How Does an Air Classifier Work in ATH Processing

Aluminum hydroxide (ATH) is heat‑sensitive flame‑retardant filler. Screens cannot handle ultrafine ATH powder; screen mesh blinding and particle agglomeration make mechanical sieving impractical. The dynamic turbo air classifier is the core unit inside closed‑circuit ATH grinding lines, delivering sharp particle‑size cut, narrow PSD, strict over‑size control, while matching low‑temperature operating requirements for ATH production.

Core Separation Principle

The air classifier separates powder by balancing two competing forces on individual particles in the classification chamber:

1. Centrifugal force: Generated by high‑speed rotating classifier wheel, pushes larger / heavier particles outward.
2. Air drag force: From system fan airflow, pulls fine particles inward toward collection side.

• Fine qualified ATH particles: Air drag > centrifugal force → pass through classifier‑wheel blade gaps, move to cyclone and bag‑house collectors as finished product.
• Oversized coarse particles: Centrifugal force > air drag → thrown toward chamber wall, lose velocity and fall down. Secondary air performs elutriation to strip trapped fine powder from coarse fractions. Coarse particles are recirculated back to grinding chamber for re‑grinding in closed‑circuit mode.

Cut‑point (D97 target) is adjusted online by changing classifier wheel rotating speed and system air volume. Higher rotor speed produces finer ATH powder; lower rotor speed gives coarser product, no machine shutdown required.

Step‑by‑Step Operation inside an ATH Production Line

1. Powder transport: After low‑temperature grinding, ATH powder is carried by negative‑pressure airflow into the classifier classification zone.
2. Particle dispersion: Turbulent airflow breaks soft ATH agglomerates before separation; poor dispersion will cause fake coarse particles and worsen PSD.
3. Dynamic separation at classifier wheel: Rotating turbine sorts ATH particles. In‑spec fines go through wheel gaps; oversized grains are rejected.
4. Secondary air elutriation: Falling coarse material meets upward secondary air. Entrapped fine ATH particles are washed free and return to classification zone, improving classification efficiency and reducing fine‑particle loss in recycle stream.
5. Closed‑loop recirculation: Oversized ATH flows back to grinding mill for further pulverization. Only powder meeting particle‑size specification leaves the loop as final product.
6. Powder collection: Qualified fine ATH is collected by cyclone separator and pulse dust collector for downstream surface modification or packing.

Special Design Requirements for ATH Application

ATH imposes unique constraints for air classifier hardware and operation, different from common mineral fillers:

1. Strict temperature control: All air classifier components must work with the mill water‑jacket cooling system. Overall material temperature must stay below 110 °C, to avoid ATH crystal‑water loss and degradation of flame‑retardant performance. Hot spots inside classifier chamber must be eliminated.
2. Anti‑iron‑pollution option: For cable‑grade high‑purity ATH, classifier wheel, guide vanes and inner liners can adopt ceramic lining, to limit iron contamination and preserve powder whiteness and electrical properties.
3. Sharp top‑cut performance: LSZH cable‑grade ATH demands extremely low over‑size residue (typically ≤0.1 % above target D97). The classifier needs optimized flow‑channel design to minimize coarse‑particle leakage into fine product.
4. Anti‑agglomeration airflow tuning: Ultrafine ATH tends to form soft agglomerates. Well‑tuned secondary‑air flow helps de‑agglomerate, preventing agglomerates from being counted falsely as coarse particles.

Key Benefits for ATH Manufacturing

• Produces narrow particle‑size distribution critical for cable‑grade ATH; avoids broad PSD from stand‑alone grinding.
• Closed‑circuit circulation reduces over‑grinding; prevents excessive super‑fine ATH that raises oil‑absorption value and hurts polymer compounding processability.
• Online fineness adjustment, easy switching between different ATH grades (D97 2 μm ~ 8 μm).
• No screen components; eliminates mesh‑blocking trouble caused by sticky fine ATH powder.

Common Operational Pitfalls in ATH Air Classification

1. Insufficient secondary‑air volume → fine ATH trapped in coarse recycle stream, low classification efficiency and higher energy consumption.
2. Excessive system temperature → partial ATH dehydration inside classifier, permanent loss of flame‑retardant function.
3. Wrong rotor speed‑airflow matching → broad particle‑size distribution, high over‑size residue.
4. Wear of un‑lined classifier wheel → iron contamination, lower whiteness for finished ATH powder.

In ATH processing, the air classifier is not simply a sieve. It is a dynamic force‑field sorting unit running in closed‑circuit with the grinding mill. By balancing centrifugal and drag force, it strictly controls D50, D97 and over‑size content. When paired with low‑temperature cooling and anti‑contamination construction, it consistently delivers high‑quality ATH filler for low‑smoke zero‑halogen cables, rubber and plastic composites.