Jan Svoboda. Magnetic Techniques for the Treatment of Materials
Подождите немного. Документ загружается.
110 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Feed
Magnetics
Non-magnetics
Iron yoke
Coil
Pole-tip
Roll
Figure 2.53: Schematic diagram of a four-roll wet high-intensity magnetic sepa-
rator.
Table 2.5: Specification of wet high-intensity roll magnetic separators.
Parameter 5SVK ERM3 2MSM5
Throughput [t/h] 2to6 5 3to6
Roll diameter [mm] 300 300 270
Roll length [mm] 2000 2000 2000
Number of rolls 2 4 2
Magnetic field [T] 1.2 1.6 1.2
Mass of separator [t] 4.7 10.9 4.9
Water consumption [m
3
/h] 8to10 8to10 10
Input power [kW] 6 4
diagram of the four-roll separator is shown in Fig. 2.53 while an isometric view
of the four-roll separator ERM-3 is shown in Fig. 2.54. A detailed view of the
profiled roll can be seen in Fig. 2.55.
The lower pole tip contains slots through which the non-magnetic fraction
leaves the separator. Magnetic particles from the feed are attracted to the teeth
of the roll and are discharged when the roll leaves the sphere of influence of the
magnetic field.
Typical specifications of various models of wet high-intensity roll magnetic
separators are summarized in Table 2.5. A photograph of a 2-MSM-5 separator
is shown in Fig. 2.56.
These separators were successfully applied to the beneficiation of coarse (- 5
mm) manganese ore, iron ores such as siderite and limonite, and to the removal
of serpentine from magnezite. While these separators cannot treat e!ciently
2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 111
Figure 2.54: A four-roll wet high-intensity roll magnetic separator ERM-3.
Figure 2.55: A detailed view of the profiled roll of a wet high-intensity roll mag-
netic separator.
112 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.56: 2-MSM-5 wet high-intensity roll magnetic separator (courtesy of
ˇ
ZB, Czechoslovakia).
very feebly magnetic and very fine particles, their main advantage, compared to
WHIMS, is the ability to treat coarse particles and the absence of the matrix
blockage. The range of applicability is similar to that of the rare-earth drum
magnetic separators, although the field generated by these separators is some-
what higher than that by the drum separators, with similar magnitude of the
field gradient. Further advantage of wet induced roll separators is that it is pos-
sible to control the magnetic field strength. They can be e!ciently applied to
materials that fall between low-intensity magnetic separation and high-intensity
high-gradient separation.
2.4.3 Solenoid-based magnetic separators
All wet high-intensity magnetic separators already discussed have produced the
magnetic field by means of an electromagnet with iron yoke. For small work-
ing volumes and for moderately large magnetic induction, this constitutes an
e!cient and relatively cheap design, although the machines tend to be massive.
The input power needed to saturate the iron poles is modest and reasonably
high magnetic induction, of the order of 1 T, can be generated in the air gaps,
which guarantees su!ciently high throughputs (tens of tonnes per hour) of fine
weakly magnetic material. However, it follows from the discussion in Section
1.7.3 that this design suers two serious drawbacks:
a. The maximum magnetic induction that is obtainable in the working vol-
ume is limited by the saturation polarization of iron (approximately 2 T). Since
such an induction can be generated only in a narrow gap, much lower inductions
2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 113
Figure 2.57: Frantz Ferrofilter magnetic separator.
are usually created in units designed for practical large-scale applications.
b. Scale-up is very di!cult. In order to increase the throughput, the gap
between the poles must be increased. This results either in a dramatic drop in
the magnetic induction in the working volume, or in a substantial increase in
the number of ampere-turns in order to maintain the same magnetic induction.
The machine becomes extremely massive in relation to its capacity, magnetic
induction becomes increasingly limited by the width of the gap, and the cooling
of coils also becomes a problem.
