Jan Svoboda. Magnetic Techniques for the Treatment of Materials

Подождите немного. Документ загружается.

420 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

5.4 Magnetic ﬂocculation

5.4.1 Magnetic ﬂocculation of ferromagnetic materials

As discussed in Section 3.6.1, magnetic ﬂocculation of strongly magnetic materi-

als is a process commonly encountered in wet low-intensity magnetic separation

of ﬁnely ground ferromagnetic (s.l.) materials. It arose from a detailed study

of mechanisms of particle capture in wet drum magnetic separators [R20, R23]

that the dominant mechanism by which the magnetic particles are captured in

wet drum magnetic separators is the formation of ﬂocs by magnetic ﬂocculation.

This process, characterized by the rate of ﬂocculation and by the ﬂocculation

time,isthusaected by the pulp density and the volumetric ﬂowrate through

the separation zone of the separator.

Magnetic ﬂocculation is used to enhance the e!ciency of numerous other

processes. Magnetic ﬂocculators are often used, for instance, to increase the

settling rate of solids in dewatering equipment, to improve the performance

of centrifuges that remove solids from steel e"uent plants, and to reduce the

addition of ﬂocculating reagents.

Magnetic ﬂocculation of strongly magnetic materials is largely an empirical

technique. Owing to their simplicity, magnetic ﬂocculators are generally used

without any extensive research, although selection of variables such as magnetic

ﬁeld strength, solids concentration and particle size distribution is governed by

a few basic rules.

Magnetic ﬁeld strength

The onset of agglomeration of natural magnetite, as illustrated in Fig. 3.37,

appears to take place at a magnetic induction as low as 10 mT (100 G), while

40 mT (400 G) seems to be necessary to generate stable ﬂocs [B30]. In magnetite

ores of various origin, the threshold magnetic induction was found to range from

8 mT (80 G) to 80 mT (800 G) [K14], and an empirical formula to describe a

relationship between the threshold ﬂocculation magnetic ﬁeld strength K

and

the coercive force K

was proposed:

= 123

(5.22)

The strength of the magnetic ﬁeld required to induce ﬂocculation is also

determined by the residence time of a suspension in the ﬁeld and by the concen-

tration of solids. The higher the ﬂow velocity and the lower the concentration of

suspension, the higher is the magnetic ﬁeld strength required to form magnetic

ﬂocs. Benson et al. [B31] state that for eective ﬂocculation, slurries with more

than 1% of natural magnetite should be exposed to a 60 mT (600 G) magnetic

ﬁeld, for a period of not less than 0.07 s. The best results were obtained after

a residence time of 0.2 s [B30]. Rayner and Napier-Munn [R23] observed that

in a low-intensity drum separator the magnetic ﬂocs were formed in the ﬁeld of

0.2 T in less one second, a substantially shorter time than the typical residence

time in the separator.

5.4. MAGNETIC FLOCCULATION 421

Figure 5.94: Tensile strength of FeSi and magnetite ﬂocs as a function of particle

size and concentration of the magnetic fraction in a suspension

(after Karmazin et al. [K14]).

Concentration of solids

As the concentration of solids in a suspension increases, the characteristic time

of ﬂocculation w

1@2

, given by eq. (3.126), decreases and the probability of particle

collision and adhesion increases. If the retention time of particles is su!ciently

longer than w

1@2

, which is, in turn, determined by the concentration of solids

in a suspension, then a high degree of agglomeration can be expected. It was

observed [R23] that at a high solids content in the feed, the magnetic particles,

independently of their size, began to ﬂocculate rapidly on the entry into the

magnetic ﬁeld of a drum separator. At low slurry densities, for example in

secondary drum magnetic separators used to recover dense media, ﬂocculation

is slow and the ﬁnes are increasingly getting lost.

The inﬂuence of the concentration of solids and of particle size on tensile

strength is shown in Fig. 5.94. It is evident that the tensile strength of mag-

netic ﬂocs increases with increasing pulp density and particle size, the latter

dependence having the same pattern as the dependence of magnetic susceptibil-

ity on particle size. This is the consequence of the fact that the tensile strength

of a ﬂoc is determined by the magnetic moment of the interacting components

of the ﬂoc, rather than by coercive force, as claimed by Rainer et al. [R23].

