What is the role of air classification in rCB de‑ashing?

Total ash of recovered carbon black (rCB) originates from tire inorganic additives: zinc‑oxide, zinc sulfide, iron oxides, silica, calcium salts and other mineral phases. Wet acid‑alkali leaching can achieve deep de‑ashing but brings high cost and wastewater burden. As the core dry‑process unit in the JACAN rCB grinding‑classification system, air classification realizes physical ash reduction without chemicals. Based on technical data from rcb‑mill.com, this article elaborates its working principle, key functions, practical limits and matching process requirements for rCB de‑ashing.

1. Basic working principle for air classification in rCB system

After pyrolysis char is fully de‑agglomerated by ultrafine grinding, embedded mineral ash grains are liberated from carbon‑black agglomerates. In the air classifier, mixed powder is carried by high‑speed circulating airflow. Particles are subjected to centrifugal force from rotating classifier wheel and drag force from air flow.

Inorganic ash particles possess higher true density than carbon‑black aggregates. Under fixed operating parameters, high‑density mineral‑rich particles receive stronger centrifugal effect and are rejected as coarse tailings; lower‑density carbon‑black powder passes through the classifier wheel and becomes finished rCB fraction. Size‑density joint sorting separates ash‑rich components from carbon matrix.

Critical precondition: Effective air‑classification de‑ashing relies on sufficient liberation by ultrafine grinding. Ash locked inside unbroken char agglomerates cannot be separated by airflow sorting.

2. Core roles of air classification for rCB de‑ashing

2.1 Separate high‑density mineral‑rich tailings

This is its primary function. Liberated coarse zinc‑bearing, iron‑bearing and silica‑rich mineral particles are concentrated into tailings stream and continuously discharged out of the system. Multi‑stage cascaded classification further improves rejection efficiency of heavy‑ash fractions, cutting overall ash content of dry‑processed rCB.

2.2 Optimize particle‑size distribution and reject composite ash‑bearing granules

Some particles are composite agglomerates consisting of carbon black tightly bonded with fine mineral ash. Air classification rejects these high‑density composite granules as tailings, preventing composite ash‑carbon particles from entering final powder. It narrows particle‑size distribution of finished rCB simultaneously.

2.3 Complement upstream magnetic separation

Magnetic separation mainly removes metallic iron fragments and ferromagnetic iron particles, yet cannot handle non‑magnetic iron‑oxide, zinc‑sulfide and silica ash. Air classification undertakes removal of non‑magnetic inorganic minerals, forming combined impurity‑removal workflow together with magnetic separator.

2.4 Reduce load for subsequent wet purification

For projects targeting ultra‑low‑ash rCB below 1.5 %, air classification completes pre‑de‑ashing in dry state. By discharging large quantity of mineral tailings upfront, it lowers ash load for downstream acid‑alkali leaching modules, reduces chemical consumption, improves leaching efficiency and decreases wastewater volume.

2.5 Avoid secondary contamination compared with wet methods

Whole dry separation process produces no wastewater. When equipped with anti‑wear rotor and liners, air classification will not introduce extra ash impurities, which is suitable for large‑capacity continuous rCB production.

3. Key parameters affecting de‑ashing performance

1. Classifier rotor speed: Higher rotor speed creates stronger centrifugal field, sharpens separation cut‑point, rejects more fine heavy‑ash particles and achieves lower product ash, accompanied by certain yield loss.
2. System air volume: Matched air drag force balances centrifugal force; improper air volume causes disordered particle sorting and deteriorates de‑ashing effect.
3. Feeding rate: Over‑feeding leads to particle crowding inside classification chamber; minerals cannot be fully separated. Stable low‑rate feeding benefits ash rejection.
4. Grinding liberation effect: Insufficient grinding leaves ash encapsulated inside carbon agglomerates; even well‑tuned classifier cannot separate locked‑in mineral impurities.

4. Inherent limitations of air classification for rCB de‑ashing

1. Cannot remove ultra‑fine mineral inclusions: Nano‑scale or sub‑micron ash particles share similar aerodynamic behaviour with fine carbon‑black particles. They will flow through classifier together with rCB powder and remain in finished product.
2. Upper limit of dry‑process ash reduction: Even with optimized multi‑stage air classification, typical dry‑process rCB ash stabilizes around 2.0‑3.0 %. It cannot stably reach ash below 1.5 %. Fine embedded zinc sulfide and silica are the main residual ash sources.
3. Yield‑ash trade‑off: Pursuing extremely low ash by raising classifier rotor speed will significantly reduce rCB finished yield, increasing unit production cost.
4. No chemical dissolution function: Air classification only achieves physical separation. It cannot dissolve or decompose mineral ash phases.

5. Typical complete process configuration with air‑classification de‑ashing

1. Raw pyrolysis char → multi‑pass high‑intensity magnetic separation for metallic iron removal
2. Anti‑wear ultrafine grinding: break char agglomerates and liberate embedded mineral ash
3. Multi‑stage air classification: discharge heavy‑ash tailings to obtain dry‑processed rCB
4. Two branches for different product grades:
   • General‑grade rCB: directly powder collection and packaging
   • Premium low‑ash rCB: output from air classification enters wet acid‑alkali leaching for deep de‑ashing

Air classification is an indispensable core unit for dry‑type rCB de‑ashing. Depending on density‑size aerodynamic sorting, it rejects most liberated high‑density mineral‑rich tailings, cooperates with magnetic separation to remove inorganic impurities, and effectively reduces processing burden for follow‑up wet purification.

Nevertheless, air classification is a physical separation technology with clear performance ceiling. It cannot eliminate sub‑micron embedded mineral impurities. To produce ultra‑low‑ash rCB, air classification pre‑treatment must combine with wet chemical leaching. JACAN rCB grinding‑classification system supports multi‑stage air classification layout, and parameters can be adjusted flexibly to balance ash index and finished‑product yield for different rCB grades.