How to Prevent Over‑Grinding of Aluminum Hydroxide

Over‑grinding of ATH means producing excessive ultra‑fine fractions beyond target specification. Consequences include: higher oil‑absorption value, severe particle agglomeration, sharp drop in throughput, increased energy consumption, elevated wear‑parts loss, and higher risk of thermal stress / local over‑heating. For cable‑grade ATH, over‑grinding deteriorates compounding performance in halogen‑free cable formulations. Over‑grinding occurs in ACM, ring‑roller mill and jet‑mill circuits; prevention requires control of feed, classifier settings, airflow, circulating load, and process monitoring.

1. Understand root causes of ATH over‑grinding

1. Classifier wheel speed set too high, forcing excessive cutting of fine particles.
2. Insufficient system airflow; particles stay too long inside grinding loop, repeated re‑grinding.
3. Too low feed rate; few new particles enter mill, existing powder circulates many times.
4. Excessive grinding pressure / impact energy (ring‑roller spring pressure, jet‑mill nozzle pressure).
5. High circulating load; large quantity of material trapped in closed‑circuit loop.
6. Feed already contains large amount of fine pre‑ground material.
7. Wear‑parts degradation: worn rollers, rings, hammer or classifier blades distort actual cut‑point.

2. Optimize dynamic air classifier settings (primary control for closed‑circuit mills)

Applicable for ACM, ring‑roller mill, fluidized‑bed jet mill.

1. Do not blindly increase classifier rotor speed for finer D97. Higher rotor speed not only cuts D97, but also generates more super‑fine fractions. Tune rotor speed together with airflow to achieve target D97 while minimizing over‑fine tails.
2. Verify classifier actual performance: test full PSD distribution, not only D50/D97. Monitor volume percentage of particles below 1 μm as over‑grinding indicator.
3. Timely inspect classifier wheel wear. Eroded or chipped blades cause disordered classification, producing broad PSD with excess ultrafine fractions, even if parameters are unchanged. Replace wear‑parts once deviation appears.

3. Match system airflow and feed rate

1. Maintain rated design feed rate. Running at very low feed rate is a common cause of over‑grinding. With little fresh feed, material circulates repeatedly inside grinding chamber. Avoid long‑time light‑load operation.
2. Adjust total fan air volume collaboratively with classifier speed. Too‑small air flow extends particle residence time in grinding zone, increases over‑grinding risk. Do not reduce airflow only for chasing finer fineness.
3. Keep negative‑pressure pipeline, cyclone and bag‑house clean. Powder buildup raises system resistance, reduces effective air volume and prolongs residence time.

4. Adjust grinding energy input

Ring‑roller mill

• Avoid excessive spring‑loading grinding pressure. Higher pressure produces more fine tails. Set spring pressure matched to target product fineness, do not over‑pressurize to pursue higher output.

ACM air classifier mill

• Avoid excessively high rotor speed of grinding hammer rotor. High hammer speed delivers stronger impact energy and generates more sub‑fine particles. Balance hammer speed, classifier speed and throughput.

Fluidized‑bed jet mill

• Do not run with unnecessarily high jet‑nozzle pressure. Higher gas pressure enhances particle collision and yields excessive ultra‑fine fractions. Optimize nozzle pressure to just meet target PSD.

5. Manage feed material characteristics

1. Control feed particle‑size distribution. If feed already contains abundant fine powder, reduce grinding intensity; otherwise these fines will be further over‑ground.
2. Remove hard tramp impurities (quartz, iron grit). Hard impurities create random extra comminution events and generate surplus ultrafine particles. Install magnetic separation and sifting before mill feeding.
3. Feed moisture must comply with specification. Sticky caked material creates uneven grinding bed, unstable circulating load and inconsistent over‑grinding tendency.

6. Control circulating load

High circulating load is typical for closed‑circuit grinding. Too‑high circulating load indicates mismatch between grinding capacity and classifier cut‑point, leading to multiple re‑grinding of same particles.

• Practical indicator: monitor cyclone/bag‑house collection efficiency and mill current draw. Sustained abnormally high host‑motor current with low finished output signals excessive circulating load and potential over‑grinding.
• Adjust feed rate, classifier speed and airflow to bring circulating load back to equipment recommended range for ATH.

7. Process monitoring & quality feedback loop

Do not rely only on mill operating parameters; use powder test data as feedback.

1. Monitor full PSD curve, not only D97. Pay attention to fraction <1 μm volume percentage and oil‑absorption value. Rising oil‑absorption is a very practical field indicator for ATH over‑grinding.
2. Record motor current, classifier speed, air volume, feed rate, oil‑absorption, PSD trend log per shift.
3. If oil‑absorption increases without changing target D97, it is clear sign of over‑grinding. Mitigate: slightly reduce classifier speed, increase feed rate appropriately, or increase system airflow.
4. Periodically inspect wear‑parts. Wear‑part deterioration can cause hidden over‑grinding without obvious parameter change.

8. Operational pitfalls to avoid

1. Running mill at low throughput for long‑time production of medium‑coarse ATH.
2. Only focusing on D97 index while ignoring fine tail fractions and oil‑absorption.
3. Tuning only classifier speed without adjusting airflow and feed rate together.
4. Continuing production when classifier wheel, rollers or hammers are heavily worn.

Troubleshooting quick reference

Preventing ATH over‑grinding is multi‑dimensional: set reasonable classifier cut‑point, avoid light‑load operation, match airflow‑feed‑grinding‑energy, control circulating load, and use oil‑absorption together with full PSD curve as key feedback indicators. Over‑grinding cannot be judged only by D97; excess ultra‑fine fractions will degrade downstream cable‑compounding performance even if D97 meets specification.