# Eddy Current Separator

## Eddy Current Separator

Electrical currents are induced in all conductors when exposed to an alternating magnetic field. The induced current generates a magnetic field in the conductor that opposes that of the alternating magnetic field. Presented in Figure 2 is a schematic illustration of generated eddy currents. In this particular example, the alternating magnetic field is produced by a series of permanent magnets mounted on the circumference of a rotor. The permanent magnets have alternating polarity. As the rotor revolves, an alternating magnetic field is produced and the rate of revolution determines the frequency of the alternating magnetic field. When a conductor, such as a metallic disc in the illustration, is placed in the alternating magnetic field, a closed loop current flow occurs.

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### Description

The current loop in the conductor produces a magnetic field that is a mirror image of the alternating magnetic field. At any point in time, as shown in Figure 2, the induced magnetic field in the conductor directly opposes the alternating magnetic field. This opposition of magnetic fields produces an instantaneous repulsion in the conductor. The conductor is consequently repelled from the rotor. The alternating magnetic field has no effect on non-metallic materials as they pass through the magnetic field and discharge the rotor in a natural trajectory.

(1) Unit capacity of TPH/ft of rotor width

The repulsive forces in a rotating permanent magnetic field have been described in many publications. In generalized terms, the repulsive forces can be classified as either machine dependent or component dependent variables. The repulsive forces dependent on the machine characteristics are as follows:

Fr (machine) ∝ H²f………………………………………..(1)

Where H is the magnetic field intensity and f is the frequency of the alternating magnetic field. In the above example of a rotating permanent magnetic field, the frequency f can further be described as:

f = n p/2…………………………………………………….(2)

where n is the revolution of the rotor and p is the number of magnetic poles. The above relationship indicates that the repulsive force of the separator can be maximized utilizing extremely high magnetic fields in combination with high frequencies.

It was demonstrated that the relationship between the frequency of the alternating magnetic field in a rotating disc separator and the magnitude of rejection of closely sized aluminum is practically linear. Note that the frequency can also be expressed as the linear velocity of the magnets relative to the particle. This expression has been used in certain derivations.

Any given piece of material (referred to as component) has specific characteristics relating to repulsion. The repulsive forces dependent on the component characteristics are as follows:

Fr (component) ∝ m σ /ρ S………………………………( 3)

where m is the mass, σ is the electrical conductivity, ρ is the density, and S is the shape of the material. The above relationship indicates that the repulsive force on a component is maximized when the electrical conductivity is high and the density is low. Presented in Table 1 is the σ/ρ factor for a variety of different metals. The relatively low specific gravity of aluminum results in a factor an order of magnitude higher than many common metals. Equation 3 also indicates that the overall mass of the component positively effects the force of repulsion.

The shape of the conducting component has a profound effect on the force of repulsion. The shape influences both the induced current loop induced and the proximity of the center of gravity of the component to the magnetic field. The force of repulsion increases exponentially approaching the drum surface. Although it is extremely difficult to accurately quantify the effect of random shapes to the force of repulsion, approximations have been made. Disc shaped conductors as well as cylindrical shaped conductors respond very favorable due to the relatively large induced current loops that are established. Small randomly shaped conductors, such as metallic particles in a crushed slag, also respond very favorable due to the close proximity to the magnetic field. A sphere responds with a relatively low force of repulsion due the small current loop that is induced with respect to mass. Multiple induced current loops can be established in conductors with irregular bends. The magnetic fields generated from these current loops counteract one another lowering the net repulsive force. Laminates of metallic sheets or plates will each have an individual interacting current loop resulting in a very low net repulsive force.

The repulsive force of the system is the combination of the two dependent equations and again in generalized terms can be expressed as follows;

Fr ∝ H²f m σ/ρ S……………………………………………….( 4 )

The above equation indicates that the repulsive force and the subsequent separation efficiency for any given conductor is complex and is dependent on several interacting variables.

