KONA Powder and Particle Journal No. 34 (2017) 241–247/Doi:10.14356/kona.

2017012 Original Research Paper

Eddy Current Separation of Nonferrous Metals

Using a Variable-Frequency Electromagnet †

Nakul Dholu, James R. Nagel, Dave Cohrs and Raj K. Rajamani *

1
Department of Metallurgical Engineering, University of Utah, USA

Abstract
We present a novel technique for sorting nonferrous metal scrap by using eddy current separation. However,
rather than vary the magnetic field with a spinning rotary drum, our system utilizes a fixed electromagnet excited
by an alternating electric current. The technique requires no moving parts other than a feeding mechanism and
has the capacity to operate at excitation frequencies up to 50 kHz and beyond. Sorting results are demonstrated
using various combinations of metal spheres, which resulted in nearly perfect performance in terms of grade and
recovery. We also demonstrate sorting of aluminum alloys from other aluminum alloys, with consistent grade and
recovery between 85–95 %.

Keywords: eddy-current separator, electromagnet, nonferrous, metals, alloys

1. Introduction permanent magnets mounted into specialized conveyors

and collectors (Weiss, 1985). This typically leaves behind
Metal recycling is a multibillion dollar industry with a stream of nonferrous metals like copper, aluminum and
tremendous economic opportunities and positive environ- brass, mixed with other nonmetallic fluff like rubber, tex-
mental impact. In 2015 alone, global aluminum produc- tiles, and plastic. Recovery of nonferrous metals then typ-
tion from mined ore reached well over 58 million metric ically occurs through the use of standard eddy current
tons, selling at an average price of $0.88 per pound separators, which consist of a spinning drum of perma-
(USGS, 2016). Global copper production likewise reached nent magnets placed under a moving conveyor belt
over 18 million tons with an average price of $2.77 per (Schloemann, 1975; Lungu and Rem, 2003; Lungu, 2005).
pound. Nearly all of this material eventually ends up in As metallic particles pass over the magnets, electrical
municipal waste streams wherein local scrap recyclers eddy currents are induced by the alternating magnetic
generate significant wealth by collecting and selling the field, which in turn produces a net magnetic force that al-
valuable metals. In 2013, for example, 55 % of all alumi- ters their trajectories.
num produced in the United States came from recycled Once the nonferrous metals have been separated from
scrap (USGS, 2015). Copper, in comparison, was pro- the fluff, it is desirable to further separate the base metals
duced from 33 % recycled material. In principle, these into purified streams. For example, aluminum and copper
values could reach much higher if not for the great diffi- are two common metals that might exist in relative mass
culty in sorting various materials into more pure streams. concentrations of 90 % Al to 10 % Cu. Since copper is a
Consequently, there is a strong economic pressure to find distinctly red metal that contrasts against the gray of alu-
new ways to efficiently sort municipal waste streams into minum, one common sorting method might utilize com-
their basic, elemental components. puterized optical sorting via basic color recognition
To meet this challenge, many industrial machines and (Kutila et al., 2005). Alternatively, one could exploit the
processes have been engineered specifically to sort valu- fact that copper is nearly three times more dense than alu-
able materials from a mixed stream of waste (Gaustad et minum, thereby utilizing the popular technique of dense
al., 2012). For example, separation of ferrous metals, such medium separation (Weiss, 1985). In many developing
as iron and steel, is readily accomplished through large, countries, the cost of human labor is so low that metals
are simply sorted directly by hand (Spencer, 2005). All of
†
Received 3 May 2016; Accepted 14 June 2016 these methods have various costs, capabilities, and
J-STAGE Advance Publication online 9 July 2016 throughputs associated with them, and individual recy-
1
Salt Lake City, UT 84112, USA cling firms must generally weigh the pros and cons within
* Corresponding author: Raj k. Rajamani
E-mail: raj.rajamani@utah.edu their own economic niche.
TEL: +1-801-581-3107 In this paper, we will introduce a new method for sepa-

