Bulk Equipment, Bulk Technology

Picking the right suspended magnet

One of the most commonly installed magnetic separators in heavy industry applications is the suspended magnet. Magnet specialist Eriez explains how they work. 

One of the most commonly installed magnetic separators in heavy industry applications is the suspended magnet. Magnet specialist Eriez explains how they work. 

Suspended magnets are installed above belt conveyors, apron feeders, and vibratory pan feeders to reliably remove large and potentially damaging tramp metal from the material flow. 

The first step when selecting a suspended magnet is to determine the required suspension height of the magnet, or the distance from the bottom of the magnet to the bottom of the material-carrying conveyor. To determine the magnet suspension height, first consider the material burden depth. This can be calculated by looking at data such as throughput capacity, material bulk density, material particle size range, conveyor belt speed, and conveyor belt width. Once the burden depth has been established, you can determine the required magnet suspension height that will provide for maximum efficiency of tramp removal.

Suspended permanent magnets vs suspended electromagnets

Suspended magnets are available in permanent and electromagnetic designs, each serving distinct purposes. The magnetic fields produced by permanent and electromagnetic magnets differ in shape and depth, which is a key consideration when selecting the most suitable magnet design for the specific application. 

Permanent suspended magnets use a permanently charged magnetic circuit that provides a continuous and uniform magnetic field across the magnet’s face. This can prove useful when dealing with wider conveyor belts carrying relatively low burdens of material. 

Aside from its powerful, uninterrupted magnetic field, there are many other advantages to using a permanent suspended magnet. Since the magnet is permanent, it requires no power connection and is relatively light and installed easily. As a result, suspended permanent magnets have a lower operating cost compared to suspended electromagnets that require constant power. 

Related stories:

While they do not offer the same depth of magnetic field as their electromagnetic counterparts, suspended permanent magnets are a reliable and cost-effective way to remove tramp iron at lower suspension heights. Suspended permanent magnets are most effective when installed at suspension heights of 30.5cm or lower and are suitable for applications like rock quarries, recycling facilities, and mobile crushers/shredders. 

For applications that require magnet suspension heights greater than 30.5cm, the use of suspended electromagnets is recommended. Suspended electromagnets employ insulated wire coils installed around a solid steel core, topped with a steel backbar, and enclosed in a steel box to create a powerful U-shaped magnetic field. This parabolic magnetic field shape makes suspended electromagnets ideal for installations above troughed belt conveyors.  

Suspended electromagnets offer a greater depth of magnetic field than permanent magnets and certain models can be installed at higher suspension heights of up to 76.2cm. This makes them the best option for installations in high-tonnage applications, such as mining operations. Electromagnets require a constant direct current power source from a rectifier control that converts AC plant power to DC power to the magnet. This allows the electromagnet to be easily turned off for cleaning and maintenance. 

Suspended magnet features

Both suspended permanent magnet and electromagnet designs are available in either manual cleaning or self-cleaning configurations. If tramp iron is infrequent in the product flow, the manual clean design is often sufficient.  Manual-clean magnets require an operator to physically clean them of ferrous debris. Manual-clean permanent magnets can also be fitted with drawer-style stripper plates that can be pulled open to separate the recovered ferrous contaminants from the magnet face. With manual-clean electromagnets, the magnet is simply turned off for cleaning. 

If the magnet is expected to regularly encounter tramp iron, a self-cleaning design is better suited. Self-cleaning magnets use a small, low-voltage motor and pulleys to rotate a belt around the magnet which automatically cleans the face of the magnet without interrupting the material flow. These self-cleaning units can be supplied with zero speed switches and pulley guards for improved long-term maintenance and safety purposes. 

Depending on the application, several different styles of self-cleaning belts can be installed on these magnets, including: 

• Standard two-ply rubber belts with bolted stainless-steel cleats, which are suitable for most applications from aggregates to light recycling applications.

• Armour-clad belts with bolted stainless-steel plates along the entire belt length, which are suitable for large, heavy pieces of ferrous metal to prevent damage to self-cleaning belts.

• Urethane belts with molded cleats, which offer superior puncture resistance and are suitable for applications with sharp ferrous objects.

Hazardous duty applications

Suspended magnets are often installed in classified hazardous areas where combustible dust may be  present, such as coal mines, chemical plants, and grain processing facilities. For these installations, certain features are recommended, including:

• high fire point transformer oil or air-cooled electromagnet

• dust proof / ignition proof (NEMA 9) junction boxes

• hazardous duty motors (for self-cleaning models)

• anti-static self-cleaning belts (for self-cleaning models) 

Suspended magnets are generally utilised to recover large and infrequent pieces of damaging tramp iron. In situations where a large volume of ferrous metals is expected, a magnetic drum separator may be the more suitable option. 

Send this to a friend