How to Control the Top Cut (D97) of Kaolin Powder

D97 top cut means 97% of kaolin particles are smaller than the target micron value, and only 3% exceed this size. Strict D97 control eliminates coarse grit that causes scratches, speckles and poor gloss in coatings, paper and plastics. The top cut is mainly regulated by adjusting the air classifier, grinding circuit parameters and raw material conditions, with systematic control methods as below.

1. Adjust the rotating speed of the classifier wheel (Most Direct Control Factor)

The classification rotor generates centrifugal force to block coarse particles, which directly determines D97 value.

• Increase rotor speed: Higher centrifugal force intercepts larger particles. The D97 becomes smaller, finer top cut, less coarse residue.
• Reduce rotor speed: Weaker centrifugal force allows bigger particles to pass through the wheel, D97 rises (coarser top cut).
  Operation tip: Fine-tune frequency converter by 5–10 Hz each time, test particle size after stabilization to avoid over-adjustment.

2. Regulate secondary classification airflow

Air drag force carries fine powder through the wheel gap, balancing centrifugal force of the rotor.

• Raise secondary air volume: Stronger airflow drags relatively large particles through the classifier, D97 increases.
• Lower secondary air volume: Weak airflow can only carry tiny fines, coarse grains are rebounded back to mill, D97 decreases.
  Note: Keep primary grinding airflow stable; only adjust secondary air for D97 trimming.

3. Optimize system negative pressure balance

Unstable negative pressure creates turbulent airflow, disturbing particle separation and drifting D97 index.

• Maintain slight stable negative pressure inside classifier and mill chamber;
• Clear blocked dust filter bags regularly to prevent air resistance rise, which causes abnormal D97 fluctuation.

4. Control the feed rate of raw kaolin

Excessive feeding overloads the classifier, the wheel cannot fully intercept coarse particles, leading to higher D97.

• High feed volume: More mixed coarse aggregates pass through → D97 becomes larger.
• Low, steady quantitative feeding: Sufficient sorting time for each particle → tighter D97 top cut.
  Use screw quantitative feeder to keep constant feeding tonnage for stable D97.

5. Adjust grinding intensity to reduce oversized agglomerates

If many large unground kaolin lumps enter the classifier, the circulation load surges and D97 drifts high.

• For ceramic-lined ball mill: Adjust filling rate of ceramic beads, prolong grinding residence time to break coarse agglomerates.
• For vertical ultrafine mill: Raise grinding roller pressure to fully crush raw ore before classification.
  Fewer pre-existing coarse particles greatly reduce the burden on the classifier and stabilize D97.

6. Control circulating load of closed grinding circuit

Circulating load = mass of returned coarse material / mass of finished fine powder.

• High circulating load: Too many coarse particles repeatedly enter the classifier, partial coarse grains escape, D97 widens.
• Moderate circulating load (standard range 120%–200% for kaolin): Balanced sorting efficiency to lock target D97.
  If circulating load is too high, slightly cut feed rate or lift classifier speed.

7. Raw material pretreatment to remove oversized impurities

Raw kaolin with large hard quartz, feldspar blocks will push up D97 permanently:

1. Pre-crush raw ore below 8–10 mm before grinding;
2. Remove coarse gravel impurities via pre-screening and magnetic separation;
3. Homogenize raw material moisture (2%–5%) to eliminate hard dry agglomerates.
   Uniform feed particle size avoids sudden D97 jump during production.

8. Online particle size monitoring for real-time closed-loop correction

Install laser particle size analyzer at finished powder outlet to track D97 continuously:

• When D97 exceeds target value: Boost classifier rotor speed or decrease secondary air flow;
• When D97 is too small (over-fine, high oil absorption): Reduce rotor speed or increase secondary air.
  Automatic linkage between particle size tester and classifier frequency motor realizes unmanned stable D97 control.

Practical Operation Example

Target: D97 ≤ 2 μm cosmetic grade kaolin

1. Set classifier wheel to high frequency (high speed);
2. Reduce secondary classification air volume moderately;
3. Adopt low steady feeding speed;
4. Strengthen grinding pressure to break all kaolin aggregates;
5. Maintain stable negative pressure and clean dust collectors.

The primary lever to adjust kaolin D97 top cut is classifier rotor speed, assisted by secondary airflow, feed rate and circuit circulating load. Combined with raw material pretreatment and online particle size feedback, manufacturers can precisely lock the D97 index, completely eliminate oversize coarse particles and produce high-standard ultrafine kaolin for high-end industrial uses.