Magnets, Electromagnets, and Eddy Current Separators

Credit: Dings

Editor’s note: This article first appeared in the January/February 2016 issue of MSW Management.

Recycling is not an easy thing to do, and its difficulties increase with scale. For large communities and cities, industrial scale recycling operations are required. This further requires the adoption of industrial methods utilizing specialized machinery that can extract specific materials from the wastestream with sufficient capacity to manage incoming waste loads. Technically, the process of recycling MSW presents a series of challenges and problems.

The first is the nature of the material itself. MSW collected at the curbside and hauled by collection trucks to the landfill or material recovery facility (MRF) is extremely heterogeneous with waste materials of all types and characteristics (density, water content, chemical composition, electromagnetic potential, and sizes), mixed and compacted together into a mass that makes separation and extraction inherently difficult.

And not every article of waste is composed of a single unique material. A significant proportion of the wastestream consists of composite materials that combine two or more types. This can include something as simple as glass jars with screwed on metal lids and pasted on paper labels, or complex composite packaging like children’s drink boxes that use a layer of aluminum foil sandwiched between layers of plastic and paper. It is easy to imagine how difficult (if not practically impossible) it is to economically recycle the individual components of composite packaging materials.

External factors can indirectly limit the amount of waste being recycled. Though they are definitely “doing well by doing good” by promoting sustainability and preserving natural resources, it is no secret that private recyclers (like any other business) are in business to make money. Even publicly owned and operated recycling agencies strive to show a positive annual budget balance. So, if there is a lack of market demand for a recycled material, this lack of demand will reduce the amount generated by recycling due to the iron law of supply and demand.

A lack of locally available recycling facilities can create another external bottleneck to higher recycling rates. Whether expanding existing facilities or building new ones, the recycler requires capital investment. And investors need an adequate return on their investment. If economic conditions depress demand (and demand growth) for recycled materials, investors will find other uses for their money.

The material recovery facility is the one type of facility that meets all of these needs: efficient separation of materials by means of specialized equipment, the ability to manage large tonnages of incoming waste, providing quality products that meet market demands, and ensuring efficient employment of capital investment. Traditionally, MRFs were simple affairs where materials already presorted by homeowners and businesses were received and manually organized and stockpiled for shipment to market. The type of labor-intensive MRF is usually referred to as a multi-stream, or “clean” MRF.

The second type of MRF accepts a mixed wastestream direct from waste collection and hauling trucks. The mixed waste mass is a process by machinery designed for efficient, sequential removal and segregation of each type of waste material. This type of capital-intensive MRF is called a single-stream, or “dirty” MRF. It is this type of facility that processes most of our recycled waste and utilizes electromagnetic and eddy current separators for metal recovery. These separators represent both the most technically advanced and simple to operate machines used at a MRF, and are the perfect examples of technological solutions applied to the problems of recycling . . . and the most valuable.

And, given the profitability and consistent market demand for ferrous and non-ferrous metals of all kinds, scrap metal remains the crown jewel of recycling.

Recycling Rates
According to EPA (2012), the wastestream consists (on average) of the following material categories. The total amount of MSW generated by the US (2012) came to about 251 million tons (prior to recycling). Recycling and composting removed 86.6 million tons of waste (34.5% of the total) from the wastestream, leaving 164.4 million tons to be landfilled (53.8%) or incinerated to generate electricity (11.7%). Recycling over one third of the wastestream over all is a significant accomplishment, one obtained by achieving the recycling rates for each type of material.

Combining these two data sets gives the recycling rate for each type of material. “Other” would include items like car batteries and old tires that are often banned from landfilling, resulting in very high recycling rates. “Yard Trimming” recycling rates represent extensive composting operations that are performed at other locations than MRFs. “Paper and Paperboard” have always had high recycling rates for bulk newsprint, office paper, and old cardboard.

