Crumb Rubber Production: From Scrap Tyres to Granulated Material

By:     Conor Murphy

What Crumb Rubber Is and Why It Matters

Crumb rubber is the granulated rubber material produced by reducing scrap tyres through mechanical or cryogenic processing. It is the primary output of the tyre recycling industry by volume and the raw material for a wide range of downstream applications spanning sports surfaces, road construction, moulded rubber products, and playground safety surfacing.

The significance of crumb rubber production extends beyond its commercial applications. It is the principal mechanism by which the UK’s approximately 55 million annual used tyres are diverted from landfill and from illegal dumping into productive use. Every tonne of crumb rubber produced represents approximately 100 car tyre equivalents converted from a waste material into a commercial product.

Understanding the production process, the equipment involved, and the quality factors that determine crumb rubber value is relevant to tyre processors planning or operating production facilities, to businesses generating significant tyre volumes who want to understand where their tyres go, and to procurement teams buying crumb rubber for specific applications who need to specify the right material grade.

Gradeall International, based in Dungannon, Northern Ireland, manufactures the front-end tyre processing equipment that feeds crumb rubber production lines. Sidewall cutters, rim separators, and tyre processing equipment from Gradeall’s tyre recycling range prepare tyres for shredding and granulation at recycling facilities globally. With nearly 40 years of manufacturing experience and equipment in over 100 countries, Gradeall’s team understands the full tyre processing chain from collection through to finished crumb rubber output.

Tyre Composition: What Crumb Rubber Production Must Separate

A passenger car tyre contains approximately 45 to 47 percent rubber compound (a blend of natural and synthetic rubber with carbon black, sulphur, oils, and chemical additives), 15 to 25 percent carbon black, 10 to 15 percent steel (in the bead wires and belt reinforcement), 5 to 10 percent fabric (nylon, polyester, or rayon textile reinforcement in the carcass), and the balance in processing chemicals and minor additives.

Crumb rubber production must separate these components: the rubber compound becomes the crumb rubber product, the steel is recovered as scrap metal, and the textile fibre is separated as a by-product that has limited commercial value but that must be removed to produce acceptable-quality crumb rubber. The quality of the separation at each stage determines both the commercial value of the crumb rubber and the recovery value of the steel by-product.

Truck tyres have higher steel content than car tyres due to the additional belt layers needed for load-carrying capacity. OTR tyres used in mining and earthmoving have the highest steel content of all, with multiple reinforcing steel layers that represent significant scrap metal value. The steel recovery from tyre processing is a meaningful by-product revenue stream that contributes to overall processing economics.

Front-End Processing: Preparing Tyres for the Production Line

Before tyres enter the primary shredder, two front-end processing steps significantly improve the efficiency and output quality of the crumb rubber production line.

Rim separation. Tyres arriving with steel or alloy wheels attached must have the rims removed before processing. A tyre with its rim still attached cannot pass through a primary shredder designed for tyre rubber; the rim would damage shredder blades and contaminate the steel recovery stream with the different alloy composition of the wheel compared to the tyre’s belt steel. Gradeall’s tyre rim separator handles car and van tyres; the truck tyre rim separator removes wheels from HGV and truck tyres. The separated wheels are sold as scrap aluminium alloy or steel separately from the tyre processing line’s steel output.

Sidewall cutting. For truck and OTR tyres, sidewall cutting before primary shredding improves shredder throughput and reduces energy consumption per tonne processed. A whole truck tyre fed into a primary shredder presents a large, stiff, circular object that resists engagement with the shredder blades; the same tyre with sidewalls removed is more compliant and feeds through the shredder more efficiently. Gradeall’s truck tyre sidewall cutter and OTR tyre sidewall cutter perform this pre-processing step, reducing energy costs and improving throughput at the shredding stage.

The Ambient Mechanical Processing Route

Ambient mechanical processing is the dominant crumb rubber production method globally. The process operates at ambient temperature, using mechanical cutting, shredding, and granulation forces to progressively reduce tyre rubber to the target particle size.

Stage one: Primary shredding. Whole or pre-processed tyres enter the primary shredder. High-torque twin-shaft shredders are the standard technology at this stage; the counter-rotating shafts with interlocking blades tear the tyres into large chips of 50 to 100mm. The primary shredder must be robust enough to handle the steel wire and fabric in the tyre without excessive blade wear; shredder blade specification and replacement interval are significant operating cost factors in primary shredding.

