The ACM fine impact mill uses an internal aerodynamic recirculation loop to continuously return off-spec oversized particles back to the grinding zone, which is the key mechanism for tight particle size distribution. Recirculation happens entirely inside the mill body without external material conveying.
1. Separation of qualified fines and off-spec particles at classifier wheel
After raw material is impacted and shattered in the grinding chamber, all particles are swept upward toward the high-speed rotating classifier wheel by the system airflow.
Two forces act on each particle at the classifier:
- Centrifugal force from classifier rotation: pushes particles outward. Larger, heavier off-spec particles experience much stronger centrifugal force.
- Air drag force: pulls particles inward through the classifier wheel gaps. Small fine particles are dominated by air drag.
Oversized off-spec particles cannot pass through the classifier wheel. They are thrown radially outward by centrifugal force to the inner wall of the classification housing. Qualified fine particles pass through the wheel and travel with airflow to the pulse dust collector as finished product.
2. Internal drop path for off-spec particles
The mill housing has a designed annular passage around the classifier wheel. Off-spec particles hitting the housing wall lose velocity and fall down along this annular channel by gravity. They drop back into the central grinding rotor area, rejoining new incoming raw material.
This is the core recirculation route: grinding chamber → classifier inlet → classifier separation → off-spec particles fall back → return to grinding zone.
3. Re-grinding and repeated classification cycle
Once returned to the grinding zone, off-spec coarse particles are re-exposed to high-speed impact, shear and particle collision from the grinding rotor. They are comminuted into smaller fragments. The newly generated particle mixture is again carried by airflow up to the classifier wheel for another size screening.
The cycle repeats until particles are reduced small enough to overcome centrifugal force and pass the classifier. Only particles meeting the cut-point exit the grinding-classification loop.
4. Process parameters affecting recirculation efficiency
- Classifier speed: Higher classifier speed increases centrifugal force, more particles are rejected into recirculation, final product becomes finer and recirculation load rises.
- System airflow: Airflow must be balanced. Too high airflow can carry some coarse particles through classifier and contaminate finished product; too low airflow reduces upward transport, weakens recirculation.
- Feed rate: Overfeeding increases recirculation load, raises chamber temperature and reduces grinding efficiency. A stable low feed rate helps maintain consistent recirculation.
5. Benefits of internal off-spec recirculation
- No external return conveyor required; recirculation is built inside the ACM unit, saving footprint.
- Continuous closed-loop processing, one-pass feeding to get qualified powder.
- Eliminates oversized particles escaping into final product, achieving narrow PSD.
- Maximizes material utilization, reduces waste.
ACM recirculates off-spec particles through centrifugal rejection at the dynamic classifier wheel. Oversized particles are thrown to the mill housing wall, drop back into the grinding chamber by gravity for repeated impact grinding, then go back to classification. This internal closed cycle continues until particles reach the target cut size and pass through the classifier to collection. Adjusting classifier speed and airflow controls the recirculation load and final particle size.
