Surface Structure of Pb-0.5%Ag Anode Used in ... - ThaiScience

Chiang Mai J. Sci. 2006; 33(1) : 67-77

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Surface Structure of Pb-0.5%Ag Anode Used in ZincSurface Structure of Pb-0.5%Ag Anode Used in ZincSurface Structure of Pb-0.5%Ag Anode Used in ZincSurface Structure of Pb-0.5%Ag Anode Used in ZincSurface Structure of Pb-0.5%Ag Anode Used in ZincElectrowinningElectrowinningElectrowinningElectrowinningElectrowinningSurassawadee PSurassawadee PSurassawadee PSurassawadee PSurassawadee Paliphot [a], Taliphot [a], Taliphot [a], Taliphot [a], Taliphot [a], Torranin Chairuangsri* [a], Noppadol Yorranin Chairuangsri* [a], Noppadol Yorranin Chairuangsri* [a], Noppadol Yorranin Chairuangsri* [a], Noppadol Yorranin Chairuangsri* [a], Noppadol Yottawee[b]ottawee[b]ottawee[b]ottawee[b]ottawee[b]

and and and and and Suwit Uawanichkul [b]Suwit Uawanichkul [b]Suwit Uawanichkul [b]Suwit Uawanichkul [b]Suwit Uawanichkul [b]

[a] Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, 50200, Thailand.

[b] Zinc Refinery Plant, Padaeng Industry Public Co., Ltd., Tak province, 63000, Thailand.

* Author for correspondence, e-mail : [email protected]

Received : 18 April 2005

Accepted : 7 September 2005

AAAAA BSTRACTBSTRACTBSTRACTBSTRACTBSTRACT

Surface structure of the Pb-0.5%Ag plate after used as anode in zinc electrowinning for15 days and effect of pretreatment by dipping the lead alloy plate in a hot acidic potassiumpermanganate solution on the surface structure were studied by scanning electron microscopyand x-ray diffractometry. Formation of compound layers including PbO

n, PbSO

4 and

MnOOH-MnO2 were observed on the lead alloy plate after used in zinc electrowinning for

15 days, with a thickness ratio of about 1:100:900. The observation supports the Pavlov�smodel of Pb/PbO

n/PbSO

4, where 1<n<2, and the ECE reaction model proposed by Paul

and Cartwright for the formation of MnO2 via an insulating intermediate MnOOH. Treatment

by dipping the lead alloy plate in the hot acidic potassium permanganate solution resulted inthe formation of a compound layer containing PbSO

4 and Pb

2MnO

4. This layer could act as

a barrier layer decreasing a dissolution rate of lead ions and reducing the contamination of Pbat the cathode during zinc electrowinning.

Keywords : lead anode, Pb-0.5%Ag, surface structure, zinc electrowinning.

1. I1. I1. I1. I1. INTRODUCTIONNTRODUCTIONNTRODUCTIONNTRODUCTIONNTRODUCTION

Zinc aqueous electrowinning cellscomprise of two kinds of electrodes. One isthe cathode where deposition of zinc metaltakes place on the electrode surface by areduction of zinc ions supplied fromelectrolyte; Zn2+

(aq) + 2e- = Zn

(s). The other is

the anode where mainly an oxidation of water,H

2O

(aq) = 2H+

(aq) + 1/2O

2(g) + 2e-, occurs on

the electrode surface releasing oxygen gas andhydrogen ions to the electrolyte. In the zincrefinery plant, Padaeng Industry Public Co.,Ltd., Tak province, Thailand, aqueous solutionof sulphuric acid has been used in ore leaching.After purification, the leaching solutioncontaining ions of zinc and other impuritiesis fed into the cells. Plates of 99.5wt%

aluminium alloy have been used as thecathodes, while plates of Pb-0.5wt%Ag havebeen used as the anodes [1]. Afterelectrowinning for 36-48 hours, stacks ofaluminium alloy plates are lifted out of thecells and the zinc layer covering on their surfacewas mechanically stripped off as zinc sheets.The zinc sheets are melted and cast into ingots.The aluminium alloy plates are then cleanedand reused. Pb-0.5wt%Ag plates arepretreated by dipping in the aqueous solutionof 5.27x10-2 M potassium permanganate and0.125 M sulphuric acid at 90 oC for 8 hoursbefore using as the anode. This is believed toform a protective layer to prevent thedissolution of Pb into the electrolyte whichwill contaminate with zinc deposition at the

9.0 cm6.0 cm

(a) (b)

(a) (b)

Figure 1. A comparison between the surfaces of the new Pb-0.5wt%Ag plate (a) and theplate after used as the anode in the zinc electrowinning cell for 15 days (b).