Copyright © 2017 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open access
article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 241
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rating nonferrous metals from other nonferrous metals, an alternating electric current. This significantly changes
including nonferrous alloys. Dubbed electrodynamic sort- the mathematical nature of the physics involved as now
ing (EDX), our system utilizes a time-varying magnetic the relative velocity between the magnet and particle is no
field generated by an alternating electric current within a longer a significant factor. We may therefore begin with
fixed electromagnet. The principle was originally pio- Faraday’s law of electromagnetic induction, which states
neered by Saveliev to extract gold particles from rocky that a time-varying magnetic field B will give rise to an
ore (Saveliev, 1998) but has since remained relatively un- electric field E in accordance with (Jackson, 1999)
noticed and unrefined. The process is very similar to tra-
B
ditional eddy current separators mentioned earlier, but  E  . (1)
without the need for mechanically intense spinning of any t
heavy, rotary magnets. Another distinct advantage to our If we next assume sinusoidal steady-state operation at an-
approach is the ability to tune the frequency of excitation gular frequency ω, all time derivatives can be replaced
to as high as 50 kHz and beyond. This allows for much with ∂/∂t = –jω. The phasor form of Faraday’s law is
more active recovery of smaller particles in the realm of therefore written as
1.0 cm and below, which can be especially difficult for
E j B . (2)
rotary-based separators and human separators to work
with. Our system also has the distinct advantage of being Now let us consider a metal particle placed somewhere in
completely dry, thus avoiding the wet slurry contamina- the magnetic field B. Because of the induced electric field
tion commonly used in many flotation-based separators. E, electric charges within the metal are accelerated ac-
Beginning in Section 2, we discuss the basic physical cordingly, thus giving rise to an eddy current density J.
mechanism of eddy current separation as a derived prin- The relation between E and J is then given by the point
ciple of Maxwell’s equations. Section 3 then follows with form of Ohm’s law, written as
a high-level description of the EDX system, including a
J = σE , (3)
mechanical diagram and electrical circuit model. Sorting
data is then presented in Section 4, wherein metal spheres where σ is the electrical conductivity of the metal. Chang-
of various sizes and conductivities are separated by our ing magnetic fields therefore give rise to electric fields,
system. Section 5 finally concludes with a discussion of which in turn give rise to eddy currents in metallic ob-
further applications and research goals for EDX. jects.
Next, we note that electrical currents also give rise to
magnetic fields of their own in accordance with Ampere’s
2. Theory of eddy current separation law,

  Be 
µ0J , (4)
Eddy current separation is a highly mature technology
with many practical applications in the recycling industry. where Be indicates the magnetic fields produced just by
The basic principle is derived from the fact that when a the eddy current density J. This implies more changing
permanent magnet passes over a conductive metal object, magnetic fields that must be accounted for in Eqn. (1),
electric charges within the metal tend to experience a net which in turn cause changes in the resultant eddy current
magnetic force (Rem et al., 1997). This causes the charges J. Thus, a complete solution for J, E, and B requires us to
to flow around in distinct swirling patterns, commonly solve Eqns. (1)–(4) simultaneously. While such a process
referred to as eddy currents or Foucault currents. If the extends well beyond the scope of this article, we can at
magnetic field is strong enough and the relative motion least note that once a solution for J is finally obtained, it
quick enough, this force can significantly accelerate the becomes possible to calculate the net Lorentz force F act-
entire metal particle. Thus, a common design theme with ing on the metal particle. This is given by the magnetic
modern eddy current separators is a large, rotary drum force law,
implanted with a series of permanent magnets. When the
drum is mechanically rotated at a very high velocity, F
  J  B dV , (5)
nearby metal particles tend to deflect along distinct kine-
matic trajectories away from other nonmetallic fluff. A where the integral is carried out over the volume defined
physical barrier is then placed somewhere between the by the space within the particle. Thus, if the excitation
two trajectories, thus separating material into two distinct field B is strong enough and the frequency ω great
streams. enough, the net force F will be great enough to signifi-
For the case of electrodynamic sorting, we completely cantly deflect arbitrarily large particles of metal.
do away with the spinning permanent magnets, replacing
them instead with a single fixed electromagnet excited by

