Glossary of terms

The air gap is the distance between the surface of a magnet and the ferromagnetic object the magnet is meant to capture or hold. Air conducts magnetic flux far worse than steel, so even a tiny gap markedly reduces the flux density and the force — in holding applications a gap of 0.2 mm can cut the holding force to as little as a quarter of the catalogue value.

The gap is also made up of things commonly forgotten — the stainless steel sleeve of a rod, build-up of separated material, a protective cover or a layer of dust. Guide the material in a thin layer as close as possible to the magnetic surfaces and clean the surfaces regularly. Measure the flux density with a gaussmeter at the real distance of the material flow, not on the bare magnet; the difference between the two values decides whether the separator captures fine metal abrasion or lets it through. With fine particles, every additional millimetre of gap means an order-of-magnitude drop in capture.

Anisotropy of a magnet means the material has a preferred "easy" axis built in during production, along which it can be magnetized to full performance — it is created by pressing the powder in a magnetic field, which aligns the grains in one direction. An isotropic magnet has no preferred axis and can be magnetized in any direction, at the cost of noticeably lower strength.

Practically all high-performance magnets in separators — sintered NdFeB as well as anisotropic ferrites — are anisotropic; cheap ferrite mouldings and magnetic foils remain isotropic. The consequence for operation: an anisotropic magnet cannot be "re-magnetized" into a different direction, and if it is turned the wrong way in an assembly, you will not get full performance out of it.

The Barkhausen effect is the jerky, discontinuous character of the magnetization of a ferromagnet: as the external field is smoothly increased, the domains do not flip gradually but in small avalanches. Heinrich Barkhausen demonstrated it in 1919 — the jumps could be heard as crackling in a loudspeaker — providing the first direct proof of the existence of magnetic domains.

The jump-like remagnetization generates measurable Barkhausen noise, which responds sensitively to the structure and mechanical stresses in steel. Non-destructive testing therefore uses it to evaluate surface layers, for example after grinding or hardening. For magnetic separation the effect is more of a theoretical backdrop — it explains why the magnetization of a material is not a perfectly smooth process.

Coercivity is the strength of the oppositely oriented magnetic field needed to completely demagnetize a magnetized material. It is denoted Hc and expressed in amperes per metre (A/m). The higher the coercivity, the better the magnet resists demagnetization by external fields, shocks or temperature.

Distinguish between the normal coercivity HcB and the intrinsic coercivity HcJ, which manufacturers state in datasheets as the measure of a magnet's resistance to demagnetization. Coercivity decreases with rising temperature — which is why a cheap standard-grade neodymium magnet in a hot plant irreversibly loses part of its performance even if the temperature stays below the catalogue maximum. For thermally demanding applications, choose NdFeB grades with higher coercivity (suffixes H, SH, UH).

The Curie temperature is the temperature above which a ferromagnetic material loses its ferromagnetic properties — thermal motion destroys the ordering of the magnetic moments and the magnet becomes an ordinary piece of metal. Iron has a Curie temperature of approximately 770 °C, nickel 360 °C and NdFeB neodymium magnets around 320 °C.

A magnet does not, however, only weaken at the Curie temperature. Standard neodymium magnets lose performance permanently above just 80 °C, which is why we work with the maximum operating temperature of a magnet, not its Curie point. For hot processes — dryers, furnace outlets, hot granulates — choose ferrite magnets, which withstand operating temperatures up to 250 °C, or special high-temperature NdFeB grades. Exceeding the operating temperature is one of the most common reasons why a separator "mysteriously" stops capturing contaminants after some time.

The demagnetization curve is the part of the hysteresis loop lying in the second quadrant — it describes how a permanent magnet loses flux density when an oppositely oriented field acts on it. The two fundamental magnet parameters come precisely from this curve: the remanence Br on the vertical axis and the coercivity Hc on the horizontal axis.

The magnet's operating point lies on this curve, determined by the design of the magnetic circuit and the working air gap; its position defines the flux density the magnet will actually deliver in a specific device. Manufacturers publish curves for several temperatures — with rising temperature the curve drops and develops a knee, and if the operating point falls below the knee of the curve, the magnet becomes permanently partially demagnetized.

Demagnetization is the process of removing residual magnetism from a ferromagnetic object. It is most often performed with an alternating magnetic field of gradually decreasing amplitude, which breaks up the aligned arrangement of the magnetic domains in the material; the object then stops attracting metal particles.

Residual magnetism arises during machining, magnetic clamping or the storage of parts near strong magnets. In production it causes trouble: magnetized tools, bearings and workpieces pick up chips and complicate assembly as well as subsequent measurement. Hand, table and tunnel demagnetizers are used for degaussing — hand-held units demagnetize to a depth of 20–40 mm, tunnel units are deployed in automated lines. Note that an ordinary demagnetizer cannot "switch off" the permanent magnet of a separator: thanks to its high coercivity, it is designed to retain its magnetization permanently.

Diamagnetism is the weakest form of magnetic behaviour: diamagnetic substances are very slightly pushed out of a magnetic field. They include copper, gold, silver, water, graphite and bismuth. The phenomenon arises in all substances, but it is observable only where it is not masked by stronger paramagnetism or ferromagnetism.

Even in the strongest diamagnetic materials, the repulsive force is orders of magnitude weaker than the attractive force the field exerts on ferromagnetics, and under industrial separation conditions it is practically unmeasurable. For plant operation the conclusion is simple: copper, brass or bronze can be neither captured nor repelled by a magnet. They are removed from the material stream by an eddy current separator, and their presence in the finished product is revealed by an industrial metal detector. The diamagnetic behaviour of water also explains why the moisture of a material does not magnetically interfere with separation itself.

The Earth's magnetic field is the natural field generated by the movements of molten iron in the Earth's core. At the surface it reaches a mere 25 to 65 microtesla; it protects the planet from the charged particles of the solar wind and orients the compass needle. The magnetic poles drift slowly and do not coincide exactly with the geographic ones.

The Earth's field is roughly a hundred thousand times weaker than the field on the surface of a neodymium separator, so it has no effect whatsoever on industrial separation and disappears into rounding in normal gaussmeter measurements. It is useful for one thing only: it makes the compass work, with which you can determine the polarity of an unmarked magnet — the north end of the needle points to its south pole.

Eddy currents are electric currents induced in an electrically conductive material whenever the magnetic field around it changes. In accordance with Lenz's law, they create their own magnetic field opposing the change that caused them — a repulsive force thus arises between the conductor and the magnet.

This is exactly the phenomenon non-ferrous metal separation is built on. An eddy current separator has a rapidly rotating rotor with rare-earth magnets in the head of a belt conveyor; it induces eddy currents in particles of aluminium, copper or brass, and the repulsive force ejects them from the rest of the material. Recycling lines thus obtain clean plastic, glass or regrind free of non-ferrous metals. The condition for effectiveness: the material must first be stripped of ferromagnetic particles by a magnetic separator, otherwise the rotor risks damage. Eddy currents are also used by induction brakes and metal detectors.

An electromagnet is a coil with a core of magnetically soft steel that creates a magnetic field only while electric current flows through it. The field strength grows with the current and the number of coil turns; when the current is switched off, the field practically disappears. This fundamentally distinguishes an electromagnet from a permanent magnet, whose field is permanent.

Its main advantages are switchability and power — electromagnets achieve great depth of reach, which is why they are used in suspension separators above conveyors carrying deep burdens of material, in scrap-handling electromagnets for moving metal waste, and in holding systems. The price is continuous energy consumption, the need for cooling and the behaviour during a power failure, when the load and the captured contaminants are released. For most separation tasks, maintenance-free permanent magnet systems are therefore sufficient.

Electromagnetic induction is the phenomenon in which a change in magnetic flux generates an electric voltage in a conductor; it is described by Faraday's law of 1831. Do not confuse it with magnetic flux density B — that is a quantity describing field strength, while electromagnetic induction is a process.

Induction underlies generators, transformers and induction heating — and, in magnetic separation, the whole principle of eddy current separators. Their rapidly rotating magnetic system induces eddy currents in non-ferrous metals, whose field flings particles of aluminium or copper out of the material stream. The same phenomenon is used by a gaussmeter when measuring alternating fields and by a metal detector when searching for non-magnetic contaminants.

