Indian Journal of Engineering & Materials SciencesVol. 5, June 1998, pp. 130-135

Magnetic-gravity separation of iron ore

P A Usachyov & S Yu KorytnyMining 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 developedfor separation of magnetite ore. It provides separation of mineral complexes according to magneticproperties, density and size. Magnetic-gravity separators (MGS) have been designed and theirparameters are defined, It has been shown that MGS enables to obtain 1.5-3.0% improvement in Fecontent in concentrates and to produce superconcentrate (72% Fe, 0.2-0.3% Si02). MGS also providesfor intensification of thickening and desliming of ferromagnetic suspensions.

Drum separators with permanent magnets are themost commonly used ones in the beneficiation ofiron ores. In these separators the magnetic forcessignificantly overcome the dynamic forces of pulpflow and gravity forces experienced by the mineralgrains. Magnetic systems are used to createmagnetic fields with intensity, H=60 to 100 kAlmand high inhomogeneity. In such cases, due tostrong magnetic interaction of magnetic mineralgrains between themselves and with magnetic fieldthe tloccules of magnetic mineral grains are formedon the surface of the separator's drum which alsocontains non-magnetic mineral grains and theiraggregates with magnetic minerals in it. As a resultreduction of the intensity of magnetic field up to36-40 kAlm in drum separators the improvement ofconcentrate grade occurs, accompanied by largerlosses of Fe with non-magnetic product in whichFe is combined not only with magnetite aggregatesbut with fine grade magnetite also. This is causedby the fact that on reducing the size by <50 mk, themagnetic susceptibility of magnetite particlessharply decreases and becomes comparable or lessthan that of aggregates of magnetite and rock-forming minerals. That is why beneficiation of ironore follows complex route of crushing, grinding,classification, magnetic separation, flotation andslime treatment. However the investments forimprovement of the flowsheet for low grade, finephenocrystal ore having complex mineralcomposition is not cost efficient. The increase infmer grinding not only sometimes makes theseparation more expensive and metal losses' higher

but also complicates the production of high-gradeconcentrate as a result of strong magnetic andadhesion interaction of fine particles.

Magnetite concentrates are polydisperse productsin which free particles of magnetite have a wide sizerange and aggregation of magnetite with rock-forming minerals occur, mainly, in coarse fraction(Tab"le I). The particles of magnetite concentrate,although having similar magnetic properties, stilldiffer in their density and mass. Taking into accountthe dispersity, mineral composition and physicalproperties of magnetite particles, the improvementof magnetite concentrate grade should be based on a

,process providing for selective classification of themost grainy fraction, composed of the aggregates ofmagnetite with rock-forming minerals.

Experimental ProcedureBased on the studies of physical and mechanical

properties of ferro-suspensions in magnetic fields, anew magnetic-gravity method for separation hasbeen suggested providing for high selectivity ofseparation of mineral complexes according to theirmagnetic properties, density and size'". Similar

:principle of separation is presented by Shattacharyyaand Sali3.

The essence of magnetic-gravity separation(MGS) is that, by applying electromagnetic fieldhaving the intensity H=4 to 16 kAlm andinhomogeneity gradient up to 4 kAlm2 per meter ofsuspension, in required hydrodynamic regime, theferromagnetic particles, as a result of the effect ofmagnetic and gravity forces, pass into the lower

USACHYOV & KORYTNY: MAGNETIC-GRAVITY SEPARATION OF IRON ORE 131

Table l-Composition of magnetite concentrates of the Kostomuksha combinat

Size class, mk Output,% Content,% Degree of magnetiteFe Si02 summary magnetite liberation,%

magnet. non-ore aggregatesII stage separation concentrate

+50 38.5 37.6 42.2 13.9 1.8 84.3 28.0-50 61.5 65.9 7.6 66.4 4.7 8.9 95.7

Total 100.0 55.0 20.9 58.5 3.6 37.9 78.4III stage separation concentrate

+50 8.2 41.8 37.8 34.4 0.2 65.4 59.9-50 91.8 68.9 3.1 95.4 0.6 4.0 99.0

Total 100.0 67.6 5.6 90.4 0.5 9.0 97.0

Fig. I-Maximum speed of ascending water flow (V)excluding the removal. from ferromagnetic suspension ofmagnetite particles of different grade sizes (d): curve 1-without magnetic field; curve 2 - with magnetic field with 4kAlm intensity.

