Jan Svoboda. Magnetic Techniques for the Treatment of Materials
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40 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT
Figure 1.27: Magnetic susceptibility meter KLF-4 (courtesy of Agico Instru-
ments).
Magnetic induction methods
In the simplest application of the induction principle, a sensing coil is wound
around a sample. A second winding is used to generate variable magnetic field,
which produces a magnetization. There are several instruments based on the in-
duction principle available commercially. They are essentially induction bridges
that measure magnetic susceptibility of a sample of any shape and form while the
mass of the specimens can range from a few grams to several tens of grams. Fig-
ure 1.27 illustrates such an automatic computer-controlled inductivity bridge,
which is suitable for rapid laboratory measurement of magnetic susceptibility
of rocks, soils, minerals and other materials. Sensitivity of the instrument is
1×10
6
(SI) and the measurement is conducted at the magnetic field range
from 10 A/m to 300 A/m (0.12 G to 3.8 G). A similar instrument with resolu-
tion of 2×10
6
(SI) is shown in Fig. 1.28.
An even more sensitive instrument is the KLY-3 Kappabridge shown in Fig.
1.29. It employs a fully automatic zeroing system with autoranging and auto-
matic compensation of the thermal drift and its sensitivity is specified as 2×10
8
(SI).
While the above described laboratory susceptibility meters could be of con-
siderable interest and use for practitioners and researchers in magnetic sepa-
ration, they might not be easily aordable and su!ciently portable in plant
operation. For field operation there are several types of portable susceptibil-
ity meters commercially available. Figure 1.30 shows such an easily aordable
pocket-size susceptibility meter. The instrument weighs 0.18 kg and has the
measurement sensitivity of 1×10
7
(SI). The field data can be stored in 250
memory registers and recalled later.
The advantages of the induction bridge susceptibility meters are their high
accuracy, fast measuring rate and automated computer-controlled mode of op-
eration. High sensitivity enables one to measure the magnetic susceptibility of
1.6. MEASUREMENT OF MAGNETIC PROPERTIES 41
Figure 1.28: Magnetic susceptibility meter MS2 (courtesy of Bartington Instru-
ments).
Figure 1.29: Magnetic susceptibility meter Kappabridge KLY-3 (courtesy of
Agico Instruments).
42 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT
Figure 1.30: Pocket-size magnetic susceptibility meter SM-30 (courtesy of ZH
Instruments).
Table 1.12: Magnetic susceptibility of pyrrhotite measured by an induction
bridge [S8].
Magnetic field [G] Volume magnetic susceptibility ([SI]x10
3
)
0.0256 186
0.9 233
5.36 357
materials ranging from diamagnetic to ferromagnetic.
The main drawback from the magnetic separation point of view is that these
instruments operate at very low magnetic field and do not allow measurement of
the susceptibility as a function of the magnetic field strength. The measurement
of susceptibility at a low magnetic fields is only acceptable for paramagnetic and
diamagnetic materials for which the susceptibility is field-independent. For ma-
terials with ordered structures, such as ferromagnetic (v=o=) materials, magnetic
permeability and magnetic susceptibility are strongly dependent on the exter-
nal magnetic field, as has been demonstrated in Figs. 1.9 and 1.15. Table 1.12
illustrates the dependence of the initial magnetic susceptibility on the magnetic
field strength, as measured by an induction bridge.
It can be seen from Figs. 1.9 and 1.15 that the initial permeability or
susceptibility is generally smaller than the eective permeability at the magnetic
field at which magnetic separation takes place, i.e.
l
?
hii
= If the external
magnetic field is su!ciently high then $ 1 and $ 0, and the magnetic
force on a particle will be determined by the saturation magnetization P
v
as
follows from eq. 1.44. The value of P
v
can be high or low, depending on the
1.6. MEASUREMENT OF MAGNETIC PROPERTIES 43
Figure 1.31: Vibrating sample magnetometer (courtesy of LDJ, Inc.).
type of material, irrespective of what the value of the initial permeability (or
susceptibility) is. It is thus clear that evaluation of separability of ferromagnetic
(v=o=) materials based on the measurement of the initial magnetic susceptibility
can lead to erroneous conclusions.
