D90 <10 μm means 90% of the powder by volume has agglomerates smaller than 10 μm. This target is widely used for standard rubber and masterbatch-grade rCB. It can be achieved by proper pre-treatment, matched mill selection, closed-circuit turbo air classification, and stable process control.
Important note: This is the size of rCB agglomerates, not the nanoscale primary carbon particles. The grinding process only breaks porous pyrolysis char clusters, not the original nano carbon spheres.
Step 1: Rigorous pre-treatment (foundation)
- Pre-crush & screen raw char:
If incoming feed is 6 mm lumps, crush and screen to max 2–5 mm for ACM mill or ≤1–2 mm for fluidized bed jet mill. Oversize lumps cannot fully de-agglomerate in one pass and leave large clusters that push D90 higher. - High-intensity magnetic separation:
Remove embedded steel fragments after crushing. Steel grit damages hammers, liners and turbo blades, reduces grinding efficiency and creates coarse grit. - Drying:
Dry coarse char before fine grinding, control moisture ≤1.0% (preferably 0.3–0.8%). High moisture causes sticky agglomerates that resist de-agglomeration and stick to classifier blades.
Step 2: Mill selection
Option A: ACM Air Classifier Mill (mainstream, cost-effective)
Best for mass production to hit D90 <10 μm.
- High-speed hammers apply impact and shear to break porous char agglomerates.
- Built-in turbo classifier wheel controls the cut size and recycles coarse clusters back for re-grinding.
Option B: Fluidized Bed Opposed Jet Mill (premium)
Uses particle‑on‑particle collision. Delivers cleaner powder with tighter PSD and lower metal contamination. Preferred when you also need low D97 for high-performance rubber. Higher energy cost and lower throughput than ACM.
Step 3: Tune turbo classifier and process parameters
This is the core adjustment to hit D90 <10 μm:
- Increase turbo classifier wheel speed: Higher rotational speed creates stronger centrifugal force, rejects more coarse agglomerates, lowers D90. If speed is too low, borderline large agglomerates leak through and D90 rises.
- Optimize airflow: Match airflow to classifier speed. Too low airflow cannot carry fine particles; excessive airflow pulls oversized agglomerates through the classifier, failing the D90 target.
- Reduce feed rate: Lower feed loading increases particle residence time inside the grinding zone, improving de‑agglomeration. Trade-off: lower throughput.
- Closed-circuit recirculation: All coarse agglomerates rejected by the turbo classifier fall back continuously for repeated de‑agglomeration, preventing coarse material from escaping to the final product.
Step 4: Monitor quality and maintain equipment
- Use laser particle size analysis for routine PSD testing (D50, D90, D97). Use online PSD feedback for automatic process adjustment if available.
- Inspect wear parts periodically: worn hammers, liners or turbo blades degrade grinding/classification performance and cause D90 to drift upward.
- Keep the classifier blades clean; sticky rCB buildup distorts the centrifugal field and leads to coarse bypass.
Common pitfalls that prevent reaching D90 <10 μm
- Feeding oversized lumps into the main mill
- High moisture causing re-agglomeration
- Overloading the mill with high feed rate
- Worn turbo blades or hammer liners
- Unstable airflow or fluctuating classifier speed
Typical operating target example
For ACM mill:
- Pre-treated feed ≤2–5 mm
- Turbo classifier speed increased
- Optimized airflow and reduced feed rate
- Result: D90 <10 μm, D97 usually around 12–18 μm (depending on tuning)
To achieve D90 <10 μm for rCB: prepare clean, dry, size-limited feedstock, select ACM or fluidized bed jet mill, and increase turbo classifier speed while balancing airflow and feed rate. Coarse agglomerates are recycled in closed circuit for repeated de‑agglomeration. Continuous PSD monitoring and regular wear-part inspection maintain stable D90 performance.
