Fixed carbon (FC) represents the non‑volatile solid carbon residue after removing moisture, volatile matter and ash from pyrolysis char. High fixed‑carbon pyrolysis char is the preferred feedstock for high‑quality recovered carbon black (rCB). Higher fixed‑carbon means lower proportion of tar residues and organic impurities, which helps improve rCB reinforcement performance and reduces odor risk.
Fixed carbon is restricted by tire feedstock composition, pyrolysis thermal profile, post‑treatment and impurity removal. Based on technical insights from rcb‑mill.com, this article describes practical industrial approaches to raise fixed‑carbon value of waste‑tire pyrolysis char, mechanism, trade‑offs and process limitations.
1. Understand composition constraints of raw waste‑tire feedstock
Fixed‑carbon upper limit is partially determined by incoming tire material.
- Prioritize whole‑passenger / truck tire rubber; minimize mixed feed containing plastic, rubber hoses, padding and other miscellaneous waste. Foreign polymer feed brings extra volatile organic fractions and lowers final fixed carbon.
- Remove steel cords, textile fiber and large dirt contaminants before pyrolysis. Textile fibers generate extra ash and volatile components, diluting fixed‑carbon proportion in char.
Important note: Tire itself contains inherent inorganic fillers (ZnO, silica, calcium carbonate). These form ash, they do not contribute to fixed‑carbon value. Even with perfect pyrolysis, ash will always occupy part of char mass.
2. Optimize pyrolysis process parameters to boost fixed carbon
Fixed‑carbon increases by driving off tar, light hydrocarbon and other volatile organic substances from solid char matrix.
2.1 Pyrolysis temperature
- Too low temperature (<450 °C): incomplete decomposition, large amount of heavy tar remains adsorbed on char surface; volatile matter stays high, fixed‑carbon remains low.
- Moderate‑high temperature range 550–650 °C: continuous cracking and volatilization of tar components; volatile matter drops and fixed‑carbon rises. This is the preferred operating window for rCB‑oriented pyrolysis char.
- Excessively high temperature (>700–750 °C): fixed‑carbon will no longer increase significantly. Instead, severe sintering of carbon‑black aggregates occurs, degrading DBP and BET properties for downstream rCB.
2.2 Solid residence time
Sufficient holding time at target temperature allows secondary cracking of adsorbed tar inside char. Longer residence time removes more heavy organics and raises fixed‑carbon.
- Trade‑off: Over‑long residence time accelerates carbon sintering. Balance fixed‑carbon improvement versus rCB aggregate structure damage.
2.3 Vapor residence time
Shorten vapor residence time to avoid secondary tar re‑condensation back onto hot char surface. When oil vapor stays too long inside reactor, heavy tar may re‑adsorb onto solid char, increasing volatile matter and suppressing fixed‑carbon. Optimize reactor gas‑phase flow path for fast vapor discharge.
2.4 Inert atmosphere
Maintain oxygen‑free inert environment. Trace oxygen will cause partial char combustion and mass loss; burn away solid carbon and reduce fixed‑carbon yield. Strictly prevent air leakage into pyrolysis reactor.
Practical pyrolysis outcome: Well‑optimized tire pyrolysis can lift char fixed‑carbon typically to 75 ~ 84 wt% (dry basis, before de‑ashing). Further growth is restricted by inherent ash content.
3. Post‑pyrolysis upgrading technologies to increase fixed‑carbon
Even after pyrolysis, char surface still carries residual heavy tar and organic deposits. Post‑treatment can further upgrade fixed‑carbon.
3.1 Mild thermal post‑calcination / tempering
Subject raw pyrolysis char to secondary moderate‑temperature heat treatment under inert atmosphere at 500‑620 °C. Surface‑adsorbed heavy tar undergoes thermal cracking and volatilization. Volatile matter decreases, fixed‑carbon percentage increases.
- Merits: Effectively strip residual tar; reduce odor of char.
- Drawbacks: Additional energy consumption; risk of aggregate sintering if temperature exceeds threshold. Must strictly control temperature and holding time.
3.2 Physical separation to reduce ash dilution
Fixed‑carbon is a calculated index:
Fixed‑carbon = 100 − moisture − volatile matter − ash
High ash content directly dilutes fixed‑carbon reading. Even if carbon substance itself does not change, removing mineral ash will mathematically raise fixed‑carbon percentage.
Workflow:
- Ultrafine grinding liberate mineral impurities
- Magnetic separation + multi‑stage air classification remove part of ash‑rich tailings (dry route)
- Optional wet acid‑alkali leaching for deep ash removal
Key concept: This is enrichment by impurity removal, not generating more carbon substance. It is one of the most effective industrial means to obtain high fixed‑carbon finished rCB powder.
3.3 Solvent washing (lab‑scale, rarely mass production)
Organic solvent washing dissolves surface‑sticky tar components from char. Volatile‑matter decreases and fixed‑carbon rises. High solvent cost and recovery complexity limit large‑scale application.
4. Critical unavoidable trade‑offs
- Fixed‑carbon vs sintering risk
Higher pyrolysis temperature / longer calcination improves fixed‑carbon, but causes sintering of original carbon‑black aggregates. BET drops, DBP oil absorption decreases, and rCB reinforcing performance deteriorates. Do not blindly pursue maximum fixed‑carbon value. - Fixed‑carbon vs char yield
Driving off tar and organics reduces total char mass. Higher fixed‑carbon is accompanied by lower char mass yield from tire feedstock. Project economy needs to be balanced. - Fixed‑carbon test logic
Dry‑basis fixed‑carbon index is affected by ash content. Two char samples with identical true carbon content can show very different fixed‑carbon readings simply due to ash difference. Always test ash, volatile matter, BET and DBP together for comprehensive quality evaluation.
5. Recommended complete industrial workflow for high fixed‑carbon char
- Feedstock sorting: clean waste tire feed, exclude miscellaneous plastic/rubber impurities, remove textiles and large dirt.
- Main pyrolysis: control reactor temperature 550‑650 °C, optimize solid and vapor residence time under oxygen‑free inert atmosphere; rapidly export oil vapor to prevent tar re‑condensation.
- Optional mild inert‑atmosphere post‑calcination for raw char to strip residual surface tar.
- Cooling under inert condition, avoid char oxidation by air.
- Dry upgrading: magnetic separation + anti‑wear ultrafine grinding + multi‑stage air classification for ash removal, to further enrich fixed‑carbon in rCB powder.
- Optional wet‑leaching for high‑grade product for deep ash reduction and higher fixed‑carbon finished powder.
Conclusion
To increase fixed‑carbon of pyrolysis char: first control clean tire feedstock, then optimize pyrolysis thermal profile (temperature, residence time, fast vapor escape, inert atmosphere) to crack and remove tar‑like volatile organics. Mild post‑calcination can further strip surface‑adsorbed organics.
It should be emphasized that ash dilution heavily influences fixed‑carbon index. Removing mineral impurities via grinding‑classification or wet‑leaching is an effective way to enrich fixed‑carbon in final rCB.
Meanwhile, excessive thermal treatment will sinter carbon‑black aggregates and damage reinforcing properties. Fixed‑carbon must be balanced against BET, DBP and overall process yield. JACAN grinding‑classification system performs dry‑mode ash enrichment for pyrolysis char and cooperates with upstream pyrolysis units to produce high‑fixed‑carbon rCB feedstock.
