How to Prevent Agglomeration During NCM Grinding

Agglomeration frequently occurs in NCM jet‑mill dry grinding process, divided into sinter‑hard agglomerates formed in calcination, process‑induced soft agglomerates generated during grinding, and moisture‑triggered secondary agglomeration. Unremoved agglomerates lead to abnormal Dmax, wide PSD span, electrode coating defects and battery safety hazards. Hard agglomerates originate upstream, while soft agglomerations are caused by particle static electricity, moisture, local over‑heating and particle collision adhesion. Preventing agglomeration requires joint control of precursor, calcination, grinding‑classification parameters, electrostatic elimination, low‑dew‑point environment and equipment flow‑field optimization.

1. Classification and root causes of agglomeration in NCM grinding

1. Hard sintered agglomerates (from calcination)
   Formed by over‑sintering, uneven lithium mixing or excessive saggar loading. Sintering necks tightly bond multiple secondary particles. Jet‑mill collision can hardly break them without damaging intact spherical particles. This type belongs to upstream origin.
2. Process soft agglomerates (generated inside grinding loop)
   Freshly fractured NCM particles carry high static charge after high‑speed collision. Electrostatic attraction makes fine particles adhere onto coarse particle surfaces; fine fractions also stick together to form loose agglomerates. These soft agglomerates can be dispersed, yet may pass through classifier and enter finished product under poor flow‑field condition.
3. Moisture‑induced secondary agglomeration
   High‑nickel NCM contains residual lithium. When contacting humid air, lithium compounds absorb water, become sticky, and particles bond together. This agglomeration may happen in feeding zone, pipeline or sample testing stage.

Important note: Jet‑milling cannot eliminate hard sintered agglomerates completely. Prevention should start from precursor and calcination, not rely entirely on grinding process.

2. Eliminate hard sintered agglomerates from upstream

2.1 Precursor optimization

• Stabilize co‑precipitation conditions (pH, ammonia, temperature, stirring rate) to reduce precursor hard agglomerates.
• Optimize aging, filtration and drying process, avoid irreversible hard agglomeration of hydroxide precursor.
• Perform PSD inspection for each precursor batch; reject heavily agglomerated precursor before calcination.

2.2 Calcination process control

• Strictly control heating rate, peak temperature and holding time. Avoid local over‑sintering which generates fused hard lumps. High‑nickel NCM has an extremely narrow sintering window.
• Ensure uniform mixing of NCM precursor and lithium source; local lithium enrichment causes partial over‑sintering and hard agglomerates.
• Standardize saggar loading thickness and weight, guarantee homogeneous heating inside saggar.
• Check calcined clinker before grinding. If large hard agglomerate ratio rises, adjust calcination parameters instead of raising grinding pressure blindly.

3. Suppress soft electrostatic agglomeration in jet‑mill grinding

3.1 Optimize grinding parameters to reduce fine‑particle generation

Massive ultrafine fines are the main material basis for electrostatic agglomeration.

• Use minimum feasible grinding pressure to achieve de‑agglomeration. Excess pressure produces large amount of fine fragments which are easy to aggregate by static charge. Especially critical for high‑nickel NCM.
• Avoid excessively low feed rate. Too low particle concentration leads to violent particle collision, generates more fines and increases static accumulation. Maintain reasonable gas‑solid ratio.
• Control closed‑loop circulation ratio. Excessive circulation causes repeated wear of particles, produces more fine powder and aggravates electrostatic adhesion.

3.2 Electrostatic elimination design for equipment

• Install static‑elimination grounding for jet‑mill chamber, classifier, cyclone separator and all powder‑contact pipelines. Ensure reliable grounding to release static charge accumulated by high‑speed particle friction.
• For high‑nickel material, conductive ceramic or anti‑static lining can be adopted to reduce static accumulation on inner wall.
• Avoid dry cold nitrogen with ultra‑low humidity that amplifies static electricity; balance dew‑point requirement and static risk.

3.3 Optimize air classifier flow‑field

• Optimize secondary‑air volume and guide‑vane angle. Secondary air produces strong shear force, disperse soft electrostatic agglomerates before classification, prevent loose agglomerates passing the turbine wheel into finished product.
• Avoid particle over‑concentration caused by over‑feeding. Dense particle cloud increases probability of collision‑adhesion and soft‑agglomerate formation.
• Regularly inspect and replace worn turbine blades. Distorted flow‑field reduces shear dispersion capacity for soft agglomerates.

4. Prevent moisture‑caused agglomeration

Moisture‑induced agglomeration is prominent for high‑nickel NCM. Residual lithium reacts with water vapor to form sticky lithium hydroxide / lithium carbonate, bonding particles together.

• Whole grinding system (feeding hopper, jet mill, classifier, cyclone, finished‑product bin) runs under nitrogen protection. Strictly control system dew‑point, avoid humid air ingress.
• Minimize exposure time of calcined clinker before feeding into mill. Do not store sintered clinker under ambient atmosphere for long time.
• During PSD laboratory testing, shorten sample air‑exposure time. Moisture‑agglomerated samples will show falsely high D97 and Dmax, mislead process judgment.

5. Equipment operation and maintenance measures

• Regularly clean grinding chamber, classifier volute, return pipeline and cyclone. Remove wall‑adhered powder layer. Accumulated powder may peel off and enter finished product in form of large agglomerates.
• When switching different material grades, perform full equipment cleaning to prevent cross‑contaminated agglomerates mixing into new batches.
• Prevent local over‑heating inside grinding loop. High temperature accelerates residual‑lithium surface reaction and makes particles sticky, promoting agglomeration. Control system exhaust temperature.

6. Distinguish real agglomerates from test‑caused false agglomeration

• Laser dry‑dispersion PSD test: set proper dispersion air pressure. Too low dispersion pressure cannot break soft agglomerates, test data shows false high Dmax.
• When abnormal Dmax appears, use SEM to verify: distinguish real hard sintered agglomerates, soft electrostatic agglomerates or moisture‑induced false agglomerates. Avoid blind adjustment of grinding parameters due to measurement error.

7. Troubleshooting table for NCM grinding agglomeration

Agglomeration during NCM grinding includes hard sintered agglomerates from calcination, electrostatic soft agglomerates generated in grinding loop and moisture‑induced agglomeration. Hard agglomerates must be controlled at precursor and calcination stage. Soft agglomerates are reduced by suppressing ultrafine‑fines generation, reliable static elimination and classifier secondary‑air shear dispersion. Strict low‑dew‑point nitrogen protection prevents moisture‑triggered particle adhesion. Meanwhile, regular equipment cleaning and correct PSD test method avoid misjudgment. Comprehensive measures can effectively reduce agglomerate content in finished NCM powder, improve particle‑size stability and guarantee downstream battery performance.