Selective Nickel electrowinning

RI 8749

Bureau of Mines Report of Investigations/1983

Selective Nickel ElectrowinningFrom Dilute Electrolytes

By G. R. Smith, W. R. Thompson,and P. E. Richardson

UNITED STATES DEPARTMENT OF THE INTERIOR

Report of Investigations 8749

Selective Nickel Electrowinning

From Dilute Electrolytes

By G. R. Smith, W. R. Thompson,

and P. E. Richardson

UNITED STATES DEPARTMENT OF THE INTERIOR

James G. Watt, Secretary

BUREAU OF MINESRobert C. Horton, Director

,l('|2

*^*

This publication has been cataloged as follows:

Smith, Gerald R., 1940-

Selective nickel electrowinning from dilute electrolytes.

(Report of investigations ; 8749)

Bibliography: p. 20.

Supt. of Docs, no.: I 28.23:8749.

1. Nickel— Electrometallurgy. 2. Electrolytes. I. Thompson, W. R.

(William Richard), 1957- . II. Richardson, Paul E. III. Title.

IV. Series: Report of investigations (United States. Bureau of

Mines) ; 8749.

-W2-M^43~ [TN799.N6] 622s [622'. 348] 82-600223

CONTENTSPage

Abstract 1

Int roduc t ion 2

Experimental 3*" Channel cell 3

Equipment 4

Vj Preparat ion of solut ions 4Synthetic electrolyte 4Leach electrolyte 5

Deposit evaluation 5

Results 6

Synthetic electrolyte 6

Electrochemical studies 6

Deposit evaluation 105 g/1 Ni 2+ electrolyte 101 g/1 Ni 2+ electrolyte 13

Leach electrolyte 13Leaching 13Purification 15Nickel electrowinning 16

Conclusions 18References 20

ILLUSTRATIONS

1

.

Schematic of channel cell and flow system 3

2. Velocity profiles for channel cell 4

3. Top view of channel cell with I-V measuring system 5

4. I-V curves "at several velocities—5 g/1 Ni 2+ ,50° C 7

5. Tafel slopes— 5 g/1 Ni 2+ ,50° C 8

6. Velocity versus limiting current density—5 g/1 Ni 2+ ,50° C 9

7

.

Arrhenius plot—5 g/1 Ni 2+ 9

8. I-V curves at several velocities— 1 g/1 Ni 2+ ,50° C 10

9. Cross section photomicrographs of nickel deposits—5 g/1 Ni 2+ ,

50° C 1110. SEM photographs of nickel deposits—5 g/1 Ni 2+ ,

50° C 1211. Photomicrographs and SEM photographs— 1 g/1 Ni 2+ ,

50° C 1412. Pressure leaching curves for gabbro ore flotation concentrate .... 15

13. Photomicrograph of electrowon copper 15

14. Fluidized bed apparatus for copper removal 16

15. Copper removal rate through fluidized bed of nickel powder 16

*9*

TABLESPage

1. Composition of gabbro ore flotation concentrate 6

2. Nickel electrowinning—dilute leach electrolyte 173. Nickel electrowinning—dilute leach electrolyte, deposit composi-

tion versus electrolyte concentration 184. Nickel electrowinning—dilute leach electrolyte, velocity effect

on deposit composition 18

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

A/cm2 ampere per squarecentimeter

lim micrometer

mm millimeterA/m2 ampere per square

meter Mohm-cm megohm-centimeter

° C degree Celsius m/s meter per second

cm2 /s square centimeterper second

mv/s millivolt per second

ohm-cm ohm-c ent ime t e r

g/1 gram per liter

pet percentg/ton gram per ton

ppm part per millionhr hour

psi pound per square inch

K kelvinrpm revolution per minute

kcal/mo le kilocalorie per moleV volt

kw kilowatt 1

v (SHE) volt, standard hydrogen1 liter electrode

1/min liter per minute wt-pct weight -percent

min minute

SELECTIVE NICKEL ELECTROWINNING FROM DILUTE ELECTROLYTES

By G. R. Smith, ' W. R. Thompson, 2 and P. E. Richardson 3

ABSTRACT

Critical and strategic metals are often present insmall quantities in low-grade domestic ores. When theseores are leached, the resulting solution usually containsthe metals in very dilute quantities. Selectivee lee t r owinning from dilute electrolytes was investigated bythe Bureau of Mines. A metal deposit containing 84 petnickel was electrowon from the leach solution of a complexdomestic ore bulk flotation concentrate originally contain-ing approximately 2 wt-pct nickel. Key to achieving accele-rated deposition rates, in the case of dilute solutions, is

the rapid movement of the electrolyte through the electro-winning cell. Pure, synthetic nickel electrolytes were usedto establish deposition parameters, and to optimize electro-lyte velocity or mass transfer rates. Hydrodynamic ande lee t

r

odepos i t ion parameters for both the synthetic andleach electrolytes are presented and the experimental re-search described. Selective e lee t r owinning appears to offera viable alternative to physical separation methods toobtain separate metal "concentrates" from low-grade ores.

^Supervisory research chemist.•'Chemist

.

-'Supervisory research physicist.All authors are with the Avondale Research Center, Bureau of Mines,Avondale, Md.

INTRODUCTION

Increasing the relative motion of

the electrolyte with respect to theelectrode surface has long been recog-nized as a means by which the masstransport of ions to the electrode canbe increased to allow operation at

higher current densities. Basictheories and early experiments werediscussed by Nernst ( 12) ,^ Brunner ( 4_)

,

Lin, Denton, Gaskill, and Putnam (11),

Eisenberg, Tobias, and Wilke (6), and

Levich ( 10 )

.