Ferromagnetism is the strongest form of magnetism: the ability of a substance to strongly amplify a magnetic field and to be attracted by a magnet, thanks to domains in which the magnetic moments of the atoms are aligned. A ferromagnet is a substance with this property — at room temperature iron, nickel, cobalt and their alloys, including common steels and cast iron.

Magnetic separation captures precisely these ferromagnetic particles — steel abrasion, chips, bolts, wire. Aluminium, copper and brass will not be caught by a separator. Stainless steel is a special chapter: austenitic steel 1.4301 (AISI 304) is practically non-magnetic in the annealed state, but cold working creates weakly magnetic martensite in it. Abrasion from stainless steel processing equipment is therefore captured only by separators with very high flux density — bear this in mind when specifying equipment for food and chemical plants.

The hysteresis loop is a graphical representation of how the magnetic flux density B of a material depends on the strength H of the external magnetic field during a complete cycle of magnetization and demagnetization. It is also known as the BH curve. Two key values are read from its shape: the remanence Br and the coercivity Hc.

The shape of the loop tells an engineer what the material is good for. A narrow loop means a magnetically soft material — easy to magnetize and demagnetize, with low losses — which is why such materials are used for the cores of transformers and electromagnets. A wide loop indicates a magnetically hard material that retains its magnetization permanently — the material permanent magnets for separators are made of. The area of the loop, moreover, corresponds to the energy lost in one magnetization cycle. When selecting a magnet, therefore, do not compare strength alone but the manufacturer's full BH characteristic.

The initial magnetization curve, also called the virgin curve, is the course of the very first magnetization of a material from the unmagnetized state up to saturation. It starts at the origin of the coordinates; its steepness corresponds to the permeability of the material and it ends in saturation, from which the material moves along the hysteresis loop during any further field changes.

A magnet passes along the virgin curve only once — during initial magnetization at the factory or in a magnetizer. The essential point is to reach full saturation: an under-magnetized magnet permanently delivers lower performance than its grade declares, and the difference cannot be seen by eye, only by measurement. In complaints about weak magnets, therefore, compare the total moment or flux density with the values in the material datasheet.

Irreversible loss of magnetization is the permanent weakening of a permanent magnet that persists even after the cause has passed. It is most often brought about by exceeding the maximum operating temperature, and further by strong external magnetic fields or hard mechanical shocks. Unlike reversible temperature changes, the magnet does not recover by itself after cooling down.

The more the temperature exceeds the operating limit, the greater the share of performance the magnet loses permanently; with standard neodymium the risk begins above just 80 °C. If the material structure has not been altered by exceeding the Curie temperature, the magnet can be recharged with a strong external field — with the encapsulated magnetic cores of separators, however, remagnetization is generally impracticable and is solved by replacement. Suspected permanent loss is confirmed by gaussmeter measurement and comparison with the values recorded at installation. Prevention lies in choosing a material with a sufficient temperature margin for operating peaks.

Lenz's law is the physical law according to which an induced electric current always acts against the change that produced it. Heinrich Lenz formulated it in 1834 as a complement to Faraday's law of electromagnetic induction — nature "resists" a change in magnetic flux.

The entire separation of non-ferrous metals rests on Lenz's law. The rapidly rotating magnetic drum of an eddy current separator changes the flux in passing pieces of aluminium or copper, the induced eddy currents in them create a field opposing the change — and the particle is flung away from the drum, out of the material stream. The same law explains why a magnet falls conspicuously slowly through a copper tube: it is braked by the field of its own induced currents.

The Lorentz force is the force a magnetic field exerts on a moving electric charge; it is perpendicular both to the direction of the charge's motion and to the direction of the field. The faster the motion and the stronger the field, the greater the force — a stationary charge feels no force from a magnetic field.

The Lorentz force is the microscopic cause of the phenomena on which both measurement and separation are built. It deflects the electrons in the semiconductor chip of a Hall probe, creating the voltage a gaussmeter measures; it curves the paths of charged particles in accelerators and mass spectrometers; and it acts on the electrons that form the eddy currents in eddy current separators. Everyone who measures magnetic fields or separates non-ferrous metals encounters it — even without seeing it directly.

Magnet ageing is the gradual, spontaneous loss of magnetization of a permanent magnet over time. In modern materials it is very small and slows down further: ferrite magnets are the most stable over time, followed by neodymium and SmCo, while AlNiCo alloys age fastest. A quality neodymium magnet thus spontaneously loses only a fraction of a percent of its performance over the entire service life of a separator.

If a device weakens faster, the cause is usually not age but operating conditions — exceeding the working temperature, a strong external field, corrosion under a damaged housing or hard impacts. Only regular measurement with a gaussmeter, using the same procedure at the same spot on the surface, reveals the true condition; compare the values with the installation report. An annual check as part of a magnetic separation audit catches the decline before it shows up as metal contaminants in the product. For new equipment, request the measurement report at delivery.

A magnetic circuit is the path along which magnetic flux closes: it consists of the magnet, ferromagnetic guiding parts (cores, pole pieces) and air gaps. A closed circuit conducts the flux mainly through steel; an open circuit closes through air — with heavy losses, because air conducts flux far worse.

Every separator is, at its core, an engineered magnetic circuit. The designer chooses the shape of the magnets and steel parts so that the largest possible share of the flux passes through the working zone where the material flows. A lone magnet without a circuit works "idle" and its field disperses; the same magnet in a well-designed circuit achieves markedly higher flux density at the working point. That is why the performance of a device cannot be estimated from the magnet material alone.

The magnetic dipole is the basic unit of magnetism: the pair of north and south poles formed by every magnet and every atom with a magnetic moment. An isolated pole — a magnetic monopole — does not exist in nature; dividing a magnet only ever produces smaller dipoles.

Related to the dipole nature of magnetism is multipole magnetization, in which a series of alternating poles is created side by side on a single piece of material. A short distance between poles means a high field gradient at the surface — and it is the gradient that decides the force with which a magnet attracts a small particle. That is why the cores of magnetic rods are assembled from a series of magnets in opposing orientation, not from one long magnet.

Magnetic domains are microscopic regions inside a ferromagnetic material in which the magnetic moments of the atoms are aligned in the same direction. In an unmagnetized piece of steel the domains point randomly and their effects cancel out externally; an external field gradually rotates them into one direction until the material reaches saturation.

The domain structure explains how materials behave in service: in magnetically soft steel the domains largely return to a random state once the magnet is removed, while in magnetically hard materials they stay aligned — a permanent magnet is created. Domain flips during magnetization occur in jumps (the Barkhausen effect), which some steel diagnostic methods exploit. The practical takeaway is why steel parts near separators gradually become magnetized and why demagnetization helps: an alternating field of decaying amplitude "scrambles" the domains again.

A magnetic field is the region of space in which magnetic forces act on ferromagnetic substances, moving charges and current-carrying conductors. It is represented by field lines that emerge from the north pole of a magnet and return to the south pole; its source is a permanent magnet, or an electric current in a conductor or coil.

In magnetic separation, the shape and reach of the field are what count: the field lines are densest at the poles, where the field is strongest, and the field strength drops very quickly with increasing distance from the magnet. A separator therefore captures only particles that pass close enough to the magnet surface — a metal contaminant a few centimetres further away passes through unnoticed. This yields the basic design rule: the material must be guided in a thin layer as close as possible to the magnetic surfaces, for example between the rods of a magnetic grate, not in a large cross-section far away from them.

Magnetic field lines are imaginary lines used to visualize the course of a magnetic field. Outside a magnet they run from the north pole to the south pole, inside it they close back on themselves; they never intersect, and the more densely they lie together, the stronger the field at that point. They can be made visible with iron filings sprinkled on paper above a magnet.

A field line picture quickly reveals where a separator's active zones are and how its structure shapes the field. But field line density falls off very steeply from the surface, so an illustrative diagram with long loops says nothing about the real reach. When evaluating offers, do not settle for a drawing of the field — request flux density values measured with a gaussmeter at a specific distance from the surface, i.e. at the point where your material will actually flow. This steep field decay is precisely why grid magnets work with a thin layer of material guided closely around the rods.

