July/August 1992
Shredder operators and others are applying new eddy current technology in improving their products and turning some of what might have been waste into profits.
By Jeff Borsecnik
Jeff Borsecnik is assistant editor of Scrap Processing and Recycling.
To the nontechnical mind (like mine), using magnets to separate nonmagnetic materials is a pretty unusual—if not almost magical—concept. To those in the know, however, it's simply a promising, practical application of a phenomenon that won Thomas Edison a patent in 1889.
That phenomenon has since made its way into the scrap recycling industry: eddy current separators (ECS) use rapidly alternating magnetic fields to repulse nonferrous metals, removing them from a mixed stream of materials. In addition, because eddy currents have no effect on nonmetallics and powerfully attract ferrous items, systems can actually separate three types of materials.
As such, ECSs can be used in a variety of scrap processing operations, such as separating aluminum cans from plastic bottles in material recovery facilities, concentrating precious metals in shredded computer scrap, extracting fragmented aluminum caps and pull tabs from ground plastic or glass cullet, and reclaiming valuable metals from bottom ash at mass-burn incinerators.
Probably the most popular applications for ECSs among scrap recyclers, however, is use in automobile and appliance shredding operations, where they've found two niches: upgrading the nonferrous stream remaining after shredded material has been magnetically separated, and retrieving from shredder fluff residual metals that have traditionally been lost to disposal.
Getting to this point wasn't particularly quick. Following Edison 's early efforts, eddy current technology advanced fitfully, with most efforts at practical application taking place only in the last several years.
Eriez Magnetics (Erie, Pa.), which manufactures magnetic and vibratory equipment for a variety of industries, took some steps to develop the technology in the United States in the late 1960s, but found little interest, says Al Gedgaudas, the firm's manager of resource recovery equipment. While enthusiasm for eddy current applications in theUnited States has lagged behind that in Europe , says Gedgaudas, a growing number of manufacturers are now bringing the technology to the U.S. market.
The timing coincides with dramatic new improvements in magnetics that have been put to use in some ECSs. Some ECS manufacturers, for example, are employing in their products rare earth magnets (a name derived from the rare earth elements—those with atomic numbers of 58 through 71, such as neodymium—that are part of the brew used to create them), powerful permanent magnets that may be five to seven times as strong as earlier generations of magnets.
Applying Magic
The heart of an ECS system is a rotor or drum whose surface is covered with rows of magnets arranged lengthwise so that the outward-facing surface of each is a row of poles that are opposite in charge to those of the row next to it. This drum is spun at high speed, which creates a magnetic field of rapidly alternating polarity. Current flow is generated within nonferrous particles that enter the field—in much the same way as current is generated in a motor—and this current in turn creates a magnetic field around the particle that is, at any instant, of the same polarity as the field generated by the eddy current rotor. Because like poles repel, the particle is forced away from the eddy current unit.
In a typical ECS, the eddy current rotor is located within another—somewhat larger—drum that acts as the end of the feeding conveyor. Nonferrous metallic particles conveyed into the eddy current field nearly leap off the end of the conveyor, propelled by the repelling force, and over a barrier. Nonmetallic particles are not affected by the field, so they simply tumble off the end of the conveyor as they normally would, falling short of the barrier, and separation is achieved.
Although ferrous material is generally removed by a magnetic separator prior to introduction into an ECS, ferrous particles that remain in the mixed material flow sent through an ECS are attracted by the powerful magnetism of the eddy current rotor, so they stick with the belt for a greater portion of the circumference of the pulley than the other two materials, finally moving off at a different angle than nonferrous or nonmetallic items.
Some manufacturers build complete ECS systems, including belts, feeders, and conveyors, while others depend on the complementary skills of other firms to provide some of the equipment. Perhaps the biggest question surrounding the design of ECS systems for shredding operations, however, is whether to install the system in-line or as a stand-alone unit.
According to more than one manufacturer, operators are often reluctant to put an ECS system in-line, fearing it will be unable to keep up with the shredder. "They don't want the tail wagging the dog," as Marlin Bills, president of Osborn Engineering (Tulsa, Okla.), an ECS maker, puts it. Nevertheless, in-line systems are said to offer material-handling cost savings and may feature an optional bypass in case of a problem and/or to allow use of system as a stand-alone operation when needed (such as for upgrading the nonferrous stream at the end of a shift in which the ECS was in-line processing fluff, suggests Bills).
Stand-alone units, on the other hand, allow use of the ECS when the shredder is down, as long as stockpiled material is available, notes Rusty Manning, inside salesperson for Newell Industries Inc. (San Antonio), which manufactures shredders and separation systems. (ECS systems are manufactured by Newell Engineering Ltd., a subsidiary firm located in Redditch, England.) In addition, most manufacturers recommend that those who wish "mine" existing shredder residue stockpiles purchase an off-line, batch system.
