Over-grinding is a common issue in kaolin closed-circuit grinding-classification systems. It produces excessive submicron fines, broadens particle size distribution, raises specific energy consumption, and worsens powder agglomeration. The core of over-grinding prevention is to stop qualified fine particles from staying repeatedly inside the mill, and keep only true coarse fractions circulating back for regrinding. Control measures cover classifier tuning, system circulation management, feeding control, equipment selection and process monitoring.
1. Optimize turbo air classifier parameters to release fines promptly
The classifier is the first line of defense against over-grinding. Its job is to extract qualified fine kaolin out of the circuit immediately once formed.
- Set a proper rotor speed matched to target cut size. Excessively high rotor speed rejects too many qualified fines back to the mill, forcing them to be ground again.
- Tune secondary air for deagglomeration. Kaolin soft agglomerates often carry fine particles into coarse return flow. Sufficient secondary air strips attached fines before they return to the mill, avoiding unnecessary regrinding.
- Maintain suitable solid-gas ratio. Overloaded powder concentration inside the classifier causes particle collision and misclassification, sending valid fines back to the grinding chamber.
2. Control circulating load and return material properties
Circulating load is the mass ratio of returned coarse material to finished product. Too high circulating load is the main trigger for over-grinding.
- Keep circulating load within the designed range for kaolin ultrafine milling. Too high circulation means fines are trapped in the loop and repeatedly processed.
- Monitor PSD of return material regularly. If return material contains large amounts of fine particles instead of only genuine coarse fractions, your classifier is misclassifying and will cause over-grinding.
- Avoid sudden spikes in return material flow, which increases material residence time inside the mill.
3. Stabilize feeding rate and raw material quality
Unstable feed creates uneven retention time inside the mill.
- Maintain continuous, constant feeding. Low or intermittent feeding makes material stay longer in the grinding zone, leading to over-grinding.
- Pre-treat raw kaolin: remove coarse gangue and control moisture. High-moisture sticky kaolin agglomerates easily, disrupting classification and increasing recirculation of fine agglomerates.
- Keep consistent raw ore hardness and mineral composition; raw material variation will shift grinding behavior.
4. Adjust mill operating conditions and grinding media
- Control material residence time in the grinding chamber. For JACAN ceramic-lined ultrafine mills, adjust internal flow resistance to avoid particles being trapped for excessive cycles.
- Maintain grinding media size and filling ratio. Too small media or overfilled media increases fine particle fracture and over-grinding tendency. Replace worn media and liners on schedule; worn components produce unpredictable fine fractions.
- Do not run the mill empty. Empty operation creates unnecessary media collision and generates useless ultra-fine dust.
5. System airflow and pressure stabilization
Closed-circuit grinding relies on stable negative pressure and airflow balance.
- Stabilize induced draft fan speed and system negative pressure. Pressure fluctuation changes classifier cut point, causing fines to be periodically returned to the mill.
- Reduce powder accumulation inside pipes and classifier chamber. Sudden collapse of deposited powder causes irregular material surges and unstable grinding cycles.
6. Online monitoring and closed-loop process control
- Track real-time PSD (D50, D97 and span). A rising amount of submicron fines is an early warning sign of over-grinding.
- Monitor power consumption, circulating load and finished product yield. If energy consumption rises while throughput does not increase, over-grinding is likely occurring.
- Build automated parameter linkage: adjust rotor speed, fan airflow and feeding rate together instead of single-point manual tuning.
To prevent over-grinding in kaolin closed-circuit grinding systems, the key principle is: extract qualified fines as soon as they are generated, and only return genuine coarse particles to the mill. Optimizing classifier rotor speed and secondary air, controlling circulating load, stabilizing feed rate and maintaining balanced airflow are essential. Combined with PSD online monitoring, operators can avoid excess submicron fines, reduce energy consumption and maintain narrow PSD of kaolin products.
