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Table 2. The maximum allowable quantities of contaminants permitted in wood-based panels manufacture [38]. Contaminant Limit (g/kg) Arsenic (As) 0.025 Cadmium (Cd) 0.050 Chromium (Cr) 0.025 Copper (Cu) 0.040 Lead (Pb) 0.090 Mercury (Hg) 0.025 Fluorine (F) 0.100 Chlorine (Cl) 1.000 PCP 0.005 Creosote 0.0005 3.3. Screening and Cleaning for Wood-Based Panel Mills Recycled wood in the UK is frequently shredded as the first stage in conversion to a feedstock for use. The shredded material can then be segregated by size and ferrous metals removed using an overband magnet. This works well for the larger and better liberated metal components, but small ferrous metal pieces may remain if well embedded into wood particles. In some systems these are detected visually to return into the shredder or removed using technology such as the rare earth drum magnet and eddy current separator [39]. Entrapped metals are removed using the drum magnet, whereas non-ferrous metals can be removed using the eddy current separator (with an alternating magnetic field). A further metal detector may be installed above the belt for transport to the secondary shredding step. In many systems, sieving occurs using a variety of screens and sifting tables. Vibrating conveyors may also be used to segregate material by size during movement through the process. In some mills, it is important to ensure that plastics and low-density materials are also removed from the recycled wood feedstock. This can be achieved using various technologies, some based on screens with rollers to flick the lightweight, bulky plastics off the top of the chip. Other systems can use advanced sensor technology to detect plastics optically and eject them from the current of chipped wood. One example is the Cyclops system from PAL [40]. Garcia et al. [8] report the combinations of X-ray, near-infrared (NIR), and hyperspectral sensing technologies, where both NIR and hyperspectral methods are combined with principal component analysis (PCA) to aid recognition. In Italy, where 50% of panels are to be made from recycled material by 2030, Fantoni have demonstrated separation technologies on an MDF line—where excellent segregation and cleaning is required [41]. This used a quarter of a million tons of furniture-grade recycled material and a system developed by Steinert. A wide range of separating and cleaning technologies exist—some are applied at the dry end, and others may be used at the wet end. For example, a wet system may use density difference to separate stones, grit, and metals from the wood chips, which float across the surface of a tank of water, while the denser contaminants drop to the bottom. This can be a suitable step to elevate furnish moisture content shortly before resination and pressing, or prior to refining (which uses wet chips). Dry-end systems might be used as the chips leave the shredding and screening stage. Technologies can include kinetic systems, using momentum and air resistance to govern segregation; pneumatic systems, agitating particles through a vertical system of screens; and gravimetric systems, based on density within a cyclone-type structure [42].
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