Grinding time is a key operating parameter controlling kaolin particle size distribution in both wet and dry milling circuits. Within a reasonable range, longer grinding time reduces particle size; however, excessive grinding will not continuously improve fineness and may damage kaolin’s natural lamellar structure while raising energy consumption.
1. Initial stage: Rapid particle size reduction
At the beginning of grinding, large kaolin agglomerates and coarse lumps are broken quickly.
- Particle D50 and D97 drop sharply as grinding time increases.
- Loose kaolin clusters separate into individual platelets. Coarse grits are removed by classification.
- In this phase, extending grinding time is highly efficient for fineness improvement.
2. Intermediate stage: Slower size reduction
After coarse particles are broken down, the grinding rate slows significantly.
- Only small kaolin platelets and minor hard impurities remain. Collisions mainly refine fine fractions rather than breaking large chunks.
- Particle size decreases gradually, and the particle distribution curve becomes narrower with stable PSD.
- This stage is the target time window for most commercial coating and filler-grade kaolin. Operators balance fineness, energy cost and particle morphology.
3. Over-grinding stage: Little fineness gain, obvious side effects
Once reaching the limit of grindability, further extending grinding time brings almost no reduction in D50 or D97.
- Kaolin’s thin plate-like crystals are fractured into small, irregular fragments. The aspect ratio drops.
- New ultra-fine submicron fractions increase, raising specific surface area. These fine particles tend to re-agglomerate.
- Energy consumption rises continuously without quality benefits.
- For high-end coating kaolin, broken platelets reduce gloss, barrier performance and rheological properties.
Difference between wet grinding and dry grinding
- Wet grinding: Water disperses kaolin platelets, so particle breakdown is gentler. The transition to over-grinding takes longer. Wet circuits often use closed-loop classification; qualified fines are separated continuously instead of relying only on static grinding time.
- Dry grinding: Particle collision impact is stronger. Over-grinding occurs faster. Dry milling needs stricter time control to avoid platelet damage and powder agglomeration.
Interaction with other parameters
Grinding time does not work independently. Its effect is affected by:
- Grinding media size and filling ratio
- Slurry solid content (wet grinding) or air flow rate (dry grinding)
- Feed particle size and hard impurities such as quartz
- Mill rotational speed
In industrial closed grinding circuits, operators do not adjust grinding time alone. They combine residence time control with classifier cut point to stabilize PSD, instead of extending grinding time blindly.
Practical production guidance
- Too short grinding time: Large residual coarse particles, high D97, poor coverage performance.
- Proper grinding time: Target D50/D97 achieved, intact lamellar shape, low energy consumption.
- Too long grinding time: No further fineness improvement, broken kaolin platelets, powder agglomeration, higher power cost.
Within a certain range, longer grinding time reduces kaolin particle size. The rate of size reduction slows as particles get finer. Beyond the optimal point, extra grinding time causes over-grinding: kaolin platelets fracture, re-agglomeration appears, energy cost increases, and filler performance deteriorates. Industrial kaolin production uses closed grinding-classification loops to control effective residence time and avoid static over-grinding.