68 Chiang Mai J. Sci. 2006; 33(1)

cathode. After using for 10-15 days in theelectrowinning cells, it was found thatunknown products were formed as flakinglayers on the surface as shown in Figure 1. Asa result, the resistance of the electrowinningcells and hence the applied cell potential wereincreased, which is a lost of the electricalpower. Cleaning is therefore needed to removethese products out of the anode surface every

10-15 days.In this work, the surface structure of the

Pb-0.5%Ag anode used in the zinc refineryplant, Padaeng Industry Public Co., Ltd., Takprovince, Thailand, was investigated includingtype of products and thickness of layersformed after pretreatment in the potassiumpermanganate solution and after using as theanode in the zinc electrowinning cells.

2. M2. M2. M2. M2. MAAAAATERIALSTERIALSTERIALSTERIALSTERIALS

Pb-0.5%Ag plates of 90 cm width x 150cm length x 1.27 cm thick were supplied fromthe zinc refinery plant, Padaeng Industry PublicCo., Ltd, in two forms;

(i) as-cast(ii) after used as anode in the zinc

electrowinning for 15 days.

The surfaces of the as-cast plates wereditched to increase the surface area (see Figure1(a)). For a good circulation of the electrolyte,they were punched to make arrays of roundholes with about 2 cm in diameter. After usedas anode for 15 days, layers of compounds

formed on their surfaces, especially at thebottom of ditches and inside holes as seen inFigure 1(b).

3. M3. M3. M3. M3. METHODSETHODSETHODSETHODSETHODS

3.1 Surface Structure of the Pb-0.5%Ag

Plate After Used As Anode in Zinc

Electrowinning

Specimens of 1.50 cm width x 1.50 cmlength x 1.27 cm thick were cross-sectionedfrom the lead alloy plate after used for 15days. They were cold-mounted in epoxy resinby exposing out the cross-sectional surface,as schematically shown in Figure 2. Standardgrinding by silicon carbide papers and

1.50 cm

1.50 cm

1.27 cm

Cold mounting in resin

Figure 2. Schematic of the cross-sectionalspecimen.

Chiang Mai J. Sci. 2006; 33(1) 69

polishing by alumina paste were thenperformed, following by etching withconcentrate hydrochloric acid for 3 minutesat room temperature. Conductive coating wasdone in a gold sputter coater at 18 mA for 30seconds. Specimens were investigated in aJEOL JSM-5910LV scanning electronmicroscope operated at 15 kV equipped withultrathin window, energy dispersive x-rayspectrometer, INCA, Oxford Instrument.

The compound layer formed on theouter-most of the lead alloy plate after usedfor 15 days is dark-brownish in colour. It isquite brittle and fell out from the lead alloyplate as flakes. These flakes were ground bymortar, screened to less than 0.125 mm, andthen heated at 100 oC for 3 hours to get ridof moisture. The powders were finallypressed into the holder and studied by usingan x-ray diffractometer (Philips) PW303/60X� Pert PRO Console with CuKa source at ascanning rate of 0.02 degree per second.

3.2 Effect of Pretreatment on Surface

Structure of the As-cast Pb-0.5%Ag Plate

Square specimens of 1.5 cm x 1.5 cmwere cut from the as-cast Pb-0.5%Ag plateand thinned to a thickness of 1 mm. Thethinned specimen were dipped into a solution

of 5.27x10-2 M potassium permanganate and0.125 M sulfuric acid in distilled water at atemperature of 90 oC for 8 hours. Afterdipping, the specimens were washed in seriesof beakers containing distilled water until thereis no change in the colour of the water,following by rinsing with methanol and dryingin a desiccator. Type of compounds formedon the surface of those specimens was thenstudied by SEM-EDS and XRD.

