A new continuous recycling system has been developed at Argonne National Laboratory of Argonne, IL, which can separate auto shredder residue into distinct material streams that can eventually be reprocessed into useful products. The lab signed its first license agreement with N.V. Salyp, a recycler in Eiper, Belgium. The company plans to incorporate the system into a demonstration facility for recycling vehicles and will sublicense the Argonne technology to automobile shredders worldwide. Argonne's technology separates previously ‘unrecoverable’ residue—a mass of plastic, rubber, glass, fibers, and dirt—into useful product streams of polyurethane foam, thermoplastics, and inorganic fines. In the wash section of Argonne's foam cleaning operation, a conveyor moves the foam forward, then compresses and releases it to remove entrained oils and dirt. The company plans to incorporate Argonne's separation and foam cleaning technology into the demonstration plant, but is opting for a mechanical separation technology to separate mixed thermoplastics. Salyp plans to license a technology for mechanical separation of mixed plastics. The demonstration plant will also include a bumper shredding facility.
A junked automobile typically contains approximately 550 to 750 pounds of a jumbled mass of plastics, metal fines, rubber, glass, fibers, dirt, and oils that are left behind after the vehicle has been stripped of reusable parts and reclaimable metals. Each year in North America, landfills swallow up some 3 million to 5 million tons of this " unrecoverable" solid waste, known as auto shredder residue. Comparable quantities of auto residue are sent to landfills in Europe and Asia.
Today's automobile recyclers focus their efforts on reclaiming metals, which account for about 75 percent of a vehicle's weight. Once automobiles have reached the end of their useful life, they begin their final journey, during which re cyclers peel away every reusable remnant. In North America, the first stop is at one of 12,000 auto dismantlers, which recover reusable parts. The stripped down cars are shipped to one of about 185 auto shredding operations that use large hammermills to smash the hulks into smaller chunks of metal , which are sold back to the iron and steel and nonferrous metal industries.
Economical separation and recovery of the residue, however, has proved elusive until now. A new continuous recycling system has been developed at Argonne National Laboratory of Argonne, Ill., which can separate auto shredder residue into distinct material streams that can eventually be reprocessed into useful products.
Earlier this year, the lab signed its first license agreement with NV Salyp, a recycler in Eiper, Belgium. The company plans to incorporate the system into a demonstration facility for recycling vehicles and will sublicense the Argonne technology to automobile shredders worldwide.
The arrangement is atypical because, as a rule, Argonne tries to license its technology in the United States first, before marketing it abroad, said Paul Betton of Argonne's Industrial Technology Development Center, who is handling the marketing of this process. However, severe landfill restrictions that European auto shredders will face over the next several years have made the technology particularly attractive there, he explained. Betton said Argonne has received interest from several U.S. companies and is negotiating with one U.S. auto shredder.
After the shredders have finished recovering the metals from a vehicle, the residue is a very heterogeneous mixture, and it is hard to separate, noted Bassam J. Jody, project manager of the Energy Systems Division at Argonne, who led the team that developed the process. "Everything you can think of, you can find in there;' he said. While the focus of much 'research on finding a use for the residue has been directed at recovering its energy value, Argonne's approach is to separate the mass into distinct product streams-flexible polyurethane foam, thermoplastics, and inorganic fines-that can be cleaned or further refined for eventual reuse.
Initial separation of the auto shredder residue into the three streams is accomplished with a two-stage trommel. Separation of inorganic fines is accomplished in the trommel's first stage. The fines, which include metal oxides, glass, dirt, and some plastics and fibers, are the heaviest fraction, accounting for up to 60 percent of the residue's weight. The first stage of the trommel is equipped with a quarter-inch mesh screen. As the residue passes through the trommel, small particles are shaken loose and drop through the mesh. These are conveyed off for further separation downstream.
In the next stage of the trommel, the residue passes through a staggered arrangement of longitudinal slots that remove plastic pieces. As the mixed material passes to this stage, flat plastic pieces fall through the slots and are conveyed to a magnetic pulley, which pulls out ferrous pieces that may have been separated with the plastics.
