The ECS is at risk of costly damage from ferrous contamination : IN DEPTH: Eddy Current Separators - Protecting Your Investment

The role of the Eddy Current Separator (ECS) is to effectively remove high-value non-ferrous metals, such as aluminium, copper and brass from the waste stream. As Rob Jones, managing director of ECS manufacturer Magnapower explains, the technology behind an ECS is simple, but effective.

“The head drum of an ECS conveyor contains a high speed rotating magnet system which spins rapidly with alternating polarity,” he tells WMW. “This alternating magnetic field generates eddy currents of electrical energy in non-ferrous metals, causing movement within them and consequently repelling them away from other materials and allowing their separation and recovery from other material which simply fall off the belt due to gravity.”

First developed commercially in the 1980s, ECS units have the capacity to separate non-ferrous metals from 1mm in size (fines) and above to larger shredded scrap metal. According to Jones this ability to recover even the smallest pieces of valuable fractions has made them an essential addition to any modern MRF in terms of maximising efficiency and the bottom line.

The problem

While an ECS is highly efficient at flinging non-ferrous metals from the belt as they pass through its magnetic field, any ferrous metals reaching the ECS conveyor are attracted to the rotor by its magnetic force, and get very hot very quickly. As such ferrous metal which reaches the ECS can cause significant damage to the belt.

According to Jones the most usual method of stopping ferrous metals reaching an ECS is the use of magnetic separators to remove them prior to the ECS, but these are unlikely to be 100% effective, especially for smaller pieces of ferrous metal. This is most often because these smaller pieces, e.g. metal shards and nails, are easily obscured by other materials, causing them to be missed by the magnetic separator.

“Even the tiniest shard of this missed ferrous metal reaching an ECS unit through the belt can result in considerable damage to the rotor cover which, if left, can result in the rotor being destroyed completely,” he says.

That’s a thought echoed by Tristam Hubbard-Miles, technical sales manager at the Recycling Division of magnetic sorting equipment manufacturer, Eriez.

“What happens with the ferrous metals, because they’re magnetic, when the get to the eddy current end you’ve got high speed spinning magnets and they’re inducing the eddy currents. If it’s non-ferrous and not magnetic the ECS induces an eddy current that has a corresponding magnetic field and its thrown away from the rotor. If it is magnetic, it ends up sticking and being thrown away so bounces up and down and inducing eddy currents while heating up further and further. It gets to the point where it’s hot enough to melt through the belt and eventually the parts underneath,” he tells WMW.

According to Jones, the consequences of this can be far-reaching. Firstly, he says, it can be dangerous from a health and safety perspective because the high speed rotating shaft can be exposed, risking injury to operators.

He also notes that ECS units are most usually installed within the processing line, meaning that any failure can cause the complete plant to shut down whilst repairs and/or replacement works are carried out. This will have a severe impact on a plant’s operations and revenue, as well as its reputation as a reliable MRF.

Finally, an ECS unit not working to its maximum efficiency due to damage will also lead to a loss of income from the non-ferrous metal stream, as well as downgrading any metal it does recover due to contamination.

“For example,” says Jones. “If an ECS is processing one tonne of aluminium per hour in a 40 hour week, even a 1% variation in performance can equal 400 kg per week or 20 tonnes per year of missed fractions. This could equate to up to £14,000 in cash terms.”

The solution

According to Jones the problems caused by ferrous metals reaching an ECS unit is fairly simple. There are two main methods which MRF operators can implement to minimise the risk of ECS damage.

“The first is to install a ferrous protection magnet on the processing line prior to the ECS to remove any potentially harmful pieces of ferrous metal,” he says. “These magnets tend to be top fed, such as a magnetic head pulley in the preceding conveyor or a top fed drum magnet prior to the ECS.”

“This solution is relatively low in cost,” adds Jones. “It’s likely to cost less than the price of a repair to a significantly damaged rotor, cover and belt. As the ferrous material is presented to the high intensity surface of the magnet, it is more likely to be extracted than with an overband separator.”

Hubbard-Miles has a similar message: “We recommend a minimum of at least a permanent suspended magnet to remove any tramp iron, and then ideally either an option with a magnetic pulley within the conveyer prior to the eddy current, or even better a magnetic drum separator prior to the eddy current.”

However, the selection of pre-sorting equipment also depends on the material being processed.

“For mixed bag waste, where most of the items are fairly large – 50mm and above –an overhead magnet should suffice,” says Hubbard-Miles. “If you’re dealing with a finer product, such as woodchip or incineration ash, you might want a magnetic drum in there to get rid of some of the finer particles.”

Jones’ second solution is to install a unit with an eccentric ECS rotor, which he says will always provide a more reliable rotor solution as it is less prone to damage from ferrous metal.

He explains that this is because eccentric rotors release ferrous metal due to their magnetic field being focused in the separating area. This field then reduces as the belt moves around the rotor, allowing the ferrous metal to drop off.

Conclusion

According to Tristam Hubbard-Miles, the majority of the ECS damage seen by Eriez’s service centres is caused by misuse and poor protection from ferrous contamination. However, he says that when properly maintained and protected Eddy Current Separators are essentially a hard wearing piece of kit, with a useful service life in excess of 10 years.

Jones backs this up and asserts that protecting ECS units is crucial to prolonging the effectiveness and lifespan of these vital pieces of MRF equipment, as well as protecting operators against the risks and disruption that can arise from ferrous metal damage.

“The beauty of the additional ferrous protection suggested here,” concludes Jones. “Is that it can often be retro-fitted, reducing the risk of ferrous metal damage quickly and efficiently, and at less cost and inconvenience than having to stop MRF operations to repair or replace ruined equipment.”

For an operator the ECS may only be one cog in the complex wheel that is a material recycling facility, but it is cog that can bring wheel to a costly grinding halt. While the ECS may not always be the first thing on the operator’s mind, following some basic best practice from the manufacturers is surely a sound decision.