How to Reduce Iron Contamination in Aluminum Hydroxide Powder

Iron impurities are the primary factor causing yellow discoloration, decreased whiteness, and unstable quality of ground aluminum hydroxide (ATH). Iron sources fall into two major categories:

1. Inherent iron impurities inside raw ATH (iron oxide minerals, soluble iron salts)
2. Wear iron generated by friction between materials and metal equipment components during crushing and grinding

Magnetic separators can remove metallic iron particles, yet they cannot eliminate iron oxide contained in raw ore. Therefore, reducing iron pollution requires combined control of raw materials, equipment upgrading, process configuration and standardized operation.

1. Raw Material Pre-Control: Cut Inherent Iron at the Source

1. Select high-grade crude ATH with low iron content
   Prioritize raw materials with Fe₂O₃ ≤ 0.02% for high-whiteness flame retardant grade. Reject raw materials mixed with dark iron-bearing mineral impurities.
2. Pre-screening and preliminary impurity removal
   Remove scrap iron, stones and foreign sundries before feeding into the crushing system.
3. Control raw material moisture
   Dry raw ATH to moisture ≤1.5% at 80–110°C. Wet material adheres to metal surfaces, aggravating abrasion and rust shedding.

2. Equipment Retrofit: Avoid Wear Iron (The Most Effective Measure for Grinding Lines)

A large proportion of iron contamination for ultrafine ATH comes from mechanical abrasion. This is the core improvement scheme recommended for dry grinding production lines.

Dry Grinding System (Ring Roller Mill / ACM Air Classifier Mill)

1. Adopt full ceramic lining for the grinding cavity
   Replace carbon steel liners, guard plates with high-purity alumina ceramic or zirconia ceramic. Prevent direct contact between ATH and metal components.
2. Upgrade all material contact parts
   Change carbon steel hoppers, chutes, pipelines and screw conveyors to SS304 / SS316 stainless steel to avoid rust flaking.
3. Use ceramic-coated classifier rotors
   High-speed rotating turbine is prone to wear; ceramic coating reduces metal shedding.

Wet Grinding System (Horizontal Bead Mill)

1. Use zirconia grinding beads instead of steel beads. Steel beads will produce massive iron pollution.
2. Select fully ceramic grinding cylinder and ceramic agitator disc to eliminate metal abrasion.

Practical production data: After full ceramic transformation of the grinding host, finished ATH whiteness can increase by 2–4 degrees.

3. Configure Multi-Stage Magnetic Separation Equipment

Magnetic separators capture free metallic iron produced by equipment abrasion. They must be arranged at key positions of the whole production line:

1. Primary iron remover: Installed at raw material feeding inlet to remove large iron fragments.
2. Secondary high-strength magnetic separator: Installed between quantitative feeder and grinding machine to intercept fine iron particles.

• Recommended type: High-gradient permanent magnetic iron remover for dry powder continuous operation.
  ⚠️ Important limitation: Magnetic separation cannot remove iron oxide impurities existing inside ATH crystal particles. It only works for free metallic iron.

4. Optimize Operating Parameters to Minimize Equipment Wear

Improper operation accelerates component abrasion and generates extra iron impurities:

1. Maintain stable and uniform feeding rate
   Underfeeding leads to direct collision between grinding rollers/rings without material buffer, sharply increasing wear. Overfeeding causes uneven material layer and local abrasion. Use constant-speed screw feeders.
2. Avoid long-term no-load operation
   Never run the mill empty during startup, shutdown or standby. No-load friction causes serious metal loss.
3. Maintain low-temperature grinding condition
   High temperature accelerates metal oxidation, corrosion and abrasion. Keep material outlet temperature ≤110°C via water jacket cooling.
4. Regular inspection of wearing parts
   Timely replace cracked, worn ceramic liners. Once the metal base is exposed, a large amount of iron will be mixed into powder quickly.

5. Advanced Purification Technology for High-End Electronic-Grade ATH

For submicron ultra-high-purity ATH adopting wet grinding process:

1. Slurry washing process
   After wet milling, perform solid-liquid washing to dissolve and remove soluble iron ions on the surface of ATH particles.
2. Wet high-gradient magnetic separation
   Install magnetic separators in slurry circulation pipeline to trap fine iron oxide particles suspended in water.
3. Filter and separate impurities before drying, preventing iron impurities from entering finished powder.

6. Avoid Secondary Cross-Contamination in Post-Processing

1. Keep production workshop clean; isolate ferrous raw materials to prevent iron dust drift.
2. Thoroughly clean the whole pipeline and mill cavity when switching from low-grade ATH to high-white ATH production.
3. Use sealed conveying pipelines to stop external dust and rust from entering the system.

Comparative Summary of Improvement Methods

Reducing iron contamination in ATH powder requires layered control:

1. Choose low-iron raw materials as the foundation;
2. Apply ceramic anti-wear equipment to cut abrasion iron (core measure);
3. Deploy multi-stage magnetic separators to capture free iron particles;
4. Standardize operation management to slow component wear and avoid secondary pollution.

For mainstream large-scale dry grinding lines producing flame-retardant ATH, the optimal configuration is: low-iron crude ATH + multi-stage magnetic separator + water-cooled ceramic-lined ultrafine ring roller mill closed-circuit system, which can stably control iron impurities and maintain high whiteness of finished powder.