what is the impurity removal method for graphite anode raw materials

Two raw material systems: natural flake graphite concentrate and artificial graphite (calcined petroleum/needle coke). Hazardous impurities for LIB anodes are categorized as:

1. Silicate gangue: SiO₂, mica, feldspar, quartz (Si impurity, causes cell gas swelling)
2. Metal oxides: Fe, Al, Ca, Mg, Cu, Ni, Mn (trigger electrolyte decomposition, micro short circuits)
3. Magnetic iron debris: Equipment wear iron powder (critical safety risk)
4. Volatile organics, refractory fragments, plastic/wood foreign bodies
5. Residual soluble salts, acid/alkali residues from pretreatment

Industrial impurity removal is divided into physical separation (front-end rough removal), chemical leaching (core deep purification for C>99.9%), ultra-high temperature thermal purification (ultra-high purity finishing), and auxiliary combined processes.

1. Physical Impurity Removal (Upstream Pre-Treatment, Low Cost, Mass Production)

Applied before crushing/milling/purification; removes bulk foreign bodies, magnetic metals, coarse gangue without chemical consumption.

1.1 Flotation (Only for Natural Graphite Ore Raw Ore)

• Principle: Graphite hydrophobicity vs gangue hydrophilicity; collector + frother generates bubbles to float graphite flakes, sink silicate rock
• Standard process: Rough flotation → multiple cleaning flotation
• Parameters: Pulp concentration 15–25%, pH 7–9, collector (diesel), frother (pine oil)
• Purification limit: Raw ore C 5–15% → flotation concentrate C 94–96%
• Advantages: Low cost, large throughput; Limitation: Cannot remove micro-inlaid silicate/metal impurities, cannot reach battery-grade >99.9% carbon

1.2 Multi-Stage High-Gradient Magnetic Separation (Mandatory for All Graphite Lines)

Target impurity: Iron powder, magnetite, steel wear debris (most harmful magnetic impurities)

• Equipment: High-gradient magnetic separator (magnetic field ≥12000 Gs NdFeB permanent magnet)
• Full-process layout:
  1. Before pre-crushing: Remove calcination furnace refractory iron chunks
  2. After crushing/pre-sieving: Eliminate crusher liner wear iron
  3. Post chemical purification, post spheroidization: Final trace iron removal
• Control standard: Total Fe impurity reduced from 200–500 ppm to ≤10 ppm for battery-grade graphite
• Advantages: No chemical pollution, continuous online operation; Limitation: Cannot remove non-magnetic Si, Al, Ca impurities

1.3 Multi-Stage Sieving & Shredding

• Initial coarse sieving (8–16 mesh): Remove bulk rock, refractory brick, plastic, wood lumps
• Ultrasonic fine screening post-grinding: Separate hard fine gangue gravel, agglomerated impurity clusters
• Wet shredder after filter press: Break wet graphite cake to expose wrapped mineral impurities for subsequent leaching

1.4 Closed Air Classification Gravity Separation

Differentiate graphite powder and high-density mineral gangue via airflow density difference; removes fine silicate particles before chemical purification, reduces acid consumption by 20–30%.

2. Chemical Purification Methods (Core Technology to Reach Fixed Carbon ≥99.9%)

Dissolve silicate and metal oxide impurities into liquid phase via acid/alkali reaction, separated by filter pressing and multi-stage countercurrent washing. Two mainstream industrial routes.

2.1 Mixed Acid Leaching (HF+H₂SO₄+HCl, Most Widely Used for Natural Graphite)

Reaction Principle

1. HF dissolves silica/silicate: (SiO_2 + 6HF = H_2SiF_6 + 2H_2O) (soluble fluorosilicate)
2. HCl dissolves Fe₂O₃, Al₂O₃, CaO, MgO into soluble metal chlorides
3. H₂SO₄ strengthens leaching acidity, accelerates reaction kinetics

Standard Process

1. Spheroidized graphite powder (D50 12–22 μm) vacuum dried (moisture <0.5%)
2. Sealed PVDF reactor, liquid-solid ratio 3:1, acid volume ratio H₂SO₄:HCl:HF=3:2:1, 50–70°C stirring 6–10 h
3. Filter press separation, 5–8 stage countercurrent deionized water washing until filtrate pH=6–7, no free fluoride ions
4. Vacuum drying + secondary magnetic separation

Purity Performance

• Single leaching: Fixed carbon 99.90–99.93%
• Double cycle leaching: Stable C≥99.95%, total metal impurities ≤20 ppm, residual Si ≤5 ppm

Pros & Cons

✅ High impurity removal efficiency, stable mass production, low equipment investment
❌ HF hazardous waste liquid generation; needs lime neutralization for fluoride sludge treatment