That leaves “Metals” as the material with the second-highest MRF-related recycling rates at over one third. And there are several good reasons for this relatively high recycling rate. The recycling and sale of scrap metals tends to be profitable on a per ton basis. From 2003 to 2008, the average price of ferrous and non-ferrous scrap metals nearly doubled. The market crash of 2008 set back these price advances. Prime scrap steel fell from a peak of $700 per ton, to a low of $120. Scrap aluminum fell from $0.60 per pound, to $0.20. The price of copper, a material so much in demand that criminals risked electrocution by ripping copper wire out of live substations, fell from $3, to $1 per pound. But with the economic recovery came the return of higher scrap metals prices, and the value of scrap metal markets have nearly recovered to pre-recession levels. This increased value of scrap metal refocuses interest in efficient scrap metal removal at MRFs.

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Scrap Metal and the Recycling Process
Scrap metal removal is just one stage of the recycling process performed by a MRF. Waste shows up at a local MRF in the back of a waste collection truck. Regional MRFs may receive waste from large open top trucks, baled loads of waste, or even from railroad cars. Upon arrival, the comingled wasted is dumped onto the MRFs tipping floor. From there, the waste is usually pushed by wheeled dozers or front end loaders onto conveyor belts or hoppers. Conveyor belts then carry the waste from receiving to sorting. The belts themselves come in various types (slider-belt, chain-driven, roller-type, steel-belt, or rubber-belt) and often come with sides that flip over the top of the mounded waste to prevent loss during transport.

The sorting stage could be as simple as a manual sorting platform. This labor-intensive operation relies on humans to pull out and extract certain types of waste as it passes by their station, and place it into a receiving hopper. Though many single-stream MRFs include a manual sorting stage, most rely more on sorting machinery for automated waste extraction. Often, the comingled waste passes through shredders and pulverizes to reduce the object and particle sizes to facilitate removal. In some of the newer plants, organics (paper, paperboard, food waste, yard trimmings, etc.) is subject to saturated steam pulping which renders this material into a pulp suitable for feeding a gas producing digester.

The workflow sequence and separation stages vary from MRF to MRF. But typically, the following steps occur. Non-recyclable material is removed as much as possible up front for disposal in the landfill. Machines called disc separators can remove large cardboard items (sheets stock, boxes, etc.), newspaper, and mixed paper. The extracted materials are sent to hoppers for bailing and stockpiling. The remaining waste gets fed into a sorter line where large containers (mostly plastic) are removed. Electromagnetic separators extract ferrous metals and eddy current separators remove non-ferrous metals. This leaves the wastestream’s residual glass, which is removed and sorted by color.

The above is a relatively simple MRF operation. More complicated and sophisticated MRFs rely on a wider assortment of separation and sorting machines-and less on labor. These machines utilize various methods that match the waste materials physical characteristics. Magnetic and eddy current separators extract metals based on their electromagnetic characteristics. Disc screens and rotating trommels separate containers and paper or paperboard based on size and shape. Air classifiers and air knives sort paper by weight. Optical sensors sort and separate plastic and glass by color.

Once separated and sorted, the recycled materials enter the last stage of the recycling process. Here, the extracted products are then sent through compactors, can flatteners, and baling systems to increase their density prior to stockpiling. From stockpiles, the materials are loaded on to trucks or railcars that ship them to market.

Scrap Metal Separation by Electromagnetic Characteristics
As mentioned above, the potentially most lucrative portion of the recycled wastestream is its metals component, both ferrous and non-ferrous. But how is this done? More importantly, how is this done in the most cost-effective and profitable manner?

Electromagnetic separators are simple in design and operation. They extract ferrous metals from the wastestream by means of magnetic attraction. Configurations can vary from overhead to under the conveyor belt. Overhead electromagnets pulls waste up out of the wastestream as it travels underneath on the conveyor belt.

Belt separators incorporate electromagnets installed directly under the belt. As the conveyor belt moves to its last roller and turns under itself, all of the waste except the ferrous metal falls off at the turn under point into a waiting receptacle or another conveyor belt. The ferrous metal, on the other hand, sticks to the underside of the belt due to magnetic attraction. It then continues to a scraper blade, which pries off the ferrous metal so it can fall into a designated hopper.

A different version of a magnetic separator combines electromagnets with a rotating trommel. Essentially a rotating drum set at an angle, a trommel usually has holes perforating in its surface to allow the escape of small particles and objects. This configuration lines the drum surface with electromagnets. As waste passes through the trommel most of the waste continues out the bottom while ferrous metals cling to the interior wall. Eventually the weight of the accumulated ferrous metals overcomes the force of magnetic attraction ad the ferrous metals work their way out of the drum into a collection chute and storage bin.

