Optical separators TYPES The technologies used in this type of equipment with applications in light package sorting plants are the following: • Separation by near infra-red (NIR) The reading is generally made between the wavelengths of 1,100 and 1,900 nm. This reading determines the nature of the polymer. If the sensor can detect and increase the spectrum to between 700 and 1,900 nm, it is possible to differentiate paper and wood board (cellulose). Its main application in sorting plants is for separating materials according to their nature (PET, HDPE, mixed plastic, beverage cartons and P/C). • Separation by colorimetry or visible spectrum The reading is performed in the 400 to 800 nm wavelength range. These wavelengths allow determining the colour of the polymer regardless of the material it is made of. Its use in sorting plants is to separate material of the same nature by colour: Natural / Colour HDPE • Separation by X-ray densitometry Although it is not normally applied for light package sorting plants, there are units currently installed in solid urban waste plants to separate unsolicited materials in the organic fractions (inert materials, glass, etc.). • Mixed separation The techniques discussed above can be combined in a single unit, allowing sorting of the material not only by its nature, but also by its colour, nature or shape. CHARACTERISTICS The main parameters that need to define an optical sorting unit can be summarised as follows: • Useful width of the conveyor belt. The useful width determines the treatment capacity of the optical separator. Generally, the material arriving at optical separators has a density of 50-70 kg/ m³ (depending on pre-treatment and quality of the input material). In view of this density and the material treated in package sorting plants, the capacities as a function of useful width are shown in Table 13.
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