Jan Svoboda. Magnetic Techniques for the Treatment of Materials

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130 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

oer a number of advantages:

a) Superconducting magnets can produce high magnetic induction in large

volumes at low energy consumption. The power is required mainly for cooling

rather than to compensate for ohmic losses in the coil of the magnet. Moreover,

the cost of cooling is independent of the magnetic ﬁeld generated, unlike resistive

magnets where the cost of cooling is proportional to the square of the ﬁeld

intensity. The economic e!ciency of the superconducting magnets thus increases

with increasing ﬁeld strength. In some applications the increased magnetic ﬁeld

allows the ﬂow rate of the slurry to increase and thus the throughput of the

separator increases.

b) Since iron cladding is generally not needed, the machine can be made

physically smaller.

c) The superconducting coils can be arranged in such a way that a su!-

ciently large ﬁeld gradient can be generated without using a matrix, while still

maintaining high magnetic force.

Two types of magnetic separators that use superconducting coils have been

introduced into industrial practice, namely the matrix separators and open-

gradient magnetic separators (OGMS). The matrix separators fall into two dis-

tinct design categories, namely cyclic vertical ﬂow units operating with liquid

nitrogen and helium liqueﬁer, and reciprocating HGMS.

As a simple extension of the matrix-based high-gradient magnetic separators,

the conventional resistive solenoid magnet was replaced by a superconducting

coil. These separators operating in the cyclic mode in the switch on/o regime

are based on short, large diameter, coils, processing at slow slurry velocities

with fairly infrequent ﬂushing.

In order to minimize the energy consumption and the running costs of su-

perconducting magnet systems, the magnet should be as passive as possible.

The persistent mode option allows the magnet to remain energized with no ad-

ditional power input. Elimination of the switch on/o regime would also result

in a longer duty cycle and thus in higher throughput of the separator.

These issues of a passive magnet were addressed by the reciprocating concept

[S15, W4, R3]. In this operation, one canister with the matrix processes the

slurry while the second canister is cleaned in a region of zero magnetic ﬁeld.

This technology, ﬁrst proposed by English China Clays, was not viable until the

magnet technology caught up with the operational requirements in the early

nineties [R4].

The second type of superconducting magnetic separators, namely OGMS, is

based upon deﬂection of magnetizable particles in a non-homogeneous magnetic

ﬁeld, according to their magnetic susceptibility and size. Instead of employing

a matrix to generate a magnetic ﬁeld gradient, the geometry of the magnet

winding is used to create the ﬁeld gradients and magnetic force that deﬂects

the magnetizable particles. These open-gradient magnetic separators include a

linear multipole separator and a drum magnetic separator.

2.5. SUPERCONDUCTING MAGNETIC SEPARATORS 131

Slurr

out

Slurr

uifier

LHe Dewar

LHe stand

erconductin

net

LHe

GHe

LHe

GHe

Figure 2.79: Schematic diagram of a cyclic superconducting HGMS.

2.5.1 Cyclic superconducting HGMS

The cyclic resistive high-gradient magnetic separators were exclusively designed

for puriﬁcation of kaolin. A resistive magnet of diameter of 2100 mm (84”)

generated the ﬁeld of 1.8 T at a power input of up to 400 kW. Bigger units

with a solenoid diameter of 3000 mm and weighing up to 625 tonnes consumed

600 kW. In order to reduce the energy consumption, the Huber Corporation

(USA) decided to replace the resistive solenoid by the superconducting coil.

A schematic diagram of a cyclic superconducting HGMS is shown in Fig.

2.79. The ﬁrst such unit, designed and built by Eriez Magnetics and shown in

Fig. 2.80, was installed at Huber Corp. in May 1986 [S16]. The ﬁlling factor

of the steel wool matrix is about 6% and the matrix is 500 mm deep. The clay

slurry is pumped from holding tanks into the distributor header; it ﬂows upward

through the matrix to the return header. A typical ﬂowrate is between 1 and 2

/min, and typical retention time ranges from 50 to 90 seconds [W5].

After a period of time, the matrix accumulates enough magnetic material

so that the e!ciency of separation drops o. At this point the slurry feed

is stopped, the magnet is de-energized, and the steel wool is washed in both

directions with clean water. The magnet is then re-energized and the feed

slurry is re-started. A typical duty cycle is 26 minutes separation, 1 minute of

de-energizing, 2 minutes of ﬂushing and rinsing, and 1 minute of re-energizing

[W5].

