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.Hydrometallurgy 59 2001 5567

www.elsevier.nlrlocaterhydromet

Electrowinning of copper from sulfate electrolyte in presenceof sulfurous acid

B. Panda, S.C. Das)

Council of Scientific and Industrial Research, Regional Research Laboratory, Bhubaneswar 751 013, Orissa, India

Received 16 January 2000; received in revised form 21 July 2000; accepted 27 July 2000

Abstract

The electrowinning of copper from acidic sulfate solution in the presence of sulfurous acid using a graphite anode was

investigated. The effects of operating variables including sulfur dioxide concentration, copper concentration, sulfuric acid

concentration, current density and temperature on the cell voltage, anode potential, power consumption, current efficiency,

deposit quality, surface morphology, crystallographic orientations and polarisation characteristics were studied. Other anode

materials such as PbrSb, PbrAg, Ti and Ti IrO were also used to examine their effects on electrocatalytic activity for2

oxidation of SO and deposit quality. A rectangular stainless steel cathode of length 8 cm, width 5 cm and thickness 2 mm2

was used for copper electrowinning. Increases in SO concentration, copper concentration, sulfuric acid concentration and2

temperature lower the power consumption. These variables have no effect on the current efficiency of copper deposition.

The presence of SO in the copper electrolyte affects both the cathodic and anodic polarisation curves. In addition, it causes2

. . . . . . . .shifting of preferred orientations from 220 111 200 311 to 111 200 220 311 . Changes in crystallographic

orientation are also seen in the surface morphology of deposited copper. It is found that minimum power consumptiontogether with maximum current efficiency and improved surface morphology can be achieved using a graphite anode.

q2001 Elsevier Science B.V. All rights reserved.

Keywords:Electrowinning; Copper; Sulfate electrolyte

1. Introduction

Copper extraction has undergone many develop-

ments in the past 20 years. Both pyro- and hydromet-allurgical processes have been improved and totally

)

Corresponding author. Tel.: q91-674-581750; fax: q91-674-

581750. .E-mail address: [email protected] S.C. Das .

w xnew processes have been reported 1 . The techno-

economic problem associated with the SO2emissions from copper smelting in pyrometallurgical

processes has prompted the development of hy-drometallurgical processes for recovery of copper

from sulfide concentrates. The unit operations gener-

ally involved in these hydrometallurgical processes

are roasting, leaching and electrowinning. In recent

years, there has been a marked increase in the pro-

duction of copper by electrowinning. A major draw-

back is the high energy requirement associated with

0304-386Xr01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved. .P I I : S 0 3 0 4 - 3 8 6 X 0 0 0 0 1 4 0 - 7

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( )B. Panda, S.C. Das rHydrometallurgy 59 2001 55 6756

the copper electrowinning compared to electrorefin-

ing of copper. The process requires nearly 810 .times more power 2.0 kW hrkg than electrorefin-

. w xing 0.25 kW hrkg 2 . This high energy require-

ment in copper electrowinning has prompted a num-

ber of investigations for reducing the energy con-w xsumption 3 . One such approach is to substitute an

alternative anodic reaction in the place of oxygen

evolution. The other possible alternative anode reac-w xtions suggested 3 may be given as follows:

2Clys Cl q 2e EB s y1.36 V 1 .2 SHE

Fe2qs Fe3qq e EB s y0.77 V 2 .SHE

Cuqs Cu2qq e EB s y0.15 V 3 .SHE

H SO q H O s SO2yq 4Hqq 2e2 3 2 4

EB s y0.18 V. 4 .SHE

.The above reactions, except reaction 1 occur at

lower potentials than the oxygen evolution reaction. .However, reaction 4 is of more interest to the

w xinvestigators 5 . Oxidation of dissolved SO on2carbon and graphite anodes has been studied by

w x w xseveral investigators 6 . Wiesener 6 pointed out

that carbon anodes are not suitable for the anodicw xoxidation of SO . Pace and Stauter 7 found a2

power consumption of 1 kW hrkg of Cu produced

as against twice that amount for conventional prac-w xtice. Bharucha et al. 8 succeeded in designing a

novel graphite anode for use in copper electrowin-

ning. A mixture of air and 1215% SO was sparged2through a porous graphite anode. However, the work

did not proceed beyond a limited number of trials at

a laboratory scale.