Vibrating sample magnetometer
A vibrating sample magnetometer (VSM), shown in Fig. 1.31, is one of the most
popular techniques in magnetometry [F3]. The sample is vibrated at small
amplitude with a known frequency and phase in a uniform applied magnetic
field. The field distortion produced by the sample varies in an a.c. manner
and is detected by a pick-up coil. Using a lock-in amplifier to filter out signals
at frequencies other than the sample vibration frequency, VSM is able to make
highly sensitive measurements. The magnitude of the signal is dependent on the
magnetic properties of the sample. The external magnetic field can be produced
by superconducting magnets, various types of electromagnets or Helmholtz coils,
providing excitation fields from a fraction of milliTesla to many Tesla.
For all magnetizable materials, the VSM test results are displayed as mag-
netization curves. For ferromagnetic (v=o=) materials the results are in the form
of the simple hysteresis loop, with values provided for all of the critical para-
meters, such as coercive force, remanent and saturation magnetization, applied
magnetic field and others. VSM can be used to test magnetic properties of
metal alloys, minerals, permanent magnets, thin films, magnetic fluids etc. A
typical hysteresis curve, as obtained by VSM, is shown in Fig. 1.32.
For purposes of evaluation of magnetic separability of materials, VSM is
a very useful, albeit rather expensive instrument. Since susceptibilities of dia-
magnetic and paramagnetic materials do not depend on the strength of the
44 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT
Figure 1.32: The hysteresis curve for mild steel, as obtained by VSM.
external magnetic field, information obtained using inductive bridges is su!-
cient to access separability of such materials. For ferromagnetic (v=o=) materials,
however, the magnetic susceptibility is strongly dependent on the field strength
and separability of such materials is determined by the magnetic properties at
the operating magnetic field strength of a given magnetic separator.
Vibrating sample magnetometer allows the determination of the magnetic
susceptibility at any point on the hysteresis curve (Fig.1.32), as the tangent to
the curve. It is thus possible to determine the magnetic susceptibility at any
value of the magnetic field. This procedure is useful only in the linear part of
the magnetization curve. Once the material begins to saturate magnetically,
the susceptibility approaches zero and the magnetic force on a particles is de-
termined by the saturation magnetization. A typical field dependence of the
magnetic susceptibility as determined from the magnetization curve obtained
using VSM is shown in Fig. 1.33.
As has been stated earlier, in magnetic separation many paramagnetic and
diamagnetic materials, and minerals in particular, contain ferromagnetic (v=o=)
admixtures. These admixtures can significantly influence the values of magnetic
susceptibility as a consequence of its dependence on magnetic field, temperature
and particle size. A correct evaluation of magnetic susceptibility in terms of eq.
(1.44), under given experimental or operational conditions, can only be eected
with instruments such as VSM. However, VSM is only rarely accessible to a
practitioner or even researcher in magnetic separation and detailed information
about magnetic properties of minerals available in literature is limited. An
exception is the work of Dahlin and Rule [D2] who investigated the magnetic
susceptibility of numerous minerals as a function of the magnetic field strength.
1.7. SOURCES OF MAGNETIC FIELD 45
Figure 1.33: Volume magnetic susceptibility of magnetite and ferrosilicon 270D,
as a function of magnetic field, as measured by VSM [S8].
A vibrating sample magnetometer with a superconducting magnet capable of
producing a magnetic field of 7.16×10
6
A/m(90kOe)wasused. Theresults
showed that magnetic susceptibility of most minerals that were investigated
was essentially constant in magnetic fields above those needed to saturate the
ferromagnetic constituents, in agreement with eq. (1.44). Figure 1.34 illustrates
some of the results.
Hysteresisgraphs
Hysteresisgraphs are systems for testing and analyzing hysteresis properties (B-
H loop) of hard and soft magnetic materials. The commercially available units
are usually precision personal computer-controlled systems. Designers of per-
manent magnet-based magnetic separators can use such instruments to test,
analyze and quality-control hard magnetic materials such as ferrites, Alnico,
samarium-cobalt and neodymium-iron-boron alloys. Figure 1.35 illustrates such
a hysteresisgraph.
