minimizing lead contamination copper ew

8/17/2019 Minimizing Lead Contamination Copper EW

1/16

iRIi

s934

Bureau

of

Mines

Report of Investigations

19B5

Minimizing Lead

Contamination

in Copper Produced by Solvent

Minimizing Lead ontamination in opper Prod

U

NITED ST TES DEP RT

MENT OF THE

INTERIOR

~

MINES 75TH A


2/16

Report of Invest

igations

8 934

M i

nimizi

ng Lead Contamination

in Coppe

r Produced

by

So lvent

Extraction

-El

ectrow

inning

By

T. H. Jeffers

and R

D. Groves

UNITED STATES

DEPARTMENT

OF THE INTERIOR

Donald Paul Hodel Secretary

BUREAU OF MINES

Robert C. Horton Director

M Y

0 2 98

Vj J

-

l.

e

,- - -

esear

LIBR R


3/16

Library

of

Congress

Cataloging n

Publication

Data

Jeffers,

T.

H. Thomas

H.

Minimizing lead con taminati

on In

copper

produced by solvent

cx tracti

on -e lectrowinn in g.

Report

of

inve

stiga

tions / United States Dept. of

the

Interior,

l3ureau

of

Mines; 8934)

Bibliography: p . 10-11.

Su pt. of Docs. no.: I 28.23:8934.

1. Copper -E l

ec

t rome

tallurgy.

2. Copper-Lead cont en t. 3 . Ele ctro

l

yte

so

lutions.

4. Anodes-Corrosion.

l.

Groves, R. D. Rees

D.).

II.

Titl e .

III.

Series: Report of

investigations Un

ited

States.

Bureau o f

Mine

s)

;

8934.

TN 23.

U4

3 [TN780] 622 s [669 .31

84-23011


4/16

CONTENTS

Abstract

II :

Introduction

o

,.

Materials equipment

and

procedure

Experimental

resul ts • .

Effect of

cobalt concentration

• • • • • • • • •

co

Effect

of

entrained

organic extractant • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Maximum

current dens i ty

Diluent

variat ions

Effect

of

current density.

copper concentra

t ion

acid

concentration

and

temperature

Effect

of

ele t rolyte impurit ies

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Effect of anode composition • • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Conclusions.

References ••

1 .

2.

ILLUSTR TIONS

Photograph

of

solvent

extraction-electrowinning

unit • • • • • • • • • • • • • • • • • • • • •

Schematic

diagram

of

solvent extraction-electrowinning

unit

• • • • • • • • • • • • • • • •

T BLES

1

Effect

of electrolyte cobalt concentration on

lead content

of electrowon

1

2

3

6

6

6

7

7

8

8

9

9

1

4

5

copper • • • • • • • • • • • . . • . . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . . • • • • • • • • • • • • • • • 6

2. Effec

t

of en

t

rained organic ext

r

ac

t

ant on

lead content

of electrowon

3.

copper • • • • • • • • • • • . • •• • • • • • • • • • • • • • • • • . • . • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • . • 7

Effect of current density

temperature copper

concentration

and

ele tro-

lyte acid concentration on

lead

content of

electrowon

copper

• • • • • • • • • • • • • •

8


5/16

UNIT OF

ME SURE BBREVI TIONS

USED

IN THIS REPORT

A ft

ampere

per

square

foot h

hour

D

degree

Celsius

in

inch

f t

square

foot

pct percent

g

gram

ppm

par t

per mill ion

giL

gram

per l i t r

vol pct volume

percent

gpm ft

gallon per

minute

vol t

per square foot


6/16

MINIMIZING

LEAD

CONTAMINATION IN COPPER PRODUCED BY

SOLVENT

EXTRACTION ELECTROWINNING

By 1 H. Jeffers 1 and

R

D Groves 2

BSTR CT

The Bureau of Mines

conducted

a

l abora to ry

i nves t iga t ion of

copper

e lec t rowinning from e lec t ro ly tes produced

by

solvent

extrac t ion .

The

purpose

of the research was to gain a b e t t e r

understanding

of

the

fundamental

re la t ionship

between

copper

electrowinning

condi t ions

anode corros ion and cathode

pur i ty .

