Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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350 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.20: The eect of the demagnetizing magnetic induction on FeSi vis-

cosity in a cone circuit at Premier Diamond Mine, South Africa

(adapted from [C17]).

It is common practice at DMS plants to install demagnetization coils on the

magnetic separator return lines. Although the need to demagnetize media has

been recognized in the static-bath processes, the usefulness of demagnetization

coils in cyclone plants has been a subject of dispute. The views range from

outright rejection to unconditional acceptance. These polarized views are part

of the DMS plant folklore, although there is an overwhelming scientiﬁc and en-

gineering evidence of the beneﬁcial eect of media demagnetization. In general,

the installation of demagnetization coils in DMS cyclone plants is recommended

(e.g. [N8]).

The inverse relationship between the magnetic ﬁeld strength in a demagneti-

zation coil and the viscosity of the circulating medium FeSi medium in the cone

circuits is illustrated in Fig. 5.20. A similar beneﬁcial eect of FeSi demagneti-

zation, in a cyclone plant, was reported by Napier-Munn and Scott [N7]. They

observed that the coil had a rapid and substantial eect on the viscosity and the

results are summarized in Fig. 5.21. The viscosity had started to drop shortly

after the coil was activated and to rise again after the coil was de-activated.

Measurements of the magnetic susceptibility of magnetized samples of fer-

rosilicon showed that the values of magnetic susceptibility were higher than

those of unmagnetized samples [B29]. After demagnetization by a coil generat-

ing the a.c. magnetic induction of 0.06 T, the magnetic susceptibility returned

to its original value.

Practical aspects of demagnetization are discussed in Section 5.5.

5.3. WET MAGNETIC SEPARATION 351

Figure 5.21: The eect of demagnetization on the viscosity of 150D FeSi medium

at the cyclone plant of the Mount Isa Mines lead-zinc concentrator,

Australia, (adapted from [N8]).

Wet magnetic drum separators for ore concentration

Wet low-intensity drum magnetic separators are also used for the concentration

of magnetite ore or other strongly magnetic minerals. Most of the low-intensity

magnetic separation technology used today was developed during the 1960s

with no signiﬁcant changes taking place. The most noticeable changes over the

years have occurred with the increased diameter and length of the drum. With

each increase of the drum diameter, the process capacity of units increased and

throughputs as high as 160 t/h per meter length of the drum were recorded.

However, tests with 1500 mm diameter drum separators could not reproduce

this trend for the capacity increase, and currently not many concentrators of

this size have been installed [S13].

The improved design and more e!cient permanent magnetic materials al-

lowed the magnetic ﬁeld rating of the drum magnetic separators, traditionally

expressed as the magnetic induction at 50 mm (or 2 inches), to be increased,

from 0.05 T to about 0.12 T available today.

In the ore concentration applications, the separators are designed for three

basic operations, namely as cobbers, roughers (cleaners) and ﬁnishers.

Cobbers Drum separators used in cobbing operations are designed to obtain

maximum rejection of a non-magnetic fraction and maximum recovery of the

magnetic material. Most iron-ore concentrating drum separators are applied in

multi-drum, usually con-current, conﬁgurations so that the high-grade magnetic

concentrate is obtained. In cobbing operations, two drums are typical, but as

352 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

many as four drums have been used. The diameter of the drums is either 900

or 1200 mm and the magnetic ﬁeld rating, at 50 mm, of 0.1 T is standard for

cobbers.

Roughers Drum magnetic separators used in roughing or cleaning are applied

after the size of the cobber concentrate has been further reduced. Because the

cobbing has signiﬁcantly upgraded the ore, magnetic loading of the drum is

usually increased considerably. The objective of roughing is to obtain a high-

grade magnetic concentrate, while maintaining high recovery. Double-drum

separators, in con- or counter-current modes of operation are typically used in

roughers and the required magnetic ﬁeld at 50 mm is, as a rule, 0.07 T to 0.09

Finishers Finishing drum magnetic separators are designed to produce the

highest possible iron content in the concentrate. The feed tank and feed arrange-

ments are usually of semi-counter-current design, with the objective of dispers-

ing the feed particles in order to obtain maximum rejection of non-magnetic

particles. Both 760 and 900 mm-diameter drums are used in the ﬁnisher appli-

cations, with the magnetic ﬁeld strength at 50 mm lower (e.g. 0.05 T) than in

a rougher, to improve selectivity of separation. The feed size is usually reduced

to less than 75 mor45m.

