Kaolin is one of the most widely‑used non‑metallic minerals, applied in papermaking coatings, ceramics, rubber‑plastic fillers, paints, cosmetics and advanced materials. Raw kaolin ore contains quartz, mica, iron‑titanium oxides and organic impurities, so multi‑stage processing is required to obtain qualified powder products with target fineness, whiteness and particle morphology.
There are two mainstream industrial routes: dry process and wet process. The dry route is simple and low‑cost for general‑grade fillers; the wet route delivers higher purity, better platelet retention and superior whiteness for high‑end coating‑grade kaolin. Ultra‑fine grinding and closed‑loop classification are core sections in both workflows, directly determining final PSD and production efficiency.
1. Dry process flow (for low‑impurity raw ore, filler‑grade kaolin)
Dry processing consumes no water, featuring fewer process units and lower operating cost. It fits raw ore with low sand content and natural high whiteness, mainly producing 325‑2500 mesh filler powder for ceramics, rubber and plastics.
- Raw ore crushing
Mined kaolin lumps go through jaw crusher and fine crusher, reduced to 10‑50 mm feeding size. Gangue and large waste rock are removed by pre‑screening. - Pre‑purification & magnetic separation
High‑gradient magnetic separator removes partial ferromagnetic iron‑bearing impurities. No water is added in this stage. - Drying
Moisture of raw material is reduced to below 2 % by rotary dryer or flash dryer, preventing agglomeration during milling. - Closed‑circuit ultra‑fine grinding & classification (core step)
Dried kaolin is fed into ball mill or vertical roller mill. Integrated air classifier continuously separates qualified fine powder; coarse fractions circulate back to grinding chamber for re‑grinding. Ceramic liners and ceramic grinding media are commonly adopted to avoid metal contamination and preserve kaolin platelet structure. - Surface modification (optional)
For polymer‑filling applications, mechanochemical surface modification is carried out inside the mill system with silane or stearic acid modifiers to improve powder dispersibility in plastic and rubber matrix. - Product collection and packaging
Qualified powder is collected by cyclone separators and bag‑house dust collectors, then screened, tested and packed into finished goods silos.
Limitation of dry process: limited impurity‑removal capacity; hard to eliminate ultra‑fine iron‑titanium impurities, so final brightness cannot reach the level of wet‑processed kaolin.
2. Wet process flow (high‑end coating‑grade kaolin, high‑purity product)
Wet processing delivers excellent delamination effect, high whiteness and narrow particle‑size distribution. It is dominant for paper‑coating, high‑grade ceramic and cosmetic‑grade kaolin, yet consumes large volume of water with higher plant investment.
- Crushing and blunging (slurry preparation)
Crushed kaolin ore is mixed with water and dispersant in high‑speed blunger tank to form homogeneous kaolin slurry, liberating clay platelets from gangue minerals. - Degritting and gravity classification
Vibrating screens and hydrocyclones remove coarse sand, quartz and mica particles from slurry. Large‑size gangue is discharged as waste residue. - Deep purification
- High‑gradient magnetic separation: remove fine paramagnetic iron‑titanium impurities to raise product whiteness.
- Flotation / selective flocculation: separate fine‑grained impurity minerals.
- Chemical bleaching (optional): reduce soluble iron for further brightness improvement for high‑brightness requirements.
- Wet grinding & delamination (key stage)
Ceramic‑lined wet ball mill exfoliates stacked kaolin crystal layers without destroying sheet‑like morphology. Hydrocyclone groups form closed‑circuit loop: coarse slurry returns to mill for re‑grinding while fine slurry flows downstream. - Thickening & dewatering
Purified fine slurry is concentrated in thickener tank. Filter press or vacuum belt filter squeezes water out and produces filter cake with moisture around 25‑35 %. - Drying and depolymerization
Filter cake is dried by spray dryer, flash dryer or rotary dryer. Hard agglomerates formed during drying are broken through secondary dry milling to restore primary particle characteristics. - Calcination (optional for calcined kaolin)
For special‑application calcined kaolin, material is heated in rotary kiln at 900‑1050 °C. Crystal water is removed to gain high‑opacity metakaolin for paint and refractory industries. - Final classification, quality control and packaging
Finished powder passes precision air classification. PSD, whiteness, moisture and impurity index are inspected. Qualified products are packed for delivery.
3. Key controlling parameters in kaolin production
- Platelet protection: Choose proper grinding intensity; avoid over‑grinding which breaks kaolin sheet‑shaped particles and degrades product performance. Ceramic media is strongly recommended for high‑purity lines.
- Closed‑circuit performance: Both dry and wet lines rely heavily on classifier efficiency. Circulating load directly affects yield, energy consumption and final particle‑size distribution.
- Impurity control: Iron and titanium content determines whiteness; magnetic separation, flotation and bleaching should be matched according to raw ore properties.
- Moisture management: Excess moisture causes sticking on grinding media and liners, lowers grinding efficiency and forms hard agglomerates in final powder.
Kaolin powder production selects dry or wet workflow according to raw‑ore conditions and target product specifications. Dry process is economical for general‑purpose fillers. Wet process enables deep purification and good delamination for high‑value coating‑grade kaolin. Grinding‑classification closed‑circuit system is the core of both production flows, which determines product fineness, morphology, energy cost and overall plant capacity. Proper equipment configuration and parameter tuning can maximize kaolin product quality and economic benefits.
