Classification and post-treatment is the downstream finishing section after spheroidization, surface coating and carbonization of graphite anode powder. Its core goals are: trimming particle size distribution, removing ultrafine fines and oversized agglomerates, eliminating ferromagnetic metal impurities, breaking soft agglomerates, stabilizing powder morphology and meeting battery-grade PSD, tap density, impurity and morphology specifications. This article introduces the full industrial sequence, equipment, key control parameters and quality inspection for graphite anode classification and post-treatment.
1. Process Sequence Overview
Standard flow:
Spheroidization / carbonization output → Low-energy de-agglomeration → Multi-stage air classification → Magnetic separation → Vacuum drying → Online sampling inspection → Packaging
Off-spec coarse or fine fractions are recycled back to spheroidizer or coating system for reprocessing.
2. Step-by-Step Process Description
2.1 Low-energy De-agglomeration
After spheroidization or high-temperature carbon coating, graphite particles easily form soft agglomerates. These clusters are not real large particles but multiple primary particles stuck together.
- Equipment: low-speed impact de-agglomerator or jet de-agglomerator. High shear grinding is forbidden, to avoid breaking spherical graphite particles, increasing SSA and damaging the amorphous carbon coating layer.
- Purpose: separate bonded soft agglomerates back into discrete primary spherical particles, without destroying particle sphericity.
- Control target: check SEM and PSD after treatment; no new fine powder generation.
2.2 Multi-stage Air Classification (Core Unit)
Air classifiers cut powder by aerodynamic drag, separating material into coarse fraction, target product fraction and ultrafine fine fraction. For graphite anode, usually 2 or 3 classification stages in series.
- First stage: remove oversized particles / hard agglomerates
Oversized material returns to JCSM spheroidizer or de-agglomerator for rework. - Second stage: strip ultrafine powder (<1~2 μm) Ultra-fine graphite has high specific surface area, consumes electrolyte, reduces ICE and causes gas swelling in cells. This fine stream is collected separately as by-product.
- Middle cut fraction: qualified target graphite anode powder.
Key control parameters:
- Classifier rotor speed: adjust cut point of particle size. Higher rotor speed = finer cut size.
- Carrier gas: dry air, O₂＜100 ppm, dew point controlled. Prevent static discharge and oxidation.
- Feed stability: constant feeding rate to guarantee consistent PSD batch to batch.
- Target PSD: match customer requirement (typical D50:15–22 μm, narrow span).
2.3 Magnetic Separation (Metal Impurity Removal)
Two-pass magnetic separation is adopted for battery-grade graphite.
- Purpose: remove ferromagnetic wear debris (iron, steel) generated from spheroidizer rotor, furnace, conveying pipelines and de-agglomerator. Metal contaminants cause internal short risk and catalyze electrolyte side reactions.
- Equipment: high-intensity permanent magnetic separator / electromagnetic separator.
- Spec requirement for high-end anode: Fe impurity down to ppb level.
2.4 Vacuum Drying
Even after carbonization, graphite powder can absorb trace moisture from air during classification and conveying.
- Condition: 120–160 °C under vacuum.
- Target residual moisture ≤0.15%.
- Function: eliminate adsorbed water; water will hydrolyze LiPF₆ electrolyte and produce HF in lithium-ion cells.
2.5 Online Sampling & Quality Testing
Before packaging, continuous batch sampling for physical and electrochemical indicators:
- Particle size distribution (PSD), D50, D97
- Sphericity, tap density, loose density
- Specific surface area (SSA)
- Moisture, ash, metal impurities
- Partial batches: electrochemical half-cell test (ICE, cycle performance)
Powder failing any index is sent back to rework.
2.6 Powder Packaging
Closed inert-gas packaging. Sealed aluminum foil bags with nitrogen purging to prevent moisture absorption during storage and transportation.
3. Difference Between Natural Graphite And Artificial Graphite Post-Treatment
- Natural spherical graphite: mainly post-spheroidization classification + magnetic separation before coating; after coating, lighter de-agglomeration and fine removal. More focus on trimming broad PSD from flake spheroidization cycles.
- Artificial graphite: after graphitization and carbon coating, agglomeration is more serious. Requires stronger low-energy de-agglomeration, stricter multi-stage classification and double magnetic separation for ultra-low metal impurity.
4. Recycled Material Management
Two recycle streams:
- Oversized particles: return to JCSM spheroidizer for edge polishing.
- Ultrafine fines: collected separately for low-grade anode or other carbon applications, not blended directly into finished product.
5. Common Defects & Troubleshooting
- PSD span too wide, D97 out of spec
Root cause: unstable classifier speed or fluctuating feed rate.
Solution: stabilize feeding and tune classifier rotor speed. - Tap density drops after classification
Root cause: over-de-agglomeration generating new fines.
Solution: reduce de-agglomeration intensity. - High iron impurity
Root cause: single-stage magnetic separation, worn equipment liners.
Solution: upgrade to double magnetic separation, replace ceramic lining for contact parts. - Batch-to-batch sphericity fluctuation
Root cause: unseparated poorly rounded coarse particles mixed in finished powder.
Solution: tighten coarse cut point.
Classification and post-treatment for graphite anode is a precision finishing process consisting of low-energy de-agglomeration, multi-stage air classification, magnetic separation, vacuum drying and quality inspection. It screens out oversized agglomerates and harmful ultrafine fines, removes metal contamination and stabilizes powder physical properties. This step locks final product quality after spheroidization and surface modification and directly impacts anode tap density, SSA, impurity level and battery electrochemical performance.
