Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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310 CHAPTER 4. DESIGN OF MAGNETIC SEPARATORS

Design procedure

We will assume that we want to design the pole proﬁles for a ferrohydrostatic

separator operating with a ferroﬂuid. In order to obtain selective and accurate

separation of the feed material into two density fractions, the apparent density

of the ferroﬂuid in the working volume of the separator must be constant. This

condition will be met if the ﬁeld gradient along the vertical axis is constant,

i.e. if conditions (4.61) are met. Therefore, the proﬁle pattern must satisfy eq.

(4.77).

The practical procedure of designing the pole proﬁle and the magnetic ﬁeld

characteristics is as follows:

1. Determine, from eq. (3.145), what ﬁeld gradient CK@C| is needed to obtain

the required density of separation, i.e. the apparent density of the ferroﬂuid.

2. Using CK@C| =2D (eq. (4.76)) calculate D=

3. From K =2Du (eq. (4.71)) calculate K, for a given value of u= The length

of the pole tips and the width of the gap between the pole tips are determined

by u. The simplest approach would thus be to take u to represent the radius

vector of the point of the closest distance O between the pole tips.

4. Based on the condition X = Du

sin 2 = frqvw., it is possible to calculate

the constant from

frqvw

(sin 2

)

1@2

(4.78)

where u

is the starting radius vector and 

is the starting azimuthal angle, as

shown in Fig. 4.55. Using this constant, the pole proﬁle is then given by

u =

frqvw

(sin 2)

1@2

(4.79)

A similar procedure can be used to construct other pole proﬁles, for instance

the isodynamic shape.

Theroleofthemirrorplate

It follows from the pioneering work of Kaiser and Mir [K18] that a ferromagnetic

mirror plate simulates the role of the upper half of the hyperbolic surfaces.

Computer modelling of the hyperbolic pole proﬁles has shown [F20] that the

presence of the mirror plate negatively aects the constancy of the vertical ﬁeld

gradient for the isodynamic ﬁeld pattern. On the other hand, the constancy of

the ﬁeld gradient in a magnetic system with constant vertical gradient improved

by including the mirror plate.

The position of the plate was found to have an inﬂuence on the magnetic

characteristics of the circuit, particularly the constancy of the vertical ﬁeld

gradient. For the GMUO FHS-40M separator, shown in Fig. 4.60, the optimum

distance of the plate from the top of the pole pieces was about 28 mm. On the

other hand, the thickness the plate did not aect the magnetic parameters.

4.8. DESIGN OF A FERROHYDROSTATIC SEPARATOR 311

Figure 4.60: GMUO FHS-40M ferrohydrostatic separator with a mirror plate.

4.8.4 Scale-up of ferrohydrostatic separators

The application of ferrohydrostatic separation on production scale, particularly

for the recovery of non-ferrous metals from automobile or electronic scrap, re-

quires that separators with throughputs of at least 1 t/h, but preferably more,

are available. In existing models of FHS, that are based on the iron-yoke con-

cept with the air gap between the pole pieces, the increase in throughput can

be accomplished by increasing the gap between the pole pieces. As has been

discussed in Section 4.3, the increase in the length of the air gap results in an

increase in the ampere-turns and thus of the mass, size and cost of the magnet.

With ferrohydrostatic separators, the situation is aggravated even further

by the fact that in order to maintain the required ﬁeld gradient, the necessary

magnetic ﬁeld increases linearly with dimensions of the air gap. In other words,

since

=2D for p = 2 (the constant gradient ﬁeld pattern), the magnetic

ﬁeld strength, given by K =2Du (eq. (4.71)), required to maintain this gradient,

increases with u= The scale-up of a ferrohydrostatic separator by increasing the

width of the air gap thus results not only in an increase in the magnetic ﬁeld,

with attendant uneconomical increase in the volumes of iron and copper, but

alsointheincreaseoftheheightofthepolepieceswellbeyondtheheightof

the pool of the ferroﬂuid required for e!cient separation.

