How to Prevent Oversized Particles in NCM Powder Production

Oversized particles (high D99 / Dmax / killer particles) mainly originate from: sinter‑bonded hard agglomerates in calcination, incomplete de‑agglomeration in post‑processing, hard cake chunks, and false agglomerates from precursor drying. The best strategy is source prevention across precursor, sintering, and post‑processing, rather than only fixing oversized particles after they form.

1. Precursor co‑precipitation stage (first‑line prevention)

Even before calcination, abnormal large precursor particles will carry over into final NCM.

1. Maintain stable CSTR reactor parameters: strictly lock pH, ammonia concentration, temperature, stirring speed and solid content. Parameter drift causes burst secondary nucleation and abnormal giant agglomerates.
2. Adopt seed‑feeding and multi‑stage series reactor design to separate nucleation and particle‑growth zones, suppress generation of over‑size precursor spheres.
3. Optimize filtration, washing and drying conditions. Avoid hard secondary agglomeration during dehydration; these pseudo‑large particles survive calcination and appear as oversized material.
4. Install coarse overflow discharge for precursor slurry to remove occasional oversized precursor agglomerates before filtering.

2. Lithium mixing & calcination / sintering (most critical source of hard sinter‑bonded agglomerates)

Most real hard oversized killer particles are formed here, when adjacent secondary spheres fuse together via sinter necks. Post‑processing can only break these necks with limited efficiency.

1. Improve mixing homogeneity of precursor and lithium source (LiOH·H₂O). Local lithium enrichment lowers melting point and accelerates inter‑particle sinter‑bonding. Use high‑efficiency dry mixing or high‑shear wet mixing. Keep lithium excess ratio within reasonable range (1.03‑1.05 for NCM811).
2. Optimize sintering temperature profile:
   • Avoid over‑sintering: excessive peak temperature or over‑long holding time creates heavy sinter‑bonded agglomerates.
   • Apply two‑step sintering: low‑temperature decomposition stage, then high‑temperature lithiation. Reduces inter‑sphere fusion.
3. Control material bed thickness in sintering furnace / roller hearth kiln. Too thick bed leads to uneven temperature distribution, local over‑sintering and large sinter cake clumps.
4. Gentle pre‑crushing of sintered cake: use low‑energy coarse crusher to break big sinter blocks before feeding to jet‑milling line. Prevent huge bulk chunks entering fine grinding loop. Do not use high‑impact crushing which generates excessive fines.

3. Post‑processing: de‑agglomeration + closed‑loop air classification (block oversized particles from entering final product)

Even with good upstream control, minor sinter‑bonded agglomerates still occur. This step stops oversized particles from going into finished powder.

1. Jet mill operation principle: de‑agglomerate, do not over‑grind
   • Use fluidized‑bed opposed‑jet mill under N₂ inert atmosphere for high‑nickel NCM.
   • Set nozzle pressure and feed rate to just break sinter necks; avoid insufficient milling (unbroken agglomerates remain oversized).
   • Jet mill internal single‑stage classifier is not enough for strict EV‑grade Dmax requirements.
2. Independent multi‑stage turbo air classifier closed‑loop system (key barrier against oversized particles)
   • First‑stage classification sets coarse cut‑point; all oversized particles are separated and circulated back to jet mill for re‑de‑agglomeration, not sent to finished product.
   • Second‑stage classification further polishes PSD and tightens D99 / Dmax specification.
   • Adjust classifier wheel speed, system air flow and feed concentration to stabilize cut‑point.
3. Final safety screening: ultrasonic vibratory sieve as last‑pass guard. It catches rare residual oversized agglomerates. Note limited throughput and mesh blinding risk; cannot be used as main‑process solution.

Important: Do not try to solve oversized particles only by raising jet‑mill energy. Over‑milling shatters intact secondary spheres, produces massive ultra‑fines, raises residual lithium and degrades high‑nickel material performance.

4. Process monitoring & troubleshooting

1. Monitor full PSD, focus on D99 and Dmax, not only D50. Deploy on‑line laser particle size detectors at classifier outlet for real‑time feedback.
2. If oversized particles keep appearing:
   • Check sinter cake morphology first: if many hard fused agglomerates exist, root cause is sintering, not grinding.
   • Check jet‑mill feed rate: over‑feeding causes incomplete de‑agglomeration.
   • Check classifier loop: confirm coarse fraction is fully recirculated, not bypassed to finished tank.
3. Stabilize batch‑to‑batch consistency: control precursor quality, sintering thermal profile, and post‑processing parameter lock‑in.

5. Summary workflow for preventing oversized NCM particles

Stable narrow‑PSD precursor → homogeneous lithium mixing → optimized two‑step sintering to minimize inter‑particle sinter bonding → gentle pre‑crush sinter cake → mild jet‑mill de‑agglomeration → multi‑stage closed‑loop turbo air classification to reject oversized fractions → optional final ultrasonic sieve safety check.

Core rule: Prevent oversized particles from forming in precursor and sintering is far more effective than removing them in post‑processing. Post‑processing is a correction barrier, not a fix for poor upstream quality.