A comparative study of the dissolution of nickel and copper in acidified CuSO4-acetonitrile-H2O and CuCl2-NaCl-H2O solutions

--_.:

..

..

.-

-1‘. -‘--- .. -: J. Electron~aL’Chkm,.l68(i984)

.:.

:

.-

i63--;IjB.:1

:

:

.’

-.

163

'EIs&-ierSequdiaS.A.,Lausan~e-Pri&&inTheNetherlands _.

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. .’ A

.COMPARATIVE STUDY OF THE-DISSOLUTION OF NICKEL AND COPPER .IN ACIDIFIED ACETONITRILE-H20 .and .CuCl2-NaCl-H20 SOLUTIONS

CuS04-

: Z.Y..LU,~D.M. School

MUIR and

I.M.

RITCHIE.

of Mathematical and qhysical Sciences,

Western

Aus&?

ia

6150

(Au&-al

Murdoch

University,

Murdoch,

ia)

ABSTRACT The electrochemistry of the dissolution of copper and nickel rotating disc 0 solutions was investiCuS04-AN-H 0 and CuCl -NaCl-H electrodes in acidified zed from el zctroch e&-?cal measurements gated. The rate constants calcula 0 and CUSO~-AN-H2Q indicate that the dissolution .of copper ih both CuCl -NaCl-H diffusion a;id nickel in CuCl -NaCl-H 0 soIutions ar g a71 Cu P II) solutions, controlled. However, the-disgolution’of nickel in acidified CuSO -AN-H 0 solutions was shown to be controlled by the formation of a nicke140xide2fi?m_ Rate constants for nickel dissolution were measured by three methods. Good agreement was obtained between the mixed potential predicted from polarization measurements and that observed in the dissolution process.

These results of two comparable from a segregation

contribute to a fundamental understanding of the reactivity leach systems proposed for the processing of calcines derived of dead roasted cha?copyrite/pentlandi te concentrates _

INTRODUCTION There using

is

an

brine

increasing

solutions

interest

or

in

sulfate

the

leaching

saltsinaqueous

because of the high stability of Cu(I) in requirements has

been

for

carried

nickel

Cg-111

acidic

aqueous

out in

is

known about

in

aqueous

the electrolytic recently

acidic

on the

chloride

solutions the

has

~221

who reported

Fe(III)

in acidified

of

also

been or

for that

the

the

two

investigated

of

AN-H20 solutions

is

of

of

the

copper of

of

energy

low

work

C3-83

and

nickel

in

However,

C12-191.

copper

solutions

Extensive

kinetics

Pang

dissolution

and

passivation

dissolution

of

(AN)

[?,21.

kinetics The

work

electrowinning

systems

copper

electrode

solutions.

electrochemistry

AN so?ution-except

Ritchie

these

recovery

and

acetonitrile

1 i ttl

copper

and

et aZ_ C201 and Couche copper

controlled

metal

by

the

by Cu(I1)

diffusion

e

nickel and or

of

the

oxidant_

cou7d

in

brine

be used

roasted

at

35oOc.

The

to

750°C

solutions leach

and

Cu(II)

sulfate

a chalcopyrite/pentlandite

followed

calcine

by segregation.

consisted

of

mainly

roasting copper

~. 0022-0728/34/$03.00

that

by Lu et aZ- C223. it was repobted

.In an,ear?ier paper chloride

~1984ElsevierSeq~oiaS~.

-in

both Cu(I1)

aqueou‘s

acetonitrile

calcine

which

with metal,

salt nickel

and

.

solutions

had

been

coal

-at

metal,

dead 670-

nickel

.I

164 :

oxide- and magnetite _ copper

recovery

with the least

Both 1 each systems: gave similar

and between l-69% nickel

amount of nickel

leached nickel

most nickel leached temperatures favour A fundamental leach

.‘ _: _ _:.

.~

with-the

leached

resul ts:with’88-95%. The large

copper depended onthe

variation

segregation

-in the

conditions.

with 5% coal at 670 ‘C -and-the It is believd'that: higherafter segregation at 870 ‘C. : the formation of Ni metal rather than NiO. .-.

after- segregation

study of the dissolution

systems was therefore

conditions

.recovery.

