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 Separation of plastic scraps


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.
The systems available are very many but when it goes to costs, the range decreases a lot; anyway here a list of what's available.
- Air classifiers
- Mechanical classifiers
- NIR (Near Infrared Rays)
- Laser spectral analysis
- Polarized light
- UV light
- Colours by fluorescent light
- Electrostatic
- By melting or softening point
- Other methods
- Sink-float tanks
- Hydrocyclones
- Sink-float by preferred solvent absorption
- Sink-float by hydrophobicity
- Froth flotation
- Selective dissolution
- Hydrolisys
- Glycolysis, hydroglycolysis
- Pyrolysis
You 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.
This 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 by size.
In some cases, flakes with different shapes can be taken into consideration by the fact one kind slides faster than another one.
Classifiers can be circular, flat, inclined, with slow or high frequency vibrations etc.
- 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.
Its 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.
Not 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.
The limit of this method is the fact bottles need to lined up one by one therefore production rate will be pretty low.
- UV light is used to separate polymers that exhibit different UV absorption or UV induced fluorescence.
To human eyes, PET will stay clear while PVC turns black therefore this is a very common way to manually sort bottles.
- Fluorescent light is used to sort different colours; a camera check the frequency of response and diverse different colours.
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.
One 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.
Normally 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 lighter.
The 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.
But we are very close to chemical separation here.