Tales of a bulk-solids-handling engineer…
If you have ever been involved in a bulk-solids-handling process in your plant, you have surely come across the issue of metal contamination inside your incoming raw materials. Regardless of whether your product is limestone, cocoa powder or recycled PET flakes, you really cannot afford to miss this article.
Everything you always wanted to know about Magnetic Separators but were afraid to ask
Magnetic separators are “machines” designed to catch foreign ferrous particles found inside your processing materials. The performance of these magnets depends on how powerful they are and how close you can place them inside your bulk solids flowing materials (without blocking their free flow).
The backbone of choosing a magnetic separator always starts with the principle question any 10-year-old could ask you about a permanent magnet: Ferrite or Neodymium? Ferrite is a low-cost solution with medium strength, whereas neodymium is expensive but 5 times more effective. Depending on the value of your end-product and on the cost of potential damage, both solutions will get you through.
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Potential damages from iron impurities in your process
Receiving raw materials has always been a challenge. Suppliers from all over the globe with different quality standards (but very low prices) can give you a headache in no time: Non-flowing raw materials, foreign particles, agglomerates, etc. Speaking of powders and granules in bulk bags, 25-kg bags or silo-trucks, one cannot be too careful.
Damages of iron particles bigger than 1mm in your production line might be visible at your screw conveyor with dents and scratches. Your Z-type-bucket elevator could get derailed or have a blockage and get several plastic buckets crashed. Moving on to your hammer mill rotating at 1500 rpm you can sense serious mechanical damage coming. No need to talk about your expensive extruder lines…
“Remember that metal impurities are not only imported by your exotic raw materials received, but they might also be produced by the malfunctioning of your own equipment”
But smaller iron particles, not cleared away properly, can also damage your process. If they are smaller than 1mm, they might not be able to cause any mechanical damage, but they might jeopardise your quality.
If you are active in the food or pharma sector, imagine the impact of having flour, oregano or baby food sold at your local store with minor iron particles inside! It is obvious that you should protect your production line by removing unwanted metal contaminants. Let’s focus on choosing the right type of magnetic separator for that.
Types of magnetic separators available (permanent magnets)
We will be focusing on bulk solids handling plants producing powders for sectors like food, pharma, fertiliser and chemical, cement and building materials, plastic pellets, etc. The idea here is to remove all iron impurities. (On the other hand, if we wanted to discuss dedicated recycling units, then the idea would have been to concentrate valuable magnetic material. They both need magnetic separators.)
Grate magnets or magnetic grids are designed for installation inside inlet hoppers. There are plane grids or grids with removable boxes, circular or rectangular grids, just to name a few. It is a low-budget solution.
The operator throws them inside the hopper. In order to clean it, the operator has to bend down, take the magnetic grid out of the hopper at the outer surface, clean it manually and then throw it back in.
To simplify the cleaning process special magnetic bars were designed. These magnetic bars for a down-stream pipeline were designed in a drawer-type box, which makes it easier to be cleaned manually by the operator.
You can see here a bespoke Magnetic Grate Separator we manufactured for oregano. It consists of a hopper, a stirrer, a first stage magnetic drawer with 5+6 magnetic bars of 12’000 Gauss, a second stage magnetic drawer with 5+6 magnetic bars of 12’000 Gauss and, a vibrator with shock absorbers.
Magnetic pipe or “torpedo magnetic separator” is designed to catch the iron particles due to its aerodynamic shape. It can only be installed in a vertical position. The hinged door makes manual cleaning easier.
Similar is the pneumatic magnetic separator that can be installed in-line in a pneumatic conveying pipeline. It combines a torpedo and a straight hump magnet in an air-tight version.
Hump magnets or zig-zag magnets are also popular. The zig-zag magnetic separator in the left picture has 2 permanent mixed fixed outside the 2 hinged doors. The hump magnet in the right picture is very effective. It has 2 magnets fixed on both doors that attract simultaneously the falling material from both sides.
Suspended magnetic plates are frequently installed above belt conveyors. To get good separation results, this powerful magnet should be covering the whole width of the conveyor belt. The first crucial factor is the distance between the magnet and the product. The ideal distance would be just 25 mm!
The second crucial factor is how thick the layer of the product on that conveyor belt is. The third crucial factor is how quickly the belt is moving. Magnetic plates perform better on flat belts than on hollow belts. They can be installed horizontally but also inclined on the side close to the outlet of the belt conveyor. The operator must manually clean this permanent magnet.
“The backbone of choosing a magnetic separator always starts with the principle question any 10-year-old could ask you about a permanent magnet: Ferrite or Neodymium?”
The heavenly blessing of self-cleaned magnetic separators
The magnetic drum or magnetic roller is frequently used with or without housing at a free-flowing product stream. Although the drum is rotating, the semi-cylindrical permanent magnet inside is not rotating, thus ensuring that all ferrous material will be removed at the side. This is a perfect example of an automatic self-cleaned magnetic separator.
The pulley separator is part of a belt conveyor. This special roller drum (pulley) replaces the outlet pulley of the belt conveyor. It is a full permanent magnet, and it rotates driven by the gear motor of the belt conveyor.
You can see a bespoke magnetic pulley separator we manufactured for a light bulb recycling plant. The video below shows how the ferrous parts are being separated.
The over belt magnet is a belt conveyor that has a hidden magnetic plate installed under its moving belt. This separator makes all ferrous particles attracted to the conveyor belt. The conveyor belt conveys the ferrous particles a bit further, and afterwards (when the magnetic plate ends), it releases them to fall to their proper waste bin destination.
The “metal detector and separator” is a more sophisticated machine. The upper part contains a metal detector. The lower part consists of an automatic asymmetrical 2 way-valve. Once the contaminated product is detected, the valve switches and pushes the contaminated product (together with a lot of the good product) aside. These detectors can sense both ferrous and non-ferrous metals (stainless steel, aluminium, brass, etc.)
Although we are barely scratching the surface of this topic, if you are still reading this article with both eyes open, it means that you might be interested in some specific application. There is a myriad of different types of magnetic separators, but one question always remains.
Where on earth should you install them?
A good place is always at the reception stations for raw materials. The intention is to remove the bigger iron particles that are likely to damage your processing equipment. When receiving flour by a silo truck, then you should consider placing a pneumatic magnetic separator D100mm vertically at the inlet of each silo filling pipe. When receiving bulk bags, you might want to install a drawer grid separator at the outlet of each bulk bag unloader. Same stands for a bag tipping station.
During your bulk-solids handling process, there are critical places to worry about. At the inlet of down-sizing equipment (hammer mill, roller mill, crusher, shredder, rotary grinder, etc.). Sometimes at the inlet of a mixer would be useful, although mixers tend to have multiple inlets (which multiplies the cost). Always consider installing a magnetic separator before the pellet press or the extruder.
Remember that metal impurities are not only imported by your exotic raw materials received, but they might also be produced by the malfunctioning of your own equipment. Having said that, you should try to find possible high friction / abrasion / wear places in your process that need extra attention.
Finally, you should consider all places right before the end product is being packaged for the end-user. 20 packaging machines deserve 20 magnetic separators. Look only for vertical installations, since gravity will help you avoid blockages. If inclined installation cannot be avoided, then you might want to consider a hump magnet.
I think it was Coco Chanel that said, “less is more”, but in our case if you want to protect your production plant you should install as more as you can.
