To push iron content below 100 ppm, a combined physical and chemical purification flow is required. Single magnetic separation or acid leaching alone usually cannot reliably hit this target. The whole workflow includes ore pre-treatment, liberation, multi-stage magnetic separation, flotation, acid leaching, anti-contamination grinding, and final quality control.
1. Raw Ore Selection & Pre-Treatment
Start with low-iron vein quartz as feed ore. Avoid heavily iron-stained quartzite.
- Calcination + water quenching: Heat quartz to 800–950 °C then rapidly quench. Thermal shock creates microcracks, unlocking encapsulated iron-bearing mineral inclusions and greatly improving later impurity removal efficiency.
- Scrubbing and desliming: Remove surface iron coatings, clay slimes and adsorbed fine iron particles. Slime coating is a major reason residual iron remains high.
2. Liberation: Zero-Iron Grinding
Traditional steel grinding media will add iron contamination, making it impossible to reach <100 ppm.
- Use ceramic-lined mills with high-purity alumina or zirconia grinding beads.
- Grind to the target particle size to fully liberate iron mineral intergrowths, but avoid over-grinding which creates ultra-fine slimes.
- Closed-circuit air classification removes fine clay and iron-rich slime fractions.
3. Multi-Stage High-Gradient Magnetic Separation (HGMS)
Quartz is non-magnetic; hematite, limonite, siderite and iron silicate are weakly magnetic.
- Use high-gradient magnetic separators with 1.0–2.0 T magnetic field.
- Apply roughing → cleaning → scavenging three-pass magnetic separation. Thin-layer feeding improves capture of fine weakly magnetic iron-bearing particles.
- This step removes most free iron oxide minerals before chemical treatment, lowering acid consumption.
4. Reverse Flotation
Flotation removes feldspar, mica and iron-bearing silicate minerals that magnetic separation cannot capture. Adjust pulp pH and use selective collectors to float impurity minerals away from quartz. This cuts residual iron locked in silicate phases.
5. Acid Leaching (Critical Step For Final Iron Reduction)
Use mixed acid (oxalic acid + HCl) at 70–90 °C with stirring; ultrasonic assistance can boost leaching efficiency. Oxalic acid is highly selective for iron oxides with minimal attack on quartz matrix.
- Leaching time: 4–8 hours.
- After leaching, repeatedly wash with deionized water until filtrate conductivity is stable, to eliminate dissolved iron ions and residual salt.
Note: Acid leaching can only dissolve surface/exposed iron oxides. It cannot remove iron ions substituted inside quartz crystal lattice.
6. Drying & Post Treatment
- Dry purified quartz powder in clean, non-ferrous equipment to prevent secondary iron contamination.
- If ultra-low iron (<20 ppm) is required, high-temperature chlorination (1100–1300 °C) can volatilize lattice-bound iron impurities, though this process uses corrosive gas and requires strict safety and exhaust treatment.
7. Strict Cross-Contamination Control
All equipment liners, pipelines, hoppers and contact surfaces must be non-ferrous (ceramic, high-purity polyurethane or quartz). Even minor wear from steel parts will introduce iron and break the 100 ppm target.
8. Testing & Quality Assurance
Use ICP-OES or ICP-MS for iron content testing at each stage. Monitor iron after magnetic separation, flotation and acid leaching to adjust parameters.
Common Pitfalls
- Insufficient mineral liberation: iron locked inside quartz grains cannot be accessed by magnet or acid.
- Grinding media contamination: steel balls introduce new iron.
- Poor washing after leaching: precipitated iron hydroxide re-adsorbs onto quartz powder surface.
- Slime coating: fine clay covers iron minerals and blocks magnetic capture.
The proven industrial route to get quartz powder iron below 100 ppm is: calcination-quenching → scrubbing → zero-iron grinding & classification → multi-stage high-gradient magnetic separation → reverse flotation → acid leaching → DI water washing and drying. Physical separation removes bulk iron impurities, and acid leaching eliminates surface iron oxides. For lattice-substituted iron, high-temperature chlorination is needed for deeper purification.
