Classifier mill (also known as ACM air classifier mill) is the most widely used dry grinding equipment for large-scale recovered carbon black (rCB) production. It integrates mechanical impact grinding and dynamic air classification inside one machine, forming a closed-circuit milling system. It de‑agglomerates pyrolysis char from waste tire pyrolysis and controls particle size distribution to produce rCB powder for rubber, plastic and masterbatch applications.
1. Pre‑Treated Feedstock Feeding
Before entering the classifier mill, crude rCB char undergoes essential pre‑treatment: high‑intensity magnetic separation removes steel wire and iron fines, crushing reduces large lumps, and drying lowers moisture to prevent caking.
The clean, dry rCB is continuously metered into the grinding chamber by a quantitative screw feeder. Stable feeding rate ensures consistent particle size and avoids overloading the rotor.
2. Mechanical Impact & Shear Grinding
Inside the grinding chamber, a high-speed rotor fitted with hammers or pins spins rapidly. The incoming rCB agglomerates are struck violently by rotating hammer tips. Particle‑to‑particle collision and wall shear break the porous pyrolysis char clusters.
The objective is de‑agglomeration, not smashing primary carbon black spheres. It breaks weak bonds between aggregated carbon particles while preserving the original carbon microstructure and reinforcement performance of rCB.
3. Air Transport & Dynamic Classification
A stream of process air flows through the grinding zone, carrying ground rCB particles upward toward the built-in high-speed classifier wheel.
The rotating classifier wheel creates a strong centrifugal force field. Two particle separation outcomes happen:
- Fine particles meeting the target PSD: Gas drag force overcomes centrifugal force. These fine rCB particles pass through the gaps of the classifier wheel and flow out with the air stream to the collection unit.
- Coarse, incompletely de‑agglomerated agglomerates: Higher mass gives stronger centrifugal force. They are rejected by the classifier, drop back to the grinding zone, and are re-hammered repeatedly.
Operators adjust the classifier wheel speed and air volume to tune the cut point. Increasing classifier speed produces finer rCB; reducing speed allows coarser powder to pass through.
4. Powder Collection & Gas Circulation
The air carrying qualified fine rCB enters the baghouse filter. The filter bags separate solid rCB powder from process air. Finished powder falls into the product hopper, while purified air is recycled or discharged.
Online quality checks can be added after collection to monitor residual steel content and particle size.
Key Advantages for rCB
- Compact integrated design: Grinding and classification in one unit, saving plant footprint and investment cost compared with separate mill + external classifier systems.
- High continuous throughput: Suitable for mass commercial rCB production with stable output.
- Flexible size adjustment: Quickly switch between different rCB grades by changing rotor speed, classifier speed and feed rate.
- Closed negative pressure operation: Minimizes powder leakage and dust emission. Nitrogen inert gas can be used for explosion protection for high-volatile rCB feedstock.
Limitations
High-speed hammer/rotor contact creates metal wear. Even with upstream magnetic separation, long-run operation may introduce trace iron contamination. For ultra-fine and ultra-low metal specification rCB, fluidized bed jet mills are often selected instead.
A classifier mill processes recovered carbon black by combining high-speed mechanical impact de‑agglomeration and built-in dynamic air classification. Coarse agglomerates are continuously recirculated for regrinding, while qualified fine powder is separated and collected. It is the workhorse for mainstream rCB plants, balancing throughput, energy consumption and controllable particle size for standard rubber and polymer grades.
