KNOWN WAYS FOR PREWASHING PLASTIC SCRAPS
First,
let's see what is useful to pre-wash and what's useless divided by scrap
category:
(You
can go directly to the one is of interest to you)
Pre-washing
is provided mainly when granulator blades can wear rapidly because of the
amount and type of dirt, but here a single shaft shredder is the machine to use
and wearing is not critical.
As
we said it its dedicated page, the problem here is mainly separation and two,
sometimes three, sink-float tanks are required to deeply separate, at least,
olefins from sinkable plastics.
Just because
the best way to achieve results is using clean water, each tank should
"separated" from the following one by a washing/drying step so, while
material goes forward, water goes backwards.
This apply to
almost all systems, surely to all "no chemical" washing lines but, in
this particular case, it is very important.
Back to the
pre-washing matter, it goes by itself this system already has a pre-washing and
nothing else is needed, at least from cleaning point of view. (while the
separation problem remains)
For
film scraps coming from collection we do not suggest a pre-washing
treatment of any kind because amount of dirt is generally little and granulator
should handle this pretty easily.
If
contamination is not this "little" but still scraps are coming from
scraps collection, we do suggest a single-shaft shredder that will last very
many days without problems.
If film scrap
is agro-film, silage film or stuff like that, the pre-washing step should be
seriously taken into consideration.
The film we
just mentioned is clearly contaminated by sand, hearth, stones, you name it and
any blade of any cutting machine wears out in no time therefore a pre-washing
of some sort is kind of compulsory.
Specially for film,
there are many ways to carry out this depending by which material you're
feeding in and what you need at the end. (pellets to blow film, pellets for
pipes or profiles extrusion, blow molding, injection molding, etc.)
For
agro-film, for example, a low energy consumption wet trammel will do the job
and removes 95% of dirt just spraying some water in a tumbling, perforated
screen; with a washer/dryer just after it, dirt will be almost completely gone
and for the rest of the washing line it will be like eating a piece of cake.
In case of
silage film, where contamination is not only sand but also grass, hay, leaves
and other floating contamination, it will be better to use a pre-washing
machine that makes friction as well to release most of this dirt.
This machine
will consume more energy but it is the only way to get rid of floating
contamination.
Of course,
unless scrap bottles are not coming from a landfill pick up or something
similar to this and unless we are not talking about oil jars.
If this is
the case, we do suggest to go with a double shaft shredder that with little
energy opens and shred the bales of bottles (or lose) and doses pieces to a
pre-washing machine that will remove surface dirt, most of paper labels and
most of soap, milk oil or whatever dirt is inside. (the most, not all the dirt)
This, once
again, only in the case bottles are more than dirty because for every machine
you add to the system, there will be a bigger investment cost, maintenance,
energy etc. (it means, do not complain if at the end the system costs too much
!)
For the pre-washing of the PET
bottles washing line,
we need a little bit of your patience to go through what it means.
Let's make an
introduction first:
It is quite
important the way PET bottles are cut; this effects the way line will perform,
the amount of fines, the amount of PET flakes floating on to of water and some
other few things.
Therefore a
sharp edge flakes will make the washing line to run much better than a jagged
one.
All this at
the end means blades of granulator should be sharp all the time. Period.
As everybody
knows, PET is abrasive by itself, paper isn't less and dirt doesn't help much
so how we do it ?
A lot
depends, once again, by the way bottles are arriving at the very beginning.
If you
operate in a country where it never rains, we do suggest to skip the pre-washing
process all together and put a second granulator so you can switch from
one to the other and have the line running 24/7 with no downtime.
Why this ?
Because a second dry granulator cost less than the pre-washing, takes less
space, consume less energy, is more productive than the wet process,
granulator will never rust and all these good things.
After this
pre-washing of flakes is much easier, more effective and trouble-free.
But we should
be talking about pre-washing, isn't it ?
Because most
of countries have rain, snow etc. pretty often and bottles are already wet,
most of the times, we do suggest to go with a wet process all the way.
Keeping water
very hot the process has the big advantage to turn PVC bottles opalescent,
so way easy to recognize and remove from the PET stream.
More than
this, being in a country where electricity doesn't cost a fortune, a second
trammel, running with hot air, can get to the point to shrink all bottles,
making the average thickness higher and easier to wash, dry etc..
Does this
sound good ? Yes it does also because, back to PVC bottles, they will definetly
get brown and very easy to recognize and remove.
After all
this, your bottles will be pretty clean, shrunk, labels and dirt free and
without any PVC content so granulator blades will last very long and its
maintenance cost not high at all.
On the other
side you should keep in mind that hot water cost a bit and hot air even more so
you should run the numbers in the country where you are and see if this is
convenient to you.
Each piece of
the equipment cost money from investment and operating costs point of view.
Keep
this always in mind.
- HDPE scrap bottles washing and pelletizing
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 undersatnd.
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 itself.
The choice for the right granulator is a thought one.
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 water.
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” true.
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.
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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 of. 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 plastics.
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.
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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.
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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.
- Starting a plastic recycling business ,
- How much do you know about machinery for plastic recycling?
Post-consumer scraps are available in huge quantities at a low price, sometimes for free. (at least it use to be this way)
Some information about clean, already sorted. post-industrial scraps and common methods to put them back into the production process.
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