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Chemical Engineering and Processing 46 (2007) 757–763

The effect of additives on anode passivation in electrorefining of copper

M. Ojaghi Ilkhchi a,∗, H. Yoozbashizadeh b, M. Sadegh Safarzadeh c,d

a Faculty of Materials Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iranb Department of Materials Science and Engineering, Sharif University of Technology, P.O. Box 11365-9466, Tehran, Iran

c Department of Materials and Metallurgical Engineering, Iran University of Science and Technology, Narmak, Tehran, Irand Iranian Zinc Mines Development Company, IZMDC, Zanjan, Iran

Received 17 June 2006; received in revised form 10 September 2006; accepted 4 October 2006Available online 10 October 2006

bstract

In copper electrorefining process, some additives are added to the electrolyte to improve the morphology of cathode deposits as well as theuality of products. In the present investigation, the effects of thiourea, glue and chloride ions (as additives) on the passivation of industrial coppernodes under high current densities have been reported. Experiments were conducted at 65 ◦C; using a synthetic electrolyte containing 40 g/l Cu2+

nd 160 g/l H2SO4. Results obtained from chronopotentiometry experiments showed that increasing the concentration of chloride ion leads to

ncrease in passivation time. The results also indicated that from a certain level on, namely 2 ppm, the increase in thiourea and glue concentrationsecreased the passivation time. Thiourea and glue in low concentrations, i.e. ≤2 ppm, retarded passivation, compared to an electrolyte withoutdditives.

2006 Elsevier B.V. All rights reserved.

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eywords: Copper electrorefining; Anode passivation; Thiourea; Glue; Chlorid

. Introduction

Copper electrorefining in acidic copper sulfate electrolytesas been widely used to produce high purity copper. Accordingo Faraday’s law, there are two approaches to enhance the pro-uctivity of electrorefining process; i.e. increasing the currentensity and surface area of the electrodes both result in moreroductivity. Increasing the surface area of the electrodes per seecessitates the enlargement of the cell dimensions and hencelectrolyte volumes. However, increasing current density, with-ut increasing electrolyte volume, results in higher productionapacities, but has some disadvantages such as lower purity andndesired morphology of cathode and detrimental phenomenonf anode passivation. Dissolution of anodes decreases or eventops due to passivation phenomenon and subsequently; it resultsn high energy consumption as well as increments in residue

crap and voltage levels. The precipitation of a non-conductingopper sulfate layer on the anode surface, due to supersaturationf the electrolyte has been considered as a reason for passiva-

∗ Corresponding author. Tel.: +98 412 3444334; fax: +98 412 3444333.E-mail address: m ojaghi@sut.ac.ir (M.O. Ilkhchi).

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255-2701/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2006.10.005

; Chronopotentiometry

ion [1]. Passivation of anodes is affected by the compositions ofhe anode and electrolyte as well as temperature and electrolyteirculation rate. However, impurities in copper anodes, whichre usually noticeable amounts of Se, Te, Bi, As, Sb, S, Ni andrecious metals such as Ag and Au, affect the anode passivation.

Anode passivation has received increasing attention and theactors influencing it have been extensively studied. The studies2–4] show that electrolyte composition and temperature play anmportant role in passivation phenomenon. Anode passivationas shown to be aggravated by increasing the factors such asopper concentration, the sulfuric acid concentration and currentensity, as well as decreasing the temperature. These factorsncrease the probability of copper sulfate precipitation. Also its found that the time of passivation decreases with an increasen Sb3+, As3+, Bi3+ and dissolved O2 in the electrolyte [5]. Thisehavior is observed to increase the slime formation. Abe et al.1] concluded that passivation time decreased with increasingickel as nickel sulfate.

