Fluidized‑bed opposed‑jet mill is the mainstream de‑agglomeration equipment for NCM cathode powder, typically operated under nitrogen inert atmosphere for high‑nickel grades. Jet milling relies on high‑velocity gas‑stream particle‑to‑particle collision, not metal‑blade impact. Its core function is to break sinter‑bonded hard agglomerates and soft agglomerates, while ideally preserving intact original NCM secondary spherical particles. It does not pulverize individual secondary spheres unless over‑milled.
1. Core particle‑size changing mechanisms
- Breakage of inter‑sphere sinter bonds (desirable effect)
Sintered NCM cake contains multiple secondary spheres fused together during calcination. High‑speed collision fractures only the sintered neck between spheres. Large agglomerates split into discrete original secondary particles.
→ D99 / Dmax drops significantly; D50 shifts slightly downwards; coarse tail of PSD is reduced. This is the main target for NCM jet milling. - Secondary sphere fracture (undesirable over‑milling effect)
When jet energy is too high, or feed rate is too low, collision force exceeds the mechanical strength of polycrystalline secondary spheres. Whole secondary spheres crack and shatter into small fragments and abundant ultra‑fine particles.
→ D50 decreases sharply, D10 becomes much smaller, PSD span widens, specific surface area rises rapidly. For NCM811, this brings higher residual lithium, moisture sensitivity and side reactions.
Single‑crystal NCM is more brittle; jet mill parameters require extra careful tuning to avoid excessive fine fragments.
2. Key jet mill parameters and PSD influence
(1) Nozzle grinding pressure
- Increase pressure: higher gas velocity, stronger collision energy. More sinter agglomerates broken; Dmax and D99 reduce. Risk: excessive pressure shatters secondary spheres and generates massive fines.
- Decrease pressure: mild de‑agglomeration. Less new fines generated, but residual sinter‑bonded agglomerates remain → high D99.
(2) Feed rate
- Lower feed rate: fewer particles in grinding chamber, higher single‑particle collision probability. Better removal of coarse agglomerates. Risk of over‑grinding.
- Higher feed rate: particle crowding cushions collision impact. Less secondary‑sphere damage, but incomplete de‑agglomeration, residual coarse tail.
(3) Internal classifier wheel speed (integrated inside jet mill)
Most industrial jet mills embed an internal turbine classifier:
- Higher wheel speed: smaller cut‑point; coarse particles are rejected back into grinding zone for re‑processing; finished powder gets finer D50 and lower Dmax. Too high speed will also return qualified medium‑size particles and over‑process material.
- Lower wheel speed: coarser cut‑point; less re‑circulation, lower grinding intensity.
(4) System gas flow
Higher gas flow increases particle transport speed and throughput; residence time shortens, reducing grinding intensity.
3. Typical PSD trend for NCM during jet milling
- Well‑tuned mild jet milling (ideal operating window)
- D99 / Dmax drops obviously;
- D50 shifts slightly downward (typically 0.5‑1.5 μm shift only);
- D10 increases moderately;
- PSD span gets smaller;
- Secondary‑sphere morphology remains intact.
- Over‑milling condition (too high pressure / too low feed rate)
- D50 drops drastically;
- D10 falls sharply, large amount of sub‑micron fines appear;
- Span becomes larger instead of narrower;
- Tap density decreases, SSA increases.
- Insufficient milling
- D50 barely changes;
- D99 / Dmax remains high because sinter‑bond agglomerates are not fully disassembled.
4. Important limitations of jet milling for NCM
- Jet milling cannot completely rewrite the original precursor PSD. If incoming powder already has very broad particle distribution, jet mill can only trim the coarse tail, cannot turn a wide‑PSD feed into extremely narrow‑PSD product. Multi‑stage external air classification is required additionally.
- Jet mill is for de‑agglomeration, not for arbitrary size reduction. You should not use jet mill to greatly lower D50 of NCM.
- High‑nickel NCM must use inert N₂ atmosphere during jet milling. Air environment creates new surface lithium impurities from freshly fractured particle surfaces.
5. Practical rule‑of‑thumb for NCM jet‑milling operation
Set parameters just enough to break sinter necks between agglomerated secondary spheres; avoid applying extra energy to break the secondary spheres themselves. After jet milling, further external multi‑stage air classification is used to optimize span and remove residual oversized particles.
