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Recovery Improvement of Fine MagneticParticles by Floc Magnetic SeparationSubrata Roy aa Aditya Birla Science and Technology Company Ltd., Taloja, NaviMumbai, IndiaAccepted author version posted online: 27 Jun 2011.Version ofrecord first published: 23 Jan 2012.

To cite this article: Subrata Roy (2012): Recovery Improvement of Fine Magnetic Particles by FlocMagnetic Separation, Mineral Processing and Extractive Metallurgy Review: An International Journal,33:3, 170-179

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RECOVERY IMPROVEMENT OF FINE MAGNETICPARTICLES BY FLOC MAGNETIC SEPARATION

Subrata RoyAditya Birla Science and Technology Company Ltd., Taloja, Navi Mumbai,India

The performance of floc magnetic separation (FMS) has been compared with wet

high-intensity magnetic separator (WHIMS). This study was performed on low-grade iron

ore slime contained 59.58% Fe with 4.57% silica and 3.78% alumina. Detailed characteriza-

tion data indicated that a substantial amount of the slime was below 20lm in size.

Beneficiation studies indicated that the FMS process is effective to recover fine hematite

and goethite particles, compared with the conventional magnetic separation. In conventional

magnetic separation, the extent of the fluid drag force exceeds the magnetic force exerted on

ultrafine particles. Thus, ultrafinemagnetic particles were usually not recovered effectively by

magnetic separators, resulting in the loss of valuable ultrafine slime particles. The FMS pro-

cess significantly increases the magnetic force on the ultrafine iron ore in the form of hydro-

phobic flocs in a magnetic field, thus the ultrafine particles can be picked up effectively as

magnetic concentrates. The FMS process improved the Fe recovery from 37.35% to 79.60%.

Keywords: beneficiation, characterization, floc magnetic separation, iron ore slime, magnetic separation

INTRODUCTION

Wet high-intensity magnetic separation (WHIMS) is widely used for treating fineweakly magnetic iron (Fe) minerals. The forces acting upon particles in a magnetic sep-arator are magnetic, hydrodynamic drag, gravity, and friction (Wills 1985; Svobodaand Fujita 2003). Each of these forces varies with design of magnetic separator. Whilemagnetic forces attract magnetic particles, gravity and drag forces work against mag-netic forces. For ideal separation of particles in the magnetic separator, magnetic forcesmust overcome the hydrodynamic drag forces. However, for ultrafine magnetic parti-cles, the liquid drag force is greater than the magnetic force and separation efficiencycollapses in the fine size range. The poor efficiency of magnetic separation of weaklymagnetic mineral fines can be improved through increasing magnetic field gradient,field intensity, and particle size. The field gradient and field intensity have beenincreased using high-gradient magnetic separators and superconducting magneticseparators, respectively (Song, Lu, and Lopez-Valdivieso 2002). However, the particlesize can be increased using flocculation techniques for treating in low- andhigh-intensity magnetic separation. This process is termed as floc magnetic separation

Address correspondence to Subrata Roy, Aditya Birla Science and Technology Company Ltd.,

Taloja, Navi Mumbai 410 208, India. E-mail: roy.subrata11@yahoo.com

Mineral Processing & Extractive Metall. Rev., 33: 170–179, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0882-7508 print=1547-7401 online

DOI: 10.1080/08827508.2011.562948

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(FMS). Flocculation is commonly used in mineral industries for particle aggregation.Aggregation by flocculation is accomplished by the bridging of many mineral particlestogether by using flocculant (Wills 1985). Very little work has been done onFMS (Song,Lu, and Lopez-Valdivieso 2002; Arol and Aydogan 2004). The results have confirmedthat the poor recovery response ofmineral fines can be greatly improved by this process.

