Kaolin drying is a critical step in kaolin powder production. Unstable moisture will affect powder flowability, whiteness, particle dispersion and final product quality. Too high moisture causes agglomeration and caking; over-drying increases energy consumption and may trigger dust problems. Precise moisture control runs through raw material preparation, dryer operation, temperature & air volume adjustment, and real‑time sampling detection.
1. Pre‑treatment: Stabilize inlet kaolin moisture before drying
The foundation of stable outlet moisture is consistent feed moisture.
- For filter cake kaolin after wet beneficiation: Use filter press or vacuum dehydration to reduce initial moisture to a controllable range, normally 30%–38% for kaolin filter cake. Avoid large fluctuations in cake thickness and solid content.
- Homogenize filter cake by stirring and mixing. Mix batches with different water contents to reduce feed deviation. Remove large lumps to prevent uneven drying inside agglomerates.
- Add quantitative feeding equipment such as screw feeder to maintain constant material feeding rate into the dryer. Sudden material surge is one main reason for moisture fluctuation.
2. Adjust dryer operating parameters
Hot air temperature
Hot air temperature directly controls evaporation rate.
- For kaolin ultrafine powder, keep inlet hot air temperature within the recommended range. Excessively high temperature will not only over-dry kaolin but also risk thermal damage to whiteness. Too low temperature leads to incomplete water removal and high residual moisture.
- Control outlet gas temperature as a key reference indicator. Outlet temperature correlates closely with final kaolin moisture. Set target temperature according to required finished moisture and keep it stable.
Air volume and air velocity
- Increase air volume when higher evaporation capacity is needed. Hot air carries water vapor away from kaolin surface.
- Avoid excessive air speed that blows fine kaolin particles into the dust collector prematurely, resulting in incomplete drying. Too low air velocity accumulates water vapor inside the drying chamber and slows evaporation.
- Maintain negative pressure inside the drying system to prevent moisture‑laden hot air leakage and ensure smooth vapor discharge.
Retention time
Control how long kaolin stays inside the drying chamber.
- Adjust rotating speed for rotary dryers or classifier speed for flash dryers to modify residence time.
- If finished moisture is higher than target: extend retention time or raise inlet temperature slightly. If moisture is too low: shorten retention time or reduce hot air temperature.
3. Real‑time moisture monitoring and closed‑loop control
- Install online moisture sensors at the dryer discharge outlet for continuous automatic measurement. The system can feed signals back to adjust feeding speed or hot air heater power.
- Combine offline laboratory sampling: collect kaolin samples at regular intervals and test moisture content by oven drying method to calibrate online sensors and avoid sensor drift.
- Set moisture upper and lower alarm thresholds. Trigger alerts once values go beyond specifications for immediate operator intervention.
4. Post‑drying handling to prevent reabsorption of moisture
Kaolin powder is hygroscopic. Even well‑dried products can pick up moisture from ambient air.
- Cool dried kaolin powder before packaging. Hot powder sealed directly inside bags will condense water vapor inside packages and cause secondary caking.
- Control workshop humidity. Keep low relative humidity in finished powder storage area.
- Use moisture‑proof packaging bags and seal quickly after discharging.
5. Common troubleshooting
- Kaolin moisture unstable: Check feeding uniformity first. Uneven filter cake is the most frequent cause.
- Local over‑wet agglomerates: Break lumps before drying; improve material dispersion inside drying chamber.
- Finished moisture always lower than target: Reduce hot air temperature or increase feed rate properly to cut energy waste.
- Finished moisture always higher than target: Verify whether hot air heating capacity is sufficient or check if exhaust duct is blocked and water vapor cannot escape.
Controlling kaolin moisture in drying process relies on four core links: stable feed pretreatment, matched hot air temperature / air volume / residence time, continuous moisture monitoring, and anti‑moisture absorption after drying. A well‑controlled drying system helps kaolin manufacturers get consistent powder quality, reduce energy cost and avoid caking and whiteness loss, supporting high‑grade kaolin for paper coating, ceramic and filler applications.
