Suspended Magnetic Separators for Tramp Metal Removal
By Dan Norrgran, Market Manager-Heavy Industries • Eriez

When addressing the magnetic collection of tramp metal, the Suspended Electromagnet (SE) is the industry workhorse. The SE magnet, providing tramp metal collection of conveyed materials, is a widely used magnetic separator. The electromagnet is mounted or suspended over a conveyor belt to remove relatively large pieces of tramp metal that represent a potential hazard to downstream crushers, mills, pulverizes and grinders. The magnet can also be mounted over feeders or chutes.

Components
The electromagnet consists of an electromagnetic coil with a cylindrical steel core. This assembly is completely enclosed in a steel “box.” The coil magnetically includes the steel core that projects a magnetic field for the collection of tramp metal. The coil and core are submerged in transformer oil to dissipate heat. (The major components of a suspended electromagnet are detailed in Figure 1.)

The coil actually consists of a series of individual coils. The coils are spaced on the core with fiberglass spacers to provide adequate convection cooling. The coils are typically wound from round aluminum conductors with Nomex insulation. The size of the conductor is selected from the design to provide the proper amperage draw and magnetic field projection, as well as adequate cooling capacity.
The electromagnetic coil is housed in a steel box which has the dual purpose of protecting the coil and providing a steel circuit for the magnetic flux path. The housing is fabricated from a steel plate and the cylindrical core is machined from a solid block of steel. The bottom of the housing is a heavy manganese steel plate that provides protection and abrasion resistance when collecting tramp metal.

The magnet housing has an expansion tank that allows for the expansion of the oil and the removal of condensate.

A power supply is used to energize the magnet and it converts the alternating current input to direct output to energize the coils.

Magnetic Circuit Design
There have been recent technological advancements in the design and modeling of magnetic circuits. Precise magnetic circuit modeling and optimization is now carried out using multi-dimensional finite element analysis. The input is a scale design of the magnetic circuit; the output is a contour plot of the generated magnetic field intensity and the magnetic field gradient. This methodology provides an accurate depiction of the magnetic field configuration.

The coil design and the subsequent power draw are the two first order variables in the design of the electromagnet and the projected magnetic field. The coil must be sufficiently large in combination with minimized power to allow for convection cooling. Further, this design extends the life of the conductor insulation and subsequently the life of the electromagnet.

The Eriez design maximizes the coil to minimize the power required and the subsequent heat dissipation.

Applications of Suspended Magnetic Separators
The SE magnet removes ferrous tramp metal from moving conveyors to protect downstream equipment such as crushers, mills, shredders and presses. The largest market for SE magnets is in coal mining, hard rock mining and aggregate products removing shovel teeth, cable, tools and bolting prior to crushing and grinding. A large piece of ferrous tramp metal such as a shovel tooth or rail track will not yield in a crusher and may damage the drive system. In the worst case, the crusher shaft is bent requiring complete replacement. Not only is this type of repair costly, it also results in significant downtime.

Suspended Electromagnet Selection
The foremost factor in SE magnet selection is the burden depth of the material on the conveyor belt. (Note that the belt speed, width, capacity and bulk density are all factors in the material burden depth on the conveyor belt.) The burden depth determines the suspension height of the magnet and consequently the effective magnetic field strength at the belt surface. Conveyer belt idlers elevate the edges of the belt forming a trough. The effect of idlers must be accounted for in the burden depth calculation and the height of the idlers must be noted for the magnet suspension height.

SEs are mounted in one of two positions over a conveyor belt (as shown in Figure 2). In Position One, the magnet is mounted just over the stream of material leaving the head pulley. This position utilizes the full potential of the magnet as it reacts with the material in suspended trajectory. Tramp metal is easily pulled through the suspended burden. Further, the flow of material is directed toward the magnet face. Collection of the tramp metal from the material flow does not necessitate a change in direction. At conveyor belt speeds of less than 350 fpm, the suspended trajectory of the material is minimal and becomes near vertical. In this case, the magnet must be shifted to a position approaching directly over the head pulley.

In Position Two, the magnet is mounted over the conveyor belt prior to the head pulley. This position requires higher magnetic field strengths to attract the ferrous component, shift the direction of momentum and pull it through the bed of material.

