Full length articleA novel MagLev-based separation approach for heavy metal recycling
Graphical abstract
Introduction
Heavy metals are metallic chemical elements that have a specific density of 5.0 or greater, and usually are toxic to organisms (Shahid et al., 2017; Dang et al., 2021; Tahir et al., 2021). Owning to their biotoxicity, widespread use and potential values, recycling could be a most promising solution to mitigate the heavy metals that are released to the environment (Awasthi and Li, 2017; Gu et al., 2019a; Chen et al., 2020). One of the major challenges in recycling heavy metals lies in their concentrations, which are extremely low even in waste electrical and electronic equipment (WEEE) (Zeng et al., 2016; Gu et al., 2019a, 2019b). Therefore, separation (mainly refers to physical separation or mechanical separation) plays a crucial role in recycling systems (Cui and Forssberg, 2003; Huang et al., 2009; Wang and Xu, 2015); effective separation could enrich the concentration levels of heavy metals, and thereby would greatly reduce the consumptions of energies and chemicals that are used in subsequent hydrometallurgical and pyrometallurgical processes (Lu and Xu, 2016; Awasthi and Li, 2017; Gu et al., 2019a), as well as their environmental impacts (Amato et al., 2017; Gu et al., 2019a). Besides, removing heavy metals via separation could boost the environmental friendliness of recycled materials (Mao et al., 2020).
However, to the best of our knowledge, there are few methods that are competent to physically separate heavy metal contents from metallic mixtures. Most of the reported separation processes, such as floatation separation (Hanafi et al., 2012), electrostatic (Silvas et al., 2015), pneumatic jigging (Wang et al., 2015), vibratory separation (Wang et al., 2016), eddy-current separation (Li et al., 2017), fluidization separation (Zhao et al., 2017), magnetic projection (Zhang et al., 2019; Zhao et al., 2020, 2021), focus solely on separating metals from non-metals. One primary difficulty in physically separating heavy metal contents can be attributed to their physical properties such as density and conductivity, the differences of which might be too small to be exploited by the known separation processes. In addition, the low concentration levels of heavy metals also propose immense difficulty for the extant separation technologies (Gu et al., 2019a).
In this study, we propose a novel density-based separation approach that is based on magnetic levitation, or hereinafter referred to “MagLev”, to recycle heavy metals from solid mixtures. Conventional MagLev configuration is based on a simple configuration of two identical square magnets with like-poles facing each other (Mirica et al., 2009; Nemiroski et al., 2016), and have been used to separate materials with different densities (Atkinson et al., 2013; Zhao et al., 2018). However, although the MagLev approach is highly sensitive to tiny density difference (Mirica et al., 2008, 2009; Xie et al., 2016; Xia et al., 2017, 2021), the configuration has limited separation range; based on the two-magnets configuration, the technology can only handle materials with densities no more than 3.00 g/cm3 (Ge et al., 2020). The modified MagLev configurations such as axial magnetic configuration (Ge and Whitesides, 2018; Zhang et al., 2018, 2020) and magnetic projection configuration (Zhang et al., 2019; Zhao et al., 2020, 2021) suffer the exact same weakness. Therefore, these processes are not applicable to heavy metal recycling.
In our proposed MagLev configuration, an array of magnets (can be 2x, 4x, 6x, …) are employed to replace the original magnet pair, and are set beneath the containers, i.e., an inner container and an outer container, as shown in Fig. 1a. A light source is placed beside the containers, and a tilted reflecting mirror is positioned opposite to the light source; this setting ensures sufficient brightness for the microscope that is used to observe the separation of particles, see Fig. 1a. The physical separation takes place in the inner container (topless and filled with paramagnetic solution, usually MnCl2 aqueous solution), the position of which is adjustable, ensuring a larger operational space for particle separation and manipulation. Glass baffles are inserted to grant the inner container with extended space for separation. The particular design aims to preserve heavier particles in the inner container and to drain lighter particles into the outer container with paramagnetic solution via adjusting the position of the inner container and removing the inserted glass baffles. The separation procedure is portrayed in Fig. 1b. In the following content, the container refers to the inner one, which is responsible for particle separation and extraction.