The removal of non-ferrous metals requires a more complicated process. Lacking direct magnetic attraction characteristics, non-ferrous metals like aluminum need to have a current induced in them. This is accomplished by a rapidly rotating rotor mounting magnets with alternating polarity. The rotating magnetic fields create alternating currents within the on-ferrous metals. In turn, these alternating currents create electromagnetic fields of their own. These opposing magnetic fields repel each other, causing the non-ferrous metal to jump off the conveyor belt into adjacent collection bins.

Major Suppliers of Magnetic and Eddy Current Separators
Bulk Handling Systems LLC (BHS) is noted for its complete MSW recycling systems. For example, BHS has developed the first MSW recovery system in the US to keep 70% of waste out of the landfill, including the recovery of 90-99% of high value commodities like PET plastics-on the first pass. Included with these high value materials are non-ferrous metals like aluminum cans that are extracted by eddy current separators. In addition to high recovery rates and productivity, their compact-sized eddy current separators are built for durability and minimal maintenance.

The CP Group supplies both eddy current and electromagnetic separators. The first is designed primarily for the extraction of aluminum cans from the wastestream. Utilizing a powerful rare earth magnetic drum enclosed in a heavy-duty frame, the CP Eddy Current Separator provides a durable, cost-effective solution to aluminum recovery. Its low-profile design allows for integration even into system with limited space. Utilizing 230- or 460-V three-phase power, it is driven by a shaft-mounted TEFC belt gear reducer motor, which allows for variable speeds ranging from 250 to 400 feet per minute. At these rates, it can process 4 to 13 tons per hour, depending on belt width (which varies from 30 to 60 inches).

The CP Drum Magnet separator has a heavy-duty design for long operational lifetimes and minimal maintenance costs (no cooling oil is required). Its non-magnetic exterior drum shell rotates around a fixed magnet center, which pulls ferrous metals out of the wastestream. The placement of the magnets can be customized allowing the separator to drop off metal over, under, or down from the drum. Its Magnetic Separator (which can use either a permanent or electromagnet), on the other hand, is designed for speed and efficiency. Set cross-angle to the main conveyor belt, the overhead electromagnet extracts steel, which then adheres to the upper belt. This allows for continuous removal of steel scrap. Utilizing a 2-hp TEFC drive motor, it is self-cleaning and equipped with 10-gauge mild steel plate and stainless steel side plates.

Bunting Magnetics Co. manufactures an extensive line of magnetic separation equipment designed to remove metal contaminants from a wide variety of media (dry particulates, liquids, and slurries). Their magnetic systems are designed to remove metal contamination from gravity, mechanical, or pneumatic conveying systems. As such, they find wider applications than just in traditional MRFs. Their extensive product line can be modified for specific needs while still meeting demanding quality standards. For example, their sanitary grade metal separators are the industry’s first equipment to earn USDA AMS certification. All models meet NSF/ANSI/3-A14159-1 2002 specification. These standards ensure that food processors, chemical producers, and pharmaceutical developers produce pure meat, poultry, drugs, and medicines. A few examples will suffice to illustrate the diversity of their product line: drawer filter magnets, center flow in line magnets, magnetic liquid traps, plate and drum magnets, suspended magnets, grinder magnets, wedge magnets, torpedo magnets, gravity in line magnets, and such. This wide variety of magnetic products is incorporated into electromagnetic and magnetic separation equipment used throughout the scrap metal industry.

Bunting’s Permanent Magnetic Crossbelt Separation Conveyors are used and mounted across the upstream in-feed end of a pick conveyor to automatically remove all “ferrous metal” contamination from the material stream. Optimum installation requires a large very strong permanent magnet designed for at least 12 inches of magnetic influence. Placement should be as low as possible depending on the size of the largest trash object in the wastestream. The lower the magnet, the better. The lower pick conveyor should have none magnetic components (fiberglass, wood, etc.) that would interfere with the working of the upper extraction magnet. In addition to height, width is an important consideration with the upper magnet being at least 6 inches longer than the width of the lower conveyor belt. This configuration will ensure maximum extraction of ferrous metals.
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General Kinematics Corp. specializes in vibratory material separators but has applied electromagnetic technology in a unique fashion. Their line of scrap metal processing equipment reflects this integrated approach. Their eddy current separator magnetic feeders are especially useful in extracting non-ferrous metals from recycling shredded automobiles. The integration of electromagnetic and vibratory separation technologies is applied to their PARA-MOUNT II and balanced PARA-MOUNT IV. These are vibrating feeders that enhance the presentation of recycled materials to magnets and eddy currents for increased recovery.

