How to Prevent Thermal Decomposition of Aluminum Hydroxide (ATH) During Grinding

Aluminum hydroxide (Al(OH)₃) undergoes endothermic dehydration reaction when temperature exceeds 130°C: crystal water separates out, forming boehmite or alumina.

2Al(OH)3−>Al2O3⋅H2O+2H2O↑(>130∘C)

Thermal decomposition brings severe quality defects:

• Loss of flame retardant property (core function of ATH disappears)
• Powder turns yellow, whiteness drops obviously
• Oil absorption rises sharply, poor compatibility with plastic & rubber
• Particle surface morphology changes, poor filling performance

Heat inside grinding systems mainly comes from: friction & impact between material and grinding parts, particle collision, compressed air heat, insufficient airflow heat removal. Below is a systematic set of industrial control measures for dry and wet grinding lines.

1. Clear Temperature Control Target (Core Index)

Stable material outlet temperature ≤ 110°C, leave a safety margin below the critical decomposition temperature of 130°C.
For high-end ultrafine ATH, control temperature below 95°C for long-term continuous operation.

2. Equipment Cooling Modification (Dry Grinding System – Most Widely Used)

2.1 Water jacket cooling for main mill

Applicable for ring roller mill, ACM air classifier mill

• Install closed circulating water jacket on grinding chamber housing. Use cooling tower circulating water to continuously take away friction heat.
• Monitor cooling water inlet & outlet temperature; ensure water flow is sufficient. Avoid scale inside the jacket which reduces heat exchange efficiency.

Note: Do not spray water directly into the grinding chamber; it will cause material agglomeration and blockage.

2.2 Optimize ventilation & cold dry air circulation

All closed-circuit dry grinding systems rely on airflow to carry heat away:

1. Match reasonable induced draft fan air volume. Insufficient airflow leads to heat accumulation inside the mill.
2. Introduce low-temperature dry outdoor air into the system; avoid recycling hot exhaust air in a fully closed loop.
3. Install air heat exchanger if working in high-temperature summer environment to cool inlet air.
4. Regularly clean pipeline and classifier blade dust buildup; dust deposits hinder heat transfer.

2.3 Avoid over-grinding & excessive energy input

Long retention time inside grinding chamber accumulates heat continuously:

• Set proper dynamic classifier rotating speed, form efficient closed-circuit classification. Discharge qualified fine powder quickly, prevent over-circulation of particles.
• Do not blindly pursue ultra-fine powder by raising grinding pressure/rotor speed; higher mechanical force generates extra friction heat.

3. Standardized Production Operation Parameters

3.1 Stable and uniform feeding rate

• Overfeeding: material layer thickens, airflow cannot penetrate, poor heat dissipation, local hot spots appear.
• Underfeeding: direct collision between grinding roller/ring without material buffer, sharp temperature surge.
  Use quantitative screw feeder to keep constant feeding capacity.

3. Strictly control raw material moisture

Raw ATH moisture should be controlled at 0.5% ~1.5% after low-temperature pre-drying (drying temperature ≤110°C).

• Too high moisture: material sticks on inner wall of mill, forms material crust. The crust has extremely poor heat dissipation and creates persistent hot zones.
• Pre-drying temperature must never exceed 120°C to avoid pre-decomposition before grinding.

3. Continuous temperature online monitoring

Install temperature sensors at: mill outlet, classifier housing, powder collection pipeline.
Set interlock alarm: when material temperature exceeds 115°C, trigger alarm; automatically reduce feeding speed for emergency cooling.

4. Process Route Advantages: Wet Grinding Inherently Reduces Thermal Risks

If ultra-fine submicron ATH is required, wet bead grinding is naturally friendlier to ATH:

• Water medium has high specific heat capacity; friction heat is quickly absorbed by slurry. Slurry temperature rarely exceeds 60°C under normal operation.
  Key control points for wet route:

1. Avoid long-time slurry heating during subsequent drying; adopt low-temperature spray drying / flash drying (<110°C).
2. Prevent local overheating on the dryer wall from causing partial dehydration of ATH filter cake.

5. Common Wrong Operations That Accelerate ATH Thermal Decomposition

1. Running the mill without load (no material) for a long time during startup or shutdown
2. Recirculating all hot exhaust air without fresh cold air supplement
3. Ignoring scale blockage of mill water cooling jacket
4. Improper pre-drying with temperature over 130°C
5. Continuously increasing grinding intensity to pursue finer powder without upgrading cooling system
6. Serious material adhesion and caking inside the grinding chamber without timely cleaning

6. Comprehensive Standard Process Flow to Avoid Thermal Decomposition (Dry Line)

Coarse ATH raw material → Low-temperature drying (<110°C) → Magnetic iron removal → Constant-rate feeding → Water-jacketed cooled ultrafine mill + high-speed air classifier (cold air supplementary ventilation)
→ Hot powder conveyed to cyclone and bag collector → Natural cooling silo → Inspection & packaging

Conclusion Summary

Preventing thermal decomposition of ATH during grinding relies on three core principles:

1. Take heat away timely: water jacket cooling + sufficient cold ventilation
2. Control heat generation: stable feeding, avoid over-grinding and no-load operation
3. Eliminate hot spots: control raw material moisture, regularly clean inner wall adhesion

For most large-scale production lines producing D50 3–15 μm flame retardant ATH, the water-cooled closed-circuit ring roller mill system is the most mature solution to keep material temperature stable below 110°C and fully protect the complete crystal structure of aluminum hydroxide.