1.7 Sources of magnetic field in magnetic sep-
aration equipment
The aim of magnetic separation is to recover or remove particles with sizes rang-
ing from several tens of millimeters down to a fraction of micrometre, with a
wide spectrum of magnetic susceptibilities, from ferromagnetic to weakly para-
magnetic. Furthermore, it is frequently necessary to separate two or more ma-
46 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT
Figure 1.34: Magnetic susceptibilities of various minerals, measured by VSM, as
a function of magnetic field strength (adapted from [D2]).
Figure 1.35: Hysteresisgraph Permagraph (courtesy of Magnet Physik GmbH)
for quality control of permanent magnets.
1.7. SOURCES OF MAGNETIC FIELD 47
terial species, the magnetic susceptibilities of which do not dier very much, of
a wide range of particle sizes. The magnetic field strength needed to recover
such particles, therefore, varies substantially from one application to another
and dierent ways of generating the magnetic field are employed.
As can be seen from eq. (1.8), the magnetic force required to separate a
magnetizable particle depends not only on the physical properties of a material,
and on the magnetic field strength, but also on the value of the gradient of
the magnetic field. Therefore, to achieve a selective separation of a valuable
material from unwanted ones, a carefully selected combination of the magnetic
field strength and its gradient must be used to warrant a high recovery and
simultaneously a high grade of the concentrate. Magnetic separation can be
considered successful only if it produces the right quality of the concentrate
with the right recovery.
In some applications of magnetic separation, for instance in magnetic filtra-
tion, the ”brute force” approach is often used to ensure a high degree of removal
of magnetizable solids from suspension. Consequently an eort is usually made
to maximize the external magnetic field or the field gradient, or both. This ap-
proach is also employed in some applications in mineral processing, for instance
in removal of magnetic contaminants from industrial minerals. Such a combi-
nation of high magnetic field and field gradient often results in a high-quality
non-magnetic product, at the expense of losses of the non-magnetic compo-
nent into the magnetic fraction. Undue emphasis on the magnetic field strength
and/or field gradient sometimes results in reduced recovery of the valuable com-
ponents and reduced selectivity. Excessively high capital and operating costs are
additional consequences of such an approach. In the majority of applications,
only a moderate magnetic field, combined with an optimized field gradient, are
required to achieve the optimum results.
The desired magnetic induction and the field gradient are thus a function
of the type of application, required metallurgical performance and economic
criteria. In the following section the basic methods of the generation of magnetic
fields and their gradients, as used in magnetic separation, will be reviewed.
1.7.1 Permanent magnets
Recent years have seen a strongly increasing industrial demand for permanent
magnet materials. Permanent magnets have become essential components in
many electric, electronic and electromechanical devices, including magnetic sep-
arators. Although the earliest permanent magnet, lodestone, was known in an-
cient times, the greatest strides in magnet development have occurred only over
the past hundred years. Each advance has been connected with the discovery of
a new class of materials, characterized by ever more desirable properties. This
evolution has been monitored generally by one figure of merit for a permanent
magnet, the maximum energy product (EK)
max
, which provides a measure of
the field that can be produced outside a unit volume of magnet material. The
so-called theoretical maximum energy product, the largest value realizable in
48 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT
Table 1.13: Magnetic properties of Alnico alloys.
Material B
u
[T] H
f
[Oe] H
fl
[Oe] (BH)
max
[MGOe
(kJ/m
3
)]
Alnico 5 1.27 640 645 5.5 (44.0)
Alnico 5-7 1.34 740 745 7.5 (60.0)
Alnico 5DG 1.33 685 690 6.5 (52.0)
Alnico 8B 0.9 1600 1640 6.75 (54.0)
Alnico 9 1.05 1500 1515 10.5 (84.0)
principle, is an intrinsic quantity defined by
(EK)
max
=
1
0
(
M
v
2
)
2
(1.49)
where M
v
is the saturation polarization (in [T]).
This maximum energy product (in [J/m
3
]) can be achieved only if the magnet
retains M
v
in a reverse field at least as large as M
v
@2 [H4]. The maximum energy
product is thus limited, to perhaps 1.2×10
6
kJ/m
3
(150×10
6
MGOe), by the
maximum saturation polarization of elements and alloys of approximately 2.5 T.