.-

of the leaching

of copper and nickel

undertaken to ascertain process

in the two

the mechanism-and-optimum

and to compare the electrochemistry

of

copper and nickel corrosion in chloride and sulfate media, in an attempt to achieve more se1 ective 1caching under specified conditions_ This type of study is also of importance to other metallurgical processes. In the dissolution of copper and nickel iti copper(I1) chloride-brine solutions or in copper(I1)

sulfate-aqueous acetonitrile solutions, the cathodic of copper(I1) and the anodic oxidation of copper(O) and nickel(O) are processes. Accordingly, these half reactions were investigated and

reduction important

compared in typical leach solutions. The diffusion another fundamental parameter which was investigated differ

significantly

concerning

in chloride

and sulfate

the mechanism of the dissolution

constructing

coefficient since it

media.

Valuable

reaction

may also

an Evans diagram C231 from the separate

of copper(I1) is likely to

is

information be obtained

polarization

by

curves.

Mixed potentials predicted from the polarization diagrams were compared with those measured during the leaching reaction and the corrosion currents.were compared with rate constants

determined

by kinetic

measurements.

EXPERIMENTAL Solutions

All

chemicals

used in the preparation

water was purified 40 MS2 cm_

by a double deionizing

analytical

grade reagents.

system and had a resistivity

The of about

Acetonitrile was distilled over KMn04 and the middle fraction was

retained (b.pt. 81 ‘C).

was free

were

of

impurities

This fraction within

showed no UV absorbance

the limits

of detection

above 220 nm and

of gas chromatograph.v_

Electrodes The platinum disc electrode

teflon cylinder diamond paste. adsorbed distilled cycling

and polished

(area 0.196 cm’) to a mirror

was mounted in a 1.5 cm diameter

smooth finish

using 10 nm and 1 urn

The electrode was then degreased with chloroform and cleaned of organic matter by immersion in chromic acid and washing with double water. Any surface films were removed by repeated anodic/cathodic

between 1.6 V to -0.90

The nickel wet-and-dry

V in 0.5 M H2S04 (us Hg2S04/Hg)_

electrode (purity : 99.99%) was prepared by abrasion Carborundum paper and washed thoroughly with deionized

with 1200 water and

:-

: ..

_..

‘. :

.~

:. ..

165

.'

.

it was ;etched

:Alter.natively,solution

for -about---1’.min ‘in. a_n equal -

HNGB-and.H2S04. then Gashed thoroughly

-.volume mixture_ of :concentraied .tiater and the test ._ .-~

.

..::_ :

'.

_-

--thk‘ t&+solution,:

_y-

:-

-

I

just

prior

to use..

1..

with -_.

_~

(purity : .99.9%) was prepared’_ by abrasion with 1200 wetCarborundum paper. : It -was.etched. in a.-solution ‘of 50 volume %

The .cGpper el tiiytrode

and-dry nitric

acid,

then rinsed

-‘The counter- electrode

with deionized

water and acetone.

was a platinum wire-with.

a surface

area of about 20

times that bf the working electrode. ,. For the.Cu(II)

chloride

SHE)was used with a saturated aqueous Cu(II)-acetonitrile

a calomel

system:

reference

electrode

(0.245

V US

For the potassium chloride solution salt bridge_ a Hg2S04/Hg reference electrode (O-655 V

system,

US SHE in water) was used with-a saturated potassium sulfate solution salt bridge_ In each solution, the bridge was connected to a Luggin capillary which was placed with its tip about O-5 mm from the centre of the working electrode_ All potentials in this paper are quoted vs the SHE in water. to facilitqte a were made for the liquidcomparison between the two leach systems. Corrections junction

potential

between aqueous acetonitrile

sulfate

solutions

and a

saturated K2S04 bridge using the values determined by Senanayake and Muir E241. Equipment and technique Polarization curves for the reduction of Cu(II)- and the oxidation of copper and nickel

were measured on a PAR Electrochemical

system Model 170.

Viscosities

relative to water were determined using an Ostwald viscometer. The kinetics of nickel dissolution was determined by chronopotentiometry described

by Barth- et aZ. C251.

