Drivers and Pathways for the Recovery of Critical Metals from Waste-Printed Circuit Boards

Abstract

The ever-increasing importance of critical metals (CMs) in modern society underscores their resource security and circularity. Waste-printed circuit boards (WPCBs) are particularly attractive reservoirs of CMs due to their gamut CM embedding and ubiquitous presence. However, the recovery of most CMs is out of reach from current metal-centric recycling industries, resulting in a flood loss of refined CMs. Here, 41 types of such spent CMs are identified. To deliver a higher level of CM sustainability, this work provides an insightful overview of paradigm-shifting pathways for CM recovery from WPCBs that have been developed in recent years. As a crucial starting entropy-decreasing step, various strategies of metal enrichment are compared, and the deployment of artificial intelligence (AI) and hyperspectral sensing is highlighted. Then, tailored metal recycling schemes are presented for the platinum group, rare earth, and refractory metals, with emphasis on greener metallurgical methods contributing to transforming CMs into marketable products. In addition, due to the vital nexus of CMs between the environment and energy sectors, the upcycling of CMs into electro-/photo-chemical catalysts for green fuel synthesis is proposed to extend the recycling chain. Finally, the challenges and outlook on this all-round upgrading of WPCB recycling are outlined.

Keywords: artificial intelligence; catalysts; critical metals; green metallurgy; hyperspectral sensing; waste printed circuit board.

Figure 1

Periodic table showing the common CMs designated by three governing bodies (the US, China, and EU), elements present in PCBs, and metals that have been commonly recycled.

Figure 2

Component composition and the corresponding CM constitution in a typical PCB. Non‐CMs, Pb and Fe, are excluded from the display although they are commonly used in various ECs and bare boards.

Figure 3

a) Metal contents in WPCBs. Values are averaged from various literatures.^{[ 37} , ^{42 ]} b) Metal contents in individual types of ECs dismantled and sorted from WPCBs. MPCs, SLCC, and MLCCs are abbreviations of metalized polypropylene film capacitors, single‐layer ceramic capacitors, and multi‐layer ceramic capacitors, respectively. Plotted from reference.^{[ 37f ]}

Figure 4

a) A tree diagram illustrates the metal species and their interrelationships recovered from WPCBs through current industrial recycling. The size of the circle where the metal is located approximately reflects how often it is recycled by industries. b) A Sherwood plot shows metals’ concentration in PCBs in relation to their market prices in March 2024.

Figure 5

a) A concrete example of LeNet‐5 CNNs. In the architecture, each plane in the convolution layers is a feature map and at each input location, six different types of features are extracted by six units in identical locations in the six feature maps. b) A hyperspectral acquisition setup for ECs’ identification and its workflow of object detection algorithm using a GOL‐based Faster‐RCNN (left). Comparison of the resulting identification performance on the same PCB between the RGB model and the RGB+HSI model (right). Adapted with permission.^{[ 61 ]} Copyright 2020, IEEE. c) Schematic illustration of the fusion of RGB and X‐ray images for capturing both internal and external features of iPhones and predicting their models. iCAM stands for Iterative Class Activation Mapping network strategy. Adapted with permission.^{[ 64a ]} Copyright 2022, IEEE. d) X‐ray transmission images of crushed PCBs in 16‐bit gray scale to show the detection of IC chips by visual and automated means. Reproduced with permission.^{[ 66 ]} Published under a CC‐BY 4.0 license. Copyright 2020.

Figure 6

a) Schematic diagram of spectrum region of X‐ray, photoelectric absorption process, and corresponding spectrum with K‐edge marked. b) K‐edge value of selected CMs and other recycled metals as a function of their atomic numbers. Replotted according to the tabular K‐edge data.^{[ 80 ]} The light red shadow indicates the detectable range of K‐edge set by the state‐of‐the‐art photon counting multi‐energy detector (19.2‐160 keV). c) Schematic illustration of MEXRT detection (left), ME‐XRT mapping image of the detected capacitors (middle), and MEXRT spectra for three representative types of capacitors (right). Adapted with permission.^{[ 79 ]} Published under a CC‐BY‐NC license, Copyright 2022. d) Schematic flowchart of metal enrichment through the “dismantle and sort” strategy. Reproduced with permission.^{[ 37f ]} Copyright 2023, Elsevier.

