IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 49, NO. 6, NOVEMBER/DECEMBER 2013
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Contact System Design to Improve Energy Efficiency in Copper Electrowinning Processes Eduardo P. Wiechmann, Senior Member, IEEE, Pablo Aqueveque, Member, IEEE, Guillermo A. Vidal, and Jorge A. Henriquez
Abstract—To improve energy efficiency in copper electrowinning, different technologies have been developed. These include electrode positioning capping boards and 3-D grids, electrode spacers, and segmented intercell bars. This paper introduces a design concept to avoid electrode open circuits and reduce contact resistances. The design is based on a female tooth shape for the contacts on the intercell bar. This leads to improved electrode alignment, reduced contact resistances, easier contact cleaning, and ensured electrical contact for the electrodes. It results in lower operational temperature for the electrodes, reduced plant housekeeping, increased lifespan for capping boards, and higher rate of grade A copper production. The comparative results presented should be a useful guideline for any type of intercell bar. Improvements in production levels and energy efficiency should reach 0.5% and 3%, respectively. A 3-D finite-element-based analysis and industrial measurements are used to verify the results. Index Terms—Carbon footprint, cell voltage, current density, current efficiency, dispersion, electrowinning (EW), intercell bar, specific energy.
I. I NTRODUCTION
T
HE relevance of copper electrowinning (EW) (CuEW) to obtain grade A copper has increased with bioleaching of sulfides. Now, both oxides and sulfides can be treated without smelting and electrorefining. This is a major development to further boost the actual 6-million-ton annual world production via CuEW. The electric energy worldwide consumption of this process is estimated as 12 000 GWh/year with a carbon footprint of 12 million ton of CO2 per year [1], [2]. Therefore, a continuing effort to improve specific energy efficiency is under way. A number of improvements in the process include enhanced anodes, capping boards and spacers, and currentmode intercell bars [3]. These efforts have been quite successful in balancing process currents, limiting and virtually eliminating short circuits, and improving specific energy to a 1880-kW/ton
Manuscript received June 30, 2011; revised December 21, 2012; accepted March 18, 2013. Date of publication June 17, 2013; date of current version November 18, 2013. Paper 2011-MIC-328.R1, presented at the 2011 IEEE Industry Applications Society Annual Meeting, Orlando, FL, USA, October 9–13, and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY A PPLICATIONS by the Mining Industry Committee of the IEEE Industry Applications Society. E. P. Wiechmann, P. Aqueveque, and J. A. Henriquez are with the Department of Electrical Engineering, University of Concepción, Concepción 4070386, Chile (e-mail:
[email protected]; pablo.aqueveque@ ieee.org;
[email protected]). G. A. Vidal is with Zigbar Ltd., Concepción 4070138, Chile (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIA.2013.2269035
level [4]. However, an open circuit’s occurrences offer an unresolved challenge. They are difficult to detect and clear because open circuits do not produce overtemperatures or overcurrents. Therefore, thermographic cameras and gauss meters are ineffective. The use of electronics to detect open circuits based on voltages is an alternative [5], [6]. However, CuEW plants are an extremely harsh industrial environment for electronics. The concept introduced in this paper goes beyond detection. It is a contact design to ensure electrical contact between electrodes and intercell bars. On top of ensuring electrical contact, the voltage drop of the contacts is minimized. Finally, female contact shape and position should be designed for proper spacing of electrodes. These three factors constitute the design keystones. With this in mind, different shapes are researched and compared in this work. These include hangers with circular, rectangular, and trapezoidal cross sections and their respective female-shaped intercell bar contacts. Industrial data obtained with specific shapes are used. Additionally, a 3-D finite-element model tuned with these data provides information related with contact resistance performances. Finally, this paper includes results obtained throughout mechanical and electrical tests with an industrial anode aged 10 000 h (industrial site average). II. P ROCESS E LECTRICAL M ODEL Due to the slow dynamics of the electrochemical process, electrical currents can change by 10% in 30 min, and a steadystate model represents the voltage distribution behavior with enough accuracy [7], [8]. In EW, the electrolyte resistance produces a relevant voltage component. This voltage can be reduced by lowering the space between electrodes. However, as the distance is reduced, the plant exhibits electrolyte circulation problems and becomes sensitive to electrode misalignments. The additional voltage components required are as follows: cathode and anode polarization and the overpotential for the current to flow. For CuEW, these polarization voltages are approximately 1.230 Vdc. Finally, a voltage drop is produced by the contacts between the intercell bar and the electrodes. These contacts are produced by hanging the anodes and cathodes on the intercell bar. Ideally, low identical contact resistances and perfect electrode alignments produce balanced currents with high energy efficiency (see Fig. 1). However, industrial sites are characterized by high dispersion in contact resistances, unsure electric contacts, and bad electrode alignments. For worst, the shape of the intercell capping boards increases the occurrence of open circuits.
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Fig. 1. CuEW electrical model.
