Research on Iron Removal Processes in Kaolin Beneficiation

Kaolin, a crucial industrial raw material, is widely used in ceramics, papermaking, and coatings. However, naturally occurring iron and titanium impurities significantly reduce its whiteness and performance. Thus, iron removal is a core step in kaolin purification. Current mainstream methods include physical, chemical, and microbial processes, with innovative technologies like superconducting magnetic separation significantly enhancing efficiency.

1. Physical Iron Removal Processes

1.1 Magnetic Separation

This method exploits the weak magnetism of iron minerals (e.g., hematite, pyrite) in kaolin.

·         Conventional Magnetic Separation: Uses permanent or electromagnetic equipment to remove coarse iron particles but struggles with fine impurities.

·         Superconducting Magnetic Separation: Employs cryogenic superconducting separators (e.g., CGC-300) with magnetic fields exceeding 5.5T. Experiments show that at 5.5T, Fe₂O₃ content drops from 1.30% to 0.56%, whiteness increases from 60.2% to 89.2%, and optimal slurry velocity (1.0cm/s) balances efficiency and concentrate yield (79.72%).

1.2 Flotation

Separates impurities via differences in mineral surface hydrophobicity.

·         Standard Flotation: Activates ore pulp (40%~60% concentration) with calcium ions and removes anatase via high-energy scrubbing, yet shows limited efficacy for iron oxides.

·         Carrier Flotation: Adsorbs Fe₂O₃ onto lime carriers, generating iron-rich foam for targeted removal.

1.3 Coagulation/Flocculation

Adds flocculants (e.g., polyacrylamide) to settle kaolin particles, leaving iron-titanium impurities suspended. Subsequent dewatering is required to eliminate residual chemicals, suitable for low-concentration pulp.

2. Chemical Iron Removal Processes

2.1 Acid Leaching

·         Simple Acid Leaching: Uses sulfuric or hydrochloric acid to dissolve iron oxides (e.g., hematite) into soluble Fe³⁺ but fails for sulfide minerals.

·         Oxidation-Reduction Bleaching: Oxidizes Fe³⁺ to Fe²⁺ with H₂O₂, then stabilizes it with reductants (e.g., sodium dithionite), significantly improving whiteness.

2.2 Oxidation and Reduction

·         Oxidation: Degrades iron mineral structures using strong oxidants, ideal for organically bound iron.

·         Reduction: Converts Fe³⁺ to soluble Fe²⁺ under strict pH control (2~3) to avoid reprecipitation.

3. Microbial Iron Removal Process

Utilizes acid-producing microbes (e.g., Acidithiobacillus ferrooxidans) to dissolve iron minerals. For instance, pyrite is oxidized to sulfuric acid and soluble Fe²⁺, enabling eco-friendly removal. This method is sustainable but time-consuming, requiring optimized microbial activity.

4. Integrated Process Optimization

Combined methods enhance efficiency:

·         Pre-Magnetic + Superconducting Separation: Coarse electromagnetic separation followed by superconducting processing (5.5T) reduces Fe₂O₃ to 0.49% and achieves 93.6% whiteness.

·         Flotation-Chemical Bleaching: Carrier flotation for preliminary removal, coupled with oxidation-reduction for deep purification.

Conclusion

Current iron removal technologies integrate physical, chemical, and biological methods, with superconducting separation and hybrid processes demonstrating high efficiency and cost-effectiveness. Future efforts should focus on low-energy reagents, selective microbial strains, and industrial-scale bioprocessing to maximize kaolin utilization.