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The growing global demand for rare-earth elements (REEs), particularly neodymium (Nd), praseodymium (Pr), and dysprosium (Dy), is driven by their vital role in technologies such as electric vehicles, wind turbines, and electronics. As primary sources of REEs become increasingly depleted and environmentally taxing to mine, recycling REEs from end-of-life (EoL) NdFeB magnets presents a promising path toward sustainable resource management. However, the recycling rate of REEs remains low due to a combination of technological, economic, and infrastructural barriers. The development and optimization of diverse recovery methods, including hydrometallurgical, pyrometallurgical, and biometallurgical, are critical to addressing these challenges.
REE recycling not only conserves natural resources but also minimizes the environmental footprint associated with mining. However, the sustainability of recycling processes depends on both their environmental impacts and economic viability.
Bioleaching is the most environmentally friendly approach, primarily due to its low carbon footprint and minimal generation of hazardous waste. Using microorganisms to leach REEs from waste magnet material, bioleaching reduces chemical and energy use. However, its slow kinetics and low extraction efficiency limit its large-scale application.
Hydrometallurgy involves the use of aqueous chemistry, typically acids, to leach REEs from magnet waste. Recent innovations have improved its sustainability, particularly through closed-loop acid recycling systems. One such method uses oxalic acid to precipitate REEs, thereby regenerating hydrochloric acid, which is then reused. This significantly reduces fresh acid requirements and minimizes waste.
A comparative Life Cycle Assessment (LCA) conducted by Becci et al. examined three recovery methods: hydrometallurgical using HCl, solid-state chlorination, and pyrometallurgical oxidation-carbothermic reduction. The hydrometallurgical method was found to be the most environmentally favorable, owing to its lower energy consumption, simpler setup, and beneficial byproducts such as Fe₂O₃, which contribute to environmental credits.
Recent innovations in metallurgy focus on reducing energy demand while improving efficiency. One such advancement is selective chlorination, which employs ammonium chloride (NH₄Cl) at temperatures between 300 and 420 °C. This method has demonstrated an impressive extraction efficiency of 99.8% for rare earth elements (REEs), all while minimizing energy consumption and chemical waste.
Another promising approach involves using waste tire rubber-derived carbon (WTR-DC) as a reducing agent. This strategy not only effectively isolates high-purity rare-earth oxides (REOs) but also repurposes waste materials, thereby reducing carbon residue.
Additionally, the short-loop calcium reduction diffusion technique offers a significant breakthrough by circumventing the traditional multi-step processes typically required. This method allows for the direct conversion of NdFeB magnet sludge into reusable magnetic powders. Operating at moderate temperatures, this innovation substantially reduces CO₂ emissions and energy consumption, thereby facilitating on-site recycling at magnet manufacturing facilities.
Despite advancements in recycling techniques, the industry faces several persistent challenges. One significant issue is the material heterogeneity found in magnet waste streams. These streams are often compositionally diverse and can be contaminated with substances such as oxygen, carbon, and other metals, complicating the consistency and efficiency of recovery processes.
Additionally, the separation complexity of rare earth elements (REEs) presents a major hurdle. The similar chemical properties of these elements make selective separation difficult, and technologies capable of efficiently isolating individual aspects, such as dysprosium (Dy), neodymium (Nd), and praseodymium (Pr), remain under development.
Another critical challenge lies in maintaining the magnetic properties of recovered materials, particularly in short-loop recycling processes. Factors such as altered microstructure, grain size, and crystallographic texture can significantly affect performance. However, there is hope on the horizon, as the reapplication of grain-boundary diffusion (GBD) processes using elements such as terbium (Tb) has shown promise in restoring coercivity and thermal stability, which are essential for the effective use of recycled materials.
A sustainable, circular economy for rare earth elements (REEs) must prioritize advanced understanding of thermodynamics and kinetics. This entails gaining deeper insights into the high-temperature behaviors, solubility, and phase equilibria of REEs, which are crucial for optimizing selective extraction processes and minimizing waste generation.
In addition, there should be a focus on comprehensive economic and lifecycle analyses of various recovery methods. These evaluations should account for reagent costs, energy requirements, emissions generated, and the market value of recovered materials.
Moreover, integrating green energy sources, such as solar, wind, and hydropower, into energy-intensive pyrometallurgical operations can significantly reduce their environmental impact. By embracing these strategies, we can work toward a more sustainable future for the rare-earth elements industry.
The recovery of rare-earth elements from magnet waste is a critical step in addressing global resource scarcity and advancing green technology. While no single method currently meets all economic and environmental criteria, a hybrid approach that combines the environmental advantages of hydrometallurgy, the efficiency of pyrometallurgy, and the ecological safety of biometallurgy offers the best path forward. Continued research and innovation, supported by policy and infrastructure development, are vital to unlocking the full potential of REE recycling and achieving a sustainable future.