Not every magnetic separator is designed for use in standard MRFs for the extraction of metals and cans. Van Dyk Recycling Solutions (VDRS) provides the TITECH finder system, designed to separate high purity metal fractions from even the most difficult fractions in terms of composition, grain size, and mix from mixed waste and metal streams. Utilizing new technology like Van Dyk’s DEEP DATA, and proven technology such as the SUPPIXX image processing capability developed by TOMRA Sorting, the TITECH is able to immediately identify the smallest conductive particles mixed in with other bulk waste. SUPPIXX then enhances the resolution of these minute sensor signals, and these enhanced digitized signals are fed to DEEP DATA, which generates the information that enables the TITECH finder to detect and sort metal objects by shape, size, and signal intensity.

Working in tandem, these technologies provide continuous monitoring of the material presented on the conveyor belt below. According to Scott Jable, Midwest Regional Manager for VDRS, this improves product quality and purity up to 15%, while enhancing sorting stability and maximizing metal recovery. “The TITECH can detect even the smallest metal fragments and separate them from the main waste steam with jets of compressed air. This allows for the maximum recovery of the high-quality recycled mixed metals in the stream,” he says.

In addition to industrial shredders, Vecoplan LLC provides magnetic separators. In particular, their cross belt magnet separators provide large collection areas and self-cleaning operation. A relatively simple, but extremely useful, design feature is the “Bridge Breaker.” This feature prevents “bridging,” the accumulation of material that gets wedged into position between the sidewalls. There it remains suspended unable to be processed. The Bridge Breaker clears away this accumulation to allow for normal operations.

Zimmer America Recycling Solutions manufactures sophisticated metal detection and separator systems for plastic recycling. These systems remove all types of ferrous and non-ferrous metals (stainless steel, aluminum, copper, brass, and lead), no matter if they are pure, bonded, coated, insulted, or embedded. S+S metal detectors identify potentially damaging metal particles during reprocessing of production waste (in-house recycling) or in specialist recycling facilities. Installing them into feeder conveyors can prevent damage to downstream production machinery.

The Dings Company Magnetic Group manufactures a wide variety of magnetic separators: overhead electromagnets, overhead permanent magnets, eddy current separators, drum separators, and magnetic head pulleys. Their permanent magnetic removal systems feature patented flux control circuits, which provide a stronger and more uniform magnetic field. Their unique construction results in lower weight, which allows for the use of smaller magnets. And they can be adjusted to different strengths to match different suspension heights for easy integration into MRF systems. Magnetic head pulleys are a variation on the standard under belt magnets. These are the actual pulleys that impart motion to the conveyor belt. As the belt conveys the wastestream to the pulley at the terminal end of the belt, ordinary waste falls over the edge into a waiting hopper while ferrous metals cling to the pulley until it is cleaned off the belt for collection in their own separate storage bin.

Since 1981, Magnetic Products Inc. (MPI) has designed and manufactured a range of magnetic separators. These separators have been improved over the years to provide maximum ferrous and non-ferrous extraction while keeping pace with ever-increasing demand for scrap metal and expanding MRF capacity. In addition to improvements in strength and extraction rates, MPI has improved its design to maximize operating life with minimal downtime, even under harsh MRF conditions. Increased productivity and reduced operating costs result in maximum ROI for the MRF operator.

MPI’s most recent development is a recently patented magnetic separator that includes optional add on modules of conventional MRF magnets, such as suspended cross belt magnets for higher capacity systems, and eddy current separators. This modular combination allows for the efficient removal of both high grades steel, as well as weakly magnetic ferrous metals. This new technology offered by MPI can separate 99% pure ferrous metals with the added optional features. This provides a real benefit in MRFs that grind and/or pelletize combustible waste for alternative energy applications. The application of this system is not limited to MRFs. They can provide efficient metals extraction for electronics recycling, automotive recycling, automotive shredded residue, and plastics and tire recycling.