High mass transport cells also per-mit electrowinning from dilute solu-tions. The ability to electrowin metalsfrom dilute solutions acquires particu-lar significance as a result of recent

trends to use hydrometallurgical proc-esses as a means to avoid pollutionproblems associated with smelting and to

recover metals from low grade ores by in

situ leaching. Increasing interest hasalso been shown by the electroplatingindustry in conserving metal valuesordinarily lost through "drag out" of

solution on the plated parts. Barkerand Plunkett (2) studied the electro-lytic recovery of nickel at concentra-tions of 0.001 to 0.02 g/1 Ni 2+ in bothplanar electrode and fluidized bed elec-trode systems. Bettley, Tyson,Cotgreave, and Hampson (3) evaluated a

combination forced flow, fluidized bedelectrowinning cell as a method for re-covering nickel from dilute electroplat-ing effluents having concentrations of

0.3 to 1.25 g/1 Ni 2+ . Landau (9_) inves-tigated the distribution of copper ion

^Underlined numbers in parentheses re-fer to items in the list of referencesat the end of this report.

transport rates along planar electrodesat concentrations of 0.32 to 6.3 g/1Cu 2+ and electrolyte flow rates up to

4.6 m/s (Reynolds number (Re) =

100,000). Kovacs (8) studied the masstransfer and hydrodynamic phenomena onplanar electrodes during the electro-winning of copper from 3 g/1 Cu 2 +

solution circulated at 9.2 m/s. Skarboand Harvey (14) and Harvey, Miguel, Lar-son, and Servi (!) applied air agitationto increase mass transport in copper

electrowinning from dilute solutions andshowed that a current density of 32 A/m2

was possible for each gram per liter of

copper in solution.

The principal objectives of the re-search described in the present reportwere (1) to establish the hydrodynamicand electrodeposit ion parameters for

electrowinning nickel in a channel cellutilizing synthetic, dilute (1 to 5 g/1Ni 2+ ) electrolytes, and (2) to evaluateelectrolyte purification procedures re-quired to electrowin nickel in a channelcell from an actual dilute leach elec-trolyte. The latter solution was pre-pared from a bulk sulfide flotation con-centrate of Duluth gabbro ore which con-tained significant concentrations of Cu,

Ni , and Fe ; and lesser quantities of Co,

Pb , Zn, and precious metals. Studieswith this concentrate and related leachsolutions represented an introduction of

the metal recovery process at an earlystage in the mineral processing proce-dure. Successful implementation of a

process to recover metals from dilutesolutions by electrowinning is an alter-native to pyrometallurgical processingand hydrometallurgical solvent extrac-tion stages.

EXPERIMENTAL

CHANNEL CELL3

The channel cell used in this study

is shown schematically in figure 1.

Electrolyte was circulated through the

cell with a 95 1/min positive dis-placement pump from either a 30- or a

225-1 iter-capacity reservoir corre-sponding to the use of either leach or

synthetic, dilute solution. A 400-mm-long, 200-mm-diam cylindrical chamberpreceded the electrolysis channel and

served to minimize turbulence eddiesassociated with the transition from the

32-mm-diam connecting pipe and the

1 , 200-mm-long, 13-mm-wide, 50-mm-highchannel electrolysis section. Thecylindrical chamber contained threesuccessively finer screens (8, 14, and

20 mesh) positioned perpendicular to the

flow to act as manifolds for distribut-ing the flow evenly across the entranceto the channel. A shorter chamber (200

mm) containing no screens was positionedafter the exit from the channel . Each

^obert Associates, Inc., Bowie, Md.

,

assisted in the design and hydrodynamiccharacterization of the channel cell.

end of the channel was tapered and

projected about 75 mm into the adjacentcylinder to further aid in minimizingeddy effects. Electrolyte velocitythrough the test cell was computed fromthe volumetric flow that was monitoredwith a meter in the return section ofthe circuit. The rate of flow wasadjusted with in-line valves as well as

with a bypass circuit that also con-tained a gas venting chamber. Velocityprofiles in the channel were measuredwith a laser-Doppler anemometer (19).Figure 2 shows two velocity profilesmeasured at a cross section 20 hydraulicdiameters from the entrance and at mid-height. They represent linear flowrates of 0.10 and 1.4 m/s correspondingto Reynolds numbers of 3,000 and 43,000,respectively. The corresponding shapeof each curve is similar to that whichis typically obtained under fullydeveloped laminar and turbulent flowconditions. Velocity distributionacross the channel becomes more uniformwith increasing turbulence because of

decreasing viscous effects and increas-ing inertia forces in the bulk solution.

Immersion heater

/Bypass

! i i! I i J

#1Pump /

IS

Gas vent

chamber

- Flowmeter

Screens

EL

Reservoirir/

Electrodes

"Z 5^

_&

Entrance^cylinder

Channel cell >

FIGURE 1. - Schematic of channel cell and flow systerru

Horizontal center of channel -

DISTANCE FROM CATHODE, mm

FIGURE 2. - Velocity profiles for channel cell.

Velocity measured at cross section 20 hydraulic

diameters from entrance and at midheight.

The electrolytic cell was con-structed of 25-mm-thick polyacrylicplastic. The lid was connected to the

cell body with steel reinforced poly-acrylic clamps, and sealed with a

neoprene 0-ring gasket. Materials of

construction for the cylinders and the

bypass chamber were 6-mm-thickpolyacrylic and PVC, respectively.

Quartz immersion heaters (up to 3.75kw) were mounted within the reservoir to

thermostatically control the electrolyteat desired temperatures up to 50° C.

opposite that of the cathode. The

Luggin capillary passed through this

wall and a small hole in the Pb-6Sbanode. Three individual nickelcathodes, 20 mm high and 45 mm long,

were mounted in the sidewall 20, 30, and

40 hydraulic diameters from the channelentrance. This permitted threeindependent potent iodynamic sweepmeasurements or electrolytic tests to be

made simultaneously at a chosen velo-city. Fully developed flow is generallyaccepted to exist between 25 and 75

hydraulic diameters. Three separateanodes were mounted on the opposite walland each had a surface area 50 pet

larger than the cathodes. The edges of

the removable cathodes were sealed witha rubber based filler. Electricalconnection to the cathodes and anodes

was made through 3.2-mm-diam brass rodsthreaded into the back of the elec-trode .

All potent iodynamic sweep measure-ments were made at a position near the

center of the cathode (22 mm downstreamfrom the leading edge) and representedonly an average polarization in view ofthe fact that the convective diffusionprofile changes along the length of an

electrode in a flowing electrolyte.

