Air classification is a key dry separation technology integrated inside ACM classifier mills and fluidized bed jet mills for recovered carbon black (rCB). It separates fine, fully de‑agglomerated rCB powder from coarse incompletely broken agglomerates using airflow and centrifugal force. Coarse clusters are recycled back into the grinding zone for re‑processing, while qualified fine powder moves to collection. It controls the particle size distribution (especially D97, the coarse tail), which defines rCB grade and market value.
Core Working Principle
Air classification relies on the balance of two opposing forces acting on rCB agglomerates:
- Drag force from process air: Pulls particles upward toward the classifier wheel. Smaller, lighter particles are easily carried by air.
- Centrifugal force from rotating classifier wheel: Pushes particles outward. Larger, heavier agglomerates experience stronger centrifugal force.
Particles pass through the classifier wheel only when air drag overcomes centrifugal force. Heavier coarse agglomerates are rejected and fall back for re‑grinding.
Step‑by‑Step Air Classification Process for rCB
- Particle transport by air flow
After impact or jet milling, rCB agglomerates are suspended in the process air stream and carried upward to the high‑speed dynamic classifier wheel. - Size separation at the classifier wheel
The rotating wheel has many radial blades creating a strong centrifugal field.
- Fine agglomerates meeting target PSD: Air drag > centrifugal force → pass through blade gaps and flow to baghouse filters.
- Oversized coarse agglomerates: Centrifugal force > air drag → thrown outward, drop back to the grinding chamber for repeated de‑agglomeration.
- Closed‑circuit recirculation
Rejected coarse material continuously re-enters the grinding zone. This loop ensures particles cannot exit the system until de‑agglomerated to meet the cut point.
Key Adjustable Parameters to Tune rCB Fineness
- Classifier wheel speed: The primary control parameter. Higher speed = stronger centrifugal force, finer cut point, lower D97. Lower speed allows larger particles to pass, increasing throughput but raising D97.
- Air volume / airflow rate: Higher air flow increases drag force, carrying more fine particles. Too much air can pull oversized particles through and degrade D97.
- Feed rate: Higher feed rate increases particle loading, which may cause agglomerate clustering and worsen classification efficiency.
Difference Between Static Air Classifier and Dynamic Classifier
For rCB milling, dynamic wheel air classifiers are used (integrated inside ACM and jet mills). Static air classifiers have no rotating wheel and are less precise, so they are rarely used for high-spec rCB requiring tight D97 control.
Why Air Classification Is Critical for rCB
- It controls the coarse tail (D97), the most important specification for high-value tire-grade rCB.
- It enables continuous closed-circuit grinding, improving de‑agglomeration efficiency and reducing energy waste.
- It avoids over-grinding primary carbon nanostructures, preserving the reinforcing properties of rCB.
Limitations
Air classification performance is sensitive to moisture. High moisture makes rCB sticky, agglomerates clump together and distort particle separation, resulting in higher D97. Hard steel contaminants also damage classifier wheel blades and reduce classification precision.
Air classification for rCB uses a rotating dynamic classifier wheel to balance air drag and centrifugal force. Fine qualified powder passes through to collection, while coarse agglomerates are returned to the grinding zone for re‑de‑agglomeration. Operators adjust classifier speed and airflow to precisely control D97 and D50, producing rCB powder that meets high-value tire and rubber specifications.
