ATH (aluminum hydroxide) is widely used in white and light‑colored flame retardant coatings. Whiteness reduction is mainly caused by iron impurities, organic contamination, thermal discoloration, particle agglomeration, and grinding/media wear. Improving ATH whiteness requires raw material control, mineral purification, optimized milling/classification, low-temperature processing, and anti-contamination design.
1. Select high-purity, low-iron bauxite raw ore
Iron oxide (Fe₂O₃) is the primary culprit for yellowish or off-white ATH.
- Choose bauxite ore with ultra-low iron content. Strictly limit total Fe impurity.
- Control other colored heavy metal impurities such as manganese and chromium.
- Use high-quality Bayer-process ATH feedstock instead of lower-grade mineral sources.
2. Purification before grinding
- Apply chemical bleaching / iron removal treatment on the raw ATH slurry for wet-process ATH.
- Magnetic separation to remove ferromagnetic iron particles introduced during ore processing.
- Filter and wash to reduce soluble colored impurities.
3. Use non-metallic lining and grinding media to avoid metal contamination
This is critical for dry ultrafine grinding lines (JACAN ACM / jet mill + JSGX modification mill).
- Replace steel liners and metal rotors with ceramic or polyurethane liners, ceramic rotors.
- Use ceramic grinding beads instead of steel balls to prevent iron abrasion.
- Regularly inspect equipment for wear. Worn metal parts release fine iron powder, which turns ATH yellow and reduces whiteness.
4. Optimize grinding and classification process
- Avoid over-grinding. Excessive impact and shear can create surface defects on ATH crystals and increase light absorption, lowering whiteness.
- Stable air classification removes dark coarse impurities and black foreign particles.
- Control particle size distribution: well-dispersed ATH particles reflect light evenly. Hard agglomerates scatter light unevenly and appear dull.
5. Strict temperature control during surface modification
ATH will turn slightly yellow if exposed to local hot spots above 110 °C during modification.
- Maintain JSGX modification mill chamber temperature below 110°C, ideally 80–100°C.
- Prevent localized friction overheating. Multi-point temperature sensors and cooling air control are required.
- Avoid modifier thermal decomposition. Degraded organic residues stain the powder surface and reduce whiteness.
6. Control moisture and organic contamination
- Keep ATH free moisture at 0.2–0.5%. Excess moisture can cause discoloration during storage and processing.
- Use clean, low-color modifiers. Some cheap stearic acid has yellow tint and will reduce final powder whiteness. For high-whiteness coatings, select high-purity white-grade silane or stearic acid.
- Prevent oil, dust, rust and foreign debris from entering the powder conveying system. Maintain negative pressure sealed pipelines.
7. Post-treatment and packaging
- After modification, cool powder rapidly to stop thermal aging.
- Use inert clean air for conveying and collection.
- Package in moisture-proof bags to avoid oxidation and moisture absorption during storage.
Quick quality check
Whiteness can be measured with a whiteness meter. High-grade coating ATH usually reaches whiteness ≥94%. Compare powder samples side-by-side under standard light source for visual screening.
To improve ATH whiteness for coatings, start with low‑iron raw material and iron removal purification. Equip the grinding and modification line with ceramic anti‑wear components to avoid metal contamination. Prevent over-grinding and local overheating during JSGX surface modification, use high-purity colorless modifiers, and keep the whole production system clean and sealed. This delivers bright high-whiteness ATH ideal for white and light-colored flame retardant coatings.
