Kaolin Iron Removal Process and Technical Workflow

Kaolin, a critical industrial mineral, requires high whiteness for premium applications, with iron impurities being the primary cause of discoloration. Depending on iron occurrence forms, current methods include physical, chemical, microbial, and combined processes. Key workflows are detailed below:

I. Physical Iron Removal

Magnetic Separation

Workflow:

Pretreatment: Crush and grind raw kaolin to<45μm.

Separation:

Strongly magnetic minerals (e.g., magnetite): Direct separation via conventional magnetic separators.

Weakly magnetic minerals (e.g., limonite): Roast at 600–800°C to convert to Fe₃O₄, then separate using high-gradient magnetic separators (>2T).

Dehydration: Concentrate, filter, and dry the kaolin concentrate.

Features: Suitable for magnetic minerals; high equipment cost for weak magnetism.

Flotation

Workflow:

Slurry Preparation: Adjust pulp density to 40–60%.

Surface Activation: Add calcium ions and scrub under high-speed agitation.

Carrier Flotation: Introduce lime to adsorb Fe₂O₃, then use collectors (e.g., tall oil) to float iron-loaded carriers.

Features: Effective for titanium impurities but requires pH control (8–11).

Selective Flocculation

Workflow:

Dispersion: Adjust pulp pH to 8–11 with Ca²⁺/Mg²⁺ for iron coagulation.

Flocculation: Add weak anionic flocculants (e.g., PAM) to settle kaolin; remove iron-rich supernatant.

Secondary Treatment: Further purify residuals via magnetic separation.

Features: Suitable for ultrafine particles but demands post-dewatering.

II. Chemical Iron Removal

Acid Leaching

Workflow:

Acid Treatment: Heat pulp with oxalic acid (5–10%) at 100°C for 1.5–2h to dissolve surface Fe³⁺.

Filtration: Wash out soluble iron salts.

Features: Ideal for hematite; strong acids may damage kaolinite structure.

Oxidation-Reduction Combined Method

Workflow:

Oxidation: Add H₂O₂/NaClO to oxidize Fe²⁺ (e.g., pyrite) to Fe³⁺.

Reduction: Apply Na₂S₂O₄ or NaBH₄ at pH=2.5–4 to reduce Fe³⁺ to soluble Fe²⁺.

Stabilization: Use EDTA/oxalate to chelate Fe²⁺ and prevent re-oxidation.

Features: Versatile but generates heavy metal wastewater.

Acid-Hydrogen Reduction

Workflow:

Reduction: Add zinc powder to HCl to generate H₂, reducing Fe³⁺ to Fe²⁺.

Filtration: Remove Fe²⁺ via filtration.

Features: Suitable for coal-series kaolin (Fe₂O₃>2.1%).

III. Microbial Iron Removal

Workflow:

Bacterial Cultivation: Inoculate Acidithiobacillus ferrooxidans in Fe²⁺-rich medium.

Bio-Oxidation: Oxidize pyrite to Fe³⁺ and H₂SO₄.

Separation: Centrifuge to remove iron-rich solution.

Features: Eco-friendly but time-consuming (10–15 days).

IV. Combined Processes & Trends

Typical Workflow: Magnetic separation (pre-removal) → Flotation (titanium removal) → Oxidation-reduction (deep purification).

Innovation: Superconducting magnetic separation for weak magnetism; hybrid microbial-chemical methods.

Conclusion

Kaolin iron removal requires tailored processes: physical methods for roughing, chemical methods for refining, and microbial methods for eco-efficiency. Future advancements lie in multi-technology integration to balance efficiency, cost, and sustainability, unlocking high-end applications of kaolin.