D10 represents the fine‑particle tail (10 % volume of particles are smaller than this value); D90 reflects the coarse tail (90 % volume are smaller than this value). D10 is mainly affected by over‑grinding and generation of fine fragments; D90 is dominated by residual hard agglomerates and coarse bypass from poor classification. D50 alone cannot represent particle‑size distribution quality. Stable control of D10 and D90 requires joint control of raw‑material quality, jet‑mill grinding energy, classifier performance (rotor speed, main‑air, secondary‑air), feeding stability and circulation ratio.
Target reference for typical polycrystalline NCM:
- D50:6‑12 μm
- D10:1.5‑4 μm
- D90:12‑22 μm
- Span=(D90‑D10)/D50：0.8‑1.2
1. Understand what drives D10 and D90
D10 (fine tail)
- Decrease of D10 → more ultrafine fine fragments; early warning signal for over‑grinding.
- Root causes: excessive grinding pressure, too low feed concentration, too high circulation ratio, fragile under‑sintered secondary spheres being crushed.
D90 (coarse tail)
- Increase of D90 → more coarse components: real hard sintered agglomerates or soft electrostatic agglomerates (fake coarse).
- Root causes: unbroken calcination hard agglomerates, classifier coarse bypass, insufficient secondary‑air, classifier overload, rotor wear, seal leakage.
Important: Soft agglomerates will push D90 higher, but ultrasonic dispersion test makes D90 drop significantly; hard agglomerates remain unchanged after sonication.
2. Raw‑material pre‑control (foundation for stable D10/D90)
- Stabilize calcination quality
- Avoid large fused hard agglomerates, otherwise D90 is hard to lower; do not simply raise grinding pressure (will pull D10 down).
- Avoid under‑sintered fragile secondary particles, which easily break and make D10 too low.
- Pre‑screen calcined clinker to remove oversized chunks before jet‑mill feeding. Homogenize batch feed to keep consistent grindability.
- Reduce precursor hard agglomerates upstream.
3. Jet‑mill grinding parameters control (main influence on D10)
- Grinding pressure (use minimum effective pressure)
- Higher pressure: breaks hard agglomerates (D90 ↓), meanwhile generates fine fragments (D10 ↓).
- Lower pressure: less fine generation (D10 ↑), risk of residual hard agglomerates (D90 ↑).
- Operation logic: increase pressure just enough to eliminate real hard agglomerates, stop further pressure rise to protect D10.
- Stabilize feeding rate & gas‑solid ratio
- Too low feed rate: low particle concentration → violent particle‑to‑particle collision, D10 drops sharply.
- Surge / bridging of gravimetric feeder: instant over‑feed causes incomplete de‑agglomeration, D90 spikes intermittently.
- Keep feed within optimal range, avoid extreme low‑load operation.
4. Classifier tuning (core for both D10 and D90)
Classifier rotor speed, main airflow, secondary‑air work together; single‑parameter adjustment cannot independently fix D10 or D90.
4.1 Rotor speed
- Increase rotor speed: cut‑point becomes finer → D50 ↓, D90 ↓; circulation ratio rises, risk of repeated grinding makes D10 ↓.
- Decrease rotor speed: cut‑point coarser → D50 ↑, D90 ↑; circulation ratio reduces, D10 tends to increase (less over‑grinding).
Warning: Simply raising rotor speed to reduce D90 often causes D10 to become too small.
4.2 Main airflow
- Increase main airflow: higher drag force, more coarse particles pass through rotor → D90 ↑; circulation ratio reduces, D10 may rise.
- Decrease main airflow: drag force weakens → D90 ↓; fine loss increases, higher circulation risk → D10 ↓.
4.3 Secondary‑air (critical for fake coarse from soft agglomerates)
- Increase secondary‑air: disperse soft electrostatic agglomerates, apparent D90 drops (without breaking real spheres).