Fe ore slimes and fines are generated during the course of mining operations.Slimes are generally waste materials below 150 mm size and are separated from the Feore in washing plant. It is sludgy with high water volume and low Fe content. Finesare coarser than slimes (150 mm) and finer than calibrated lump ore (8mm). Slimelosses are common to all mineral processing operations (Prakash et al. 2000; Roy,Das, and Venkatesh 2008). Fe ore is no exception, especially the soft laminatedand lateritic ores of India that are quite soft and friable in nature (Roy and Das2008, 2009). Slime accumulation in most of the Fe ore bodies is due to weatheringand decomposition of ore and certain rock bodies associated with it (Roy, Das,and Mohanty 2007). Apart from the primary slime formation, subsequent secondaryslimes are produced during mining processing, handling, and comminution of Fe oreto its liberation size (Roy 2009). It is possible to minimize the formation of secondaryslime formation. However, formation of undesirable secondary slime is unavoidable.These slimes are of relatively low grade and cannot be utilized directly in blast fur-nace. They are generally dumped into the tailing ponds due to lack of proper proces-sing technology. However, such dumping may cause serious environmental hazards(Roy, Das, and Mohanty 2007) over a prolonged period. Today, increasing need isbeing felt for their beneficiation and utilization. Thus, beneficiating the low-grade Feores and slimes to remove the gangue minerals and enhancing their grades are pro-spective proposition today (Srivastava and Kawatra 2009). The issue of the utiliza-tion of Fe ore slimes, however, is fairly complex owing to the extremely small size ofthe individual mineral particles and has not met with great success until now. How-ever, many of the other issues related to mineralogical characterization, physical lib-eration and mineral dissemination are common to both these kinds of problems.

In the present study, low-grade Fe ore slime was collected for detailed character-ization and possible beneficiation. The effect ofWHIMS has been compared with FMSprocess. WHIMS has become a powerful technique for the recovery of weakly mag-netic Fe minerals (Svoboda 1994). Many theoretical and experimental investigationsofWHIMShave been reported (Svoboda andRoss 1989; Tucker 1994). The equipmentvariables were fixed and the effects of particle size and magnetic field strength werestudied simultaneously. The aim of this work was to investigate the effect of a propersize enlargement process on the recovery of ultrafine particles in WHIMS. This mightbe a substitution for high-gradient magnetic separations in fine mineral processing.

EXPERIMENTAL

Characterization of Iron Ore Slime Sample

Characterization of the Fe ore slime consisted of various studies such assize analysis, chemical analysis, scanning electron microscopy (SEM) with energydispersive spectroscopy (EDS), and microscopic analysis. These steps are describedin detail in the following sections along with the observations.

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Particle size measurements of the Fe ore slime were carried out using theVibratory Laboratory Sieve Shaker ‘‘Analysette3.’’ Graphical representation ofthe size analysis data of the slime sample is shown in Figure 1. From the chemicalcompositions, it has been seen that the Fe ore slime sample had a feed grade of59.58% total Fe with 4.57% silica and 3.78% alumina.

Micromorphological and mineralogical characterization studies have beenconducted using SEM with EDS micro analyzer (JSM 840 A=EDS) at the NationalMetallurgical Laboratory (NML), Jamshedpur. This study allowed the identificationof mineralogy, micro-morphology of individual particle, and elemental compositionof the slimes. The results of this investigation were compiled in the form of photo-micrographs. SEM analysis with EDS has been done with the slime head sample.

Beneficiation Studies Using WHIMS

The magnetic separation was conducted with a WHIMS (model type JONES P40, made by JONES-Ferro Magnetic, Inc., Switzerland) at NML, Jamshedpur. Theseparation box of the WHIMS was equipped with four grooved plates of 6 cm heightand 0.8 cm width. Feed slurry of about 5% solid by weight was chosen. The sampleweight of each test was 500 g. The slurry was fed at the same flow rate (500ml=min).To reduce the adverse influence of hydrodynamic force during the test process, thewashing water in 250ml=min flow rate was controlled carefully. A number of testswere conducted with variable magnetic field strength; results are given in Table 1.

