| Techniques for waste automatic sorting |
Hydraulic separation is based on differences in particle motion in a liquid according to the simultaneous action of gravity and one or more other forces such as resistance to penetration of particles in the liquid. This separation technique uses liquid density, viscosity and the ability of the body to float or sink in this fluid. In the case of flotation, the liquid is stationary: the separation "float / cast" is made according to the principle of buoyancy. The solids are immersed in a liquid having a intermediate density between those of solids to be separated. The heavy particles will sink while the lighter will float.
Hydraulic separation is based on differences in particle motion in a liquid according to the simultaneous action of gravity and one or more other forces such as resistance to penetration of particles in the liquid. This separation technique uses liquid density, viscosity and the ability of the body to float or sink in this fluid. In the case of flotation, the liquid is stationary: the separation "float / cast" is made according to the principle of buoyancy. The solids are immersed in a liquid having a intermediate density between those of solids to be separated. The heavy particles will sink while the lighter will float.
The dissolved air flotation allows the separation of plastic materials according to their ability to float or sink in a liquid and depending on their wettability by a liquid. That is to say that the surface properties of materials make it hydrophobic or hydrophilic. The waste is immersed in tanks containing a liquid in which air is forced vertically in ascending or descending. The wettability of different particles implies adherence to different bubbles: the light particles and hydrophobic (low wettability) stick to the bubbles that lead, allowing their recovery. Heavy particles and hydrophilic (high wettability) remain in the cells and flow for withdrawal thereof.
The material to be recycled is paid continuously, after grinding on a rotating drum or on a conveyor belt. The particles undergo loading resulting in the appearance of positive or negative charges by electron transfer. The phenomena of electrostatic attraction / repulsion of charged particles allow the separation. It is used in particular the more or less rapidly than is the material to take or lose static surface charges. There are two charging modes: ion bombardment or high-voltage corona charging and triboelectric charging. In the case of ion bombardment, the material is subjected to the action of an electric field (15,000 to 30,000 V) that loads each particle. Conductive materials (metals) lose their charge very quickly and fall off the drum by gravity. On the contrary, non-conductive materials lose their charge very slowly, adhere to the drum and follow its rotation.
Near infrared spectroscopy is based on the analysis of a reflection spectrum whose signature reveals the structure of molecules. It is able to recognize materials. The object is illuminated and reflects a certain amount of light. This information is retrieved by a read head which sends it to a spectrometer, which breaks the light signals into different wavelengths and then sends this information to the computer that processes it and performs the calculations. The nozzles eject objects, which are sorted into two or three categories.
The principle of fluorescence is to send X-rays produced by a CRT, on the waste to be analyzed. These "primary" X-rays excite the atoms, which emit radiation X "secondary" in turn (atoms store energy then release it as light to another wavelength) whose spectrum is analyzed by a spectrometer. The reissue spectrum is typical of the atomic composition of the sample and elemental mass concentration of each element. The identification by X-ray fluorescence is mainly used for negative sorting: it is to remove undesirable products such as brominated or chlorinated PVC, of a pre-sorted feed.
The object is illuminated by halogen lamps and reflects a certain amount of light. This reflected light is detected by a sensor (camera or spectro colorimeter) located in the read head. After that, this information is sent to the processing unit which analyzes the information and control valves. The latter eject objects that are found in two or three categories, by color families. |
