How to Measure the Moisture Content of ATH Powder in Real‑Time During Production

Real‑time moisture monitoring of ATH (aluminum hydroxide) powder is critical during ultrafine grinding, classification and surface modification. Excess free moisture causes powder agglomeration, poor dispersion in polymers, bubbles in sealants/adhesives and accelerates ATH surface hydroxyl bonding. Importantly, the measurement must only detect surface free moisture, not crystal water inside ATH crystals, which is released above ~150 °C and should not be counted as moisture. This article references ATH processing technology from ath‑mill.com and JACAN powder engineering.

1. Core measurement principle consideration for ATH

Standard loss‑on‑drying (LOD) lab testing heats samples to 105 °C. This temperature safely removes free surface moisture without driving off crystal water, and acts as the reference benchmark.
For online real‑time sensors: the instrument must distinguish free surface moisture from inherent crystal water. Avoid high‑temperature probe styles that trigger partial dehydration and produce false high readings.

2. Main real‑time online measurement technologies for ATH powder

(1) Near‑Infrared (NIR) Spectroscopy — Preferred for ATH continuous production

NIR sensors mount on pneumatic conveying lines, screw chutes or at the outlet of the drying/modification reactor.

• Principle: Water has characteristic near‑infrared absorption peaks. The sensor scans powder surface and calculates free moisture continuously.
• Advantages: Non‑contact, fast response (milliseconds), continuous real‑time data; can integrate with PLC for automatic feedback control of dryer temperature or feed rate. Works well for fine, flowing ATH powder.
• Limitations: Sensitive to powder packing density, layer thickness and dust. Requires regular calibration against offline LOD reference samples. Changes in ATH particle size or BET will shift calibration curves.
• Typical application point: After de‑agglomeration, before powder packaging.

(2) Microwave Moisture Sensor

• Principle: Microwave signal attenuation correlates with water content in bulk powder. Measures bulk moisture through the whole powder stream rather than only the surface layer.
• Advantages: Less affected by powder color than NIR; suitable for high‑flow dense powder streams. Can be installed on screw conveyors or chute walls.
• Limitations: Sensitive to bulk density variations. ATH’s high mineral dielectric constant requires dedicated calibration for ATH grades.

(3) Capacitance / RF Moisture Probes

• Principle: Water changes the dielectric constant of the powder bed. Probes inserted into the powder stream output a moisture signal.
• Advantages: Low equipment cost, simple installation.
• Limitations: Highly sensitive to powder compaction, particle size and conductivity changes. Less accurate for ultrafine ATH with variable flow. Best used as trend indicator rather than precision measurement.

(4) In‑line gravimetric loss‑on‑drying (automatic continuous sampling system)

Automated sampler extracts small powder aliquots at fixed intervals, automatically dries at 105 °C and calculates weight loss.

• Advantages: Directly matches standard laboratory LOD method, very reliable for ATH.
• Limitations: Not truly instantaneous; cycle time of tens of seconds to several minutes. Higher maintenance cost. Suitable for critical quality control points before packaging.

3. Recommended installation positions in ATH production line

1. After flash drying / de‑agglomeration unit: main control point to adjust drying air temperature and air flow.
2. Outlet of surface modification reactor: verify moisture rise after coating process.
3. Final product conveying line before packaging: final quality interlock; auto‑trigger alarm or divert off‑spec powder if moisture exceeds limit (normally >0.3–0.5%).

4. Critical operational rules for ATH online moisture measurement

1. Calibration against lab LOD (105 °C):
   All online sensors must be calibrated with ATH samples of known free moisture. Never calibrate using water‑spiked samples alone; use actual production ATH powder.
2. Avoid overheating the sensor:
   Sensor contact temperature must stay far below 150 °C. Any probe heating above this threshold will release ATH crystal water and cause completely wrong readings.
3. Control powder flow and dust:
   Dust buildup on optical windows (NIR) will drift readings. Install air purge to keep lens clean. Maintain stable powder bed thickness over the sensor window.
4. Temperature compensation:
   Process air temperature fluctuations influence both NIR and microwave readings. Enable built‑in temperature compensation for the sensor.
5. Periodic cross‑check:
   Take grab samples hourly for offline LOD testing to validate online readings and recalibrate when PSD or BET of ATH product grade changes.

5. Typical control threshold

For flame‑retardant ATH for rubber, sealants, LSZH cables and coatings: target free moisture ≤0.3%, alarm trigger at 0.4%, automatic reject at >0.5%.

6. Common pitfalls

1. Confusing crystal water with free moisture: High temperature gravimetric sensors set above 120 °C will start decomposing ATH and report falsely high moisture.
2. Poor calibration after product grade switch: Changing ATH fineness or BET changes powder packing and surface area, invalidating old calibration curves.
3. Ignoring dust fouling on optical sensors: gradual drift leads to undetected high‑moisture batches.

For real‑time ATH moisture measurement in production, NIR is the most versatile non‑contact choice for continuous monitoring, while microwave sensors are suitable for bulk flow measurement. Automatic in‑line LOD samplers deliver high‑accuracy reference verification. All sensors must be calibrated against standard 105 °C lab loss‑on‑drying and must avoid temperatures that release ATH crystal water. Combined with PLC closed‑loop control, real‑time moisture monitoring stabilizes powder quality, reduces agglomeration and eliminates high‑moisture off‑spec ATH entering polymer formulation customers.