Processing var iables were eva l -

uated

in a

cyc l i c

semicontinuous system

in which the deple ted e l e c -

t ro ly te was used to s t r ip

copper

from loaded organic

ex t rac tan t . For

the

production of high qua l i ty copper

cathodes the e lec t ro ly te was

dosed

with cobal t and s t r ipped of organic

solvent ex t rac t ion reagent .

An

optimum cobal t

addi t ion

of 60 ppm and removal of the en t ra ined

ex t rac tan t

with ac t iva ted

carbon provided

the bes t condi t ions for

e lec t rowinning

copper

conta in ing the l eas t

amount of lead

and

con

t r o l l i ng anode

corros ion. As

littl as

13 ppm of

the en t ra ined

organic ex t rac tan t LIX 64-N s ig n i f i c a n t ly

acce le ra ted

anode

cor ro -

s ion. However the

addi t ion of 1 000 ppm of organic di luent

kerosine

did not af fec t anode corros ion. After cobal t dosing

and

removing the

entra ined

ex t rac tan t

cathodes

conta in ing

l e s s

than

2 ppm

lead

were

cons i s t en t ly

electrowon a t

widely var ied

current

dens i t i e s

e lec t ro-

l y t e

copper

and

acid concentra t ions and temperatures.

Dosing

of the

e lec t ro ly te

with

se lected impur i t ies including

magnesium aluminum

manganese

and i ron

did

not a f fec t

the

cathode puri ty . However 0.05

gi

of chlor ide ions markedly

increased

anode

cor ros ion and

the sub

sequent t r a n s f e r of lead to the cathode copper .

C

hemical

engineer .

2Supervisory

meta l lu rg is t .

Sa l t Lake City

Research Center

Bureau of Mines

Sa l t

Lake Ci ty UTe


7/16

2

INTRODUCTION

The

e l ec t r i c a l

indust ry

i s

the

l a rges t

consumer of ref ined

copper

in

the

United

Sta tes ,

and near ly

75

pct of

the

ref ined

copper

production i s used to make wire.

Elect roref ined , r a the r than electrowon,

copper

i s preferred for wire

drawing be

cause

of

i t s r e l a t i ve l y high

pur i ty .

Impur i t ies adverse ly a f fec t the wire

drawing

cha rac t e r i s t i c s

and product r e

s i s t i v i t y Quant i ta t ive data , however,

re l a t ing impuri ty

l eve l s and

t he i r

e f fec t s are

l ack ing . ASTM

standards

only

speci fy a minimum pur i ty of

99.9

pct

Cu,

s i lver

being counted as

copper (1-2) .3

Tolerable

l eve l s

of other

impur i t i e s -a re

not

spec if ied.

Electrowon copper

i s

deposited

on

cop

per s t a r t i ng sheets

cathodes) from

cop

per bear ing so lu t ions . Insoluble anodes,

genera l ly lead a l loys , cons t i tu te

the

other requ i red

e lec t rodes . H is tor ica l ly ,

copper e l ec t ro l y t e was

obtained

by l each

ing copper

ore , and

soluble impuri t ies

report ing in

the

e lec t ro ly te

contaminated

the deposited copper. Recent

development

of solvent ext rac t ion-e lec t rowinning SX

EW

technology not

only

provided for

con

cen t ra t ing

copper

from

di lu t e

process

streams,

but provided

a

re l a t ive ly

pure

copper e l ec t ro l y t e .

However,

because

lead al loy

anodes

are used, lead

contam

ina t ion of

the electrowon

copper remains

a problem. Although some companies

pro

duce electrowon copper of su f f i c i en t

pur

i t y

and

qua l i ty to be used as e l ec t ro

ref ined

copper

without

fur ther

t rea tment ,

the r e la t ionsh ip

between

electrowinning

condit ions and cathode puri ty has not

been completely determined.

The

Bureau of Mines t es ted a copper SX

EW

process

to ga in a

be t t e r

understanding

of the

fundamental r e la t ionsh ip

among

electowinning condi t ions , anode cor

ros ion , and lead contaminat ion

of

the

cathode copper . The

e f fec t s

of occasion

a l

curren t in te r rupt ions , cobal t

dosing,

3Underl ined

numbers

in

parentheses re

fe r

to i tems in

the

l i s t

of

re ferences

a t

the

end of

t h i s repor t .

organic entrainment,

e lec t ro ly te

pur i ty ,

and

current

dens i ty

were

inves t iga ted,

and the

use of

antimony-lead and calcium

lead

al loy

anodes were compared.

