Iron and titanium-bearing minerals are the main impurities that reduce kaolin brightness. Common forms include hematite, goethite, pyrite, ilmenite and rutile. The selection of removal technology depends on the occurrence state of impurities, ore grade and target product brightness. Multiple methods are often combined in kaolin beneficiation circuits.
1. Physical separation methods
High-intensity magnetic separation
Iron oxides and titanium minerals are paramagnetic, while pure kaolin is non-magnetic. High-intensity wet magnetic separators capture fine magnetic impurity particles.
- Suitable for liberated iron oxide minerals such as goethite and hematite.
- Operating parameters: adjust magnetic field strength, slurry concentration and flow velocity.
- Limitation: Poor effect for ultra-fine iron impurities encapsulated inside kaolin particles.
Gravity separation
Gravity separation (including hydrocyclone, spiral classifier) separates minerals based on density difference. Ilmenite and rutile have much higher specific gravity than kaolin.
- Used to remove coarse, dense titanium minerals before fine grinding.
- Often applied in the pre-desliming stage to reduce impurity load for downstream processes.
- Not effective for fine-grained iron and titanium.
Flotation
Selective flotation separates titanium minerals (rutile, ilmenite) from kaolin.
- Add tailored collectors to attach to titanium impurity surfaces and form froth for removal.
- Usually applied for kaolin where titanium is the main brightness-limiting factor.
- Requires strict pH control and reagent dosage optimization.
2. Chemical bleaching (reductive leaching)
Reductive bleaching is widely used for removing fine iron oxides, especially surface-stained iron. It is the key step to boost kaolin brightness for high-end paper coating grades.
- Principle: Under acidic conditions, reducing agents convert insoluble ferric iron into soluble ferrous ions, which are then washed away.
- Common reagent: Sodium dithionite (hydrosulfite).
- Control factors: pH value, reaction temperature, bleaching time, reagent dosage and slurry solid content.
- Limitations: Higher chemical cost; wastewater contains residual reducing agents and dissolved iron that needs treatment. It cannot remove titanium minerals.
Oxidative bleaching
Used when pyrite exists in kaolin. Oxidants oxidize pyrite into soluble iron compounds for removal. Often used before reductive bleaching.
3. Selective leaching / acid washing
Acid leaching dissolves iron oxides. Dilute sulfuric acid or oxalic acid can dissolve iron stains.
- Oxalic acid forms stable soluble iron complexes and has good performance for surface iron.
- Disadvantage: High reagent cost, corrosive to equipment, strict wastewater neutralization required.
4. Calcination
Calcined kaolin is heated to high temperature. This method does not physically remove iron and titanium, but changes their crystal form.
- Iron and titanium oxides develop color at high temperature. If their content is too high, calcination will cause yellowing or reddening.
Note: Calcination is not an impurity removal method; it is only used to produce structured calcined kaolin for special fillers.
5. Combined process flow (typical industrial route)
- Scrubbing and dispersion: break kaolin aggregates to liberate iron and titanium minerals.
- Desliming by hydrocyclones: remove coarse heavy titanium minerals.
- High-intensity magnetic separation: remove liberated iron oxide particles.
- Flotation: further remove residual rutile/ilmenite when titanium is high.
- Reductive bleaching: eliminate fine surface iron to raise brightness.
- Washing and filtration: remove dissolved iron ions and residual chemicals.
Key considerations
- If iron is encapsulated inside kaolin crystals, physical separation and bleaching cannot remove it. Such ore cannot produce high-brightness kaolin.
- Each method has cost trade-offs: physical methods have low chemical consumption, while bleaching delivers the highest brightness improvement but increases operating cost.
- Wastewater from bleaching needs proper treatment to meet environmental discharge standards.
Iron impurities are mainly removed by high-intensity magnetic separation and reductive bleaching. Titanium minerals such as rutile and ilmenite are removed via gravity separation and flotation. Industrial kaolin beneficiation usually combines several physical methods plus reductive bleaching to meet brightness requirements for coating-grade kaolin.