312 CHAPTER 4. DESIGN OF MAGNETIC SEPARATORS

Feed

Solenoid

winding

Float

Sink

Separation

chamber with

ferrofluid

Figure 4.61: Schematic diagram of a ferrohydrostatic separator with a solenoid

magnet [S56].

Solenoid-based FHS

It is clear that the design of a large-throughput ferrohydrostatic separator must

not be based on a simple increase of the air gap between the pole pieces. A

natural approach to the scale-up of FHS would thus be to replace an iron-core

magnet by a solenoid magnet as was proposed by Svoboda [S56]. Figure 4.61

illustrates such a solenoid-based FHS separator. A solenoid, and particularly an

iron-clad solenoid, has numerous advantages compared to an iron yoke magnet.

While it is necessary to increase the air gap in an iron yoke magnet in order to

achieve an increase in throughput of material to be separated, with the attendant

drawbacks mentioned above, the throughput of a solenoid-based separator can

be increased by merely increasing the relevant transverse dimension (diameter)

of the solenoid. The axial length (along the | - axis, as shown in Fig. 4.61) of

the air gap and thus the number of ampere-turns required to generate a given

magnetic ﬁeld remain constant.

A solenoid allows the design of the magnetic ﬁeld pattern to be achieved in

a simple and accurate manner. Such an approach facilitates the provision of a

magnetic ﬁeld gradient which is constant, thereby enabling close control to be

maintained over the apparent density of the ferroﬂuid and accordingly over the

cut point density. This can be achieved, for example, by precisely designing the

winding of the solenoid, or by varying the current density at dierent positions

in the winding, or by using a multiple winding arrangement. This is in contrast

4.8. DESIGN OF A FERROHYDROSTATIC SEPARATOR 313

Feed

Float 1

Sink 1

Float 2

Sink 2

FHS 1

FHS 2

Figure 4.62: A two-stage ferrohydrostatic separator with solenoid windings

[S56].

to iron-core magnets, where approximate solutions of the Laplace equation are

used to design approximate shape of pole tips, a task that is further complicated

by non-linear magnetization characteristics of iron.

The magnetic ﬁeld across the transverse dimension of a solenoid (along the

{ - axis) is homogeneous in a large region within the solenoid, which means

that the same constant apparent density of ferroﬂuid can be achieved across the

essentially full transverse dimension. Therefore, almost the entire diameter of

the solenoid can be used for separation. This is in contrast to iron-core magnets

where the constant gradient exists only at the plane deﬁned by the | -axisand

considerable deviations from a constant gradient exist in the proximity of the

pole tips.

Because of the relatively small mass and size of a solenoid compared to an

iron-yoke magnet, it is possible to arrange for two or more FHS units to provide

for multi-stage separation, as shown in Fig. 4.62. In this arrangement the top

unit, operating, for example, at the apparent density of 8000 kg/m

> can ﬂoat

zinc and sink copper and lead, while copper could be separated from lead into

the ﬂoating fraction in the bottom separator operating at the apparent density

of 9500 kg/m

Computer modelling of the solenoid-based FHS [S8] showed that it is fairly

simple to achieve ﬁeld gradients of the order of 3 T/m in su!ciently large vol-

umes of ferroﬂuid. It was also shown that the use of steel cladding considerably

314 CHAPTER 4. DESIGN OF MAGNETIC SEPARATORS

Grad B

Conventional U-dipole

FHS

Grad B

C-dipole FHS

Figure 4.63: U-dipole magnetic circuit versus C-dipole circuit.

increased this volume in both axial and radial directions. The modelling also

conﬁrmed the technical and economic advantages of this design by the use of a

superconducting solenoid.

C-dipole-based FHS

The scale-up of a conventional iron-yoke-based ferrohydrostatic separator can

also be accomplished by re-orienting the U-shaped magnet (Fig. 3.53), to form

a C-dipole circuit, as shown in Fig. 4.63. Svoboda [S57] proposed the use of C-

dipole magnet to generate a vertical magnetic ﬁeld, the pattern of which can be

controlled by appropriate design of the magnetizing coils on upper and/or lower

legs of the C-dipole, and/or by controlling the relative polarity of the electrical

current ﬂowing through these coils. Appropriate shaping of the pole-tips of the

upper and lower pole pieces can also be employed to create a required ﬁeld

pattern.

