Indian Journal of Engineering & Materials Sciences
Vol. 5, June 1998, pp. 130-135
Magnetic-gravity separation of iron ore
P A Usachyov & S Yu Korytny
Mining Institute, Kola Science Centre, Russian Academy of Sciences, 24 Fersman str., 184200
Apatity, Munnansk region, Russia
Received 17 October 1997; accepted 30 April 199'S
In order to produce high-grade iron ore concentrates a magnetic-gravity method has been developed
for separation of magnetite ore. It provides separation of mineral complexes according to magnetic
properties, density and size. Magnetic-gravity separators (MGS) have been designed and their
parameters are defined, It has been shown that MGS enables to obtain 1.5-3.0% improvement in Fe
content in concentrates and to produce superconcentrate (72% Fe, 0.2-0.3% Si02). MGS also provides
for intensification of thickening and desliming of ferromagnetic suspensions.
Drum separators with permanent magnets are the
most commonly used ones in the beneficiation of
iron ores. In these separators the magnetic forces
significantly overcome the dynamic forces of pulp
flow and gravity forces experienced by the mineral
grains. Magnetic systems are used to create
magnetic fields with intensity, H=60 to 100 kAlm
and high inhomogeneity. In such cases, due to
strong magnetic interaction of magnetic mineral
grains between themselves and with magnetic field
the tloccules of magnetic mineral grains are formed
on the surface of the separator's drum which also
contains non-magnetic mineral grains and their
aggregates with magnetic minerals in it. As a result
reduction of the intensity of magnetic field up to
36-40 kAlm in drum separators the improvement of
concentrate grade occurs, accompanied by larger
losses of Fe with non-magnetic product in which
Fe is combined not only with magnetite aggregates
but with fine grade magnetite also. This is caused
by the fact that on reducing the size by <50 mk, the
magnetic
susceptibility
of magnetite particles
sharply decreases and becomes comparable or less
than that of aggregates of magnetite and rockforming minerals. That is why beneficiation of iron
ore follows complex route of crushing, grinding,
classification, magnetic separation, flotation and
slime treatment. However the investments for
improvement of the flowsheet for low grade, fine
phenocrystal
ore
having
complex
mineral
composition is not cost efficient. The increase in
fmer grinding not only sometimes makes the
separation more expensive and metal losses' higher
but also complicates the production of high-grade
concentrate as a result of strong magnetic and
adhesion interaction of fine particles.
Magnetite concentrates are polydisperse products
in which free particles of magnetite have a wide size
range and aggregation of magnetite with rockforming minerals occur, mainly, in coarse fraction
(Tab"le I). The particles of magnetite concentrate,
although having similar magnetic properties, still
differ in their density and mass. Taking into account
the dispersity, mineral composition and physical
properties of magnetite particles, the improvement
of magnetite concentrate grade should be based on a
,process providing for selective classification of the
most grainy fraction, composed of the aggregates of
magnetite with rock-forming minerals.
Experimental Procedure
Based on the studies of physical and mechanical
properties of ferro-suspensions in magnetic fields, a
new magnetic-gravity method for separation has
been suggested providing for high selectivity of
separation of mineral complexes according to their
magnetic properties, density and size'". Similar
:principle of separation is presented by Shattacharyya
and Sali3.
The essence of magnetic-gravity
separation
(MGS) is that, by applying electromagnetic field
having the intensity H=4 to 16 kAlm and
inhomogeneity gradient up to 4 kAlm2 per meter of
suspension, in required hydrodynamic regime, the
ferromagnetic particles, as a result of the effect of
magnetic and gravity forces, pass into the lower
131
USACHYOV & KORYTNY: MAGNETIC-GRAVITY SEPARATION OF IRON ORE
Table l-Composition
Size class, mk
of magnetite concentrates of the Kostomuksha combinat
Output,%
Fe
+50
-50
Total
38.5
61.5
100.0
37.6
65.9
55.0
+50
-50
Total
8.2
91.8
100.0
41.8
68.9
67.6
Si02
Content,%
summary
magnet.
non-ore
II stage separation concentrate
1.8
42.2
13.9
4.7
7.6
66.4
3.6
20.9
58.5
III stage separation concentrate
37.8
3.1
5.6
Fig. I-Maximum
speed of ascending water flow (V)
excluding the removal. from ferromagnetic suspension of
magnetite particles of different grade sizes (d): curve 1without magnetic field; curve 2 - with magnetic field with 4
kAlm intensity.
70
34.4
95.4
90.4
0.2
0.6
0.5
Fig.2-Influenceof
magnetic field intensity (H) upon the
efficiency (E) of MG-separation of magnetite concentrates of
curve 1 - 56% grade class -45 mk; curve 2 - 92% grade class 45 mk..
suspension flow, thus, creating a concentrated
mobile layer.
