Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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140 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.87: Davis Tube magnetic separator.

Figure 2.88: The 3D model of the Davis Tube electromagnet [M24, M10].

2.6. LABORATORY MAGNETIC SEPARATORS 141

Figure 2.89: Frantz isodynamic separator.

Schultz [S20] suggested that a magnetic induction of 0.4 T or greater be-

tween the magnet poles should be used. However, Steinert and Boehm [S21]

claimed that the current practice was to conduct the Davis Tube tests at a

magnetic induction equal to that on the surface of the drum of the magnetic

separator. Since it is the magnetic force or the force index, and not the magnetic

ﬁeld strength, that are decisive in the operation, it is unlikely that this practice

would yield correct information.

Computer modelling of the pattern of the magnetic force generated by drum

magnetic separators and a Davis Tube (shown in Fig. 2.88) was carried out

by Murariu and Svoboda [M10]. It was found that in the case of conventional

ferrite drum separators, with a standard gap of 25 mm between the drum and

the tank, Davis Tube tests should be conducted at the magnetic induction of

0.1 T. On the other hand, correct assessment of performance of a rare-earth

drum can be obtained by operating a Davis Tube at about 0.3 T or higher,

depending on the design of the magnetic systems of the drum.

2.6.2 Frantz isodynamic separator

The Frantz isodynamic separator is a magnetic device that allows fractionation

of materials based on their magnetic properties. This separator, shown in Fig.

2.89 consists of an electromagnet having two long pole pieces with a long narrow

air gap between them. The pole design is such that the magnetic ﬁeld proﬁle is

isodynamic, so that the magnetic force density across the width of the air gap

142 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

litter

Magnetic

fraction

Non-ma

netic

fraction

Figure 2.90: Forces acting on a particle in a Frantz isodynamic separator.

is constant, i.e.

EuE = frqvw= (2.1)

Material to be separated enters the magnetic ﬁeld at point A, as is illustrated in

Fig. 2.90. If the force of gravity I

and the magnetic force I

on a particle are

balanced in such a way that the particle moves along the central line of the air

gap, this particle has a 50 % chance of reporting to a magnetic or non-magnetic

fraction. The balance of forces can be expressed as:

j sin  =





EuE (2.2)

where p

and Y

are the mass and the volume of the particle, respectively, j

is the gravitational constant and  is the transverse slope of the air gap. By

re-arrangement we obtain:

" =



EuE

sin  (2.3)

It can be seen that, because of the isodynamic pattern of the ﬁeld, the

action of the separator on a particle is determined solely by its mass magnetic

susceptibility, and is independent of the size and the mass of the particle.

The magnitude of the term EuE is determined by the electric current L in

the electromagnet so that

" =

sin 

(2.4)

where q and n are constants [M11]. With an electric current of 1.8 A, the mag-

netic induction at the narrowest part of the gap is approximately 2 T. In order

2.6. LABORATORY MAGNETIC SEPARATORS 143

Table 2.9: Chemical compounds used for calibration of magnetic susceptibility

instruments.

Compound Formula " [10

·m

/kg]

Mohr’s salt Fe(NH

)

(SO

)

·6H

O 4.1

Nickel sulphate NiSO

·7H

O 2.0

Copper sulphate CuSO

·5H

O 0.74

Cobalt sulphate CoSO

·7H

O 4.6

Ferrous sulphate FeSO

·7H

O 5.2

Manganese chloride MnCl

15.4

Manganese pyrophosphate Mn

12.9

to determine the exact values of these constants, each Frantz separator must be

calibrated with compounds having well deﬁned values of magnetic susceptibil-

ities. Chemical compounds usually used for calibration are listed in Table 2.9,

together with their mass magnetic susceptibilities.

Mass magnetic susceptibility " can thus be obtained from eqs. (2.3) or (2.4)

by plotting the cumulative per cent recovery of the material into the magnetic

fraction, versus the current. Typical magnetic proﬁles, or partition curves, are

shown in Fig. 2.91. The mean magnetic susceptibility of a sample can be deﬁned

as the value of " for which 50 % of the sample reports into the magnetic fraction.

While the sample represented by curve 1 in Fig. 2.91 has a narrow distribution

of magnetic susceptibility, the sample described by curve 2 contains a wider

spectrum of the values of magnetic susceptibility.

