Pyrolysis char generated from end‑of‑life tires, plastics and biomass is the raw feedstock for producing recovered carbon black (rCB). However, this pyrolysis char inherently contains notable iron and zinc‑based impurities: iron mostly comes from residual steel wires and mechanical wear components, while zinc mainly originates from tire‑formula zinc‑oxide curing agents.
Excess iron and zinc impurities severely degrade rCB performance. Iron triggers rubber aging and reduces anti‑oxidation capacity of rubber compounds. Zinc‑rich ash raises total ash content, weakens reinforcement performance, worsens particle‑size distribution and limits high‑end applications such as tire rubber and battery‑material fillers. To produce high‑quality rCB with D90＜10 μm particle size and low‑ash specifications, integrated physical pre‑treatment, grinding‑classification and optional chemical purification workflows are required to strip iron and zinc contaminants from pyrolysis char.
1. Physical removal of iron impurities: pre‑treatment & magnetic separation
Iron impurities exist in two forms inside pyrolysis char: large‑size steel‑wire fragments from waste tires, and fine iron‑bearing granular inclusions mixed within char particles. High‑intensity magnetic separation serves as the front‑line physical iron‑removal step in industrial rCB processing lines.
In JACAN’s standard rCB processing workflow, raw pyrolysis char firstly goes through raw‑material pre‑treatment with high‑intensity magnetic separators. Powerful magnetic fields trap steel wire debris and ferromagnetic iron particles before grinding. This step protects downstream grinding and classification equipment from hard metal‑fragment damage, while cutting bulk iron content significantly.
Limitations of magnetic separation: magnetic separation can eliminate metallic iron fragments efficiently, but cannot remove non‑magnetic iron‑oxide embedded inside char agglomerates. In addition, new iron contamination may be introduced via grinding‑equipment abrasion. To mitigate secondary iron pollution, wear‑resistant high‑alloy grinding components are adopted to minimize iron‑wear ingress during ultra‑fine pulverization.
After magnetic separation, the material enters ultra‑fine grinding and de‑agglomeration units. Grinding breaks compact char agglomerates, liberating embedded iron‑rich mineral inclusions. Subsequent high‑precision air classification separates high‑density iron‑containing coarse fractions by aerodynamic density‑size sorting, further lowering residual iron levels in finished powder and narrowing particle‑size distribution.
2. Zinc‑impurity challenge: why physical processing alone is insufficient
Unlike iron, zinc inside tire‑pyrolysis char mainly presents as zinc oxide, zinc sulfide and other zinc‑bearing inorganic compounds, tightly embedded within carbon‑black aggregate structures. Most zinc‑based impurities are non‑magnetic and share similar particle‑density ranges with carbon‑black particles.
Magnetic separation, grinding and air classification can only remove a small portion of free‑state zinc‑rich coarse ash particles. Physical dry processing cannot dissolve or separate zinc compounds locked inside carbon‑black agglomerates. For high‑grade rCB specifications requiring ultra‑low zinc and ash content, supplementary purification approaches are necessary.
3. Main industrial routes for zinc removal from pyrolysis char
3.1 Dry physical processing (grinding + air classification system)
As deployed in JACAN’s full‑scale dry‑processing system for rCB, multi‑stage grinding and high‑precision air classification realize partial zinc‑ash removal by separating coarse high‑ash mineral‑rich fractions.
Workflow:
- Pre‑treated char passes ultra‑fine grinding for thorough de‑agglomeration, releasing zinc‑rich mineral phases encapsulated inside carbon aggregates.
- Specialized air classifiers perform density‑based separation. High‑density zinc‑bearing coarse particles are diverted as tailings.
- Multi‑parameter intelligent adjustment of feed rate, air velocity and classifier frequency optimizes cut‑point, maximising removal of zinc‑rich heavy‑phase fractions while preserving rCB structural integrity and reinforcement properties.
Merits: Fully dry process, zero wastewater generation, easy integration with existing rCB production lines, low operating cost.
Drawbacks: Only free coarse zinc‑ash can be eliminated; cannot remove fine‑embedded zinc compounds, so limited purity ceiling. Suitable for mid‑grade rCB products.
