By R.D. Pascoe, University of Exeter | ISBN 1-85957-235-9 | Published Rapra Review Reports, Vol. 11, No. 3, Report 124, 2000 | Paperback | 108 pp
Recycling is a major issue for most plastics processors and waste disposal authorities in the new century. Legislation is putting increased pressure on industry to generate products which can be reused or reclaimed. Plastics recycling technology has been well studied and plants have been established for several years in different parts of the world. Both mechanical recycling and chemical recycling methods can be used. However, in order to recycle mixed plastics waste some form of material separation is generally required.
This review describes the processes being developed to identify plastic components from mixed plastic waste and sort them into material types. It provides a good, straightforward overview of the topic, describing machinery, methodology and applications in clear terms. The author of this overview, Dr Richard Pascoe, is an engineer specializing in separation techniques. He is currently based at Exeter University.
Topics covered in this review include:
- Plastic preparation prior to sorting;
- Methods for identifying plastics;
- Automated and manual sorting systems;
- Examples of plastics separation in practice.
Plastics are often contaminated when collected. Hence methods have been developed for dry cleaning of plastics, washing of plastics and the removal of coatings using abrasives. The form of the material may be inconvenient for use with identification and sorting equipment, hence waste shredding, grinding and cutting techniques are often employed. The comminution process may also aid in the separation of different types of plastics from mixed plastic components. Several methods are currently available for the identification of waste plastics to facilitate separation. These include color recognition systems, bar code reading systems, X-ray fluorescence techniques, Near infra-red spectroscopy, Fourier transform mid infra-red spectroscopy, fluorescent dye markers, Raman spectroscopy, laser desorption-ion mobility spectrometry and ultrasonic and photo-acoustic methods. Manual sorting techniques can be used where the plastic components are large enough to justify the time and effort involved. Visual identification of the materials can be used, or mobile analysis equipment. The variations in physicochemical properties of plastics can be used to separate them. For example, contact charging (triboelectrification) has been used to separate plastics based on a difference in dielectric constants. Techniques used in other industries, such as the mineral processing sector, have been adapted for use with waste plastics. Hydrocyclones and centrifuges separate materials by density. Froth flotation has also been used; this is based on the differences in surface properties of plastics.
This Rapra Review Report comprises a concise, expert overview, supported by a bibliography of around 400 abstracts compiled from the Rapra Abstracts database on the topic of identification and sorting of waste plastics. This bibliography provides extensive additional information.