The problem with stray metal
Loose pieces of metal turn up in product streams more often than people expect. Bolts shake free of equipment, wire and fasteners come in with raw materials, and worn parts shed fragments. This is usually called tramp metal. In a food, pharmaceutical or chemical line it can contaminate product. In a grinder, crusher or pump it can cause serious damage. Plants therefore often remove metal early in the process and check again near the end.
Two families of equipment do the job. Magnetic separators physically pull out magnetic metal. Metal detectors sense metal of various types and trigger some kind of rejection, usually without removing it by magnetism.
How magnetic separators work
A magnet attracts ferrous material, meaning iron and many steels. A separator places a magnetic field where product passes. Stray ferrous pieces are held while the product carries on. The magnets are usually permanent, built from strong rare-earth or ceramic materials, though some large units use electromagnets.
Their ability to hold metal depends on magnet strength, how close product passes to the magnet, how fast it moves, how deep the layer is, and the size and shape of the metal pieces. Fine ferrous particles are harder to catch than larger lumps. Very fine or weakly magnetic particles may need stronger magnets or repeated exposure.
Common styles
- Grate and grid magnets. Rows of magnetic tubes sit in a chute, hopper or duct. Free-flowing dry products fall past them, and metal sticks to the tubes. They are common in powders and granules, and in many food and plastic lines.
- Plate magnets. A flat magnet is mounted in a chute or above a flow, suited to lumps and larger volumes.
- Drum separators. A rotating drum with a fixed magnet inside carries ferrous pieces out of the stream and releases them into a separate path. They allow continuous self-cleaning.
- Overband (suspended) magnets. A belt moves across a magnet suspended above a conveyor, lifting tramp iron off and discarding it to the side. They are common over conveyors in mining, recycling and aggregates, and are sometimes placed ahead of crushers to protect them.
- Pulley magnets. A magnetic head pulley on a conveyor holds ferrous metal on the belt longer so it drops at a different point.
- Liquid-line magnets. Magnetic bars or filters installed in pipes protect pumps and capture fine ferrous particles from liquids and slurries, sometimes alongside line strainers.
Material often matters: stainless steel housings are typical in food and hygienic environments. Some stainless steel alloys are weakly magnetic or not magnetic at all, so a magnet will not always catch them.
What magnets miss, and where detectors help
Magnets only attract certain metals. Aluminum, copper, brass and many types of stainless steel are not strongly magnetic, so they pass straight through a typical magnetic separator. Metal detectors fill this gap. They generate an electromagnetic field and sense disturbances caused by metal passing through. Many can detect ferrous, non-ferrous and some stainless metals, though sensitivity differs among metal types and depends on product characteristics. Wet, salty or conductive products can create signals that mimic metal (“product effect”), and detectors are tuned to account for this.
When a detector finds metal, a reject system acts, such as a diverter, air blast, push-arm or conveyor stop. Detection systems are generally checked using test pieces at set intervals as part of a quality system. Some operations also use X-ray inspection systems, which can find non-metal foreign bodies like dense glass or stone and are covered more under inspection sensors.
Where to place them
Position matters. Common practice is to put magnets before equipment that is vulnerable, such as crushers and mills, and at points where product is handled before packing. A detector often sits at the end of the line, so it can catch contamination from the line itself. The product needs to pass the magnet in a thin, evenly spread layer; a thick stream can hide metal in the middle of the flow. Slowing the flow or adding a vibrating feeder can help.
Cleaning and checking
Magnets do their work by collecting metal, so they need regular cleaning or they stop being effective. Manual types are cleaned by hand at set intervals. Self-cleaning designs use belts, drums or release mechanisms. Cleaning frequency depends on how much metal arrives, and any metal recovered is useful evidence: a sudden increase may point to wear or failure upstream, and it makes a good starting point for root cause analysis.
Magnet strength can decline over time, especially with heat, impacts or age, and should be tested as the manufacturer recommends. Quality records matter in regulated industries, and manufacturing quality control programs often specify checks and documentation.
Safety
Strong magnets can pinch fingers, pull tools toward them and interfere with some medical implants and electronic devices. Manufacturer instructions should be followed for handling and removal. Use lockout and tagout style procedures in general terms when working near moving equipment, and have trained, certified personnel do the work. Local regulations and site quality rules apply. This article is general information, not engineering advice.