Electrolysis circuits for the three

electrode pairs consisted of a constantcurrent power supply, a shunt for cur-

rent measurements, and an ampere-hourmeter. Anode-cathode and cathode-calomel potentials were measured withhigh input impedance voltmeters.

PREPARATION OF SOLUTIONS

EQUIPMENT Synthetic Electrolyte

Figure 3 shows a detailed schematicof the electrolytic cell configurationand associated electrochemical measuringsystem. The reference electrode wasconnected via a salt bridge to a 0.5-mm-diam Luggin capillary probe positioned1.3 mm from the nickel 200 alloy cathodesurface. Accurate positioning of theprobe was accomplished by means of a

micrometer mounted on the outside wall

Synthetic dilute nickel electrolyteswere prepared using reagent grade salts

and deionized water ( p =10 Mohm-cm)

.

Nickel sulfate hexahydrate was used to

supply the nickel ions in concentrationsranging from 1 to 5 g/1 Ni^+ . The sup-porting electrolyte consisted of 35 g/1sodium sulfate (Na2S04) and 20 g/1 boric

acid (H3BO3).

Luggin capillary

JL^WV-^r Cathode

:MK\\\\^.^X\\\\<i^_ Anod;

Channel —' Sidewall -

Capillary

positioning device -

"Brass connection

uSalt bridge-

Saturated

/ calomel

'.'KJ

-• *

Potentiostat

Voltage programer

FIGURE 3. - Top view of channel cell with l-V measuring systertit

Leach Electrolyte

To prepare a representative leach

solution, a bulk flotation concentrateof the Duluth (Minn.) gabbro ore obtain-ed from the Twin Cities (Minn.) ResearchCenter was leached following closely the

procedure of Vezina (16). The concen-trate was composed primarily ofpyrrhotite (FeS) and chalcopyrite(CuFeS2), with lesser amounts ofpentlandite (FeNi)qSg. Its elementalcomposition is listed in table 1. The

concentrate was ground to 100 pet minus20 mesh, mixed with a 20 g/1 H2 S04 , 20

g/1 H3BO3, 35 g/1 Na 2S04 solution to a

pulp density of 17 pet solids, and thenleached in an autoclave, under 100 psi

oxygen pressure at a temperature of 110°

C, for 5 to 8 hr. H3BO3 and Na 2 S04 wereutilized in the leaching procedure to

provide supporting electrolyte in subse-quent electrowinning experiments. Theconcentrate and solution were continous-ly mixed during leaching by means of 75-

mm-diam stainless steel paddles rotatedat 575 rpm. Decomposition reactions

that occur during leaching are expressedby equations 1 through 3 (16)

.

The postleaching procedure includedshutting off the external oxygen pres-sure, slowly releasing the internal

autoclave pressure to the vapor pressureof the solution, cooling to -60° C, and

finally disassembling and filtering the

resulting slurry through acid resistant

paper in a Buchner type funnel.

DEPOSIT EVALUATION

Cathodes were prepared for deposi-

tion by polishing with 240-grit alumina

cloth, rinsing with water, wiping with

ethyl alcohol, and drying in air. At

the completion of an electrolysisperiod, nickel deposits were removedfrom the cell, washed in water, airdried, and weighed to obtain currentefficiency values. For physical

(FeNi) 9 S 8+ 4.50 2

+ 9H2S04 + 4.5NiS04 + 4.5FeS04 + 8S° + 9H2 (1)

FeS + 0.50 2 + H2S04 ->• FeS04 + S° + H2 (2)

CuFeS 2+

2+ 2H

2 S04 + CuS04 + FeS04 + 2S° + 2H2 (3)

evaluation, the deposits were strippedfrom the cathode and examined usingphotomicrography and scanning electronmicroscopy (SEM). For determining grainstructure and the sizes and distribu-tions of voids, cross sections were cut

from the geometrical center of the de-posits, mounted in clear plastic, sandedto a 600-grit surface, polished with 20-

then 5-)jm alumina powder, and finallyetched at 50° C in a sulfuric acid-hydrogen peroxide solution for 5 to 10

min. Photomicrographs were obtained at

magnifications up to 600. SEMexaminations were used to evaluate the

uniformity of deposit growth,crystalline size, and evidence of the

initiation of nodular formations.

TABLE 1. - Composition of gabbro oreflotation concentrate

Component, wt-pct

:

Concentration

Iron 37.9Sulfur 26.6Copper 12.1

SiO 10.0A1

2 33.7

Nickel 2.1CaO 1.7MgO 1.7TiO 15

Cobalt 11

Lead 10

Zinc 06Manganese .04

Cadmium .02

Component, g/ton:

Silver 30.51Palladium 2.40Gold 68

Platinum 31

RESULTS

SYNTHETIC ELECTROLYTE

Electrochemical Studies

In order to determine the effect of

electrolyte flow rate on the electro-chemical parameters for nickel deposi-tion from dilute solutions, slow (5

mv/s) potentiodynamic sweeps from theopen-circuit potential to approximatelythe hydrogen evolution potential werecarried out at a number of flow velo-cities between and 2.1 m/s. The elec-trolytes contained either 5 or 1 g/1Ni^+ , and the temperature was 50° C.

Immediately prior to the potentio-dynamic sweeps, a thin coating of nick-el was deposited on the blank nickel 200

alloy cathode to ensure that the result-ing polarization curves would be repre-sentative of Ni-Ni^"1" deposition reac-

"Experimental data were obtained byCarl Goldsmith, engineering technician,and William Kolodrubetz, physical sci-ence aid, Avondale Research Center,Avondale, Md.

tion. Following this procedure, the

open-circuit potential still remainedsomewhat unstable, varying betweenand 0.1 v (SHE). This instability canprobably be attributed to the lack of a

truly reversible equilibrium for the Ni-Ni2+ couple and to the existence of

other electrochemical reactions such as

Ni02 + 4H+ + 2e~ = Ni 2+ + 2H2 (4)

and 2 + 4H+ + 4e_

=£ 2H2 0, (5)

so that the open-circuit assumed a mixedpotential value.

