Preprocessing of end-of-life tires (ELT) is designed to remove non-rubber contaminants, reduce particle size, homogenize feedstock, and prevent equipment damage inside the pyrolysis reactor. Poor preprocessing creates unstable pyrolysis conditions, higher impurities in rCB, increased reactor wear and frequent operational downtime. The full workflow follows a sequential order: inspection & sorting → bead wire removal → primary shredding → magnetic separation → fiber removal → secondary size reduction → screening & cleaning → stockpile feeding.
1. Feedstock inspection and sorting
This is the first upstream quality control step.
- Separate tire types: passenger tires, truck tires, OTR tires. Each type has different rubber, steel, fiber and filler ratios, which affect rCB quality.
- Reject heavily contaminated tires: tires with excessive dirt, stones, metal attachments, foam inserts or sidewall damage filled with debris.
- Remove non-tire foreign objects: rims, valves, weights and plastic attachments. Mixed foreign material increases ash and metal contamination in final rCB.
- Segregate damaged or weathered tires. Severely aged tires contain oxidized rubber that can negatively impact pyrolysis behaviour.
2. Bead wire removal
Tire bead wire is thick, high-tensile steel at the tire rim.
- Thick bead wire cannot be efficiently shredded; it will jam shredders and damage cutting blades.
- Bead extraction machines cut and pull bead wires out before shredding.
- This step removes the largest steel fraction upfront and reduces the load on downstream magnetic separators.
3. Primary shredding
Shred whole tires into coarse rubber chips, typically 50–150 mm.
- Purpose: reduce bulky tires into manageable pieces, expose embedded steel cords and textile fibers for separation.
- Output quality matters: uneven oversized chips cause poor heat transfer inside the pyrolysis reactor, leading to inconsistent devolatilization. Too fine chips may be entrained and carried away by pyrolysis vapors.
- Limitation: shredding only breaks the tire; steel and fiber remain embedded within rubber chips at this stage.
4. Multi-stage magnetic separation for steel removal
Magnetic separators extract steel cords and steel filaments from shredded rubber chips.
- Primary magnet: captures large exposed steel cords.
- Secondary magnetic separation: removes finer steel fragments released during secondary shredding.
- Critical goal: minimize residual steel entering the pyrolysis reactor. Unremoved steel creates local hot spots in the kiln, catalyses unwanted vapor cracking, and leaves iron oxide impurities in raw rCB char.
- Even with multi-pass magnets, micro steel fines trapped inside rubber chips often remain and carry through to pyrolysis.
5. Textile fiber removal
Tires contain polyester, nylon or rayon textile cords.
- Fibers do not respond to magnets, so air classification / zig-zag air separators are commonly used. Low-density fibers are lifted by airflow while denser rubber chips fall through.
- If fibers enter the pyrolysis reactor: they decompose into tar and ash, increasing volatile content and ash in rCB. Fibers may also melt and stick together, blocking reactor internals or discharge systems.
- Fine fiber residues are difficult to fully eliminate and remain one source of trace ash in final rCB.
6. Secondary shredding / granulation
Coarse rubber chips are further reduced to smaller, uniform granulate (typically 10–50 mm), matching the reactor’s feed specification.
- Smaller, consistent particle size improves heat transfer uniformity during pyrolysis, helping stabilise heating rate and residence time.
- This step liberates more embedded micro steel and fiber, which can then be captured by additional magnetic and air separation steps.
- Over-granulation is avoided to prevent excessive dust generation before pyrolysis.
7. Screening and final cleaning
Vibratory screens separate oversize material, fine dust and residual loose contaminants.
- Oversize chunks are recirculated back into granulators.
- Fine mineral dirt, sand and stone are removed. These inorganic particles directly increase ash content in rCB.
- Final visual and online inspection to check residual steel and fiber levels.
8. Homogenization and controlled feeding
Clean rubber granulate is blended to stabilise batch composition.
- Blend different tire streams to keep consistent rubber / filler ratio for repeatable rCB quality.
- Store material in silos with sealed, metered screw feeders. Controlled continuous feeding prevents air ingress into the pyrolysis reactor, which would burn carbon and reduce rCB yield.
Why preprocessing directly impacts rCB final quality
| Poor preprocessing issue | Consequence for rCB |
|---|---|
| Incomplete steel removal | Iron contamination, reactor hot spots, high mill wear, poor rCB ageing resistance |
| Unremoved textile fiber | Higher ash and residual volatiles, unstable surface properties |
| Oversized rubber chips | Non-uniform pyrolysis, partial under-pyrolysis, inconsistent char |
| Sand/stone contamination | Elevated ash; rCB limited to low-grade filler applications |
| Mixed tire types without sorting | Batch-to-batch variation in DBP, ash and reinforcing performance |
Waste tire preprocessing for rCB pyrolysis is a staged purification and size-reduction sequence: sorting → bead wire removal → primary shredding → multi-stage magnetic steel separation → fiber air separation → secondary granulation → screening → homogenised feeding. Its core objective is to remove steel, textile and mineral contaminants while producing uniform rubber granulate. Effective preprocessing lowers ash and metal impurities in rCB, protects pyrolysis reactor hardware, stabilises thermal conditions, and reduces the purification burden on downstream char grinding and classification. Even an optimised pyrolysis reactor cannot compensate for feedstock contaminated with steel, fiber or high mineral dirt.
