Rock Products

MAR 2013

Rock Products is the aggregates industry's leading source for market analysis and technology solutions, delivering critical content focusing on aggregates-processing equipment; operational efficiencies; management best practices; comprehensive market

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MATERIAL HANDLING CONVEYING ��� These large magnets also remove sharp metal that can damage or tear expensive conveyor belts, especially at transfer points. ��� Removing Tramp Metal Using Conveyor Magnets By Bill Dudenhoefer Removing damaging tramp metal from ag��� gregate processing is one of the most impor��� tant procedures to not only enhance product purity, but also protect expensive down��� stream equipment. The suspended electromagnet (SE) ��� some��� times called the ���workhorse��� in metal sepa��� ration ��� is one powerful piece of equipment widely used in the aggregates industry. In��� stalled, used and maintained properly, an SE magnet can operate efficiently for years under harsh conditions. Understanding guidelines for SE magnet se��� lection, optional components, magnet posi��� tioning 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. 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. 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 �����in. hex nut. The larger the mass, the more likely the SE magnet will be to pick up the object, even through maximum bur��� den depth. How does tramp metal enter the production stream? Any place heavy machinery is in��� volved 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 enter��� ing chutes and crushers. 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 be��� comes more difficult to remove ferrous com��� ponents. 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 44 ROCKproducts ��� MARCH 2013

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