Air classifier achieves particle classification based on the counterbalance between centrifugal force from rotating rotor and aerodynamic drag force from circulating process air, the core physical principle for splitting fine/coarse rCB in closed-circuit grinding lines. Separation performance is heavily governed by classifier rotor speed and system airflow, two core parameters already linked to rCB fineness control in prior topics.
1. Core Force Balance Principle (Fundamental Separation Logic)
Two opposing forces dominate particle movement inside classification chamber, deciding whether particles exit as finished fine product or return for regrinding as coarse rejects:
- Centrifugal Force (Fc, outward pushing force, generated by spinning classifier rotor)
Proportional to particle mass and square of rotor speed. Coarse rCB grains, ash-rich hard agglomerates and oversized grit own larger mass → strong centrifugal force overcomes air pull, thrown away from rotor toward outer casing. - Air Drag Force (Fd, inward pulling force, from circulating induced draft air)
Proportional to particle surface area and system air volume. Tiny fine rCB has large surface-to-mass ratio → drag force exceeds centrifugal resistance, pulled through blade gaps into central fine air channel.Cut-point size: The critical D50 particle diameter where centrifugal force equals drag force; particles smaller than cut point turn into finished fines, larger ones become coarse return feed for regrind.
2. Key Structural Parts & Their Separation Functions (Typical vertical air classifier for rCB)
| Component | Core Role in Classification |
|---|---|
| Fixed inlet guide vanes | Straighten chaotic incoming airflow into stable spiral flow, eliminate turbulent short-circuit that causes coarse leakage into fine product |
| High-speed bladed classifier rotor | Core separating component; variable RPM via VFD adjusts centrifugal magnitude to shift cut fineness |
| Outer annular casing + coarse return chute | Rejected coarse particles drop along casing inner wall by gravity, flow back into grinding mill for secondary crushing |
| Central fine air outlet duct | Collect air-carried fine rCB, deliver powder-air mixture to pulse bag filter for finished rCB collection |
3. Step-by-Step Separation Flow in rCB Closed Grinding Circuit
- Mixed ground powder (blended fine rCB, coarse agglomerates, high-ash grit) is lifted by hot circulating air from grinding cavity into the annular classification zone between guide vanes and rotating rotor.
- Suspended particles rotate with spiral airflow:
- Coarse fraction: Powerful centrifugal drive flings particles onto casing wall, airflow cushion disappears, coarse material slides down return pipeline back to grinder feed inlet.
- Fine fraction: Air drag dominates, tiny particles penetrate gaps between classifier rotor blades and enter central air duct.
- Fine-powder laden air flows out of classifier into dust collector; qualified fine rCB is trapped as final product, purified exhaust air loops back into grinding chamber to form fully closed airflow circulation.
4. Main Tunable Parameters Regulating Separation Effect (Critical for rCB PSD control)
① Classifier rotor speed (primary fineness adjuster)
- Raise rotor RPM: Higher centrifugal force tightens cut point → finer finished rCB, more medium-sized grains rejected back as coarse;
- Lower rotor RPM: Weaker centrifugal barrier allows larger particles to pass → coarser final product.
② Circulating system airflow volume (secondary matching parameter)
- Increase air flow: Stronger drag force pulls bigger particles inward → product becomes coarser;
- Reduce air flow: Weakened air pull requires higher rotor speed to maintain same fine cut, commonly applied for high-grade ultra-fine rCB production.
③ Inlet feed rate
Excessive feeding creates dense particle suspension inside classification zone; mutual particle encapsulation traps coarse grit inside fine clusters, leading to coarse leakage and wide particle distribution. Optimized stable feed keeps dilute particle concentration for high separation efficiency.
④ rCB feedstock property (unique for recovered carbon black)
High moisture (>3%) causes fine rCB to agglomerate with coarse ash grit; bonded clumps are wrongly rejected as oversized coarse and reduce fine yield. Pre-drying feed below 3% moisture removes agglomeration to stabilize classification, consistent with maintenance optimization measures mentioned previously.
5. Classification Efficiency Index for rCB Production
Standard qualified air classifier achieves ≥85% fine recovery efficiency (target-sized fine rCB successfully collected in finished goods); excess coarse leakage (<80% efficiency) indicates mismatched rotor-airflow parameters or abnormal feed agglomeration needing on-site adjustment.