The scale-up of such a C-dipole-based separator can be accomplished by

merely increasing the length of the magnet O

, and maintaining a constant

length of the air gap O

between the pole-pieces, as shown in Fig. 4.64. The

magnetic ﬁeld along the length of the C-dipole is homogeneous, and the same

magnetic ﬁeld pattern and apparent density of the ferroﬂuid, along the vertical

axis, can be maintained along the full length of the magnet. One possible

embodiment of such a concept of the C-dipole FHS is shown in Fig. 4.65.

4.8. DESIGN OF A FERROHYDROSTATIC SEPARATOR 315

Figure 4.64: Schematic diagram of a ferrohydrostatic separator based on a C-

dipole magnet [S57].

Figure 4.65: Schematic diagram of a ferrohydrostatic separator based on a C-

dipole magnet [S57].

316 CHAPTER 4. DESIGN OF MAGNETIC SEPARATORS

Figure 4.66: Magnetic circuit of an FHS separator with across-the-pole feeding

system.

Figure 4.67: Separation process in the across-the-pole FHS concept [K19].

4.8. DESIGN OF A FERROHYDROSTATIC SEPARATOR 317

Figure 4.68: Layout of the ATP ferrohydrostatic separator [K19].

Figure 4.69: A pilot-plant ATP ferrohydrostatic separator (De Beers Consoli-

dated Mines (Pty.) Ltd.)[K19].

318 CHAPTER 4. DESIGN OF MAGNETIC SEPARATORS

"Across-the-pole" FHS

An even simpler concept for overcoming the throughput limitation of the con-

ventional U-pole design of FHS was developed at De Beers Consolidated Mines

(Pty.) Ltd. [S8, K19]. By rotating the feed direction through 90

, the through-

put increase could be achieved by lengthening the pole pieces rather than in-

creasing the width of the air gap. Figure 4.66 gives a three-dimensional impres-

sion of this across-the pole (ATP) concept, while Fig. 4.67 gives a schematic

description of the separation process. As can be seen, instead of feeding the

material across the width of the air gap of the magnet, the particles are intro-

duced into the ferroﬂuid across the poles. The ﬂoat fraction passes over one of

the pole pieces, while the sink fraction is collected underneath in the bottomless

separation chamber. An illustration of the layout of the separator is shown in

Fig. 4.68.

The width of the air gap is determined by the required residence time of

material in the ferroﬂuid and remains the same for any length of the pole-pieces,

and, therefore, for any throughput of the separator. The magnetic pattern and

the apparent density of the ferroﬂuid are determined by the proﬁle of the pole

tips, in the same way as in conventional FHS units. A view of the pilot-scale

ATP separators is shown in Fig. 4.69.

Chapter 5

Practical Aspects of

Magnetic Methods for

Materials Treatment

Magnetic separation is used in a wide spectrum of areas of material handling

whose requirements dier considerably. In general terms, magnetic methods of

separation are used in several distinct processes:

• Magnetic ﬁltration

- recovery of liquid (the magnetizable solids are discarded)

- recovery of magnetizable solids (liquid is discarded).

• Concentration of a valuable "non-magnetic" component of a mixture by

discarding the magnetizable component recovered into the magnetic frac-

tion.

• Concentration of magnetizable component into the magnetic fraction and

rejection of the "non-magnetic" waste.

• Concentration of non-ferrous metals by discarding the non-metallic com-

ponent of a mixture, and separation of individual non-ferrous metals into

individual fractions.

• Separation of materials into individual density fractions.

The selection of the most appropriate magnetic technique to be used in a

given application is a formidable task, and clear deﬁnition of the objective of

the process is an important prerequisite for successful choice of the separation

technique.

319