Visit our new Website/Visita il nostro nuovo sito

Leda plastic recycling  Plastic recycling washing line 
 

HOME

 Post-consumer PET bottles washing lines

BACK

PET bottles recycling system for Food applications

Facts and figures:
If you’re thinking about a recycling system to go from kerbside PET bottles collection all the way to pellets for food application, here some explanation to make you better understand what it is all about.
Flakes for food purposes must have, at least, the following characteristics:
 

·        Flow ability:   

Free flowing

 

·        Bulk density    

Kg/m3

350-400

·        Intrinsic Viscosity

dl/g          

0,75 +/- 0,03 dl/g

·        CIE (b) colour (Gardner method) 

-

< 1

·        Fines (< 0,5 mm)

%

< 0,3

·        Particle size distribution

mm

6-12 mm

·        PH value 

7 +/- 0,5

·        Moisture content

%

< 0,7

·        Chemical contamination (wash residuals)

ppm

< 50

·        Floatable contamination

ppm

< 60

·        PVC contamination

ppm

< 30

·        Glue contamination

ppm

< 10

·        Metal contamination

ppm

< 20

·        Other non-melting particles

ppm

< 45

·        Green Pet contamination  

ppm

< 500

·        Light blue contamination

%

< 10

·        Black PET etc.   (e.g. serving trays or material with similar density to PET)

ppm

< 5

·        Glass contamination

ppm

< 5

This as far as washing concern, while the final product to be used into food application should be further pelletized and IV upgraded through a Polycondensation process, in flake form, it can be used right away to spin fibres, 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).
It goes by itself that material (PET flakes) should be perfectly clean before going to the last steps of the process and hereafter we will talk extensively about the different steps to achieve perfectly clean flakes; by this we want you to understand why the line has so many machines and the purpose of each one.
Let’s start dividing the line into main categories that we define as:
Sorting  - Pre-washing - Washing – Drying

Sorting

We assume the worse case, saying bottles coming from kerbside collection, meaning the collection of transparent bottles.
Into bales, that’s normally the way scrap is coming to the recycling facility, we will therefore have PET bottles, clear, green, blue and some other colours, some clear PVC bottles that are still around, few HDPE bottles, an Aluminium can be found here and there, may be a glass bottle, that has been crushed by the baler, or not, a polystyrene tray or coffee cup (always present) and, why not, a pair of socks.
Because, of course, what has to arrive to granulation phase of the process MUST be only PET, a pretty sophisticated sorting system should be set up.
If you are going to buy already sorted bottles, situation doesn’t change much, if not for the fact sorting colours will be much easier, the amount of PVC bottles will be way less, it will be practically no glass bottles and, if your supplier is really serious, not even the socks but you better be sure that nothing but PET reach the granulator.
The very first machine of the line is a bales breaker, a machine that will deliver single bottles out a compressed bale where bottles are stick together.
The lay-out of this machine is pretty simple and it looks like this:

 

 This machine, like the other that will follow, can be, and normally is, customized according customer needing, depending on average size of bales, production rate and space available on the building.
Because we are trying to give you as much as possible information here, keeping in mind economics anyway, bales breakers are useful when production rate of the line is over 500 Kg/hour and/or bales are very heavy and compressed.
Bottles are therefore collected by a conveyor belt equipped with a permanent magnet mounted on a perpendicular small conveyor belt, to automatically extract steel and/or a metal detector to stop this conveyor in case some other (non ferrous) metals are present.
As you will see, like in this case, there are many options available because if your suppliers swears to you it will never be metal parts into bales, a simple, inexpensive, metal detector can be enough while if you get material from unknown source, to be on the safe side, the conveyor needs to be equipped with both, and may not be the end of it.
This conveyor has also the task to elevate bottles to the inlet of the trammel.
The trammel is a big rotating screen having quite many purposes:
First one is to get rid of small pieces that can be broken like glass bottles, broken PVC bottles that get brittle after being stored outside under the sun (because of UV rays), stones, few loose caps, and other stuff that will not be PET anyway, so something to get rid of.
One more thing this equipment does, is the opening of the many bottles that may still trapped together, and this because the trammel is equipped with paddles that lift bottles up and let them fall down several times before going out.
Another very important job carried out by this device, is the fact that it normalize the flow of material coming from bales breaker and this is very important for the equipment that follows.
At this point it’s difficult to better explain this concept but, if you like to go head with the reading, we will get to the point where this will make a lot of sense.
The machine looks like this:

