Hydrometallurgy 127–128 (2012) 178–186

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Hydrometallurgy

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Dissolution windows for hydrometallurgical purification of metallurgical-gradesilicon to solar-grade silicon: Eh–pH diagrams for Fe silicides

Eunyoung Kim a,b, Kwadwo Osseo-Asare a,⁎a Dept. of Materials Science and Engineering and Department of Energy and Mineral Engineering, Penn State University, University Park, PA 16802, USAb Metallic Resources Technology Lab., LS-Nikko Copper Inc., 618 Sampyeong-dong, Bundang-gu, Seongnam-si, Gyeonggi-do, 463–400, South Korea

⁎ Corresponding author. Tel.: +1 814 865 4882; fax:E-mail address: ako1@psu.edu (K. Osseo-Asare).

0304-386X/$ – see front matter © 2012 Published by Edoi:10.1016/j.hydromet.2012.05.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 February 2012Received in revised form 29 April 2012Accepted 11 May 2012Available online 26 May 2012

Keywords:MG-Si (metallurgical-grade silicon)Solar-grade siliconSilicidesLeachingPurification

Potential vs. pH diagrams for the systems Fe–Si–(Cl−)–(F−)–H2O were generated using the HSC5.0 softwareand applied to purification of metallurgical-grade silicon (MG-Si) by hydrometallurgical methods. The dia-grams for the Fe–Si–H2O system show that with increasing electrochemical potential (Eh) the order of ap-pearance of Fe silicides is FeSi2, FeSi, and Fe3Si. Further, from a thermodynamic standpoint, Si in the Fesilicides (FeSi2, FeSi, and Fe3Si) would be easier to oxidize than Fe, due to the lower potentials of thecorresponding silicides, compared with Fe. The early formation of SiO2 during dissolution may prohibit or re-tard further dissolution of Fe, as verified by previous experimental results. This inhibitive effect is expected toincrease with increasing Si content in the silicides. The diagrams for the Fe–Si–Cl−–H2O system indicate thatdissolution of Fe silicides with relatively low Si content, such as FeSi and Fe3Si, is enhanced by adding chlorideions to acidic solutions. In the presence of HF, formation of fluoro-complexes enlarges the stability domains ofdissolved iron and silicon, which increase with increasing {F} and/or decreasing temperature. The trendsobtained for the Fe–Si–F−–H2O system suggest that the dissolution of Fe silicides would be enhanced by de-creasing temperature or removing FeF2(s) (e.g., via ultrasonication) during the leaching process.

© 2012 Published by Elsevier B.V.

1. Introduction

Photovoltaic (PV) technology or solar electricity is attracting in-creasing attention as the ultimate source of sustainable, green energy.Currently silicon (Si) is the semiconductor material used in most PVapplications (~90%). The first step in the commercial upgrading of sil-icon is production of metallurgical grade silicon (MG-Si) viacarbothermic reactions (Luque and Hegedus, 2003). During solidifica-tion of moltenMG-Si, the metallic impurities, such as Al, Fe, Ca, Mg, Ti,Mn, and Cu, segregate to the grain boundaries, forming intermetallicphases with silicon, i.e., silicides (Dong et al., 2011; Juneja andMukherjee, 1986; Santos et al., 1990; Xiaodong et al., 2009).

Metallurgical-grade silicon is upgraded for application in solarcells by removing these impurities and conventionally this has beenperformed at high temperatures by the Siemens process; theresulting product, called semiconductor-grade silicon, typically has~9 N purity (Luque and Hegedus, 2003), a value that is higher thanthe required purity for solar-grade silicon (Si purity, b99.995%)(Sarti and Einhaus, 2002). Due to the high cost and energy consump-tion associated with this process, acid leaching via hydrometallurgicalmethods has been introduced and investigated as an alternative. Thismethod, based on low temperature processing (temperatures not

+1 814 863 4718.

lsevier B.V.

higher than 100 °C, as opposed to 1150 °C for the Siemens process )(Luque and Hegedus, 2003), has the potential advantage of lower en-ergy requirements.

Acid leaching for purification of MG-Si has been investigated bymany researchers using various acids, such as H2SO4, HCl, HNO3,aqua regia (mixture of HCl and HNO3), and HF, as summarized inTable 1. These studies do not agree on the range of optimum leachingconditions. For example, use of fine (Lu et al., 2011) or coarse particlesize (Santos et al., 1990) has been reported and an unequivocably ef-fective leaching reagent has not emerged among the several that havebeen investigated, such as HF (Dietl, 1983; Juneja and Mukherjee,1986; Xiaodong et al., 2009), HCl (Juneja and Mukherjee, 1986; Lianet al., 1992), HF+HCl (Boulos, 1983; Lee et al., 2009), and aquaregia (Chu and Chu, 1983). Santos et al. (1990) and Dietl (1983)reported opposite leaching behavior of Fe in HF solution, as presentedin Table 1. In MG-Si, silicides as impurities mainly contain iron(~1.0 wt.%), aluminum (~0.7 wt.%), and calcium (~0.6 wt.%) (Donget al., 2011; Luque and Hegedus, 2003; Santos et al., 1990; Sarti andEinhaus, 2002). Among these silicides, calcium silicides such asCa2Si, CaSi, and CaSi2 react with dilute acid and dissolve readily(Bailar et al., 1973). Aluminum has no compounds in the Si–Al binarysystem, but silicides do exist for ternary or quaternary systems, suchas AlFeSi, AlCaSi, and CaAlFeSi (Anglézio et al., 1990; Bailar et al.,1973; Margarido et al., 1994). The aluminium silicides are reportedto dissolve more easily than Fe silicides (Margarido et al., 1993a,1993b, 1994, 1997). Therefore, among the metal silicides present in

Table 1Hydrometallurgical investigations for purification of silicon.

