Narrow particle size distribution (PSD) is a critical quality indicator for high-end kaolin products, such as paper coating, plastic filler, ceramic raw material and functional mineral powder. A narrow PSD means fewer oversized coarse particles and fewer ultra-fine sub-fines, delivering consistent whiteness, gloss, viscosity and packing density. Achieving tight PSD relies on raw material pretreatment, matched milling technology, high-precision turbo air classification, system airflow control and post-process stabilization.
1. Raw material pretreatment to eliminate pre-existing agglomerates
Raw kaolin often contains soft agglomerates, coarse gangue and variable moisture, which are the root causes of wide PSD.
- Remove coarse impurities and gangue via screening and magnetic separation before grinding. Hard impurity particles cannot be ground uniformly and will create oversized tails.
- Control feed moisture strictly. High moisture causes kaolin to stick together and form agglomerates; these clumps will break randomly during milling and produce scattered ultra-fine fractions.
- Pre-disperse sticky kaolin clusters. Dry pre-dispersion breaks up soft agglomerates before entering the mill, avoiding uneven particle fracture.
2. Select suitable ultrafine grinding equipment and stable milling conditions
The grinding machine defines the basic particle fracture mode, which directly affects PSD width.
- Choose low-impact, uniform grinding equipment with ceramic lining. Traditional ball mills tend to produce a broad PSD with both over-ground nano-fines and under-ground coarse particles. JACAN’s kaolin mill realizes selective particle fracture, minimizing over-grinding.
- Maintain constant and stable feeding rate. Fluctuating feed volume leads to variable material retention time inside the mill. Under-fed material gets over-ground; over-fed material leaves uncrushed coarse particles.
- Keep consistent grinding medium condition. Replace worn ceramic media and liners periodically. Worn components cause irregular particle impact and broaden PSD.
3. Precision turbo air classification is the core for narrow PSD
Classification is the key step to cut off oversized particles and control the particle size window.
- Use high-precision turbo air classifier with adjustable rotor speed and secondary air. Rotor speed sets the cut point, while secondary air washes agglomerated kaolin, stripping fine particles attached to coarse surfaces and preventing misclassification.
- Fine-tune the matching between rotor speed and system fan airflow. Mismatched parameters either let coarse particles leak into finished powder or waste qualified fine particles into return material.
- Control solid-gas ratio in the classification chamber. Too high powder concentration increases inter-particle collision, re-agglomeration and misclassification, destroying PSD sharpness.
- Run multi-stage classification if ultra-narrow PSD is required. Two-stage turbo classification can sharply reduce oversized residues and remove excess submicron fines for high-grade kaolin.
4. Optimize closed-loop system airflow and material circulation
Kaolin ultrafine lines work in closed-loop grinding-classification circulation. Poor airflow stability widens PSD.
- Stabilize negative pressure of the whole system. Pressure fluctuation changes particle drag force, shifting the actual cut size continuously.
- Reduce dead zones and powder deposition inside pipes and classifier chamber. Built-up powder can suddenly fall into the airflow and create unexpected coarse spikes in PSD.
- Optimize return material ratio. Too high circulation load leads to repeated over-grinding; too low circulation leaves residual coarse particles.
5. Post-processing and online monitoring
- Avoid secondary agglomeration after classification. Surface modification should be controlled at proper temperature and dosage. Improper modification may cause re-agglomeration and distort PSD test results.
- Install online particle size monitoring. Continuously track D50, D97 and span value. Adjust rotor speed or feed rate in real time once PSD drifts out of target range.
To obtain narrow PSD kaolin, the priority sequence is: qualified raw material pretreatment → uniform and controlled grinding → high-precision turbo air classification with optimized secondary air → stable closed-loop airflow and circulation. The span of PSD can be greatly reduced by combining JACAN’s ceramic-lined mill and turbo classifier. Multi-stage classification is adopted when extremely sharp particle size cut is required for high-end kaolin applications.