This inves t iga t ion evolved from

a study

of

nat ive

copper

process ing

by

ammoniacal

leaching ,

solvent extrac t ion,

and

e l ec

t rowinning.

Although

leach ing and so l

vent

extrac t ion

were

inc identa l

to

t h i s

s tudy ,

they were

necessary to main ta in

the

e lec t ro ly te copper

content . Although

some electrowinning

da ta

were prev iously

presen ted ,

t h i s report

consol idates the

resu l t s

and presents new f indings i-i).

The

source of

lead

in

electrowon copper

i s the

insoluble par t i cu la t e corros ion

products of

lead

al loy

anodes (5) .

Lead

oxide i s

formed

on

the

anodes, s loughs

off ,

and i s

physica l ly

entrapped

by

the

depos i t ing

copper .

Evidence

of t h i s was

provided in an inves t iga t ion 6) in which

each

anode

was enclosed in a porous fab

r i c bag. Cathodes

conta in ing

only 1 ppm

Ie-ad were

pro

-

du

·ced b-ecause anode corro

s ion products

were re ta ined in

the

bags.

Without

bags,

the cathode lead con ten ts

ranged from

10

to 43 ppm. Anode

bagging,

however,

i s

expensive

and

requi res

p e r i

odic

maintenance.

Although

lead al loy

anodes

are major

cont r ibutors to cathode impuri ty , they

are

used

for

copper

e lec t rowinning

be

cause of

low cost ,

durab i l i t y ,

ease

of

fabr ica t ion, low maintenance,

and exce l

l en t

conduct iv i ty .

The

lead al loys

tha t

have

had widespread use include antimony-

·

lead al loy

conta in ing

3

to

8

pct

ant imo

ny)

and

calcium-lead

al loy with

0.01

to

0.10 pct calcium). Several a l t e rna t ive

anodes

have

been

inves t iga ted,

including

lead al loyed with s i l ve r , t i n , or arsenic

(6-8) . The cor ros ion

res is tance

of some

o f

these

al loys was impressive, but they

were

not

adopted because of t he i r

high

cost .

Titanium

coated with a conduct ing

l ayer of

noble

metal or metal oxide has

been

considered for e lec t rowinning an

odes. In laboratory t e s t s

lead dioxide

on a

t i tan ium

subs tra te was used and very


8/16

littl cor ros ion

was noted

(I).

However,

a f t e r 40

days of

cont inuous use, the lead

dioxide separa ted from the subs t r a t e and

the

anodes became inac t ive .

The

presence of small amounts

of

cobal t

in

the

e lec t ro ly te

decreased the

lead an

ode

cor ros ion

ra te

(5 , 9) and

reduced

the

lead

content of electrowon copper (5 ,

10) . In addi t ion , the

phys ical

form

o f

the

lead oxide corros ion

products

was

a l

te red .

Without

cobal t

in

the e l ec t r o

ly te , the anode sca le

sloughed off ;

with

cobal t , the

scale adhered

to the

anodes.

Apparent ly,

the reduced amount of pa r t i c

u la te lead

suspended in

the e lec t ro ly te

lowered

lead

contaminat ion

of

the depos

i t i ng

copper . Most

commercial

SX EW

plants

add

cobal t

to t he i r e l ec t ro ly te to

take

advantage of

th i s

benef ic ia l

e f f ec t .

3

In SX EW plants ,

the

e lec t ro ly te co n

ta ins

some en t ra ined

extrac tant

from the

solvent ex t r ac t ion operat ion . This en

t ra ined ex t rac tan t

cons i s t s

of

very small

drople ts of

suspended

LIX 64N-kerosine,

as wel l

as some soluble

organic mater i a l .

Studies

on

the

e f fec t

of

en t ra ined

ex

t r ac tan t

on deposited copper determined

tha t

the ex t r ac t an t

was

coalesced

and

concentrated

by

anode gassing and

was

subsequent ly

adsorbed

on the

cathode s u r

face 8) . The

re su l t ing copper was

dark

co lored , granular , and loose ly adherent

to the

cathode . This phenomenon

i s

f re

quently observed in

i ndus t r i a l

tankhouses

and i s

commonly

r e f e r r ed to

as

organic

burn. The

burned

depos i ts are l e ss

pure

than

smooth

adherent

copper deposi t s .