422 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

5.4.2 Magnetic ﬂocculation of weakly magnetic minerals

In contrast to the magnetic ﬂocculation of strongly magnetic minerals, the

application of this process to feebly magnetic materials has been limited to

laboratory-scale investigations. The ﬁrst experimental observation of the mag-

netic ﬂocculation of weakly magnetic minerals was reported by Hencl and Svo-

boda in 1979 [H16]. It was observed that magnetic ﬂocculation of ﬁne siderite

particles (51% - 20 m), with a magnetic susceptibility of 0.75×10

6

/kg,

took place at a magnetic ﬁeld as low as 0.4 T, and the rate of agglomeration

increased with increasing ﬁeld strength.

Experimental investigations into the magnetic ﬂocculation of paramagnetic

and diamagnetic particles were subsequently carried out by Watson [W26] and

Parker et al. [P8, V5], while a more detailed systematic study of agglomeration

of hematite was conducted by Ozaki and Matijevic et al. [O6, O7].

An extensive and thorough theoretical and experimental investigation of

ﬂocculation of weakly magnetic minerals in a magnetic ﬁeld was conducted by

Wang, Forssberg and Pugh [P10, W17, W18, W19]. The results illustrated in

Figs. 3.43 and 3.44 conﬁrm that ﬂocculation of feebly magnetic materials takes

place at moderate magnetic ﬁelds.

The possibility of industrial applications of magnetic ﬂocculation

Selective chemical ﬂocculation of mineral slimes provides an e!cient means of

recovering very ﬁne particles. However, this method is rather costly and envi-

ronmentally problematic. However, no chemical agents are required in magnetic

ﬂocculation and the ﬂocculation time can be reduced by increasing the magnetic

ﬁeld. Magnetic ﬂocculation would thus allow creation of clusters of very ﬁne

particles that could be readily recovered in a high-gradient magnetic separa-

tor. Detailed analysis of the potential of incorporating magnetic ﬂocculation

into magnetic separation circuits was presented by Svoboda [S79, S1]. Figure

5.95 illustrates two possible proposals for the inclusion of magnetic ﬂocculation

in a magnetic circuit for the recovery of very ﬁne, weakly magnetic minerals.

However, commercial large-scale exploitation of this technique will require in-

volvement of industry with academia and their commitment to develop further

innovative ideas and transfer them into production.

5.5 Demagnetization

Exposure of ferromagnetic (s.l.) materials to external magnetic ﬁelds often

results in the formation of magnetic ﬂocs that interfere with their further ma-

nipulation. In order to break the ﬂocs and to improve dispersion of magnetic

particles in suspension, the material must be demagnetized. The demagneti-

zation process is often applied, for instance, to preparation of feed for sizing,

ﬁltration, magnetic separation and for heavy media suspensions. It is usually

not possible to demagnetize fully a ferromagnetic (s.l.) material, such as mag-

5.5. DEMAGNETIZATION 423

Magnetic

flocculation

Magnetic

separator

Non-

magnetics

Magnetic

separator

Magnetic

flocculation

Non-

magnetics

Magnetic

concentrate

(a) (b)

- 40 µm

Magnetics

- 40 µm

Magnetics

+10 µm

Magnetics

+ 10 µm

Non-

mags+mags

- 10 µm

- 40 µm

-40+10 µm

Figure 5.95: Possible applications of magnetic ﬂocculation to the beneﬁciation

of weakly magnetic ores (adapted from [S1]).

netite or ferrosilicon, at temperatures below the Curie temperature (570

Cfor

natural magnetite).

There are three main methods of demagnetization. Thermal demagnetiza-

tion, commonly used with permanent magnet materials, consists of heating a

material to a temperature above its Curie temperature and then allowing it

to cool in zero magnetic ﬁeld. Caution must be exercised to avoid chemical

changes or sintering induced by temperature [L16, H23]. This rather impracti-

cal method is being used as a benchmark to compare the eectiveness of simpler

demagnetization methods.

