A study of the influence of copper on the gold ... · electrowinning process when present in high concentrations relative to that of gold or when ... current density for the copper

▲177The Journal of The South African Institute of Mining and Metallurgy APRIL 2004

Introduction

Electrowinning is best suited to the partialrecovery of gold from solutions with high goldtenors. Furthermore, no foreign ions areintroduced into the solution and it can also beefficiently operated at higher temperatures.High grade gold can be produced that requiresno further purification, provided that thesolution to be treated does not contain metalions that will be plated during electrowinning.Unfortunately, this is an idealized assumptionbecause base metals such as silver, copper,nickel, cobalt and iron typically occur with goldin the electrolyte as cyanide complex ions andmay be plated out during electrowinning. Amajor disadvantage of the gold electrowinningprocess is its inability to produce discardabletailings. This is mainly due to the cathodicshift in the reduction potential of the Au(CN)2

-/Au half reaction when the gold insolution decreases, resulting in preferentialdeposition of base metals, and also a decreasein the mass transfer rate of aurodicyanide tothe cathode surface, which may cause asignificant decrease in the efficiency of theelectrowinning process.

Base metals may influence the goldelectrowinning process when present in highconcentrations relative to that of gold or whentoo high overpotentials are applied. Basemetals can adversely affect the electrowinningprocess in terms of decreased currentefficiencies, poor bullion fineness, passivationof the cathode surface1 and by influencing themorphology of the plated gold.

Theoretical background

Thermodynamics of gold electrowinning

The following electrochemical reactions are ofrelevance during the gold electrowinningprocess (E values quoted for metal ion concen-trations of 10-4 mol.dm-3, NaCN concentrationof 0.2% and NaOH concentrations of 2%):

[1]

[2]

[3]

[4]

The gold, which is present in the solutionin the form of aurodicyanide (Au(CN)2

-), isreduced to metallic gold, according to Reaction[1] at potentials more negative than thereversible potential. Reaction [2], representingoxygen reduction in alkaline solutions, isanother cathodic reaction competing with golddeposition. This is mainly because the

Cu CN e Cu CNE VSHE

( ) + ↔ += −

− − −32 3

0 760;

.

2 2 20 809

2 2H O e H OHE VSHE

+ ↔ += −

− − ;.

O H O e OHE VSHE

2 22 4 40 419

+ + ↔=

− − ;.

Au CN e Au CNE VSHE

( ) + ↔ += −

− − −2 2

0 672;

.

A study of the influence of copper onthe gold electrowinning processby J. Steyn† and R.F. Sandenbergh*†Paper written on project work carried out in partial fulfilment of B.Eng(Metallurgical Engineering) degree

Synopsis

Electrowinning is an attractive method to recover gold from therelatively concentrated aurodicyanide solutions produced by theelution of loaded carbon, provided that the plated gold can be easilyremoved using high pressure water sprays. In practice, difficultywith the removal of the gold is occasionally experienced and hasbeen correlated with the presence of high concentrations of copperin the eluate. In this work, the influence of copper on the electro-plating of gold was further investigated using a laboratoryelectrowinning cell with a rotating disc cathode, polarization, andadhesion tests. It was found that the reduction of copper willcompete with gold electrowinning at high copper tenors and that theadhesion of the precipitated gold is also increased under theseconditions.

Keywords: reduction, adhesion, morphology, current efficiency,limiting current.

* Department of Material Science and MetallurgicalEngineering, University of Pretoria.

© The South African Institute of Mining andMetallurgy, 2004. SA ISSN 0038–223X/3.00 +0.00. Paper received Jan. 2004; revised paperreceived Mar. 2004.

A study of the influence of copper on the gold electrowinning process

electrolyte is likely to be saturated with oxygen due to theoxygen evolution occurring at the anode. Reaction [3]represents the evolution of hydrogen, which occurs at asignificant rate at potentials more negative than -0.96 VSHEwhen working at pH values above 10.1 The evolution ofhydrogen should be under kinetic control at a significantrange of potentials more negative than -0.96 VSHE andtherefore consumes a high proportion of the cathodic current.The major anodic reaction is the oxidation of water tooxygen, also indicated by Reaction [2].

