Around the world, recycling plants rely on Eddy Current Separators to separate and recover various types of conductive non-ferrous metals. An Eddy Current Separator works by utilizing high strength magnetic forces that repel and eject non-ferrous metals. Examples of materials separated by an Eddy Current Separator include aluminum soda cans, aluminum foils in discarded packaging, and different sizes of scrap metal and wire from ASR and ICW automotive, appliance, and electronic recycling applications.
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The first Eddy Current Separator was developed by the Bird Group, a UK-based metal recycler, in . The development of the Eddy Current Separator was so significant that it resulted in the Bird Group being presented with the Prince of Wales' Award for Technology and Innovation and the Tomorrow's World Award to Technological Development and Innovation. The design of the Eddy Current Separator permanently changed the landscape of metal recycling by making it much easier and more efficient to separate and recover highly valuable non-ferrous metals like copper and aluminum. In the years that followed its initial invention, Eddy Current Separator technology has continued to grow and evolve.
In order to successfully separate non-ferrous metals with an Eddy Current Separator, there are several key criteria to consider. This technical review assesses five of the key design characteristics of the Eddy Current Separator, and aims to dispel some common myths about this piece of equipment. The five design characteristics we will go over are:
An Eddy Current Separator's design is based around a magnetic rotor that has alternating magnetic poles (north/south) which are continuously rotating within a non-metallic shell that is also rotating, but at a slower pace. A urethane feed belt rotates at the same speed as the non-metallic outer shell, and this feed belt conveys material into the higher rotating speed magnetic field that the rotor generates. Here, non-ferrous metals become induced by magnetically-driven eddy currents. In the non-ferrous metal particle, the eddy currents generate an electric current. Then, the non-ferrous metal particle will produce its own magnetic field. This new magnetic field will then react with the rotor's magnetic field, creating a repulsive effect that causes the conductive non-ferrous metal to be repelled and, therefore, separated from the original product stream.
This separation is in accordance with two laws:
Faraday's Law of Induction: This law relates to where electrical currents are induced when conductors enter a rotating magnetic field. For example, a conductor may be a non-ferrous metal such as a piece of aluminum or copper.
Lenz's Law: In this law, the induced eddy currents create a magnetic field that opposes the magnetic field that initially created it. As a result, the conductor metal will be repelled away from the magnetic source.
Theoretically, if there was an increase in the number of polarity charges per second, there would be a positive effect on separation efficiency (i.e., more flux change would generate a greater reactive magnetic field in the non-ferrous metal). However, tests investigating this theory have shown that as rotor speed increases (and the rate of change of polarity changes increases), the actual displacement or thrown distance of a non-ferrous metal particle reaches a peak. Once it has reached this peak, the displacement of the non-ferrous metal particles will either be sustained or will even fall. This fall can be significant in the case of smaller particles.
When the relationship between magnetic rotor speed and the throw distance of a non-ferrous metal particle was investigated further, it was found that the amount of repulsive energy induced into a non-ferrous metal particle is proportional to the dwell time in the field. A shorter dwell time results in less induced energy. Therefore, the reduced energy leads to a reduction in the displacement or throw of a non-ferrous metal particle.
Other considerations to make in terms of rotor speed include maintenance and wear. When the rotor speed is increased, key components such as bearings experience a higher wear factor.
In practice, every magnetic rotor design has an optimum rotational speed that will produce the best level of non-ferrous metal displacement for each application. Commonly, this speed is between 2,000 and 5,000 RPM. There is a perception that an increased rotor speed or frequency will always result in a higher level of non-ferrous metal separation, but this perception is misguided.
' Eddy Current Separator Technical Overview by Prof Neil Rowson
An Eddy Current Separator's belt is the mechanism that conveys material into the head pulley magnetic rotor's rotating magnetic field. The belt's speed is the factor that will determine the conveyor's burden depth, the material's dwell time in the magnetic field, and what the trajectory of the material will be when leaving the belt.