In the present work, an attempt was made to

examine the effects of sulfurous acid on the copper

electrowinning from sulfate electrolyte. Sulfurous

acid was used as the source of SO because it may2be easier to trap the environmental formidable emis-

sions of SO as sulfurous acid and transport to the2copper cell house for their conversion to SO 2y and4simultaneously reducing energy consumption in the

copper electrowinning. Other common compounds .like Na SO and NH SO were avoided to pre-2 3 4 2 3

vent the formation of their respective sulphates in the

copper electrowinning cell, which may ultimately

affect the copper electrowinning process. A graphite

anode was used and the effects of sulphurous acid on

the cell voltage, anode potential, energy consump-

tion, deposit quality, crystallographic orientations and

surface morphology of cathode deposit and polarisa-

tion behaviour during copper electrowinning were

studied. Other anode materials such as PbrSb,

PbrAg, Ti and Ti IrO were used for comparison.2

2. Experimental methods

2.1. Apparatus and material

The electrolytic cell used in the present work was

a 250-ml pyrex beaker. It was covered with a per-

spex lid with a provision to insert anode and the

cathode. The cathodes used were rectangular stain-

less steel sheets having the following dimensions;length 8 cm, width 5 cm and thickness 2 mm. For

electrical connection to the cathodes, strips of the

same material having dimensions; length 11 cm,

width 1 cm and thickness 2 mm were welded at the

center of the top edge of the rectangular sheets. The .anodes used consisted of graphite, PbrSb 7% ,

.PbrAg 1% , Ti and TiIrO . The anodes were of2the same dimensions as those of the cathode. A

.saturated calomel electrode SCE was used as the

reference electrode. A rectifier with a maximum

capacity of 4 A and 30 V was used as the DC source.The cell voltage and anode potential were measured

by inserting precision voltmeters in the circuit. A

thermostat was used for maintaining the desired elec-

trolyte temperature.

The electrolytic solution was prepared from .reagent grade copper sulfate CuSO , 5H O , sul-4 2

phuric acid, sulphurous acid and double-distilled wa-

ter. The addition of sulfurous acid to the copper

electrolyte was made by adding the appropriate vol-

ume of the reagent. The concentration of SO in the2stock sulfurous acid solution was analysed before

preparing the electrolytic solution for each experi-

ment.

2.2. Electrolysis

Unless otherwise stated, the electrowinning exper-

iments were carried out for 2 h at room temperature

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( )B. Panda, S.C. Das rHydrometallurgy 59 2001 55 67 57

. 230"18C and at a current density of 150 Arm

using a bath containing 20 grL copper, 30 grL

H SO and 4.16 grL SO and graphite anode. In all2 4 2the experiments, one cathode and one anode were

taken and kept face-to-face 3 cm apart. During the

electrowinning experiments, the cell voltage and an-

ode potential were measured at 1-h interval. After

electrolysis, the cathode was removed and thor-

oughly washed with tap water followed by distilled

water and air dried after washing it in acetone. The

current efficiency was calculated from the weight

gained by the cathode.

2.3. Polarisation measurements

.Linear sweep voltammetry LSV was used to

examine the cathodic and anodic polarisation be-

haviours during copper electrowinning in the pres-

2 .ence and absence of SO . Platinum 1 cm and2 2 .graphite 1 cm were used as working electrodes. A

platinum wire and a SCE were used as counter

electrode and reference electrode, respectively. Ex-

periments were conducted taking 100-mL solution of

different compositions. A PAR 175 Universal Pro-

grammer was used to drive a PAR 173 Potentiostat.

The cathodic and anodic polarisation were studied

between 0.0 to y1.6 V and q0.2 to q2.0 V,

respectively, at a scan rate of 10 mVrs. The polari-

sation curves were recorded using a model RE0091

XY recorder.

2.4. Deposit examination

X-ray diffractometry was carried out to determine

the crystallographic orientations and the surface mor-

phology of the deposits was examined by SEM.

