Jean M. Hoffman
Consider the mix of materials that go into household appliances and automobiles. The good news, say researchers at Argonne National Laboratory (Argonne), is that over 95% of the over 50 million vehicles scrapped globally each year enter a comprehensive recycling infrastructure. Nearly 75% of the weight of the vehicles is metals and the metals are profitably recycled through direct reuse, component remanufacturing, and scrap processing or shredding. Other components such as batteries, automotive fluids, some windshield glass, starters, alternators, and other dismantled parts are also recycled.
The percentage of recycled materials from vehicles is about to go up. Argonne, working with the Vehicle Recycling Partnership (VRP) of the United States Council for Automotive Research (USCAR), a partnership between General Motors, Ford Motor Co., and Chrysler, and the American Chemistry Council-Plastics Div. (ACC-PD), is developing technology for recycling polymeric materials from shredder residue.
Scrap processors use giant 3,000 to 8,000-hp hammer mills to shred both vehicles and other obsolete metal-containing products. Household appliances, industrial scrap, and demolition debris are all candidates for being turned into fist-sized chunks as a means of liberating the metals. Processing-unit operations vary but the basic process involves air classification of the “lights” fraction followed by one or more stages of magnetic separation to recover the ferrous metals.
Trommels and screens are then used to remove particles smaller than about 5/8 in., followed by one or more stages of eddy-current separations to recover the nonferrous metals.
Once the metals are gone, what’s left is called shredder residue. It’s typically a mix of polymers (plastics, rubber, and polyurethane foam), a “fines” fraction that includes metal oxides, glass, and dirt, as well as residual amounts of ferrous and nonferrous metals. For each ton of metal a shredding facility recovers there is roughly 500 lb of shredder residue. Unfortunately, this byproduct has historically ended up in landfills. That’s because recycling efforts were driven by the value of the metal — the single largest source of recycled ferrous scrap for the iron and steel industry is obsolete automobiles.
In the last 15 years, however, both automobiles and white goods have increasingly used polymers and composites. Existing separation technologies for polymer recovery rely on differences in density to separate solid particles. These methods will work on certain thermoplastics. But shredder residue content has overlapping densities and shapes which make it difficult to get enough material with the right purity for scrap processors.
Argonne’s new process is designed to efficiently separate polymers of equivalent densities. Called froth flotation, it was originally developed to separate acrylonitrile- butadiene-styrene (ABS) from high-impact polystyrene (HIPS); a mixture of plastics that is typical of that recoverable from obsolete appliances. A large-scale (1,000 lb/hr) pilot plant was built and operated at the Appliance Recycling Center of America’s Minneapolis facility to confirm process economics and the effectiveness of Argonne’s froth-flotation process.
The pilot plant recovered ABS at purities in excess of 99% and at yields of more than 80%, reports Argonne’s Energy Systems Div. Director Ed Daniels. ABS recovered from this operation was successfully used to injection mold automotive parts, thus confirming the feasibility of using obsolete postconsumer plastics (in some cases, plastics more than 15 years old) to meet the performance requirements of parts produced for this industry today.
“Argonne has since adapted the basic process to separate plastics from other mixed-waste streams including polyolefins, styrenics, and rubber from the mixed plastics found in consumer electronics and auto-shredder residues,” says Energy Systems Div. Project Manager Process Engineering and Analysis Group Bassam J. Jody.
“The complexity of shredder residue makes it impractical to recover recyclable materials in a single step. So Argonne developed a mechanical- separation system that employs commonly us e d shredder equipment to isolate polymers and metals into preconcentrates that are more manageable. The pilot plant processes up to 2 tons of shredder residue hourly and can recover 90% of the polymers (pieces larger than 2 mm) in the shredder residue and over 90% of the residual ferrous and nonferrous metals (larger than 2 mm),” says Jody.
The plant can separate materials in different sequences. But a typical one starts with the manual removal of large metallic and polyurethane foams (PUFs), as well as rocks. (Full-scale shredders screen out large pieces by a trommel and/or a destoner).
Breaking the PVC chain PVC (or vinyl) recycling in the U.S. likely will continue to be based on mechanical methods of grinding material into flakes and powder, and then reusing it to manufacture products much like the originals (pipes to pipes, for example). This method does not break the polymer chain. The Vinyloop process developed by Solvay SA, Belgium, is one recycling technology that does break the polymer chain. Vinyloop makes possible separation of PVC from polyester fiber, glass fiber, natural textiles, polyurethane foam, metals, rubber and many other materials through selective dissolution.
Physical processes including washing and cutting, grinding, milling and a homogenization step turn the waste into a form that can be fed into the Vinyloop. The need for these operations and their sequence depend on the waste’s composition.
