Apropos K

Separation and efficient sorting of black plastics

Quelle: istock / MiguelMalo

Up to now, it has not been possible to maintain the value of black plastics of the kind used for dashboards when they are recycled: due to their absorption properties, they are simply invisible to the standard analytical systems. A new technology now developed by Fraunhofer research scientists is able to separate and sort black plastics efficiently – at affordable prices.

The sorting machine with the conveyor belt that transports the plastic waste after it has been shredded into small pieces. (© Fraunhofer IOSB)

It doesn’t take long to fill a dustbin with plastic waste after a shopping trip to the supermarket: juice, meat, fruit and other food products are packaged in plastic. Enormous amounts are involved here – about 5.7 million tonnes of such waste are produced per year in Germany alone. Although most people are conscientious about collecting packaging in the yellow bins provided for this purpose, only about 42 per cent of this waste enjoys a second life as nappies, fleece pullovers, stuffed toys etc. The rest is used for energy recovery: it ends up in refuse incineration plants. This is primarily the case with black plastics, which it has not been possible to separate and sort efficiently up to now. This is because existing sorting systems operate in the near-infrared range, which generally enables plastic to be classified. However, what works well with most plastics fails with black plastics: the soot that gives the plastic its dark colour absorbs most of the signal, so that the optical system is unable to see these materials. And it is becoming increasingly important to recycle dark plastics too: if the agreed EU limits for the recycling of cars are to be observed, there is no alternative to the recycling of black plastics.

Affordable system separates black plastics too

Research scientists from the Fraunhofer Institutes for High Frequency Physics and Radar Techniques FHR in Wachtberg, of Optronics, System Technologies and Image Exploitation IOSB in Karlsruhe, and for Intelligent Analysis and Information Systems IAIS in Sankt Augustin are now providing a solution to this problem. “For the first time, we have developed an affordable sorting system that detects not only black but also all other coloured plastics – in real time and in large quantities: blackVALUE”, confirms Professor Thomas Längle, Departmental Manager at IOSB. 

The heart of the system is the radar camera. How it works: the plastic waste – shredded into small pieces – is transported on a conveyor belt, at the end of which the plastic flakes drop at a speed of two to three metres a second. The radar camera transmits terahertz rays – which are between infrared rays and microwaves - through this flow of falling flakes. On the other side of the beam, the system analyses the way in which the individual flakes have changed the beam – and decides on the basis of the sprectra determined what plastic is involved. Within 35 milliseconds, a decision is taken whether the flake is catapulted out of the flow of plastic via a targeted blast of air or whether it can continue on its way. A colour camera provides additional information about the shape of the object, in order to open the blowing nozzles at the right time.

98 to 99 per cent sorting efficiency – at an affordable price

“The higher the frequency with which such cameras operate, the more accurately they measure – although their price increases with the accuracy level too” is how Dirk Nüßler, spokesman for the production business unit at FHR, describes the challenge. “Radar or THz line scan cameras that are required to measure at a speed of, for example, three metres a second – just under 10 km/h – can easily cost up to one million euros. That is prohibitively expensive for recycling centres. We therefore tried to find a compromise between accuracy and affordability.” Ingenious algorithms developed by IAIS help to square this circle; they detect even the smallest of differences in the spectra. And since they are self-learning, they operate increasingly precisely in the course of time. The results are impressive: the camera that operates at 90 gigahertz achieves the 98 to 99 per cent sorting efficiency level that is required and it is a comparative bargain at a price of the same order as a hyperspectral camera. 

The applications for the terahertz camera are by no means limited to recycling alone. “Our development is a key technology that is a viable option for numerous different applications – whether rolling steel or manufacturing food is involved.” In order to be able to adapt the camera to different requirements, the research scientists have given it a modular design. Different frequency extensions can, for example, be added like lenses: extensions for 120 and 240 gigahertz are currently in progress. The camera is to be available to recycling centres in early 2017 and is to be launched on the market at the end of 2017. GD