Separation in Action
An off-line system may also be the best choice for companies that operate a wet shredder, such as Yorke Doliner and Co. (Cocoa, Fla.), which uses its ECS to upgrade the nonferrous stream flowing out of its shredder. Because eddy currents are said to work better on dry material, the recycling firm batch feeds its ECS separately from the shredder, which allows the flow to dry out before eddy current treatment, says Daniel Smith, the company's manager. (One manufacturer—Buffalo, N.Y.-based Wendt Corp., which manufacturers wet shredder separation systems—has produced a press to squeeze the water out of wet shredder residue to prepare it for eddy current separation.)
Despite the fact that Yorke Doliner does not rigorously presort the material headed to its ECS system by size (eddy currents work best on like-sized material), only removing 1/2-inch-and-smaller [MAKE REAL FRACTION] particles and very large ones, Smith notes that the company's eddy current operation produces a product that is 92-percent nonferrous metal by weight. If a particular market made it worthwhile to put in the extra effort and equipment to carefully presort the material, Smith believes the system could be "fine tuned so well we'd get a 99-percent rate."
Yorke Doliner's ECS system replaced a heavy-media operation, an easy conversion, Smith says, that required only the addition of the ECS unit, a few conveyors (whose speed had to be adjusted to keep things running smoothly), and a shelter to keep things dry. Payback was achieved in about a year, he reports, and the system works as well as the heavy media system did, while using much less labor and energy.
"The thing is working great!" he asserts. The production rate is lower than that of the previous separation operation, but one could buy three ECS packages for the price of the heavy media unit, says Smith, who adds that the firm has also considered installing an ECS for separation of the fluff generated by a dry shredding operation at the facility.
The ECS at Chaparral Steel Co. (Midlothian, Texas), is assigned fluff duty. The system processes about 20 tons per hour of residue collected by an air separation system, reports Johnny Lee Morgan, the company's manager of scrap processing. Now in its third year of operation, the ECS system produces a product that is 95-percent nonferrous metals, and paid for itself in about a year, says Morgan. In the Chaparral system, shredder residue is thoroughly separated by size and the fines are removed, then each sized segment of the stream flows under magnets that pull out ferrous metals and onto shakers that spread it evenly and send it to four eddy current units running parallel.
Improving Profits
The percentage of nonferrous reportedly produced by the ECS systems at Yorke Doliner and Chapparal shouldn't be ignored. According to Osborn's Bills, the nonferrous stream flowing from a dry shredder operation's air classification system might typically be 40- to 60-percent metal, and prices reflect that content. As a rule of thumb, he says, every 10-percent increase in the purity of that material can result in a penny more paid per pound, so eddy current treatment that results in a 40-percent-to-90-percent-jump in metal content can raise the price a nickel or more a pound.
Those that use ECS systems to mine fluff headed for disposal, may not see the same kind of profits, but they are squeezing revenue from a waste stream. "It's like the scrap recycler getting everything out of the car but the honk of the horn," says Denny Schreck, district manager for Universal Engineering Corp. (Cedar Rapids, Iowa), a manufacturer of shredders, separation systems, and related equipment.
Other benefits of ECSs, say manufacturers, include the following.
- ECS enable scrap recyclers to determine the worth of their own material, rather than depending on a recovery firm to tell them. ("It's not unusual for three different recovery plants to give you three differing responses on the quality of the material," says Mark Mullins, regional sales manger for Texas Shredder Inc. (San Antonio), who estimates that such figures vary by as much as 5 percent. "Five percent over a year is a lot," he adds.)
- Because most ECS systems include a trommel or screen, mining fluff with an ECS can satisfy some state environmental rules for "treating" the material before disposal, notes Mullins.
- It's generally easy to fit an ECS system into an existing operation.
- Removing metals from fluff reduces the volume of waste that needs to be disposed.
- The systems do not generate pollution.
- ECS systems can be adjusted to handle different materials by such maneuvers as changing the belt speed and adjusting the separation barriers.
Eddy's Not Perfect
Despite these selling points, ECS systems aren't the answer to every scrap recycler's needs. Eddy currents work best on highly conductive, lightweight nonferrous metals, such as aluminum. In fact, to an ECS, stainless steel might as well be rock. This generally nonmagnetic oddball will stay with the nonmetallic residue, requiring hand sorting for retrieval, along with coated wire, note processors and manufacturers.