Beneficiation Studies Using FMS

In this study, weakly magnetic fine minerals are formed into hydrophobic flocsand then were subjected to WHIMS. This process is known as FMS. The FMS

Figure 1 Graphical representation of size distribution of iron ore slime.

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process consists of three steps, i.e., dispersion, hydrophobic flocculation, andmagnetic separation.

Flocculation was carried out on slime samples. Sodium oleate has been selectedas a hydrophobic flocculent for the size enlargement processes. The slime sample wasdiluted and sodium silicate (1 kg=ton) was added. Dispersion was achieved in thisstep. After that, the slurry was transferred to a mixing tank equipped with the stir-ring shaft. The slurry was first adjusted for pH at 9.5 by using sodium hydroxide sol-ution and then was strongly conditioned at 800 rev=min for a given time whilesodium oleate was added, leading to the formation of hydrophobic floc of Fe miner-als. The Fe ore floc was then subjected to WHIMS in the flow rate of 500ml=min and10% solid concentrate and washing water in 250ml=min flow rate was used.

Earlier research works (Song and Lu 1994; Song, Lopez-Valdivieso, and Ding1999; Song, Lu, and Lopez-Valdivieso 2002) have shown that a strong hydrophobicflocculation of hematite fines in aqueous suspensions could be induced by sodium ole-ate upon its adsorption on the particles under a strong slurry conditioning. In thepresent study, different concentrations of sodium oleate have been used for hydro-phobic flocs of Fe minerals. The Fe ore flocs are then subjected to WHIMS to seethe effect of size enlargement in magnetic separator. The results are given in Table 2.

RESULTS AND DISCUSSION

Detailed particle characterization of Fe ore slime (see Figure 1) revealed thatthe slime is very fine in nature. A substantial amount of the slime is below 10 mm(64% by weight). D80 of the distribution is about 16 mm.

Table 2. Floc magnetic separation result under different concentration of sodium oleate

Sodium oleate (kg=ton) Product wt % Fe grade (%) SiO2 (%) Al2O3 (%) Fe recovery (%)

Feed 100 59.58 4.57 3.78 100

1.5 Concentrate 61.9 66.78 1.23 1.16 69.33

Tailing 38.2 47.90 9.98 8.03 30.67

2 Concentrate 64.5 66.53 1.35 1.21 71.97

Tailing 35.6 46.98 10.41 8.45 28.03

2.5 Concentrate 67.4 66.12 1.78 1.58 74.77

Tailing 32.6 46.07 10.34 8.33 25.23

3.5 Concentrate 72.8 65.14 2.87 2.34 79.59

Tailing 27.2 44.70 9.11 7.64 20.41

Table 1. WHIMS test results of iron ore slime in different field intensity

Field intensity (Tesla) Product wt % Fe grade (%) Fe recovery (%) SiO2 (%) Al2O3 (%)

Feed 100 59.58 100 4.57 3.78

1.06 Concentrate 28.45 67.15 32.06 1.21 1.13

Tailing 71.55 56.57 67.94 5.9 4.84

1.4 Concentrate 31.67 67.11 35.67 1.25 1.26

Tailing 68.33 53.16 60.97 6.11 4.95

1.52 Concentrate 33.18 67.06 37.35 1.31 1.27

Tailing 66.82 55.87 62.66 6.19 5.03

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The SEM–EDS analysis results, shown in Figure 2, indicate that the slime sam-ple contains variable quantities of gangue along with Fe. Among the gangue miner-als, kaolinite is most abundant and it occurs as limonitic kaolinite having elementsSi, Al, and Fe (Roy and Das 2008). This kaolinite is the main source of aluminain the slime.