Several recent studies have focused on the effect of addi-

ives on passivation in laboratory scale. Jin et al. [6] showed thathe addition of amino acid chelating agents could inhibit pas-ivation. In particular ethylenediaminetetraacetic acid (EDTA),iethylenediaminepentaacetic acid (DTPA), and triethylenedi-

7 ering and Processing 46 (2007) 757–763

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Table 1Chemical composition of commercial copper anode used in experiments

Element wt.% Element wt.%

Cu 98.64 Zn 0.0317Na 0.283 Ag 0.0316Cl 0.193 Se 0.0292Si 0.180 Fe 0.0144Al 0.179 As 0.0135Hf 0.105 Ni 0.0084Ca 0.0831 Pb 0.0074K 0.0625 Sb 0.0070PM

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Fig. 1 illustrates the chronopotentiogram of tested anodein the standard electrolyte. This chronopotentiogram shows apotential jump from the 960th second on that contributes to pas-sivation of the anode. As can be seen from the curve, potential

58 M.O. Ilkhchi et al. / Chemical Engine

minehexaacetic acid (TTHA) show some benefit and all ofhem exhibit an optimum concentration for extending passi-ation times. Jin and Ghali [7] studied the effects of sev-ral aromatic nitro compounds. The most beneficial reagentso inhibit passivation were 3,5-dinitrosalicylic acid and 3,5-initrobenzoic acid. Jin and Ghali [8] investigated the effect oflue, thiourea, avitone, percol and chloride ions on the passi-ation of copper anodes. Hiskey and Cheng [9] illustrated thatime to passivation is a function of thiourea. For small addi-ions, i.e. 1–5 ppm, thiourea improved the passivation behavior,hereas for large additions, namely greater than 15 ppm, pas-

ivation times decreased. Moats and Hiskey [10] examined theole of glue, thiourea and chloride ions on passivation of coppernodes.

Sarcheshmeh Copper Complex in Iran uses the electrore-ning process to produce the high purity copper cathodes.hemical composition of Sarcheshmeh anodes is different from

he anodes that have been studied previously. Some additivess grain refining agents are added in order to improve the mor-hology of cathode deposits in the electrorefining of copper.hiourea, glue and chloride ion are also used in most refineries.sing these additives will result in smooth and nodulelesseposits. On the other hand, these additives affect the tendencyf anodes to the passivation. In this investigation, these additivesere evaluated at various concentration using industrial anodes

rom Sarcheshmeh Copper Complex. Some experiments werearried out to detect the effects of additives and their interactionn the passivation of anodes, which are described below. Inhis work, chronopotentiometry, which is an electrochemical

ethod, was employed to plot chronopotentiogram curves fornvestigating passivation effect.

. Experimental

.1. Materials and methods

A synthetic electrolyte was utilized for all electrochemi-al tests. The electrolyte was prepared by using sulfuric acidMERCK, Analytical grade); commercial hydrated copper sul-ate and distilled water. The commercial additives (glue andhiourea) were supplied by the Sarcheshmeh Copper Com-lex, Iran. Chloride ions added in the form of hydrochloriccid (MERCK, Analytical grade). An electrolyte compositionf 40 g/l Cu (0.63 M) and 160 g/l H2SO4 (1.63 M), as a stan-ard electrolyte, was used at a temperature of 65 ◦C. Glue andhiourea that are well-known to decompose in the electrolyteere added just prior to heating the electrolyte.The working, counter and reference electrodes were consid-

red as a cylinder shape from an industrial anode with a lengthf 4 cm, made of high purity copper rod and a standard calomellectrode (SCE 0.24 V versus SHE), respectively. The workingnd counter electrodes were prepared by polishing with siliconarbide sandpaper and washing with de-mineralized water. Then

hey transferred to the electrolytic cell containing 100 cm3 ofynthetic electrolyte, held at 65 ◦C and allowed to reach steadi-ess with the system by being held in the electrolyte at openircuit potential for 5 min.

0.0648 Th 0.0039g 0.051 Ti 0.0028

Working electrodes were cast in the form of cylindrical rodsnd machined carefully into 1.1 cm diameter and 1 cm heightods. Soldering a copper wire to the base of each cylinderrovided electrical contact. All sides of the electrodes were insu-ated with nail polish excluding 1 cm2 of circular face for theorking electrodes and 10 cm2 of the same area for the counter

lectrodes. Chemical composition of the commercial coppernode utilized in this investigation is presented in Table 1.

.2. Instrumentation

Chronopotentiometry was utilized to observe the effectf electrolyte additives on the passivation response duringnodic dissolution. A constant current density of 3820 A m−2

as applied in the experiments using a 273 A Potentio-tat/Galvanostat EG&G equipment controlled by 352 Corrosionnalysis software. Based on previous works by other researchers

11], a 3820 A m−2 current density was selected.

. Results

.1. Additive free electrolyte

Fig. 1. Chronopotentiogram of industrial anode in the standard electrolyte.

