What is the difference between a magnetic separator and a metal detector?

A magnetic separator physically removes metal from the product stream. A metal detector detects metal and stops the conveyor or triggers a reject flap but does not automatically remove it. Both technologies are often combined: the magnetic separator proactively removes most particles, while the metal detector serves as a final control and stops the process if metal is detected.

How strong should a magnetic separator be?

The required magnetic field strength depends on the application. For coarse parts such as screws, ferrite systems with around 2,000 to 4,000 Gauss are often sufficient. For food safety and fine iron, neodymium systems with 8,000 to 14,000 Gauss at the active surface are required. Even higher field strengths are needed for stainless steel abrasion.

Can magnetic separators also filter stainless steel?

Yes, but only to a limited extent. Cold-worked or abraded stainless steel becomes slightly magnetic and can be captured by high-gradient neodymium magnets with over 10,000 Gauss. Fully austenitic, soft stainless steel is paramagnetic and usually not captured. Very high field strengths or alternative technologies are required for such applications.

Does a magnetic separator consume electricity?

Permanent magnet separators do not consume electricity for the magnetic field. Electromagnetic separators, used for extremely large volumes or switchable fields, require electrical energy. However, most industrial applications use permanent magnets due to their cost efficiency and reliability.

How do I check the function of my magnetic separator?

Inspection is performed using a teslameter or gaussmeter to measure flux density at the pole surface. Test rods with defined iron spheres are also used to simulate breakaway force. In the food industry, these inspections should be carried out regularly—at least annually—and documented.

What is the difference between ferrite and neodymium magnetic separators?

Ferrite magnets are cost-effective and temperature-stable up to 250°C but have lower field strength. They are suitable for coarse iron. Neodymium magnets, which belong to the rare-earth magnet group, provide extremely high holding forces for the finest particles but are more temperature-sensitive and expensive. They are essential for fine iron and stainless steel abrasion.

How often must a magnetic separator be cleaned?

Cleaning intervals depend on the contamination load. Automatic systems clean continuously. Manual systems must be cleaned before magnet saturation is reached to prevent particle detachment. A saturated magnet loses much of its capture effectiveness. Cleaning intervals should be determined based on the specific application.

What happens if a magnetic separator does not function properly?

If a magnetic separator does not function properly, metal particles can enter the product, leading to quality issues, machine damage, or—in the food industry—safety risks. Regular maintenance and inspection are therefore essential. If problems occur, a specialist should be consulted immediately.

Can magnetic separators also be used for liquids?

Yes, there are special magnetic separators for liquids, known as liquid traps or liquid filters. They are designed for use in pressure pipelines and can remove fine iron from viscous media such as chocolate, syrup, or paints. However, pressure loss in the system must be considered.

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Technical Guide

Magnetic Separator – Definition, working principle and industrial application

Q: Can magnetic separators be used in explosive atmospheres?

Yes, ATEX-compliant magnetic separators are available for use in explosive atmospheres. They must be grounded to prevent static discharge and carry appropriate certification. When selecting, ensure the device is approved for the specific zone classification.

A comprehensive technical guide to magnetic separators. Learn how magnetic separators work, which designs are available and what matters in selection and engineering.

Comprehensive guide

Technical details

Practical applications

Selection criteria

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Magnetic separator diagram - Alexander Magnettechnik

Summary

A magnetic separator, also referred to as a magnetic separator system, is a process engineering component used to remove ferromagnetic foreign bodies from product streams. These devices are essential for machine protection—preventing damage caused by metal parts—and for quality assurance, particularly product purity. As metal separators using magnets, they are primarily used in the food industry, pharmaceutical industry, and recycling industry. Separation efficiency largely depends on the magnetic field strength, the field gradient, and the flow mechanics of the product. Modern magnetic separators use permanent magnets made of neodymium-iron-boron or ferrite to extract ferromagnetic particles such as iron, nickel, and cobalt from bulk materials, liquids, or gases.

Magnetic Separators – Short Definition

A magnetic separator is a technical system that uses permanent or electromagnetic fields to extract magnetically conductive particles—usually iron metals—from a non-magnetic carrier medium such as bulk solids, liquids, or gases. It functions as a filter medium that selectively acts on the magnetic susceptibility of particles contained in the material flow. The operating principle is based on magnetic separation, in which ferromagnetic materials are attracted by an inhomogeneous magnetic field and removed from the product stream.

