How To Control The Key Particle Size Indicators Of Graphite Anode During Classification

During air classification of lithium-ion battery graphite anode, the core particle size indicators include D10, D50, D97, Dmax, span and residual oversize. Precise control of these parameters directly impacts tap density, specific surface area, electrode processing performance and battery electrochemical performance. This article explains the control logic, operational adjustment strategies, online monitoring and typical troubleshooting for each key particle size index, applicable to both spheroidized natural graphite and artificial graphite anode powder.

1. Definition of Core Particle Size Indicators for Graphite Anode

• D50 (median particle size): The most critical index, representing the median diameter of powder particles. It determines electrode coating thickness and compaction density.
• D97: 97% of particles are smaller than this value; the main index to eliminate harmful coarse tails.
• D10: Reflects the fine particle fraction; excess fine particles raise BET and reduce initial coulombic efficiency.
• Dmax: Maximum particle size. Oversized particles may pierce the separator and trigger internal short circuit risk.
• Span: Span = (D90-D10)/D50, used to evaluate width of particle size distribution. Lower span means narrower and more uniform PSD.

2. Control Mechanism via Air Classifier Main Parameters

The particle size indicators are mainly regulated by four core variables: classification wheel speed, total air volume, secondary rinse air and feed rate.

2.1 Control D50

D50 is primarily tuned by matching classification wheel speed and total air flow.

• Increase classifying wheel speed: stronger centrifugal force → finer cut point → D50 decreases.
• Reduce classifying wheel speed: weaker centrifugal force → coarser cut point → D50 increases.
• Increase total air volume: higher drag force, more medium particles enter fine product → D50 drops slightly.
• Operation tip: Adjust wheel speed first for coarse D50 correction, fine-tune air volume for minor offset. Keep feeding stable; fluctuating feed rate will cause D50 drifting.

2.2 Control D97 & Dmax (Eliminate coarse tail)

D97 and Dmax are the highest priority safety indicators for battery anode.

• High residual coarse particles (D97 / Dmax out of spec):
  1. Raise classification wheel speed to enhance centrifugal interception of large particles.
  2. Increase secondary rinse air to break soft agglomerates; avoid agglomerates being mistaken as fine particles.
  3. Check for rotor blade fouling: powder buildup on the wheel will form uneven airflow, letting oversized particles escape.
• If D97 is too small: Excessive interception reduces yield and increases fine powder loss. Reduce rotor speed moderately.
• Critical rule: Dmax is usually verified by offline sieve test, which complements laser PSD results.

2.3 Control D10 & ultrafine fraction

D10 reflects submicron ultrafine powder content. Too low D10 means excessive ultrafines.

• Excess ultrafine particles (D10 too small, high BET): Reduce total air volume or lower rotor speed to cut submicron fraction entering fine product.
• Too few fine particles (D10 too large): Tap density and slurry fluidity get worse. Appropriately increase airflow or rotor speed to introduce a small amount of fine powder.
• Note: Ultrafine fraction strongly impacts first-cycle efficiency; high-end graphite anode requires strict cap on submicron content.

2.4 Control Span (narrow PSD distribution)

Span characterizes classification sharpness. Low span indicates uniform particles, favorable for high tap density.

• High span (wide distribution): Root causes are agglomerated feed powder, insufficient secondary air, unstable feeding, moisture-induced particle adhesion.
• Optimization: Increase secondary rinse air to disperse agglomerates; stabilize loss-in-weight screw feeder; ensure feed moisture ≤0.2%; clean classification wheel regularly.

3. Pre-classification Raw Material Control

The classification unit can only optimize PSD, and cannot completely fix poorly distributed incoming material.

1. Fully deagglomerate spheroidized graphite before feeding to break soft agglomerates.
2. Remove metal impurities via magnetic separator, prevent rotor damage and local airflow disturbance.
3. Strictly control moisture below 0.2% to avoid particle sticking and false coarse/ultrafine signals.
4. Limit the PSD range of incoming material; too broad raw distribution increases classification difficulty and worsens span.

4. Closed-loop Online Monitoring Strategy

1. Install online laser particle size analyzer at fine product outlet, continuously collect D10, D50, D97 data.
2. Link PSD signals to PLC control system. When indicators deviate from set window, the system automatically adjusts classification wheel frequency and total air volume.
3. Periodically perform offline verification: laser particle test + wet sieve for Dmax inspection, to eliminate measurement deviation from online equipment.
4. Set batch homogenization after classification to reduce PSD difference between front and rear of one production batch.

5. Common Off-spec Conditions & Solutions

6. Key Operation Principles

1. One-factor adjustment principle: Change only one parameter each time, wait for system stabilization before reading PSD data, avoid overlapping interference.
2. Priority sequence: Dmax / D97 > D50 > D10 > Span. Coarse tail is the highest priority risk indicator.
3. Parameter matching: Rotor speed and air volume must be matched. Single extreme adjustment leads to low classification sharpness and yield loss.
4. Post-check: After classification, use magnetic separation and sieve inspection as final quality gate before carbon coating.

Keep the whole classification system under nitrogen closed loop. Control oxygen concentration to prevent graphite dust explosion. Static grounding and regular rotor dynamic balance inspection are required for continuous stable production.