Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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460 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

S N

Overflow “non-

magnetic” particles

Feed

Magnet 1 -

0.2 T

Underflow

Strongly

magnetic

flocs

Medium

magnetic

flocs

Magnet 2 -

0.15 T

Figure 5.123: Magnetic cyclone for beneﬁciation of iron ores (adapted from

Yurov [Y5, Y6]).

5.7.1 Magnetic cyclone

The concept of a magnetic cyclone was developed in the late sixties of the last

century as a natural extension of a conventional hydrocyclone with the objective

of providing an additional external force to supplement the gravitational and

centrifugal forces that cause classiﬁcation and separation. Iron ore beneﬁciation

and heavy media manipulation are two areas in which the magnetic cyclone was

applied on production scale. A review of some of the concepts and applications

of a magnetic cyclone can be found in [F29].

Iron ore beneﬁciation

Probably the ﬁrst magnetic cyclone was developed and applied in mineral process-

ing on a production scale in the former Soviet Union [Y5, Y6]. A schematic

diagram of such a magnetic cyclone is shown in Fig. 5.123. A slurry of roasted

magnetite passing through the ﬁrst ferrite permanent magnet system became

magnetized and the ﬂocs thus formed reported into the underﬂow. In the vicin-

ity of the overﬂow, moderately magnetic unﬂocculated particles were exposed to

another magnet. Additional ﬂocs thus created also reported into the underﬂow.

The diameter of the cyclone was 2500 mm and a throughput of up to 190 t/h

of solids was achieved. At least 24 such cyclones were in operation in 1986 at

the CGOK plant.

Freeman et al. [F30] emulated the above approach simply by replacing the

5.7. MAGNETISM IN OTHER AREAS OF MATERIAL HANDLING 461

Overflow

Feed

Underflow

Solenoid

netic force

Figure 5.124: Schematic diagram of a heavy media magnetic cyclone for the

control of density dierential [S84].

ferrite magnets with rare-earth NdFeB permanent magnets. Improved recovery

of magnetite, compared to a standard hydrocyclone, was reported.

A straightforward improvement of the Russian concept was proposed by

Watson et al. [W27]. The magnetic circuit used in this cyclone consisted of

a two-coil 0.2 T electromagnet which attracted the magnetizable particles to

the walls of the cyclone from where they were directed to the underﬂow. The

system was found to separate magnetic particles e!ciently into the underﬂow

and reject "non-magnetic" particles to the overﬂow only for small-diameter (?

100 mm) cyclones. For larger-diameter cyclones, the results were disappointing.

Fricker [F28] proposed a dierent system for the magnetic circuit with the

objective of attracting magnetizable particles to the centre of the cyclone and

then through the overﬂow. An electromagnet of horseshoe cross-section, with

the magnetic ﬁeld gradient increasing radially inwards, was used and the hy-

drocyclone section was ﬁtted between the two poles of the magnet. The device

was applied, with modest success, to the beneﬁciation of iron sands.

Boxmag-Rapid Ltd. have improved the above described designs by em-

ploying a quadrupole electromagnet [A34, A35]. The quadrupole arrangement

generates a higher magnetic ﬁeld gradient and thus a stronger magnetic force

compared to the dipole conﬁguration. However, theoretical analysis of the mul-

tipole magnetic cyclones [G22] revealed that the quadrupole arrangement is not

su!ciently powerful for successful separation and the eight-pole conﬁguration

is optimum.

The Boxmag-Rapid device was designed as a thickener for de-watering the

462 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

dilute medium from the rinse screens in heavy media recovery plants, where

the higher recovery of the magnetic cyclone can lead to a smaller size of the

wet drum magnetic separators. In iron-ore treatment the magnetic cyclone was

intended to replace conventional large-diameter thickeners and cyclones, to give

higher unit throughputs and magnetic recoveries.

In order to increase the magnetic force in a cyclone, Savitzky et al. [S83]

proposed to incorporate ferromagnetic rings on the outside of the conical part

of the cyclone, along its vertical axis. These rings become magnetized by the

externally positioned coils and generated, therefore, a higher magnetic ﬁeld and

ﬁeld gradient. The presence of several coils and the ferromagnetic rings, allowed

optimization of the patterns of the magnetic force in dierent zones along the

axis of the cyclone. A similar design, based on permanent magnets, was used,

on laboratory scale, for beneﬁciation of beach sands [P24].

Despite these eorts, the concept of the magnetic cyclone as applied to iron

ore beneﬁciation and thickening has not been accepted by the mining industry.

