what is the drying standard of natural flake graphite for lithium battery anode

This standard covers four critical drying stages in the whole production line, including moisture limit, temperature, vacuum, atmosphere, uniformity, equipment requirements, national standard basis (GB/T 24533-2019, JC/T 2315-2016), and quality control criteria.

1. Core Hazards of Excessive Moisture in Graphite

1. Dilutes mixed acid during leaching, reduces impurity removal efficiency, causes unstable purity
2. Moisture reacts with lithium salt electrolyte (LiPF₆) to generate HF, leading to cell swelling, capacity decay and low first-cycle Coulombic efficiency
3. Powder agglomeration, blocking screens, jet mills and silos; uneven coating during battery slurry mixing
4. Residual soluble salt enrichment in graphite pores, raising trace metal impurity levels

2. Stage-by-Stage Drying Standard (Full Process)

Stage 1: Post-Flotation Filter Cake Drying (Raw Flake Graphite, C=90%–95%)

Raw material: Wet filter cake after flotation, moisture 15%–35%

Two-Stage Gradient Drying (Mandatory)

1. Hot nitrogen convection pre-drying
   • Medium: Dry nitrogen (dew point ≤ -40°C, O₂＜500 ppm to avoid graphite oxidation)
   • Temperature: 120–130°C, material layer thickness ≤20 mm
   • Target moisture after pre-drying: ≤3%
2. Vacuum deep drying
   • Vacuum degree: -0.08 ~ -0.09 MPa
   • Temperature: 105–110°C, holding time 4–6 h
     Final standard before spheroidization: Moisture ≤0.5%

Uniformity control

Multi-point sampling (top/mid/bottom of dryer tray) moisture deviation ≤±0.05%

Stage 2: Pre-Leaching Drying (Spheroidized Graphite Before Acid Purification)

Spherical graphite after spheroidization & screening, absorbs ambient moisture during conveying

• Equipment: Closed vacuum tray dryer / continuous vacuum belt dryer
• Vacuum: ≥-0.09 MPa, temp 105°C, dwell time ≥4 h
• Strict moisture limit: ≤0.5%
• Key rule: If moisture exceeds 0.5%, acid solution will be diluted, Si impurity removal rate drops by 20%+

Stage 3: Post-Acid Leaching Washing Drying (Most Critical Step)

Graphite filter cake after 5–8 stage countercurrent water washing, contains free water and adsorbed water with residual fluoride/salts

Double-stage vacuum drying process

1. Primary low-temp vacuum drying: 110–120°C, remove free water, moisture down to ≤1%
2. Deep vacuum drying: -0.09 MPa, 105°C, 6–10 h, eliminate pore-bound water

Moisture grading standards (Industrial Mass Production)

Stage 4: Finished Product Secondary Low-Temp Drying (Before Silo Homogenization)

Prevent moisture re-absorption during screening and conveying

• Low-temperature nitrogen drying: 80–90°C, normal pressure dry nitrogen circulation
• Control target: No moisture rebound, maintain moisture ≤0.09% for power grade

3. General Technical Drying Process Specifications

3.1 Temperature Control Rules

• Max safe temperature ≤130°C: Over 130°C will cause slight oxidation of graphite surface, increase BET and reduce ICE
• Temperature fluctuation inside dryer ≤±2°C; avoid local hot spots
• Slow heating rate ≤5°C/min to prevent graphite layer cracking and fine powder generation

3.2 Vacuum & Atmosphere Standard

1. Vacuum drying: Working vacuum ≥-0.08 MPa; ultimate vacuum ≥-0.09 MPa
2. Protective atmosphere (all drying systems): Closed nitrogen isolation
   • Nitrogen dew point ≤ -40°C
   • Oxygen content inside drying chamber ≤500 ppm
   • Purpose: Prevent graphite oxidative weight loss and surface defects
3. No open-air hot air drying allowed for battery-grade graphite (air moisture & oxygen cause secondary pollution)

3.3 Material Pavement & Residence Time

• Single layer thickness ≤20 mm; thick stacking leads to internal incomplete drying and moisture stratification
• Vacuum drying holding time cannot be shortened arbitrarily; extend 2 h if cake thickness exceeds standard
• Continuous belt dryer: Adjust belt speed to guarantee minimum 4 h heating residence time

3.4 Uniformity Acceptance Standard

After drying, take 5 samples from different positions of the same batch; moisture difference between any two samples ≤0.03%

4. Auxiliary Drying Quality Standards

1. No secondary metal contamination
   All contact parts of dryers: zirconia ceramic, PU, 316L stainless steel; carbon steel is forbidden to avoid iron shedding
2. No residual fluoride salt enrichment
   Uneven drying causes local salt agglomeration; after drying, multi-point conductivity test of graphite leachate is required
3. No powder agglomeration
   After drying, pass through 400 mesh screen; hard agglomerate residue = 0

5. Common Drying Defects & Adjustment Schemes

1. Moisture out of limit (>0.1% for power grade)
   Cause: Insufficient vacuum degree, short holding time, over-thick material layer
   Solution: Boost vacuum to -0.09 MPa, extend drying time by 2–4 h, reduce single layer thickness
2. Batch moisture inconsistency
   Cause: Unstable nitrogen supply, uneven heating in dryer
   Solution: Calibrate heating system, stabilize nitrogen flow, increase mixing before drying
3. Graphite oxidation, high BET value after drying
   Cause: High oxygen in drying chamber, temperature over 130°C
   Solution: Increase nitrogen purge flow, lower drying temperature to 105–110°C
4. Hard salt agglomerates after drying
   Cause: Incomplete washing before drying, uneven cake spreading
   Solution: Add one countercurrent washing stage, break filter cake before feeding dryer

6. Summary of Core Drying Moisture Thresholds

1. After flotation pre-drying: ≤3%
2. Before acid leaching (spheroidized powder): ≤0.5%
3. Post-leaching finished battery-grade graphite:
   • Basic energy storage grade: ≤0.10%
   • EV power battery mainstream grade: ≤0.09%
   • High-rate fast-charging ultra-high purity grade