Crumb particle

This technology (Benko and Beers, 2002a,b,c) utilizes a solvent to treat (devulcanize) the surface of rubber crumb particles of sizes within about 20-325 meshes. It is a variation of earlier disclosed technology (Hunt et al., 1999). The process is carried out at a temperature range between 150°C and 300 C at a pressure of at least 3.4 MPa in the presence of solvent selected from the group consisting alcohols and ketones. Among various solvents, the 2-butanol exhibited the best ability to devulcanize sulfur-cured SBR rubber. Duration of the process is above 20 min. 

In this process, which has been in existence as a concept for at least 30 years and is often referred to as a microbial process, chemolithiotrophic bacteria in aqueous suspensions in the presence of oxygen can be nsed to selectively attack the sulfur crosslinks in the snrface of waste rnbber crumb particles. The process can also be regarded as a type of biodegradation process, the by-prodncts of which can inclnde elemental sulfur, sulfates, sulfides and snlfnric acid. Specific examples of the bacteria that have been used to devulcanise rubber in this way inclnde species of Thiobacillus, e.g., T. thiooxidans. 

Crumb particles are activated by the process, which leads to matrix interactions... 

A tensile strength above 10 MPa and an elongation at break above 250% are possible in the final sintered products when chemical agents are added to improve the interfacial interaction between the crumb particles. 

The opening between the wires of a screen, commonly used to describe particle size, e.g., of crumb rubber. The finer the mesh the more openings it will have per unit of width, i.e., 30 mesh describes 30 holes/inch. 

The PAT process has been unable to stabilize organic materials such as rubber crumbs or wood chips because these substances have a memory effect and rebound to their original size after compression. The process requires low moisture content in the waste stream because waste loading is sensitive to moisture content. For hazardous and mixed wastes, influent particle size must be less than 0.25 inches in order to achieve proper brick formation and adequate stabilization. 

Eventually, the latexes are dewatered and recovered in solid form by freeze-coagulation and centrifugation, or by mechanical isolation. Mechanical isolation consists of shear coagulation of the latex to form a paste. The paste is then heated and sheared to form a crumb. Finally, the crumb is mechanically dewatered and ground to the desired particle size. A key feature of this method is the relatively low energy consumption of this process (17). 

In order to minimise air separation from the venturi walls the unit was designed with a 3.3 1 area expansion ratio with a 6° included angle on the divergent side. The implication of this limitation is that a second stage of processing is likely to be necessary to reclaim the smaller rubber fraction lost to the filters in the main venturi separator. The primary rubber crumb product has a particle size range of approximately 6 mm. The testing with the venturi separator effectively produced a main product of -6, +1 mm. The fraction below 1 mm was lost to the filters with the liberated fabric. However, with the fabric teased away from the steel inclusions it is expected that a simple cyclone could be tuned to recover the majority of the 1 mm rubber crumb. 

Application of these equations for the test data obtained does not provide a highly accurate match as the equations are based on particles with a constant Cd. The raw rubber crumb contained a range of particle shapes and corresponding Cds which introduced significant variances into the computations. However, the equations clearly indicate the operational background of the unit. 

The use of a vertical venturi separator was successfully used to clean steel and synthetic fibres from rubber crumb produced from an automotive tyre recycling plant. The major advantage of the venturi separator was an inherent positive feedback loop that exists at the throat. If more material was introduced to the throat then a higher velocity was generated at the throat to help retain particles (dependant upon air mover characteristics). The more conventional cleaning units observed had a negative feedback inherent in their design, with an increased local load of product the air velocity would... 

In the process, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is included if desired. Polymerization proceeds in bulk with precipitation of polymer, and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. Raw copolymer is obtained as fine crumb or flake containing imbibed formaldehyde and trioxane that are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

If the amounts of volatiles to be degassed are too high, gas velocities in the back vent and forward vent may also be too high and may drag out melt or crumbs at the same time. Once the polymer cools to below melt temperature after the flash it may form smaller particles that could be discharged via the back vent if gas velocities are too high. 

Poly(vinyl chloride) films are produced in two main forms—unplasticized and plasticized—and over the years different machines have been manufactured to handle the two types. When calendering unplasticized PVC there is a tendency for small particles, usually referred to as crumbs , to fall away from the edges of the film and from the feed nip. Such crumbs then could fall on to the finished film, where they would stick and form defects. To avoid this, producers of unplasticized film usually prefer an L configuration in which the product travels up the stack and surface contamination of this kind is prevented. With plasticized PVC the problem of crumbs does not occur to any great extent and, as it is an advantage to have good access to the part of the calender where the finished film is made, an inverted L configuration is the most popular. 

Slurry is transferred from reactor to a stirred flash drum where it is mixed with steam and hot water (=60° C, 1.5 bar). Methyl chloride and unreacted monomers are flashed off overhead and recycled, whereas particles agglomerate as coarse crumbs, the size of which is controlled by addition of zinc stearate. The suspension is then stripped to eliminate traces of volatiles, and the rubber is separated by filtration, dewatered by extrusion, dried and sent to a finishing unit for baling, packaging, and weighing. 

All of the solution processes require high efficiency in recovering the solvent. The schematic in Fig. 2 shows how a typical solution polymerization plant is arranged. The most widely used process consists of termination of the polymerization and the addition of antioxidant to the polymer solution. The solution may be treated to remove initiator or catalyst residue and then transferred into an agitated steam-stripping vessel in which unreacted monomer and solvent are flashed off, leaving the rubber as particles (crumb) in water. The water/crumb slurry then is dewatered and dried. The recovered monomer/solvent is recirculated to a series of distillation columns to recover monomer and purify the solvent. As both the anionic and the coordination catalyst systems are highly sensitive to impurities such as water, the purification system is very critical for satisfactory process control. 

In this series of experiments, whole tire reclaim rubber was ground to a particle size of 40 mesh (about 420 pm) and the surface of the ground crumb rubber was then devulcanized. The surface devulcanization was carried out in 2-butanol under the conditions of time, pressure, and temperature specified in Table 3. Then, the samples of surface devulcanized reclaimed rubber made were analyzed by a thermogravimetric... 

---