Hydrometallurgy 105 (2011) 364–369

Contents lists available at ScienceDirect

Hydrometallurgy

j ourna l homepage: www.e lsev ie r.com/ locate /hydromet

Short communication

The removal of alumina and silica from iron rejects slime by chemical leaching

S. SarkarRaw materials and coke making group, RD&T, Tata Steel Ltd, Jamshedpur, India

E-mail address: s.sarkar1@tatasteel.com.

0304-386X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.hydromet.2010.10.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 December 2009Received in revised form 21 October 2010Accepted 22 October 2010Available online 2 November 2010

Keywords:Iron ore slimeAcid-leachingCaustic leachingAluminaSilicaKinetics

A hydrometallurgical method of alumina and silica gangue removal from rejects slime of iron ore by alkali andacid leaching is proposed. Up to about 80% gangue is removed by chemical leaching with sodium hydroxide,sulphuric, hydrochloric and nitric acids. Although sulphuric acid is the most effective among the acids, itsuffers from a substantial loss of iron. Sodium hydroxide is the most effective leachant at 95 °C with anoptimum concentration of 5 M NaOH for 2 h. The iron value of the leached slime improved from 58.8% to67.5% and contained b2% Al2O3.Process parameters were optimized by circulating and staged leaching with different concentrations ofsodium hydroxide (1.25–7.25 M). The leaching kinetics shows that the principal reaction of alumina+silica isfirst order with respect to hydroxide ions.

ll rights reserved.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

A large quantity of iron ore fines are generated duringminingwhichis about 35% of the total iron ore mineral. Slimes, defined as −25 μmparticles, are an inseparable part of such operations and may containsignificant amounts of valuable iron oxideminerals with relatively highgangue content as compared to run-of-mine ore. Thus, slimes areusually discarded prior to concentration because they have a highalumina content which makes them unsuitable for direct blast furnacefeed material. Conventional techniques, such as gravity concentration,magnetic separation, flotation etc, have limitations in treating theslimes. Many synergic techniques have been developed to treat slimes;among them are shear flocculation, selective flocculation, carrierflotation and high gravity separators.

Earlier efforts have been made to reduce the alumina level by(Gujraj et al., 1983; Hanumantha Rao and Narasimhan, 1985;Mahiuddin et al., 1989; Das et al., 1992) who primarily focused onflocculation techniques with limited success. A few studies have alsobeen carried out in the areas of anionic and cationic flotation,magnetic separation, roasting followed by magnetic separation,electrostatic separation, hydrogen reduction and caustic digestion(Stone, 1964; Tiemann, 1962; Show and Conley, 1950) Physicalmethods are effective when the particles are liberated, but a chemicalapproach, in which the ore slimes are leached with suitable solution,is a relatively simple process as it can directly extract the materialwithout strict requirement for particle size as compared to physicalprocesses.

Therefore, chemical beneficiation of minerals arises from thelimitation of physical beneficiation processes. Very little informa-tion has been published on the removal of gangue from iron oreslime by chemical leaching (Tiemann, 1962; Hohenstein, 1967)However, in recent years, an acid leaching approach has beenrecognized as a possible alternative route for phosphorus removalfrom iron ore (Jin et al., 2006; Zhang and Muhammed, 1989; Alafara,et al., 2005; Sarkar & Biswas, 2009)

The present investigation has been carried out to identify alaboratory scale hydrometallurgical method for removal of ganguematerial by leaching with a suitable inorganic acid or with sodiumhydroxide to produce blast furnace grade iron ore. Sodium hydroxideselectively reacts with silica and alumina present in gangue materialto form soluble sodium silicate and aluminate with minimum ironoxide dissolution; whilst mineral acids not only react with ganguematerial but also reacts with iron oxides and goethite phase. Sulphuricacid is comparatively better due to its lower reactivity towardsgoethite and iron oxides with minimum loss of iron. However,hydrochloric acid quite readily dissolves goethite and other ironoxides even though it may have an advantage in dissolving ganguematerials more readily as soluble chloride salts. Nitric acid is notpreferred because of its low reactivity and the formation of nitratesand nitroso compounds (Yong-shi et al., 2006).

2. Experimental

2.1. Iron rejects slime analysis

The test samples of iron ore rejects were taken from the slimeponds of iron ore at Joda mines, Tata Steel Ltd, India. The iron ore

Table 1Bulk analysis of Joda iron ore rejects slime.