The aggregates formed by ferromagnetic particles
in the layer are easily destroyed
by the
hydrodynamic force. Non-magnetic particles and
their aggregates with magnetite are taken away with
water flow into the upper part of suspension and
removed as tailings. A specific feature of the method
is that the separation is done within the whole
Degree of magnetite
liberation,%
84.3
8.9
37.9
28.0
95.7
78.4
65.4
4.0
9.0
59.9
99.0
97.0
volume of suspension concentrated by magnetic
field at constantly renewed layer of ferromagnetice
particles from the initial suspension. The time of
separation can be controlled within a wide range by
changing the volume of concentrated ferromagnetic
layer. Due to magnetic particles made "heavier" by
the application of electromagnetic field it is possible
to strongly increase the speed of the ascending water
flow up to (1-2).10-2 mis, which enables to remove
coarse non-magnetic particles into tailings along
with aggregates consisting of ferromagnetic and
non-magnetic minerals (Fig. 1). The selectivity of
separation of mineral complexes according to their
magnetic properties and density is provided by
regulating the intensity of magnetic field and speed
of outcoming water flow meeting the following ratio
of the applied forces,
For magnetite particles: Fm<Fg; Fm+F.,i>Ff
For magnetite aggregates with non-magnetic
particles, Fm<Fg; Fm+Fg<Ff
For non-magnetic particles: Ff>Fg,
where Fm is magnetic force applied on the
particle, Fg is gravity force of the particles, and Ff is
the force of ascending water flow.
The values and ratio of F m and Ff depend upon
size and output of feeding product and the
requirements for the grade of the final concentrate.
To provide for maximal efficiency of MGseparation while reducing the size of initial feed, the
intensity of magnetic field should be increased
(Fig. 2).
For practical realization of the flowsheet some
alternative designs of impeller and non-impeller
MGS have been developed'<. A modified design of
non-impeller MGS (Fig.J) includes a cylindrical
case (1) made of non-magnetic material, magnetic
system (2), surrounding the case, charge (3) and
discharge (4) devices, sewing chute (5), a device for
~L---~--~5~--~6----7~--~'
H,I(A/m
magnetite
aggregates
/
\
132
INDIAN 1. ENG. MATER
wash water supply (6) with tangentially installed
sleeves (7), ferromagnetic layer level gauge (8),
block (9) for control over automated system for
concentrate discharge, electromagnetic shutter (10),
cone breaking device (11) installed coaxial under the
charging device on the same level with the gauge
(8). Magnetic-gravity
apparatus
operates
as
follows--the initial feed as suspension enters the
case (1) through the charge device (3). Under the
effect of magnetic field produced by. the magnetic
system (2) a ferromagnetic layer is formed with
clear upper border. The ascending water flow
washes the layer and rotates it by supplying water
through tangentially mounted sleeves (7). As a result
of magnetic interaction between themselves and the
applied magnetic field, the magnetic particles are
concentrated in the lower part of the case (I) and are
removed from the separator through the device (4).
Non-magnetic
particles and aggregates not or
slightly affected by ferromagnetic layer gravity are
carried away from the case by the ascending water
flow combined with the feed water into sewing
chute (5) and removed as sewage.
The automated system for discharge of magnetic
product operates as follows--the
gauge (8)
generates alternative voltage, the value of which is
proportional to the relative position of the gauge and
the upper border of the layer. Block (9) transforms
this signal into direct voltage which is supplied to
electromagnetic solenoid valve (10). At feeding the
material to the device there is no signal from the
gauge until the level of ferromagnetic lay~r reach~s
the gauge.' Direct voltage on the solenoid has Its
maximal value with the material not being
discha:ged. With the level of ferromagnetic layer
increasing voltage of the gauge starts to generate
decreasing direct voltage supplied to the solenoid of
operating mechanism. The capacity of the .sleeve
gradually increases until it gets balanced With the
amount of entering feed, while the level of
ferromagnetic layer is stabilized",
MGS-l.5 rn has the following characteristics:
Capacity, t/h .
15-20
Consumption:
electric energy, kW/h
3
water, m' concentrate
3-5
up to IJ.2
Size of material separated, mm
2.0 - 2.5
Dimensions, m
Weight, kg
1500
sci, JUNE
1998
FEED
tI
ICONCENTRATE
Fig. 3-Design of electromagnetic separator
Results and Discussion
During
the
technological
research
and
commercial testing of magnetite concentrates from
AJS Karelsky Okatysh, Olkon Lebedinsky GOK
(Russia), and AJS Sydvaranger (Norway) it was
revealed that MG-separation provides for high
selectivity of separation by removing into tailings
the main part of coarse grain fraction containing
magnetite aggregates and non-magnetic particles
with 40-60% magnetite content. This effect can be
observed for concentrates of different size grade
(Tables 2-4). Tailings obtained at MG-separation
after thickening are reasonable to be reground and
subjected to further separation in a separate cycle.