No Materials Particle size (μm) Lixiviants Experimental conditions Purity of Si (%) Comments

1 MG-SiSi (~99.5%)Fe (0.35%), Al (0.15%)

– Aqua regia, HCl,H2SO4+HNO3

(no dilution)

Time (h): 24, 100Temp: 86 °C at 1 atm200 °C at 10 atmS/L ratio=1: 1.7

Si (b99.90%)Al (320 ppm),Fe (350 ppm)

Aqua regia is the most effective.

2 3 different compositionof MG-SiSi (~99.0%)Fe (~0.5%), Al (~0.25%),Ca (~0.27%)

10 to 140 HCl, HF, H2SO4, HNO3 Time and temp:16 h at 20 °C,2 h at 80 °CS/L ratio=1:20[HF]: 0.5–5%[HCl]: 2.5%[H2SO4]: –[HNO3]: –

Si (b99.98%)Al (20 ppm),Ca (4 ppm),Fe (5 ppm)

Mixture of HCl and HF is the most effective.Dissolution of Ca and Al decreased with[HF] due to formation of insoluble fluorides,while Fe dissolution increased with [HF].

3 MG-SiSi (~98.0%)Fe (1.0%)

40 to 100 HCl, HF Time (h): 1, 12Temp: 25, 105 °CS/L ratio=1:1–1:51st HCl+HF leaching2nd HCl leaching[HCl]: –[HF]: 10%

Si (b99.90%)Al (−)Ca (−)Fe (95 ppm)

Combination of acid leaching and thermal(plasma) treatment

4 MG-SiSi (~97.0%)Fe (3–5%),Al (0.3%),Ca (0.2%)

50 to 1000 FeCl3 (+HCl)HF+HNO3

Temp: 100 °CS/L ratio=1:1–1:51st FeCl3 (+HCl)2nd HF+HNO3

[FeCl3]: 147 g/L[HCl]: 40 g/L[HF]: 2–5%[HNO3]: 5–10%

Si (b99.90%)Al (10 ppm),Fe (5 ppm)

Making alloy with Ca improveddissolution of impurities.

5 MG-SiSi (98.0%)Fe (0.56%),Al (0.3%), Ca (0.3%)

1 to 100 HNO3, HCl,HF

Time and Temp:5–20 h at 20 °C0.5–2 h at 80 °CS/L ratio=1:4[HNO3]: 5%[HCl]: 15, 20%[HF]: 2%

Si (b99.99%)Al (35 ppm),Ca (10 ppm),Fe (10 ppm)

Successive acid treatment;[HNO3]→ [HCl]→ [HF]+[HCl]

6 MG-SiSi (98.0%)Fe (1.0%), Al(0.25%), Ca (1.2%)

150 to 400 Aqua regia,HF, H2SO4

(no dilution)

Time and temp:3 h at 50 °CS/L ratio=1:5–1:10Twice leaching

Si (b99.95%)Al (100 ppm),Ca (55 ppm),Fe (25 ppm)

HF leaching is more efficient

7 MG-SiSi (98.0%)Fe (0.4%)Al (0.81%)Ca (0.088%)

30 to 126 HNO3, H2SO4, HCl, HF, Aquaregia

Time (h): 5–18Temp: 20, 50, 80 °CS/L ratio=1:10[HCl]: 2.5–16%[HF]: 2.5–5%[HNO3]: –[H2SO4]: –

Si (b99.90%)Al (800 ppm),Ca (70 ppm),Fe (180 ppm)

HCl is the most effectiveFe, Ca, and Al leaching decreased with[HF] and leaching time.Optimum particle size: 116 μm

8 MG-SiSi (98.0%)Fe (0.73%),Al (0.61%), Ca (0.25%)

b200 HCl, H2SO4, HCl+H2SO4

(no dilution)Time (hrs): 1, 2Temp: 30, 80 °CStirring speed: 540 rpm

Si(b99.98%)Fe (17 ppm),Al (56 ppm),Ca (88.4 ppm)

HCl leaching is most effective.Combination of attrition grinding and leaching

9 MG-SiSi (99.0%)Fe (0.15%), Ca (0.032%),Al (0.054%)

50 to 150 ⁎Acids A, B, C (?) Time (days): 1, 2, 3, 4, 5Temp: 20, 30, 40, 50,60, 70, 80 °CAcid (mol/L):0.5, 2, 4, 6, 8

Si(b99.80%)Fe (230 ppm),Al (140 ppm),Ca (−)

Optimum: acid C 6 mol/L, 60 °C, 4 day, 50 μm

10 Natural silica mineralSi (99.0%)Fe (0.06%),Al (0.20%),Ca (0.007%)

100 to 250 Oxalic acid, aqua regia(AR),HCl+HF, HNO3+HF

Time (h): 4, 8Temp: 80, 95 °COxalic acid: 0.2 M[HCl]+[HF]: 2.5%[HNO3]+[HF]: 1%

Si(b99.90%)Fe (262 ppm)Al (224 ppm)Ca (53.6 ppm)

Impurity removal is most effective inHCl/HF solution.

