Pitch (coal‑tar pitch or petroleum pitch) acts as critical binder and carbon‑coating precursor for artificial graphite anode. Raw commercial pitch contains quinoline‑insoluble particles (QI), ash, moisture, light volatile fractions and trace metallic impurities. Without proper pretreatment, these contaminants cause uneven granulation, poor carbon‑coating uniformity, severe gas evolution during carbonization, increased irreversible capacity and safety hazards for lithium‑ion cells. Pretreatment covers impurity purification, thermal pre‑treatment, crushing‑milling, and strict index control before mixing with calcined coke powder.
Core raw‑material index requirements before pretreatment
| Parameter | Anode‑grade pitch specification | Remarks |
|---|---|---|
| Softening point | 105‑135 °C (medium‑temperature pitch for binder) | Controls melt viscosity during kneading and granulation |
| Quinoline insoluble (QI) | ≤0.3 wt% | Reduce hard impurity particles; primary QI must be removed |
| Ash content | ≤0.05 wt% | Strict limit for metal impurities (Fe, Na, Ca) |
| Moisture | ≤0.2 % | Prevent foaming and local over‑spraying in molten state |
| Coking value | ≥45 % | Guarantee carbon yield after carbonization |
| Sulfur content | ≤0.4 % | Lower gas generation in high‑temperature treatment |
Full Pretreatment Workflow for Pitch Binder
Raw pitch block → melting & thermal filtration purification → devolatilization → cooling & solidification → crushing → fine milling & air classification → sieving → sealed nitrogen‑protected storage → feed to kneading / granulation system.
1. Melting and thermal filtration (core impurity‑removal step)
Solid pitch is melted under nitrogen inert atmosphere at 160‑210 °C. High‑precision thermal filtration removes coarse quinoline‑insoluble solid particles, coke debris and mineral ash impurities.
- Filter mesh: 200‑325 mesh high‑temperature metal‑ceramic filter elements.
- Strictly avoid air‑exposed melting to prevent pitch pre‑oxidation.
- Target after filtration: QI ≤0.3 %, ash ≤0.05 %.
Note: Unfiltered raw pitch carries coarse QI particles. These hard particles create defective points inside secondary graphite particles and degrade battery cycle performance.
2. Controlled devolatilization (vacuum thermal treatment)
After filtration, molten pitch goes through low‑pressure vacuum holding at 220‑260 °C, residence time 60‑120 min.
- Remove low‑molecular‑weight light volatile fractions. Excess light components will cause violent foaming, swelling and heavy smoke in subsequent kneading and pre‑carbonization.
- Outlet moisture controlled ≤0.2 %.
- Do not over‑heat above 280 °C, to avoid premature thermal‑polymerization which shifts softening point out of process window.
3. Cooling, solidification and coarse crushing
Purified molten pitch is cast into cooling steel belts under sealed inert condition, solidified into brittle solid sheets or lumps. Then coarse crushing down to 3‑8 mm particle size.
- All crushing equipment adopts non‑metallic lining to prevent iron contamination.
- Hot pitch cannot be exposed to humid air; otherwise moisture re‑absorption occurs.
4. Fine milling & air classification
Coarse pitch particles are fed into closed‑loop impact mill with nitrogen protection.
- Target particle‑size for binder pitch powder: D50 = 8‑15 μm; 100 % pass 200‑mesh sieve.
- Air classification removes oversize particles and trace magnetic impurities. High‑intensity magnetic separation removes ferromagnetic wear debris.
- Finished pitch powder cannot contain coarse hard particles, which would lead to local poor bonding during granulation.
5. Sealed storage requirement
Finished pretreated pitch powder must be stored in nitrogen‑purged silos.
- Avoid moisture absorption and surface oxidation by ambient air.
- Temperature of silo kept below 40 °C to prevent softening and agglomeration of fine pitch powder.
Common production pitfalls in pitch pretreatment
- Insufficient thermal filtration: Residual high QI content causes uneven carbon coating and poor initial coulomb efficiency.
- Incomplete devolatilization: Large‑amount light volatiles lead to swelling, pores and cracks inside secondary artificial‑graphite particles.
- Metal contamination during crushing/milling: Steel linings introduce iron impurities, which bring internal short‑circuit risks for finished cells. Must use ceramic or polyurethane linings.
- Moisture re‑absorption: After cooling, pitch contacts humid air; moisture causes bubbling in molten mixing stage. Whole post‑treatment system must be sealed.
- Over‑thermal‑treatment: Excessive temperature / holding time raises softening point sharply, pitch loses suitable fluidity for kneading and granulation.
For individual production lines adopting liquid pitch direct feeding: skip solid crushing‑milling. After melting‑filtration‑devolatilization, qualified molten pitch is directly transported by heat‑traced pipelines to the kneader or granulation reactor. Keep pipeline temperature 10‑15 °C above pitch softening point to avoid solidification blockage.