Significant improvements in grade of magnetite
concentrate by MG-separation at other iron ore
complexes are reported earlier='. MG-separation
enables to reduce silica, phosphorus and sometimes
sulfur content in iron concentrates---e.g.,
MGseparation enables to reduce P2~ content in Kovdor
magnetite concentrate from 0.15 to 0.09%. MGseparation
during
desliming
provides
for
acceleration of ascending water flow 2-3 times as
compared with desliming agents, which enables !o
have more efficient removal of coarse gram
aggregate fraction into sewage.
Taking into account mineral composition and
physical characteristics of magnetite concentrates
USACHYOV
& KORYTNY: MAGNETIC-GRAVITY
SEPARATION
Lebedinsky GOK
Table 2---Parameters ofMG-separation of magnetite concentrates in AlS
Output,%
Product
Content,%
Fe
sio,
Recovery,%
Fe
sto,
133
OF IRON ORE
-50 mk grade class
Content"/o Recovery'Y.
Class
Fe Class
Fe
+50 mk grade class
Content"10 Recovery%
Class
Fe Class Fe
Density of
sewage,
kg/m"
III Stage separation concentrate
8.2
91.8
100.0
Non-magnetic product
Concentrate
Feed
23.5
69.7
66.0
56.7
2.9 62.4
3.1 97.1 37.6
7.5 100.0 100.0
45.2
14.5
17.0
11.0 21.8
63.5 78.2
52.1 100.0
0.6
12.8
13.4
54.8
85.5
83.0
33.8
5.4
70.8 94.6
68.8 100.0
2.3
84.3
86.5
3200
0.3
2.1
2.4
66.7
97.3
95.6
47.2
3.9
70.7 96.1
69.8 100.0
2.5
95.1
97.6
3500
V Stage separation concentrate
5.4
94.6
100.0
Non-magnetic product
Concentrate
Feed
35.0
70.3
68.4
43.6
2.8 53.8
2.1 97.2 46.2
4.3 100.0 100.0
Table 3-MG-separation
Product,%
Output, %
33.3
2.7
4.4
13.2 40.8
56.7 59.2
38.9 100.0
from NS Karelsky Okatysh
of concentrates
Content, %
--45mk
Fe
grade class
Si02
Recovery, %
grade class, mk
Fe
--45
+45
MG-separation
H, , Wm
Y, cmls
Concentrate of III separation stage
Non-magnetic
product
Concentrate
Feed
3.6
40.7
20.1
62.9
1.6
27.3
1.1
33.3
96.4
100.0
94.1
92.2
68.5
66.8
4.7
6.8
98.4
100.0
72.7
100.0
98.9
100.0
66.4
6.5
1.9
4.5
2.4
Concentrate of II separation stage
21.8
17.6
22.3
61.9
6.9
40.8
8.3
78.5
78.2
100.0
66.7
56.0
68.2
58.3
4.8
17.2
93.1
100.0
59.2
100.0
91.7
100.0
21.5
100.0
Non-magnetic
productConcentrate
Feed
Table 4--MG-separation
Product
of magnetite concentrates from NS Olkon
Output, %
Content, %
-71 grade mk
class
Recovery,%
grade class, mk
-71
+71
Fe
Fe
Concentrate of 8-12 Sections
Non-magnetic
Concentrate
Feed
product
5.2
94.8
100.0
65.5
80.6
79.8
22.6
67.7
65.4
4.3
1.8
60.5
4.7
7.6
95.7
100.0
8.9
91.1
100.0
98.2
100.0
41.5
58.5
100.0
58.7
2.4
6.5
4.6
95.4
100.0
9.6
90.4
100.0
3.1
96.9
100.0
65.5
34.5
100.0
Combined Concentrate
Non-magnetic
Concentrate
Feed
product
7.2
92.8
100.0
30.5
59.7
47.8
(Table 1), the capability of MG-separation to
separate mineral complexes under their magnetic
properties and density (Tables 2-4) a new principle
for design of flowsheets for processing of magnetite
.ore has been developed providing for production of
open ore phase as ready iron concentrate and
processing of aggregate fraction in a separate cycle.
The kernel of the technology is as follows-feed
rough magnetite concentrate with over 55-60% Fe
content goes to classification by means of screening,
e.g., on vibration screens, thus obtaining coarse-
28.6
70.0
66.9
grain and fine-grain products. Redistribution of free
ore minerals and aggregates occurs proportional to
their content in grade classes of feed concentrate.
