A polyolefin is a polymer produced from a simple alkene, and belongs to the largest group of thermoplastics. The term polyolefin means “oil-like”, referring to the oily or waxy feel of the material. They are mostly used in consumer goods, structural plastics, food packaging and industrial products and more than 30 % of the plastic consumed in Europe consists of polyolefin. However, producing new polyolefin is remarkably resource-demanding, as the manufacturing process of 1 kg polyolefin requires around 2 kg of oil. Due to technological shortcomings in the fields of technical waste treatment and plastics separation treatment, there is no way to produce high-quality secondary raw plastics material from complex waste at a reasonable cost. Today, polyolefin recyclers focus mainly on the relatively pure post industrial waste, since these wastes can be made into high-purity product materials by existing and cost-effective process technology.
In theory, post-consumer wastes such as Construction and Demolition Waste (CDW), Waste from Electric and Electronic Equipment (WEEE), household waste and Automotive Shredder Residues (ASR) provide vast resources of the material in question; they are from five to ten times larger reservoir of polyolefin than the post-industrial wastes. However, these wastes are a much more complex mixtures of materials and hence much more difficult to recycle. The main reason is the low efficiency of the current separation technologies for the secondary plastic. Today, only 7 % of the polyolefin sold in Europe is recycled. New technologies must be relatively simple to be cost-effective, but also accurate enough to create high-purity products and able to convert a substantial fraction of the materials that are present in the waste into useful products of consistent quality in order to be economical.
Within the now-running project W2Plastics, two methods are combined into a ground-braking technology for sorting complex waste into secondary polyolefin products; Magnetic Density Separation (MDS) and Hyperspectral and Ultrasound process control.
The MDS is used for separating mixtures of plastic chips into separate streams of pure material. Density separation is normally performed in regular water, but is not applicable to separate polyolefins from each other since they are all lighter than water. This magnetic separating technique uses a magnetic liquid with nano-particles of iron oxide in combination with a magnetic field, creating different densities in the fluid. The plastic waste, containing flakes of different material, is fed into the fluid and different materials float at different heights. The Ultrasound system is used for identifying the various levels and equipment for extracting one kind of plastic can then be positioned correctly according to the indications from the ultrasound. The Hyperspectral imaging is used to check the quality of both input and output materials.
Ultrasounds sensing had never been used for plastics assessment before, but the
ultrasound imaging that is normally used for scanning babies and internal organs has some useful features within this application: It has a high resolution and a short capture time, and the image is able to show internal interfaces between tissues without being influenced by the particle shape and color. This makes the separation process flexible and continuous, and thereby faster, with higher separation efficiency. To determine the purity of the extracted layer of plastic, a continuous hyperspectral imaging process is thereafter used.
The so-performed separation process is free from hazardous residues. The technique has been tested in the laboratory and it is about to be tested in a pilot plant, giving a technology readiness level of 6 on the TRL scale, according to the scope of the seventh framework programme (FP 7) a grant funding mechanism that provides public grant partial funding for R&D. The now-running project will end in 2013 and has been supported by the European Commission under the Seventh Framework Programme and is coordinated by the Delft University of Technology (TUDelft) in the Netherlands.