In the Delft recycling lab in the faculty of Civil Engineering and Geosciences, they don’t like to talk about plastic waste. When the recycling experts pass pieces of shredded plastic through their magnetic density separator, they are left with nice piles of plastics that are ready for use as raw materials, each neatly separated according to density.
Prof. Peter Rem is the brain behind this machine, which operates with a magnetic fluid containing iron oxide. The density of this fluid can be varied from high to low by applying a magnet. This gradient causes plastics to float according to their density, making them easy to separate.
‘In 2007, we received an EU subsidy to develop this technology’, explains Rem’s colleague, Dr Maarten Bakker. ‘A Delft spin-off company has recently been trying to market the technology.’
The researchers now hope to develop acoustic sensors that will allow them to look into the fluid. ‘The fluid streams are quite complex. Turbulence should be avoided, because that mixes the particles again. An acoustic sensor could help us to improve the calibration of the machine.’
In the field of medicine, acoustics are often used to look inside bodies. ‘For our purposes, however, that technology is not usable’, notes Bakker. ‘The images that are generated require too much human interpretation. If a doctor is uncertain about what he sees, he will refer the patients to a surgeon. We don’t have any room for uncertainty. The device has to know exactly what type of plastic it sees, because everything has to be done automatically.’