How can the scrapped Ndfeb magnets be efficiently recycled? Can the magnetic properties after recycling be close to those of the original materials?

2. Recycling Technologies for NdFeB Magnets

Recycling methods for NdFeB magnets fall into two categories: long-loop (chemical extraction of REEs) and short-loop (direct reuse or remanufacturing). The choice depends on the scrap type (e.g., production waste vs. end-of-life products), cost, and environmental impact.

2.1 Long-Loop Recycling: Chemical Extraction of REEs

Long-loop recycling involves breaking down magnets into individual REEs, which are then reprocessed into new magnets or oxides. Key methods include:

• Hydrometallurgy:
  • Process: Dissolve magnets in acids (e.g., HCl, H₂SO₄), then use solvent extraction or selective precipitation to isolate REEs. For example, Santoku Corporation grinds magnets into particles <75 μm, oxidizes them in NaOH at elevated temperatures, and leaches REEs selectively.
  • Advantages: High purity (99%+ REE recovery), suitable for complex scrap.
  • Challenges: High chemical consumption, wastewater treatment costs, and energy use (e.g., heating for leaching).
• Pyrometallurgy:
  • Process: Heat magnets with fluxes (e.g., CaO, MgO) to form slag containing REEs, which are then reduced to metals. For instance, sulfate roasting and nitrification roasting extend pyrometallurgy by modifying oxidation states.
  • Advantages: Scalable for large volumes, minimal liquid waste.
  • Challenges: High energy input (1,200–1,600°C), potential air pollution from dust emissions.
• Electrochemical Methods:
  • Process: Use electrolysis to extract REEs from molten salts or aqueous solutions. This method is less common but offers precision in separating REEs.
  • Advantages: Low chemical waste, potential for selective recovery.
  • Challenges: High capital costs for specialized equipment.

2.2 Short-Loop Recycling: Direct Reuse or Remanufacturing

Short-loop recycling bypasses chemical extraction, preserving the magnet’s structure for reuse or reprocessing into new magnets. Key methods include:

• Hydrogen Decrepitation (HD):
  • Process: Expose magnets to hydrogen gas, causing them to fracture into powder due to volume expansion in the Nd₂Fe₁₄B phase. The powder is then pressed and sintered into new magnets.
  • Advantages: Energy-efficient (88% less energy than primary production), retains magnetic properties.
  • Case Study: HyProMag’s patented Hydrogen Processing of Magnet Scrap (HPMS) technology recovers NdFeB alloy powder from scrap, achieving 99.8% REE recovery efficiency.
• Magnet-to-Magnet Recycling:
  • Process: Demagnetize scrap magnets, clean them (remove coatings, glue), and reshape them into new geometries. For example, Hitachi Metals recycles over 90% of its production waste into new magnets.
  • Advantages: Minimal material loss, low cost for production scrap.
  • Challenges: Limited to magnets with intact physical properties (e.g., no corrosion or breakage).
• Direct Melting:
  • Process: Melt scrap magnets and cast them into new alloys. This method is less common due to the risk of impurity incorporation.
  • Advantages: Simple for homogeneous scrap.
  • Challenges: Requires strict quality control to avoid degradation.

3. Restoring Magnetic Properties in Recycled Magnets

The magnetic properties of recycled NdFeB magnets depend on the recycling method, scrap quality, and post-processing treatments. Key factors include:

3.1 Grain Boundary Modification (GBM)

• Principle: The magnetic properties of NdFeB magnets depend on the microstructure: the Nd₂Fe₁₄B matrix provides high magnetization, while the grain boundary phase (rich in Nd and REEs) isolates grains to reduce coercivity loss.
• Process: Add REE hydrides (e.g., DyH₃ nanoparticles) during sintering to modify grain boundaries. Liu et al. demonstrated that adding 1% DyH₃ before sintering recovers up to 89% of the original (BH)max (maximum energy product).
• Outcome: GBM enhances coercivity and remanence, making recycled magnets suitable for high-performance applications like traction motors.

3.2 Optimizing Pressure and Temperature

• Pressure: In HD and HDDR (Hydrogen Decrepitation-Disproportionation-Desorption-Recombination) processes, increasing pressure above 1 bar accelerates hydrogen absorption but reduces magnetic properties. The optimal pressure for sustainable processing is 50 kPa.
• Temperature: Sintering at 1,000–1,100°C is critical for densification. Deviations can lead to porosity or grain growth, degrading properties.

3.3 Case Studies: Performance of Recycled Magnets

• Electric Motors: A study compared two identical motors—one using recycled NdFeB magnets (via magnet-to-magnet processing) and the other using virgin magnets. The recycled magnets exhibited 7.0% higher open-circuit flux linkage and 6.4% higher torque despite having 15% lower dysprosium content.
• Industrial Applications: Recycled magnets from MRI scanners, pumps, and wind turbines showed properties similar to virgin magnets (e.g., remanence Br = 1.16–1.29 T, coercivity HcJ = 1,147–1,590 kA/m).

4. Challenges and Future Directions

Despite advancements, recycling NdFeB magnets faces challenges:

• Material Quality Variability: The condition of scrap (e.g., corrosion, coatings) affects recycling efficiency. For example, glue residues from bonded magnets require alkali roasting for removal.
• Economic Viability: Long-loop methods are costly due to chemical and energy inputs. Short-loop methods are cheaper but limited to high-quality scrap.
• Scalability: Most industrial plants (e.g., HyProMag, REEcycle) are pilot-scale. Large-scale adoption requires policy support (e.g., subsidies, extended producer responsibility).

Future Innovations:

• Microwave-Assisted Processing: Rapid, energy-efficient heating for oxidizing magnets or assisting combustion.
• Advanced Sorting Technologies: AI-powered sensors to separate magnets from e-waste by composition and geometry.
• Circular Economy Models: Integrating recycling into product design (e.g., modular devices for easy magnet removal).

5. Conclusion

Efficient recycling of scrapped NdFeB magnets is achievable through short-loop methods like hydrogen decrepitation and magnet-to-magnet processing, which preserve magnetic properties while reducing environmental impact. By optimizing grain boundary modification, pressure, and temperature, recycled magnets can match or exceed the performance of virgin materials in applications like electric vehicles and wind turbines. However, scaling up recycling requires addressing material variability, economic barriers, and technological gaps. Collaborative efforts among governments, manufacturers, and researchers are essential to transition to a circular economy for NdFeB magnets, ensuring sustainable access to critical REEs for future technologies.