This process means that two dead time periods occur during the ramp up

and down of the magnetic ﬁeld, along with a dead time while the ﬂush of

the magnetics is carried out. In order to reduce this dead time, fast ramp

132 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.80: A cyclic superconducting Eriez Magnetics HGMS for kaolin bene-

ﬁciation (courtesy of Eriez Magnetics, Inc.).

superconducting magnets are used. The coil and power supply are subjected

to rapid current cycles in order to apply and remove the magnetic ﬁeld. This

results in increased heating of the coil by eddy currents and larger and more

expensive cooling power is required. A more robust power supply to drive the

current quickly is also required. This results in signiﬁcant cyclic stresses within

the magnet itself [R4].

2.5.2 Reciprocating superconducting HGMS

Superconducting magnets can take advantage of a higher magnetic ﬁeld com-

pared to resistive magnets, and can increase the processing velocity. However,

as the slurry velocity increases, the duty cycle decreases and the economics of

separation is aected. Also, for the optimum operation of a superconducting

magnet, taking into account coil stresses, ﬁeld uniformity and power require-

ments, it is preferable to operate the superconducting HGMS in persistent mode.

The issues of long duty cycles of separation and of optimum magnet operation

are addressed by a reciprocating technology. In order to achieve economic op-

eration, the captured particles must be ﬂushed from the matrix, with the ﬁeld

on, and the capture zone must, therefore, be removed from the magnetic ﬁeld.

The train of canisters, as shown in Figure 2.81, is moved by means of a ram

that operates on a magnetically balanced canister, which houses a multi-section

separation region. As one matrix canister is engaged in separation, the second

is undergoing ﬂushing and rinsing. Reciprocation can be accomplished in 10

2.5. SUPERCONDUCTING MAGNETIC SEPARATORS 133

Iron yoke

ACTIVE ACTIVE

DUMMY

DUMMYDUMMY

LINEAR ACTUATO

REFRIGERATOR

Product

Feed

Reciprocating matrix train

Superconducting coil

Figure 2.81: Schematic diagram of a reciprocating canister HGMS.

seconds [S17].

While economic design of resistive solenoids is based on short, large diameter

coils, for superconducting coils it is far simpler to provide a coil whose length

is greater than the diameter. Such a geometrical arrangement, used in Out-

okumpu/Carpco separators, allows the use of a radial slurry ﬂow rather than an

axial ﬂow. As can be seen in Figure 2.82, the radial ﬂow allows the feed area,

and thus the throughput, to be increased.

The ﬁrst industrial superconducting magnetic separator was designed by

the Institute for Refrigeration Engineering in Prague, Czech Republic [F6, K8].

This 5 T unit, shown in Figure 2.83 was a reciprocating canister machine for

the beneﬁciation of kaolin. Although, initially, the performance of the separator

was encouraging, subsequent deterioration in e!ciency and drop in throughput

as a result of matrix blockage were given as the main reasons for closure of the

separator [A7]. It was suspected that insu!cient velocity of ﬂush water and

a strong stray magnetic ﬁeld in the ﬂush station were the main sources of the

problems.

In order for a superconducting magnet to operate, it has to be cooled below

its critical temperature. The most common way of cooling such a magnet is

to immerse it in a bath of liquid helium at 4.2 K within a vacuum-insulated

cryostat. The period between liquid helium additions can be further extended

by using thermal insulation and liquid nitrogen. Mechanical coolers, based on

the Giord-McMahon refrigeration cycle, can extend the period between helium

reﬁlls ever further. In this technique, a small volume of high-purity helium gas

recirculates through a closed loop, generating a cooling eectatthepointof

134 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.82: Diagram of an active matrix canister in reciprocating supercon-

ducting HGMS (Outokumpu Technology, Carpco Division).

Figure 2.83: The Czechoslovak reciprocating-canister superconducting magnetic

separation for beneﬁciation of kaolin.

2.5. SUPERCONDUCTING MAGNETIC SEPARATORS 135

Table 2.7: Cryoﬁlter HGMS models [S18].