An exactly

onto a platinum electrode in an electrolyte -I; Na2S04. 120 g 1-l; NiS04_7H20, 175 g 1 pH =: 5.5. The electroplating

was carried

the platinum electrode

rotating

rate,

was determined

as

was plated

with the following composition: NaCl 25 g 1-l; HSBOS,l20 g 1-l;

out at a current at 400 rpm.

mined by atomic absorption analysis acid) was 97 r 1%. The dissolution dissolution

known anWnt of nickel

density

The efficiency

of

50 A m-

of plating

of nickel after dissolution time.in the leach solutions,

with (deter-

in hydrochloric and hence the

by measuring the sudden change in electrode

potential.

RESULTSAND DISCUSSION Reduction

of Cu(I1)

For Cu(I1)

in- Cl-/H20

and S0,12-/AN/H,0 solutions

dissolutiGn .reactions, the diffusion (D) of Cu(I1) is alfundamental parameter of the process_ Itmay be conveniently.determined:at a rotating-disc electrode by measuring the coefficient

diffusion-controlled

: ’

1 imiting current density -(iL).- as -a..function of rotation speed ‘(w) for the-. .., reduction of Cu(I1) to Cu(1) on platinum, and applying the Levich equation T261:

_..-

..

166

I

-. _... -.

--:’

_.

-.

._ ~. :.:.-.

.: -:

.-

._ _. f--:.z)

,wisinrads-l,: Figure

i

chloride there and

is

shoks :th&t

-there

solution,

and

a clear

departure

in

is -a’ Very most from

good

aqueous.

Levich

relationshi-6

for

acetonitrile&lf&te

1 inearity.:in

the:case-

Cu(II-)--j-n

.solutio& of

low

-but

AN .concentratio.ns.

high.acidity.

40(

10

20

1

30

40

b-q-td4

Fig. I. Effect of rotation speed on Cu(I1) diffusion-controlled current density in AN/H 0 and NaC1!H20 solution at 25 ‘C and Cu(I1) = 0.1 M. Key: (RI)3 M Nah , 1 M HCl; (X)6 K AN, 1 M H2 SD4: (814. M AN, 0.05 M H&4, 0.5 PI Na2S04; (A)4 M AN, 1~M H2S04. .. -:

.’ .. 1.. : ..

-. ‘. ..

acetonifrilk

&der~o& 'ao~e~~le&r&i' t&&f&r' reductjon._ iti.th+potential -.r&ige of 0.45 -‘-0’:15- V; and .that. 6eldw- 0115 V; .a-‘further -reduction-takes place: leading to ammonia and ethane’as.~ final’&oducis:.-

In.this:.workf we &nimid&the effect : of AN adsorption -by Itredting~~the el e&ode. at a- positive potential before eachpotentiodynamid were erratic:’

measurement. ~.

Without- such ~pretreahnent; --the’ results

-.:

obtained .,

The effedt-df Cu(I1) ~
Itis~surpri&r&

that in the-~con~&k~ation range O.OOl---6.i .M Cu(II),

‘it

is the concentratioti-of Cu(II) rather than .the-background -soltition-and speciation of_Cu(II) that has the greatest

the value of D even for the’solutions

,_ -.-

,.

effect

on D:

of similar

There is a clear ionic strength -.

decrease

in

when the Cu(I1)

-..‘; ._.

-.

.

.

.:.

Fig_: 2_ Cu(II). diffusion .coeffi&ents -as:6..functidn of 'Cu(I1)&&entr&on -. (in AN/H&l; -NaCl’/HpO,and Hz0 at..25:‘C). .Key::_-_1_ CuSO$-in: Na2S.O at: pH 2, pi =.1.58, .ref-- [301;_. 2:...CuSO;-.in water.-.refC311:- .31 j 3 M NaC4 ; 1 M HCl;:.U-= 4.3;. -4. .--‘6MAN/H20, 1
concentration.increa&s;. Quickenden.and~Jian~~.~~D,, and.Eversole_&~+$, :-[37]_ .(wh&

~&ults_~~&e

also shown in-cig_'-Z); ;,havereporteda $imilar~.tre_nd~ for_ j :

cu(Iij in'>cjueoussulfate solutions. .Therefore,t-hetioDper.con&tration seems~._.to be a'very~.im@rtant~factor::to‘be considered-when-ma-kingcomparisons.betwee_n~, j -diffusion coefficients in d~fierent.:sclutians;~~

’

‘-.

