Automatic magnetic projection for one-step separation of mixed plastics using ring magnets
Graphical abstract
Introduction
Extensive use of plastics across the world has brought an ever-increasing burden of waste plastic management, e.g., 359 M tonnes in 2018 (PlasticsEurope, 2019). The challenge of plastic pollution has attracted unprecedented attention (Wong et al., 2020). In existing reports, mechanical recycling and chemical degradation are two main methods to manage plastic waste. Despite its high efficiency, chemical degradation (Qi et al., 2020; Xiu et al., 2020) may make secondary pollution. In contrast, mechanical recycling can manage waste plastics in an environmentally friendly and low-cost way, and therefore has been viewed as a promising method in dealing with this huge challenge (Biganzoli et al., 2015; Gu et al., 2017; Ripa et al., 2017). Recycled plastics have a comparable value to their virgin counterparts (Gu et al., 2016). However, limited by cost and technology, 25% of waste plastics are still managed through landfill or incineration (PlasticsEurope, 2019), while landfilled plastic waste is considered to be a main source of marine plastic debris (Schmidt et al., 2017). Mechanical-physical separation methods (hereinafter referred to as “separation”), that utilize the differences in the physical properties of materials, e.g., magnetic susceptibility, electric conductivity and density to separate mixed materials, have been widely used in waste management to enrich and to recycle desired materials, especially plastics (Gu et al., 2019; Zhang and Xu, 2016). However, efficiently separating multiple mixtures simultaneously is still a challenge for most reported separation methods. Most of the existing method can only deal binary separation. For instance, magnetic separation can only collects ferric metals (Zhou and Xu, 2012), corona electrostatic separation recovers metallic components from non-metals (Zhang and Xu, 2016), hydro cyclone (Wang et al., 2015; Yuan et al., 2015) and tribo-electrostatic separation (Li and Xu, 2019) can only separate two materials based on the differences in their densities or charge polarity, respectively. Therefore, the separation of multiple mixed materials can only be achieved in a step-wise manner (Li and Xu, 2019; Meng et al., 2017). To achieve total separation of multiple mixed materials, a novel method with high efficiency is highly demanded.
Magnetic levitation (MagLev) is a novel technology for density-based analysis of diamagnetic materials that are submerged in paramagnetic media sandwiched between two magnets (Subramaniam et al., 2014; Zhang et al., 2018b, Zhang et al., 2018a). MagLev also can be used to separate mineral, polymers, biological samples based on the densities of materials(Ge et al., 2020), with both high sensitivity (Ge et al., 2020) and versatility (Xie et al., 2019). Based on these advantages, MagLev could afford the one-step separation of multiple wasted plastics (Zhao et al., 2018). Comparing with the existing method, one-step separation has the advantages of higher efficiency, lower costs, and promising processing capacity. However, the collection process of MagLev-based separation is extremely limited, because of the narrow operational space between the two identical magnets (Mirica et al., 2009; Zhao et al., 2018) and the magnetic force that may converge the desired particles (Winkleman et al., 2007). In addition, the collection process still employed manual operation, which highly restricts the efficiency of MagLev.
To address this challenge, inspired by MagLev, we proposed a novel separation method denoted as “magnetic projection” (H. Zhang et al., 2019; X. Zhang et al., 2019), which consists of a single square permanent magnet placed beside a container full of paramagnetic solution. The magnetic force created by the magnet and the solution drives the diamagnetic particles from their initial position to the corresponding collection zones. Experimental results indicated that the separation efficiency of magnetic projection can reach over 95% (X. Zhang et al., 2019; Zhao et al., 2020). The magnetic projection is an effective, economic, and one-step separation method. However, magnetic projection using square magnets still remained major bottlenecks that it is hard to achieve automatic feeding, which means the separation process is not continuous. Thus, the magnetic projection cannot perform its full potential.
In this study, we pioneered automatic magnetic projection using ring magnets, with the objective to improve the maneuverability of this separation process to achieve higher efficiency and automaticity, thereby extending its prospect in industrial waste management. The experimental design of this study is summarized in a schematic diagram (Fig. 1). Simulation was applied to determine the projection distance of different materials. The device with a motor-driven input process ensured the continuous separation of the plastic wastes. The study realized the separation of six different plastics: polypropylene (PP), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyvinyl chloride (PVC), polyethylene terephthalate (PET), and polylactide (PLA). The separation results showed all recoveries could reach 95%. The separation device, utilizing the characteristics of ring magnets, has broken through the major bottlenecks of magnetic projection in maneuverability and automation, with minimal cost and environmental burden.
Section snippets
Theory
When a diamagnetic object enters the paramagnetic solution, with the influence of the magnetic field, the diamagnetic objects will be pushed away from the magnet. Meanwhile, the objects with higher density than the paramagnetic solution will decrease, and the objects with the density less than the paramagnetic solution will rise and form a projection curve. This is a projection process. The acceleration of the object during the movement is related to the density. The projection distance is
Device
The ring magnetic projection device mainly contains the 3D printed base, ring magnets and the driven part, as shown in Fig. 2. The cost of the device could be less than ¥ 500 (¥ 1 = $ 0.1527, on April. 8): ~¥ 200 for the 3D printed base, ~¥ 100 for each magnet, and ~¥ 100 for the driven part. The device can be divided into a feeding and separation area. The feeding part is comprised of an entrance, a feeding channel (15 mm wide) and a feeding pendulum driven by an actuator. Samples in the
Magnetic field
The simulation results of the magnetic field of 1–3 magnets are shown in Fig. 3. The arrows in the figure show the direction of the magnetic force. To better exhibit the magnetic force direction, the arrows' sizes were normalized to be equal, this was done because the value of the magnetic force changes sharply in some very narrow spaces (e.g. the edge of the magnets). The simulation results show that the magnetic flux intensity near the central axis of the ring magnet is greatly weakened due
Conclusion
This study presented an automatic and controllable one-step separation method based on the principle of magnetic projection. We innovatively utilized ring magnets to enable automatic feeding. The magnetic field strength of the ring magnet is weaker than that of the square magnet, however, magnets were superimposed to increase magnetic field strength. According to the simulation of the magnetic field, we analyzed the characteristics of the superimposed magnets. The trap area in the magnetic
Abbreviations
- PP
polypropylene
- ABS
acrylonitrile butadiene styrene
- PC
polycarbonate
- PVC
polyvinyl chloride
- PET
polyethylene terephthalate
- PLA
polylactide
Symbols
- F
vector of force
- g
gravitational acceleration
- V
volume of the object
- v
vector of velocity
- B
vector of magnetic flux density
- χ
relative magnetic permeability
- ρ
specific density
- m
mass
- a
vector of acceleration
- ∇
gradient operator
- μ
magnetic permeability
- x
displacement
- h
height
Subscripts
- g
gravitation
- mag
magnetic
- D
drag
- m
paramagnetic medium
- b
buoyancy
- s
sample
CRediT authorship contribution statement
Xuechun Zhang: Conceptualization, Simulation, Visualization, Writing - Original draft preparation. Weitong Zhang: Validation, Data curation. Jun Xie: Editing. Jue Fu: Device. Jianzhong Fu: Supervision. Peng Zhao: Editing and Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the financial support of the National Natural Science Foundation of China (Grant nos. 51875519, 51821093, and 51635006) and the Zhejiang Provincial Natural Science Foundation of China (Grant no. LZ18E050002).
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