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Technology reference

Removing ferrous metal before the separator

Magnetic separation pulls iron and steel out of a mixed stream with a permanent magnet or an electromagnet. On an aluminum line it comes before the eddy current separator, to protect that machine and to keep steel out of the aluminum product.

By [author name, role and employer] · Technical review by [reviewer name, role and employer] · Updated

  • Aluminum and non-ferrous
  • Ferrous
  • Fiber
  • Glass
  • Film
  • Other and residue

1What steel does at the separator

An eddy current separator throws aluminum and other non-ferrous metal forward. Steel behaves differently. The rotor's field attracts it instead of throwing it, and steel held by that field heats up. (Source: Bunting, 5 key eddy current separator operating parameters, 2020.)

Two separator makers' manuals describe the damage. Hot steel can damage the belt and the shell, and the heat can weaken the rotor magnets for good. One of the two, IMI, does not treat this "ferrous burnout" as a warranty item. (Sources: IMI, eddy current separator service manual 900505, undated; Eriez, RevX-E eddy current separator manual RM-99, 2015.)

Steel also costs recovery while the line runs. A magnetic piece disturbs the field, so conductive pieces near it are not pushed away as they should be. (Source: Bunge, Recovery of metals from waste incinerator bottom ash, OST, 2019.)

Stops carry the most risk. In an emergency stop the belt halts while the rotor coasts on, so a pinned steel piece can reach red heat and melt into the belt (Source: Bunge, 2019). Eriez's manual says to stop the feed conveyor before the separator for the same reason (Source: Eriez RM-99, 2015).

Rotor layout changes how much harm stray steel does. On a concentric rotor, steel stays on the belt at the bottom of the rotor, and when the belt stops it can burn through the belt into the shell. An eccentric rotor lets steel move out of the field and drop off, often into the non-metal stream. (Source: Bunting, 2020.) Bunge notes that an eccentric rotor limits the zone where its field acts to about 30 degrees of arc, while a concentric rotor's zone is wide and more prone to belt damage (Source: Bunge, 2019).

Neither layout replaces the magnet. Eriez designs its eccentric separator to run with a minimum of ferrous metal in the feed, and Bunting advises taking ferrous metal out first with overhead, pulley or drum magnets. (Sources: Eriez RM-99, 2015; Bunting, 2020.)

This guide follows the separator makers' manuals: magnets go ahead of the separator, and the separator is not relied on to remove steel.

The metal buyer asks for the same thing. ReMA's grades for used beverage cans, including baled cans (Taldon), require magnetically separated material with no steel. Zorba, shredded non-ferrous scrap, must have passed one or more magnets. (Source: ReMA, ReMA's ISRI Specifications, July 2026.) Aluminum scrap grades and what buyers accept covers the grades in full.

Steel has to come out before the separator

No magnet removes every piece of ferrous metal, so separator makers ask for one or more magnets upstream. IMI names a drum, a magnetic head pulley or a cross-belt magnet, alone or together. (Source: IMI 900505.)

2Magnet types used ahead of a separator

Recycling plants use three kinds of magnet for this job: overhead magnets, magnetic head pulleys and drum magnets (Source: Bunting, Which magnetic separators do recycling plants use, 2026). An overhead magnet can hang in one of two positions, so Figure 1 shows four arrangements.