Figure 7

a) Schematic illustration of FJH treatment for WPCB powder. b) Comparison of leachability of REEs in 1 M HCl from raw WPCB powders and the 50‐V FJH‐activated WPCB powders. c) Gibbs free energy change of the rare earth oxides and rare earth metals dissolution reactions in acid. a‐c are reproduced with permission.^{[ 89 ]} Published under a CC‐BY 4.0 license. Copyright 2022. d) Commercial FJH device with its sample stage (Model JH3.3‐P, Max output current = 500 A, Max output power = 20 kW, Hefei In‐situ Technology. Co. Ltd.).

Figure 8

a) Schematic diagram for photocatalytic dissolution of PGMs. b) The dissolution percentages of Al, Fe, Cu, Ag, Au, Pd, Pt, Ru, Rh, and Ir in different solvents under photocatalytic conditions (left) and the dissolution kinetics of Cu, Ag, Au, and Pt from metal catalyst matrix throughout sequential loading of acetonitrile and dichloromethane (right). Reproduced from reference.^{[ 120 ]} c) Free energy change (ΔG _dislocation) contour plot from the ternary composition diagram for various ⋅CH₂CN: ⋅CHCl₂: CH₂Cl₂ compositions coordinated to Pt atoms (upper) and the lowest energy structure for R₀‐Pt and R₁‐Pt on the Pt (111) surface (lower, Cyan: Pt; Blue: N; White: H; Gray: C; Cyan: Cl; Red: O). Reproduced with permission.^{[ 121 ]} Copyright 2022, Wiley‐VCH. d) Proposed chemical mechanism for retrieving PGMs by photocatalysis. hʋ, light illumination. Reproduced from reference.^{[ 120 ]}

Figure 9

a) Periodic classification of REEs with spheres scaled to their ionic radius and plot of upper crustal abundance of REEs against their ionic radii (left inset). Stability constant of RE(EDTA)‐complexes compared with their ionic radii (right inset). Adapted with permission.^{[ 1a ]} Copyright 2019, AAAS. b) Chemical potentials of chlorine corresponding to the equilibria between Ln and LnCl₂ and LnCl₂ and LnCl₃ at 80 °C. Values computed from estimated data with large uncertainty are indicated by dotted symbols. Reproduced with permission.^{[ 134 ]} Copyright 2000, Springer Nature. c) Oxide‐sulfide anion exchange chemistry exacerbates the thermodynamic differences between metal compounds as illustrated at 1000 °C. Reproduced with permission.^{[ 135 ]} Copyright 2021, Springer Nature.

Figure 10

a) Overview of the possible advantages and upcycling application for the recovered CMs from WPCBs. b) Prototypical volcano plot demonstrating the activity of a catalyst plotted against the free binding energy of reaction intermediate (denoted as RI) at 0 overpotential. Reproduced with permission.^{[ 171 ]} Copyright 2020, John Wiley & Sons, Inc. c) Volcano plots for transition metal catalysts with different surfaces (stepped and flat) in electrochemical NRR applications. Reproduced with permission.^{[ 172 ]} Copyright 2012, Royal Society of Chemistry. d) Volcano plot by Trasatti for catalysts assessed in HER under acid conditions. Reproduced with permission.^{[ 173 ]} Published under a CC‐BY 4.0 license. Copyright 2014. e) The volcano plot demonstrating various metal oxides used in OER applications. Reproduced with permission.^{[ 174 ]} Copyright Elsevier 2022.

Figure 11

a) Fabrication process for M‐LIG which highlights the formation mechanism in which M‐LIG is formed on chitosan film. Reproduced with permission.^{[ 181 ]} Published under a license 4.0 (CC BY‐NC‐ND). Copyright 2023. b) Schematic illustration of the recovery process of Ni, Mn, and Co from spent Li‐ion battery cathode and the synthesis process of NiMnCo‐AC catalyst via thermal radiation. Reproduced with permission.^{[ 156c ]} Published under a license 4.0 (CC BY‐NC‐ND). Copyright 2022.