Magnetic Separation Basics

Scrap processors and recyclers should be familiar with the three main methods of magnetic separation.

August 15, 2001

Magnetic separation systems began appearing in scrap yards after World War II when heavy duty shredders used for grinding automobiles started to pop up across the United States. The early magnetic separation systems were mainly electromagnets; permanent magnets began making inroads when ceramic material became available and the cost to produce them decreased significantly, providing field strengths matching those of their electromagnet cousins. In addition, permanent magnets did not have to rely on an outside power source, and did not have the overheating problems associated with the early electromagnets, which were usually expensive and bulky.

As the scrap industry evolved, magnetic separation systems evolved, too. By the end of the 1970s, three main types of magnetic separation systems were prevalent: the overhead magnet; the magnetic pulley; and the magnetic drum. And by the end of the 1980s, another form of magnetic separator, the eddy current, was becoming popular with both scrap processors and municipal recyclers. Although the eddy current will not be discussed here, its contribution to the recycling industry has been significant. An eddy imparts a magnetic charge to nonferrous metal material via a revolving, alternating-pole magnet usually under the conveying belt and in the head pulley. When the charged particle comes in contact with the field of an opposite pole, it is repelled and sorted.

Today, magnetic separation still dominates the way processors remove ferrous from nonferrous material. While permanent magnets are popular choices, advances in electromagnets have made them competitive again.

OVERHEAD MAGNETS

The first type of magnetic separation equipment is the overhead magnet. These are stationary magnets with self-cleaning belts that rotate around the magnet assembly. The cleated belt moves the attracted ferrous material and sorts it out of the magnetic field. These magnets can be configured in two main ways – parallel to the conveyor, referred to as inline; or perpendicular to the conveyer, referred to as crossbelt. Other configurations are actually variants of the overhead magnet where multiple magnets are used to transfer ferrous material from one magnet to another. These magnets are referred to as "multi-stage" magnets.

In an inline application the magnet is normally positioned at the end of the conveyor above the head pulley. The main advantage to positioning the magnet in this fashion is that entrapment of ferrous pieces and particles is reduced. Material is freed once it leaves the conveyor belt and the magnet can pluck suspended ferrous material out of the air.

If the conveyor is on an incline, the momentum of the particles leaving the conveyor belt results in an initial trajectory upward and toward the magnet. Thus, the material gets closer to the magnet and the ferrous particles have a better chance of getting picked up.

"No matter how hard a processor tries to prevent entrapment, it is always going to occur with an overhead magnet," says one manufacturer of magnetic separation equipment. "But it is not going to occur as much in an inline configuration as it is with in a crossbelt arrangement."

It is especially tough to pull out ferrous from wet, shredded wood streams with an overhead magnet, because the shreds start to interlock and clump. Suppliers say that wet wood and any other wet material is more difficult to process, and should be avoided if possible when applying magnetic separation. However, an inline configuration can free up more of the ferrous material for separation.

For inline applications, the magnet should be the width of the conveyor. Some manufacturers have square magnets. Others offer rectangular magnets where the longer length of the magnet is parallel to the conveyer, providing more coverage of the belt.

The other application for an overhead magnet is in the crossbelt configuration. This is a popular installation because placing the magnet inline over the head pulley is not always practical – there may be other equipment, such as a magnetic pulley or an eddy current separator, at the end of the conveyor. Plus, material recovery facility operators like the crossbelt configuration because the magnet can be positioned close to the hand picking stations, and because slower belt speeds increase the magnet’s efficiency.

In both the inline and the crossbelt configurations, the overhead magnet is working against gravity, so it has to work harder and normally has to be more powerful than a magnetic pulley or drum. However, the inline setup requires less field strength than the crossbelt, because it does not have to combat entrapment, nor does it have to change the direction of the ferrous material. Therefore, an inline overhead magnet can cost less than one used in a cross-belt configuration.

Variants of the overhead magnet include single- and three-stage magnets. In a single-stage magnet, ferrous material is carried through a magnetic field and offloaded onto another conveyor, while nonferrous material drops down into a container.

In a three-stage configuration, the ferrous goes through three separate magnets that are contained in a single housing. When the ferrous material is transferred from one magnet to the other, the particles are flipped and any entrapped nonferrous material falls out, resulting in a cleaner end product. Both the single- and three-stage variants are powerful magnets that can pick up heavy pieces of ferrous metal.

While many manufacturers sell both permanent and electromagnetic configurations, one manufacturer recommends that a processor use an electromagnet in the overhead position when the distance between the magnet and conveyor has to be greater than 12 inches.

Most overhead applications are with permanent magnets, but processors also like the electromagnet because they can adjust the strength and even shut it down when they don’t need to separate ferrous.

MAGNETIC PULLEYS

Another type of magnetic separator is the magnetic pulley. In this configuration the magnet is embedded in the head pulley of the conveyor. As the pulley spins, the magnetic force grabs the ferrous particles and carries them around and under the pulley until the natural belt separation from the face of the pulley forces the particles to fall in a separate bin. While suppliers are wary of recommending a pulley versus an overhead magnet unless they know the specific application, most say that, generally, a pulley will pull out finer particles of ferrous than an overhead magnet.

This better sort is possible because material is closer to the magnet, which is just under the belt. Also, the pulley has gravity working in its favor. This method, however, may not be effective in pulling off larger pieces of ferrous material or material that is trapped on top of the material stream.

Another drawback of the magnetic pulley is that the strength of the magnet is limited by the size of the pulley. Usually, a magnetic pulley can achieve only 6 to 7 inches of penetration at best, according to one supplier.

