Grinding pressure is one of the core adjustable parameters of fluidized‑bed opposed‑jet mill for NCM cathode post‑sinter processing. It determines the kinetic energy of high‑speed nitrogen jets and particle‑to‑particle collision intensity, and directly changes agglomerate breaking degree, particle‑size distribution, BET specific surface area and secondary‑particle integrity. Grinding pressure works together with classifier wheel speed, airflow and feeding rate to define final PSD. Too low pressure leaves unbroken sintered agglomerates; excessive pressure triggers over‑grinding, generates abundant ultrafine fines and damages spherical secondary particles. This article explains the influence mechanism, performance trade‑offs and practical operation guidelines for NCM production.
1. Fundamental Mechanism
In jet mill, Laval nozzles convert high‑pressure nitrogen into supersonic gas flow. Higher grinding pressure increases gas velocity and accelerates NCM particles to higher impact speed. Comminution mainly occurs through particle‑on‑particle collision and friction, not mechanical hammer impact.
- Moderate collision force fractures weak sintering necks between secondary‑particle agglomerates. Intact spherical secondary particles remain preserved.
- Excessive collision energy will crack and shatter complete secondary particles, producing large quantities of sub‑micron fine fragments.
- Low collision energy can only break soft agglomerates; hard sintered lumps from calcination survive and circulate inside the closed‑loop system.
Important note: Grinding pressure changes the particle‑size composition entering the classifier, while the classifier sets the final cut‑point. Grinding pressure alone cannot fully determine finished D50 and Dmax.
2. Effect of Increasing Grinding Pressure
When other parameters (classifier speed, airflow, feeding rate) stay unchanged:
- D50, D97 and Dmax trend to decrease. More sintered agglomerates are disassembled.
- D10 drops significantly; the fraction of ultrafine particles rises.
- BET specific surface area increases.
- Span value may become wider because large amounts of new ultrafine fractions are produced.
- Risk of secondary‑particle micro‑cracking and pulverization rises.
- For high‑nickel NCM, more fresh surface is exposed, increasing sensitivity to moisture and residual alkali.
Negative consequences of excessively high grinding pressure:
- Over‑grinding syndrome: large volume of sub‑micron powder is generated.
- Severe side‑reactions in cells, gas swelling, accelerated cycle fading.
- Even if Dmax is reduced, electrochemical performance deteriorates.
3. Effect of Reducing Grinding Pressure
- Collision kinetic energy decreases. Many sintered agglomerates cannot be fully broken, D97 and Dmax increase.
- Less ultrafine powder is generated; D10 becomes coarser, BET keeps at relatively low level.
- Secondary‑particle morphology is well preserved.
- Risk: residual hard agglomerates become oversized killer particles, bringing battery safety hazards.
- PSD span may increase due to remaining coarse agglomerates.
If grinding pressure is too low, even raising classifier wheel speed cannot eliminate hard lumps, because classifier only separates particles rather than crushing them.
4. Interaction between Grinding Pressure and Other Parameters
4.1 Cooperate with classifier wheel speed
- Higher grinding pressure produces more fine fractions; classifier needs corresponding speed adjustment to obtain target D50. If classifier speed remains unchanged after pressure rise, finished D50 will become finer than expected.
- When target is finer D50: do not only raise grinding pressure; prioritize increasing classifier speed to avoid over‑grinding.
4.2 Cooperate with feeding rate (gas‑solid ratio)
Under high feeding rate (low gas‑solid ratio), particles crowd in collision zone. The actual effect of grinding pressure is weakened. Even high pressure cannot effectively break agglomerates.
4.3 Limitation for heavily sintered hard lumps
Grinding pressure can break moderate sintered agglomerates. For heavily fused hard lumps formed by severe over‑sintering in calcination, blindly raising grinding pressure cannot fully pulverize these lumps without destroying normal secondary particles. In such case, upstream calcination optimization is required.
5. Typical Pressure‑setting Principle for Industrial NCM
- NCM523 / NCM622: 0.6–0.9 MPa
- High‑nickel NCM811: 0.5–0.8 MPa (use relatively moderate pressure to protect secondary particles and control BET)
Operation guidance:
- Start from low pressure, gradually increase pressure until Dmax meets specification. Stop pressure rise once target particle‑size is achieved, avoid excessive margin.
- Perform pressure gradient test for each material grade to find the optimal pressure window between “insufficient de‑agglomeration” and “over‑grinding”.
- Monitor not only D50 and Dmax, but also D10, span and BET when adjusting grinding pressure.
6. Troubleshooting related to grinding‑pressure deviation
| Observed phenomenon | Root cause related to grinding pressure | Recommended countermeasure |
|---|---|---|
| High Dmax, many unbroken agglomerates | Grinding pressure too low | Moderately increase grinding pressure; check nozzle wear |
| D50 becomes finer, but BET and span rise sharply | Grinding pressure too high causing over‑grinding | Reduce grinding pressure; fine‑tune classifier parameters |
| Dmax still high even under maximum grinding pressure | Hard sintered lumps from calcination | Optimize calcination temperature / holding time |
| PSD drifts batch‑to‑batch | Unstable nitrogen supply pressure | Stabilize gas source pressure; add pressure monitoring alarm |
Grinding pressure controls particle collision energy in jet milling. Higher pressure improves agglomerate breaking capacity and reduces D50 / Dmax, meanwhile brings risk of over‑grinding, elevated BET and secondary‑particle damage. Lower pressure protects particle integrity yet may leave harmful oversized agglomerates. Grinding pressure must be matched with classifier wheel speed, feeding rate and upstream clinker quality. It is not preferred to achieve target PSD only by adjusting grinding pressure. Reasonable pressure setting should balance particle‑size target, secondary‑particle morphology and electrochemical performance of NCM cathode powder.
