Cathode Quality Cmq 2006

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CANADIAN METALLURGICAL QUARTERLY, VOL 45, NO 1

ON THE PHYSICAL QUALITY OF COPPER ELECTRODEPOSITS

OBTAINED ON MESH CATHODES

L. CIFUENTES* and M. MELLA

Departamento de Ingeniera de Minas, Universidad de Chile, Tupper 2069, Santiago, Chile*[email protected]

(Received in revised form July, 2005)

Abstract The effect of operation parameters (cell current density, temperature, recirculation flow rate

and concentration) on the physical quality of copper electrodeposits obtained on a copper mesh cathode has

been studied in an electrodeposition cell based on reactive electrodialysis. The electrolyte contained 50 g/L

sulphuric acid, 3 to 9 g/L Cu and 3 g/L As. The physical quality depends on the cell current distribution and

the kinetic control for the electrodeposition reaction. Acoverage ratio for the electrodeposit has been defined.

The physical quality of the electrodeposit increased markedly with increasing cell current density, decreasingcatholyte temperature and decreasing copper concentration in the catholyte. The effect of the recirculation

flow rate of the catholyte on the physical quality was slight. It must be noted that as copper concentration

diminishes, the chemical quality of the electrodeposit worsens due to greater arsenic co-deposition.

Rsum On a tudi leffet des paramtres de fonctionnement (densit de courant de la cellule,

temprature, dbit de recirculation et concentration) sur la qualit physique de dpts lectrolytiques de

cuivre obtenus sur une cathode en maille de cuivre, dans une cellule de dpt lectrolytique base sur

llectrodialyse ractive. Llectrolyte contenait 50 g/L dacide sulfurique, 3 9 g/L de Cu et 3 g/L dAs. La

qualit physique dpend de la distribution du courant de la cellule et du contrle cintique de la raction

dlectrodposition. On a dfini un taux de couverture du dpt lectrolytique. La qualit physique du dpt

lectrolytique augmentait nettement avec une augmentation de la densit de courant de la cellule, une

diminution de la temprature du catholyte et une diminution de la concentration de cuivre dans le catholyte.

Leffet du dbit de recirculation du catholyte sur la qualit physique tait peu important. On doit noter qu

mesure que la concentration de cuivre diminue, la qualit chimique du dpt lectrolytique se dtriore cause de la plus grande co-dposition darsenic.

INTRODUCTION

Cathode Quality

In conventional copper electrometallurgy, a distinction ismade between chemical and physical cathode quality.Chemical quality is determined by the concentration ofcopper ( 99.99%) and various impurities in the resultingelectrodeposit, whereas the physical quality is given by thedegree of compactness of the sheet cathode (absence of

porosity, cracks and voids) and by the smoothness of itssurface (absence of dendrites, nodules and other protrusions).In addition, cathode samples are subject to mechanical tests.

In non-conventional copper electrometallurgical cells,cathode geometries other than the sheet are used, such ascopper granules or copper mesh. In these cases, the chemicalquality is determined in exactly the same way as in theconventional case, but the physical quality needs to beredefined.

Objectives

This work attempts to propose physical quality criteria for

copper electrodeposits obtained on copper mesh in a reactive

electrodialysis (RED) cell and to establish the effect of cell

operation parameters (cell current density, temperature,

recirculation flow rate and concentration) on the physical

quality of the copper electrodeposits.

The RED cell is made up of two compartments, the first

one containing the cathode and the catholyte and the second

one containing the anode and the anolyte. The compartmentsare separated by an anion membrane which prevents cation

transport between the electrolytes.

Previous Work

The effect of various operation parameters on cathode quality

in conventional copper electrowinning and electrorefining has

Canadian Metallurgical Quarterly, Vol 45, No 1 pp 09-16, 2006 Canadian Institute of Mining, Metallurgy and Petroleum

Published by Canadian Institute of Mining, Metallurgy and PetroleumPrinted in Canada. All rights reserved


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L. CIFUENTES and M. MELLA

been studied extensively [1-6], but there are no publishedstudies on the effect of operation parameters on the quality ofnon-conventional cathodes.

