Industrial Magnets - Removing Foreign Bodies in bulk powders and solids
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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|
|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.
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
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
|Static Magnet, gravitary feed|
|Rotary magnet, gravity feed|
|Static magnet, in-line
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
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
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
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