An ACM mill separates coarse and fine particles by dynamic aerodynamic classification inside the mill housing, relying on the force balance between centrifugal force from the rotating classifier wheel and drag force from system airflow. No metal screens are used. Qualified fine particles pass through the classifier wheel for collection, while coarse oversize particles are rejected and fall back to the grinding chamber for re‑grinding, forming a continuous closed‑loop separation workflow.
Full separation process step‑by‑step
- Particle suspension delivery to classification zone
After being crushed in the lower grinding chamber, mixed‑size particles (blending fine powder and coarse fragments) are lifted upward by negative‑pressure airflow. The airflow carries the particle cloud into the upper classification zone surrounding the high‑speed classifier wheel. All particles are airborne before separation takes place. - Two opposing forces act on every particle
As particles approach the gaps between classifier wheel blades, two competing forces act simultaneously:
- Air drag force: Generated by system fan airflow, pulls particles radially inward toward the wheel center and product outlet. Drag force is more dominant for light, small particles.
- Centrifugal force: Produced by the fast‑spinning classifier wheel, throws heavier, larger particles radially outward. Centrifugal force acts more strongly on coarse particles with higher mass.
- Two separation outcomes
- Fine particles (qualified product): Air drag ＞ centrifugal force. Fine particles overcome centrifugal effect, pass through gaps of classifier wheel blades. They are transported by airflow out of the ACM mill to cyclone separators and pulse dust collectors for final collection.
- Coarse particles (oversize rejects): Centrifugal force ＞ air drag. Coarse particles cannot penetrate the wheel. They are thrown outward, collide with the stationary baffle ring around the classifier assembly.
- Coarse particle recirculation back to grinding zone
Rejected coarse particles lose velocity, drop by gravity straight down into the lower grinding chamber without leaving the mill body. They mix with fresh incoming feed and undergo another round of impact grinding. After re‑comminution, airflow lifts them again to the classifier for re‑separation. This cycle repeats until particles meet the cut‑size requirement.
Key adjustable parameters that govern coarse‑fine separation
1. Classifier wheel rotational speed (primary parameter)
- Increase wheel speed → higher centrifugal force. Only very fine particles can pass through. More material is rejected as coarse recirculation; finished powder becomes finer.
- Decrease wheel speed → weaker centrifugal force. Larger particles can pass through the wheel; coarser final product.
Adjustment can be done online during mill operation, no shutdown is needed.
2. System air volume (secondary parameter)
- Higher air volume → stronger drag force. More particles are pulled inward through the classifier. Capacity rises, but risk of coarse particle leakage increases.
- Lower air volume → weaker drag. Only finer particles can be carried through; product gets finer while throughput drops.
Classifier speed and airflow must be matched to avoid coarse leakage or excessive internal circulating load.
3. Grinding rotor performance
The grinding rotor does not perform separation, but determines the size distribution of particles entering the classifier. If grinding intensity is insufficient, excessive coarse material floods the classification zone and impairs separation efficiency.
4. Feed concentration
Over‑feeding creates dense particle clouds inside the classifier zone. Frequent particle‑to‑particle collisions interfere with aerodynamic force balance. Some coarse particles may be dragged through together with fines, broadening particle‑size distribution. Stable feeding maintains separation efficiency.
Screen‑free separation advantage
Unlike traditional impact mills that separate particles via physical screen apertures:
- ACM aerodynamic separation achieves stepless cut‑point adjustment.
- Eliminates screen clogging, screen breakage and metal fragment contamination, which is critical for battery‑grade high‑purity powders.
- Full‑ceramic classifier wheel option keeps metal‑free condition while maintaining coarse‑fine separation performance.
Common misunderstanding
The classifier wheel does not sieve particles by physical aperture size. Separation depends on particle mass‑to‑drag ratio, not purely geometric particle dimension. Particle density will also influence separation results: high‑density particles behave like “coarse particles” under the same aerodynamic condition.
ACM separates coarse and fine particles by aerodynamic force balance inside the integrated classifier wheel. Air drag pulls fine particles through for collection; centrifugal force rejects coarse particles for internal recirculation and re‑grinding. Classifier wheel speed and system airflow jointly set the cut‑point. This screen‑free closed‑loop separation delivers narrow particle‑size distribution for mineral fillers, battery materials and other industrial powders.
