How To Control D10 D50 D90 Of Calcined Coke Graphite Anode During Classification

Calcined coke is the primary raw material for artificial graphite anode. After spheroidization, air classification is applied to tune D10, D50 and D90, narrow particle size distribution, optimize tap density and electrochemical performance. D50 represents median particle size; D90 reflects coarse tail distribution; D10 indicates fine particle fraction. This article explains the control principle, parameter tuning, pre-treatment, closed-loop monitoring and troubleshooting for these three key PSD indicators in air classification of calcined coke powder.

1. Definition & Performance Impact Of D10, D50, D90

• D50 (median particle size): The core setpoint for anode grading. It defines the main particle size, directly affects electrode coating thickness, compaction density and slurry rheology.
• D90: 90% of particles are smaller than this value. It reflects the content of relatively coarse particles in the product. High D90 means more large particles, raising risks of poor coating flatness and separator piercing.
• D10: 10% of particles are smaller than this value. It quantifies the ultrafine fraction. Too low D10 means excessive submicron fines, which increases BET surface area, consumes more electrolyte and reduces initial coulombic efficiency.

The span formula: Span = (D90−D10)/D50. Lower span means narrower, more uniform particle distribution and higher tap density.

2. Pre-classification Raw Material Preparation

Calcined coke has high hardness, low volatile content and high particle brittleness, so it easily generates extra ultrafine fines under strong collision.

1. Deagglomerate spheroidized calcined coke powder by vibration dispersion before feeding; break soft agglomerates to prevent agglomerates being counted as large particles and distorting PSD test results.
2. Control powder moisture ≤0.2%. Moisture causes particle adhesion, rotor fouling and unstable classification cut point.
3. Complete pre-magnetic separation to remove iron wear debris, avoid rotor damage and secondary metal contamination.
4. Stabilize incoming PSD. If raw feed has an excessively broad particle distribution, classification sharpness will decline and D10/D50/D90 become hard to stabilize.

3. Air Classifier Parameter Tuning For D10, D50, D90

The three main adjustable variables: classifying wheel speed, total air volume, secondary rinse air, plus stable feed rate.

3.1 Adjust D50

D50 is mainly controlled by matching classification wheel speed and total airflow.

• Increase classifying wheel speed: higher centrifugal force → finer cut point → D50 decreases.
• Reduce classifying wheel speed: weaker centrifugal force → coarser cut point → D50 increases.
• Increase total air volume: stronger drag force carries more medium-sized particles into fine product → slight drop of D50.

Operation rule: Adjust rotor speed for large D50 deviation; fine-tune air volume for minor offset. Only modify one parameter each time and wait for system stabilization before sampling.

3.2 Adjust D90 (control coarse fraction)

D90 is sensitive to secondary rinse air and rotor fouling.

• D90 higher than target (too many coarse particles):
  1. Raise classifying wheel speed to strengthen interception of medium-coarse particles.
  2. Increase secondary tangential rinse air to disperse agglomerates and avoid agglomerates passing into qualified product.
  3. Inspect classifying wheel surface; coke powder adhesion distorts airflow field and lets coarse particles escape. Clean rotor regularly.
• D90 lower than target: Excessive interception reduces yield. Reduce rotor speed moderately to allow medium particles to enter fine product.

3.3 Adjust D10 (control ultrafine fraction)

D10 is mainly affected by total airflow and rotor speed. Calcined coke is brittle; high-speed collision easily produces new submicron fines.

• D10 lower than target (too many ultrafine particles, high BET):
  Reduce total air volume or lower rotor speed, to cut submicron particles from entering qualified product. Avoid excessive particle collision inside the chamber.
• D10 higher than target (insufficient fine particles):
  Appropriately raise airflow or rotor speed to introduce a small amount of fine powder. A small proportion of fine particles helps fill gaps between large particles and lift tap density.

3.4 Feed rate influence

Unstable feeding causes continuous drifting of D10, D50, D90. Use loss-in-weight screw feeder to maintain constant mass flow.

• Overfeeding: particle crowding, particle collision interference, poor separation sharpness, D90 rises.
• Too low feed rate: longer particle residence time inside chamber, more collision breakage, D10 drops.

4. Recommended Process Sequence

Spheroidization mill → primary magnetic separation → air classification → secondary magnetic separation → secondary drying → homogenization → carbon coating

1. Coarse stream from classifier: return to spheroidization mill for re-shaping.
2. Qualified fine stream: send to subsequent purification and drying.
3. Superfine fraction: separately collected for auxiliary carbon additive.

5. Closed-loop Quality Control System

1. Install online laser particle size analyzer at qualified powder outlet, continuously monitor D10, D50, D90 and span.
2. Link PSD signals to PLC. When indicators deviate from specification window, the system automatically fine-tunes rotor frequency and air volume.
3. Offline periodic verification: laser PSD test + wet sieve inspection. Calibrate online instrument regularly to eliminate measurement error.
4. Batch homogenization after classification, to balance PSD difference between front and rear of each batch.

6. Common Off-spec Conditions & Troubleshooting

Calcined coke powder is combustible. Keep the classification loop under nitrogen inert protection, continuous oxygen monitoring, static grounding and explosion vent panels. Prevent excessive particle collision heat accumulation. Regular rotor dynamic balance inspection is required for continuous stable operation.