3. Results and discussions

3.1. Sulfur dioxide concentration

The effect of different concentrations of sulfur

dioxide on the reduction of anode potential duringw xcopper electrowinning has been studied 3 7 . Mishra

w xand Cooper 3 reported that while sparging 10%,

20%, 100% SO at a current density of 200 Arm2,2the reductions in anode potential are in the order of

100, 700 and 1000 mV, respectively. Pace and Stauter

found a power consumption of 1 kW hrkg of copper

in the presence of 1.3 grL of dissolved SO in the2copper electrolyte. In the present investigation, the

influence of SO concentration in the electrolytic2bath was studied in the range 0.2524 grL. Figs. 1

and 2 show the effect of initial concentration of SO 2on cell voltage, anode potential and power consump-

tion, respectively. It can be seen from Fig. 1 that

both the cell voltage and anode potential decrease

rapidly with increase in SO concentration up to 152grL and then remained constant with further in-

crease in SO concentration. This is because SO2 2oxidises at a less positive potential than to the oxy-

gen evolution reaction. The results obtained on cell

voltage and anode potential at the end of 1- and 2-h

electrolysis show that the cell voltage increases by

, 0.3 V and anode potential by , 0.2 V. This may

be due to decrease in SO concentration in the2copper electrolyte. The decrease in SO concentra-2tion over the period of electrolysis may be attributed

to the consumption and loss to atmosphere. Fig. 2

shows the plot of power consumption against SO 2concentration. The trend in the fall in power con-

Fig. 1. Effect of SO concentration on cell voltage and anode2potential. Cu concentration, 20 grL; H SO concentration, 302 4grL; current density, 150 Arm2 ; temperature, 308C; duration of

electrolysis, 2 h; anode substrate, graphite.

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Fig. 2. Effect of SO concentration on power consumption. Cu2concentration, 20 grL; H SO concentration, 30 grL; current2 4density, 150 Arm2 ; temperature, 308C; duration of electrolysis, 2

h; anode substrate, graphite.

sumption is similar to that shown by cell voltage or

anode potential. It sharply decreased up to a SO2concentration of 15 grL and then remained constant.

It is found that by adding SO to the copper elec-2trolytic bath, the power consumption during copper

electrowinning can be lowered by , 550 kW hrt of

Cu during 2-h electrolysis. The increase in the con-

centration of SO has no effect on cathodic current2efficiency and is found to be 98% throughout.

3.2. Copper concentration

w xPace and Stauter 7 obtained good quality cath-

odes with a current efficiency of 95% in copper

electrowinning with a copper concentration from 10

grL down to 2 grL by circulating SO sparged2w xsynthetic leach liquor at 498C. Mishra and Cooper 3

were successful to electrowin copper down to a level

of 0.5 grL in the solution from an initial concentra-

tion of 20 grL of copper by sparging N q 5% SO2 2at 200 cm3rmin between the electrodes. As per

w x 2qRobinson 1 , concentration of Cu in the elec-

trolyte affects the operating current density. In the

present work, experiments were carried out by vary-

ing the concentration of copper in the electrolytic

bath, to see its effect on cell voltage, anode potential

and power consumption during electrowinning of

copper in the presence of sulfur dioxide. The concen-

tration of copper was varied in the range 1050

grL. The cell voltage and anode potential values are

plotted against copper concentration in Fig. 3. As

expected, the cell voltage is relatively high at low

copper concentrations, i.e., up to 30 grL and is

almost constant beyond this. The anode potential

does not show any significant change and also nosignificant change in the power consumption ,

.14001424 kW hrt of Cu is observed in the range

of copper concentration studied. The current effi-

ciency is found to be , 98% in this range.