In a paper presented recently to a bulk-materials handling conference, Don Morgan, product manager of separation equipment for O.S. Walker Co. (Milwaukee), a manufacturer of magnetic products, provided a list of common nonferrous elements and their conductivity/density ratios, which act as a measure of their sensitivity to eddy currents. Aluminum topped the list, followed by magnesium and copper (though copper is a better conductor, it is far denser than aluminum, so the eddy current effect on it is not as strong). In the middle were silver, zinc, and gold, followed by brass, nickel, tin, and lead.
In addition, Morgan's paper points out, "Eddy current generation is a surface effect on the particle; thus a shape like a whole aluminum can has a good deal of surface and extremely low density. If the same can is flattened or crushed, its repulsive effort will be less because surface area has been reduced and density has increased."
For best results, the ECS requires careful metering of flow so that material is fed at as close to single-particle depth as possible. "The biggest problem," says Johnny Lee Morgan of Chaparral Steel, is that "fluff is a difficult beast to handle—it tries to gob up on you."
Ferrous particles, which are mightily attracted to eddy current magnets, also can cause problems if not carefully controlled, note several experts. Tiny ferrous particles can slide along the moving conveyor, attempting to stay near the ECS rotor, grinding away at the belt, says Bills, whose answer to this is the use of cleats to prevent the ferrous particles from hanging around on the returning belt.
Larger ferrous items can also be a problem if the feeding conveyor stops while moving these particles through the field, warns Bills, who explains that heat builds up rapidly in such particles as they move through the rapidly alternating field. A chunk of ferrous metal stopped in the field "will burn a hole in the belt," he says, thus "you better make sure when you shut that belt down it's clean." Osborn's ECS units include an "electromechanical circuit that will not allow the rotor to be turning with the belt stopped, whether power is on or off," which he says is an important consideration since the heavy, fast-spinning ECS rotor would not stop as quickly as the conveyor drum in a power interruption.
Wendt Corp's president, Thomas A. Wendt, suggests that careful separation of magnetic particles before introduction into the eddy system solves these problem before they start. Eriez's Gedgaudas recommends using powerful rare earth drum magnets upstream, but notes that his firm uses improved belt and cover designs to protect the ECS as well. He adds that regular cleanup and maintenance of the system are also important, because of the precision of the machinery.
Sorting Through the Separators
There is a basic difference in the relative positions of the ECS rotor and the exterior conveyor drum among manufacturers. Some separators use a so-called eccentric design, in which the eddy rotor does not spin on the same axis as the external drum, but on an axis above and toward the end of the conveyor. This arrangement concentrates the eddy current field on a particular section of the larger drum, allowing the unit to release ferrous particles once they move past that sector, preventing problems, say several manufacturers, including Phil Lombardo, executive vice president of Metalloxyd Inc. (Stamford, Conn.).
In the other arrangement, a concentric design, the two rotors spin on the same axis, which produces a field surrounding the entire exterior drum. Bills says this design allows use of a larger-diameter rotor, which has more surface area for magnets, creating as powerful a field as the eccentric type while spinning slower, which limits mechanical wear and problems. Gedgaudas says the concentric rotor "is ideal for repelling metals that vary in size and shape, since different pieces are repelled at different locations" and adds that the larger-diameter rotor also provides "a deeper magnetic field and more poles or frequency, which is advantageous in repelling small metallics."
In either case, an ECS can put a scrap recycler back $140,000 for a unit "not much bigger than his desk," notes one manufacturer executive. Gedgaudas says that the rare earth magnets themselves, which are made by a very small number of suppliers, make up the "lion's share" of the cost of the equipment, but other factors, such as the quality of the magnets and the sophistication of the magnetic circuit, are contributing factors, as is the difficulty of handling such powerful magnets when building the equipment.
Morgan of O.S. Walker calls ECSs "one of the most complicated magnetic devices." He suggests material costs should eventually fall, but notes that rare earth magnets are widely used in sophisticated industries like electronics, and the "nuts and bolts" magnets industry does not have a lot of influence on prices.
Where's Eddy Headed?
"In just a few years every shredder operation is going to have one," says Chaparral's Morgan. One manufacturer's representative suggests that about 10 percent of U.S.automobile-shredding operations are already running ECSs and more are looking. "The shredding industry is by far and away more competitive today than anytime in the 20 or 30 years it's been around," prompting operators to squeeze every pound of salable material they can get out of their scrap, says Universal's Schreck. "The only thing holding back almost every shredder from buying them right now is the lousy market, weak cash flow. No one argues the merits," adds Mullins of Texas Shredder.
Beyond wider use, what's next? Several of the makers suggest that while more of their customers are putting eddy to work, they will get eddy to work better, aiming, for example, to recover metallic fines generally ignored now. •
Shredder operators and others are applying new eddy current technology in improving their products and turning some of what might have been waste into profits.