11 MG-SiSi (99.0%)Al (0.27%), Fe (0.23%),Ca (0.058%)

50 to 500 HF, HNO3, HCl Time (h): 8Temp: 50 °CConcentration of acids: 2 M

Si(b99.90%)Fe (14.6 ppm)Al (461 ppm)Ca (3.6 ppm)

HF leaching is most effective.Ultrasonic stirring is more effectivethan mechanical stirring.

12 Mg-SiSi (99.0%)Fe (0.43%), Al (0.39%)

10 to 150 HF Time (min): 45, 80, 120Temp: 25, 60, 80 °C[HF]: 0.07 to 0.28 M

Si(b99.98%)Al (720 ppm),Fe (460 ppm)

Taguchi design.Affecting factors of impurity removal:Particle size>[HF]>leaching time>temp.Optimum condition;120 min, 80 °C, 46.88 to 7.5 μm

1. Chu and Chu, 1983; 2. Dietl, 1983; 3. Boulos, 1983; 4. Halvorsen, 1985; 5. Bildl et al., 1986; 6. Juneja andMukherjee, 1986; 7. Santos et al., 1990; 8. Lian et al., 1992; 9. Yu et al., 2007; 10.Lee et al., 2009; 11. Xiaodong et al., 2009; 12. Lu et al., 2011.⁎A, B, and C are acid types that were not disclosed.“–”: no information.

179E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

Table 2Fe silicide dissolution and characterization studies.

No. Materials Particle size Lixiviants Experimental conditions or methods Objective Comments

1 Industrial Fe–Si alloys and MG-SiSi (~84.7%),Fe (~12.7%), Al (~6.92%), Ca (~1.73%)

10 mm to22.4 mm

HCl, FeCl3·6H2O Time (h): 6, 12Temp:102 °C,S/L ratio=1:10,Stirring speed:100 rpm1st HCl leaching,2nd HCl+FeCl3·6H2O[HCl]: 110, 220 g/L[FeCl3·6H2O]: 300, 400 g/L

Effect of structural phase on Fe–Sialloys refining

FeSi2 is the most resistive

2 Industrial Fe–Si alloysSi (~80.2%), Fe (16.89%),Al (1.32%), Ca (0.97%)

10 mm to22.4 mm

HCl, FeCl3·6H2O Time (h): 10, 24Temp: 102 °CStirring speed: 400 rpm[HCl]: 110, 220 g/L[FeCl3·6H2O]: 300, 400 g/L

Leaching kinetics α-Fe1−xSi2 is main component-Dissolution follows shrinking core model

3 Industrial Fe–Si alloys and MG-SiSi (~84.7%),Fe (~12.7%), Al (~6.92%), Ca (~1.73%)

10 mm to22.4 mm

HCl, FeCl3·6H2O Time (h): 6, 18Temp:102 °CS/L ratio=1:10Stirring speed:100 rpm1st HCl leaching2nd HCl+FeCl3·6H2O[HCl]: 220 g/L[FeCl3·6H2O]: 400 g/L

Fe–Si alloys refining: structural effecton the kinetics of acid leaching

Reactivity: CaSi, CaSi2, CaAl2Si1.5>(Ca)–Al–Fe–Si>>FeSi2

4 Synthetic Fe–Si alloysSi (~89.0%), Fe (~20.3%),Al (~5.1%), Ca (~1.5%)

20 to 1000 HCl Time (h): 10Temp:102 °CStirring speed: 400 rpm[HCl]: 150, 220 g/L

Reactivity of Fe–Si alloys phases Ca and/or Al containing phases are solublewhile α-Fe1– xSi2 isinsoluble.

5 Industrial Fe–Si alloysSi (~80.2%), Fe (16.89%),Al (1.32%), Ca (0.97%)

Characterization by XRD and SEM/EDS Chemical and structural studyof Fe–Si phases

Ca–Al–Fe–Si phase is easier to attack thanFeSi2 phase

6 Ferrosilicide (FexSi) producedby atomizationSi (15.4%)Fe (83.2%)Al (0.01%)

10 to 100 Characterization by XRF, XPS and SEM Surface properties of atomizedFexSi as a heavy medium

The surface covered with a mixedSiO2–Fe(II/III) oxide film

7 Ferrosilicide(FexSi)Si (10, 16, 30.0%)

Phthalate, borate buffersolutions (pH 6.4, 8.5)

RRDE, Electrochemical measurement FexSi electrochemical kinetics and corrosionbehavior in neutral and higher pH

The passivation by SiO2/Fe(III) oxide filmdecreased anodic dissolution and corrosion rate.

8 Ferrosilicide (FexSi)Si (10, 16, 30.0%)

Borate buffer solution(pH 8.5)

Photocurrent spectra, Impedance andElectrochemical measurement, XPS

Characterization of passive films on FexSi Passivation layer increased with Si content

9 Fe–Si alloysSi (8–20%)

H2SO4 Temp: 25 °C[H2SO4]: 1 MPotentiodynamic polarization

Corrosion/passivity characteristicof Fe–Si alloys

The passivity of alloys:1) 14% Si is controlled by the formation of SiO2 film.2) Below 14%Si, is controlled by a passive Fe oxide film.

10 FeSi single crystal H2SO4

NaFTemp: 25 °C[H2SO4]: 0.5 M[NaF]: 50–100 mMCyclic voltammetry

Anodic dissolution of the (100) and(110) faces of iron monosilicide

Metal-enriched (100) face dissolves faster.Fluoride ions promote anodic process dueto dissolutionof the surface layer of SiO2 and Si

11 MonosilicideFeSiCoSiNiSi

H2SO4

NaFTemp: 25 °C[H2SO4]: 0.5 M[NaF]: 50 mMCyclic voltammetryAnodic polarization

Effect of the metal component onanodic dissolvability

Dissolution rate: NiSi>CoSi, FeSi

12 Pure FePure SiFeSiFeSi2

NaOH Temp: 25 °C[NaOH]: 1.0–.0 MCyclic voltammetryXPS

Anodic dissolution of iron silicidesin alkaline electrolyte

Iron silicides are highly resistant to anodic dissolution.

