Separation becomes a bottleneck of dealing with the enormous stream of waste plastics, as most of the extant methods can only handle binary mixtures. In this paper, a novel method that based on magnetic levitation was proposed for separating multiple mixed plastics. Six types of plastics, i.e., polypropylene (PP), acrylonitrile butadiene styrene (ABS), polyamide 6 (PA6), polycarbonate (PC), polyethylene terephthalate (PET), and polytetrafluoroethylene (PTFE), were used to simulate the mixed waste plastics. The samples were mixed and immersed into paramagnetic medium that placed into a magnetic levitation configuration with two identical NdFeB magnets with like-poles facing each other, and Fourier transform infrared (FTIR) spectroscopy was employed to verify the separation outputs. Unlike any conventional separation methods such as froth flotation and hydrocyclone, this method is not limited by particle sizes, as mixtures of different size fractions reached their respective equilibrium positions in the initial tests. The two-stage separation tests demonstrated that the plastics can be completely separated with purities reached 100%. The method has the potential to be industrialised into an economically-viable and environmentally-friendly mass production procedure, since quantitative correlations are determined, and the paramagnetic medium can be reused indefinitely.

Introduction

Massive global production of plastics, e.g., 322 M tonnes in 2015, has resulted in an ever-increasing stream of waste plastics (PlasticsEurope, 2016). Owning to less environmental burdens and economic feasibility, mechanical recycling of waste plastics or plastic recycling is becoming a promising practise in dealing with this particular waste stream (see Gu et al., 2014, 2016a,b, 2017a; Biganzoli et al., 2015; Wäger and Hischier, 2015; Ripa et al., 2017; Zheng et al., 2017). Recycled plastics possess comparable performance and advantageous prices to their virgin counterparts (Gu et al., 2016a), and have already been applied in the manufacturing sector (Gu et al., 2016b, 2017a). Plastic recycling is also one of the major sources of environmental benefits in waste management systems (Biganzoli et al., 2015; Wäger and Hischier, 2015; Ripa et al., 2017). However, although plastic recycling enjoys a rapid growth in recent years, most of waste plastics are still sent to landfill or incineration (PlasticsEurope, 2016). In reported cases of plastic recycling, only waste plastics of certain types and sources are recycled. For example, in the municipal solid waste (MSW) management system of Naples, only polyethylene (PE), polypropylene (PP) and polyethylene terephthalate (PET) get mechanically recycled while the mix plastics are recovered as fuels or directly end up in landfill (Ripa et al., 2017). Some Chinese plastic recycler only takes sorted waste plastics from factories or dismantling sites (Gu et al., 2017a). From a brief review of the extant literature, it can be deduced that the technology of separating mixed waste plastics is highly desirable for promoting the rate of plastic recycling, as most current practises cannot handle this particular type of MSW. Since separation of mixed plastics poses a difficult challenge to promoting mechanical recycling of plastics, the field thereby attracts a great deal of attention from academics. Froth flotation is one intensively studied plastic separation technology (see Guney et al., 2013; Wang et al., 2014, 2015, 2017a,b; Zhao et al., 2015; Censori et al., 2016; Truc and Lee, 2016, 2017; Pita and Castilho, 2017; Salerno et al., 2018; Wang and Wang, 2017). Froth flotation exploits tiny density differences between materials (Censori et al., 2016), yet, this technology primarily focuses on separating desirable types of plastics from binary mixtures, e.g., separating polyvinyl chloride (PVC) from mixtures like PVC/polystyrene (PS) (Salerno et al., 2018), PVC/PET (Guney et al., 2013), PVC/acrylonitrile butadiene styrene (ABS) (Wang et al., 2017b; Wang and Wang, 2017), PVC/polycarbonate (PC) or PVC/polymethylmethacrylate (PMMA) (Wang et al., 2017a), or separating PS from PS/PMMA, PS/PET, and PS/PVC (Pita and Castilho, 2017).