Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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90 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.26: Reading induced magnetic roll separator (courtesy of Roche Min-

ing).

Figure 2.27: An induced magnetic roll separator with bottom feed (courtesy of

Outokumpu Technology, Carpco Division).

2.3. DRY HIGH-INTENSITY MAGNETIC SEPARATORS 91

Permanent magnet ring

Steel ring

Figure 2.28: Details of a permanent magnet roll.

Table 2.3: Typical values of magnetic induction on the roll surface, as deter-

mined from modelling and by measuring using a conventional Hall-

Eect probe.

Magnet B [T] - modelled B [T] -measured

Nd-Fe-B 1.9 1.0

Sm-Co 1.5 8.0

Ferrite 0.5 0.3

manent rings are arranged with the same polarity facing one another. Very

high magnetic ﬁelds and ﬁeld gradients are generated on the surface of the roll.

As discussed in Section 1.6.1, it is di!cult to measure the real magnitude of

the ﬁeld using conventional measuring equipment because the ﬁeld varies over a

scale of several hundreds of micrometers. Electromagnetic modelling software,

however, allows determination the pattern of the ﬁeld around a roll as shown

in Fig. 2.29. Typical values of the magnetic ﬁeld are shown in Table 2.3. The

values of the magnetic ﬁeld strength measured using a conventional Hall-Eect

probe are also shown.

The values of the magnetic ﬁeld strength and its gradient on the roll surface

and around the roll are strongly dependent on the conﬁguration of the magnet

and steel rings. Thin magnet and steel rings generally produce a very high

ﬁeld, the ﬁeld gradient and the magnetic force at the surface of the roll, while a

long reach of the magnetic force can be achieved using thicker steel rings. The

relative thicknesses of the magnet and steel rings generate a widely diering

pattern of the magnetic ﬁelds around the roll. While the most common ratio

of the widths of the magnet to the steel is 4:1, the correct choice of the roll

92 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.29: Pattern of the magnetic ﬂux lines around a magnetic roll.

Figure 2.30: Operation of a permanent magnetic roll separator.

2.3. DRY HIGH-INTENSITY MAGNETIC SEPARATORS 93

Figure 2.31: Permanent magnetic roll separator Permroll

(courtesy of E.L.

Bateman Ltd.).

conﬁguration is determined mainly by the magnetic properties and particle size

distribution of the material to be treated.

Operation of permanent magnetic roll separators is schematically shown in

Fig. 2.30. For easy removal of magnetic particles, the roll is covered by a belt,

which can be as thin as 0.12 mm, to allow the feed material to be as close to the

magnet as possible. The belt is supported by an idler roll. Below the conveyor

is a hopper which collects the discharging material, while adjustable splitters

divert the dierent fractions into collection bins placed beneath the hopper.

The most common diameter of the roll used to be 72 mm or 76 mm, while

rolls 100 mm and 300 mm in diameter have now become commonly available

from several manufacturers. The latter design is based on magnet segments

rather than rings. With increasing roll diameter, the residence time of particles

in the magnetic ﬁeld, the ﬁeld depth and the e!ciency of separation increase.

Alternatively, by increasing the speed of rotation, to maintain the same residence

time as with a smaller roll, a higher throughput could be achieved with the roll

of a larger diameter.

For production-scale applications, the width of the magnetic zone of the roll

is either 1000 mm or 1500 mm, although for smaller scale applications 250 mm

and 500 mm wide rolls are available. Throughput of roll separators is aected

by many factors, which include particle size, particle density, proportion of the

magnetic component in the feed and the desired quality of the ﬁnal product.

As a guideline, a throughput of 2 to 6 t/h/m can be achieved with material

smaller than 2 mm, while larger material can be processed at a throughput up

to 15 t/h/m. A modular design enables a number of passes to be arranged into

a compact unit.

94 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.32: Installation of multistage Inprosys roll magnetic separators (cour-

tesy of Outokumpu Technology Inc.).