 

 Also in this case configuration and dimensions will vary according to production rate, set up of the building, handling of garbage and so on.
We will say many times that most of machinery needs to be customized according to customer needs because of the simple reason that, not paying attention to details, customer can be in trouble with “little things” that, added together, make the line difficult to handle.
Few examples here:
If the outlets of what we call garbage, meaning whatever is not a PET bottle, are easy to reach, removing will be easy and fast as well otherwise it will take one or more person to handle only garbage and this is a cost (and a messy place where to work)
If the line will work with chemicals, you will need a safe storage place and this place should be set according to building layout.
In other words, if initial set up is taking care of all aspects of the job, line will be “easy” to work with and costs will be always the same, and something you know well in advance, otherwise surprises will be just around the corner and you’ll never know when they will show up.
So, we are at the point where our bottles are in a single form, metals free, and coming with the throughput rate we expected.
We do still have commingled bottles anyway that need to be only PET at the end so:
Bottles are falling out from the trammel to a flat conveyor belt.
This conveyor belt can be a simple one if your bottles have been pre-sorted.
But, as we where saying before, if you are getting bottles from kerbside collection, you should expect some “other plastics” contamination.
Therefore we should take care of sorting in the best possible manner.
Here, like anywhere else, it is possible to make a perfect selection, fully automatic with the guarantee of getting 100% PET bottles and nothing else but, specially if you are at the beginning, you don’t want to spend a little more than a fortune (about a million) only for sorting. Will you ?
When you reading this document, please understand it is a general one because the situation can be very different according with the country the system will be installed in; for example, if the cost of an un-skilled worker in your country is 4000 Euro a year it is a story, while if it is the same amount per month, situation definitely changes.
And you should choose the right device accordingly.
Here following a description of the most common ways of sorting methods that are widely used on a PET bottles recycling system.
If you will use some common sense, you will be the only qualified person to choose the way it should be done and how much the cost will be.
Negative and positive sorting methods:
The common meaning for “negative” is 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 you may know, 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 practically no PVC.
With negative sorting, same people can pick up other garbage of different kind, normally easy to recognize because of the different shape.

 

Negative sorting can be chosen if throughput is not more than 500 Kg/hour, otherwise the amount of bottles will be to much and this system will never give you good results.
It is normally calculated that each person can handle about 150 Kg/hour with such a method.
For “positive” is meant the fact people pick up each single PET bottle, and this way, of course, people should pay more attention, and toss 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 is, again, much more accurate.

 

If you go a head reading, please keep in mind that sorting systems can be combined together or, in other words, you can the system up in the way there is 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 persons, standing behind a 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 being a non-crystalline polymer, are discarded by an air nozzle.
Till this point, no doubts this is the way to go.
Now, let see the dark face of the medal:
The average capacity in terms of separation of these devices ranges from 94 to 96 % of contamination, so let’s run some numbers:
Because we need to get a final product with 30 PPM of PVC (or less) , meaning 0,003% and having, in the best case 96% separation (meaning 4% left into the bulk) guarantee, these are the calculations:
If initial contamination equals 1% the result is    1 x 4 / 100 = 0,04       (too much)
If initial contamination equals 0,5% the result is 0,5 x 4 / 100 = 0,02    (too much)
To make a long story short, you need to start with less than 0,07% to get to the 30 PPM and there is no way you will ever find a supplier that gives you this kind of number.
Let’s see what we get having two of these machines in series always starting from 1% that’s a common number:
1 x (4x4) / (100 x 100) =  0,016% and this is the number we want.
This means we do need two devices in series to get the result and, at this point you know how much a good quality product cost, only for PVC separation.
On my opinion, what make more sense in most cases, among all these possibilities, is a combination of  manual and automatic sorting in the way quality is achieved and costs are kept somehow low.
At this point we have our bottles separated, by type of plastic, colour, metals and foreign objects free and therefore ready to go into granulation.