Variables in low-intensity magnetic separation of ores Factors associ-

ated with the properties of the feed, to be considered in the selection of a wet

drum magnetic separation applied to the ore beneﬁciation, are [M26]:

• Feed rate (t/h/m)

• Feed particle size (max. 10 mm) and size distribution

• Feed volume (m

/h)

• Concentration of magnetic particles

• Degree of liberation

• Desired recovery into the magnetic fraction.

As shown in Table 5.17, the maximum feed rate varies with the type of

separator considered; maximum and recommended feed volumes are also given.

Typical sizes of feed particles for the three modes of operation are:

Cobbers: - 6 mm + 850 m

Roughers: - 500 m + 300 m

Finishers: - 75 m+45m.

5.3. WET MAGNETIC SEPARATION 353

Table 5.17: Feed rates and feed volumes in LIMS, as applied to ore beneﬁciation

[M26].

Application Feed rate [t/h/m] Feed volume [m

/h/m]

Max. Recommended Max. Recommended

Cobber 75 45 100 70

Rougher 60 45 100 70

Finisher 15 15 35 20

Table 5.18: Number of drums and magnetic poles used in LIMS for ore beneﬁ-

ciation [M26].

Separator Number of drums Number of poles

Cobbers 2to4 5or6

Roughers 1or2 5or6

Finishers 2or3 4to10

Magnetic content and degree of liberation The magnetic content of

iron ores to be concentrated by LIMS varies over a fairly wide range. Ores with

iron concentrations from 10% to 50% or more have been successfully treated.

Since liberation is not complete at the cobber stage, a substantial amount of

gangue is carried into the cobber concentrate. Typically, the cobber concentrate

contains 50% Fe or more and, accordingly, the magnetics content of the rougher

feed is quite high. The ﬁnisher feed contains 2% to 5% of non-magnetics and

thus has a very high concentration of the magnetic material.

Complete liberation of most iron ores that are being treated by LIMS is

usually at sizes ﬁner than 150 m, although some ores have to be reduced to -

45 m.

Magnetic ﬁeld strength and number of poles Increased magnetic ﬁeld

strength achievable in modern low-intensity drum magnetic separators allows

not only more e!cient recovery of ﬁne particles, but also the throughput of a

separator can be increased without losing recovery. Such an eect of magnetic

ﬁeld strength on throughput and recovery is illustrated in Fig. 5.22.

The number of drums used in various applications varies with the individual

ores and the choice is determined by the results of pilot-plant or laboratory

tests. In addition, various types of magnetic assemblies and pole combinations

have been successfully used. Typical magnetic pole speciﬁcations and number

of drums are shown in Table 5.18.

Throughput The feed rate in a low-intensity magnetic separator aects

its metallurgical performance, as can be seen in Figs. 5.22 and 5.23. With

increasing feed rate, the recovery of magnetic particles into the magnetic fraction

decreases, the rate of decrease being strongly dependent on the magnetic ﬁeld

354 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.22: Recovery of iron by LIMS, as a function of throughput, for dierent

values of magnetic induction at 50 mm (adapted from Sundberg

[S13]).

strength generated by the magnetic drum.

5.3.2 Wet high-intensity magnetic separation

Wet high-intensity magnetic separation is used in three dierent technological

processes, as shown in Fig. 5.24. These separators can thus be used either as

a magnetic ﬁlter to remove a single solid component from a suspension, or as a

magnetic separator to remove an unwanted magnetic component from a mixture

of materials, or for a concentration of valuable weakly magnetic materials from

a mixture.

The high intensity of the magnetic ﬁeld in these separators is generated

either by rare earth or ferrite permanent magnets (for example in RE drum

separators or in a Ferrous Wheel separator), or by resistive electromagnets or

superconducting magnets.

Magnetic ﬁeld gradient, an equally important parameter in magnetic sepa-

ration, is generated by three dierent methods. Close-gradient, open-gradient

and matrix (polygradient) techniques produce magnetic ﬁeld gradients of widely

dierent magnitudes, as can be seen in Table 1.20. As a result of these dierent

attributes of high-intensity magnetic separators, a plethora of names and terms

is being used to classify them, although the operating principle of many of them

are the same or, at worst, they can address similar separation objectives. The

resulting chaos is further magniﬁed by the advent of new magnetic materials

and advanced magnet designs that allow new approaches to high ﬁeld and high

5.3. WET MAGNETIC SEPARATION 355

Figure 5.23: The eect of feed rate on the recovery of magnetite, in a low-

intensity magnetic drum separator. Drum length: 1.2 m, drum

diameter: 900 mm (adapted from [K14]).

