Weakly magnetic impurities inside quartz mainly include hematite, limonite, goethite, biotite, tourmaline, ilmenite, chlorite and fine iron-bearing silicate inclusions. These minerals own low specific magnetic susceptibility, so High Gradient Magnetic Separation (HGMS, wet type is dominant for quartz purification) is the core technology instead of conventional low-intensity magnetic separation.
1. Fundamental Physical Principle
Magnetic force acting on mineral particles follows the formula: Fmag=χ⋅V⋅H⋅∇H
- : Volume magnetic susceptibility; Weak magnetic impurities have far lower than metallic iron/ferromagnetic magnetite; pure quartz is diamagnetic with negative tiny susceptibility and almost no magnetic attraction.
- : Background magnetic field strength; : Magnetic field gradient (key for capturing weak-magnetic grains).
Conventional low-field separators only supply uniform magnetic field with low gradient ∇H. Even raising field intensity cannot generate enough magnetic force to overcome slurry drag force on weak-magnetic particles. HGMS creates ultra-high local field gradient via ferromagnetic matrix (stainless steel wool, corrugated steel mesh) installed inside uniform background magnetic field. Sharp edges of matrix concentrate magnetic field lines, forming strong local magnetic force to trap weakly magnetic mineral grains. Non-magnetic quartz particles are washed away by slurry flow.
2. Core Equipment & Two Application Forms for Quartz
2.1 Wet High Gradient Magnetic Separator (Mainstream for quartz slurry processing)
Most widely arranged after scrubbing and wet grinding in quartz purification flow for 4N/5N raw material pretreatment.
- Structure composition: Electromagnetic coil → uniform background magnetic chamber → magnetic matrix bed (stainless steel wool is preferred for fine quartz below 0.5 mm)
- Working cycle (intermittent operation):
- Feeding stage: Quartz slurry flows horizontally/vertically through matrix bed under fixed background magnetic field; weakly magnetic impurity particles stick onto matrix surface by high-gradient magnetic force; pure quartz slurry passes through as non-magnetic overflow.
- Flushing stage: Cut off magnetic field power; high-pressure ultrapure water backwashes matrix to flush off trapped weak-magnetic impurities as tailing.
- Typical background field for quartz: 12,000–18,000 Gs
2.2 Induced Roll Magnetic Separator (Dry high-gradient, for coarse dry quartz 0.04–2 mm)
Used for dry raw quartz pre-beneficiation before crushing. Toothed magnetic roller generates high gradient on tooth tips; weak-magnetic grains adhere on roller surface and drop separately after leaving magnetic field zone, quartz falls by gravity. Limited application for ultrafine silica powder due to dust agglomeration.
3. Key Adjustable Parameters to Improve Weak-Mineral Removal Efficiency in Quartz Production
| Parameter | Recommended Setting for High-Purity Quartz | Function Description |
|---|---|---|
| Slurry solid content | 20–35 wt% | Overhigh concentration causes particle agglomeration, quartz entraps weak impurities; too low reduces equipment throughput |
| Slurry flow speed | Low linear velocity 0.8–2.5 m/min | Extend particle residence time inside matrix bed to capture micron-sized weak magnetic inclusions |
| Magnetic matrix filling rate | 6–11% steel wool filling volume | Low filling leads to insufficient capture points; overfilling blocks slurry flow |
| Feed particle size | 20 μm ~ 1 mm | Fine slime (<20 μm) easily forms flocculation and carries impurities out; coarse grains keep locked impurity in un-liberated mineral intergrowth |
4. Optimal Layout Position in Full Quartz Purification Process
Three standard staged magnetic separation configuration to remove impurities stepwise:
- 1st stage: Low-intensity magnetic separator (3,000–5,000 Gs) Remove ferromagnetic free iron fragments from crusher/grinder abrasion first, avoid big iron scraps damaging HGMS matrix.
- 2nd stage: Wet HGMS after high-intensity scrubbing Scrubbing strips iron oxide film coated on quartz surface, liberate surface weak-magnetic iron minerals for HGMS capture.
- Optional 3rd stage: Secondary HGMS after wet grinding & before acid leaching Remove newly exposed fine weak-magnetic inclusions released after particle crushing, reduce acid consumption and iron load in subsequent chemical purification.
For 5N ultra-high purity quartz: extra post-leaching secondary HGMS to eliminate tiny iron precipitates generated during acid reaction.
5. Why Ordinary Low-Field Magnet Cannot Remove Weak-Magnetic Impurities
Low-intensity magnetic devices only build homogeneous magnetic field with negligible gradient . Weak-magnetic particles have low susceptibility , calculated magnetic force is smaller than hydrodynamic drag force from water flow, so impurities escape together with quartz slurry. Only high gradient magnifies the product to generate enough capturing force.
6. Auxiliary Process Tips to Upgrade Separation Result
- Pre-heat calcination + water quenching before magnetic separation: thermal shock cracks quartz matrix, unlock encapsulated weak-magnetic mineral inclusions for full liberation.
- Add low-ion inorganic dispersant in slurry: disperse agglomerated fine quartz powder, prevent weak impurity wrapped inside quartz flocs and missed by separation.