Granulation

Everybody knows what a granulator is, so this chapter shouldn’t take much to be written and read.
We want just talk a little bit about the different technologies used for this specific job and give you our opinion about it.
Known choices are the following:
-  Pre-ground bottles with a 50 mm. grid to make the granulator blades to last longer and, after washing and separation downsize flakes to required size.  Process can be dry or wet.
-  Add water into granulator while cutting to get a kind of pre-washing, with a standard size grid.
-  Use the standard process, meaning dry cutting with a standard 12 mm. grid.
As preamble to this chapter, 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 anyway shorter than you expect and blades change takes twice as much than forecasted, all the time.
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, as we said before, a big advantage; on the other side water is the only vehicle available to make flakes to go trough the screen, and this decrease production comparing two granulators with the same size.
Of course, running the machine with water, some washing takes place, specially for paper (paper labels) and other surface contaminants and this is, again, an advantage, but plastic labels will stay into and there is no way to separate them into this phase of the process.
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) but the big advantage stays in the fact that all labels, paper and plastic, can be removed by a pretty simple air separator, making the life of the rest of washing line way more easy.
We will not be talking about the grinding of bottles with a big screen size because this is not compatible with our set up of washing line but this can be a choice if caustic soda washing takes place into what I call “ big pots” .
In our set up, we do suggest two dry granulators, as shown in the following drawing, where one is always working and the other one is stand by, or under maintenance, and ready to in operation as soon blades of first one get dull.

 

If it is true the second granulator cost money, it’s also true that the time saved, practically never being down for maintenance, pays back to cost of the granulator in no time.
Third, small granulator was set up in the line in the figure for grinding colored bottles.
We do also suggest to put granulator(s) in a pit in order to make sound-proofing more effective and less costly.
In fact another aspect of granulation is the fact that, no matter if dry or wet, the machine makes a lot of noise and sound-proofing is compulsory wherever you are.
During cutting action, granulator removes by friction some of the labels sticking to the surface of flakes and, of course, detach labels from the bottles therefore the final result is a mixing of PET flakes, some with the label still sticking on its surface, some free, some free paper labels and some out of  LDPE and PP (and sometimes expanded PS that fortunately floats as well).
Firs step is to get rig of most of labels right before flakes get anywhere; to do this the common way is to make flakes go through a “labels separator” that’s 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 lighter than PET otherwise, specially paper labels will be heavy as well and will not be separated or, worse than this, will start sticking on pipes, cyclones walls, to the hopper and so on.
At this point, flakes will fall 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 to work anyway, doesn’t matter if granulator is working or not, while with two, as suggest by us, the size of buffer can be way smaller.
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 them up but not that easy get the material out of it, specially if material is dirty, sticky and , you name it.
And don’t tell me about agitation devices to keep material moving or for aid to extraction because these stuff make everybody life miserable, specially the maintenance guy I have a lot of respect of.
Sometimes, more than one storage tank should be set up, to work with a different colour, in case of custom washing when changing material of for some other reasons.
All these considerations did make us to think two granulators are not that expensive at the end because this kind of set up makes the rest of the line more easy to handle and working for the stated number of hours all the time. (unless something crazy happens)
At the end of all this, we got our material, the way it should be, into a storage bin.
All this is fine, you may think, but why a storage bin 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 hydrocyclone doesn’t support flakes variation and drying as well.
Because granulator cut whatever quantity you like just after replacing blades while after few hours production decrease a lot or, for some reasons, feeding to 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 into bales-breaker of feeding belt and the only way to know it 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.
This to make everybody to understand that washing line itself doesn’t make miracles but washes only some kinds of dirt in a certain way and under certain conditions we will see in the next chapter.