The use of a reactive electrodialysis cell for copperelectrodeposition and various aspects of its operation haverecently been described and discussed by Cifuentes et al. [7-10].The RED cell exhibits potential for further development as lab

scale results have shown that the cathodic current efficiency, thecell voltage, the specific energy consumption and the effect ofimpurities are all superior to the results shown by conventionalcopper electrometallurgical plants. The speciation of thesolutions used in this cell (i.e. the determination of the speciespresent and their concentrations as functions of temperature, pHand the abundance of key system components) has also beenstudied by means of experiments and the development ofthermodynamic models [11-14]. Solution speciation is veryimportant in RED cells as the nature and concentration ofspecific ions determine mass transport across the membrane. Inparticular, the speciation of the aqueous CuSO4-H2SO4-Assystem was studied by Casas et al. [12].

Kinetic Controls

An electrochemical reaction is said to be under chargetransfer control when the charge transfer step (electrontransfer from electrode to ion) is considerably slower than themass transfer step (ion diffusion towards the cathode surface).The reaction rate is given by the Butler-Volmer equation.

(1)

Mass transfer control takes place when the mass transferstep is considerably slower than the charge transfer step. The

reaction rate is given by Ficks law

(2)

Mixed control occurs when the values for the masstransfer and charge transfer transport rates are about the sameorder of magnitude. The reaction rate is

(3)

where

hmixhct+ hconc (4)

Figure 1 shows an Evans diagram which depicts theelectrochemical kinetics for copper electrodeposition (Cu2+ +2e Cu0). The potential ranges for charge transfer, masstransfer and mixed control are shown. Since industrial copperelectrodeposition normally takes place under mixed control, a

distinction is made between mixed control dominated bycharge transfer and mixed control dominated by masstransfer. The diagram also shows the potential ranges whichfavour secondary and tertiary distribution of the currentdensity. This is discussed in the following section.

Potential and Current Distribution

The distribution of local current density and potential are ofparamount importance in any electrochemical processincluding copper electrodeposition. The space distribution ofthese variables in an electrochemical reactor influence localreaction rates and surface phenomena such as the growth ofdendrites and nodules in electrocrystallization. Thesephenomena are significant in determining the physical qualityof the cathode.

The distribution of these variables is given by Laplacesequation

2f = 0 (5)

and Ohms lawi = kf (6)

There are three main types of current and potential distri-bution on a given electrode in an electrolytic cell [15-17]:

1. Primary distribution. Only the electric field isconsidered. There is no overpotential on the electrode and noconcentration gradient in its vicinity; this distribution dependsessentially on electrode and cell geometry. For two parallelplate electrodes, the current density exhibits a minimum at thecentre of the electrode and it grows outwards becominginfinite at the edges. The primary distribution is independentof electrolye flow rate and therefore, it is symmetric.

2. Secondary distribution. Both the electric field andcharge transfer effects are considered. The electrodepositionreaction is under charge transfer control which generates an

i

iF

GT

F

GT

i

i

F

GT

i

i

F

GT

b amix

cmix

b

La

amix

b

Lc

cmix

=

- -

+

- -

0

0 01

exp exp

exp exp

ah

ah

ah

ah

i zFDcb

=

d

i iF

GT

F

GT

s a

ct

c

ct=

0 exp exp

10


Fig. 1. Evans diagram (logarithm of current density vs electrode potential)

for the Cu2+ + 2e Cu0 reaction showing kinetic control ranges and current

density distribution ranges.


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ON THE PHYSICAL QUALITY OF COPPER ELECTRODEPOSITS OBTAINED ON MESH CATHODES 11


overpotential on the electrode. Mass transport phenomena areneglected. The overpotential is linked to the reaction currentdensity by the Butler-Volmer Equation 1 which is used as aboundary condition. For an arrangement with parallel plateelectrodes, the secondary distribution is more uniform thanthe primary one and the infinite current density at theelectrode edges is eliminated.

3. Tertiary distribution. Electric field, charge transfer andmass transfer effects are considered. The electrodepositionreaction is under mixed control so that the concentration andcharge transfer overpotentials are linked resulting in a morecomplex mathematical Equation 3 for the boundarycondition. The reactant concentration gradients and limitingcurrent densities are important so that the current densitydistribution depends on the hydrodynamics of the electrolyte.

