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/!· INSOLUBLE ANODES lN THE ELECTROWINNING OF COPP~R*

I. H. WARREN**

D. WESLEY**

* Trabalho apresentado no II Encontro Nacional

de Tr~tamento de Minirios e Hidrometalurgia

** Department of Metallurgy - University of

British Oolumbia - Canadá

PROGRAMA DE ENGENHARIA METAL0RGICA E DE MATERIAIS

COPPE - UFRJ

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_G-1,50 LU:BLE Ai'iOD€5 I?J TRE ELEC'l'ROWTNNIJ.I-TG O"F COPPER

by

I. H. \>Ja-.rren* and D. Wensley*

Introduction

Interest in the electrowinning of copper has increased in recent years

primarily because of its application in the r_eC:?Very of copper from el,ectro­

lytes obtained from solvent extraction processing of dilute copper-containing

solutions (1). The properties required of an anode for metal electrowL>.!Üng

can be generally defined as (a) Good electrical conductivity, (b) Low o::cy~.,-.1n

overpotential, (c) High corrosion resistance, (d) Mechanical strength.

Whilst these properties are typically required of an anode for the electro­

rN'inning of copper' a process (2) has been <lescribed where depolarization of the O~Jgen evolution reaction is effected by the use of sulphur dioxide,

with the reaction [2] replacing reaction [1] as the anodic reaction.

+ H20 ~ 1/2 o2

+ 2 H + e

2 H2o + so

2 + · so

4 2- + 4 H+ + e

• [1]

[2]

In this specific instance carbon, which is normally unable to meet requirement

(c) in typical electrowinning environments, can serve satisfactorily as an

anode. In general, however, the trend is to seek anode systems for copper

electrowinning which have the increased corrosion resistance required for __ --.

acceptable performance in the highly acid electrolytes produced by solvent

extraction processing (3). ·

The alternative materials available for anodes for electrowinning are

limited both by chemistry and economics and it is for this reason that lead

and its various alloys with, for example, antimony and silver have had such

general application. A technique for decreasing the corrosion of lead anodea

by the addition of small amounts of cobalt to the electrolyte v•• dev.loped

many years ago (4). This technique is able to be used in the recovery of

* Department of Metallurgy, Uni~ersity of British Columbia, Vancouver 8,

Bri tia h Columbia, --Cànada.

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metal co.at,?d ancdes ~oncern;>:; the ro.Le cf the subst~ate m.etnl. Tha o:v-ell

such as to lead tr' the destn~ct:l.on o.( thig film, particularly in regl.ons of

porosity of the p-r,..,ciot~s mete.1 coat:ing, then corroaion and undermining of the

coating may occur~;i)In stdd:t~::!..ou to thi;;; mode of ano.f.l.e destruction a slO":v

direct d:Lssolution of th"'~ -platinum met.ul:s may occur. In summ.ary the

degradation of pr<H~ious meta1 coa.ted -alectrodes ma.y occur by the follo~.~·i.ng

mechanísms:

Ncble metal dissolut1sn due ;:o the presence o:f complexing ~'x:.5.ons or

complexing organic additives.

Noble metal dissolution due to local cooditions of high activity.

Pitting and breakdown c:f the titsmium substrata due to the pra:Jenca

of aggressive aniont'1.

Accelerated corrosion of the tit.anium ?Jubstrata duli to local cooditions

of high acidity or alkalinity,

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of gold, rhodium, pe..lladium, nic:<.el, tin, chromium, and copper .,._ffith ser1ice

lifetimes depending on the particular condition of current d'e.nsity, temper?:?.­

ture, and elactro1yte composition (10,11). Fortunately, sulphate e:ectro-

1ytes are cot'nlllonly employed, with a consequent lower possib:llity cf ccr:· c&~

sion of the nob1e metal or breakdmm of the substrate (12). The ons~t ::cf

pitting in t:!.t:anium occurs at very low potentials in solutiona contain:i.P..,g:

as little as .001 MI- or Br- (13). The presence of sulphate anions in

chloride electrolytea has been sho\vn to inhibit the breakdown of titanium

(14,15). The use of substrate materials, superior to titanium in corrosion

resistance, lms been suggested. Tantalum, niobium, or titanium alloy sub­

strates are possib11ities. For the particular case of copper electroplating,

Warne and Hayfield report that, for an electrodeposited platinum-titanium

anode (50-60 gm./m} loading) operating at 90 A/ft. 2 in 75-100 gpl H2so4 and 50-57 gpl Cu at temperaturas to 40°C, degradation occurred through coat­

ing undermining (10). Ives and Gilbert (16) recently reported rates of

loss from massive platinum anodes of approximately 10 ~g/A hour at 40°C in

150 g/1 sulphuric acid.