Master Magnets Ltd.‘s High Intensity ECS units are specifically designed to handle the separation of small and difficult to extract non-ferrous particles that require high repulsive forces for an accurate separation. It comes in Eccentric ECS, ECS “R” Type and Can Sorter models. Eccentric ECS units are designed to remove products smaller than 10 millimeters, which possesses is a high content of ferrous steel. The “R” Type is a mid range separator that incorporates features from both the smaller Eccentric ECS and the larger Can Sorter models. It is designed for general purpose recycling operations where the sizes of the non-ferrous scrap objects are larger than 20 millimeters. At the high end of the operating scale, the Can Sorter is specifically designed fro the removal of aluminum beverage cans from the wastestream. These sorters provide low cost alternatives to typical eddy current separators for MRFs with specific sorting applications that don’t require specification machines. Optional features include a wide variety of configurations for vibratory feeders, chutes, covers and a base frames; as well as variable speeds for specific waste flow rates.

National Recovery Technologies Inc.‘s NRT Metal Director Flake is a highly specialized piece of equipment, using advanced electromagnetic sensing technology to remove virtually all-ferrous and non-ferrous contaminants from plastic (PET, HDPE, PP, and PVC) flake commodities. Capable of processing 7,500 pounds of flake per hour its object-tracking algorithm identifies and ejects metal as small as 1 millimeter, making it useful for the processing metal from auto shredder residue and fluff.

Walker Magnetics National Ltd. manufactures overhead magnets as large as 100 inches in diameter. The Walker Scrapmaster D series is designed to meet the needs of scrap processing operations by maximizing the lifting capacities of standard cranes. This magnet has a durable ribbed case, and deep field aluminum wound coil combined with a manganese steel bottom plate. Operationally, its 75% duty cycle provides high productivity, and its Class H insulation ensures safe operation. In addition to specialized lift magnets, Walker manufactures mainstream magnetic seaports for MRF operations. The company’s Dynaforce permanent magnetic pullies are inherently self-cleaning and utilize Ceramax strontium-ferrite ceramic magnets that never need charging and are impervious to weather. These come in two configurations: axle pole (magnetic poles around the pulley circumference), and radial pole design (alternating magnetic poles across the pulley width). The Eddy Current Conveyor System achieves 98% recovery of aluminum cans and other non-ferrous metals from wastestreams, ash flows, and automobile fluff. Complete systems can be designed for new facilities or tailor-made to fit into existing ones. Walker’s Dynaforce Suspended Separator operates at 150 or 230 VDC, has class R insulation, and is oil cooled. Designed for rugged durability, its four-pulley design can operate at variable speeds to match incoming waste flow.

Primarily known as a manufacturer of compaction and cubing systems that compress waste materials (paper, cardboard, plastic, wood, sludge, and biomass) into high-density cubes of refuse derive fuel (RDF), Warren & Baerg Manufacturing also incorporates magnetic separator systems for metal removal. This integrated approach ensures the purity of the RDF. MSW_bug_web

Comments
  • Andrew Smittie.

    Hello, I am the CEO of Green E-Waste Miracles, LLC. We are a new start up getting into the electronic waste recycling. We are looking for magnetic and eddy current separators, but we need more detail in how well your product works. I have heard of machines that can sort non ferrous into three distinct categories, what can your product do? What percentages of materials will still be mixed at the end of the sort? What is the cost for machines that can handle one ton per hour our better kinds of loads accurately. What kind of warranties and maintenance are included, if any?

    Reply
  • Ravi Nath.

    Excellent article with enough technical details appealing to an engineer and the not-so-technically oriented.

    Reply
  • Billy.

    Thanks for this informative article – I’m currently doing research on magnetic separation for food processing as my family is getting into the manufacturing industry for some of our products. I’ve found a few magnetic separators from http://cescomagnetics.com as they were recommended to me by a nearby business we work with, but I do not have much experience in this field. Finding an article explaining these methods is helpful, but I’ll keep looking before I make any decisions! Thank you

    Reply

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