Separation Supplement -- Magnetics – A Time Tested Separating Method

Magnets can be seen everywhere across the recycling spectrum. From the most modest of MRFs to multi-million dollar automobile shredders, magnets are used to separate recoverable materials that will ultimately head to different destinations.

Although the power of a magnet to be attracted to iron was known to ancestors in previous eras, even as the year 2000 approaches advances continue to be made in magnetic technology. The recycling and scrap processing industries are among those pushing and utilizing that new technology.

When steel or iron comes into a scrap yard promptly from a factory or otherwise in an uncontaminated format, the only magnet it may come in contact with before being re-melted is a lifting magnet.

But the rest of the ferrous scrap stream—from tin-plated steel cans to auto hulks—is likely to go through one process or another that will entail magnetic separation. The magnetic equipment used comes in several forms.

DRUMS HANDLE THE LARGE STREAMS

Large drum magnets are typically found downstream from auto shredders. Often the drum magnets are used as the first separation method for the stream of material leaving the shredder. The post-shredder drum units can vary in size from 36 inches in diameter up to 72 inches in diameter and can be either permanent magnets or electromagnetic.

Large diameter (36-, 42-, 48-, 60-, and 72-inch diameter) magnetic drums have been used for many years in scrap metal yards to separate iron and steel from other materials. In addition to reclaiming ferrous materials from shredded car bodies, they are used in other scrap metal applications as well as for pulling ferrous metals from municipal solid waste, wood waste, slag, incinerator bottom ash, foundry sand, and in minerals processing applications.

A typical automobile shredder system uses magnetic drums to provide the initial separation of ferrous metal from a fragmentized auto (or frag), followed by a cross-belt magnetic separator and then a magnetic head pulley.

The large diameter magnetic drum separators consist of a fixed, but adjustable, magnetic element inside a heavy-duty outer shell with cleats that rotate around the element. The magnetic element is either a permanent or an electro-type. The ferrous material jumps to the drum shell where the cleats move it through the magnetic field and discharge it at the end of the magnetic arc.

The permanent non-electric magnetic drums are ideal for locations where electrical power is limited or unstable. The shell drive motor is the only electrical power required (typically 3 to 10 horsepower).

Most processors prefer electromagnetic drum magnets since they offer more bang for the buck. They are also safer since the magnetic field can be turned off when the plant is not operating or in a case of a ferrous jam. Permanent magnets, in comparison, are always on and require more safety precautions. Electro drum magnets are available in two designs: a radial pole shoe and an agitator type.

The electro-agitator drum magnet uses a deep field rectangular-core pick up magnet to reach out and grab the ferrous material, and a second rectangular-core agitator to flip or agitate the ferrous, cleaning it of contaminants like loose mud, paper, fluff and trash. A double drum scheme with counter rotation provides maximum cleaning.

Many magnetic drums are the older style non-agitating transfer design that uses a deep field radial pick-up magnet and pole shoes to convey or transfer ferrous around the drum shell to the discharge area. When used in top feed schemes, these drums will provide maximum recovery. This design can also be side fed for foundry type applications.

Steel mini-mills use more scrap metal than older integrated mills, but very clean scrap containing few contaminants. The older style radial pole shoe drums optimize ferrous recovery but do not provide adequate ferrous cleaning. The all electro agitator type drum magnet cleans the steel more thoroughly. Often, retrofitting this type of drum to an existing system or mag stand will provide a product acceptable to the end user. The electro agitator design provides a cleaner ferrous from wet, sticky or muddy material. It also allows a processor to turn down the air in the air cascades and still get a valuable product.

Some electro agitator drums also provide 80% to 90% ferrous loading across the drum shell width as compared to less than 50% in the older pole shoe design (recognizable by the two or three shiny wear bands on the shell). This transfers more fragmented metal (frag) and lengthens drum shell life.

The smaller diameter magnetic drums typically process between 10 to 15 tons per hour per foot (TPH/F) of drum width, while the larger diameter drums handle up to 30 TPH/F of drum width. The actual rate varies with the type of product being processed.

With the variety of drum diameters and overall widths available, as well as the various options available, drum prices range from $30,000 for a smaller size drum to $100,000 for a larger drum with specialty options. While they are more expensive than suspended belt magnets, the drums’ minimal maintenance requirements (only two bearings to grease), durable construction, and longer service life make them the preferred choice in applications processing shredded car hulks, foundry sand, steel mill slag, and co-generation bottom ash applications.