SEs are available as a manual cleaning style or a self-cleaning style. Manual cleaning magnets are best suited where only small amounts or occasional pieces of tramp metal are encountered. The manual cleaning magnet must be periodically turned off in order to remove tramp iron accumulation from the magnet face.

Self-cleaning magnets employ a cross-belt running around the magnet face to provide continuous removal of collected tramp iron. When tramp metal is attracted to the magnet face, the cross-belt intercepts it and discharges it to the side away from the conveyor belt. The self-cleaning magnet is best suited where high levels of tramp iron or large pieces of tramp iron are anticipated. An 1/8-inch thick cross-belt is standard. The cross-belt is continuously driven around the magnet on a system of four pulleys driven with a small gear motor. Optional cross-belts using thicker ply belts or armor cladding is available for severe duty applications.

In most applications the magnet is suspended three inches over the top of the burden.

Experience dictates that the operating magnetic field strength or the magnetic field strength at the belt surface should be 600 gauss for adequate ferrous collection. As the magnet width increases to accommodate the belt width, the magnetic field strength increases at any given distance. It is simply the case that a wider magnet has a larger core and coil. By design, larger coils have more amp-turns and operate at a higher magnetic field strength. Further, industry standards demonstrate that the burden depth increases as the belt width increases requiring an increased magnetic field strength for tramp metal collection.

Factors Influencing Magnetic Collection
There are exceptions to the sizing of the SE magnet. The response of the magnet is influenced by the following factors:
Belt Speed - As the belt speed increases it becomes increasingly difficult to attract and collect ferrous components. This is especially difficult when the magnet is situated in Position One where the momentum of the ferrous component has to change direction.

Burden Depth - As the burden depth on the conveyor belt increases, an increase in the magnetic field strength is required for effective collection of ferrous tramp. A ferrous component situated on the surface of the belt, buried under a heavy burden of material, requires an increased magnetic attractive force for collection. The ferrous component must be pulled through the additional weight of the burden. The idler height must also be accounted for. Conveyor belts typically utilize 35 degree idlers. The height of the idler is also a factor in the suspension height. A cross-belt or Position Two magnet must be positioned to clear the idlers.

Size of Ferrous Component -The suspension height of the magnet must be increased if relatively large ferrous components are anticipated. The suspension height of the magnet, with the ferrous component attracted to the face, must clear the burden on the conveyor belt and not impede the flow of material. If a six-inch rectangular plate is regularly anticipated, the suspension height must be in excess of six inches. The higher suspension height requires a stronger magnet to maintain the magnetic field strength at the working distance.

Shape of Ferrous Component – The shape of the ferrous component is also a factor. Steel plate, for example, has a high surface area relative to its weight. This configuration reacts with a high magnetic attractive force when exposed to a magnetic field. In contrast, a sphere has the lowest surface area relative to its weight. This configuration reacts with a minimal magnetic attractive force when exposed to a magnetic field.

All of the above exceptions will require increased magnetic field strengths of 600 to 800 gauss at the working distance specified.

Applications and Sizing
Magnetic Field Strength - The suspended electromagnet is typically configured with a cylindrical steel core. The coil includes a magnetic field through the core. The strongest point of the magnet is the center of the core. The core projects the magnetic field downward onto the belt surface. The magnetic field extends from the core out to the edges of the magnet box to complete the circuit. This magnetic field configuration approximates the cross-section profile of a conveyor belt running on idlers.

Force Index - In any application, a magnet suspended over the conveyor belt at a given distance must be capable of collecting the required tramp metal. The distance from the magnet to the conveyor belt is termed the suspension height. The magnetic collection of ferrous tramp metal at the given distance is the basis for sizing a magnet selection and sizing. A measure of a magnet’s ability to achieve this can be defined as the force index or magnetic force.

Force index is the product of the magnetic field strength and the magnetic field gradient at the point under consideration.
Since many factors may affect choice of the proper magnet, intelligent engineering practice dictates that each application be individually reviewed by your magnet manufacturer’s engineering team before selection is made.

Dan Norrgran serves as the product manager of the Minerals and Materials Processing group, as well as the Heavy Industry market manager. Dan has a bachelor’s degree in physics from Hamline University, a master’s degree in engineering from The University of Minnesota, and a master’s degree in business administration from The Pennsylvania State University. Dan can be reached at dnorrgran@eriez.com.

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