Wie Ch Mann 2006

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 3, MAY/JUNE 2006

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Current-Source Connection of Electrolytic Cell Electrodes: An Improvement for Electrowinning and Electrorefinery Eduardo P. Wiechmann, Member, IEEE, Guillermo A. Vidal, and Antonio J. Pagliero

Abstract—This paper presents a current-source connection for electrolytic cells facilities, enlarging actual production boundaries. Specifically, the aim of production-level increase, quality improvement, and costs reduction in electrowinning and electrorefining plants have been limited by the current–density (CD) dispersion. In fact, the cathodes of a plant designed to operate with a nominal 320 A/m2 CD per cathode, usually presents CDs in a range between 220 and 420 A/m2 (±31% variations). Industrial tests performed in a copper ER plant proved that the proposed connection halves dispersion. It has been estimated that a 15% earnings increase should be obtained with the increase of efficiency, quality, and production. Index Terms—Current density (CD), current efficiency, electrorefining (ER), electrowinning (EW), short circuits.

I. I NTRODUCTION

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HE WORLD production of high-purity copper and zinc is obtained by electrochemical methods, either electrowinning (EW) or electrorefining (ER). Both processes are similar and are based in the electrochemical deposition of a metal that is contained in a charged electrolyte. By forcing a dc current from an inert anode (mainly lead calcium tin anodes) through the electrolyte to the cathode, the metal ions are reduced at the cathode surface (mainly stainless steel in copper—aluminum in zinc), being deposited as metal. Impurities are not deposited at the cathode, obtaining a high-purity level metal deposit. Today processes are carried out in tankhouses with several hundreds of tank or cells. Up to 91 anodes and 90 cathodes (Phelps Dodge Miami Mining Corporation, Miami, AZ, USA) are inserted in each cell, and up to 3136 cells (Norilsk Copper Smelter, Norilsk, Russia) can be employed in one tankhouse. An average tankhouse contains around 50 000 cathodes in four groups of 250 cells with 50 cathodes each [1], [2]. Normally, Paper PID-05-31, presented at the 2004 Industry Applications Society Annual Meeting, Seattle, WA, October 3–7, and approved for publication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Mining Industry Committee of the IEEE Industry Applications Society. Manuscript submitted for review October 15, 2004 and released for publication January 26, 2006. This work was supported by Fondecyt (Chilean Fund for Scientific and Technological Research) under Project 1040473. E. P. Wiechmann is with the Electrical Engineering Department, University of Concepcion, Concepcion, Chile (e-mail: [email protected]). G. A. Vidal was with the Metallurgical Engineering Department, University of Concepcion, Concepcion, Chile. He is now with Inchalam SA, Concepcion, Chile (e-mail: [email protected]). A. J. Pagliero is with the Metallurgical Engineering Department, University of Concepcion, Concepcion, Chile (e-mail: [email protected]). Digital Object Identifier 10.1109/TIA.2006.872933

every one of these groups is fed by a 12-pulse transformer rectifier injecting 32 000 A. The process voltage ranges from 0.3 V in copper ER, 2.0 V in copper EW to 3.0 V in zinc EW. The electrochemical process is current dependent. According to Faraday’s Law, for depositing 1 mol of metal, its equivalent charge shall be applied to an electrochemical cell. Thus, for depositing 1 ton of zinc, 820 kAh shall be injected to the system. The process dynamic will vary depending on the manner in which this electric charge is transferred to the electrochemical cell. In effect, for accumulating 820 kAh, 820 kA can be transferred in 1 h, or 35 kA for the period of one day (approximately 23.4 h). The operational maximum current density (CD) is set to ensure the deposit quality. As a matter of fact, depending on the electrolyte conditions such as concentration, impurities level, temperature, and controlling agents, an optimal process dynamic is eligible. The process dynamic is given by the CD. This operational parameter is calculated as the mean current that flows through the cathodes times the cathodes’ area. Ideally individual CDs must be maintained as close as possible to the target CD. In the present days, elevated CDs for these processes are around 320 A/m2 . Important efforts are focused on increasing this value, thus reducing the required period of time to produce the same amount of metal, and permitting the facility to produce more tons of metal in a year. Consequently, the nominal plant capacity is boosted. As mentioned before, the electrochemically deposited metal quantity, and quality, depends on the applied current. Also, a modern plant will obtain its better performance when all the cathodes operate at 320 A/m2 . However, this last sentence is still a utopia, because a heavy current dispersion is observed in each plant, altering the operational conditions. Under correct operational conditions, cathodes operating with 220 and 420 A/m2 can be encountered in the same cell. This negative effect is produced by slight parameter variations that are amplified by the electrodes electrical connection. In 1901, Walker patented the electrical connection between electrochemical cells, which is known as the Walker connection (or Walker system). He proposed a small section conductive bar to transmit the current from one cell to another. This bar connects all the cathodes from one cell (outgoing current) with all the anode of the next cell (incoming current). Thus, each cell has their anodes electrically coupled and submitted to the same voltage. Also, the cathodes of each cell are electrically connected and operating with a unique cathode voltage. Therefore,

