Jan Svoboda. Magnetic Techniques for the Treatment of Materials
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80 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.14: Dry drum magnetic separator with top feed [M8].
Figure 2.15: Dry drum magnetic separator with bottom feed [M8].
2.2. WET LOW-INTENSITY DRUM MAGNETIC SEPARATORS 81
Figure 2.16: Operation of a dry drum magnetic separator (courtesy of Steinert
GmbH).
bottom feed produces a clean magnetic concentrate with little non-magnetic
carry-over and should be used to treat materials containing a high percentage
of magnetic particles.
2.2 Wet low-intensity drum magnetic separators
Wet drum magnetic separators are used to recover ferromagnetic solids from
a slurry feed. The major areas for their use are in media recovery in dense
medium separation plants and in beneficiation of magnetite ore. Figure 2.17
shows the recovery of the magnetic particles by a wet drum magnetic separator.
Physical construction of wet drum separators is slightly dierent for the two
applications, with the ore concentrator being more rugged in construction and
subject to more detailed specifications than the media recovery units.
The drum is partially submerged in a tank of water and carries the material
to be treated across the face of the magnetic system and the magnetic concen-
trate out of the tank. The size of the feed treated in these separators rarely
exceeds 6 mm while the bottom size of the feed can be as small as 20 m=
The design of the tanks for the drum separator is very important and is
determined by the objectives of the separation process. There are three basic
tank designs, based on the slurry flow:
• concurrent
• counter-current
82 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.17: A wet drum magnetic separator.
• counter-rotation.
2.2.1 Concurrent tank
The concurrent tank design is shown schematically in Fig. 2.18. The feed
is introduced at one end of the separator through the feed box and flows in
the direction of drum rotation. Magnetic material is picked up by the magnet
and the non-magnetic material is discharged at the bottom through the tailings
discharge opening. Magnetic particles continue to adhere to the surface of the
drum and pass through an opening between the drum shell and the tank wall.
The magnetic fraction is then discharged down the discharge lip.
The concurrent design of the tank is used mainly for treating coarser material
in the size range 5 mm and below. High throughput and high quality of the
magnetic concentrate are characteristic features of this design.
2.2.2 Counter-rotation tank
In the counter-rotation design of the tank (Fig. 2.19), the feed is introduced
through a special feed box to the drum, which rotates in the opposite direction
to the slurry flow. Magnetic material is picked up by the drum and discharged
almost immediately. Since the tailings flow along the entire magnetic arc of the
drum before being discharged, losses are minimum. This tank design is used
2.2. WET LOW-INTENSITY DRUM MAGNETIC SEPARATORS 83
Figure 2.18: The concurrent tank.
Figure 2.19: The counter-rotation tank.
84 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.20: The counter-current (Steenson) tank.
primarily where surges in throughput are experienced, possibly involving high
content of the magnetic material. Very high recoveries can thus be achieved
with this design, while the grade of the concentrate is of secondary importance
only. The particle size can be up to 4 mm, but is preferably smaller than 0.5
mm.
2.2.3 Counter-current tank
The counter-current tank (Fig. 2.20) is used mostly for finishing where very
thorough cleaning is required. The term ”counter-current” refers to the fact
that the tailings must flow counter to the rotation of the drum when leaving
the separator. Feed is introduced near the bottom of the drum and agitated
through wash-water jets. Tailings flow out from the opposite end of the tank
through a tailings discharge overflow, which also acts as the slurry level control.
This ensures a high recovery of magnetic material and a high quality of the
concentrate. These tanks are most suited to finely ground material of 100 m
and less.
2.2.4 Design of the drum
The separating drum consists of two main parts: a stationary magnetic circuit
and a shell, usually made of stainless steel and often protected by an anti-wear
cover. The shell rotates freely around the magnets that cover an angle up to
150
0
. The choice of the permanent magnet material and design of the magnetic
circuit are the same as those for dry drum magnetic separators as discussed
in Section 2.1.5. The drums are available in 760 mm (30”) to 1200 mm (48”)
diameter; 900 mm (36”) drum diameters being the most frequently used. The
drum width ranges from 610 mm to 3050 mm, usually in increments of 300 mm.
2.2. WET LOW-INTENSITY DRUM MAGNETIC SEPARATORS 85
Figure 2.21: Throughput of a drum magnetic separator, as a function of the
drum diameter (after [K6], [S13]).
With increasing diameter of the drum, the throughput also increases. This
is illustrated, for dierent types of tank design, in Fig. 2.21. It appears, how-
ever, that the 1200 mm diameter is the optimum size, as a further increase in
diameter has not apparently resulted in the corresponding increase in processing
capacity [S13]. The increased throughput with increasing drum diameter can
be expected as the residence time of particles in the magnetic field increases as
the length of the magnetic arc increases.
This increase in residence time results either in improved recovery of the
magnetic particles (possibly with a reduced grade of the concentrate due to
increased entrainment of non-magnetic particles) or, at constant recovery, in
increased throughput. Large diameter drums usually incorporate a larger pitch
of the magnetic poles [S13]. Such a modification of the magnetic design then
results in a deeper magnetic field around the drum and in the ability to treat
higher volumes of material. However, a larger pitch of the poles reduces the
field gradient and thus the magnetic force acting on the magnetizable particles.
This could result in a reduced recovery.
Figure 2.22 illustrates the operation of wet-intensity drum separators in an
iron-ore beneficiation plant. A more detailed discussion of practical aspects of
operation and selection of wet drum magnetic separators is given in Chapter 5.
