DE NORA’S SOLUTION – PART I, DSA® ANODES FOR COPPER

ELECTROWINNING

A. Fiorucci, A. Calderara, L. Iacopetti, F. Timpano, G. Faita, C.W. Brown, Jr., B. De

Masi, M.H. Barker

De Nora

Via Bistolfi, 35

Milan, Italy

alessandro.fiorucci@denora.it

F. Prado

Infotrol

C. Ramon y Cajal, 7

Monzon, Huesca Spain

ABSTRACT

The majority of commercial metal electrowinning (EW) practice today is based on lead

alloy anodes. De Nora’s R&D investment combined with over 40 years of expertise and

experience in developing and manufacturing DSA®

anodes for chloride based EW led to the

development of “De Nora’s Solution” for sulfate based copper electrowinning (Cu EW). De

Nora’s Solution brings added value to the Cu EW tankhouse, sustainable “Green” energy

savings, bottom line cobalt elimination savings, lead-free copper and significant improvement

in tankhouse operation. De Nora’s Solution package provides a custom engineered titanium

structure with the latest generation energy saving electro-catalytic coating, breakthrough

dendrite mitigation system for protection against short circuits, simultaneous current &

voltage monitoring, an innovative acid mist abatement system capturing oxygen bubbles from

the bulk electrolyte and improving cell conductivity and enhanced current distribution across

the electrode surfaces, generating additional voltage savings. This paper describes De Nora’s

rigorous process of testing and validation from laboratory to industrial scale, simulating actual

and extreme Cu EW plant conditions for stable operation, performance and longevity before

introduction into the commercial tankhouse. Results of dendrite mitigation, energy saving and

cathode quality will be discussed. Acid mist abatement results will be presented in part II.”

INTRODUCTION

De Nora DSA® anodes - well known in the electrochemical industry - are made of a

titanium structure coated with a proprietary and patented Mixed Metal Oxide (MMO) layer

consisting of precious metal and valve metal oxides. Since their introduction in 1960, DSA®

anodes have gradually replaced technologies not considered environmental-friendly [1],

replaced lead anodes in processes such as the production of copper foil [2-3] and are

established as a viable alternative to the chemicals used for water disinfection [4], both for

industrial and municipal water, as well as swimming pool maintenance.[ADD REF]

Additionally DSA® anodes are found in electrogalvanizing of steel with zinc, printed circuit

board manufacture, surface finishing and many others applications [5-6].

During the last 40 years DSA® anodes have demonstrated to be “Green and

sustainable” in an array of industrial practices, providing improvement in plant efficiency and

product quality, in accordance with the highest environmental and safety standards. The use in

EW of DSA® anodes dates back to 1972 for Ni EW from a chlorine leaching process [7-8].

In 2010, drawing on over 40 years experience in coating titanium anodes, De Nora

established a program for the development of a “Solution” for Cu EW centered on DSA®

anodes and aimed to solve customer stated problems that are present in tankhouses, such as

the control of acid mist and improve plant performance.

Figure 1 - Photographs of the test cell facilities at Industrie De Nora’s

R&D center in Milan. Left - a full height (1 m), narrow (15

cm) cell, right - arrays of cells under electrolysis.

The program drew upon novel implementation of technology and monitoring systems

long provided by De Nora to the chlor-alkali industry exhibiting significant benefits to EW

tankhouse operations. This is not unusual as many developments in EW have come from

using technology from other industries [9]. A test program was started in De Nora’s R&D

laboratories (Figure 1) focused on developing a novel coating tailored for the rigors of

electrowinning of non-ferrous metals [10-11]. Four pilot plants were used to pre-qualify the

Solution before testing in the field. In parallel, strategic partnerships were established with

well recognized research centers and technology providers. The main components of the De

Nora’s Solution and results achieved so far are described here.

DISCUSSION AND RESULTS

The De Nora Solution is a package developed to create significant economic value for the

hydrometallurgical industry, enhancing the traditional technologies. It bundles together the

following features:

Long lasting energy saving coating;

Sustainable long lasting recoatable structure;

Engineered mechanical acid mist elimination;

Protection against dendrite shorts, resulting in enhanced copper production;

Monitoring and control technology package;

The Coating

An intensive test program was carried out by De Nora to develop a novel coating; the

coating was specifically designed for electrometallurgical applications: improved catalytic

activity was gained by modifying the crystalline lattice and reducing the crystallite size

without changing the thermal production conditions used in the coating process, in order to

secure optimal catalyst stoichiometry and stability against corrosion.

