be automatically removed by the trammel.
If a whole glass bottles is into the bale of PET bottles, call your supplier and smash it on his head. He will remember.
PS is unfortunately pretty common into PET bottles scrap bales.
It is easy to remove because it doesn't have the shape of a bottle in any case.
If you go with the positive sorting, you'll never find  a piece at the end of the line while with negative sorting, personnel should care about this material.
It sinks like PET and it stays with it all the way till the end.
Some of it will be lost by the rinse-dryers and dryer because a lot more brittle, but some will remain and will contaminate your final product.
Now let's go to the last one, not the last in order of importance:
PVC is the most dangerous contaminant for the PET, and everybody knows this.
Being the ideal quantity equal to zero but a fair amount that can be tolerated is 10 PPM (parts per million) so let's see if there is any way to reach this quantity.
We said NIR automatic detectors are doing a good job separating different plastics so let's run some numbers:
The average quantity of PVC found into bottle bales is about 1% -AVERAGE-
Even taking in consideration this as maximum quantity, we do have as starting point 10.000 PPM (1%)
With the NIR detector, with a sorting capacity of 95%, that's the best case, we therefore remove 9.500 PPM but what remains is 500 PPM that's 50 times more than what we need.
As we said at the beginning of this page, if the NIR detector didn't recognize the PVC bottle in the first chance, it will be difficult it will do it in a second one.
This can happen because of many factors but here we'll be talking about the fact we still have PVC into our PET stream and not way it is there.
Another good sorter is the (old) x-ray detector; old because it was the very first detector used in plastic recycling and not widely used any more because it recognize only PVC.
But is isn't this exactly what we want ?
An x-ray detector has an higher sorting capacity that can go up to 99% and because it uses a different scanning method, more than likely it will discard PVC bottles the first machine didn't recognize.
And 1% of the 500 we had before makes 5 PPM that's a good number indeed, meaning that even if contamination at the beginning has a peak up to 2% we still are on the safe side.
Everything almost perfect, till the time you get the offer of the two scanners and start scratching your head because we are talking about expensive devices.
Let's see instead what's available to get to the same result without spending a couple of 200.000 dollars or more.
(We forgot to mention colors sorting but this shouldn't be big deal for anybody)
Polarized and UV light is something available at a way cheaper price (if we do not consider the people involved)
Polarized light is nothing but a white light going through a piece of plastic sheet that polarize the light (let the light to go in only one direction) and if operator wears a pair of polarized glasses, will easily recognize PVC bottles, that turn dark, from PET remaining clear.               Sounds easy ? Wait till the end.
UV light is the second chance to sort out PVC bottles.
You build up a box, because it shouldn't be any other light with different frequency than UV range, and your PVC bottle will turn rainbow color while PET bottles remain clear.
Let's go to the other face of the medal.
In a 1.000 Kg/hour system, because the average weight of our bottles is 35 grams each, we'll be talking about 29.000 bottles per hour to scan.
Because scanning, doesn't matter if using polarized light or UV, should be "mono-layer" meaning bottles cannot be piled up on the conveyor belt and because it takes a certain time to check every single bottle, we'll be talking about a lot of persons and/or multiple conveyor belts to get the job done.
Like everything in this world, there is also something in between, meaning a first manual sorting (positive sorting if possible) and a final automatic machine for the final control.
This actually what most of everybody does and it has been proven to be the cheapest and safer way to go.
Few more words about what we said in the pre-washing section; if bottles will go through an hot water trammel and specially into the hot air trammel for bottles shrinking, PVC bottles will turn slightly brown and manual sorting will be easier and faster.
On the other side, energy consumption will be pretty high.
We will not go to any conclusion here; the best we can say is each system requires its own sorting method depending, as you sure understood, by quite many factors.
And only yourself know your reality.