In the segmented intercell bar [3], a single cathode of an upper cell is connected to a single anode of a lower cell. This “current-source” connection of the load forces the current flowing through a cathode to flow through an anode (see Fig. 1). With this connection, slight voltage differences among cell electrodes partially compensate contact resistance dispersion and electrode misalignment sensitivity. This connection provides an intrinsic capability to withstand parameter deviations by forming paths to the current regardless of the deviations of the resistance parameters [3]. Furthermore, the arrangement generates preferred paths for the electrical current or current channels. These channels share similar circuit equivalent resistances, producing balanced currents throughout the cell. Each equivalent circuit resistance is composed of a number of contact and electrolyte resistances in series. This also means that resulting path resistances will be more balanced as the number of series cells increases. Industrial results prove that segmented bars produce a better performance than Walker bars [4]. This paper presents the results obtained for segmented bars. This choice is done considering that this technology is already better than the other ones, so making an improvement over it can be very difficult. III. C ONTACT S HAPES The electrical and mechanical tests have been realized with different contact designs. Hanger bar male cross sections and respective female bar recipient contacts include circular, rectangular, and trapezoidal shapes. A. Circular Hanger Bar and Matching Female Contact (CC) A circular hanger bar is put over a matching female contact on the bar [see Fig. 2(a)]. This shape produces surface contacts with low pressure. Initial positioning produces canted (not vertically aligned) electrodes. B. Rectangular Hanger Bar and Line Contact (RL) This is the most used contact [see Fig. 2(b)]. A rectangular hanger bar is put over a circular or triangular male shaped tooth on the bar. This connection produces a line contact with high
Fig. 2. Different contact types: (a) Circular hanger and contact recipient (CC), (b) rectangular hanger and line contact recipient (RL), (c) rectangular hanger and flat contact recipient (RF), and (d) trapezoidal hanger and contact recipient (TT).
pressure. This alignment is less preferred due to the high lateral tolerances. Alignment of the electrode is obtained only if the electrode positioning system is accurate. C. Rectangular Hanger Bar and Flat Contact (RF) A rectangular hanger bar is put over a flat contact on the bar [Fig. 2(c)]. The main disadvantage of this shape of contact is “dirt” on the contact surface. This shape produces high contact resistances and current dispersion. A position error also produces a bad alignment. D. Trapezoidal Hanger Bar and Matching Female Contact (TT) The trapezoidal hanger bar is put over a trapezoidal female shaped tooth on the bar [Fig. 2(d)]. This shape produces a high contact surface with low pressure. The “more vertical” orientation of contact surfaces reduces dirt. The “V” shape of the male acts as a penetrating spear ensuring electrical contact. The alignment produced by this shape is superior. It allows up to 12 mm of initial error positioning, ensuring self-relocation of the hanger bar to a “perfect” position. The shape also ensures a vertical alignment of the electrodes. The best property of this contact system is to stay in position under external disturbance forces. These external forces are produced by human or equipment moving over the electrodes during maintenance or harvest operations of neighbor cells. IV. E XPERIMENTAL R ESULTS A set of tests was done to evaluate the sensibility of different contact shapes to external disturbance forces. The first test, shown in Fig. 3, evaluates the force necessary to vertically misalign an electrode. The worst result is exhibited by the circular CC type. Only a 2-N force was necessary to offset in 20 mm the bottom position of a 1mt2 electrode when the circular type is used. The best result is exhibited by the TT type where a force of 60 N is required to produce the same offset.
WIECHMANN et al.: CONTACT SYSTEM DESIGN TO IMPROVE ENERGY EFFICIENCY IN CuEW PROCESSES
Fig. 3.
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Mechanical tests done with electrodes.
Fig. 5. Temperature on surface and line contacts. TABLE I P ROPERTIES OF D IFFERENT C ONTACT T YPES
Fig. 4. Thermal simulation of different contact shapes. This simulation was realized by a 3-D model finite-element-based analysis. (a) Circular hanger and matching contact recipient (CC), (b) rectangular hanger and line contact recipient (RL), (c) rectangular hanger and flat contact recipient (RF), and (d) trapezoidal hanger and matching contact recipient (TT).