Dissolution takes place using a solvent that selectively dissolves the PVC compound and not the secondary material. It is carried out at a temperature which is adapted to the material and its composition, always in the absence of air in a closed process.
Separation techniques are determined by the nature of the insolubles. They include centrifuging, decanting, or cycloning. This is because the behavior of fibers from coated fabric differs from that of rubber in waste from cables, for instance. After separation, the secondary material is washed with pure hot solvent to eliminate virtually all the dissolved PVC compound, then stripped with steam to recover all the solvent, and then discharged.
Precipitation of the PVC begins when additives are introduced. This is a specific feature of this recycling process and allows the properties of the output to be adjusted. At Vinyloop Ferrara SpA, in Italy, a plasticizer is added to adjust the shore hardness. Steam is injected to evaporate the solvent completely.
The PVC compound formulation is recovered as an aqueous slurry. All components of the original PVC resin formulation are recovered in the regenerated compound, not just PVC (except in special cases or in voluntarily designed separation). The slurry from the precipitation (a mixture of process water and regenerated PVC compound) is dried. The process water is treated to reach the required purity before discharge. And the regenerated PVC compound is packed and ready for use.
Solvents run in a closed loop. Over 99.9% of the solvent is recovered and separated from the water in a multistep process using condensation and density separation. A final gas effluent treatment lowers the solvent concentration in the flue gas to meet legal requirements.
The Ferrara Vinyloop handles five major categories of materials (cables, automobile, flooring, tarpaulins, and rigid product). The process can recycle PVC composites with any PVC concentration.
The remaining material is shredded to about 1-in. chunks and then conveyed to a two-stage trommel. The first stage removes fines. The second removes thin planar and semiplanar pieces through adjustable slots. This portion is primarily plastics, rubber, and small amounts of metal, foam, and fiber pieces. This residue passes over a magnetic pulley to recover the ferrous metals and over an eddy-current separator to sift out nonferrous metals.
Oversized material exits the trommel and consists primarily of PUF that gets squeezed down so it exits the shredder as a larger piece. Also in this stream are fabrics, fibers, and plastics, as well as metals that are too big to go through the trommel slots. The oversized material also passes over a magnetic pulley and eddy-current separator that extract metal alloys.
“About one-third of the shredder residue — the plastic-intensive portion — is recovered as a polymer concentrate, says Jody. “This gets granulated to an average particle size of 1/4 to 3/8 in. It is processed on a vibrating screen to remove fines and an air stream removes residual foam pieces, dust, and other light materials. Next comes the froth flotation process. Here the plastic concentrate goes into a solution that selectively enhances or retards the hydrophobicity (repelled by water) or hydrophilicity (attraction to water) one or more of the targeted plastics so that they can be separated from the mixture.”
“The process uses a series of six separation tanks,” explains Daniels. “The chemistry of the solutions in each tank controls the separation effectiveness of the overall process. So-called ‘light’ materials, including polyolefins, float in the first tank. At this stage the process can also be set to force over 90% of the wood to float so it can be dealt with only once. The ‘light’ fraction also contains appreciable amounts of different rubber species. Any sinkers at this stage become the ‘heavies’ and include metals, glass, rocks, rubber, and glass-filled nylons.”
As the concentrate progresses from one tank to the next, Daniels continues, the polyolefins are removed from the “lights” followed by targeted polymers such as ABS, PC, ABS/PC, PS, and PVC. Final processing includes polishing and cleaning steps to remove dirt and contaminants for better economics and marketability.
Though the PUF is only 5% of shredder residue, it amounts to over 30% of its volume, says Jody. “The market for recycled foam in North America continues to grow. Foam rebond industries import millions of pounds of scrap foam from Europe and Asia. The imported scrap supplements the more than 1.75 billion pounds of virgin foam used to produce such foam products as residential and commercial carpet padding, automotive carpet padding and headliners, car-seat cushions, and other consumer and automotive products.”
But the viability of foam recovered from shredder residue for the foam-rebond market depends on two key factors: Development of an economical process for recovering foam from shredder residue, and confirmation that the recovered foam meets quality requirements, says Daniels. Unsurprisingly, Argonne finds the highest-quality foam comes from dismantling and then washing the foam from seats. But manual separation of the foam is not economical.
Researchers from several of the organizations involved developed a continuous process for retrieving flexible foam from shredder residue. The continuous foam-washing and drying system was pilot tested at a shredder facility. Economic analysis of the process indicates a potential payback of less than 2 years for a plant producing about 1,000 tons/yr of foam.