EXPERIMENTAL

A lab scale reactive electrodialysis (RED) cell was used to

carry out the experiments. It was made of 15 mm thick acrylicplates and consisted of two compartments, one for the anodeand the anolyte and a second one for the cathode and thecatholyte. In the present case, the reactions are copperelectrodeposition at the cathode (Cu2+ + 2e Cu0) and wateroxidation at the anode (2H2O O2 + 4H

+ + 4e), butalternative anodic reactions (such as Fe2+ Fe3+ + e) can alsobe used. The cell was held together by seven 5 mm diameterstainless steel bars bolted at both ends. Figure 2 shows aschematic of the RED cell.

The cathode was made of four layers of 44 cm2 coppermesh as shown in the micrographs below. The anode was a44 cm2 lead sheet with 0.1 cm thickness. The anode back

face was masked with epoxy resin. Compartment dimensionswere 7.510.52.5 cm3.

The RED cell was operated, in all cases, with an anolytecomposition of 50 g/L sulphuric acid which provided theconductivity for the occurrence of the anodic reaction.

The base catholyte composition was 9 g/L copper,50 g/L sulphuric acid and 3 g/L arsenic. This composition was

chosen because it is analogous to a copper electrorefiningelectrolyte at 1:4 dilution and RED cells could be applied tothe treatment of such electrolytes.

The source chemicals were analytical grade CuSO45H2O, As2O5 and 96% (w/w) sulphuric acid. The catholytecomposition was varied in order to establish its effect oncathode quality. No additives were used.

The anion membrane, which separated anolyte fromcatholyte, was an Ionac MA-3475. It was fitted in a 44 cm2

window cut in the acrylic plates between the compartments.This membrane was selected on the basis of availability andperformance in previous works. It was kept in place by 2 mmthick rubber seals. Membrane properties and the treatment towhich they were subjected before each experiment have been

given elsewhere [9].The electrolytes were separately recirculated by two

Watson-Marlow 505S peristaltic pumps. Two one litrerecirculation glass tanks were used. The inner diameter of therecirculation inlet and outlet tubes fitted to the cell was 1 cm.Total volume for anolyte and catholyte was 600 cm3 each. Thetemperature of both electrolytes was kept constant by a Julabothermostatic bath.

Constant cell current densities were provided by a 2 A,30 V Idisa rectifier. The tests lasted for four hours. The cellvoltage was continuously monitored during each experiment.

Arsenic in the electrodeposits was analyzed by atomicabsorption with hydride generation [18,19]. A copper electrodeposit sample was dissolved in aqua regia (HNO3 + HCl),

the solution was left for two hours at 100 C and then put ina hydride generator unit attached to an atomic absorptionspectrometer. NaBH4 was used as a reducing agent. Theresulting hydride was then fed into the spectrometer andanalyzed.

Micrographs were obtained with a digital PanasonicLC40 photographic camera attached to a Nikon Labophotmicroscope with a 5 to 40 magnification capability. In allcases, the front face of the cathode (i.e., the cathode sidewhich faces the anode) was pictured. The influence of cellcurrent density (150, 225 and 290 A/m2), temperature (22 and50 C), recirculation flow rate (450 and 950 cm3/min) and Cuconcentration (3 and 9 g/L) were studied. Table I shows thevalues of all variables for each experiment conducted.

RESULTS AND DISCUSSION

Results for cell voltage, coverage ratio and arsenic concen-tration in the electrodeposit are shown in Table II for allcases studied. Electrodeposit morphologies are presented asmicrographs (Figures 3 to 8). In all cases, the arrow on theFig. 2. Schematic of the RED cell.


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micrograph indicates the entry point of the electrolyte intothe cell.

Basic Concepts

In order to discuss the results, the following concepts areproposed:

1. The coverage ratio (rcov) is defined as the fraction (%)of the apparent cathode surface area covered by theelectrodeposit after 2 g of copper have been deposited.

(7)

2. A physically good deposit is one which presents ahomogeneous distribution of crystals and a high coverageratio. A homogeneous deposit indicates that the depositionconditions do not favour a secondary distribution of thecurrent density which means that the electrode potential forthe deposition must be far from the charge transfer control

potential range. In other words, a good deposit should be theresult of a tertiary distribution with mixed control dominatedby mass transfer (Figure 1).

