PET bottles recycling system for Food
- 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.
for food purposes must have, at
least, the following characteristics:
(b) colour (Gardner method)
contamination (wash residuals)
(e.g. serving trays or material with similar density to PET)
- 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
- 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:
- Pre-washing - Washing – Drying
- 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
- 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
- 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”
- 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
- 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
- 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
- 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
- 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
- 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
- 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.
- 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
- Known choices are the following:
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.
water into granulator while cutting to get a kind of pre-washing, with a
standard size grid.
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.
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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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
- 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:
- 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
- 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
- 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
- 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