Textile fibers (polyester, nylon, rayon) are embedded within tire rubber as reinforcement layers. Unlike steel, they are non-magnetic and cannot be captured by magnetic separators. Fiber removal relies mainly on density-based air separation, supported by mechanical liberation, screening and supplementary cleaning steps. Unremoved fibers entering pyrolysis decompose into ash, tar and char contaminants, raising residual volatiles and ash content in rCB, and may block reactor internals.
1. Mechanical liberation: shredding and granulation first
Fibers are tightly bound inside the tire matrix, so they must be physically exposed before separation.
- Primary shredding breaks whole tires into coarse chips, tearing the rubber matrix and partially releasing textile filaments.
- Secondary granulation further reduces rubber into smaller particles (10–50 mm). This step liberates more short fiber segments from rubber.
- Liberated fibers are lightweight, thin and fluffy, while rubber granulate is dense and granular. This density difference becomes the foundation for air separation.
Without sufficient granulation, fibers remain trapped inside rubber chunks and cannot be separated, no matter how good the air classifier is.
2. Zig-zag air classifier (the primary fiber removal technology)
Zig-zag air separators are the most widely used equipment for tire fiber removal in rCB preprocessing lines.
- Working principle: Material falls vertically through a zig-zag shaped chamber. Controlled upward airflow counteracts gravity.
- Dense rubber granulate has higher specific gravity, so it falls downward through the air stream and exits as clean heavy fraction.
- Low-density textile fibers are lifted by the upward air and carried out as light reject stream.
- Adjustable air velocity is critical: too low and fibers fall with rubber; too high and usable rubber granulate is lost together with fibers, reducing feed yield.
- Multiple passes through zig-zag units are often deployed for higher fiber removal efficiency, especially for fine short fibers.
3. Other auxiliary separation technologies
Air table / gravity separator
Uses vibration plus low air flow. Rubber particles settle on the vibrating deck; light fibers migrate to the top and are blown away. Good for fine fiber fractions after granulation. Often used as a polishing step after zig-zag separation.
Cyclone dust collection
Captures fine fiber dust carried in the air stream from zig-zag separators. Recovers usable rubber fines and prevents fiber dust from escaping into the environment.
Screening
Vibrating screens separate by particle size. Long fiber strands tend to form fluffy agglomerates that do not pass through defined mesh sizes, so they can be screened out. Screening works well for long fibers but is less effective for micro short fibers.
4. Process sequence integration
Typical fiber removal workflow after steel separation:
- Granulate rubber chips to liberate embedded textile fibers
- Feed granulate into zig-zag air classifier for primary fiber stripping
- Use air table or secondary zig-zag unit to remove residual fine fibers
- Cyclones collect fiber dust and recover rubber fines
- Screen to remove remaining fiber bundles before silo storage and feeding to pyrolysis reactor
5. Challenges and limitations
- Short micro-fibers: After repeated granulation, fibers are chopped into tiny fragments. These small fibers have similar aerodynamic behaviour to fine rubber particles and are hard to fully separate. Trace fiber residues are almost inevitable.
- Moisture impact: Wet fibers stick to rubber surfaces, increase bulk density and drastically reduce air separation efficiency. Feedstock drying may be required in humid environments.
- Fiber-rubber entanglement: Fibers can wrap around rubber particles, forming hybrid agglomerates that are not sorted by airflow. These carry over into pyrolysis and become a source of ash.
6. Impacts of incomplete fiber removal on rCB
When textile fibers enter the pyrolysis reactor:
- Polyamide and polyester fibers thermally decompose, generating volatile organics and leaving inorganic ash.
- Ash content in raw pyrolysis char rises, which lowers the achievable grade of final rCB.
- Fiber decomposition also creates extra tar, which deposits on rCB agglomerates, making de-agglomeration harder during downstream grinding.
- Melted fiber residues can stick to reactor screws, internal lifters and discharge points, causing blockages and unstable solid residence time.
Textile fiber removal from tire feedstock depends on mechanical liberation via shredding and granulation, followed mainly by zig-zag air classification that leverages density differences between rubber and fiber. Air tables, cyclones and screening act as polishing steps to reduce residual fiber. Complete elimination of micro-fibers is not economically feasible in industrial lines. The target is to reduce fiber carryover to a minimum threshold, preventing excessive ash and tar formation during pyrolysis and protecting final rCB quality.
