Wetting Technologies for High-Accuracy Sink-Float Separations in Water- Based Media

B. Hu*, 1, N. Fraunholz2, P. Rem1
1 Delft University of Technology, Faculty of Civil Engineering and Geosciences, Section Materials and Environment/Recycling Technology, Stevinweg 1, 2628 CN, The Netherlands
2 Recycling Avenue, Delft, The Netherlands

© 2010 Hu et al.

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 Department of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands; Tel: +31-15-27 82897; Fax: +31-15-27 88162; E-mail:


The accuracy of sink-float separations in water-based media such as Magnetic Density Separation is compromised by tiny air-bubbles attached to the surface of hydrophobic materials in the feed. Separations of plastics mixtures with accuracy better than 10 kg/m3 require a total elimination of air-bubbles by pre-wetting the surface of the feed particles with a thin film of water. Theoretical and experimental results are compared for pre-wetting processes based on condensation from the gas phase and direct liquid-solid contact for both shredded waste and virgin polymers. The tests show that the wettability of most polymers improve slightly by steaming for about 1 minute, but the best wetting results are obtained with immersion in boiling tap water. The success of wetting in boiling water can be explained by the deposition of a thin layer of calcium carbonate. Shredded plastics are easier to be wetted than virgin polymers, probably because a relatively rougher surface is caused by the shredder process. A new wetting process to eliminate air bubbles was developed on the basis of these results, which carries a minimal amount of heat and water with the feed to the MDS process liquid.

Keywords: Plastics, polymers, polyolefin, wetting technology, wettability.