Jan Svoboda. Magnetic Techniques for the Treatment of Materials
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150 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.99: Material separation in eddy-current separator (courtesy of Steinert
GmbH).
N
S
S
N
N
S
N S
S
N
Configuration A Configuration B
T
yp
e 1 T
yp
e 2
Figure 2.100: Two designs of drum eddy-current separators (after [N3]).
2.7. EDDY-CURRENT SEPARATORS 151
Table 2.10: Manufacturers of eddy-current separators [N3].
Manufacturer Type Configuration
Steinert (D) 2 A
Lindemann (D) 1+2 A
Eriez Magnetics (USA) 1 A
Huron Valley (USA) 1 AorB
Andrin (F) 1 A
Bakker Magnetics (NL) 1 A
Raoul Lenoir (F) 1 A
Newell Industries (USA) 1 A
Rutherford Light Metals 1 B
mately to damage the drum. In eccentric design, the ferromagnetic material is
removed automatically from the region of the magnetic field generated by the
rotating drum.
The repulsive force acting by eddy currents on a small particle [S22, Z4] is
proportional to:
I
uhs
p
s
s
s
g
i
E
2
(2.6)
where p
s
and g are the mass and diameter of the particle, respectively, E is
the magnetic induction and i the frequency of its change, and
s
and
s
are
the electrical conductivity and density, respectively, of the particle. It can be
seen that the e!ciency of separation is determined, amongst other factors, by
the ratio of material conductivity and density. The larger the dierence in the
values of this ratio for the materials to be separated, the more selective the
separation will be. Table 2.11 lists this ratio for materials of interest in eddy-
current separation. It is evident that aluminium and magnesium can be rather
e!ciently separated from semi- and non-conductors, while lead and stainless
steel will not respond to eddy currents.
Selection of dierent models of eddy-current separators is usually dictated
by particle size distribution of the feed material and the required throughput.
The width of the belt ranges from 500 mm to 2000 mm and nominal throughput
is of the order of 10 m
3
/h/m. Magnetic induction on the surface of the shell
ranges from 0.25 T to 0.38 T. Particles in the size range - 100 + 5 mm are
usually treated in standard ECSs.
Separation of fine particles Although separation of particles smaller than
5 mm poses a problem [Z5, R6], several innovative designs have successfully
reduced the minimum particle size to 0.1 mm. The Steinert model 6119 [H11]
and Raoul Lenoir model Eddybox, are claimed to be able to separate e!ciently
particles as small as 1 mm. The Eddybox separator is based on free-fall of
metallic particles through a magnetic structure, has no moving parts and is
claimed to achieve a throughput as high as 1 t/h.
152 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Table 2.11: The material parameter for eddy-current separation [N3, S22].
Material @ [10
3
×s×kg
1
×m
2
]
Aluminium 13.0
Magnesium 12.2
Copper 6.3
Silver 6.0
Zinc and Zamac 2.4
Gold 2.1
Brass 1.3 to 1.8
Tin 1.2
Lead 0.45
Stainless steel 0.18
Glass 0
Plastics 0
Figure 2.101: Eddy-current separator Fourmi for separation of fine particles
(Raoul Lenoir).
2.8. SEPARATORS WITH MAGNETIC FLUIDS 153
Figure 2.102: Schematic diagram of an eddy-current separator based on rotation
of fine particles (courtesy of N. Allen).
The design of Fourmi
R
°
eddy-current separator (Raoul Lenoir), shown in
Fig. 2.101, is based on a horizontally rotating polar disc with radially mounted
NdFeB permanent magnets. An alternating magnetic field generates eddy cur-
rents in metallic particles that are fed on a stationary separation disc placed
above the polar disk. The conductive particles, being acted upon by repulsive
forces, become fluidized and are removed by air jets. Particles as small as 0.1
mm are claimed to be e!ciently separated.
It will be shown in Section 3.9.3 that with decreasing particle size the torque
on a particle becomes an important force. Particles in ECS then respond to the
field changes by rotation rather than by repulsion. Allen [A21, A22] proposed
the use of particle rotation as the separating mechanism of fine metal particles.
A schematic diagram of such an ECS based on particle rotation is shown in Fig.
2.102.