To generate su!ciently large magnetic induction at large volumes, it is nec-
essary to use an iron-clad solenoid, as has been analyzed in Section 1.7.3.
Frantz Ferrofilter
The first magnetic separator that used the iron-clad solenoid was the so-called
Frantz Ferrofilter, introduced by S.G. Frantz in 1937 [F1]. This separator, shown
in Fig. 2.57, became the conceptual basis of cyclic high-gradient magnetic sep-
arators. It consists of an iron-bound solenoid magnet generating a magnetic
field of about 0.15 T, while the matrix is formed by screens fashioned from thin
sharp ribbons of magnetic stainless steel, as shown in Fig. 2.58.
The device operates intermittently; the flow is interrupted, the magnetic
field is reduced to zero and the magnetic particles are backwashed. Then the
field is switched on again and the entire cycle is repeated.
Because of the low demagnetizing factor of the ribbon elements that are
114 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.58: A ribbon matrix used in a Frantz Ferrofilter.
oriented parallel with the direction of the magnetic field, the magnitude of
the background magnetic induction was su!cient for the recovery of fine fer-
romagnetic particles. However, in order to magnetize the randomly oriented
filamentary matrix with very high aspect ratio, a much higher magnetic field is
needed. As a means of achieving a high degree of retention of quasicolloidal, fee-
bly magnetic impurities from kaolin, Frantz’s concept was extended so that more
powerful iron-clad solenoids were used in conjunction with randomly oriented
steel wool. The early developments of cyclic high-gradient magnetic separators
are described in detail in references [O3, K5, K7, I2, I3].
Modern cyclic high-gradient magnetic separators
The first industrial high-gradient magnetic separator, which grew from Frantz’s
concept and from investigations into the removal of weakly magnetic discolour-
ing agents from clays was built in 1969 [O4]. Figure 2.59 shows schematically
the essential elements of operation of such a separator. The system comprises
a canister filled with a matrix formed of compressed mats of magnetic stain-
less steel wool, although other types of matrices can be used. The canister is
placed in an iron-clad solenoid which generates a magnetic induction up to 2 T.
The matrix creates a high degree of non-homogeneity of the magnetic field and
produces a large magnetic force acting on the magnetizable particles.
The slurry is fed to the canister either vertically downwards or upwards
through the matrix with the magnetic field switched on. The magnetic particles
from the slurry are trapped into the surface of the magnetized matrix, while
the non-magnetic particles pass through the canister. When the matrix has
been loaded with the magnetic particles, the flow is halted, the magnetic field
2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 115
Figure 2.59: Schematic diagram of operation of a cyclic HGMS.
Figure 2.60: An automated small capacity cyclic Sala-HGMS
WP
116 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.61: High-gradient magnetic filter for e"uent control in the steel indus-
try (courtesy of Metso Minerals, Ltd.).
is reduced to zero and the magnetic fraction is flushed from the matrix.
Such cyclic devices are useful in applications where a slurry to be processed
contains a small fraction of magnetic particles. These machines have found a
widespread application in the kaolin industry and in waste water treatment.
A range of cyclic high-gradient magnetic separators is manufactured by sev-
eral companies. The solenoid diameter ranges from 1 m to 3 m, input power
from 100 to 500 kVA and throughput as high as 200 t/h is claimed. Figure 2.60
shows a small-scale cyclic HGMS while Fig. 2.61 presents a Metso Minerals
cyclic HGMS for waste water filtration. A production-scale Eriez Magnetics
HGMS for purification of kaolin is shown in Fig. 2.62.
Dry Allis-Sala HGMS Dry matrix high-gradient magnetic separation was
developed by Allis Sala [J7] in order to extend applicability of dry separation
to such a fine particle range that cannot be easily beneficiated by roll or drum
dry separators. Schematic diagram of the separator is shown in Fig. 2.63.