Figure 2 - Novel DSA® coating for electrowinning. Left photo shows a MMO surface

morphology, right photo shows a micro-structural cross-section

The coating was subjected to various electrolytic conditions in order to verify

durability, stability, resistance to reverse current, as well as resistance to various solution

impurities such as Mn, leveling agents, and other organics. Accelerated life tests were

conducted according to internal De Nora test protocols (1, 3, 5, 10 and 30 kA/m2). In parallel,

tests were carried out at current densities more representative of industrial Cu EW tankhouse,

in a range of 300-600 A/m2. The results from the most significant tests are summarized below.

Stability & durability of the DSA® coating

The goal is to provide a 300 mV saving at an anodic current density of 500 A/m2, with an

operating life of five years. The results achieved to date are based on test data carried out

under accelerated life tests at 1 kA/m2. The main observations have been (i) a limited loss of

MMO with a well defined initial break-in period. The term break-in for DSA®

anodes carries

a similar meaning to conditioning of traditional lead anodes, though for DSA® anodes break-

in is dramatically shorter than lead anode conditioning; (ii) a low wear rate (expressed as the

loss of the electro-catalytic coating during the entire operating life) equivalent to the best

projection from accelerated life testing as shown in Figure 3; (iii) a stable single electrode

potential (SEP) for the DSA® anode discussed in the next section.

Figure 3 – Plot of the wear rate of a DSA® coating tested at a current density of

1 kA/m2. For reasons of industrial sensitivity of the data, the time

axis is given in arbitrary units

The coating has also been tested in industrial pilot cells, simulating test conditions

representative of industrial Cu EW practice. A constant anode potential was achieved in all

the tests with an average voltage saving of ≥ 300 mV over Pb-alloy anodes. No drift in the

voltage was evident, even after 1 year of operation in synthetic electrolyte solutions

representative of those tankhouse conditions.

Tolerance of Manganese

Manganese ions (Mn) are typically present in copper tankhouse electrolytes and

deposit on the anode as manganese oxide (usually written as MnO2), the aim was to produce a

DSA® coating which is insensitive to a Mn concentration of 100 ppm in continuous operation

and tolerant to Mn upsets of as high as 500 ppm (0.5 g/dm3).

The quantity of MnO2 anodically deposited depends on a number of factors:

concentration, time of operation, temperature and iron concentration. The effect on DSA®

coatings of up to 100 ppm of Mn in continuous operation was investigated in De Nora mini-

tankcells for more than 7000 hours. Figures 4 and 5 compare the behavior of the conventional

and novel DSA® coatings in presence of Mn ions. The novel coating showed no drift in the

SEP and limited MnO2 pickup on the surface at a current density of 600 A/m2. On the basis of

the data obtained at high current density, it is estimated that at a current density of 300 A/m2

the rate of MnO2 deposition will be halved. Data in figure 5 demonstrates that the behavior of

the novel coating is not influenced by the presence of Mn up to 100 50 ppm.

Figure 4 – A plot of the change in SEP for novel and conventional

DSA® coatings in the presence of Mn (100 ± 50 ppm).

T= 40°C, j = 600 A/m2, [H2SO4] = 170 g/dm

3

Tests were carried out to simulate the impact on the DSA® coating in the case of Mn

concentration fluctuations. Tests were carried out on the 2 dm3

scale with periodic addition of

Mn(II), 170 g/l H2SO4, T = 40°C using a hydrogen evolving Zirconium cathode. The novel

DSA®

coating showed a tolerance against Mn upset in laboratory cells up to 5000 ppm in acid

solution for a duration of 5 weeks and full performance was restored by cleaning. If a

maximum SEP drift of 50 – 100 mV is permitted, then figures 4 and 5 show that in the case of

the conventional DSA® anode coating, washing is required to restore the initial voltage after

200-250 days of operation. In the case of the novel DSA® anode coating the washing intervals

can be much longer, at least in a 3× range.

Figure 5 shows that the pickup of manganese as MnO2 on the DSA® surface was an

order of magnitude lower with the novel coating after 6 months. In other parallel tests, some

of the samples were cleaned of MnO2 using a ferrous sulfate wash (FeSO4), and returned to

testing. The XRF manganese count was found to return to zero for the samples which had

been cleaned. Figure 6 shows that the percentage variation in SEP for the novel coating was

much lower than the targeted 10% and generally ≤2%.

