how to pretreat natural flake graphite with carbon content 90%-95% for graphite anode

Graphite with 90%–95% fixed carbon carries high embedded silicate, metal oxide gangue, and uneven impurity distribution. Its pretreatment is a two-stage physical pre-upgrading + standard anode forming pretreatment system, which first lifts carbon to ≥94.5% to reduce acid consumption and stabilize final purification consistency, then follows spheroidization, screening, drying and magnetic separation for acid leaching feed.

1. Core Problems of 90%–95%C Raw Graphite

1. Low carbon (90–94%): Massive interlayer gangue embedded in flakes; direct spheroidization + acid leaching causes 30% higher HF consumption, severe batch purity fluctuation, finished carbon hard to hit ≥99.90%
2. 94–95% medium-carbon concentrate: Minor wrapped fine silicate; can skip re-flotation but needs enhanced gravity classification & multi-stage magnetic separation
3. Uneven particle segregation: Coarse flakes carry more gangue, fine powder enriches soluble metal impurities
4. Wet filter cake agglomeration traps internal gangue, leading to incomplete impurity exposure

2. Full Pretreatment Flow (Two Technical Branches: 90–94% Low-Carbon / 94–95% Medium-Carbon)

Route A: 90%–94% Low-Carbon Flake Graphite (Mandatory Re-Flotation Pre-Upgrading)

Step 1 Raw Material Receiving & Batch Classification

1. Incoming full-element test: Fixed carbon, ash, Si, Fe, moisture; separate batches by flotation source and carbon grade
2. Manual pre-sorting + primary high-gradient magnetic separation (12000 Gs): Remove bulk rock, refractory blocks, steel wear iron
3. Layered silo homogenization: Stack horizontal layers ≤50 cm, cross-cut reclaiming ≥12 h to balance ash fluctuation

Step 2 Stage Grinding + Multi-Stage Re-Flotation (Key Pre-Upgrading to ≥94.5% C)

Target: Lift fixed carbon from 90–94% to stable 94.5–95.5% before spheroidization

1. Light regrinding (rod mill, protect large flakes): Grind coarse flakes to -100~+200 mesh, dissociate gangue wrapped in graphite interlayers; avoid over-grinding to prevent fine flake loss
2. Flotation circuit: 1 roughing → 1 scavenging → 4–5 cleaning flotation
   • Pulp concentration: 18–22%, pH adjusted to 8–9 with sodium carbonate
   • Collector: diesel 300–450 g/t; frother: pine oil 25–40 g/t
3. Flotation index control: Concentrate fixed carbon ≥94.5%, recovery ≥90%; discard low-carbon tailings
4. Vacuum filter press dewatering: Filter cake moisture controlled 15–25%

Step 3 Two-Stage Uniform Drying

1. Hot nitrogen convection pre-drying (120–130°C): Reduce moisture to ≤3%
2. Vacuum deep drying (-0.09 MPa, 105°C): Final moisture ≤0.5%, eliminate pore-bound water; multi-point moisture deviation ≤±0.05%

Step 4 Jet Milling & Air Gravity Classification (Pre-Remove High-Density Gangue)

1. Grind dried flake to D50 12–18 μm
2. Closed airflow classifier: Separate high-density silicate gangue fine particles; cut impurity load by 25–30%
3. Double magnetic separation after classification to remove grinding iron powder

Step 5 Spheroidization Shaping

1. Multi-cycle spheroidization (6–8 cycles, higher than standard 95%+ feed): Fully peel off surface gangue, smooth sharp flake edges
2. Target sphericity ≥0.88, tap density ≥0.82 g/cm³
3. Oversize coarse particles loop back for re-shaping

Step 6 Multi-Layer Ultrasonic Fine Screening

Double-layer screen layout:

• Upper 200 mesh (75 μm): Remove oversized unshaped flakes (residue ≤0.5%)
• Lower 325 mesh (44 μm): Intercept ultra-fine impurity-rich dust (fines pass ≤15%)

Step 7 Final Pre-Leaching Homogenization

Nitrogen-sealed silo air-lift circulation ≥4 cycles, static homogenization ≥12 h; multi-point sampling fixed carbon fluctuation ≤±0.2%
→ Output: Qualified spheroidized powder for mixed acid leaching

Route B: 94%–95% Medium-Carbon Flake Graphite (Skip Re-Flotation, Enhanced Physical Pre-Dedoping)

No secondary flotation; strengthen gravity separation and magnetic removal to cut excess gangue load