Industrieller Kontext

In an industrial context, a primary distinction is made between machine protection—removing coarse parts such as screws or nails—and quality assurance, which focuses on removing the finest iron abrasion or paramagnetic stainless-steel particles. While machine-protection magnetic separators primarily capture large metal parts that could cause machine damage, quality-assurance magnetic separators are designed to remove the smallest particles that could impair product quality.

History and Development

The use of magnetic forces to separate materials has a long history. As early as antiquity, people used natural magnets, known as lodestones, to separate iron-containing materials. Modern magnetic separation, however, only developed with industrialization in the 19th century. The first industrial magnetic separators were used in the mining industry to separate iron ore from rock. With the development of strong permanent magnets made of Alnico in the 1930s, magnetic separators became interesting for other industries as well. The introduction of ferrite magnets in the 1950s made magnetic separators more cost-effective and widely available. The breakthrough came with the development of neodymium-iron-boron (NdFeB) magnets in the 1980s, which offer up to five times the magnetic field strength of ferrite magnets. This development enabled the separation of very small particles and weakly magnetic materials such as stainless steel.

Functionality and Physical Principles

The function of a magnetic separator is not based solely on magnetic attraction, but on the interaction of several physical quantities.

To successfully separate a particle from a product stream, the magnetic force must be greater than the sum of opposing forces, including gravity, flow resistance, friction, and inertia. This complex interaction of various forces makes the design of magnetic separators a demanding engineering task.

The Physics of Separation

The magnetic force acting on a particle is described by the following relationship:

𝐹 𝑚 = 𝑉 \cdot 𝜒 \cdot 𝐻 \cdot 𝑑 𝐵 𝑑 𝑥

Wobei:

V: Volume of the particle
χ: Magnetic susceptibility of the material
H: Magnetic field strength
dB/dx: Field gradient (change in flux density over distance)

Important for users: When purchasing, attention is often paid only to the flux density measured in Tesla or Gauss at the surface. For long-range effectiveness, i.e., the capture depth, however, the field gradient is decisive. A magnetic separator requires a strongly inhomogeneous field to attract particles. A homogeneous field would only align the particle but not move it. The field gradient describes how quickly the magnetic field strength decreases with increasing distance from the magnet. A steep gradient means the field weakens rapidly, which can be problematic for separating particles at greater distances.

The Separation Process

Entry

The contaminated material enters the effective range of the magnetic field. The particles move through the system with the product stream. The speed and direction of the product stream significantly influence the efficiency of separation.

Capture

Ferromagnetic particles are accelerated toward the magnetic poles by the Lorentz force and magnetic attraction. The strength of this attraction depends on the magnetic field strength, the field gradient, and the magnetic properties of the particle.

Retention

The particles adhere to the pole surface or extractor tube and must be held there against the flow force of the downstream product. This requires a sufficiently high magnetic holding force that must be greater than the acting flow forces.

Discharge

The separated metal parts are removed from the system either manually or automatically, for example by a scraper system. Modern systems feature automatic cleaning cycles that enable continuous operation.

Magnetic Properties of Materials

Not all metals are captured by magnetic separators. Magnetic susceptibility determines how strongly a material reacts to a magnetic field. Ferromagnetic materials such as iron, nickel, and cobalt are strongly attracted. Paramagnetic materials such as aluminum and platinum are weakly attracted, while diamagnetic materials such as copper and zinc are even slightly repelled. Austenitic stainless steel is normally paramagnetic but can develop ferromagnetic properties through cold working or abrasion.

Types of Magnetic Separators Compared

The design of the magnetic separator is determined by the conveying medium (dry or liquid), the conveying method (free-fall, pressure, or belt), and the required level of purity.

There are various designs of magnetic separators, each optimized for specific applications. Selecting the right type depends on numerous factors, including the type of material to be processed, particle size, throughput, and purity requirements.