- Without sufficient secondary‑air: soft clusters are counted as coarse particles, D90 falsely high, circulation ratio increases and indirectly pulls D10 down.
Secondary‑air adjusts apparent D90 caused by soft agglomerates, cannot reduce real hard sintered agglomerates.
Practical tuning logic
- First use secondary‑air to eliminate soft‑agglomerate‑caused high D90.
- If real hard agglomerates remain (D90 still high after ultrasonic dispersion), slightly raise grinding pressure, monitor D10 simultaneously.
- Adjust rotor speed + main‑air in pairs to reach target D50, keep circulation ratio low to prevent D10 dropping.
- If D10 is too low (too many fines): reduce grinding pressure, lower classifier rotor speed, increase feed rate properly.
5. Equipment factors affecting D10 & D90 stability
- Classifier rotor wear / deformation: flow‑field distortion, coarse bypass → D90 periodically rises; higher circulation also pushes D10 down. Inspect ceramic rotor regularly.
- Classifier over‑loading (>80 % rated throughput): particle interference, coarse bypass → D90 unstable. Reduce throughput load.
- Seal leakage (shaft‑seal / housing): unclassified raw powder directly enters finished product → D90 spikes. Check purge‑gas seal condition.
- Powder accumulation inside volute and pipeline: periodically peeling lumps cause intermittent D90 spike. Implement regular cleaning.
- Nitrogen pressure and airflow fluctuation: leads drifting cut‑point, batch‑to‑batch variation for D10/D90. Install pressure‑stabilizing station.
6. Environment control
- Nitrogen dew‑point ≤‑40 ℃: high dew‑point aggravates soft agglomerates, D90 false high. Prevent air ingress.
- Full‑system static elimination: reduce soft agglomeration induced by static.
7. Typical fault‑shooting table
| Observation | Root cause | Correction |
|---|---|---|
| D90 too high; D90 drops after ultrasonic dispersion | Soft electrostatic agglomerates | Increase secondary‑air; check grounding; lower dew‑point |
| D90 too high, no change after ultrasonic dispersion | Residual hard sintered agglomerates | Slightly raise grinding pressure; check feed clinker quality |
| D90 high, meanwhile D10 very low | Hard agglomerates + partial over‑grinding | Do not blindly raise pressure; optimize calcination clinker; reduce circulation ratio |
| D10 too low (excessive fine tail), D50 still OK | Over‑grinding: high pressure or high circulation ratio | Reduce grinding pressure; lower classifier speed; increase feed rate |
| D10 too high (lack fine fraction), D90 OK | Insufficient grinding energy | Slightly raise grinding pressure, monitor D90 |
| D10 & D90 fluctuate batch‑by‑batch | Feeder surge; nitrogen instability; rotor wear | Stabilize feeding; stabilize gas source; inspect rotor & seals |
8. Key operation principles
- Never tune only by D50; must track D10 and D90 together. D10 is your most sensitive early warning for over‑grinding.
- Distinguish real hard agglomerates vs soft agglomerates via ultrasonic PSD test before modifying grinding pressure.
- Rotor speed and main‑air must be matched; avoid only changing rotor speed.
- Control circulation ratio: high circulation will pull D10 down even if grinding pressure is reasonable. Sample return stream regularly.
- Combine PSD with BET and SEM: when D10 decreases while BET rises, it confirms secondary‑particle fracture occurs.
D10 is mainly controlled by grinding energy and circulation ratio; D90 is affected by hard agglomerates, classifier coarse bypass and soft electrostatic agglomerates. Stable D10‑D90 control starts from calcined clinker quality, applies minimum‑effective grinding pressure, optimizes the triple parameters of classifier rotor speed/main‑air/secondary‑air, limits circulation ratio, and maintains classifier hardware integrity. Always differentiate real coarse agglomerates and soft agglomerates, avoid solving D90 problem by blindly increasing grinding pressure and causing D10 to deteriorate.
