Factors to consider when selecting electromagnets

By Kevin Yanik| February 22, 2024

Milton

Crushers are designed to crush rock – not metal.

Still, managing the tramp metal that can navigate a production system is part of the job of quarry operators everywhere.

“When producers blast, they’re generating metal and they may have rebar or rock bolts in their material,” says Darrell Milton, global market manager of heavy industry at Eriez. “You may also have bucket teeth that break off the loaders. These sorts of things need to be recovered prior to crushing.”

One way to capture such material is with a magnet. An even better way to prevent metal from damaging a production system is to pair a magnet with a metal detector, according to Milton.

“A lot of machinery utilizes manganese components,” he says. “In almost every application you should consider having a magnet to remove ferrous metals followed by a metal detector that senses metals you may miss. That metal detector will sense that piece of metal and shut down the system so the metal can be removed.”

Selection and installation

An over-the-head-pulley installation is generally considered a manufacturer’s preferred way of installation, but such an approach isn’t always possible. Photo: Eriez

For quarry operators, Milton says electromagnets are a good starting point to best manage tramp metal. Determining what type of magnet is best for the many applications in aggregates, however, is easier said than done.

For instance, when Eriez sizes an electromagnet for a quarry application, several factors rise above all others: belt width, belt speed, idler degree and burden depth are among those. The typical size and type of metals targeted for removal is also critical information for equipment vendors.

“Why that’s key is different sizes and shapes of metal react differently to a magnet,” Milton says. “The smaller the piece of metal, the more difficult it is to remove because there’s not much attraction force or mass to it.”

The metal’s shape also matters.

“A piece of rebar is easier to remove than a bolt or a nut,” Milton says. “Something rod-shaped is easier to remove than something spherical due to surface area of the metal.”

Another metric that matters with magnet sizing is suspension height.

“Suspension height is determined by the burden depth,” Milton says. “The burden depth is the actual depth of the product on the belt. The magnet is suspended over the top of the product burden, attracting the metal out of the product flow and discharging it away from the magnet or is manually removed.”

As Milton describes, suspension height is the factor that matters most when installing an electromagnet. But where a magnet resides along the conveying system is important, as well.

“Do you want it cross belt along the length of the conveyor so it’s at 90 degrees over the belt,” he says. “Or, do you want it over the top of the head pulley.”

Milton contends the most effective location to install a magnet is over the head pulley.

“When the material discharges the head pulley, it fans out and becomes dynamic,” he says. “It can be much easier to pull a piece of metal through that dynamic burden to attract it to the magnet face.”

While an electromagnet should capture the majority of tramp metal in a production system, a metal detector positioned downstream can serve as a security blanket.

Maintenance

Keeping a magnet free of debris is vital to its performance, as well.

“As metal is attracted to a manual magnet over time, it needs to be cleaned,” Milton says. “If it’s an electromagnet, the metal needs to be turned off. If it’s a permanent magnet, it has to be cleaned by hand or by using a cleaning plate.”

Keeping a manual-cleaning permanent magnet clean can be more of a chore for producers than they originally anticipate.

“When they buy the manual-cleaning magnet, they’ll say we attracted so much metal in the morning, but it didn’t attract any metal after lunchtime,” Milton says. “Well, the magnet can only accumulate so much metal. It comes down to how often you clean it. Purchasing a self-cleaning magnet can take these concerns out of the equation.