The Frantz magnetic barrier separator

While the Frantz isodynamic magnetic separator employs the isodynamic pat-

tern of the magnetic force density, expressed by eq. (2.1), the operation of the

barrier separator is based on the presence of the anadynamic and katadynamic

force patterns [F7]. In the katadynamic region, the magnetic force increases with

the increasing ﬁeld strength, while in the anadynamic region the force decreases

with the increasing magnetic ﬁeld:

EuEA0 (katadynamic),

EuE?0 (anadynamic) (2.5)

As can be seen in Fig. 2.92, the magnetic force density between the pole

tips of the separator is bell-shaped and creates a barrier in the isodynamic

plane, between the katadynamic and anadynamic regions [J2]. Depending upon

whether paramagnetic or diamagnetic susceptibility is to be exploited, a mixture

of particles is fed into the anadynamic or katadynamic region. As the mixture

moves under gravity towards the isodynamic plane, the magnetic force slows

the particles of the susceptibility to be exploited and accelerates the particles

having susceptibility of the opposite sign. In the vicinity of the barrier, where

144 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.91: Magnetic proﬁles, or magnetic partition curves, of two types of

materials, as obtained by Frantz isodynamic separator.

Figure 2.92: Superimposed graphs of the magnetic induction and magnetic force

density in the barrier separator, showing katadynamic (k) and ana-

dynamic (a) regions.

2.6. LABORATORY MAGNETIC SEPARATORS 145

Figure 2.93: Rotating disk isodynamic magnetic separator.

the magnetic force has the maximum value, particles having susceptibility such

that the magnetic force equals or exceeds the opposing force of gravity, are

deﬂected in the direction parallel to the isodynamic plane. Particles having

susceptibilities lower, or of the opposite sign, pass through the barrier. The

barrier separator thus allows selective separation and fractionation of materials

based on small dierences in magnetic susceptibility [R5].

Disk isodynamic magnetic separator

Frantz isodynamic separator is a selective device that allows separation of mix-

tures based on dierences in magnetic properties. This is in contrast to most

other magnetic separators (such as roll, drum and matrix separators) which use

the katadynamic ﬁeld pattern and are thus non-selective from ﬁrst principles.

In these machines the separation is based on dierences in magnetic properties

as well as on particle size.

Frantz separator is a laboratory unit that does not lend itself to scale-up. In

order to overcome limitations of the Frantz separator and, at the same time to

retain its high selectivity, Gerhold et al. [G29, G30, G31] designed and built a

rotating disk isodynamic magnetic separator shown in Fig. 2.93. The separator

consists of a horizontally rotating disk and magnetic system with pole-pieces

that generate an isodynamic magnetic ﬁeld in the separation gap. The magnetic

force created by the magnetic system acts radially inwards while the competing

force is the centrifugal force. This force can be adjusted to the values ranging

from ﬁve to ten times the force of gravity.

146 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Tails collectorRotating disc

Rotating disc

Tails collector

Feed

Tails

Mags

Pole

Tails

Figure 2.94: Schematic diagrams of operation of a dry rotating disk isodynamic

separator, with two dierent proﬁles of the magnetic ﬁeld (adapted

from Gerhold [G30]).

Particles fed onto the disk assume the rotational velocity of the disk. The

weakly magnetic particles are deﬂected outwards and are collected in a non-

magnetic bin. More strongly magnetic particles are deﬂected inwards in a form

of a rotating ring and are removed before they each the feed region. The opera-

tion of the separator, with two dierent proﬁles of the magnetic ﬁeld, is shown

schematically in Fig. 2.94.

A pilot-scale separator with a throughput of 2 t/h, maximum magnetic in-

duction of 2 T and ﬁeld gradient of 10 T/m, was built [G29] and selective sepa-

ration of ferrimagnetic and strongly paramagnetic materials in the size ranging

from 3 mm to 50 m was obtained [G30].

2.6.3 Separators with a rotating magnetic ﬁeld

Particle separation in rotating or alternating magnetic ﬁelds was proposed in

the early years of development of magnetic separation technology [T3, D1]. It

appears that this idea has not made its mark on the mineral processing industry.

However, in many magnetic separators, the particle rotation is present as a side-

eect rather then a deliberate attempt to rotate the particles. For instance,

magnetic clusters in low-intensity magnetic separators are rotated slowly as they

are carried past successive magnetic poles of opposing polarity. In doing so, the

ﬂocs release mechanically or magnetically entrained ”non-magnetic” particles,

improving thus the grade of the magnetic concentrate.

