Steel content originates mainly from steel cords and steel bead wires in radial tires. It does not become part of the final recovered carbon black chemical structure, but it imposes major impacts on pre-treatment, pyrolysis reactor operation, char post-processing, equipment wear, safety and final rCB purity. If steel is not adequately removed before pyrolysis, it creates cascading problems across the whole rCB production line.
1. Impacts on pre-pyrolysis shredding and separation
Tire steel is tough and ductile. During primary shredding:
- Steel wires wrap around shredder rotors, causing winding, blade binding, frequent jams and unplanned downtime.
- Wrapped steel reduces shredding efficiency and produces uneven rubber chip size. Poorly sized feed leads to inconsistent heat transfer inside the pyrolysis reactor.
- Magnetic separation is the standard removal step. Large steel beads and thick cords are easy to capture; fine steel filaments are much harder. Micro steel fragments remain embedded inside rubber chips and survive into the pyrolysis stage.
- Higher steel fraction increases the load on magnetic separators, requiring multi-stage magnetic separation to lower residual steel levels.
2. Effects inside the pyrolysis reactor
Steel that enters the high-temperature pyrolysis zone changes reactor behaviour:
- At pyrolysis temperature (400–550 °C), steel conducts heat far faster than rubber. Local hot spots form around steel fragments, accelerating over-cracking and local carbon gasification. This reduces rCB yield and creates inconsistent devolatilization across the feed bed.
- Steel can catalyse secondary cracking of pyrolysis vapors, increasing gas yield, lowering oil yield and promoting extra coke deposition on char surfaces. The resulting rCB agglomerates become harder and more difficult to de-agglomerate in downstream milling.
- Long steel strands may tangle inside rotary kilns, blocking material flow, damaging internal lifters and causing uneven material movement and residence time distribution.
3. Post-pyrolysis char purification and metal contamination in rCB
After pyrolysis, residual steel transforms into fine iron/iron oxide particles mixed with rCB char.
- Fine iron contaminants cannot be fully eliminated by single-stage magnet separation. These iron impurities directly raise the metal content of finished rCB.
- Iron oxides act as pro-oxidants. In rubber compounds, they accelerate thermal ageing and oxidative degradation, which is unacceptable for tire-grade rCB. Specifications for premium rCB set strict limits on iron content.
- Iron impurities also interfere with colour consistency; higher iron content darkens or creates speckles in polymer and rubber products.
4. Equipment wear in grinding and classification circuits
Fine steel and iron oxide particles are highly abrasive.
- They accelerate wear on grinding rotors, liners, classifier wheels and cyclone internals. Maintenance cycles shorten, and spare-part costs rise sharply.
- Hard metallic fragments can create sparks inside the grinding/classification system. Since rCB dust is combustible, this increases the risk of dust explosion, requiring enhanced inerting, grounding and explosion protection design.
- Metallic grit also damages particle-size distribution: it creates coarse outliers and prevents the production of a tight, targeted PSD for high-grade rCB.
5. Operational and economic trade-offs
Tire types differ in steel loading:
- Truck radial tires: high steel cord content, thicker wires. More pre-treatment steps required, but the rubber fraction yields high-performance rCB.
- Passenger car tires: lower steel mass fraction, finer steel filaments; micro steel contamination is trickier to fully remove.
- OTR tires: heavy bead steel and thick cords; shredding and steel removal become more capital-intensive.
Higher steel feedstock demands:
- Multi-stage shredding + multiple magnetic separators
- Extra screening and air classification steps to capture steel fines
- More frequent inspection and maintenance on all processing equipment
- Additional quality testing for iron content in final rCB batches
Mitigation strategy
The best practice is to remove steel as early as possible in the pre-treatment stage: primary shredding → bead wire extraction → multi-pass magnetic separation → secondary fine shredding, before feeding rubber chips into the pyrolysis reactor. This minimizes steel carryover into the kiln and downstream powder processing. Even with optimized separation, trace iron still remains and must be controlled via post-pyrolysis magnetic refining to meet commercial rCB metal limits.
Steel itself is inert to the rCB carbon structure, but residual steel in tire feedstock disrupts the whole production workflow. It causes shredder jams, creates reactor hot spots, catalyses unwanted secondary pyrolysis reactions, introduces harmful iron contamination into finished rCB, increases mill abrasion and elevates dust explosion hazards. Premium rCB production relies on robust upstream steel removal; incomplete separation sets an upper limit on final product grade, even with well-designed pyrolysis reactors and high-performance grinding-classification systems.