A typical eddy current separator is shown in Figure 5-30. This device is a modification of a linear induction motor in that it generates a sine wave of magnetic intensity, which travels down the length of the motor with alternating north- and south-pole components. As the metal-rich concentrate passes over the linear induction motor, eddy currents are induced in an electrical conductor that appears on the surface of the table. The induced magnetic fields associated with the eddy currents in the metals interact with the moving field generated by the motor, which pushes the conductors (nonferrous metals) along the linear motor. All that is necessary to achieve removal is to orient the motor transverse to the direction of the feed, so as to repel the metal away from the main direction of travel. The mixed material is fed to one end of the nonmagnetic belt, which travels over the linear induction motors positioned on the underside of the belt. Recover)- of the metal concentrate is on the top side of the belt, where the material to be removed is ejected by the linear induction motor against the retaining wall and into the extract area. The rejects are not affected by the eddy currents and therefore flow along the lower portion of the belt area. Table 5-4 shows the performance of a typical linear induction motor in separating aluminium from a shredded refuse from which ferrous material has been removed.

Using the numbers in Table 5-4, the eddy current separator was able to achieve a recover) of only little more than 50% with a purity of only 89%—not generally acceptable to secondary materials dealers. Thus, a hand removal of contaminants or additional screening is required after the eddy current separator has been used.

 Material is fed onto the conveyor belt of the eddy current separator, which moves it across the magnetic rotor where separation occurs. The two streams of material discharge into a housing. The housing has a splitter to divide the nonferrous metal from the nonmetallic material, such as paper, plastic, wood or fluff. The key component of the eddy current separator is the magnetic rotor, which has a series of permanent rare earth magnets mounted on a support plate attached to a shaft. The magnetic rotor is surrounded by (but not attached to) a wear shell which supports the conveyor belt. This allows the rotor to spin independently and at a much higher speed than the wear shell and belt. When a piece of nonferrous metal, such as aluminium, passes over the separator, the magnets inside the rotor rotate past the aluminum at high speed.

The eddy current separator is the most suitable technology’ for recycling nonferrous metals such as copper, aluminum, and others from industrial wastes and municipal solid wastes (MSW). Nowadays, eddy current separation is extensively used in recycling industries for automobiles (ELV), electronics (WEEE), demolition (D&CW), bottom ashes, and MSW, and for the material processing of nonferrous metals. According to equipment makers, about 500 such separators have been installed around Europe, and most of them within the past decade.This growth appears set to continue in the coming years. The materials treated by this technique must be free of ferrous metals to maintain separation efficiency and equipment. For this reason, most new eddy current separators are coupled with low-intensity magnetic separators as shown in Fig. 3.11.

The principle of eddy current separation is that an electric charge is induced in a conductor by changes in magnetic flux cutting through it. Such changes in magnetic flux can be achieved by using a rotating permanent magnet, and magnetic flux intensity can be controlled by using an electrical conductor. The effect of such currents is to induce a secondary magnetic field around the nonferrous particles. This field reacts with the magnetic field of the rotor, resulting in a combined driving and repelling force that literally ejects the conducted particle from the stream of mixed materials. This repulsion force is in combination with the product belt speed and the optimisation of the product.

An Eddy current separator respond to the problem of separating nonferrous metals from the remainder of refuse and depend on the ability of metals to conduct electrical current. If the magnetic induction in a material changes with time, a voltage is generated in that material, and the induced voltage will produce a current, called an eddy current. The feed to an eddy current separator might be the reject component from air classifiers from which the ferromagnetic components (steel cans mostly) have been removed.

Many eddy current separators are inclined tables. Underneath the table are several large magnets that produce an electrical field. If a particle that conducts electricity slides down the inclined table, the electrostatic forces push it in a direction perpendicular to its path. Only those particles that conduct electricity (as the particles move down the inclined table and come under the influence of the charge field) are laterally displaced. Nonconductors are not affected by the charge field and drop straight down.

In recent years, the strength of permanent magnets has increased several fold. Rare earth permanent magnetic circuits now rival electromagnetic circuits in magnetic field strength without power consumption. This evolution of permanent magnets has provided a cost effective alternative for the generation of high intensity magnetic fields and has led to the successful re-introduction of the eddy-current separator.

Empirical testing has demonstrated that there are numerous metal recovery processes conducive to eddy-current separation. The repulsive force and the subsequent separation efficiency for any given conductor is complex and is dependent on several interacting variables. Quantitative studies have demonstrated that the shape and weight of the conducting component is an important parameter and necessitates the need for empirical testing.