1. Margarido et al., 1993a; 2. Margarido et al., 1993b; 3. Margarido et al., 1994; 4. Margarido et al., 1997; 5. Magarido, 1988; 6. Williams and Kelsall, 1989; 7. Kelsall and Williams, 1991a; 8. Kelsall and Williams, 1991b; 9. Omurtag and Doruk,1970; 10. Shein and Kanaeva, 2000; 11. Shein, 2001; 12. Shein et al., 2007.

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Table 3Selected thermodynamic data at 25 °C.

Species ΔGof,298 (kcal/mol) Species ΔGo

f,298(kcal/mol)

H2O −56.7 [1] FeCl2+ −86.27 [2]F− −66.63 [1] Fe2+ −18.84 [2]HF(aq) −70.94 [1] Fe3+ −4.11 [2]HF2− −138.16 [1] FeSi −18.75 [2]FeOOH −116.93 [2] FeSi2 −18.74 [2]Fe2O3 −177.11 [2] Fe3Si −22.61 [2]Fe3O4 −242.65 [2] Fe5Si3 −59.59 [2]Fe(OH)2 −117.46 [2] FeSiO3 −267.07 [2]Fe(OH)3 −169.45 [2] Fe2SiO4 −329.91 [2]Fe 0 [2] Si 0 [2]HFeO2

− −95.35 [2] Si(OH)4 (aq) −314.67 [2]FeO2

− −92.64 [2] SiO2 (am) −203.32 [1]FeF2+ −89.62 [2] HSiO3

− −242.78 [2]FeF+ −90.70 [2] SiO3

2− −211.40 [1]FeF2 −160.21 [2] SiOH)3O− −292.40 [1]FeF3 −221.07 [2] SiO3(OH) 3− −267.78 [1]FeCl2 −72.15 [2] Si(OH)2O2

2− −281.33 [2]FeCl3 −79.39 [2] Si4O8(OH)44− −948.80 [1]FeCl2+ −37.50 [2] Si4O6(OH)62− −975.09 [1]FeCl+ −53.03 [2] SiF62− −525.67 [1]

[1] Wagman et al., 1982, [2] HSC Chemistry 5.0.

181E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

MG-Si, Fe silicides are most insoluble and have the highest chemicalresistance. For the Fe–Si binary system, four different compounds –

FeSi2, FeSi, Fe5Si3, and Fe3Si – have been identified as stable Fe silicide

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Eh

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2.0

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0.0

-0.5

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Eh

(V)

Si

HSi

O3-SiO2

SiO

3(O

H)3-

Fe

Fe3O4

Fe(OH)2

FeOOH

Fe3+

Fe2+ Fe(OH)4-

Fe(OH)3-

(a)

(c)

Fig. 1. E–pH diagrams for the Si–H2O and Fe–H2O systems at 25 °C: (a)

phases (Bailar et al., 1973), in addition to the Fe2SiO4 silicate phase inthe Fe–Si–H2O system (Kelsall and Williams, 1991a). Thus, the focusof the present work is on these silicides.

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(V)

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0.0

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Eh

(V)

Si

SiO2

SiO

3(O

H)3-

Fe

Fe3O4

Fe(OH)2

FeOOH

Fe 3+

Fe2+

(b)

(d)

{Si}=10−4, (b) {Si}=10−2, (c) {Fe}=10−4, (d) {Fe}=10−2 M.

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Eh

(V)

FeSi2

Fe

Fe3O4

FeOOH

Fe2SiO4

FeSi

Fe3Si

Fe(OH)2

Fe3+

Fe2+

FeSi2

Fe

Fe3O4

FeOOH

Fe2SiO4

FeSiFe3Si

Fe(OH)2

Fe3+

Fe2+

SiO2

SiO

3(O

H)3-

Si

(a)

(b)

Fig. 2. Eh–pH diagrams for the systems (a) Fe–Si–H2O, (b) superimposed (a) and Si–H2O at 25 °C. ({Fe}={Si}=10−2 M).

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Eh

(V)

FeSi2

Fe

Fe2O3

Fe3O4

Fe2SiO4

FeSiFe3Si

Fe3+

Fe2+ Fe(

OH

) 4-

Fe(OH)3-

Si

SiO2

HSi

O3-

SiO

3(O

H)3-

Fig. 3. Eh-pH diagram for the Fe–Si–H2O system at 90 °C. ({Fe}={Si}=10−2 M).

182 E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

The aqueous reactivity of Fe silicides has been investigated by sev-eral researchers due to the high chemical resistance of thesematerials,as shown in Table 2, a property that is used in corrosion protection,steelmaking, and foundry technology (Kirk-Othmer, 2007). Iron sili-cides are also utilized in gravity separation processes as a medium(S.G 6–7, Si 13–16%) which offers corrosion and abrasion resistancefor the recovery of minerals, coal, and scrap metal (Kelsall andWilliams, 1991a, 1991b, 1991c; Williams and Kelsall, 1989) and thisapplication too has inspired some research into the chemical stabilityof these silicides. These earlier studies (Kelsall and Williams, 1991a,1991b, 1991c; Margarido et al., 1993a, 1993b, 1994, 1997; Omurtagand Doruk, 1970; Shein, 2001; Shein and Kanaeva, 2000; Shein et al.,2007; Williams and Kelsall, 1989), based on different Fe–Si phasesprovide a useful foundation for the current work.