Burden Depth
To achieve optimum separation, there must be a monolayer of material that is uniformly thin. A two-stage feed system produces the ideal feed characteristics. As the initial step in the process, a vibratory feeder will evenly spread feed material across the entire width of the tray. The width of this tray will be slightly narrower than the Eddy Current Separator's belt width. Then, this material will be fed onto the Eddy Current Separator's faster moving belt, which then spreads the material out further so that it attains a single particle depth. This lowering of the burden depth to a monolayer increases separation efficiency by reducing the risk that non-ferrous metal particles will collide with non-metallic materials.
Dwell Time
The dwell time of a particle in the rotating magnetic field is dictated by the speed of the belt the particle is traveling on. If there is a longer dwell time, more energy will be induced into the conductive non-ferrous metal particle. In an extreme example, if aluminum is allowed to remain in the rotating field for too long, it will become exceptionally hot and anneal. At the other end of the spectrum, a belt speed that is too high will transfer material through the magnetic field too quickly, thus reducing the separation effect.
Material Trajectory
The belt speed of any conveyor will create a trajectory of material as it leaves the belt. When in operation, higher belt speeds result in longer and higher projections of conveyed material being sent away from the head pulley. In order to achieve successful separation of non-ferrous metals on an Eddy Current Separator, it is essential to understand all forces that are impacted on the particles, including belt speed.
The forces a non-ferrous metal particle is subject to upon entering the Eddy Current Separator's rotating magnetic field include:
' Eddy Current repulsion force from the magnetic rotor
' Conveyor propulsion that is proportional to the belt speed
' Gravity
Combined, these three forces produce a resultant force that then will determine the non-ferrous metal particle's trajectory.
The Eddy Current repulsion'here, force A'will only generate a reaction in a conductive non-ferrous metal particle. Meanwhile, forces B and C affect all of the material on the conveyor belt. This means that if the size, shape and weight of every particle on the conveyor was the same, and there was no magnetic rotor to factor in, the throw trajectory would be the same. When Eddy Current repulsion is factored in, the trajectory of the non-ferrous particle is altered, propelling it upward and away from the magnetic rotor.
When an Eddy Current Separator is being commissioned, technicians will often initially run non-metallic material over the unit so that they can check the material trajectory in order to correctly position the splitter. Adding non-ferrous metal fraction allows the two trajectories to be compared, and then the optimum setting of the splitter can be determined. This splitter enables non-metallic and non-ferrous metal constituents to be separated from each other.
However, in the case that Force B'the forward force from the feed belt'is increased, while all other forces remain constant, the resultant force vector will flatten and reach a point where the trajectories of non-metallic materials and non-ferrous metal particles either cross each other or become almost identical. In this situation, the limited difference between the two trajectories will prevent optimum separation from being achieved.
Because different Eddy Current Separators will have different designs of magnetic rotors, and because the shape, size, and other characteristics of non-ferrous metal particles can vary so widely, there is no such thing as a one-size-fits-all belt speed. To determine the optimum belt speed, the application and installation must first be taken into account.
The conveyor belt length of an Eddy Current Separator can vary significantly. Part of this is due to claims that longer belts will improve the level of separation achieved. To determine the belt length, engineers analyze the function of the belt and how it will interact with the other parts of the greater separation system.
The job of a conveyor belt is simple'to transport the material into the rotating magnetic field, also referred to as the 'separation zone.' In order to achieve optimum separation, material will be settled and ideally still on the conveyor belt prior to entering the separation zone. Longer conveyor belts allow a longer dwell time for material to settle.
However, material particles settle quicker when they are fed onto the belt via a vibratory feeder. The vibratory feeder action evenly spreads material across the tray's width before it is gently dropped onto the more rapidly moving belt. This causes the material to be splayed even more. But, minimizing the drop height from the vibratory feeder to the belt minimizes product bounce and settles the stream faster, which must be settled before the stream hits the magnetic field. In practice, utilizing a vibratory feeder eliminates the need for Eddy Current Separators to have long conveyor belts.
In some specialized applications, material being fed directly from a vibratory feeder (with a non-metallic trap or tray tip) into the separation zone can remove the need for any belt at all. However, the belt is still important, as it functions as a cleaning device that transports any attracted magnetic material out of the product stream and away from the Eddy Current Separator's rotor shell.
Even though the Eddy Current Separator is a system based on magnetic principles, there is often confusion regarding the separation of ferrous metals.