Wet high-

intensity

magnetic

separation

Filtration

< 1% solids

> 1% solids

Cyclic

Continuous

< 1% solids

> 1% solids

Cyclic

Continuous

Removal

from

mixture

High recovery

High grade

High grade and

recover

Continuous

separation

Concentration

from mixture

Figure 5.24: The range of applications of wet high-intensity magnetic separation

(adapted from Svoboda [S1]).

356 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Table 5.19: Nomenclature for wet high-intensity magnetic separators.

Separator Acronyms

Rare-earth drum separator REDS, REDMS, MIMS

Linear multipole open-gradient separator OGMS

Matrix continuous separator WHIMS, HGMS, HIMS

Matrix cyclic separator HGMS

Matrix cyclic magnetic ﬁlter HGMF, HIMF

gradient generation. A summary of the terms most frequently used to describe

wet high-intensity magnetic separators is given in Table 5.19.

A schematic diagram of dierent technological processes, in which wet high-

intensity magnetic separators are used, is shown in Fig. 5.24. It is clear that

conditions for successful separation in these three categories can dier widely.

Their correct selection must be based upon proper identiﬁcation of the target

of the process and the deﬁnition of criteria that would distinguish successful

separation from unsuccessful. The decision-making process in such a selection

is outlined in Fig. 5.25 and its detailed discussion will be presented in subsequent

sections.

Wet rare-earth drum magnetic separators

Although dry rare-earth drum magnetic separators have been readily accepted

by the mining industry, it has taken somewhat longer to develop and implement

wet RE drum separators. One of the reasons for reluctant acceptance of wet

RE drums by the industry is the onerous problem with the removal of strongly

magnetic material from the drum, even when these particles are present in trace

concentrations. Conventional high-pressure spraying or scraping methods are

not su!ciently eective in overcoming the strong magnetic force holding the

particles on the drum, and new approaches to this problem are being investi-

gated [A23].

Magnetic ﬁltration

Magnetic ﬁltration is used primarily for the removal of ﬁne and quasi-colloidal

particulates from dilute ﬂuid suspensions. Removal is carried out by passing sus-

pensions through a magnetized matrix, which leads to the retention of particles

throughout the matrix. The ﬂuid stream may be either liquid or gas.

The process of magnetic ﬁltration is analogous to that of deep bed ﬁltration,

the dierence being that an additional external force, namely the magnetic

force, contributes to the ﬁltration e!ciency in magnetic ﬁltration. Although

the magnetic force does not signiﬁcantly increase the probability of particle col-

lision with the matrix [S31, S33], the enhanced e!ciency of particle attachment

caused by the presence of strong, static dipolar magnetic interaction between

the matrix and a colliding particle increases signiﬁcantly the e!ciency of the

5.3. WET MAGNETIC SEPARATION 357

HGMS

Single solid

component?

Magnetic

filtration

Magnetic

component

valuable?

Continuous

High

Recovery?

High

grade?

<1%

solids?

Continuous

High

grade?

Moderate B

high v

coarse

matrix

Unlikely

scenario

Careful

optimization

required

High B

low v

fine matrix

Cyclic MS

Loss of

nonmags

critical?

Low B

high v

coarse

matrix

High B

low v

fine matrix

<1%

solids?

Cyclic

Continuous

High B

low v

fine matrix

Yes

Figure 5.25: Selection of the operating conditions in wet high-gradient magnetic

separation (adapted from Svoboda [S1]).

358 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

particle capture. The similarity between magnetic ﬁltration and deep bed ﬁltra-

tion indicates that the basic rules common to deep bed ﬁltration are applicable

to magnetic ﬁltration.

As the aim of magnetic ﬁltration is the removal of a single solid component

from a suspension, a brute force approach can be used so that all particles

are collected on the matrix. Therefore, the largest possible magnetic force,

compatible with the magnetic properties of the material to be removed, should

be used. Furthermore, the maximum magnetic ﬁeld, in combination with the

ﬁnest possible matrix, compatible with the particle size distribution, should be

employed.

Simultaneously, in order to ensure the highest degree of extraction of solids,

the ﬂuid shear stress on the deposited particles should be kept as low as possible.