Four ways to take steel out ahead of the separator Four side views. 1: an overhead magnet hangs over the head of a conveyor, angled over the path material takes as it leaves the belt; steel is lifted onto the magnet's own belt and dropped clear. 2: an overhead magnet hangs over the run of a conveyor, across the belt; steel is pulled up out of the burden onto its belt. 3: a magnetic head pulley holds steel to the belt as it wraps the pulley and drops it underneath, behind a divider, while everything else leaves on its normal path. 4: a vibratory pan feeds a drum magnet; steel is carried round under the drum and released behind a divider, while everything else falls off the front. A marks the gap between the magnet face and the material; B marks the depth of material. 1 A 2 A B 3 4 Four ways to take steel out ahead of the separator Four side views. 1: an overhead magnet hangs over the head of a conveyor, angled over the path material takes as it leaves the belt; steel is lifted onto the magnet's own belt and dropped clear. 2: an overhead magnet hangs over the run of a conveyor, across the belt; steel is pulled up out of the burden onto its belt. 3: a magnetic head pulley holds steel to the belt as it wraps the pulley and drops it underneath, behind a divider, while everything else leaves on its normal path. 4: a vibratory pan feeds a drum magnet; steel is carried round under the drum and released behind a divider, while everything else falls off the front. A marks the gap between the magnet face and the material; B marks the depth of material. 1 A 2 A B 3 4
  1. 1Overhead magnet, in-line. Hangs over the head of the conveyor, angled over the path material takes as it leaves the belt. Its own belt carries the steel clear.
  2. 2Overhead magnet, cross-belt. Hangs over the run of the conveyor. Its belt runs across the conveyor and carries the steel off to one side.
  3. 3Magnetic head pulley. Takes the place of the ordinary head pulley. Steel stays on the belt round the pulley and drops behind a divider.
  4. 4Drum magnet. A shell turning round a fixed magnet. Steel is carried round the drum and released behind a divider.
  5. AGap to the material. Section 5 covers how small to make it.
  6. BDepth of material. How deep the material lies on the belt.
Figure 1. Four ways to take steel out ahead of an eddy current separator, side views. Schematic, not to scale; sizes, gaps and belt speeds come from each maker. The drum is drawn with material passing over it; drums also run the other way up, lifting steel out of material passing underneath. Sources: Eriez SM-305, 2012; Eriez SM-240, 2011; STEINERT, Magnetic drums, undated.

Lifting steel from above the belt

An overhead magnet hangs above the belt. Eriez's manuals cover self-cleaning units, which carry captured steel away on their own belt, and manual-cleaning units. (Source: Eriez, suspended permanent magnet manual SM-305, 2012.) In-line and cross-belt mounting are compared in section 4.

Catching steel at the head of the conveyor

A magnetic head pulley takes the place of the conveyor's ordinary head pulley. Steel rides round the pulley on the belt and drops away behind the normal path.

Remcon Equipment, the publisher's affiliate, compares a head pulley with a magnet hung above the belt on its magnetic head pulley page.

Carrying steel round a drum

A drum magnet is a shell that turns round a fixed magnet. Drums run with material passing over them, or lift steel out of material passing underneath. (Source: STEINERT, Magnetic drums, undated.)

In bottom ash, Bunge calls drums the usual choice for magnetic pieces under about 80 mm. A deflecting drum catches even steel locked in mineral, which gives high recovery and a poorer grade, and is often used ahead of the eddy current separator. An extracting drum gives cleaner scrap but loses some iron. (Source: Bunge, 2019.)

What the publisher's affiliate builds

Remcon Equipment, the publisher's affiliate, builds overhead magnet units around magnets bought from other makers, and builds magnetic head pulleys. Its overhead magnetic separators and magnetic head pulley pages describe what it builds. Industrial Magnetics, Inc. (including its Walker brand) and Dings Magnetic Group have supplied it magnets, and their directory listings say so.

3Choosing the magnet

Eriez lists what sets the size of a suspended magnet: belt width, belt speed, burden depth, the material, and the size, shape and frequency of the tramp steel. Faster belts and deeper burdens need stronger magnets. Small or round pieces under a deep burden are the hardest to catch. (Source: Eriez, suspended magnet brochure SB-305, 2021.)

Shape matters as well. Flat plates and rods come out more easily than spheres or cubes (Source: Eriez Magnetics Europe, suspended electromagnet brochure SB320UK7, undated). At one auto shredder, large, heavy, round pieces such as knuckles and gears were hard for magnets to lift (Source: Argonne National Laboratory, All Auto Shredding, 2011).

Permanent magnets are now common in modern separators. Ferrite suits low-intensity duty, under about 0.2 T at the magnet surface. Neodymium-iron-boron (NdFeB) magnets give high intensity, above 0.2 T and often about 0.6 T. (Source: Bunge, 2019.)

The field can come from permanent magnets or from an electric coil. The table sets the two side by side on criteria the sources state. It is not a ranking.