The aim of this paper is to construct Eh–pH diagrams for Fe sili-cides and to use these to guide the hydrometallurgical purificationof MG-Si. The emphasis is on the dissolution behavior of the impuri-ties from different phases and with different acid types. Previous at-tempts to explore purification processes of solar cell materials bymeans of Eh–pH diagrams have been limited and focused only onthe dissolution of CdTe and related separation processes in aqueous

systems (Mezei et al., 2008a, 2008b). Other relevant research in-cludes applications of Eh–pH diagrams to the chemical etching of sil-icon in aqueous HF solutions (Osseo-Asare et al., 1996) and removalof contaminants on wet silicon surface (Norga and Kimerling, 1995).

2. Thermodynamic data

The thermodynamic data used in this work are listed in Table 3.Most of the data were taken from the HSC 5.0 software databaseand some values unavailable in HSC 5.0 were adopted from varioussources as shown in the table.

3. Results and discussion

3.1. The Si–H2O and Fe–H2O systems

Fig. 1 presents potential-pH diagrams for the Si–H2O and Fe–H2Osystems. In each diagram, the upper and lower dotted lines representthe O2/H2O and H2O/H2 stability boundaries, respectively. In the Si–H2O system (Fig. 1a and b), the stable species are SiO2 from acidic toneutral pH (Kelsall and Williams, 1991a; Osseo-Asare et al., 1996;Pourbaix, 1966) and HSiO3

− and/or SiO3(OH)3− in alkaline solution.As can be seen, elemental Si is unstable relative to oxidation to SiO2

above ca. −0.9 VSHE at pH 0, an Eh value that is far below the waterstability line, which indicates that Si is easily oxidized to SiO2. The di-agrams also show that the dominant stability phase of Si in water issolid SiO2 except in highly alkaline condition (>pH 10) when the dis-solved species HSiO3

− (10−4 M {Si}), and SiO3(OH)3− appear. In theFe–H2O system, the trends in species and stability regions displayedin Fig. 1c and d are in agreement with previous Eh–pH diagrams inthe literature (Kelsall and Williams, 1991a; Osseo-Asare, 1979;Osseo-Asare et al., 1983; Pourbaix, 1966). The ionic species Fe2+ andFe3+ are stable in acidic conditions. Also it can be seen that Si oxida-tion is more thermodynamically favorable compared with Fe, becausethe oxidation potential of Fe to Fe2+ (Fig. 1c and d) is higher than thatof Si to SiO2 (Fig. 1a and b).

3.2. The Fe–Si–H2O system

Several researchers previously investigated acid leaching for MG-Si purification using H2SO4, HNO3, and aqua regia, as summarized inTable 1. The corresponding leaching conditions may be exploredwith the aid of the Eh–pH diagrams for the Fe–Si–H2O system,shown in Figs. 2 and 3. The Fe–Si–H2O and Si–H2O systems are

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Eh

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Fe

Fe3O4

Fe(OH)2

FeOOH

Fe2+

FeCl2+

Fig. 4. Eh–pH diagram for the Fe–Cl–H2O systems at 25 °C ({Fe}=10−2 M, {Cl}=1.0 M).

2.0

1.5

1.0SiO2 3(

OH

)3-

183E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

superimposed upon each other in Figs. 2b and 3; the predominanceareas for Fe species are represented by black lines while the regionsfor Si species are depicted with red lines. Fig. 2 shows the Eh–pH di-agram for the Fe–Si–H2O system at 25 °C. The trends and stability re-gions of solid species such as Fe2SiO4, Fe3O4, Fe, Fe3Si, FeSi, and SiO2

depicted in Fig. 2 are in general agreement with the previous workof Kelsall andWilliams (1991a). The order of appearance of the Fe sil-icides, starting from the most reducing potential (Eh) conditions isFeSi2bFeSibFe3Si. Comparing the Fe–Si–H2O system (Fig. 2) withthe Si–H2O and Fe–H2O systems (Fig. 1), it can be seen that Fe2SiO4

and Fe silicides (FeSi2, FeSi, Fe3Si) appear in the Fe–Si–H2O system(Fig. 2). Eqs. (1) to (3) represent the decomposition reactions of Fesilicides under acidic to neutral pH conditions.