In short, the Eddy Current Separator has two pulleys and a belt. One of these pulleys is magnetic. Because of the laws of physics, the magnetic head pulley (or rotor) attracts ferrous metal. Therefore, the system would separate non-magnetic metals from ferrous magnetic metals. However, there are some limitations associated with this.
Concentric Rotors
An Eddy Current Separator using a concentric magnetic rotor design has a magnetic rotor that maintains the same gap between the outer non-magnetic shell and the internal magnets throughout the entire circumference of the rotor. The high-strength magnetic rotor is constructed with powerful neodymium rare earth magnets that hold ferrous metal tightly in place'at the expense of making it difficult to discharge from the belt. Additionally, because of the high-speed rotating magnetic field, ferrous metal that cannot be discharged instead remains on top of the belt at the bottom point of the magnetic rotor. Because the magnetic field is constantly rotating and changing, the metal vibrates and becomes very hot. As a result, once the conveyor belt stops, this red hot ferrous metal can easily burn a hole through the belt and onto the non-metallic shell's surface. Over time, continuous wear resulting from ferrous metal will lead to deterioration and holes on the non-metallic shell's surface. Then, attracted ferrous metals will pass through these holes and attach to the rotor magnets, leading to catastrophic failure of equipment. So, in these scenarios it is important to have maximum magnetic separation of the material upstream before the stream lands on the Eddy Current belt.
Eccentric Rotors
An Eddy Current Rotor utilizing an eccentric magnetic rotor design mounts a smaller rotor in the top quadrant of the non-metallic shell. In this design, attracted magnetic material will move through a diminishing magnetic field until it ultimately reaches a point where it falls away from the belt. This ferrous metal then often discharges into the non-metallic fraction. But, even in this scenario, upstream magnetic separation is critical to minimize the amount of ferrous material coming in contact with the Eddy Current field.
Best Practice
In order to achieve optimum metal recovery and separation, a multi-staged approach must be implemented, with specific materials being recovered at different stages. Installing upstream magnetic separation such as a magnetic crossbelt, pulley magnets, and drum magnets allows for successful recovery of valuable ferrous metals prior to material being introduced to the Eddy Current Separator. Most systems will utilize dual magnetic separation with a magnetic crossbelt, and then either a neo magnetic head pulley or a neo magnetic drum separator before the stream hits the Eddy Current belt. This then allows for safe non-obstructive separation and recovery of the non-ferrous metals on the Eddy Current Separator.
Earlier, we saw the laws of Faraday and Lenz suggest that the strongest rotating magnetic field would be able to produce a greater repulsive effect. In operation, however, this has been proven not to be the case.
An Eddy Current Separator's rotor is constructed from several strong permanent magnets. These magnets may be either neodymium rare earth or ceramic ferrite, and are attached to a steel carrier. The permanent magnet's dimensions'both its arc length around the rotor as well as its block thickness'will dictate the throw of the magnetic field.
' A magnet that is longer and thicker will produce a deeper magnetic field.
' A magnet that is shorter and thinner will produce a shallow, intense magnetic field.
Regardless of if a magnet is long or short, the maximum magnetic intensity will occur on the pole (or surface) of the magnet that is mounted on the magnetic rotor's carrier. A key parameter in design focuses on minimizing the distance between the surface of the belt and the magnetic pole, as the strength of a magnetic field decreases as distance increases. This distance is influenced by four variables:
1. Carbon fiber wrap: Magnets are positioned to sit on the rotor's magnet carrier, which independently rotates within the non-magnetic shell. To securely hold the magnets to the carrier, carbon fiber tape is wrapped around the magnetic rotor assembly.
2. Air gap: There must be a sufficient air gap between the surface of the carbon fiber tape and the inside of the shell to ensure no contact occurs, even when there may be a small degree of flexing when the rotor is spinning at high speeds.
3. Shell thickness: In order to withstand wear and tear caused by punishing work environments, the shell must be sufficiently thick to resist deflection across wider conveyors.
4. Belt thickness: Belts also must be designed to withstand difficult working conditions, and some applications will require thicker, more robust feed belt designs and others can utilize thinner urethane belts to minimize the gap.