The reduction reaction of cuprous cyanide (Cu(CN)32-) to

metallic copper is indicated in Reaction [4]. The morenegative potential for the reduction of cuprous cyanide tometallic copper than that for the reduction of aurodicyanideto metallic gold, signifies that gold should plate preferentiallyto copper at these conditions. However, copper may co-deposit with gold if high overpotentials are applied, or whenthe copper concentration is high relative to that of gold.2

Polarization diagrams

The kinetics of electrochemical reactions are typically studiedby conducting polarization experiments in which the currentrequired to change the potential of the electrode may bedetermined. If the potential of the electrode is changed at acontinuous rate, so-called potentiodynamic polarizationdiagrams are obtained.

While it is relatively easy to obtain polarization data, theinterpretation is often difficult, as more than one reactionmay contribute to the currents measured. Thus, it ispreferable to first simplify the investigation of industrialsystems by characterizing a purer electrolyte and thensequentially adding species that may contribute to furtherelectrochemical reactions. In this way the contributions of theindividual reactions may be isolated, if it can be assumedthat interaction between the species will be minimal.

Morphology and adhesion of the plated gold

From an industrial point of view, it is essential to obtainloosely adherent precipitates that can be easily removed withhigh pressure water. If this cannot be achieved, some goldwill get locked up inside the electrowinning cells, which canresult in electrowinning being uneconomical. The electrolytetemperature, current density, and conductivity of theelectrolyte have typically proved to be the main variables thatinfluence the adhesion of gold precipitates.3 Co-deposition ofbase metals, such as copper, may also have an effect on themorphology and adhesion of the precipitate.

Experimental

The electrodeposition of gold in industrial electrowinningcells is typically operated under mass transfer control. Torealistically simulate this and to obtain repeatable results, itis necessary to control the rate of mass transfer to theelectrode. This can be conveniently done using a rotating discelectrode for which mass transfer correlations are wellknown. The rate of mass transport to the electrode surface isa function of the angular velocity of the rotating disc asindicated by the Levich equation below:

[5]

where, iL = limiting diffusion current density (A/cm2)n = number of electrons transferredF = Faraday’s constant (96 485 C/mol electrons)v = kinematic viscosity (m2/s)D = diffusion coefficient (m2/s)C = concentration (mol/ dm3)ω = angular velocity (rad/s).

From Equation [5] it is evident that a direct relationexists between the limiting current and the square root of theangular velocity. The angular velocity was varied and thesubsequent limiting current measured, in order to determineif the reaction is mass transfer controlled. On basis of this, anangular velocity of 1200 rpm. was selected for all test work.

For the rotating disc tests, a 500 ml Perspex beaker, inwhich the rotating disc was placed together with two carboncounter electrodes symmetrically on either side of therotating disc, was used as cell. The working electrode,counter electrodes and reference electrode were connected toan ACM Gill AC potentiostat.

The cathode was an 8 mm diameter, 304 stainless steelrod with length of 7.5 mm, mounted in a non-conductiveplastic cylinder, leaving an exposed area of 0.5 cm2. Thecathode was polished, to a mirror finish, with 0.25 microndiamond paste. Electrical connection to the electrode wasachieved through a threaded rod screwed into the back of thecylinder. Contact between the sides of the electrode and theelectrolyte was prevented by fitting it into a Teflon sleevewith a diameter of 15 mm. As reference electrode, asilver/silver chloride electrode with a Luggin tube was used,the end of the Luggin tube placed just beneath the rotatingdisc. The Luggin tube salt bridge was filled with a saturatedpotassium chloride solution. Impedance tests were donebefore each test to determine the resistance of the electrolytebetween the cathode and the tip of the Luggin probe, and thepotentials were adjusted using Ohm’s law.

Synthetic solutions were prepared by AngloGold’s WestWits assay laboratories to simulate plant conditions. Theelectrolyte was prepared by adding 2 per cent caustic sodatogether with 0.1 per cent CN, which simulates conditionsnormally encountered in practice. Chemical compositionstypically ranged from low to high copper and gold concen-trations in order to determine at which concentrations theinfluence of copper became problematic. The solutiontemperature was kept constant at 25°C for all test work byimmersing the Perspex beaker in a water bath. The influenceof gold and copper at various concentrations was studied byconducting potentiodynamic tests using an ACM Gill ACpotentiostat. Potentiodynamic scans were done from 0 to -2.5V at a scan rate of 3 mV/s. The applied potential and theresulting current were stored on a personal computer usingdata acquisition software.

