How to remove impurities from tire pyrolysis char?

Crude pyrolysis char (raw recovered carbon black / rCB) from waste tire pyrolysis contains multiple impurity fractions, which cause specks, poor dispersion, higher ash content, unstable color, reduced reinforcing performance and equipment abrasion. Common impurities include: ferrous metal fragments, non-ferrous metal particles, nylon fiber residues, sand & soil grit, inorganic mineral ash (ZnS, SiO₂, CaCO₃), residual tar & volatile organics.

Based on practical rCB upgrading experience from rcb-mill.com, this article systematically classifies industrial purification technologies, sorts the optimal process sequence, compares dry physical separation (mainstream mass production) and chemical demineralization (high-end premium grade), and delivers troubleshooting guidance.

Important principle reminder
Physical separation can remove discrete foreign impurities (metal, fiber, grit). It CANNOT eliminate inherent inorganic ash embedded inside carbon aggregates (Zn, silica originally formulated in tires). To drastically lower total ash content, chemical washing demineralization is required.

1. Classification of Impurities & Corresponding Removal Technologies

2. Standard Industrial Purification Route – Dry Physical Purification (For Rubber & Masterbatch Grade rCB)

This is the most widely adopted, cost-effective flow integrated on rcb-mill complete processing lines, suitable for large continuous production.

Step 1: Primary high-intensity magnetic separation (Front-end, critical protection)

• Equipment: High-gauss electromagnetic separator (8000–16000 Gauss)
• Target: Remove large steel fragments before grinding, prevent rotor & classifier abrasion.
• Operation tip: Install magnetic separation before coarse crushing, and deploy a secondary magnetic separator after grinding to capture micro iron particles.

Step 2: Coarse crushing & vibrating screening

Crude char bulk lumps are crushed; vibrating sieve removes long fibers, large grit and unpyrolyzed rubber chunks.

• Mesh selection: 60–120 mesh for primary screening.
• Limitation: Cannot separate fine impurities mixed with powder.

Step 3: Thermal pretreatment (Drying + tar removal)

Flash dryer / indirect heating dryer (130–180°C, avoid >220°C oxidation):

1. Remove adsorbed moisture to prevent powder adhesion inside grinding system
2. Volatilize surface tar; eliminate sticky substances that bind impurities and carbon aggregates together

Step 4: Ultrafine closed-circuit grinding + dynamic air classification (Core purification unit)

This step separates lightweight fine rCB from dense coarse mineral impurities:

1. Grinding breaks sintered pyrolysis agglomerates
2. Air classifier separates material by particle size and density
3. Heavy inorganic grit, large unopened impurity clusters are rejected into circulating coarse material and discharged regularly

Working logic: Mineral ash particles have higher density; they tend to be thrown outward by centrifugal force inside the classifier and excluded from finished fine powder.

Step 5: Homogenization silo & final inspection

Blend powder to avoid impurity segregation during pneumatic conveying; online sampling for ash content and particle size monitoring.

Full dry process flow summary
Crude pyrolysis char → Primary magnetic separation → Coarse crushing + vibrating screen → Drying & tar removal → Closed-circuit ultrafine grinding + air classification → Secondary fine magnetic separation → Homogenization → Finished refined rCB

✅ Advantages: Continuous production, low water consumption, no wastewater, low operating cost
❌ Limitation: Can only reduce ash moderately; cannot remove micro inorganic ash locked within carbon aggregates. Typical final ash: 10–16%

3. Advanced Purification: Wet Chemical Demineralization (For Ink, Coating, Premium Conductive Grade rCB)

If customers require ultra-low ash (≤3–5%), physical separation alone fails; acid-alkali two-stage washing is adopted.

Process principle

1. Acid leaching (Dilute HCl)
   Dissolve zinc compounds (ZnS, ZnO), calcium salts and soluble metal oxides.
2. Alkali leaching (Dilute NaOH)
   React with silica (SiO₂) to remove white carbon black ash.
3. Multi-stage filtering & repeated washing with deionized water until neutral pH
4. Deep vacuum drying to obtain low-ash refined rCB

✅ Advantages: Dramatically reduce total ash content, improve jetness and dispersion
❌ Disadvantages: High investment, generates wastewater, longer production cycle, higher energy cost; must verify surface property changes after chemical treatment for downstream applications.

4. Auxiliary Supporting Technologies

4.1 Gravity air separation

Can be arranged after screening to separate heavy grit from light carbon black powder, often used as pre-treatment before grinding.

4.2 Inert atmosphere thermal purification

Low-oxygen heating removes heavy tar without carbon surface oxidation, improving wettability in polymer matrix.

4.3 Ultrasonic dispersion (Lab / small batch)

Used in wet demineralization to unlock ash wrapped inside carbon agglomerates, improve washing efficiency.

5. Common Operational Mistakes & Troubleshooting

Problem 1: Still visible metal speckles in finished rCB

Root cause: Only one-stage magnetic separation; micro fine iron particles escape; magnetic strength insufficient.
Solution: Two-stage magnetic separation (coarse + fine powder stage); upgrade to high-gauss electromagnetic separator.

Problem 2: Ash content cannot be lowered further by dry processing

Root cause: Most ash is embedded inside carbon aggregates, not discrete free mineral particles.
Solution: If lower ash specification required, add chemical demineralization process.

Problem 3: Fiber and soft impurities block classifier wheel

Root cause: Insufficient front-end screening; residual tar makes fibers sticky.
Solution: Strengthen pre-screening; optimize thermal tar removal.

Problem 4: Impurity particle size fluctuates batch by batch

Root cause: Unstable waste tire feedstock (mixed passenger tires/truck tires, soil-contaminated raw tires).
Solution: Segregate feedstock; avoid random mixing of dirty waste tires.

6. How to Choose the Suitable Purification Route

1. Rubber compound, ordinary plastic masterbatch (General market)
   👉 Dry physical purification process (rcb-mill standard line)
   Balanced cost, stable quality, meets mainstream requirements.
2. High-end coatings, conductive inks, special composites (Low ash demand)
   👉 Dry pre-purification + wet chemical acid-alkali demineralization
3. Fuel-grade crude char (Low requirement)
   👉 Simple magnetic separation + screening only, no ultrafine grinding.

7. Conclusion

Impurity removal for tire pyrolysis char follows a layered control strategy:

1. Remove large discrete foreign impurities (metal, fiber, grit) via magnetic separation and screening at the front end;
2. Remove tar and moisture by thermal pretreatment to avoid impurity agglutination;
3. Use closed-circuit ultrafine grinding and dynamic air classification to separate dense mineral impurities, realizing primary upgrading;
4. For ultra-low ash premium grades, deploy supplementary wet chemical demineralization.

The integrated rCB processing system designed by rcb-mill.com combines magnetic impurity removal, drying, ultrafine de-agglomeration grinding and precision air classification, systematically removing external impurities and stabilizing ash level for refined recovered carbon black.