Oxides % dry Elements %

Fe3O4 b 0.1 Fe 58.78Fe2O3 80.72 Cu b 0.01FeO 3.00 Zn b 0.01Al2O3 6.65 S 0.08SiO2 5.74 P 0.153P2O5 0.35 Cl 0.02K2O 0.12 F 0.10Na2O 0.11 Moisture: 2.3%

Particle size : −25 μmDensity : 4800 kg m−3

MgO 0.10CaO 0.20MnO 0.10TiO2 0.30V2O5 0.10Total 97.49

Table 2Experimental conditions for the leaching of iron ore reject slime.

Experimental parameters Variables

1.Temperature, °C I 50 °C for HCl, H2SO4, HNO3,

II. 30 °C, 50 °C, 70 °C, 90 °C for NaOH2. Concentration I. 10 % w/v HCl, H2SO4, HNO3

II. 1.25 M, 2.5 M, 5.0 M, 6.25 M & 7.25 M NaOH3. Stirring Speed , rpm I. 800–900 rpm for HCl, HNO3 H2SO4,

II. 200, 400,800, 1000 rpm for NaOH4. Particle size (micron) −25 micron5. Reaction time I. 120 mins for HCl, H2SO4, HNO3

II. 10, 30, 60, 90,120,180, 240, 300, 320 min for NaOH6. Slurry ratio 1:10 for HCl, H2SO4, HNO3

1:5, 1:10, 1:15 for NaOH

Table 3Analysis of feed slime and leached slime by different acids and sodium hydroxide.

Materials %Fe %SiO2 %Al2O3 %P %Recoverya

Feed (wt%) cs 5.74 6.65 0.15 -10% H2SO4 63.20 2.65 2.50 0.04 75%10% HNO3 62.80 3.35 4.11 0.04 75%10% HCl 61.50 3.54 3.98 0.04 65%5 M NaOH 67.81 2.12 1.25 0.030 90%

a based on iron value.

365S. Sarkar / Hydrometallurgy 105 (2011) 364–369

slimes were in situ sieved and analysed with 94.4% present as b25microns particles. The sub-25 micron fraction contained 58.8% Fe,5.74% SiO2, 6.65% Al2O3 and 0.15% P. The bulkmineralogical analysis of

Position [°2Th

20 30 40 50

Cou

nts

0

100

400

900

1600

Kao

linite

1A H

emat

ite

Hem

atite

Hem

atite

; Kao

linite

1A

Mag

netit

e lo

w

Hem

atite

; Kao

linite

1A

Hem

atite H

emat

ite

1

Fig. 1. XRD of different minerals

iron ore rejects slime is shown in Table 1 which contained about 13%silica and alumina gangue minerals. It was used as received. Allchemicals used were LR grade.

2.2. Methods and equipment

A three neck round bottom flask of 1 litre capacity fitted with aspiral condenser was used as a reactor and placed in an oil bathmaintained ±2 °C. The leaching conditions for the experimentscarried out with different variable parameters are summarised inTable 2.

Typically for acid leaching, 400 ml of acid solution was chargedinto the reactor and heated to temperature before adding 40 g. ironore slime and reacting with continuous stirring for 2 h. The reactionmixture was filtered by a pressure filter followed by a water wash tocomplete the removal of acid. With caustic leaching, the slurry ratiowas varied from 1: 5 to 1:15 over a temperature range from 30 to90 °C (Table 2).

Some experiments were carried out with circulating leaching.In this method, solution was continuously recycled at a constantflow through the extractor until the gibbsite-goethite-quartz phasein the ore was dissolved at equilibrium. The reaction parameterswere varied for different runs in order to investigate the effect of

eta] (Copper (Cu))

60 70 80

Hem

atite

;

Hem

atite

; ; K

aolin

ite 1

A

Hem

atite

; Mag

netit

e lo

w; K

aolin

ite 1

AH

emat

ite; K

aolin

ite 1

A

Hem

atite

; Kao

linite

1A

Hem

atite

; ; M

agne

tite

low

; Kao

linite

1A

Hem

atite

; Mag

netit

e lo

w; K

aolin

ite 1

A

Hem

atite

; Kao

linite

1A

Hem

atite

;

Hem

atite

Hem

atite

phase of feed iron ore slime.

Table 4Optimum extraction after 2 h with varied NaOH concentration and slurry ratio at 90 °Cand 800 rpm stirring.

