How to prevent overheating during silica grinding

During precipitated silica grinding, heat mainly comes from mechanical impact friction, particle‑particle collision, air compression‑expansion and high‑speed rotation of rotor and classifier. Excessive temperature will cause multiple problems: silica agglomeration, material caking on internals, BET drop, filter bag aging, bearing failure, and even safety risks. ACM air classifier mill generates much more frictional heat than fluidized‑bed jet mill (jet mill benefits from Joule‑Thomson cooling effect). Below are systematic preventive measures for silica processing.

Main heat sources in silica grinding system

1. ACM main hammer rotor: high‑speed mechanical impact and friction produce major heat.
2. High‑speed rotating classifier wheel: air shearing and friction generate heat.
3. Air compression and turbulent flow inside pipeline and cyclone.
4. Friction from high‑velocity silica particles rubbing against liners.

Note: Fluidized‑bed jet mill: compressed air expands at nozzles, creates cooling effect, overheating rarely occurs. Overheating risk is primarily for ACM mill systems.

1. Optimize grinding process parameters (most cost‑effective measure)

‑ Reduce unnecessary main rotor peripheral speed. Follow the “weak‑impact, strong‑classification” principle for silica. Too‑high hammer speed not only generates massive heat, but also fractures primary silica aggregates and degrades BET. Adjust fineness mainly via classifier wheel speed instead of raising hammer speed.
‑ Avoid over‑feeding. Overload causes material accumulation inside grinding chamber, increases friction residence time and sharp temperature rise. Keep feed rate within the equipment’s rated capacity.
‑ Avoid long‑time no‑load or low‑load running. Idle high‑speed rotation generates heat without material taking away heat. Shut down if no feeding.
‑ Control internal recycle ratio. Too high recycle ratio means massive coarse silica circulates repeatedly inside high‑energy grinding zone, accumulates heat.

2. Optimize system air volume and cold air supplement

Airflow acts as the main heat‑carrying medium for air‑classifier mills.
‑ Maintain sufficient induced‑draft fan air volume. Enough process air takes frictional heat out of grinding chamber, cyclone and pipeline. Insufficient airflow is one of the most common causes of overheating.
‑ Properly supplement ambient cold fresh air at the mill inlet. Do not run with fully closed air inlet. Fresh cold air dilutes system temperature.

Warning: For precipitated silica, do not introduce humid hot air. Ensure inlet air has low humidity; otherwise cold‑air intake will bring moisture and trigger silica caking. If workshop air humidity is high, use dehumidified make‑up air.
‑ Check for pipeline blockage and material deposition. Caked silica blocks airflow channel, reduces heat dissipation and causes local hot spots. Regularly clean inner wall build‑up.

3. Bearing and rotating component temperature control

High‑speed bearings for main rotor and classifier wheel are frequent overheating failure points.
‑ Strictly follow lubrication specification: select correct high‑temperature grease; do not over‑grease or under‑grease. Over‑filling grease will also cause bearing overheating.
‑ Install real‑time temperature sensors on bearing housings, set alarm threshold (normally <75 °C). Trigger alarm and interlock shutdown when temperature exceeds limit. ‑ Check shaft‑purge air sealing system. Continuous dry cooling purge air cools bearing assembly and prevents silica powder intrusion into bearing gap. Blocked purge‑air line leads to bearing heating and metal contamination. ‑ Regularly check rotor dynamic balance. Unbalanced rotor brings extra vibration and friction heat.

4. Equipment material & structural improvements

‑ Ceramic‑lined internals help reduce frictional heat compared with bare metal. Smooth ceramic surface lowers particle friction heat generation, meanwhile reduces silica adhesion and caking‑induced hot‑spots.
‑ For high‑fineness silica (D50 3‑5 μm) with high heat generation, add external water‑cooling jacket on grinding chamber body and classifier housing. Indirect water cooling cools shell without contacting powder. Do not use direct water spray, which will cause silica agglomeration.
‑ Select high‑temperature resistant filter bags for dust collector. When system temperature rises, prevent bag melting or aging failure.

5. Raw‑material condition control

‑ Keep feed moisture ≤0.8~1.0 %. High‑moisture silica sticks to chamber walls, forms thick caking layers. Caking hinders heat transfer, creates local hot‑spots, and may even bake silica. Dry raw material before grinding if moisture is too high.
‑ Pre‑break large lumps. Big chunks increase impact friction heat inside grinding chamber.

6. Operation & troubleshooting for overheating phenomenon

Typical signs of overheating:
‑ Mill shell temperature obvious rise; filter bag house temperature climbs; frequent material caking inside chamber; bearing temperature alarm.

Emergency handling steps:

1. Reduce feeding rate or stop feeding temporarily, keep fan running to ventilate and cool down the whole system.
2. Check whether air‑inlet is blocked, pipeline is clogged, purge‑air fails.
3. Check bearing vibration and temperature. Do not open inspection door when mill is hot to avoid hot‑air scalding.
4. After temperature drops, inspect inner wall for silica caking, remove accumulated material.

Difference between ACM and fluidized‑bed jet mill on heating

‑ Ceramic‑lined ACM mill: High risk of overheating; all above measures must be implemented.
‑ Fluidized‑bed opposed jet mill: Expansion of compressed air produces Joule‑Thomson cooling effect. Grinding chamber temperature is usually below ambient temperature. Overheating seldom occurs; only pay attention to classifier wheel bearing heating.

Common mistakes to avoid

1. Solve fineness problem only by raising hammer rotor speed: generates large amount of heat plus BET loss.
2. Close air‑inlet to increase internal circulating air, leading to insufficient heat dissipation.
3. Ignore shaft‑purge air, cause bearing heating and secondary metal contamination.
4. Deal with overheating by spraying water directly into mill chamber, resulting in silica agglomeration and caking.

To prevent overheating in precipitated silica grinding (mainly for ACM air classifier mill):

1. Adopt “low‑speed rotor + high‑speed classifier” process strategy to reduce mechanical friction heat source.
2. Guarantee sufficient process air volume and reasonable cold dry fresh‑air supplement for heat removal.
3. Monitor bearing temperature, maintain air‑purge sealing and standardized lubrication.
4. Control feed moisture and avoid inner‑wall material caking which creates hot‑spots. Add water‑cooling jacket for high‑fineness silica when necessary.
5. Jet mill has intrinsic cooling effect, over‑heating risk is very low.