How to improve the volatile matter content in milled rCB

Volatile matter (VM) of milled recovered carbon black (rCB) mainly originates from surface‑adsorbed hydrocarbons, oxygen‑containing functional groups (‑COOH, ‑OH, C=O), residual light tar fractions and partial organic deposits on particle surfaces. For rubber compounding, appropriate volatile matter improves filler‑rubber wettability and mixing processability. Over‑high VM causes scorch risk and odor; excessively low VM leads to poor dispersion and weak interfacial bonding.

After conventional grinding‑classification processing, high‑temperature friction and hot gas flow often strip surface light organics of rCB, pushing volatile matter down to 1.5‑3.0 wt%, lower than many virgin carbon‑black grades. Based on technical experience from rcb‑mill.com, this article explains feasible industrial approaches to raise volatile matter of milled rCB, mechanism, process constraints and common pitfalls.

1. Root causes for low volatile matter in milled rCB

1. Pyrolysis feedstock baseline: Over‑pyrolyzed char (high temperature + long residence time) already loses most light volatile components, setting a low upper limit for final VM.
2. Grinding thermal effect: High‑speed mechanical grinding generates local frictional heat; hot circulating air flow carries away surface light hydrocarbon and partial oxygen‑rich groups during classification. Jet milling with high‑temperature compressed gas aggravates this devolatilization effect.
3. Post‑processing heating: Drying, de‑ashing thermal steps further remove surface volatile fractions.
4. Wet‑leaching purification: Acid washing dissolves and strips partial surface organic functional groups, generally lowering volatile matter after drying.

Important note: Milling itself cannot generate new volatile components; it only removes existing surface volatiles. Improving VM for milled rCB mainly relies on feedstock tuning, mild process control and post‑milling surface functionalization / coating.

2. Feedstock control: set reasonable baseline volatile matter

Volatile‑matter adjustment should start from pyrolysis char selection before milling.

• Select moderately pyrolyzed tire char; avoid over‑cooked char produced under excessive temperature and long holding time. Moderate pyrolysis preserves proper light tar and surface functional residues.
• Reject char subjected to secondary high‑temperature calcination, which has extremely low initial volatile matter and is hard to upgrade in downstream milling circuit.
• Keep raw char volatile matter within 3.5‑6 wt% as starting condition. If raw char VM is below 2 wt%, physical milling alone can hardly reach target volatile‑matter level.

3. Optimize grinding‑classification parameters to prevent over‑devolatilization

The core idea: reduce thermal stress on rCB particles during milling and classification, minimize loss of native surface volatiles.

1. Control grinding temperature
   Adopt air‑cooled grinding chamber design; limit material outlet temperature below 110 °C. Avoid continuous high‑temperature frictional heating which burns off surface light organics.
2. Tune system air volume and gas temperature
   Reduce excessive hot‑air circulation in classification loop. Use ambient‑temperature cooling air instead of pre‑heated process air where possible. Too‑large air flow blows away adsorbed light‑weight volatile fractions.
3. Avoid over‑grinding
   Excessive grinding intensity not only breaks carbon‑black aggregates and drops DBP value, but also creates massive fresh high‑energy surfaces, accelerating volatile loss. Match rotor speed and feed rate to achieve target fineness without unnecessary over‑processing.
4. Optimize collection system: Reduce high‑temperature residence time inside cyclone and bag‑house; cool finished powder rapidly after classification to lock surface adsorbed components.

Limitation: Parameter optimization can only preserve original volatile components from feedstock. It cannot create extra volatile matter. If target VM is significantly higher than raw char baseline, post‑surface treatment is mandatory.

4. Post‑milling surface treatment to increase volatile matter

4.1 Mild oxidative surface functionalization

Controlled mild oxidation introduces oxygen‑containing groups (carboxyl, hydroxyl, carbonyl) on rCB surface; these surface functional groups contribute to higher volatile‑matter test value under thermogravimetric conditions.