In d.c. ﬁeld demagnetization, a particle must be exposed to a magnetic ﬁeld

greater than its intrinsic coercivity K

(Fig. 1.7), which, when the magnetic

ﬁeld is returned to zero, will result in zero magnetization. Practically, however,

because the ﬁeld inside a single particle depends in a rather complicated manner

on the form of individual particles, the shape of the clusters, and the density

of the slurry, it is not possible to demagnetize powdered material by use of a

constant demagnetizing ﬁeld [L16].

The practical method of demagnetization is by use of a time-varying magne-

tizing ﬁeld of gradually decreasing amplitude, produced by a coil energized by

an a.c. power supply. As a result, the particle magnetization traces a series of

424 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

minor hysteresis loops and ultimately converges towards zero. The magnetiz-

ing ﬁeld must start at a value appreciably greater than the coercive force and

be gradually reduced with each alteration until it reaches a very low value. A

well established rule of thumb is that the ﬁeld amplitude should be ﬁve times

the coercive force and the rate of decrease should be about 10% per cycle [H23].

For rough demagnetization, at least four alterations should be applied, although

when demagnetizing di!cult materials, as many as a hundred alterations may

be needed [M19].

Demagnetization of magnetized particles by an a.c. ﬁeld is based on the

assumption that the body has a ﬁxed position in the ﬁeld. In practice, however,

the particles and clusters are free to move in the slurry. When exposed to a

rotating magnetic ﬁeld, under suitable conditions, particles can rotate rather

than become demagnetized. This problem becomes particularly pronounced for

materials with a high value of intrinsic coercive force unless the strength of the

demagnetizing ﬁeld is greater than the coercive force and the particles are non-

spherical. An approximate criterion for particle rotation is that if the external

rotating ﬁeld K is smaller than the coercive force K

, the particles will rotate. If

K is greater than K

and K

, the magnetic moment of the particle will rotate

but the particle will probably not. Whether a particle will rotate or not depends

also on the viscosity of the slurry: high viscous drag can prevent particles from

rotating fast enough to achieve a synchronous motion. This phenomenon will

be even more pronounced at high frequencies of the applied demagnetizing ﬁeld.

Several observations, essential for e!cient demagnetization, can thus be sum-

marized:

1. The ﬂow rate may be high but should preferably be laminar. This follows

from the observation that the probability of rotation of particles is smaller for

laminar ﬂow than for turbulent ﬂow, as a result of rod-like clusters of particles

being aligned along the streamlines in the laminar ﬂow.

2. The minimum residence time of the slurry in the magnetic ﬁeld should be of

the order of 0.24 s to 0.28 s, which corresponds to 12 to 14 cycles of the a.c.

magnetic ﬁeld.

3. For feebly coercive materials, such as natural magnetite or ferrosilicon, a

magnetic induction ranging from 0.03 T to 0.06 T is usually su!cient.

4. For highly coercive materials (e.g. maghemite, titano-magnetite ores) a ﬁeld

of high frequency (400 Hz to 600 Hz) and high magnitude (0.07 T to 0.1 T)

should be used. Some magnetite ores could not be demagnetized even at a

magnetic induction of 0.18 T [K14].

5. Demagnetization of very ﬁne particles, close to single-domain size, is

increasingly di!cult. This observation is related to a trend of increasing

coercive force with decreasing particle size, as shown in Fig. 1.13 and

expressed by eq. (1.45).

5.6. SEPARATION IN MAGNETIC FLUIDS 425

Demagnetizing coils

The demagnetizing unit is usually an air-core a.c. solenoid wound on a pipe,

preferably arranged vertically, through which the slurry ﬂows. The pipe section

through the coil is made from non-metallic material to eliminate the shielding

of the slurry from the magnetic ﬁeld and the eddy current generation. The

sample to be demagnetized may be gradually withdrawn from the solenoid, or

the sample can be left in a position with a damped sinusoidal ﬁeld applied.

