Natural flake graphite has a sheet-like crystal structure and good intrinsic conductivity, but raw flake ore and spheroidized spherical graphite contain ferromagnetic iron impurities. Iron contaminants mainly originate from mine gangue, crushing/spheroidizing mill lining wear, classification wheel and pneumatic conveying pipelines. If iron residue is high, lithium-ion cells suffer self-discharge, micro-short circuits and accelerated capacity decay. Magnetic separation is the core dry purification step to reduce iron content for natural flake graphite anode. This article covers material characteristics, process layout, equipment selection, parameter control, quality inspection and troubleshooting for flake graphite iron removal, before and after spheroidization.
1. Special Characteristics Of Natural Flake Graphite For Magnetic Separation
- Flaky morphology: Flat flakes easily overlap and wrap fine iron mineral inclusions or metal wear debris. Agglomerated stacked flakes shield iron particles and reduce capture efficiency.
- Raw ore contains magnetic gangue: Unlike artificial graphite precursor coke, natural flake graphite may have native magnetite, pyrrhotite and weakly magnetic iron silicate embedded inside flakes. These embedded iron compounds are harder to remove than free metal wear debris.
- Good lubricity: Graphite flakes are slippery, easy to slide through magnetic matrix gaps; poor dispersion leads to short residence time in magnetic field.
- Static agglomeration: Thin flake particles build static charge during conveying, forming flake clusters that trap iron impurities inside.
2. Recommended Multi-stage Process Sequence
Two typical application positions:
- Pre-spheroidization magnetic separation (raw flake graphite)
Raw flake graphite → coarse crushing → primary permanent magnetic separation → fine grinding → spheroidization mill
Purpose: Remove large iron fragments from ore and crushing equipment before shaping, protect spheroidization rotor. - Post-spheroidization magnetic separation (spheroidized natural graphite anode, main iron removal stage)
Spheroidization mill → primary magnetic separation → air classification → secondary electromagnetic separator → homogenization → high-gradient magnetic separation → carbon coating
This three-stage series layout is standard for battery-grade spherical natural graphite.
3. Magnetic Separator Equipment Selection
3.1 Permanent magnetic separator (1st stage rough removal)
Magnetic field strength: 300–600 mT
- Function: Capture large free iron scraps, steel fragments and coarse magnetite particles.
- Advantage: Low energy consumption, simple structure.
- Limitation: Cannot capture fine weakly magnetic iron and submicron iron wear debris.
3.2 Electromagnetic dry magnetic separator (2nd stage intermediate purification)
Magnetic field: 0.8–1.2 T
- Function: Remove medium-sized ferromagnetic wear impurities generated during spheroidization and air classification.
- Advantage: Adjustable magnetic intensity, suitable for fluctuating incoming iron content.
3.3 High-gradient magnetic separator (HGMS, final polishing stage for high-end anode)
Magnetic field: 1.0–1.8 T, fine stainless steel wool magnetic matrix
- Function: Trap submicron weakly magnetic iron oxides and fine alloy wear particles. Critical to hit ppm-level iron specification for EV-grade natural spherical graphite.
- Note: For natural flake graphite, magnetic matrix must adopt thicker media compared with artificial graphite, to avoid flake bridging and blockage.
4. Critical Pre-treatment Before Magnetic Separation
These steps are more important for flake graphite than granular calcined coke:
- Full deagglomeration & dispersion
Use vibration feeder and dilute-phase nitrogen conveying to spread stacked graphite flakes into single particles. Overlapped flakes hide iron impurities and greatly reduce iron removal efficiency. - Moisture control ≤0.2%
High moisture makes flakes stick together, cling to magnetic matrix and cause chamber clogging. Dry raw powder if moisture exceeds limit. - Static elimination
Install static eliminator on feeding pipeline. Static charge causes flake graphite clustering. All equipment must be fully grounded. - Control feed temperature
Keep powder temperature below 50 ℃ to reduce static generation.
5. Key Operating Parameters Tuning
- Magnetic field strength
- Rough removal: 0.3–0.6 T
- Intermediate purification: 0.8–1.2 T
- Final HGMS polishing: 1.0–1.8 T
Avoid over-high magnetic field: natural flake graphite can be adsorbed onto magnetic matrix, causing material loss and blocking media gaps.
- Feed rate & residence time
Flaky particles flow faster than spherical graphite. Reduce feed speed moderately to extend residence time inside magnetic zone. Too high throughput leads iron particles escape without capture. - Magnetic matrix cleaning cycle
Natural graphite flakes tend to bridge and deposit on matrix. Set automatic periodic purge: cut excitation current, use nitrogen blast + mechanical vibration to flush captured iron impurities and stuck graphite flakes. Cleaning interval shorter than artificial graphite line. - Inert gas environment
Whole magnetic separation loop runs under closed nitrogen protection. Control oxygen concentration <0.5% to prevent graphite dust explosion and oxidation.
6. Quality Control & Testing
- Standard battery natural graphite: total iron ≤50 ppm
- High-end spherical natural graphite for EV: total iron ≤10 ppm
- Inspection methods: ICP-OES for total elemental iron; magnetic impurity tester for ferromagnetic residue.
- Sampling points: inlet of primary magnet separator, outlet of final HGMS unit.
- Alarm threshold: trigger adjustment when iron reaches 80% of upper specification limit.
Important limitation: Magnetic separation only removes free ferromagnetic and weakly magnetic iron minerals. Iron embedded inside graphite crystal flakes cannot be removed by dry magnetic separation. For such embedded iron, wet acid leaching is required as supplementary purification.
7. Common Problems & Troubleshooting
| Problem | Root Cause | Solution |
|---|---|---|
| Residual iron out of specification | Flake stacking wraps iron; feed rate too fast; matrix blocked | Enhance pre-dispersion; reduce feed throughput; shorten cleaning cycle |
| Graphite loss on magnetic matrix | Magnetic field too high; static adhesion | Lower magnetic intensity; install static eliminator |
| Magnetic matrix bridging & clogging | Flake agglomeration, high moisture | Reduce powder moisture; optimize vibration dispersion; increase nitrogen purge |
| Batch iron instability | Ore source fluctuation, unstable feeding | Strict incoming ore inspection; adopt loss-in-weight feeding |
| New iron contamination | Unlined carbon steel chamber | Replace powder contact parts with PU or ceramic lining |
8. Safety & Operational Notes
- Natural flake graphite fine powder is combustible dust. Static grounding, oxygen online monitoring and explosion vent panels are mandatory.
- Check magnetic matrix clogging every shift. Flake bridging will change magnetic field distribution and degrade iron removal performance.
- Separated iron waste must be sealed and isolated, prevent back-mixing into qualified graphite product.
- After magnetic separation, send powder directly to secondary drying and homogenization under nitrogen atmosphere, avoid moisture absorption.