Insu!cient understanding of the theoretical principles of magnetic cycloning

and rather unsophisticated design of the magnetic circuits are the main reasons

for the disappointing performance of such devices. Typically, the performance of

magnetic cyclones has been characterized by ine!cient mineral recovery, unde-

sirable ﬂocculation of magnetic particles, poor concentrate grades, and product

accumulation in the cyclone.

Heavy media manipulation

In order to aect the distribution of the particles of the magnetic medium

and thus the density dierential (dierence between the underﬂow and over-

ﬂow medium densities) within the cyclone, Svoboda [S84] proposed to apply

a vertically oriented magnetic ﬁeld generated by a solenoid wound around the

cyclone. The arrangement is shown in Fig. 5.124. By adjusting the magnetic

ﬁeld strength and by suitable positioning of the magnet, it is possible to control

the density distribution of the heavy media and to set an optimum density dif-

ferential, cut-point density and Ep for the Tromp curve of the cyclone. Figure

5.125 illustrates the eect of the magnetic ﬁeld on the density dierential in the

cyclone. It can be seen that, by applying the magnetic ﬁeld, the density dier-

ential between the overﬂow and underﬂow decreases until it reaches a minimum.

With a further increase in the ﬁeld, the density dierential begins to rise. This

is the result of the onset of magnetic ﬂocculation of the ferrosilicon particles,

increased settling of the magnetic ﬂocs and distortion of the ﬂow pattern within

the cyclone. The greatest reduction in the density dierential was achieved with

the magnet close to the overﬂow because the closer the magnet is to the over-

ﬂow, the more uniform the distribution of ferrosilicon within the cyclone. It is

even possible to induce inversion of the density, resulting in a negative density

dierential.

The ability to vary the density dierential enabled the density dierential

to be related to the mean probable error. By determining the minimum Ep

it was possible to identify the density dierential at which the cyclone should

5.7. MAGNETISM IN OTHER AREAS OF MATERIAL HANDLING 463

Figure 5.125: Density dierential as a function of the magnetic ﬁeld strength,

for various positions of the magnet and for two feed densities.

Ferrosilicon: 270D (adapted from [S85]).

operate to provide the highest selectivity of separation. It can be seen in Fig.

5.126 that the minimum value of Ep, for 270D ferrosilicon, occurred at a density

dierential of 250 kg/m

, in agreement with the plant experience [F31].

Subsequent test work was carried out on production scale by Myburgh [M31].

She observed that the magnetic ﬁeld had a similar eect on the operation of a

small (100 mm) and large (510 mm) cyclone, shown in Fig. 5.127. It was also

found that the eect of the ﬁeld was the same whether medium alone or medium

plus diamondiferous gravel feed was used. It was observed that by applying a

magnetic ﬁeld, the medium was stabilized, which resulted in a reduction in the

underﬂow density with increasing magnetic ﬁeld strength. It was also found that

by direct manipulation of the underﬂow density, the cut point can be directly

controlled. As a consequence, the yield to the concentrate was reduced with

concomitant reduction of the volume of material to be processed downstream.

Additional pilot-plant scale test work was conducted at De Beers [V8], in a

well-deﬁned fully instrumented dense medium system, shown in Fig. 5.128, to

verify observations acquired in a long-term industrial plant environment, The

feed, underﬂow and overﬂow densities were recorded on-line and the eect of

the magnetic ﬁeld on the latter two densities is clearly noticeable in Fig. 5.129.

464 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.126: Mean probable error Ep and density dierential, as a function of

magnetic induction, for 270D ferrosilicon (adapted from [S85]).

Figure 5.127: A cyclone with solenoid in a bottom position.

5.7. MAGNETISM IN OTHER AREAS OF MATERIAL HANDLING 465

hoppe

vibrating feeder

mixing box

dewatering screen

magnetic separator

correct medium sump

cyclone

underflow sieve ben

Figure 5.128: The fully instrumented DebTech (De Beers Consolidated Mines,

Ltd.) pilot-scale DMS plant, equipped with 100 mm cyclone [V8].

Figure 5.129: The on-line observation of the eect of the application of the mag-

netic ﬁeld on the underﬂow and overﬂow densities.

466 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.130: Comparison of performance of various processes for the beneﬁci-

ation of magnetite ore (30.4 % Fe, 94% - 44 m), (adapted from

Yalcin [Y7]).

The results conﬁrmed the previous pilot-plant and production-scale obser-

vations of the eect of the magnetic ﬁeld on the density dierential, cut-point

density and Ep. It was also observed that for a speciﬁc selection of the magnetic

ﬁeld strength, to give a suitable density dierential, the yield to the concentrate

was reduced [V8].

