Automated Macrosorting Systems
Much macrosorting, such as separating PET bottles from HDPE bottles, nylon carpet from polyester carpet, and so on, is still done by hand, often by workers picking materials off conveyor belts and placing them in the appropriate receptacle. However, mechanized means of sorting to make the process more economical and reliable continue to become more prevalent. The various devices commercially available to separate plastics by resin type typically rely on differences in the absorption or transmission of certain wavelengths of electromagnetic radiation, on machine vision systems that recognize materials by shape or color, or on some combination thereof. Many of these systems can separate plastics by color as well as by resin type.
These sorting systems can be divided into singulated-feed and mass-feed systems. In a singulated-feed system, objects are fed individually to the sensor, which then identifies each object (such as a plastic bottle) by type and directs it to the appropriate stream. In a mass-feed system, the recycling stream is spread out in a single layer across a wide belt. Singulated-feed systems can operate with a single sensor, although multiple lines (and therefore multiple sensors) are often required to increase capacity. The sort purity rate is typically 98 to 99 percent. In a mass-feed system, a separate sensor is required for each type of plastic to be sorted. A second sensor in a series can be used to increase the sorting purity. If a second sensor is not used, manual quality control is required as the final step. Such systems typically generate a purity of 90 to 95 percent, somewhat lower than singu-lated-feed systems.65
The Environment and Plastics Industry Council (EPIC) of Mississauga, ON, and Corporations Supporting Recycling commissioned a study of plastic sorting technologies in 2002. An update to the original study was released in 2005. It reported that the main manufacturers of mass-feed plastic bottle sorting equipment for the North American market
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were Magnetic Separation Systems (MSS), National Recovery Technologies Inc. (NRT), Pellenc Selective Technologies Inc., S&S Separation and Sorting Technology GmbH, and TiTech.66
Magnetic Separation Systems (MSS) of Nashville, TN, developed a system that sorts two to three plastic bottles per second, separating by resin type and color, using four sensors and seven computers. X-ray transmission is used to detect PVC, an infrared light high-density array separates clear from translucent or opaque plastics, a machine vision color sensor identifies bottle color (even ignoring the label), and a near-infrared spectrum detector identifies resin type.67 A later MSS high-capacity plastic bottle separator uses a single sensor for both color and resin identification.68 According to the EPIC report, the MSS Sapphire model can sort specific plastic resins, aseptic cartons, paper, and mixed plastics from a commingled stream of mixed plastics and paperboard, with the material positioned in a single layer on a conveyor. The system uses a combination of near-infrared detectors, high-speed microprocessors, and air jets. It cannot distinguish between colors or shapes. At each stage, the incoming stream is split into two output streams. It can handle 1500 to 3000 kg/hr. Reportedly, 18 such systems were installed in Germany, but none in North America. Typical product purity is more than 90 percent. The Aladdin system, designed for high-capacity MRFs, has all the capability of the Sapphire plus an integrated color sensor to allow separation between natural and colored containers. It can divide an input stream into three output streams and can count bottles separated by type and size. Seven such systems, two in Switzerland and five in North America, were reported to be in operation at four different facilities. Capacity is reported to be up to 4000 kg/hr.66
National Recovery Technologies, Inc. sells the MultiSort IR system, which uses near-infrared (NIR, also called shortwave infrared, SWIR) sensors to sort a designated polymer type from a commingled stream of mixed plastics and paperboard, with a throughput rate up to 4545 kg/hr. Like the MSS Sapphire system, it performs only one sort at a time and cannot sort by color. The company's MultiSort ES system can sort colors into groups at rates up to 3630 kg/hr. It also performs only one sort at a time. NRT's VinylCycle system was one of the original systems for separating PVC from PET, dating back to 1991. It is available in six bottle per second or ten bottle per second capacity and is widely used.65,66 More recently, the NRT system was shown to be capable of sorting polylactide (PLA) bottles from PET bottles.69
