Jan Svoboda. Magnetic Techniques for the Treatment of Materials
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380 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS
Figure 5.51: The eect of the rod diameter on magnetic separation of siderite
ore (adapted from Hencl et al. [H22]).
Figure 5.52: Separation of fine ilmenite (- 45 m) by SLON magnetic sepa-
rator with rod matrices of dierent diameters. Comparison with
expanded metal matrix is also shown (adapted from Xiong [X3]).
5.3. WET MAGNETIC SEPARATION 381
Table 5.22: Grooved plates used as a matrix in Jones magnetic separators.
Plate type Specification Applications
4R 4 grooves per inch For coarse material (- 3 mm),
gap width 5 mm
8R 8 grooves per inch Medium size material (- 0.5
mm), gap 1.2 - 2.5 mm
12R 12 grooves per inch Very fine material (- 100 m),
gap width 0.5 mm
Table 5.23: Beneficiation of oxidized quartzites (Krivoy Rog) by magnetic sep-
aration, using dierent matrices [M28].
Matrix Recovery [% Fe] Grade [% Fe]
Grooved plates, gap 4 mm 59.8 52.1
Grooved plates, gap 2.2 mm 66.4 52.4
Groovedplates+expandedmetal,
gap 6 mm
67.9 54.0
incorporated expanded metal sheets between the grooved plates. A considerable
increase in the recovery of iron was observed when this matrix was applied to the
beneficiation of feebly magnetic, finely ground (80% - 45 m) Krivoy Rog (the
Ukraine) and (90% - 74 m) Baikal (Russia) iron quartzites, as is summarized
in Tables 5.23 and 5.24.
Another approach was taken by Turkenich [T2] and Ulubabov and Karmazin
[U2] who proposed to use inclined grooved plates so that the slurry passes over
the plate surfaces in the form of a film. These plates were designed in such
a way that the grooves on opposite sides of the gap were inclined in dierent
directions, as is shown in Fig. 5.53. Central sections of the gap are thus free of
the slurry and the e!ciency of separation is determined by the thickness of the
Table 5.24: Beneficiation of Baikal oxidized quartzites by magnetic separation,
using dierent matrices [M28].
Matrix Ore Recovery [% Fe] Grade[%Fe]
Grooved plate Siderite 84.0 33.2
Grooved
plate+expanded
metal
88.2 32.8
Grooved plate Limonite 52.6 54.8
Grooved
plate+expanded
metal
68.2 54.0
382 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS
Figure 5.53: Film-flow grooved plates with inclined ridges and grooves. (1)
plates, (2) spacers [T2].
slurry film on the surface of the plate. The distance between the plates can be
increased to eliminate the matrix blockage.
This approach to the film-flow matrix has been employed in 6-ERM-35/315
magnetic separators, described in Section 2.4.2. A comparison of the perfor-
mance of conventional vertical plates and inclined plates is shown in Fig. 5.54.
Fundamental theoretical issues of the film slurry flow in this matrix were dis-
cussed by Turkenich [T2].
The optimum geometry of a matrix
The criteria for selection of the optimum shape and geometry for matrix col-
lectors are even more vaguely defined than those for the size of the collectors.
The choice of the matrix depends on the characteristics of the slurry undergoing
the separation process and is often dictated by personal beliefs, experience and
the availability of a suitable matrix type. Ideally, the matrix should oer max-
imum collection e!ciency, minimum magnetic reluctance and fluid impedance,
together with good ability to be cleaned. Various shapes of a matrix are used,
such as grooved plates, balls, rods, wire mesh, expanded metal and steel wool.
Besides the size of a collector, a matrix is characterized by its filling factor
i, or by porosity (or void fraction) :
i =
volume of matrix
volume of separation chamber
(5.7)
and
=1 i (5.8)
5.3. WET MAGNETIC SEPARATION 383
Figure 5.54: Comparison of the performance of vertical and inclined grooved
plates, as a function of the width of the gap (adapted from [U2]).
Table 5.25: Filling factors of various matrices.
Matrix Filling factor i
Spheres 0.50 to 0.64
Steel wool 0.01 to 0.06
Expanded metal 0.12 to 0.28
Wire mesh 0.15 to 0.25
Grooved plates 0.50 to 0.85
The porosity of the matrix bed changes with time, as particle accumulation
within the matrix increases:
=
l
g
(1
g
) (5.9)
where
g
is the specific deposit (volume of deposited matter per unit volume of
matrix),
g
is the porosity of a deposit and
l
is the porosity of clean matrix.
Typical values of the filling factor for assorted types of matrices are given in
Table 5.25, while Figs. 5.55 and 5.56 illustrate some more frequently used
matrices.
