An internal classifier mill (widely known as Air Classifying Mill, ACM) integrates mechanical impact grinding and dynamic internal air classification inside one single machine housing. Instead of sending ground material to an external classifier, the built‑in rotating classifier wheel performs particle‑size separation within the mill body. Oversized grains automatically fall back for re‑grinding, forming a continuous closed‑loop dry‑grinding workflow under negative‑pressure airflow.
Core machine layout
The mill body is vertically divided into two stacked functional zones:
- Lower grinding zone: Equipped with high‑speed grinding rotor (hammers / pins) and wear‑resistant liners for particle size reduction.
- Upper classification zone: Houses an independently driven, frequency‑controlled classifier wheel as the internal classification core.
Negative‑pressure airflow, generated by the downstream induced‑draft fan, runs through both zones to transport particles, dissipate grinding heat and drive classification separation.
Step‑by‑step working principle
1. Metered feeding and impact grinding
Raw material is fed continuously into the grinding chamber via a screw or pneumatic feeder. The grinding rotor spins at high tip speed (60‑120 m/s). Materials are comminuted by three combined forces:
- Direct impact from rotor hammers
- Collision against chamber liners
- Inter‑particle shear and attrition
Brittle feedstock breaks rapidly into mixed fine‑and‑coarse particle fractions.
2. Airborne transport toward internal classifier
Circulating upward airflow lifts the ground particle mixture from the grinding zone straight into the upper internal classification zone, with no intermediate conveying or external transfer equipment.
3. Dynamic internal classification: force balance inside the classifier wheel
This is the core of the internal‑classifier principle. Every particle entering the rotating classifier wheel is subjected to two opposing aerodynamic forces:
- Air drag force: Pulls particles inward through wheel blade gaps toward the product outlet.
- Centrifugal force: Generated by the fast‑spinning classifier wheel, throwing heavier particles outward.
Two outcomes happen instantly:
- Qualified fine particles: Air drag ＞ centrifugal force. Fine powder passes through the classifier wheel and exits the mill, moving to cyclone separators and pulse dust collectors for final collection.
- Oversized coarse particles: Centrifugal force ＞ air drag. Coarse grains are rejected by the classifier wheel, drop by gravity straight back into the lower grinding chamber for re‑grinding.
4. Continuous internal closed recirculation
Rejected coarse particles return directly to the grinding zone without leaving the mill housing. Fresh feed and returned oversize material are ground together continuously. This “grind‑classify‑re‑grind” cycle runs non‑stop during production.
No metal screens are used. Particle cut‑size depends purely on aerodynamic separation, eliminating screen clogging and metal‑fragment contamination risks.
How to adjust finished‑particle size
The internal classifier wheel uses an independent variable‑frequency drive, adjustable online without stopping the mill:
- Increase classifier‑wheel speed → stronger centrifugal force → finer cut‑point, finer finished powder.
- Decrease classifier‑wheel speed → weaker centrifugal force → coarser allowed particle size.
Final fineness is fine‑tuned by coordinating classifier‑wheel speed, grinding‑rotor speed and system airflow volume.
Key technical benefits of internal classifier design
- Simultaneous grinding & classification: Two process stages inside one unit; compact footprint, fewer auxiliary machines.
- Suppresses over‑grinding: Qualified fines are extracted immediately once formed; only oversize grains keep re‑grinding. Produces narrow particle‑size distribution.
- Screen‑free operation: Avoids screen wear, blockage and metal contamination, critical for battery‑grade and high‑purity mineral powders.
- Online grade switching: Change product fineness by modifying classifier frequency without shutdown.
- Closed negative‑pressure system: Airflow removes grinding heat, suitable for many brittle and moderately heat‑sensitive materials.
Material‑friendly upgrade option
For battery‑grade metal‑free processing, all material‑contact parts (grinding liners, hammers, classifier‑wheel blades, elbows) can be constructed with zirconia or alumina ceramic components to eliminate iron‑contamination risks, while retaining the same internal‑classifier working principle.
