Removing metal contaminants in powder and bulk solids materials
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Permanent magnets are used in process industries to separate foreign bodies from the bulk of good products. Foreign bodies, typically metallic parts like bolds, screws or just some metal shavings, can be detrimental to the process, by damaging (rotating) equipment, or to the final product. Implementing magnetic separation at different points of the process is an efficient solution to detect the presence of a metallic foreign body, remove it, or at least isolate a production suspected with contamination.
The article focuses on bulk solids handling and foreign matter control, but the notions introduced on magnets are still perfectly valid in liquid phase.
A foreign body is a solid piece of material of different nature than the product being processed. It can be anything from a plastic pen, to jewellery, to metallic pieces coming from an equipment that suffered a breakdown and released parts in the product flow (bolts, nuts, metal shavings if a rotating part has scratched a static one).
Foreign matters can have dire consequences for a factory. There
presence in the product being manufactured can lead to :
- Further mechanical damages : for example if a bolt gets caught in an Airlock rotary Valve the rotor tip may be damaged and scratch on the stator, leading to the breakdown of the valve, or even to the generation of metal shavings that constitute other foreign bodies
- Safety concerns for the process : the introduction of a metal part in a high speed rotating equipment like a Mill can lead to sparks and to dust explosion if the conditions are met (see ATEX)
- Safety concerns for the consumer : if the product is sensitive, like food or pharmaceuticals, the presence of a foreign body can endanger the health of the consumer (ingestion of a metal part for example)
Magnetic separation of metal contaminants can be used to protect the process, the product and the consumer. A good understanding of magnet designs and magnet efficiency is necessary to efficiently select and use those equipment. Magnets are a central layer of defense for foreign bodies removal in process industries.
Many permanent magnet design exists, however, only few of them are really useful in process industries
|Grate Magnets / Drawer Magnets||This is one of the most widespread efficient design for magnetic separation. Several rods of magnets are disposed side by side, often with 2 layers. Product is free falling to the magnet. The denomation "drawer" means that the magnetic rods can easily be removed by pulling on them, like a drawer.|
|Rotary Magnets / Drum Magnets||Drum magnets are made of magnetic rods disposed in concentric circles. The magnet is rotating and is supplied by material falling through it by gravity. Rotating magnets are often used to overcome bridging of powder over the rods. They have also generally a higher efficiency than static drawer magnets.|
|Bullet magnets / inline magnets||Metal separation is often carried out in the powder free falling by gravity (see drawer magnets, rotary magnets) but it can be necessary to perform the metal detection in line a pneumatic conveying pipe. For this purpose, manufacturers have developped "bullet" magnets where a large magnetic rod (bullet) is positionned at the center of the pipe, the material having to flow around it.|
|Plate magnets (chute magnets)||Magnets can also be designed as plates. They are sometimes used prior to some machines (grinders especially) and often are in 2 parts taking a Z shape. The objective here being to force the product to flow as a thin layer and increase the area of contact with the plates.|
Magnetic separation can be used on powder / solids raw materials or on semi-finished product at any point of the process.
For a magnet to actually separate metal contaminants, it must be designed to generate a strong magnetic field. The 1st design criteria is therefore the nature of the magnetic material. The material should be in rare earth and most specifically in Neodymium (NdFeB). Such material can generate up to 13500 Gauss (bare magnet), much higher than ferrite magnets (3000 Gauss) that should not be recommended for sensitive process industries.
The second important design criteria is the geometry of the magnet. Magnetic field decreases very quickly with the distance from the magnet. Therefore the magnet must be designed to ensure that the product flow remains very close to the magnet. For drawer magnets for example, it is achieved by having several tubes aligned closely (typically 50 mm center / center) and over 2 layers. Thus way it is ensured that the product will actually encounter a magnetic field strong enough to separate metals.
The combination of the magnet strengh and the geometry will result in
a certain efficiency for the magnet. This efficiency is not the same
for every metals since it will depend on the magnetic properties of
the material to be separated. In a properly designed and operated
static magnet, the following efficiencies should be expected :
- Carbon Steel : 100%
- Stainless Steel 304 : min 85%
- Stainless Steel 316 : min 70%
Efficiencies with a rotating magnet are usually higher ; with an inline magnet (conveying line), it can be lower.
It may be required to validate the strength of a magnet for compliance reasons, or to follow a potential loss of strength over time (especially if the magnet is operated at high temperature).
2 types of tests are commonly admitted : the pull test, in which the force required to remove a piece of metal from the magnet is measured ; and the use of a Gauss meter allowing to read the value of the magnet field.
Magnetic filtering will allow to catch different contaminants in bulk solids industries, removing foreign bodies.
Magnets are a key components in a powder process to ensure reliability, safety of the installation and safety of the product. Magnetic separators allow to remove metal contaminants from bulk solids flows. Different design exist including rotary magnets or magnet drawers easily accessible for cleaning.
Please follow the link to get access to magnet theory and design details : All you need to know on industrial magnets for powder checking
|Powder properties||Unit operations||Equipment handbook|
Flow of solids
Mass and funnel flow silos
Particle Size Distribution (PSD)
Dosing and weighing
Solid Gas Separation
Grinding and Milling
Solid Liquid Suspension
Checking (sieve and magnets)
|Big Bag Tipping Stations
Airlock rotary Valve
Rotary Valves (pneumatic)