Magnetic pulleys can also be configured in conjunction with an overhead magnet. These combinations are recommended when the material stream contains a preponderance of ferrous metals. When this is done, make sure that the two types of magnetic devices are adequately separated – by 8 feet or even more in some cases – in order to avoid magnetic interference.

 

Calculating Burden Depth

In order to determine the optimal type and position of a magnet, it’s useful to calculate burden depth. Several factors must be considered before the calculation can be made. The operator must know capacity in cubic feet per minute (C); belt width in feet (W) and belt speed feet per minute (V); and the burden depth factor (F). The F factor is needed to compensate for the normal dip in the center of the conveyor; and to compensate for the tilt angle of the magnet if it is positioned over the head pulley at the end of the conveyor.

The equation for calculating burden depth is: De = F(C/WV).

For example, consider an operation which has a 3-foot wide conveyor belt with outside idlers at 35-degree angles. The speed of the conveyor is 500 feet per minute, and the capacity of the conveyor is 800 tons per hour of material.

First, the capacity of 800 tons per hour needs to be converted into cubic feet per minute. In order to accomplish this, the material density of the main medium must also be known. Let’s say the material is 3-inch minus in size, with a density of 50 pounds per cubic foot. In this case the capacity in cubic feet per minute would be: (800 tons per hour)(2,000 pounds/1 ton)(1 hour/60 minutes)(1/50 pounds per cubic foot) = 533 cubic feet per minute.

Second, determine the F factor. This figure can be obtained from the magnet installer. In this case, F equals 22.6.

Third, fill in the information into the burden depth equation: (22.6)(533 cubic feet per minute/3 feet x 500 feet per minute) = 8 inches.

Therefore, in this case, the material being carried by the conveyor has average depth of about 8 inches, and the magnet has to be positioned so that it can pull material through that burden depth.

 DRUM MAGNETS

Drum magnets are similar to pulley magnets; however, in the drum magnet, the magnetic element is stationary and positioned only on one side of the drum with a maximum of 180 degrees of arc. While the outer casing of the drum rotates, material is pulled through the magnetic field.

Drum magnets can be positioned for three methods of feed: up-and-over feed; down-and-under feed; and top feed. In an up-and-over configuration, ferrous is lifted out of the stream and carried up and over the magnet while the nonferrous material drops off the feeder. This application is commonly used in auto shredders, ash handling and other high-ferrous content streams.

In down-and-under feed, ferrous is carried under the drum and dropped on the other side. It has the shortest and most direct transfer area for the ferrous and is usually used for streams with larger ferrous pieces.

Finally, in the top-feed configuration, material cascades off the front side of the drum and the ferrous is carried through the magnetic field and separated. This type is used mainly for material streams that contain ferrous with weak magnetic properties.

The preponderance of drum magnets used today are in the scrap industry and on auto shredders. They are normally fed by a vibratory feeder or conveyor, and the speed of the drum can be adjusted to match the incoming feed. As with all types of magnetic separation equipment, the incoming feed must be controlled so that it does not overwhelm the ability of the magnet to pull out ferrous.

Drum magnets also come in two types: axial- and radial-pole. In an axial-pole drum magnet, the alternating poles are situated along the circumference of the drum. This configuration results in the same polarity across the width of the drum. With the same polarity across the width, there aren’t any dips in the magnetic field. So, axial-pole drum magnets are recommended for pieces that are 1 inch or less in size.

Radial-pole drum magnets have the same polarity along the circumference of the drum, which gives alternating polarity across the width. This results in dips in the magnetic field across the width of the drum. Therefore, radial-pole drum magnets are recommended for material pieces of 1 inch or greater.

Again, these types of magnets can be permanent or electromagnet. One manufacturer recommends that auto shredder operators considering adding a drum magnet install an electromagnetic one because it is hard to work around a permanent magnet in that configuration.

Optimum performance of a drum magnet depends on position of the magnetic arc, particle size, rate of feed, the gap between the feeder and drum surface, and other factors.

AREAS TO CONSIDER

There are several areas to consider before buying a magnetic separation device. Processors must consider the depth of material that will be processed (the burden depth); the range of particle size; conveyor troughing; speed, width and overall capacity of the conveyor; and the density of the material stream.

Conveyor troughing applies only to overhead magnets because conveyors normally run in a concave fashion so that material does not fall off when the conveyor is moving. Therefore, the overhead magnet field must be able to reach into the trough of the conveyor to pull out material. Idlers on the end of the conveyor are normally inclined at 20, 35 or 45 degrees to create the trough.

Burden depth is the average depth of the material on the conveyor belt. Calculating burden depth is useful to determine the maximum amount of material that the magnetic field must penetrate, and to position the magnet optimally over a conveyor (see sidebar). Many suppliers will give processors a chart that has the burden depths and other data for the different streams that a company may run, and for different throughputs.

Magnets are usually positioned for the most difficult situation. "That is the first thing we ask is what is the range of materials being processed," says one supplier. "We want to make sure that the magnet is large enough to pick up the target material in the most demanding scenario possible."

While most overhead magnets are adjustable, some scrap companies and recyclers have built special platforms for the overhead magnet so that it can be adjusted to the optimum height more quickly. One company that was processing a wide range of materials needed to constantly adjust its overhead magnet, so it built a hydraulic platform for the overhead magnet that could be easily raised or lowered depending on the application

Another supplier recommends that buyers considering purchasing an electromagnet should check to see if the magnetic circuit is balanced and provides a uniform magnetic field and the appropriate depth of field. Unbalanced electromagnets can cause excessive power drains .

In the end, processors must remember that magnets, no matter the type, aren’t 100 percent effective, according to another supplier.

The author is managing editor of Recycling Today.