3. Aphysically bad depositis one which presents a hetero-geneous distribution of crystals (which may includedeposit-free zones) and a low coverage ratio. The depositmorphology is characteristic of a secondary distribution of

the deposition current density with little or no deposit at thecentre of the cathode and most of it near the edges. Anelectrode potential close to the charge transfer controlpotential range would favour this outcome. In other words, abad deposit should be the result of a secondary distributionwith mixed control dominated by charge transfer (Figure 1).

Base Case

For the base case (current density = 290 A/m2, temperature =50 C, flow rate = 450 cm3/min and concentration = 9 g/Lcopper and 3 g/L arsenic), the morphology of the deposit isshown in Figure 3.

There is abundant nucleation and growth of crystals inthe areas near the electrode edges. This is indicative of asecondary distribution of the deposition current density andmixed control dominated by charge transfer for the electrode-position. The coverage ratio is 94%.

Effect of Current Density

The effect of current density on deposit morphology is shownin Figures 4 and 5.

Figure 4 shows a deposit obtained with a depositioncurrent density of 225 A/m2. There are deposit-free zones atthe centre of the cathode and greater nodulation and crystalgrowth near the edges. As in the base case, this is indicativeof a secondary distribution and mixed control dominated by

rcov

cov=A

Atotal

100

12


Table I Experimental conditions1

Experiment Current density Temperature Flow rate [Cu](studied variable) A/m2 C cm3/min g/L

Base case 290 50 450 9Current density 225 50 450 9Current density 150 50 450 9

Temperature 290 22 450 9Flow rate 290 50 950 9Cu concentration 290 50 450 3

1 Arsenic concentration in the catholyte was 3 g/L in all experiments.

Table II Results

Experiment Cell voltage Coverage Arsenic in(studied variable) V ratio deposit1

% ppm

Base case 2.86 94


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charge for the deposition, but the distribution is now more

marked than in the base case. The coverage ratio is 80%,which is less than the base case.

Figure 5 shows the deposit obtained at 150 A/m2. Thereare large areas without deposit at the centre and sharpnodulations at the cathode edges. The coverage ratio is 65%which indicates that the secondary distribution is even moremarked than in the 225 A/m2 case. This confirms that thecoverage ratio decreases as the current density decreases.

Effect of Electrolyte Recirculation Flow Rate

Figure 6 shows the effect of increasing the catholyte flow ratefrom 450 to 950 cm3/min. The morphology is similar to theone presented by the base case, but the crystal size and distri-bution appear more homogeneous and there is an increase incopper deposition near the entry point of the catholyte andsmall deposit-free areas on the cathode. The coverage ratio is92% which is similar to the base case. The recirculation flowrate appears to have little effect on the physical quality.



Fig. 3. Base case. Experimental conditions for all the figures are in Table

I. The arrow shows the entry point of the catholyte to the cell.

Fig. 5. Deposit obtained at a current density of 150 A/m2.

Fig. 4. Deposit obtained at a current density of 225 A/m2.

Fig. 6. Deposit obtained at a catholyte recirculation flow rate of 950

cm3/min.


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Effect of Electrolyte Temperature

Figure 7 shows the effect of decreasing the electrolytetemperature from 50 to 22 C. The deposit is homogeneous,without deposit-free areas and without nodulation. Thecoverage ratio is 100%, which points to a tertiary distributionof the current density with mixed control dominated by masstransfer for the deposition reaction.

Effect of Copper Concentration

Figure 8 shows the effect of decreasing copper concentrationfrom 9 to 3 g/L. Copper concentration now equals the arsenicconcentration in the catholyte. The deposit is similar to the oneshown in Figure 7, without nodulation at the cathode edges,indicating tertiary distribution with mixed control dominatedby mass transfer. The coverage ratio is 100%. However, thedeposit looks darker (opaque brown rather than bright red)which suggests that the electrode surface is covered by acompound rather than a metallic deposit. This could beindicative of co-deposition of arsenic species such as Cu3As.

Arsenic Co-deposition

Arsenic was analyzed in all the produced electrodeposits. Asshown in Table II, its concentration was very similar in allcases but one which was the deposit obtained at a reducedcopper concentration in the catholyte (3 g/L). In thisparticular case, arsenic concentration increased dramaticallyto over 450 ppm which shows that a higher As/Cu concen-tration ratio in the catholyte greatly favours the co-depositionof this element. This result confirms the consideration putforward in the previous section.

