Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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400 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.74: Concentration of impurities in the non-magnetic kaolin concentrate,

as a function of ﬂow rate. Matrix: 2 to 3 mm balls, E

=1T

(adapted from [S62]).

Figure 5.75: Brightness of the kaolin concentrate, as a function of ﬂow velocity,

at dierent values of the applied magnetic induction. Matrix: 75

m steel wool, matrix length: 500 mm (adapted from [S60]).

5.3. WET MAGNETIC SEPARATION 401

Figure 5.76: The eect of slurry velocity on the recovery of Cu and Pb, and the

grade of the magnetic concentrate from Tsumeb ﬂotation tailings.

Matrix: mesh, E = 1.2 T (adapted from [S67]).

Figure 5.77: The eect of ﬂow velocity on recovery and grade of magnetic con-

centrate of siderite ore (- 75 m). Matrix: 3 mm rods, matrix

length: 150 mm, E = 0.5 T (adapted from [H22]).

402 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.78: The e!ciency of removal of solids from rolling mill e"uent, as a

function of feedrate (adapted from [F23]).

small. Recovery often does not depend on the slurry density over a wide range

of densities, although in some cases recovery has been found to increase with

increasing slurry density. The grade of the magnetic concentrate frequently, but

not always, decreases with increasing slurry density.

As the eect of slurry density on the metallurgical performance of a mag-

netic separator is small, it is advantageous to use the highest possible slurry

density, compatible with permissible matrix loading and feed rate. As a rule, in

beneﬁciation of ores, it is possible to vary slurry density between 20% and 50%

solids, while with ﬁne clays the upper limit is probably about 30% solids.

5.3.7 Feed rate

The feed rate, which is closely associated with matrix loading, is an important

variable in determining the cost-eectiveness of a magnetic separator. The

higher the feed rate, the higher the tonnage treated, resulting in lower capital

and operating costs per tonne treated. At low and moderate feedrates, where

the matrix is not ﬂooded and the continuity of the slurry ﬂow has not yet been

established, the interstitial velocity of the slurry and the erosion force acting

on deposited particles are high. With increasing feed rate recovery into the

magnetic fraction decreases and the grade of the magnetic product increases.

This trend is shown in Fig. 5.78, which illustrates the eect of the volumetric

ﬂow on the ﬁltration e!ciency of rolling mill e"uent water. Figure 5.79, on

the other hand, shows the eect of feed rate on the reduction of impurities in

ﬂuorspar by a continuous WHIMS machine.

At high feed rates, when the matrix is ﬂooded, interstitial velocity is reduced

5.3. WET MAGNETIC SEPARATION 403

Figure 5.79: Recovery of P

from ﬂuorspar, and the grade of the non-

magnetic product, as a function of feed rate. Matrix: 7 mm balls

(adapted from [J6]).

due to the resistance of the matrix, and recovery can increase with increasing

feed rate. However, the grade of the magnetic product decreases as a result of

the reduced selectivity of the process and of the carry-over of gangue into the

magnetic fraction.

5.3.8 The eect of particle size

In Chapter I, the performance of a magnetic separator was shown to be deter-

mined by the interplay of the forces acting on a particle, namely between the

magnetic dipolar force on the one hand and the ﬂuid drag, inertial, gravitational

and centrifugal forces on the other. All these forces depend, in various ways,

on particle size which is a crucial factor that limits the applicability of various

types of magnetic separators. The approximate particle-size ranges of dierent

classes of magnetic separators are depicted in Fig. 5.80. It can be seen that

each magnetic separator operates in a limited particle size range and the upper

and lower limits of this range are determined by the relative importance of the

magnetic and competing forces.

The upper size limit of particles that can be treated is determined also by the

construction characteristics of the separator and, in WHIMS/HGMS machines,

by the type of matrix used. Of considerable relevance is the establishment

of the lowest size limit of particles that can be recovered, and of the role of

slimes since the e!ciency of particle capture decreases with decreasing particle

size. The lower size limit of particles that can be recovered aects not only

the recovery and the selectivity of the magnetic separation process, but has

a signiﬁcant impact on the e!ciency of the overall technological ﬂowsheet, of

404 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.80: Particle size and magnetic susceptibility ranges of magnetic sepa-

rators.