The present study is concerned with the behaviour of anodes of platinum­

iridium alloy coatings on titanium in solutions of sulphuric acid and sulph­

uric acid and copper sulphate.

Experimental

Experimenta with 70% platinum/30% iridium-coated titanium electrodes

(prepared by a therma1 decomposition method by Imperial Metal Industries

Ltd. (17)) were conducted in neutral and acid chloride electro1ytes,

sulphuric acid solutions, and a simulated electrowinning electrolyte con­

taining 2M H2so4 + O.SM Cuso4• All experiments were made at 20°C, with

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Experiments :ve:r·e ccndc:'.cted :a.t :1.ncdic :?o+:en.tia.ls between 1.8 and 2.0 ·~

volts (n.iLe., or w1th .:~nodi:: cur:r;::::ú.ts up ~~o 350 :ru\Ím, L.. Electroly.3is :í..n

A ma:rked differer.ce :J..n. the ch,:ang!'l •J'€ the ,üeci:rochem!ca11y active sur-

face area of the a nodes was not~d bet't'7'<:1lt1 the ;;r..:tlphate- '?.nd chlor :l.de-contain-

ing electrolytes.

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cond'l tl:::-n ·\> i 1.c:./r.ve"i ~r:) lc; :'~ :\: ... .:.:...~::

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as can be asc·~rtained ;:;~' comparison "Nith t:;.e appea;cB:nce of the 1.mc.cat:ed

titanium base (Figurs 4) ., Cperation in neutral chlo::cid~ environ~ent pro-

duces a rot!gh, nodclldr grnr,,~~.h in suriac•~ pits (Figure 5). Polariz<1 tic'': if,

copper sulphate electrolyt:~ produced a crystalline deposit on the •el<::·~t::. , ,_.,;:

surface, which was identified as Pb02

(Figure 6). The deposit was. t>.::t:ti:L .. '.:;Üy

adherent, and anodic pola:r:tzation in. ccpper-free 2M H2so

4 failed to si.gr.i­

ficantly remove it. Ho~~ev,:er, treatme·nt for several minutes at open-circu:H:

in lN HCl solution remov;;:::d the Pbo2

f les.ving a surface again characteri}ltic

of a n2v' anode ('Tisually~ electrochemica.1ly and ccmposition-wise).

Coating lasses w~~re detennined for cJ~periments performed in 2M H2so

4,

with and lvithout the addition of O.SM Cuso4

(Figure 7). Lasses in 2M H2so4 were small, and depended on the total time of electrolysis more than the

magnitude of the applied current,.

in. 2 , the coating loss ~.vas '2.4%,

For example, after 97 hours at 350 EA/ ?

After 205 hours at 105 1:fA/in.-, the loss

was 5 .O%. In conper sulpl-.a te-eontaining elec~rolytes~ it wa.s initially

observed that the rate of loss of coating was apparently much higher - at

least six times as much. However, discovery and subsequent removal of the

Pb02

surface deposit showed that the X-ray fluorescence method for determin­

ing coating thickness w-as giving erroneously low results when the Pb02

deposit was present. After removal of the deposit, c.oating lasses "lere

found to be of the same arder as in copper-free electrolyte~

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metal itse:.f. Init!a:i..lf" ''activa'' di;J.::;:Jh1.tion may occur (especially L,n.

