How to Prevent Over‑Grinding of NCM Cathode Material

Over‑grinding of NCM means excessive mechanical impact and shear force shatter intact spherical secondary particles, generating abundant angular fine fragments. Typical consequences: sharp BET increase, low tap density, broad particle‑size distribution (low D10), poor compaction density, high metal impurity risk, deteriorated cycle performance and higher nitrogen energy consumption. Over‑grinding mostly occurs in jet‑mill closed‑loop circulation systems, also seen in ball/bead‑mill processes. Prevention requires upstream raw‑material control, reasonable equipment selection, precise parameter tuning, circulation load management and multi‑indicator quality monitoring, rather than relying only on single‑parameter adjustment.

1. Identify typical signs of NCM over‑grinding

Process monitoring should combine multiple indicators; D50 alone cannot detect over‑grinding.

1. BET value rises obviously, far higher than baseline of qualified material.
2. PSD: D50 may still be within target range, but D10 drops significantly, span becomes high (>1.2), many ultrafine fractions appear.
3. SEM observation: large quantity of angular broken fragments, cracked secondary spheres.
4. Tap density / compaction density decreases.
5. Circulation ratio is high; large amount of qualified powder repeatedly circulates inside grinding loop.

Note: Soft electrostatic agglomeration does not belong to over‑grinding; ultrasonic dispersion test and SEM can distinguish.

2. Upstream raw‑material optimization (reduce grinding burden at source)

1. Stabilize calcination process
   Avoid severely over‑sintered huge hard agglomerates. If large fused lumps exist, operators have to raise grinding pressure to break them, which destroys normal spherical particles. Meanwhile avoid under‑sintered clinker with low mechanical strength; fragile secondary spheres break easily under moderate collision energy.
2. Pre‑crush and screen calcined clinker before jet‑mill feeding, remove oversized chunks. Homogenize clinker batch‑to‑batch to keep consistent grindability.
3. Reduce precursor hard agglomerates during co‑precipitation step.

3. Jet‑mill parameter control (core prevention measures for mass production)

3.1 Grinding pressure: use minimum effective pressure

Only apply enough pressure to break sintered necks between bonded agglomerates.

• For high‑nickel NCM typical window: 0.5–0.8 MPa.
• Do not keep excessive safety margin pressure. Every unnecessary pressure increment increases particle impact energy.
• Adjust step‑by‑step: raise pressure until hard agglomerates disappear in SEM / PSD D90, then stop increasing.

3.2 Stabilize feeding and gas‑solid ratio

1. Use loss‑in‑weight gravimetric feeder, eliminate hopper bridging, rat‑hole and periodic material surging.
2. Strictly avoid extremely low feed rate: low particle concentration leads to violent particle‑to‑particle collision, secondary spheres fracture easily.
3. Avoid severe over‑feeding which causes incomplete de‑agglomeration; do not compensate poor de‑agglomeration by raising grinding pressure. Maintain optimal gas‑solid ratio for each NCM grade.

3.3 Control circulation ratio (very critical for closed‑loop jet‑mill)

Repeated circulation is the most common root‑cause of over‑grinding in actual production.

1. Follow minimum sufficient classifier speed principle: set the lowest rotor speed that meets D50, D90 and Dmax requirements. Excessively high classifier speed sends plenty qualified medium‑size particles back into grinding chamber for repeated impact.
2. Match classifier rotor speed with main airflow. Do not only tune rotor speed. Reasonable speed‑airflow combination improves classification sharpness, reduces fine‑particle loss into return stream, lowers circulation load.
3. Optimize secondary‑air: sufficient secondary‑air disperses soft electrostatic agglomerates, prevents intact spheres from being wrongly rejected back to grinding zone. Do not use grinding pressure to solve soft‑agglomerate problem.
4. Periodically sample return coarse stream: if large volume of already‑qualified particles exist in return flow, circulation ratio is too high, re‑optimize speed‑airflow recipe.
5. For high‑nickel grades with strict requirements, adopt two‑stage series classification: first stage removes oversized agglomerates; second stage fine‑tunes particle size, reduces single‑loop circulation burden.

4. Equipment‑related prevention

1. Jet‑mill selection: adopt fluidized‑bed opposed‑jet mill (particle‑against‑particle collision). Avoid high‑energy impact mills that produce strong shear force.
2. Classifier maintenance
   • Inspect rotor blade wear regularly. Worn rotor reduces classification sharpness and increases circulation ratio.
   • Keep volute, guide vanes and pipelines clean. Powder buildup distorts flow‑field, causes mis‑classification and higher circulation.
   • Check shaft‑seal and housing sealing to prevent bypass flow.
3. Nitrogen gas source: stabilize pressure and airflow fluctuation. Pressure swing forces operators to adjust parameters frequently, causing intermittent over‑grinding.
4. Static elimination: reliable grounding for mill and classifier to reduce soft agglomerates. Control circulating nitrogen dew‑point ≤‑40 ℃ to reduce moisture‑induced agglomeration.

5. If using media‑based milling (ball mill / bead mill, lab or single‑crystal process)

1. Prefer zirconia grinding media; prohibit steel media.
2. Select proper bead size: avoid over‑large beads with excessive impact energy.
3. Control bead‑to‑material ratio; do not over‑fill grinding media.
4. Strictly control milling time; stop milling once target particle‑size is reached. Extended milling time continuously breaks secondary spheres.
5. Regularly screen out cracked and worn beads.

6. Operation & quality monitoring strategy

1. Build complete grade‑based process recipe library: lock grinding pressure, feed‑rate, classifier speed, main‑air, secondary‑air. Eliminate operator habit of “set higher pressure for safety”.
2. After any parameter change, wait for full steady‑state, avoid frequent short‑term tuning.
3. Multi‑dimensional quality check, not only D50:
   • PSD: monitor D10, D90, span; drop of D10 is early warning of over‑grinding.
   • BET: sharp rising is sensitive early warning signal.
   • SEM morphology: directly observe broken secondary‑particle fragments.
4. Use online PSD monitoring for early drift warning, avoid producing large volume of over‑ground off‑spec batches.

7. Common mistakes to avoid

Preventing NCM over‑grinding focuses on avoiding unnecessary mechanical energy input and reducing repeated particle circulation. Stabilize calcined clinker quality, apply minimum effective grinding pressure, stabilize feeding, optimize classifier speed‑main‑air‑secondary‑air matching to lower circulation ratio. Monitor D10, BET together with SEM morphology as early warning signals, instead of judging quality only by D50. Two‑stage classification can further reduce circulation load for high‑nickel NCM, protecting spherical secondary‑particle integrity while meeting particle‑size specification.