When carbon fiber wrap, internal air gap, shell thickness, and belt thickness are all accounted for, there will be several millimeters of space between the belt's surface and the point of maximum magnetic strength. After these parameters have been set, the next step is to determine what the expected performance of rotors with short or long magnets will be.
Shorter magnets produce an intense magnetic field, but the field is shallower in height. The magnetic intensity drops off quickly as you get to the surface of the belt'and the magnetic field on the belt surface is lower than on the face of a magnet. Comparatively, longer magnets are able to throw a deeper magnetic field. When applying this information in practice, it means that a magnetic rotor with short, yet exceptionally strong neodymium rare earth magnets may ultimately produce a weaker magnetic field on the belt surface compared to a rotor constructed from longer standard strength ferrite magnets at the point of separation (i.e., on the surface of the belt).
A shallow or deep magnetic field has a big effect'it means that classifying a magnetic rotor as 'strong' or 'weak' will ultimately be completely dependent on where the measurement supporting that description is taken. For example:
ROTOR DESIGN ON THE SURFACE OF THE BELT 10MM ABOVE THE SURFACE OF THE BELTRotor 1 ' Short, exceptionally strong Rare Earth Neodymium Magnets with a shallower field
Strongest WeakerRotor 2 ' Long, standard strength Ferrite Magnets with a deep field
Weakest StrongRotor 3 ' Long, exceptionally strong Rare Earth Neodymium Magnets with a deep field
Strong StrongestTable 1: Eddy Current Separator rotor comparisons
Ultimately, the application an Eddy Current Separator is being used in will always dictate the selection of magnetic rotor design. For example, if the aim is to project a magnetic field into the center of a large non-ferrous particle, such as an aluminum can, a rotor with longer magnets that can produce a deep magnetic field will be best. As another example, if the goal is the separation of small non-ferrous metal particles, such as those found in ground plastics, it is better to use a rotor with shorter magnets and a shallower, more intense magnetic field.
It is vitally important to understand these principles when considering what design and type of Eddy Current Separator will best suit a specific application. It is not enough to be told that the magnetic field is the 'strongest on the market,' as this will not determine if that Eddy Current Separator rotor design is truly able to offer the optimum level of separation.
To select the optimum non-ferrous metal separator, understanding the application it will be used in is key. Our engineers consider the following:
Material:
' What the material is, and what form it is in
' How material is being fed
' Has ferrous material been removed from the stream?
' Particle size range (typically between 3mm and 50mm)
' Non-ferrous metal particles
' Shape of non-ferrous metal particles (e.g. angular, wire, spherical, etc.)
' Moisture level of the feed
' Separation objective
' % recovery objective
' % purity objective
Installation:
' Feed conveyor width, or how the material will be fed
' The type of magnetic ferrous separation to utilize, or if magnetic ferrous separation is already installed upstream
' Flow capacity desired by the customer
' Type of feeding to utilize to get the speed and burden depth desired
Separation technology plays a major role in the recycling of waste materials. Metals are recovered using one or a combination of metal separators including Magnetic Separators, Eddy Current Separators and ElectroStatic Separators. Eddy Current Separators and ElectroStatic Separators play an important role in maximising metal recovery and revenue from complex streams presently processed by metal recyclers. Such technology enables the recovery and purity enhancement of valuable non-ferrous metals, with both technologies having certain key operational and separation benefits.
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This technical review compares and contrasts the two separation technologies by assessing:
The Eddy Current Separator (ECS) is used widely in the recycling industry to either recover or remove non-ferrous metals from non-metallic materials. Different designs enable the separation of small (down to 3mm) and large non-ferrous metals.
The range of Bunting Eddy Current Separators includes both concentric and eccentric rotor configurations to suit a wide range of separation objectives. The range also includes Laboratory and Pilot-Plant scale Eddy Current Separators, which are used for testing purposes at Bunting's UK-based Centre of Excellence.
The Eddy Current Separator utilises magnetic forces to physically repel non-ferrous metals and enable a separation from non-metallic materials. The Eddy Current Separator is often supplied as part of a metal separation module along with a first-stage Drum Magnet removing ferrous metals.