4. R4. R4. R4. R4. RESULESULESULESULESULTSTSTSTSTS



Electrowinning

Observation was done at the bottomregion of ditches as schematically shown inFigure 3(a). Figure 3(b) shows a backscatteredelectron image of compounds formed on thePb-0.5%Ag plate after used for 15 days atlow magnification (x30). Three different areaswere observed. The corresponding EDSspectrums are also given inset. Area A is thebulk of the lead alloy. EDS spectrum fromarea A contains peaks of Al and O due topenetration of alumina powders into the leadalloy during polishing. The EDS spectrumfrom area B contains peaks of Pb, S, O andZn. This layer could be lead sulfate containingzinc ions from the electrolyte. The thicknessof this layer is 555±124 mm. Finally, spectrumfrom area C contains peaks of Mn, O and S.This layer could be manganese oxidecontaining sulfate ions from the electrolyte.The average thickness of this outer-most layeris 3,743±100 mm. Higher magnification imagein Figure 3(c) was taken from the region withinthe white border in Figure 3(b). It revealed acomparatively thin layers with bright contrast,marked D, between the bulk of the lead alloy(marked A) and the outer layer of lead sulfate(marked B). From EDS spectrum, whichcontains peaks of Pb and O, this layer couldbe lead oxides with the average thickness of4±1 mm. The transition zone, which couldcontain both lead sulfate and lead oxides, wasalso observed as marked T in the figure. Dark,fine particles densely dispersed within the bulk

Pb-0.5%Ag

resin

area of observation

(a)

Lead sulfate

Zn

area marked B

B

C

A(b)

Lead-Silver anode

area marked A

Lead-Silver anode

area marked A

Lead sulfate

Zn

area marked B

Manganese dioxide orHydrate manganese oxide

S

area marked C

Lead oxide

A

B

D

T

(c)

area marked D

Figure 3. (a) Schematic drawing shows the area of observation in SEM at the bottom ofditches on the lead alloy surface. (b) and (c) are backscattered electron images at magnificationx30 and x4,000, respectively, showing layers of compounds formed on the lead alloy plateafter used as anode in zinc electrowinning for 15 days. Corresponding EDS spectrums aregiven inset.

70 Chiang Mai J. Sci. 2006; 33(1)

of the lead alloy are alumina particles penetra-ted during polishing. It should be noted thatthese particles did not penetrate into the areasof lead oxides and lead sulfate due probablyto the higher hardness of these compounds.This gives a beneficial effect that the boundaryof the lead alloy and the lead oxides can be

seen clearly and the thickness of the lead oxidelayer can be measured. Porosity was observedwithin the lead sulfate layer as indicated byarrows in the figure.

The XRD pattern of the compoundfrom the outer-most layer of the lead alloyplate after use in zinc electrowinning for 15

Position (O2 Theta)

MnO2

MnOOH

Figure 4. The XRD spectrum taken from the outer-most compounds of the lead alloy plateafter used in zinc electrowinning for 15 days.

Table 1. A list of corresponding phases and hkl planes of the outer-most compounds of thelead alloy plate after used in zinc electrowinning for 15 days.

2θ (degree) phase hkl planes Bravais lattice18.222326.912528.920837.788742.217650.089755.927260.461365.6824

MnO2

MnO2

MnO2

MnO2

MnO2

MnOOHMnOOH

MnO2

MnOOH

200210310211301222412521004

Tetragonal *

Tetragonal *

Tetragonal *

Tetragonal *

Tetragonal *

Monoclinic **

Monoclinic **

Tetragonal *

Monoclinic **

* Tetragonal (a = 9.748 Å, c = 2.863 Å)** Monoclinic (a = 8.880 Å, b = 5.250 Å, c = 5.710 Å)

Chiang Mai J. Sci. 2006; 33(1) 71

days is given in Figure 4. Most of peaks couldbe matched to the standard spectrums of (i)tetragonal MnO

2 (JCPDS file No. 44-0141)

with lattice parameters a=9.748Å andc=2.863Åand (ii) monoclinic MnOOH

(JCPDS file No. 74-1632) with latticeparameters a=8.880Å, b=5.250Å andc=5.710Å. Corresponding 2θ and hkl planesfrom both JCPDS files are given in Table 1.