In

tankhouse pract ice ,

coalesced

ex t rac tan t

i s

skimmed

from the e lec t ro ly te ce l l s to

cont ro l

the organic burn.

MATERIALS EQUIPMENT AND PROCEDURE

The

e lec t rowinning i nves t iga t ion was

conducted

in

a semicontinuous

c i r cu i t

u t i l i z ing cyc l i c leaching and

solvent ex

t r ac t ion

to

provide copper-enriched

e lec

t ro ly te .

A photograph of

the

equipment

i s shown in f igure

1,

and

a

schematic i s

shown in f igure 2. Solvent , 5 pct LIX-

64N4

disso lved

in

keros ine,

was

mixed

with leach l iquor in

a

packed column, and

the phases were separated in a s e t t l e r .

The

copper-bearing organic phase (denoted

by

0 in

f igure

2) was then washed with

water the aqueous phase,

An)

to remove

entra ined

ammonia.

Copper

was

s t r ipped

from the

ex t rac tan t

with high-acid

copper-depleted e lec t ro ly te and,

in some

t e s t s ,

entra ined

extrac tant was removed

from the

enriched

e lec t ro ly te using a c t i

vated

carbon.

The

copper

electrowinning

ce l l

con ta in

ed s ix , 6- in

2

lead anodes

3/16

in t h i ck ,

and f ive ,

6- in

2

copper cathodes

0.005 in

th ick . Cathode spacing was 1 in . The

elect rodes

were immersed to

a

depth of

3.5 in ; e f fec t ive elect rode

surface areas

4

Re

ference to spec i f ic

not imply endorsement by

Mines.

products

does

the Bureau

of

for

the anodes and cathodes

were

1.9

f t

2

and

1.5 f t

2

r espec t ive ly . E lec t ro ly te

flow

was 0.1 gpm/f t

2

of

cathode

area .

Cast lead anodes conta in ing 3.6 pct Sb

were used in most of

the

t e s t s , but Ca-Pb

anodes were

also

used. The

Ca-Pb

anodes,

which were machined from

a

commercial

an-··

ode,

contained 0.052 pct

Ca

and

0.01

pct

Sb.

The

t e s t

procedure

cons is ted

of

e l e c

t rowinning for 8-h per iods , and

5

to

8

e lec t rowinning periods were completed

for

each var iable

inves t iga ted .

A cathode

from

one of

the ce l l

posi t ions

was remov

ed.

for

analysis

and replaced with

a

new

s t a r t i ng shee t

a t

the beginning of each

per iod .

Before

e lec t rowinning , the

s t a r t i ng sheets were l i gh t ly coated with

a

lanol in-base

wax

and

deposited

copper

was peeled

off

to

obta in a

sample for

chemical

analysis . After

each

t e s t

se r i e s , the

anodes were

cleaned

of oxide

coating

by

abras ion with

a

wire

brush

to

prevent

the

re su l t s being

biased

by anode

his tory . In some

t e s t s ,

a f t e r several

cathodes

had

been produced

using

a spe

c i f i c

se t of

operat ing condi t ions , the


9/16

4

FIGURE 1 Photograph of solvent extraction electrowinning unit.


10/16

ACID-SETTLING

SECTION

COPPER-STRIPPING

SECTION

ACID-WASHING

SECTI

ON

COPPER-LOADI N

SECTION

r - - - - - - - - - - - - - - - - - ~ - - - - - - - - - - - - - - - - _ - - - - - - - - - - - - - - - - - - - - _ - - - - - - - - - - - - - - - - - - ~ ~ - P r e g n a n t

o

- - - - - - f4 l

A

Sell

ier

o

- - - - - -

A

Sel l ier

So lution

bleed

liquor from

\ leach

/,

;

/ \

Column

contactor

/ 7

/

/

\ \

/ \

/

/,

Settler

Activated-

carbon

column

Organic

surge

tank

Heater

Copper

Return

to leach

FIGURE 2. - Schematic diagram of

solvent

extraction-electrowinn

i

n

unit.

anode corrosion products tha t had s lough

ed off

were

co l l ec ted . Addit ional ly , the

conduct ive adherent

anode

corrosion

layer

was

scraped of f . These corrosion prod

uc ts

and coat ings were dr ied , weighed

and analyzed

to

determine

the amounts and

types

of

anode corrosion

occurr ing

with

various processing

condi t ions .