A photograph of a two-stage production-size unit is shown in Fig. 2.31, while

Fig. 2.32 illustrates the installation of multistage machines.

One of the most important issues that aects operational ﬂexibility, ease of

maintenance and cost e!ciency is the belt tracking. Tracking, tensioning and

changing the belts can be extremely time-consuming, in addition to a high cost

of replacement of damaged belts. Several proprietary tracking and tensioning

systems and roll support structure are available and it is claimed that it is

possible to replace a belt in less 5 minutes [D8].

The magnetic force generated by the permanent magnetic roll is of the same

order of magnitude as that in induced magnetic rolls. A spectrum of minerals

that can be successfully treated by either magnetic separator is practically iden-

tical. Permanent roll separators, however, possess numerous advantages such

as low energy consumption, and smaller mass and ﬂoor space. The absence of

an air gap allows treatment of particles as large as 25 mm. A few drawbacks

include belt wear and inability to control the magnetic ﬁeld strength. While

permanent roll separators are predominantly dry machines, wet units are being

used, for instance, for the concentration of diamonds [R2].

2.3.4 High-intensity drum separators

Recent progress in the availability and aordability of rare-earth permanent

magnets has resulted in design and construction of powerful high-intensity drum

magnetic separators (REDS). Eriez Magnetics were probably the ﬁrst to replace

2.3. DRY HIGH-INTENSITY MAGNETIC SEPARATORS 95

Figure 2.33: Operation of a dry NdFeB drum magnetic separator (courtesy of

Outokumpu Technology Inc.).

standard ferrite magnets with NdFeB permanent magnets and nearly all current

suppliers of REDS adhere to this conventional design philosophy [A13]. Figure

2.33 illustrates the operation of a rare-earth dry drum magnetic separator.

Asomewhatdierent approach [M13, W6, U1] to the design of the high-

intensity drum separator was based on a plurality of small magnet blocks whose

direction of magnetization changed in small increments, as can be seen in Fig.

2.34. This system which was claimed to require less magnetic material in com-

parison to a conventional system [W6] to achieve the magnetic induction of 0.7

T on the drum surface, was incorporated into the Permos separator of KHD

Humboldt Wedag AG.

The density of the magnetic force (or the force index) EuE of a rare-earth

drum magnetic separator is much smaller than the force index generated by

most roll separators, usually not exceeding 20% of EuE for an NdFeB roll (see

Table 1.1). The main reason is that the magnetic ﬁeld gradient in REDS is

much lower than that in a roll separator, as a result of the geometry of the

magnetic system. On the other hand, the reach of the magnetic force can be

extended to greater depths, allowing eective treatment of coarse particles. The

current consensus is that roll magnetic separators are not only more powerful

than drum magnetic separators, but they also seem to oer greater selectivity

and ﬂexibility [A13].

Generation of eddy currents in the rotating shell of the drum separator can

be a serious problem because of limited temperature tolerance of NdFeB perma-

nent magnets. The level of these currents depends on the magnetic induction,

the number of magnetic poles, the speed of rotation, the shell thickness and the

96 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.34: The magnet structure of the Permos separator [U1].

conductivity of the shell material. Heat will develop in proportion to the drive

power that is required for the shell to overcome the braking eect of the eddy

currents [A13]. The heat generation, sometimes aggravated by high tempera-

ture of the feed material represents, therefore, a problem for standard NdFeB

materials, for which a maximum operating temperature of 80

is recommended.

In such situations, more advanced magnet materials with a higher temperature

tolerance must be used.

2.4 Wet high-intensity magnetic separators

2.4.1 Dry separation versus wet separation

Although dry high-intensity magnetic separators have been successfully applied

to the beneﬁciation of a large spectrum of minerals, several basic disadvantages

exist, which prohibit a wider use of a dry process. For dry magnetic separation to

be successful, the ore must be completely dry and it almost invariably requires

the ore to be sized into several fractions, each of which must be spread in a

monolayer over the separators.

