Recovered carbon black (rCB) originates from the pyrolysis of end‑of‑life tires. Crude pyrolysis char contains agglomerated carbon clusters, residual ash, fine steel fragments and volatile matter. The grinding and milling process breaks down large carbon agglomerates, controls particle size distribution, removes residual impurities, and delivers powder with consistent fineness for rubber, plastic and composite applications. The whole workflow is a closed‑loop combination of pre‑treatment, ultra‑fine milling, air classification, dedusting and product collection.
1. Pre‑Treatment Before Milling
Raw pyrolysis char cannot directly enter the mill. Pre‑treatment protects milling equipment and improves milling stability.
- High‑intensity magnetic separation: Ferromagnetic steel wire and iron fragments are extracted. This prevents hard metal particles from wearing grinding rotors, liners and classifier wheels.
- Crushing & sieving: Large char lumps are crushed and screened to remove oversized non‑carbon impurities, ensuring uniform feed size for consistent feeding.
- Drying: Moisture in crude char is reduced. Wet material causes agglomeration, sticking inside the mill and blocking air classification.
2. Feeding & Ultra‑Fine Milling
Homogenized dry rCB feed is metered by a quantitative screw feeder into the grinding chamber. The most commonly used equipment for rCB is the ACM impact mill or air classifier mill, which integrates grinding and primary classification in one unit.
Inside the grinding chamber, high‑speed rotating hammers or pins strike carbon agglomerates. Particle‑to‑particle collision and shear force break the tough pyrolysis char clusters into fine primary carbon particles.
For rCB, the milling parameters are strictly controlled: rotor speed, feed rate and material bed thickness directly determine the final particle size. The process is operated under closed negative pressure to avoid powder leakage and dust emission.
3. Air Classification (Critical Step)
Ground powder flows with the air stream into the integrated classifier. The rotating classification wheel creates a centrifugal force field.
- Fine particles that meet target fineness pass through the classifier wheel and move to the collection system.
- Oversized coarse agglomerates are rejected by the classifier, fall back to the grinding zone and are re‑milled.
This closed‑circuit recycling ensures stable PSD (particle size distribution), such as D90 or D50 targets required by customers. Multi‑stage classification can be applied for ultra‑fine rCB grades.
4. Dust Collection & Product Collection
The fine rCB powder carried by air enters the baghouse filter system. Filter bags separate solid carbon powder from the carrier air. Purified air is discharged after treatment.
The collected fine powder drops into a product hopper. At this stage, optional secondary inspection can be applied: online metal detection to check residual steel content, and sieve screening to eliminate occasional coarse grit.
5. Post‑Processing & Packaging
Depending on application requirements, finished rCB powder may go through surface modification / surface treatment to improve dispersion in rubber or polymers.
After homogenization, the qualified rCB is automatically packed into bulk bags or small sacks for shipment.
Process Control & Quality Monitoring
Key indicators are monitored continuously during milling:
- Particle size distribution (D50, D97)
- Residual steel/iron content
- Moisture, ash content
- Bulk density and dispersibility
If residual steel exceeds limits or particle size drifts out of specification, the system triggers alarms or adjusts classifier speed and feeding rate automatically.
Why This Milling Workflow Is Unique For rCB
Crude pyrolysis char has a porous, highly agglomerated structure. Simple ball milling tends to produce wide particle size distribution and low efficiency. The air classifier mill (ACM) closed‑circuit process is preferred for rCB because it efficiently de‑agglomerates carbon clusters while maintaining energy efficiency, and it fits well with the upstream magnetic separation pre‑treatment.
The grinding and milling process for recovered carbon black starts with pre‑treatment including magnetic metal removal and crushing, followed by closed‑circuit ultra‑fine impact milling and air classification. Oversized particles are recirculated for re‑grinding, fine powder is captured by bag filters, and final quality inspection ensures consistent rCB powder. This milling sequence converts coarse pyrolysis char into high‑purity, size‑controlled recovered carbon black suitable for industrial reinforcement applications.