The magnetic field is generated by an iron-clad water-cooled solenoid and the
matrix is placed in the bore of the coil. Particle transport through the matrix
is accomplished by a compressed air-driven vacuum pump. The pump sucks the
air through the heater into the separation circuit. The air inlet is positioned at
the feed point, where the pre-heated air and particles are mixed before entering
the matrix.
The economic expansion of high-gradient magnetic separation to numerous
mineral processing applications requires the utilization of fully continuous mag-
2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 117
Figure 2.62: Eriez Magnetics cyclic HGMS for kaolin beneficiation (courtesy of
Eriez Magnetic, Inc.).
Feed
Air heater
Air Filter bed with matrix
Vacuum pump
Solenoid
Figure 2.63: Schematic diagram of dry Sala HGMS.
118 CHAPTER 2. REVIEW OF MAGNETIC SEPARATORS
Figure 2.64: Continuous Sala high-gradient magnetic separator.
netic separation. Cyclic devices are limited in their use to separations in which
the magnetic fraction is a small portion of the feed and to the feeds that must
be passed through the device at controlled slow feedrates. Continuous solenoid
high-gradient magnetic separators have eliminated the limitations of the cyclic
machines and several advanced devices have been built.
Sala-HGMS carousel separator
The Sala-HGMS carousel separator was developed from the cyclic separator to
meet the need for a continuous industrial machine capable of processing ores
containing a high proportion of magnetics [O5, M9, A5]. As can be seen in
Fig. 2.64, a double-saddle iron-clad coil surrounds the carousel ring. Seals
mounted on the ring allow independent control of feed and flush velocity to
enhance the separation and to increase the flushing e!ciency of the magnetics.
The operational sequence is similar to that used in conventional continuous
iron-core-type high-intensity separators as is evident in Fig. 2.65.
Further advantage of such a magnetic separator is that it can be scaled up
to treat high volumes of material. Magnetic field is parallel to the slurry flow.
When increased throughput is required, the width of the magnet is increased,
while its height remains constant. The surface area of the matrix can, therefore,
be enlarged without increasing the width of the air gap and thus the number of
ampere-turns to generate the required magnetic field.
The iron return frame reduces the magnetic flux leakage to places outside the
magnetic head. In high-intensity magnetic separators with iron yokes, the stray
2.4. WET HIGH-INTENSITY MAGNETIC SEPARATORS 119
Figure 2.65: The operational sequence in a continuous Sala HGMS [A5].
magnetic field is so high despite a reversal of polarity of two adjacent magnetic
heads that the magnetic induction in the flush station area usually amounts to
about 20 mT. This is su!ciently high to hamper the e!cient flushing of the
matrix. With iron-clad solenoids, the magnetic induction outside the magnet is
as low as 2 mT, depending on the matrix used.
Sala HGMS are manufactured under the Metso Minerals label in various
models. The number of magnetic heads ranges from one to four and magnetic
induction varies from 0.5 T to 2 T. Throughput of 200 t/h per magnet head
is specified for the largest model 480 ABCD. The matrix is, typically, a com-
partmented stack of expanded metal screens. A photograph of a single-head
Sala-HGMS unit, with nominal throughput of 15 t/h is shown in Fig. 2.66. The
largest model that is presently being oered by Metso Minerals is the HGMS
350 shown in Fig. 2.67. This unit can be fitted with up to three magnetic heads
and can reach a throughput up to 300 t/h.
VMS magnetic separator
A novel concept for replacement of a horizontal rotor or carousel by a vertical
wheel was incorporated in the VMS magnetic separator. The VMS magnetic
separator, the isometric view of which is shown in Fig. 2.68, was developed in
the Ore Research Institute in Prague, in former Czechoslovakia [H9, H10, C7].
A magnetic field is generated by the horizontally oriented solenoid wound
with a water-cooled hollow conductor. The coil is enclosed in steel cladding,
the upper and lower pole-pieces of which are perforated to allow passage of the
slurry. The slurry is fed into the separator via a feed box and it then enters