Magnetic field strength is the quantity describing a magnetic field independently of the medium in which it acts. It is denoted H and expressed in amperes per metre (A/m); the older CGS system uses the oersted (1 Oe ≈ 79.6 A/m). It is linked to the magnetic flux density B by the permeability of the medium: B = μ·H.

The difference between the two quantities is practical: H describes the "imposed" field, for example from the coil of an electromagnet, while B tells you what field actually arises in a specific material. That is why coercivity — a magnet's resistance to demagnetization by an external field — is quoted in A/m or oersteds, whereas separator performance is described by flux density in gauss on the surface. When reading a magnet datasheet, remember: values in kA/m refer to the material's resistance to demagnetization, values in gauss or tesla to the strength of the field the device applies to the contaminants.

Magnetic flux is the quantity expressing the total amount of magnetic field passing through a given area. It is denoted Φ, expressed in webers (Wb), and calculated as the product of the magnetic flux density and the area the field passes through perpendicularly: Φ = B · S. One weber corresponds to a flux density of 1 tesla over an area of 1 m².

For separator design, flux is key when engineering the magnetic circuit: steel pole pieces and backing plates conduct the flux from the magnets to the point where the material flows, much as a conductor carries electric current. A well-designed circuit concentrates the flux into the working gap and increases the flux density exactly where the contaminants are captured; a poorly designed one lets it "leak" uselessly into the structure. It is precisely the quality of the magnetic circuit that often distinguishes two separators with identical magnets inside.

Magnetic flux density, also called magnetic induction, is the physical quantity describing the strength of a magnetic field at a specific point. It is denoted B and expressed in tesla (T); in the magnetic separation industry the older unit gauss is also commonly used (1 T = 10,000 G). The higher the flux density on the separator surface, the smaller and more weakly magnetic the particles it can capture.

Common ferrite separators reach 1,500–3,000 G on the surface, neodymium rods in grid magnets up to 16,000 G. Always compare the value on the magnet surface — flux density falls off steeply with distance. It is the flux density at the point where the material actually flows, not the "paper" value from a catalogue, that determines separation efficiency. Verify the values with a gaussmeter fitted with a Hall probe; when comparing offers, request a measurement report — manufacturers often state the flux density inside the magnet, not on the surface of the housing.

Magnetic force is the force a magnetic field exerts on ferromagnetic objects or other magnets. It cannot be described by a single number: it depends on the flux density and its gradient, on the material, size and shape of the attracted object, and it falls very steeply with distance from the magnet.

In manufacturers' data, therefore, distinguish two quantities. Flux density in gauss describes the field on the surface; the holding (breakaway) force in newtons or kilograms states what the magnet holds in direct contact with a steel plate — and even a small gap reduces it by an order of magnitude. For separation, what matters is not how much a magnet can "lift", but the force with which it attracts a small particle at the real distance of the material flow. Always compare the same quantity measured by the same method.

The magnetic moment is a vector quantity expressing the "strength" and orientation of an elementary magnet — an atom, a molecule or an entire body. It arises from the motion and spin of electrons; the sum of the moments in a volume of matter gives its magnetization, and for a whole magnet its total moment.

The total magnetic moment is the most objective parameter of an individual magnet, because it does not depend on where on the surface it is measured. It is determined with a fluxmeter and a Helmholtz coil and is used in outgoing quality inspection of magnets — two magnets of the same size and grade must have the same moment. A spot measurement with a gaussmeter on the surface is no substitute for it; the two methods complement each other.

The magnetic north and south poles are the two regions of a magnet where its field is strongest and where the field lines leave and re-enter the magnet. Poles always exist in pairs: cutting a magnet does not create a lone pole but two smaller magnets, each with its own north and south. Like poles repel, unlike poles attract.

You can determine the polarity of an unmarked magnet with a compass — the north end of the needle points to the magnet's south pole — or with a simple polarity indicator. Ferromagnetic contaminants are attracted equally by both poles, so pole orientation has no effect on a separator's capture; the arrangement and alternation of the poles, however, define the shape of the field on the device surface. Polarity matters when assembling magnets into arrays and in holding systems, where incorrectly oriented magnets weaken each other. Poles cannot be told apart by eye, which is why manufacturers usually mark the north pole with paint or a dimple.

Magnetic reluctance is the quantity describing how strongly a given medium opposes the passage of magnetic flux; it is the analogue of electrical resistance. It is expressed in units of H⁻¹, alternatively A/Wb. The relationship between magnetomotive force, flux and reluctance is described by Hopkinson's law — the counterpart of Ohm's law for magnetic circuits.

Magnetic flux always closes along the path of least reluctance. Mild steel opposes it about a thousand times less than air, which is why separator designs use steel cores and pole pieces that concentrate the flux into the working zone, while every air gap weakens it. In practice this means a steel sheet between the magnet and the material effectively shields the field, whereas non-magnetic stainless steel is almost transparent to the flux — one more reason separator housings are made precisely of non-magnetic stainless steel. When designing a circuit, the reluctances of the individual sections add up like resistances in series.

Magnetism is a physical phenomenon in which substances and electric currents act on each other through a magnetic field. It has been known since antiquity thanks to natural magnetite; today it underpins electric motors, sensors and industrial magnetic separation. According to their behaviour in a magnetic field, substances are classified as diamagnetic, paramagnetic and ferromagnetic.

What matters in practice is that only ferromagnetic substances respond strongly — iron, nickel, cobalt and most steels. Diamagnetic materials (copper, plastics) and paramagnetic materials (aluminium) are practically not attracted by an ordinary magnet. That is why a magnetic separator reliably removes steel abrasion or bolts, but for an aluminium chip you need a different technology. When designing a separation solution, always start from the actual composition of the contaminants in your material, not from the general notion that "a magnet catches metal".

Magnetization is the process by which a ferromagnetic material becomes magnetic under the influence of an external magnetic field — the magnetic domains rotate into the direction of the field. At a certain field strength all the domains align and the material reaches saturation; a stronger field will not increase the magnetization any further.

Industrial permanent magnets are magnetized by a short, powerful pulse in a coil. Ordinary steel parts in a plant are also subject to magnetization: tools, screens or bolts near strong separators become magnetized over time, hold abraded metal on their surfaces and carry it further down the line. This unwanted residual magnetism is removed by demagnetization. Check the degree of magnetization with a gaussmeter — where it causes problems, demagnetize the parts with a hand, table or tunnel demagnetizer.

The maximum energy product is the principal measure of the performance of a permanent magnet material. It is denoted (BH)max, expressed in kJ/m³ or in MGOe (1 MGOe ≈ 8 kJ/m³), and corresponds to the point on the demagnetization curve where the product of flux density and field strength is highest — that is, the amount of magnetic energy stored per unit volume.

It is (BH)max that hides in the grade designations of neodymium magnets: grade N35 has roughly 35 MGOe (about 279 kJ/m³), N52 approximately 414 kJ/m³. The higher the value, the smaller the magnet needed for the same performance. When comparing separators, however, remember that (BH)max describes the material, not the device — capture is decided by the flux density on the housing surface at the point of material flow, which also depends on the circuit design and the thickness of the protective tube. A higher material grade also goes hand in hand with lower temperature resistance.

Paramagnetism is a weak form of magnetism: paramagnetic substances are drawn into a magnetic field only very slightly and retain no magnetization after it is removed. They include aluminium, magnesium, titanium, chromium and liquid oxygen. The force acting on paramagnetic materials is on the order of a thousand times weaker than on ferromagnetic ones.

In practice this means an ordinary magnetic separator will not capture an aluminium chip, even though it is metal — the attraction is so weak that material flow and gravity overcome it. For aluminium and other non-ferrous metals, recycling therefore relies on separators working on the eddy current principle, which actively repel non-magnetic conductive particles out of the material stream. Final product inspection for all metals, including paramagnetic ones, is then handled by an industrial metal detector at the end of the line.

Permeability is the quantity expressing how easily a material conducts a magnetic field — a kind of magnetic counterpart to electrical conductivity. It is denoted μ; relative permeability μr, referenced to vacuum, is commonly used. Air has a μr of approximately 1, ordinary structural steel on the order of thousands, which is why steel strongly amplifies and conducts a magnetic field.