2.2 Alkali-Acid Combined Purification (Low HF Green Upgrade Route)

Principle

1. High-temperature alkali fusion first decomposes silicates without high HF dosage: Graphite mixed with NaOH (mass ratio 100:20–30), 550–700°C heat preservation 2–3 h; (SiO_2+2NaOH=Na_2SiO_3) (soluble sodium silicate, hot water washed away)
2. Secondary dilute mixed acid leaching removes residual Fe, Al, trace silica with minimal HF

Process Flow

Graphite powder + solid NaOH → high-temperature alkali fusion → hot water washing filter cake → dilute H₂SO₄+HCl+trace HF leaching → multi-stage washing → drying

Purity & Environmental Advantages

• One alkali-acid cycle: C≥99.90%; two cycles: C up to 99.96%
• HF consumption reduced by 70%, fluoride hazardous sludge output greatly decreased

Shortcomings

High-temperature alkali furnace equipment cost, higher energy consumption, longer production cycle

2.3 Green HF-Free Leaching (New Low-Carbon Industrial Route)

Ammonium fluoride electrolyte electrochemical leaching / organic weak acid leaching (ascorbic acid) replaces pure HF; avoids toxic hydrofluoric acid, wastewater treatment simplified, purity reaches 99.9–99.95%. Currently applied in small-scale low-emission production lines, not yet fully popularized for large capacity.

3. Ultra-High Temperature Thermal Purification (Deep Finishing for Ultra-High Purity ≥99.99%)

Used as final deep purification after chemical leaching for high-power fast-charging anode graphite; works for both natural and artificial graphite precursors.

Core Principle

Under inert argon shielding, heat graphite to 2600–3000°C; all inorganic impurities reach boiling point and vaporize, separated via exhaust gas extraction:

• Silicates decompose into SiO gas and volatilize
• Metal oxides reduced to low-boiling metal vapor (Fe, Al, Mg, Ca, Cu)
• Trace insoluble residual impurities that acid leaching cannot eliminate are completely removed

Standard Process Parameters

1. Pre-purified graphite (C≥99.90%) loaded into closed graphitization furnace
2. 2800–3000°C constant temperature holding 12–24 h, continuous argon circulation to carry impurity vapor out
3. Slow cooling under inert atmosphere, crushing & classification

Purification Index

Fixed carbon ≥99.99%, total impurity content <10 ppm, trace heavy metals undetectable

Pros & Cons

✅ Highest purity, removes all refractory trace impurities, improves graphite crystallinity
❌ Extremely high electricity cost, large furnace footprint, only applied to high-value premium anode materials

4. Special Auxiliary Purification Method: Chlorination Roasting (Lab/Small Batch Only)

• Principle: 800–1200°C feed Cl₂ gas; metal impurities react to form volatile metal chlorides, separated by gas extraction
• Limitation: Chlorine gas highly toxic, severe equipment corrosion, strict safety protection required; rarely used in commercial lithium battery graphite mass production

5. Complete Combined Industrial Impurity Removal Workflow (Stable C>99.95%)

Natural Spherical Graphite Anode Full Impurity Removal Chain

Raw graphite ore → flotation rough purification (C94–96%) → pre-crushing + multi-stage magnetic separation → spheroidization shaping → mixed acid two-cycle leaching → countercurrent washing → vacuum drying → secondary magnetic separation → optional 2800°C thermal deep purification → finished high-purity graphite (C≥99.95~99.99%)

Artificial Graphite (Calcined Coke) Impurity Removal Chain

Calcined petroleum coke lump → pre-crushing + double magnetic separation → jet milling grading → coke-pitch kneading granulation → pre-carbonization → 2800–3000°C graphitization (thermal purification built-in) → post-graphitization pickling light leaching → magnetic separation → finished artificial graphite (C≥99.98%)

6. Quality Acceptance Standard After Full Impurity Removal

7. Common Impurity Exceedance Troubleshooting

1. Excess silicon impurity: Insufficient HF leaching time / low alkali fusion temperature → extend leaching holding time or increase NaOH addition ratio
2. Iron over-limit: Worn metal equipment lining, insufficient magnetic separation → replace with ceramic/PU lining, add multi-stage high-gradient magnetic separators
3. Residual soluble salt impurities: Incomplete washing after leaching → increase countercurrent washing stages, test filtrate conductivity online
4. Trace heavy metal residual: Single chemical leaching cannot remove → add ultra-high temperature thermal finishing purification

Summary

1. Front-end physical removal (flotation, magnetic separation, sieving) eliminates bulk foreign bodies and magnetic iron, reducing load for downstream deep purification;
2. Mixed acid leaching or alkali-acid combined process is the mainstream industrial chemical method to stably reach carbon content over 99.9% for power battery graphite;
3. Ultra-high temperature thermal purification is the finishing process to achieve 99.99% ultra-high purity graphite for high-end fast-charging applications;
4. Full-process matching of physical pre-removal + chemical leaching + final magnetic separation ensures all impurity indicators meet lithium battery anode material standards.