An eddy current separator is a dual pulley conveyor system, where the non-metallic head pulley houses an independently rotating high-speed magnetic rotor. Separation occurs when a non-ferrous metal particle (e.g. aluminium, copper or zinc) is conveyed into the magnetic zone. The non-ferrous metal particle is exposed to rapidly changing magnetic polarity. In accordance with Faraday's Law of induction, electric currents are induced when conductors enter a rotating field (e.g. non-ferrous metals such as aluminium). By Lenz's Law, the induced eddy currents create a magnetic field that opposes the magnetic field that created it, thus resulting in the conductor being repelled away from the magnetic source.
The two magnetic fields oppose against each other (i.e. North vs North pole repulsion), causing the repulsion of the non-ferrous metal particle and change in trajectory. The measured positioning of a splitter enables the separation of non-ferrous metals from non-metallic material due to the altered and unaltered material trajectories.
The speed of the magnetic rotor is dependent on the number of poles on the magnetic rotor and the application.
A mix of non-magnetic material, within a governed particle size range, is fed via a conveyor or feeder onto the Eddy Current Separator belt. This runs at a higher speed than the material feed allowing the material to spread out, ideally into a mono-layer. The material is conveyed into the head-pulley separation zone, where non-ferrous metals are repelled out of the non-metallic host material. The rejected non-ferrous metals are propelled over a strategically placed splitter and onto a collection conveyor or into a bin.
For a separation to occur, the eddy current forces produced by the separator need to induce enough energy to cause a reaction. The amount of energy induced into a non-metallic metal is proportional to the metal's conductivity, size, shape and weight/density. The ability to impart a separation is also dependent upon the characteristics of the hosting non-metallic material.
The conductivity/density ratio of non-magnetic materials (Table 1) indicates how well a particle reacts. Metals with a high ratio react well and have good separation characteristics.
In practice, the most powerful Eddy Current Separators inject enough energy into particles with a high conductivity/density ratio to produce a reaction on particles as small as 3mm. However, due to the particle size, the energy produced is low. A separation is possible if the particle size range of the hosting material is tightly controlled between 1 and 5mm. As the particle size range broadens, the ability to separate such small sized non-ferrous metals diminishes as there is not enough energy to move the particle a sufficient distance to enable a separation.
The particle shape is another separation variable determining the ability to produce a separation. Flat particles respond well, whereas spheres or wires are more difficult. This is due to the reaction within the particle. The eddy current force causes the particle to turn and then spin. Flatter particles flip and lift whilst circular particles simply turn without being repelled away from the magnetic rotor.
To inject the maximum amount of repulsive energy into a non-ferrous metal particle there needs to be a dwell time when the two opposing fields meet. If this time is too short then not enough injected energy is generated. In operation, very high rotation speeds do not allow for sufficient dwell time to inject the necessary energy to produce a good separation.
In practice, the optimum rotational speed is unique to each design of Eddy Current Separator magnetic rotor and can vary between and RPM.
When deciding on an optimum rotational speed, there are mechanical considerations such as maintenance and wear. As the magnetic rotor speed increases, there is a higher wear factor on key components such as bearings. In operation, the ideal magnetic rotor speed is commonly set at between and RPM, balancing separation performance and wear.
As Eddy Current Separators have different designs of magnetic rotor and the variety of shapes, sizes and characteristics of non-ferrous metal particles is extensive, there is no one belt speed that suits all and each time the application needs to be studied and understood to enable the best level of separation.
For optimum separation, the material needs to be 'settled' on the conveyor belt before entering the 'separation zone' and this is one of the reasons for a long belt.
However, the best level of separation is achieved when material is fed onto the conveyor belt via a Vibratory Feeder. The Vibratory Feeder evenly spreads the material, ideally into a mono-layer, before dropping onto the conveyor belt. As the flow of the material is controlled by the vibration of the Vibratory Feeder, and the end of the feeder tray is close to the surface of the belt, the material settles down quickly and hence a shorter conveyor belt can be used.