The high permeability of steel is exploited in the magnetic circuits of separators: pole pieces and backing plates concentrate the field into the working gap. Conversely, permeability close to one explains the behaviour of austenitic stainless steels — in the annealed state they typically have a μr of up to 1.02, they hardly affect a magnetic field and a magnet will not capture them; with cold working their permeability rises as martensite forms. The μr value is thus a quick tool for estimating from a material datasheet whether magnetic separation will capture a given metal.

Remanence is the residual magnetic flux density that remains in a ferromagnetic material after it has been magnetized and the external magnetic field removed. It is denoted Br, expressed in tesla or gauss, and read from the hysteresis loop. The higher the remanence, the stronger the permanent magnet that can be made from the material.

Neodymium magnets of grade N35 have a remanence of about 1.17 T (11,700 G), grade N52 about 1.42 T; ferrite magnets reach values several times lower. Beware of a common confusion, though: Br is a property of the material, not the flux density on the separator surface — that depends on the design of the whole magnetic circuit and is usually considerably lower. Remanence also has an unwanted face: tools and steel parts magnetized during production attract chips and complicate assembly. There, degaussing with a tunnel or hand demagnetizer is the remedy.

Saturation, or magnetic saturation, is the state in which a ferromagnetic material is fully magnetized — all the domains are aligned and further increases in the external field no longer raise the magnetization. In everyday separation language, the same word also describes a separator filled up with captured contaminants.

A saturated grate is the most common cause of "non-functioning" separation. The layer of captured metal on the rods shortens the reach of the field, and the material flow starts tearing contaminants back off the surface — the separator then lets metal through even though the magnets are fine. The remedies are regular cleaning intervals set according to the actual capture rate, automatic cleaning for heavily contaminated materials, or a version with saturation indication that itself alerts the operators when it is time to clean.

Superconducting magnets are electromagnets with windings made of a superconductor, which loses its electrical resistance at temperatures close to absolute zero. Enormous currents can then flow through the coil without losses, creating fields unattainable by any other technology — magnetic resonance imaging works with 1.5 to 3 tesla, laboratory systems exceed even 45 tesla.

The price of this performance is liquid helium cooling and complex infrastructure, which is why superconducting magnets belong to medicine, research and accelerators, not production halls. In industrial separation you will meet them only exceptionally, in special high-gradient systems for extremely fine raw materials; ordinary plants are fully served by permanent neodymium magnets — with no energy consumption and no maintenance.

The temperature coefficient of a magnet states by how many percent its parameters change reversibly with each degree Celsius. For the remanence of neodymium magnets it is roughly −0.11 % per degree, for samarium-cobalt only −0.03 to −0.05 % — which is why SmCo retains more performance in heat. The change is reversible: after cooling, the magnet returns to its original values.

The coefficient explains why a separator in a hot plant captures less well even without exceeding the permitted temperature — at 80 °C neodymium reversibly delivers about 6 % less than at 20 °C. A curiosity of ferrites: their coercivity does not fall with temperature but rises, so they are harder to demagnetize when hot. Always relate gaussmeter measurements to the temperature at which they were taken; comparing winter and summer values without correction can lead to a false alarm.

A fluxmeter is an instrument for measuring magnetic flux. It works by integrating the voltage induced in a measuring coil: when a magnet is inserted into or withdrawn from the coil, the change in flux produces a voltage pulse, from which the fluxmeter calculates the flux in webers, or the total magnetic moment of the measured piece.

The fluxmeter and the gaussmeter complement each other and must not be confused. A gaussmeter measures the flux density at one point on the surface — suitable for checking separators in operation. A fluxmeter with a Helmholtz coil measures the total moment of a magnet regardless of how it is oriented in the coil — which is why it is the standard for incoming and outgoing inspection of individual magnets. A complaint about a weak magnet is best supported by a fluxmeter report; a spot measurement can be challenged over probe position.

A gaussmeter, also called a teslameter, is an instrument for measuring magnetic flux density. Its heart is a Hall probe — a semiconductor sensor on which a magnetic field creates a measurable voltage. The instruments measure both DC and AC fields and display values in gauss, tesla or amperes per metre.

For a meaningful result, place the probe directly on the magnet surface at a pole — even a distance of a few millimetres reduces the reading by an order of magnitude. Always measure with the same procedure at the same spot; only then are values comparable year on year. Regular gaussmeter measurement reveals the weakening of a separator caused by exceeding the operating temperature or by mechanical damage before it shows up as metal contaminants in the product; it is therefore a standard part of a magnetic separation audit.

The gauss (G) is the unit of magnetic flux density commonly used in the magnetic separation industry. The conversion is 1 tesla = 10,000 gauss. Although the SI system prefers the tesla, separator manufacturers traditionally state device performance in gauss, and separators are compared accordingly.

Indicative values: ferrite separators reach 1,500–3,000 G on the surface, common neodymium rods exceed 10,000 G, and special versions for capturing weakly magnetic abrasion exceed 20,000 G. When comparing offers, however, always clarify where the value was determined: marketing figures often describe the flux density inside the magnetic core, whereas separation efficiency is decided by the flux density on the housing surface at the point of material flow. Request a gaussmeter measurement report taken on the device surface — the difference between the two figures can be severalfold.

The Hall probe is a semiconductor sensor for measuring magnetic flux density based on the Hall effect, described in 1879 by Edwin Hall: on a thin current-carrying chip, a transverse voltage proportional to the flux density arises in a magnetic field. The probe is the measuring element of gaussmeters and teslameters as well as contactless position and speed sensors.

The probe measures only the field component perpendicular to the chip, so its orientation fundamentally affects the result. The transversal type is laid flat on the magnet surface; the axial type measures the field along its axis, for example in coil bores; thin transversal probes exist for the slots between the rods of grid magnets. When checking a separator, always hold the probe in the same orientation, perpendicular to the pole surface, and measure repeatedly at the same spot — only then are the values mutually comparable. Protect the probe from bending and impact — a damaged sensor invalidates the entire measurement.

A Helmholtz coil is a pair of identical coaxial coils placed at a distance equal to their radius. In this arrangement the fields of the two coils combine so that an exceptionally homogeneous magnetic field arises in the space between them — equally strong throughout the working volume.

The homogeneity has two main uses. An energized Helmholtz coil creates a defined field for calibrating gaussmeters and testing sensitive instruments; connected the other way round, as the sensing coil of a fluxmeter, it measures the total magnetic moment of an inserted magnet regardless of its exact position. It is part of the basic equipment of magnetic laboratories and the inspection stations of magnet manufacturers; you will not encounter it in separation operations themselves.

Measuring magnetic flux density is the basic way to verify the actual performance of a magnet or separator. It is carried out with a gaussmeter fitted with a Hall probe, directly on the device surface at the poles; the result is stated in gauss or tesla. A complementary method is measuring the breakaway force with a test piece (pull test).

Magnets weaken over time — most often through exceeding the operating temperature, mechanical damage or corrosion. The weakening cannot be seen: the separator looks the same, it just captures less. So measure regularly, ideally once a year, always with the same procedure at the same spots, and record the results in a report for year-on-year comparison with the values at installation. For new equipment, request from the supplier a measurement report for the specific unit taken on the housing surface — the catalogue value of the material is not the same thing.

The oersted (Oe) is the unit of magnetic field strength H in the CGS system, named after Hans Christian Oersted. Its SI counterpart is the ampere per metre; the conversion is 1 Oe ≈ 79.6 A/m. The oersted is paired with the gauss — in vacuum, a field with a strength of 1 Oe corresponds to a flux density of 1 G.

Today you will meet oersteds mainly in the magnet datasheets of overseas manufacturers, who traditionally state coercivity in kilo-oersteds (kOe), while European datasheets use kA/m; the conversion is 1 kOe ≈ 79.6 kA/m. When comparing materials from different catalogues, always check the unit — numerically, values in kOe and kA/m differ by a factor of about eighty, which at first glance looks like a vast difference in quality.

The breakaway force is the force needed to pull a steel test piece away from the surface of a magnet. It is determined by the so-called pull test: a test piece of defined shape is placed on the measured spot and pulled away steadily with a force gauge until it breaks free; the recorded value is stated in newtons.