Constituents Fe Al2O3 SiO2 P Recovery

% % % % %

Feedmaterials

58.91 6.65 5.74 0.15

1.25 M NaOH Slurry ratio 1:10 63.56 5.12 5.16 0.081:15 64.23 4.48 5.00 0.08 88%1:20 65.45 4.12 4.98 0.07

2.5 M NaOH Slurry ratio 1:10 65.01 4.00 4.25 0.041:15 65.45 3.89 3.96 0.03 89%1:20 66.12 3.11 3.45 0.04

5.00 M NaOH Slurry ratio 1:10 66.50 1.69 2.95 0.041:15 67.80 1.20 2.22 0.03 92%1:20 67.88 1.23 2.01 0.04

7.25 M NaOH Slurry ratio 1:10 67.81 1.25 2.12 0.0301:15 67.78 1.21 2.20 0.031 90%1:20 67.74 1.23 2.11 0.032

366 S. Sarkar / Hydrometallurgy 105 (2011) 364–369

the circulating leach condition and solution samples were collectedat specific time intervals for the kinetic study. It was observed thatcirculating leaching was more effective than staged addition ofleachate.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100 120 140 160 t (min)

frac

tio

n o

f A

l 2O

3 le

ach

ed

(a) (

(c) (d

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 20 40 60 80 100 120 140 160

t (min)

frac

tio

n o

f p

ho

s le

ach

ed

800

400

200

800400200

Fig. 2. Effect of stirring and time on leaching with 5 M NaOH at 95

2.3. Analysis

The elements in the sample were determined by atomic emissionspectroscopy (ICP-AES). The total alkali and acid concentration in thesolution was measured by wet-chemical as well as potentiometrictitration.

The XRD patterns of feed material are presented in Fig. 1. It isobserved that hematite and kaolinite are the main phases togetherwith some magnetite.

2.4. SEM preparation

About 1 g sample of iron ore slime was taken in a mould. A bindingmixture was thoroughly mixed to remove any air bubbles present andthen allowed to dry. After the dried sample was polished, it was goldcoated in vacuum for 40–60 s to make it conductive for scanningelectron micrograph.

3. Results and discussion

3.1. Effect of different leachants (HCl, H2SO4, HNO3, and NaOH)

The effect of different acid and caustic leachants on slimeextraction has been studied under a range of different experimental

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100 120 140 160 t (min)

frac

tio

n o

f S

iO2

leac

hed

b)

)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 20 40 60 80 100 120 140 160

t (min)

frac

tio

n o

f F

e va

lue

enri

ched

200

800

400

800400

200

°C. (a) Al2O3 ; (b) SiO2 ; (c) phosphorus; (d) iron enrichment.

367S. Sarkar / Hydrometallurgy 105 (2011) 364–369

conditions and concentrations as summarised in Table 2. Table 3shows the results of leaching iron, aluminium, silica and phosphoruswith 10% acids and 5 M NaOH which indicates that a significantamount of iron is lost by acid leaching compared to caustic leachingand that sulphuric acid is the most effective among the acids.

3.2. Optimization of NaOH concentration

The leaching results for four different concentrations of sodiumhydroxide are given in Table 4 which shows that about 50% of aluminaand silica was removed from the slime in 2.5 M NaOH at 90 °C after2 hr and a maximum of about 75% was removed with 5 M NaOHstirred at 800 rpm. There was little effect of slurry density below 10%w/v nor any effect of increasing stirring speed on the rates ofextraction.

3.3. Leaching characteristics with 5 M NaOH solution

The dissolution of alumina, silica and phosphorus vs. leachingtime is shown in Fig. 2a–d. It shows that the rate of dissolution ofsilica and alumina is similar but the rate of phosphorus dissolutionis much faster. This indicates phosphorus may be present in theslime other than in the apatite phase. The dissolution reaction ofgibbsite, quartzite and clay with sodium hydroxide are shown inEqs. (1)–(4).

AlðOHÞ3 þ NaOH→NaAlO2 þ 2H2O ð1Þ

SiO2 þ 2NaOH→Na2SiO2 þ H2O ð2Þ

(a)

(c)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 20 40 60 80 100 120 140 160t (min.)

frac

tio

n o

f A

l 2O

3 le

ach

ed 90

70

50

30

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0 20 40 60 80 100 120 140 160t (min.)

frac

tio

n o

f p

ho

s. le

ach

ed

90

7050

30

Fig. 3. Effect of temperature on the rate of leaching with slurry of 1:15 at

Al2O3 þ 2NaOH→2NaAlO2 þ H2O ð3Þ

3Al2Si2O5ðOHÞ4 þ 8NaOH→½Na8ðAl6Si6O24ÞðOHÞ2� þ 9H2O ð4Þ

For the dissolution of alumina or silica, the concentrationderivative with respect to time can be expressed as a first orderreaction, according to:

rA = − dCc

dt= −k C

α

c

where dCc is the quantity of remaining alumina present in g/L, k is therate constant and ά is the reaction order. Clearly, Fig. 1 shows that theeffect of NaOH concentration on alumina and silica dissolution issimilar and a plot of log C NaOH vs. log ( k ) gave an approximatestraight line with a slope around 1 indicating that the dissolution ofalumina and silica is first order reaction with respect to (OH−).