2.8 Separators with magnetic fluids
2.8.1 Basic concept
Separation in magnetic fluids is a sink-and-float technique, which exploits dier-
ences in the densities of materials to be separated. In this technique, a magnetic
fluid, placed in a non-homogeneous magnetic field, exhibits an apparent den-
sity dierent from its natural density [R7, R8]. This density can be controlled
through a wide range of values, exceeding densities of all known elements and
materials. While the scope of the conventional sink-and-float technique is re-
154 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
stricted by a limited range of densities of the separation medium, the upper limit
of ferrosilicon medium being approximately 3500 kg/m
3
, and that of Clerici so-
lution about 4200 kg/m
3
at room temperature, magnetic fluids allow densities
as high as 20 000 kg/m
3
or even higher to be achieved.
The apparent density
d
of a magnetic fluid placed in a non-homogeneous
magnetic field can be written (in SI units) as:
d
=
0
+
i
0
j
EuE (2.7)
where
0
is the physical (true) density and
i
is the volume magnetic suscepti-
bility of the fluid.
The apparent density of a magnetic fluid is thus dependent on the magnetic
properties of the fluid, and the magnetic field strength and its gradient. A
magnetic fluid, placed in a non-homogeneous magnetic field, is attracted to the
regions of the highest magnetic field. This magnetic force produces an opposite
force within the fluid that tends to expel an immersed non-magnetic body. This
results in additional, magnetically generated, levitation, which causes particles
to float.
In the early days, solutions of paramagnetic salts (such as MnCl
2
) were used
as a magnetic fluid [A12, A1]. As a result of their low magnetic susceptibility (of
the order of 6×10
7
m
3
/kg) very high magnetic fields and high gradient were
required to achieve densities greater than those obtained using heavy liquids.
For instance, in order to achieve an apparent density of the paramagnetic liquid
equal to 3500 kg/m
3
, the second term on the right-hand side of eq. (2.7) must
be equal to 5×10
4
T
2
/m. For a magnetic induction of 2 T, a gradient as high
as 2.5×10
4
T/m is required. These are unrealistically high values, particularly
for meaningful production-scale applications of this technique.
In the late sixties of the last century methods of preparation of stable sus-
pensions of nano-sized magnetite particles in hydrocarbon carrier fluids were
developed [P5, R10]. These ferromagnetic fluids (ferrofluids) are considerably
more magnetic than the paramagnetic liquid and become magnetically saturated
at moderate magnetic fields. Equation (2.7) can then be re-written as:
d
=
0
+
P
i
j
uE (2.8)
where P
i
is the magnetization of the fluid (in [A/m]). If magnetic polarization
M
i
, (in [T]) is used, then eq. (2.8) is expressed in the form
d
=
0
+
M
i
0
j
uE (2.9)
In fjv units eq. (2.8) can be re-written as
d
=
0
+
P
i
4j
uE (2.10)
where P
i
and E are given in [G].
2.8. SEPARATORS WITH MAGNETIC FLUIDS 155
N
S
FEED
MAGNET
MAGNETIC FLUID
PRODUCTS:
FLOAT
SINK
Figure 2.103: Schematic diagram of a separator with magnetic fluids.
Figure 2.104: Laboratory ferrohydrostatic separator FGS-40 (courtesy of
GMUO, the Ukraine).
156 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Table 2.12: FHS separators manufactured by GMUO [G4].
Model Feed size [mm] Throughput
[kg/h]
Density range
[kg/m
3
]
FGS-1A -50+0.2 60 1200 - 11 000
FGS-70 -25+2.5 200 6500 - 10 000
FGS-40 -10+0.2 5 4000 - 9000
FGS-40/3 -5+0.2 2 2500 - 3500
FGS-200 - 100 + 10 4000 3000 - 6000
It can be seen from eqs. (2.8) to (2.10) that in order to achieve a constant
apparent density of separation, in a magnetically saturated ferrofluid, the gradi-
ent of the magnetic field must be constant. This is achieved by suitably shaping
the pole tips of the magnetic system. A ferrohydrostatic separator (FHS), which
uses a ferrofluid as a working medium, thus produces two fractions, namely a
float and a sinks products. Figure 2.103 illustrates the concept of a separator
with magnetic fluids.
The e!ciency of ferrohydrostatic separation decreases with decreasing size
of particles. In order to overcome this ine!cient separation of fine particles in
gravity-based FHS, a rotation-based separation Magstream process was devel-
oped [W7].
2.8.2 Ferrohydrostatic separators
AVCO Corporation and NASA (USA) pioneered the FHS technique in 1973
by employing a kerosene-based ferrofluid to separate automobile scrap [M12].
