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Dismantling and separation procedures

Vincent Semetey (CNRS Senior Scientist at Chimie ParisTech) describes the different dismantling and separation procedures of WEEE.
In this lecture, I will present the physical separation and the thermal treatments used in e-waste recycling. The main physical separation processes used in e-waste treatments are divided into two categories. Dry separation methods include manual dismantling to recover all hazardous components. Shredding of e-waste into small particles in order to liberate metals from plastics. Magnetic separation to recover ferrous metals. Eddy current separation to recover nonferrous metal. Electrostatic sorting to separate and recover plastics. Optical sorting to recover printed circuit board or to separate brominated from nonbrominated plastics. We can find also wet separation metals with the flotation operation to recover different type of plastics and metals.
Manual dismantling remove valuable components present in devices such as printed circuit board or motors with high metal contents by hand using tools usually and ensure to the removal of substances and also ensure the removal of toxic substances, such as mercury, batteries, or CFC gases in order to depollute e-waste. More and more automated dismantling allows intensive work while improving safety and reducing cost.
Smasher is used for the initial opening of e-waste and separation of component without crushing and destroying harmful substances and valuable material. Shredder, a toothed roll crusher, is used for reducing the size of all kinds of material after their depollution. The size separation equipment is splitting the incoming crushed materials according to their size through their screen apertures. Typical equipment for this separation are the rotating drum screen or the vibrating screen. The iron scraps can be easily recovered by using low-intensity magnetic separation, which is commonly used in e-waste recycling process. The magnetic materials stick onto the magnetic poles and are easily separated from the incoming e-waste stream.
The separation is usually associated to the eddy current separation where the remaining material passes through another belt magnet to the eddy current separator to remove non-ferrous metals such as aluminium. In the electrostatic separation the waste is uniformly and simultaneous charged using friction in a rotating drum. Positively or negatively charged particles pass through an electric field where they are deflected towards an electrode and they are separated from each other. This separation technique is particularly useful to separate plastics ABS from polystyrene, polyethylene from polypropylene. The densimetric table separator is a combination of fluid bed technology and inclined vibration table. The injected air floats the lighter density particle while the vibrating table transports upward the table the heavier particles.
The simple principle of density separation called float and sink in water with specific density has proved to be effective in separating plastics with different densities and also to separate metals from plastics. The heavier particles sink to the bottom of the tank while the lighter plastic fractions float to the surface.
Sorting of metal and plastics may be performed by using an optical sorting system using different sensor units to identify material and air jets subsequently sort them out into different compartments. Different sensors you need can be used. The colour camera is used for the separation of different coloured plastics and circuit boards, while the X-ray Fluorescence sorting system is used to separate materials according to their composition and it is particularly used for the removal of brominated flame retardants containing plastics. The near infrared sorting system separates numerous selected plastics from the mixed stream according to their infrared spectrum. Thermal treatments can be used in e-waste recycling.
Pyrolysis is used to degrade the organic part of the printed circuit board in the absence of oxygen facilitating their recycling. However halogenated hazardous substances resulting from the thermal degradation of brominated epoxy resins are produced, associated with high load of halogens, which may result in corrosive effects. Consequently gases must be treated using for example postcombustion, activated charcoal posttreatments. Pyrolysis requires specific plants.
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