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3. Description of electrodynamic sorting our system usually only requires about 350 W to operate.
At the output of the power amplifier, we employ a se-
The basic configuration of our EDX system is depicted ries RLC circuit to drive the core. Because the primary
in Fig. 1. The core of the system is a wire-wound ferrite electromagnet is comprised of a wire-wound ferrite, it
toroid, excited by an alternating electric current. A spe- naturally manifests as a lumped inductance L with values
cialized gap is then cut out from the toroid and serves as typically ranging between 1–10 mH. In series with the in-
the concentration point of time-varying magnetic fields. ductor is an equivalent resistance R, representing the in-
As particles enter the gap, eddy currents are induced in ternal thermal loading of the magnet that occurs during
accordance with various factors, like conductivity, geom- operation. Typical values may reach as high as 20 Ω, but
etry, B-field variation, and frequency of excitation. In can vary strongly with the size of the magnet, the mag-
particular, particles with very high conductivity will tend netic material, and the frequency of operation. However,
to experience very strong forces, thus pushing them away this value is usually dwarfed by the reactive impedance of
from their natural kinematic trajectories. Dense particles, the inductor, which can easily reach many hundreds of
however, have greater inertia and will therefore resist be- Ohms when excited to several kilohertz. We therefore
ing accelerated. Where exactly the particles land will placed a lumped capacitance C in series with L, thus cre-
therefore depend on a combination of both factors, which ating a resonant RLC circuit. When excited at the reso-
are fortunately varied enough to produce highly distinct nant frequency f0 = 1/( 2π LC ) the capacitive reactance
trajectories among most materials of interest. Thus, a perfectly negates the inductive reactance, thus leaving
simple mechanical splitter is all that is required to finally only the series resistance R to impede current.
separate materials into their corresponding bins. Fig. 3 shows an isometric view of the magnetic core it-
Fig. 2 shows an electrical circuit diagram for the EDX self, which has been shaped into a square toroid. The di-
drive electronics. A small-signal voltage Vs is produced by mensions are given by an outer radius of 440 mm, inner
a standard signal generator, which determines the fre- radius of 240 mm, and height of 30 mm. The gap is a spe-
quency and amplitude of the excitation signal. This signal cialized double-cut, which is used to funnel flux into a
is then fed to the high-power amplifier U1, which is tight volume of space and serves as the feeding point for
needed in order to drive the core with enough current to small metal particles. At the point of inner radius, the gap
saturate the magnet. For this particular system, we used separation starts at 10 mm with a flare angle of 10°. The
the AE Techron model 7796, which is rated to deliver up- flare then extends radially outward for 30 mm before
wards of 5 kW of continuous power. In practice, however, opening up to a widened flare of 40°.
Considering that the core must channel strong magnetic
fields at high frequency, the specific choice of material is
very important. In particular, electrical conductivity must
be very low, or else eddy currents induced within the
core, itself tend to result in heavy thermal dissipation. It is
also important that the material be magnetically soft,
meaning that hysteresis effects must be small as well.
NiZn ferrite is well-known for satisfying both of these
properties (Leary et al., 2012) and was therefore used to
construct our EDX magnet.
Fig. 1  Schematic diagram of the electrodynamic sorting sys-
Finally, the core must be wound with electrical wiring
tem.
and excited by an alternating current. The exact number
of turns in the windings is somewhat arbitrary, though
many practical issues must be carefully weighed. In prin-
ciple, if the drive current I is held constant, then a dou-
bling of the number of turns N will subsequently double

Fig. 2  Equivalent circuit model for EDX drive electronics. Fig. 3  Ferrite core geometry.

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the magnetic field intensity B within the gap. However,

this also increases the inductance of the coil by a factor of
four, thus quadrupling the corresponding voltage at a
given frequency. If this voltage gets too large, then the in-
sulation around the wiring itself can potentially break
down and halt the system. Likewise, we can reduce the
value of N to maintain a reasonable voltage, but only by
reducing the current density around the core. This causes
B to fall accordingly, which can only be compensated for
by increasing I. After careful trial-and-error, we eventu-
ally settled on a value of N = 300 turns, which was
enough to saturate the magnet at a peak current of
roughly I = 4.5 A.