The adhesion of the precipitates was determined by apull-off test using Scotch tape. The degree of adhesion wasrated on a scale of 1 to 5, with 1 being very loosely adherentand removable with slow flowing water, 2 fully removablewith Scotch tape, 3 partially removable with Scotch tape(more than 50 per cent removed), 4 partially removable withScotch tape (less than 50 per cent removed), and 5 highlyadherent and only removable with 600 grid sanding paper.i nFv D CL = −0 62 1 6 2 3 1 2. / / /ω

▲

178 APRIL 2004 The Journal of The South African Institute of Mining and Metallurgy

The morphology of the deposited metal was studied usinga scanning electron microscope.

Results and discussion

Polarization diagramsThe influence of the gold tenor on the polarization behaviourof the stainless steel electrode is shown in Figure 1.

A positive shift in the potential, where the gold reductioncould be noticed, from -1.09 VSSE to -0.95 VSSE at a currentdensity of 0.85 mA/cm2 can be observed when the gold insolution concentration is changed from 5 ppm to 1 000 ppm.This signifies that, during the electrowinning process, thepotential for gold reduction will also shift to more negativepotentials as the gold is removed from the solution. This mayresult in a change in the rate-limiting step of the electro-chemical reaction from diffusion control to chemical controlwhen a constant potential is applied. A decrease in thecurrent density at an applied potential of -1.0 VSSE can alsobe observed when the gold in solution concentrationdecreases, as indicated in Figure 1.

As mentioned, copper may co-deposit with gold whenpresent in high concentrations relative to the concentration ofgold. From Figure 2 it is evident that copper, present inconcentrations of 200 ppm and higher, will plate preferen-tially to gold present at 5 ppm. At these concentrations thecurrent density for the copper reduction reaction is higherthan that for the gold reduction reaction at an appliedpotential of -1.0 VSSE. Copper will therefore plate at a fasterrate than gold under these conditions.

Although the current efficiency for gold electrowinning isexpected to decrease as copper co-deposits with gold, the rateof gold electrowinning should remain the same, whethercopper is present in the solution or not. This is because, at aspecific applied potential, the limiting current for goldreduction should be independent of the copper concentration,assuming that the electrode potential will be such that therate-determining step for gold deposition will be the masstransfer of aurodicyanide to the reaction interface.

Influence of the metal ion concentration on theadhesion of the precipitate

Very loosely adherent precipitates were obtained when goldwas plated from solutions containing only gold. These precip-itates were easily removable with Scotch tape. On the otherhand, copper plated from pure copper containing solutionsproved to be moderate to highly adherent. Table I gives anindication of the degree of adhesion of the precipitates.

As indicated in Table I, some of the precipitates came offin two layers, with the bottom layer being more adherentthan the top layer. A good example of this is the 2 000 ppmCu precipitate where the top layer came off extremely easily(removable with flowing tap water) while a highly adherentbottom layer remained, which had to be removed with 600grid sanding paper. The morphologies of these layers areshown in Figure 3:

From Figure 3 it is evident that the top layer plated in amore dendritical fashion, while the bottom layer depositedwith a basal structure. The highly adherent bottom layerdeposited as very small grains, packed tightly together. Incontrast to this, much larger grains were observed for theloosely adherent top layer. The two different morphologiesare possibly due to a change in the fluid dynamics during theelectrowinning cycle. The hypothesis for this phenomenon isthat at the beginning of the cycle, mass transfer to thecathode might have been slow. This allowed ample time forthe copper atoms to reach the lowest energy locations on thecathode, resulting in a fine grained, basal and very adherentprecipitate. As copper started to precipitate, the cathodesurface became rougher, thereby causing an increase inagitation and subsequently an increase in the rate of masstransfer, resulting in a more dendritic morphology for the toplayer of the precipitate.