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 42, NO. 3, MAY/JUNE 2006

between the 51 anodes and the 50 cathodes, it is only one applied voltage and several paths to transfer the current. Thus, any oddness in the process will lead to a current unbalance. The CD unbalance is one of today’s process limitation. In fact, it reduces the process quality, restricts the CD level, and diminishes the process efficiency. Thus, a proposal to avoid these unbalances is well appreciated by this industry. This paper proposes an electrical load rearrangement obtained by a new segmented intercell bar and presents results obtained from an industrial trial. II. B ACKGROUND : W ALKER C ONFIGURATION The Walker configuration was patented by Arthur L. Walker in 1901, U.S. Patent 687 800 [3]. This invention has been widely used and is up to this day the most common industry practice. It is based in a distributing intercell bar of relative small cross-sectional area located between adjacent cells. The cathodes of one cell and the anodes of the next succeeding cell make electrical contact with such distributing bar by resting on it, and are equally distributed throughout its length. As noted by Walker, the distributing intercell bar do not require to carry more than a fraction of the entire process current, “for the reason that the current passing from each cathode to the distributing bar will find its path of least resistance to be through one or more of the immediately adjacent anodes of the next tank.” Because of this, he comments, “the distributing bar will have a current capacity in total equal to that of a conductor bar (bus bar) having a cross-sectional area equal to the sum of the cross-sectional areas of the distributing bar between adjacent cathodes and anodes.” The observation made by Walker also shows the configuration’s main disadvantage. Specifically, due to the fact that the current finds its path through the least resistance, the current distribution in each cell is highly dependent to the resistance path. Electrically, each cell with the Walker configuration can be seen as nth parallel-connected resistances, each representing an anode–cathode pair, and all of them are fed by a unique voltage. Obviously, the current flows through the less resistance paths, impairing the CD of each anode–cathode couples. Among the factors affecting these resistances, which are worth mentioning: electrode positioning and separation, electrolyte conductivity, and contact resistance between electrodes and the distributing bar. Critical factors for this configuration are as follows. 1) Electrode contact: each electrode is connected to the electrical circuit by a single contact. If contact quality varies significantly, the current distribution between electrodes is also compromised. 2) Electrode spacing: electrodes should be placed carefully to ensure proper spacing unless a special capping board is used. Fig. 1 shows a tankhouse with the cells and electrodes. III. C URRENT -S OURCE C ONNECTION The current-source connection of electrochemical cell electrodes presented in this paper was developed and designed by us

Fig. 1.

Electrolytic tankhouse cells and electrodes.

Fig. 2. Current-source connection by segmenting the intercell bar. The cathodes are straight bars, while the anodes are curved.

for our company Optibar, a Chilean research company focused on design and commercialization of an innovative technology for electrometallurgy plants. The aim of this invention was to inhibit a short circuit formation and to equalize the current distribution between electrodes [4], [5]. Short circuits are formed on the cathode surface when a higher CD is observed. In this case, the metal deposit grows until it reaches the anode, establishing an electrical (not electrochemical) path between anodes and cathodes, thus highly reducing the process efficiency. The segmented intercell bar (Optibar) resembles a common multipole electrical connector. It uses as many double contact segments as anode–cathode couples are to be connected. The main technical claim is “since the deposit is current dependent, a current-source load arrangement scheme should be the best solution approach.” The proposed connection scheme is showed in the Fig. 2, in which it can be seen that the current outgoing from one cathode is directly injected to one anode of the next cell. In this figure, cathode hangers are straight bars, while anodes ones are curved. In the Walker arrangement, a reduction of the resistance between an anode and a cathode results in a higher current flow through these electrodes. An identical resistance variation results in a smaller current deviation when using the InterCell Plus current-source-based bar. This connection provides an intrinsic capability to withstand parameter deviations. Furthermore, the arrangement generates preferred paths for the electrical current or current channels. These channels share

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Fig. 5. Short-circuit occurrence in ten cathode cycles.

Fig. 3.

Segmented intercell bar in operation.

Fig. 4.

