Separation of magnetic and non-magnetic metals in a municipal incineration plant (case study)

The needed separation system should be fully automatic (i. e., requiring no human operators) and its control system had to be fully compatible with the control system of the entire separation line.

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Solution

The first step was to remove ferromagnetic particles:

That is why we proposed to use a self-cleaning overband magnet suspended over the conveyor belt. The advantage of this type of magnetic separator is that it lifts and automatically removes ferrous metal from the conveyed material.  

We have chosen the most powerful model of the DND-AC Mx3 F BEAST overband magnet, whose unique properties include:

• Suspension height up to 450 mm
• Max. conveyor belt speed up to 3 m/s

The second step was to separate the non-ferrous metals:

The basic condition for the successful separation of non-magnetic metals is the previous removal of iron particles (this was done in the 1st step).

The non-ferrous metals can be separated by special magnetic separators based on eddy currents. That is why these separators are called eddy current separators (or non-ferrous metal separators). They include a magnetic cylinder (rotor) with very strong magnets, which at high revolutions (when the magnetic poles alternate rapidly) creates a magnetic field that magnetizes even non-ferromagnetic metallic materials for a short term.

The magnetic polarity of the particles magnetized in this way is the same as the polarity of the magnetic field created by the separator rotor. Therefore, magnetized materials are repelled and automatically ejected out of the conveyor belt.

There are two main types of eddy current separators:

• Separators with concentric magnetic rotors (ECS-C RAM) - These separators are designed for the separation of larger metal pieces (from 50 mm). They are therefore suitable for the treatment of e. g. municipal waste. The advantage is their lower price and high processing capacity. The rotor comprises 4 magnetic poles.
• Separators with eccentric magnetic rotors (ECS-E BULL) - Thanks to their design, they allow the induction of a strong magnetic field and are thus suitable for the separation of even very small metal particles (from 5 mm).

Since it was necessary to separate non-ferrous metal pieces larger than 50 mm, we chose the ECS-C RAM separator with a concentric magnetic rotor.

A precondition for the successful separation of non-ferrous metals is a uniform distribution of the treated material on the ECS separator belt. Therefore, we designed a 1,500 mm wide separator that should on the one hand reduce the thickness of the material layer and on the other hand better spread the material on the separator belt.

Benefits of using the eddy current separator ECS-C RAM together with the overband magnet DND-AC Mx3 F BEAST:

By using the most powerful overband magnet DND-AC Mx3 BEAST, we were able to separate ferromagnetic particles and achieve the material purity needed for the downstream application of the eddy current separator.  

The eddy current separator ensures the separation of non-ferrous metals from municipal waste. Thanks to the favorable price of ECS-C RAM, the return on investment is very attractive.

The customer is satisfied with our solution - we were able to separate ferrous as well as non-ferrous objects from municipal waste, thereby enabling further efficient sorting of this waste.

Applied equipment:

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The eddy current separator ECS-C RAM and the overband magnet DND-AC Mx3 F BEAST

Advantages of the eddy current separator ECS-C RAM

• Separation of metal particles from 50 mm
• Concentric rare earth rotor
• Magnetic rotor comprising 4 poles
• Working widths from 500 to 2 000 mm

Advantages of the self-cleaning overband magnet DND-AC Mx3 F BEAST

• Automatic cleaning without the interruption of the material flow
• Suspension height up to 450 mm
• Separation of magnetic particles from 0,5 mm
• Max. conveyor belt speed: 3 m/s
• 10-year warranty
• All-stainless steel design
• No dead spots between the magnetic core and separator cover
• Weight from 440 kg - easy installation
• Magnetic flux density of G (at the distance of 200 mm from separator belt)
• Ferrite as well as light neodymium variants

Often, Bunting will receive questions asking if a magnetic separator can be designed in a specific size. This is a simple enough request. However, things become more complicated when the enquiry also requests a specific Gauss value.

It is understandable why customers would inquire about Gauss, as it is frequently quoted by companies supplying magnetic separators. It is also the standard used by HACCP, which is the most widely used program for ensuring food safety. However, customers are not always provided with information about the specific relevance of Gauss. What is Gauss? Is 100,000 Gauss better than 10 Gauss? In this blog, we will answer some of these questions.

“Gauss” is defined as the cgs (centimeter-gram-second) unit of measurement of a magnetic field. This is also known as the “magnetic induction” or “magnetic flux induction.” One gauss is equal to one Maxwell per square centimeter.

Unfortunately, this definition does not necessarily help the customer, and may only add to their confusion. Additionally, this definition does not truly assist the customer in understanding if one magnetic separator is truly stronger than another. It is more effective when customers have set parameters to work within, such as the Food Safety Modernization Act stating food processors are required to have 10,000 Gauss.

There are several inherent problems associated with purchasing a magnetic separator based on the “Gauss” value alone:

1) How can you be certain the magnetic separator you are purchasing truly has the level of Gauss quoted?

2) Do you have a Gauss meter to measure and verify the Gauss yourself? Have you been trained to use this tool? It can be difficult for an untrained person to use a Gauss meter, and can lead to some wild results being obtained. It is important that a plant operator is able to get consistent results in order to avoid the mistake of replacing magnets that are still performing optimally.

3) Different levels of Gauss will be found at various different points on the magnetic separator. Do you know where the highest Gauss reading is located on the magnetic separator?

4) Higher Gauss magnetic separators do not necessarily provide the best metal separation, as the ability of the magnetic separator to capture metal particles is not a function of Gauss alone.

Fortunately, Bunting has answers. There is a quick and simple way to measure and assess magnetic attractive force on magnetic separators such as grate magnets, plate magnets, and tube magnets. By performing this physical test, the “Gauss” reading is at least partially removed from the equation. The process is simple and utilizes one of Bunting’s magnetic pull test kits.

First, record the pull value when the magnetic separator is new, or get the value from your supplier. You can use that number as your baseline as you do your test. This will allow you to compare the percentage loss of pull strength as the same amount of percentage loss for Gauss.

To perform a pull test, use a spring balance with a 6 mm 0.5” steel ball on the end. Place the steel ball on the surface of the magnetic separator. Then, the steel ball will be pulled away and off of the magnetic separator. A reading will be taken on the spring balance—this is the pull force. Then, repeat this test at several different locations on the magnetic separator.

After conducting this test, you will then have a real, meaningful, tangible measurement to guide you. With a magnetic pull test kit, you can conduct this same test on various other magnetic separators, and you will be able to independently confirm which magnetic separator requires the greatest amount of force to extract the steel ball from the magnetic field.

As part of an internal auditing system, this test can be repeated at regular intervals as a way of ensuring the magnetic separator is not losing strength. This is a very simple and very effective method of assessing the magnetic strength of smaller magnetic separators. This method does not need extensive training, and does not require high tech, costly equipment.

If magnetic force is important when requesting the price for a magnetic separator, it is recommended that you do not only state the Gauss level. Rather, you should also ask for the pull force measurement. After receiving the magnetic separator, you can then conduct your own pull test to ensure that the purchased equipment meets the ordered specification.

Contact us to discuss your requirements of Hydraulic Automatic Straw Baler Supplier. Our experienced sales team can help you identify the options that best suit your needs.