!-.

.:

1

Diffusion coefficients of Cu(II);.Cu(I)and Fe(II1) in -'aqueous~solutions

containing various concentrationsof.Cl-, AN and acid are listed in Tab1 e'l..., A direct comparison of these values is difficult not only because of changes in the concentration of Cu(I1) but also because of changes in the speciationof Cu(I1) in chloride and sulfate systems. In acidified sulfate solutions, there . . 2will be further changes dependent on the pH and relative concentrationsof SO4 2and HSO;. Copper(I1) forms a weak complex with SO4 ) KI = 4-O C32; and a slightly stronger complex with Cl-, K1 = 4.0, K2 = 4.7 C331. According-to the Eh-pC1 diagram for copper, Cu(I1) primarily exists.as CuC12 at high chloride f concentrations.whereas it exists as CuCl at low chloride concentrations C3L

When the diffusion coefficient of Cu(I1) iscompared with those of Cu(1)

TABLE 1 Diffusion coefficients of copper species at 25 'C_ Electroactive DElectrolyte lo-ID/m2 s-l species 0.1 M CuSO4, 4 M AN, 4,4 Cu(I1) 0.05 M H2SO4. 0.5 M 0.1 M CuSO 6 M AN,Na2S04 4.5 Cu(I1) 0.05 M 0.1 M 0.1 M CuSO4, H2S8;,6 M AN,Na2SO4 1 M H2SO 4-4 Cu(I1) 0 05 M ftSO 6MAN 5.82 0.0002-0.003M CuSO Cu(I1) Cu(I1) 0.05 M CuSO 10 M $4 0 05 M -5 Cu(I1) 0.001 M CuSd;, 0 1 M icSo 62Sg4 7.1 5-54 0-l M CuC12, 3 M-NaC1.21 8 HCl Cu(I1) 0.1 M CuC12, 4 M NaCl, 0.02 M HCl 5.52 0.1 M CuCl2,4M NaCl, 1.0 M HCl 0.02 M CuCl 3 M HCl 0.0002-0.009 M CuCl, 4.0 M NaCl 7.3 0.1 M CuCl, 4-O M NaCl, 1 M HCl Cu(I) Fe(II1) 0_OlMFe2(S04)3, 6.0 M AN, .4.2 1.0 M H2S04

Ref.

this work this work this work ~281 CZOI E281

this work this work c351 c31 c281 r351 c211

and Fe(III), it is found to be much lower than that of Cu(1) .butmuch the same 2+ as Fe(II1). This is because the strongly hydrated ions, CU(H~O)~ and Fe(H20)63+, are large compared with the weakly hydrated Cu(AN)$or CUCKOOfrom species. Furthermore, nitrile molecules are reported to displatie.water the outer solvation .shellof Cu(I1) ion and make it less mobile than in aqueous solutions E341. If allowance is made for the pronounced effect of ion-pairing in concentratedCu(I1) solutions, as well as the influence-of ionic

strength

.: ....:--:

...,,.

_:-

.,._

._

-I.:. .;.:. .;-.-, I

.‘: .

‘.

__.i_. -, ‘._“ .. ..-.-’ >. -.‘..:and Cl_y[ANlconcentration;-:etc; -_ . .I1 . .- .

: ._..I

:~

._

-,

.. .

‘.

:I..

.

_..

_.

, ‘.it -will..-. ..-I .z_:. be.pbserved .that the d.iffusion .!of. Cu-i.II)..det‘~rrmned- in this’.work are surprisingly similar-‘.

. . ~do&&iekts

are inuch”greil:ter~~~ftErendes’ in thereported..values RDE-techniques, and .widely .~differing’ ~&n&ntrations_

difftision

coefficient

of. &13~-~

was -slightly

. 169 .’ :