Histogram of calculated CDs.

similar circuit equivalent resistances producing balanced currents throughout the cell. Each equivalent circuit resistance is comprised of a number of contact and electrolyte resistances in series. This also means that resulting resistances will be more balanced as the number of series cells increases. This effect reduces the system sensitivity to parameter variations. IV. I NDUSTRIAL T EST FOR O PTIBAR The performance of this current-source connection was tested in an industrial facility. For direct comparison, a tankhouse section compromising 20 cells was implemented with one half of the cells connected under the conventional scheme, and the other half-employing Optibar for the current-source connection. Different current, voltage, temperature, and electrochemical measurements were taken three times a day (one per shift), for a period of four months. These data have been used to evaluate both systems. The industrial test bar is shown in the Fig. 3. A. CD Dispersion The current that flows out from the cathode through the hanger bar was measured in each of the 50 cathodes of each cell, accumulating around 210 000 values. The CD was calculated, dividing the measured value by the cathode area. The Fig. 4 shows a comparative histogram of the obtained CDs for both connections. With the Walker configuration, the system exhibits CDs between 240 and 400 A/m2 . The main difficulties of operating

with this CD dispersion are that the cathode copper quality and weight varies greatly. The current efficiency, determined as the percentage of the really deposited versus theoretical copper weight (according to Faraday’s Law), is reduced when the cathodes operates with offset to its optimal CD. For worst, cathodes with elevated CDs tend to form dendrites and short circuits. To overcome these, a reduction of the CD dispersion becomes mandatory. The use of Optibar reduces the CD dispersion. The minimum changes from 240 to 290 A/m2 and the maximum from 400 to 350 A/m2 . Naturally, operational problems were highly reduced. On top, short circuit formation was inhibited further enhancing the current efficiency and product quality. B. Short Circuit Formation With the trend of boosting the process current the occurrence of short circuits is a main issue. An abnormal CD increase will form an oversized deposit, thus reducing the distance between anode and cathode. This will reduce the equivalent resistance between these two electrodes, further increasing the CD. This vicious process will progress until the deposit contacts the anode, establishing a metallic contact between the electrodes. With the conventional Walker system, the short-circuit-current availability is the whole process current, up to 32 kA. This current increase is only controlled by the voltage produced by the high current flowing through the equivalent resistance. The Optibar current-source connection establishes a series connection of electrodes inhibiting this vicious process. Therefore, as shown in Fig. 5, the occurrence of short circuits is substantially reduced. Moreover, remaining short circuits produced by electrometallurgical or alignment reasons are limited in magnitude since the new bar does not interconnect electrodes of the same cell. V. I MPACT ON P ROCESS P ERFORMANCE The enhancement in CD dispersion and short circuit formation affects directly the process performance. Effectively, after cathode harvest, the copper deposit was weighted and physically and chemically analyzed. With the copper weight, the current efficiency for both systems was determined. The system operating with the Walker connection exhibits a mean 95.0% current efficiency, while with the Optibar system the mean current efficiency increases up to 96.5%. This 1.5%

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TABLE I PARAMETERS IMPROVEMENT WITH OPTIBAR

Fig. 6. CD distribution with Walker (W) and Optibar (O) intercell bars. The segmented intercell bar allows to operate with a 5% and 10% CD increase.

current efficiency difference is a direct production-level increase, mainly by losses control. The physical and chemical quality was also improved. With the Walker system, the cathode rejection due to a low-quality level was 4.8%. This copper cannot be sold as a high-grade copper, thus reducing its price. Usual problems were a highgranular level (physical quality) and elevated impurities content (chemical quality). The current-source connection exhibits a reduced 1.5% cathode-rejection level. None of the affected electrodes compromises physical quality, and the chemical quality was affected by anodic slimes. Another important customer requirement is to get a homogeneous copper weight. The conventional Walker system produces copper with a weight varying from 125 to 175 kg. This variation of ±25 kg was reduced to ±15 kg with the currentsource connection. VI. CD I NCREASE The current distribution enhancement reduces the highest CD in the cells from 380 to 350 A/m2 , giving an extra operational margin of 10% in CD increase. Fig. 6 shows the current distribution obtained with a CD increase of 5% and 10%. The narrower dispersion in CD allows in increasing the process current without exceeding the highest CD obtained with the conventional Walker connection. VII. E CONOMIC I MPACT The process parameter improvements discussed above are directly related to an economical impact. Table I resumes all the mentioned improvements and is the basis for the next economic evaluation. Direct impact on the process revenue is given by the increase in the current efficiency and in the CD, and by the cathoderejection reduction. The better current efficiency allows producing more copper (or the process metal) with the same energy injection. Thus, without any further costs, the incomes are raised. The same effect is applied to the cathode rejection by low quality. This metal can be now sold as a high grade and must not be disposed as a scrap. With the CD-increase capability, the process shall provide more energy, thus incurring in a higher costs. However, the whole process will increase its earnings (income minus costs) in the same percent amount. Evaluating Table I into the economical impact, the process earnings can be raised up to 15% (2% from the current effi-

