Magnets - Removing Foreign Bodies

Section summary
1. Types of Magnets
2. Extractors
3. Magnet Design and Sizing
4. Position of Magnets
5. Tracking of Foreign Bodies

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 magnetisable materials

Bare magnets can reach the following Gauss levels

Table 1 : Magnetic material

Magnet type Bare Magnet strength
Ferrite 3000 Gauss
Rare Earth (Neodymium NdFeB) >13000 Gauss

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.

2. Extractors

Magnets can be used as such in the process, they will trap material. 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

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

Position Magnetic strength at surface of magnet
Bare magnet >13000 Gauss
Magnet encapsulated in S/S rod >10000 Gauss
On extractor tube >8000 Gauss

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. 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 particles

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

Magnet type Drawing
Static Magnet, gravitary feed - Static Magnet fed by gravity
Rotary magnet, gravity feed - Static Magnet fed by gravity
Static magnet, in-line
Conveying line
Liquid processing - Static Magnet in-line

4. Positions of magnet

Magnets can either be fed by gravity or positionned 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 avdantages

Table 4 : Position of magnets in the process

Magnet place in the process Pros Cons
Gravity Simple installation and check

No pressure inside the magnet, less riks of having leakages of product (specifically for powders

Efficiency a priori higher than in line (material cross the magnet at lower speed)
If multiple product streams, need to have 1 magnet / stream

Requires a dosing equipment to control the flow prior to the magnet

May be necessary to use a rotary magnet for cohesive powders

SAFETY : one must be careful to rotating equipments before and after the magnet
In conveying line Allows to have 1 magnets checking different product streams - savings

Will allow to gain some heights
May be less efficient than magnets fed by gravity since the speed of product will be higher

Suitable for lean phase but not for dense phase conveying

May damage the product being conveyed by impact

Operators to be aware that the magnet inside may be under pressure

It is preferrable 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

Magnets are placed in the process in order to prevent the entry of magnetic foreign bodies in the process, 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

Other related sections

Sifter / Vibrating sieves

Metal Detector

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