FeSi2 þ 4H2O ¼ 2SiO2 þ Fe þ 8Hþ þ 8e

− ð1Þ

FeSiþ 2H2O ¼ SiO2 þ Fe þ 4Hþ þ 4e

− ð2ÞFe3Siþ 2H2O ¼ SiO2 þ 3Fe þ 4H

þ þ 4e− ð3Þ

Based on Eqs. (1)–(3) and the above diagrams, it is expectedfrom a thermodynamic standpoint that the Si in the silicideswould be easier to oxidize than Fe, due to their lower electrochem-ical potential compared with Fe. The early formation of SiO2 (as in-soluble surface films) during silicide decomposition (Eqs. (1)–(3))will most likely prohibit or retard further dissolution of Fe. Previ-ously reported leaching results for Fe silicides in MG-Si supportthis idea. It was found by Margarido et al. (1993b) and Omurtagand Doruk (1970) that Fe dissolution from Fe silicides declinedwith increasing Si content. Shein and Kanaeva (2000) studied theanodic dissolution behavior of different faces – (100) and (110)orientation – of iron monosilicide in a sulfuric acid electrolyte.They reported that the metal-enriched (100) face dissolves faster andthat FeSi is much more resistant to anodic polarization than pure iron;this behavior was attributed to the increasing binding energy of Siwith increasing Si contents in Fe silicides (Gomoyunova and Pronin,2010) and the high chemical stability of the SiO2 surface filmwhich ap-pears with increasing Eh in Fig. 2. Also, Omurtag and Doruk (1970) in-vestigated the corrosion behavior of Fe silicides in 1 M H2SO4 at 25 °Cand concluded that the passivity of silicides with 14% Si was controlledby the formation of a surface SiO2film. Below14% Si, the dissolutionwascontrolled by a passive Fe oxide film. According tothese results, dissolu-tion of Fe from Fe silicides is decreased by SiO2 and/or Fe oxide films,depending on the Si content. Silicon which is present above 98% inMG-Si is susceptible to oxidation and is stable as SiO2 except in alkalineconditions (Figs. 1 and 2). Table 4 presents chemical reactions (Eqs.(4)–(6) expected during the early stages of the aqueous decompositionof Fe silicides (based on FeSi as a model Fe silicide). The Si(s) in theseequations is oxidized to SiO2 depending on the pH; Fe species alsovary with pH and Eh, and include passivation layers consisting of mate-rials such as Fe2O3, Fe3O4, and FeSiO4.

Table 4Dissolution reactions of Fe silicides.

Equations

Iron dissolution in Fe silicides FexSiy+2xH+=ySi(s)+xFe2++xH2(g) (4)2Fe2++1/2O2(g)+2 H+=2Fe3++H2O (5)FexSiy+xFe3+=ySi(s)+2xFe2+ (6)

Acidic solution Si+2H2O=SiO2+2 H2(g) (7)Neutral solution Si+2H2O=SiO2+2 H2(g) (7)

2Fe2++SiO2+2H2O=Fe2SiO4+4 H+ (8)Fe2++2Fe3++4H2O=Fe3O4+8 H+ (9)2Fe3++3H2O=Fe2O3+6 H+ (10)

Alkaline solution SiO2+2OH−=SiO3(OH)3−+H+ (11)2Fe3++3OH−=Fe2O3+3 H+ (12)Fe2++2Fe3++4OH−=Fe3O4+4 H+ (13)Fe2++2OH−=Fe(OH)2 (14)

It is known that the high corrosion resistance of the silicides influoride-free acid media is due to the surface layer of SiO2 which pre-vents selective dissolution of iron (Rakityanskaya and Shein, 2006).Also, Fe(III) ion is stable in acidic media below pH 1 at 25 °C (Fig. 2)and the corresponding aqueous stability region decreases with in-creasing temperature (e.g., to 90 °C; Fig. 3). Thus, additionally con-tinuing Fe dissolution is prevented by formation of Fe(III) oxideswhen oxidizing agents, such as HNO3 and H2O2, are present in theleaching solution in the absence Fe(III)-complexing agents. In alkalineconditions (pH>13) both Fe and Si can be solubilized as Fe(OH)3−

(or HFeO2−), Fe(OH)4− (or FeO2

−) and SiO3(OH)3− with increasingtemperature (Fig. 3). This may provide a pathway to dissolve Fe andSi simultaneously from Fe-silicides at elevated temperatures. Sheinet al. (2007) studied the anodic dissolution of iron and its silicides(FeSi, FeSi2) and pure Si in 0.1 to 5.0 N NaOH solution at room tem-perature. They reported that the iron silicides are highly resistant toanodic dissolution due to the protective properties of the complexoxide surface films such as FeOOH, Fe2O3 while no pronounced pas-sivation of silicon is observed. This is in agreement with the aqueousstability field for SiO3(OH)3− located in the alkaline region (pH>11.5) of the Eh-pH diagram for the Fe–Si–H2O system, as shownin Fig. 2. Also, it would be expected that the removal of Fe oxide

14121086420

0.5

-0.5

-1.0

-1.5

-2.0

pH

Eh

(V)

FeSi2

FeFe3O4

FeOOH

Fe2SiO4

FeSi

Fe3Si

Fe(OH)2

Fe2+

FeCl2+

Si

0.0

SiO

Fig. 5. Eh–pH diagram for the Fe–Si–Cl–H2O system at 25 °C ({Fe}={Si}=10−2 M,{Cl}=1.0 M).

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1.0

0.5

0.0

-0.5

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pH

Eh

(V)

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0.0

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pH

Eh

(V)

Si

SiO2

SiF62-

SiO

3(O

H)3-

Fe

Fe3O4

Fe(OH)2

FeOOH

FeF2+

FeF+Fe2+

(a)

(b)

Fig. 6. Eh–pH diagrams for the systems (a) Si–F–H2O and (b) Fe–F–H2O at 25 °C. ({Fe}={Si}=10−2 M, {F}=1.0 M).

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1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

pH

Eh

(V)

FeSi2

Fe

Fe3O4

FeOOH

Fe2SiO4

FeSiFe3Si

Fe(OH)2

Fe2+

FeF2+

Si

SiO2

SiO2 SiF62-

SiO

3(O

H)3-

Fig. 7. Eh–pH diagram for the Fe–Si–F–H2O system at 25 °C. ({Fe}={Si}={F}=10−2 M).

