- What's available for the
separation of plastic scraps
- In this section we'll
be talking about separation of different plastic materials
and not from other contaminants.
systems available are very many but when it goes to
costs, the range decreases a lot; anyway here a list
of what's available.
NIR (Near Infrared Rays)
Laser spectral analysis
Colours by fluorescent light
By melting or softening point
Sink-float by preferred solvent absorption
Sink-float by hydrophobicity
will find more details about what we consider to be
feasible in common recycling but here few words about
the different systems:
Air separation is used to separate different plastics,
or even the same plastic, by the difference of the ratio
between the surface of the flake and its mass; in other
words thin thickness flakes from tick ones.
is done by an air counter-flow, that means an air flow
lifting up light stuff and let down the rest by gravity.
Mechanical classifiers are mostly used to separate flakes
some cases, flakes with different shapes can be taken
into consideration by the fact one kind slides faster
than another one.
can be circular, flat, inclined, with slow or high frequency
NIR are Near Infrared Rays gives a certain quantity
of energy to every single piece and measure the response;
this happens in terms of milliseconds.
limit is the fact it can be used only on transparent
items (mainly to sort PVC from PET bottles and flakes)
Laser spectral analysis penetrates the surface and measure
emission spectra which depends upon
heat capacity and thermal conductivity
so colour doesn't matter.
largely used because response time is pretty long.
Polarized light is used to check difference of crystallinity
therefore it applies to sort PVC from PET bottles
or anyway a mixture of two components.
limit of this method is the fact bottles need to lined
up one by one therefore production rate will be pretty
- - UV light is used to separate polymers
that exhibit different UV absorption
or UV induced fluorescence.
human eyes, PET will stay clear while PVC turns black
therefore this is a very common way to manually sort
Fluorescent light is used to sort different colours;
a camera check the frequency of response and diverse
An array of detectors can
be used to sort more than one colour.
Electrostatic separation is a system to attract or repulse
different plastics according to their charge; this makes
different plastics to make different movements.
Polymers can be sorted by the difference of melting
point; it's normally used to sort two polymers by means
of an hot conveyor belt or a hot roll working on a conveyor
belt where a mono-layer of flakes is laying down.
kind will stick while the other falls down by gravity.
There are some more methods for classifying different
plastics in a dry environment, by the fact some are
more brittle than others, by the coefficient of elasticity,
by some other differences in shape etc, and this needs
to be evaluated case by case.
Sink-float tanks are the most common and easy way to
separate plastics by density.
plain water is used to separate olefins from other thermoplastics
and sometimes water is added with salts to make it heavier
than one, and make to float some polymers while the
rest sinks etc.
Hydrocyclones enhance the difference of specific weight
by centrifugal force so materials with little difference
can be separated as well.
Sink-float by preferred solvent absorption is used when
two polymers with same specific weight need to be separated; a
solvent (alcohol, ketone, etc) makes one of the two
big problem is flammability of solvents and their recovery.
Hydrophobicity is the dislike of water; some polymers
react in a different way when going into water under
certain conditions (air bubbles, mechanical stirring,
turbulence, etc.) therefore separation becomes possible.
- - Froth flotation means air bubbles attach one solid
promoting floating in a liquid and leave
the other(s) to sink.
- Air bubbles have high affinity for
water repellent surfaces.
Selective dissolution means certain polymers dissolve
into certain solvent under specific condition while
others remain unaffected.
we are very close to chemical separation here.