The performance of various shapes of matrix collectors depends primarily
on the particle size distribution and magnetic properties of the solids to be
separated. Fine matrices, such as steel wool, are suitable for the removal of
very fine and very feebly magnetic particles but in most applications in mineral
processing these matrices are not suitable owing to their non-selectivity and
susceptibility to clogging.
For the majority of applications in materials treatment, coarse matrices (e.g.
balls, expanded metal, grooved plates) are the best choice. There is, quite
384 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS
Figure 5.55: Matrices used for magnetic separation: (from right to left ) grooved
plate, expanded metal 1.9 mm, Knitmesh.
Figure 5.56: Matrices used for magnetic separation: mesh 1.6 × 5mm(top),
mesh 2.5 × 10 mm (bottom).
5.3. WET MAGNETIC SEPARATION 385
Figure 5.57: Recovery of uranium from leaching residues (70% - 44 m) by as-
sorted matrices (adapted from [S61]).
Figure 5.58: The grade of the magnetic concentrate of uranium leaching
residues, for dierent matrices (adapted from [S61]).
386 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS
Figure 5.59: The eect of matrix length on the removal of iron from kaolin, for
various volumes of the processed slurry (adapted from [N9]).
understandably, no universal type of a matrix that meets all requirements in
all applications. Each type of matrix has its advantages and disadvantages
and detailed laboratory tests should be carried out to identify the optimum
matrix for a given application. Figures 5.57. and 5.58 illustrate the performance
of various matrices as applied to recovery of gold and uranium from leaching
residues. Although fine matrices, such as steel wool, gave high recoveries, their
selectivity was very poor. With frequently used matrices of grooved plates and
expanded metal, unsatisfactory recoveries and grades were obtained. On the
other hand, steel balls and mesh matrices gave the best results and similar
observations were made in other applications of these matrices.
The length of the matrix
The length of the matrix is an important parameter that determines the reten-
tion time of particles in the magnetic field, the probability of their collision with
the matrix collectors, and thus the probability of their capture. From numerous
experimental studies, and from theoretical description of particle capture in a
matrix, as outlined in Section 3.4.2, it is evident that the matrix length can
have an impact on the e!ciency of separation at least as great as the magnetic
field strength.
In several early WHIMS or HGMS machines, the length of the matrix was 80
to 100 mm, clearly to the detriment of their e!ciency. On the other hand, the
length of grooved plates in Jones WHIMS units was 220 mm. The application of
such a long matrix is understandable in view of poor filtration properties of the
grooved plates. In current cyclic or continuous HGMS separators, the matrix
5.3. WET MAGNETIC SEPARATION 387
Figure 5.60: The eect of matrix length on the brightness of kaolin at dierent
values of magnetic induction. Slurry velocity y = 0.5 cm/s (adapted
from [S60]).
length is often about 200 mm, although in very demanding applications, such
as kaolin brightening, the length is as much as 500 mm. The beneficial eect
of increasing matrix length to 500 mm, on kaolin brightening, is illustrated in
Figs 5.59 and 5.60.
For the recovery of moderately weakly magnetic minerals, a matrix length of
200 mm or less is usually su!cient, as shown in Figs. 5.61 and 5.62. Figure 5.61
also illustrates a well established fact that the grade of the magnetic concentrate
often does not depend on the matrix length.
Although it is clear than a longer matrix is beneficial for overall metallurgical
performance of a matrix separator, it is important that the matrix should be
kept as short as possible, consistent with the required metallurgical results. By
reducing the matrix length, the size of pole tips of an electromagnet, or the
length of a solenoid coil can be reduced. This will result in the reduction of the
mass and the costs of the separator, and in lower input power.
Matrix loading
The matrix is able to carry only a limited amount of material particles before its
capture e!ciency begins to deteriorate. Quantitatively, this capability is usually
expressed through the matrix loading. There are several ways to express this
quantity, none of them strictly correct, but acceptable if used in a consistent
manner.
In kaolin brightening, the matrix loading is often expressed through the num-
ber of bed (or canister) volumes processed in the separator. Other definitions
388 CHAPTER 5. PRACTICAL ASPECTS OF MAGNETIC METHODS
Figure 5.61: The recovery of iron and the grade of the magnetic concentrate,
as a function of matrix length, for a matrix of rods of dierent
diameters (adapted from [H22]).
Figure 5.62: The recovery of uranium from uranium-gold leaching residues, as
a function of matrix length, for various matrices (adapted from
[S61]).
5.3. WET MAGNETIC SEPARATION 389
Figure 5.63: The e!ciency of iron removal from kaolin, as a function of the
number of bed volumes of slurry processed (adapted from [N9]).
Figure 5.64: The concentration of iron oxide in the non-magnetic concentrate of
silica sand, as a function of matrix loading (after [S29]).