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1.5

1.0

0.5

0.0

-0.5

-1.0

-1.5

-2.0

pH

Eh

(V)

FeSi2

Fe

Fe3O4

FeOOH

Fe2SiO4

FeSiFe3Si Fe(OH)2

FeF+

FeF2+

Fe2+

Si

SiO2SiF62-

SiO

3(O

H)3-

Fig. 8. Eh–pH diagram for the Fe–Si–F–H2O system at 25 °C. ({Fe}={Si}=10−2 M,{F}=1.0 M).

184 E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

films would enhance the dissolution of Fe silicides; this may beachieved by increasing temperature in alkaline conditions, as shownin Fig. 3.

3.3. The Fe–Cl−–H2O and Fe–Si–Cl−–H2O systems

Fig. 4 shows a potential-pH diagram for the Fe–Cl–H2O system.With Cl− as a complexing agent, the stability region of Fe(III) ion, i.e.FeCl2+, is wider than in the simple Fe–H2O system (Fig. 1c and d) andthe potential of the Fe(III)/Fe(II) couple is lowered from 0.75 to 0 V.In contrast, chloride ions do not form complexes with Si(IV) andthus, the Eh–pH diagram for the Si–Cl–H2O system is unchangedfrom that presented previously for the Si–H2O system (Fig. 1a and b).These trends have important implications for the purification of MG-Si.

Santos et al. (1990) and Dietl (1983) reported that HCl leachingwas effective in refining MG-Si. This observation can be rationalizedby referring to the Eh-pH diagram for the Fe–Si–Cl−–H2O system dis-played in Fig. 5. Comparing Fig. 5 with the Fe–Si–H2O system (Fig. 2),it can be seen that the stability region of Fe(III) ion increases to pH 5.5in chloride media due to complexation (Fig. 5), while the SiO2 regionis not changed. From this figure, it can be expected that the

dissolution of Fe silicides with relatively low content Si such as FeSiand Fe3Si will be enhanced by adding chloride ion into acidic solution,e.g., as aqua regia (Chu and Chu, 1983; Juneja and Mukherjee, 1986),HCl (Aulich et al., 1984; Dietl, 1983; Xiaodong et al., 2009), and FeCl3in HCl (Margarido et al., 1993a, 1993b, 1994, 1997). Also, it isreported that a high content of Fe silicide such as FeSi2 remained inthe residue after FeCl3 and HCl leaching. Therefore, it seems that theeffectiveness of chloride media for purification of Si, as reported byDietl (1983) and Santos et al. (1990) depends on the Fe silicidephases, i.e., silicides with a lower Si content (FeSi, Fe3Si) are more re-sponsive to Fe removal in chloride media.

3.4. The Si–F–H2O, Fe–F–H2O, and Fe–Si–F–H2O systems

It is reported that the silicon oxide layer can be removed in fluo-ride acid media and that this enhances leaching efficiency of impuri-ties in MG-Si (Dietl, 1983; Juneja and Mukherjee, 1986; Santos et al.,1990). Fig. 6 shows Eh–pH diagrams for the Si–F–H2O (Fig. 6a) andFe–F–H2O (Fig. 6b) systems. Comparing Figs. 1 and 4, it can be seenthat the presence of F− ions introduces a stability region for theionic species SiF62− (Fig. 6a) and FeF2+, FeF+ (Fig. 6b). The featuresdepicted in Fig. 6a are in agreement with the previous work ofOsseo-Asare et al. (1996) indicating that the SiF62− stability domain

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Eh

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FeSi2

Fe

Fe2O3

Fe3O4Fe2SiO4

FeSi

Fe3Si

Fe2+

FeF2+

Fe(

OH

) 4-

Fe(OH)3-

Si

SiO2

HSi

O3-

SiO

3(O

H)3-

Fig. 9. Eh–pH diagram for the Fe–Si–F–H2O system at 90 °C. ({Fe}={Si}={F}=10−2 M).

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pH

Eh

(V)

FeSi2

Fe

Fe2O3

Fe3O4Fe2SiO4

FeSiFe3Si

FeF2+

FeF+

Fe(OH)3-

Fe2+

Si

SiO2 HSiO3-

SiF62-

SiO

3(O

H)3-

Fig. 11. Eh–pH diagram for the Fe–Si–F–H2O system at 50 °C. ({Fe}={Si}=10−2 M,{F}=0.1 M).

185E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

is nested between two SiO2 stability fields in 10−2 M {F−} at 25 °C. Forthe same fluoride activity in the Fe–Si–F–H2O system Fig. 7 indicatesthat Fe leaching can also improve between these two SiO2 regions.When {F−} is increased to 1.0 M (Fig. 8), the stability fields of SiF62−

and Fe ions, i.e. Fe2+, FeF+, FeF2+ are wider, extending to the more al-kaline region. This means that both Fe oxide and SiO2 layers will be re-moved by increasing fluoride concentration. It has been reported thataddition of fluoride (0.05 M) into 0.5 MH2SO4 solution led to a 60 foldincrease in the dissolution rate of FeSi (Shein and Kanaeva, 2000). Thisresult reflects the ability of fluoride ions to remove the passivatingSiO2 layer.

