May/June 2011
Magnet type and size are just two of many variables that factor into the selection of this essential piece of ferrous scrap separation equipment. The right design and installation can be the key to more productivity and a longer magnet life.
By Chelan David
Magnetism was first identified in ancient Greece and China more than 2,000 years ago, but the use of magnets for separation in industrial production is a slightly more recent development. One company dates its entry into the business to 1941, when an agricultural equipment salesman saw a mechanic use a magnet to pick up small automobile parts from a garage floor. Inspired by that simple demonstration, the salesman realized he could apply the power of magnetism to the grain-processing equipment he had been selling for many years. Using permanent magnetic horseshoes made of the then-relatively-new material Alnico (an alloy of aluminum, nickel, and cobalt), he developed a flat magnetic product that could remove the tramp iron that contaminated processed grain and damaged milling equipment. Other industry veterans say both flat and drum magnets were used for scrap processing even earlier than that. Today magnets are an integral part of scrap processing, and drum magnets are an indispensable part of shredder downstream separation systems.
The Heart of the Magnet
Over the years, the basic design of the scrap processing drum magnet hasn’t changed much. It consists of stationary magnetic materials encased in a rotating drum shell made of nonmagnetic metal, plus the motor, gearbox, and belt or chain that turn the drum. The shredded scrap moves toward the drum on a conveyor that’s either above or below it. The magnet pulls ferrous pieces off the conveyor onto the drum (from an underfeed setup) or pulls ferrous out of the material that falls over it (from an overfeed setup). As the drum’s shell rotates, the ferrous material moves away from the magnetic zone and falls onto a conveyor or into a chute or bin.
The biggest innovation since the first use of magnets in scrap processing is almost certainly the development of electromagnets. In the late 1950s and early 1960s, electromagnet models with radial-pole magnetic fields came onto the market, providing greater strength than that era’s permanent magnet models. Radial-pole magnets alternate polarity across the width of the drum, developing magnetic bands that resemble a roll of Lifesavers. Also, they were produced in larger drum diameters, which improved processing capacity and recovery performance.
More recently, computer-enhanced coil designs led to the development of axial-pole electromagnets, says the vice president for nonferrous equipment at one shredder company. Axial-pole drum magnets alternate polarity around the circumference of the drum, but the polarity remains the same across the width of the drum face. This design offers several advantages over radial-pole styles, explains a magnet company sales manager: A radial-pole drum magnet produces a vertical magnetic field, which remains more toward the center of the drum. This can produce “a definite vertical wear pattern, which looks like a stripe [around the drum] because a majority of the steel recovered by the drum hits the wear shell in that specific spot,” he says. An axial-pole drum, in contrast, produces a horizontal magnetic field, with magnet strength that’s spread more evenly along the length of the drum, to within a few inches of ends of the drum. “You will not see [the vertical] wear pattern on axial pole drum magnets because the field maintains its full magnetic strength all the way out to the ends of the drum,” he says. “The steel is attracted evenly across the entire drum face, which also reduces wear on the cylinder.”
Axial-pole drum magnets also produce a stronger magnetic field than older radial-drum designs, the sales manager says; thus, they pick up a higher proportion of ferrous. As long as the axial drums are properly positioned at the manufacturer’s recommended gap setting (the distance from the drum to the conveyor), they can recover a majority of the “knuckles,” dense pieces of steel which radial-pole drum magnets often miss. Knuckles require the stronger, more intense magnetic field the axial-pole magnet provides to attract and hold them on the drum shell, he says. Another company’s product manager also considers axial-pole magnets the better electromagnet design. “It will recover more ferrous and also clean the ferrous,” he says, because the drum’s shell carries material past alternating magnetic fields, causing the ferrous material to flip repeatedly, allowing trapped nonferrous material to fall away.
Radial-pole magnets still have their fans, however. They hold ferrous metals more consistently to the drum face, ensuring their removal, says the magnetics division president of one equipment firm. They excel at picking up elongated scrap, such as rebar or roof bolts, adds another magnetic separation specialist. The choice between radial-pole and axial-pole might come down to the material and separation requirements—many companies produce both.
Though electromagnets have dominated the industry for many years, permanent magnets have evolved as well. The original Alnico material has not been used for decades, says the magnetic separation specialist. Permanent drum magnet buyers now can select ceramic or rare earth magnetic elements. Ceramic magnets, made of strontium ferrite, come in several grades, the strongest of which is Ceramic 8. They cost less, weigh less, and resist demagnetization better than the alternatives, making them a popular choice in the industry, says the magnetics division president.
Rare earth magnets are made from alloys of rare earth elements, typically neodymium alloyed with iron and boron. Developed in the 1970s and 1980s, they’re the strongest permanent magnets manufactured today, but they’re also exceptionally brittle and susceptible to corrosion, so they are usually plated or coated to protect them against breaking and chipping. One company debuted a magnet last year that’s made of both ceramic and rare earth materials, producing some unique magnetic fields. “The rare earth’s deeper field gets great iron recovery, and the ceramics transfer the iron fragments,” says the company’s product manager. One concern is that China holds a near-monopoly on rare earth elements right now, notes the magnetic separation specialist, a situation that has driven up their prices and could affect the cost of drum magnets that contain them.
Beyond the Magnet
If the magnetic material is the heart of a drum magnet, the rotating drum might be its skeleton and skin all in one. The drum takes a beating from constantly being struck by the shredded material, not to mention its exposure to heat, cold, rain, snow, dirt, and debris. Letting the drum shell get worn out is one of the biggest mistakes people make, says the president of one large scrap company. A worn-out shell allows steel to get inside the magnet, ultimately destroying the coil on an electromagnet. “You have to replace your manganese shell on a regular basis,” he advises. And manganese is the material of choice for large, heavy-duty drum shells in the scrap industry. Manganese can stand up to the rigors of being pounded constantly by large pieces of tramp iron and steel, says the magnetics division president. The magnetic separation specialist concurs. “[When] the drums are suspended above the discharge conveyors, impact resistance is important. Manganese is the ideal shell material because it is significantly more wear resistant than stainless steel,” which is prevalent in lighter-duty applications. A manganese drum shell 3/8-inch thick will normally last three to five years, he says; thicker and replaceable shells will increase life.
