Pet

In flake form, it can be used to spin fibers, to extrude sheets for packaging and for all other "non food" applications.

Here will be focusing only the washing process and, at the end of this document, we will be talking a bit about pelletizing and Polycondensation (SSP) required to achieve a quality suitable for food applications.

We will talk extensively about the different steps to achieve perfectly clean flakes as we want you to understand why the line has so many machines and the purpose of each one.

Let’s start  by dividing the line into blocks that we define as:

Sorting  - Granulation - Pre-washing – Olefins separation - Washing – Drying

 

Sorting

 

We consider the worst case, which is bottles coming from curbside collection, meaning the collection of transparent bottles.

Normally the scrap is coming to the recycling facility pressed into bales, and inside our bale, we will have PET bottles (clear, green, blue and some other colors) some clear PVC bottles that are still around, few HDPE bottles, Aluminium cans found here and there, may be a glass bottle, crushed or not, a polystyrene tray or a coffee cup (always present) and, why not, a pair of socks.

To the granulation phase of the process MUST arrive only PET, and therefore, a pretty sophisticated sorting system should be set up.

If you are going to buy already sorted bottles, sorting colours will be much easier, the amount of PVC bottles will be less, no glass bottles will be present ,and, if your supplier is really serious, not even the socks but you better be sure that nothing but PET reaches the granulator.

Negative and positive sorting methods can be used:

In“negative”, bottles are running on a conveyor belt and people standing besides it are picking up whatever is recognized to be something different than PET.

As known, PVC bottles, after being folded by the baler, are turning opalescent at the edges while PET remains clear, and this is the way people can recognize PVC bottles.

Of course, this apply to clear bottles because on green and blue there is no PVC.

With negative sorting, same people can pick up other different type of garbage, normally easy to recognize because of the different shape.

Negative sorting can be chosen if throughput is not more than 500 Kg/hour.

It is normally calculated that each person can handle about 150 Kg/hour with such a method.

In “positive”, people pick up each single PET bottle, and in this way, of course, they should pay more attention, tossing them on a side conveyor belt going to granulator.

This means that whatever stays on conveyor belt is considered “ garbage”.

This is far more accurate than negative sorting because every single bottle has been picked up and seen by the operator.

This is good also when colours should be divided as well on a third conveyor.

With positive sorting each person will handle from 80 to 100 Kg/hour of bottles, so this one requires a little more personnel than the previous one but, again, is much more accurate.

 

Sorting systems can be combined together or, in other words, the system can have a positive sorting first and a conveyor with polarized light second or an automatic sorting machine and so on.

With a low to medium worker cost, the best and safe way to get rid of PVC bottles is a conveyor belt with a polarized light; a set of white light bulbs are placed at the bottom of the conveyor and the operators, standing behind lens, will recognize PVC bottles because colour turns black while all PET remains transparent.

Actually the system is not as simple as it looks because of some other involvements, but it is pretty simple and economically affordable.

For high capacity system and where workers just cost too much, the only way to go is a fully automatic NIR sorting machine(s).

NIR stays for “ Near Infrared Rays” and when non-PET bottles are scanned, having a different response from the others, are discarded by an air nozzle.

The average capacity in terms of separation of these devices ranges from 94 to 96 % of contamination, therefore in most cases, among all these possibilities, a combination of  manual and automatic sorting will be the way quality is achieved and costs are kept somehow low.

At this point we have our bottles free from foreign objects, separated by type of plastic, colour, metals  ready to go into granulation.

 

Granulation


As a preamble, we want you to know that, normally, granulation is the weak part of any system because, does not matter what you do, blades will wear out in a time shorter than you expected and blades’ change will take twice as much than forecasted.

Blades should be sharp as much as possible all the time, otherwise the edge of the flake will be indented and it may trap some air making it floating into separation tanks and causing some other small problems.

Knowing this facts, let’s see which are advantages and disadvantages between wet and dry granulation.

Going with wet granulation, meaning running the machine together with water, blades last a bit longer and this is, a big advantage; on the other side water is the only vehicle available to make flakes to go through the screen, and this decrease production comparing granulators with the same size.

Going with a dry granulator, blades life is a bit shorter, no doubts about it, but granulator can be equipped with a powerful aspirator to increase production (always comparing it to the wet process) and the big advantage is that all labels, paper and plastic, can be removed by a pretty simple air separator, making the life of the rest of washing line more easy.

We do also suggest to put granulator(s) in a pit in order to make sound-proofing more effective and less costly.

This due to the fact that granulation, no matter if dry or wet, makes a lot of noise and sound-proofing is compulsory wherever you are.

 

While cutting, granulator removes by friction some of the labels sticking to the surface of flakes and, of course, detach labels from the bottles. The final result is a mixture of PET flakes, some with the label still sticking on the surface and some free from labels, paper and/or other plastic labels in LDPE ,PP and sometimes PS.

First step is to separate most of the labels right before flakes go anywhere. To do this the common way is to use a “labels’ separator” which is a device working with an air counter-flow where everything light (in our case labels) are carried by the air while heavy pieces (PET flakes) fall down to the discharge hopper.

