Light-Emitting Diodes (LEDs) have become ubiquitous in modern lighting and displays owing to their remarkable energy efficiency. However, their escalating use presents significant end-of-life management challenges, particularly concerning the valuable and critical metals embedded within them. Establishing effective recycling methods is imperative, not merely for resource conservation but also to prevent environmental pollution and mitigate potential health risks associated with some of the constituent materials.
LEDs incorporate several metals deemed critical for modern technology. Indium (In) serves as a key component, primarily utilized in the production of Indium Tin Oxide (ITO) for liquid crystal displays and Indium Phosphors within the LED chips themselves, often combined with other III-V semiconductor compounds like 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, alongside findings that long-term occupational exposure can elevate Indium levels in the bloodstream.
Beyond Indium, another vital group of materials present in LEDs are 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 portion of this market, responsible for 57% of primary production and an even more dominant 85% share in the crucial refining stage.
The path towards efficient LED recycling is obstructed by several inherent difficulties. 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 focused on defining the most appropriate management strategies for these lamps once they reach their end-of-life. Consequently, common disposal methods like landfilling or incineration lead directly to the irreversible loss of valuable metals and contribute significantly to environmental pollution.
Recognizing these challenges, researchers globally have investigated a variety of strategies aimed at recovering 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 mixture of water and ethanol at 300°C.
Hydrometallurgical routes have also been extensively studied; Zhou et al. employed pyrolysis followed by leaching with different acids like 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 under elevated temperature and pressure, have been noted generally for their effectiveness in 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 like 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 Cerium (Ce), Europium (Eu), and Yttrium (Y) recovery 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 designed 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 utilizes PVC (polyvinyl chloride), either virgin or sourced 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 becomes concentrated within the collected liquid product, particularly accumulating in the aqueous phase where concentrations between 3 ppm and 70 ppm were measured. From this enriched aqueous solution, metallic Indium can be retrieved using established methods such as electrolytic extraction, possibly preceded by an evaporation step to further increase the Indium concentration.
This volatilization approach operates at a considerably lower temperature compared to the 1373 K vacuum separation method previously mentioned and 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 a leaching process using 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 through precipitation, typically achieved by adding a base like 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 temperature requirements compared to 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 to recover indispensable metals like 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.