Rubber / Tire Recycling Plants

Carbon black: Finely divided elemental carbon produced by partial combustion/thermal decomposition of hydrocarbons. Primary particle size 10-500 nanometers (aggregates/agglomerates 0.1-10 micron). Used as reinforcing filler in rubber (30-35% of tire weight—strengthens, improves wear resistance). Dust hazards: (1) Fire/explosion: Dry carbon black dust suspended in air is combustible. Kst 200-400 bar-m/sec (St-2 severe explosive class). Adequate ignition source (hot surface >500°C, electrostatic discharge) can ignite causing explosion. (2) Health: Inhalation of fine carbon black particles. IARC classifies as Group 2B (possibly carcinogenic). Exposure limit 3.5 mg/m³ inhalable. Causes respiratory irritation, reduced lung function with chronic exposure. (3) Housekeeping nightmare: Carbon black is intensely black, ultra-fine, adheres electrostatically to everything. Uncontrolled handling coats entire facility, workers, product black (contamination). Control measures: Enclosed handling: Pneumatic conveying from bulk bags/railcars to mix room in sealed pipes preventing dispersion. Local exhaust: Bag dumping stations with downdraft ventilation capturing dust at source. Dust collection: Bag/cartridge filters with conductive (anti-static) media preventing static buildup. Explosion venting or suppression. Housekeeping: HEPA-filter industrial vacuums (never dry sweeping which re-suspends), washdown-capable equipment. Wet systems: Some plants use carbon black slurry (30-50% solids in water) eliminating dust but requiring mixing energy and producing wet rubber crumb.

Process: Vulcanization (curing) heats rubber with sulfur/accelerators/activators to 140-180°C creating cross-links. Fume composition: Complex mixture of volatiles released during heating. Includes: Carbon disulfide (CS2), sulfur dioxide (SO2), hydrogen sulfide (H2S), amines, nitrosamines (carcinogenic), unreacted accelerators, breakdown products. Hazards: (1) Acute: Eye/respiratory irritation, headaches, dizziness, nausea ("rubber fume fever"). H2S is deadly at high concentrations. (2) Chronic: Studies link rubber industry work to higher cancer rates (bladder, lung, stomach) due to nitrosamines and solvents. (3) Odor: Sulfur compounds have extremely low odor thresholds (ppb range) causing neighborhood complaints. Control: Local Exhaust Ventilation (LEV): Canopy hoods over autoclaves/presses capturing rising hot plume. Enclosure: Curtains around continuous curing lines. Treatment: Scrubbers (chemical neutralization of sulfur), activated carbon (odor/VOC removal), or thermal oxidizers (destruction).

The Mixer: Banbury/Internal mixer combines raw rubber (bales), carbon black (fillers), oils, and chemicals. Dust sources: (1) Charging: Dumping bags of chemicals/powders into mixer hopper. (2) Discharge: Drop door opening releasing hot batch + puff of dust/fume. Fume sources: Heat generated by shearing action (120-160°C) vaporizes oils and volatiles. Control system: (1) Charging extraction: High-velocity slot hood or enclosure ring at mixer throat capturing dust during loading. Airflow 3,000-8,000 CFM depending on mixer size (No. 3 to No. 27). (2) Discharge extraction: Hood below mixer drop door capturing thermal plume from hot batch. (3) Dust Collector: Pulse-jet bag filter. *Critical:* Carbon black is sticky and explosive. Use PTFE-membrane bags for release, anti-static media, explosion venting. (4) Oil mist: If significant oil vapor, use scrubber or coalescing filter downstream of dust collector.

Process: Warm rubber dough passed through series of rollers (calender) to form sheets or coat fabric/steel cord. Hazards: Hot rubber (80-100°C) provides large surface area for volatile emission (plasticizers, tackifiers, solvents). System design: (1) Canopy hoods: Overhead hoods spanning entire width of calender rolls. (2) Enclosure: Side baffles improving capture efficiency. (3) Airflow: Face velocity 100-150 FPM at hood opening. Total volume typically 10,000-30,000 CFM per line. (4) Make-up air: Massive exhaust requires equal fresh air supply (preferably tempered) to prevent negative building pressure and worker discomfort. (5) Filtration: Generally exhausts straight to atmosphere if VOCs low, or to control device (carbon/oxidizer) if using solvent-based adhesives.

Curing press: Molds heating green tire to 150-180°C under pressure (steam/hot water). Emission point: Opening the press after cycle releases massive "puff" of steam + aerosolized rubber fumes + mold release agents. Challenges: Hundreds of presses in a large plant creates a hazy, smelly environment. Solutions: (1) Point source capture: Individual canopy hood over each press (difficult due to crane access/maintenance). (2) General ventilation (most common): Roof exhaust fans capturing rising thermal plumes. Air change rate 15-20 ACH (Air Changes per Hour). (3) Automated hoods: Moving hoods that position over press when it opens (high complexity). (4) Mold release agents: Switching from solvent-based to water-based release agents reduces VOCs significantly.

Uses: Cleaning rubber surfaces (tackifying) before splicing, cleaning tools/molds, adhesive preparation. Common solvents: Toluene, Heptane, Naphtha, MEK. Hazards: (1) Flammability: Low flash points, vapor explosion risk. (2) Health: Neurotoxic (dizziness, memory loss), dermatitis, respiratory irritation. Toluene is reproductive toxin. Control: Substitutes: Water-based adhesives where possible. Ventilation: Downdraft tables for small part cleaning (pulls vapor away from breathing zone). Lateral slot hoods for conveyorized application. Storage: Flammable storage cabinets. PPE: Organic vapor respirators where engineering controls insufficient.

Risk: Rubber dust (from grinding/retreading/buffing) is highly combustible (Kst 100-200 bar-m/sec) and burns hot. Fires are difficult to extinguish. Dust collector protection: (1) Spark detection: Infrared sensors on duct detecting sparks from grinder, triggering high-speed water mist quenching. (2) Explosion venting: Panels aimed to save area. (3) Fire suppression: CO2 or Argon gas suppression inside collector (water can create mud/cleanup issues but is cheaper). (4) Duct velocity: Maintain >4,500 FPM ensuring no settling in ducts (settled rubber dust propagates fire rapidly).

Odor profile: "Burnt rubber," sulfury, skunky. Persistence is high (travels kilometers). Technologies: (1) Thermal Oxidizer (RTO): Heating exhaust to 750-850°C destroys odor molecules. 99% effective but high fuel cost (natural gas). (2) Wet Scrubber: Chemical scrubbing (oxidizing agent like hypochlorite or permanganate) neutralizes sulfur compounds. 80-90% effective, lower cost. (3) Biofilter: Microorganisms digest odor compounds. Low energy, large footprint. (4) Activated Carbon: Adsorption. Good for dilute streams. Plasma/Ozone injection: Injecting ozone into ductwork to oxidize odors (variable effectiveness). Best practice: Capture at source (highest concentration), treat only the odorous stream, use general ventilation for heat removal only (discharge high to disperse).