Jan Svoboda. Magnetic Techniques for the Treatment of Materials
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70 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.1: Magnetic pulley (courtesy of Steinert GmbH).
2.1.1 Magnetic pulleys
An easy and simple way to remove tramp iron from material on a conveyor
belt is by means of a magnetic head pulley. These are available both in perma-
nent magnet and electromagnetic construction. They are relatively inexpensive
and easy to install, and accomplish continuous tramp iron removal. The mag-
netic pulley, shown in Figure 2.1, usually replaces the head pulley or the tail
pulley of a belt conveyor, converting the conveyor into a magnetic separator.
Material carried on the conveyor passes over the magnetic pulley which holds
the magnetic particles until they leave the region of the magnetic field while the
non-magnetic material is discharged over the pulley. A typical installation is
shown in Fig. 2.2, while Fig. 2.3 illustrates an actual process of separation by
the pulley.
The permanent magnetic pulleys can be manufactured with axial poles in
which the polarity alternates along the circumference, or radial poles, in which
the polarity changes across the width the of the belt. For large particles and
elongated shapes, the radial pole design is more suitable. For finely divided
material, the uniformity of the field across the belt width makes the axial pole
design more advantageous. Strontium ferrite magnets are usually used. Elec-
tromagnetic pulleys are used where control of the magnetic field is required.
The magnetic pulleys are manufactured in a range of diameters, from 500
mm to 1250 mm. The width of the magnetic pulley should be selected to match
the belt width. The throughput of magnetic pulleys of dierent dimensions
is shown in Figure 2.4. Standard belt speeds for which the throughputs were
calculated are shown in Table 2.1. Inclined belts provide additional areas of
contact with the magnetic field of the pulley and can tolerate higher capacities.
2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 71
Figure 2.2: Principle of operation of a magnetic pulley.
Figure 2.3: Operation of a magnetic pulley (courtesy of Steinert GmbH).
72 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Table 2.1: Standard belt speed for magnetic pulley operation.
Pulley diameter [mm] Belt speed [m/s]
500 1.1
630 1.45
800 1.7
1000 2.0
1250 2.2
Table 2.2: Recommended diameters of magnetic pulleys.
Feed size [mm] Pulley diameter [mm] Throughput [t/h/m]
50 - 100 750 - 900 250 - 400
25 - 50 450 - 600 165 - 250
6-25 300 - 450 75 -135
Magnetic pulleys of special design are used in the concentration of magnetite
and other ferromagnetic minerals. For the best results, the feed should be
screened into sized fractions and each fraction treated on a separate pulley.
Typical feed sizes would be - 100 mm + 50 mm, - 50 mm + 25 mm and -
25 mm + 6 mm. The magnetic pulley is not suited for the treatment of - 6
mm material. Pulley diameters recommended for various feed sizes are given in
Table 2.2 [M7].
2.1.2 Plate magnets
Where the amount of tramp iron contained in a product is reasonably small
and where automatic removal is not required, plate and grate magnets can be
incorporated in chutes and ducts to remove both tramp iron and iron of abrasion.
Such a plate magnet is shown in Fig. 2.5.
The plate consists of a series of alternating poles, uniform across the chute
width and alternating along the chute length. As the material passes over the
magnet face, tramp iron is trapped on the magnet face while the remaining non-
magnetic material proceeds down the chute. The plate magnets must be cleaned
and inspected daily since they are not eective if the accumulated tramp iron
is allowed to remain on the face.
The magnetized plates are usually of permanent magnet type, either ferrite
or rare earth. The eective depth of the magnetic field can be as high as 200 mm,
depending on the type of the magnetic material and on the shape of particles
to be removed. A chute angle not exceeding 45
0
is recommended [M7].
2.1.3 Grate magnets
The grate magnet shown in Figs. 2.6 and 2.7, consists of a series of magnetic
tubes, which have poles alternating between one another and along their length.
2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 73
Figure 2.4: Throughput of magnetic pulleys of various diameters, for dierent
widths of the conveyor belt. Standard belt speeds were assumed
(after [M7]).
These units are used for granular free-flowing materials finer than 12 mm and
are typically mounted at the discharge of a hopper. A number of rows of tubes
provide several passes of finely divided material and can remove essentially all
the magnetic particles. The permanent magnets used in the grate magnets
are either ferrites, which are suitable for temperatures up to 150
0
C, or Nd-
FeB magnets, with operating temperature up to 90
0
C. For higher temperature
applications, up to 350
0
C, the Alnico magnetic grates are also manufactured.
2.1.4 Suspended magnets
Suspended magnetic separators have been used over many decades to improve
material purity and to protect processing machinery. Particularly when the belt
speed is high and/or if large tramp iron objects are to be removed, suspended
magnets are recommended. Suspended magnets are available in both circular
and rectangular construction. The rectangular magnet is most commonly used
and permits installation of self-cleaning construction. Units based on perma-
nent magnets or electromagnets are manufactured, with the permanent magnet
design being restricted to lighter burdens. Simple suspended magnets, designed
to remove small amounts or occasional pieces of tramp iron, employ the manual
cleaning mechanism. They must be periodically turned o in order to remove
74 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Plate ma
g
net
Cleaning
position
S
N
Figure 2.5: Plate magnet.