THE WASHING LINE

Here the matter will be a little more difficult due to the fact that final product can be used for many different applications and, it is true everybody wants the best possible quality, but costs are directly proportional to quality level so you may want to think about it.
We will always be talking about PPM (parts per million) even in the worse case but few PPM here and there is what makes the big difference for the final application of flakes.
Categories are internationally recognized as “for fibres production” “ for strapping, spun bond and packaging sheet” and finally “for food applications”.
Few words about the most “simple” line for fibres (monofilaments) production.
Out of storage system flakes come dosed to what we call “wash-dryer” that’s nothing but an horizontal centrifuge working with water in the first part and as a dryer in the second one; this makes most, if not all, paper to convert to pulp and go trough the final screen, so flakes come out almost paper and surface dirt free before going to first sink-float tank for olefins separation.
This first wash-dryer has a low power consumption because residence time is not much indeed but it does a good job because flakes at the outlet look pretty clean (but they aren’t).
Now everything goes into a sink-float tank to remove floatable that, in our case, are LDPE and PP labels.
Because olefins have a specific weight less than 1, all labels are supposed to float; unfortunately doesn’t go this way.
First we still have labels glued to the surface of PET and, when together, the combined 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: convey 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 this because this line will not have any other separation than sink-float tanks therefore they have to work in the most efficient way.
After separation from olefins, flakes go to the washing machine that’s a machine specialized in 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.
This is the result of a long study that my grand-grandma did in the past.      And still works pretty well.
Washing under heavy friction does stress the polymer so time is important as well.
After washing, all remaining labels are detached from PET and come loose therefore a second sink-float tank is required to complete the job.
Drying is the final step of the process.

THE WASHING LINE FOR FOOD APPLICATIONS

While the general concept remains the same a lot more attention should be paid to each step of the process to achieve the results required by this kind of set up and this is the way we do it:
We get flakes, as said before, from a screw conveyor driven by an inverter, meaning a stated constant flow of material; this is more than very important for all machines.
Out of this screw conveyor we go into a pre-washing machine to get rid of surface dirt, doesn’t matter how much it is, paper that’s converted into pulp, most of glue, because we do use hot water, and whatever else is sticking on the surface of flakes.

 

Material goes then to a rinse-dryer machine just to be sure that all dirt is gone and conveyed to first sink-float tank.
Till this point we did talk very little about water and the importance this has on the washing process but, from now on, please pay attention to it because water should be used the best possible way to make sure all process is kept under a stated cost.
It may be a little difficult to understand how everything works, as far as water concern, because the inlet point of clean (filtered) water is at the very end of the system and it goes backwards to the beginning that’s the point we are just talking about.
So, the washing machine creates heat by the friction developed and discharge heated water to a filter where all solids (no chemical contamination) are removed and goes back to the washing machine.
On a close loop system, water becomes more and more hot and washing effect improves a lot, specially as far as glue contamination concern.
For countries where landfill costs are expensive, we do provide for a “compactor” so the sludge is almost water free and discharge costs consequently less.
At the end of  “pre-washing” our flakes look very clean, with some plastic label left so all we have to do is to remove labels by sink-float and the remaining glue.
And this last one is not going to be an easy job.
As we said before, flakes go to a first sink float tank to separate floating labels from PET flakes.
To do this, we “force” flakes under the surface of water and by means of an agitator, we do remove surface tension that keeps flakes together and “spray” them onto the tank in the way whatever floats will go up to surface and sinkable will fall to the bottom of the tank.

 

Because we are talking about millions of flakes per hour and, as we constantly repeat 100% in recycling doesn’t exist, chances that few flakes that are supposed to float will sink are quite many.
To better explain the concept there is a huge difference between 100% and 99,9999; 100% means nothing left while 9,9999 means 10 PPM and sometimes 10 PPM are too many.
Back to our sink-float tank, this one separates most of labels the way we said before but, to improve separation efficiency we do provide for an hydrocyclone.
Don’t be scared by the word itself; hydrocyclone is a pretty simple device that improves the difference of specific weight of two, or more, materials.
It is nothing by a normal cyclone working with water.