A different type of drum is an electrostatic separator, which provides a temporary magnetic charge to ferrous particles to separate them from non-iron bearing particles. The separators are often used for streams with a mix of steel and plastic particles, since the two different materials hold the electrostatic charge for different lengths of time, making it possible to separate them.

BELTS A COMMON FORMAT

Suspended belt magnets are large permanent or electro-magnetic separators that typically hang above belt conveyors or vibrating pan feeders in either an in-line or crossbelt position to recover ferrous metals from the material burden as it passes under the magnet. They are used in many industries to remove tramp iron or large pieces of ferrous metal from the material burden, protecting crushers and other processing equipment from damage. For the recycling industry, their task is to reclaim large amounts of ferrous material to be sold as a product.

Typical applications in recycling include removing ferrous cans from commingled household recyclables and municipal solid waste (MSW) at material recovery facilities (MRFs); sheet ferrous from automobile shredder fluff; iron from foundry sand; metals from construction and demolition (C&D) debris; and ferrous metals from boiler bottom ash.

Suspended box magnets for recycling are self-cleaning units consisting of the box magnet, a frame, pulleys, a motor drive, and a belt with cleats around the magnet to discharge the recovered ferrous material – hence the term “belt magnet.” As the ferrous metal is attracted to the magnet, the cleats on the magnet’s belt move the iron out of the magnetic field where it is discharged (often over a splitter or divider) into chutes, roll-off bins, or take-away belt conveyors. Most belt magnets have a hemispherical shaped magnetic field, stronger in the center and weaker at the edges, which is ideal for trough belt conveyors and aids in discharging the collected ferrous material.

The box of the magnet can either have a permanent or electromagnetic element inside. Permanent magnets are built in various sizes good for suspension heights or magnet gaps of 4 inches to 14 inches, while electros are built in various sizes good for magnet gaps of 4 inches all the way up to 30 inches. Electromagnets are often preferred since they typically provide more magnet per dollar, especially at magnet gaps of more than 8 inches. The electros can also be turned off when the plant is not running or to free ferrous material if a jam occurs, making them safer in these situations.

Belt magnets provide a reasonably clean ferrous sort since they attract the steel or steel tin through gravity, allowing paper, trash or non-magnetics to fall free in the process. Magnetic pulleys, by comparison, will discharge any trash caught between the ferrous material and the pulley magnet.

MAINTENANCE

The maintenance item on belt magnets is the belt and self-cleaning system. Can lids; sharp ferrous metal; large, heavy pieces; and long angle, pipe or bars can cut, abrade, puncture, or rip the magnet’s self-cleaning belt. But measures can be taken to prolong the belt service life. Thicker belts (but not too thick or they can overload the bearings), urethane belts or urethane centerlane belts, and armor clad belts (belts with stainless steel slats and cleats bolted to the belt face) are often incorporated to prolong service life.

Since the iron gathers around the core area when it gets to the belt, only the center one-third to one-half of the belt needs to be protected from abrasion, cutting, and puncture. If wires and can lids are commonly processed, it may be a good idea to use molded or vulcanized rubber rather than stainless steel cleats, so that wire and lids are less likely to catch under the slats or cleats.

Also, having more belt cleats (i.e., cleats on 18-inch centers) reduces belt wear. Having fewer belt cleats (i.e., cleats on 36-inch centers) provide a cleaner ferrous sort, since the ferrous material tumbles in the magnetic field, allowing some trash to fall out before the cleat pushes the steel into the discharge zone.

Many recycling applications use picking belts operating at 40 to 60 feet per minute (FPM). In these applications, it is useful to have a cross-belt magnet installed between the belt conveyor idlers discharging ferrous material perpendicular to the conveyor belt travel. If the magnet is located over the discharge area, or “in-line”, and the conveyor speed is less than 300 FPM, a non-magnetic (typically stainless steel) head pulley must be used because a steel pulley will be magnetized, lowering ferrous recovery.

In general, magnetic steel components such as chutes, framing, idlers, and pulleys should not be used within two to three feet of the magnet’s face and 1/2 to one foot of the magnet sides and top. Instead, non-magnetic manganese or stainless steel can be used.