Potentiodynamic current-voltage (I-

V) curves for a nickel electrolyte con-taining 5 g/1 Ni 2+ are shown in figure

4. Similar results were obtained at all

three electrode locations along the

length of the cell. Each curve has beencorrected for the resistive voltage ( IR)

component of the electrolyte using a

measured resistivity of 19.53 ohm-cm and

a Luggin probe distance of 1.3 mm.

1,500

1,200

900

600

300

-0.4 -0.6 -0.8

CATHODE POTENTIAL, v (SHE)

FIGURE 4, - l-V curves at several velocities (meters

per second)-5g/l Ni 2+, 50 uCt Letter designations in-

dicate hydrodynamic and electrodeposition parameters

for nickel deposits shown in figures 9 and 10.

The onset of nickel deposition occurs at

0.65 to 0.75 v. The thermodynamicpotential for nickel deposition at this

concentration is =-0.31 v (5_) , and the

relatively large overvoltage (0.34 to

0.44 v) can probably be attributed to

a large activation overvoltage (2).

The I-V curve for zero flow exhi-bits a well-defined limiting currentdensity (ij) of approximately 100 A/m^extending from --0.65 to -0.9 v as

expected for deposition under diffusionlimited conditions. At potentials morenegative than -0.9 v, the rise in

current density corresponds to hydrogenevolution, as evidenced by visible

gaseous evolution at the electrode sur-face .

With flow rates in the laminar range(0.05 and 0.10 m/s), somewhat less

defined plateaus are observed. Aboveflow rates of 0.10 m/s, the currentdensity over the diffusion limitedregion develops a finite slope that

increases with increasing velocity.Finite slopes over this region alsohave been observed for copper depositionin channel type cells (9, 13). This

characteristic of the diffusion limited

region has been attributed to a largercurrent density (thinner diffusionlayer) towards the leading edge of the

electrode than towards the trailingedge. Under controlled potential con-ditions, this current density gradientalong the electrode obscures the plateaunormally associated with a diffusionlimited current.

Analysis of the I-V curves using the

relationship n = a + b log i (where ri is

overvoltage, a and b are Tafel con-stants, and i is current density) for

the 5 g/1 Ni2+ electrolyte at velocitiesof 0.85, 1.4, and 2.1 m/s (fig. 5)

yields a Tafel slope (b) of - 0.13, anexchange current density (ig) of -5 X10~-> A/cm , and a transfer coefficient(a) of =0.24. These values arereasonably close to values reported in

the literature (18).

At the onset of the diffusion limit-ed region (fig. 4), the deposition rate

is controlled by the flux of nickel ions

to the electrode surface. This flux in-

creases with increasing velocity owingto a thinning of the diffusion layerand, thus, increases the range of cur-rent densities where nickel can be depo-sited. At low velocities, in the rangeof laminar flow, the limiting currentdensity (ij) an<i the diffusion layerthickness (6) for deposition onto a

plate electrode at a position x from the

leading edge of flow are given (1) by

iL(x) = (l/3)ZFU

1/2v-

1/6D2/3

Cx-1/2

(6)

and 6(x) = 3x1/2

tf1/2

v1/6

D1/3

, (7)

UJ

O<l-_io>UJ>O

0.6

.5

.4

.3

.2

.1

II

i

i

KEY

— • 0.85m/s

1.40 m/s

2.10m/s

•/A

yI

-5 -4 -3 -2 -1

LOG i, A/cm 2

FIGURE 5. - Tafel slopes-5 g/l Nj2+, 50° C.

where U is the velocity of the electro- mental values of ii( x ) were found to

lyte parallel to the electrode plane at depend on U*'^ and to extend signi-an infinitely large distance from it, v

is the kinematic viscosity of the elec-trolyte, D is the diffusivity, C is the

concentration of nickel ions in the

bulk solution, Z is the number of elec-

ficantly into the turbulent region.

Using equation 6 and the slope

obtained from figure 6, the diffusivityvalue for the nickel ion was calculated

trons transferred, and F is the Fara- to be 3.7 X 10"^ cm^/s. A diffusivityvalue of 3.9 X 10"^ cm^/s was obtainedusing a rotating disk electrode, furthersubstantiating the reliability of the

experimental iT(x) values even though

clearly defined limiting currentplateaus were not observed with increas-ing velocity.

day constant.

Figure 6 shows a plot of theexperimental ij( x ) values versus U^'^where the ij(x)'s represent those ob-tained at the onset of the diffusionlimited region (fig. 4). The laminarand turbulent flow regions are definedin terms of Reynolds number (Re) which In another series of potent io-

is related to factors such as cell dynamic sweeps with the 5 g/l Ni^+

electrolyte, the activation energy (E^)

for the deposition of nickel was deter-mined from the I-V curves at 30°, 35°,

and 40° C using a velocity of 0.85 m/s.

The effect of temperature on the rate(i) of an electrochemical reaction is

expressed by the Arrhenius equation

geometry, flow rate, and kinematicviscosity of the solution. Re valuesunder 3,000 are generally considered to

be indicative of laminar flow, whereasRe values above 11,000 are consideredto be characteristic of fully turbulentflow. A mixed transition region existsbetween these two values. The experi-

ln i = (EA-ZaHF)/RT, (8)

REYNOLDS NUMBER

11,000 43,000

FIGURE 6. - Velocity versus limiting current den-

sity-5 g/l Ni 2+, 50° C

and E» must be determined at constantovervoltage (n). An Arrhenius plot (In

i verses 1/T) at a constant overvoltageof 0.41 v, assuming a shift of -0.01 vin reversible electrode potential from30° to 40° C (5), shows the expectedlinear relationship (fig. 7) and yieldsan activation energy of =19.8 kcal/mole.A value of 21 kcal/mole has been re-

ported for a 0.5M NiS04 electrolyte uti-

lizing reversible potential data ob-

tained at 25° and 45° C U8)

.

