How Does an Air Classifier Mill Work for rCB?

Air classifier mill (ACM mill) is an integrated grinding + dynamic classification all-in-one equipment, the core unit for upgrading crude tire pyrolysis char into qualified recovered carbon black (rCB). It combines high-speed impact pulverization and centrifugal airflow separation in a single closed chamber, solving two major rCB processing tasks simultaneously: breaking sintered char agglomerates and removing heavy ash/oversize impurities. Below is its full working principle tailored to rCB production, plus process stages, key parameters and unique advantages for recycled carbon black.

1. Overall Structural Layout for rCB Special ACM

The mill system is matched to pyrolysis char characteristics, including 6 core sections:

1. Feeding unit: sealed quantitative screw feeder (prevents carbon dust leakage)
2. Primary grinding chamber: high-speed rotating pin rotor
3. Dynamic classification wheel (vertical high-speed classifier impeller)
4. Ventilation & air circulation system: induced draft fan, air duct
5. Two discharge outlets: fine finished rCB outlet, coarse ash/oversize return outlet
6. Dust collection: pulse bag filter for finished rCB collection

2. Step-by-Step Working Process for rCB

Step 1: Feeding of pre-treated pyrolysis char

Crude rCB char after magnetic separation (steel wire removed) is continuously fed into the grinding chamber via screw feeding. Raw char contains three components:

• Sintered carbon black agglomerates (target to grind fine)
• Heavy inorganic ash: CaCO₃, silica, ZnO, metal oxide grits
• Oversize unpyrolyzed rubber chunks
  Negative pressure inside the mill pulls materials into the grinding zone steadily without dust escape.

Step 2: High-speed impact grinding & de-agglomeration

Inside the grinding chamber, a multi-row pin rotor spins at high linear velocity (80–120 m/s).

• Hard sintered rCB granules collide violently with rigid pins, liner walls and inter-particle impact.
• Thermal-bonded carbon agglomerates formed during tire pyrolysis are shattered into discrete carbon black aggregates, releasing encapsulated ash particles trapped inside char clumps.
  Critical note: ACM relies on mechanical impact shear to break char blocks, which liberates inorganic impurities that were locked inside agglomerates — a prerequisite for subsequent ash removal by classification.

Step 3: Airflow material lifting and pre-separation

Circulating process air blown by the induced draft fan flows upward through the grinding zone, lifting all ground particles into the classification zone at the top of the chamber.
Two types of particles split under airflow drag force at this stage:

1. Light fine carbon black particles: low density, easily carried upward by air
2. Heavy coarse ash + unbroken large char chunks: high specific gravity, insufficient air buoyancy, fall back to grinding chamber for re-pulverization

Step 4: Dynamic centrifugal classification (core function for rCB quality control)

The high-speed vertical classification wheel rotates opposite to airflow direction, generating strong centrifugal force. Two competing forces act on mixed particles:

1. Centrifugal force from classifier wheel: pushes heavy/large particles outward
2. Air drag force: pulls fine light carbon particles inward through wheel gaps

Classification screening logic for rCB:

• Qualified fine rCB: small particle size, low density. Air drag overcomes centrifugal force; particles pass through gaps of classification wheel and flow to the finished product collection system.
• Oversize carbon fragments + mineral ash grits: large size or high density (silica, calcium carbonate ash). Centrifugal force exceeds air drag, thrown to the outer chamber wall, slide down back into the grinding zone for repeated impact crushing.

By adjusting the rotation speed of the classification wheel, manufacturers freely set the cut-point particle size of finished rCB, commonly stabilized at D90 < 10 μm for commercial grade rCB. Higher classifier speed = finer finished powder; lower speed allows slightly larger particles and higher throughput.

Step 5: Finished rCB collection and closed air circulation

Fine rCB carried by airflow enters pulse dust collector, where carbon powder is captured and discharged as finished product. Clean air is recycled back into the ACM mill to form a closed-loop airflow system, reducing heat loss, preventing carbon black dust emission and lowering explosion risks of ultrafine carbon powder.

3. How ACM Solves Unique Processing Pain Points of rCB

Compared with separate grinding + independent jet classifier lines, the integrated air classifier mill has targeted advantages for tire-derived recovered carbon black:

1. One-step de-agglomeration + ash removal
   Grinding liberates embedded ash, classification separates dense mineral impurities simultaneously, cutting one whole process unit and lowering investment cost.
2. Adjustable fineness to match rCB application grades
   Tune classifier wheel speed to produce different specifications:
   • Low-speed classifier: D90 8–12 μm for general rubber filler
   • High-speed classifier: D90 3–6 μm for premium low-ash rCB for tire sidewall
3. Closed negative pressure system for carbon black safety
   rCB is flammable ultrafine powder; full negative circulation avoids external dust leakage, equipped with inert gas protection options for high-volatile pyrolysis char.
4. Removes high-density ash efficiently
   Virgin carbon black has almost no heavy mineral particles, but rCB carries tire fillers. Centrifugal classification continuously discharges dense ash fractions, effectively reducing total ash content of finished powder.
5. Low footprint for medium-scale rCB production lines
   All grinding and classification functions integrated into single unit, suitable for 0.5–5 t/h small and medium pyrolysis char upgrading plants.

4. Limitations of Air Classifier Mill for Nano-Grade rCB

It is important to distinguish ACM from jet mills when pursuing nano fineness:

1. Mechanical pin impact creates local hot spots; excessive high-speed rotation may slightly damage carbon black aggregate structure, lowering DBP absorption value if over-ground.
2. The smallest stable cut-point of standard ACM is around D90 ≥ 3 μm; it cannot reach sub-micron nano-level de-agglomeration like fluidized bed jet mills.
3. Pin wear will introduce tiny iron impurities over long-term operation, slightly raising ash content — so multi-stage magnetic separation must be matched before feeding.

5. Standard Process Matching for rCB Production

Complete rCB processing flow with air classifier mill as core:
Raw pyrolysis char → magnetic separator (remove steel fines) → screw feeder → ACM air classifier mill → pulse dust collector (finished rCB)
Coarse ash/oversize circulating inside ACM for regrinding; high-ash tailings are periodically discharged from the mill bottom.

An air classifier mill processes rCB via integrated high-speed impact grinding and dynamic centrifugal air classification. It first shatters sintered pyrolysis char to release trapped inorganic ash, then separates lightweight fine carbon black from heavy mineral grits and oversized fragments through competing airflow drag and centrifugal force. Adjustable classifier wheel speed precisely controls the particle size of final rCB. It is the mainstream economical equipment for medium-fineness commercial rCB production, ideal for general semi-reinforcing rubber-grade recovered carbon black, while fluidized bed jet mills remain the better choice if nano submicron ultra-fine low-ash rCB is required.