The pull test complements gaussmeter measurement where flux density alone is not enough: gauss values describe the field strength at one point, while the breakaway force tells you how well the device actually holds a captured particle. It is measured repeatedly at defined spots — on grids and rods above the pole pieces, where the field is strongest — and the result is the average of several measurements. Year-on-year comparison of the reports reliably reveals magnet degradation. For lifting magnets, an annual verification of the breakaway force with a report is also recommended for occupational safety reasons.

The tesla (T) is the SI unit of magnetic flux density, named after the inventor Nikola Tesla. The conversion is 1 T = 10,000 gauss; weaker fields are stated in millitesla (1 mT = 10 G). For perspective: the Earth's magnetic field at the surface is roughly 0.00005 T, a common fridge magnet around 0.005 T. One tesla corresponds to one weber of magnetic flux per square metre of area.

Medical magnetic resonance imaging works with fields of 1.5–3 T, and the neodymium rods of separators reach up to 1.6 T at the surface. In the magnetic separation industry, however, performance is traditionally stated in gauss, so you will meet the tesla mainly in standards, measuring instrument datasheets and international literature. Conversion is easy — multiply the value in tesla by ten thousand. When reading parameters, always check the unit: confusing millitesla with tesla means an error of three orders of magnitude.

The weber (Wb) is the SI unit of magnetic flux, named after the physicist Wilhelm Weber. One weber corresponds to the flux of a homogeneous field with a flux density of one tesla through an area of one square metre; thus 1 Wb = 1 T·m², equivalently 1 V·s.

You will meet webers mainly in fluxmeter measurements, where the total flux of a magnet is stated in milliwebers, and in the design calculations of magnetic circuits. Separator catalogues do not work with them — device performance is described by flux density in gauss or tesla, i.e. the density of the flux, not the flux itself. The link between the two quantities is useful to know: the same flux squeezed by a pole piece into a smaller area means higher flux density at the working point.

An AlNiCo magnet is a permanent magnet made from an alloy of aluminium, nickel, cobalt and iron. Its main strength is thermal stability: it works at temperatures up to 450 °C and its Curie temperature of around 850 °C is the highest of the common magnetic materials. It resists corrosion and acids well, but it is considerably weaker than neodymium magnets.

The weakness of AlNiCo is its low coercivity — the magnet is easily partially demagnetized by a stronger external field or unsuitable storage, so it does not belong loose next to neodymium magnets. It is used in measuring technology, sensors and wherever high temperature rules out both neodymium and SmCo. In magnetic separation it plays a marginal role; for ordinary hot processes, ferrite or high-temperature NdFeB grades usually suffice, and AlNiCo remains the choice for temperature extremes.

A ferrite magnet, also known as a ceramic magnet, is a permanent magnet produced by sintering iron oxide with barium or strontium carbonate. Compared to neodymium it is considerably weaker; its advantages are low cost, corrosion resistance without any surface coating, and temperature resistance up to 250 °C.

The choice between ferrite and neodymium is decided by the application: ferrite separators reach 1,500–3,000 G on the surface and reliably capture larger steel particles — bolts, nuts, wire. For fine abrasion below a tenth of a millimetre, neodymium with a flux density above 10,000 G is needed. Choose ferrite for hot processes such as dryers or furnace outlets, for damp and outdoor environments, and wherever capture of coarser contaminants at a lower price is sufficient. The brittleness of the ceramic material demands careful handling — a knock or a fall easily chips the magnet.

A ferromagnetic material is a substance strongly attracted by a magnet, and thus removable by magnetic separation. It includes iron, carbon steel, cast iron, nickel, cobalt and most alloys of these metals — that is, the vast majority of metal contaminants in industrial plants.

Conversely, a magnet will not capture aluminium, copper, brass, bronze, zinc, lead or non-metals such as plastics and glass. Stainless steels are the borderline case: austenitic grades (AISI 304, 316) are practically non-magnetic in the annealed state and only partially capturable after cold working, whereas ferritic and martensitic stainless steels (e.g. AISI 430) can be separated. When deciding whether magnetic separation will remove your contaminants, do not rely on general tables — have a real sample of your material and contaminants tested.

The Halbach array is a way of assembling permanent magnets with alternately rotated magnetization directions that strengthens the field on one side of the assembly and almost cancels it on the other. It was devised by physicist Klaus Halbach in the 1980s for particle accelerators; its basis is a repeating set of five differently oriented magnets.

A one-sided field means maximum performance where it is needed, with a minimum of magnetic material — and a weak field on the back, where it would only interfere. The arrangement is used in electric motors, magnetic bearings, clamping technology and holding systems; the principle of concentrating the field into the working side is also exploited in the design of plate magnets. You do not need the term for everyday separator selection, but it explains why modern magnet assemblies outperform a plain block of the same size.

A magnet coating is a protective layer shielding the magnet from corrosion and mechanical damage. The most common is the three-layer nickel–copper–nickel plating (NiCuNi), sufficient for dry indoor environments; for damp, outdoor and chemically aggressive plants, encapsulation in epoxy resin or zinc plating is used. It is essential above all for neodymium magnets, whose alloy corrodes quickly without protection.

The layers are thin so as not to reduce magnetic strength, and correspondingly vulnerable — a knock or a chipped edge opens a path for corrosion into the magnet. In the food industry, plating alone is therefore not considered sufficient: the magnetic cores of separators are enclosed in welded stainless steel housings, which do not contaminate the material and withstand sanitation. When selecting a magnet for a damp or acidic environment, always state the operating conditions — standard NiCuNi is not enough there.

Magnetite is a mineral of iron(II,III) oxide, Fe₃O₄, the most strongly magnetic natural material. Its naturally magnetized pieces, known as lodestone, were humanity's first magnets — they turned the needles of the earliest compasses, and the whole field takes its name from the ancient region of Magnesia.

Magnetite is not just history, though. It is ferrimagnetic and reliably captured by a magnetic separator, which is exploited in iron ore beneficiation and in cleaning foundry and glassmaking sands of iron-bearing minerals. Fine magnetite is also a frequent component of the natural contamination of raw materials — the black dust on an inspection magnet is usually just that. Laboratory magnetic separation therefore also uses it as a model material in separability tests.

Sintering is the powder metallurgy process used to produce the strongest permanent magnets: a fine alloy powder is pressed in a magnetic field, which orients the grains along the easy axis, and then fused at a high temperature below the melting point. This is how sintered NdFeB, SmCo and anisotropic ferrite magnets are made.

Sintering gives the material maximum density and performance, but also a ceramic nature — sintered magnets are hard and brittle, they cannot be drilled or machined with ordinary tools, and they chip when struck. The shape is finished by grinding with diamond wheels at the manufacturer; order magnets in their final dimensions and protect them during assembly from shocks and from snapping into each other, which easily chips the edges.

A neodymium magnet is a permanent magnet made from an alloy of neodymium, iron and boron (Nd₂Fe₁₄B). It is the strongest mass-produced magnetic material: in a small volume it delivers many times the strength of ferrite. It is manufactured by sintering the powdered alloy in vacuum and is designated by grades N35–N52 according to magnetic strength.

Standard grades work up to 80 °C; for higher temperatures there are series with the suffixes M, H, SH, UH, EH and AH, resistant up to 240 °C. The alloy corrodes easily, so the magnets are given a surface coating, most often nickel or zinc plating. In magnetic separators, neodymium magnets are the standard today — only their high flux density captures fine steel abrasion and weakly magnetic particles. The higher the grade, the stronger the magnet but the lower its temperature resistance; choose the grade according to the temperature of your process, not just by the number.

A permanent magnet is a material that, once magnetized, permanently retains its own magnetic field without any supply of electrical energy. It is made from magnetically hard materials; four main types are used in industry: neodymium magnets (NdFeB), ferrite, samarium-cobalt (SmCo) and AlNiCo.

Neodymium is the strongest material available, but standard grades work only up to 80 °C and corrode without a surface coating (nickel, zinc). Ferrite is inexpensive, resistant to corrosion and to temperatures up to 250 °C, but considerably weaker. SmCo combines strength with temperature resistance at a higher price; AlNiCo is thermally stable but easily demagnetized. Neodymium magnets dominate in magnetic separators today for maximum capture of fine particles; for hot processes, choose ferrite or high-temperature NdFeB grades. Always select the material according to the temperature and environment of the specific plant.

