How to separate silica from recovered carbon black

Silica (SiO₂) in recovered carbon black (rCB) mainly comes from tire rubber formulations, including reinforcing silica fillers, anti‑aging agents, and trace mineral contaminants from raw rubber. During tire pyrolysis, silica remains in pyrolysis char without decomposition or volatilization. Silica particles feature high hardness, high density and chemical inertness. Excess silica increases total ash content, raises abrasiveness of rCB powder, damages processing equipment, and weakens rubber reinforcement performance.

Unlike zinc‑based impurities, silica is resistant to most dilute‑acid leaching. Based on technical practice from rcb‑mill.com, this article introduces physical separation, chemical dissolution, combined process routes, limitations and production notes for silica removal from rCB.

1. Occurrence characteristics of silica in pyrolysis char

1. Partial free coarse silica particles mixed in char;
2. Most silica exists as fine micro‑inclusions tightly embedded inside carbon‑black agglomerates;
3. Chemically stable: insoluble in common dilute hydrochloric acid, sulfuric acid and phosphoric acid. Conventional acid leaching for zinc and iron removal cannot dissolve silica.
4. Higher true density compared with carbon black, which creates conditions for density‑based physical separation.

These properties determine that coarse free silica can be partially removed by physical methods, while embedded fine silica is difficult to eliminate. Deep silica reduction requires special chemical reagents.

2. Physical separation methods

2.1 Liberation by ultrafine grinding

Before separation, grinding is essential. JACAN ultrafine grinding system breaks sintered char agglomerates, liberating silica grains wrapped inside carbon matrix. Without sufficient liberation, silica inclusions stay locked in carbon particles and cannot be separated.

Important note: Adopt ceramic or high‑alloy anti‑wear liners. Silica is highly abrasive; improper equipment materials will generate extra iron contamination during long‑time milling.

2.2 Multi‑stage high‑precision air classification

Silica has significantly higher specific gravity than carbon black. After liberation, air classification separates high‑density silica‑rich fractions as tailings by aerodynamic density‑size sorting.

• Optimize classifier rotor speed, system air volume and stable feeding rate to sharpen the separation cut‑point.
• Multi‑cascade classification improves rejection rate for dense silica‑rich particles.

Limitations:
Physical classification can remove most free coarse and medium‑sized silica. For fine silica below several micrometers with particle density close to rCB aggregates, separation efficiency drops sharply. Dry physical routes cannot completely eliminate fine embedded silica. Typical effect: physical processing reduces silica content partially, but cannot push silica to very low levels.

2.3 Wet gravity separation (hydraulic classification / hydrocyclone)

After grinding, rCB is dispersed into water slurry. Hydrocyclones utilize density difference to separate high‑density silica into underflow tailings.

• Merits: Good performance for coarse‑to‑medium silica particles.
• Drawbacks: Produces large slurry volume; ultra‑fine silica still hard to separate; additional filtration and drying steps increase energy cost. Mainly used as auxiliary unit rather than main process.

3. Chemical dissolution for deep silica removal

Since silica is insoluble in common mineral acids, alkaline etching is the mainstream chemical approach for deep silica removal.

3.1 Hot strong alkali leaching

Sodium hydroxide (NaOH) hot leaching reacts with silica to generate soluble sodium silicate.
Reaction equation: SiO₂ + 2NaOH → Na₂SiO₃ + H₂O

Typical operating conditions: high concentration NaOH solution, elevated temperature (90‑130 °C), sufficient stirring. After reaction, soluble sodium silicate is removed by solid‑liquid filtration and counter‑current water washing.

Merits: Effective dissolution for both free and exposed silica particles, greatly reduces silica content in rCB.
Drawbacks:

1. High temperature and strong alkali bring heavy equipment corrosion; reactors require anti‑corrosion lining.
2. Produces high‑salinity alkaline wastewater containing silicate, demanding dedicated wastewater treatment.
3. Strong alkali may alter rCB surface functional groups and partially affect rubber‑compounding performance; process parameters must be strictly controlled to avoid over‑etching.

Process sequence tip: For low‑ash rCB production combining Zn, Fe and SiO₂ removal: acid leaching (remove Zn, Fe) → filtration & washing → hot alkali leaching (remove silica) → re‑washing → drying. Single‑step acid‑alkali mixed leaching cannot work.

3.2 Hydrofluoric acid (HF) system

HF can chemically react with silica to form soluble fluorosilicate. It achieves excellent silica‑removal efficiency.
Drawbacks: Extremely toxic, highly corrosive, strict safety requirements. It is almost avoided in industrial rCB mass production, only applied for laboratory small‑sample treatment.

4. Complete industrial combined process options

Option 1: All‑dry process for general‑grade rCB

1. Raw char pre‑treatment & magnetic separation for iron removal
2. Anti‑wear ultrafine grinding for mineral liberation
3. Two‑or‑three‑stage air classification, discharge silica‑rich heavy tailings

Outcome: Remove most coarse silica. Fine embedded silica remains inside product. Suitable for ordinary rubber and filler grades where ultra‑low silica is not mandatory.

Option 2: Acid‑alkali two‑step wet purification for high‑end low‑silica rCB

1. Pre‑treatment and dry grinding‑classification for preliminary impurity rejection
2. Acid leaching → filtration & washing: remove zinc, iron and other acid‑soluble ash
3. Hot sodium‑hydroxide alkaline leaching → multiple counter‑current washing: dissolve and remove silica
4. Filtration, drying, powder collection

Outcome: Achieve deep removal of silica together with zinc and iron. Meet strict low‑ash specifications for premium‑grade rCB. Higher capital and operating cost, plus wastewater‑treatment obligations.

5. Key practical challenges

1. Silica embedded inside carbon aggregates: Even alkali leaching cannot reach silica completely enclosed inside intact carbon domains. Therefore feed‑source control is important; select tire pyrolysis char with inherently low silica input.
2. Yield loss: Both dry classification tailings discharge and wet chemical treatment cause product yield reduction. Balance target purity versus economic yield.
3. Sequence of leaching steps: Do not reverse acid and alkali steps. Remove acid‑soluble metals first, then perform alkali desilication.
4. Equipment abrasion risk in dry‑process line: Silica is highly abrasive. Without ceramic liners, grinding rotors and classifier wheels will wear rapidly and introduce secondary iron impurity.

Coarse free silica in recovered carbon black can be partially separated by ultrafine liberation plus multi‑stage air classification or wet hydrocyclone gravity separation. Nevertheless, fine embedded silica cannot be fully eliminated by purely dry physical technologies.

For high‑end products requiring low silica content, a two‑step wet‑purification workflow is required: acid leaching for zinc‑iron removal followed by hot sodium‑hydroxide alkaline etching for silica dissolution. JACAN grinding‑classification equipment completes pre‑liberation and preliminary silica rejection, and can be matched with acid‑alkali wet‑processing modules to realize low‑silica rCB production according to different application requirements.