Magnetic separator for fine contamination — swarf, mill scale and metallic dust
Particles below 0.5 mm need a different rod than bolts and wire — higher induction and a steeper field gradient
For contamination below 0.5 mm we use Ø32 mm rods in N52 grade, with induction of approximately 10,000 Gs. A standard Ø25 mm rod in N42 grade delivers about 6,500 Gs and is selected for bolts, washers and wire — on swarf and mill scale its efficiency drops noticeably. If the contamination is 304 stainless steel swarf produced by machining, Ø32 N52 is the only option: such steel is paramagnetic and requires both high induction and a steep field gradient right at the rod surface.
Why a separator catches bolts but lets fine particles through
The force a magnet exerts on a particle depends not only on induction, but also on the mass of that particle and on how quickly the field changes across space. An M8 bolt weighs several grams and effectively falls into the field on its own. A 0.3 mm particle weighs a fraction of a milligram, so the force acting on it is smaller by orders of magnitude — while the material stream flowing past carries it along.
The second factor is the gradient, meaning how fast the field rises at the rod surface. A large item only needs to be attracted; a fine particle has to be pulled out of the stream within a fraction of a second, over a distance of a few millimetres. This is why two rods with similar surface induction can perform differently on fine fractions.
The third is that mill scale and swarf from machining stainless steel have reduced magnetic susceptibility. As delivered, 304 steel is practically non-magnetic; only cold work from turning, grinding or cutting makes it weakly magnetic. This is contamination that a separator selected "for bolts" will not capture at all.
What to select for fractions below 0.5 mm
Ø32 mm in N52 grade. A larger diameter means more magnet volume per unit of length, and therefore higher surface induction — around 10,000 Gs against 6,500 Gs for Ø25 N42. Moving from N42 to N52 at the same diameter adds roughly 2,000 Gs.
Denser coverage of the cross-section. With fine fractions what matters is how much magnetic surface falls on the cross-section of the installation. This is why multi-rod assemblies use a 40 mm pitch (centre to centre) rather than 50 mm — a particle then has a better chance of meeting a rod on its way through.
Housing. AISI 304 or 316L, depending on the product passing through the separator — there is no rule that food processing always means 316L. All rods are sealed.
What not to change: adding length will not help. A Ø25 rod 275 mm long will not replace a Ø32 on a fine fraction, because the problem is not contact time but the value of the field and its gradient.
Selecting a rod by size and type of contamination
Induction thresholds at which a given fraction is retained:
| What has to be captured | Size | Recommended rod | Induction |
|---|---|---|---|
| Bolts, washers, wire | above 1 mm | Ø18–Ø25, N42 grade | approx. 6,500–8,000 Gs |
| Swarf from machining structural steel | 0.5–1 mm | Ø25 N52 | approx. 8,500 Gs |
| Mill scale, metallic dust | below 0.5 mm | Ø32 N52 | approx. 10,000 Gs |
| 304 stainless steel swarf from machining | paramagnetic | Ø32 N52, 40 mm pitch | approx. 10,000 Gs and steep gradient |
Induction values are given for the rod surface, with a tolerance of ±5%. For every unit supplied we measure magnetic force and issue a certificate of measured parameters. We do not hold HACCP or ATEX certification — our designs are built in accordance with the requirements of those standards. A Ø25 rod in N52 grade will also cope with fractions below 0.5 mm; Ø32 N52 is the safer choice for them, as it delivers higher induction and a steeper field gradient at the surface.
Pitch, installation and operation
Rod pitch. We use two: 40 mm for the narrow arrangement and 50 mm for the wide one, measured centre to centre. The narrow pitch gives denser coverage of the cross-section and is the right choice for fine, free-flowing powders and dusts. The wide pitch passes more material at lower resistance and is used for granulates and for damp, greasy or sticky materials — with such feedstock a narrow pitch risks bridging above the rods and stopping the flow. Adjacent rods must be polarised so that their fields close rather than repel; set the other way round they create dead zones between them.
Cleaning determines efficiency. With fine fractions the layer of captured particles builds up faster than with bolts and shields the field — further particles settle on the layer of swarf rather than on the rod steel, where the gradient is far weaker. A drop in efficiency after a few weeks of operation almost never comes from magnets weakening, but from the separator not being cleaned.
Temperature. Up to 80 °C as standard. A neodymium magnet loses about 0.11% of its induction per degree, and above 80 °C the loss stops being reversible. For hot lines — plastic granulate after a dryer, for example — we make H, SH and UH versions to order, stable up to 120 °C.
Length. Up to 275 mm from stock. Up to 500 mm to order; that is the maximum for a single element.
Frequently asked questions
Engineer-led selection — free of charge
Tell us what passes through the installation, how large the contamination is and whether it comes from machining stainless steel. We will select the diameter, grade and pitch, and quote a price. Phone +48 22 499 98 98, Monday to Friday 9 am – 5 pm.
+48 22 499 98 98 Contact usMagnetic rods from our range
Rods in diameters Ø18–Ø32 mm, lengths from 100 to 275 mm from stock, M6, M8 or M10 thread, AISI 304 or 316L housing. For fine fractions look for Ø32 in N52 grade.