During the 16-h period when

e lec t rowin

ning was not in progress , ce l l voltage

was reduced to 1.7 V from a normal p la t

ing vol tage

of 2.1

V to

maintain

the

an

odes

in

an anodic

condi t ion . Copper

dep

os i t ion was neg l ig ib le a t 1.7 V. Without

an appl ied e l ec t r i ca l po ten t ia l , par t of

the

anodic

coat ing of lead

oxide

conver t

ed to

lead

su l fa te .

The

loosely adhering

lead su l fa te would

detach

from

the

anodes

and

deposi t with

the. copper when pla t ing

was

resumed

and cause an abnormally high

lead content .

Applying

the vo l tage dur

ing

inact ive

ce l l

t ime prevented lead

su l fa te

format ion and

more

closely

s imu

l a ted

indus t r i a l

prac t ice .

Consis ten t

re su l t s

were

obtained by

dosing

the

e lec t ro ly te

with cobal t and

removing

en t ra ined

ex t rac t ran t p r io r to

elec t rowinning . Cobalt su l fa te was added

to

the

e lec t ro ly te to give a cobal t con

cen t r a t ion of

60

to

100

ppm.

Entra ined

ex t rac tan t was

removed

by e lec t ro ly te

f i l t r a t i o n

through 10- to

40-mesh ac t i

va ted ca r b on. Electrowon copper

with

l e s s

than 2 ppm lead was cons i s t en t ly produced

a t a

current

dens i ty of 16 A/f t2 and a

ce l l voltage

of

2.1 .

Research

eva lua t ion

was

s t rongly

depen

dent upon

the accuracy of the cathode


11/16

lead

An

atomic

of ±l

and

±O.3 ppm

a t

levels of 10 and 1

resul ted

in confidence l imi t s

ppm Pb, ( . ).

EXPERIMENTAL RESULTS

EFFECT

OF

COBALT CONCENTRATION

To

determine

the

ef fec t s

of

cobalt

con

centra t ion

on

anode

corrosion.

s ix ser i es

of

t es t s

were

a t

a

cathode

cur-

rent

of 16 A/f t

2

ce l l vol tage of

2.1 . and of C using Sb-

Pb

anodes.

contained

30

g/L

Cu, 150 H ~ 0 4

and

0 to 1,000 ppm

Co. Results in

tab le 1

show the marked

decrease in both anode and cathode

lead content tha t

resul ted when

cobal t

was added to the cobal t - f ree

No

advantage

was

gained by

cobalt concentrat ion above 60 ppm.

TABLE 1. - Effect of

cobalt concentrat ion

on lead

content of electrowon copper

Pb Anode

content

of

Co.

ppm

electrowon

Cu

ppm

o .......... .... .

38

1.6

20 4.1 .2

40

2.1


12/16

7

TABLE 2. Effect of

en t ra ined

organic ex t r ac tan t on lead content of

electrowon

copper

Extractant

Lead

content

of

ind iv idua l

Cu Average

Pb

in

Anode

cathodes ppm

content

e lec t ro - s p a l l Cath--

Cath- Cath-

l y t e g/40

h ode

1

ode

2

ode

ppm

O• • • • • • • • •


13/16

8

EFFECT

OF CURRENT

DENSITY

COPPER

CONCENTRATION ACID CONCENTRATION

AND TEMPERATURE

To

determine

the

effects of

variat ions

in

operating

parameters, several series

of copper cathodes were electrowon under

a

wide

range of

conditions.

Tests were

made with v r i t i o n ~

in current densi ty,

electrolyte copper

and

acid concentra

t ion,

and

electrolyte

temperature.

Antimony· lead

anodes

were employed in an

electrolyte

containing

100 ppm Co

that

was f i l tered

through carbon

to remove the

entrained

organic.

The

variat ions in

conditions

and tes t resul ts

are

presented

in table

3.

