Sorting Black Plastics according to Type
blackValue – Recycling Black Plastics
Using optical procedures, black plastics, as known from dashboards, are hard to sort according to the type of plastic material. Their absorption behavior simply makes them invisible to the usual analysis systems. A new system has now been developed by Fraunhofer researchers to sort black plastics according to type.
Back from shopping in the supermarket: plastics refuse ends up in the waste bin. Juice, meet, fruit and other food is packed in plastics. In Germany alone, each year approx. 5.7 million t of this waste is produced. Only about 42% are reused for products such as diapers, fleece pullovers or stuffed toys. The remaining plastics are reused energetically, being taken to incineration plants. This is true for black plastics mainly, because they used to be hard to separate by optical methods.
Conventional sorting systems operate within the near infrared range, which generally makes it possible to classify plastics. However, what works well with most plastics, does not work with black variants: The soot which gives the plastic its dark color largely absorbs the signal. The optical system is unable to see these materials. Recycling the dark plastics, too, is constantly growing more and more important, though. In order to meet the upcoming EU threshold values concerning vehicle recycling, there will be no way around reusing black plastics.
System to Separate Black and other Plastics Ready for Serial Production
Researchers at the Fraunhofer Institutes for High Frequency Physics and Radar Techniques FHR in Wachtberg, for Optronics, System Technologies and Image Exploitation IOSB in Karlsruhe, and for Intelligent Analysis and Information Systems IAIS in Sankt Augustin, all in Germany, now offer a solution to this problem, as stated by Fraunhofer. The sorting system is named „blackValue“. „For the first time, we have developed an affordable sorting system that detects every plastic color, including black, both in real time and in large quantities. We call this system blackValue,” says Prof. Thomas Längle, IOSB head of department.
At the heart of the system there is a radar camera. The shredded plastics particles are placed onto a conveyor belt at the end of which they fall down in a wide arc at two to three meters per second. The radar camera emits terahertz radiation through this stream of falling flakes. On the opposite end of the beam, the system analyses the way in which the individual flakes changed the beam. Based on the spectra received, the system determines the type of plastic material. Within 35 ms, the system decides whether or not to blow the particle out of the plastic stream by a well-targeted blast of air. A color camera provides further information on the shape of the object, so that the blower nozzles can open at the right time.
98 to 99 % Sorting Accuracy
“The higher the frequency used by the camera, the more precise the measurements – but greater precision comes at a higher price,” says Dirk Nuessler, business unit spokesman for production at FHR. Radar or THz line scan cameras designed to measure at, e.g., three meters per second (almost 16 miles/10 km per hour) belt speed can easily cost up to EUR 1 million. This is too expensive for recycling centers. “This is why we tried to find a compromise between affordability and precision.”
Algorithms from IAIS are due to help find the right balance; they can even detect the slightest differences in spectra. And because they are self-learning, their precision keeps growing over time. The results obtained from the 90 gigahertz camera are impressive: It achieves 98 to 99 % of the required sorting accuracy.
By no means is application of the terahertz camera restricted to recycling. “This development of ours is a key technology that is suitable for a number of applications, from steel rollers to food production,” states Nuessler. Recognizing the need to adapt the camera to different requirements, the researchers gave it a modular design. For instance, different frequency extension components can be mounted in a way similar to lenses. Extensions for 120 and 240 gigahertz are currently being developed. The camera is expected to be available to recycling centers in early 2017 and will be ready for the market at the end of 2017.