Figure 5 – Manganese pickup for novel and conventional coatings. j =

600 A/m2, [Mn

2+] = 100 ppm, T = 40°C

Figure 6 - Percentage deviation of the single electrode potential (SEP)

vs. time in the presence of [Mn2+

] = 100 ± 50 ppm, without

washing. j = 600 A/m2, T = 40°C, [H2SO4] = 170 g/l

The structure

The DSA® anode structure consists of electrical conductor bars, connectors and

screens designed for a current density of 500 A/m2, low ohmic drop (≤50 mV) and with a 20

years operating life which allows 4 recoating cycles. The De Nora Solution’s DSA® anode

design structure, shown in figure 7 left as a sketch and right as a photograph, comprises of:

1. Two DSA® screens activated with a novel MMO (mixed metal oxide) coating tailored

for the Cu EW application

2. Ti clad Cu rods, for good current distribution & mechanical support of DSA® screens

3. Ti cups, to protect the conductive core from contacting the electrolyte solution

4. Ti C-profiles, used as spacers in order to allow multiple recoating cycles (every 5

years) without exposing the copper core of the rods (which should last 20+ years) and

to guarantee anode planarity

5. Hanger bar, as per Cu EW industrial practice (designs vary from plant to plant)

Figure 7 – left - sketch and right - photograph of a De Nora DSA® anode structure designed

for a 500 A/m2 Cu EW application

The connection of the Ti clad copper rods to the hanger bar was performed via

brazing, in order to allow for high specific current feed (1kA total current through three rods –

333 Amps per rod), the crevice between hanger bar and conduction rod is then sealed to

prevent crevice corrosion phenomena, a typical weak point for other type of mechanical

couplings. The other joints were either TIG or resistance spot welded to give good current

distribution on the DSA® surface. The DSA

® anode production cycle draws on well-

established production processes for diaphragm anodes taking advantage of De Nora’s long

history and expertise in production of anodes for the chlor-alkali industry.

A three-dimensional finite element analysis (3D FEA) was performed on the selected

design in order to make a preliminary verification of the compliance of the model with the

designed current inputs, and to screen alternative designs for costs optimization purposes. The

model was constructed using Comsol™ software package and contained: (i) the anodic

structure, (ii) a continuum media that mimics the ohmic voltage drop (iR drop) in the solution,

accounting for an accumulation of gas from the bottom to the top of the anode and (iii) the

cathodic structure. The model was used to predict the voltage drop of the overall structure

(figure 8), and to map the current distribution for copper deposition (figure 9) and also to map

the heat distribution on the structure (not shown). Single and double contact bars for current

feed were considered for the study. Subsequently the final anode designed was tested in full

scale size and the experimental results were compared with those of the model, showing good

agreement for all parameters analyzed.

The model applied to the selected design shows that, imposing a current feed of 500

A/m2 as per input, showed better performance in the case of double contact hanger bar and the

ohmic drop of the structure did not exceed 2% of the total cell voltage, which was in

alignment with the design target of 50 mV (figure 8, left). Experimental measurements

obtained using a dummy structure show a good agreement with the model (fig. 8, right).

Figure 8 - Left - modelled voltage drop deviation for the anodic structure by FEA whilst

passing a current of 1kA, equivalent to j = 500 A/m2 . Right – experimentally

measured voltage drop for the anode (mV)

The FEA model was used to predict the current distribution on the anodes, which

mirrors the current distribution on the cathodes and consequently the distribution of the

deposited copper. The current distribution is a function of the ohmic characteristics of the

anode, the solution and the overpotentials. Analysis was performed both in the cases of no

bubble void in the solution (homogenous solution conductivity) and with a linear gas

distribution coherent with oxygen evolution at the anode.

The model results are shown in figure 9, in the case of Pb alloy or MMO anodes

without gas confining bags, the condition that is closest to the reality of the tankhouse is a

condition between the two extremes, with oxygen bubbles formed at the anode dispersed and

trapped in the electrolyte solution (especially finer O2 bubbles, with low buoyancy force),

whilst the case that with no bubbles in the bulk electrolyte solution is representative of when

gas confining skirts are installed on the anodes.

Figure 9 - Current distribution by FEA, using Comsol(TM)

software.

Simulated conditions: j = 500 A/m2, T = 50°C, [H2SO4]

= 180 g/l, [Cu]=43 g/l

It can be concluded that the effect of “well functioning” single or double contact bus-

bars is marginal in all the cases and current distribution is uniform across the anode. In case of

bubbles distribution, the effect of anode voltage drop is no longer visible as a current

distribution pattern on the cathode and the bottom part carries more current, whilst in the case

of the mist capture skirt and confined bubbles the trend is the opposite. Moreover, the analysis

indicates that in the absence of an oxygen bubble void (representative of the De Nora Solution

with the mist capture skirt), the maximum vertical deviation of current distribution is within ±

1 % which is favourable. In the case of oxygen bubbles in the bulk electrolyte, the range is ca.

