How to Purify Graphite Anode Raw Materials to Fixed Carbon >99.9%

Natural flake graphite flotation concentrate typically has fixed carbon of only 92%–98%, containing silicates (SiO₂), iron oxides, alumina, calcium/magnesium salts, and trace heavy metal impurities. These impurities trigger severe side reactions, increase cell internal resistance, and shorten battery cycle life. For lithium-ion battery anode materials, the mandatory threshold is fixed carbon ≥99.90%, while premium power/automotive graphite requires ≥99.95%.

Based on industrial purification technologies from graphite-mill.com production lines, this article systematically introduces four scalable purification routes ranked by purity output, including process principles, operating parameters, impurity removal targets, advantages, limitations, and supporting waste treatment protocols. Combined multi-stage purification achieves stable fixed carbon above 99.9%.

Key Background: Impurity Types That Restrict Carbon Purity

1. Silicate minerals (quartz, feldspar, mica): Main impurity, insoluble in single strong acid; only HF or high-temperature alkali fusion can dissolve silica.
2. Metal oxides/hydroxides (Fe₂O₃, Al₂O₃, CaO, MgO): Soluble in inorganic strong acids (HCl, H₂SO₄).
3. Trace heavy metals (Mn, Cu, Ni, Pb): Dissolved by mixed acid leaching.
4. Insoluble refractory gangue: Removable by high-temperature thermal purification.

1. Mixed Acid Leaching Purification (Most Widely Used Mass Production Route)

Core Principle

Compound mixed acid (H₂SO₄ + HCl + HF) synergistically dissolves all inorganic impurities:

• HF reacts with SiO₂ to form soluble fluorosilicate (H₂SiF₆);
• HCl dissolves iron, aluminum, calcium, magnesium metal oxides;
• H₂SO₄ provides high acidity to accelerate leaching kinetics.
  Single acid cannot simultaneously remove silica and metal impurities.

Complete Industrial Process Flow

1. Preprocessing Preparation
   Spheroidized graphite powder (D50 12–22 μm) dried to moisture ≤0.1%; fine particle size increases acid-solid contact area for full impurity dissolution.
2. Mixed Acid Formulation & Leaching Reaction
   • Acid volume ratio: H₂SO₄:HCl:HF = 3:2:1
   • Liquid-solid mass ratio: 3:1 ~ 3.5:1
   • Reaction temperature: 50–70°C, constant stirring leaching 6–10 h under sealed corrosion-resistant PVDF reactor
   • Reaction mechanism:
     SiO₂ + 6HF = H₂SiF₆ + 2H₂O
     Fe₂O₃ + 6HCl = 2FeCl₃ + 3H₂O
3. Multi-Stage Countercurrent Washing
   After leaching, filter press separates waste acid; continuously rinse graphite cake with deionized water until filtrate pH = 6–7 (no residual free acid/fluoride ions). Residual acid will degrade subsequent carbon coating performance.
4. Drying & Post Magnetic Separation
   Vacuum drying at 120–150°C under N₂ atmosphere to moisture ≤0.1%; high-strength magnetic separator removes iron precipitates generated during leaching.

Purity Output & Control

• Single mixed acid leaching: Fixed carbon reaches 99.90%–99.93%
• Two-cycle repeated mixed acid leaching: Stable fixed carbon ≥99.95% (automotive battery grade)
• Impurity limit after purification: Total metal impurities ≤20 ppm, Fe ≤10 ppm, residual silicon ≤5 ppm

Pros & Cons

✅ Low capital investment, continuous mass production, high impurity removal efficiency for silicates and metal oxides
❌ Generates HF-containing hazardous waste liquid; requires dedicated lime neutralization and fluoride sludge stabilization disposal

2. Alkali-Acid Combined Purification (Low HF Consumption, Environment-Friendly Upgrade Route)

Core Principle

High-temperature alkali fusion first decomposes silica gangue without high HF dosage; secondary acid leaching removes residual metal impurities, drastically cutting fluoride waste generation. Ideal for factories with strict hazardous waste emission limits.

Process Steps

1. Alkali Fusion Stage (Silica Primary Removal)
   Mix graphite powder with solid NaOH (mass ratio graphite:NaOH = 100:20–30); heat to 550–700°C in sealed nickel alloy furnace for 2–3 h.
   SiO₂ + 2NaOH = Na₂SiO₃ + H₂O (soluble sodium silicate, washed away by hot water)
   Hot water washing to remove soluble silicate salt, filter to obtain alkali-treated graphite cake.
2. Dilute Mixed Acid Secondary Leaching
   Use low-concentration H₂SO₄+HCl + trace HF for secondary leaching to strip residual iron, aluminum, trace silica; leaching time 3–5 h.
3. Countercurrent Washing, Dehydration, Vacuum Drying
   Same washing and drying standards as mixed acid process.