Field source: what the makers state (stated criteria, not a ranking)
CriterionPermanentElectromagnet
Power for the fieldNone. Dings describes overhead permanent magnets as needing no external power, and Eriez suggests permanent drums where power is limited or unstable.An external power supply.
Published reachEriez suspended units: about 10 in maximum suspension (CP model) and about 12 in (TP, a stronger dual-pole design), on belts under 36 in. Eriez permanent scrap drums: a field reaching up to 15 in.Sold for deeper fields and larger gaps. Bunting's electro cross-belt magnets are its strongest and suit the larger gaps of deep troughed belts; IMI markets its self-cleaning electromagnet for higher suspension and deep burdens. [Electromagnet reach figure: needs a maker's published suspension height and burden depth for a named model; none was verified]
HeatNo coil to heat.The coil runs hot: about 185°F (85°C) on the outside of an oil-cooled Eriez unit and about 160°F (70°C) on an air-cooled one.
Uses named by the makersLighter burdens and narrower belts (Dings, on construction and demolition lines); mobile plants and smaller installations (Bunting).Heavier burdens and higher throughput (Dings, on construction and demolition lines); deep troughed belts (Bunting).

Sources: Dings, Magnetic separators for construction and demolition recycling, undated; Eriez, Magnetic scrap drum separators, undated; Eriez, CP and TP suspended permanent magnets, undated; Bunting, Choosing the right permanent crossbelt magnets for recycling, 2024; IMI, Suspended electromagnet, overhead self-clean, undated; Eriez, suspended electromagnet manual SM-323, 2011.

Who publishes this. Recyclers Guide is published by [LLC name], an affiliate of Remcon Equipment, Inc. Remcon designs and builds conveyors and eddy current separator systems around rotors or complete separators made by others, builds overhead magnet units, magnetic head pulleys and trommels, resells screens and feeders, and integrates equipment discussed on this site.

4Placing the magnet

An overhead magnet hangs in-line, over the head pulley, or cross-belt, over the run of the belt.

Eriez's preferred position is in-line, over the path material takes as it leaves the belt. There the burden opens up in flight and moves toward the magnet face, which Eriez's literature calls the most efficient arrangement. (Sources: Eriez SM-305, 2012; Eriez Magnetics Europe SB320UK7.)

STEINERT also favours in-line placement, because material is loosened in free flow. It notes that cross-belt placement needs more magnetic force but less space. (Source: STEINERT, Overband magnets, undated.)

For in-line mounting, Eriez sets the magnet face about 2 in above the high point of the trajectory, or 3 in for self-cleaning units. At about 350 fpm or slower, material follows the pulley round, so the magnet's centreline should pass through the pulley's centre. A mild steel head pulley under the magnet hurts performance, so a non-magnetic stainless one may be needed. (Source: Eriez SM-305, 2012.)

Eriez calls cross-belt mounting a harder separation that usually needs a stronger magnet. It advises against it above about 350 fpm (107 m/min), and on deep burdens where small tramp steel must come out. (Sources: Eriez SM-305, 2012; Eriez Magnetics Europe SB320UK7.)

In bottom-ash plants, Bunge puts the overhead magnet over the free-fall path after the head pulley. He warns against cross-belt magnets over coarse ash: buried iron is missed, and spikes can jam, pierce and slash the belt underneath. (Source: Bunge, 2019.)

Keep other steel out of the field

Carbon steel near a magnet becomes magnetised and lowers separation. Eriez names idlers, slider beds and side frames within about 3 ft, or more on large magnets. It suggests 304 or 316 stainless, aluminum, plastic or wood near the magnet, and no steel idlers under a cross-belt magnet. (Source: Eriez SM-305, 2012.)

Belt speed limits by pulley diameter

On a magnetic head pulley, keep the belt and its lagging as thin as practical, so steel rides close to the pulley face, and use non-magnetic lacing. Eriez sets a maximum belt speed for each pulley diameter. (Source: Eriez, magnetic pulley manual SM-240, 2011.)

Maximum belt speed by magnetic pulley diameter (one maker's figures, for dry, free-flowing material)
Pulley diameterMaximum belt speed
8 in140 fpm
12 in210 fpm
15 in240 fpm
18 in270 fpm
20 in300 fpm
24 in340 fpm
30 in400 fpm
36 in440 fpm

Source: Eriez, Magnetic Pulleys installation, operation and maintenance manual SM-240, 2011. For wet, sticky, dense or tangled material, Eriez says to reduce the burden depth.

