how to purify natural flake graphite by acid method for lithium battery anode

1. Overview of Acid Purification Principle & Target Impurities

Natural flake graphite after flotation & spheroidization has fixed carbon only 94–96%. Harmful impurities for lithium anodes:

1. Silicate gangue (quartz, mica, feldspar: SiO₂, Al₂O₃) → causes cell gas swelling
2. Metal oxides (Fe₂O₃, CaO, MgO, CuO) → triggers electrolyte decomposition & micro short circuits
3. Trace sulfide, carbonate embedded in graphite flakes

Mixed Acid System (Industrial Mainstream: HF + H₂SO₄ + HCl)

• HF: Dissolves silicate impurities to soluble fluosilicate
  SiO2+6HF=H2SiF6+2H2OSiO2​+6HF=H2​SiF6​+2H2​O
• HCl: Dissolves metal oxides into soluble metal chlorides; suppresses insoluble fluoride precipitate formation
  Fe2O3+6HCl=2FeCl3+3H2OFe2​O3​+6HCl=2FeCl3​+3H2​O
• H₂SO₄: Boosts leaching acidity, accelerates reaction kinetics, inhibits HF volatilization

Purification Target

Single-cycle mixed acid leaching: Fixed carbon ≥99.90%; double-cycle leaching: ≥99.95%; total metal impurities ≤20 ppm, Si ≤5 ppm (EV battery standard)

2. Full Pre-Leaching Pretreatment (Mandatory Foundation)

Only spheroidized natural spherical graphite (D50=12–22 μm) enters acid leaching; flat flake cannot fully contact acid and leads to uneven purification.

1. Multi-stage magnetic separation
   High-gradient magnetic separator (≥12000 Gs) removes equipment wear iron powder; Fe ≤10 ppm before leaching.
2. Two-stage uniform vacuum drying
   Nitrogen convection pre-drying + vacuum deep drying; final moisture ≤0.5% to avoid diluting acid and reduce leaching efficiency deviation.
3. Fine screening to remove coarse gangue clusters
   300 mesh ultrasonic screen eliminates large silicate agglomerates; reduces acid consumption by 25%.
4. Batch homogenization
   Sealed silo air-lift circulation ≥4 cycles to eliminate particle segregation and impurity stratification.

3. Industrial Mixed Acid Leaching Core Process (Full Standard Parameters)

3.1 Equipment Requirement

• Reactor: Sealed PVDF lined stirred tank (anti-HF corrosion), jacket heat conduction oil heating, negative pressure exhaust for HF vapor recovery
• Auxiliary: Automatic loss-in-weight acid feeding system, vacuum filter press, 8-stage countercurrent washing tank, waste acid neutralization system

3.2 Standard Acid Formula & Liquid-Solid Ratio

• Concentrated acid volume ratio: H₂SO₄ (98%): HCl (36%): HF (40%) = 3:2:1
• Liquid-solid ratio L/S = 3:1 ~ 3.5:1 (acid solution mass : graphite powder mass)
• Total acid concentration in slurry: 30–35 wt%

3.3 Leaching Temperature & Time Control

1. Heating temperature: Constant 60–70°C (temperature fluctuation ≤±2°C via multi-point sensors)
2. Stirring speed: 80–120 rpm, fully suspend graphite powder without dead zones
3. Single leaching holding time: 6–10 h
   • Short time: Incomplete silicate dissolution, residual Si over limit
   • Overlong time: Graphite surface slight etching, low first-cycle Coulombic efficiency
4. Double-cycle leaching (for high-end power batteries): Repeat leaching procedure after first washing & drying

3.4 Full Leaching Step-by-Step Operation

1. Feed dried spherical graphite into sealed reactor, inject pre-mixed acid solution automatically
2. Start stirring, heat up to 65°C, hold constant temperature for full leaching cycle
3. Negative pressure exhaust continuously collects HF vapor into absorption tower for recycling
4. After reaction finishes, discharge graphite-acid slurry to vacuum filter press for solid-liquid separation
5. Waste acid filtrate transferred to lime neutralization tank to precipitate fluoride sludge for hazardous waste treatment

4. Multi-Stage Countercurrent Washing (Critical Step to Remove Residual Acid & Soluble Salts)

Unwashed residual F⁻, metal salt ions cause poor cycle performance and gas generation in batteries.