• Gas‑phase mild oxidation: Treat milled rCB at moderate temperature (120‑180 °C) with low‑concentration oxygen‑containing hot‑air flow.
  Merits: No chemical liquid residue; adjustable oxidation degree.
  Drawbacks: Strict temperature and oxygen concentration control; over‑oxidation degrades carbon‑black structure and raises surface acidity.
• Wet‑phase weak oxidation: Use dilute hydrogen‑peroxide solution for short‑time surface oxidation before drying. Suitable for small‑batch premium‑grade rCB.

4.2 Dry surface coating / modifier doping (most widely‑used industrial solution)

Atomized low‑volatile organic modifiers are sprayed onto cooled milled rCB powder in high‑speed dry modifier reactor. Small‑molecular organic coating agents contribute measurable volatile‑matter content while improving dispersion performance.

Common modifier selections:

• Fatty acids, fatty acid esters
• Low‑molecular‑weight wax fractions
• Selected coupling‑agent formulations

Typical operating window:
Material temperature: 80‑110 °C; modifier dosage: 0.5‑2.5 wt%. Keep temperature low enough to prevent modifier itself from evaporating during modification.

Merits: inline connection after grinding‑classification system, full‑dry workflow, large throughput, simultaneously tune volatile matter and polymer compatibility.
Drawbacks: Excessive dosage will bring blooming, odor and negative influence on rubber mechanical properties. Must strictly control addition amount.

4.3 Controlled light‑tar re‑adsorption (special‑grade process)

Introduce trace amount of atomized low‑molecular‑weight pyrolysis‑oil fractions into cooled rCB powder flow for controlled physical adsorption. This method can lift volatile‑matter obviously.
Drawbacks: Difficult precise dosage control; high risk of unpleasant odor and scorch in rubber; seldom adopted in mainstream rubber‑grade rCB production, mainly for low‑end filler applications.

5. Complete industrial process workflow for improving volatile‑matter of milled rCB

1. Select moderately‑pyrolyzed tire‑derived char with proper initial volatile matter, reject over‑calcined feedstock.
2. Pre‑treatment: magnetic iron removal, coarse‑ash separation.
3. Cool‑condition ultrafine grinding + multi‑stage air classification, control material temperature, avoid over‑grinding and over‑hot‑air stripping. Rapidly cool finished milled rCB powder.
4. Optional unit A: mild gas‑phase surface oxidation to increase oxygen‑containing surface groups.
5. Optional unit B: inline dry surface modification by spraying suitable organic modifiers to raise volatile matter and improve dispersion.
6. Powder collection, cooling, sealing packaging, test volatile matter, BET and DBP for quality verification.

6. Critical practical risks & constraints

1. Balance volatile matter versus rubber performance: Higher VM does not equal better product. For most rubber‑grade rCB, target volatile‑matter range is 3‑5 wt%. Too high volatile matter will cause rubber scorch, odor, compression‑set deterioration.
2. Wet‑leaching conflict: If low‑ash acid‑leaching is required, volatile matter will drop after acid treatment and drying. You need to implement surface modification after drying to compensate volatile‑matter loss.
3. Mislead from test value: Organic coating agents raise volatile‑matter test index, but do not restore native carbon‑black surface properties. Both volatile‑matter index and real rubber lab compound test are required for final qualification.
4. Storage influence: Finished high‑VM rCB shall adopt sealed packaging. Long‑term open storage will lose surface volatile fractions again.

Milling and classification tend to reduce volatile‑matter content of rCB by thermal stripping effect. Optimized grinding‑classification parameters can preserve native volatile components from pyrolysis char but cannot generate extra volatile matter.

To effectively improve volatile‑matter for milled rCB, industrial solutions combine proper feedstock selection, low‑thermal‑stress milling operation, plus optional mild surface oxidation or dry organic coating modification. JACAN grinding‑classification system supports low‑temperature milling configuration and can match inline dry modification reactor to tune volatile‑matter within target window for different rCB grades. Always correlate volatile‑matter index with real‑world rubber‑compounding performance instead of pursuing high VM value blindly.