 

We’ll not be going to go in deep technical details about this one, but only tell you the hydrocyclone receives water and flakes together and spinning into a cylinder ending with a cone, by centrifugal and centripetal force, separates floatable by sinkable.
Sinkable will go out trough one outlet and floatable will take a different way, back to sink-float tank in our case.
Because material and water are collected, and pushed, by an high speed pump, material will have another “washing step” because whatever makes friction will wash our flakes; and this is not bad at all.
At the outlet of the cone of the hydrocyclone, we will have only PET flakes, with some PPM of olefins left.
The water of this tank is kept clean by the flow it receives from the second sink-float tank where clean water is going in.
We’ll talk later about water quantities required by the whole system because it will be stated by the kind and quantity of glue still present on PET flakes surface.
At the outlet of hydrocyclone, because we do have water and flakes together, a dewatering device is needed and, after separation flakes are ready for washing. (again)
Yes, washing because we already did remove dirt, paper and other contaminants but what everybody beware the most, is glue.
To get rid of it there are very many systems available
Our philosophy is “make it easy, fast and continuous” and here the way we do it.
We do use the same machine we did use for pre-washing using a caustic soda solution or an appropriate detergent to “dissolve” the glue.

 

Let me give you some figures to make you better understand what we are talking about: every single bottle has an average amount of 1,5 grams of glue to keep the label onto.
In a system working one ton per hour (at the outlet point) means a feeding (at the inlet) of 35000 bottles, times 1,5 grams equals to a little more than 50 Kg/hour of glue to get rid of.
It is true not all bottles have a glued label but, still, we are talking about a big amount indeed.
Caustic soda together with fat (our glue) under agitation in a hot environment makes soap, and this is something everybody knows and, again, we do use a method that’s been invented few millenniums ago.
Flakes go into washing machine where and high agitation occur, together with friction heating up water with few % caustic soda, or detergent, and glue starts the process of “saponification”
To know more about how this exactly works please go to:

http://chemistry.about.com/library/glossary/bldef825.htm

This machine keeps flakes under washing for a period of time that can be variable and set by operator; time is important because, as we said before, a too short time means to not let the caustic soda detach the glue while a too long one means to let the caustic to react with the polymer and degradation occur.
“Saponification” is in fact the opposite of “esterification”
The right time 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.
Out of washing machine a proper rinsing should occur and it does with a rinse-dryer that delivers flakes with a mere 0,08% of caustic.
0,08% means 0,0008 grams per litre that unfortunately is not as little as it looks; this amount of caustic is still in position to 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.

 

Let’s talk for a bit about the treatment of this caustic soda solution because, as we always say, it’s easy to put into any system any chemical but to get rid of it is another story.
Out of the washing machine we have some dirt, some polyester fines, some “soap”, glycol and some un-melted glue plus some residual salts resulting from the chemical reaction.
On top of this, to save energy, we do need our caustic soda solution back to the washing machine without any glue and/or other contamination.
Searching among few thousands of filtration systems, we finally found the way to do it in an economical way and without having extra sludge, pretty expensive to get rid of.
It is a multi-step filtration process that can re-use a little more that 2/3 of water because a certain amount of clean water, coming from rinse-dryer, should be added anyway.
Flakes go then to a second sink-float tank for rinsing purposes and, just in case, for final separation of some labels that can still be around.
The water of this tank should look clear all the time, and it does, because all dirt is gone and only rinsing is provided.
To increase the rinsing effect, PET flakes are extracted from the bottom of the tank by means of an high speed pump, to “brush” the surface of flakes.
After water separation flakes are clean, with a neutral PH and therefore ready for drying.
This operation is carried out by a mechanical spin-dryer that delivers material with a 0,7% moisture content..

 

Before going to final storage, two more steps are normally needed; one is separation of fines, and for fines we mean small particles under 3 mm. size that may create troubles during hot drying, and one more fine control that no metals reach the final stage by means of a “pipe metal detector” .
Flakes take the final way to storage silo for further processing.