Pole pieces are components of magnetically soft steel that guide and shape the flux of a permanent magnet to where it is needed. Soft steel offers the flux minimal reluctance, so a pole piece can concentrate the field into a smaller area — and thereby raise the flux density at the working point above the value on the surface of the magnet itself.

Pole pieces are the quiet design trick of most magnetic devices. In separators they concentrate the field onto the working edges and surfaces; in holding systems they form contact faces resistant to the wear a brittle magnet could not withstand. They also explain why two devices with identical magnets perform differently: the resulting flux density at the capture point is decided precisely by the geometry of the steel parts around the magnets.

Rare earths in magnets are elements of the lanthanide group that give modern magnets their exceptional strength. The most important is neodymium, the basis of NdFeB magnets; dysprosium and terbium are added to the alloy for higher coercivity and temperature resistance. This is precisely why NdFeB and SmCo magnets are called "rare earth" magnets.

Rare earths are not actually scarce in the Earth's crust, but their mining and above all their processing are concentrated in China, so magnet prices react sensitively to trade policy, geopolitical tension and supply disruptions. For separator buyers this means two things: the prices of neodymium equipment can fluctuate considerably over time, and longer lead times for stronger grades are not unusual. Do not postpone enquiries for new equipment to the last minute, and for critical plants consider a service stock of spare magnetic cores.

A rubber magnet, also called a flexible or bonded magnet, is a composite material of magnetic powder — most often ferrite — dispersed in a flexible polymer or rubber matrix. It is produced by calendering or extrusion into foils, tapes and profiles that can be cut with scissors, trimmed and bent.

A rubber magnet is markedly weaker than sintered magnets; its strengths are freedom of shape and low cost. Typical uses: magnetic foils and tapes for shelf labelling and exchangeable signs, refrigerator door seals, promotional magnets, holding light objects. It has no place in separation — it is too weak to capture contaminants. Bonded magnets with neodymium powder also exist, stronger than the ferrite ones, but they still do not reach the performance of sintered magnets.

A samarium-cobalt magnet is a rare-earth permanent magnet produced in the compositions SmCo₅ and Sm₂Co₁₇. After neodymium it is the second strongest magnetic material, but it withstands substantially higher temperatures — 250 to 300 °C continuously — and resists corrosion without any surface coating. It also excels in high coercivity, making it hard to demagnetize.

The temperature coefficient of SmCo is only −0.03 to −0.05 % per degree Celsius, roughly a third of the neodymium value, so the magnet retains considerably more performance in heat. The price is the higher cost of cobalt and considerable brittleness — the magnets chip easily and require careful assembly. Choose SmCo where high temperature meets a demand for strength or an aggressive environment; typically sensors, motors and applications where high-temperature neodymium is no longer enough and ferrite is too weak.

The temperature resistance of a magnet is the highest operating temperature at which the magnet suffers no permanent loss of performance. Standard neodymium magnets work up to 80 °C, the temperature series M, H, SH, UH, EH and AH up to 100–240 °C. Ferrite magnets withstand 250 °C, samarium-cobalt around 250–300 °C and AlNiCo up to 450 °C.

With rising temperature, a magnet's strength decreases reversibly even below the operating limit — for NdFeB by roughly 0.1 % per degree. Once the maximum operating temperature is exceeded, the loss is permanent and the magnet does not recover even after cooling down. When selecting a separator for dryers, extruders or furnace outlets, therefore, work from the temperature of the separated material at the magnet location, not the ambient temperature, and allow a margin for temperature peaks. A performance drop in an overheated separator is verified by gaussmeter measurement and comparison with the installation report.

Manual and automatic cleaning are the two ways of removing captured metal particles from a magnetic separator. In the manual version (MC), the operator withdraws the magnetic cores from the tubes during a shutdown and shakes off the contaminants; in the automatic version (AC), the cleaning cycle is handled by a pneumatic system without operator intervention, usually without interrupting production.

The manual version is cheaper and sufficient where contaminants accumulate slowly and shutdowns do not matter. Automation pays off in continuous operations, with heavily contaminated material, and wherever cleaning reliability must not depend on operator discipline — neglected cleaning is the most common reason a separator stops capturing. The decision is made easier by versions with saturation indication (ALARM), which report when cleaning is actually needed.

Ferromagnetic contaminants are iron-based metal particles — steel abrasion, chips, weld fragments, bolts, nuts or wire — that get into the processed material through machine wear, during maintenance, or already with the raw material from the supplier. These are exactly the particles magnetic separation can capture.

In production they do double damage: they harm downstream equipment such as mills, injection moulding machines and granulators, and they degrade the product — in the food industry they additionally pose a direct risk to the consumer and grounds for complaints or a batch recall. Their size ranges from visible pieces to micron-scale abrasion invisible to the eye; it is the fine particles that are the most frequent cause of findings. Whether and how the contaminants in your material can be removed is reliably shown by a test of a real sample on a specific separator type.

High-gradient magnetic separation (HGMS) is a technique for capturing very fine and weakly magnetic particles beyond the reach of ordinary separators. The material flows through a matrix — a cartridge filled with magnetized stainless steel wool or meshes — where extreme local field gradients arise at the edges of the wires, attracting even particles around one micrometre in size.

HGMS captures not only ferromagnetic abrasion but also paramagnetic admixtures, for example iron compounds in ceramic raw materials that a conventional grate lets through. It is used where trace concentrations of iron decide quality — kaolin, feldspars, glassmaking sands, pharmaceutical and chemical raw materials. Before deployment in production, have the material tested on a laboratory matrix separator; the test result shows the achievable purity and the appropriate operating parameters of the device.

Magnetic filtration of liquids is the removal of ferromagnetic particles from process liquids by means of a magnetic field — without filter cartridges that would clog. Typically these are coolant emulsions and cutting oils from machining, grinding sludges and washing media, in which fine metal abrasion accumulates.

Metal particles in an emulsion destroy pumps, nozzles and machined surfaces and shorten the life of the liquid. A magnetic filter captures even abrasion of around one micrometre, which paper and bag filters let through, thus considerably extending their replacement intervals. The captured sludge is simply wiped off the magnetic core during cleaning; with magnetic coolant separators the contaminant discharge runs continuously and automatically. The benefit is easy to calculate: lower emulsion consumption, less filter waste and longer tool life.

Magnetic separation is a technological process in which the magnetic field of permanent magnets or electromagnets removes ferromagnetic contaminants from a material stream — from bulk mixtures, liquids and material on conveyors. It is used wherever metal particles endanger machinery or product quality.

Separation performs two roles in a plant. It protects equipment — mills, crushers, injection moulding machines or filling lines — from damage by metal particles, and at the same time it ensures product purity in the food, plastics, ceramics, chemical and recycling industries. Quality neodymium separators capture particles from a size of around 1 micron. The type of device is chosen according to the form of the material and the mode of conveying: powders in pipelines are separated differently from granulate in a hopper or material on a belt. Always verify effectiveness with a test on a real sample of your material, not just with catalogue parameters.

A magnetic separation audit is a systematic review of all the magnetic separators and detectors in a plant. It covers checking the placement of the equipment against the risk points of the line, measuring magnetic flux density with a gaussmeter on the magnet surfaces, assessing the condition and method of cleaning, and comparing the measured values with the documentation. The output is a report with measurement records and recommendations.

An audit is worth commissioning after several years of separator operation, before an IFS or BRC certification audit or a customer audit by a retail chain, after a metal find in the product, or when the raw material or technology changes. It uncovers the typical weak spots: separators weakened by exceeding the operating temperature, poorly placed or undersized equipment, and missing records that the auditor requires. Regular repetition builds a demonstrable measurement history.

A magnetic separator is a device that uses a strong magnetic field to remove ferromagnetic contaminants from bulk materials, liquids or material conveyed on belts. The field source is permanent magnets, most often neodymium or ferrite, or an electromagnet; quality neodymium separators capture particles from a size of around 1 micron.

The type of separator is chosen according to the form of the material and the mode of conveying: grid magnets and magnetic rods serve bulk mixtures; pipeline and in-line magnetic separators handle liquids and pneumatic conveying; magnetic drums and suspended separators cover belt conveying; plate and finned designs cover special applications. Effectiveness is decided by the flux density on the magnet surface at the point of material flow and by the cleaning method — manual or automatic.