3.2 Acid‑leaching chemical de‑zinc (wet purification)
Acid washing is a mature solution for deep zinc‑impurity reduction. Mineral acids such as hydrochloric acid, sulfuric acid or phosphoric acid dissolve zinc‑oxide and zinc‑sulfide impurities into soluble zinc salts, which are separated via solid‑liquid filtration and washing steps.
Process overview: Ground pyrolysis char powder mixes with dilute‑acid solution under controlled temperature and solid‑liquid ratio. Zinc‑bearing minerals leach into liquid phase. Multiple rounds of water‑washing remove residual acid and dissolved zinc ions, followed by drying to obtain low‑zinc rCB powder. This method can achieve very high zinc‑removal efficiency.
Merits: Excellent removal rate for zinc and companion metal impurities, delivers very low‑ash rCB for high‑end markets.
Drawbacks: Produces wastewater containing zinc salts; requires waste‑liquid treatment facilities; increases capital and operational expenditure; improper acid‑process parameters may damage carbon‑black aggregate structure and degrade rubber‑reinforcement performance.
3.3 Thermal fuming treatment
Under high‑temperature reduction conditions (1100‑1300 °C), zinc‑oxide compounds transform into zinc vapour, which can be captured by dust‑collection systems, separating zinc from solid‑phase carbon char. This technology is mostly used in large‑scale metallurgical‑type processing, with high energy consumption and limited adoption in mainstream rCB grinding‑classification lines.
4. Integrated industrial‑process configuration for iron & zinc removal
For commercial‑scale rCB projects, the most practical solution combines multi‑stage physical dry processing, and optionally adds chemical‑leaching units for premium‑grade output:
- Raw‑material pre‑treatment & high‑intensity magnetic separation: Remove bulk steel‑wire and metallic‑iron impurities upfront, protect downstream equipment and cut major iron sources.
- Ultra‑fine grinding & de‑agglomeration: Crush char granules, break agglomerates to liberate embedded iron‑ and zinc‑rich mineral inclusions, control product fineness D90＜10 μm while keeping carbon‑black primary‑aggregate structure intact.
- High‑precision aerodynamic air classification: Separate high‑density coarse particles containing iron‑oxide and zinc‑rich ash, optimise cut‑point via intelligent multi‑parameter adjustment to improve impurity rejection rate and product yield.
- Optional chemical‑acid‑leaching module: Deploy wet‑acid‑washing workflow when ultra‑low zinc and ash are required for high‑end tire‑grade or battery‑grade rCB applications.
- Post‑treatment: Filtration, washing, drying (for chemically‑treated material), and final powder collection.
5. Key considerations for production practice
- Balance impurity‑removal yield and product structure: Over‑aggressive grinding or over‑acid‑leaching may destroy rCB aggregate structure, reducing DBP‑absorption value and reinforcement capability. Equipment parameters must be tuned for different pyrolysis‑char feedstock sources (tire‑pyrolysis char, plastic‑pyrolysis char, biomass‑pyrolysis char).
- Control secondary iron contamination: Select wear‑resistant grinding‑chamber materials to avoid iron‑ion contamination generated by equipment‑part abrasion during pulverization.
- Process‑route selection principle: For general‑purpose rCB, adopt full‑dry magnetic‑separation + grinding + air‑classification solution with low cost and no waste‑water discharge. For high‑end low‑zinc‑low‑iron rCB requirements, add wet‑chemical leaching while configuring corresponding waste‑water‑treatment systems.
Iron impurities in pyrolysis char can be effectively eliminated by high‑intensity magnetic separation combined with grinding‑air‑classification workflows. Zinc impurities are more challenging: dry physical processing can remove partial free‑state zinc‑ash, while deep zinc reduction relies on acid‑leaching wet‑purification technology.
JACAN’s integrated grinding‑and‑air‑classification systems provide a robust dry‑processing foundation for impurity removal of pyrolysis‑char‑derived rCB, supporting real‑time parameter optimisation for variable feed‑stock properties, and delivering stable D90＜10 μm fine powder output. When paired with optional chemical‑purification units, the platform can satisfy diverse purity requirements from general‑grade to high‑end‑grade recovered carbon black.