70

~L---~--~5~--~6----7~--~'H,I(A/m

Fig.2-Influenceof magnetic field intensity (H) upon theefficiency (E) of MG-separation of magnetite concentrates ofcurve 1- 56% grade class -45 mk; curve 2 - 92% grade class -45 mk..

suspension flow, thus, creating a concentratedmobile layer.

The aggregates formed by ferromagnetic particlesin the layer are easily destroyed by thehydrodynamic force. Non-magnetic particles andtheir aggregates with magnetite are taken away withwater flow into the upper part of suspension andremoved as tailings. A specific feature of the methodis that the separation is done within the whole

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volume of suspension concentrated by magneticfield at constantly renewed layer of ferromagneticeparticles from the initial suspension. The time ofseparation can be controlled within a wide range bychanging the volume of concentrated ferromagneticlayer. Due to magnetic particles made "heavier" bythe application of electromagnetic field it is possibleto strongly increase the speed of the ascending waterflow up to (1-2).10-2 mis, which enables to removecoarse non-magnetic particles into tailings alongwith aggregates consisting of ferromagnetic andnon-magnetic minerals (Fig. 1). The selectivity ofseparation of mineral complexes according to theirmagnetic properties and density is provided byregulating the intensity of magnetic field and speedof outcoming water flow meeting the following ratioof the applied forces,

For magnetite particles: Fm<Fg; Fm+F.,i>FfFor magnetite aggregates with non-magnetic

particles, Fm<Fg; Fm+Fg<FfFor non-magnetic particles: Ff>Fg,where Fm is magnetic force applied on the

particle, Fg is gravity force of the particles, and Ff isthe force of ascending water flow.

The values and ratio of F m and Ff depend uponsize and output of feeding product and therequirements for the grade of the final concentrate.To provide for maximal efficiency of MG-separation while reducing the size of initial feed, theintensity of magnetic field should be increased(Fig. 2).

For practical realization of the flowsheet somealternative designs of impeller and non-impellerMGS have been developed'<. A modified design ofnon-impeller MGS (Fig.J) includes a cylindricalcase (1) made of non-magnetic material, magneticsystem (2), surrounding the case, charge (3) anddischarge (4) devices, sewing chute (5), a device for

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132 INDIAN 1. ENG. MATER sci, JUNE 1998

wash water supply (6) with tangentially installedsleeves (7), ferromagnetic layer level gauge (8),block (9) for control over automated system forconcentrate discharge, electromagnetic shutter (10),cone breaking device (11) installed coaxial under thecharging device on the same level with the gauge(8). Magnetic-gravity apparatus operates asfollows--the initial feed as suspension enters thecase (1) through the charge device (3). Under theeffect of magnetic field produced by. the magneticsystem (2) a ferromagnetic layer is formed withclear upper border. The ascending water flowwashes the layer and rotates it by supplying waterthrough tangentially mounted sleeves (7). As a resultof magnetic interaction between themselves and theapplied magnetic field, the magnetic particles areconcentrated in the lower part of the case (I) and areremoved from the separator through the device (4).Non-magnetic particles and aggregates not orslightly affected by ferromagnetic layer gravity arecarried away from the case by the ascending waterflow combined with the feed water into sewingchute (5) and removed as sewage.

The automated system for discharge of magneticproduct operates as follows--the gauge (8)generates alternative voltage, the value of which isproportional to the relative position of the gauge andthe upper border of the layer. Block (9) transformsthis signal into direct voltage which is supplied toelectromagnetic solenoid valve (10). At feeding thematerial to the device there is no signal from thegauge until the level of ferromagnetic lay~r reach~sthe gauge.' Direct voltage on the solenoid has Itsmaximal value with the material not beingdischa:ged. With the level of ferromagnetic layerincreasing voltage of the gauge starts to generatedecreasing direct voltage supplied to the solenoid ofoperating mechanism. The capacity of the .sleevegradually increases until it gets balanced With theamount of entering feed, while the level offerromagnetic layer is stabilized",

MGS-l.5 rn has the following characteristics:Capacity, t/h . 15-20Consumption:

electric energy, kW/hwater, m' concentrate

Size of material separated, mmDimensions, mWeight, kg

33-5up to IJ.22.0 - 2.51500

FEED

tI

I-

CONCENTRATE

Fig. 3-Design of electromagnetic separator

Results and DiscussionDuring the technological research and

commercial testing of magnetite concentrates fromAJS Karelsky Okatysh, Olkon Lebedinsky GOK(Russia), and AJS Sydvaranger (Norway) it wasrevealed that MG-separation provides for highselectivity of separation by removing into tailingsthe main part of coarse grain fraction containingmagnetite aggregates and non-magnetic particleswith 40-60% magnetite content. This effect can beobserved for concentrates of different size grade(Tables 2-4). Tailings obtained at MG-separationafter thickening are reasonable to be reground andsubjected to further separation in a separate cycle.