Purity Performance

• One alkali fusion + one dilute acid leaching: Fixed carbon ≥99.90%
• Two alkali-acid cycles: Fixed carbon up to 99.96%

Pros & Cons

✅ HF consumption reduced by 70%, less fluoride hazardous sludge, lower waste treatment cost
❌ High-temperature alkali furnace equipment investment, longer production cycle, higher energy consumption

3. High-Temperature Thermal Purification (Graphitization Purification, For Ultra-High Purity ≥99.99%)

Core Principle

At ultra-high temperature (2600–3000°C) under inert argon atmosphere, all inorganic impurities undergo thermal decomposition or vaporization and escape as gas:

• Silicates decompose into SiO gas and volatilize;
• Metal oxides reduce to low-boiling-point metal vapor and separate from graphite;
• Removes trace residual impurities that acid leaching cannot eliminate.

Applicable Scenario

Used as final deep purification after acid/alkali leaching, for ultra-high-end graphite (premium fast-charging power batteries, silicon-graphite composite precursors).

Process Parameters

1. Pretreated graphite (after acid purification, C ≥99.90%) loaded into graphitization furnace;
2. Furnace temperature: 2800–3000°C, constant temperature hold 12–24 h under argon shielding;
3. Cooling, crushing, classification to obtain thermally purified graphite.

Purity Output

• Thermal deep purification after acid leaching: Fixed carbon ≥99.99%, total impurity content <10 ppm

Pros & Cons

✅ Achieves highest purity, removes trace insoluble impurities that chemical leaching cannot eliminate
❌ Extremely high electricity cost, large equipment footprint, only for high-value ultra-pure graphite grades

4. Chlorination Purification (Special Ultra-High Purity Auxiliary Route)

Principle

At 800–1200°C, feed chlorine gas into graphite powder; inorganic impurities react with Cl₂ to generate volatile metal chlorides, separated via gas extraction.

Limitation

Chlorine gas is highly toxic and corrosive, strict safety protection required; rarely used for large-scale battery graphite production, only for lab small-batch ultra-high purity carbon materials.

5. Combined Industrial Purification Workflow for Stable C >99.95%

Standard commercial production line integrated process (natural spherical graphite anode):

1. Raw flotation graphite (C 95%–98%) → Jet milling + spheroidization shaping
2. Primary mixed acid leaching → multi-stage washing → drying (C ~99.90%)
3. Secondary low-concentration alkali-acid leaching cycle → deep washing
4. Optional: 2800°C high-temperature graphitization thermal purification (automotive high-power grade, C ≥99.99%)
5. Final magnetic separation, screening, vacuum drying → finished high-purity graphite

6. Critical Process Control to Guarantee Carbon >99.9%

1. Particle size control before leaching
   Avoid oversized graphite particles; D50 controlled 12–22 μm. Overlarge flakes have internal wrapped impurities that acid cannot penetrate, limiting purity upper limit.
2. Complete washing management
   Insufficient residual acid washing leaves soluble salt impurities inside graphite pores, lowering fixed carbon and triggering battery gas generation. Must test filtrate fluoride and metal ion concentration online.
3. Prevent secondary iron contamination
   All leaching, stirring, conveying equipment use PVDF, zirconia, silicon carbide anti-corrosion materials; install multi-stage magnetic separators after each purification step to intercept metal wear debris.
4. Strict temperature & time holding
   Short leaching/alkali fusion time leads to incomplete impurity dissolution; insufficient temperature slows silica reaction rate.

7. Waste Treatment Supporting Purification (Mandatory Compliance)

1. Spent acid wastewater: Neutralize with slaked lime to precipitate insoluble CaF₂ fluoride sludge, adjust pH to 7–9, flocculation sedimentation, filter press solid-liquid separation.
2. Fluoride heavy metal sludge: Stabilize with cement curing agent, deliver to licensed hazardous waste landfills.
3. Washing wastewater supernatant: Adsorb residual fluoride via activated alumina, test to meet industrial discharge standards before draining.

8. Purity Standard Comparison of Different Purification Processes

To purify graphite anode raw materials to fixed carbon over 99.9%, the mainstream industrial mature solution is mixed acid leaching or alkali-acid combined purification. For mass-produced lithium battery anodes, two cycles of mixed acid leaching stably reach ≥99.95% carbon content, meeting automotive battery requirements. When ultra-high purity (>99.99%) is needed, add high-temperature thermal graphitization as deep finishing purification.