How to Improve the Whiteness of Ground Aluminum Hydroxide

Aluminum hydroxide (ATH / alumina trihydrate) is widely adopted as halogen-free flame retardant and white inorganic filler for low-smoke cables, artificial stone, engineering plastics, coatings and electronic packaging. Whiteness is one of the core commercial indicators of finished ATH powder. Even high-quality raw ore will suffer obvious whiteness drop after ultrafine grinding if the process is not properly controlled.

According to ATH deep processing technology summarized on ath-mill.com, the main causes of reduced whiteness after grinding include iron contamination from equipment abrasion, thermal discoloration and partial dehydration, particle agglomeration, raw material impurity, and cross-contamination in production lines. This article systematically introduces feasible industrial solutions to steadily raise and stabilize the whiteness of ground ATH powder.

1. Raw Material Pre-Selection & Pre-Treatment (Source Control)

Whiteness limitation often originates from raw materials. All downstream grinding optimization cannot offset excessive impurities in crude ATH.

• Select high-whiteness ATH feedstock with low iron oxide content; control Fe₂O₃ ≤ 0.02% for premium-grade products. Iron oxide is the primary impurity triggering yellowish/grey discoloration.
• Remove foreign debris, ferrous scraps and dark mineral impurities before feeding. Install multi-stage magnetic separators ahead of the crushing system to eliminate ferromagnetic impurities at the front end.
• Strictly control raw material moisture: dry coarse ATH at 80–110°C to moisture ≤1.5%. Excess moisture leads to material sticking inside the mill, local overheating and color deterioration.
• Avoid mixing different batches or different grades of ATH to prevent uneven whiteness across finished lots.

2. Eliminate Iron Contamination During Grinding (Most Critical Step)

Metal abrasion inside grinding equipment is the biggest factor causing whiteness decline during dry grinding of ATH, as clearly stated in process cases on ath-mill.com.

Equipment upgrading solutions:

1. Replace steel liners and steel rotors with full ceramic lining (alumina ceramic / zirconia ceramic) for ring roller mills, ACM air classifier mills and ball mills.
2. Choose zirconia grinding beads instead of steel beads if using wet horizontal bead mills.
3. Avoid ordinary carbon steel pipelines, hoppers and screw conveyors; adopt stainless steel 304 / 316 material for all material contact surfaces.
4. Install high-strength permanent magnet / electromagnetic iron removers between feeding hopper and main mill for secondary iron removal.

Practical data: Proper ceramic transformation can lift finished ATH whiteness by 1.5–3.5 degrees compared with traditional steel grinding systems.

3. Strict Low-Temperature Grinding Control to Prevent Thermal Discoloration

Aluminum hydroxide begins to lose crystal water above 130°C, generating boehmite. Partial dehydration will make powder turn slightly yellow and permanently reduce whiteness.

• Equip the ultrafine ring roller mill with a water cooling jacket to keep the material outlet temperature consistently below 110°C.
• For ACM grinding lines: introduce cold dry air circulation inside the grinding chamber to reduce heat accumulation from impact friction.
• Prevent long-time over-grinding: matched closed-circuit air classification avoids particles staying in the grinding chamber for excessive cycles.
• Do not operate the mill under overloaded feeding conditions; overload causes poor airflow and sharp temperature rise.

4. Optimize Closed-Circuit Classification & Particle Size Distribution

Poor particle distribution and hard agglomeration visually lower powder brightness:

• Deploy high-precision dynamic turbine air classifiers to remove coarse dark impurity particles.
• Maintain reasonable classifier rotating speed to achieve narrow particle size distribution (PSD). Uniform fine powder shows better whiteness performance than powder mixed with oversized grit.
• Prevent ultra-fine ATH secondary agglomeration. Severe agglomerates scatter light unevenly and appear dim. Optional low-dose surface dispersants can improve powder looseness.
• Collect powder through cyclone + pulse baghouse; avoid long-term storage of hot powder inside collectors. Cool powder naturally before packaging.

5. Wet Grinding Whitening Technology for High-End Submicron ATH

For electronic-grade and super-high-whiteness ATH (D50 <2 μm), the wet grinding route provides better whiteness stability:

1. Prepare ATH slurry with deionized water, add environment-friendly water-based dispersants.
2. Adopt fully ceramic horizontal bead mills with zirconia media to achieve zero iron pollution.
3. Complete sufficient washing after wet grinding to remove soluble alkali metal impurities (Na₂O). Residual free alkali easily causes yellowing after drying.
4. Use low-temperature spray drying or flash drying (<110°C). High-temperature drying must be forbidden.
5. Install fluidized bed de-agglomeration equipment after drying to break dried particle clusters.

6. Production Line Management to Avoid Secondary Pollution

Many manufacturers ignore cross-contamination, resulting in unstable whiteness batch by batch:

• Perform thorough line cleaning when switching between common ATH and high-white ATH orders.
• Regularly inspect lining wear status; replace worn ceramic components timely to prevent sudden iron mixing.
• Keep the workshop environment dry and clean; isolate dark-colored mineral raw materials to prevent dust cross-mixing.
• Seal all transfer pipelines to stop external dust from entering the powder circulation system.

7. Optional Post-Processing for Further Whiteness Improvement

For customers pursuing ultra-high whiteness requirements:

• Mild slurry bleaching treatment in wet process (industry-standard inorganic whitening agents) to reduce the influence of trace variable-valence metal ions.
• Moderate surface modification: select high-purity stearic acid or silane coupling agents. Low-purity modifiers will introduce organic impurities and darken powder.
• Online whiteness detection device can be installed after the dust collector to realize real-time process adjustment.

Summary of Recommended Process Route (Dry Grinding High-White ATH)

High-whiteness crude ATH → Low-temperature drying → Multi-stage magnetic iron removal → Quantitative feeding → Water-jacketed ceramic-lined ultrafine ring roller mill + high-precision air classifier closed circuit → Powder collection → Cooling → Inspection → Packaging

Improving ATH whiteness is a systematic project covering raw material screening, anti-pollution equipment configuration, temperature control, classification optimization and site management. According to practical production experience shared by ath-mill.com:

1. For mass production of flame-retardant ATH: ceramic-lined water-cooled ring roller mill closed-circuit system is the most cost-effective solution to stabilize whiteness above 94.
2. For small-batch multi-specification high-white powder: adopt ceramic-type ACM air classifier mill with enhanced iron removal.
3. For submicron super-high-purity ATH: wet zirconia bead mill production line is the first choice.

If iron abrasion and overheating problems are solved fundamentally, manufacturers can achieve stable whiteness index and significantly raise the market grade and selling price of ground aluminum hydroxide powder.