86 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.22: Wet drum magnetic separators in an iron-ore beneficiation plant.
2.3 Dry high-intensity magnetic separators
Weakly magnetic minerals can only be removed from a material stream in mag-
netic fields much greater than those available in low-intensity magnetic sep-
arators. Dry high-intensity magnetic separation has been used commercially
since the beginning of the last century, while wet high-intensity high-gradient
magnetic separators were developed only relatively recently.
Five types of dry high-intensity magnetic separators are in use:
• cross-belt separators
• induced magnetic roll separators
• permanent magnetic roll separators
• magnetic filters
• rare-earth drum magnetic separators.
2.3.1 Cross-belt magnetic separator
The cross-belt magnetic separator is one of the oldest types of separators used
to concentrate moderately magnetic ores. The operating principle is shown
in Fig. 2.23. Dry material is fed in a monolayer onto a conveyor belt and is
carried between the poles of the magnetic system. The belt, with its load, passes
2.3. DRY HIGH-INTENSITY MAGNETIC SEPARATORS 87
Figure 2.23: Cross-belt magnetic separator.
between the poles of two or more electromagnets. The lower pole-pieces are flat
and immovable, while the corresponding upper pole-pieces are shaped and may
be raised or lowered as required for the material undergoing treatment. An
endless cross-belt runs around a series of pulleys and across each upper pole-
face surface at right angles to the conveyor belt. As the material passes, the
magnetic particles are attracted to the cross-belt, carried with it towards the
discharge side and fall into a bin.
The material may be passed successively between several pairs of magnet
poles when it is desired to make several products in one pass. Non-magnetic
tailings discharge as the conveyor belt passes over its end pulley. Machines
with two to eight pairs of poles are available and each pair can be adjusted
independently by regulating the position of the upper poles and by altering the
current through the coils. The height of the upper pole-pieces above the belt
should be set at least 2.5 times the size of the largest particle in the feed.
The conveyor belt width ranges from 100 mm to 600 mm and particles rang-
ing from 4 mm to 75 m may be treated, preferably sized into several fractions.
Depending on the gap between the poles and on the current setting, the oper-
ating magnetic induction ranges from 0.2 T to 2.0 T. The main advantage of
the cross-belt separator is that several types of magnetic products can be re-
covered in one pass. However, low throughput is a major drawback. A five-pole
cross-belt separator is shown in Fig. 2.24.
88 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.24: A cross-belt separator (courtesy of Roche Mining).
2.3.2 Induced magnetic roll separator
The induced magnetic roll (IMR) separator shown in Figs. 2.25 and 2.26 consists
of a revolving laminated roll formed of alternate magnetizable and non-magnetic
discs. The roll is placed between shaped pole-pieces of an electromagnet. The
electromagnet induces a magnetic field in the magnetic lamination of the roll
and generates regions of high magnetic field gradient.
Material to be treated is fed in a controlled thin stream by a vibratory feeder
to the top of the roll. As the roll revolves, the material passes through a narrow
gap between the pole of the magnet and the roll. Non-magnetic particles are
discharged from the roll while the magnetic particles are attracted to the roll
and report into a separate chute when they enter a region outside the sphere
of influence of the magnet. Ferromagnetic material should be removed with a
separate magnetic scalper before feeding to IMR, to prevent plugging the gap.
The gap between the feed pole and the roll is adjustable and also the setting
of the splitter is of great importance. Depending on the width of the separation
gap and the current in the magnetizing coils, the magnetic induction in the gap
can approach 2 T. The material to be treated must be dry, free-flowing and for
best results it should be in the size range - 2 mm + 100 m. The gap should
be set to at least 2.5 times the average grain size. The throughput can range
from 1 to 3 t/h per meter length of the roll.
IMR separators have been extensively used for the beneficiation of beach
sands and for the removal of weakly magnetic impurities from glass sand, an-
dalusite, feldspar, wollastonite, magnezite and other industrial minerals. IMRs
can also be used for concentration of materials such as chromite, monazite and
wolframite.
IMRs possess several limitations, namely relatively low throughput, limited
particle size range and large mass. In recent years, much simpler and easier to
operate permanent magnetic roll separators have replaced IMRs in numerous
2.3. DRY HIGH-INTENSITY MAGNETIC SEPARATORS 89
Figure 2.25: Schematic diagram of an induced magnetic roll separator (courtesy
of Roche Mining).
applications.
An induced magnetic roll with bottom feed was developed by Carpco, Inc.
It is oered as a laboratory or pilot-scale separator for treatment of particles
up to 1 mm in size, in throughputs up to 50 kg/h. The separator, shown in
Fig. 2.27, achieves highly selective separation by magnetically lifting weakly
magnetic particles against gravity. As no entrapment of unwanted material
takes place, the separator is suitable in those applications where the grade of
the magnetic concentrate or the recovery of the non-magnetic concentrate are
of prime importance.
2.3.3 Permanent magnetic roll separator
Although the idea of a permanent magnetic roll separator, consisting of perma-
nent magnet disks or rings interleaved with mild steel disks, as shown in Fig.
2.28, dates back to 1968 [A4], it was only the advent of rare-earth permanent
magnet materials that spurred the rapid development of these machines. A
patent by E.L. Bateman Ltd. [Y1] became a starting point for a productive
period of competitive development of a wide array of roll separators. Initially
Sm-Co permanent magnets were used but they were quickly replaced by NdFeB
magnets when they became commercially available.
The active part of this separator is a roll consisting of disks or rings of
permanent magnets, sandwiched between mild steel rings. The adjacent per-