What magnetic separation does
A magnetic separator captures ferrous contamination — tramp iron, wear fragments, weld spatter, fastener debris — from a product stream before it damages downstream equipment or reaches the customer.
- Equipment protection — keeping metal out of grinders, extruders, injection molds, and pumps.
- Product purity — food safety, pharmaceutical integrity, and clean recycled feedstock.
- Reclaim — recovering ferrous value from a mixed waste stream.
The magnet's job is to generate a field strong enough, and reaching far enough into the stream, to overcome the drag and momentum of the flow and hold captured metal against it.
Separator types
The stream — dry bulk, powder, slurry, or conveyed — sets the separator geometry:
| Type | Where it sits | Best for |
|---|---|---|
| Grate / hopper | Tube array in a gravity flow or hopper throat | Free-flowing powders & granules; fine capture |
| Plate | Suspended over or lining a chute | Chute and conveyor streams; larger tramp |
| Drum | Rotating shell over a stationary magnet arc | Continuous auto-cleaning; high-volume bulk |
| Pulley | Replaces a conveyor head pulley | Inline auto-discharge on belts |
| Liquid-line / trap | In-pipe cartridge in a housing | Slurries, syrups, liquids |
| Suspended / overband | Above a moving belt | Large-object tramp removal, recycling |
Grate and cartridge units capture the finest particles because product passes close to every magnet tube; drums and pulleys trade some fine capture for continuous self-cleaning at high throughput.
Field strength vs reach
Two field numbers matter and they trade off:
- Surface gauss — field at the magnet face. High surface gauss grabs fine and weakly magnetic particles that touch the surface.
- Depth of field (reach) — how far the useful field extends into the stream. Deep reach pulls contamination out of the middle of a thick flow.
Bigger magnet volume buys reach; grade and pole arrangement buy surface intensity. A high-gauss surface with shallow reach misses metal in the core of a deep bed; deep reach with modest surface may pass fine particles. Match both to the stream depth and the smallest fragment you must catch.
State the smallest particle size and its magnetic character, and the maximum bed depth or flow velocity. Those three drive the required surface gauss and depth of field — a single “high strength” number specifies neither.
Ferrite vs rare earth
- Ferrite (ceramic) — the historical separator material: low cost, corrosion-proof, stable, and adequate for coarse tramp in shallow streams (ceramic magnets).
- Rare-earth (NdFeB) — dramatically higher surface gauss and gradient in the same envelope. Worth the premium when you must capture fine or weakly magnetic particles, run high flow, or need a compact unit (material comparison).
Separators on hot product (dried grain, molten-adjacent lines, hot recycling) can exceed standard NdFeB's class and permanently weaken it. Specify a high-temperature grade or SmCo for elevated-temperature duty — see the temperature guide.
Food, pharma & HACCP
- Validated field strength — audited programs require a certified gauss value per unit, re-verified on a schedule. Specify the certificate and the re-test interval up front.
- Cleanable construction — 316 stainless housings, sanitary finishes, quick-clean grates; the magnet is fully sealed from product (field verification).
- No bare NdFeB in contact — the product side is stainless; corrosion and fragmentation risk are engineered out.
- Traceability — material and field-strength documentation to support HACCP, retail, and regulatory audits.
Weakly magnetic contamination
The hardest metal to catch isn't iron — it's work-hardened 300-series stainless. Austenitic stainless is nearly non-magnetic when annealed, but machining, forming, and wear make it faintly ferromagnetic. Capturing these fragments demands the highest available surface gradient, which is squarely a rare-earth job.
- Use the highest surface gauss and steepest field gradient you can package.
- Keep the product in intimate contact with the magnet (grate/cartridge geometry) — reach alone won't grab a weakly magnetic particle passing at a distance.
- Reduce flow velocity where these particles are the concern; residence time matters.
Design & spec pitfalls
| Mistake | Consequence | Fix |
|---|---|---|
| Speccing surface gauss only | Metal passes through the core of a deep bed | Specify depth of field for the bed depth too |
| Ferrite for fine/weak particles | Fine and stainless fragments slip through | Rare-earth for fine or weakly magnetic capture |
| Ignoring product temperature | NdFeB permanently weakens on hot lines | High-temp grade or SmCo, load-line checked |
| No auto-clean on high tramp | Blinding, bypass, contamination breakthrough | Drum/pulley or self-cleaning grate |
| Undefined capture target | Wrong strength and geometry chosen | State smallest particle, its magnetism, bed depth, velocity |
| No re-verification plan | Field decay goes unnoticed in audits | Certified gauss + scheduled re-test |
Specifying a separator
For a separator magnet or assembly, beyond the RFQ basics:
- Stream: product, form (dry/powder/slurry), flow rate, bed depth, and velocity.
- Capture target: smallest particle and its magnetic character (iron vs weakly magnetic stainless).
- Field requirement: required surface gauss and depth of field, with a certification and re-test interval where audited.
- Temperature & environment: product and ambient temperature (grade selection), wash-down, chemical exposure.
- Geometry & cleaning: separator type, envelope, and manual vs automatic cleaning.
- Compliance: food/pharma construction, stainless grade, and documentation.