Potent iodynamic I-V curves for a

nickel electrolyte containing 1 g/l

Ni 2+ with a thermodynamically reversiblepotential of =-0.33 v (_5) are shown in

figure 8. Similar results were againobtained at all three electrodelocations. Each curve was also correct-ed for the IR component of the electro-lyte (resistivity = 18.80 ohm-cm). Awell-defined limiting current densityplateau was observed only at zero flowand the onset of nickel deposition was

3.14 3.343.24

1/Tx10 3, K

FIGURE 7. - Arrhenius plot-5 g/l N i 2^ 50° C.

essentially in the same voltage range as

for the 5 g/l Ni 2 + electrolyte.Analysis of the I-V curves for the 1 g/l

Ni 2+ electrolyte at several velocitiesyielded a Tafel slope of 0.13, an

exchange current density of - 3 X 10-^

A/cm2, and a transfer coefficient of

"--0.24.

The Tafel region extended to currentdensities of 1,400 and 200 A/m2 for 5

and 1 g/l Ni 2+ electrolytes, respec-tively, using the maximum flow rate of

2.1 m/s. Accuracy of the exchangecurrent density value determined for

these electrolytes is limited by the

accuracy of the Tafel slope and the

reversible potentials at each Ni 2+ con-centration. There is some uncertaintyin the Tafel slope because of theexistence of some diffusion control and

codeposition of hydrogen in the Tafelregion for nickel deposition.

10

CATHODE POTENTIAL, v (SHE)

FIGURE 8, - l-V curves at several velocities (meters

per second)— 1 g/l Ni 2 +, 50°C. Letter designations in-

dicate hydrodynamic and electrodeposition parameters

for nickel deposits shown in figure 11.

Deposit Evaluation

5 g/l Ni 2+ Electrolyte

Nickel electrodeposits were preparedfor evaluation at flow rates and currentdensities corresponding to the letters Athrough F on the I-V curves (fig. 4).

Cross section photomicrographs of the

respective deposits are shown in figure9. Excellent deposits exhibiting a

coarse-grained columnar structuretypical of sulfate electrolytes wereobtained using deposition conditionswhere essentially no mass transportlimitation of the nickel ions existed,that is, the deposition regioncharacterized by the sharp increase in

current density near -0.65 v.

Deposits B, D, and F (240 A/m2 , 0.15m/s), (530 A/m2

, 0.60 m/s) , and (800A/m 2

, 1.40 m/s), respectively,correspond to deposits prepared in this

region. Deposit A produced at 320 A/m2

and 0.15 m/s was nonconsolidated andexhibited large voids and nodularformations, reflecting growth in thelimiting current region. Deposit C

produced at 750 A/m2, 0.60 m/s was a

more compact deposit than A containingsmall voids. It was also produced in

the mass transport limiting region but

under less stringent polarization con-ditions. Current efficiencies fordeposits prepared in the mass transportlimiting and the activation controlleddeposition regions ranged from 80 to 95

pet

.

The nonconsolidated character of

deposit E, (1,400 A/m2, 1.40 m/s) indi-

cated that it was prepared at condi-tions outside of the activation con-trolled region, although the transitionfrom activation control to mass trans-port limiting conditions was difficultto detect from the I-V curves. Appar-ently, at this high flow rate thegradient in current density along the

length of the electrode almost totallyobscures the diffusion limited current.SEM photographs of the surface ofdeposits A, C, E, and F are shown in

figure 10. Deposit F shows the expectedsmooth, consolidated surface whereasdeposits A, C, and E show varyingdegrees of nodular formation and crystalgrowth.

In addition to the poor physicalquality of deposit E, the current effi-ciency decreased to 65 pet, indicatingthat the overvoltage for hydrogenevolution decreases with increasing flowrate. Decreases in current efficiencywere also observed for deposits preparedat 450 and 600 A/m2 (not designated on

figure 4) when the flow rate wasincreased from 0.85 to 1.40 m/s,although for these deposits the micro-structure remained consolidated and

columnar

.

Yeager (17) also reported a decreaseof 0.35 v in the overvoltage forhydrogen evolution on a nickel substratewhen ultrasonic waves were used to

agitate solution in the region near the

11

f w*-*m^mmi^^mmmm

FIGURE 9. - Cross section photomicrographs of nickel deposits—5 g/l Ni 2+, 50 C\

12

w *

£ «» ^iMfcw f

-

';% ;

* nL^Wi»

.^gp

f ---"^3

,, ,

^•

-

201^ ** * .' V-

*:- «^ * .

'

Pi' ' ^^^ite*.* ]^M

FIGURE 10. - SEM photographs of nickel deposits-5 g/l Ni 2+, 50° C.

13

electrode surface. In comparison, there

appeared to be a decrease of =0.075 v in

the overvoltage for hydrogen evolutionwhen the velocity was increased fromto 2.10 m/s (fig. 4) in the high masstransport channel cell.

Numerous theories have been advanc-ed regarding the rate-determining mech-anism of hydrogen gas discharge at a

solid electrode surface. These includethe combination of atomic hydrogen into

H2 molecules, the formation of hydrogenbubbles, the diffusion of H+ to the

electrode, as well as adsorption-desorption phenomena associated withhydrogen atoms, molecules, and ions.

The kinetics of one or all of these

mechanisms could be affected in the

transition from free convection to

laminar to turbulent flow in the elec-trowinning cell, but it is beyond the

scope of this report to discuss thesekinetic factors. However, the practicalsignificance of a decrease in hydrogenovervoltage is to limit the range ofacceptable deposition currents.

1 g/1 Ni 2+ Electrolyte

Electrodeposits were also produced

for physical evaluation from an elec-trolyte containing only 1 g/1 Ni 2+ .

Photomicrographs and SEM photographsfor two electrodeposits produced at Gand H (fig. 8) are shown in figure 11.

These deposits were prepared using a

velocity of 1.40 m/s. Deposit G, pro-duced in the activation controlled de-

position region at 210 A/m2 was wellconsolidated while deposit H, producedat conditions where significant masstransport limitation existed (450A/m2

), was poorly consolidated showinglarge voids and dendritic growth. Adecrease in current efficiency from 76

to 68 pet was observed as the currentdensity was increased from 210 to 450A/m2

.

The rates of deposition of satis-factory nickel deposits from a 1 and 5

g/1 Ni 2+ electrolyte flowing at 1.40and 0.15 m/s, respectively, are similarto those obtained for industrial nickel

electrowinning from a 60 g/1 Ni 2+ elec-trolyte operated under essentially sta-tic conditions.