The result of WHIMS (see Table 1) at 1.06 Tesla (1 Amp) field strength for Feore slime shows that a relatively higher grade is obtained by magnetic separation.Comparative result shows that different magnetic field strengths slightly affect theseparation process. With increasing field strength, Fe recovery is slightly increasing.It can be seen from Table 1 that Fe distribution in WHIMS concentrate at 1.06 Tesla(1 Amp) is 32.06%. With increasing field strength from 1.40 (2 Amp) to 1.52 Tesla (3Amp), Fe distribution increases from 35.67% to 37.35%, respectively. However, in allcases, recovery of Fe is very low. While assay percentage of Fe remained relativelyhigh, more than 90% particles for sizes finer than 15 mm have been lost. The reasonbehind this can be demonstrated by following theoretical considerations.

Figure 2 SEM photomicrograph with energy dispersive spectroscopy (EDS) of iron ore slime.

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In magnetic separation, the force on a particle toward increasing field intensity(Fm) is expressed by (Svoboda and Fujita 2003) the following equation

Fm ¼ k

l0VBDB; ð1Þ

where k is the volumetric magnetic susceptibility of the particle, m0 is the magneticpermeability of vacuum, V is the volume of the particle, B is the external magneticinduction, and DB is the gradient of magnetic induction. The magnetic force is thusproportional to the product of the external magnetic field, the field gradient, and hasthe direction of the gradient.

In a magnetic separator, apart from the magnetic force, several competingforces act on a particle. These are, among others, the force of gravity, the inertialforce, the hydrodynamic drag force, surface force, and inter particle forces. How-ever, among the competing forces, gravitational and hydrodynamic drag forces arethe major competing forces.

The force of gravity (Fg) is expressed (Svoboda and Fujita 2003) as follows:

~FFg ¼ qV~gg; ð2Þ

where q is the density of the particle, while g is the acceleration due to gravity.The hydrodynamic drag (Fd) is given by the following equation:

~FFd ¼ 6pgbnp; ð3Þ

where g is the dynamic viscosity of the fluid, b is the particle radius, and np is therelative viscosity of the particle with respect to the fluid.

The magnetic forces and the competing forces are different for hematite andgoethite particles. To pick the magnetic particle from the slurry, Fm must be largerthan Fd. Graphical representation of magnetic force versus liquid drag force forhematite particle in different size fractions is given in Figure 3.

It can be seen that for constant field strength and field gradient, both the mag-netic and liquid drag forces are decreasing with decreasing particle size. For certainfiner size fractions, the liquid drag force is greater than the magnetic force. FromFigure 3, it can be seen that at 1.06 Tesla, hematite particles smaller than 25 mm sizeare lost due to higher liquid drag force compared with magnetic force.

However, with increasing field strength, magnetic forces increase and dominateover liquid drag forces. With increasing field intensity at 1.40 and 1.52 Tesla, thethreshold size limit for being carried away by the fluid drag decreases to 22 and21 mm size, respectively (see Figure 3).

Similar conditions can be observed for the goethite particles. Graphical rep-resentation of magnetic force versus liquid drag force for goethite particle for differ-ent size fraction is given in Figure 4. It can be seen that at 1.06 Tesla, goethiteparticles lower than 27 mm size are lost due to higher liquid drag force. At a fieldintensity of 1.40 and 1.52 Tesla, the thresheold size limit decreases to 24 and

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23 mm size, respectively, for goethite. Loss of fine-sized goethite is relatively higherthan fine hematite.

In this study, the Fe ore slime is very fine in nature (see Figure 1), where 80% offines are below 20 mm. WHIMS results (see Table 1) of Fe ore slime show that due torelatively higher drag forces, a significant amount of Fe is lost into the tailings. At1.06 Tesla, hematite particles less than 25 mm (see Figure 3) and goethite particle less

Figure 3 Graphical representation of magnetic force vs. liquid drag force for hematite particle in different

particle size range.

Figure 4 Graphical representation of magnetic force vs. liquid drag force for goethite particle in different

particle size range.