5Magnet height and the depth of material

Magnetic pull falls off steeply with distance from the poles (Source: Bunge, 2019). So the depth of material between the magnet and a steel piece decides whether the magnet can catch it.

A magnet's rated suspension height is a maximum, not a target. Eriez says to hang it as close to the burden as possible without material plowing into it. Typical clearance is 3 in for self-cleaning units and 2 in for manual-cleaning units. (Sources: Eriez SM-305, 2012; Eriez SM-323, 2011.)

Eriez ties performance to control of burden depth. A plow or leveller ahead of a cross-belt magnet knocks down high spots and surges. Ahead of a magnetic head pulley, Eriez says to avoid surges and level the burden with a plow bar. (Sources: Eriez SM-305, 2012; Eriez SM-240, 2011.)

On a recycling line, paper is part of the burden. A 2009 review for a Florida county found that steel cans are captured more efficiently when large pieces of newspaper are not lying over them. That is one reason single-stream MRFs split fiber from containers before the container line. (Source: Kessler Consulting for Pinellas County, Materials Recovery Facility Technology Review, 2009.)

The depth of material follows from the feed rate, belt width, belt speed and bulk density. The belt feed-rate calculator shows how they relate.

6Where the magnet sits in an aluminum line

In a single-stream MRF, screens split flat fiber from containers. On the container line, magnets pull the ferrous metal, and the eddy current separator typically follows them. (Source: Gershman, Brickner & Bratton for the Can Manufacturers Institute, Aluminum Beverage Can: Driver of the U.S. Recycling System, 2020.) Aluminum recovery on MRF container lines follows that line from end to end.

MRF operators interviewed for a 2021 industry study were confident that magnets recover most steel. They said overhead magnets can lift even full or partly full cans. (Source: Resource Recycling Systems for CMI, PFI and HCPA, 2021.)

One magnet maker describes where each machine usually sits in a recycling plant (Source: Bunting, 2026):

  • overhead magnets after shredders, screens or primary conveyors
  • drum magnets after shredding, before density or optical sorting
  • the eddy current separator after ferrous recovery
  • stainless steel separators after the eddy current separator
  • magnetic head pulleys as a second clean-up at discharge points

One documented auto shredder ran magnetic separation straight after the shredder, then screening in a trommel, then non-ferrous recovery and an inductive sorter for stainless steel. A magnetic head pulley on the trommel's infeed conveyor made a ferrous-rich fraction that could be fed back. (Source: Argonne National Laboratory, 2011.)

Some separator makers build a second magnet into the machine. Goudsmit offers its separator with an infeed module that carries a drum magnet or a magnetic head pulley, to strip the remaining ferrous metal before the rotor. (Source: Goudsmit Magnetics, EddyXpert eddy current separators, undated.)

Fine fractions raise a trade-off. Weakly magnetic particles can spoil fine non-ferrous separation, which argues for a high-intensity magnet ahead of a fines separator. But nearly half of bottom-ash feed can be weakly magnetic, and stripping it all loses the copper and precious metal attached to it. (Source: Bunge, 2019.)

The aluminum recovery guide shows one common arrangement, with a magnetic head pulley ahead of the separator. Designing an aluminum recovery line puts every machine in order. Why the magnet goes ahead of the separator, set out side by side: Eddy current against magnetic separation.

[First-hand example: what sits upstream of the separator on the affiliate's jobs, and what gets past it. To come from the shop, once Sean approves publishing it (D-01)]

7What to tell the manufacturer

Ask each magnet maker the same questions in the same order. These are the inputs the makers' own literature sizes a magnet on:

  • belt width, belt speed and troughing angle, and whether the belt is inclined
  • burden depth, including surges, and bulk density
  • head pulley diameter
  • the tramp steel: its size, its shape and how often it comes
  • moisture, and whether the material is sticky or tangled
  • whether steel idlers, slider beds or frames sit near the magnet
  • power supply and ambient temperature

Sources: Eriez SB-305, 2021; Eriez Magnetics Europe SB320UK7; Eriez SM-240, 2011; Eriez SM-305, 2012.