4. Sort demo and results

In order to quantify our sorting data, it is first neces-

Fig. 4  Primary EDX components (power amplifier not shown).
sary to define the quality metrics for a given sorting pro-
cess. We begin by considering two initial masses, m0 and
M0, of differing materials mixed together at the input of Table 1  Electrical conductivity and mass density for various
the sorter. At the output are two bins, labeled bin A and metals and alloys under consideration
bin B. The goal of the sort process is for all of mass m0 to
Material Conductivity [MS/m] Density [g/cm3]
collect in bin A and for all of mass M0 to collect in bin B,
with no errors either way. We therefore define mA as the Copper 58.5 9.0
total mass from material m0 that falls into bin A, with mB Brass 15.9 8.5
as the total mass falling into bin B. Likewise, we can de- Al-1100 34.4 2.7
fine MA and MB as the total masses from M0 that fall into
Al-6061 24.6 2.7
their respective bins.
Let us now define the recovery of bin A as the fraction Al-2024 17.3 2.8
of initial material that collects in its proper bin:
mA
RA  . each sort. To demonstrate the EDX system, we mixed to-
m0 (6)
gether various batches of metal spheres with contrasting
Likewise, the recovery of bin B is defined as material properties and then sorted them accordingly. In
all cases, the number of spheres was at least 100 for each
MB
RB  . material being sorted.
M0 (7)
Table 1 lists the materials under consideration for this
Next, we define the grade of bin A as the fraction of total study as well as their relevant physical properties. In par-
mass in bin A comprised of its target material. This is ticular, we were interested in two distinct groups of trials.
written as The first case focused solely on the dissimilar metals Al/
mA Cu, Cu/Brass, and Al/Brass, using uniform spheres with
GA  , 6.0 mm diameter. In this case, the magnet was excited to
mA  M A (8)
a frequency of 6.5 kHz and driven to a peak current of
with the grade of bin B likewise following 4.5 A. The second case then focused on sorting metal al-
mB loys, including Al-1100, Al-6061, and Al-2024. In this
GB  . case, the sphere diameters were slightly larger at 12.5 mm
mB  M B (9)
and the frequency was lowered down to 1.9 kHz.
Thus, in a perfectly ideal world, both R and G would ap- Table 2 shows a summary of our sorting trials. For the
proach 100 % for each bin. case of dissimilar metals, the disparity in trajectories was
Fig. 4 shows a photograph of our experimental test bed somewhat dramatic and highly consistent. For example,
with the feeder, magnet, and sort bins all visible. The di- upon reaching the separator, the disparity between alumi-
vider is fixed at a distance of 60 cm below the magnet but num spheres and copper spheres was well over 6 cm (10
could be translated horizontally to an optimal position for diameters), with random variations of only 1–2 cm. As a

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consequence, both recovery and grade were consistently come is further depicted in Fig. 5c, which shows the alloy
100 % over multiple runs. Similar results were also mani- mixtures before and after the sort.
fest with Cu/Brass and Al/Brass, likewise giving perfect
outcomes. Figs. 5a and 5b show samples of typical sort-
ing results both before and after the sort. 5. Discussion and conclusion
Only when we attempted to sort aluminum alloys from
other alloys did separation errors begin to emerge. In this As the results in Table 2 show, the principle of electro-
case, the mass density of the particles is virtually identi- dynamic sorting exhibits a powerful capacity for sorting
cal, leaving only small variations in conductivity by nonferrous metals from other nonferrous metals. This ca-
which to sort. Thus, the separation distances were typi- pacity was also extended to metal alloy separation, being
cally in the range of only 3–4 cm (3 diameters), with al- demonstrated with various combinations of aluminum al-
most as much variance causing errors. Even so, the results loys. While we only limited ourselves to particles on the
were still consistently very good, leading to both grade order of 0.5–1.0 cm, the basic principle could easily be re-
and recovery of well over 85 %. A typical sorting out- designed for both larger and smaller geometries. The sys-
tem utilizes no moving parts other than those needed to
Table 2  Grade and recovery results under various sorting
physically feed material into the magnet and requires no
conditions special chemical treatment or slurry in order to process
material.
Materials (A/B) RA [%] RB [%] GA [%] GB [%]
While such results are initially very impressive, it is
Al/Cu 100 100 100 100 important to remember that our particles were perfectly
Al/Brass 100 100 100 100 uniform spheres. This provided a tremendous level of
consistency that would likely not exist under real-world
Cu/Brass 100 100 100 100
conditions. It also has the added advantage of being math-
Al-1100/Al-2024 100 100 100 100 ematically symmetric, thus lending itself to closed-form
Al-1100/Al-6061 99 91 92 99 analytic solutions to the resulting force equations (Lohofer,
Al-6061/Al-2024 91 86 87 91
1989). This is invaluable in identifying the primary fac-

