Aluminum hydroxide is thermally sensitive; temperature must stay below ~120 °C to avoid dehydration into boehmite or alumina. Two mainstream industrial routes: Dry Mechanical Grinding (mass production) and Wet Milling (submicron high-purity grades).
1. Full Dry Grinding Process (Most common for flame-retardant ATH powder)
Suitable target fineness: D50 = 1 μm ~ 45 μm (325 mesh – 2500 mesh)
Step 1: Raw Material Pre-Treatment
- Crush bulk coarse ATH lumps to feed size <10–20 mm.
- Pre-drying: Dry raw material to moisture ≤ 1.5–2%. High moisture causes agglomeration, material sticking inside mills.
Drying temperature: 80–110 °C (strictly avoid >120 °C). - Magnetic separation to remove iron impurities and guarantee high whiteness.
Step 2: Closed-Circuit Grinding & Air Classification (Core Section)
Choose matched equipment according to capacity & fineness:
- Option A: Vertical Roller Mill / Ultrafine Ring Roller Mill
Best for large-scale production (5–30 t/h), stable for D50 3–15 μm, low energy consumption. - Option B: ACM Air Classifier Mill
Compact unit; produces D50 2–10 μm, widely used for medium-small capacity flame retardant powder. - Option C: Jet Mill (Air Jet Pulverizer)
For high-end ultrafine grades: D50 0.8–3 μm, narrow particle distribution; low contamination. - Option D: Ceramic-lined Ball Mill + External Air Classifier
Zero iron pollution; ideal for high-whiteness electronic-grade ATH.
Closed-loop working principle:
Coarse ATH is fed into grinding chamber → impact/shear force pulverizes particles → airflow carries powder to high-speed turbine classifier.
✅ Qualified fine powder is separated and collected.
❌ Oversized coarse particles automatically circulate back into the mill for regrinding.
Critical control: Maintain grinding chamber temperature <70–90 °C. Install cold dry air intake or water jacket cooling.
Step 3: Powder Collection
Fine powder is captured by cyclone separators + pulse bag dust collectors; recovery rate >99.5%.
Step 4 (Optional): Surface Modification
If used as plastic/rubber filler: add silane coupling agent, stearic acid during grinding to improve dispersibility and reduce oil absorption.
Step 5: Sieving & Automatic Packaging
2. Wet Milling Process (For submicron / ultra-fine high-purity ATH)
Target: D50 0.3–2 μm; mainly for electronic packaging, high-end solid surface materials.
- Prepare ATH slurry: mix dry ATH with deionized water; solid content = 35–55 wt%.
- Add water-based dispersant (polyacrylate, polyphosphate) to prevent particle agglomeration.
- Continuous grinding by horizontal bead mill (zirconia beads as grinding media).
- Filter and wash slurry after milling.
- Low-temperature spray drying (≤110 °C) to obtain dry fine powder.
- Fluidized bed de-agglomeration to eliminate dried particle clusters.
3. Equipment Selection Reference Table
| Target Fineness | Recommended Equipment | Typical Application |
|---|---|---|
| 20–45 μm (325–800 mesh) | Raymond Mill, Vertical Roller Mill | Common flame retardant, raw material for aluminum salt |
| 3–15 μm (1250–2500 mesh) | Ultrafine Ring Roller Mill / ACM Mill | General plastic, cable ATH filler |
| 0.8–3 μm | Jet Mill / Ceramic ball mill + classifier | High whiteness, low oil-absorption flame retardant |
| <1 μm (submicron) | Horizontal Zirconia Bead Mill (Wet Process) | Electronic thermal conductive filler, premium solid surface |
4. Key Technical Risks & Preventive Measures
- Thermal dehydration risk
Al(OH)₃ loses crystal water above ~130 °C. Control grinding temperature continuously; do not over-grind. - Severe agglomeration of ultra-fine particles
Use closed-circuit classification; add dispersant (wet milling) or surface modifier (dry milling). - Iron contamination & whiteness drop
Use ceramic lining, zirconia grinding media instead of steel; install magnetic iron removers. - Blockage inside the mill
Strictly control feed moisture below 2%; keep feeding rate uniform.
5. Simplified Standard Dry Process Flow Chart
Raw coarse Al(OH)₃ → Pre-crushing → Low-temperature drying → Magnetic iron removal → Quantitative feeding → Ultrafine Mill + Dynamic Air Classifier (closed circulation) → Cyclone & bag collection → (Surface modification) → Finished fine powder → Packaging