In mineral processing, a method of continuously demagnetizing particles in a

ﬂowing slurry is preferred. The particles in the slurry experience a decreasing

a.c. ﬁeld as they ﬂow out of the solenoid.

Semi-empirical equations were developed [B20] to calculate the design para-

meters of a demagnetizing coil:

K =

1=03QL

[(d + f)

+ e

]

1@2

(5.23)

7=88Q

(d + f)

(3d +9e +13f)

×10

6

(5.24)

where K is the magnetic ﬁeld strength (in [D@m]), O

is the inductance (in [H]),

d is the inside diameter of the winding (in [m]), e is the length of the coil (in

[m]), f is the radial depth of the winding (in [m]), Q is the number of turns and

L is the current in the coil (in [D]).

5.6 Separation in magnetic ﬂuids

Separation in magnetic ﬂuids is one of those techniques that oer a unique ap-

proach to material manipulation over a wide spectrum of applications. This

sink-and-ﬂoat separation technique exploits dierences in densities of the mate-

rials to be separated. As has been explained in Sections 2.8 and 3.8, a magnetic

ﬂuid placed in a non-homogeneous magnetic ﬁeld exhibits an apparent density

diering from its natural density. This apparent density can be controlled in a

wide range of values, exceeding the densities of all known elements and materials.

An important feature of this method is the very high selectivity of separation,

whereby mixtures comprising particles with very small density dierential can

be separated from each other.

Density is a unique physical property of materials and, in contrast to other

physical properties, it is, in most cases, well-deﬁned for each mineral or material.

Because of this, and in spite of the rather limited sophistication of gravity

separation equipment, separation of materials based on density is a universal

and fundamental technique, and is one of the oldest forms of mineral processing.

In principle, density separation is a very accurate way of material processing,

particularly if one can operate in a wide range of densities and if the technique

is capable of distinguishing a small density dierence.

The e!ciency of separation of particles having several properties of similar

magnitude (for instance density, magnetic susceptibility, electrical conductivity

426 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.96: Mass magnetic susceptiblity and magnetic polarization of paramag-

netic liquids, as a function of their densities. The polarization was

determined at the external magnetic induction of 0.25 T [S8].

and others) can be increased by simultaneous exploitation of two or more of these

properties. The precision and the separation rate can be increased in comparison

with conventional processes, in which a single property is exploited. Separation

in magnetic ﬂuids is based on such a combination of two widely dierent physical

properties of materials and oers a unique technique for material manipulation.

5.6.1 Magnetic ﬂuids

Paramagnetic liquids

Magnetic ﬂuids can be divided into two broad classes, namely solutions of para-

magnetic salts and ferroﬂuids. Paramagnetic liquids, as the name indicates, are

paramagnetic in their behaviour: their magnetization increases linearly with

increasing magnetic ﬁeld and their magnetic susceptibility is quite low, of the

order of 5×10

7

/kg (40×10

6

/g), and ﬁeld-independent. The values

of magnetic susceptibility and magnetic polarization, for two types of paramag-

netic liquids, as a function of their density, are shown in Fig. 5.96. The values

of magnetic polarization were measured at the applied magnetic induction of

0.25 T, which is a typical magnetic ﬁeld strength used in laboratory-scale fer-

rohydrostatic separators to achieve the apparent density of approximately 4000

5.6. SEPARATION IN MAGNETIC FLUIDS 427

Table 5.32: Magnetic susceptibilities and viscosities of manganese chloride and

holmium nitrate solutions (adapted from [A33]).

Density 1400 kg/m

Density 1850 kg/m

MnCl×4H

" [m

/kg] 6×10

7

 [Pa×s] 1.5×10

3

Ho(NO

)

×5H

" [m

/kg] 7.2×10

7

14.2×10

7

 [Pa×s] 2×10

3

12×10

3

Figure 5.97: Magnetization curve of a kerosene-based ferroﬂuid, with average

particle size of magnetite of 9 nm [K23].

kg/m

. It can be seen that polarization is very low (2×10

4

Tor2G).

