Magnetic Separators in Food Processing

Assessment and removal of metal contaminants is becoming common practice in the food processing industry. Many retailers make metal detection a critical control point (CCP) requirement in their supplier's HACCP plans, to ensure finished products meet established specifications.

Metal pieces in food products pose a safety risk to consumers and can damage processing equipment. Sources of metal contamination include:

  • incoming ingredients (contaminated during processing or transportation of the ingredient)
  • processing equipment (grinding, crushing processes or general abrasion or vibration causing the loss of nuts and bolts etc.)
  • inadequate personnel practices and environmental causes

Selecting a Magnet

Equipment that detects and removes metal contaminants is important in food processing and magnets are one approach that works successfully. Selecting the proper magnetic separator requires an understanding of magnetic properties, product characteristics and the specific environmental factors in each food processing plant.

Magnetic separators are available in a wide range of designs, including: bar, plate, grate, liquid line trap, pneumatic line and suspended.

  • Bar magnets are permanent non-electric magnetic units that can be used in a wide range of applications. They are used to remove ferrous contaminants present in small, shallow quantities of flowing powder, granules, fibres and liquids.
  • Plate magnets are used in the bottom of an inclined chute or suspended above conveyor belts or stainless steel vibratory feeders. They are used to remove ferrous contaminants that occur occasionally in products lines, particularly large pieces of metal, such as nuts, bolts or staples, from dry products.  
  • Grate magnets have magnetic tubes designed in a grid to allow the flow of material to cascade though a grate. They spread magnetic protection through cross-sectioned areas of equipment, such as pipes or hoppers. They can be used to remove fine or relatively large pieces of metal contaminants.
  • Liquid line trap magnets are traps with tube magnets inside them, designed with an inlet port to match existing pipelines. The magnet collects metal pieces like baling wire or staples.
  • Pneumatic line magnets draw metallic pieces from products such as starch, milk powder and flour as they flow through pneumatic lines.
  • Suspended magnets hang above conveyor belts and remove metal fragments from the material as they pass under the magnet. They remove large pieces of metal and can protect equipment, such as crushers, from damage.

Magnet Materials

  • Alnico magnets are made from aluminium, nickel cobalt and iron. They are economical magnet sources used in applications that have high temperatures (>204 °C). Alnico is comparable in strength to ceramics and is used to remove relatively large metal pieces, such as bolts or nuts.
  • Ceramic magnets are low-cost and made from a composite of iron oxide and barium/strontium carbonate. They are used to remove relatively large pieces of ferrous metal such as nuts, bolts, nails and other metal objects of that size.
  • Rare earth magnets generate an extremely strong magnetic field, allowing them to remove fine or weakly magnetic contamination such as rust or work-hardened stainless steel from product flow. Extensively used by the food industry, rare earth magnet types include:
    • samarium-cobalt magnets are more expensive and have a weaker magnetic field. They perform well in corrosive or high temperature environments.
    • neodymium magnets are second generation rare-earth magnets made from neodymium, iron and small amounts of boron. They are the most powerful and most  affordable of the rare-earth magnets.

Factors Affecting Magnet Performance

There are several factors that can affect the effectiveness of a magnet's performance:

  • Temperature: Magnetic materials lose strength when exposed to elevated temperatures. When magnets are heated beyond certain temperatures (which depend on their specific material), they lose strength that cannot be recovered by cooling. When using magnets in a process that involves high temperatures, make sure you are using the correct material.
  • Flow characteristics: Many food products exhibit different flow characteristics when damp or moist. For instance, sugar with high moisture content can form large particles that may plug the opening of a magnetic separator. This can stop the product from flowing the tubes of the magnet.
  • Equipment design: The spacing and number of tubes in magnetic equipment affect  the strength of the magnetic field it generates. Closer spacing and more tubes mean a stronger magnetic field and higher efficiency.
  • Product characteristics: The characteristics of the food being processed greatly affect the effectiveness of magnetic separators. Food products can be categorized in three groups: dry, liquid or moist.
    • Dry products — range from small food grains flowing down a chute to large rock-like products moving along high-speed conveyor belts. Each one requires different separation equipment. If the material is small and free-flowing, a grate magnet may be best. Plate magnets do not disturb the flow of the product nor cause it to build up when it cascades down a sloped chute. Suspended magnets work efficiently when dry products are transported on a conveyor belt.
    • Liquid products — a liquid or slurry state require a magnetic trap, either in a grate or plate configuration. Traps are similar to grates, tube magnets are arranged perpendicular to the flow inside an enclosed vessel to trap any ferrous material passing through.
    • Moist products — such as flour or starch do not flow through grate magnets because of product build up. The best option is to use a powerful magnet that is completely out of the product flow. A magnetic rotation system outside the product line eliminates product build up and allows product to flow freely.

Evaluating Magnet Performance

Processors who use magnets should not take them for granted. Magnets can lose strength over time and should be tested at least once a year. Pull tests and the use of Gaussmeters are two types of measurements that evaluate magnets.

Pull Test: The pull test is an easy and repeatable test to evaluate the performance of a magnet. It measures the force required to remove a standardized piece of metal from a magnet, using a spring scale. Commercial test equipment for testing the relative strength of magnet separation equipment can be found on the market and can help monitor the efficiency of a separator by measuring the holding force of a magnet.

Gaussmeter: A Gausmeter provides standard measurement for evaluating a magnet design. However, it is not practical for assessing the relative effectiveness of a magnetic separator. The effectiveness (strength) depends on the magnet material but also the size and weight.

Pull Test Kit
Pull Test Kit

Useful Links

Reference

Stier, R.F. (2005) Control by monitoring. Food quality and safety activities shift from human inspection of individual pieces to automated, statistically driven monitoring and improved control. Baking and Snack, 1:57-60.

For more information, email the CVO/Food Safety Knowledge Centre or call 204-795-8418 in Winnipeg.