Removing damaging tramp metal when recycling concrete and other aggregates is an important procedure to effectively remove metal contamination and protect expensive downstream equipment, especially crushers.
The suspended electromagnet (SE) – sometimes called the “workhorse” in metal separation – is widely used in the aggregates industry. Installed, used and maintained properly, the SE magnet can operate efficiently for years under the harshest of conditions.
Both manual and self-cleaning SE magnets are typically mounted or suspended over a conveyor belt to remove large pieces of tramp metal such as rebar, nuts, bolts and other steel contaminants that represent a hazard to downstream crushers. These large magnets also remove sharp metal rods (ie pokers) that can damage or tear expensive conveyor belts, especially at transfer points.
Properly choosing and installing a suspended magnet will provide the necessary tramp metal protection. Understanding benefits of each optional component and following best maintenance practices will result in long-term optimum performance.
Positioning SE magnets
An SE magnet can be tailored for a quarry, mobile equipment or other aggregate recycling application. However, it is critical to size an SE magnet correctly.
The key factors in the selection of an SE magnet are burden depth, belt width, suspension height, position and tramp iron size. Other factors include belt capacity (tonnes per hour or m3 per hour), belt speed and incline, material density, head pulley diameter and altitude.
SE magnets are designed to capture pieces of tramp metal with a large mass (ie a large screwdriver, hammerhead, bucket tooth, steel shank), but can be designed to capture smaller metal pieces such as an M6 hex nut. The larger the mass, the more likely the SE magnet will pick up the object, even through a maximum burden depth.
{{image2-a:r-w:200}}Magnet strength must increase as the conveyor belt width and burden depth increase. For instance, a 1066mm square
SE magnet suspended at a maximum 330mm can pull tramp metal through 280mm of burden depth. A larger belt requiring a 1980mm square SE magnet suspended at a maximum 762mm can pull tramp metal through 711mm of burden depth. Eriez designs and builds SE magnets for belt widths up to 2440mm, which require even higher suspension heights.
Standard vs custom
SE magnets consist of several standard components necessary to provide the magnetic force required to collect both large and small pieces of steel contamination. These include the coil, core, back bar and steel enclosure, which together provide an efficient and effective magnetic circuit for collecting the tramp metal.
Ninety-nine per cent of SE magnets from Eriez 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 from the magnet to the outer surfaces. This keeps the coil relatively cool, ensuring the strength of the magnet. Eriez is the only manufacturer to incorporate an expansion tank, to keep the coils fully submerged, which prevents heat damage and water ingress.
A small percentage of SE magnets are air-cooled and contain no interior coil or oil. This type of SE magnet is generally used in mining or quarrying areas designated as hazardous, where the interaction between the tramp metal and the magnet may cause sparks. Permanent suspended magnets (non-powered) can also be used for small applications where the overall strength and deep reach of an electromagnet is not required.
Manual clean vs self-clean
Some mining and quarrying operations prefer the more economical manual clean SE magnet if there is a low volume of tramp metal found in the process. With a manual clean model there is a requirement to remove the magnet from service, such that operators can turn off power to the magnet, move the magnet away from the conveyor line, and then clean any accumulated tramp.
Because of this downtime and the need to run around the clock, the trend today is to spend slightly more capital dollars upfront to install a self-cleaning SE magnet, which can constantly operate and provide ongoing protection while continually being cleaned of tramp metal. Self-cleaning models have a short belt conveyor built around the magnet to automatically remove tramp iron from the magnet face. Since many factors may affect the choice, installation and operation of an SE magnet, it is best to consult an experienced SE magnet manufacturer and review design components needed for the job.
Eriez has been supplying magnetic protection equipment for nearly 75 years, and has more than 23 SE magnet sizes, 69 standard models and hundreds of special designs available for unique applications.
Jaisen Kohmuench is the general manager at Eriez Magnetics in Epping, Victoria.