At the present time, one third of this maximum has been achieved with NdFeB
material. The chronological trend of the maximum energy product achieved in
various permanent magnet materials is shown in Fig. 1.1. It can be seen that
there are four major families of permanent magnet materials: Alnico, ferrite,
rare-earth-cobalt and rare-earth-iron.
Alnico
A major advance in permanent magnet technology began in 1932 with the dis-
covery of excellent magnetic properties of an aluminium-nickel-iron alloy, Alnico.
Numerous subsequent investigations resulted in many variations in composition
and properties. Alnico magnets are expensive when compared to ferrite mag-
nets since they contain 20% to 40 % of cobalt in addition to other components.
Alnico alloys have desirably high remanent magnetization, good thermal stabil-
ity, but comparatively low coercivities. Curie temperature is about 900
0
Cand
the maximum recommended operating temperature is 500
0
C. Alnico magnets
are very resistant to corrosion, nearly as resistant as stainless steel. Only slight
discolouration of the surface is encountered when a magnet is heated in air to
450
0
C [P4]. The physical properties of Alnico alloys are rather poor. High
coercivity is closely accompanied by extreme hardness and brittleness. Forming
is by casting or sintering as close as possible to the desired size and shape. For
close tolerance it is necessary to cut or wet-grind the magnet [P4]. Magnetic
properties of several grades of Alnico are summarized in Table 1.13.
1.7. SOURCES OF MAGNETIC FIELD 49
Table 1.14: Magnetic properties of the ferrite magnets.
Material B
u
[T] H
f
[Oe] H
fl
[Oe] (BH)
max
[MGOe (kJ/m
3
)]
Ferrite 5 0.395 2200 2230 3.6 (29.0)
Ferrite 7B 0.38 3250 3800 3.3 (26.0)
Ferrite 8A 0.39 2950 3000 3.5 (28.0)
Ferrite 8D 0.40 3100 3000 3.8 (30.0)
Ferrite 8C 0.43 4100 2200 4.3 (34.0)
Ferrite magnets
A new class of permanent magnetic materials was announced in 1952. The
materials were based on the crystal anisotropy of barium oxide. This class of
magnets is generally known as ferrite, but is sometimes referred to as oxide or
ceramic magnets. Today, ferrite magnets are by far the most widely used mag-
nets. The success of ferrites is due to several reasons. The raw materials are
inexpensive and non-strategic. Their high coercive force combined with reason-
ably high magnetic induction has allowed permanent magnets to be incorporated
into many types of devices. Ferrites are mechanically hard and brittle and their
magnetic properties are characterized by relatively low remanent magnetization
while the coercive force is satisfactory. The Curie temperature is 450
0
C and the
recommended maximum operating temperature is 250
0
C. At the same time,
surfaces of ferrite magnets are stable and are not subject to oxidation.
Ferrites are produced by powder metallurgy. Their chemical formula can
be expressed as MO·6(Fe
2
O
3
), where P is Ba, Sr or Pb. Strontium ferrite
has a higher coercive force than barium ferrite and is produced on a larger scale
[P4]. Ferrite magnets are available in isotropic and anisotropic grades. Magnetic
properties of various grades of ferrites are shown in Table 1.14.
Samarium-cobalt-based rare-earth magnets
The family of rare-earth permanent magnets (REPM) has evolved in the last
35 years. The materials quickly surpassed all earlier magnets, with currently
best values of energy product and coercivity, both being 5 to 10 times those of
Alnico and ferrites. While they are rather costly for universal use, the REPM,
together with the hard magnetic ferrites, are rapidly broadening applications
of permanent magnets in general [S9]. Two major subgroups are being dis-
tinguished: rare-earth Co-based magnets (particularly Sm-Co) and rare-earth
Fe-based magnets represented by Nd-Fe-B. The latter third-generation REPM,
in rapid development since 1983, use cheaper, more plentiful raw materials than
Sm-Co and oer still higher energy density at room temperature.
Sm-Co-based rare earth magnets are of two types: the SmCo
5
group and
Sm
2
Co
17
group. These two groups constitute the first and second generation
rare-earth magnets, respectively [H5]. SmCo
5
alloys are the most basic materials
that make up rare-earth magnets. More advanced Sm
2
Co
17
alloys were devel-