At the same time, however, the ﬂow velocity of a suspension through the matrix

bed should be su!ciently large to ensure a high throughput.

Matrix Although it is usually desirable to use a ﬁne matrix in magnetic ﬁl-

tration, mainly to increase the probability of particle collisions with the matrix,

it is also important to avoid mechanical straining. Straining leads to the forma-

tion of a surface mat of deposit on the matrix. This causes a high resistance to

the ﬂow and ultimately the clogging of the matrix. In order to avoid straining,

the ratio d@e of the matrix element radius d to the particle radius e should be

of the order of 50 or larger. Coarser matrices with d@e = 100 to 500 often give

results comparable to those obtained with very ﬁne matrices.

Flow velocity The choice of the ﬂow rate is usually based on the trade-

o between the purity of the e"uent and the throughput of the ﬁlter. For

the removal of ferromagnetic impurities from steel-mill waste water, where high

hydrodynamic erosion can be tolerated, interstitial velocities of up to 0.3 m/s are

used. In applications where ﬁne weakly magnetic particles are to be collected,

the ﬂow rate is often as low as 1×10

3

m/s.

Removal of magnetic impurities from mixtures

The removal of unwanted magnetic components from mixtures is a well estab-

lished application of magnetic separation and a wide spectrum of materials can

therefore be upgraded by the application of e!cient and cost-eective dry mag-

netic separators. Although dry separation is an answer in many applications,

ﬁnely ground feebly magnetic materials can be beneﬁciated only by wet high-

intensity magnetic separation. The brightening of kaolin and dolomite and the

puriﬁcation of glass sand and other industrial minerals by wet high-intensity

high-gradient magnetic separation are well proven and established applications.

Matrix The common feature of all methods for the removal of magnetic ad-

mixtures is that, as in magnetic ﬁltration, the brute force approach can, in

principle, be used. This means that a high magnetic ﬁeld, ﬁne matrix and low

5.3. WET MAGNETIC SEPARATION 359

ﬂow velocities should be used to achieve e!cient removal of impurities. This

approach can lead, however, to matrix blockage and to a loss of the valuable

non-magnetic product due to its entrainment in the magnetic fraction. In most

applications, the excessive loss of the non-magnetic product into the magnetic

waste is not acceptable and careful selection of the operating conditions is re-

quired. The matrix should be su!ciently coarse to avoid mechanical straining

but it ought to be su!ciently ﬁne to guarantee high recovery of the magnetic

impurities.

The concentration of materials

The concentration of valuable, weakly magnetic minerals from ﬁnely ground

ores is the largest application of wet high-intensity magnetic separation and

of high-gradient magnetic separation. The objectives of the concentration of

minerals by WHIMS and HGMS are threefold, as shown in Fig. 5.25.

a) High recovery of a valuable metal into the magnetic concentrate. In this

case a high magnetic ﬁeld, in combination with low ﬂow velocity should be

used. A ﬁne matrix would be a natural choice, although a coarse matrix often

performs equally well or even better.

b) High quality of the magnetic concentrate. If the recovery is of secondary

importance, the operating conditions should be chosen in such a way that

selectivity of the separation process is high. Only a moderate magnetic ﬁeld

combined with a high ﬂow velocity should be used. An excessively high

magnetic ﬁeld causes the formation of ﬂocs, comprising both magnetic and

non-magnetic particles, and also increases the possibility of the formation of

the surface mat of deposit. Correct choice of the matrix is the most critical

step in achieving selective separation.

c) High recovery and high grade. This rather natural requirement poses a

problem of considerable complexity, since often contradictory requirements are

imposed on the operating conditions. The selection of the matrix, the strength

of the magnetic ﬁeld and the ﬂow velocity must be carried out in experimental

programme, in which other variables that inﬂuence the separation process (e.g.

pulp density, dispersion etc.) are simultaneously investigated.

Cyclic or continuous separation?

The choice between cyclic and continuous high-gradient magnetic separators is

dictated mainly by the concentration of the magnetically recoverable particles

in the feed. If the mass yield into the magnetic fraction is smaller than, for

example, 1%, a cyclic separator can be suitable. However, in the majority

of applications of HGMS in mineral processing, concentration of the magnetic

component is larger than 1%; furthermore, high ﬂow rates are usually required

to achieve a high grade of the concentrate and a high throughput. Under these

conditions the matrix would be loaded within a matter of seconds and the duty