How To Control The Iron Content Of Graphite Anode Below 50ppm

Iron contamination in graphite anode mainly originates from grinding, spheroidization, air classification, powder conveying and equipment wear. Excess iron will induce self-discharge, micro-short circuit and capacity attenuation of lithium-ion cells. Controlling total iron content below 50 ppm is a basic quality requirement for commercial graphite anode. This article introduces raw material control, multi-stage magnetic separation, equipment anti-contamination design, process parameter tuning and inspection verification system for stable iron control under 50 ppm, applicable for both artificial graphite and spheroidized natural graphite.

1. Source Control: Prevent Iron Introduction At The Source

Source prevention is more critical than post removal. Most iron impurities come from equipment abrasion.

1. Raw material incoming inspection
   Check iron content of raw coke or natural graphite ore before processing. Reject feedstock with initial iron >30 ppm. Use ICP test for each incoming batch.
2. Wear protection for all powder contact parts
   All equipment in contact with graphite powder must avoid bare carbon steel:

• Spheroidizing mill, air classifier, pipeline, cyclone, hopper: adopt polyurethane lining, high alumina ceramic lining or silicon carbide lining.
• Rotor, classifying wheel, feeding screw: use wear-resistant non-ferrous alloy or ceramic coating.
• Regular thickness inspection of lining. Replace lining before wear-through to prevent metal substrate abrasion.

3. Avoid foreign iron mixing
   Install simple permanent magnet traps at raw material feeding inlet to block large iron fragments from packaging, conveyor and maintenance debris.

2. Multi-stage Dry Magnetic Separation Process Layout

Adopt series magnetic separation configuration to gradually reduce iron, instead of single one-time treatment.
Recommended sequence:
Raw material crushing/spheroidization → Primary permanent magnetic separator → Air classification → Secondary electromagnetic magnetic separator → Batch homogenization → Final high-gradient magnetic separator → Surface modification / carbon coating

Stage 1: Primary rough magnetic separation

Permanent magnet (300–600 mT). Function: capture large iron chips generated during spheroidization. Prevent coarse metal scraps entering air classifier and damaging rotor.

Stage 2: Secondary electromagnetic magnetic separation

Magnetic field 0.8–1.2 T. Remove medium-sized ferromagnetic wear debris produced in classification and pneumatic conveying. This stage undertakes the main iron removal task.

Stage3: Final high-gradient magnetic separation (HGMS)

Magnetic field 1.0–1.5 T. Capture fine weakly magnetic iron particles below 1 μm. This step acts as quality insurance to guarantee final iron below 50 ppm.

3. Key Operating Parameters For Magnetic Separation

1. Powder dispersion
   Fully deagglomerate graphite before entering magnetic separator. Graphite agglomerates can wrap fine iron particles inside, making magnetic capture impossible. Use vibration feeder and dilute phase conveying to ensure single particle exposure to magnetic field.
2. Moisture control
   Material moisture ≤0.2%. High moisture causes graphite adhesion on magnetic matrix, reduces separation efficiency and increases cleaning frequency.
3. Feed rate
   Do not overfeed. Lower throughput extends particle residence time in magnetic field. For high iron incoming material, reduce feeding speed properly.
4. Magnetic matrix cleaning cycle
   Set automatic purge cycle. When magnetic medium is saturated, stop feeding, demagnetize, then use nitrogen blowing + vibration to flush captured iron impurities. If cleaning interval is too long, iron will drop back into graphite powder.
5. Closed nitrogen environment
   Whole magnetic separation system runs under nitrogen closed loop. Static grounding for all units to eliminate static agglomeration.

4. Auxiliary Purification Measures (Backup When Magnetic Separation Is Insufficient)

Magnetic separation cannot remove non-magnetic iron compounds. If iron still exceeds 50 ppm after multi-stage magnetic separation:

• Wet acid leaching: dilute hydrochloric or sulfuric acid washing to dissolve iron oxide impurities, followed by filtering, washing and drying. Suitable for raw materials with high embedded iron.
• High-temperature thermal purification: heat treatment above 2400 ℃ for graphitized artificial graphite, volatilize or decompose part of iron impurities.

Note: Acid leaching will introduce wastewater, increase production cost and may affect powder morphology and sphericity, so it is only used as supplementary option.

5. Online And Offline Quality Monitoring

1. Sampling points: raw material, after primary magnet separation, after final high-gradient magnetic separation.
2. Test method: ICP-OES to measure total iron element content. Magnetic tester to detect magnetic impurity content.
3. Set alarm threshold: set alert at 40 ppm. Once iron exceeds 40 ppm, adjust feed rate or increase magnetic field strength before product exceeds 50 ppm.
4. Batch homogenization: homogenize qualified powder after magnetic separation to avoid local high iron spots caused by uneven impurity distribution.

6. Common Abnormalities & Troubleshooting

7. Production Operation Guidelines

1. Do regular wear inspection for all powder contact equipment on daily/weekly basis.
2. One-factor adjustment: adjust only one parameter each time (feed rate or magnetic intensity), wait for stable sampling result.
3. Maintain proper magnetic field strength. Excessively high magnetic field will trap graphite powder and reduce product yield.
4. Isolate collected iron waste to prevent back-mixing into qualified graphite.

Graphite ultrafine powder is combustible. Maintain nitrogen closed circulation, oxygen monitoring, static grounding and explosion relief panel for magnetic separation line.