LEACH ELECTROLYTE

After establishing the hydrodynamicand electrodeposition parameters for

producing satisfactory nickel depositsfrom synthetic electrolytes, the resultswere used to test the feasibility ofutilizing the high mass transport cellto electrowin nickel from a dilute leachsolution. These experiments involved(1) preparing a bulk quantity of leachelectrolyte by pressure leaching, (2)

removing some of the metal ionimpurities from the electrolyte usingappropriate purification procedures, and

(3) conducting preliminary nickelelectrowinning experiments with the

resultant leach liquor.

Leaching

Typical leaching results for Ni, Cu,

and Fe are shown in figure 12. In a 7-

hr leaching test, 80 pet of the nickeland approximately 20 pet of the copperwere leached. As acid was consumedduring leaching the pH increased from 1

to 1.7. Correspondingly the dissolvedFe 2+ was oxidized, hydrolyzed, andprecipitated as ferric oxide(Fe203 'x^O) , which decreased theiron content of the solution to 0.3

g/1. Using a solution- to -ore -concen-trate weight ratio of 5, the nickel and

copper leached from the ore correspondedto concentrations of 3.4 and 4.8 g/1,respectively. Between 30 and 40 percentof the Co, Zn, and Mn was also dissolvedduring this leaching period, as well as

2 to 5 pet of the Ag, Pb , and Cd

.

After combination of several leach

liquors and dilution with Na2S04~ H3BO3solution to obtain the desired volume of

45 1, the final concentration of elec-trolyte was, in g/1: 1.0 Ni, 1.5 Cu,

0.084 Fe, 0.05 Co, 0.06 Zn, 0.01 Mn,

0.004 Pb, 0.003 Cd, and 0.0001 Ag.

14

FIGURE 11. - Photomicrographs and SEM photographs-1 g/l Ni2+, 50° C.

15

2 4 6

RETENTION TIME, hr

FIGURE 12. - Pressure leaching curves for gabbro

flotation concentrate.

Purification

Although the leach solution obtainedfrom the complex bulk flotation concen-trate contained several metallic ion

impurities, studies to remove iron and

copper prior to nickel electrowinningexperiments were given particular atten-

t ion

.

Controlled potential electrowinningwas studied as a method for removal of

copper from the leach solution. The

channel cell system was utilized withthe only modification being an increasein electrode size for increasing the

total current through the cell. One

large titanium cathode and an equallylarge Pb-6Sb anode spanning the heightand length of the electrolysis sectionreplaced the three independent smallerelectrode pairs used for studies withsynthetic electrolyte. An electrolyteflow rate of 1.40 m/s was maintained.

Sixty-five percent of the initial1.5 g/1 Cu in the leach solution wasremoved by pot ent iostat ical ly elec-trowinning (-0.28 v (SHE)) at an averagecurrent density of 120 A/m^ and a cath-

ode current efficiency of 80 pet. Thephysical quality of the electrowon cop-per was excellent (fig. 13), and the

purity of the copper was about 99.9 pet.The deposit contained 25 ppm (0.0025pet) Ag. Copper remaining in solutionwas reduced to 0.003 g/1 in an addition-al electrolysis at an average currentdensity of 48 A/m^, but the quality ofthis material was dark and powdery andit was deposited at much lower currentefficiencies.

FIGURE 13.- Photomicrograph of electro-

won copper.

The iron impurity remaining in solu-tion after leaching was removed by a

subsequent procedure that consisted ofoxidation of the iron with 02> air, oranother suitable oxidizing agent such as

H2O2 , adjustment of the pH to 3.5 withcalcium carbonate or caustic, andfinally heating the solution to 85° C

to effect a more complete hydrolysis and

precipitation. The leach electrolyteinitially containing 0.084 g/1 Fe was

decreased to 0.020 g/1 Fe using this

procedure. Some hydrolysis andcoprecipitat ion of nickel also occurred,resulting in a 7-pct decrease of nickelin solution.

In addition to the purificationsteps conducted on the leach solution,associated tests were conducted with a

synthetic solution to establish a rapid,

efficient method for removing low con-centrations of copper. A solution con-taining 5.0 g/1 Ni and 1.0 g/1 Cu wasdecreased to 0.0002 g/1 Cu after 30

cycles of the solution through a fluidi-zed bed (fig. 14) of active nickel pow-der ( ^100 pm) , using a flow rate of

0.003 m/s. A weight ratio of 10:1

nickel powder to total copper was

16

employed. Most of the copper was re-

moved after 17 cycles as illustrated in

figure 15.

The removal of small quantities of

lead was accomplished using controlledpotential electrolysis in the channelcell. Electrodeposits containing 70 pet

Pb were obtained at a potential of -0.6

v from a synthetic solution containing 3

g/1 Ni and 0.005 g/1 Pb flowing at a

velocity of 1.40 m/s. The potential for

lead deposition was about 0.05 v less

electronegative than for deposition of

nickel. The flow of electrolyte in-

creased the limiting current density for

lead deposition so that a practicalcurrent density of 20 A/rn^ could be

obtained

.

; j s

- Filter cloth

-Fluidized bed Flowmeter -

Nickel Electrowinning

Electrowinning experiments wereconducted to recover nickel from the

dilute leach electrolyte that had beenpurified of copper and iron to the

levels of 0.003 and 0.02 g/1, respec-tively.

After a few preliminary electrolysesan electrodeposit containing 84 pet Niwas attained from a solution containing0.75 g/1 nickel ion (table 2). Majorimpurities were Zn, Co, Cu, and Fe. Thecathode current efficiency for Ni reduc-tion was -65 pet. This deposit was pro-duced at 200 A/m2 using an electrolyteflow rate of 1.40 m/s. These conditionswere similar to those established forsatisfactory deposition from a syntheticelectrolyte containing 1.0 g/1 Ni.

10 20CYCLES THROUGH FLUIDIZED BED

FIGURE 14. - Fluidized bed apparatus for copper

removal.

FIGURE 15. - Copper removal rate through flu-

idized bed of nickel powder.