3.3. Sulfuric acid concentration

The effect of sulfuric acid concentration during

copper electrowinning was studied in the range 30

150 grL. The results on cell voltage and anodepotential are given in Table 1. The cell voltage and

anode potential decrease with increase in acid con-

centration up to 150 grL. However, the decreases in

the voltages are marginal. The voltages at the end of

1 h for each sulfuric acid concentration studied are

Fig. 3. Effect of copper concentration on cell voltage and anode

potential. SO concentration, 4.16 grL; H SO concentration, 302 2 4grL; current density, 150 Arm2 ; temperature, 308C; duration of


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Table 1

Effect of H SO concentration on cell voltage, anode potential and power consumption2 4

H SO Cell voltage Anode potential % Current Power consumption Deposit morphology2 4 . . . .grL V V efficiency kW hrt of Cu

a a30 1.62, 1.59 1.52, 1.51 98 1399 Bright, smooth, compacta a50 1.62, 1.56 1.52, 1.51 98 1399 Bright, smooth, compacta a100 1.56, 1.52 1.51, 1.49 98 1350 Bright, smooth, compacta a

150 1.56, 1.52 1.51, 1.48 98 1350 Bright, smooth, compact

Cu concentration, 20 grL; SO concentration, 4.16 grL; current density, 150 Arm2 ; bath temperature, 308C; anode substrate, graphite;2duration of electrolysis, 2 h.

aData obtained in 1 h of electrolysis.

less compared to those at the end of 2 h. The

variation of sulfuric acid concentration has no signif-

icant effect either on current efficiency or power

consumption. Similar results are also reported byw xMishra and Cooper 3 . They electrowon copper

from solutions containing a concentration of sulfuric

acid as high as 170180 grL with only a slight dropin current efficiency.

3.4. Temperature

The effect of temperature during electrowinning

of copper from a solution containing iron and SO 2

Fig. 4. Effect of temperature on cell voltage and anode potential.

Cu concentration, 20 grL; H SO concentration, 30 grL; SO2 4 2concentration, 4.16 grL; current density, 150 Arm2 ; duration of


w xhas been studied by Cooper 9 . He concludes that

temperature plays a major role in deciding the qual-

ity of cathode deposit. In the present study, the

influence of temperature was studied in the range

30608C. Both cell voltage and anode potential de-

crease with increase in bath temperature where the

.fall is sharp beyond 508C Fig. 4 . No variation inthe current efficiency is observed in the temperature

range studied and is found to be 98% throughout.

Fig. 5 shows the effect of temperature on power

Fig. 5. Effect of temperature on power consumption. Cu concen-

tration, 20 grL; H SO concentration, 30 grL; SO concentra-2 4 2tion, 4.16 grL; current density, 150 Arm2 ; duration of electroly-

sis, 2 h; anode substrate, graphite.

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consumption. A significant decrease in the power

consumption is observed with increase in tempera-

ture. The power consumption decreases almost lin-

early with increase in bath temperature. Higher tem-

perature is found to improve the quality of the

deposit. This is in good agreement with the resultsw xreported by Cooper 9 .

3.5. Current density

The variation in current density during copper

electrowinning was studied in the range of 100300

Arm2 and its effects on cell voltage, anode poten-

tial, power consumption and current efficiency were

observed. Fig. 6 reports the effect of current density

on cell voltage and anode potential. The results

indicate that both the cell voltage and anode poten-

tial during copper electrowinning increase with in-

crease in current density. The increase in the cellvoltage and anode potential may be attributed to the

increase in both cathodic and anodic polarisations.

The power consumption is found to increase with .increase in current density Fig. 7 . The current

. 2efficiency remains constant 98% up to 200 Arm

Fig. 6. Effect of current density on cell voltage and anode

potential. Cu concentration, 20 grL; H SO concentration, 302 4grL; SO concentration, 4.16 grL; temperature, 308C; duration2of electrolysis, 2 h; anode substrate, graphite.

Fig. 7. Effect of current density on power consumption. Cuconcentration, 20 grL; H SO concentration, 30 grL; SO con-2 4 2centration, 4.16 grL; temperature, 308C; duration of electrolysis,

2 h; anode substrate, graphite.

and powder deposit is formed when current density

is further increased. This may be probably due to

exceeding of critical current density as reported byw xMishra and Cooper 3 .