Also say which mounting positions you have room for, and where the captured steel must go. Tell the eddy current separator maker how ferrous metal is removed upstream, and how many magnets do it.

8Questions

Do I still need a magnet if my eddy current separator has an eccentric rotor?

The manuals cited on this page say yes. An eccentric rotor lets stray steel drop away more easily than a concentric one, which reduces the harm it does. Its makers still ask for ferrous metal to be removed upstream. Sources: Eriez RM-99, 2015; Bunting, 2020; Bunge, 2019.

Why remove steel before an eddy current separator?

Steel held in the rotor's field heats up. It can damage the belt, the shell and the rotor magnets, and heat damage to the magnets is permanent. Steel also disturbs the field around nearby aluminum. And the buyers' grades for cans and for Zorba require material that has been through a magnet. Sources: IMI 900505; Bunge, 2019; ReMA, July 2026.

How close to the material should the magnet sit?

As close as it can without material plowing into it. A magnet's rated suspension height is a maximum, not a target. For its suspended magnets, Eriez gives typical clearances of 2 in for manual-cleaning units and 3 in for self-cleaning units. Sources: Eriez SM-305, 2012; Eriez SM-323, 2011.

Is one magnet enough?

Often not. No magnet removes all ferrous metal, so separator makers ask for one or more magnets upstream of the separator. Source: IMI 900505.

Read the aluminum recovery guide Find manufacturers

9Sources

Manufacturers' figures are their own published claims; this site has not tested the equipment.