Upon exiting the trommel, the remaining material passes over a gate designed to separate odd-shaped solid pieces from the flexible polyurethane foam. The separated foam passes over a magnetic pulley that pulls out ferrous pieces. By the end of the line, the resulting streams of fines, plastics, and foam~ have been largely separated and are ready for further processing.
Scrubbing up Foam
Of the three product streams, flexible polyurethane foam is furthest along in being returned to the market as useful material. After the flexible foam leaves the trommel, it enters the next stage for cleaning. Polyurethane foam accounts for only 5 percent by weight but up to 30 percent by volume of the shredder residue.
As much as half the weight of the foam recovered is due to moisture, dirt, automotive fluids, metal dust, metal oxides, and glass, according to Jody. After passing over the magnetic pulley to remove pieces of ferrous metal, the foam enters a wash, rinse, and drying process.
The foam is conveyed to a shredder that reduces it to a heated air. Residence time in the drier is 15 minutes. The cleaned and dried foam is then fed into a baler for densification and shipment.
One company that has evaluated the quality of the foam produced by Argonne's process is Woodbridge Foam Corp. of Woodbridge, Ontario. G.R. Blair, senior manager of corporate quality testing, said that the recycled foam has satisfactory qualities for certain applications. He pointed out that there is a shortage of in-plant scrap in North America for use in manufacturing carpet underlay.
Argonne sent samples of clean, fist-size pieces of flexible polyurethane foam to Woodbridge for evaluation. Woodbridge took the samples and converted them to carpet underlay, also known as rebond. The rebond from Argonne's samples had "reasonably" good properties, said Blair, meeting requirements of deodorization and cleanliness. Subsequently, Blair took the cleaned foam and used it for seat cushions in a fleet of police vehicles, as a field test. The material is performing well after a year and a half of service, he said.
Post-consumer seat rebond polyurethane foam also has good acoustical properties, making it suitable for use as carpet underlay material in automobiles, according to a study published by the Society of Automotive Engineers in 1998. Polyurethane foam was evaluated from two sources: dismantled foam from scrap vehicles, and foam reclaimed and recovered using Argonne's technology. The research was conducted by Kolano and Saha Engineers Inc. of Waterford, Mich., with cooperation from the Vehicle Recycling Partnership, a consortium under the United States Council for Automotive Research in Southfield, Mich.
The mixed thermoplastics stream resulting from the initial separation process contains several different types of thermoplastics in significant quantities. Argonne is developing a separate process based on density separation and a modified froth flotation process to separate the thermoplastics from each other.
Conventional sink/fTDAt methods are used to separate lighter materials of foam, polypropylene, and polyethylene. Sink/fTDAt methods are also used to separate the heavier materials, such as PVC and some residual metals.
Selected pairs of thermoplastic types are separated in a froth - flotation method that has been adapted to thermoplastics. Argonne has patented a froth-flotation process for separation of ABS and HIPS, which is the middling fraction left over after the lighter and heavier fractions have been removed by the sink/fTDAt method. The froth flotation process is capable of separating ABS and HIPS from the mixture in purities greater than 98 percent, according to Argonne. A demonstration of the process for separating ABS and HIPS has been conducted at Appliance Recycling Centers of America in Minneapolis.
In the froth flotation process, ABS and HIPS, two plastics that are incompatible but have similar densities, are placed in an aqueous solution of a certain density, surface tension, and pH.
The solution renders one of the plastics hydrophilic and the other hydrophobic. Air bubbles pass through the solution and adhere to the HIPS, carrying the HIPS to the surface. ABS remains wetted and sinks to the bottom. A similar strategy, incorporating solutions of different compositions, may be used to separate other pairs of plastics. For example, Argonne has experimentally verified the feasibility of using froth flotation to recover polyethylene at a purity of 98 percent from a polyethylene/polypropylene mixture.
What Happens to Polyurethane Foam in a Landfill?