3.6. Anode substrates

Anode materials play an important role in thew xelectro-oxidation of SO 8 . In the present work, the2

effects of different anode materials on electrowin-

ning of copper in the presence of SO were investi-2gated. The results are reported in Table 2 both in the

absence and presence of SO for comparison. Fig. 82illustrates the trend in cell voltage for four different

anodes such as: PbrSb, PbrAg, TiIrO and2graphite. In this set of experiments, electrowinning

was carried out at 508C. Since in all the experiments

bath composition, temperature, electrode distance,

current density and cathode material were kept con-

stant, the differences observed in the cell voltage

may be attributed to the nature of different anode

materials. The behaviour of each anode material is

given below.

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Table 2

Effect of anode substrates on cell voltage, anode potential, current efficiency and power consumption

Electrode Cell voltage Anode potential % Current Power consumption Deposit morphology . . .material V V efficiency kW hrt of Cu

In the absence of SO2a aGraphite 1.64, 1.64 1.52, 1.53 98 1412 Bright, smooth, compactaPbrSb 1.74, 1.75 1.66, 1.66 98 1498 Bright, smooth, compacta

PbrAg 1.71, 1.72 1.61, 1.62 94 1536 Bright, smooth, compacta aTiIrO 1.53, 1.55 1.46, 1.47 97 1338 Bright, smooth, compact2

In the presence of SO2a aGraphite 1.50, 1.48 1.45, 1.44 98 1292 Bright, smooth, compactaPbrSb 1.69, 1.65 1.61, 1.59 96 1485 Non-uniform, granular deposit,

adhering to the surfacea aPbrAg 1.68, 1.65 1.63, 1.59 96 1476 Non-uniform, granular deposit,

adhering to the surfacea aTiIrO 1.54, 1.55 1.46, 1.49 97.7 1333 Non-uniform, granular deposit,2

adhering to the surface

Cu concentration, 20 grL; H SO concentration, 30 grL; SO concentration, 4.16 grL; current density, 150 Arm2 ; bath temperature,2 4 2508C; duration of electrolysis, 2 h.

a

Data obtained in 1 h of electrolysis.

The graphite anode shows better performance in

comparison to the other anodes used in this investi-

gation. The cell voltage and anode potential are

Fig. 8. Effect of anode substrates on time-dependence cell voltage

and anode potential. Cu concentration, 20 grL; H SO concentra-2 4tion, 30 grL; SO concentration, 4.16 grL; current density, 1502Arm2 ; temperature, 308C; duration of electrolysis, 2 h; anode

substrate, graphite.

.found to be 1.64 and 1.53 V Table 2 , respectively,

in the absence of SO . In the presence of 4.16 grL2SO , the cell voltage and the anode potential drop to21.012 and 0.945 V, respectively, but they increase

gradually during the electrowinning period up to 1 h .and then remain almost constant Fig. 8 . The in-

crease in both the cell voltage and anode potential

with time may be attributed to the consumption and

escape of SO during the electrowinning process.2The cathode deposits obtained either in the presence

or absence of SO are smooth and bright.2The cell voltages and anode potentials with both

PbrSb and PbrAg anodes are found to be higher

than that with graphite both in the presence and .absence of SO Table 2 . This may be due to higher2

oxygen over-potential with these anode materials. In

the presence of SO , no change in either anode2potential or cell voltage is observed. Fig. 8 shows the

behaviour of cell voltage with time with these an-

odes. The trend in both the anodes is similar to that

of graphite. The cathode copper in the presence ofSO with the anodes is completely different to2graphite. In these cases, the deposits are non-uniform

and granular when SO is present.2The use of Ti as anode substrate is found to be

very unsatisfactory due to passivation of titanium

under the experimental conditions.

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The IrO -coated Ti anode behaves differently to2that of Ti alone. This anode is found to give better

results compared to graphite, PbrSb and PbrAg in

the absence of SO . But no reduction in the cell2voltage or anode potential is observed in the pres-

.ence of SO Table 2 . The plot of cell voltage with2time is completely different in this case compared to

.the other anodes Fig. 8 . Non-uniform granular

deposits are also formed in the presence of SO with2TiIrO as for PbrSb and PbrAg anodes.2

The power consumption during electrowinning of

copper in the presence and absence of SO with2these anodes is given in Table 2. The order of power

consumption in the absence of SO with different2anode materials is PbrAg)PbrSb)graphite)

TiIrO . But in the presence of SO , the order2 2changes to PbrSb)PbrAg)TiIrO )graphite.2

3.7. Polarisation behaiour

The anodic and cathodic polarisation behaviour of

SO during electrodeposition of copper on platinum2

and graphite anode was investigated using LSV.

