Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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60 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT

Figure 1.42: Pattern of the magnetic ﬁeld around a suspended magnet.

not only a strong magnetic ﬁeld but also a non-uniform ﬁeld.

A large variety of designs have evolved over the years to render the magnetic

ﬁeld non-uniform and to produce suitable magnetic gradients. Separators based

on these modes of the ﬁeld gradient generation can be classiﬁed into three basic

categories:

• Open-gradient separators

• Closed-gradient separators

• Matrix (polygradient) separators

In open-gradient separators the ﬁeld gradient is generated in several

ways:

a. As a result of the natural non-homogeneity of the ﬁeld caused by the

dependence of the ﬁeld on the distance from its source. Typical examples are

overband, suspended, plate and grate magnets. The pattern of the magnetic

ﬁeld around a suspended magnet as shown in Fig. 1.42 illustrates the formation

of the ﬁeld gradient in the separation zone below the magnet.

b. By special arrangements of permanent magnets or of the windings of elec-

tromagnets so that the patterns of alternate polarity are generated. Magnetic

rolls, pulleys and drum separators use this type of the ﬁeld gradient. Multipolar

windings of conventional or superconducting coils with alternating polarity are

employed in open-gradient magnetic separators (OGMS). Fig. 1.43 illustrates

the open-gradient magnetic system of a drum magnetic separator while Fig.

1.44 shows the 3D pattern of the ﬁeld around a permanent magnet roll. The

magnitude of the gradient and the reach of the magnetic force are determined,

among other things, by the dimensions of the permanent magnet elements or

1.7. SOURCES OF MAGNETIC FIELD 61

Figure 1.43: Pattern of the magnetic ﬁeld around a magnetic drum [U1].

electromagnetic winding relative to the dimensions of the spacers (either air or

steel) between these magnetic elements. A schematic view of a linear multipole

OGMS system is shown in Fig. 1.45.

In closed-gradient separators a high ﬁeld gradient is usually achieved by

a suitable shaping of the pole pieces of the opposing magnetic poles. Magnetic

ﬁeld is thus focused on the working element of the separator and a high ﬁeld

gradient is created. Assorted geometrical arrangements of poles and counter-

poles is shown in Fig. 1.46.

These dierent geometries, such as a hyperbolic pole facing a ﬂat pole (case

A), or a grooved pole (case B), triangular pole and multi-tooth arrangement

generate various patterns of the magnetic ﬁeld and its gradient [D4, S12]. The

gradient of the ﬁeld and the reach of the magnetic force are the function of the

curvature of the poles and the counter-poles, by the pitch of the teeth and by

the width of the separation gap. Induced magnetic roll and wet high-intensity

magnetic roll separators employ this type of closed-gradient arrangement. Fer-

rohydrostatic separators with magnetic ﬂuids also generate a closed gradient by

a suitable shaping of the pole tips [R1].

Multipolar windings or multipolar arrangements of permanent magnets also

generate closed gradient of the magnetic ﬁeld. A quadrupole geometry was used

to build a magnetic separator for biomedical applications [Z2, H8]. A sextupole

arrangement is employed in Magstream separator with magnetic ﬂuids [W9],

while quadrupole magnets, either resistive [A3] or superconducting [C6] were

also tested in magnetic separation. Figure 1.47 shows a schematic diagram

of a pure quadrupole while the coil and pole arrangement in a conventional

quadrupole magnet for physical experiments are illustrated in Fig. 1.48.

62 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT

Figure 1.44: Pattern of the magnetic ﬁeld on the surface of a permanent mag-

netic roll.

Figure 1.45: A multipole open-gradient magnetic system.

1.7. SOURCES OF MAGNETIC FIELD 63

Figure 1.46: Shapes of poles and counter-poles in magnetic separators with

closed gradient.

N pole

S pole

N pole

S pole

Figure 1.47: Magnetic ﬁeld and forces inside a quadrupole magnet. Particles

are entering the paper.

64 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT

Figure 1.48: A quadrupole resistive magnet for high-energy physics experiments

(courtesy of Danfysik A/S).

Figure 1.49: Magnetization curves for bulk steel and steel wool (after [K5]).

1.7. SOURCES OF MAGNETIC FIELD 65

Table 1.20: Typical values of the gradient of the magnetic ﬁeld in magnetic

separators.

Separator Gradient of the magnetic ﬁeld

T/m kG/cm

Suspended magnet 0.8 0.08

Nd magnetic roll 200 20

Nd drum separator 80 8

Davis tube 10 1

Frantz isodynamic 20 2

Belt separator 1-10 0.1 - 1

Steel wool (HGMS) 2.5×10

2.5×10

Steel balls (WHIMS) 1×10

100

Expanded metal 4×10

400

Grooved plates 2×10

200

Split pair OGMS 40 - 100 4-10

FHS 2 0.2

Magstream 30 3

In matrix (polygradient) magnetic separators, the gradient of the

magnetic ﬁeld is generated by means of induced magnetic moment in the ferro-

magnetic bodies (called the matrix) and placed in the external magnetic ﬁeld.

Strong magnetic ﬁeld gradient is generated in the close vicinity of these bodies

and its magnitude depends on the size and the shape of the matrix elements.

Ferromagnetic bodies of various shapes have been used in high-intensity mag-

netic separators (such as balls, grooved plates, mesh, expanded metal, steel wool,

rods etc.) and a proper choice of the matrix is one of the crucial prerequisites

for successful separation.

Since the ﬁeld gradient is inversely proportional to radius d of the matrix

element, i.e. uE  M@d, the magnetic force acting on the magnetizable particles

is maximized by using as ﬁne a matrix as possible. On the other hand, the

dimensions of the matrix elements must be selected taking into account particle

size of the material to avoid straining and to ensure the selectivity of separation.

