How to improve the pH value of recovered carbon black?

Recovered carbon black (rCB) from mining tire pyrolysis char often shows low, acidic pH (pH 4–6). This acidity comes mainly from surface carboxyl/phenolic acidic groups, residual sulfur compounds, tar and PAH deposits, and soluble acidic salts trapped on carbon aggregates. Raising rCB pH targets neutralizing surface acidic functional groups while preserving its carbon-black reinforcing structure for rubber, masterbatch and composite applications. Multiple industrial routes are available, from dry thermal treatment to wet alkaline neutralization.

1. Thermal devolatilization / high-temperature inert calcination (preferred dry industrial method)

This is the primary dry route to lift pH without introducing liquid waste, widely adopted in rCB plants referenced at rcb-mill.com.

• Principle: Under inert nitrogen atmosphere at 800–900 °C, thermal treatment decomposes and removes surface acidic oxygen-containing groups (–COOH, lactones, phenolic groups), residual tar, volatile sulfur species and organic acids. Fewer acidic surface sites directly increases rCB pH.
• Operation notes:
  • Control oxygen strictly; excess air causes carbon burnout and loss of reinforcement performance.
  • Hold time: 30–90 min. Longer residence time removes more acidic groups and pushes pH higher.
  • Sweet spot for tire-grade rCB: 850 °C. It raises pH from ~5.0 up to 7.0–7.8, without severe sintering of rCB aggregates.
• Advantage: Fully dry process, compatible with the magnetic separation → grinding & classification workflow for mining tire pyrolysis char; no wastewater generation.
• Limitation: Cannot reach very high pH (>8.2) alone, because inherent ash minerals still control surface chemistry.

2. Wet alkaline neutralization / alkaline washing (for targeted higher pH)

For projects requiring pH >7.8, wet alkaline treatment is used after grinding and classification.

• Reagents: Dilute NaOH or KOH solution (0.1–0.5 mol/L). Avoid over-concentrated strong alkali which can damage carbon aggregate structure and increase ash.
• Process: Prepare rCB slurry, add alkaline agent under stirring at 50–80 °C, hold 1–3 h, filter, wash with deionized water to remove excess free alkali, then dry at 110–130 °C.
• Mechanism: Alkali neutralizes surface carboxylic and phenolic groups and converts acidic surface sites into salt forms, increasing measured pH.
• Key risk: Over-washing or excessive alkali dosage will leave residual inorganic salts on powder, raising ash content and hurting rubber reinforcement properties. Final rinsing is critical.

3. Dry alkaline doping / surface coating (continuous powder treatment)

Suitable for inline continuous production after air classification and before pelletizing.

• Method: Fine atomized dilute alkaline solution is sprayed onto rCB powder inside a mixer or pelletizer, followed by low-temperature drying (120–180 °C).
• Benefit: Minimal water consumption, easy integration with rCB pelletizing. Adjust the alkali spray rate dynamically to stabilize pH within target range (7.0–8.0).
• Typical use case: Masterbatch-grade rCB, where moderate pH elevation is needed while maintaining good powder flowability.

4. Pre-removal of acidic impurities by water leaching

A low-cost pre-step before thermal or alkaline treatment.
Hot deionized water leaching removes water-soluble acidic salts and residual acidic sulfur compounds from crude pyrolysis char. It partially lifts pH and reduces alkali consumption in downstream modification. This is often applied after magnetic separation and before ultra-fine grinding.

Recommended sequence for mining tire rCB to raise pH

1. Magnetic separation and screening to remove steel and coarse mineral impurities
2. Hot water leaching (optional) to strip soluble acidic salts
3. Ultra-fine de-agglomeration grinding + air classification
4. Inert-atmosphere thermal devolatilization at ~850 °C (main pH-boosting step)
5. Optional mild wet alkaline washing or dry alkali spray if higher pH target (>7.8) is required
6. Drying, pelletizing and online pH QC testing

Critical quality trade-offs

1. Do not over-treat: Excessively high temperature (>950 °C) or strong concentrated alkali will degrade rCB aggregate structure, reduce tint strength and lower rubber reinforcement.
2. Ash balance: Alkaline treatment may increase ash; always monitor ash, pH and BET surface area together.
3. Application matching:
   • Tire / heavy-duty rubber: target pH 7.0–7.8, prefer thermal devolatilization only
   • Engineering plastic & color masterbatch: pH 7.5–8.2, thermal + mild alkaline adjustment

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