D90 <10 μm is a common target specification for medium-fine recovered carbon black (rCB) grades used in rubber compounds and masterbatch. Achieving this PSD requires properly matched pre‑treatment, mill selection, closed‑circuit classification, and continuous process control. The goal is complete de‑agglomeration of porous pyrolysis char clusters, while preventing oversized agglomerates from escaping the grinding loop.
1. Rigorous Pre‑Treatment Before Grinding
Poor pre‑treatment is the most frequent cause of high D90 values.
- Pre‑crushing and screening: Raw pyrolysis char lumps are reduced to ≤2–5 mm for ACM mills, or ≤1–2 mm for fluidized bed jet mills. Large lumps cannot be fully de‑agglomerated in one pass and leave residual coarse clusters.
- High‑intensity magnetic separation: Removes steel wire and iron fragments. Metal contaminants damage rotors, liners or nozzles, reducing grinding efficiency and creating coarse grit.
- Drying: Moisture is controlled below 1–2%. Wet rCB forms sticky agglomerates that resist de‑agglomeration and block classification.
2. Mill Selection & Mechanical De‑agglomeration
Two main mill types can reach D90 <10 μm for rCB:
ACM Air Classifier Mill (Mainstream Option)
The pre‑treated dry rCB enters the high‑speed impact rotor. Hammers apply impact and shear forces to break porous char agglomerates. This separates bonded carbon clusters without destroying the primary carbon black nanostructure.
Fluidized Bed Opposed Jet Mill (Premium Option)
Supersonic opposed gas jets accelerate particles for particle‑on‑particle collision. It delivers finer, cleaner powder with a tighter PSD, and is preferred when residual metal contamination must be minimized.
3. Dynamic Classifier Tuning (The Core Control for D90)
The integrated high‑speed classifier wheel is the primary tool to control the coarse tail and get D90 <10 μm.
- Increase classifier wheel speed: Higher centrifugal force rejects more coarse agglomerates, sending them back to the grinding zone for re‑processing. This lowers the D90 value.
- Adjust air volume: Optimize process air flow to carry fine particles efficiently. Too low air flow traps fine powder; too high air flow allows oversized particles to pass through the classifier.
- Reduce feed rate: Lower feed increases residence time inside the grinding chamber, improving de‑agglomeration and reducing coarse tail. However, this reduces production throughput.
Operators balance classifier speed, air volume and feed rate to hit D90 <10 μm without sacrificing too much output.
4. Closed‑Circuit Recirculation
Coarse agglomerates rejected by the classifier fall back continuously into the grinding zone for repeated de‑agglomeration. Particles cannot exit to the baghouse collector until they meet the cut point. This closed loop eliminates coarse material leakage and keeps D90 below 10 μm.
5. Continuous Quality Monitoring & Feedback
Laser particle size analysis is used for batch or online PSD testing.
- If D90 drifts above 10 μm, operators increase classifier speed or reduce feed rate.
- Routine inspection of hammers, liners or jet nozzles is required. Worn mill components reduce grinding intensity and cause D90 to rise over time.
Key Limitations
Pushing D90 below 10 μm increases energy consumption and reduces throughput. Over‑tuning the classifier may also generate excessive fine sub‑micron particles, changing bulk density and powder flow properties of rCB.
D90 less than 10 μm in rCB grinding is achieved by preparing clean, dry, size‑limited feedstock, using ACM or fluidized bed jet milling for de‑agglomeration, and adjusting the dynamic classifier wheel speed and air flow in a closed‑circuit system. Coarse agglomerates are recirculated for re‑grinding until they meet the particle size requirement, with laser PSD monitoring to maintain stable D90 performance.
