Crumb rubber, also referred to as ground rubber, asphalt rubber or size-reduced rubber, is most commonly derived from recycled automobile and truck tires. During the tire recycling process, all non-rubber materials such as dirt, bead wire and fluff, are removed. The remaining scraps of rubber are reduced to a more manageable size, usually by a mechanical grinder. Continued processing reduces the size of the rough grind into crumb rubber particles, which have various classifications, based on crumb rubber particle size and color.

The trend towards use of recycled materials has increased the demand for crumb rubber. The manufacture of asphalt rubber is the largest single application of crumb rubber, consuming an estimated 220 million pounds, or approximately 12 million tires annually. Crumb rubber can be blended with asphalt to improve the properties of the asphalt used in highway construction.1

Crumb rubber has other applications, including ground cover under playground equipment, running track material, and as a soil additive in sports and playing fields. Other uses for crumb rubber include molded rubber products (e.g., carpet underlay, dock bumpers, patio decks, railroad crossing blocks and movable speed bumps); new tire manufacturing; automotive parts such as brake pads and shoes; as an additive to injection molded and extruded plastics and as agricultural and horticultural applications/soil amendments.2

Characteristics and Challenges:
As noted, there are assorted types of crumb rubber and these variations often feature widely disparate characteristics. On average, crumb rubber has a bulk density of 22 lb/cu ft 3, depending on the grade. Crumb rubber often has irregular shapes and may come in a variety of grades in the same shipment. To prevent crumb rubber from agglomerating and to promote flow, some processors add calcium carbonate or talc (to a maximum
of four percent weight) to the crumb rubber particles.

While this blending promotes flow, further care must be taken if pneumatic conveying is part of the processing equation. An incorrect combination of air velocity and grade of crumb rubber may cause the materials to segregate. Should that occur, the crumbs could agglomerate and form solid plugs. To avoid this, and form solid plugs. To avoid this, consultation with an experienced designer of pneumatic conveying systems will help determine the proper combination of air speed and the grade of crumb rubber.

If the crumb rubber arrives at the processing facility in bulk bags, the frames used to discharge the bags should ideally be equipped with additional accessories to completely empty the crumb rubber from the bag. This includes features such as spring-loaded frames that will stretch the bags as they empty and lighten, making them rigid and removing any pockets of crumb rubber cornered in the bags. Bag activating devices are usually effective in agitating the crumb rubber, dislodging collections of the material closer to the bag spout, promoting a better flow. Some of these bag activating devices also serve as an airtight seal between the bulk bag and the receiving hopper.

Because a bulk shipment of crumb rubber may contain particles of irregular sizes and shapes which tend to agglomerate and block conveying lines, the addition of a particle reduction device may be required. If installed between the bulk bag spout and the receiving hopper inlet, these devices are proven effective in eliminating the larger particles and improving the downstream flow of the crumb rubber.

As the crumb rubber fills a receiving hopper beneath the bulk bag, the air inside the vessel is forced out. While not as notorious a dust creator as some other materials, crumb rubber is a "dirty" substance that requires containment during processing. This need becomes more pronounced if the crumb rubber

Crumb Rubber
has been blended with a light density flow aid such as talc, which is highly prone to dusting. A dust collector mounted on the discharger frame will contain the particles inside the conveyance system.

The hoppers between the discharge bags and the conveying line may need to incorporate devices such as vibrators to remove interlocked crumb rubber granules, as well mechanical agitators so as ensure the proper flow of crumb rubber to downstream processes such as a conveyor. Geometries of the hoppers must also be considered and applied appropriately.

For manual unloading of smaller bags of crumb rubber, a bag breaking station with a dust hood is likely sufficient. The placement of filtration devices and a reverse pulse jet which pneumatically cleans the filter media, blowing the trapped dust back into the receiving hopper, are recommended. A hopper screen above the receiving vessel will help protect the operators if the crumb rubber is being dumped manually into a hopper that is equipped with agitating devices.

As previously described, crumb rubbers have a wide range of physical characteristics. As this can result in a wide variability of material flow and compression properties, when handling crumb rubber with a flexible screw conveyor, it is important that a range of spiral geometries are available in order to optimize the performance of the conveyor. This will allow for the proper pairing of powder properties with equipment design.

Should your crumb rubber application feature the loading the material into bulk bags, the bag capacity will be maximized by use of a vibratory densification deck to de-aerate the crumb rubber as it fills the bag and by the application of load cells to ensure the desired weight. Seals and other dust containment devices will ensure a dust-tight operation.

Flexicon Applications:

Consultation with a Flexicon specialist will help you decide if a flexible screw or pneumatic solution best fits your crumb rubber application.

Flexicon's expert design and engineering staff will weigh each parameter and recommend the best solution for you. Upon request, Flexicon's test lab will simulate your crumb rubber handling functions before the system is installed in your plant.

Flexicon's product line of advanced flow promotion conveyors, high flow hoppers, deaeration/densification decks and a host of other components and accessories are proven performers that promote flow while reducing degradation, dusting and/or the separation of blends comprised of disparate particles.

See how and why the oldest and most successful fine-mesh crumb rubber producer in the US converted from manual dumping to automated discharging from bulk bags and conveying to cracker mills, using six automated BULK-OUT® BFC Bulk Bag discharger systems with integral flexible screw conveyors from Flexicon.

1United States Environmental Protection
3Source: Powder and Bulk. Com - Bulk Density Chart
Sources: Where noted. All other material courtesy of the Flexicon Corporation.


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