HDPE scrap bottles washing PRINTABLE
- As written somewhere else in these pages, HDPE
bottles washing is under the category of “easy to set up and run” systems.
- And this is why:
- HDPE is a polymer of olefins family and like
other olefins floats in water (while all other plastic sinks).
- This practically means separation of this
polymer from other plastics is easy and inexpensive.
- Let’s start from the very beginning of what
should be the right set up.
- A bales breaker is almost compulsory if
production rate is over 500/700 Kg/hour (1000/1500 lb/hour).
- The reason for this is kind of easy to
- If you are in a country where operators cost is
not very high, you can choose to go with manual bales opening even for higher
throughputs but this is your choice.
- Just remember a machine never get sick (unless
it breaks), it works 24 hours a day without complains, doesn’t ask for days off
and so on.
- Consider also this when you take decisions.
- After bales is opened and bottles are
“singularized” onto a conveyor belt, one operator should check for “strange”
items, meaning a pair of shoes or a dead cat not to go to granulator, and this
can be done with negative sorting.
- It is only a “just in case” precaution.
- So, all plastic bottles go under a Metal
Detector to be sure no metal gets to the granulator and then to granulator
- The choice for the right granulator is a
- HDPE, like most of plastics, wears out steel
pretty easily and together with paper and dirt things are not any easier; this
is the very first consideration.
- A wet granulator is suggested, from our point
of view, because of this reason first, and also because of the fact it is a
good “pre-washing” machine, like any machine making friction in presence of
- You may like better a dry granulator for the
reason labels can be removed by an air separator way easier if dry. Keep going, we’ll show you this is “almost”
- Second, we have to choose the size of the holes
grid; this doesn’t automatically means you should go with the size you need at
the end to feed the extruder.
- The other choice is to run this granulator with
a bigger screen, let’s say twice as much than you need, run this size flakes
through the line and then go into a “refining” granulator to downsize your
flakes to where you want.
- The objection could be two machines cost more,
consume more energy, more maintenance and so on; not quite.
- With a bigger screen on first one, the
production rate increases a good 70/80% (with holes size double than whatever
dimension it was decided to be) and this means the size of the machine itself
can be a lot smaller, less energy consumption, less number of cuts and
therefore less wearing.
- Are you following ?
- Another advantage is the fact the bigger the
flakes (the less the number of cuts) the less amount of fines will be lost in
the washing line.
- And because this difference it can be as much
as 2% it means, in a 1000 Kg/hour system, 20 Kg/hour, times 24 hours times 300
days, time the price you can sell your material, figure it out.
- Never think about it ? Well, you better start now.
- Anyway, we got our bottles converted into
flakes now and here where the washing line starts.
- Actually, using a wet granulator the washing
process is started already because out of this machine we receive flakes with a
lot of surface dirt already loose and most of paper converted to pulp by the
combination of water and friction.
- So it will be matter to separate this dirty
water from the our valuable plastic.
- The very best way to do this is by a
centrifugal dryer; drying material and then put it back into water doesn’t make
a lot of sense you would say, but look at the advantages of this step.
- A centrifugal dryer, once again, is a machine
that makes a lot of friction, for few seconds, true, but still friction is and,
again, together with some water.
- Separating material from (dirty) water simply
means to get rid of dirt right at the beginning of the line or, at least, of
most of it.
- The contamination we will find in our bottles
will be, mainly, soap, milk, detergents and few other craps that will be
soluble in water anyway, and this is what the filtration unit should take care
- And because we set up the very first part of
the system to remove most of dirt, we should have a dedicated filtration unit
for this part of the system.
- We’ll talk about water filtration at the end of
this page even if the subject is not the last one for importance.
- Washing line now.
- It may sound very obvious but a washing line
MUST have a machine that washes plastic flakes.
- As written quite many times elsewhere, washing
plastic should be a combination of friction (the same way your wife brushes
your T-shirt when very dirty), water, better if at least warm, and a certain
amount of time under these conditions.
- And, if this machine can provide also for water
change during this time, you got what you where looking for.
- To make this concept a little more clear, it is
the difference between taking a bath or a shower.
- In a bath you can wash yourself and then you
need to get a rinse somehow otherwise it doesn’t work out well while with the
shower system you get washed and rinse at the same time.
- Isn’t it ?
- Back to our washing line, we take our
pre-washed flakes and fill them into this washing machine to “brush out” all
sticking dirt that hasn’t previously been removed, convert all paper into pulp,
and take it out with the “shower” we were talking about a minute ago.
- This “brushing” should be as long as it takes
to get the job done.
- In other words if material is heavily
contaminated, or with some very difficult to remove dirt, time must be longer
and this can be done with our machine just increasing one of the parameters.
- At this point we have our flakes shining but it
is not the end of the plant yet.
- Two more steps to go.
- First is separation of HDPE flakes from other
- As we said before, all plastics other than
olefins sink so separation into fresh water does the job pretty easily and
because material has been (strongly) washed already, foamed materials are gone
already because pulverized by the granulator, the centrifuge and the washing
machine at the end.
- The very last step, at this point is drying
that, for bottle flakes, is not very hard to get.
- We do suggest a powerful dryer that
mechanically can remove all water and leave material with a mere 0.5% moisture
content, without using any expensive hot air system.
- Any vented extruder in this world can handle,
at least. 1% humidity and therefore you are on the safe side.
- End of washing line ? Almost.
- The point is a washing line, any washing line,
removes contaminants from the surface of flakes and this is what we were
looking for, but dirt doesn’t disappear and it simply goes into water.
- Got the point ?So all the machinery we have
installed are giving us a very good material together with a lot of problems
with authorities that don’t like we discharge our craps into the sour.
- In other words, washing lines go together with
a filtration unit, or more than one.
- First step in fact, is to remove solids from
water, and for solids we mean paper pulp, plastic fines, dust, you name it.
- Second is to remove soap, detergents and
whatever soluble contamination was mixed together the scrap bottles.
- And may be also what we did use to remove foam,
for example, because, as you know, soap makes foam and washing line should foam
free to run well.
- Because you are smart, you did use the simplest
way to “de-foam” you water adding a glass of Diesel, and this works pretty
well, doesn’t cost almost anything and makes your material even more shining
after extrusion but the problem is you just cannot discharge it. Period.
- So, whatever you put in, needs to be removed
before discharge, so you need to be very careful saying you found the way to
skip the problem adding this and that.
- The solution of course does exist and is a
water treatment system that needs to be chosen according to each country rule,
the cost of water, cost of chemicals that go with it and so on.
- There is not a final answer to the problem yet
because each situation is different and needs a “customized” approach and solution.