The eddy-current separator has been successfully applied in several metal sorting and recovery operations. Through an improved understanding of the separation variables, the applications of the eddy-current separator have now progressed to finer materials and subsequently more selective separations. Relatively fine sized metal bearing slags and spent foundry casting sands as well as precious metal bearing electronic scrap demonstrated excellent metallurgical response to the eddy-current separator.

## Eddy Current Separator for Sale

In reference to your request for quotation, we are pleased to quote the following equipment:

One, each Eddy Current Separator, to separate non-magnetic metals such as aluminum, copper, from other less dense material, such as plastic, complete including 12.75” diameter by 20” wide magnetic rotor with high speed rotating rare earth magnets, 20 In. wide urethane belt to transport material, one 304 SS hopper with 1.25 CuFt volume, one variable speed magnetic feeder with 3″ wide x 16″ long SS feed pan, all motors, drive, gear reducers, all belts/chains enclosed in OSHA Guards, moving conveyor portion of separator covered with enclosure with Lexan windows for viewing operation, 380 V/3 Ph/50 HZ Electrical control panel, frequency inverter for variable speed of conveyor, apx. dimensions of 155” x 78” x 65”(H). Pilot Plant size, with approximate capacity range of 600 Lbs. to 4,000 Lbs. per hour, depending upon material processed. Ship Weight : 1,600 Lbs. Ship Volume: 450 CuFt

Splitter assembly, to cut metal material separated from stream by eddy current, and channel it to a separate discharge, with adjustable splitter, Lexan windows. Adds 2 Feet to length of Eddy Current Separator.

Ship Volume: 190 CuFt Ship Weight: 1,050 Lbs.

## Recommended Spare Parts for 2 years Operation

Fiberglass shell magnet cover
Conveyor belt

Prices are net and are valid for a period of 60 days, unless otherwise noted. If you find the above equipment to be of interest, require additional information, or if we may be of further assistance, please contact us.

Eddy Current Separators remove nonferrous metals such as aluminum, die-cast metal, and copper from nonmetallic material. MPI’s proven eddy current technology ensures high separation rates. State-of-the- art system controls and features combined with technological advances result in improved nonferrous metal removal, particularly when trying to separate smaller particles such as aluminum and brass from non-conductive material.

An ultra-high-strength magnetic rotor that houses rare earth magnets spins at a high RPM. The magnetic rotor, attached to a motor driven shaft, spins independently and at a much higher speed than the conveyor belt pulley, creating a high frequency reversing magnetic field. Commingled material flows along the belt. When nonferrous product, such as aluminum, passes over the rotor, the spinning magnets generate an eddy current. This causes the aluminum to be repelled and “thrown” by the separator. Product, such as plastic, glass or other materials which are not conductive, simply fall off the end of the separator into a bin.

The term “secondary recovery” has recently brought on a new meaning in view of the depletion of natural resources, energy conservation, plant optimization, and the environmental consciousness that has led to hazardous waste management, recycling, and secondary processing. Many secondary recovery processes are being developed applying mineral processing technology. Gravity separation, magnetic separation, and flotation have all emerged as implicit methods for recovering residual values from various process streams and hazardous constituents from waste streams.

Recently, a technique for concentrating metallics has been successfully re-introduced. Although the concept of the eddy-current separator was developed over a century ago, (Patent 400317 issued to Thomas Edision on March 26, 1889) it was not until this last decade that it gained prominent acceptance.

The evolution of permanent magnets has provided a cost effective alternative for the generation of high intensity magnetic fields. High energy permanent magnets have substantially reduced the captial and operating costs over the electromagnetic circuits employed in antecedent eddy-current separators. Specifically, in recent years, the strength of permanent magnets has increased several fold with neodymiun-boron-iron rare earth magnets now providing an energy product of 35 million gauss-oersted. Figure 1 shows the evolution in the strength of permanent magnets. The development of these rare earth magnets has led to the design of circuits possessing a magnetic force an order of magnitude greater than that of conventional permanent magnetic circuits.

In recent years the eddy-current separator has been successfully applied in several metal sorting and recovery operations. Most common is the sorting of metal from shredded automobile scrap and municipal waste. The separator has however advanced to the point where it has direct application in the beneficiation of fine sized metals. Relatively fine sized metal bearing slags and spent foundry casting sands as well as precious metal bearing electronic scrap demonstrated excellent metallurgical response to the eddy-current separator. The applications of the eddy-current separator have progressed to finer materials and subsequently more selective separations.

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