Cryoﬁlter Model Application Throughput

5T/200 Pilot 400 kg/h

5T/360 Industrial up to 15 t/h

5T/460 Industrial up to 50 t/h

5T/1000 Industrial up to 150 t/h

Figure 2.84: Superconducting Cryoﬁlter HGMS (courtesy of Outokumpu Tech-

nology, Carpco Division).

forced gas expansion. Carpco Cryoﬁlter systems are using this concept of a

combination of a liquid helium bath and mechanical cooling, which ensures a

reﬁll period of 1 year in normal processing operations [K9].

Further developments in the Oxford Instruments Cryofree

technology now

allow superconducting magnet systems to be totally cooled by mechanical means,

removing the requirement for helium completely [K9, A8].

Outokumpu Technology, Carpco Division, produce a range of the Cryoﬁl-

ter HGMS employed mainly in kaolin puriﬁcation. Figure 2.84 illustrates the

Outokumpu/Carpco Cryoﬁlter HGMS, while Table 2.7 lists Cryoﬁlter models.

While the Cryoﬁlter supply package depends on the application, the primary

items and options include [S18]:

• Basic Cryoﬁlter with integral iron sheet, reciprocating canister and stan-

dard control package.

136 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Table 2.8: Comparison of resistive and superconducting cyclic magnetic systems

for HGMS, with reciprocating superconducting separators [K9].

Magnet type Magnetic

ﬁeld [T]

Mass [t] Power

[kW]

Capacity

2100 mm resistive

electromagnet

1.8 270 270 to 400 C

3000 mm resistive

electromagnet

1.8 625 300 to 400 2C

2100 mm super-

conducting mag-

net

2.5 270 25 1.25C

3000 mm super-

conducting mag-

net

2.5 625 25 2.5C

5T360 Cryoﬁlter 5 12 ? 10 0.4C

5T/460 Cryoﬁlter 5 24 ? 10 1.3C

5T/1000 Cryoﬁl-

ter

5 120 ?12 4C

• Slurry handling system.

• Advanced process control panel and SCADA monitoring system.

• Detergent matrix wash system.

Table 2.8 compares various types of matrix-based superconducting magnetic

separators used for kaolin beneﬁciation [K9].

2.5.3 Open-gradient superconducting magnetic separators

The presence of a ferromagnetic matrix in high-gradient magnetic separators

brings about a problem associated with keeping the matrix clean and passable

under industrial conditions. One possible approach to eliminate the problems in-

herent in matrix machines is to employ a system in which magnetic particles are

continuously deﬂected in a non-uniform magnetic ﬁeld, according to their mag-

netic properties and size. These open-gradient magnetic separators, designed

for the treatment of ﬁne feebly magnetic materials, must be equipped with su-

perconducting coils in order to provide su!ciently strong magnetic force. The

ﬁeld gradients generated by a suitable combination of the conductor ﬁlaments or

by coils of varying polarity are much weaker compared to the gradients created

by a matrix. Magnetic induction exceeding 2 T is thus required to achieve ad-

equate magnetic force. Figure 2.85 illustrates two dierent modes of operation

of an open-gradient magnetic separator.

2.5. SUPERCONDUCTING MAGNETIC SEPARATORS 137

Figure 2.85: Dierent systems of open-gradient magnetic separators: linear

multipole system [(a) and (b)] and drum separator (c).

Linear multipole OGMS

The design of the linear multipole system, as shown in Figure 2.85 (a) and (b),

is based on the concept of the ancient Edison’s deﬂection separator [T3]. It

consists of an array of vertically positioned coaxial superconducting coils that

are energized in opposite directions [S19]. In its simplest form, Figure 2.85(a),

the dry ore is permitted to fall vertically as a curtain around the circumference of

the magnet. The magnetic force directed radially inwards diverts the magnetic

particles inwards while the non-magnetic particles fall vertically, unaected by

the magnetic force. The method failed to meet expectations mainly because the

separation was taking place away from the region of the highest magnetic force.

As a result of low depth of the magnetic ﬁeld, its gradient and interparticle

collisions, the metallurgical results were unsatisfactory even at low feedrates.

An improvement was achieved by bringing the feed point close to the magnet

and introducing a ramp, as is shown in Figure 2.85(b). The performance of

thesystemwassu!ciently encouraging for Cryogenic Consultants Ltd. (CCL)

to design and build a production-scale Cryofos OGMS unit for beneﬁciation

of the pyroxenite ore in South Africa [G3]. In order to circumvent scale-up

problems of magnets of cylindrical geometry used up to that time for OGMS,

Good [G3] abandoned the cylindrical geometry in favour of a linear geometry.

