New frontiers

Black plastics have been challenging to sort by resin type using automated methods. While electrostatic separation has presented one option for sorting this material, which is commonly found in electronic scrap and in automobile shredder residue (ASR) streams, it has its limitations, as do heavy-media sink/float systems, which also have been employed for this task.

Felix Hottenstein, sales director for Nashville, Tennessee-based MSS Inc., a division of the CP Group, San Diego, says, “Electrostatic separation has been around for a long time and works OK, but only for certain types of plastics, and it requires multiple passes to generate acceptable quality.”

Frank van de Winkel, business development manager, metals, at Norway-based TOMRA Sorting Recycling, says one of the drawbacks of electrostatic separation is that it involves a “long process of prehandling and preprocessing.”

Ralph Uepping, technical director, recycling, at TOMRA Sorting Recycling, adds, “Electrostatic works quite well if you have a two-to-three material mixture. The more complex the mixture gets, the more difficult, and the more steps you need.” He says the particle size also must be smaller when using electrostatic separation.

Regarding heavy-media sink/float systems, Hottenstein says several plants in Europe employ the technology, such as Galloo. While he says these heavy-media systems can work well, “they are only feasible if very large volumes of material are available.”

However, recent advances in sensor-based sorting are offering recyclers additional options for sorting black plastics by resin type.

Overcoming limitations

Black plastics have proven challenging to sort using sensor-based technology because of their physical properties.

“Black plastics contain carbon black that results in a very high light absorption of the material,” says Robin Gruna, research group manager, multivariate image processing, Fraunhofer Institute of Optronics, System Technology and Image Exploitation, Karlsruhe, Germany. For this reason, optical approaches (such as NIR, or near-infrared, hyperspectral imaging) have problems generating a usable signal through light reflection, he adds.

John Green, founder and president of Green Machine LLC, Hampstead, New Hampshire, says, “Black plastics are just like a black hole. They absorb the majority of light required in reflectance measurements. This is the same for transmittance.”

However, as high-precision optical components have become more affordable, sensor-based technology has become a viable alternative for sorting black plastics by resin type, sources say. This technology is making it possible to recycle plastics that previously were going to landfills or to energy recovery.

With its blackValue project, Fraunhofer Institute has developed a hyperspectral imaging sensor that works in the terahertz range. The system uses a radar camera that emits terahertz radiation. This technology combined with a color camera to determine shape and additional properties of the plastic allow the system to analyze individual particles to determine plastic type. It can then choose which individual pieces to knock out of the stream with blasts of air.

“Furthermore, the sensor operates in transmission modes, which allows it to identify plastics composed of multiple layers,” Gruna says.

He adds that because the system operates in transmission, material is best measured when it is in flight.

Even with the recent advances in sensor-based sorting, “Black plastic sorting is still challenging and most have not yet reached the technical maturity of conventional approaches.” – Robin Gruna of Fraunhofer Institute

Steinert, based in Cologne, Germany, with U.S. offices in Walton, Kentucky, launched the Unisort Black Eye at IFAT 2016 in May, which can sort black and other dark plastics by polymer type using new camera technology, says RTT Steinert Group Managing Director and Steinert Group Director of Recycling Sales Hendrik Beel. In 2014 the company introduced the Unisort Black, which sorts dark and black objects and other residual materials that upstream technology misses, according to the company. The Unisort Black, which “is more or less a standard near-infrared unit,” Beel says, can be used to create a stream of black and dark plastics that can be further sorted by polymer type using the Unisort Black Eye.

The detection units of both Steinert systems are located above the conveyor belt and are equipped with hyperspectral imaging (HSI) technology, Beel says. With the Unisort Black Eye, a source of light illuminates the plastic flakes on the conveyor belt, while a camera system analyzes the reflected light. Beel says the system’s camera doesn’t scan the conveyor belt pixel by pixel but simultaneously scans 320 pixels across the entire belt width, enabling detection of variations in the NIR spectrum.

TOMRA also has technology capable of sorting black plastics. Its Autosort sensor-based sorters are used by Monoworld Recycling Ltd.’s $12 million plastics recycling facility (PRF) in Rushden, Northamptonshire, U.K., to sort mixed plastics into PET (polyethylene terephthalate), HDPE (high-density polyethylene) and PP (polypropylene) streams. The PRF can handle postconsumer pots, tubs, trays and mixed bottles, and TOMRA’s technology can sort black plastics from the postconsumer trays, the company says.

While Green Machine has yet to place a machine in the field that is tasked with sorting black plastics by resin type, Green says, “With further investments in our patented hyperspectral technology, we are very optimistic that the majority of high-value polymers can be identified and sorted at rates of 8-plus TPH (tons per hour).”

He continues, “We are confident that by summer 2017 we will have completed a fully automated solution for black plastics. We are currently working with ASR customers, and we are having great results. We expect to be in a position to accept orders for production units as early as December this year.”

Down to size

Steinert’s Black Eye has been designed to sort flake material ranging from 10 millimeters to 30 millimeters, or roughly 1/3-inch to 1 inch, in size, Beel says. It works with plastics that contain mineral fillers, which might not be sorted effectively based on density, he adds.

However, some sensor-based sorting systems can work with larger particle sizes.

For instance, Green says Green Machine’s technology works with 2-inch-to-8-inch particle sizes.

TOMRA’s Uepping says one of the advantages of the company’s technology is that, unlike with electrostatic separation, a small particle size is not required. The technology also can process more complex mixtures of resin types, he says.

Reality but realistic?

Despite the advances in automated sensor-based sorting, recovering and sorting black plastics from complex material streams may be most feasible under specific conditions, Hottenstein says, as in cases of high disposal costs or when legislation mandates a certain level of recovery.

He says a primary challenge of sorting black plastics by resin type “is that it’s difficult to generate a ‘ready to recycle’ quality in one step.”

The small particle sizes that commonly result from electronics and automobile recycling applications also present a challenge in that they require sensor technology with high resolution, Hottenstein adds. “This makes for a very expensive machine, which will probably be prohibitive for almost all recyclers,” he says. “It’s one of those things where many people are interested in it, but once they hear a price range, it comes off the table immediately.”

For these reasons, he says, developing a sensor-based sorting solution for black plastics is not an R&D priority for MSS.

Even with the recent advances in sensor-based sorting, “Black plastic sorting is still challenging and most have not yet reached the technical maturity of conventional approaches,” Gruna says.

Van de Winkel says the technology may not pencil out for recyclers in the U.S. that can inexpensively landfill this material but that may not have a reliable outlet for the sorted plastics, given the variety found in the ASR stream in particular. “Just being able to separate the resin doesn’t mean you have something that is sensible to recycle. Each application needs to be evaluated.”

Hottenstein also mentions the variety that exists within plastics found in ASR, which commonly use fillers or are fiber reinforced, which can make finding a consumer of this material difficult. Beel is optimistic, however, saying he “really believes in this technology and the potential.”

The author is managing editor of Recycling Today and can be contacted via email at dtoto@gie.net.