How to Control Particle Size of High‑Nickel NCM

High‑nickel NCM (NCM811, NCM9 series) features fragile secondary spherical particles, high residual alkali and strong moisture‑sensitivity. Its particle‑size control is far more challenging than medium‑nickel ternary materials. Excessive grinding energy easily shatters secondary particles and generates abundant ultrafine fines, raising BET, triggering moisture absorption and cell gas generation. Insufficient grinding leaves hard sintered agglomerates, forming oversized killer particles which bring safety risks. Particle‑size control of high‑nickel NCM requires full‑chain management: precursor co‑precipitation, calcination, jet‑mill dry grinding, air‑classifier optimization, closed‑loop circulation and strict environmental protection, to achieve target D50, narrow span, controlled fines and qualified Dmax.

1. Special characteristics of high‑nickel NCM for particle‑size processing

1. Secondary particles have relatively low mechanical strength, easy to crack under high‑energy particle‑to‑particle collision, generating large amounts of sub‑micron fines.
2. Freshly crushed surfaces expose more residual lithium compounds, quick moisture absorption in ambient air, easily form false soft agglomerates and interfere with PSD test accuracy.
3. Hard fused agglomerates formed during calcination cannot be crushed without careful parameter tuning; blindly increasing grinding pressure will break normal spherical particles.
4. Strict requirements: narrow PSD span, low ultrafine fraction, and rigid D99 / Dmax limit to eliminate oversized killer particles.

Core principle for high‑nickel NCM grinding: de‑agglomerate sintered necks, avoid destroying intact secondary spherical particles.

2. Precursor control: source of particle‑size foundation

1. Stabilize co‑precipitation parameters (pH value, ammonia concentration, reaction temperature, stirring speed, feedstock flow). Adopt seed‑crystal process to produce dense, compact spherical precursor, avoid loose and fragile secondary structures. Fragile precursor particles are prone to pulverization in subsequent jet milling.
2. Suppress abnormal secondary nucleation to reduce ultra‑fine precursor fragments, prevent wide original PSD.
3. Standardize aging, filtration and drying to minimize irreversible hard agglomerates of precursor.
4. Carry out full PSD inspection for each batch precursor (D50, D97, span). Do not feed out‑of‑spec precursor into calcination.

3. Calcination optimization: reduce hard agglomerates and improve particle strength

1. Strictly control heating rate, peak sintering temperature and holding time. High‑nickel material has narrow sintering window. Under‑sintering causes weak secondary‑particle strength; over‑sintering generates heavily fused hard lumps which are difficult to break.
2. Guarantee uniform lithium mixing homogeneity. Local lithium enrichment leads to partial over‑sintering and hard agglomerates.
3. Standardize saggar loading thickness and weight to ensure consistent sintering status across the batch.
4. Inspect calcined clinker before grinding. If large hard agglomerate proportion rises, adjust calcination rather than raising jet‑mill grinding pressure.

4. Jet mill parameter tuning for high‑nickel NCM dry grinding

4.1 Grinding pressure

Adopt moderate‑low grinding pressure window, avoid excessive pressure margin.

• Typical operating range for NCM811: 0.5–0.8 MPa.
• Increase pressure only until Dmax meets specification, stop further pressure rise. Higher pressure will crack secondary particles and produce massive fines.
• Perform gradient pressure test for each grade to balance de‑agglomeration and over‑grinding risk.

4.2 Feed rate and gas‑solid ratio

• Use gravimetric screw feeder with anti‑bridging device to maintain stable mass feed rate. Material surging or accidental low feed rate will cause serious over‑grinding.
• Avoid extremely low feed rate: low particle concentration extends residence time, particles receive excessive collision energy.
• Do not pursue maximum throughput blindly under high feed rate. Over‑feeding triggers particle interference effect, causes coarse bypass and Dmax spike. Find balance between capacity and grading precision.

4.3 Nitrogen source stability

Maintain stable nitrogen supply pressure. Pressure fluctuation leads to inconsistent collision energy and batch‑to‑batch PSD drift. Install pressure monitoring and alarm.

5. Air classifier optimization for high‑nickel NCM

5.1 Turbine speed & total airflow matching

Do not adjust turbine speed alone. Synchronously tune wheel speed and total airflow.

• Excessively high classifier speed causes large‑volume medium‑size powder back‑circulation, longer residence time and severe over‑grinding. Use the lowest feasible wheel speed that meets D97 / Dmax specification.
• Properly reduce total airflow to improve classification sharpness; prevent coarse particle bypass caused by excessive drag force.

5.2 Secondary‑air adjustment

Optimize secondary‑air volume and guide‑vane angle to reduce fine powder mixed in returning coarse fraction, improve separation sharpness and lower span value.

5.3 Two‑stage series classification (recommended for high‑nickel grades)

• First‑stage classifier: intercept super‑large killer particles.
• Second‑stage classifier: precisely adjust D50 and remove partial ultrafine fines.
  Two‑stage grading significantly reduces oversized risk and narrows PSD span compared with single‑stage classifier.

5.4 Mechanical requirements

• All contact parts (turbine wheel, volute, guide vane, lining) must adopt full ceramic or non‑metal material to avoid metal contamination.
• Regularly inspect blade abrasion. Worn rotor distorts flow‑field and causes random coarse leakage; replace according to running hours.
• Regularly clean classifier volute and return pipeline to eliminate powder accumulation dead zone.

6. Closed‑loop circulation management

• Control circulation ratio. Excessive circulation leads to repeated impact, gradually wears secondary particles and generates ultrafine fines.
• Periodically sample circulating coarse powder for PSD test. If circulating material contains plenty of fine fragments, it indicates over‑grinding; reduce grinding pressure or classifier speed in time.
• If unbreakable hard sintered lumps continuously exist in circulating stream, the root cause is calcination, not grinding equipment.

7. Environmental control and quality monitoring

1. Moisture‑proof protection: High‑nickel NCM easily absorbs moisture and forms temporary soft agglomerates. Keep grinding, conveying and sampling environment under low dew‑point nitrogen protection. Minimize air exposure time of samples for PSD testing, otherwise D97 and Dmax test results will be falsely high.
2. Key monitoring indicators (do not only focus on D50):
   • D10: reflect ultrafine fine‑particle content
   • D50: median particle size
   • D97, D99, Dmax: control oversized killer particles
   • Span: reflect particle uniformity
   • BET specific surface area: indirect indicator of over‑grinding
3. Deploy inline online PSD monitor at finished‑product outlet, set alarm thresholds for Dmax and span.
4. Use SEM for auxiliary verification: check secondary‑particle integrity, confirm whether agglomerates are real hard lumps or test‑caused soft agglomerates.

8. Typical troubleshooting for high‑nickel NCM particle‑size control

Particle‑size control of high‑nickel NCM is a full‑process system engineering. Dense precursor and reasonable calcination improve secondary‑particle strength and reduce hard agglomerates. Jet mill adopts moderate grinding pressure and stable feed rate to realize de‑agglomeration without crushing spherical particles. Optimized air classifier, preferably two‑stage series grading, strictly intercepts oversized particles and narrows PSD. Control circulation ratio to avoid over‑grinding. Strict low‑dew‑point environment prevents moisture‑induced false agglomeration. Monitor complete PSD indexes together with BET, so as to obtain high‑quality high‑nickel NCM powder with target particle size, low fine‑particle content and good uniformity.