PET BOTTLES SORTING

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.