Double shells are common in the scrap industry, says one shredder company representative. Instead of having one 3/8-inch shell, you might have two ¼-inch shells, for example. “When you see a hole in the outer one, you change it,” he says. “This beats changing the whole shell [because you can] change it quickly” and get the magnet back in operation. Another scrap equipment expert seconds that recommendation. Despite the additional cost, he explains, the second shell makes sense because it can easily be replaced in the field and provides an additional layer of protection for the internal windings. “Once the drum shell has been compromised, the cost of repair goes up significantly,” he says.
Most drum shells feature protuberances—called cleats, sweeps, lifters, or knock-offs—that provide some impact resistance and help move the ferrous material away from the magnetic field and toward the back side of the drum, where it lands on the proper conveyor or chute. Properly designed lifters “are vital on axial [magnets], much more than on radials, as during the flipping, improper lifters actually strip the long pieces from the magnet,” says the shredder representative. The pieces “fall off and are picked up many times before some other pieces usually carry them over.”
Selecting the Right Magnet
Drum magnet buyers have an array of choices before them: permanent magnet or electromagnet? Axial or radial pole? What width and diameter? No single magnet design is superior in every case, says the shredder representative. “A good permanent magnet, properly sized, can be just as strong as an electromagnet, and a radial magnet can be just as strong as an axial one.” Proponents of electromagnets say they’re best for picking up heavy pieces of ferrous metal. An electromagnet has a more powerful magnetic gradient—the ability to attract or move material—than a permanent magnet, says the shredder representative. “To create an attraction as great as one gets from an electromagnet, you must have a larger permanent magnetic drum,” he says. Electromagnets also have safety advantages, says the magnet sales manager. They can be switched off, making it safer to perform maintenance in their vicinity. Permanent drum magnets supporters point out that they don’t require electricity to energize the magnetic field, which can save money, and even those who prefer electromagnets note that permanent magnets require less maintenance. Most companies warranty their permanent magnets for life against loss of strength.
There is one point of agreement among these magnet experts: Don’t try to make a magnet purchasing decision on your own. The variables to consider include the type of application, processing volume, mix and type of materials (as well as their weights and densities), and conveyance equipment (including belt width and speeds). In terms of dimensions, “drum width is based on the feed presented to the magnet,” says the magnetic separation specialist, with some companies offering widths up to 120 inches on radial-pole and permanent axial-pole designs, and even wider options on some axial-pole electromagnets. Diameter can vary, with typical widths between 32 inches and 72 inches. In general, the larger the diameter, the more the magnet recovers. “As total drum surface area increases, so does capacity,” says the magnetic separation specialist. “Drum speed can also manipulate capacity to a degree, but there are limits to how fast—or slow—you can turn a drum.” If you’re debating between two sizes of magnet, go for the larger one, says the shredder representative. “There is no such thing as a drum magnet that is too big,” he says. “There are plenty that are too small.” The sales manager seconds that idea. “Don’t shortchange yourself on the drum magnet size. A bigger diameter drum gives you more adjustment, will offer better performance, and give you some room to grow.” With so many variables, these experts recommend collecting as much information as possible and submitting it to the drum magnet manufacturer or seller for analysis and recommendations. At least one magnet seller performs tests on its clients’ materials to ensure that the separation meets the client’s expectations.
When comparing models from different companies, ask the manufacturer to give you the magnetic strength (measured in gauss) of its model at a specific distance from the drum face, the sales manager suggests. For example, what’s the strength at 10, 12, or 14 inches from the drum face in both cold and hot operating conditions? “All manufacturers overstate the proper gaps,” the shredder representative asserts. “For instance, on a 72-inch-diameter magnet, we find the gap should be [no more than] 12 inches”—not the 18 inches the manufacturer states—“if the user wants to get 99 percent of the knuckles, assuming proper loading, feed, and drum speed.”
Further, compare features such as thickness of the wear shell, and ask whether the purchase includes things like drive units, wear covers, side skirts, and mounting bearings. The cost of a drum magnet varies depending on the size and model. Drums used for large scrap applications can range from $25,000 to more than $135,000, according to one company’s sales manager.
Boosting Life Span
Different parts of drum magnets have differing life spans, says the magnet sales manager. “Wear shells’ [life span] depends on how much material passes over the top of them,” he says. “Roller bearings typically have 100,000 hours of life span. [Electromagnetic] coils can last for many years depending on voltage input to them and run time.” To avoid costly repairs, keep drum magnets in peak condition, and meet the requirements of the warranty, be sure to follow the manufacturer’s maintenance instructions. Always ensure the bolts between the end caps and the outer shell are tight, for example, to prevent water and magnetic dirt from getting inside the drum and damaging internal components. Inspect the outer shell for wear every month, and be careful about overgreasing the roller bearings. Also, for electromagnets, monitor the electrical measurements: the voltage going to the drum and amps during operation. With electromagnet warranties of one to 10 years and permanent magnet warranties of one year to a lifetime, proper maintenance will ensure a scrapyard’s drum magnet remains effective for a long time.
Chelan David is a writer based in Overland Park, Kan.
Magnet type and size are just two of many variables that factor into the selection of this essential piece of ferrous scrap separation equipment. The right design and installation can be the key to more productivity and a longer magnet life.