Type	Operating Principle	Typical Application	Advantages	Disadvantages
Tube Magnet / Grid Magnet	Grid of magnetic rods in the product stream flowing by gravity. The magnetic rods are arranged in a staggered pattern to ensure maximum contact surface.	Powders, granulates, grain, plastics, free-flowing bulk materials.	High field strength in the contact area, easy to retrofit, cost-effective, maintenance-friendly.	Risk of bridging with poorly flowing products, limited capture depth.
Overband Magnet	Magnet block above a conveyor belt pulls ferromagnetic parts upward out of the material stream.	Recycling, mining, wood chips, waste incineration, coarse bulk materials on conveyor belts.	Continuous cleaning possible, no product congestion, high capture depth, suitable for large throughputs.	Less efficient for very fine particles below the layer thickness, higher space requirement.
Plate Magnet	Flat magnetic plate installed in chutes or under conveyor belts to attract ferromagnetic particles.	Coarse bulk materials, fibrous materials, materials with high flow velocity.	No flow resistance, no clogging, wear-resistant, easy to install.	Lower field strength deep in the flow compared to grids, limited separation performance for fine particles.
Drum Magnet	A magnetic drum has a rotating shell around a stationary magnetic core. Material is conveyed over the drum, with ferromagnetic particles adhering to the drum until they are discharged outside the magnetic field.	Recycling, shredder material, food processing, continuous high-throughput processes.	Fully automatic, continuous separation, high efficiency, suitable for large volumes.	More complex installation, moving parts require maintenance, higher acquisition costs.
Liquid Filter (Liquid Trap)	Magnetic rods inside a closed pressure vessel through which the liquid flows.	Chocolate, syrup, paints, cooling water, viscous liquids, pressure pipelines.	Removes fine iron from viscous media, suitable for pressure applications, high separation efficiency.	Pressure loss in the system must be considered, cleaning requires downtime, higher complexity.
Rotary Magnetic Separator	The magnetic rod cage rotates inside the housing to loosen the material while simultaneously capturing iron particles.	Poorly flowing powders, sticky materials, products prone to bridging.	Prevents bridging, improves material flow, continuous cleaning possible.	Moving parts, higher energy consumption, more complex design.

Expert tip: For poorly flowing powders prone to bridging, rotary magnetic separators are recommended. Here, the magnetic rod cage rotates inside the housing to loosen the powder while capturing iron. This design is particularly suitable for moist or sticky materials that would cause blockages in conventional grid magnets.

Comparison of Magnet Materials

Choosing the right magnet material is crucial for the performance of a magnetic separator

Various magnet materials are used in magnetic separators. Each material has specific properties that make it suitable for certain applications.

Ferrite Magnets (Ceramic)

Ferrite magnets, also known as ceramic magnets, are the most cost-effective option for magnetic separators. They are made of barium or strontium ferrite and offer good temperature stability.

Eigenschaften:

Low acquisition cost
Temperature-resistant up to 250°C
Corrosion-resistant
Suitable for coarse iron and large particles
Lower field strength than neodymium magnets
Larger dimensions required for equivalent performance

Anwendungen:

Coarse metal parts, screws, nails, recycling applications, high-temperature applications

Neodymium Magnets (NdFeB)

Neodymium-iron-boron magnets belong to the group of rare-earth magnets and offer the highest available magnetic field strength. They are up to five times stronger than ferrite magnets.

Eigenschaften:

Extremely high field strength (up to 14,000 Gauss)
Compact design possible
Standard temperature range up to 80°C
Special variants available up to 200°C
Essential for fine iron and stainless steel
More temperature-sensitive than ferrite
Corrosion protection required

Anwendungen:

Fine iron particles, stainless steel abrasion, food industry, pharmaceutical industry, applications with the highest purity requirements

Samarium-Cobalt Magnets (SmCo)

Samarium-cobalt magnets offer high field strength with excellent temperature stability but are significantly more expensive than neodymium magnets.

Eigenschaften:

High field strength similar to neodymium
Temperature-resistant up to 350°C
Very good corrosion resistance
Higher cost than neodymium
Less widely available

Anwendungen:

High-temperature applications, corrosive environments, specialized industrial applications

Fields of Application

Selection Criteria & Technical Specifications

When designing a magnetic separator—also known as sizing—engineers must consider numerous parameters

Correctly selecting a magnetic separator is crucial for application success. Incorrect sizing can result in insufficient separation performance, product quality issues, or even machine damage.

Magnet Material

Choosing the magnet material is one of the most important decisions when designing a magnetic separator.

Ferrite (Ceramic)

Ferrite magnets are inexpensive, provide a deep magnetic field, and are temperature-resistant up to 250°C. They are well suited for coarse iron and large particles but are insufficient for fine particles or stainless steel.

Neodymium (NdFeB)

Neodymium magnets are up to five times stronger than ferrite magnets and are essential for fine iron and weakly magnetic stainless steels. The standard temperature range is up to 80°C, with special variants available up to 200°C. However, corrosion protection is required.