At the same time, the dimensions of the matrix bodies must be chosen in such

a way that the magnetic force is su!ciently strong to recover the magnetic

fraction. On the other hand, the magnetic force must not be too strong to

cause the matrix blockage and retention of the ”non-magnetic” component.

In order to magnetize a ﬁne matrix, su!ciently strong external magnetic

ﬁeld is required. This is because the ﬁlling factor of the matrix is often low

and the matrix does not conduct magnetic ﬂux very well. Typically, the ﬁlling

factor ranges from 0.04 (4%) for steel wool matrix to about 0.20 (20%) for

expanded metal. The exception is the ball matrix where the ﬁlling factor can

be as high 0.50, and grooved plates with the ﬁlling factor up to 0.80. Figure

1.49 illustrates that a background magnetic induction of at least 0.7 T is needed

to saturate the steel wool matrix, compared to about 0.2 T that is necessary to

66 CHAPTER 1. PRINCIPLES OF MAGNETIC TREATMENT

Figure 1.50: The magnetic ﬁeld lines around a cross-section of a magnetized

steel sphere or steel wire.

saturate the bulk material. The large background induction needed to saturate

steel wool is a result of demagnetization eects. A large proportion of ﬁlaments

in the randomly orientated steel wool is arranged along the di!cult axis of

magnetization and exhibits thus a large demagnetization factor. The pattern

of the magnetic ﬁeld around a magnetized steel wire or steel sphere is shown in

Fig. 1.50. Typical values of the gradient of the magnetic ﬁeld for various types

of magnetic separators are given in Table 1.20.

Chapter 2

Review of Magnetic

Separation Equipment and

Techniques

In reviewing magnetic separators, it is necessary to examine hundreds of patents

and papers and it is not possible to give adequate coverage to all of these. More-

over, there appear to be more models and modiﬁcations of magnetic separators

than there are applications. Since each embodiment of a magnetic separation

technique possesses distinct merits as well as limitations, no single technique is

universally applicable to all types of magnetic separation.

Various classiﬁcation schemes exist by which magnetic separator can be sub-

divided into categories. These schemes are based upon the dominant physical

or technological feature. Magnetic separators can thus be grouped as follows:

a) Based upon the medium carrying the ore:

• dry

• wet

b) Based upon the requirement of the system:

• removal of iron and similar materials for the protection of machinery sub-

jected to damage or wear

• extraction of valuable magnetic constituents

• removal of deleterious magnetic impurities

• separation of materials based on properties other than magnetic (e.g. den-

sity, conductivity)

• material handling

68 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

c) Based upon the way the magnetic ﬁeld is generated:

• permanent magnets

• electromagnets with iron yoke

• resistive solenoids

• superconducting magnets

d) Based upon the magnitude of the magnetic ﬁeld and its gradient:

• low-intensity magnetic separators

• high-intensity magnetic separators

• high-gradient magnetic separators.

Low-intensity separators are primarily used for the manipulation of ferro-

magnetic materials or paramagnetic materials of high magnetic susceptibility,

and/or of su!ciently large particle size. These separators can operate either in

dry or wet modes.

High-intensity separators are used for treatment of weakly magnetic mate-

rials, coarse or ﬁne, in wet or dry modes.

Very ﬁne, feebly magnetic materials can be treated in high-gradient magnetic

separators.

From the practical point of view, classiﬁcation according to the requirement

of the system is the most illustrative. For example, the application of magnetic

separation to iron removal is usually dictated by the material handling system

involved. The most commonly used magnetic separators include:

• Magnetic pulleys, applied in belt conveyor systems as head pulleys

• Suspended magnets, applied over belt conveyor chutes or feeders

• Magnetic drums

• Plate magnets, applied in chutes

• Grate magnets, applied in product streams.

For concentration or removal of magnetizable particles, a wide spectrum of

magnetic separators is used. These include:

• Magnetic pulleys

• Magnetic low-intensity or high-intensity drums

• Induced magnetic rolls

• Permanent magnet roll separators (IMR)

2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 69

• Cross-belt and disk magnetic separators

• Magnetic ﬁlters

• Wet high-intensity magnetic separators (WHIMS)

• High-gradient magnetic separators (HGMS)

For magnetic material handling, the magnetic systems usually applied are:

• Magnetic conveyors

• Magnetic rolls

• Lifting magnets

Magnets are also used in numerous special applications:

• Separators with magnetic ﬂuids, in which material is separated mainly on

the basis of its density (FHS, MHS)

• Eddy-current separators for separation of non-ferrous metals (ECS)

• Dense-medium separators, in which a magnet is used to control magnetic

heavy medium

• Demagnetizing coils

• Magnetic ﬂocculators.

2.1 Dry low-intensity magnetic separators

Dry low-intensity magnetic separators are usually used to remove tramp iron, to

concentrate coarse strongly magnetic iron ores, and to recover iron values from

blast furnace and steel mill slag. Removal of strongly magnetic impurities from

a variety of materials is also accomplished with dry low-intensity separators.

Tramp iron magnetic separators are used to protect handling and processing

equipment such as crushers and pulverizers. These separators are usually used

on dry material or on material which contains only surface moisture.

Tramp iron comes in many shapes and sizes. By conventional deﬁnition,

tramp iron means that a particle has a signiﬁcant size, which is su!cient to cause

damage. Iron of abrasion, or particles smaller than 3 mm in diameter, usually

do not cause equipment damage although they can discolour or contaminate the

product [D7]. As far as the top size is concerned, large pieces as long as 2 m can

be successfully removed. The size and shape of the tramp iron, together with

the material handling system used must be considered in selecting the protective

magnet.