There

for.Cu.( I) .using:it’ and. f?&r: c353 ‘found that the

lower. than that

?f Fe(CN)i3-

measured by’RDE under the same-conditions and reported a value relative to the . . standard value of Fe(CN)63-. ‘_ This work-~shows that the-diffusion coefficient of Cu(IJ) in our chloride system ii about 515 x iO-lo m2 s-l , which’ls__. slightly of 4.4, x 10-l’ m2 s-l in our. sulfate-AN-H20 system. controlled

conditions,

we wouldianticipate

higher than the mean value Thus, under diffusion-

slightly

faster

leaching

using CuCl2

in brine compared with CuS04 in, aqueous acetonitrile, Anodic dissolution of copper and nickel. Quite different~behaviour can.be.expected for the anodic or nickel in chloride and sulfate media. In dilute chloride known

to form CuCl which can cover the copper surface

reaction. nickel

The formation

_

sulfate

On the

other

solutions

of

hand,

such

an insoiuble

nickel

is

C12-197, whilst

layer

be ieadily

copper passivates

of copper

media,

and limit

chloride

known to

reactions

is

topper

unlikely

with

aqueous current den-

passivated

only at-high

is

the anodic in

sities. It is therefore particularly relevant to compare the anodic dissolution of nickel and copper in strong brine soluti_ons in which Cu(1) is qui.te soluble .as cuc12- and CuC132- 3 and in aqueous acetonitrile-sulfate solutions where nickel passivation‘is likely to -influence the leaching process. Figure 3 shows the potentiodynamic polarization curves for a copper and-a nickel

electrode

solution.

in both aqueous AN-sulfate

solution

and aqueous chloride.

It can be seen that in a 6 M aqueous acetonitrile

solution,

both-

_

nickel and~copper passivate at +0.4 V and +0.7 V, respectively. However, the peak current density -obtained with copper was about 50’times higher than that. obtained

with nickel .and wassimilar

at PH 3as well

as at PH.0 with 1 M H2S04.

The passivation of nickel in aqueous-sulfate solution is caused by the formation of a nickel oxide film and has been widely studied Cl3-191_ The passivation of nickel ism.

in sulfate

solutions.containing

Wi.th regard--to

copper,_as

AN would probably

the current

density

have a similar

‘and concentration

mechanof Cu(1)

at the surface. of the electrode increases, the -concentration of free AN in the double-layer decreases and Cu(Jj becomes .unstable, Hence, the formation of- an insoluble

copper(I1)

cu4(oH)6s04, C361.

salt,Rassive

film

is.possible.

CU~(OH)~SO~or Cu(OH)2arethe

However .at .pH -< 2; passivation

CuSO4.on the anode surface ..,

__.*

is

-At.pH 5 3 and Eh > 300 mV,

thermodynamically likely

stable

species

to be caused by saturation

of.

X371_ .-

Fig3. Anodic potentiodynamic polarisation curves for Cu and Ni electrodes in,AN/H20 and NaCl/H20 at 2.5 OC. Full lines 6 M AN; 1 M H$O4; dashed line 3 M NaCl, I M HCl; scan rate 10 mV S-l; stationary condition. In

practical

O-70

V to

This

is

copper about

systems,

0.2

-

0.3

much more

+0.4

important

is

copper of

about

formation Benari of

in

in

of

to

ratio

than

the

peak

density of

the

the

in

solutions

Cu(II):

potential

after

nickel

leach

of

for

nickel

vary

Cu(1)

in

copper

passivation

is

peak

very

low

active

is

that

in

state

a 3 M NaCl,

even

at

1 M HCl

a current

solution

the

density

at a potential of about +0.2 V with -2 A m . In this case.the passivation of

of

4000

a peak

current

2300

copper

is

a CuCl

et at.

film

[381.

density

60 mV per

-

SO

occurs

(Fig.

solution

passivates

of

0.65

passivation,

passivation

AN-H20-sulfate

from solution.

at

3).

especially

processes. 3 shows

an

the

However,

current

Fig.

still

potentials

unlikely.

passivation

leaching

The measured about

the

the

passivation

current

is

V and

By contrast, rode

negative

passivation

Consequently

the

V according

as

But

will

not

is

much

Tafel decade

discussed

again, occur

slope for

under with

higher

than

within the

by Braun

and

practical

Nobe

leaching

C61,

chloride

concentrations

the

1 imiting

current

100

dissolution

mV of of

the

rest

copper

Hyde

density potential,

electwhile

density

caused

by the

i3J

conditions,

high

Ni

A m-‘,

and this

because of was

in both chloride

kind the

peak

Cu(II). found

to

solutions

be

.

_.

