Raw pyrolysis char (also called pyrolysis carbon black or crude char) is the solid output directly from the tire pyrolysis reactor. It is not a saleable rCB product. Finished recovered carbon black is obtained after multi-stage purification, de-agglomeration, grinding and air classification of raw char. The two materials differ dramatically in particle state, impurities, surface properties, processability and end-use performance.
1. Particle and agglomerate structure
Raw pyrolysis char
- Consists of large, hard agglomerates. Primary carbon black particles are fused and bonded together by residual tar and secondary coke formed during pyrolysis.
- Agglomerate size ranges from tens of microns up to millimetre-level lumps.
- Original primary carbon black aggregates from the tire are locked inside coke/tar bonds. These bonds cannot be broken by simple sieving.
- Wide, uncontrolled particle size distribution.
Finished rCB
- Ultra-fine powder after de-agglomeration and air classification. Target PSD is typically D90 < 10 μm or finer, depending on grade.
- The grinding circuit breaks coke bridges between primary aggregates while preserving the native carbon black aggregate structure (DBP absorption).
- Narrow and tightly controlled particle size distribution, meeting customer specification.
2. Impurity content
Raw pyrolysis char
- High ash content: contains inorganic residues from tire fillers, ZnO, silicates, calcium carbonate, plus ash from textile fibers.
- Residual iron: fine iron/iron oxide particles from incompletely removed tire steel.
- Residual volatiles (tar): heavy hydrocarbon compounds adsorbed on char surfaces or trapped inside agglomerates.
- Trace contaminants: mineral dirt, minor sulfur compounds from rubber vulcanization.
Finished rCB
- Ash is reduced by magnetic refining, air classification and optional dry/wash purification steps.
- Iron content is strictly controlled via post-pyrolysis magnetic separation to avoid accelerating rubber ageing.
- Residual volatiles are removed either by controlled reactor devolatilization or low-temperature post-treatment.
- Impurity levels are certified to match commercial specifications for rubber or plastic compounding.
3. Surface chemistry and reinforcing properties
Raw pyrolysis char
- Surfaces are covered with coke and tar deposits. These coatings block the active surface sites on carbon black aggregates.
- DBP absorption is artificially low, because the aggregate network is locked inside coke. It cannot fully disperse in rubber.
- Poor compatibility with rubber and polymer matrices. If used directly, it delivers very low reinforcement.
Finished rCB
- Coke/tar layers are partially removed and agglomerates are de‑clumped. The active surface of carbon black aggregates is exposed.
- DBP absorption, BET surface area and structure are stabilized and tested to match target grades.
- Good dispersion in rubber compounds; delivers reinforcing performance that can partially substitute virgin carbon black.
4. Flowability, handling and safety
Raw pyrolysis char
- Contains irregular hard lumps and sticky tar-bound agglomerates; poor powder flow.
- Higher residual tar can create sticking, caking and odour issues during storage and transport.
- Not a free-flow powder.
Finished rCB
- Free-flowing fine powder. Bulk density is controlled for pneumatic transport and automatic dosing in compounding lines.
- Low tar content, stable storage behaviour, compliant with industrial powder handling standards.
5. Suitability for end applications
Raw pyrolysis char
- Cannot be directly used in tires, hoses or high-performance rubber.
- Only suitable for low-value applications like solid fuel or low-grade filler after simple treatment.
Finished rCB
- Graded for different uses: tire compounds, industrial rubber, plastic masterbatch, coatings. Premium grades can replace virgin carbon black in tire formulations.
Comparison table
| Parameter | Raw Pyrolysis Char | Finished Recovered Carbon Black |
|---|---|---|
| Agglomerate state | Large, coke-bonded hard lumps | De-agglomerated fine powder |
| Particle size | Very broad, mm-scale chunks | Controlled PSD (D90 <10 μm typical) |
| Ash content | High | Reduced and tightly controlled |
| Iron contamination | Higher residual iron | Low, controlled by magnetic polishing |
| Surface condition | Covered with tar and secondary coke | Cleaned, exposed active carbon surface |
| DBP absorption | Low, masked by coke | Measurable, stable reinforcing structure |
| Dispersion in rubber | Poor | Good for qualified grades |
| Commercial status | Intermediate reactor product, not saleable | Final saleable powder product |
Raw pyrolysis char is a crude, agglomerated, impurity-rich solid byproduct from pyrolysis. Its carbon black primary aggregates are trapped inside coke and tar, so it has almost no usable reinforcement properties. Finished rCB is produced by purifying raw char, removing metal/mineral impurities, and applying ultra-fine grinding and air classification to break coke bonds while preserving carbon black aggregate structure. This downstream processing is mandatory to convert crude pyrolysis solids into a high-performance filler comparable to virgin carbon black.