Cell Voltage

The cell voltage is given by

(8)

Table II shows cell voltage values for all the conditionsstudied. The base case is 2.86 V (at 290 A/m2) whichdecreases to 2.58 V at 225 A/m2 and 2.19 V at 150 A/m2. Thisis to be expected from Equation 8, as both the anodic andcathodic overpotentials decrease with decreasing cell currentdensity and also do the ohmic potential drops in catholyte,anolyte and membrane.

The cell voltage increases with decreasing temperaturebecause ion diffusivities and the electrical conductivity ofcatholyte, anolyte and membrane all decrease with decreasingtemperature. The electrical resistance of the electrolytesvaries with conductivity as

(9)

which means that as the conductivity decreases, theresistances and the cell voltage increase.

Vcell also increases with decreasing copper concentrationbecause a lower concentration of charge carriers (cupric ions)causes a decrease in the conductivity of the catholyte, whichis an increase in its resistance with the corresponding increasein theIRcath term in Equation 8.

The cell voltage decreases with increasing catholyteflow rate which is to be expected because a higher flow rateimplies a higher value for the limiting current density forcopper deposition which causes a decrease in the cathodicoverpotential (hc) in Equation 8.

Rd

A=

1

k

V E I R R Rcell e a c cath an m= + + + + +( )D h h

14


Fig. 8. Deposit obtained at a copper concentration of 3 g/L.

Fig. 7. Deposit obtained at a temperature of 22 C.


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CONCLUSIONS

1. The physical quality of copper electrodeposits obtainedon copper mesh cathodes in a reactive electrodialysiscell can be explained in terms of current density distri-bution, kinetic controls and coverage ratio.

2. The physical quality of the electrodeposit increases as

the cell current density increases.3. The electrodeposit becomes slightly more homogeneous as

the recirculation flow rate of the catholyte increases, butthere is increased deposit near the entry point of the catholyteto the cell and small deposit-free areas on the cathode.

4. The physical quality of the electrodeposits increaseswith decreasing catholyte temperature.

5. The physical quality of the electrodeposits increaseswith decreasing copper concentration in the catholyte,but arsenic co-deposition also increases which markedlylowers the chemical quality of the deposit.

FURTHER WORK

Considering that the physical quality of the electrodeposit isstrongly linked to the predominant type of current distribution andtherefore, to the kinetic control of the Cu2+ + 2e Cu0 reaction,the electrochemical kinetics of copper deposition in RED cellsshould be studied further in order to quantify these relationships.This would allow for the development of a predictive model ofthe physical quality of copper mesh cathodes.

ACKNOWLEDGEMENTS

This work was funded by the National Committee for Scienceand Technology (CONICYT, Chile) via FONDECYT project

No. 101 0138. Continued support from the Department ofMining Engineering, Universidad de Chile, is gratefullyacknowledged. Financial support from Placer Dome to theChair of Environmental Studies in Mining is alsoacknowledged. Thanks are due to Gloria Crisstomo for herhelp with the production of this paper.

LIST OF SYMBOLS

A surface area of cross section perpendicular to migrationpath, m2

Acov apparent surface area of cathode covered by deposit, m2

Atotal total apparent surface area of cathode, m2

cb

reactant concentration in the bulk solution, mol/m3

F Faradays constant, C/eqD diffusivity, m2/sd length of migration path, mG gas constant, J/mol/K

I cell current, Ai current density, A/m2

i0b exchange current density in terms of reactant concen

tration in the bulk solution, A/m2

i0s exchange current density in terms of reactant concen

tration at the electrode surface, A/m2

iLa, iLc limiting current densities, anodic and cathodic, A/m2

R electrical resistance, OhmRcath, Ran, Rm electrical resistence of catholyte, anolyte and

membrane, OhmT absolute temperature, K

Vcell cell voltage, Vz charge numberaa, ac charge transfer coefficients, anodic and cathodicd thickness of the diffusion layer, mDEe difference between the equilibrium potential of the

anodic and cathodic reactions, Vha overpotential of anodic reaction, V.hc overpotential of cathodic reaction, V.hconc concentration overpotential, V.hct charge transfer overpotential, V.hmix mixed control overpotential, Vj electric potential, Vrcov coverage ratiok electrical conductivity, Ohm-1 m-1

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