Stage two: Steel separation. The output of the primary shredder is a mixture of rubber chips, liberated steel wire, and fragments still containing embedded steel. Magnetic drum separators or overband magnets positioned across the conveyor extract free steel from the rubber chips. Some steel-rubber fragments that were not fully liberated at primary shredding continue to the secondary granulator; steel remaining embedded in the rubber will be liberated at subsequent stages and separated by downstream magnets.

Stage three: Secondary granulation. The steel-separated rubber chips pass through a granulator that further reduces particle size to 10 to 20mm. The granulator uses rotating blades or mills operating at higher speed than the primary shredder to achieve finer size reduction. Additional magnetic separation follows the secondary granulator to remove any steel liberated at this stage.

Stage four: Tertiary milling. Further milling reduces granulate to the target crumb rubber particle size. For coarse crumb rubber (4 to 8mm for playground surfaces or equestrian surfaces), two stages of size reduction may be sufficient. For fine crumb rubber (1 to 4mm for artificial turf or rubber-modified asphalt), additional milling passes are needed. For rubber powder (under 0.5mm for compound substitution applications), further fine milling is required using high-speed disc mills or similar equipment.

Stage five: Fibre separation. Throughout the size reduction process, the textile reinforcement fibres from the tyre carcass are liberated and must be removed. Aspiration systems (air separators) pass the crumb rubber through an air stream that entrains the lightweight textile fibres while the denser rubber particles fall through. Multiple aspiration stages are typically needed to achieve acceptable fibre content in the finished crumb rubber.

Stage six: Screening and classification. The finished crumb rubber is screened to remove oversized particles (which are returned to the mill) and classified into size fractions by vibrating screens. The classified fractions are stored and sold separately, as different applications require different particle size specifications.

The Cryogenic Processing Route

Cryogenic processing uses liquid nitrogen to cool tyres to approximately -120°C, below the glass transition temperature of the rubber compound, making it brittle. The embrittled rubber is then fractured by mechanical impact (typically in a hammer mill or similar impact device) into fine particles, and the steel and fibre are separated by screening and aspiration.

Advantages of cryogenic processing. The primary advantage is the quality of the separation between rubber, steel, and fibre. At cryogenic temperatures, the rubber fractures cleanly at the interfaces between rubber and the reinforcing materials, producing a cleaner separation than ambient mechanical processing. The resulting crumb rubber has lower steel and fibre contamination than ambient-processed crumb of equivalent particle size. The particle shape produced by cryogenic fracture is different from that produced by ambient shredding; cryogenically processed particles tend to have a more angular, irregular surface that some application specialists argue provides better bonding characteristics in certain rubber compound applications.

Disadvantages. The energy cost of producing liquid nitrogen and maintaining the cryogenic processing environment is significant. Cryogenic processing is typically more expensive per tonne than ambient processing, which limits its use to applications where the quality premium justifies the additional cost. The equipment capital cost is also higher than ambient processing lines of comparable throughput.

Crumb Rubber Quality: The Factors That Determine Commercial Value

Not all crumb rubber commands the same price. The commercial value of a specific batch of crumb rubber depends on several quality parameters that buyers specify and test:

Particle size distribution. The particle size specification is the primary commercial variable. Different applications require different size ranges; a buyer specifying 1 to 4mm crumb rubber for artificial turf infill will reject material with significant content outside this range.

Steel content. Crumb rubber with high residual steel content (small wire fragments that passed through the magnetic separation stages) is a problem in most applications. In sports surfaces, steel fragments represent an injury risk. In rubber compound applications, steel affects compound properties. Maximum acceptable steel content is specified by buyers; contaminated material is discounted or rejected.

Fibre content. Residual textile fibre in crumb rubber affects its bulk density and flow properties, and can be visually apparent in finished products. Maximum fibre content specifications are common in buyer contracts.

Moisture content. Wet crumb rubber affects bulk handling properties and can affect the performance of applications that require dry material. Maximum moisture content specifications are typically 1 to 2 percent.

The quality of the crumb rubber coming off the production line is directly determined by the quality of the front-end processing,” says Conor Murphy, Director of Gradeall International. “Tyres that have had rims removed cleanly and have been appropriately pre-processed produce better crumb rubber with less contamination than tyres that go through the primary shredder with rims still attached or without appropriate size reduction. Getting the front end right pays dividends all the way through the line.”

Contact Gradeall International for front-end tyre processing equipment that improves crumb rubber production quality and throughput.

FAQs

← Back to news