Capabilities of Ultrasound for Monitoring and Quantitative Analysis of Polyolefin Waste Particles in Magnetic Density Separation (MDS)

M.C.M. Bakker*, S.A. Sanaee
CITG, Recycling Engineering, Delft University of Technology, Stevinweg 1, 2628 CN, The Netherlands.

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© 2010 Bakker and Sanaee.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the CITG, Recycling Engineering, Delft University of Technology, Stevinweg 1, 2628 CN, The Netherlands; Tel: +31 (0)15 27 85219; E-mail:


A study was conducted into the potential of ultrasound as a qualitative and quantitative tool for the Magnetic Density Separator (MDS), where an ultrasound sensor array will operate from inside the MDS ferrofluid. The desired applications of the ultrasound are monitoring of polyolefin particles in flow during the separation process, and performing throughput measurements and quality inspection. The possibility to adapt off-the-shelf medical imaging technology was investigated with an eye towards the real-time requirements for industrial application. It is shown that a medical imager can be adapted for monitoring the flowing particles at speeds up to 30 cm/s, and the imaging quality can be very good under optimum viewing angle conditions. Ultrasound imaging may also allow for quantitative through-put analysis provided the flowing particles are sufficiently spaced, which puts a limit to the maximum measurable throughput. If viewing conditions are not optimum the medical imager relies on the human operator to improve image quality. This situation is undesired for industrial operation where there may be little or no time for operator intervention. To anticipate these occurrences a second imaging method is proposed that could act in conjunction, which is effective since it is based on a different physical principle. Ultrasound quality inspection is a very different technique, and its feasibility depends first of all on whether different types of polyolefin are acoustically distinctive. Using experiments and 3D acoustic computer modelling to determine wave speed and material attenuation, the potential of ultrasound for polyolefin identification under immersed conditions was investigated. With a calibrated measurement setup it is shown that polyolefins may potentially be classified into acoustically distinctive groups. The possible errors in throughput measurement and quality inspection and the requirements for these techniques to perform under operational MDS conditions are also addressed.

Keywords: Waste plastics, polyolefins, magnetic density separation, ultrasound monitoring.