In closed‑loop jet‑mill systems for NCM cathode powder, airflow carries particles into the turbine classifier, governs particle transportation, separation cut‑point, dispersion of soft agglomerates and circulation load. Classifier rotor speed defines centrifugal force; airflow provides drag force. The balance between centrifugal force and aerodynamic drag determines which particles pass to finished product and which return for re‑grinding. Many production teams only tune rotor speed while ignoring airflow, leading to unstable D50/D90, poor classification sharpness, residual soft agglomerates and excessive circulation ratio. This article explains core functions, parameter coupling, process behaviour and mass‑production practical notes for EV‑grade NCM.
1. Fundamental physical principle
Inside dynamic turbine classifier:
- Rotating classifier wheel generates centrifugal force, pushing larger / heavier particles outward toward coarse return path.
- System airflow (main air + secondary air) generates inward drag force, pulling fine particles through rotor gaps into finished‑product collection.
Separation logic:
- Large particles: centrifugal force ＞ drag force → rejected, flow back to grinding chamber.
- Fine particles: drag force ＞ centrifugal force → pass through rotor as finished powder.
Classifier speed sets centrifugal magnitude; airflow magnitude sets drag magnitude. Cut‑point is determined by the balance of both, not rotor speed alone.
2. Core roles of airflow in NCM classification
2.1 Transport and deliver particles to classification zone
Main system airflow carries ground NCM powder from grinding chamber into classifier housing. Insufficient airflow causes particle sedimentation, powder buildup inside volute and pipelines, intermittent surging and PSD fluctuation. Too‑high airflow increases total gas volume, raises nitrogen compression consumption.
2.2 Adjust classification cut‑point (coarse‑fine separation size)
Under fixed classifier rotor speed:
- Increase total airflow: higher drag force, more medium‑size particles can overcome centrifugal force and pass through rotor → D50 and D90 become coarser.
- Decrease total airflow: lower drag force; only finer particles can pass → D50 and D90 become finer.
Critical production fact: You can get identical D50 with multiple sets of “speed‑airflow” combinations.
- Higher rotor speed + higher airflow
- Lower rotor speed + lower airflow
But classification sharpness, circulation ratio and energy consumption will differ greatly between the two groups.
2.3 Improve classification sharpness
Well‑matched airflow reduces overlap: minimize over‑size in finished product and minimize fine‑particle loss into return coarse stream.
- Poor airflow matching: broad PSD tail, high D90 even if D50 is within target range; many qualified fine particles go back to grinding chamber, increasing circulation ratio and energy consumption.
2.4 Secondary‑air function: disperse soft electrostatic agglomerates
Secondary air injects shear airflow before classifier inlet.
NCM (especially high‑nickel grade) forms soft electrostatic agglomerates: multiple complete spherical particles stick together by static charge. Without shear dispersion, these loose clusters behave like one large “fake coarse particle”.
- Secondary‑air shear breaks soft agglomerates into discrete individual spheres before classification. Prevents intact qualified spheres from being incorrectly rejected back to grinding loop, reduces unnecessary re‑circulation and avoids over‑grinding.
Secondary air cannot break hard sintered agglomerates formed in calcination.
2.5 Control circulation ratio
Total airflow directly impacts how much material is recirculated.
- Excessively high airflow at fixed rotor speed: more coarse agglomerates penetrate into finished product → D90 and Dmax out‑of‑spec.
- Too low airflow: most particles get rejected back to mill, circulation ratio rises sharply, specific energy increases, risk of secondary‑particle fracture from repeated collision.
3. Parameter coupling in mass NCM production
- When you change classifier speed, airflow must be matched accordingly.
- Target finer D50: increase rotor speed; moderately reduce total airflow. If airflow stays high, strong drag will pull coarse particles through, D90 will drift high.
- Target coarser D50: reduce rotor speed; properly lower airflow to suppress coarse bypass. If airflow remains too high, coarse tail deteriorates.
- Secondary‑air adjustment
- Too low secondary air: poor de‑agglomeration, many soft agglomerates enter finished powder.
- Too high secondary air: disturbs main flow‑field, reduces classification efficiency, increases total gas consumption.
- Common mistake: fix airflow constant and only adjust rotor frequency.
Result: When rotor speed shifts far from design operating point, classification sharpness degrades. You can hit target D50 but get poor D90 / broad span and high circulation load.
4. Typical production‑site phenomena caused by improper airflow
| Phenomenon | Root cause related to airflow | Countermeasure |
|---|---|---|
| D50 OK but D90 too high, many over‑size particles | Excessive total airflow, drag force pulls coarse clusters through rotor | Reduce total airflow; increase secondary‑air for soft‑agglomerate dispersion |
| High circulation ratio, high energy consumption, BET rising | Insufficient airflow; most particles rejected back to grinding | Raise total airflow, re‑match classifier speed |
| PSD fluctuates batch‑to‑batch | Unstable system airflow; powder deposition caused by low local flow velocity | Stabilize fan output; clean volute and pipelines |
| Soft agglomerates visible under SEM, Dmax unstable | Insufficient secondary‑air shear | Increase secondary‑air volume within reasonable range |
5. Practical operation guidelines for NCM
- Each NCM grade has an optimized operating window of total airflow paired with classifier speed. Store complete speed‑airflow‑secondary‑air recipes in process library.
- Prefer operating point with good classification sharpness: achieve target D50/D90 at lower circulation ratio, saving nitrogen energy and lowering over‑grinding risk.
- Distinguish main airflow and secondary‑air functions: main airflow controls overall cut‑point; secondary‑air mainly disperses soft electrostatic agglomerates.
- Monitor not only PSD, also circulation return stream. If large amount of already‑qualified fine particles circulate back, it signals mismatched speed‑airflow combination.
- Maintain equipment: powder buildup inside classifier housing changes actual flow‑field even if instrument shows fixed airflow set‑value. Scheduled cleaning is required.
Airflow provides aerodynamic drag force, counterbalancing centrifugal force from classifier rotor wheel. It controls particle transport, classification cut‑point, separation sharpness, soft‑agglomerate dispersion and circulation load. Rotor speed alone cannot fully define PSD output. For NCM production, especially high‑nickel material, correct matching between classifier speed, total airflow and secondary‑air is essential to obtain stable D50, controlled coarse tail (D90/Dmax), lower circulation ratio, reduce energy consumption and protect spherical secondary‑particle morphology.