Pellenc Selective Technologies (PST), based in France, has 100 machines installed around the world for various types of waste separation. It has two plastics sorting systems. The Mistral uses NIR to identify all materials in a single pass. The Sirocco uses a vision system to identify objects by location, shape, transparency, and color. Purity levels of 90 to 98 percent can be obtained; output varies from 2 to 10 tonnes/hr. The systems can handle plastic, paper, metal, and multilayer aseptic packaging. Ottawa recently installed a Mistral system in their MRF.66
Separation and Sorting Technology GmbH, based in Schonberg, Germany, has formed a joint venture with Tectron Engineering of Laguna Hills, CA. It has 500 machines around the world for sorting of plastics, glass, and metals. The Varisort system is designed for whole bottle sorting and accommodates a variety of sensors such as near-infrared, color-analysis, metal-detection, and X-ray. Different sensors can be placed on a single separation unit. Purity achieved is 99.5 percent. The company also sells SPEKTRUM color sorters.66 TiTech manufactures the TiTech Autosort beverage-carton sorter, originally developed to separate plastic-laminated paperboard. It uses near-infrared spectroscopy, particle detection, and selective impulses of compressed air. The position, size, and shape of the object are determined, in addition to determining the resin type. As with the systems described above, at each stage, only one sort is performed. Throughputs can be as much as 6000 kg/hr. Reportedly, two North American facilities are now using these systems.65,66
PLASTICS RECYCLING AND BIODEGRADABLE PLASTICS 8.33
The only singulated sorting system identified in the EPIC report was the RapidSort system from Rofin Australia Pty. Ltd. It is reported to be capable of sorting both commingled and contaminated single polymer streams, achieving contamination levels of less than 50 ppm for PVC in PET, using high-resolution near-infrared and visible spectroscopy in a one-stage sensor. Each bottle is scanned several times to detect dirt, tops, labels, and other contaminants as well as to identify the bottle resin. Objects that are identified as nonplas-tics or that are not identified are separated from the streams of positively identified containers. The capacity of the system is 5 bottles per second, for a total of 800 to 900 kg/hr. Multiple lines can be used to increase overall throughput. Two facilities are using this system in Australia.66
A variety of other companies have developed similar equipment, some targeted at bottles and some at other plastics sorting tasks such as for carpet, automotive plastics, electronics, and so on.
Peter Walker Systems markets the Polyana and Tribopen systems in continental Europe. The Polyana system uses FT-IR spectroscopy to identify plastics. A special optical cell is used that identifies the plastic in a period of 4 to 6 s. It will identify plastics of all colors and can also identify blends by comparing the spectroscopic fingerprint with the spectra in its integrated database. The database can be modified, with compounds added or removed to suit the demands of a particular application. Two models are available, the portable 420 and the stationary 460 system, which provides more rapid identification. The Tribopen system, patented by Ford, identifies plastics by measuring the triboelectric (static) charge generated when its head is rubbed against the plastic to be identified. A single pen can be used only to separate two plastic groups from each other, but using two or three different pens can allow more complex separations to be made. Identification is negative only—that a tested polymer is not a specified plastic.70
Hamos GmbH of Penzberg, Germany, also has an IR-based plastics identification system for recycling applications. The company also manufactures equipment for color sorting that can be used in recycling processes.71
Sorting of black plastics has posed a difficult problem, as standard spectrometers cannot be used. Their high carbon content causes black plastics to absorb light to such a degree that, when intense light sources such as lasers are used to analyze them, they heat up and can emit light or even ignite. While Raman spectroscopy can be used to identify dark and intensely pigmented plastics, it requires low laser power, resulting in long measurement times on the order of 10 ss. In early 2001, SpectraCode, Inc., a manufacturer of spec-trographic plastic identification devices, announced it had developed a new device that provides for instantaneous identification of postconsumer black plastics. It contains a modified probe that uses a sampling technique to test black samples at full laser power with no burning, allowing identification in half a second or less.72 SpectraCode has targeted automotive and electronics plastics recycling for its technology.73
One way to simplify the identification task is to put taggants into the plastic, making it easy for automated systems to recognize them. Microtrace LLC, of Minneapolis, MN, manufactures Microtaggant identification particles. Currently, they are targeted mostly at preventing counterfeiting but also may be of use in plastics recycling.74
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