Metal contaminants in food are metal fragments that enter the product during production — from worn screens, mixers, conveyors, knives or bolted joints, or already with the raw material. Under HACCP methodology they rank among the physical hazards a food producer must systematically control.

A metal find at the consumer means complaints, fines and the recall of a whole batch, so prevention is built in two stages: magnetic separators capture ferromagnetic particles, including fine abrasion, directly in the raw material stream, and an industrial metal detector at the end of the line reveals even the stainless steel and non-ferrous metals a magnet cannot capture. The effectiveness of both measures must be regularly verified and documented — retail chain auditors and certification schemes require records of cleaning, measurement and tests with reference test pieces.

Separation of non-ferrous metals is the removal of aluminium, copper, brass and other non-magnetic metals from a material stream. An ordinary magnetic separator will not capture these metals — they are not ferromagnetic. The eddy current principle is used instead: an eddy current separator (ECS) with a rapidly rotating magnetic rotor induces currents in the non-ferrous metal particles, whose field ejects them from the material stream.

The typical application is recycling: recovering aluminium and copper from shredded waste, cleaning plastic regrind, glass or refuse-derived fuels. The condition for effectiveness is an upstream magnetic separator that first removes the ferromagnetic particles — otherwise the ECS rotor risks damage. Non-magnetic stainless steel, however, is not reliably ejected by eddy currents; an industrial metal detector serves to check for it.

Tramp metal is the established English term for foreign metal objects that have made their way into a material stream — bolts, nuts, tool fragments, wires, loader bucket teeth. In Czech one speaks of foreign metals or metal contaminants, but the term tramp metal is in common use here as well, mainly in the mining, recycling and woodworking industries.

While fine ferromagnetic abrasion threatens mainly product purity, tramp metal destroys machinery — a single bolt can damage a crusher, mill or granulator, or puncture a belt. The protection is a separator placed upstream of the sensitive machine: a suspended magnetic plate or an electro suspension magnet above the conveyor, or a hump magnet on gravity pipe runs. It pays to combine it with a metal detector, which also catches non-magnetic metals such as aluminium or stainless steel.

Wet and dry magnetic separation are the two basic modes of removing metal contaminants according to the physical state of the material. Dry separation cleans bulk solids — granules, powders, grain, regrind — using grates, plates, rods and drums. Wet separation captures ferromagnetic particles from liquids, slurries and viscous mixtures, where pipeline and in-line separators or magnetic filters are used.

The chosen mode determines the equipment design. With bulk solids, even distribution of the material around the rods is decisive; with liquids, it is tightness, flow velocity and easy sanitation — which is why wet separators have welded stainless steel housings and hygienic designs. Beware of viscous and sticky mixtures such as chocolate or syrups: they require a special magnet arrangement so that the flow of material actually sweeps around the magnetic core. Choose the mode according to the point in the process where the contamination risk is highest.

A drum magnetic separator is a device with a rotating non-magnetic shell, inside which sits a stationary magnetic segment. The material is fed onto the drum: ferromagnetic particles cling to the shell, the rotation carries them beyond the reach of the magnetic field, where they drop off by themselves into a separate fraction. The clean material continues along its own trajectory.

Its main advantage is continuous self-cleaning without operator intervention — the drum does not saturate, so it handles large volumes and heavily contaminated materials. Typical deployment: recycling, crushing lines, foundry sands, processing of bulk raw materials with a high iron content. The drum width is chosen according to line throughput and the magnet material according to the fineness of the contaminants: ferrite for coarse iron, neodymium for fine abrasion. For belt conveying, the alternative is a suspended magnet above the conveyor.

An electromagnetic separator, known in the industry as an overband, is a suspension separator above a belt conveyor whose field is created by an energized coil instead of permanent magnets. An electromagnet achieves a greater depth of reach than a permanent plate, so it captures metals even from deep layers of material — the reach extends up to 400 mm.

The overband is hung across or along the belt and continuously discharges the captured metal off the conveyor with its own short belt. Choose the electro version for thick layers and large pieces of iron — recycling, crushers, coal, aggregates; the price is energy consumption, higher weight and the need for a power cabinet. For shallower layers and economical operation, a permanent suspended plate with no power supply is enough. The decisive parameters are the suspension height, belt width, speed and character of the material.

A hump magnet is a hump-shaped pipeline separator in which material falling through a gravity pipe changes direction twice, striking a magnetic plate each time. The two separation stages in series increase the probability of capturing ferromagnetic contaminants even at high throughputs.

The hump magnet is designed for high-capacity gravity conveying of bulk solids — feed, grain, granules, wood material — where a single plate across the full pipe cross-section would miss part of the contaminants. The angled shape also slows and spreads the falling material, so contaminants pass closer to the magnets. It is made with manual cleaning (MC), where the operator swings the plates open and wipes them, and with automatic cleaning (AC) for continuously running plants. When selecting, state the throughput, pipe diameter and character of the material.

An inspection magnet is a hand-held magnetic tool for spot-checking the presence of ferromagnetic contaminants in raw materials and products. It most often takes the form of a magnetic rod with a handle or a tip, which is dipped or pushed into a sample — flour, granulate, feed — and after withdrawal shows what is circulating in the material.

The inspection magnet is the cheapest entry into the magnetic protection of a plant. A quick test reveals metal contamination in a delivered raw material before goods receipt, gives the supplier proof that a complaint is justified, and ongoing sampling downstream of a separator verifies that the installed protection actually works. A find on an inspection magnet is also the best argument for purchasing an in-line separator — it shows the type, quantity and size of the particles the device should be sized for.

A magnetic conveyor is a conveyor with a magnetic system hidden under the belt or a stainless steel deck, which holds ferromagnetic objects in place during transport. Sheets, pressings and swarf therefore do not slip even on steep inclines, and the conveyor can run them at an angle, vertically or even upside down.

Magnetic conveyors solve the handling of parts an ordinary belt cannot manage — sharp, oily swarf from machine tools and presses, small pressings, lids and sheet metal blanks. Magnetic holding saves space (a steep angle instead of a long inclined run) and improves safety, because sharp pieces do not fly off. Unlike a separator, the conveyor's job is not to sort metals but to move them safely; the two functions can, however, be usefully combined in a line.

A magnetic grate is a separator formed by an array of magnetic rods in a frame; the material falls through between the rods and the ferromagnetic contaminants remain captured on their surface. It is installed in hoppers, discharge chutes and bins where bulk material passes by gravity — typically granulate, powders, grain products.

It is made in fixed and telescopic-core versions, with manual or automatic cleaning, or in a housed version for enclosed lines. The rod pitch is a compromise: a denser grate captures more contaminants but slows the flow of poorly flowing materials. Effectiveness is decided by the flux density on the rod surface at the point of flow — quality neodymium grates exceed 10,000 G. For poorly flowing and bridging materials, choose a vibrated version or a different separator design; suitability is verified by a sample test.

A magnetic head roller is a separator that replaces the drive or return pulley of a belt conveyor. It holds ferromagnetic contaminants in the conveyed material against the belt, while the clean product falls freely off the end of the conveyor; the belt carries the captured metal under the roller, where it is released into a separate discharge chute once it leaves the magnetic field.

The roller is an elegant solution where there is no room in the line for another device — separation runs continuously and automatically right in the existing conveyor, unattended and with no space requirement. The difference from a magnetic drum: the drum is a standalone machine with its own drive and housing into which the material is fed, whereas the roller simply replaces the conveyor's existing pulley. Effectiveness depends on the thickness of the material layer on the belt — for deep layers, combine it with a suspended plate magnet above the belt.

The magnetic rod is the basic building block of magnetic separation: a stainless steel tube with neodymium magnetic cores, on whose surface ferromagnetic contaminants are captured from the material flowing past. Quality rods achieve a surface flux density above 10,000 G, top versions above 13,000 G.

It is used on its own in small hoppers and chutes, but more often as a component of grid magnets and housed grate separators. Selection is decided by the flux density on the sleeve surface, not the catalogue value of the core, along with the rod diameter and the temperature resistance of the magnets used. Captured abrasion stays on the rod even as material flows past, but it must be wiped off regularly — manually by withdrawing the core, or by automatic cleaning. A weakening rod is verified by gaussmeter measurement and comparison with the delivery report.