3.4. Effect of temperature

Fig. 3a–d shows the effect of temperature on the rate of alumina,silica and phosphorus removal with 5 M NaOH. It indicates that theremoval of gangue minerals significantly decreases with lowertemperatures and attained close to equilibrium level after 120 min.

3.5. Mineralogical effect

Fig. 4a shows the SEM-EDS of feed material which indicates thatmajor part of alumina is present in the goethite and gibbsite phases

(b)

(d)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 20 40 60 80 100 120 140 160

t (min.)

frac

tio

n o

f S

iO2

leac

hed

90

70

50

30

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0 20 40 60 80 100 120 140 160 t (min.)

frac

tio

n o

f F

e va

lue

enri

ched

800 rpm. (a) Al2O3 ; (b) SiO2 ; (c) phosphorus; (d) iron enrichment.

Fig. 4. SEM image with microprobe analysis results at different points of slime (a) before treatment ; (b) after treating with 6.25 M NaOH.at 95 °C.

368 S. Sarkar / Hydrometallurgy 105 (2011) 364–369

whereas silica is in the quartzite phase. Fig. 4b shows the SEM-EDS ofthe leached material which indicates that alumina is mostly removedfrom gibbsite and silica is largely absent in goethite and partiallyleached from the quartzite phase. The XRD of the slime after causticleaching showed only the hematite phase still remained.

4. Conclusions

The main contaminant of the fine particle (b25 micron) hematiteslime is kaolinite. Alumina and silica can be selectively removed fromiron ore rejects slime by leaching with sodium hydroxide whereasleaching with mineral acids dissolves significant amounts of ironoxides. Sodium hydroxide simultaneously removed alumina, silicaand phosphorus with negligible loss of iron. This reaction is sensitiveto the concentration of NaOH and temperature as well as the flowvelocity of the leaching solution.

The following optimum conditions were identified for single stagepercolation leaching:6.25 M NaOH; liquid/solid: 1:15; 95 °C; 800–900 rpm; 180 min.

Clearly, the leached ore has to bewashed to recover the caustic andthe recovery and recycling of sodium hydroxide is necessary to reducethe cost of the process.

Acknowledgements

The authors would like to thank the R&D and Scientific Division ofTata Steel Ltd, India operation for providing the research opportunityand publication permission.

References

Alafara, A., Baba, A., Adekola, F.A., Folashade, A.O., 2005. J. Appl. Sci. Environ.Mgt. Vol. 9 (3),15–20 and citations therein.

Das, B., Prakash, S., Mohapatra, B.K., Bhaumik, S.K., Narasimhan, K.S., 1992. Beneficiationof iron ore slimes using hydrocyclone. Miner. Metall. Process. 9 (2), 101–103.

Gujraj, B., Sharma, J.P., Baldawa, A., Arora, S.C.D., Prasad, N., Biswas, A.K., 1983.Dispersion—flocculation studies on hematite-clay systems. Int. J. Miner. Process. 11,285–302.

Hanumantha Rao, K., Narasimhan, K.S., 1985. Selective flocculation applied to Barsuaniron ore tailings. Int. J. Miner. Process. 14, 67–75.

369S. Sarkar / Hydrometallurgy 105 (2011) 364–369

Hohenstein, P.P., 1967. Erzmetall 20, 217.Yong-shi, Jin, Tao, Jiang, Yong-bin, Yang, Qian, Li., Guang-hui, Li., Yu-feng, Guo, 2006.

J. Cent. South Univ. Technol. 06−0673−05.Mahiuddin, S., Bandopadhyay, S., Baruah, J.N., 1989. A study on the beneficiation of Indian

iron ore fines and slime using chemical additives. Int. J. Miner. Process. 11, 285–302.Sarkar, S, Biswas, P.P., 2009. “Extraction of pure silica from coal and other siliceous

minerals” Indian Patent Application , number to be allotted.Show, M.L., Conley, J.E., 1950. Laboratory tests on percolation leaching of silica from

bauxite,”, US Bureau of Mines, Report of Investigation RI 4649.

Stone, R.L., “The mechanism and rate of extraction of silica from low-grade siliceousiron ores by digestion in sodium hydroxide solutions”, Ph.D. Thesis, Dept.Metallurgical and Mineral Engineering, University of Wisconsin, 1964.

Tiemann, T.G.,1962, “Caustic extraction of silica from low grade siliceous iron ores,”Trans. SME/AIME, June and citations therein.

Zhang, Y., Muhammed, M., 1989. The removal of phosphorus from iron ore by leachingwith nitric acid. Hydrometallurgy 21, 255–275 and citations therein.