In conclusion, the length of belt depends on how the material is being fed:
The Eddy Current Separator is a conveyor with a magnetic rotor acting as a magnetic head pulley. Due to the laws of physics, the Magnetic Rotor will attract ferrous metal and so could be used to separate magnetically susceptible materials. However, there are some limitations:
However, irrespective of the rotor configuration, it is good practice to separate ferrous metals using an appropriate Magnetic Separator prior to the Eddy Current Separator. Plants achieving optimum metal recovery and separation focus on specific materials at key points in the process, including the ferrous and non-ferrous metal fractions.
In theory, magnetic rotors with stronger magnetic fields produce better levels of separation could be true, but in practice the manufacturing and design process mean that there are limits to any magnetic rotor. All Eddy Current Magnetic Rotors are constructed from permanent magnets attached to a carrier pulley. The size of the permanent magnet (both in length around the rotor and thickness) dictates the throw of magnetic field. Longer and thicker magnets produce deeper magnetic fields than shorter and thinner magnets.
Irrespective of whether the magnet is long or short, the maximum magnetic intensity is on the pole (surface) of the magnet. Therefore, in practice, there has already been a loss of magnetic intensity by the time the field reaches the surface of the feed conveyor belt as it has passed through:
If very short magnets are used, a very shallow and intense magnetic field is produced on the pole. However, this field may only just reach the surface of the belt. In contrast, longer magnets produce a lower field on the pole, but throw a deeper magnetic field.
This means that you could have a magnetic rotor with short and exceptionally strong Rare Earth Magnets producing a weaker magnetic field at the point of separation (i.e. on the surface of the conveyor belt) than one constructed from longer standard strength Ferrite Magnets.
Such criteria also makes it very difficult to classify any magnetic rotor as being 'strong' or 'weak' as this definition is totally dependent on where the measurement to support that description is taken. For example:
Magnetic rotor selection is dependent on the metals requiring separation. For large non-ferrous particles (e.g. aluminium can), requiring a magnetic field projected into the centre of that particle for maximum separation effect, then a deep magnetic field with longer magnets is preferable. For small non-ferrous metal particles (e.g. granulated cable), then a shallow magnetic field with shorter more powerful magnets will produce the best separation.
Eddy Current Separators are successfully used to recover and remove non-ferrous metals in a wide-range of recycling applications.
The separation performance and capacity varies according to the application and characteristics of the material (Table 2). The operating criteria for any single project are often optimised with controlled testing at the Bunting Centre of Excellence. The physics of Eddy Current Separator design limits the effective particle size that can be processed on the separator. Typically, conducting particles (even copper and aluminium) struggle to be thrown far enough as the eddy current forces and induced energy are too low, compromising metal recovery and revenue optimisation.
To maximise the separation and recovery of non-ferrous metals, Bunting has developed four different eddy current separator designs:
The Electrostatic Separator separates conducting from non-conducting materials in a size range of +45 microns to 4mm. The technology enables the recovery of metals commonly difficult to separate on an Eddy Current Separator and, subsequently, lost to waste.
Every material has electrostatic properties that potentially allow separation in an external electrostatic field. For metal recycling applications, materials are categorised into conductors and non-conductors (dielectric) materials. When an electrostatic field is applied the particles behave differently, thus enabling a separation.
Conductor materials (mostly metals) will lose the charge to an earthed surface, whilst non-conductors (plastics, glass, and stone) will retain the charge and be pinned to the earthed surface. This effect is known as 'image charge' and is the operating principle of the ElectroStatic Separator. This effect can be utilised to separate highly conductive metals from lower conductivity metals (e.g. aluminium from copper) or conductors from non- conductors (granulated copper cable from plastic insulation).
ElectroStatic Separation exploits the difference in electrical conductivity between various materials in a feed material to produce a separation. The ability to produce a separation depends on a number of key material characteristics including conductivity, moisture content and size range. In many applications, often due to the fine particle size, the ElectroStatic Separator is the only technology that enables a separation. The technology also replaces less environmentally-friendly separation processes such as froth flotation.
The technology is used to either attract or repel particles, both enabling a separation. When using attractive forces, conducting particles stick to an oppositely charged object, such as a metal drum, thereby separating them from other particles. When the type of beneficiation uses repelling force, it is normally employed to change the trajectory of falling objects to sort them into different fractions.
There are three main methods of particle charging/electrification.