Pole Spacing & Rod Diameter

The distance between magnetic poles and the diameter of the magnetic rods significantly influence separation performance. Fine powders require thin rods with close spacing to achieve a high field gradient. Coarse chunks such as ore or coal require large block magnets with deep fields capable of capturing particles at greater distances.

Throughput & Flow Velocity

Product flow velocity is a critical factor. If the product flows too fast—for example, over 2 meters per second in free fall—the magnetic holding force may be overcome. This phenomenon is known as the wash-off effect. In such cases, baffle plates or step magnets are required to reduce speed and increase dwell time in the magnetic field.

Product Properties

Moisture and viscosity influence bridging and material flow. Wet or sticky materials require special designs such as rotary magnetic separators.
Material abrasiveness requires hardened surfaces or coatings such as tungsten carbide on magnet tubes to minimize wear.
Particle size determines the required magnetic field strength and field gradient. Very fine particles require high-strength neodymium magnets.
Product temperature influences the choice of magnet material. High temperatures require ferrite magnets or special high-temperature neodymium variants.
pH value and chemical composition can affect magnet corrosion resistance and require appropriate protective measures.

Installation Location and Space Constraints

Available space and position in the process chain influence the choice of magnetic separator. End-of-line installations ensure the highest product purity, while in-line installations improve machine protection.

Cleaning Requirements

Cleaning frequency and method influence the choice between manual, semi-automatic, and fully automatic systems. Continuous processes require automatic cleaning systems.

Installation, Operation & Maintenance

Correct installation and regular maintenance are crucial for the efficiency and service life of a magnetic separator

A magnetic separator can only function optimally if it is correctly installed, operated, and maintained. Installation errors can lead to significant performance losses.

Installation

Install the magnetic separator as close as possible to the end of the process chain—also referred to as end-of-line—to ensure final product purity. This ensures that all contaminants generated throughout the process are captured.
Avoid direct mounting on heavily vibrating machines, as vibrations can promote detachment of fine iron particles. Use vibration dampers or flexible connections if necessary.
Warning: Ferromagnetic components in close proximity can short-circuit the magnetic field—a phenomenon known as shunting. Maintain minimum clearances of at least 50 millimeters from ferromagnetic materials.
Ensure that the magnetic separator is easily accessible for maintenance and cleaning. This significantly reduces downtime.
When installing in pressure pipelines, pressure loss must be considered. Consult the manufacturer’s technical data.

Maintenance & Cleaning

A magnetic separator is only as good as its maintenance condition. A saturated magnet—one fully covered with iron particles—loses much of its capture effectiveness. Cleaning intervals depend on the contamination load and should be regularly reviewed.

Manual Cleaning

During manual cleaning, iron particles are removed by hand. This requires system downtime. Observe occupational safety: There is a risk of cuts from sharp metal shavings. Wear appropriate protective gloves.

Easy-Clean / Quick-Clean

With Easy-Clean or Quick-Clean systems, the sleeves remain in the product stream while the magnet cores can be pulled out. The iron particles then fall off without fully dismantling the system. This significantly reduces downtime.

Automatic Cleaning

Pneumatic or hydraulic cycles automatically clean the magnet at defined intervals, for example during 24/7 operation. These systems enable continuous operation without manual intervention.

Common Errors and How to Avoid Them

Demagnetization: Heat—for example, welding on the housing without first removing the magnets—or strong impacts can demagnetize magnets. Avoid temperatures above the Curie temperature of the magnet material.
Wear: Worn-through sleeves in abrasive media can cause the product to contact the magnets and become contaminated, or magnet material to enter the product. Regular inspection and timely replacement of worn components are essential.
Contamination: A contaminated magnet loses effectiveness. Ensure cleaning intervals are adhered to.
Incorrect installation: An incorrectly installed magnetic separator cannot function optimally. Follow the manufacturer’s installation instructions precisely.

Regular Inspection

Regular inspections should be carried out to detect wear early. Check magnetic field strength using a teslameter or gaussmeter, inspect sleeves for wear, and ensure all seals are intact.