Figure 5(a) shows a lower magnificationsecondary electron image of the compoundsformed on the as-cast Pb-0.5%Ag platedipped in the solution of 5.27x10 -2 Mpotassium permanganate and 0.125 M sulfuricacid in distilled water at a temperature of 90oC for 8 hours. A secondary electron imageat higher magnification with correspondingEDS spectrums inset is given in Figure 5(b).Two morphologies of crystals were observed.

One is facetted particles, marked A, containingPb, S and O. These relatively large particlescould be lead sulfate. This was confirmed bythe XRD spectrum in Figure 6, in which someof strong peaks could be matched toorthorhombic PbSO

4 (JCPDS file No. 44-

0268) with lattice parameters a=8.450Å,b=5.380Å and c=6.930Å.

The other is apparently aggregatedcrystals, marked B, containing Pb, Mn, S andO. As can be seen in Figure 6, some of strongpeaks could be matched to tetragonal

(a)

Lead manganese oxide

Lead sulfate

area marked BA

B

area marked A(b)

Figure 5. Secondary electron images at magnification x1,000 (a) and x5,000 (b) shows twomorphologies of crystals formed on the surface of the as-cast Pb-0.5%Ag plate after dippingin the hot acidic potassium permanganate solution. Corresponding EDS spectrums are giveninset.

Position (o2 theta)

KMnO4

PbSO4

Pb2MnO4

Figure 6. The XRD spectrum taken from the compounds formed on the surface of the as-cast Pb-0.5%Ag plate after dipping in the hot acidic potassium permanganate solution.

72 Chiang Mai J. Sci. 2006; 33(1)

2θ (degree) phase planes hkl Bravais lattice

19.57521.02522.17523.27523.92526.62527.72528.07529.72530.27531.22533.27534.87535.82537.27541.42543.67545.07545.77547.57550.97554.77560.67568.42583.225

KMnO4

PbSO4

Pb2MnO4

Pb2MnO4

KMnO4

PbSO4

KMnO4

Pb2MnO4

PbSO4

KMnO4

KMnO4

PbSO4

Pb2MnO4

Pb2MnO4

PbSO4

KMnO4

PbSO4

PbSO4

PbSO4

Pb2MnO4

PbSO4

Pb2MnO4

Pb2MnO4

PbSO4

PbSO4

011200177226002210211311112211321020002510401121122401410422303641801105532

Orthorhombic*

Orthorhombic **

Tetragonal ***

Tetragonal ***

Orthorhombic*

Orthorhombic **

Orthorhombic*

Tetragonal ***

Orthorhombic **

Orthorhombic*

Orthorhombic*

Orthorhombic **

Tetragonal ***

Tetragonal ***

Orthorhombic **

Orthorhombic*

Orthorhombic **

Orthorhombic **

Orthorhombic **

Tetragonal ***

Orthorhombic **

Tetragonal ***

Tetragonal ***

Orthorhombic **

Orthorhombic **

Table 2. A list of corresponding phases and hkl planes of the compounds formed on thesurface of the as-cast Pb-0.5%Ag plate after dipping in the hot acidic potassium permanganatesolution.

** Orthorhombic (a = 8.45 Å, b = 5.38 Å, c = 6.93 Å)*** Tetragonal (a = 12.77 Å, c = 5.142 Å)

Chiang Mai J. Sci. 2006; 33(1) 73

Pb2MnO

4 (JCPDS file No. 76-0437) with

lattice parameters a=12.7Å and c=5.142Å.The results suggest that the aggregated crystals,marked B, could be mainly lead-manganese

oxide. Corresponding 2θ and hkl planes aregiven in Table 2. Thickness of the compoundlayer measured from cross sectional specimenin Figure 7 is about 20 mm.

lead anodecompoundlayer

resin

Figure 7. Secondary electron image shows a cross section of the compound layer formed onthe surface of the as-cast Pb-0.5%Ag plate after dipping in the hot acidic potassiumpermanganate solution.

74 Chiang Mai J. Sci. 2006; 33(1)

5. D5. D5. D5. D5. D ISCUSSIONISCUSSIONISCUSSIONISCUSSIONISCUSSION



Electrowinning

In copper electrowinning [2], lead anodeswere also found to cover with lead and apassive layer of lead (IV) dioxide, but, whenthe cell current was switched off, lead (II)sulfate were spontaneously formed by twocoupling reactions;

−−+→+ ePbSOSOPb saqs 2)(4)(

2

4)( (1)

)(2)(4

2

)(4)()(2 224 aqsaqaqs OHPbSOeSOHPbO +→+++−−+

....(2)These reactions led to anode corrosion,

cathode contamination and financial loss asreported in the case of zinc electrowinning inthe present work.