The Bunting ElectroStatic Separator uses Corona charging to generate an electrostatic field as this is the most efficient method for metal recycling and mineral processing applications at tonnes per hour capacities.
Feed material (e.g. granulated copper cable) is fed via a controllable vibratory feeder onto the revolving earthed roll in a monolayer. A voltage is applied to the static electrode (typically 20-30 KeV) which generates an electrostatic field as the particles fall onto the roll. Conducting particles (e.g. copper) instantly lose their charge to the earthed roll and are throw off by the imparted centrifugal force generated by the revolving roll. Insulating particles (e.g. plastic) will keep their charge and create an image force that pins it to the roll.
If the image force is greater that the potential centrifugal force generated by the revolving roll the particle will remain pinned to the roll and is discharged by a brush into a separate chute.
Hence a separation takes place on a continuous basis at tonnes per hour capacity for free flowing materials.
The processing history of a sample dictates the ElectroStatic Separator settings. Controlled laboratory tests at Bunting's Centre of Excellence identify the key parameters needed to optimise and then meet the separation objective. The key control variables for separation are:
The ElectroStatic Separator works effectively with a particle size range of between plus 45 micron and minus 4mm. Therefore, this technology enables processors to recycle fine particle size fractions that cannot be effectively treated on an Eddy Current Separator.
As with all physical separation processes, closer particle size range distribution increases separation efficiency. This enables the roll speed and splitter plate positions to be optimised with greater precision, thereby increasing grade and recovery of material being processed.
As with any physical separation, processing excess fine material (under 45 microns) can inhibit the separation efficiency and cause flow issues on the feeding process. Inclusion of this fraction also compromises feed capacity, as a consistent monolayer of material is needed to achieve optimum grade and recovery of valuable metals.
Surface moisture inhibits the separation efficiency of the unit meaning that the feed must in a relatively dry state and free flowing.
For most recycling applications an applied voltage of 20-30 KeV to the tungsten electrode is sufficient. Separation control is achieved by changing the applied voltage in this range to change the quantity of insulator material that is pinned; subsequently adjusting the grade and recovery of the material. The higher the applied voltage- the greater the image force and the more material is pinned. This is particularly useful where there is a proportion of none liberated metal/ plastic scrap material.
The rotational speed (i.e. the centrifugal force) of the roll throws the conducting material (copper/aluminium) from the earthed roll. Balancing the applied voltage, which pins the insulators (plastics), and the roll speed, allows for the creation of separate streams of materials for continuous separation.
Adjustable splitter plates enable the separation of the conducting/non conducting fractions. In some applications, an additional splitter produces a third 'middlings' fraction of non-liberated material, which is often returned to an earlier point of the process for further granulation or shredding to improve liberation.
Surface moisture adversely affects some more sensitive separation and, therefore, the Bunting ElectroStatic Separator has an optional heating unit to control the relative humidity.
The position of the tungsten electrode is adjustable to optimise sensitive separations between materials with similar conductivity.
ElectroStatics is a long-standing separation technology in the minerals industry dating back to the late s. However, the necessity of recovering or separating smaller metal particles from waste products has resulted in many companies investing in ElectroStatic Separations. Typical applications include:
Capacities per meter width for metal recycling applications are commonly 2-3 tonnes per hour, although any capacity and separation are both dependent upon the material characteristics. Such performance criteria are proven with controlled tests at the Bunting Centre of Excellence.
Technology holds the key to increasing the proportion of waste materials reclaimed, reused and recycled. Used in combination, and alongside magnetic separators, the Eddy Current Separator and ElectroStatic Separator enable the recovery of a high proportion of the metallic (ferrous and non-ferrous) materials from any waste.
In practice, the Eddy Current Separator is ideally suited for recovering or separating non-ferrous metals above 3mm. ElectroStatic Separators are perfectly suited for the smaller fraction, separating and recovering conductive metals from 4mm down to +45 microns. ElectroStatic Separators also have the ability to separator different conductors, such as copper from aluminium on tightly-sized fractions.
For additional information on Eddy Current Separation or ElectroStatic Separation, including tests at Bunting's UK Centre of Excellence, please contact us on:
: Gordon Kerr at
Via the website
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