  1. Bunting. 5 key eddy current separator operating parameters (blog). November 10, 2020. https://buntingmagnetics.com/blog/5-key-eddy-current-separator-operating-parameters. Read 2026-10-06.
  2. Industrial Magnetics, Inc. (IMI). Eddy current separator service manual, endless belt, 900505. Undated. https://assets.magnetics.com/assets/files/manuals/900505_ECS_Service_Manual_Endless_Belt.pdf. Read 2026-10-06. Industrial Magnetics supplies eddy current separators and magnets to Remcon Equipment, the publisher's affiliate.
  3. Eriez Magnetics. RevX-E eddy current separator installation, operation and maintenance manual, RM-99. April 2015. https://www.eriez.com/Documents/Literature/IOMs/Metals-Recycling/RM-99-Eriez-RevXE-Eddy-Current-Separator-IOM.pdf. Read 2026-10-06.
  4. R. Bunge, UMTEC, OST Eastern Switzerland University of Applied Sciences. Recovery of metals from waste incinerator bottom ash (technical monograph). June 2019. https://www.ost.ch/fileadmin/dateiliste/3_forschung_dienstleistung/institute/umtec/fachartikel/2019/19-03_recovery_of_metals_from_waste_incinerator_bottom_ash.pdf. Read 2026-10-06.
  5. Recycled Materials Association (ReMA). ReMA's ISRI Specifications. July 2026. https://www.isrispecs.org/wp-content/uploads/2026/09/2026-July-ReMA_ISRI_Specifications.pdf. Read 2026-10-06.
  6. Bunting. Which magnetic separators do recycling plants use to maximize metal recovery? (blog). June 11, 2026. https://buntingmagnetics.com/blog/which-magnetic-separators-do-recycling-plants-use-to-maximize-metal-recovery. Read 2026-10-06.
  7. Eriez Magnetics. Suspended permanent magnets installation, operation and maintenance manual, SM-305. 2012. https://www.eriez.com/Documents/Literature/IOMs/Magnetic-Separation/SM-305-Eriez-Suspend-Permanent-Magnet-IOM.pdf. Read 2026-10-06.
  8. Eriez Magnetics. Magnetic pulleys installation, operation and maintenance manual, SM-240. 2011. https://www.eriez.com/Documents/Literature/IOMs/Magnetic-Separation/SM-240-Eriez-Magnetic-Pulley-IOM.pdf. Read 2026-10-06.
  9. STEINERT. Magnetic drums (product page). Undated. https://steinertglobal.com/us/sorting-systems/magnetic-separation/magnetic-drums/. Read 2026-10-06.
  10. Remcon Equipment, Inc. (the publisher's affiliate). Overhead magnetic separators. https://www.remconequipment.com/overhead-magnetic-separators. Read 2026-10-06.
  11. Remcon Equipment, Inc. (the publisher's affiliate). Magnetic head pulley. https://www.remconequipment.com/magnetic-head-pulley. Read 2026-10-06.
  12. Eriez Magnetics. Suspended magnets brochure, SB-305. 2021. https://www.eriez.com/Documents/Literature/Brochures/Products/Magnetic-Separation/SB-305-Eriez-Suspended-Magnet.pdf. Read 2026-10-06.
  13. Eriez Magnetics Europe. Suspended electromagnets brochure, SB320UK7. Undated. https://www.eriez.com/Documents/Literature/Brochures/Products/Magnetic-Separation/SB-320-UK-Eriez-Europe-Suspended-Electromagnet.pdf. Read 2026-10-06.
  14. C. Duranceau and J. Spangenberger, Argonne National Laboratory. All Auto Shredding: evaluation of automotive shredder residue generated by shredding only vehicles, ANL/ES-C0201801. July 11, 2011. https://publications.anl.gov/anlpubs/2011/09/70931.pdf. Read 2026-10-06.
  15. Dings Magnetic Group. Magnetic separators for construction and demolition recycling applications. Undated. https://dingsmagnets.com/magnetic-separators-for-construction-amp-demolition-recycling-applications/. Read 2026-10-06. Dings has supplied magnets to Remcon Equipment, the publisher's affiliate.
  16. Eriez Magnetics. Magnetic scrap drum separators (product page). Undated. https://www.eriez.com/Products/Metals-Recycling/Ferrous-Recovery/Magnetic-Scrap-Drums. Read 2026-10-06.
  17. Eriez Magnetics. CP and TP suspended permanent magnets (product page). Undated. https://www.eriez.com/Products/Magnetic-Separation/Magnetic-Separation-for-Tramp-Metal/Large-Magnetic-Separation/CP-TP-Suspended-Permanent-Magnets. Read 2026-10-06.
  18. Bunting. Choosing the right permanent crossbelt magnets for recycling (blog). April 2, 2024. https://buntingmagnetics.com/blog/choosing-the-right-permanent-crossbelt-magnets-for-recycling. Read 2026-10-06.
  19. Industrial Magnetics, Inc. (IMI). Suspended electromagnet, overhead self-clean (product page). Undated. https://www.magnetics.com/products/suspended-electromagnet-overhead-self-clean. Read 2026-10-06. Industrial Magnetics supplies eddy current separators and magnets to Remcon Equipment, the publisher's affiliate.
  20. Eriez Magnetics. Suspended electromagnets installation, operation and maintenance manual, SM-323. 2011. https://www.eriez.com/Documents/Literature/IOMs/Magnetic-Separation/SM-323-Eriez-Suspend-Electromagnet-IOM.pdf. Read 2026-10-06.
  21. STEINERT. Overband magnets (product page). Undated. https://steinertglobal.com/us/sorting-systems/magnetic-separation/overband-magnets/. Read 2026-10-06.
  22. Kessler Consulting, Inc., for the Pinellas County (Florida) Department of Solid Waste Operations. Materials Recovery Facility Technology Review. September 2009. https://www.nswai.org/docs/MATERIALS%20RECOVERY%20FACILITY%20TECHONOLOGY%20REVIEW.pdf. Read 2026-10-06.
  23. Gershman, Brickner & Bratton for the Can Manufacturers Institute. Aluminum Beverage Can: Driver of the U.S. Recycling System. June 23, 2020. https://www.cancentral.com/wp-content/uploads/2023/03/GBB-Report-Aluminum-Can-Drives-U.S.-Recycling-System-Final-2020-0623.pdf. Read 2026-10-06.
  24. Resource Recycling Systems for CMI, PFI and HCPA. Surveying state of MRF and end market barriers to recycling steel and aluminum aerosols and pet food cans. February 2021. https://www.cancentral.com/wp-content/uploads/RRS_CMI_Results_Feb2021_FINAL.pdf. Read 2026-10-06.
  25. Goudsmit Magnetics. EddyXpert eddy current separators (product page). Undated. https://www.goudsmitmagnetics.com/en-us/products/belt-conveyor-magnetic-separators/eddyxpert-eddy-current-separators. Read 2026-10-06.

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