How to conduct laser diffraction particle size analysis for graphite

Laser diffraction (dry/wet dispersion) is the standard test for graphite D10/D50/D90, critical for spherical anode graphite, flake graphite, expandable graphite. Graphite has unique challenges: high oil absorption, static agglomeration, light absorption (low refractive signal), easy flake stacking. Below is standardized operating workflow, dispersion solutions, parameter settings & troubleshooting.

1. Pre-Test Preparation

1.1 Sample pretreatment

1. Drying: Graphite contains adsorbed water, residual pitch solvent from carbon coating.
   • Dry at 120 °C under nitrogen for 2 h, cool to room temp; moisture < 100 ppm.
   • High-moisture samples cause agglomeration & false large particle readings.
2. De-agglomeration: Loose lumps by gentle mortar grinding (no steel balls, avoid iron contamination); screen through 100 mesh sieve to remove hard aggregates.
3. Sample quantity:
   • Wet method: 0.1–0.3 g graphite per test
   • Dry method: 0.5–1.0 g graphite

1.2 Key optical parameter setup (graphite fixed constants)

Graphite strongly absorbs laser light; wrong RI leads to distorted PSD curves.

• Refractive index (RI): Real = 2.40, Imaginary (absorption coefficient) = 0.80–1.0
• Particle model: Irregular / non-spherical (flake graphite); spherical model only for shaped spherical anode graphite
• Calculation algorithm: Mie scattering (mandatory, Fraunhofer is inaccurate for sub-20 μm graphite)

2. Two Test Methods: Wet Dispersion (Preferred for Most Graphite) vs Dry Dispersion

Part A: Wet Laser Diffraction (Industry Standard for Battery Spherical Graphite)

Step 1: Select dispersant solvent

Graphite is hydrophobic; pure water causes severe floating/agglomeration. Choose one system:

1. Ethanol (anhydrous) – most common
   • Advantages: fast wetting, low surface tension, easy cleaning, no swelling of coated carbon layer
2. Isopropanol (IPA) – for ultra-fine D50 <5 μm graphite
3. Water + surfactant (0.1% sodium hexametaphosphate / SDS) – low-cost, avoid for pitch-coated graphite (may dissolve surface carbon)

Step 2: Instrument liquid cell pre-check

1. Fill circulation tank with clean dispersant, run circulation, purge bubbles completely.
2. Background measurement: Zero baseline until light attenuation <0.5% (bubbles = fake fine particle peaks).
3. Check cell window: No graphite residue from previous batches; wipe with lint-free cloth.

Step 3: Sample dispersion procedure

1. Pre-disperse graphite in a small beaker with 10 mL dispersant.
2. Ultrasonic treatment (critical to break graphite flake stacks & static agglomerates):
   • Power: 300–400 W, ultrasonic time: 3–5 min
   • Do NOT over-sonicate (>8 min): will crush graphite into extra ultrafines, falsify D10/D50
3. Drop suspension into circulating liquid cell slowly, monitor obscuration rate:
   • Target obscuration: 8%–15%
   • <5%: insufficient signal; >20% multiple light scattering, distorted distribution
4. Keep circulation pump speed medium (600–1000 rpm) to prevent particle sedimentation.

Step 4: Test run & data collection

1. Stabilize 30 s after adding sample to eliminate transient bubbles.
2. Collect 3 consecutive replicate measurements (10–30 s each).
3. Accept test only if D50 RSD ≤ 3% between repeats; re-sonicate if deviation is large.
4. Record D10, D50, D90, span value [(D90-D10)/D50].

Step 5: Post-test cleaning

• Flush circulation system 3× with pure ethanol to remove residual graphite (graphite adheres strongly to plastic/tubing).
• Empty tank and dry to prevent cross-contamination for next batch.

Part B: Dry Laser Diffraction (For Expandable Graphite, Large Flake Graphite; No Solvent Residue)

Wet solvent may alter expandable graphite interlayer structure; use dry route:

1. Instrument purge: Purge sample feeder with dry compressed air (dew point ≤ -40 °C) to eliminate moisture static.
2. Dispersion pressure setting (core parameter):
   • Flake graphite (high aspect ratio): 1.0–1.5 bar (low pressure to avoid breaking flakes)
   • Spherical anode graphite: 2.0–2.5 bar (higher pressure to split soft agglomerates)
3. Feed rate: Slow uniform feeding to maintain obscuration 6%–12%.
4. Anti-static setup: Add ion air blower at sample injection port; graphite static causes particle adhesion to pipeline walls.
5. Limitation: Dry method shows wider span than wet test; use wet method as arbitration standard for quality control.

3. Critical Control Points Specific to Graphite

1. Avoid flake stacking artifacts
   Un-dispersed stacked flakes are detected as single large particles → falsely high D50. Ultrasonic + low-surface-tension solvent solves this.
2. Static elimination
   Fine graphite generates heavy static; agglomerates shift PSD toward larger sizes.
   • Wet: ethanol instead of water
   • Dry: ionized dry air, anti-static coating on feed lines
3. Prevent particle crushing
   Over-sonication (wet) / over-high dispersion pressure (dry) produces artificial ultrafines, D10 drops sharply, span widens.
4. Light absorption correction
   Do not set imaginary absorption coefficient to 0; graphite absorbs most transmitted laser light, wrong RI leads to undercount fine particles.
5. Sedimentation prevention
   Graphite density ~2.2 g/cm³, fast sinking. Maintain continuous circulation during wet testing.

4. Common Failures & Troubleshooting

5. Standard Reporting Items for Graphite PSD

1. Test method (wet ethanol / dry dispersion)
2. Optical parameters (RI real & imaginary value)
3. Dispersion conditions: ultrasonic power/time or dry air pressure
4. D10, D50, D90 (μm)
5. Span = (D90 − D10) / D50
6. Obscuration, replicate RSD
7. Sample type: flake graphite / spherical coated anode graphite / expandable graphite

6. Standard Compliance Reference

• International: ISO 13320 Laser Diffraction Particle Size Analysis
• Domestic: GB/T 19077, GB/T 30835 (lithium ion anode graphite particle size test)
• Battery factory QC arbitration rule: Wet ethanol laser diffraction as official test method.