M.O. Ilkhchi et al. / Chemical Engineering and Processing 46 (2007) 757–763 759

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Fig. 4. Chronopotentiogram of industrial anode in the electrolyte containing5 ppm thiourea.

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ig. 2. Chronopotentiogram of industrial anode in the electrolyte containingppm thiourea.

scillations went on to the end of the test and anode remainedn passive state.

.2. Effect of thiourea

Figs. 2–6 illustrate the chronopotentiogram curves of testednode in the electrolyte containing 1, 2, 5, 15 and 30 ppmhiourea, respectively. According to the results, increasinghiourea concentration up to 2 ppm, leads to increase in pas-ivation time; however, by increasing thiourea concentrationo ≥5 ppm level, the passivation time decreases. As can beeen, adding 1 ppm thiourea to the electrolyte increases theassivation time of anode. Fig. 7 shows the passivation timef the anode in terms of thiourea concentration. Thioureappears to have a positive effect at low concentrations and viceersa an adverse effect at high concentrations. No depassiva-ion effect was observed during the tests. Chronopotentiogramslso showed that potential of the active dissolution region

ecreased, which claims that the dissolution becomes easier inhe presence of thiourea, i.e. it has a depolarizing effect on thenode.

ig. 3. Chronopotentiogram of industrial anode in the electrolyte containingppm thiourea.

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ig. 5. Chronopotentiogram of industrial anode in the electrolyte containing5 ppm thiourea.

.3. Effect of glue

Figs. 8–12 illustrate the chronopotentiograms of tested anode

n the electrolyte containing 1, 2, 5, 15 and 30 ppm glue, respec-ively. According to the results, increasing glue concentrationp to 2 ppm, leads to increase in passivation time; however, by

ig. 6. Chronopotentiogram of industrial anode in the electrolyte containing0 ppm thiourea.

760 M.O. Ilkhchi et al. / Chemical Engineering and Processing 46 (2007) 757–763

Fig. 7. Passivation time of anode against thiourea concentration.

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Fig. 10. Chronopotentiogram of industrial anode in the electrolyte containing5 ppm glue.

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ccentrations and an adverse effect at high concentrations. Again,

ig. 8. Chronopotentiogram of industrial anode in the electrolyte containingppm glue.

ncreasing glue concentration to ≥5 ppm level, the passivationime decreases, which is completely consistent to the resultsttained from the effect of increasing thiourea concentrationn the passivation time. As can be seen, adding 1 ppm glue

o the electrolyte, increased the passivation time of the anode.owever, that time decreases by increasing glue concentration.ig. 13 shows the passivation time of anode in terms of glue

ig. 9. Chronopotentiogram of industrial anode in the electrolyte containingppm glue.

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ig. 11. Chronopotentiogram of industrial anode in the electrolyte containing5 ppm glue.

oncentration. Glue appears to have a positive effect at low con-

o depassivation effect was observed during the tests. The glueimplifies the dissolution of anode, concerning the fact that theotential of active dissolution region is small compared to its

ig. 12. Chronopotentiogram of industrial anode in the electrolyte containing0 ppm glue.

M.O. Ilkhchi et al. / Chemical Engineering and Processing 46 (2007) 757–763 761

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Fig. 15. Chronopotentiogram of industrial anodes in electrolyte containing40 ppm chloride ion.

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Fig. 13. Passivation time of anode against glue concentration.

uantity without the presence of additive and consequently itight be concluded that the glue has a depolarizing effect on

he anode as well.

.4. Effect of chloride ions

Chloride is almost the only non-organic electrolyte additivehich is used in copper electrorefining. Normally added amountf chloride ion in refineries varies between 20 and 70 ppm. Thehronopotentiograms of industrial anodes in electrolyte contain-ng 20, 40, 60, 80 and 100 ppm chloride ion have been illustratedn Figs. 14–18, respectively. Fig. 19 shows the passivation timef the anode in terms of chloride ion concentration. As can beecognized, the addition of chloride ion to electrolyte increaseshe passivation time of anodes and this period is extended withncreasing the chloride ion. The potential of the active dissolu-ion region in the presence of chloride ion is more negative than

he situation in which chloride ion is not present, which stateshe depolarization effect of chloride ion on the anode. That is toay, the presence of chloride ion simplifies the dissolution of thenodes.

ig. 14. Chronopotentiogram of industrial anodes in electrolyte containing0 ppm chloride ion.

