Light-Emitting Diodes (LEDs) have become ubiquitous in modern lighting and displays owing to their remarkable energy efficiency. However, their increasing use poses significant end-of-life management challenges, particularly regarding the valuable and critical metals embedded in them. Establishing effective recycling methods is imperative, not only for resource conservation but also to prevent environmental pollution and mitigate potential health risks associated with some constituent materials.
LEDs incorporate several metals deemed critical for modern technology. Indium (In) serves as a key component, primarily used in the production of Indium Tin Oxide (ITO) for liquid crystal displays and Indium Phosphors within LED chips, often combined with other III-V semiconductor compounds such as Gallium Arsenide. The global refinery output of Indium stood at an estimated 920 tonnes in 2021, with China possessing the largest refining capacity at 530 tonnes. A point of concern arises from studies linking compounds such as Indium Arsenide and Indium Phosphide to proliferative lesions, as well as findings that long-term occupational exposure can elevate Indium levels in the bloodstream.
Beyond Indium, another vital group of materials present in LEDs is the Rare Earth Elements (REEs). This group comprises 17 chemically similar elements indispensable for numerous advanced applications, including LEDs, high-performance magnets, and automotive catalytic converters. In 2020, the global primary production of rare earth oxides amounted to 214,000 tonnes. China commands a significant share of this market, accounting for 57% of primary output and an even more dominant 85% share in the crucial refining stage.
Several inherent difficulties obstruct the path towards efficient LED recycling. Firstly, the wide diversity of lamp types means that metal content varies considerably, complicating the development of standardized, one-size-fits-all processing techniques. Secondly, the presence of hazardous substances, such as lead, within some LED components can impede straightforward recycling workflows and necessitate careful handling. Furthermore, there is a widely acknowledged deficit in mature, efficient recycling technologies specifically tailored for the complex material mix found in LEDs. Compounding this is a lack of comprehensive research to define the most appropriate management strategies for these lamps at the end of life. Consequently, common disposal methods such as landfilling or incineration result in the irreversible loss of valuable metals and contribute significantly to environmental pollution.
Recognizing these challenges, researchers worldwide have investigated a variety of strategies to recover metals from EoL LEDs. These explorations span thermal, chemical, biological, and physical methods. For instance, Zhan and colleagues explored pyrolysis to remove organic components, followed by physical separation and high-temperature vacuum vaporization (1373 K) to recover Gallium (Ga) and Indium (In). The same research group also demonstrated high recovery rates for Ga, In, Arsenic (As), and Silver (Ag) using extraction with a subcritical water-ethanol mixture at 300°C.
Hydrometallurgical routes have also been extensively studied; Zhou et al. employed pyrolysis followed by leaching with various acids, including oxalic acid, achieving up to 83.42% Gallium recovery. Murakami et al. focused on Gold (Au), leaching it with aqua regia at 80°C and subsequently purifying it using selective adsorption. Hydrothermal methods, which utilize water at elevated temperature and pressure, are generally effective at dissolving otherwise insoluble materials under relatively mild, low-pollution conditions.
Bio-hydrometallurgy offers another avenue, exemplified by Pourhossein and Mousavi, who successfully used the bacterium Acidithiobacillus ferrooxidans to leach Copper (Cu), Nickel (Ni), and Gallium (Ga), achieving high efficiencies under optimized conditions. Other approaches include combining chemical and mechanical processes, as demonstrated by Nagy et al. for Gallium recovery, and the use of specialized solvents, such as ionic liquids, by Van den Bossche et al. to extract Gallium and Indium.
Conceptual process designs, such as the one proposed by Ruiz-Mercado et al. for the recovery of Cerium (Ce), Europium (Eu), and Yttrium (Y) from flat screens, and mechanical processing focused on disassembly, studied by Martins et al., further illustrate the breadth of research efforts.
Addressing the specific need for comprehensive recovery, one study detailed a novel, integrated two-stage process to reclaim both Indium and the heavy rare-earth element Lutetium (Lu) from EoL LED materials.
The first stage focuses on Indium recovery through chloride volatilization. This technique cleverly uses PVC (polyvinyl chloride), whether virgin or derived from plastic waste, as an inexpensive chlorinating agent. When heated to 650°C (923 K), the PVC thermally decomposes, releasing hydrogen chloride (HCl) gas. This reactive gas interacts with the metals present in the LED material, forming metal chlorides. Indium Chloride (InCl3) is notably volatile at this operational temperature.
The gaseous metal chlorides, including InCl3, are then transported away from the solid residue and subsequently condensed. Experimental data confirmed that Indium is concentrated in the collected liquid product, particularly in the aqueous phase, where concentrations of 3-70 ppm were measured. From this enriched aqueous solution, metallic Indium can be recovered using established methods, such as electrolytic extraction, which may be preceded by an evaporation step to increase the Indium concentration further.
This volatilization approach operates at a considerably lower temperature compared to the 1373 K vacuum separation method previously mentioned. It is proposed to offer a faster recovery route than some traditional hydrometallurgical processes.
The second stage of the process targets Lutetium, which remains in the solid residue. Lutetium compounds, being non-volatile under the 650°C conditions of the first stage, are retained within the solid coke material left after halogenation. This Lutetium-bearing coke is then subjected to leaching with a solution containing 32% sulfuric acid and hydrogen peroxide. This leaching step effectively dissolves the Lutetium, with reported experimental efficiencies reaching up to 71%.
Following leaching, Lutetium is selectively extracted from the acidic aqueous solution into an organic phase. The study found that Tributyl Phosphate (TBP) dissolved in 1-octanol serves as a highly effective extractant system, confirmed by favorable partition coefficient measurements. Finally, pure Lutetium can be recovered from this loaded organic phase by precipitation, typically achieved by adding a base such as sodium hydroxide (NaOH), consistent with standard practices in rare-earth metallurgy.
This integrated process highlights a potentially effective strategy for reclaiming multiple critical metals from the complex waste stream generated by EoL LEDs. Its significance lies in several key advantages. It facilitates the simultaneous recovery of both Indium and Lutetium from the same initial waste material.
Furthermore, the ingenious use of PVC waste as a chlorine source not only reduces reagent costs but also presents an opportunity for synergy between plastic waste management, by addressing chlorine content issues in waste incineration, and e-waste recycling.
Critically, this work represents the first documented method for recovering the valuable heavy rare earth element Lutetium specifically from EoL LED sources. The process also potentially offers efficiency gains through faster Indium recovery and lower operating temperatures than some alternative technologies.
The development and widespread implementation of robust recycling technologies for EoL LEDs are clearly paramount for sustainable resource management. Innovative approaches, such as the described combination of chloride volatilization and subsequent leaching/extraction, offer viable pathways for recovering indispensable metals such as Indium and Lutetium. Success in these endeavors will be crucial for conserving finite resources, mitigating environmental pollution from electronic waste, and advancing towards a more circular economy.