Wiart et al. [3-5] gave an extensiveliterature review on kinetics of Pb and Pb-Ag anodes that, in lead-acid batteries, first aninsulating layer of PbSO

4, then a conductive

layer of PbO2 were successively formed on

the electrode. Different crystalline phases of

PbO2 (a and b) can be formed. The PbO

2

layer was a result from the oxidation of PbOsublayer so that the PbO sublayer was coverby a non-stoichiometric PbO

n oxide. Pavlov�s

model of Pb/PbOn/PbSO

4, where 1<n<2,

has been developed [6].For anodes used for zinc electrowinning,

Rerolle and Wiart [3] reported that the surfacestructure of the Pb and Pb-Ag anode wereconsistent with the Pavlov�s model. In thiswork, direct observation by SEM-EDS inFigure 3 support the Pavlov�s model of Pb/PbO

n/PbSO

4 and is in agreement with the

previous work [3]It has been reported [3] that Ag was

oxidised and Ag ions were incorporated intothe PbO

n layer where they decreased both the

activation energy and the oxygen overvoltage.They also enhance the formation of b-PbO

2

[7]. However, the results from SEM-EDS inthis study did not found the presence of Agin the PbO

n layer, but this could be due to the

detectability limit of the EDS technique.Porosity observed in PbSO

4 layer in

Figure 3(c) is due to the higher Pilling-

Chiang Mai J. Sci. 2006; 33(1) 75

Bedsworth ratio of PbSO4/Pb than that of

PbOn/Pb. The molar volume ratio of

PbSO4:PbO

2 is 3.8:1 as lead sulfate molecules

are larger than lead (IV) dioxide ones [2].Since electrolyte in zinc electrowinning

contained Mn2+ ions, the formation of aconductive layer of MnO

2 was reported [4,8].

Two possible models have been proposedfor overall deposition reaction of MnO

2;

−++++→+ eHMnOOHMn aqsaqaq 242 )()(2)(2

2

)( (3)

Firstly, Paul and Catwright [9] proposedthree steps of reactions including;

−+++→ eMnMn aqaq

3

)(

2

)( (4)

+++→+

3

)()()(2

3

)( 32 aqsaqaq HMnOOHOHMn (5)

+−++→ )()(2)( aqss HeMnOMnOOH (6)

Reaction (4) and (6) are electrochemicalstep (E), while reaction (5) is a chemical step(C). This model is so called an ECE reactionmodel. MnOOH is a poorly conductive layer.Diffusion of Mn2+ ions through the layer ofMnOOH controls the overall rate of thereaction.

Secondly, Petitpierre et al. [10] proposedthat MnO

2 is formed by a hydrolysis reaction

with steps including;

−+++→ eMnMn aqaq

3

)(

2

)( (4)

++++→

4

)(

2

)(

3

)(2 aqaqaq MnMnMn (7)

+++→+ )()(2)(2

4

)( 42 aqsaqaq HMnOOHMn (8)

In this work, direct observation inFigures 3-4 found the presence of MnOOHwith MnO

2. The results therefore support the

Paul and Cartwright model. Studies ofoxidation of manganese (II) to MnO

2 in

sulfuric acid on Pt [11] and Au [12] electrodesalso support the formation of an insulatingintermediate MnOOH. Mass transfer controlwas confirmed by enhancement of the

oxidation rate with high temperature andstirring [11].



Pretreatment by dipping the as-cast Pb-0.5%Ag plate in the hot acidic potassiumpermanganate solution resulted in theformation of PbSO

4 and Pb

2MnO

4 on its

surface. Figure 8 schematically shows possiblesequence of PbSO

4 and Pb

2MnO

4 formation.

First, lead could be oxidized and lead (II) ionsdissolved out into solution. They were thenprecipitated together with available counterions in the solution, both SO

42- and MnO

4-, as

PbSO4 and Pb

2MnO

4. These products could

act as a barrier decreasing the dissolution rateof lead ions and hence reducing the cathodecontamination as observed in the plantoperation.