complexin;;: media)~ fo1lu~.\1•::!d by "pas::;ive'1 dissolution of surface nob1,::: n!eta1

oxides. 'I'he dissolution ·;,~l1aviou:r is lJ.kcly to change 'N'ith time, a.,'J ~.~

properties of the passt'T2 layers 0:1 no"b:l.e meta la change with potent:L.:· L

Irre•Jersible cxidat:Lm of the t:it:aniu.:n substrate also occurs du:c"..:m~

anodic pob:dzation, ·;qith the quali~:f of the titanium oxide fil:m depende.nt

on th~ eL,~ctTolyte syst.::.<In in çuest:i.,)n, In hal:l..de media, the electrocnemi-

ca.lly act:17;::: surf::v:e a.::::ea -.v-.23 signific.a·:;.dy decreased, ;'!s opposed to the

negligible ~ffect in sulphate electrolytes. It is known that the breakdo·wn

voltage o.ê T:i.0 2 films is signifícantly d:;creased in halide media, and that

sulphate, eonversely, inbibits t:ltaniu:m. pitting, hence it is not unreason­

able te e:xpect diffe:nmt degrees of substrate attack, depend;ing on the

system ir;. question. Th~ morJ_:Jhclogy of the degre.dation of anodes, as

observed by means of scaar:i;:tg rüec:::ron mic:roscopy, showed che formation of

irregular masset·' of titaoJ.um o:.xide i'nodul.es 11 in "pits" on the sur:face o.f

anodes used in ne'.1tral ch1oride el·=ct·::ol::rtes. Ir. sulphuric acid electrolytes,

extended anodic electrolys:ts produced clean pits which ?..xposed the titanium

substrate beneath_t,l}_e noble metal coatiz1g, clearly suggesting that the loss

of noble metal may occur through undermining. Undermining if:! also suggested

in the case of chJ.oride electro.lytes, as the electrical isolation of the

coating (as observed by the decrease of surface area of visually unaffected

anodes) could indicate that a poorly-conducting film forma between the

substrate and parts of the coating. No electrical .isolation of the surface

is observed in sulphate electrolytes.

Anodes used in th.,e electrowinning of copper show similar behaviour to

those used in sulphuric acid 8olution9 having no apparent change in surface

areat and similar corrosion rates. Tne appearance of the Pb02 depoait on

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the elac trolyt8. 1 t l.s not un·reasonable to a:cpect the lead to mi,;::: ate to

the anoda as an anionic. complz:t. The ?bC., crystals are most abundant at oí.

the platinum-rich sites (i. a. cond,-1cting site.s) indicating that tb·air

appearance is indeed an electrochem:tcal phencmenon. The behaviour of the

Pbo2-covered anodes is not significantly differ2nt from. that of Pb02-free

anodes, and hence it is not surprisinz that this phencmenon has not been

reported previously. The maaking of the true platinum values by tbis

deposit may account for the reports of accelerated coating loas, in copper

aulphate me~ia, made by other authora. It is interesting to note that: t:w

production of a titanium-noble metal~laad dioxide anode has been describ~ó

by Antonov and Stepanenko (18), whersby lb02 is deposited anodically over

the noble metal coatings from a lead nitrate-containina solution.

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Po+entia.l v~ lime curvt.s for- galvo.nosto.t1c.. elec+r·oly5iS at 3So rr.Ajin 2 : (A) in 2M HzS04-

(8) in 2M H2 S04 + .5 M Cu.S 04-

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Chcu'19e of Sl.\rfo.c.e are(). vs fi me for po+ennos+o.tl c. pola.l"'iza.+lon ct+ /800mV'(n.h.e.) ti"\ var·.ovs e I e c i-r-clytes a+ 20 °.

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Figure 3: S.E.M. view of anode surface after 500 hr. at 1900 mv (n.h.e.) in

2M H2so

4• Final current density r.vaE; 220 mA/in

2. (1000 x)

Figure 4: S.E.M. view of etched titanium substrate, prior to application of

noble metal coating. (1000 x)

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Figure 5: S.E.H. vie1v of anode which was used in industrial preparation of

chlorate. (1000 x)

Figure 6: S.E.M. view of anode surface after 670 hr at 1800 mv (n.h.e.) in 2M

H2so4 + .SM cuso4 . Final current density was 25 mA/in2 . Surface

deposit was identified as Pbü2

• (1000 x)

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Electrolyte:: 2rv~--H2-S04 ·-1 o 1.900rnV(n.h.e.) · 0 1800 r~N(n.h.e.) • 350~h2.

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----SI ec+rolyte: 2M H2S04-+.5/v1 CuS04

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Chanqe in pla+J~uM surfa.cQ. loadinq üS a r-esul+ of gG~Ivonosi-c.fic and po+ent;os+o.i-lc... elec:l-ro/ysH; ln 2tvt H2S04 , w;+h and L.u·,tho'-á 1-he o.dd1t1on of 0.5M CuS04-.

