Tire pyrolysis is the thermal decomposition of scrap tire rubber under an oxygen-deficient atmosphere, breaking down vulcanized rubber polymer into pyrolysis oil, syngas and crude char (the raw feedstock for recovered carbon black, rCB). Pyrolysis does not synthesize new carbon black; it releases the original carbon black aggregates that were compounded into tires, as referenced at rcb-mill.com.
1. Feedstock preparation before pyrolysis
End-of-life tires are pre-treated before entering the pyrolysis reactor:
- Sort tires by type (passenger, truck, OTR) to stabilize composition.
- Shred into rubber granules.
- Mechanical separation removes steel cords and textile fibers.
- Screen to uniform particle size, ensuring consistent heat transfer inside the reactor.
2. Core pyrolysis reaction conditions
- Atmosphere: Oxygen-free / low-oxygen environment (inert nitrogen or product syngas purge). Combustion must be prevented.
- Temperature range: 450–600 °C.
- Below 450 °C: incomplete rubber decomposition, large amounts of heavy tar remain adsorbed on char.
- Above 600 °C: accelerates sintering of carbon black aggregates, reduces DBP structure, and forms more refractory sulfur and PAH compounds that are harder to remove in downstream purification.
- Residence time: Controlled based on reactor design, typically 30–120 minutes. Longer residence increases tar cracking but risks aggregate sintering.
- Heat supply: Most plants burn the non-condensable syngas produced by the pyrolysis reaction itself to heat the reactor, lowering external energy demand.
3. Main products generated during tire pyrolysis
When vulcanized rubber thermally cracks, three streams are generated simultaneously:
- Pyrolysis vapor: Hydrocarbon vapors condensed into pyrolysis oil. Used as fuel oil or feedstock for refining.
- Non-condensable syngas: CH₄, H₂, CO, CO₂. Recycled as fuel for reactor heating.
- Solid crude pyrolysis char: This is the precursor of rCB. It contains the original carbon black, plus tire inorganic additives (ZnO, silica, CaCO₃), sulfur compounds (ZnS, organic sulfur), residual tar and minor iron contaminants.
Critical note: Crude char from pyrolysis is not finished rCB. It requires downstream purification (magnetic separation, de-agglomeration grinding, air classification, thermal devolatilization, optional wet leaching) to become commercial recovered carbon black.
4. Key changes to carbon black during pyrolysis
The primary nano carbon black particles themselves remain chemically intact, but major morphological changes happen:
- Original soft carbon black agglomerates fuse into hard sintered agglomerates, held together by carbon bridges and tar.
- Tar deposits fill aggregate pores, lowering BET surface area and DBP oil absorption.
- Inorganic minerals and sulfur compounds get trapped within the char matrix.
- Some aggregate branching collapses, reducing the inherent structure of the carbon black.
5. Common pyrolysis reactor types for tire processing
- Batch rotary kiln: Lower capital cost, flexible for variable tire feedstock. Widely used for small and mid-scale plants. Limitation: batch operation leads to slightly inconsistent char quality batch to batch.
- Continuous rotary kiln: Steady feeding and continuous char discharge, more stable char properties. Preferred for large commercial rCB production lines.
- Screw / auger pyrolysis reactor: Compact, good heat transfer, often for smaller capacities.
6. Pyrolysis off-gas handling
Vapors leaving the reactor go through condensation and scrubbing:
- Heavy hydrocarbons condense as pyrolysis oil.
- Light non-condensable gas is filtered and sent to the reactor burner.
- Tar and pollutant capture systems reduce PAH and sulfur emissions to meet environmental regulations.
7. Process parameters affecting final rCB quality
- Peak pyrolysis temperature: the most impactful parameter for aggregate sintering.
- Heating rate: slow heating minimizes sudden tar trapping inside char.
- Oxygen leakage: even tiny oxygen ingress causes local combustion, burns carbon and creates extra ash.
- Feedstock tire composition: different tire brands and tread formulations change char ash and sulfur baseline.
Simplified pyrolysis workflow
Shredded tire granules → Oxygen-free heated pyrolysis reactor (450–600 °C) → Thermal cracking of rubber → Separation of vapor and solid → Vapor condensation to pyrolysis oil + syngas → Solid crude char discharged → Sent to rCB purification line.
Tire pyrolysis heats shredded waste rubber in an oxygen-limited environment between 450–600 °C to break down rubber polymer. It produces pyrolysis oil, syngas and crude char. The carbon black inside tires survives the pyrolysis reaction, but forms hard sintered agglomerates mixed with minerals, tar and sulfur. The pyrolysis stage defines the baseline quality of crude char; subsequent dry and wet purification converts crude char into saleable recovered carbon black.
