How to Choose the right Permanent Overband Magnet

Permanent Overband Magnets are commonplace in most mines and quarries, removing tramp ferrous metal and protecting crushers, screens and conveyors against damage. However, there are different designs of such magnets developed to suit specific applications.

According to Bunting, one of the leading designers and manufacturers of magnetic separators, eddy current separators, metal detectors and electrostatic separators, understanding an installation is key to selecting the right overband magnet. The company designed and built the first magnets of this type in the early 1980s and has since supplied thousands to companies operating worldwide. “Although the basic technology has not changed, advances in magnet materials and manufacturing techniques have significantly enhanced the ferrous metal separation performance,” the company with European manufacturing facilities in Redditch and Berkhamsted (both in the United Kingdom) underlined. “Overband magnets lift and automatically remove tramp ferrous metal from conveyed mined or quarried rock. The permanent design features a magnet block mounted in a frame, with two or four pulleys, and a revolving self-cleaning rubber belt.”

In operation, mined or quarried rock is conveyed underneath the overband magnet, which attracts, lifts and then removes damaging tramp ferrous metal. “The size and type of magnetic system (permanent or electro) is dictated by the conveyor width, depth of material on the conveyor and the nature of the tramp ferrous metal. Permanent overband magnets are commonly found on mobile plants such as crushers, screens and shredders, and in lower volume installations.”

Models and Applications
Bunting’s range of permanent overband magnets includes four models to suit different installations. The heavy-duty PCB model suits quarry and small mining operations, operating at suspension heights of up to 400 millimeters (mm) on conveyors between 300 mm and 2,000 mm wide.

As pointed out, the PCB-C compact and lightweight model suits mobile plant installations, such as crushers and screens, and operations where space is limited. The PCB-C operates at suspension heights up to 250 mm, above 600 mm to 1,500 mm wide conveyors.

For installations where the magnet is located in a difficult location for regular maintenance, Bunting designed the QBC quick-belt change model. As the model implies, the self-cleaning belt is easy and quick to change, reducing downtime. This model operates at suspension heights up to 300 mm on conveyors between 600 mm and 1,500 mm.

The Tri-Polar overband magnet produces a different shaped magnetic field with increased power. Although commonly heavier, the deeper magnetic field means that the suspension height of the Tri-Pole is higher, up to 400 mm. The shape and depth of the magnetic field make this model better suited for separating smaller or long and thin (e.g. nail-like) tramp ferrous metal.

Overband magnet Selection
Selecting the correct overband magnet, in terms of permanent or electro and the specific model, is dictated by the application. “Bunting’s applications engineers assess the ferrous metal separation objective (i.e. the plant being protected); the nature of the tramp ferrous metal (i.e. shape, size, etc.); and the conveyed rock (i.e. size range, burden depth),” the company informed. These criteria would assist the team in selecting the optimum magnet for any given application.

(Published in GLOBAL RECYCLING Magazine 1/2024, Page 46, Photo: Bunting)