Some belt magnets use multiple magnets inside the self-cleaning belt. The additional magnets clean the ferrous material by agitating or flipping it and shaking out trash. The additional magnets can also be designed to extend the ferrous discharge point. This is advantageous to convey ferrous material beyond disk screens and other adjacent equipment.

COSTS

Self-cleaning belt magnets vary in cost by the width and depth of the magnetic field required. Smaller sized permanent belt magnets can be purchased for less than $10,000; typical electro belt magnets for medium-sized MRFs cost approximately $15,000 to $20,000; and larger belt magnets can cost $25,000 to $40,000 and up. The belt magnet’s advantage is that it focuses all the magnetic field generated within the box through the bottom plane or face of the magnet, allowing deep magnetic fields at reasonable prices.

If a belt magnet is undersized, this does not mean it will not reclaim ferrous material; it just means it is not reclaiming all of the available ferrous. Often magnetic head pulleys are installed downstream of belt magnets to remove the smaller ferrous materials – such as bottle caps, can lids, and wires. Magnetic head pulleys replace the drive pulley on a belt conveyor and are available in diameters of 8, 12, 15, 18, 20, 24, 30, 36, and 42 inches and belt widths from 8 inches to 72 inches. A small magnetic head pulley with shaft costs approximately $1,000, while the majority of magnetic head pulleys are priced between $3,000 and $8,000. Larger pulleys may cost as much as $30,000.

A REPELLING DEVELOPMENT

The “oddball” of the magnetic sorting family is the eddy current separator. The eddy current is technically a magnetic piece of equipment that was not designed with steel in mind.

Rather, the eddy current removes nonferrous metals from a stream that also contains a significant amount of nonmetallic material. A magnetic rotor that is encased in a nonmetallic shell spins quickly when the polarity produced by a passing piece of aluminum (or more accurately, its magnetic field) occurs. The end result is that aluminum is repelled away from that rotor, which allows aluminum items to “leap” off the end of a conveyor that has passed under the eddy current separator, while other materials simply drop off the conveyor.

Since their introduction to the recycling industry, eddy currents have become increasingly sophisticated and have also been designed to handle more material.

High-frequency eddys are gaining popularity due to their ability to separate small and medium-sized nonferrous metals from auto shredder residue fines, foundry sand, bottom ash, computer scrap, glass cullet and plastics.

One way for eddy currents to handle a greater flow of material is to be built wider. 60-inch feed width units are being made that send their material to a 66-inch eddy current rotor.  RT

The sections on drum magnets and belt magnets are by Al Gedgaudas, recycling equipment manager for Eriez Magnetics, Erie, Pa. Other text is by Brian Taylor, editor of Recycling Today.

Analysis Via Gamma Rays

Steel mills and scrap processors have been forming cooperative efforts for decades to help ensure the right blend of steel melt chemistry begins with scrap shipments to the mill.

While parties on both sides have pronounced satisfaction with many of these arrangements, most mills would still welcome any development that helps them sort and separate scrap to achieve a least cost-suitable charge blend of metallics on every melt.

Systems Alternatives Inc. (SAI), Maumee, Ohio, has the scrap and steel industry marketing rights to a device that may play a key part in optimum melt chemistry procedures at mills in the future.

The Bulk Scrap Analyzer (BSA) uses “Prompt Gamma Neutron Activation Analysis” technology to measure the chemical composition of scrap as it travels on a conveyor. The BSA provides the composite analysis of scrap traveling beneath it—not just the surface but to a depth several inches thick—the company points out.

The BSA has been field tested, says SAI president John Underwood, and can analyze ferrous scrap at a rate of 200 tons per hour. The device can detect the percentage of copper, chromium and other elements present in a stream of shredded scrap or in turnings or busheling. In the future, it may have the potential to analyze a greater range of scrap grades when redesigned with a larger aperture.

There has been interest in technology from both scrap processors and steel mills, Underwood says. Processors may find the BSA ideal for analyzing and “certifying” the tramp element content of scrap produced from a shredder, while mills can analyze their incoming metallics as a means of lining up least cost -suitable charge melts.

One of the next steps in the development will be to perfect separation methods downstream of the BSA, so that material that is analyzed with an undesirable amount of, for instance, copper, can be diverted from the rest of the stream.

“The analyzer provides a method for processors and consumers to add value and guarantee products, control quality, design blends, use low-cost material and reduce off-specification melts,” Underwood states.