The leaching behavior of MG-Si has also been investigated in hightemperature media (~90 °C) in an effort to increase reactivity (Bildl etal., 1986; Dietl, 1983; Juneja and Mukherjee, 1986; Santos et al.,1990). It has been reported that increase of HF concentration and/ortemperature has negative effects on the impurity removal. Figs. 9and 10 show Eh–pH diagrams for the Fe–Si–F–H2O system at 90 °Cand 10−2 M and 1.0 M {F−}, respectively. For the lower fluoride con-centration, 10−2 M {F−} (Fig. 9), the stability region of SiO2 is domi-nant. Comparing this diagram to the same condition at lowtemperature (25 °C, Fig. 7), it is seen that the SiF62− region has dis-appeared. This may be due to decreasing stability of the fluoride com-plex with increasing temperature. When the concentration of fluoride

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Eh

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FeSi2

Fe

FeF2

Fe2O3

Fe3O4Fe2SiO4

Si

FeSiFe3Si

FeF2+

Fe(

OH

) 4-

Fe(OH)3-

Fe2+FeF+

SiO2

HSi

O3-

SiF62-

SiO

3(O

H)3-

Fig. 10. Eh–pH diagram for the Fe–Si–F–H2O system at 90 °C. ({Fe}={Si}=10−2 M,{F}=1.0 M).

ion is increased to 1.0 M (Fig. 10), the stability field of SiF62− is extend-ed to pH 6, whereas Fe(II) forms a solid phase, FeF2 with fluoride,reflecting decreasing solubility. These phenomena, such as decreasingstability of soluble fluorocomplexes and formation of insoluble spe-cies (such as FeF2) would result in decreasing leaching efficiencywith increasing temperature and fluoride concentration. Therefore,considering the reactivity and solubility of the Fe and Si species itwould seem that the optimum condition for Fe silicide dissolutionin fluoride media would require a more moderate temperature com-pared with the 90 °C used in Fig. 10. Fig. 11 presents the Eh–pH dia-gram of the Fe–Si–F−–H2O system in 0.1 M {F−} at 50 °C. As can beseen, the ionic stability regions of Si (SiF6−2) and Fe(III) are almostsimilar (~pH 4) but the solid phase, FeF2, has disappeared, comparedwith the same system at 90 °C (Fig. 10). Xiaodong et al., 2009 intro-duced a microwave stirring method in 2.0 M HF and 90 °C to enhanceimpurities removal. They found that the ultrasonic vibration in-creased Fe removal as well as Al and Ca release and they rationalizedthese results by proposing that ultrasonication increased diffusion ofHF, generated heat and introduced a cavitation effect. It is likely thatthe ultrasonic vibration would also help to disrupt any FeF2(s) filmsformed on the dissolving surfaces.

3.5. Effect of crystal structures and other impurities in MG-Si

Anglézio et al. (1990) and Margarido et al. (1994) characterizedMG-Si. The main phases were Si–Fe, Si–Fe–Al, Si–Fe–Ca–Al as Fe sili-cides. Margarido et al. (1993a) also analyzed the effect of structuralcomposition on the purification of Fe–Si alloys by a two-step acidleaching with HCl, HCl+FeCl3 at 102 °C. The results show that Si–Fe–Al and Si–Fe–Al–Ca phases are susceptible to leaching, while Fe–Si silicides, such as tetragonal and orthorhombic FeSi2, remain assolid residues. This means the leaching is also affected by structuralphases. Therefore, making alloys to change structural phase fromacid resistance materials to more soluble structures by adding Ca orAl is also a possible way to enhance impurities removal from MG-Si(Margarido, 1988; Margarido et al., 1993a, 1993b, 1994, 1997).

4. Conclusions

In the present work Eh-pH diagrams for the Fe–Si–(Cl−)–(F−)–H2O systems were generated using the HSC5.0 software. Based onthe diagrams discussed above, the stability regions of various species

186 E. Kim, K. Osseo-Asare / Hydrometallurgy 127–128 (2012) 178–186

related to MG-Si purification can be predicted. The main findings maybe summarized as follows:

- Si–Fe–H2O system: the stability field of SiO2 is dominant in theacidic-to-neutral pH regime. Fe(III) ion is stable in acidic mediabelow pH 1 at 25 °C and the region decreases with increasing tem-perature (up to 90 °C in this work). The order of oxidation state ofFe silicides is FeSi2bFeSibFe3Si. However, leaching results for Fe sil-icides in MG-Si, as reported in the literature, show that Fe dissolu-tion is retarded with increasing Si content not by the oxidationorder. The Eh–pH diagrams reveal that this trend is attributable tothe dominance of SiO2 in the Si–H2O system. The implication hereis that selective dissolution of Fe from MG-Si may be improved bydecreasing temperature and lowering the Si content of Fe silicides.

- Si–Fe–Cl−–H2O system: Fe(III) ion is stable as FeCl2+ and the stabil-ity region of Fe(III) ion increases to pH 5.5 in chloride media due tocomplexation, while the SiO2 region remains unchanged. Dissolu-tion of Fe silicides with relatively low Si content, such as FeSi andFe3Si, is enhanced by adding chloride ion in acidic solution.

- Si–Fe–F–H2O system: the introduction of fluoride ions into theaqueous system enhances the stability domains of the ionic spe-cies of iron and silicon at low temperature. The stability regionof Fe ion is decreased with increasing temperature and concentra-tion of fluoride ion due in part to formation of insoluble FeF2(s). Forsilicon dioxide dissolution, the region of SiF62− is enhanced by in-creasing {F−}, which results in increased dissolution of impurities.Introducing ultrasonication and converting the impurities intomore soluble phases, as reported by some researchers, may behelpful in enhancing the dissolution efficiency of the impurities.

Acknowledgements

The first author is grateful for support by the National ResearchFoundation of Korea Grant under the Korean Government (Ministryof Education, Science and Technology, Project No. NRF-2010-357-D00154).

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