NdFeB (Neodymium-Iron-Boron) is the strongest permanent magnetic material available today. And It is widely used in EV traction motors, wind turbines, consumer electronics, and more. In its manufacturing process, the pulverization step is critical because it directly affects particle-size distribution, oxygen content, and the final magnetic performance. Traditional mechanical pulverization methods (such as шаравая млын or jaw crushing) cannot meet the stringent requirements of NdFeB, while the рэактыўны млын has become the industry standard.
This article will systematically explain the necessity of air jet mills from several dimensions, including material properties, process requirements, and reasons for the failure of alternative solutions.
Material Characteristics of NdFeB Make Pulverization Highly Challenging
High hardness and brittleness
The main phase Nd₂Fe₁₄B has a Mohs hardness of 6–7, with extremely high brittleness. However, the presence of a small amount of Nd-rich soft phase causes localized plastic deformation instead of clean intergranular fracture during mechanical crushing.
Extremely high chemical reactivity
Nd is a highly reactive rare-earth element with a standard electrode potential of –2.32 V and oxidizes instantaneously upon exposure to oxygen. Smaller powder particles dramatically increase the oxidation rate. Studies show that NdFeB powders <10 μm can gain 500–1000 ppm oxygen within 30 seconds when exposed to air.
Magnetic properties are highly sensitive to grain orientation
Sintered NdFeB requires monocrystalline particles (typically 3–5 μm). Any micro-cracks or grain-boundary damage resulting from improper crushing will disrupt magnetic anisotropy, reducing Br by 5–10%.
Fatal Limitations of Traditional Mechanical Pulverization
| Pulverization Method | Main Problems | Impact on NdFeB |
|---|---|---|
| Jaw/Hammer Crusher | Local hot spots (>200°C), metal contamination | Nd-rich phase melts and smears; Fe contamination >500 ppm |
| Шаравой млын (Dry/Wet) | Media wear, oxygen diffusion, long exposure time | Oxygen >3000 ppm, particle-size sigma >2 |
| Disc/Ролікавы млын | Uneven shear, heat accumulation | Higher defect density, HcJ decreases by 15% |
Mechanical pulverization inevitably generates frictional heat and metal wear debris. Since NdFeB has an extremely low oxidation threshold (ΔG for Nd₂O₃ ≈ –1700 kJ/mol), even minor temperature rise can trigger irreversible oxidation.
Unique Advantages of the Рэактыўны млын for NdFeB
А рэактыўны млын uses supersonic airflow (Mach number > 2) to accelerate particles and cause self-collision, providing the following unmatched advantages:
No media and no contamination
- No mechanical moving parts in the milling chamber, eliminating Fe, Cr, Zr contamination.
- Typical impurity increase is <50 ppm, compared with 500–2000 ppm in mechanical mills.
Low-temperature operation
- Gas expansion produces a Joule–Thomson cooling effect, keeping the milling process near-isothermal.
- Oxygen increase is only 80–150 ppm under inert-gas protection.
This is a key advantage that traditional milling technologies can never achieve.
Narrow particle-size distribution for high-end magnet materials
- Particle relative velocity >300 m/s ensures clean grain-boundary fracture.
- Extremely narrow PSD: D50 = 3.5 ± 0.5 μm, Span < 1.2, monocrystalline rate >95%.
- Jet mill + precision ceramic classifier enables D50 1–10 μm, with excellent morphology and narrow distribution.
This fully meets the requirements of advanced high-coercivity NdFeB magnets.
Integrated deoxygenation and classification
- Cyclone + bag collector keep oxygen content <50 ppm online.
- Closed-loop nitrogen circulation limits powder exposure to <1 second.
Suitable for automated, continuous production with high consistency
NdFeB magnets require extremely stable and consistent powder quality. Jet mill systems support:
- Automatic feeding
- Real-time particle-size control
- Continuous stable operation
- Closed-loop grinding + classification
This ensures uniform quality across batches, ideal for large-scale industrial production. Thus, the jet mill has become the standard equipment for high-end NdFeB powder manufacturers.
Industrial Validation Data
- Over 98% of the world’s top three NdFeB producers rely on jet-milling technology.
- Jet-milled powder: O content = 900–1200 ppm → sintered magnet: Br = 14.2 kG, HcJ = 18 kOe
- Mechanically milled powder: O content >3500 ppm → Br = 13.6 kG, HcJ = 14 kOe (same formulation)
Заключэнне
The requirement for jet mill in NdFeB production is not the result of technological monopoly, but rather the inherent demands of high-performance permanent magnets:
- precise particle-size uniformity
- ultra-high хімічны purity
- low oxidation and safe processing
- high consistency and cost-effectiveness
The jet mill, through its unique self-impact mechanism and inert-gas protection capability, has proven to be the most reliable and effective industrial solution for meeting these stringent requirements.
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— Posted by Jason Wang