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than 27 mm (see Figure 4) could not be recovered in the concentrate. Table 1 showsthat with an increase in intensity to 1.4 Tesla, Fe loss in tailing product decreasedas the threshold sizes of hematite (see Figure 3) and goethite (see Figure 4) areincreased. Relatively higher concentrations can be obtained in low field strength of1.06 Tesla (1Amp). With an increase in the field intensity to 1.4 Tesla (2Amp),the percentage of Fe in the ore decreased and yield increased. With an increase inthe field intensity, fine particles are partially recovered in the concentrate.

However, in all cases, recovery of Fe is very poor and recovery decreased as theparticle size decreased. It was thought that the drop in the recovery of fine hematiteand goethite particles in a magnetic separator can be prevented by a proper sizeenlargement process.

Flocculation is commonly used for particle aggregation. Aggregation by floc-culation is accomplished by the bridging of many mineral particles together by usingflocculant. For this study, sodium oleate, when used as the flocculant in the selectiveflocculation of Fe ores, causes the formation of hydrophobic flocs of Fe minerals.The Fe ore floc is then subjected to WHIMS at 1.52 Tesla field strength.

The effect of sodium oleate addition on the recovery and the grade of the Feconcentrate obtained have been investigated in this study. The results of the testswith four different sodium oleate concentrations are presented in Table 2. Theimprovement in the Fe recovery can be seen in Figure 5. Compared with the conven-tional magnetic process where Fe recovery is 37.35%, Fe recovery has been improvedto 79.6% by the FMS process. It can be seen from Figure 5 that recovery in the con-centrate sharply increases with addition of sodium oleate. The concentrate graderemains almost the same (see Tables 1 and 2) with lower dosage (1–2mg=l) anddecreases abruptly with higher dosage (3.5mg=l). The oleate ions cause hydrophobi-sity to hematite and goethite particles upon their adsorption. More addition ofsodium oleate causes stronger adsorption, and stronger hydrophobisity causes

Figure 5 Graphical representation of recovery percentage of Fe and grade percentage of Fe in conven-

tional magnetic separation and floc magnetic separation method.

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higher recovery of hematite and goethite particles. However, higher dosage ofsodium-oleate cases the bridging of ferruginous clay with hematite and goethiteparticles. These ferruginous clays, due to their Fe content, increase the Fe recoverybut at the same time, Al2O3 and SiO2 contents decrease the grade of the concentrate.

CONCLUSIONS

Characterization and beneficiation of low-grade Fe ore slime have been studiedin the present work having 59.58% total Fe, 4.57% silica, and 3.78% alumina.Mineralogical observations indicate that hematite and goethite are two mainFe-bearing minerals, while kaolinite is found to be the major gangue minerals.The Fe ore slime has been subjected to magnetic separation by WHIMS. Resultsshow that the grade of the ore has been improved considerably up to 67.15% totalFe, 1.21% silica, and 1.13% alumina by conventional magnetic separation method.

It has been seen that for the finer sized particles, liquid drag force is greaterthan the magnetic force and hematite and goethite particles below 21 and 23 mm size,respectively, are lost in the tailings. To recover these weakly magnetic mineral fines,the FMS process, has been studied. The FMS process shows that the magnetic sep-aration of hematite and goethitic fines can be considerably improved by selectivelyaggregating the magnetic fines through the hydrophobic flocculation induced bysodium oleate. The FMS process improved the Fe recovery from 37.35% to 79.6%in conventional method. However, higher dosage of sodium oleate decreases thegrade by bridging the ferruginous clay particle during the flocculation process.

ACKNOWLEDGMENT

The author thanks the editor and the reviewers for their comments that signifi-cantly improved the manuscript.

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Roy, S., 2009, ‘‘Recovery improvement of fine iron ore particles by multi gravity separationtechnique.’’ Open Mineral Processing Journal, 2, pp. 17–30.

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