Industrial magnets are used in process industries to trap foreign body materials (metal contaminants such as tramp metals). This page is focusing on inline magnets for powder or liquid processes, such as inline magnets for food processing.

1. Types of Magnets

Powerful magnets are made of 2 families : ferrite magnets and rare earth magnet. The strength of a magnet is expressed in Gauss, the higher the Gauss, the stronger will be the magnet and higher will be the chances of catching magnetizable materials. Neodymium magnets are typically strong magnets.

What is the magnetic strength of magnetic material ?

Bare magnets can reach the following Gauss levels

Table 1 : Magnetic material

It is important to notice that these values are those of the bare magnets. Actually, such magnets are then encapsulated in a Stainless Steel tubes (in the case of magnet rods, a convenient magnet type for process industries). The Gauss value at the surface of the tube is therefore lower than the bare magnet. For NdFeB, the strength will drop around 10000 Gauss. If the magnet is equipped with an extractor, to facilitate the cleaning, it will drop further to 8000-8500 Gauss. The drawing below is giving an illustration of the phenomena.

How to measure the strength of a magnet ?

To measure the magnetic force of a magnet, it is possible to use an electronic equipment, a Gauss Meter, which will directly measure the magnetic field strength, or a mechanical equipment, like a pull strength test which will convert the strength necessary to remove a probe from a magnet into a magnetic force.

2. Extractors (easy cleaning)

Magnets can be used as such in the process, they will trap tramp metals. However, the attraction force is so strong that it may be difficult to clean the magnet. To overcome this issue, manufacturers of magnets have designed some extractors. The magnet is actually sliding in such extractor. Separating the extractor from the magnet is sufficient to let the metal trap falls in a place where it can be collected. This design of magnetic trap is especially useful in food industries where collecting the foreign body helps to understand its origin.

The extractor has as a consequence to cause the magnetic strength drop further, it will reach around 8000 Gauss at the surface of the extractor in contact with the product, it is still a considerable magnetic strength

Table 2 : The different level of magnetic strength on magnet bar

Figure 1 : The different level of magnetic strength on a magnet bar

3. Magnet design and sizing

The most practical magnet type for the Dry Processes industries is the multi-rods magnets (grate magnets), very often used as magnetic trap in food industry.  This kind of magnets is sometimes also referred as grate magnets. Having rods allows generally to let the powder pass through and have a large surface area in contact with the product, which facilitates the removal of metal scraps.

Rods should not be spaced with too large pitch, otherwise some places of the open areas will not be reached by the magnetic field which is decreasing quickly when going away from the magnet. A pitch of 50 mm, center/center, appears to be a maximum

There is no formula to assess the maximum flow that a magnet can accomodate. For free flowing powders (with d~0.5), 5/6 t/h can be reached. More cohesive powders may not go through a static magnet and a rotary model may be considered

Schematic representation of the most common magnet types are given below for reference

Table 3 : The different design of industrial magnets

4. Positions of magnet

Magnets can either be fed by gravity or positioned in a pneumatic conveying line. The most common way to proceed is to have the magnet fed by gravity but having the magnet in a conveying line can have some advantages :

Table 4 : Position of magnets in the process

It is preferable to use magnets to control the raw materials when they enter the process. They are particularly used at tipping stations.

5. Tracking of Foreign Bodies

How are magnet used to remove foreign body / metal contaminants ?

Magnets are placed in the process in order to prevent the entry of magnetic foreign bodies in the process such as tramp metals, thus guaranteeing the quality of the final product. However, it is important for the plant operator to control at very regular intervals the magnets in order to assess if foreign bodies have been trapped. 1st reason is that, at a certain extent, trapped foreign bodies will prevent a good flow of product and can even be released to the process, 2nd because the plant operator must be aware of any sign of contamination by foreign bodies.

For this purpose, it is necessary to record precisely the findings on every magnets. A factory should have in place quality controls asking to check the magnets every x hours - typically once per shift - and tables recording the type of findings and the weight. Trending those data allows to easily recognize quality incidents at a supplier, and then feedback to him, or in the own operator process, thus triggering maintenance activities to find out the source of the foreign bodies.

Extractors on magnets make it easy to clean the magnet, collect the foreign bodies, and track them.

Figure 2 : Principle of magnet cleaning

6. Magnets manufacturing companies

When buying a magnet for industrial purposes, it is crucial to contact a company that has a long experience in producing strong magnets. As mentionned above, the design of powerful magnets for protecting industrial processes require specific care in the design, from the selection of strong magnet, typically Neodymium, to a proper arrangement of the magnets in rods, implementation of extractors for easy cleaning. Always ask for references and question your commercial contacts thanks to the infos in this page, if they can show good experience and answer your questions, then you have more chances to be trustful !

Neodymium magnet supplier :

• Lenoir magnetic systems (raoul-lenoir.com)

(Note that PowderProcess.net has no link with this company)

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