With dry methods, care must also be taken to ensure control of a possi-

ble dust hazard, an expensive precaution both in capital and operating costs.

Furthermore, dry separators have considerably lower throughputs than wet ma-

chines. While dry separators frequently yield an excellent separation on material

coarser than 75 m, on unsized material containing a large portion of ﬁnes, the

wet process is the only acceptable one. Considerable advantages must, therefore,

be oered by dry separation to justify its use in preference to wet separation.

In order to extend the process of magnetic separation to ﬁne weakly mag-

netic minerals, wet magnetic separators generating su!ciently high magnetic

2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 97

Figure 2.35: The principle of operation of a continuous wet high-intensity mag-

netic separator.

force in the working space must be used. A major development was achieved

in this direction in 1955 by Jones [J1], who combined a high magnetic ﬁeld

strength with Frantz’s idea of a magnetized matrix [F1]. By this approach, the

magnetic force acting on magnetizable particles was increased by several orders

of magnitude compared to dry magnetic separators. As shown in Table 1.1, a

typical value of the product EuE for an Nd-Fe-B roll separator is 300 T

/m,

while with the magnetized steel wool matrix, this value is as high as 5×10

Jones’s wet high-intensity magnetic separator (WHIMS) became the starting

point of a remarkable progress in this ﬁeld and resulted in the development of

a wide array of high-intensity high-gradient magnetic separators. This new

type of separators extended the application of magnetic separation to classes of

materials otherwise considered untreatable by magnetic means.

A schematic diagram of operation of a continuous wet high-intensity mag-

netic separator is shown in Fig. 2.35.

2.4.2 Iron core-based wet high-intensity separators

Jones high-intensity magnetic separator

The production-size Jones wet high-intensity separator is shown in Fig. 2.36

The magnetic ﬁeld in the working gap is generated by a conventional iron-

core electromagnet and the ﬁeld gradient is enhanced by a matrix consisting of

grooved plates of salient pole or of triangular groove plates, placed in the air

gap. The plates, shown in Fig. 2.37, are vertically arranged in plate boxes that

are placed around the periphery of the rotors.

98 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.36: Perspective view of a Jones continuous wet high-intensity magnetic

separator (courtesy of Humboldt Wedag AG).

Figure 2.37: Grooved plates of Jones WHIMS.

2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 99

Figure 2.38: Flush device for the magnetic fraction in the Jones WHIMS.

The slurry is gravity-fed onto the matrix where the magnetic particles are

captured onto the plates, while the non-magnetic fraction passes through and is

collected in a trough below the magnet. The feed points are at the leading edges

of the magnetic ﬁeld. Feeding is continuous due to the rotation of the plate boxes

in the rotors. Each rotor has two symmetrically disposed feed points. Before a

plate box with captured particles leaves the magnetic ﬁeld, any entrained non-

magnetic particles are washed out by low-pressure water and these middlings

are collected in a separate launder under the rotor.

When the plate boxes reach the demagnetized zone half-way between the

two magnetic poles, where the magnetic induction changes its polarity, and the

magnetic ﬁeld is essentially zero, the adhering magnetic particles are washed

out with high-pressure water sprays. A ﬂush device is shown in Fig. 2.38.

The particles in the slurry feed should be smaller than 1 mm and this de-

termines the gap between the grooved plates. As a rule of thumb, the gap

width, measured between two grooved plate tips, should be 1.5 to 2 times the

maximum particle size [B8]. Particle size distribution also aects selection of

the pitch of the grooves. Salient pole plates are used when a high grade of the

magnetic concentrate is required, while triangular groove plates are used when

high recovery into the magnetic fraction is important. The height of the plates

is 190 mm and the ﬁlling factor of the plate box can range from 50% to 80 %,

depending on the type of plates and the gap between them.

The maximum magnetic induction in the empty separation space does not

exceed 1 T, although values up to 2 T are often stated. Such values can be