± 9 %.

The tests of copper deposition performed in an industrial pilot at 300 and 450 A/m2

have confirmed the good distribution of current density with both Pb-alloy and DSA® anodes

[12]. The maximum deviation in cathode deposit mass over the surface was 5% for both Pb

and DSA® anodes. This is somewhat lower than the values predicted by the FEA model, but

still higher than the 1% measured with the De Nora Solution.

Temperature of the structure during operation was investigated to predict any possible

aging/deactivation of the components of the structure. In particular the components

considered to be most sensitive to potential damage by thermal stress including the connection

between the hanger bar and the interface between the conductive copper rod and its clad

material. The maximum temperature measured on the anode hanger bar was just 5°C higher

than the electrolyte temperature, this was in agreement with the numbers obtained from the

FEA and far below the temperature levels which might cause contact aging.

Dendrite mitigation & monitoring system

A problem in Cu EW processes is the formation of nodules or dendrites on the cathode

surface, that can decrease copper quality/purity. In addition, if the dendrite reaches the anode,

it leads to short-circuiting events, decreasing the plant’s current efficiency and affecting the

copper production. Although several systems to guarantee anode-cathode spacing are

available on the market and leveling agents (also referred to as inhibitors) are almost

universally used to decrease dendrite growth, at present there are no state-of-the-art

technologies able to avoid short circuits in Cu EW cells.

Because the formation of dendrites is an operational issue related to the cathode and

the electrolyte solution not controlled by the anodes, De Nora’s goal was the development of

a system able to limit the effect of a short. With this scope, specific studies were conducted,

stimulating dendrite growth and investigating the effect on the DSA® anode. A test campaign

was carried out in a transparent Cu EW pilot cell installed in Milan; a 0.15 m2 electrode pair

was used, tracking the current flowing to the anode and the cathode.

The tests were performed at a fixed potential in order to reach the desired current

density and then deliberately seeding a dendrite on the cathode surface, as seen in Figure 10

left. Tracking the dendrite during its evolution was challenging, the contact of the dendrite

with the anode can be seen in Figure 11 at 0:04 minutes, the peak current at 0:07 minutes

represents the point at which the current interlock cut the power to the cell. The corresponding

damage to the DSA® anode structure is seen in Figure 10 right.

Figure 10 - Left stimulated dendritic growth in full height Cu EW tests. j

= 465 A/m2, T = 47°C, [H2SO4] = 170 g/l, [Cu]= 42 g/l. Right

the corresponding damage on the DSA® anode mesh.

Figure 11 - Current vs. time in during deliberate shorting event

between cathode and DSA® anode

A patent pending dendrite mitigation system (DMS), has been developed by De Nora.

The system, complete with the sensor, allows detection of the presence of a dendrite and halts

it before it reaches the anode surface, allowing the plant personnel to take action in reasonable

time. The DMS system protects the De Nora Solution and its components from damage

caused by shorts and also prevents the decrease in current efficiency which is typical during

the shorting event. The DMS is coupled with a new monitoring system designed and provided

by Infotrol S.L. (De Nora’s partner), capable of measuring the inflowing current and detecting

short events for each individual anode. Infotrol's monitoring system is included in the anode

structure, therefore it requires no modification of the cell. Each measurement device

communicates with a junction box which in turn communicates with the tankhouse control

room. Infotrol’s monitoring system has been demonstrated to work with the same precision

for both single and double contact bus-bar systems.

CONCLUSION

Drawing upon more than 40 years experience in mixed metal oxide coated titanium

anode technology, De Nora has developed an innovative value Solution with new DSA®

anodes for copper EW, including:

an anode coating, capable of providing a 300 mV voltage saving at 500 A/m2 over the

full operating life and with low sensitivity to solution impurities, with the best

utilization of noble metal oxide loading (patent pending)

a structure with a 20 year operating life, able to provide low ohmic drop (≤50 mV at

500 A/m2), with good current distribution, ± 1% deviation between top and bottom

an acid mist capture system that substantially decreases the acid mist entering the

tankhouse atmosphere (patent pending) a dendrite mitigation system able to protect the

De Nora Solution from damage caused by shorts

electronic monitoring and supervision of the cell capable of measure the current

flowing in every single DSA®

anode bar and detect any impending shorting event

(patent pending).

REFERENCES

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30.06.2013

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