Selection of grade class at classification i~according
to the level of ore mineral liberation in the feed
concentrate and requirements for the grade of final
concentrate. Fine grain product of classification is
MG-separated with obtaining final concentrate and
product (sewage), presented mainly by magnetite
aggregates. Coarse grain product of classification is
MG-separated in a separate cycle with removing
-134
INDIAN 1. ENG. MATER.
grain fraction (sewage) into tails. Magnetic product
obtained with thickened sewage of MG-separation
fine-grain product is reground, dressed in drum
magnetic separators and MG-separated with final
concentrate and dump tails produced. Derrick screen
with polyurethane grid can be used for classification
of rough magnetite concentrate providing for over
80% efficiency of beneficiation for 100 mk class.
These screens are used at some iron processing
plants in the U.S. and Norway.
In Figs 4 and 5 the principal flowsheets for
regrinding of rough magnetite concentrates at
Lebedinsky GOK and Karelsky Okatysh are shown.
Pre-classification of 70(100) mk class by fine
screening with the following MG-separation of
classification products of magnetite concentrate of
the first stage in Lebedinsky GOK enable to produce
final concentrate (over 40% of I stage concentrate)
and 14.8% tails of over 70(100) mk. Regrinding is
about only 40% of the feed concentrate (Fig. 3). By
eliminating the third stage of dressing and obtaining
coarse grain tails (45-50% of -50 mk class) under
the suggested flowsheet 20-30% reduction of energy
consumption is possible as compared with the
operating one [II]. From the V separation stage
concentrate, MG-separation enables to get high
grade iron concentrate (about 70% of Fe and not
more than 3% of silica for electrometallurgical
production of steel (Table 2).
Similar technological and engineering solutions
for regrinding of concentrate of the second
concentration stage in A/S Karelsky Okatysh,
provide for production of concentrate with 70% Fe
of 92% -50 mk grade and coarse grain tails (62% 50 mk grade). Less than 40% is sent to regrinding
from the second stage of concentration. Thus,
without extra costs for regrinding it is possible to
increase Fe content in Karelsky Okatysh from
67.5/70% (Fig. 5).
MG-separators enable to establish commercial
production of superconcentrates (72% Fe, 0.2-0.3%
silica) in A/S Olkon (Russia) and A/S Sydvaranger
(Norway). At Olkon, MG-separators are used also
for combined thickening up to 65% of solid
hematite-magnetite concentrate at simultaneous 0.30.5% improvement of concentrate Fe grade.
sci, JUNE
1998
'·1--
lOO.OJi,i
100.0
~A1lON
"·~·<l·
AItA1lON
1lON
••.r-l'.Z•7
-r"~
59.6
00.<
6.7-&
".8~
'7.Z
,.~9.0
55.'
<.,
".8~
"'.2
Z5.Z~
5.8
c-
T••••
Fig. 4--Flowsheet of processing I concentration stage
magnetite concentrate with MG-separation being used at A/S
Lebedinsky GOK.
9.I1~::
'S.9~:~.
Con<en_
T••••
25. ~.£a262
5.1
?4.37C•3-':,::'Cj2
_(%)"
l~E-~"
"'.9
, .•••••
F.
,-auIpIl;~,
~""""y;
P... ~.••.•••
Fe, 1i1ica_·50 ••••• tI_
••.•
·Fe~
Fig. 5--Flowsheet of processing II concentration stage
magnetite concentrate with MG-separation being used at A/S
Karelsky Okatysh.
content, intensify the process of beneficiation of
Magnetic-gravity method and separators with low fines and slimes of magnetite-containing ores and
intensity of magnetic field enable to produce high desliming and thickening of ferromagnetic
grade iron concentrates with minimal impurities suspensions. MG-separators used at production of
Conclusion
USACHYOV
& KORYTNY:
MAGNETIC-GRAVITY
materials for blast furnace and electrometallurgical
production of metal do not require regrinding of 2
rough concentrates, provide for 1.5-2.0 times
reduction of magnetic separation operations and for 3
20-30% less energy consumption.
References
1 Usachyov P A, Magnetic rheology of mineral separation
in
SEPARATION
OF IRON ORE
l35
suspensions (Nauka, Leningrad), 1983.
Usachyov P A & Opalev A S, Magnetic-gravity
mineral
processing (Kola Science Centre RAS, Apatity), 1993.
D N & Sali V T, Indian J Technol, Vol 23
Shattacharyya
(1985) 21.
4 Usachyov P A, Opalev A S & Pershukov
1540088 (Mining Institute KSC RAS),1987.
5 Usachyov P A, Gorn Zh, N 12, (1993), 22.
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\
A A, Rus Pal