Considerable eort to develop this technology was subsequently expended in
Japan and the USA. Hitachi [N4], Nittetsu Mining Co. [S51], Tohoku University
[S50] and US Bureau of Mines [R9] further confirmed the unique features of the
FHS technology. Numerous pilot-plant-scale separators were designed and built,
with throughputs up to 500 kg/h. Although extensive tests confirmed viability
of this technique, particularly when applied to the recovery of non-ferrous metals
from automobile scrap, production-scale application did not take place in the
Western World at that time [F8]. Comprehensive review of the history of FHS
in Japan and the USA was given by Fujita [F8].
Ferrohydrostatic separation was also researched and used, in approximately
the same time period, on a production scale in the former Soviet Union. The
Gipromashugleobogashcheniye Institute (GMUO) in Lugansk, the Ukraine, de-
veloped a comprehensive range of models of ferrohydrostatic separators for dif-
ferent applications and throughputs. These separators were used in such diverse
applications as densimetric analysis of coal, recovery of gold from gravity con-
centrates, diamond concentration and separation of non-ferrous scrap. Table
2.12 summarizes various models of FHS units manufactured by GMUO [G4].
Figure 2.104 illustrates the versatile FGS-40 separator suitable for laboratory-
scale separation of various minerals in the density range from 4000 kg/m
3
to
2.8. SEPARATORS WITH MAGNETIC FLUIDS 157
Figure 2.105: Production-scale ferrohydrostatic separator FGS-70 for separation
of non-ferrous metals (courtesy of GMUO, the Ukraine).
158 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.106: Ferrohydrostatic separator Shlikh-1 with permanent magnets for
recovery of alluvial gold (Too Geos Co.).
9000 kg/m
3
. The FHS unit designed to separate non-ferrous metals in the den-
sity range between 6500 kg/m
3
and 10 000 kg/m
3
, with particle size up to 25
mm, is shown in Figure 2.105.
As a result of geopolitical changes in the nineties of the last century, most
markets for GMUO ferrohydrostatic separators in the former Soviet Union disap-
peared and the separators have not been innovated to meet the current standard
of technology.
Ferrohydrostatic separators based on permanent magnets are produced by
Too Geos Co. (Russia). These small-scale low-throughput machines have been
designed for the recovery of alluvial gold from gravity concentrates. Throughput
of the Shlikh-2 model is approximately 50 kg/h, while a laboratory model Shlikh-
1, shown in Fig. 2.106 can treat about 10 kg/h.
All models of the Shlikh FHS separator are supplied together with a vibrating
screen and disk and roll magnetic separators, as shown in Fig. 2.107. These
units allow the classification of the material and the removal of strongly and
medium magnetic materials from the feed into the FHS unit.
Since FHS is a unique technique for material manipulation in areas such as
diamond-processing industry, De Beers Consolidated Mines (Pty.) Ltd. (South
Africa) developed a range of selective FHS separators. These fully automated
computer-controlled, hands-o units are capable of treating particles in the size
2.8. SEPARATORS WITH MAGNETIC FLUIDS 159
Figure 2.107: Shlikh-1 system comprising a vibrating screen, disk and roll mag-
netic separators and a ferrohydrostatic separator (Too Geos Co.).
range - 25 + 0.3 mm and can distinguish a density dierence of at least 20
kg/m
3
(0.02 g/cm
3
). Units with a throughput exceeding 1 t/h, depending on
the feed density and size, have been built [S23]. Figure 2.108 shows a two-
stage automated and secure ferrohydrostatic separator built by De Beers for
the recovery of diamonds. The ferrohydrostatic separator Rhomag
R
°
, shown in
Figure 2.109, is used for separating fine fractions of exploration samples.
2.8.3 Magstream separator
The main limitation of ferrohydrostatic separation, in which the important com-
peting force is the force of gravity, is ine!cient separation of fine particles. Ef-
forts were made by Revnetsev [R11] and Andres et al. [A14] to replace the force
of gravity by the centrifugal force. This proposal resulted in a rotation-based
separation process Magstream [W7] developed by Intermagnetics General Corp.
The essential features of the Magstream separation process are shown in
Fig. 2.110. Particles to be separated are combined with a ferrofluid and passed
downward through a long, vertically oriented annular duct where they rotate
with the flow guide and duct about the duct axis. The duct is surrounded by a
multipole magnet that produces a non-homogeneous magnetic field. A radially
inward displacement force, which results from the radially outward attraction
of the magnetic fluid, is felt by all particles. This force is opposed (or aided)
by a radially outward (or inward) magnetic force acting on paramagnetic (or
diamagnetic) particles, and by the centrifugal force.
At a given speed of rotation, the inward displacement forces are greater on
light particles, which then move radially inwards. Heavier particles experiencing