Table I also indicates that gold that was deposited fromsolutions containing high gold concentrations, typically 1 000 ppm Au and 500 ppm Au, proved to very looselyadherent while the precipitates obtained from low goldconcentrations, typically 100 ppm Au and 5 ppm Au,



Figure 1—Potentiodynamic results for various gold concentrations (1200 rpm; 25°C; 2% NaOH; 0.1% CN)

Gold polarization diagrams


deposited moderately adherent. The morphology of thedeposited gold from solutions containing 1000 ppm Au and 5 ppm Au is shown in Figure 4.

From Figure 4 it is clear that the gold that precipitatedfrom the 1 000 ppm Au solution deposited as much largergrains than the gold that was precipitated from solutioncontaining 5 ppm Au. It is therefore apparent that precip-itates with a fine grained structure will be highly adherent,while more loosely adherent precipitates are expected forcoarse grained precipitates.

Observations made in industrial electrowinning cellsindicated that a thin adherent metallic layer remained after

washing the cathodes with high pressure water. Taking theabove-mentioned scenarios for gold and copper into account,the following explanation can be given for this occurrence:When copper is present in very high concentration in theelectrolyte, typically 2 000 ppm, while gold is present inconcentrations of less than 500 ppm, the reduction potentialfor cuprous cyanide to metallic copper is more positive thanthat for the reduction of aurodicyanide to metallic gold.Consequently, copper will co-deposit with gold at asignificant rate. During this period a precipitate similar tothat indicated in Figure 3(b) may be obtained, resulting in ahighly adherent initial precipitate. As the copper concen-

▲


Figure 2—Polarization diagrams illustrating the effect of high and low copper concentrations (1200 rpm; 25°C; 2% NaOH; 0.1% CN)

Figure 3—SEM images of the precipitate produced from the solution containing 2000 ppm copper, (a) top layer (b) bottom layer. (-1.0 VSSE, 2% NaOH,

0.1% CN, 1 200 rpm, 25°C)

Table I

Indication of the adhesion of the obtained precipitates (-1.0 VSSE, 2% NaOH, 0.1% CN, 25°C, 1 200 rpm,

14-hour cycles)

Adhesion 1 000 ppm Au 500 ppm Au 100 ppm Au 5 ppm Au 2 000 ppm Cu 1 000 ppm Cu 500 ppm Cu

Top layer 1 1 2 3 1 2 4Bottom layer N/A 2 N/A N/A 5 3 N/A

(a) (b)

tration in solution decreases, the current efficiency for goldelectrowinning will increase. The adherent metallic layer thatremains on the cathode surface after washing may thereforebe because of a small grained, tightly packed copper. It ispossible that the co-deposited gold may be encapsulated bythis highly adherent copper layer, which will result in goldlock-up inside the electrowinning cells. This effect wascharacterized by adding 1 000 ppm Cu to a solution initiallycontaining only 100 ppm Au. This resulted in a change in theadhesion of the precipitated metal from a level 2 to a level 4.A ‘patch-like’ precipitate was obtained where most of thegold and copper plated separately in certain areas of thecathode. An SEM image of this precipitate is shown in Figure5, where the lighter areas indicate the precipitated gold andthe darker areas the precipitated copper.

Influence of applied potential on the morphology ofthe precipitate

Different morphologies are expected when gold is depositedat different applied potentials. Figure 6 illustrates thedifference in the morphology when gold was deposited froma solution containing 1 000 ppm Au at an applied potential of-1.0 VSSE (Figure 6a) and -1.4 VSSE (Figure 6b). It is evidentthat a more dendritical precipitate is obtained when apotential of -1.4 VSSE is applied.

The reason for the different morphologies is most likelybecause of the difference in the mass transfer rate for the two

applied potentials where hydrogen evolution was much morecopious at -1.4 VSSE than at -1.0 VSSE. Gold that plated at anapplied potential of -1.0 VSSE had ample time to diffuse tomore favourable sites on the cathode than gold that plated at-1.4 VSSE. The dendritical precipitate obtained at an appliedpotential of -1.4 VSSE was easily removable with slow flowingwater (level 1 adhesion) while the fine grained precipitateobtained at -1.0 VSSE proved to be only removable withScotch tape (level 2 adhesion). It can therefore be expectedthat a more loosely adherent precipitate can be obtainedwhen a large overpotential is applied.