5.7.2 Magnetic gravity separation, ﬂotation and vacuum

ﬁltration

Although the initial eorts to combine the force of gravity with a magnetic force

in spirals and shaking tables date back to the middle of the last century, success

has been limited. The modiﬁcation of a gravity separator usually consists of

placing permanent magnets under the separating surface. The magnets can be

arranged so that an intermittent or a moving magnetic ﬁeld permits removal

of the magnetic material to the concentrate discharge slot [M32]. In spite of

obvious advantages and often promising results, the technique has not found a

wide-spread application in material processing. A more sophisticated approach

to the understanding of the physical principles of the process and to magnetic

circuit design is required for this method to fulﬁl its potential.

The application of a magnetic force to column ﬂotation was investigated

by Yalcin [Y7]. The device consisted of a conventional ﬂotation cell with a

column attached to one side. The column was ﬁtted with an array of rare-earth

permanent magnets. The top of the ﬂotation cell was sealed o and the froth was

forced to ﬂow over the magnets. Magnetic particles that may be carried with the

5.7. MAGNETISM IN OTHER AREAS OF MATERIAL HANDLING 467

froth were then captured and returned to the cell. This technique, designed to

prevent the recovery of magnetic materials to froth products while non-magnetic

minerals were ﬂoated, produced a high-grade magnetite concentrate at high

recovery, compared to alternative processes, as illustrated in Fig. 5.130.

Watson et al. [W28, W29] demonstrated that a magnetic force can enhance

solid/liquid separation of magnetic slurries in drum or disk vacuum ﬁlters. A

laboratory vacuum drum ﬁlter, equipped with rare-earth permanent magnets

(with a magnetic induction of 0.16 T at the cloth), was applied to the ﬁltration

of magnetite pulp and steel plant waste sludge. It was observed that the capacity

of the ﬁlter could be increased by up to 300% without signiﬁcantly increasing

the moisture content of the cake.

The ﬁltration results using an industrial disk ﬁlter equipped with ferrite

permanent magnets (magnetic induction at the ﬁlter cloth of 0.035 T), demon-

strated that the ﬁlter capacity could be increased by a factor of two, with a

slight increase of 1% to 4% in the cake moisture content.

5.7.3 Magnetic jigging

The basic idea of combining gravitational and magnetic ﬁelds in the jigging

process for separation of ﬁne particles was proposed and the physical principles

of such a process outlined by Lin et al. [L20]. A mathematical model of a batch

magnetic jig was developed, and the bed dynamics were analyzed and compared

with the theoretical predictions [L20]. Lin et al. [L21, L22] further expended

the theory of magnetic jigging and described particle motion and separation in

a continuous magnetic jig

This analysis indicated that the superposition of gravity and relatively weak

magnetic forces principally changes the character of jigging, and enables the

separation of mixtures that are di!cult to beneﬁciate. It was shown that the

introduction of additional magnetic force I

in the range of 0.2 to 0.4 of the

magnitude of the gravitational force I

(i.e. I

= 0.2 to 0.4I

) allows separa-

tion of particles of small density dierential for particle diameter smaller than

0.04 mm. For larger particles, with diameters in the range - 0.2 + 0.04 mm, the

additional magnetic force providing the separation increases to 0.5 to 0.8I

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Chapter 6

Industrial Applications of

Magnetic Methods of

Material Treatment

During the last forty years of the 20th century, magnetic separation evolved from

a simple technology for manipulation of strongly magnetic coarse materials into

a powerful technique for the treatment of weakly magnetic, ﬁnely dispersed par-

ticles. Development of magnetic separators based on powerful new permanent

magnets, construction of reliable superconducting magnets, design of e!cient

eddy-current separators and production of magnetic carriers and low-cost mag-

netic ﬂuids enabled the extension of this technology from mineral processing to

diverse areas of science and engineering. The spectrum of applications of the

magnetic techniques to material manipulation is formidable. The concentration

of various ferrous and non-ferrous minerals has become an important applica-

tion, as has the removal of low concentrations of magnetizable impurities from

industrial minerals. Removal of impurities from waste water, the recycling of

metals from industrial wastes and the concentration or removal of biological

objects in medicine and biosciences, have become important areas of applica-

tion of magnetic technology. In view of the enormous wealth of information on

various current applications of magnetic techniques to material handling, only a

cursory review of these applications can be given in this monograph. A detailed

description of some of the earlier industrial applications of magnetic separation

to minerals beneﬁciation can be found in [S1].

469