Figure 5.81: The eect of particle size on the recovery of iron from goethite ore,

at dierent values of the magnetic induction,  =2×10

3

(SI),

(adapted from [S68]).

5.3. WET MAGNETIC SEPARATION 405

Figure 5.82: Recovery of iron by Readings WHIMS, from a hematite ore, as a

function of particle size (adapted from [M29]).

which magnetic separation usually is a part. The reduced ability of a magnetic

separator to recover ﬁne particles is illustrated in Figs. 5.81, 5.82 and 5.83.

5.3.9 Pre-treatment of slurry

When an ore is to be beneﬁciated by magnetic separation, it often requires

preliminary treatment in order to improve the e!ciency of separation. The

most useful operations used to prepare the ore are magnetic scalping, dispersion,

preliminary magnetization, magnetizing roasting and desliming.

Magnetic scalping

Strongly magnetic impurities which are often present in ores, and other material

mixtures, cause blockage of the matrix in WHIMS and HGMS machines and of

the separation gap in induced magnetic roll separators. Adhesion of such impu-

rities to the roll in belt roll separators causes misalignment and early wear of the

belt. The presence of these impurities also impairs the grade of the magnetic

concentrate as a result of entrainment of gangue and unliberated particles in

the ferromagnetic clusters.

The detrimental eect of ferromagnetic impurities and tramp iron can be

reduced by magnetic scalping, usually using low-intensity drum or roll mag-

netic separators. On a laboratory scale, magnetic scalping of very ﬁne strongly

magnetic impurities can be carried out using a high-gradient magnetic separator

equipped with a paramagnetic matrix, e.g. aluminium.

406 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.83: Recovery of U

from leach residues, for dierent size fractions,

as a function of magnetic induction (adapted from [S69]).

Dispersion

The degree of selective magnetic separation that can be achieved with ﬁnely dis-

persed particles depends on the liberation of individual particles from each other.

Therefore, complete dispersion of all particulate components is an essential pre-

requisite for the selective separation that follows. In this way, non-selective ﬂocs

originally present in the slurry must be broken down and the colloidal stability

ofthedispersedstatemustbeestablished.

As has been discussed in Section 3.6, selective interactions between particles

of valuable material in a mixture with other material components are based on

the following criteria:

• Particles of valuable material should carry an electrical charge of the same

sign and magnitude as the gangue material, so that the probability of

ﬂocculation between them is minimized.

• The electrical charge on the particles of the valuable material should be

such that the repulsive interaction energy between them will be smaller

than the attractive energy.

Dispersion of natural ores is often di!cult, owing to interference from dis-

solved ions and because it is not possible to draw reliable conclusions for selective

aggregation or dispersion behaviour of a slurry from results obtained with pure

minerals [S70]. Although dispersion is an inherent part of most ﬂotation and

ﬂocculation ﬂowsheets, its application to magnetic separation is limited mainly

to kaolin puriﬁcation by HGMS. Dispersion by sodium hydroxide and sodium

silicate is a vital part of the pulp preparation.

5.3. WET MAGNETIC SEPARATION 407

Figure 5.84: The grade of the magnetic concentrate from the -12 m fraction of

uranium-gold cyanidation residues, as a function of the concentra-

tion of dispersants (adapted from [S69]).

Dispersion has been found to be very beneﬁcial in the upgrading of uranium-

gold leaching residues by WHIMS [S69]. The use of a dispersant increased the

grade of the magnetic concentrate by 35% for the full - 150 m sample, and by

55% for the fraction smaller than 12 m. Figure 5.84 illustrates the e!ciency

of various dispersants and the role in their concentration of the grade of the

uraninite concentrate.

Preliminary magnetization

Several experimental studies have indicated that preliminary magnetization of

magnetizable materials can result in improved recovery and selectivity of mag-

netic separation. This phenomenon is related to the increase or decrease of

magnetization with time, in a constant magnetic ﬁeld, the so-called magnetic

viscosity. The change of magnetization P is generally proportional to the log-

arithm of time w that has elapsed since the last change in the ﬁeld [D15].