Fig. 5  (a) Sorting results between 6.0 mm copper and brass spheres. (b) 6.0 mm aluminum and copper spheres.
(c) 12.5 mm aluminium alloys. Al-6061 spheres are painted black with Al-2014 spheres painted white.

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tors which contribute to particle forces as well as their M0 initial mass of material 2 (kg)
varying significance when compared to each other. In
mA mass of material 1 in bin A (kg)
practice, however, real-world scrap is highly variable in
MA mass of material 2 in bin A (kg)
size, geometry, and even composition, and will naturally
exhibit much greater variance in trajectories during the mB mass of material 1 in bin B (kg)
sorting process. We are therefore currently in the process MB mass of material 2 in bin B (kg)
of collecting real-world scrap material and testing our N wire turn number
systemʼs performance accordingly. The results of these
R electrical resistance (Ω)
investigations, as well as the challenges we overcome,
RA recovery of sort bin A (%)
will be the subject of future publications.
One particular feature that requires further research is RB recovery of sort bin B (%)
the problem of throughput. In practice, it will be neces- RLC resistor, inductor, capacitor
sary to process many hundreds of kilograms per hour in V volume (m3)
order to gain any interest from industrial recyclers. How-
σ electrical conductivity (S/m)
ever, due to the bottleneck inherent to the magnetic gap,
µ0 permeability of free space (H/m)
our current system can only process perhaps 50–100 kg
per hour when pushed to its limits. Increasing throughput ω angular frequency (rad/s)
also tends to add greater chaotic variation to the feed pro-
cess, thereby reducing grade and recovery in the outcome.
Further design iterations must therefore increase through- References
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Author’s short biography

Nakul Dholu

Nakul Dholu completed his undergraduate degree in Metallurgical Engineering from

Gujarat Technological University, Gandhinagar, India, in 2013. He graduated with MS
degree in Metallurgical Engineering as well from the University of Utah, USA, in 2016
under the supervision of his Professor Dr. Raj Rajamani. Currently he is working as a
Metallurgist-I at the Freeport-McMoRan Inc. in Arizona, USA. His research interests are
in the field of light metal scrap recycling techniques and developing energy efficient
mineral processing operations.

James Nagel

Dr. James Nagel completed his undergraduate degree in 2004 and MS degree in 2006,
both in Electrical Engineering, from Brigham Young University in Provo, Utah. He then
completed his PhD in 2011, also in Electrical Engineering, from the University of Utah
in Salt Lake City, Utah and was awarded the Stockham Medal of Excellence for Con-
spicuously Effective Teaching. He now works as a research associate for the University
of Utah, where his research focuses on applied electromagnetics with a specialty in nu-
merical methods.

Dave Cohrs

Dave studied Environmental Science with emphasis on Analytical Chemistry at Uni-

versity of Maryland, Baltimore County in Maryland. He managed water quality labora-
tories at public aquariums for 16 years and has consulted for aquariums, zoos, and
academia on water chemistry and life support topics. He now works for the University of
Utah as a research associate.

Raj K. Rajamani

Raj Rajamani has been on the faculty of the Metallurgical Engineering Department of
the University of Utah, Salt Lake City, Utah, USA since 1980. Currently he holds the po-
sition of professor. His research interests include population balance modeling of tum-
bling mills, computational fluid dynamics of hydrocyclones, discrete element modeling
of semi-autogenous grinding mills, eddy current sorting of metallic particles and model-
ing of high pressure grinding rolls. He received the Antoine M. Gaudin Award, presented
by the Society of Mining, Metallurgy and Exploration Engineers Inc. in the year 2009.

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