Aluminum hydroxide (ATH / gibbsite) readily agglomerates in dry ultrafine grinding. As particle size falls below ~10 μm, surface energy rises sharply; fine particles stick together by van der Waals forces. Excessively high grinding speed creates excessive impact, generates ultra-fine fines, raises powder temperature and worsens agglomeration. Too low speed delivers insufficient particle breakage and low throughput. The optimal grinding speed depends on mill type, paired with closed‑circuit air classification and strict temperature control. This article references ATH processing technology from ath‑mill.com and JACAN powder engineering.
1. Key principle linking grinding speed and ATH agglomeration
High rotor speed → higher collision energy → more ultra‑fine particles, larger BET, greater inter‑particle attraction. Heat from high‑speed impact also increases surface hydroxyl activity and promotes soft agglomeration.
For ATH flame‑retardant grades, the target is attrition‑dominated gentle grinding, not high‑velocity impact grinding. The goal is to break coarse particles by rubbing and shear instead of violent impact, minimizing the generation of excess submicron fines that drive agglomeration. Also, material temperature must stay ≤110 °C to avoid crystal water loss.
2. Optimal speed range by mill type for dry ATH ultrafine grinding
Vertical roller mill (common for large‑volume ATH production)
Main mill rotational speed: 21–23 rpm.
This low speed creates a stable material bed for inter‑particle extrusion and mild attrition. It avoids explosive fine generation, produces blocky particles with low BET, and greatly reduces agglomeration tendency. Coarse material is recirculated back to the grinding table by internal air classification.
Turbo / air classifier mill (ACM, widely used for ATH fine powder)
Grinding rotor speed: 3000–5000 rpm (for D50 2–8 μm flame‑retardant ATH).
Do not run above 5500 rpm for ATH. Higher rotor speed quickly builds up submicron fines, increases BET and triggers heavy agglomeration.
Classifier wheel speed is separate: typically 2000–4000 rpm. The classifier removes excess ultra‑fine fractions, which is essential to stop agglomeration even at fixed grinding rotor speed.
Closed‑circuit ball mill (dry grinding)
Optimal rotation speed: 65–70% of the mill critical speed.
This setting gives cascading, attrition‑preferred motion. If speed rises above 75% critical speed, cataracting high impact dominates; excessive fines and agglomeration appear. Ceramic grinding media and ceramic liners are used to avoid iron contamination.
3. Speed alone cannot prevent agglomeration — supporting critical controls
- Closed‑loop air classification is mandatory
Even at ideal grinding speed, ultra‑fine fines will form. The classifier cuts off the fine tail and returns oversize particles back to the mill. Without classification, fines accumulate inside the system and re‑agglomerate continuously. - Feed rate matching
At fixed grinding speed, too low feed rate means particles stay inside the grinding chamber too long, over‑grind and agglomerate. Too high feed causes incomplete comminution. - Temperature control
High speed generates frictional heat. Hot ATH particles have more active surface hydroxyl groups and stick more easily. Maintain material temperature below 110 °C via cooled process air. - Minor surface coating additive during grinding
Small dosage of stearic acid or dispersant can coat fresh particle surfaces during milling, suppressing particle‑particle adhesion and soft agglomeration.
4. Common operational mistakes
- Running ACM mill at maximum rotor speed to chase finer D50: this creates huge amounts of submicron fines, high BET and severe agglomeration.
- Removing the classifier or lowering classifier speed to increase yield: fines accumulate, powder loses flowability.
- Ignoring temperature rise: heat not only promotes agglomeration but also risks ATH dehydration and permanent loss of flame retardancy.
5. Practical recommendation for flame‑retardant ATH
For most commercial ATH grades used in rubber, sealants, coatings and LSZH cables (D50 2–6 μm):
- Use low‑impact grinding speed; prioritize attrition over impact.
- Pair grinding mill with high‑precision turbine air classification to strip excess submicron fines.
- Monitor PSD, BET and powder flowability as speed is adjusted. If agglomeration appears, reduce grinding rotor speed or raise classifier wheel speed to remove fine fractions.
There is no universal single speed value; optimal speed is mill‑specific. For ATH to avoid agglomeration, select a moderate grinding speed that delivers mild attrition rather than violent impact. The primary strategy is to limit over‑production of submicron fines, control temperature, and use closed‑circuit classification. Matching grinding speed with classifier parameters and feed rate is the complete solution to reduce soft agglomerates while maintaining target particle size for flame‑retardant composites.