The

data

show

that

low-lead

cathode

copper

can

be produced under a wide range

of

operating

condit ions. With

current

densit ies of 8 to 24 A/ft2 copper concen

t rat ions of 20 to 50 giL acid concentra

t ions

of

50 to 150

giL

and electrolyte

temperatures

of 25° to 45° C 50 cathodes

were

produced

of which only 5 contained

more

than

2

ppm Pb. However when the

electrolyte acid concentration

was in -

creased

to

200 giL the

cathode

lead con

tent

increased

to

an

average

value

of 8.1

ppm. Observations during the l a t t e r t es t

indicated that more

than

normal

amounts

of

anode

spall

were produced.

EFFECT OF ELECTROLYTE

IMPURITIES

The impuri t ies tested and amounts

used,

in

grams per

l i t e r

were 4.5

Mg

1.9 Al,

1.4

Mn

2.8 Fe,

and 0.05

CI. Impurity

selections and amounts

used

were

based

on

analyses

of electrolytes

from

commercial

operations. In addit ion

to the

selected

impurity,

the

electrolyte

contained 30

giL

Cu 150

giL

H

2

S0

4

and

100

ppm

Co.

For each

tes t

s ix to eight cathodes were

produced

with 8 h

of

electrowinning a t

16

A/ft2

of cathode surface.

Calcium-lead

alloy anodes

were

used,

and the electro-

ly te

temperature

was

25°

C.

The resul ts showed

that

aluminum, mag

nesium, and manganese had no effect on

the electrowinning process.

Low-lead

cathodes were

produced

when the electro-

lyte contained these impuri t ies and


14/16

of anode spa l l was col lec ted

a f t e r

40 h

of

electrowinning.

I ron i n the e l ec t ro

l y t e did not

a f fec t

e i t he r anode corro

s ion or the

t r ans fe r

of

lead

to the

ca th

ode hut there

was

a 2.3-pct decrease

in

the curren t e f f ic iency .

EFFECT

OF NODE

COMPOSITION

Calcium-lead

and

antimony-lead

anodes

are curren t ly employed

i n commercial

tankhouses. Therefore

t ~ · ] ser i es of

t e s t s

were conducted to

compare

these an

odes under

various electrowinning condi

t ions .

In

the f i r s t t e s t se r ies

the

e l ec t ro l y t e contained 30 giL Cu 150 giL

Hi304 and 80 ppm Co. A curren t densi ty

of 16 A/ft 2 was used and the elec t rowin

ning temperature

was 25°

C.

Five

ca th

odes containing

1.3 1.0 0.6 0.5

and

0.9

ppm Ph

were

produced with the

ant imony-lead anodes and f ive cathodes

containing

0.5

0.4 0.3

1.0

and 1.0 ppm

Ph were electrowon with the calcium-lead

anodes. These

r e su l t s show tha t

cathodes

with low l evels of lead contaminat ion

were

produced with

hoth se t s of anodes.

The

phys ical cha rac t e r i s t i c s of a l l the

cathodes were

s imi l a r and

only

ins ign i f

i can t amounts of

anode

corros ion

products

were found

a f t e r

each ser i es of t e s t s .

In the second

t e s t

s e r i e s s imilar

electrowinning condi t ions were employed

9

except

tha

e lec t ro ly te

concained 40 ppm

of ent ra ined organic

ext rac t an t which

i s

s imi la r to leve ls

found

i n i ndus t r i a l

e l ec t ro l y t e s . Indiv idual

cathodes ~ -

trowon with

the ant imony-lead

anodes con

ta ined

3.3

1.7

10 .5

1.4

and 21 ppm Pb

for an average of

7.6

ppm

Pb.

Cathodes

obtained with

the

calcium-lead anodes

contained

14 .0

6.8 10.0 0.4 and

4.0

ppm Pb for an average of 7.1 ppm Once

again s imilar

cathodes were

produced

with

each se t

of

anodes.

However

the

amount

of anode spa l l col lec ted

a f t e r

t es t ing

with the ant imony-lead

anodes was

0.7 g but only 0.3 g

was

obtained a f t e r

electrowinning

with

the

calcium-lead an

odes. In th i s

case

the cathode lead

content was not di rec t ly r e la ted to

the

ra t e

of

anode

corros ion.