The split pair was replaced by a linear dipole of rectangular cross section, each

side having a separation length of 3 m. The separator could be utilized along

almost its entire length on both sides of its axis. Unfortunately, the magnet and

the cryogenic system proved to be di!cult to manufacture and the separator

never reached operational status [S1].

138 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS

Figure 2.86: Superconducting drum magnetic separator Descos (Humboldt

Wedag AG).

Further development at CCL and at Carpco Inc. addressed the issues of

the optimized feeding close to the region of the strongest magnetic force and

of the removal of the magnetic particles from the walls of the separator. The

concept of a ﬂat-faced magnet inclined at an angle with respect to the feeder

evolved into the Cryostream

series of OGMS units marketed by Outokumpu

Technology, Carpco Division [K9, S17]. In spite of considerable design progress,

the concept of Cryostream has not yet enjoyed commercial success.

Descos OGMS

The drum magnetic separator developed by KHD Humboldt Wedag AG em-

ployed a multipole magnet and combined the advantages of a superconducting

magnet with those of a conventional drum separator. As can be seen in Fig.

2.86, eight poles of the magnet system were arranged along one third of the

circumference of the drum. Four race-track coils were placed in a liquid helium

tank and generated 3.2 T on the surface. The drum of the separator was made

of carbon-ﬁbre reinforced plastic. The diameter of the drum was 1200 mm and

its length 1500 mm. Commercialization of the product proved to be di!cult

as a result of its high cost, in spite of its reportedly satisfactory metallurgical

performance.

2.6 Laboratory magnetic separators

A rule that applies to magnetic separation as much as to most physical separa-

tion techniques, is that the quickest and surest way of ﬁnding out how a material

2.6. LABORATORY MAGNETIC SEPARATORS 139

will behave in a separator is to run it through the machine. Theoretical models

of the process, which would help to predict the e!ciency of low-intensity mag-

netic separation are scarce and mostly too simpliﬁed to be of practical value. A

wealth of theoretical descriptions of high-intensity and high-gradient magnetic

separation applies mainly to idealized situations, with limited signiﬁcance for

practical problems. The information obtained from these models can sometimes

even be misleading and this, combined with the fact that magnetic and some

physical and chemical properties of the ores and other materials are di!cult

to ascertain, indicates that laboratory tests should be carried out before con-

clusions can be drawn about the e!ciency of material treatment by magnetic

means.

A wide spectrum of laboratory devices and bench-scale magnetic separators

have been built to simulate a particular process in a particular production-scale

machine. Generally, the usefulness of these small-scale machines is limited. For

instance, in magnetic separators based on permanent magnets, such as magnetic

pulleys, overband magnets and drum magnetic separators, the magnetic ﬁeld

generated is a function of the volume of the permanent magnets used in the

manufacture of the separator. A small-scale separator will thus generate a

lower magnetic ﬁeld, often of dierent pattern, compared to a production-scale

machine.

Moreover, physical dimensions of the separators, such as the diameter of a

pulley or a drum, determine the residence time of material in the magnetic ﬁeld

and thus the e!ciency of separation. Also, a small width of a magnetic roll or a

drum introduces signiﬁcant end eects into the results. In laboratory-scale tests

of high-gradient magnetic separation, it is essential to keep the matrix depth

and ﬂow velocity of the slurry through the matrix the same as in the industrial

conditions. Quite often it is not possible. Overall, tests on laboratory-scale

units can thus be misleading and practical experience in evaluating the results

of small-scale tests, on small, not necessarily representative samples, is essential.

Commercially available laboratory-scale magnetic separators, such as Davis

Tube and Frantz isodynamic separator can, however, be used for a reliable

analysis of the separability of materials.

2.6.1 Davis Tube

A Davis Tube (DT), shown in Figure 2.87, is a laboratory instrument designed

to separate small samples of strongly magnetic ores into strongly magnetic and

weakly magnetic fractions. It has become a standard laboratory equipment used

for the assessment of the separability of magnetic ores by low-intensity magnetic

separators [S20, S1]. It was developed in 1921 and no signiﬁcant changes have

been made in its design since then. The separator consists of an inclined tube

25 mm in diameter, placed between pole-tips of an electromagnet. Material to

be separated is passed through the tube; the strongly magnetic fraction is held

by the magnetic ﬁeld while the weakly magnetic material is washed down the

tube.