The classifier cage, also known as the classifier wheel rotor, is the core rotating assembly inside an ACM mill’s classification zone. It adopts an open cylindrical cage construction, mounted coaxially above the grinding rotor and driven by an independent high-speed shaft. Air-particle mixture flows radially inward through blade gaps from the outer annular classification zone, while centrifugal force rejects oversized particles back to the grinding chamber. Its complete assembly consists of upper flange, lower flange, classification blades, central hub, sealing structure and optional internal diversion parts.
1. Upper and lower flange (end rings)
These two parallel circular support rings form the top and bottom frame of the cylindrical cage. The blades are clamped vertically between upper and lower flanges to maintain fixed blade spacing and axial alignment.
- The upper flange connects to the classifier drive shaft. It transfers torque from the motor to the whole cage assembly.
- The lower flange keeps blade bottoms fixed and maintains consistent radial clearance between cage bottom and stationary baffle shroud.
Both flanges are precision-machined and dynamically balanced after assembly to reduce vibration during high-speed rotation.
2. Classification blades / vanes
Blades are the most critical functional part, arranged uniformly around the cage circumference between the two flanges. Blade geometry directly defines classification performance.
- Radial straight blades: Extend outward along radial lines, simple structure, low air resistance, suitable for medium-coarse cuts and high throughput.
- Backward-canted blades: Angled opposite to rotation direction, smoothing inter-blade airflow, suppressing vortex and delivering sharp cut points for ultra-fine powder.
- Forward-canted blades: Angled toward rotation direction, strong particle scrubbing effect for agglomerated powder, but higher pressure drop.
Blade quantity varies by cage diameter: more blades create narrower flow channels and tighter PSD, while fewer blades reduce blockage risk for coarse or sticky materials. Blade surfaces can be hardened, carbide-coated or fully ceramic for abrasive or metal-free processing.
3. Central hub assembly
The hub sits at the cage centre and attaches the entire cage to the classifier shaft. It is the torque transmission core. The hub includes positioning shoulders and locking nuts to secure the cage axially and radially. For modular designs, the hub uses bolted connections so worn blade segments can be replaced individually instead of changing the whole cage.
4. Internal diversion cone (optional)
Some high-precision or double-layer classifier cages install a diversion cone mounted on the hub inside the blade cylinder. It guides fine powder airflow converging toward the central product outlet, reduces internal turbulence and prevents fine particle recirculation inside the cage. This structure improves classification sharpness for ultra-fine grades.
5. Static shroud, baffle and labyrinth air seal (mating stationary parts)
Though not part of the rotating cage itself, these stationary components form the classification gap together with the rotating cage.
- Baffle shroud: Separates grinding zone and classification zone, guiding particle-laden air evenly into the outer perimeter of the classifier cage.
- Labyrinth / air purge seal: Installed at the top and bottom clearance between rotating cage and stationary housing. It blocks short-circuit flow, preventing coarse particles from bypassing the blade gap and contaminating finished fine powder. This seal gap is a key dimension for stable classification.
6. Single-layer vs double-layer nested cage structure
- Single-layer open cage: Standard ACM configuration. One ring of blades between upper and lower flanges. Simple, easy disassembly and cleaning, widely used for most mineral, food and chemical powders.
- Double-layer nested cage: Outer blade ring plus inner secondary blade ring. It provides two-stage classification inside one assembly, minimising coarse leakage and producing extremely narrow particle size distribution. Applied for battery materials, high-purity ceramics and premium chemical powder. It increases airflow resistance and maintenance complexity.
Working flow inside the cage
Mixed air and particles enter the annular space outside the classifier cage. Fine particles follow airflow drag force, pass through gaps between rotating blades, enter the inner cavity of the cage and flow to the product outlet. Oversized particles gain enough centrifugal force from the rotating cage, bounce off blade outer surfaces, drop down along the baffle shroud and return to the hammer grinding zone for reprocessing.
The ACM classifier cage is an open cylindrical rotor framed by upper and lower flanges, circumferentially fitted with classification vanes and mounted on a central hub. Blade profile, blade count, cage height and optional diversion cone determine separation precision. It works in combination with stationary baffles and labyrinth seals to control the critical classification gap. Single-layer cages serve general production, while double-layer nested cages are adopted for high-spec ultra-fine powder applications.