_--..

:

-_

i

_,-, :.

__

. _y

..

(

.-

:.

-.

‘:;

.._:y

:

f.

:

;.

:

;

:

-.

-_:, :

17%

.. .; : and AR-Hz’) ._‘sUl;fate.~sol Utions ;. This- did not differ .over a range’ of AN concen‘trati& .fG& .i ‘_ 14 k and does no~tcompare with a-.Tafel slope of 37-4d I% reported ‘for the anodic .. sol&ions. i35j;. Hhie, sulfate

solu‘tions-

dissolution

of copper

the addi.tion

in deaerated

aqueous sulfate

of even 1ow:concentrations

of AN to

changes the mechanism .and ‘Tafel. slope.

In’pure- water the ‘anodic dissolution .ofcopper is a. two-step process with Cu+‘to CL? being rate dettknin~ng C40; 411.. Eut.from these r<s-it would appear.that the copper(I)-acetonitrile complex is in reversible-equilibrium with copper meta. at the electrode surface; and that the slow step is the mass transfer of ‘.the species Cu(AN),,+- into the bulk of the solution. Since the current density (i). is given by the following Levich equation when the bulk concentration is much less than the surface concentration i

0.62FD

=

-u6

2'3

u2

v

w

CCU(AN)n+l,

(2)

where square brackets denote concentrations, and the subscripts refers to the surface, we can then apply the Nernst equation and obtain the'following relationship: E=E”

+ (RT/F)lni

Thus it

is clear

- (RT/F)ln(0.62FD

from equation

(3)

2'3

v

-we

that a Tafel

l/2

w

slope

.-‘j

) of 59 mV will

be obtained

if the reaction is under diffusion control It also follows from ;j;ation (3) that there should be a linear relationship between log i and 1og w at a fixed potential which will have a slope of 1. As shown in Fig. 4, a linear relationsh i p between log i and log w112 was indeed found, but the slopes are about 0.7 _ 0.8 for sulfate solutions containing AN, and about 0.9 for the chlor de solutions. Hence the anodic dissolution lled

of copper

to a large

extent

in both aqueous AN sulfate

current

slow irreversible rate.

density

solutions

is contro-

by the rate of

from the electrode surface. relationship for the anodic anodic

and brine

the diffusion of the copper(I) complex By contrast, there is no linear log i us log w1’2 dissolution of nickel in these two systems and the

is hardly affected

kinetics

with possibly

by the rotation NiO formation

speed.

governing

This implies the reaction

Fig, 4, Effect of rstatinn speed Qn medic current dansifias of copp@r in AN/H$ and NaCl/H 8. Key: 1. 10 M AN, 1 M H2SQ 3 E = Q.QQ V: 2, I'M ANI 1 M H2$Qq? R = tQ.QQQV; 3, 3 M N&l', 4 M HCl, fi = =Q,Q2 V; 4. 4 M NaCl, Q.Q2 M HCl, E = -Q.QE V. Evans diagram, corrosion

current

and.mixed

potential

The corrosion current density-and mixed po;et-&ialcan be obtained using an Evans Diagram.

Such diagrams,

as shown in Figs.

5 and 6 plot

the

cathodic

polarization curves for Cu(II)/Cu(I) on platinum and the anodic polarization curves of copper and nickel in the solutions without Cu(II);

can be seen

It

that for both copper and nickel in chloride solution, and copper in aqueous ANsulfate

solution,

the dissolution

similar

corrosion

current

rates

densities

at

are

Cu(I1)

various

diffusion

rotation

controlled

with

The slight

speeds.

difference observed is attributed to the variation in the diffusion coefficients of Cu(I1)

in the leach solutions as discussed above.

dissolution of nickel in aqueous AN-sulfate current

density

the presence to 50 ‘C, from about

at

the

passivation

of chloride

the

peak current

800 A m-2

M H2S04,

the

When the

density

for

temperature

nickel

is more complex by temperature, is

increased

and the

acidity

and

from 25 ‘C

in 6 M AN and 1 M H2S04 increases

(Fig. 5) to 2400 A mm2 and nickel changes from a passivated

state to an active state. to 1.0

ions.

solutions

peak is affected

On the other hand, the

Similarly, when the acidity

peak.current

density

increases

changes

about

ten

from 0..05 M H$S04

times

(Fig.