6. C6. C6. C6. C6. CONCLUSIONSONCLUSIONSONCLUSIONSONCLUSIONSONCLUSIONS

6.1 Observation of the surface structure ofthe Pb-0.5%Ag plate after used as anodein zinc electrowinning for 15 daysrevealed the formation of compoundlayers including PbO

n, PbSO

4 and

MnOOH-MnO2. The thickness of PbO

n,

PbSO4 and MnOOH- MnO

2 layers are

about 4 microns, 555 microns and 3,743microns, respectively.

6.2 Direct evidences from the observationsupports the Pavlov�s model of Pb/PbO

n/PbSO

4, where 1<n<2, and the

ECE reaction model proposed by Pauland Cartwright for the formation ofMnO

2 via an insulating intermediate

MnOOH.6.3 Treatment by dipping the as-cast Pb-

0.5%Ag plate in the hot acidic potassiumpermanganate solution resulted in theformation of a barrier containing PbSO

4

and Pb2MnO

4 on the surface, which

could decrease a dissolution rate of leadions and reduce the contamination of Pbat the cathode during zinc electrowinning.

(a)

Pb-0.5%Agplate

aqueous solution of potassium

permanganate and sulfuric acid

MnO4

-

SO4

2-

MnO4-

MnO4-

SO4

2-

(b)

(c)

Pb-0.5%Agplate

Pb2+

MnO4

-

Pb2+

Pb2+

Pb2+

MnO4-

SO4

2-

SO4

2-



Pb-0.5%Agplate PbSO

4

Pb2MnO4



MnO4-

Figure 8. Possible sequence of PbSO4 and Pb

2MnO

4 formation on the Pb-0.5%Ag plate.

76 Chiang Mai J. Sci. 2006; 33(1)

AAAAACKNOWLEDGEMENTSCKNOWLEDGEMENTSCKNOWLEDGEMENTSCKNOWLEDGEMENTSCKNOWLEDGEMENTS

The authors would like to thank theElectron Microscopy Research and ServiceCenter (EMRSc) for electron microscopyfacilities. The Department of Physics, Facultyof Science, Naresuan University is thanked forthe XRD facilities.

RRRRREFERENCESEFERENCESEFERENCESEFERENCESEFERENCES

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Science, 1998; 40: 225-234.

[3] Rerolle C., and Wiart R., Kinetics of Pband Pb-Ag Anode for Zinc Electrowin-ning-I. Formation of PbSO

4 Layers at

Low Polarization, Electrochimica Acta,1995; 40: 939-948.

[4] Cachet C., Rerolle C., and Wiart R.,Kinetics of Pb and Pb-Ag Anode forZinc Electrowinning-II. OxygenEvolution at High Polarization,Electrochimica Acta, 1996; 41: 83-90.

[5] Rerolle C., and Wiart R., Kinetics ofOxygen Evolution on Pb and Pb-AgAnode During Zinc Electrowinning,Electrochimica Acta, 1996; 41: 1063-1069.

[6] Pavlov D., Processes in Solid State atAnodic Oxidation of a Lead Electrode

Chiang Mai J. Sci. 2006; 33(1) 77

in H2SO

4 Solution and Their Dependence

on the Oxide Structure and Properties,Electrochimica Acta, 1978; 23: 845-854.

[7] Monahov B., Pavlov D., and Petrov D.,Influence of Ag as Alloy Additive on theOxygen Evolution Reaction on Pb/PbO

2

Electrode, Journal of Power Sources, 2000;85: 59-62.

[8] Wark I. W., Electrodeposition of Zincfrom Acidified Zinc Sulphate Solution,Journal of Applied Electrochemistry, 1979; 9:721-730.

[9] Paul R.L., and Cartwright A., TheMechanism of the Deposition ofManganese Dioxide Part II. ElectrodeImpedance Studies, Journal of

Electroanalytical Chemistry, 1986; 201: 113-122.

[10] Petitpierre J.-Ph., Comninellis Ch. andPlattner E., Oxydation Du MnSO

4 en

dioxyde de manganese dans H2SO

4 30%,

Electrochimica Acta, 1990; 35: 281-287.[11] Nijjer S., Thonstad J., and Haarberg G.

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Surface Structure of Pb-0.5%Ag Anode Used in ... - ThaiScience