Figs. 9 and 10 show the anodic polarisation be-

haviour of SO on platinum and graphite electrodes,2respectively. Fig. 11 shows the depolarisation effect

of SO on the kinetics of electrodeposition of copper2on platinum electrode.

3.7.1. Anodic polarisation behaiour of SO2The anodic polarisation behaviour of SO on2

platinum and graphite anodes was studied between

q0.2 and q2 V vs. SCE. Fig. 9 shows the effect of

SO on platinum anode with electrolyte of different2compositions. The oxidation of SO in 30 grL2

.H SO curve b, Fig. 9 shows a large plateau2 4region, i.e., between q0.45 and q0.85 V compared

.to SO oxidation in CuSO curve d or CuSO q2 4 4 .H SO system curve f . The plateau region ob-2 4

tained for curve d is between q0.8 and q1.1 Vand for curve f is between , 0.8 and 1.0 V. These

plateau regions correspond to the formation of sul-

fate ions from sulfite ions which varies with the

Fig. 9. Anodic potentiodynamic curves for electrolytes of different composition. Working electrode, Pt; scan rate, 10 mVrs; temperature, . . .308C. Key: a H SO concentration, 30 grL; b H SO concentration, 30 grL q SO concentration, 4.16 grL; c Cu concentration, 202 4 2 4 2

. . .grL; d Cu concentration, 20 grL qSO concentration, 4.16 grL; e Cu concentration, 20 grL q H SO concentration, 30 grL ; f C u2 2 4concentration, 20 grLq H SO concentration, 30 grLq SO concentration, 4.16 grL.2 4 2

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Fig. 10. Anodic potentiodynamic curves for electrolytes of different composition. Working electrode, graphite; scan rate, 10 mVrs; . . .temperature, 308C. Key: a H SO concentration, 30 grL; b H SO concentration, 30 grL q SO concentration, 4.16 grL; c Cu2 4 2 4 2

.concentration, 20 grLq H SO concentration, 30 grL; d Cu concentration, 20 grL q H SO concentration, 30 grLq SO concentra-2 4 2 4 2 . .tion, 4.16 grL; e Cu concentration, 20 grL; f Cu concentration, 20 grL q SO concentration, 4.16 grL.2

w xelectrolytic composition 10 . The results also indi-

cate that oxidation of SO takes place at relatively2higher potential and is also inhibited in the presence

of Cu2q ions. The increase in current starts at around

q0.4 V for curves d and f whereas for curve bthe same starts at around q0.32 V. The current

decreases as potential increases for curves b, d

and f. This decrease in current may be due to the

formation of PtO, which inhibits the oxidation ofw xsulfur dioxide 10 . The current increases again

.around , q1.34 V curve f when the oxygen

evolution begins. For curve b, the current decreases

to 0 at q1.4 V, whereas for curves d and f some

current is indicated indicating that the reaction .oxidation of SO is not fully inhibited in the2presence of copper ions until oxygen evolution be-

gins.

When carbonrgraphite electrodes are anodically

polarised in a supporting electrolyte, evolution of a

mixture of gases containing O , CO and CO may2 2w xtake place at the electrode surface 11 . The mecha-

nism of formation of these gaseous substances has

not been clarified, however, most studies mention

the anodic charge transfer of H O or OHy prior to2w xgas evolution 12 . Fig. 10 shows the anodic polarisa-

tion behaviour of SO with graphite substrate with2electrolyte of different compositions. It is seen from

Fig. 10 that the anodic current for SO oxidation at2graphite electrode increases and does not show any

inhibition of oxidation reaction as in the case of .platinum substrates curves b, d and f . The current

starts increasing from q0.2 V and continues to

increase with increase in potential. The initial in-

crease in current in this case may be due to self-dif-

fusion of electrons within the substrate followed byy w xthe anodic charge transfer of H O or OH 12 . In2

the present case, a weakly established plateau is

observed in all the electrolyte compositions studied.