Significant improvements in grade of magnetiteconcentrate by MG-separation at other iron orecomplexes are reported earlier='. MG-separationenables to reduce silica, phosphorus and sometimessulfur content in iron concentrates---e.g., MG-separation enables to reduce P2~ content in Kovdormagnetite concentrate from 0.15 to 0.09%. MG-separation during desliming provides foracceleration of ascending water flow 2-3 times ascompared with desliming agents, which enables !ohave more efficient removal of coarse gramaggregate fraction into sewage.

Taking into account mineral composition andphysical characteristics of magnetite concentrates

USACHYOV & KORYTNY: MAGNETIC-GRAVITY SEPARATION OF IRON ORE 133

Non-magnetic productConcentrateFeed

Table 2---Parameters ofMG-separation of magnetite concentrates in AlS

Output,% Content,% Recovery,% +50 mk grade classFe sio, Fe sto, Content"10 Recovery%

Class Fe Class Fe

III Stage separation concentrate

23.5 56.7 2.9 62.4 45.2 11.0 21.8 0.6 54.8 33.8 5.4 2.369.7 3.1 97.1 37.6 14.5 63.5 78.2 12.8 85.5 70.8 94.6 84.366.0 7.5 100.0 100.0 17.0 52.1 100.0 13.4 83.0 68.8 100.0 86.5

Lebedinsky GOK-50 mk grade class

Content"/o Recovery'Y.Class Fe Class Fe

Density ofsewage,kg/m"

Product

8.291.8

100.0

3200

V Stage separation concentrate

Non-magnetic productConcentrateFeed

5.4 35.0 43.6 2.8 53.8 33.3 13.2 40.8 0.3 66.7 47.2 3.9 2.594.6 70.3 2.1 97.2 46.2 2.7 56.7 59.2 2.1 97.3 70.7 96.1 95.1

100.0 68.4 4.3 100.0 100.0 4.4 38.9 100.0 2.4 95.6 69.8 100.0 97.6

3500

Product,%

Table 3-MG-separation of concentrates from NS Karelsky Okatysh

Output, % Content, % Recovery, %--45mk Fe Si02 grade class, mk Fe

grade class --45 +45

MG-separationH, , Wm Y, cmls

Non-magneticproductConcentrateFeed

3.6

Concentrate of III separation stage

40.7 20.1 62.9 1.6 27.3 1.1 33.3

94.1 68.5 4.7 98.4 72.7 98.9 66.492.2 66.8 6.8 100.0 100.0 100.0

Concentrate of II separation stage

17.6 22.3 61.9 6.9 40.8 8.3 78.5

66.7 68.2 4.8 93.1 59.2 91.7 21.556.0 58.3 17.2 100.0 100.0 100.0 100.0

6.5 1.9

96.4100.0

Non-magneticproduct-ConcentrateFeed

21.8

78.2100.0

4.5 2.4

Product

Table 4--MG-separation of magnetite concentrates from NS Olkon

Output, % Content, % Recovery,%

-71 grade mkclass

Concentrate of 8-12 Sections

Fegrade class, mk-71 +71 Fe

Non-magnetic product 5.2 65.5 22.6 60.5 4.3 8.9 1.8 41.5Concentrate 94.8 80.6 67.7 4.7 95.7 91.1 98.2 58.5Feed 100.0 79.8 65.4 7.6 100.0 100.0 100.0 100.0

Combined Concentrate

Non-magnetic product 7.2 30.5 28.6 58.7 4.6 9.6 3.1 65.5Concentrate 92.8 59.7 70.0 2.4 95.4 90.4 96.9 34.5Feed 100.0 47.8 66.9 6.5 100.0 100.0 100.0 100.0

(Table 1), the capability of MG-separation to grain and fine-grain products. Redistribution of freeseparate mineral complexes under their magnetic ore minerals and aggregates occurs proportional toproperties and density (Tables 2-4) a new principle their content in grade classes of feed concentrate.for design of flowsheets for processing of magnetite Selection of grade class at classification i~according.ore has been developed providing for production of to the level of ore mineral liberation in the feedopen ore phase as ready iron concentrate and concentrate and requirements for the grade of finalprocessing of aggregate fraction in a separate cycle. concentrate. Fine grain product of classification isThe kernel of the technology is as follows-feed MG-separated with obtaining final concentrate andrough magnetite concentrate with over 55-60% Fe product (sewage), presented mainly by magnetitecontent goes to classification by means of screening, aggregates. Coarse grain product of classification ise.g., on vibration screens, thus obtaining coarse- MG-separated in a separate cycle with removing