Microscopic

analyses of the anode

spa l l

from the

calcium-lead anode ind ica ted a

smaller

average par t ic le

s ize when

compared with

spa l l

obtained

with

the ant imony-lead an

odes. Apparent ly the

smaller pa r t i c l e s

remained suspended in

the

e l ec t ro l y t e for

a longer per iod of

t ime

and were

more

readi ly entrapped

by the deposi t ing

cop

per . Thus comparable leve is of cathode

contaminat ion were

obtained with

both

antimony-lead and calcium-lead anodes

but the anode corros ion r a te was lower

with

the

calcium-lead

anodes

and

a

longer

serv ice

l i f e

would be expected.

CONCLUSIONS

The inves t iga t ion showed tha t cobal t

dosing of

e l ec t ro l y t e

from

copper

solvent

ext rac t ion c i rcu i t s

mater ia l ly

re tarded

lead t ransfer

to the

copper

cathodes and

decreased the

anode corros ion

ra t e .

The

optimum cobal t l eve l was 60 ppm Cobalt

addi t ion

a l t e red

the composition of the

conduct ive

coating

formed

on

the

anodes.

With cobalt-dosed e l ec t ro l y t e

the anode

coating

contained 40

pct

Pb0

2

and 60 pct

PbO.

With

coba l t - f r ee e l ec t ro l y t e the

coat ing

contained

85

pct

Pb02 and 15 pct

PbO

The

PbO was a dense adherent coa t

ing whereas

Pb0

2

was a th ick loose coat

ing tha t readi ly f laked offo

The

anode corros ion

ra t e

and

lead

im

pur i ty leve ls i n

the cathodes were

proport ional to entrainment of the LIX

64N

solvent

ext rac t ion reagent . Entra in

ed organic extrac tant also

a l t e red

the

composition of

the

lead oxide coat ing on

the

anode.

With an e l ec t ro l y t e dosed

with cobal t and

containing

18 ppm of or

ganic the anode coat ing contained 55 pct

Pb02

and

45

pct

PbO;

whereas

with organ

i c excluded

the

coating

contained

35 pct

Pb02

and 65 pct PbO Thus

conditions

tha t

favor

the

formation of

PbO on

the

anode also favor low-lead

cathode copper

and decreased anode corros ion.

With two except ions sa t i s f ac t o ry cop

per cathodes were produced over a

wide

range of condit ions and impuri ty accumu

la t ions af te r the e l ec t ro l y t e was f reed


15/16

10

of

organic and

dosed with

cobal t .

Elec

t ro ly te ac id i ty of 200 gi

and

a chlor ide

content of only

0.05 gi

increased anode

corros ion and lead

in the

cathode.

Comparative t e s t s with an e lec t ro ly te

containing 40 and 80

ppm of organic

extrac tant

and

cobal t respect ive ly

showed

tha t anode ( orrosion was greater

with ant imony-lead

anodes than with

calcium-lead anodes. However,

the lead

contamination of the copper cathodes

was

essent i a l ly the same. In t h i s case, the

lead t r ansfer

was

not

in

propor t ion to

the anode corros ion ra te . Microscopic

examinat ion

revealed

tha t the

spa l l

from

the calcium-lead anodes

was smaller s ized

than the

spa l l

from the

ant imony-lead an

odes.

Apparently,

the

smaller

par t i c l es

remained

suspended in the

e lec t ro ly te and

provided

more oppor tuni t ies for entrap

ment

by

depos i t ing

copper.

Thus, the use

of calcium-lead anodes does not

decrease

the

cathode

lead contamination, but a

longer

service l i f e

is expected

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for

Testing and

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Standard

Speci f ica t ions for

Electro ly te

Copper

Wire

Bars,

Cakes,

Slabs,

Bi l l e t s

Ingots,

and

Ingot

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B 5-73

in 1974

Annual Book of ASTM Stan

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Part

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( Including Elec t r i ca l Conductors) . Phi l

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2.

Standard

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Elec t ro ly t i c Cathode Copper. B

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1974 Annual Book of ASTM

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Par t

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1974,

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D.

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Electrowinning

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a t

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16/16

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INT . BU.OF MI N ES P GH . P

A 7

898

minimizing lead contamination copper ew

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