Similar observations are also reported by Hunger andw x .Lapicque 12 . In the case of d Fig. 10 , i.e.,

oxidation of SO in CuSO and H SO electrolytes2 4 2 4two plateau regions, i.e., between 0.250.45 and

0.71.3 V are observed. An oxide layer is also

expected to be formed on the graphite electrodew xduring anodic polarisation 10 . However, the experi-

mental results show that the oxide layer formed does

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Fig. 11. Cathodic potentiodynamic curves for electrolytes of dif-

ferent composition. Working electrode, Pt; scan rate, 10 mVrs; . .temperature, 308C. Key: a H SO concentration, 30 grL; b2 4

.H SO concentration, 30 grLqSO concentration, 4.16 grL; c2 4 2 .Cu concentration, 20 grLqH SO concentration, 30 grL ; d C u2 4

concentration, 20 grLqH SO concentration, 30 grLqSO2 4 2concentration, 4.16 grL.

not disturb the SO adsorption on the electrode2w x .surface 10 . For curve b Fig. 10 , i.e., oxidation

of SO in H SO alone the plateau is observed in2 2 4between 0.25 and 1.2 V. The single plateau in the

.case of curves a, b and c Fig. 10 may be due

to simultaneous discharge of OHy ions and SO2 .oxidation. For curves e and f Fig. 10 , i.e., in

CuSO and CuSO q SO , respectively, the increase4 4 2in current is negligible compared to curves a, b,

.c, and d Fig. 10 and no distinct plateau is

observed. It is also observed that SO oxidation is22q inhibited in the presence of Cu curves c, d, e

.and f and also the presence of H SO activates2 4 .SO oxidation curves b and d .2

3.7.2. Cathodic polarisation .Curve a Fig. 11 shows the hydrogen evolution

at a platinum electrode in H SO electrolyte. H2 4 2starts evolving at a potential of 0.45 V vs. SCE.

When SO solution is added to H SO , the H2 2 4 2 .evolution reaction is depolarised curve b , thus

shifting cathode potential to a more positive value,

i.e., y0.3 V vs. SCE. Besides, a plateau is observed

in the potential range y0.55 to y0.65 V. This

plateau may be ascribed to the reduction of H SO2 3w xto S, leading to H S formation 13 according to the2

following reactions:

H SO q 4Hqq 4e S q 3H O2 3 2

EB s 0.45VSHE

S q 2Hqq 2e H S aq . .2

E

B

s 0.141V.SHE

The formation of sulfur and H S was also ob-2served during the experiment. The polarisation for

deposition of Cu is shown in curve c where the

limiting current density plateau for copper deposition

Table 3

Cyrstallographic orientations under various experimental conditions using a graphite anode

. .Cu H SO SO Current density Temperature Peak intensity IrI %2 4 2 02 . . . . .grL grL grL Arm 8C . . . .111 200 220 311

20 30 0 150 30 74 47 100 4320 30 4.16 150 30 100 32 29 25

20 30 8.0 150 30 100 36 21 20

20 30 15.30 150 30 100 22 33 27

50 30 4.16 150 30 100 51 32 30

20 150 4.16 150 30 100 46 30 27

20 30 4.16 200 30 100 36 27 26

20 30 4.16 150 50 100 32 45 27

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starts at a potential of , 0.6 V vs. SCE. When SO2is added to the CuSO q H SO electrolyte, two4 2 4current plateaus are observed, the first one, i.e.,

, 0.800.85 V vs. SCE refers to limiting current

density for Cu2q ion reduction, whereas the second

one, i.e., y0.9 to y1.0 V vs. SCE refers to the

.reduction of H SO curve d . In the presence of2 3SO , the cathodic reduction of copper is depolarised2and black deposits are found to be formed on the

cathode at a more negative potential, i.e., beyond

y1.0 V, which may be attributed to the formation ofw xCuS. Lichusina et al. 14 also observed the depolar-

. . .Fig. 12. SEM photomicrographs of copper deposits. Key: a SO concentration, nil; b SO concentration, 4.16 grL; c SO2 2 2 . . . .concentration, 15.3 grL; d Cu concentration, 50 grL; e H SO concentration, 150 grL; f temperature, 508C; g current density, 1502 4

2 . 2Arm ; h current density, 200 Arm .