5).

o-

l

-0

-__-_

61

Q2

-____

$3

8.6

\

______._

Cl!%

.-06

($7

E (v) Palarisatlen diagram fsr the dlssslution sf copper and nickel with E:?;I:‘l” B M AN/H Cl CuSQ = 8 1 M; Na SQ = S mva ssl; Q;; t$ H$Qq = 1 M (full lines) erQ.Q5 M (dashed l&v&); scdn rate .

1

3

Polarisation Fig. -6. Cu(I1) -in 3 M NatJ,

diagram 1, M HCl;

for CuCY2

the =

dissolution of copper and nickel with 0.1, M; .-scan rate = 5 mV s-l; 25.oC..

:

: ,174

..

..

-..

bf chloride

The addition

ion also accelerates. the break-down’of. _.__ increases_ the’ peak current density.

.~_ ‘a-rid greatly

filth

~.

.. -. ..

‘. :. : of .nfcke7 is a. very ‘-1. -. 5; active nickel has a

is -of interest to note -that. the mixed potential sensitive indicator of the prdcess. .-As shown in’Fig. It

potential

‘of about O-3-0-4

the diffusion current

controlled

density

V and its anodic-curve

region’

(points

A-E, Fig.

the mixed potential

is too low,

&pass.ive-

crosses’ 5).

the cathodic

Curve in

However, if-the.

.will

shift.to

anodic

the-lower

part of

occurs with a. change of the curve (points PI a.nd P2, Fig. 5j and. passivation about 0.35 V in the value of the mixed potential. The mixed potential of nickel dissolution therefore is a function of the When the nickel disc is rotation speed and the addition of chloride ions. stationary. the value increases passivates (Fig. only at rotation increased to 0.1

measured potential is about 0.35 V, but upon rotation, the’to about 0.72 V within a~few seconds as the electrode-surface 7). With the addition of 0.01 M Cl-, the passivation occurred speeds higher than 600 rpm. When the addition of Cl - was M, no passivation occurred and the mixed potential remained

at about 0.36 V even at very high speeds.

Thus, there by polarization

between the mixed potentials predicted measured experimenta 7y (Fig, 7) _ It is noted that the mixed potentials systems are virtually and anodic lals

reactions

appear constant

of copper

independent

of rotation

are diffusion

controlled.

(rr”. points

Table 2 shows the current

is an excel lent agreement curves (Fig. 5) and those

in both leaching

speed,

because

solution

both the cathodic

As a result, the mixed potentA, B and points F, G in Fig. 5).

densities,

mixed potentials

and Tafel

slopes

of copper

and nickel for several chloride and su7fate solutions_ Both measured va7 ues = and values calculated from the Evans diagrams are included. The good agreement of the data for the dissolution of copper in both leaching solution systemi, and the dissolution reactions

of nickel

are Cu(I1)

AN-sulfate solution temperature (50 ‘C)

in chloride

diffusion

solutions,

controlled.

is further

The dissolution

evidence

that these

of nickel

cou7d become Cu(II) diffusion controlled and in strongly acidic solution.

only

in aqueous

at elevated

Kinetics The dissolution

kinetics

of a-copper rotating

aqueous AN solutionscontainingCu(I1) chloride

solutions

containing

Cu(If)

disc

in various

CZOI and Fe(II1) [3J,

acidified

C217, as well

has been described-and

as in

good agre&ent

between the kinetic constants determined by rate measurements and electrochemical methods has been reported. In this work, the rate constants of nickel dissolution predicted from Evans diagrams were confirmed by kineticmeasurements. The dissolution of nickel in the region of its passiration is complicated by the

presence

of an oxide

film and so the kinetic

measurements.were carried

-out only

WO0-10M

;

I 030-I

c--

-

-:

-----

200

0

Cl-

800

1200

1600

Rotation speed ( r.p.m.)

