Sulfur in recovered carbon black (rCB) originates mainly from tire vulcanization systems. It exists in multiple forms: inorganic sulfides (primarily zinc sulfide ZnS), elemental sulfur, and stable organic‑bound sulfur (thiophenic structures locked within carbon aggregates). High‑sulfur rCB causes rubber scorch risk, poor ageing performance, unpleasant odor, and limits high‑end tire‑grade applications. Dry grinding and air classification alone cannot remove sulfur‑bearing components, so chemical reagents are essential for deep desulfurization. Based on technical content from rcb‑mill.com, this article sorts common industrial and lab‑scale chemicals for rCB desulfurization, working principles, advantages and limitations.
1. Acid‑leaching reagents (wet acidic desulfurization)
Acid treatment is widely combined with de‑ashing and zinc removal in rCB wet purification workflows. It mainly decomposes inorganic sulfides, while showing limited effect on stable organic‑bound sulfur.
Common acid chemicals
- Hydrochloric acid (HCl)
Most‑used mineral acid for rCB wet processing. Reacts with zinc sulfide to generate hydrogen sulfide gas; soluble zinc and iron salts go into aqueous phase during solid‑liquid separation.
- Reaction:
ZnS + 2HCl → ZnCl₂ + H₂S↑ - Merits: Low cost, good dissolution performance for metal sulfides; simultaneously removes iron‑based ash impurities.
- Drawbacks: Releases toxic H₂S gas requiring sealed exhaust treatment; cannot break thiophenic organic sulfur; corrosive to equipment.
- Phosphoric acid (H₃PO₄)
Milder alternative mineral acid. Less volatile than HCl, lower corrosiveness. Dissolves part of inorganic sulfides while modifying rCB surface chemistry.
- Merits: Less hazardous fume; less aggressive damage to carbon‑black aggregate structure.
- Drawbacks: Higher unit cost; incomplete removal for refractory sulfide phases.
- Nitric acid (HNO₃) / mixed acid (HNO₃‑H₂SO₄)
Oxidative acid system. Oxidizes sulfide sulfur into soluble sulfate salts, reducing hazardous H₂S emission.
- Merits: Oxidative action converts sulfur species into dissolved sulfate ions; partial removal for certain organic sulfur fractions.
- Drawbacks: Strong oxidation may destroy rCB carbon‑black aggregates, drop DBP oil‑absorption value; high corrosion; strict waste‑water handling requirement; seldom used for large‑scale industrial rCB production.
General limitation for acid leaching: Acid reagents efficiently eliminate inorganic sulfide sulfur, yet most organic‑bound thiophenic sulfur remains inside carbon matrix. Final sulfur content can rarely drop below 0.3 wt% purely by acid washing.
2. Alkaline desulfurization reagents
Alkali‑based chemicals work via hydro‑thermal reaction or high‑temperature molten‑salt system, targeting both inorganic sulfides and partial organic sulfur compounds.
Common alkaline chemicals
- Sodium hydroxide (NaOH, caustic soda)
Core reagent for alkaline desulfurization. Used in hot aqueous leaching or high‑temperature molten‑salt condition. Under heating, NaOH reacts with ZnS and transforms sulfur into soluble sulfite/sulfate salts. Often compounded with calcium‑based additives to boost desulfurization efficiency.
- Merits: Good reactivity toward zinc sulfide; also dissolves silicate ash components.
- Drawbacks: High operating temperature requirement for deep sulfur removal; produces salty alkaline wastewater.
- Potassium hydroxide (KOH)
Similar function to NaOH, stronger alkalinity. Frequently applied in lab‑scale rCB activation‑desulfurization combined process.
- Merits: Excellent impurity dissolution, certain capacity to decompose organic sulfur.
- Drawbacks: Much higher material cost than NaOH; rarely adopted for bulk industrial rCB.
- Sodium carbonate (Na₂CO₃)
Common auxiliary alkali, usually blended with NaOH in molten‑salt formula. Acts as buffering agent and sulfur‑fixing medium to capture oxidized sulfur into sodium sulfate salts. - Calcium‑series additives (CaO, Ca(OH)₂)
Typical sulfur‑fixing agents in alkaline molten‑salt system. Calcium compounds cooperate with NaOH, capture converted sulfur and form stable calcium‑sulfur salts, improving overall desulfurization rate up to 90 %+ under optimized thermal conditions.
3. Oxidative desulfurization chemical agents
Oxidative desulfurization converts insoluble sulfur‑containing substances (elemental sulfur, sulfide, partial organic sulfur) into water‑soluble sulfate, which is then separated by water washing.
- Hydrogen peroxide (H₂O₂)
Mild liquid oxidizer, usually added into acidic or alkaline leaching slurry. Oxidizes H₂S and metal sulfides directly into sulfate without large‑volume toxic gas release.
Merits: Reaction by‑product is water; relatively friendly operating environment.
Drawbacks: High consumption cost; decomposition under heating; limited effect on thiophenic organic sulfur. - Sodium hypochlorite (NaClO)
Liquid oxidative reagent for wet slurry treatment.
Drawbacks: Residual chlorine contaminates rCB; risk of producing toxic chlorinated by‑products; not recommended for rubber‑grade rCB.
4. Reagents for thermal‑gas‑phase desulfurization (non‑liquid‑leaching)
These are not wet‑bath chemicals but reaction atmospheres applied in high‑temperature rotary kiln for rCB thermal desulfurization:
- Steam (H₂O vapor)
High‑temperature water vapour reacts with surface‑bound sulfur species, releasing H₂S for exhaust collection. Often combined with alkaline sorbent filling. - Hydrogen‑containing reducing gas
Hydro‑desulfurization: sulfur converts into H₂S under hydrogen atmosphere. Very high equipment investment, mainly research‑stage technology for rCB.
5. Practical industrial process selection and critical constraints
- For standard‑grade rCB: Combine magnetic separation, grinding‑classification + acid leaching (HCl / phosphoric acid). Remove most zinc‑sulfide‑origin inorganic sulfur. Organic‑bound sulfur remains as unavoidable residual.
- For low‑sulfur premium‑grade rCB: Adopt alkali‑molten‑salt composite system (NaOH + Na₂CO₃ + CaO) with controlled high temperature, achieving high total‑sulfur removal rate.
- Hard‑to‑remove fraction: Thiophenic organic sulfur embedded in carbon framework cannot be eliminated by conventional wet acid‑base chemicals. If ultra‑low‑sulfur specification is required, thermal hydro‑desulfurization must be considered.
Key risks in chemical desulfurization
- Toxic H₂S gas must be fully absorbed and treated for acid‑leaching lines.
- Over‑strong acid or alkali will degrade rCB aggregate structure and hurt rubber reinforcement performance.
- Wastewater loaded with zinc, heavy‑metal ions and salt needs complete waste‑liquid treatment to meet environmental compliance.
For recycled carbon black wet purification, hydrochloric acid and phosphoric acid are dominant acidic desulfurization chemicals targeting inorganic sulfide sulfur. Sodium hydroxide compounded with calcium‑based additives is the most promising alkaline reagent for higher desulfurization efficiency covering partial organic sulfur. Oxidants such as hydrogen peroxide serve as auxiliary additives.
It should be noted that chemical reagents cannot fully remove thiophenic organic‑bound sulfur locked in carbon matrix. JACAN grinding‑classification equipment delivers pre‑purified rCB feedstock, which can connect with matched wet‑chemical desulfurization modules to reach target sulfur index for different‑grade recovered carbon black.