Advantages & Limitations

Magnetic separators offer numerous advantages but also have certain limitations that must be considered

Advantages of Magnetic Separators

Cost efficiency: Permanent magnet separators consume no ongoing energy costs for the magnetic field, unlike electromagnetic systems. This makes them particularly economical for continuous processes.
Durability: With proper handling, neodymium magnets lose less than one percent of their charge per ten years, resulting in a very long service life with minimal performance loss.
Safety: Magnetic separators prevent costly machine downtime caused by metal particles and significantly reduce the risk of product recalls. In the food industry, they can even be life-saving.
Environmental friendliness: Since no energy is required for the magnetic field, permanent magnet separators have a very low CO2 footprint.
Ease of maintenance: Modern magnetic separators are designed to be maintenance-friendly and require minimal servicing.
Versatility: Magnetic separators can be used in a wide range of applications, from dry bulk solids to viscous liquids.

Limitations and Restrictions

Non-magnetic metals: Copper, aluminum, brass, and other non-ferromagnetic metals are not captured by standard magnetic separators. Eddy current separators or all-metal detectors are required for these applications.
Stainless steel: Austenitic stainless steel, such as V2A, is paramagnetic and difficult to capture. This requires extremely high field strengths above 12,000 Gauss, achievable only with neodymium magnets. Even then, separation performance for fully austenitic stainless steel is limited.
Temperature limits: Standard neodymium magnets lose magnetization at temperatures above 80°C. Special variants or ferrite magnets are required for high-temperature applications.
Particle size: Very small particles below 10 micrometers can be difficult to capture and require special high-gradient magnets.
Product velocity: At very high flow velocities, magnetic holding force can be overcome, resulting in a wash-off effect.

Safety and Standards Requirements

Compliance with industry standards is essential for the safe and lawful operation of magnetic separators

Magnetic separators must comply with numerous standards and regulations that vary depending on the application. Adherence to these standards is not only legally required but also essential for the safety of personnel and consumers.

Machinery Directive 2006/42/EC

The European Machinery Directive governs the general safety of machinery. Magnetic separators must meet the requirements of this directive and bear CE marking.

ATEX Directive 2014/34/EU

For use in potentially explosive atmospheres, such as dust explosion hazard environments or near flammable gases, magnetic separators must be ATEX-compliant. They must be grounded to prevent static discharge and carry appropriate certification.

Food Contact

FDA CFR 21: Materials that come into contact with food must comply with the requirements of the U.S. Food and Drug Administration. This includes the use of approved materials and documentation of compliance.
EC 1935/2004: The European regulation on materials in contact with food governs requirements for materials that come into direct or indirect contact with food. Magnetic separators for the food industry must comply with these requirements.

HACCP (Hazard Analysis and Critical Control Points)

In the food industry, magnetic separators must be documented as Critical Control Points (CCP) within HACCP systems and regularly validated. This includes annual inspections using a gaussmeter to verify magnetic field strength and documentation of all maintenance and cleaning activities.

RoHS Directive

The RoHS Directive (Restriction of Hazardous Substances) restricts the use of certain hazardous substances in electrical and electronic equipment. While permanent magnet separators contain no electrical components, the materials used must still be RoHS-compliant.

ISO 9001

Many manufacturers of magnetic separators are ISO 9001 certified, ensuring continuous quality assurance. This is an important quality indicator when selecting a supplier.

Comparison with Alternative Technologies

Nachteile:

Very expensive, requires radiation protection, complex maintenance

Economic Efficiency and Cost-Benefit Analysis

Investment in a magnetic separator is generally very economical

The cost of a magnetic separator is usually very low compared to potential damage caused by metal particles. A cost-benefit analysis should consider several factors.

Acquisition Costs

Acquisition costs vary depending on the type and size of the magnetic separator. Simple tube magnets start at a few hundred euros, while complex automatic systems can cost several thousand euros.

Operating Costs

Permanent magnet separators have virtually no operating costs, as they require no energy for the magnetic field. Only maintenance and cleaning costs apply, which are minimal.

Savings Potential

Savings from a magnetic separator can be substantial: prevention of machine damage, reduction of product recalls, improvement of product quality, and increased customer satisfaction. In the food industry, the cost of a product recall can amount to millions of euros.

Payback Period

The payback period of a magnetic separator is usually very short—often less than one year—especially in critical applications such as the food industry.

Weiterführende Fachartikel

Vertiefende Informationen zu speziellen industriellen Trennprozessen.

Metallsortierung

Eignung, Grenzen und typische Einbausituationen in industriellen Trennprozessen.

Frequently Asked Questions

Answers to the most important questions about our products and their applications.

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