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ce~:;;.:;.~a j ~ ..:-\:o.':&re!~'?" f.-'Ia7 2 J .31--4.],

Hyvarin;:.n, O., 'f'he Eff~t::·'" ,rf Silvar anil. Co'J.J..'!.\1'.: on the 0:ty3en Zvol::,,:L:;~:~. ~ .. t. Le(:~d à.r:.cd.:::s~

!isr:ren, I .,~:L,, :::·i: :.3.:L ~ 11-.::'i:::J.·l E.lectrooe.:: f.o.':' Cblorato \!ell Op·~art1ci~~ ;:res~~n.t~-d :~'t ~:·.nc~ F.c·u: .. "-~1:·. ~-::·o .!.nt 1~1:-:?..m:lc.s2. lt4~:to.t.Eh·.:.:ci:n~ Ccnf ~ of C~ S._;,(;{) '2d1 ~]., A.I.C.~.s., s~~t. 1931.

I--iopk~.ns:· t,7.,1l.) ~ 1s·~ :a1.~., ~1~~.-JI.:.t:.tot~Ji.t;.~~ins :JÍ Ccpp~~ =::cm Solv,ent htra,':".ti,o·:: EJecr.::olyt.e8 - ?roi-;L~c:m 'll.1.d Po'3s-l.b:i.Liti~:;; in Iat·&n;.:H:3..ona.l Sym,;_;..>O~:i~:ll'.

on H:,;·d":om~:.t;.2.lhtrgy ~ .:L L!YLE. » Chict1go~ J.:Y73~ 127-.L.Sl~~

Cotton~ .J, D1) ~ .:\nod.te ·p:,j1.;J.:rj_zatio::t of ·r:it:?.rLil.Im~ f!l:~:<;;~:L~ Ind .. , Lond~·n.-'11

68-69.

Knodkevicb, S.D.~ el: :1L, Ccn:-rosio:1 Re::rb':ô.n.ce o:f. th.n Tit"!::-lium Sa;:>e under a Plati:num coa.t:Lng Ü! r•üation t:; t:h~ pll o.~ the AnoJ.yta~ :;nektrokhi:mi:r:i ~ .§..~ 1970, 1.35-8 ..

1var.ne 9 r'L.à .• and Hayfield, P .c.s., Dur:,:~h!..lity of l.':itanium Anodes in Electroplatin.g, T:l:''3.ns. Tust. I'futal '!finish:Lng, />5, 1967, 83-92.

Haley ~ A.J. ~ use oi Ii.l2lt~luhüi! .:~u::d2:tary P..:nod(!S, 1!?.C.glei1ard Ind. !'~1:1 .. Bull., ]_, 1967, 157-61; .•

12.. Juchniewicz ~ R., e-:.: al. ~ In.f.luence of Som~ Factor til on Anodic hharviour of Platinizad..T.itaniu:m., Zasz. Nauk. Po:i.:ltach, Gd.anak., Chem., llio .. 20:. 1970, 119-33.

13. Mikhcolova, L.A., et al.~ El!llctrochemi:a1 Millsrie.ti:r of the Titaai.um Base of a Plati1"1ized 'I'ita:nium Anode durio,g l!:lect::olysia of Nat"W:'al Waters, Elektrokhimiya~ 2_, 1973, 825-9.

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18.

Dugdale, I. ar.d Cottcn. J.B.~ '::'h::; Anodic P:üa:rízati_on of T:i.i:,:~~·üu.P. i~ Ealide Solutions, Corr. Sei., 4, 1964, 397-411.

C2.rn.y) N., Behaviour of Platinized Titaniu.'ll A:ncdes in Rive::: \;í::.t~rs" Pr;-;c, J!'ourth Int. Con5. -Het. Corr.) 1969, 721-3.

IY"es, J .G. and Gilbert, J .R.B., :Factors Affecting the Life cf :::robL: Metal Treated Titanit..'1ll Anodes, presented at A. I.M.E. Annual Conierence,. Dallas, Feb. 1974.

Angel, C.H. and Deriaz, 1-i.G., Brit. Pat. 885,819.

Antonov, S.P. and Stepanenko, V.G., Production of Titanium-Palladium (rhodium, platinnm)-lead dioxide-type Anodes and their Corros:icrn Resistance in Sulphate Solutions, Korroz. Zashch. Metal., 1972, 6:'~·71.