What Is the Function of an Air Classifier in ATH Processing

Air classifier (dynamic turbine classifier) is the core unit in closed‑circuit dry grinding lines for ATH (ACM mill, ring‑roller mill, jet mill system). It separates particles by aerodynamic drag and centrifugal force, controls finished‑product particle‑size distribution, returns coarse over‑size particles back to grinding zone for re‑grinding, and removes qualified fine powder. For heat‑sensitive, cable‑grade ATH, it directly determines D50, D97, over‑size residue, over‑grinding degree and oil‑absorption value.

Core functions for ATH processing

1. Precisely control finished particle‑size (D50, D97, top‑cut / over‑size residue)

High‑speed rotating classifier wheel generates centrifugal force; airflow brings particles toward wheel.

• Fine particles: airflow drag overcomes centrifugal force → pass through classifier wheel, become finished product.
• Coarse particles: centrifugal force dominates → rejected, fall back into grinding chamber for re‑grinding.

By adjusting classifier rotor speed and system air volume, you tune cut‑point. This is how cable‑grade ATH achieves strict D97 (e.g. 5 μm, 8 μm, 10 μm) and low over‑size residue requirement. Without air classifier, mill output will have broad PSD with large coarse tails and cannot meet LSZH cable specifications.

Important: Screen‑type hammer mill uses screen for size control; it cannot achieve this precise fine‑cut for ATH.

2. Build closed‑circuit circulating grinding loop

Coarse rejected material circulates back to grinding zone, forms circulating load.

1. Improve grinding efficiency; avoid sending unqualified coarse material into finished product.
2. Reduce one‑pass over‑grinding risk: particles are not ground endlessly in one pass; only particles meeting cut‑point escape the grinding loop.
3. For ATH: properly controlled circulating load helps balance oil‑absorption, avoid generating excessive sub‑micron fine fractions.

3. Suppress over‑grinding of ATH (process‑quality function)

If operated correctly, the classifier limits residence‑time of already‑fine particles inside grinding zone.

• Too‑high classifier speed will produce excessive ultrafine fractions (over‑grinding, higher oil‑absorption).
• Matching rotor speed with airflow can achieve target D97 while minimizing fine tails, which is critical for halogen‑free cable‑ATH compounding performance.

4. Remove foreign debris and tramp hard impurities

Large hard contaminants (quartz grit, metal fragments) have high mass; strong centrifugal force rejects them back into grinding loop. Most heavy impurities will eventually discharge through mill reject port, reducing hard‑impurity content in final ATH powder, lowering wear‑part erosion and iron contamination.

5. Collaborate with airflow for heat dissipation (special for heat‑sensitive ATH)

Air classifier works within negative‑pressure air circuit. Process airflow carries away grinding‑friction heat along with qualified fine ATH powder.

• Air volume matched with classifier speed prevents heat accumulation inside mill chamber, helps control material temperature ≤110 °C to protect ATH crystal water.
• Too‑low airflow for a given classifier setting will prolong particle residence time, cause both over‑grinding and temperature rise.

Key failure modes of air classifier affecting ATH quality

1. Classifier wheel blade wear / chipping: Distorts cut‑point. D97 becomes larger, over‑size residue rises, PSD broadens, even if operating parameters unchanged. For ceramic‑grade ATH, chipping may introduce ceramic debris.
2. Material caking on classifier wheel housing: Uneven airflow field, unstable PSD, batch‑to‑batch fluctuation.
3. Improper speed‑airflow matching: High rotor speed + insufficient air volume → serious over‑grinding, oil‑absorption spikes.

Differences in different ATH grinding systems

1. Ring‑roller mill: Top‑mounted dynamic air classifier; controls product fineness, coarse returns to grinding turntable.
2. ACM air‑classifier mill: Integral classifier on mill housing outlet; core finishing control for cable‑grade ATH.
3. Fluidized‑bed jet mill: Built‑in turbine classifier; achieves ultrafine dry ATH D97 1.5‑2.5 μm; no grinding rotor, classification is dominant size‑control mechanism.

For ATH processing, air classifier is not simply a “sieving device”. Its main functions:

1. Accurately set particle‑size cut‑point to meet D97 and over‑size requirements for cable‑grade ATH.
2. Realize closed‑circuit circulation, return coarse particles for re‑grinding.
3. Mitigate over‑grinding risk and control oil‑absorption.
4. Assist airflow heat removal to prevent ATH thermal decomposition.
5. Reject heavy tramp impurities.

Performance of air classifier largely determines final ATH product quality; even good grinding host cannot produce qualified cable‑grade ATH with poorly‑performing classifier.