Magnetic Density Separation of Polyolefin Wastes

Abstract

Polymer recycling has significantly positive impacts on the environment, economics and society. Using recycled polymer to replace virgins reduces the growing amount of post-consumer polymer wastes, decreases the demand for fossil fuel and creates local jobs. As the most used polymer types, polyolefins certainly have attracted attention of recycling. Mechanical sorting has been pointed out as the preferred route for the recovery of relatively clean and homogeneous plastic waste streams. Whereas washing and compounding technologies for polymer scrap have dramatically improved over the last decade, there is as yet no sorting technology commercially available that is both cheap and effective. Consequently, the amount of recycled post-consumer mixed plastics remains low and the recycled polymer value is even worse. Innovative recycling technologies for post-consumer wastes should be extremely powerful, simple, cost-effective and accurate enough to create high quality products. Magnetic density separation (MDS) is such a technology. Early research showed that MDS has the potential to turn around the poor image of polyolefin recycling. However substantial research was needed to improve the separation process before high quality secondary polyolefins could be obtained from complex waste mixtures. This thesis starts with the characterization of the main resources of secondary polyolefins to determine the proper input streams for MDS. Concerning the lifetimes and the total consumption of polyolefin applications, household packaging and car components are two interesting streams for MDS. To reach the necessary separation accuracy for gaining high quality secondary PP and PE in one single step, MDS needs to be sensitive to very small differences in the physical properties of the input materials. Therefore, three main aspects influencing the separation process were studied: wettability of polymers, quality of the process liquid and flow turbulence. For this thesis, a new wetting process to eliminate the effect of air bubbles on the apparent polymer density was developed, a simple method to avoid the segregation of the process liquid was provided, and the sources of turbulence were studied and reduced. Based on all of these achievements, two MDS setups were designed and their performance on the post-consumer polyolefins was evaluated. For both setups, over 98% of the polyolefin waste can be successfully recycled with sufficient separation accuracy: 10 kg/m3. The output products of the MDS were analysed to assess their quality: composition, rheological and mechanical properties. According to the analyses, the derived products from MDS were high purity PP and PE. Based on the mechanical analyses, the separated household wastes showed good and consistent properties, independently from their origin, which were comparable with neat polymers. A model that simulates the MDS process was constructed on the basis of the fundamental principle of MDS and the major factors affecting the process. The study concluded that the separation accuracy of MDS in polymer recycling is determined by the flow turbulence and the uneven magnetic field. The combined effect of these two factors indicates that for most of the polymers turbulence is the major cause of an inaccurate separation. Both the experiments and the simulations concluded that MDS can accurately separate polyolefin mixtures based on their density. With such high accuracies in density, polyolefin mixtures can be separated into different polymers according to their type by MDS in a single step.