Figure 2.6: Grate magnet.
2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 75
Figure 2.7: Grate magnet.
Figure 2.8: Principle of operation of an in-line suspended magnet.
76 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.9: Installation of a cross-belt suspended magnet (courtesy of Boxmag-
Rapid Ltd.).
iron accumulated on the face of the magnet. Where a large amount of tramp
iron is expected, or where access to the magnet for cleaning is di!cult, self-
cleaning magnets should be used. A belt driven across the magnet face allows
continuous automatic removal of tramp iron.
Suspended magnets can be installed at many points in a material handling
system: at any point along a conveyor belt, at the discharge end of feeders or
screens and above chutes or launders. The preferred location for a suspended
magnet is at an angle over the conveyor belt as shown in Fig. 2.8. In this
orientation the tramp iron is moving into the face of the suspended magnet and
the load is breaking open, allowing easier tramp iron removal. Self-cleaning
magnets can be operated across the belt, as shown in Fig. 2.9, and as in-
line units as illustrated in Fig. 2.10. In the cross-belt mode of operation, the
magnet belt moves perpendicularly to the conveyor belt. Because the tramp
iron must be attracted and turned by 90
0
from the movement of the conveyor
belt, a cross-belt magnet is usually larger and stronger than an in-line magnet.
Typical values of the magnetic field as a function of suspension distance for
oil-cooled suspended electromagnets, for dierent lengths of the magnet, are
shown in Fig. 2.11 [M7]. The selection process for suspended magnets will be
discussed in Chapter 5.
2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 77
Figure 2.10: Installation of an in-line suspended magnet (courtesy of Steinert
GmbH).
Figure 2.11: Magnetic field as a function of the suspension distance for oil-cooled
suspended electromagnets of dierent lengths.
78 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.12: Pole configurations in drum magnetic separators [M8].
2.1.5 Drum magnetic separators
Drum magnetic separators are probably the most widely used type of magnetic
separation equipment. The design of drum separators is predominantly based
upon permanent magnet technology, although electromagnetic drums can also
be occasionally found in industrial practice. Drum separators are applied to
the treatment of particle sizes, which range from several centimeters down to
several micrometers, and operate either in a dry or wet mode. While ferrite
magnets are used in conventional low-intensity drum separators, recent progress
in availability and aordability of rare-earth permanent magnets has resulted
in the construction of powerful high-intensity drum separators.
The basic design of all drum magnetic separators is essentially the same.
Blocks of permanent magnets are arranged in a stationary position inside a
non-magnetic rotating drum, as shown in Fig. 2.12.
Drums are made in several configurations depending on the application.
The magnet assembly, consisting of three to nine magnetic blocks of alternating
polarity, covers an angle usually ranging from 90
0
to 120
0
. As can be seen in Fig.
2.12, in the radial arrangement of the magnets, the polarity alternates across
the width of the drum, and is the same along the drum circumference. Such an
arrangement is suitable for the removal of elongated tramp iron or concentration
of coarse particles when high recovery is required.
In the axial arrangement, the polarity alternates along the circumference
of the drum and is uniform across the width of the drum. This arrangement
2.1. DRY LOW-INTENSITY MAGNETIC SEPARATORS 79
Figure 2.13: A pattern of the magnetic flux around a magnetic drum.
provides an agitating action enabling the release of entrained non-magnetic par-
ticles. The axial arrangement is used when a high quality magnetic concentrate
is required, although sometimes at the cost of reduced recovery.
The development of improved permanent magnet materials resulted in in-
creased magnetic induction available on the drum surface and at a suitable
operating distance from the drum. Conventional ferrite-based drums generate
up to about 0.22 T on the drum surface and 0.10 T at 50 mm. In NdFeB-based
drums a magnetic induction as high as 1 T can be reached on the drum surface.
A pattern of the magnetic flux around a magnetic drum is shown in Fig. 2.13.
Drums are manufactured with dierent diameters and widths. While di-
ameters of ferrite drums range from 600 mm to 1500 mm, rare-earth drum are
generally smaller, with diameters from 380 mm to 1000 mm. The most common
width of the low-intensity drums is 1500 mm, although units 3000 mm wide are
also manufactured.
Dry low-intensity drum magnetic separators are used mainly for tramp iron
removal where magnetic pulleys and suspended magnets are not feasible. With
rare-earth magnets, the drums are also able to separate stainless steel and iron-
bearing minerals. These separators can operate with top feed and bottom feed,
as shown in Figs. 2.14 and 2.15. The material to be separated is fed at the top
of the rotating drum surface, as shown in Fig. 2.16; the non-magnetic fraction
leaves the drum, while the magnetic fraction is retained by the magnetic force at
the drum surface until carried outside the eective magnetic field. In bottom-
feed drums, the magnetic component of the feed is picked up by the magnet
while the non-magnetic fraction leaves the system unaected. Drums with top
feed produce the highest magnetic removal and are suitable for materials that
contain relatively small amount of magnetic particles. However, the drum with