To verify the effectiveness of our proposed in recycling heavy metals, we employ this process separate indium from indium-tin oxide (ITO)-glass mixtures and spent liquid crystal displays (LCDs). Indium, a post-transition heavy metal (ρs = 7.30 g/cm3) with extremely limited global reserve of only 16,000 tons (USGS, 2008), and is being depleted rapidly in recent decades (Frenzel et al., 2017). At present, annual global demand of indium is estimated to be 600 to 800 tons, and this figure is expected to grow at 5% to 10% per year (Ciacci et al., 2019). Up to 85% of indium is used to produce of indium-tin oxide (ITO), an optoelectronic material that consists of indium oxide (In2O3, 90 wt.%, ρs = 7.18 g/cm3) and tin oxide (SnO2, 10 wt.%, ρs = 6.95 g/cm3), and a vital component of liquid crystal displays (LCDs) (Tolcin, 2013). Due to the ever-increasing demand and limited supply potential, recovering indium content from spent LCDs is highly important to sustain the development of relevant industries. Compared to mining, it is more economic to recover indium from spent LCDs, because the concentrations of indium in these display units (> 160 ppm) (Yang et al., 2013; Rocchetti et al., 2015) are much higher than that in natural ores (10–20 ppm) (Tolcin, 2013). Notably, ITO are slightly soluble and cytotoxic, therefore can be leaked into the surrounding environment from waste LCDs (Vchirawongkwin et al., 2014). Moreover, ITO has also been reported to being related to lung diseases (Tanaka et al., 2010; Nagano et al., 2011) and damages to male sexual organs (Ernst, 2016). Therefore, from both economic and environmental perspectives, recycling indium from LCDs is a great necessity.
Over a decade, developing technologies that aim at recovering indium from spent LCDs is a research hotspot (Zhang et al., 2015; Amato and Beolchini, 2018; Swain and Lee, 2019). Compared to the extensive literature that aims to recycle indium via hydrometallurgical (e.g., Yang et al., 2013; Rocchetti et al., 2015; Argenta et al., 2017) or pyrometallurgical processes (e.g., Ma and Xu, 2013; He et al., 2014; Wang et al., 2019), only limited efforts have been invested on separating indium content. Wang et al. (2018) used floatation separation to concentrate ITO from ultrafine LCD glass powder, using sodium hexametaphosphate as dispersant and dodecylamine as collector. Here we employ the proposed MagLev configuration (see Fig. 1a) to separate ITO from glass powder and mechanically processed spent LCDs, as well as to separate indium oxide from ITO. Before the recovery of indium contents, we use the configuration to separate two types of tricolor fluorescent powders, i.e., europium-excited barium-magnesium aluminates (BaMgAl10O17:Eu2+, blue, ρs = 3.70 g/cm3) and europium-excited yttrium oxide (Y2O3:Eu3+, red, ρs = 5.10 g/cm3). The experimental work is briefly illustrated in Fig. 1c, and is later elaborated in the section of Materials and Methods. Prior to the experimental section, in the following section we provide a piece of theoretical analysis to prove that our proposed MagLev configuration can break through the limitation of 3.00 g cm−3 (Ge et al., 2020), and can thereby effective separate heavy metals (the densities of which over 5.00 g cm−3).
Section snippets
Theoretical analysis
Prior to modeling, a Cartesian coordinate system is proposed in device: the origin of the coordinate system is the center of the two top magnets and the z-axis is straight up, i.e., being vertical to the magnets.
In general, there are three forces applied on any object in any MagLev device, namely magnetic force (Fmag), gravitational force (Fg), and buoyant force (Fb). Based on the previous literature (Mirica et al., 2009; Zhao et al., 2018), Fmag can be expressed using Eq. (1):
Device and materials
The device is constructed according to the MagLev configuration (see Fig. 1a). N52 permanent magnets (manufactured by Beijing Jiu-Jiu Co. Ltd., China) are used to compose the magnet array. Each of these magnets has a size of 100 mm • 10 mm • 5 mm. The magnetic residual flux density at the central of one such magnet's surface (the plane of 100 mm • 10 mm) is 1.47 ± 0.01 T. Both the containers are 3D printed using polylactic acid (PLA), and the thickness of the PLA plates is set to be 1 mm. The
Results of the verification tests
The results of the verification tests are displayed in Fig. 4. In Fig. 4, our proposed configuration can effectively levitate heavy metals, as the specific densities of Y2O3:Eu3+ and Cu are greater than 5.0 g/cm3. The novel MagLev configuration breaks the levitation limitation of the conventional MagLev configurations, that is, 3.00 g/cm3 (Ge et al., 2020). We offer a closer examination on the results of the verification tests to demonstrate the excellent maneuverability and controllability of
Conclusion
A novel MagLev configuration is proposed to recycle heavy metals (densities are greater than 5.00 g cm−3), which poses a difficult challenge to the extant separation processes due to their high densities and minor physical property differences. Different from the reported MagLev-based devices, which can only handle with materials with specific densities less than 3.00 g/cm3, our proposed configuration is competent to levitate heavy metals with the magnetic field provided by an array of magnets.
CRediT authorship contribution statement
Chengqian ZHANG: Data curation, Formal analysis, Investigation, Methodology, Software, Visualization, Writing - original draft. Daofan TANG: Data curation, Formal analysis, Investigation, Software, Validation, Visualization, Writing - original draft. Mingyi CAO: Investigation. Fu GU: Conceptualization, Funding acquisition, Methodology, Project administration, Resources, Supervision, Writing - original draft, Writing - review & editing. Xiangyu CAI: Investigation. Xuetao LIU: Investigation.
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. 71901194, 51875519, 51821093 and 51635006) and the Zhejiang Provincial Natural Science Foundation of China (Grant no. LY19G010009).
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The authors contributed equally.