Coating‑grade pitch for graphite anode jet milling contains quinoline‑insoluble (QI) hard particles, coke grit, mineral ash, and mechanical impurities. Without proper molten‑state filtration before crushing and jet milling, these rigid impurities will pass through jet‑milling chamber, remain in final pitch powder, scratch classifier rotors and nozzle components, generate metal wear debris, create hard defect particles in carbon‑coating layer, reduce initial coulombic efficiency and raise battery safety risks. Pitch filtration must be completed in molten liquid phase before solidification and jet‑milling feed preparation. Dry‑powder screening cannot remove embedded QI impurities.
Full Process Sequence
Raw pitch block → melting under nitrogen protection → multi‑stage thermal filtration → vacuum devolatilization → cooling & solidification → coarse crushing → vacuum drying (moisture ≤0.2 %) → pre‑screening → nitrogen‑protected jet milling → classification → finished coating‑grade pitch powder.
Critical note: Filtration happens on molten pitch, not on solid pitch powder. Jet‑milling only grinds pre‑purified solid pitch, it cannot eliminate QI and ash impurities originated from raw pitch.
Step 1: Melting conditions prior to thermal filtration
Melt pitch inside sealed heating tank with nitrogen inert atmosphere, oxygen controlled below 5 %.
- Melting temperature: 170‑210 °C, 10‑25 °C higher than pitch softening point, guarantee low viscosity for smooth filtration.
- Avoid temperature above 220 °C before filtration, to prevent premature thermal polymerization and rising softening‑point.
- Adequate stirring to suspend solid impurities, prevent sedimentation at tank bottom.
Step 2: Two‑stage thermal filtration (core purification for jet‑mill feedstock)
Adopt serial coarse‑then‑fine thermal filtration setup, high‑temperature resistant ceramic‑metal composite filter elements. Single‑stage filtration easily causes rapid element blocking and incomplete impurity removal.
- Primary coarse filtration: 100‑150 mesh high‑temperature filter. Remove large coke fragments, foreign debris and aggregated big QI clusters. Protect downstream fine filter element from rapid clogging. Operating temperature maintained 170‑210 °C with heat‑traced pipeline to avoid pitch solidification inside filter housing.
- Secondary fine filtration: 250‑325 mesh ceramic‑metal sintered filter element, the critical barrier for jet‑milling‑grade pitch. Retain fine quinoline‑insoluble particles and mineral ash.
- Target after dual‑stage filtration: QI ≤0.3 wt%, ash ≤0.05 wt%.
- Differential‑pressure monitoring: Replace filter cartridges when pressure difference exceeds 0.35 MPa, prevent impurity breakthrough.
Filter element warning: Ordinary stainless‑steel mesh is not preferred; trace iron‑chromium debris may shed into molten pitch. Ceramic‑metal sintered filter minimizes secondary metal contamination for battery‑grade material.
Step 3: Post‑filtration vacuum devolatilization
Filtered clean molten pitch flows into vacuum devolatilization reactor. Process at 220‑260 °C under vacuum −0.07~−0.09 MPa, residence time 60‑120 min. Remove light‑molecular volatile fractions. Do not exceed 280 °C to avoid over‑polymerization.
Step 4: Solid‑phase pre‑treatment before jet milling
After devolatilization, molten pitch is cast and solidified under nitrogen atmosphere.
- Coarse crushing down to 1‑3 mm particle size, non‑metallic lined crusher to avoid iron contamination.
- Vacuum inert drying: moisture controlled ≤0.2 %, temperature strictly ≤150 °C, prevent pitch softening and agglomeration.
- Pre‑sieving before jet‑mill feeding: 60‑80 mesh vibrating sieve, reject oversize agglomerates. Jet‑mill maximum allowable feed size ≤1 mm; oversized feed causes unstable feeding, nozzle blockage and poor particle‑size distribution.
- High‑intensity magnetic separation before jet‑mill feeding, remove trace ferromagnetic impurities introduced in crushing steps.
Step 5: Jet‑milling auxiliary gas‑source filtration
Compressed nitrogen for pitch jet milling also requires multi‑step purification:
- Remove oil, water and solid particulates. Residual oil or moisture will cause pitch powder agglomeration inside jet‑mill chamber. Dew‑point of working gas ≤‑40 °C.
Key Quality Targets After Full Filtration & Pretreatment Before Jet Milling
| Index | Specification for jet‑mill feed pitch |
|---|---|
| Quinoline insoluble (QI) | ≤0.3 % |
| Ash content | ≤0.05 % |
| Moisture | ≤0.2 % |
| Feed particle size to jet mill | ≤1 mm, 100 % pass 80‑mesh |
| Oxygen content | Low, no obvious pre‑oxidation |
Common Production Failures & Troubleshooting
| Problem | Root Cause | Countermeasure |
|---|---|---|
| Hard impurity particles remain in jet‑milled final pitch powder | Missing fine thermal‑filtration step; filter element broken / pressure‑breakthrough | Inspect filter cartridge integrity; monitor differential pressure; strictly implement two‑stage molten filtration |
| Jet mill nozzle blockage | Molten pitch partial pre‑polymerization; feed material oversize | Control melting temperature; strictly control jet‑mill feed ≤1 mm |
| Iron contamination increases after jet‑milling | Filter sheds metal fragments; crushing equipment steel lining | Use ceramic‑metal sintered filter; adopt non‑metallic linings for crushing unit |
| Filter element blocks rapidly | Too high raw‑pitch QI value; insufficient melting temperature | Select low‑QI raw pitch; raise melting temperature within safe range; enhance pre‑filtration |
| Pitch powder agglomerates inside jet‑mill chamber | Working gas contains moisture / oil; pitch moisture exceeds 0.2 % | Upgrade compressed‑gas drying‑filtration system; improve vacuum‑drying procedure |
Important Process Remarks
- Dry screening after jet milling cannot substitute molten thermal filtration. Ultra‑fine QI particles already embedded inside pitch matrix cannot be removed by powder‑state sieving.
- All pipelines, filter housings and transfer vessels must be heat‑traced; pitch solidification inside pipeline causes filter failure and production shutdown.
- The whole molten‑filtration section must run under inert nitrogen atmosphere; pitch oxidation generates high‑oxygen species, deteriorating anode electrochemical performance.