17

TABLE 2. - Nickel electrowinning

—

dilute leach electrolyte

Electrolyte ElectrodepositMetal composition, composition,

g/1 pet

Nickel .... 0.75 84.3Zinc .03 4.2Cobalt .023 2.9

.020 2.7

.007 .5

Manganese

.

.005 .15

Copper .003 2.7

Cadmium. .

.

.0002 .02

Deposits produced at a lower current

density (100 A/m2 ) and a higher velocity(2.10 m/s) contained 30 pet Ni , 15 pet

each of Pb and Zn, and less than 1 pet

of each of Cd, Co, Fe and Mn.

The total metal content in each of

these deposits was less than 100 pet.

The balance was apparently related to

the coprecipitation of a yellow solidduring electrowinning. Recovery of a

sample of this material and subsequentanalysis by proton induced X-ray emis-sion showed it to contain about 40 pet

Ni , 15 pet Fe , and a substantial quan-tity of O2 (likely a mixed oxide).

In a continuing series of electro-winning experiments with the leach solu-tion, further studies were conducted to

determine the deposition rate of the

remaining impurities. To conduct theseexperiments the concentration of the

nickel ions was increased from 0.75 to

2.80 g/1 by addition of nickel sulfatehexahydrate

.

Results of nickel electrowinningexperiments at 300 A/m2 with this leachsolution predictably showed, underconstant hydrodynamic conditions (0.85m/s), that the rate of codeposition ofimpurities decreased nearly propor-tionally with the concentration ofimpurities in solution (table 3). Pro-gressive purification of the solutionand gradual deposition of higher puritynickel would likely occur in subsequentelectrolysis stages; however, anoptimum nickel purity would not be

attainable by this method until the

nickel ion concentration in solutionalso had decreased significantly.

Results in table 3 also show that

coprecipitation of oxides again occurredduring these electrolyses as evidencedby the metal content of less than 100

pet. Apparently the amount of nickel as

oxide was greater in the first depositbut the reason for this difference wasnot established. It is assumed that ad-ditional measures must be taken to in-sure adequate buffering near the elec-trode surface. This would preventlocally high pH regions where the nickelion might tend to hydrolyze and precipi-tate.

Since mass transport of impuritiesas well as nickel is affected by thehydrodynamics of the solution, severalexperiments were conducted at 200 A/m2

to determine the effect of velocity onthe impurity deposition when electro-lyzing at conditions established to be

in the range of satisfactory nickeldeposition (table 4). When the velocitywas increased from 0.85 to 1.40 m/s, thepercentage of the more electropositiveimpurities, Pb, Cu, and Cd increased in

the deposit by 6, 5.5, and 4 times,

respectively, while that of ironremained essentially unchanged. The

percentage of zinc, a more electro-negative metal not normally expected to

codeposit, actually increased by a fac-

tor of 2.5 as the velocity wasincreased. The presence of zinc ions

increases the overvoltage for nickeldeposition by as much as 0.3 v, leadingto coreduction of zinc and nickel ions

(15). Apparently this increase in over-voltage is due to the adsorption of a

zinc hydroxide layer on the cathode.Zinc hydroxide may actually shift the

deposition potential for nickel slightlymore electronegative than that for zinc,since the zinc deposition rate increasedwith velocity in a manner similar to the

more electropositive impurities. Theseresults suggest that with carefulcontrol of the cathodic potential and

the electrolyte velocity, a moreselective removal of zinc could be

achieved

.

18

TABLE 3. - Nickel electrowinning—dilute leach electrolyte, depositcomposition versus electrolyte concentration

1st deposit 2d deposit

Metal Electrolytecomposition,

g/1

Electrodepositcomposition,

pet

Electrolytecomposition,

g/1

Electrodepositcomposition,

pet

Nickel ....

Cobalt

Manganese

.

Copper ....

Cadmium. .

.

2.80.025

.020

.006

.005

.004

.003

.0002

76.01.91

2.79

.80

.32

1.41

.17

.005

2.35.019

.015

.003

.004

.003

.0006

.0001

91.01.62

1.39

.38

.27

.57

.07

.002

TABLE 4. - Nickel electrowinning—dilute leach electrolyte,velocity effect on deposit composition

Electrolytecomposition,

g/1

Electrodeposit composition, pet, at

—

Metal0.85 m/s 1.40 m/s

Manganese .

.

Cadmium. . .

.

2.79.028

.025

.007

.004

.003

.001

.001

81.11.722.31

.33

.59

.80

.005

.16

53.41.17

5.60.16

.54

4.81.02

.90

Some codeposition of manganese wasobserved even though the thermodynamicpotential for Mn^+ reduction is anadditional 0.12 v more electronegativethan for the coreduction of zinc and

nickel. This may result from potentialgradients on the electrode surface of

sufficient magnitude to deposit small

quantities of manganese. Deposition ofmanganese decreased by 52 pet as the

velocity was increased from 0.85 to

1.40 m/s.

The overvoltage for cobalt deposi-tion is probably increased by the pres-

ence of zinc ions in a manner similar to

nickel. According to the results in

table 4, the percentage of cobalt in the

electrodeposit was decreased by 32 pet

as the velocity was increased, indicat-ing its retention in solution at proper-ly controlled potentials and velocities;thus allowing for its subsequent re-covery. Cobalt could, of course, be re-moved by precipitation as cobaltic hy-droxide after oxidation with chlorineor nickelic hydroxide, a common practicein purifying nickel electrorefiningsolutions

.

CONCLUSIONS

Electrochemical data for nickeldeposition from dilute solutions, in-cluding Tafel constant, exchange current

density, transfer coefficient, and acti-vation energy values, compared closelywith values reported in the literature.

19

Limiting current density plateaus are

not clearly defined on the I-V curves at

increasing velocities due to a current

density gradient along the length of the

electrode which obscures the plateau.

However, using the value obtained at the

onset of the diffusion limited region as

the limiting current density value, it

was observed to vary as the square root

of the velocity significantly into the

turbulent region.

