D50 reflects the median particle size of LFP powder, while D90 represents the particle size below which 90 % of the powder volume lies. Stable D50 and D90 are fundamental for tap density, electrode coating consistency, impedance and cycling performance of LFP‑based lithium‑ion batteries. Controlling D50 and D90 in dry processing is a joint outcome of grinding intensity, turbo classifier tuning, feeding stability, closed‑loop circulation and powder dispersion, rather than adjusting one single parameter.
Definition of D50 and D90 for sintered LFP
For post‑sinter LFP, D50 and D90 characterise de‑agglomerated powder, not primary crystal grains. Soft agglomerates formed during sintering will raise apparent D50 and D90. The processing goal is to dissociate agglomerates without crushing original primary particles, so as to hit target D50 and D90 while preserving tap density.
Key parameters to regulate D50 and D90
1. Grinding / deagglomeration intensity
Grinding energy sets the base particle‑size level for incoming powder.
- Jet mill: Adjust grinding nozzle pressure. Higher jet pressure delivers stronger particle‑to‑particle collision; both D50 and D90 decrease. Excess pressure fractures primary particles, produces large volume of ultrafines and lowers tap density. Lower pressure leaves residual agglomerates, driving D50 and D90 higher.
- Full‑ceramic air classifier mill (ACM): Adjust rotor speed. Higher rotor speed brings stronger mechanical shear, reduces D50 and D90. Operate at moderate speed for LFP; high‑speed violent impact breaks primary grains and degrades electrochemical performance.
2. Turbo classifier wheel speed (dominant for D90)
Classifier wheel speed is the most critical parameter for D90 control.
- Increase wheel speed: higher centrifugal force rejects more coarse particles. D90 drops, finished powder becomes finer.
- Decrease wheel speed: larger particles pass classification zone, D90 rises.
D90 is highly sensitive to classifier speed drift. Closed‑loop frequency control is required to stabilise rotational speed, avoiding D90 fluctuation across batches. While classifier speed mainly governs D90, it also indirectly influences D50.
3. Secondary classification airflow
Secondary air disperses soft agglomerates before classification and shapes separation sharpness.
Insufficient secondary air causes incomplete de‑agglomeration inside classifier; agglomerates behave like large particles and push D90 upward. Excessive secondary airflow may entrain partial coarse material into finished powder. Tuning secondary air optimises PSD span and helps stabilise both D50 and D90.
4. Feed rate control
Surge or unstable feeding is a common source of drifting D50 and D90.
Over‑feeding overloads the grinding chamber. Portions of LFP agglomerates escape sufficient dispersion, resulting in higher D50 and D90. Too‑low feed rate leads to over‑processing, generating excessive ultrafines and reducing D50 drastically. Loss‑in‑weight gravimetric feeder is recommended to keep constant mass throughput.
5. Closed‑loop recirculation of coarse rejects
Oversized fractions rejected by the turbo classifier circulate back to the deagglomeration zone.
Multiple mild circulation passes break soft agglomerates step‑by‑step. This approach stabilises D50 and D90 without applying destructive high‑energy grinding. If coarse material is directly discarded, yield drops sharply; if circulation is disabled, D90 will be hard to constrain.
6. System total airflow
Total circulating airflow determines powder residence time inside grinding‑classification system. Higher airflow carries powder through faster, shorter processing time tends toward higher D50 and D90; lower airflow extends residence time for more sufficient dispersion. Airflow must stay stable together with feeding rate.
Practical process tuning logic
- Set proper grinding intensity according to feedstock sintering condition, establish baseline particle size level.
- Adjust classifier wheel speed first to lock target D90.
- Fine‑tune feed rate and secondary airflow to bring D50 into specification window.
- Keep closed‑loop circulation enabled; avoid raising grinding intensity blindly when D50/D90 are out‑of‑spec.
- Deploy online laser particle‑size analyser for real‑time D50 and D90 monitoring. Feed measurement data back to classifier speed and feeding rate for automatic correction.
Typical mistakes in production
- Only adjust grinding pressure or rotor speed, ignoring classifier setting: D90 cannot be controlled effectively.
- Chase finer D50 by increasing grinding energy: primary particles fracture, tap density decreases, PSD span widens.
- Tolerate feeding fluctuation: D50 and D90 swing batch‑to‑batch even with fixed equipment parameters.
- Insufficient secondary air: agglomerates are not dispersed, measured D50/D90 are falsely high.
Controlling D50 and D90 for LFP powder is a multi‑parameter coordination work. Grinding intensity sets baseline particle size; turbo classifier wheel speed dominates D90; feed rate and secondary airflow fine‑tune D50. Together with closed‑loop circulation and online PSD feedback, manufacturers achieve stable, repeatable D50 and D90 while retaining primary‑particle morphology and tap density for battery‑grade LFP cathode powder.