--134 INDIAN 1. ENG. MATER. sci, JUNE 1998

grain fraction (sewage) into tails. Magnetic productobtained with thickened sewage of MG-separationfine-grain product is reground, dressed in drummagnetic separators and MG-separated with finalconcentrate and dump tails produced. Derrick screenwith polyurethane grid can be used for classificationof rough magnetite concentrate providing for over80% efficiency of beneficiation for 100 mk class.These screens are used at some iron processingplants in the U.S. and Norway.

In Figs 4 and 5 the principal flowsheets forregrinding of rough magnetite concentrates atLebedinsky GOK and Karelsky Okatysh are shown.Pre-classification of 70(100) mk class by finescreening with the following MG-separation ofclassification products of magnetite concentrate ofthe first stage in Lebedinsky GOK enable to producefinal concentrate (over 40% of I stage concentrate)and 14.8% tails of over 70(100) mk. Regrinding isabout only 40% of the feed concentrate (Fig. 3). Byeliminating the third stage of dressing and obtainingcoarse grain tails (45-50% of -50 mk class) underthe suggested flowsheet 20-30% reduction of energyconsumption is possible as compared with theoperating one [II]. From the V separation stageconcentrate, MG-separation enables to get highgrade iron concentrate (about 70% of Fe and notmore than 3% of silica for electrometallurgicalproduction of steel (Table 2).

Similar technological and engineering solutionsfor regrinding of concentrate of the secondconcentration stage in A/S Karelsky Okatysh,provide for production of concentrate with 70% Feof 92% -50 mk grade and coarse grain tails (62% -50 mk grade). Less than 40% is sent to regrindingfrom the second stage of concentration. Thus,without extra costs for regrinding it is possible toincrease Fe content in Karelsky Okatysh from67.5/70% (Fig. 5).

MG-separators enable to establish commercialproduction 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 alsofor combined thickening up to 65% of solidhematite-magnetite concentrate at simultaneous 0.3-0.5% improvement of concentrate Fe grade.

ConclusionMagnetic-gravity method and separators with low

intensity of magnetic field enable to produce highgrade iron concentrates with minimal impurities

'·1--lOO.OJi,i100.0

~A1lON

"·~·<l·00.<

1lON

-r"~59.6

AItA1lON

6.7-& ,.~9.0<., 55.'••. r-l•7 ".8~

'.Z '7.Z

".8~"'.2c-

Z5.Z~5.8

T••••

Fig. 4--Flowsheet of processing I concentration stagemagnetite concentrate with MG-separation being used at A/SLebedinsky GOK.

9.I1~:: 'S.9~:~.

Con<en_

?4.37C•3-':,::'Cj2_(%)" "'.9

l~E-~" , .•••••F.

T••••25. ~.£a262

5.1

,-auIpIl;~, P... ~.••.••• Fe,1i1ica_·50 ••••• tI_~""""y; ••.•·Fe~

Fig. 5--Flowsheet of processing II concentration stagemagnetite concentrate with MG-separation being used at A/SKarelsky Okatysh.

content, intensify the process of beneficiation offines and slimes of magnetite-containing ores anddesliming and thickening of ferromagneticsuspensions. MG-separators used at production of

USACHYOV & KORYTNY: MAGNETIC-GRAVITY SEPARATION OF IRON ORE l35

materials for blast furnace and electrometallurgicalproduction of metal do not require regrinding of 2rough concentrates, provide for 1.5-2.0 timesreduction of magnetic separation operations and for 3

20-30% less energy consumption.References1 Usachyov P A, Magnetic rheology of mineral separation in

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suspensions (Nauka, Leningrad), 1983.Usachyov P A & Opalev A S, Magnetic-gravity mineralprocessing (Kola Science Centre RAS, Apatity), 1993.

Shattacharyya D N & Sali V T, Indian J Technol, Vol 23(1985) 21.

4 Usachyov P A, Opalev A S & Pershukov A A, Rus Pal1540088 (Mining Institute KSC RAS),1987.

5 Usachyov P A, Gorn Zh, N 12, (1993), 22.

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