By Bill Dudenhoefer
Removing damaging tramp metal from aggregate processing is one of the most important procedures to not only enhance product purity, but also protect expensive downstream equipment.
The suspended electromagnet (SE) – sometimes called the “workhorse” in metal separation – is one powerful piece of equipment widely used in the aggregates industry. Installed, used and maintained properly, an SE magnet can operate efficiently for years under harsh conditions.
Both manual clean and self-clean SE magnets are typically mounted or suspended over a conveyor belt to remove large pieces of tramp metal that represent a hazard to downstream crushers, mills, pulverizers and grinders. These large magnets also remove sharp metal that can damage or tear expensive conveyor belts, especially at transfer points.
How does tramp metal enter the production stream? Any place heavy machinery is involved is a potential source for wayward metal. Examples are chain-driven equipment where pieces of the chain break off or drill bits that shave off and embed into the rock face. Stray bolts, nails and even tools can find their way into the rock product before entering chutes and crushers.
Understanding guidelines for SE magnet selection, optional components, magnet positioning and best maintenance practices will result in optimum performance.
Selecting and Positioning SE Magnets
The most important factors in the selection of an SE magnet are burden depth, belt width, suspension position and tramp iron size. Other factors include belt capacity (tph or cfh), belt speed incline angle, material density, head pulley diameter and altitude.
SE magnets are designed to capture pieces of tramp metal with a large mass (i.e. a large screwdriver, hammerhead, bucket tooth, steel shank) rather than smaller metal pieces such as a ¼-in. hex nut. The larger the mass, the more likely the SE magnet will be to pick up the object, even through maximum burden depth.
Magnet strength increases as conveyor belt width and burden depth increase. A 42-in.- square SE magnet suspended at a maximum 13-in. can pull tramp metal through 11-in. of burden depth. A 78-in. square SE magnet suspended at a maximum 30-in. can pull tramp metal through 28-in. of burden depth. Some manufacturers, including Eriez, design and build SE magnets for belt widths of 96-in. and larger that necessitate even higher suspension heights.
Belt speeds can also influence the size of SE magnets. As the belt speed increases, it becomes more difficult to remove ferrous components. Larger, stronger SE magnets may be required for faster belt speeds.
The shape of the tramp metal should also be considered. Steel plate has a high surface area relative to its weight versus a sphere, which has the lowest surface area relative to its weight. Flat plates and rod-shaped tramp metal are easier to remove than spherical or cube-shaped tramp metal.
SE magnets are typically mounted in one of two positions over a conveyor belt, mostly depending upon belt speed. If the belt is moving fairly slowly (300 ft. per minute or slower), then pulling tramp metal out of burden depth should not be an issue. Therefore, it is recommended that the SE magnet is positioned in a cross-belt fashion upstream of the head pulley.
When the belt is traveling faster, it’s better to have the SE magnet mounted just over the stream of material leaving the head pulley. Material coming off faster-moving belts results in a higher trajectory free-fall off the head pulley, clear of any burden. The SE magnet can capture the tramp metal as it flies off the head pulley. (A non-magnetized head pulley is recommended in this case.)
Standard vs. Custom Components
SE magnets consist of several standard components to provide the magnetic force necessary to collect large pieces of tramp metal. The coil, core, backbar and steel enclosure provide an efficient and effective magnetic circuit for collecting this tramp metal.
Ninety-nine percent of SE magnets built come with coils that are always fully immersed in cooling oil. Heat and moisture, the greatest enemies of electromagnets, are effectively controlled by the oil. The oil takes heat – generated by the coils and core – and moves it away to the outer surfaces of the magnet box where air currents circulating around the magnet will cool it.
Aluminum coils are the standard among SE magnet manufacturers. However, some customers like the idea of copper coils because they generate more power in less space. Copper coils – although more expensive – can be incorporated into the SE magnet design upon request.
A small percentage of SE magnets are air-cooled and contain no interior coil or oil; the inside of the magnet is completely dry. This type of SE magnet is generally used in mining or other dusty areas considered as hazardous locations.
The air-cooled SE magnet costs more to build and is most often rated by a third-party underwriter, such as UL or CSA. The customer must specify that the working environment is hazardous and work with the SE magnet manufacturer to ensure the air-cooled magnet meets all underwriting guidelines.
Manual Clean vs. Self-Clean
Some mining and quarrying operations prefer the less expensive manual clean SE magnet if there is a low volume of tramp metal generally found in the conveyed material. With a manual clean model, there is some downtime as operators turn off power to the magnet, move the magnet away from the conveyor line, then clean any accumulated tramp.
Because of this downtime, the trend today is to spend more upfront to install the SE magnet and continue to operate the magnet during the cleaning process. Self-clean models have a short belt conveyor built around the magnet to automatically remove tramp iron from the magnet force.
The self-clean system, like the one offered by Eriez, features a rubber conveyor belt, bearings, rugged but simple continuous channel frame, adjustable take-ups and shaft-mounted reducer with V-belt coupling to a TEFC motor.
Today’s SE magnets are built to withstand harsh weather and operate for long periods with low maintenance. Two factors that reduce magnet holding force over time are oil contamination and reduced power.
During routine inspection, operators should check the cleanliness of the oil by taking a sample, submitting it for inspection and noting any contamination. Any sort of contamination in the oil can reduce its ability to take heat away from the coils and core, thereby reducing the magnet’s ability to attract metal.
Keep up with routine lubrication and adhere to your manufacturer’s instructions on proper oil amount. (Depending on the size of the magnet, oil capacity can range from 30 gal. up to several hundred gallons.)
If the SE magnet is losing power even with clean, filtered oil, operators should check the power source to make sure the magnet is receiving an ample amount of power. In most cases, the cause is a blown fuse inside the rectifier. This simple change-out can bring the SE magnet back to optimum holding force.
Since many factors may impact the choice, installation and operation of an SE magnet, it’s best to consult with an SE magnet manufacturer and go over any design components needed for the job. Eriez, for example, has 23 SE magnet sizes, 69 standard models and hundreds of special designs available for unique applications.
Bill Dudenhoefer is manager, separation products for Eriez.