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ig. 16. Chronopotentiogram of industrial anodes in electrolyte containing0 ppm chloride ion.

.5. Effects of simultaneous addition of three different

dditives

The chronopotentiogram of industrial anodes in the elec-rolyte containing 40 ppm chloride ion, 1 ppm thiourea and

ig. 17. Chronopotentiogram of industrial anodes in electrolyte containing0 ppm chloride ion.

762 M.O. Ilkhchi et al. / Chemical Engineering

Fig. 18. Chronopotentiogram of industrial anodes in electrolyte containing100 ppm chloride ion.

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ig. 19. Passivation time of anode in term of chloride ion concentration.

ppm glue is displayed in Fig. 20. It is evident that the indus-rial copper anode in the electrolyte containing three addi-ives simultaneously, passivated after 1140 s, which shows that

here is a delay in the passivation in the presence of all kindsf additives. By comparing this timeframe with the passi-ation time in the electrolytes containing equal contents of

ig. 20. Chronopotentiogram of industrial anodes in electrolyte containing0 ppm chloride ion, 1 ppm thiourea and 1 ppm glue.

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and Processing 46 (2007) 757–763

ach additive individually, it can be concluded that the addi-ion of the chloride ion weakens the effect of thiourea andlue. This might be caused by the formation of a complexompound due to the reaction of thiourea and glue with chlo-ide ion. Also the potential of the active dissolution region isore negative in comparison with standard electrolyte, and this

emonstrates a simplification in dissolution in the presence ofdditives.

. Discussion

According to findings of Mattson and Bockris [12] the dis-olution of copper takes place by passing through two mono-lectron transfers (Eqs. (1) and (2)), the latter being slow andinetically, determines the dissolution rate:

u → Cu+ + e (1)

u+ → Cu2+ + e (2)

Consequently, concentration of Cu+ ion increases near thenode surface and disproportionation of these ions (Eq. (3)) pro-uces copper powder and Cu2+ ions:

Cu(sol)+ → Cu(sol)

2+ + Cu0 (3)

As a result, copper powder can inter the slime and increasehe copper losses; furthermore, it increases the volume of thenode slime and troubles the diffusion of the copper ions. Copperowder can moreover reduce the silver ions to metallic silver thatakes a diffusion barrier for the copper ions.In the acidic electrolyte, thiourea can be oxidized as the for-

amidine disulfide (FDS) form and thiourea and FDS both canorm complexes with Cu+ ions. In the case of thiourea, this com-lex is in the form of [Cu(Tu)4]+, i.e. each Cu+ ion complexesith four molecules of thiourea. This complex has a high sta-ility constant, which demonstrates that it is very stable. Due tohe formation of such complexes, the concentration of Cu+ ionsecreases and their detrimental effects on the anode passivations then neutralized.

Furthermore, thiourea and its decomposition products candsorb on the anode, similar to sulfur, and act as a barriergainst the diffusion of copper ions. In low concentrations, theomplexing effect of thiourea becomes prior to adsorption butith increasing the concentration, decomposition products alsoill increase and consequently, adsorption effect will appear in

dvance. As a result, the adsorbed compounds on the anode willover its active surface area and block the anode dissolution andelp to its passivation, That is to say, the passivation time of thenodes decreases.

Glue is an animal protein, which is composed of amino acids.he positive effect of glue on the passivation of anode in lowoncentrations is probably owing to the complexation of Cu+

ons with amino acids. Glue can also be adsorbed on the anode.

ith respect to decreased passivation time in high concentra-

ions of glue, probably the adsorption effect increases with anncrement in the concentration and prevails over the complexingffect.

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The positive effect of adding chloride ion on the passi-ation of industrial anodes can also be explained. Noguchit al. [5,13] showed that passivation time decreased withncreasing dissolved O2; Then again, Bombach et al. [14]emonstrated that Cu+ ions decreased by increasing dissolved2. According to these two observations, it would be con-

luded that increasing the cuprous content might be beneficialo passivation. Hence one can say chloride ions are benefi-ial for prevention of passivation effect through stabilizationf cuprous ions. However, as mentioned above, the increas-ng of Cu+ ions near the anode surface is detrimental. Mostikely chloride ions decrease the concentration of the Cu+

ons with the formation of CuCl2− that is a soluble complexon.