Hydrodynaraic and electrodepositionparameters were established for produc-ing suitable nickel electrodeposits frompure dilute (1 to 5 g/1 Ni) electrowinn-ing type solutions, utilizing a highmass transport channel cell. Cathodecurrent efficiencies for favorable de-

posits ranged from 75 to 95 pet. Excel-lent nickel electrodeposits, free of

voids and exhibiting a columnar crystal-line structure, were obtained at =j 210 and800 A/m2 for 1 and 5 g/1 Ni electrolytes,

respectively, using an electrolyte flowof 1.40 m/s. Corresponding current ef-

ficiencies were 76 and 83 pet.

Electrowinning studies conducted

with an impure dilute leach solutionprepared from the Duluth gabbro ore

flotation concentrate yielded an elec-

trodeposit containing 84 pet Ni . In re-

lated electrolyte purification studies,

copper and iron were removed effectivelyfrom the leach solution to levels as low

as 0.003 and 0.020 g/1, respectively,using electrowinning and precipitationtechniques

.

Selective electrolytic removal of

lead ions was also demonstrated to bepossible using a closely controlledcathodic potential and a rapid flow of

electrolyte to increase mass transfer of

lead ions. At an electrode potential of-0.6 v, deposits containing 70 pet Pb

were obtained using a synthetic electro-lyte.

The presence of zinc ions in solu-

tion reportedly increases the overpoten-tial for nickel electrowinning by ==0.3 vcausing the codeposition of significantquantities of zinc. Electrolytic remov-al of zinc from the leach solution wasincreased by a factor of 2.5 as the

electrolyte flow rate was increased from0.85 to 1.40 m/s, suggesting that, withcareful control of the cathodic poten-tial and the electrolyte velocity, a

more selective removal of zinc impurity

would be achieved. The deposition rate

for removing the more electropositiveimpurities, Pb, Cu, and Cd , increased by

factors of 4 to 6 when the velocity was

increased from 0.85 to 1.40 m/s.

Electrolyte purification and nickelelectrowinning studies demonstrated the

feasibility of applying dilute solutionelectrowinning technology to the hydro-metallurgical processing of complex low-

grade ore concentrates. Advances in the

technology for electrowinning from di-lute solutions would provide a methodwhereby direct recovery of metals could

be accomplished at a very early stage in

the mineral processing procedure.

20

REFERENCES

1. Antropov, L. I. (TheoreticalElectrochemistry). Mir Publishers,Moscow, 1st ed

. , 1972, translated by A.

Beknazarov, 568 pp.

2. Barker, B. D., and B. A. Plunkett.The Electrolytic Recovery of Nickel FromDilute Solutions. Trans. Inst. MetalFinish., v. 54, pt . 2, 1976, pp. 104-

110.

3. Bettley, A., A. Tyson, S. A.

Cotgreave, and N. A. Hampson. The

Electrochemistry of Nickel in theChemelec Cell. Surface Technol

., v. 12,

1981, pp. 15-24.

4. Brunner, E. Reaction Velocity in

Heterogeneous Systems. Z. Physik.Chemie, v. 47, 1904, pp. 56-102.

5. Carr, D. S. , and C. F. Bonillo.II. Nickel in Neutral Sulfate Solution.J. Electrochem. Soc . , v. 99, No. 12,

1952, pp. 475-481.

6. Eisenberg, M. , C. W. Tobias, andC. R. Wilke . Ionic Mass Transfer and

Concentration Polarization at RotatingElectrodes. J. Electrochem. Soc., v.

101, 1954, pp. 306-320.

7. Harvey, W. W. , A. H. Miguel, P.

Larson, and I. S. Servi. Application of

Air Agitation in Electrolytic Decopperi-zation. Trans. Inst. Min. and Met.,v. 84, Sect. C, 1975, pp. 11-17.

8. Kovacs, L. Diffusional MassTransfer on High Specific SurfaceElectrodes. Proc . 3d Conf. Appl

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Chem. , Unit Operations, and Processes,Veszprem, Hungary, 1977, pp. 31-37.

9. Landau, U. Distribution of MassTransport Rates Along Parallel PlaneElectrodes in Forced Convection. Ph.D.Thesis, Univ. Calif., Berkeley,Calif., 1976, 344 pp.

10. Levich,

HydrodynamicsV. G. PhysicochemicalPrentice-Hall, Englewood

Cliffs, N.J. , 1962, 700 pp.

11. Lin, C. S., E. B. Denton, H. S.

Gaskill, and G. L. Putnam. DiffusionControlled Electrode Reactions. Ind.

Eng. Chem., v. 43, No. 9, 1951, pp.2136-2143.

12. Nernst , W. Theory of ReactionVelocity in Heterogeneous Systems. Z.

Physik. Chemie, v. 47, 1904, pp. 52-55.

13. Selman, J. R. Measurement and

Interpretation of Limiting Currents.Ph.D. Thesis, Univ. Calif., Berkeley,Calif., 1971, 304 pp.

14. Skarbo, R. R. , and W. W. Harvey.Conditions for the Winning of Copper in

the Form of Coherent High-Purity Elec-trodeposits. Trans. Inst. Min and Met.,v. 83, Sect. C, 1974, pp. 213-222.

15. Vaaler, L. E. Electrolytic Puri-fication of Nickel Plating Solutions.J. Electrochem. Soc. Electrochem. Sci.

and Technol., February 1978, pp. 204-

207.

16. Vezina, J. A. Acid PressureLeaching a Pentlandite-Chalcopyrite-Pyrrhotite Concentrate. Dept . ofEnergy, Mines and Resources, Ottawa,Canada, Mines Branch Tech. Bull. 129,

1970, 28 pp.

17. Yeager, E. Acousto-Electrochemical Effects in ElectrodeSystems. Trans. Symp. on ElectrodeProcesses. John Wiley & Sons, Inc., NewYork, Ch. 6, 1959, pp. 145-159.

18. Yeager, J., J. P. Cels, E.

Yeager, and F. Hovorka. I. Codepositionof Nickel and Hydrogen From SimpleAqueous Solutions. J. Electrochem.Soc, v. 106, No. 4, 1959, pp. 328-336.

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1964, pp. 176-178.

INT. -BU. O F MINES, PGH., PA. 26720

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