Andres [A33] compared magnetic properties of aqueous solutions of rare-

earth salts with those of manganese chloride. He observed that magnetic suscep-

tibility of holmium nitrate, the most magnetic of the solutions of the rare-earth

salts, at its limit of solubility, is approximately three times higher (1.6×10

6

/kg) than that of manganese chloride (0.5×10

6

/kg).

Although the solutions of rare-earth salts do extend the range of achievable

apparent densities somewhat, their high cost compared to manganese chloride

(by a factor of 50 for pure compounds) hardly justiﬁes their industrial-scale

application. A comparison of magnetic susceptibilities and viscosities of man-

ganese chloride and holmium nitrate is given in Table 5.32.

In addition to low magnetic susceptibility, some paramagnetic liquids tend

to degrade in the presence of light; others tend to crystallize at lower temper-

428 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS

Figure 5.98: Magnetization curve of a ferroﬂuid with an average particle diam-

eter 6 nm [K23].

atures, while others decompose at elevated temperatures. All such liquids are

rather expensive and their recycling is essential, although not easy. The para-

magnetic liquids have a high surface tension and do not wet the particles surface

adequately.

Ferroﬂuids

A magnetic ferroﬂuid is a stable suspension of sub-domain magnetic particles,

often magnetite, in a carrier ﬂuid. Water, hydrocarbons and silicones are used

as carrier liquids. The particles, which have an average size of 10 nm, are coated

with a stabilizing dispersing agent, which prevents particle agglomeration, even

when a strong magnetic ﬁeld gradient is applied to the ferroﬂuid.

In the absence of an external magnetic ﬁeld, the magnetic moments of indi-

vidual particles are randomly distributed and the ﬂuid has no net magnetization.

When exposed to an external magnetic ﬁeld, the ferroﬂuid becomes magnetized

and reaches magnetic saturation at moderate magnetic ﬁelds. When the applied

ﬁeld is removed, the particles demagnetize rapidly and exhibit typical superpara-

magnetic behaviour characterized by the absence of coercivity and remanence.

A typical magnetization curve of a ferroﬂuid is shown in Fig. 5.97, while Fig.

5.98 illustrates the magnetization curve of a ferroﬂuid with a smaller average

particle size of 6 nm. It can be seen that a high magnetic ﬁeld is required to mag-

netically saturate a ferroﬂuid containing very small magnetite particles. This is

a consequence of the di!culty in aligning the magnetic moments of very small

5.6. SEPARATION IN MAGNETIC FLUIDS 429

Figure 5.99: Saturation polarization of ferroﬂuids as a function of average par-

ticlesize[K23].

particles [C23]. Although very small dimensions of magnetite particles improve

the stability of a ferroﬂuid, the value of saturation magnetization decreases with

decreasing particle size, as shown in Fig. 5.99.

In weak magnetic ﬁelds the main contribution to magnetization is made by

the larger particles which are more easily oriented by a magnetic ﬁeld, whereas

the approach to saturation is determined by ﬁne particles, the orientation of

which requires large ﬁelds [R16, C22].

Viscosity of ferroﬂuids The viscosity of a ferroﬂuid is one of the most im-

portant parameters that aects selectivity of separation in ferroﬂuid. In the

absence of a magnetic ﬁeld, the viscosity of a ferroﬂuid is greater than that

of the carrier liquid as a result of the perturbation of the streamlines by sus-

pended particles. Einstein [E5] showed that the dependence of the viscosity of

a suspension of rigid spheres on the volume fraction i is represented by

 = 

(1 + 2=5i) (5.25)

where 

is the viscosity of the carrier liquid. At volume fractions greater than

0.01, higher order terms in i have to be introduced into eq. (5.25) and the

viscosity can be described by the Vand relation [V6]:

 = 

[1 + 2=5i +7=349i

+ ====] (5.26)

On application of a magnetic ﬁeld, a torque 

× E is introduced, which

hinders the rotation of the particles about axes perpendicular to the magnetic