1. Raw material homogenization silo → pre-shred wet cake → two-stage drying (moisture ≤0.5%)
2. Jet milling + high-efficiency air classification (extended classification residence time to remove fine silicate)
3. 3-stage magnetic separation (before grinding, post-grinding, post-spheroidization)
4. Spheroidization (4–6 cycles) → 200+325 mesh ultrasonic screening
5. Finished pre-leaching homogenization silo
   → Direct feed to acid purification

3. Key Control Parameters for Pretreatment of 90–95%C Graphite

3.1 Flotation Pre-Upgrading (Only for <94% C)

• Regrinding fineness: -0.074 mm (200 mesh) accounted for 55–60%
• Flotation cleaning times: Minimum 4 stages to guarantee C≥94.5%
• Flake protection: Adopt rod mill instead of ball mill to avoid excessive fine flake generation

3.2 Drying Uniformity

• Two-stage gradient drying mandatory; single-layer material thickness ≤20 mm
• Dew point of circulating hot nitrogen ≤-40°C to prevent condensation and local impurity enrichment

3.3 Spheroidization Matching

Low-carbon graphite has more surface gangue; increase spheroidization cycles by 2–3 times vs standard 95.5%+ feed to fully expose embedded impurities for acid leaching

3.4 Screening Mesh Standard

Consistent with battery-grade anode requirement:

• Raw flotation feed: +50 mesh (300 μm) large flakes
• Pre-spheroid powder: -100~+200 mesh
• Post-spheroid semi-finished: 200 mesh upper cutoff, 325 mesh lower cutoff

3.5 Magnetic Separation Configuration

3 fixed nodes for 90–95%C graphite: pre-drying, post-grinding, post-screening; magnetic field ≥12000 Gs, Fe controlled ≤10 ppm before leaching

4. Special Pretreatment Auxiliary Technology for Low-Carbon Graphite

4.1 Pre-Acid Washing (Light Pickling Before Formal Mixed Acid Leaching)

For graphite with 90–93% C: add dilute HCl pre-leaching (50°C, 2 h) after spheroidization

• Function: Dissolve surface exposed metal oxides first, reduce main mixed acid HF consumption
• Liquid-solid ratio 2:1, filter & wash to neutral before formal mixed acid leaching

4.2 Alkali Fusion Pre-Treatment (Alternative for Ultra-Low Carbon 90–92% C)

If re-flotation cannot lift carbon above 94%: deploy low-temperature alkali fusion before acid leaching

1. Graphite + NaOH mass ratio 100:25, 550°C heat preservation 2 h
2. Hot water washing to remove soluble sodium silicate, filter and dry
3. Enter mixed acid leaching; HF dosage reduced by 70% vs direct acid leaching

5. Acceptance Standard After Full Pretreatment (Before Acid Purification)

6. Common Pretreatment Defects & Solutions for 90–95%C Graphite

1. Post-leaching Si impurity over-limit
   Cause: Insufficient re-flotation cleaning times, incomplete gangue dissociation during regrinding
   Solution: Add 1–2 flotation cleaning stages, extend light regrinding time
2. Excessive acid consumption, high fluoride waste
   Cause: Skip re-flotation for 90–94% low-carbon graphite
   Solution: Mandatory multi-stage re-flotation pre-upgrading or add alkali fusion pre-treatment
3. Batch purity inconsistency after purification
   Cause: Poor raw material homogenization, uneven spheroidization cycle
   Solution: Extend silo homogenization time, unify spheroidization cycle number per batch
4. High iron impurity
   Cause: Insufficient magnetic separation stages, worn metal equipment lining
   Solution: Add one magnetic separation node, replace carbon steel lining with zirconia ceramic

7. Simplified Full Process Summary

90–94% Low-Carbon Natural Flake Graphite

Raw graphite (90–94%C) → pre-sorting + primary magnetic separation → layered homogenization → rod mill light regrinding → multi-stage re-flotation (lift C≥94.5%) → filter press dewatering → two-stage uniform drying → jet milling + airflow gravity classification → triple magnetic separation → multi-cycle spheroidization → 200/325 mesh ultrasonic screening → nitrogen-sealed homogenization silo → pre-leaching spherical graphite powder

94–95% Medium-Carbon Natural Flake Graphite

Raw graphite (94–95%C) → pre-sorting + magnetic separation → homogenization silo → two-stage drying → jet milling + enhanced airflow classification → triple magnetic separation → spheroidization shaping → double-layer fine screening → homogenization → acid leaching feed