A magnetic sweeper is a mobile magnetic collector of metal debris from floors, roadways and paved areas. Strong permanent magnets in a low-slung body attract nails, screws, wires, swarf and sharp fragments; it is made in hand-pushed versions, as a forklift attachment for the forks, and as a tow-behind unit for vehicles.

The sweeper is above all a plant safety tool — metal fragments on roadways mean punctured forklift tyres, injured employees and metal carried into production on shoe soles. Regularly sweeping warehouse aisles, loading ramps and the surroundings of presses and machine tools significantly reduces the risk; at airports the same principle is used to prevent foreign object damage to aircraft. The collected metal is released from the sweeper by swinging open the scraper plate, with no picking by hand off the magnet.

A magnetizer is a device that magnetizes a ferromagnetic object or a permanent magnet by the brief application of a strong magnetic field. Industrial magnetizers usually work in pulses — a capacitor bank discharges into a coil and the peak field saturates the material to full magnetization.

Magnetizing is used in two ways. Small magnetizers magnetize screwdrivers and bits so they hold screws; industrial pulse systems magnetize permanent magnets only after assembly into the array, which simplifies production — unmagnetized magnets are easier to glue and fit. The magnetizer is the functional counterpart of the demagnetizer: one creates magnetization, the other cancels it with an alternating field of decaying amplitude. A weakened magnet can only be fully restored by a field considerably stronger than its own.

A pipeline magnetic separator is a device inserted directly into a pipe run that captures ferromagnetic contaminants from material conveyed by gravity, pneumatically or by pump — from powders, granulates and liquids. The design differs by medium: cone-shaped bullet magnets for falling and pressure-conveyed material, in-line separators with magnetic rods for liquids.

Pressure versions commonly withstand up to 10 bar; hygienic versions with polished surfaces and quick disassembly for cleaning exist for the food and pharmaceutical industries. When specifying, state the pipe diameter, conveying velocity, pressure and temperature of the medium — it is the flow velocity at the magnet that decides whether the field has time to capture fine particles. Effectiveness on your specific material is verified by a sample test.

A plate magnetic separator is a device with a flat magnetic plate that captures ferromagnetic contaminants from material flowing past it — in a chute, in a pipeline or on a belt conveyor. Unlike a grate, the material does not pass through the plate, so the separator does not slow the flow and there is no risk of clogging.

The plate is the choice for coarser, damp, sticky or poorly flowing materials that would clog the gaps of a grate — mash-type feed mixtures, sawdust, crushed raw materials, but also flour or cereals in chutes. Capture starts at roughly half-millimetre particles; for fine abrasion, a grate with rods in direct contact with the material is more effective. Plates are made with manual and automatic cleaning and in versions with saturation indication, which tells the operators when it is time to clean the separator.

A suspended magnet is a plate-type magnetic separator hung above a belt conveyor, which pulls lump iron out of the layer of material on the belt — bolts, nuts, wires, tool fragments. It is made with permanent magnets or as an electromagnet, and in manual or self-cleaning versions, in which a short dedicated belt carries the captured metal away (overband).

It is deployed in recycling, cement plants, and on crushing and sorting lines — wherever crushers and mills need protecting from lump iron. The decisive parameters are the suspension height above the belt and the thickness of the material layer: effectiveness falls quickly with increasing distance, so the magnet is sized for the specific conveyor. For fine abrasion a suspended magnet is not enough — that is only captured by grates or rods in subsequent separation stages.

A free-fall metal detector is an industrial detector designed for bulk materials in free fall — granules, powders, grain products. The detection coil surrounds a vertical pipe; the moment a falling metal particle disturbs its field, a fast pneumatic diverter deflects the contaminated section of the flow into a waste container within a fraction of a second. Production continues without interruption.

It is typically installed below silos, dryers and dosing units in the plastics and food industries — often as the final check before packaging or the injection moulding machine. It detects all metals, including stainless steel and non-ferrous ones, thus complementing magnetic separation, which removes the ferromagnetic share earlier in the process. Selection is decided by the detection aperture diameter and the sensitivity — stated as the diameter of the test sphere the device can still reliably detect.

An industrial metal detector is a device that electronically reveals metal particles in a material stream, including those a magnet cannot capture — stainless steel, aluminium or brass. It works on the balanced coil principle: a transmitter coil creates an alternating magnetic field, and a metal particle passing through the detection aperture disturbs the balance of the receiver coils, which the system evaluates as a find.

Unlike a separator, the detector does not remove the contaminant itself — the signal triggers a reject mechanism (a diverter flap, valve or belt stop). The proven approach is therefore a combination: the separator continuously removes ferromagnetic abrasion, and the detector at the end of the line inspects the final product. Types are chosen by conveying mode: tunnel detectors for belt conveyors, free-fall detectors for bulk materials, pipeline detectors for liquids and pneumatic conveying.

A tunnel metal detector is an industrial detector with a through aperture, through which products pass on a belt conveyor. The detection coil in the tunnel reveals metal contaminants, including stainless steel and non-ferrous metals, even inside packaged products — unlike a magnetic separator, which cannot capture non-magnetic metals or deal with packaging.

Tunnel detectors are placed at the end of the line as the final check before dispatch, typically for packaged foods, baked goods and meat products. The system automatically rejects a contaminated item with an ejector, or stops the belt and calls the operator; every event is logged for audit purposes. Sensitivity is affected by the tunnel size and the so-called product effect of moist and salty products, which is why the detector is always calibrated to the specific product using certified test pieces.

ATEX refers to the European Directive 2014/34/EU for equipment intended for potentially explosive atmospheres. In plants with combustible dust — mills, silos, pneumatic powder conveying — the areas are divided into zones 20, 21 and 22 according to how often an explosive atmosphere occurs, and every installed device must match, by its category, exactly the zone in which it operates.

Magnetic separators and metal detectors play a double role here: by removing metal particles they reduce the risk of a spark in the process, and they themselves must be in ATEX versions — with an assessment of potential ignition sources, conductive bonding and certification for the declared zone. When enquiring, therefore, always state the zone classification at the installation point and the dust parameters; the manufacturer documents the equipment's classification in the declaration of conformity.

A critical control point (CCP) is a step in food or feed production at which an identified hazard can be specifically eliminated or reduced to an acceptable level — and whose failure no subsequent step can remedy. For each CCP, critical limits, a monitoring method, corrective actions and record-keeping are established.

For metal contamination, the CCP is usually designated as the metal detector at the end of the line, because it is the last device able to demonstrably reject a contaminated product. Magnetic separators in the raw material stream tend to play the role of a preventive measure (oPRP) — they reduce the risk continuously, but without a record for every item. At a CCP the auditor requires evidence: records of regular detector testing with test pieces, procedures for handling finds and operator training. Have the division of roles between magnets and the detector confirmed in the hazard analysis.

HACCP is the system of hazard analysis and critical control points, mandatory for food and feed producers. Within it, metal particles rank among the physical hazards: the producer must assess the risk of their occurrence and put in place control measures that demonstrably manage the contamination. Magnetic separators and metal detectors are the standard technical measures for this purpose.

The equipment alone, however, does not satisfy an audit — the records decide. In concrete terms: define where separation or detection constitutes a control point, set the cleaning procedure and frequency, measure magnetic flux density regularly, test the detectors with reference test pieces and document everything. Missing records are precisely the most frequent finding of IFS and BRC auditors; a demonstrable measurement history is ensured by a regular magnetic separation audit.

IFS Food and BRCGS are international food safety certification standards that retail chains require from their suppliers. Both devote separate chapters to foreign bodies: based on a risk analysis, the producer must implement procedures against physical contamination, including metal, and treat contaminated products as non-conforming.

With magnets and detectors, the auditor wants not just their presence but proof of effectiveness — a register of all magnetic separators, regular measurement of magnet strength with reports, records of detector testing and the evaluation of finds as potential complaints. The weak spot is usually the magnet measurement itself: an internal check without a measuring instrument does not satisfy an audit. An independent magnetic separation audit with a gaussmeter and a final report covers the standard's requirement and reveals weakening cores before the customer finds them. Demand for certification always rises ahead of recertification cycles.