The rate of increase or decrease of a magnetization is determined by the

viscosity coe!cient

g ln w

(5.17)

which is strongly dependent on the particle size. It has been shown [D15] that

for iron particles



300V

(5.18)

where V

and V

are single domain and multidomain viscosity coe!cients,

408 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.85: The eect of roasting temperature on the magnetizing reduction of

two types of iron ore (adapted from [U5]).

respectively. A strong increase in the viscosity coe!cient can thus be expected

with a decrease in the grain size.

The viscosity coe!cient is also dependent on the magnetic ﬁeld strength. At

low ﬁelds the coe!cient is usually proportional to the ﬁeld, while at moderate

ﬁelds the coe!cient becomes independent of the ﬁeld.

Fraas [F24] exposed a wide spectrum of coarse (- 0.4 + 0.2 mm) magnetizable

minerals to a homogeneous magnetic ﬁeld before these minerals were separated

in a belt separator. Fraas found that the pre-magnetization of samples resulted

in increased recovery and improved selectivity of separation.

Pre-magnetization of magnetite ore by a magnetic induction of 0.03 T was

implemented at the Magnitogorsk Metallurgical Plant (Russia) to improve the

metallurgical performance of dry magnetic separators [R22]. By exposing a

weakly magnetic iron ore from Krivoy Rog (the Ukraine), to a pre-magnetizing

pulse magnetic ﬁeld of 1.9 T and pulse duration of 1.2×10

3

s , the recovery

increase and selectivity improvement were reported by Špa

cek and Do

ckal [S74].

The eect was particularly noticeable in the - 10 m size fraction.

Preliminary magnetization of ﬁne fractions of hematite ore prior to HGMS

resulted in an improvement in the recovery and grade of the magnetic concen-

trate [S75]. For a pre-magnetization time as short as 10 s, the iron recovery

from the - 12 m fraction increased from 44.5% to 49.2% Fe at 0.4 T, and the

grade of the concentrate increased from 16% Fe to 21% Fe. Although a higher

pre-magnetization magnetic ﬁeld improved recovery, selectivity of magnetic sep-

aration was better at low ﬁelds.

5.3. WET MAGNETIC SEPARATION 409

Figure 5.86: Magnetic susceptibility of roasted hematite as a function of roasting

temperature. Mass magnetic susceptibility of hematite: 0.95×10

6

/kg (adapted from [C21]).

Magnetizing roasting

Many weakly magnetic minerals can be converted to more strongly magnetic

compounds by subjecting the ore to a reducing or oxidizing atmosphere at el-

evated temperatures. Magnetizing roasting is, for example, a very eective

process in treatment of iron ores, which are poorly responsive to conventional

beneﬁciation techniques such as ﬂotation, gravity separation and magnetic sep-

aration. It involves conversion of feebly magnetic iron minerals in the ore to

a strongly magnetic form. In most magnetizing reduction systems, the ore is

reduced at a particular temperature and then quenched in water. The result-

ing maghemite (-hematite) is strongly magnetic and can then be recovered by

low-intensity magnetic separation.

The eect of the roasting temperature on the recovery of iron from roasted

iron ore (Agbaja) into the magnetic concentrate is shown in Fig. 5.85 [U5]. It

can be seen that the optimum recovery was reached at a temperature of about

500

C. A similar observation was made by Nasr and Youssef with Bahariya

Oasis (Egypt) iron ore [N10].

The variation in magnetic susceptibility of hematite (Hoyt Lake, Michigan)

as a function of reduction roasting time and temperature is illustrated in Fig.

5.86. It can be seen that at temperatures above 500

C, the magnetic suscep-

tibility increases signiﬁcantly and reaches that of magnetite at 800

C to 1000

C, depending on the charcoal dosage [C21]. However, oxidizing roasting of

hematite did not change its magnetic susceptibility.

The practice of oxidized roasting of ilmenite is well established and it is