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Technology: Infrared makes light work of sorting plastics

点击量:   时间:2019-03-02 04:16:01

By ANDY COGHLAN Recycling of plastics could become faster, cheaper and more efficient thanks to an automatic waste sorting system under development in the US. The system can distinguish automatically between six different plastics, sorting objects such as bottles and bags with almost 100 per cent accuracy. In the US, recycling operations are limited because of the difficulties and expense of sorting plastic materials by hand. Containers and packages are made from a variety of plastics, and can only be recycled if they are separated from one another. The Society of Plastics Industry is asking its member companies to stamp codes on containers and packaging to identify the polymers from which they are made. This enables people to sort the goods by hand, but the process is painstakingly slow and prone to error. ‘The nice thing about our system is that even if items don’t have a code stamped on them, it still tells you what the polymer is,’ says Suzanne Stanton, a member of the team at the Sandia National Laboratories in Albuquerque, New Mexico, which developed the new system. ‘The error rate is much lower, so you get pure streams of plastic materials when you’re done, and the sorting is much faster than by hand,’ she says. Radiation is shone onto the plastic items as they pass along a conveyor belt. The radiation is in a narrow band, the near-infrared, which ranges between 600 and 2500 nanometres in wavelength. The objects reflect the radiation, which is collected by a detector called a high-resolution Fourier transform near-infrared scanning spectrometer. This analyses the reflected radiation, and then feeds it into an artificial neural network. Stanton explains that the neural network is a computer that ‘learns’ how to recognise individual plastics from the distinctive radiation they reflect. It achieves this by running through repeated ‘training sessions’ in which it is asked to ‘guess’ the identities of samples, and is then told the correct answer. Stanton’s team has now devised a system which scans whole plastic items in one-tenth of a second. This, says Stanton, is fast enough to enable it to be used for mass sorting. The system can distinguish six types of widely used plastics. These include polyethylene terephthalate, a material from which many transparent bottles are made, and two types of polyethylene from which opaque bottles, buckets and carrier bags are made. The other three distinguishable plastics are polyvinyl chloride, polystyrene and polypropylene. Stanton says that almost all recyclable plastics items are made from these materials, so very little would go to waste in the ‘unidentifiable’ category. She says that so far, the $500 000 for the two-year project has been provided by the US Department of Energy,