. Conclusions

Based on the investigation conducted on the effect of addi-ives on anode passivation in electrorefining of copper, the fol-owing items may be concluded:

In the standard electrolyte containing 160 g/l sulfuric acidand 40 g/l copper, the Sarcheshmeh industrial copper anodespassivated at the current density of 3820 A m−2 after elapsing960 s at 65 ◦C.Adding 1–2 ppm of thiourea to the electrolyte increased thepassivation time of the anode; whereas adding more amountsof thiourea decreased that time.Presence of 1–2 ppm of glue in the standard electrolyte delaysthe anode passivation, whereas increasing the glue concentra-tion accelerates it.Chloride ion delays the passivation of industrial copper anode.However, it must be noticed that the presence of chlorideion is deleterious and will result in severe corrosion of theapparatus, subsequently, it is not useful to add a great deal ofchloride ion to delay passivation, from an economical point ofview.

The effect of thiourea and glue is related to complex forma-tion with Cu+ ions in low concentrations and the adsorptionphenomenon is the reason for their adverse effect in high con-centrations.

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and Processing 46 (2007) 757–763 763

cknowledgments

The authors wish to thank Dr. M. Ghorbani, Mr. J. Ahmadiannd Mr. A. Khalili for their kind support.

eferences

[1] S. Abe, B.W. Burrows, V.A. Ettel, Anode passivation in copper refining,Can. Metall. Q. 19 (1980) 289–296.

[2] S. Abe, S. Goto, Effect of sulfuric acid concentration in copper electrolyteupon anode passivation—studies of copper anode passivation, NipponKogyo Kaishi 98 (1982) 41–46.

[3] S. Abe, S. Goto, Effect of electrolyte temperature upon passivation of cop-per anodes—study of copper anode passivation, Nippon Kogyo Kaishi 98(1982) 113–117.

[4] J. Sedzimir, W. Gumowska, Influence of electrolysis variables on the pas-sivation time of copper anodes in copper electrorefining, Hydrometallurgy24 (1990) 203–217.

[5] F. Noguchi, T. Nakamura, Y. Ueda, Behaviour of anode impurities in copperelectrorefining. Effect of bismuth, arsenic, antimony and oxygen in copperanode, J. Min. Mater. Process. Inst. Jpn. 105 (1989) 321–327.

[6] S. Jin, H. Djellab, E. Ghali, Effect of some amino acid chelating agents onthe passivation of copper anodes in copper sulfate/sulfuric acid electrolyte,Hydrometallurgy 24 (1990) 53–65.

[7] S. Jin, E. Ghali, Effect of some aromatic nitro compounds on the passivationof copper anodes during electrorefining, J. Appl. Electrochem. 21 (1991)247–254.

[8] S. Jin, E. Ghali, Influence of some bath additives on the passivation ofcopper anodes in CuSO4–H2SO4 electrolyte, Can. Metall. Q. 31 (1992)259–267.

[9] J.B. Hiskey, X. Cheng, Fundamental studies of copper anode passivationduring electrorefining. Part III. The effect of thiourea, Metall. Mater. Trans.B 29 (1998) 53–58.

10] M.S. Moats, J.B. Hiskey, The role of electrolyte additives on passiva-tion behaviour during copper electrorefining, Can. Metall. Q. 39 (2000)297–305.

11] X. Cheng, J.B. Hiskey, Fundamental studies of copper anode passivationduring electrorefining. Part I. Development of techniques, Metall. Mater.Trans. B 27 (1996) 393–398.

12] E. Mattson, J.O. Bockris, Galvanostatic studies of the kinetics of depositionand dissolution in the copper + copper sulfate system, Trans. Faraday Soc.55 (1959) 1586–1601.

13] F. Noguchi, T. Nakamura, Y. Ueda, Behaviour of anode impurities in copper

electrorefining. Effect of lead, arsenic and oxygen in anode, J. Min. Mater.Process. Inst. Jpn. 105 (1989) 1017–1023.

14] H. Bombach, K. Hein, H. Baum, E. Rill, Studies of the electrochemicalbehavior of binary copper–arsenic alloys in copper sulfate electrolytes,Neue Huette 27 (1982) 87–90.