POLYURETHANE FOAM IS ALMOST EVERYWHERE in society, from car seats to construction materials to appliances. Disposal of these materials raises questions about the stability of polyurethane foams in landfills. Of particular interest is whether or not the breakdown of foam leads to the formation of aromatic amine, a known carcinogen, which could be released in the soil or groundwater. To find the answer, Carnegie-Mellon University's Department of Biological Sciences conducted a study, funded by the Society of the Plastics Industry's Polyurethane Division.
To simulate landfill conditions, scientists adapted a paper-based landfill simulator constructed of a series of five and six-gallon pails that replicate landfill conditions under anaerobic methanogenesis. The pails included samples of a variety of polyurethane foam types, including toluene diisocyanate-based (TDI), which is used in flexible foams, and methylene diphenyl diisocyanate-based (MOl), found in rigid foams. The canisters were sealed to create anaerobic conditions, and water was continually percolated through. The leachate was monitored to determine if there was any breakdown of the polyurethane foam.
After a period of 700 days, no aromatic amines were detected using gas chromatography mass spectroscopy, according to William E. Brown, who headed the study. Nor was any physical breakdown of the polyurethane foam cubes evident from visual inspection after the buckets were taken apart following the experiment.
In one of the controls for the experiment, Brown did a calculation of the degradation product, TDA (toluenediamine), that would have resulted if the TDA-based foam had degraded. Initially, a very small amount of TDA, about 0.001 percent of the leachate, was detected. The TDA disappeared within 100 days.
Where did the TDA go? "That's a good question," said Brown. One possibility is that it completely bound to the paper matrix. Another possibility is that it may have been converted to something else. "At this point, we don't have an answer. We know that we are not getting an aromatic amine coming off, but we don't know if it is being bound or whether it's being metabolized and converted to something else."
In a similar procedure, no sign of MDA, or methylenedianiline, an insoluble hydrophobic substance, was found.
Brown also wants to know what degradation products would result once the anaerobic bacteria ran out of cellulose in the paper and was forced to use polyurethane as a carbon source. "You've got a mixed bag of microorganisms; anyone of them could adapt to anyone of the carbons that are being put into the systems." This is beyond the scope of the SPI's funding, which terminated after 700 days. Brown said he is seeking funding to continue the experiment to see what products are created over the longer term.
Inorganic fines, less than a quarter-inch in size, are essentially by-products of the separation of flexible foam and thermoplastics. These particles account for 30 to 60 percent of the residue's weight. "The fines are very rich in iron oxide," according to Jody. "We can concentrate the ferrous fraction, by sending it through a magnetic drum." The magnetic drum is capable of pulling out roughly 40 percent by weight.
Because the fines are rich in iron and silicon oxides, they are potentially useful in cement production, according to Jody.
According to Shuaib Ahmad of the American Concrete Institute International in Farmington Hills, Mich. , careful evaluation and experimentation will be needed to demonstrate the feasibility of using the recovered fines in cement. (In fact, samples of auto residue with major amounts of silica, iron oxide, and calcium oxide are feasible for use in raw cement mixes, according to an evaluation for Portland cement manufacturing, conducted by Construction Technology Laboratories Inc. of Skokie, Ill.).
The idea of recycling auto residue has gained the interest of N.V. Salyp, which plans to incorporate the technology in a demonstration center scheduled to open at the end of 2000 or beginning of2001. A new European Union directive requires that 80 percent of a vehicle's weight must be reused or recovered by the year 2005. By 2015, 95 percent of scrapped autos must be recycled. Landfill costs in Europe are already several times the costs in the United States.
However, Ivan Vanherpe, project manager of Salyp's planned ELV (for "end-of-life vehicle") Center, sees a market for this technology in North America and elsewhere in the world as well as in Europe.
European recycling goals are not achievable by conventional dismantling, according to Vanherpe, who said that Argonne's technology for recycling polyurethane foam, together with other recycling technologies for plastics, may help the industry meet the new requirements.
The company plans to incorporate Argonne's separation and foam cleaning technology into the demonstration plant, but is opting for a mechanical separation technology to separate mixed thermoplastics. Salyp plans to license a technology for mechanical separation of mixed plastics by September. The demonstration plant will also include a bumper shredding facility.