1. Washing medium: Deionized water (conductivity ≤5 μS/cm)
2. Washing flow: 5–8 stage countercurrent washing (fresh water contacts last-stage graphite, waste water pre-washes raw acid filter cake to save water)
3. Control standards per washing stage:
   • Final filtrate pH = 6–7
   • Free fluoride ion content <1 ppm (fluoride test paper/online ion detector)
   • Filtrate conductivity ≤20 μS/cm
4. Uniform washing requirement: Spread filter cake thickness ≤15 mm on filter plates; break cake agglomerates to eliminate internal unwashed dead zones

5. Post-Leaching Deep Treatment & Homogenization

5.1 Uniform Vacuum Drying

Two-stage gradient drying to eliminate adsorbed water and residual soluble salt enrichment:

1. Hot nitrogen convection pre-drying (120–130°C) → moisture ≤1%
2. Vacuum deep drying (-0.09 MPa, 105°C, 4–6 h) → final moisture ≤0.1%
   Multi-point sampling moisture deviation ≤±0.05%

5.2 Secondary Magnetic Separation

High-gradient magnetic separator removes trace iron impurities generated by reactor lining friction during leaching; Fe ≤3 ppm for high-power fast-charging graphite.

5.3 Finished Product Homogenization

Nitrogen-sealed large homogenization silo with air-lift closed circulation ≥4 cycles, static storage ≥12 h to eliminate purity segregation caused by conveying stratification.
Acceptance standard: Top/middle/bottom sampling fixed carbon deviation ≤±0.03%, Si/Fe difference ≤1 ppm.

6. Alternative Low-HF Acid Purification Route (Alkali-Acid Combined Process, Green Upgrade)

For factories limiting HF consumption; widely used to reduce fluoride waste discharge by 70%

Step 1 High-temperature alkali fusion (pre-remove most silicates)

1. Graphite + solid NaOH mass ratio 100:25, mix uniformly
2. 550–700°C muffle furnace heat preservation 2–3 h; SiO₂ reacts into soluble Na₂SiO₃
3. Hot water washing to remove sodium silicate, filter into alkali-treated filter cake

Step 2 Dilute mixed acid secondary leaching

Low HF dosage mixed acid (H₂SO₄+HCl+trace HF), 60°C leaching 4 h to dissolve residual metal oxides & trace silicates

Performance

Single cycle achieves fixed carbon ≥99.90%; double cycle ≥99.96%; much lower HF waste liquid output than full mixed acid process

7. HF-Free Green Acid Purification (Emerging Low-Carbon Technology)

Replace pure HF with ammonium fluoride (NH₄F) electrolyte, no concentrated hydrofluoric acid storage/transport risk

1. Leaching system: H₂SO₄ + NH₄F aqueous solution, 70–90°C stirring 3–5 h
2. Mechanism: NH₄F releases F⁻ in situ to dissolve silicates; avoids high-concentration HF hazards
3. Index: Fixed carbon ≥99.90%, suitable for energy storage low-to-medium power graphite production

8. Quality Acceptance Standard After Acid Purification

9. Common Defects & Adjustment Solutions

1. Excess silicon impurity after leaching
   Cause: Insufficient HF ratio, low leaching temperature, short holding time
   Solution: Increase HF proportion, raise temperature to 68–70°C, extend leaching time to 9–10 h
2. High residual fluoride ion after washing
   Cause: Insufficient washing stages, uneven filter cake thickness
   Solution: Add 1–2 washing stages, break filter cake agglomerates before washing
3. Batch-to-batch purity fluctuation
   Cause: Unstable acid automatic feeding, incomplete pre-leaching homogenization
   Solution: Calibrate loss-in-weight acid feeder, extend pre-leaching silo circulation cycles
4. Local iron impurity over-limit
   Cause: PVDF lining wear, missing post-leaching magnetic separation
   Solution: Repair reactor lining, install magnetic separator after drying discharge

10. Complete Full Flow Summary of Mixed Acid Purification

Spheroidized natural spherical graphite → multi-stage magnetic separation → vacuum deep drying → fine screening → silo homogenization → sealed PVDF reactor mixed acid leaching (60–70°C, 6–10 h) → vacuum filter press solid-liquid separation → 5–8 stage countercurrent deionized water washing → two-stage vacuum drying → secondary magnetic separation → nitrogen sealed homogenization silo → battery-grade high-purity natural graphite anode raw material