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ising action of SO2y during electrodeposition of3copper. In addition, the reduction of H SO is also2 3

. 2qpolarised curve d in the presence of Cu .

3.8. Crystallographic orientations

The copper deposits obtained under different elec-

trowinning conditions are examined by XRD method

to determine the order of preferred crystal planes.

The results are given in Table 3. The preferred

crystallographic orientations of the copper deposit

obtained in the absence of SO are in the orders2 . . . .220 111 200 311 . But when 4.16 grL SO is2added to the copper bath, the most preferred crystal

. .plane changes from 220 to 111 and the order . . . .becomes 111 200 220 311 . Increase in SO2

.concentration 4.1615.30 grL does not change the .crystal growth significantly and the 111 plane re-

mains preferred throughout. The variables such ascopper concentration, H SO concentration, bath2 4temperature and current density do not change the

.habit of crystal growth where the 111 plane re-

mains preferred throughout. However, some minor

change in the order of planes is observed when the

current density is increased.

3.9. Deposit morphology

The morphology of copper deposits obtained from

sulfate electrolyte in the presence of sulfurous acidunder various experimental conditions are shown in

Fig. 12. The surface morphology of copper deposit

in the absence of SO is given in Fig. 12a. Fig. 12b2shows the deposit morphology in the presence of

4.16 grL SO . Addition of SO to the copper bath2 2completely changed deposit morphology. Nodular

growth is observed. Such abrupt change is also

marked from the crystallographic orientations. The . .most preferred plane changed from 220 to 111 .

The presence of SO mainly increase the crystallite2size. When the SO concentration is raised to 15 16

2 .grL Fig. 12c , the size is further increased. The

change in the deposit morphology and increase in the

crystallite size on addition of H SO to the copper2 3electrolyte may be due to the presence of SO2y,3which depolarises the electrodeposition of copper. It

w xhas been reported 15 that during electrodeposition

of copper, the polycrystalline growth is dependent on

w xthe anion present in the bath. Story and Barnes 16w xand Barnes et al. 17 , from their study on growth of

habit of electrodeposited copper, observed that the

deposit morphology of polycrystalline aggregates

changes with the over-potential. Walker and Cookw x y18 also observed that addition of Cl shifts the

morphology of copper deposits. With increasing con-

centration of Cly, the deposit appears to be coarser.w xRouse and Anbel 19 reported that an increase in

Cly concentration in the copper electrolyte decreases

the cathode polarisation. Observations of Rouse andw xAnbel 19 were also supported by Lakshman et al.

w x w x20 . In addition, these authors 20 also reported

change in the texture of the deposits. All this infor-

mation suggests that anions, which depolarise the

copper deposition process, affect the surface mor-

phology by increasing the crystallite size. This is

probably due to inhibition of nucleation of newer

crystallites by surface coverage due to the adsorptionof SO2y and thus the situation favours the increase3in size of the crystallites, which are already grown.

The present results, i.e., cathode depolarisation due

to presence of SO 2y associated with increasing crys-3tallite size, are in agreement with the findings cited

above. At higher copper concentration, the deposit .seems to be more compact and uneven Fig. 12d .

When sulphuric acid concentration in the copper .bath Cu 20 grL, SO 4.16 grL is increased, a2

major change in the deposit morphology is observed

.Fig. 12e . The crystallite size is now decreased butthe size is relatively larger than that with deposit

.obtained in the absence of SO Fig. 12a . The2morphology shown at lower current density, i.e., at

2 .100 Arm Fig. 12g is different to that at 1502 .Arm Fig. 12b and is close to those obtained at

either higher bath temperature or copper concentra- .tion Fig. 12d,f . But when the current density is

raised beyond 150 Arm2, the deposit morphology is .completely changed Fig. 12h and a leaf-type growth

is observed.

4. Conclusion

The electrowinning of copper in the presence of

sulfurous acid was studied. It was found that increase .in SO concentration in the electrolyte 1 decreased2

the cell voltage, anode potential and power consump-

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