How to reduce ash content in rCB to below 1.5%?

Recovered carbon black (rCB) produced from waste‑tire pyrolysis char faces a major bottleneck of high ash content. Ash mainly consists of zinc‑oxide, zinc sulfide, iron oxides, silica, calcium salts and other inorganic mineral residues inherited from tire formulations and pyrolysis processes. For high‑end rubber compounding, plastic filling and battery‑related applications, rCB ash must be controlled below 1.5%.

Conventional simple grinding and screening only deliver rCB ash above 3–5%. To hit the strict ＜1.5 % ash target, a combined workflow of raw material pre‑treatment, liberation by ultrafine grinding, multi‑stage precision air classification, and optional wet chemical purification is required. This article is based on process practice from rcb‑mill.com for industrial‑grade low‑ash recovered carbon black production.

1. Source control of pyrolysis char feedstock

Ash reduction starts at feedstock selection, as raw char quality sets the upper limit for final ash performance.

• Prioritize high‑quality tire‑pyrolysis char; avoid char mixed with plastic, rubber waste and other miscellaneous feed which brings extra mineral fillers.
• Remove coarse slag, un‑carbonized residues and large inorganic lumps before entering the milling circuit.
• Stabilize pyrolysis temperature and residence time: incomplete pyrolysis retains more inorganic‑organic bonded mineral phases, making ash removal far harder.

Even with optimal pyrolysis output, raw char ash commonly sits at 6‑12%, so downstream purification remains indispensable to reach below 1.5%.

2. Physical pre‑treatment: iron removal and coarse ash rejection

Iron‑bearing particles from tire steel cords and equipment abrasion form a considerable portion of total ash.

1. High‑intensity magnetic separation
   Deploy multi‑pass high‑intensity magnetic separators ahead of grinding. Metallic iron fragments and ferromagnetic iron oxides are removed. This step cuts part of iron‑based ash and protects subsequent grinding and classification rotors from metal impact damage. Magnetic separation works well for free metallic iron but has limited effect on non‑magnetic iron‑oxide embedded inside char aggregates.
2. Primary screening and air elutriation
   Remove oversized mineral slag and heavy coarse ash fractions. These fractions contain concentrated inorganic impurities and should be discharged as tailings before fine processing to avoid circulating high‑ash material into ultrafine milling.

Note: Physical pre‑treatment alone cannot push total ash below 1.5%. It serves to reduce impurity load and lower the burden for subsequent core purification units.

3. Ultrafine grinding for mineral‑carbon liberation

Most inorganic ash phases are tightly encapsulated inside pyrolysis char agglomerates. Without sufficient de‑agglomeration, mineral impurities stay locked within carbon particles and cannot be separated by air classification.

In the JACAN rCB grinding‑classification system:

• Specialized ultrafine mill breaks compact char agglomerates without excessively destroying primary carbon‑black aggregate structure, preserving critical DBP absorption reinforcement index.
• Grinding liberates embedded zinc, iron, silicate and calcium mineral inclusions, freeing inorganic ash particles for density‑based separation.
• Use anti‑wear liners and rotors (ceramic or high‑alloy material) to prevent secondary ash pollution from equipment metal abrasion. New iron contamination generated by grinding will directly raise final ash value and ruin the target below 1.5%.

4. Multi‑stage precision air classification (core dry‑process step)

After liberation by grinding, multi‑series high‑precision air classifiers perform density‑size sorting. Inorganic ash particles own higher specific density than carbon‑black fractions.
Process key parameters to tune:

1. Classifier rotor speed: raise rotating frequency to sharpen cut‑point, reject more high‑density mineral‑rich fine fractions as tailings.
2. System air volume and feed rate: low, stable feeding improves separation efficiency; excessive feed causes material crowding and deteriorates ash rejection performance.
3. Multi‑stage cascading classification: primary classification removes major heavy‑ash tailings; secondary fine polishing classification further strips residual fine mineral particles.

Dry‑process limitation: Well‑optimized full‑dry grinding plus multi‑stage classification typically achieves rCB ash around 2.0‑2.8%. It is difficult to stably and repeatedly get below 1.5% relying only on dry physical separation, because ultra‑fine zinc‑containing mineral grains are mixed at similar particle size and density with carbon‑black particles. To hit ＜1.5% ash specification, wet chemical leaching must be integrated.

5. Wet chemical leaching: indispensable for ash ＜1.5%

Acid leaching dissolves zinc oxides, zinc sulfide, iron oxides and soluble salt minerals into liquid phase, realizing deep ash reduction.
Typical workflow:

1. Dry‑milled low‑pre‑ash rCB powder is mixed with dilute acid solution under controlled temperature, solid‑liquid ratio and stirring intensity. Zinc, iron, calcium‑based inorganic ash components dissolve into liquid.
2. Solid‑liquid filtration separates carbon‑black solid phase from metal‑ion‑containing leachate.
3. Multiple counter‑current water washing removes residual acid and dissolved inorganic ions.
4. Vacuum or hot‑air drying produces finished low‑ash rCB with total ash stably below 1.5%.

Critical process risks to manage:

• Over‑strong acid condition or over‑long reaction time will damage rCB aggregate structure, drop DBP value and weaken rubber reinforcement performance.
• Leachate contains zinc and heavy‑metal ions; complete wastewater treatment system is mandatory for environmental compliance.
• Increase capital investment and operating cost compared with all‑dry process.

6. Complete industrial combined process route for ash below 1.5%

1. Feedstock screening & quality selection of qualified tire pyrolysis char
2. Multi‑pass high‑intensity magnetic separation + air elutriation for coarse ash and iron removal
3. Anti‑wear ultrafine grinding for full liberation of encapsulated mineral impurities
4. Two‑stage precision air classification for preliminary ash reduction (intermediate product ash ~2.0‑2.8%)
5. Wet acid leaching → filtration → multi‑stage water washing → drying for deep purification
6. Final fine powder collection and quality inspection for total‑ash testing

7. Practical production key points

1. Yield‑quality trade‑off: Both dry classification tailings loss and wet‑leaching material loss should be considered. Higher ash removal rate means lower finished‑product yield. Project design must balance target ash, yield and operating cost.
2. Strictly avoid secondary contamination: All contact parts in grinding, classification and slurry‑processing section should adopt anti‑wear non‑ferrous lining materials.
3. Feed fluctuation compensation: Pyrolysis char ash varies batch‑by‑batch; online ash monitoring and real‑time adjustment for classifier parameters and leaching condition help maintain stable ＜1.5% ash output.
4. Product positioning: For general‑grade rCB, use full‑dry process for ash 2.0‑4.0%. Only high‑value markets require adding wet‑leaching unit for ash below 1.5%.

Physical pre‑treatment, anti‑wear ultrafine grinding and multi‑stage air classification can greatly lower rCB ash, yet stable total ash below 1.5% cannot be achieved by dry technology alone. To reach this strict specification, dry physical purification for pre‑de‑ashing combined with wet acid leaching is the reliable industrial solution.

JACAN rCB grinding‑classification system delivers qualified pre‑purified powder, and can be matched with supporting wet‑leaching modules to produce low‑ash recovered carbon black meeting ash＜1.5% requirement for high‑end application scenarios.