Although large overpotentials yield loosely adherentprecipitates and optimum potential, in terms of the rate ofgold electrowinning and the adhesion of the precipitate,should be determined. Figure 7 shows the rate of goldelectrowinning as a function of applied potential.

From Figure 7 it is evident that a decrease in the rate ofgold electrowinning occurs when too high overpotentials areapplied. This is likely to be due to a decrease in the effectivecathode area available for electrowinning because of theenhanced hydrogen evolution, as well as dislodging of thegold from the cathode surface due to the increased surfaceturbulence caused by the evolution of hydrogen gas.

For this study, the optimum applied potential proved tobe -1.2 VSSE, since the fastest electrowinning rate wasobtained at this potential and a more loosely adherent



Figure 4—SEM images of the precipitated gold from (a) 1 000 ppm Au solution, and (b) 5 ppm Au solution (-1.0 VSSE, 2% NaOH, 0.1% CN, 1200 rpm, 25°C)

(a) (b)

Figure 5—SEM image obtained from a solution containing 100 ppm Au and 1 000 ppm Cu. (-1.0 VSSE, 2% NaOH, 0.1% CN, 1200 rpm, 25°C, 6 hours)

AuCu


precipitate can be expected than for gold plated at an appliedpotential of -1.0 VSSE.

ConclusionsAlthough copper will co-deposit with gold during theelectrowinning process, relatively high grade gold can still beproduced by controlling the potential of the cathode. Whilethe presence of copper will lead to a decrease in the currentefficiency for gold recovery, the rate at which gold isrecovered should remain unaffected if a constant potential isapplied.

Observations made during this study indicated that theinfluence of copper on the adhesion of the obtainedprecipitate is of greater importance than its influence on theelectrowinning performance in terms of the rate and currentefficiency. Highly adherent precipitates were obtained whencopper and gold were plated from solutions containing highcopper concentrations, typically at concentrations higher than500 ppm Cu. The level of adhesion of a gold precipitateobtained from a solution containing 100 ppm Au increasedsignificantly when 1 000 ppm Cu was added to the solution.Large grained precipitates were obtained when gold wasplated from solutions containing high gold concentrations.These precipitates were very loosely adherent. Lower gold

concentrations yielded fine grained, moderately adherentprecipitates.

More loosely adherent precipitates are expected whengold is plated at higher overpotentials. Although theadhesion decreased with an increase in the overpotential, adecrease in the rate of gold recovery occurs when too highoverpotentials are applied; this is probably due to theremoval of loosely adherent gold by the copious evolution ofhydrogen gas on the surface of the electrode. In general,potentials more negative than that needed to operate undermass transfer control would only serve to promote otherreduction reactions, which is typically undesirable.

AcknowledgementsThe financial and technical contribution from AngloGold andthe support of Basie Maree, Andries Swart, Lionel Sampson,all from AngloGold, is highly appreciated.

References1. STANLEY, G.G. The Extractive Metallurgy of Gold in South Africa, SAIMM

Monograph series M7, vol. 2, 1987. pp. 849–894.2. PAUL, R.L. and SOLE, K.C. Refining of gold by the selective dissolution of

silver and copper from binary gold alloys, South African Journal ofChemistry, vol. 39, 1986. pp. 101–107.

3. BARQUINS, M. and COGNARD, J. Adhesion characteristics of gold surfaces,Gold Bulletin, vol. 19, no. 3, 1986. pp. 82–86. ◆

▲


Figure 6—Morphology of the precipitated gold at an applied potential of (a) -1.0 VSSE and (b) -1.4 VSSE (1 000 ppm Au, 2% NaOH, 0.1% CN, 25°C, 1 200 rpm)

(a) (b)

Figure 7—Influence of the applied potential on gold electrowinning rate (1 000 ppm Au, 2% NaOH, 0.1% CN, 25°C, 1 200 rpm)

Gold electrowinning rate vs. applied potential

-0.8 -1 -1.2 -1.4Potential (VSSE)

Rec

ove

ry r

ate

(mo

l/s)

1.60E-08

1.40E-08

1.20E-08

1.00E-08

8.00E-09

6.00E-09

4.00E09

2.00E-09

0.00E+00

1 000 ppm Au

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A study of the influence of copper on the gold ... · electrowinning process when present in high concentrations relative to that of gold or when ... current density for the copper