In some applications it is possible to use particle rotation alone to achieve

separations not possible using conventional attraction-based magnetic separa-

tion, as has been shown by Allen [A9, A10, A11]. For instance, particles

2.7. EDDY-CURRENT SEPARATORS 147

S N S

net travel

Particle rotation

Figure 2.95: Generation of particle rotation.

having similar magnetic susceptibilities, but considerably dierent magnetic

anisotropies, can be separated from each other by their selective rotation. As

a result of magnetic anisotropy the external magnetic ﬁeld exerts a torque on

the particle in order to align its axis of easy magnetization with the direction of

the external ﬁeld. If the magnetic ﬁeld generated, for example, by permanent

magnets with alternating polarity, is moving, then the particles will rotate as is

shown in Fig. 2.95.

The concept of a rotating magnetic ﬁeld has been employed in rotating

magnetic ﬁeld (RMF) laboratory separators developed and manufactured by

Ka (Pty.) Ltd., Australia. Figure 2.96 shows an RMF lift separator that can be

employed for separation by rotating weakly ferromagnetic or even paramagnetic

particles with magnetic anisotropy. Figure 2.97 illustrates an RMF separator

for separation of paramagnetic minerals.

In these separators, the rotating magnetic ﬁeld is generated by a rotating

magnet rotor ﬁtted with magnetic poles of alternating polarity around its cir-

cumference. The rotor in Fig. 2.96, rotating in a clockwise direction, produces

a ﬁeld that appears (to a particle on the outer drum surface) to be rotating

in an anticlockwise direction. Therefore, a magnetically anisotropic particle

dropped onto the drum will respond by rotating in an anticlockwise direction.

Non-rotating particles will be carried by the drum and its belt in a clockwise

direction.

RMF rotation separation is basically a separation of ferromagnetic (s.l.)

particles from paramagnetic particles because of a large dierence in magnetic

anisotropies and thus rotabilities of these two classes of materials.

2.7 Eddy-current separators

Although the ﬁrst patent for an eddy-current separator (ECS) was granted to

T.A. Edison as early as 1889, a large-scale application of eddy-current separa-

tion started only recently, particularly in connection with the recycling of wastes

148 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.96: RMF lift separator (courtesy of Ka (Pty.), Ltd.).

Figure 2.97: RMF separator for separation of paramagnetic particles (courtesy

of Ka (Pty.), Ltd.).

2.7. EDDY-CURRENT SEPARATORS 149

Figure 2.98: Eddy-current separator (courtesy of Steinert GmbH).

containing non-ferrous metals. The ﬁrst industrial ECSs were based on an elec-

tromagnetic ﬁeld induced either by single coils or by linear motor technique. As

a result of the dramatic development of rare-earth magnets, the currently man-

ufactured ECSs employ exclusively permanent magnets. Figure 2.98 illustrates

such an eddy-current separator.

The eddy-current technique is based on the fact that if conductive particles

are exposed to an alternating magnetic ﬁeld, or move through a steady magnetic

ﬁeld, eddy currents are generated within the particles. These eddy currents are

generated in such a way that the magnetic ﬁeld they produce opposes the exter-

nal magnetic ﬁeld, according to Lenz’s law. The opposite ﬁeld then produces a

repulsive force (Lorentz force) on the particle. Particles are thus deﬂected from

the stream of material, as is shown in Fig. 2.99, depending on their conductivity,

size, shape, and the ﬁeld strength and the rate of change of the ﬁeld.

The industrial eddy-current separators consist of two drums carrying a belt,

the belt usually being driven by the rear steel drum. The front non-metallic

drum contains a rapidly rotating magnetic system. The permanent magnets

are arranged, with alternating polarity, along the circumference of the magnetic

system. The separators usually have 6 to 18 poles which rotate at frequencies

up to 3000 rpm. Depending on pole arrangement and the system speed, the

frequencies of magnetic ﬁeld change up to 1 kHz are achieved [N3, H11].

Two main designs of eddy-current separators, each with two dierent magnet

conﬁgurations, are illustrated in Fig. 2.100, while Table 2.10 reviews the designs

used by various manufacturers.

Type 2 design, which employs an eccentric magnetic system, is reported to

have several advantages [N3, H11]. In concentric systems, strongly magnetic

particles tend to accumulate between the belt and the magnetic drum and ulti-