The article was first published in Skip Hire Magazine (March 2020)
The Permanent Overband Magnet is one of the most commonly used magnetic separators. Overband Magnets are found in most recycling plants recovering steel cans and other ferrous metals from a wide range of waste materials as well as in quarries (protecting crushers & screens) and on mobile plant (screens and crushers). However, incorrect installation leads to poor levels of ferrous metal recovery.
The design of the Overband Magnet is simple with two pulleys mounted on a frame either side of a permanent magnetic block. As the name suggests, the Overband Magnet is positioned above a conveyor, either over the head pulley, or more, commonly across the belt. Hundreds of Overband Magnets are built at the Bunting-Redditch plant every year.
Unfortunately, the separation performance is often poorer than expected simply because of incorrect installation. Often installation is correct initially, but over time the positioning is changed. Here are three typical installation issues that can be easily remedied to improve the level of separation:
Every Overband Magnet is designed to operate at a specific height above the conveyor. To accommodate an occasional surge of material, the Overband Magnet can be temporarily raised and then, as the magnetic field is further away from the conveyor belt, the separation performance falls. Often, after the Overband Magnet has been raised it is not returned to the original position resulting in an ongoing poor separation performance. For installations where material surges are common, there should be consideration for a larger Overband Magnet, designed to produce a deeper magnetic field when suspended at a higher height above the conveyor;
Magnet Width vs Conveyor Width
The magnet block of an Overband Magnet is a specific size. During an evaluation of the application, the Bunting team will consider both the width of the conveyor and the ferrous metal discharge point. If the magnet block is not wide enough, ferrous metal can be lifted out of the conveyed waste and then discharged back onto the conveyor. Issues with the width of the magnet block commonly occur when an Overband Magnet is moved within a recycling plant. The new location may have a wider conveyor belt or a discharge point that is further away;
Positioning over the Conveyor
The majority of Overband Magnets feature a magnet block that has a centrally-located single pole that projects a magnetic field down towards the conveyor. Subsequently, the maximum level of attraction and separation is achieved directly under the centre of the magnet block line. Often, and especially when the Overband Magnet is too narrow for the conveyor belt, the centre line is positioned to one side of the conveyor nearer the area of discharge. However, this means that it is less likely that any ferrous metal on the opposite side of the conveyor will be separated as the magnetic field is significantly lower;
- Application story: Junction City Mining Installs Overband Magnet in US Quarry
As with most processing equipment, correct positioning ensures optimum performance. Simple and easy adjustments to the Overband Magnet suspension height and position can significantly improve ferrous metal recovery.
For additional information on Overband Magnets or for any metal separation or recovery application, please contact us on:
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