This device works very well when labels are dry, because they are lighter than PET, otherwise, if wet, they will be heavy and will not be separated or, worse than this, will start sticking on pipes, cyclones walls, to the hopper and so on.

From the label’s separator, flakes will be discharged into a buffer system.

A buffer system can be set up in many different ways and it should be shaped and sized taking in consideration what we have before it and after as well.

For example, using one granulator, we need to have a storage capacity greater than the time of changing blades, in order to have the washing line working anyway, doesn’t matter if granulator is working or not. In this case bigger doesn’t mean more expensive but only more difficult to handle because, like all accumulation devices, it’s pretty easy to fill up but not that easy to get the material out of it, particularly if material is dirty, sticky and , you name it.

And don’t tell us about agitation devices to keep material moving or as aid to extraction because this stuff make everybody’ life miserable, specially the maintenance person/s.

Sometimes, more than one storage tank should be set up, to work with a different colour, for custom washing, for material changes, or for some other reasons.

At the end of all this, we got our material, the way it should be, into a storage tank.

One may think why a storage tank is required if it is so easy to go straight from granulator to the washing line ?

First we need to fill the washing line with a certain steady quantity of flakes all the time, because washing needs a certain amount of time, because sink-float tanks performs much better if material is coming constantly, because the whole line, drying included, doesn’t support flakes variation.

Second, a granulator cut whatever quantity you like just after replacing blades while after few hours production decrease a lot, or, for some reasons, feeding of granulator is not constant all the time.

And last, but not least, operator at the beginning of the system, should know if he’s feeding too much or too little and the only way to know is to check the level of buffer tank, or put a couple of level controls on it, to tell the operator how he’s doing.

So, after material is separated from other plastics, metals, most of labels and cut, everything is ready to go into the washing line.

 

PRE WASHING


Out of buffer tank, flakes are dosed to a pre-wash system that’s a pre-washing machine working with water and a dryer.  The pre-washing machine convert most, if not all, paper into pulp which will go with the water through the final screen, while flakes will come out paper and surface dirt almost free, before going to a buffer tank to be dosed into first sink-float tank for olefins separation.

 

OLEFINES  SEPARATION

 

In a sink-float tank all LDPE and PP labels are supposed to float because olefins have a specific weight less than 1and  should be removed ,but unfortunately it doesn’t happen.

First we still have labels glued to the surface of PET and, when together, the flake sinks, and this is fine, but labels stay attached to PET flakes also because of surface tension of the water itself.

As you may know, surface tension is so strong it can keep floating a coin if you put it flat on the surface of water, and this apply to flakes as well.

So we do have to make whatever sinks to sink and floatable to float, removing the surface tension of water.

How we do this: conveying all flakes under the surface of water into the tank and “agitating” everything to open up flakes, thus removing the surface tension.

It is compulsory to do it to work in the most efficient way.

After separation from olefins, flakes go to the next step, the washing. 

 

WASHING 

 

What washing means: our meaning of washing is take a flake, or whatever else, and heavily brush it with something, for a certain period of time, better if using hot water and some detergents. A caustic solution is also needed for a better removal of contaminants such as glues and oils.

Washing under heavy friction does stress the polymer so time is important as well.

Glue itself is not a contaminant but, leaving it into, makes turning the color during extrusion to brown, because melting point is a lot lower so its removal is compulsory.

Washing is performed using a caustic solution plus the friction applied by a washing machine. This machine detaches the glue that is conveyed outside by means of the caustic solution continuously filtered by a filtration system kept at the desired processing temperature by an heat exchanger.  

The washing machine used in the process has an electronic control by which operator states the amount of friction, and thus residence time, while flakes are kept under the caustic solution.

By the friction rods of the rotor are making, water (the caustic solution) heats up increasing the effect of caustic and therefore decreasing the residence time needed to remove the glue.  All remaining labels which are still around, are detached from PET and come loose therefore a second sink-float tank is required to complete the job.

The water going in this tank is always clean and kept clean in processing.

The right residence time in the washing machine is the one needed by the caustic soda to go in touch with the glue and detach it from the surface of the polymer and, using the proper water flow (the same caustic soda solution) carry it out from the machine.

After washing, all remaining labels detached from PET, come loose therefore a second sink-float tank is required to complete the job.

 

A proper rinsing is also necessary to get rid of all caustic residues from the flakes. The residue amount of caustic can degrade the polymer when it goes to hot drying or to extrusion therefore we should get rid of ALL caustic to get a PH of 7.

This is done with a Rinsing machine which will completely remove all traces of caustic and other chemicals from the surface of flakes. While doing this, this machine also provides for further washing. A pressure pump, with an adjustable flow control, feeds water continuously and discharges it in order to trap the contaminants and convey them outside the machine.

 

DRYING

 

Drying is the final step of the process.

This operation is carried out by a mechanical spin-dryer that delivers material with a 0,7% moisture content.


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