ciency improvement, 4.5% from the cathode-rejection reduction, and 8.5% from the CD increase). In fact, as these variables are related, it is difficult to obtain the maximum benefit in the three variables together. Thus, the operator will have to decide either to operate with a maximum CD with not so good efficiency and quality, or the opposite, or something in between. Today’s process incomes are around U.S. $500/ton for zinc EW, U.S. $1000/ton for copper ER, and U.S. $1500/ton for copper EW. Thus, a 100 000 ton/year capacity plant can earn 7.5, 15, and 22.5 million of dollars in a year by employing the current-source technology. VIII. C ONCLUSION The base metals are commodities. Consequently, their price is determined by international markets and the producers shall reduce their costs and increase their production levels to improve their benefits. The segmented intercell bar Optibar is a suitable alternative for this purpose. The process improvement, i.e., CD-dispersion reduction, current efficiency rise, and the CD level increment mean a 15% earning raise. On the high-competitive base metals market, and due to the elevated production levels of the industry, the Optibar technology will provide new improved boundaries to the process and will increase the incomes of the facilities that adopt this technology first. R EFERENCES [1] T. Robinson, J. Quinn, W. Davenport, and G. Karcas, “Electrolytic copper refining—2003 world tankhouse operating data,” in Proc. Copper - Cobre, J. E. Dutrizac and C. G. Clement, Eds., Santiago, Chile, Dec. 2003, vol. V, pp. 3–66. [2] T. Robinson, J. Jenkins, S. Rasmussen, M. King, and W. Davenport, “Copper electrowinning—2003 world tankhouse operating data,” in Proc. Copper - Cobre, J. E. Dutrizac and C. G. Clement, Eds., Santiago, Chile, Dec. 2003, vol. V, pp. 421–472. [3] A. Walker, “Plant for the electrodeposition of metals,” U.S. Patent 687 800, Dec. 3, 1901. [4] G. Vidal, E. Wiechmann, and J. Pagliero, “Performance of intercell bars for electrolytic applications,” in Proc. 5th Int. Symp. Honoring Professor Ian M. Ritchie, Hydrometallurgy, C. Young, A. Alfantazi, C. Anderson, A. James, D. Dreisinger, and B. Harris, Eds., Vancouver, BC, Canada, Aug. 2003, vol. II, pp. 1381–1394. [5] ——, “Performance of intercell bars for electrolytic applications,” in Proc. Copper - Cobre, J. E. Dutrizac and C. G. Clement, Eds., Santiago, Chile, Dec. 2003, vol. V, pp. 377–390.

WIECHMANN et al.: CURRENT-SOURCE CONNECTION OF ELECTROLYTIC CELL ELECTRODES

Eduardo P. Wiechmann (S’84–M’85) received the Bachelor degree in electronics engineering from Santa Maria University, Valparaiso, Chile, in 1975, and the Ph.D. degree in electrical engineering from Concordia University, Montreal, QC, Canada, in 1985. Since 1976, has been with the University of Concepcion, Concepcion, Chile, where he is a Professor in the Electrical Engineering Department. His industrial experience includes more than 6000 h in engineering projects and consulting. He has published numerous technical papers and has coauthored technical books. His research interests are power converters, high-current rectifiers, ac drives, uninterruptible power systems (UPSs), harmonics, and power-factor control in industrial power distribution systems. Dr. Wiechmann was the recipient of the year 2000 Concepcion City Award for Outstanding Achievements in Applied Research. He is currently a Chairman for the IEEE Chilean Joint Chapter of the Industry Applications, Power Electronics, and Industrial Electronics Societies.

Guillermo A. Vidal received the Bachelor degree in electronics engineering, the M.Sc. degree in electrical engineering, and the D.Sc. degree in metallurgical engineering from the University of Concepcion, Concepcion, Chile, in 1998, 2000, and 2005, respectively. He is presently at Inchalam SA, Chile, working as a Product Developer. Dr. Vidal was awarded the Best Graduate Student of 1998 promotion.

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Antonio J. Pagliero received the Bachelor degree in chemical engineering from the University of Concepcion, Concepcion, Chile, in 1972, the DEA degree in electrochemistry from the Scientific and Medical University, Grenoble, France, in 1974, and the Dr. degree in electrochemistry from the Polytechnic National Institute, Grenoble, France, in 1976. Since 1976, has been with the University of Concepcion, Concepcion, Chile, where he is a Professor in the Metallurgical Engineering Department. He is also a Consultant in electrometallurgy. Since 1984, he has been a member of the International Corrosion Council and, since 1997, a Correspondant Étranger of the Revue Acta Technica Belgica (ATB) Métallurgie, Bélgium. From 1992 to 1994 and 2000 to 2002, he was the Chairman of the Metallurgical Engineering Department. He has published numerous technical papers and has coauthored technical books. His research interests are hydrometallurgy and surface treatments.