Fig. 7. Mixed potential of a stirred nickel electrode in 6 M AN 1 M H2SO4, 0.1 M CuSO4 at 25 'C, and in the presence of 0.01 M and O-1 M Cl'_

in the Cu(i1) diffusion controlled region. In both CuC12-brine solutions.and CuSO4-AN-sulfatesolutions, the reaction between nickel and Cu(II)_ions proceeds according to.the following reaction : 2 cu2+ t Ni -f 2Cu+ +

Ni2+

(4)

For a diffusion.controlledreac.tion,which obey& first order kinetics. the concentrationof Cu(I1) wijl initi.allydecrease according to equation ii) : _V$. =

K.A C

(where A = area and C = concefjtration)

(5)

from which it-can be shqwn that : = .-

0.6?nD..

v

-.(i5)

c&-kosion curren_~ decsitiesj Tafel.

Comparison & of lcopper

and nickel.

‘at 25 ‘C

(@(II)_=

&lo&s

Tafel slope Electrolyte

El ectrode~

4 M AN, 0.5 M H2S04, O-5 M Na2SC4. 4 M AN, 1 M H2SOq. 6 M AN, O-05 M H2SO4, 0.1 M Na2S04_ 6 M AN. 1 M H2SOa. 4 M NaCl,

0.02

M HCI.

3MNaCl.

1MHCl.

6 M, 0.05

M H2SO4,

mV per decade

CU ( Ni 1 CU Ni { cu Ni { ,“r 1 cu Ni {-cu

6 M AN, f M H2S04b

mixed

y -.,

602

3

602

3

602

3

607

3

60%

Mixed.potential. V. ‘. measured .0.05 -0.62 0.06 -0.65 0.01 -0.60 O-06 -0.70 0.06 0.27 0.05 O-27

60; 3 -100 i 10 60+ 3 75 f 10 6Oi 3

:: { Ni ;r

0.1 M Na S04.b

and-the

Oil-M; 400 -r-pm).:

potentials

{ .-.-_.:-.. ‘~~~~~~~~ density A m-2

predicted 0.04:‘. .- -0.62 0.07 4.65 0.03 -0.60 0.07 -0.70 0.07 0.26 0.05. 0.29

-0.60

-0.60

OT28

or30

220. c 5a

190 < 5?225 < 5a 220 c 5a 250 250 250 250 300 c10a 300 300

3

apassivated bat

50 OC

l/2 This oredicts a 7inear re'lationshio between K and it, , as in Fia. 8, and !vhich was confirmed by chrononotentiometry’and direct chemical analysis of nickel. Activation energies were found to lie between 11-16 KJ mol-' in-both chloride and sulfate solutions which are typical for many such processes. We could not observe any significant differences between sulfate and chloride solution.

CONCLUSIONS The rate of dissolution of copper by copper(I1) in both aqueous acetonitrilesulfate

solutions

solutions,

and in strong

is control

brine

solutions,

led by the diffusion

and of nickel

of copper(i1)

in strong

brine

to the electrode

surface.

The dissolution of nickel in aqueous acetonitrile-sulfatesolutions is mainly determined ity

by the passivation

(1 M H2S04) and elevated

- 0.1

M Cl-,

the passivating

dissolution

in these

dissolution

of nickel

diffusion.

of the electrode temperatures current

two leach

is sufficiently

solutions.

was likewise

surface.

However,:at

high.acid-

(z-50'C), or in the presence high to allow

In the activated

found to be controlled

of 0.01

normal

condition, by the rate

nickel

the of Cu(I1)

Fig. 8. Comparison of rate according to rotation speed. 50 oc; full line 4 M.NaCl,

ion measurement;

For that

the

dissolution of nickel by various techniquesKey: dashed line 6 M AN, 1 M H S04. Oil M CuSO4, -0.02 M HCl, Oil M CuC12, 25 OC. a 0 by polarisatAAS analysis. •Jq by chronopotentiometry; A A by direct

purpose.‘of

CuS04-H20-AN

selectively-leaching

solutions

NaCl , have

absence

of

selectivity

is

present

as discreet

particles.

from

particles previousiy.

coupled

are

unlikely

the

at

No such

copper

of

ability

possible

copper-nickel to

low

copper

acidity to

using

from

ambient

selectively

leach

CuC12-H20-NaCl

Clearly

there

particles

predominate

and

will due

to

nickel,

it

would

temperature, copper solutions

be selective galvanic

without if

appear

and

the

in

nickel

_

metals

leaching corros;on

the

but

such

in the.double roasted calcines studied

C223

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