How to Achieve a Narrow Particle Size Distribution for ATH

Narrow PSD for ATH means minimal coarse over‑size tail and limited excess ultrafine fine tail; critical for LSZH cable‑grade ATH. It improves flame retardancy, extrusion performance, low oil‑absorption and consistent electrical properties. Narrow PSD is achieved by equipment selection, closed‑circuit process tuning, feed quality control, classifier optimization and stable operation.

1. Select suitable grinding‑classification equipment configuration

1. Closed‑circuit mill with high‑precision dynamic turbine air classifier
   ACM mill, ring‑roller mill, fluidized‑bed jet mill with well‑designed turbine classifier are preferred.

Screen‑type hammer mill, open‑circuit single‑pass grinding cannot get narrow PSD for ATH.

2. Classifier hardware requirement for narrow PSD

• Precision balanced classifier wheel, sharp blade profile, small tip clearance between wheel and housing. Large clearance causes particle short‑circuit, broadens PSD.
• Ceramic‑coated / zirconia classifier wheel for cable‑grade ATH; worn, eroded blades destroy cut‑point sharpness.
• Avoid volute distortion and internal material caking which distort airflow field.

3. Equipment comparison for PSD sharpness

• Fluidized‑bed jet mill + integrated turbine classifier: sharpest dry PSD (D97/D50 ratio ~1.8‑2.2), for ultrafine dry ATH D97 1.5‑2.5 μm
• ACM mill with high‑precision classifier: D97/D50 ~2.2‑2.8, industrial mainstream for D97 3‑10 μm cable‑grade ATH
• Ring‑roller mill: D97/D50 ~2.5‑3.2, better for medium‑coarser ATH D97 8‑20 μm
• Wet circulation bead mill: narrowest overall PSD (D97/D50 ~1.5‑2.0), for sub‑micron ATH (slurry state)

D97/D50 ratio is simple field indicator: smaller value = narrower particle‑size distribution.

2. Optimize dynamic air classifier tuning (most critical step)

Cut‑point is determined by classifier rotor speed and system air volume, must tune together.

1. Fixed air volume, increase rotor speed → finer cut‑point, but generates more ultrafine tail (over‑grinding, PSD broadens).
2. Increase air flow together with rotor speed: improve particle transport, reduce residence time inside grinding zone, suppress excess fine generation, keep PSD narrow.
3. Do not only chase D97 target; monitor full PSD curve, not only D50/D97. Track volume fraction of particles below 1 μm to judge over‑grinding.
4. Maintain reasonable circulating load. Too‑high circulating load causes repeated re‑grinding of same particles → broad PSD. Adjust feed rate‑air‑classifier speed to bring circulating load back to equipment recommended range for ATH.
5. Prevent material short‑circuit: check seal gaps of classifier; leakage allows coarse particles bypass classification directly into finished product.

3. Control grinding intensity to avoid over‑grinding

Excessive grinding energy creates large amount of ultrafine fines and broadens PSD.

Ring‑roller mill

Do not set excessive spring pressure on rollers; higher pressure produces more fine tails.

ACM mill

Avoid running hammer rotor at unnecessarily high rotational speed.

Jet mill

Do not use higher nozzle pressure than required for target fineness.

Practical indicator: oil‑absorption rises while D97 unchanged → sign of over‑grinding and broadening PSD.

4. Strict feed material control

1. Feed PSD should be stable. Large fluctuation of feed particle‑size will make finished PSD drift. Pre‑crush large lumps, remove over‑size feed.
2. Remove tramp hard impurities (quartz, iron grit). Hard impurities produce random extra comminution, generate random coarse and fine fractions. Install magnetic separation and pre‑sieving.
3. Feed moisture strictly controlled ≤0.2 %. Damp/caked ATH creates uneven grinding bed, unstable circulating load, inconsistent PSD.

5. Stabilize process operating conditions

1. Maintain stable feed rate. Surge feeding leads to variable circulating load, broad PSD. Use loss‑in‑weight feeder.
2. Keep system air volume stable. Check fan frequency, filter bag resistance; clogged dust collector increases system resistance, reduces actual air volume and changes classification performance. Regular clean baghouse.
3. Control mill temperature. ATH over‑heating leads to particle surface alteration, caking inside mill chamber, distorts airflow and classification field. Keep material temperature ≤110 °C.

6. Prevent classifier performance degradation due to wear and fouling

1. Periodically inspect classifier wheel blades. Erosion, chipping increases tip gap, cut‑point becomes blunt, PSD broadens, even if parameters unchanged. Implement preventive replacement schedule based on wear‑part running hours and finished Fe content.
2. Prevent material caking on classifier wheel, housing and guide vanes. Caking destroys uniform airflow field, causes batch‑to‑batch PSD deviation. Regular internal inspection and cleaning.

7. Post‑classification sifting (polishing step, optional)

Install high‑efficiency turbo sifter after collecting finished powder. It can remove residual few coarse outliers, improve top‑cut performance. It cannot fix broad PSD caused by poor grinding‑classification parameters; it only removes small quantity of coarse tails.

Troubleshooting reference

Key steps for narrow‑PSD ATH:

1. Adopt closed‑circuit grinding with high‑precision dynamic turbine air classifier; avoid open‑circuit single‑pass grinding.
2. Tune classifier rotor speed and air volume collaboratively; do not only adjust rotor speed.
3. Control grinding energy input to avoid over‑grinding which brings excess ultrafine fractions.
4. Stabilize feed quality, feeding rate and system air volume.
5. Monitor classifier wear and internal caking; keep sharp classification cut‑point.
6. Evaluate PSD by full curve and D97/D50 ratio instead of only D97 index.