The second test was measuring the horizontal force necessary to move and produce misalignment of the electrode. The worst result is obtained with the rectangular RL type (20 N). Again, the best result is produced by the TT type (600 N). This last result is of extreme relevance because open circuits are produced almost always by electrode misalignment. Electrical and thermal simulations are shown in Fig. 4. The operating temperature of the intercell bar will be benefited by 6 ◦ C when a TT type is used. The contact voltage component in CuEW is 4% [4]. The anode and cathode contacts used are typically line contacts. The resistance of this contact type is about 60 μΩ. Measurements show that, when a surface contact is used, the contact resistance is reduced. The thermographic picture in Fig. 5 shows the better performance of surface contacts. The best contact resistance is generated by the trapezoidal shape, about 30 μΩ. Also, the diagonal orientation of the surface helps to reduce “dirt” produced by electrolyte contamination. The operation of a cell begins with the load of the permanent cathodes using a crane. It is characterized by dropping the
electrodes to their position. If the hanger drop error is lower than 12 mm, circular and trapezoidal shapes produce good alignments with self-positioning to the right position. However, circular shapes tend to produce canted electrodes. The friction coefficient of the contact surface prevents the rotation of the electrodes to the vertical position. In contrast, trapezoidal contacts offer excellent vertical alignment. Therefore, the shape with the best electrical and mechanical behavior is the trapezoidal TT type. The results are shown in Table I. V. CuEW P LANT P RODUCTIVITY I MPROVEMENT The TT-type improvement in electrical contact resistances is 1.5%. For a CuEW plant rated 140 000 ton/year, this represents energy savings of 4200 MWh/year. Moreover, improvements in vertical and horizontal alignments reduce the standard deviation of electrolyte resistance by 10%. Both factors produce a 22% better current distribution. Fig. 6 shows the histogram of a plant with 65 cathodes per cell connected by standard segmented intercell bar and standard contact. Fig. 7 shows the current distribution for the same plant with trapezoidal contact shape. Extrapolating the results [4], an improvement in current distribution could generate up to 1.5% better energy efficiency with 0.5% higher current efficiency. Both factors combined could produce 3.0% reduction in specific energy (Table II).
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VI. C ONCLUSION A design concept to avoid electrode open circuits and reduce contact resistances has been presented. The design is based on a trapezoidal hanger and matching female tooth shape for the intercell bar contacts. This led to improved electrode alignments, reduced contact resistances, easier contact cleaning, and ensured electrical contacts. A 0.5% higher production with 3% reduction in energy consumption was measured. R EFERENCES
Fig. 6. Current distribution in a 65-cathode-per-cell plant with segmented intercell bars and normal contacts.
Fig. 7. Current distribution in a 65-cathode-per-cell plant with segmented intercell bars and trapezoidal contact shapes. TABLE II I MPROVEMENTS O BTAINED W ITH T RAPEZOIDAL S HAPE C ONTACT
[1] Energy consumption and greenhouse gas emissions in the chilean copper industry-events of 2008, Chilean Copper Commiss., Santiago, Chile. [Online]. Available: www.cochilco.cl [2] Chilean copper industry in front of the climatic change, Chilean Copper Commiss., Santiago, Chile. [Online]. Available: www.cochilco.cl [3] E. P. Wiechmann, G. A. Vidal, and J. A. Pagliero, “Current-source connection of electrolytic cell electrodes: An improvement for electrowinning and electrorefinery,” IEEE Trans. Ind. Appl., vol. 42, no. 3, pp. 851–855, May/Jun. 2006. [4] E. P. Wiechmann, A. S. Morales, P. Aqueveque, and R. Mayne-Nicholls, “Reducing specific energy to shrink the carbon footprint in a copper electrowinning facility,” in Conf. Rec. IEEE IAS Annu. Meeting, 2010, pp. 1–5. [5] P. E. Aqueveque, E. P. Wiechmann, and R. P. Burgos, “Short-circuit detection for electrolytic processes employing Optibar intercell bars,” IEEE Trans. Ind. Appl., vol. 45, no. 4, pp. 1225–1231, Jul./Aug. 2009. [6] P. Aqueveque, E. P. Wiechmann, and A. S. Morales, “System for the measurement of cathodic currents in electrorefining processes that employ multicircuital technology,” IEEE Trans. Ind. Appl., vol. 46, no. 5, pp. 1764–1768, Sep./Oct. 2010. [7] H. Aminian, C. Bazin, D. Hodouin, and C. Jacob, “Simulation of a SX-EW pilot plant,” Hydrometallurgy, vol. 56, no. 1, pp. 13–31, May 2000. [8] G. Barton and A. Scott, “Industrial applications of a mathematical model for ther zinc electrowinning process,” J. Appl. Electrochem., vol. 24, no. 5, pp. 377–383, May 1994. [9] I. Filzwieser and P. Pashchen, “Cathodic current distribution in a Cu refining electrolysis,” METTOP, Leoben, Austria. [10] I. Filzwieser, A. Filzwieser, and R. Ofner, “Geometric surface inspection for stainless steel cathode plates and copper cathodes,” BHM Berg- Hüttenmännische Monatshefte, vol. 155, no. 1, pp. 7–11, Jan. 2010.
Eduardo P. Wiechmann (S’81–M’86–SM’94) received the B.S. degree in electronics engineering from the Federico Santa María Technical University, Valparaiso, Chile, in 1975 and the Ph.D. degree from Concordia University, Montreal, QC, Canada, in 1985. Since 1976, he has been with the University of Concepción, Concepción, Chile, where he is currently a Professor with the Department of Electrical Engineering. He has published numerous technical papers and has coauthored technical books. His research interests include power converters, high-current rectifiers, and copper electrorefining and electrowinning. His industrial experience includes more than 8000 hours in engineering projects and consulting. Dr. Wiechmann was the recipient of the 2000 Concepción City Award for Outstanding Achievements in Applied Research.
Pablo Aqueveque (S’05–M’08) was born in Santiago, Chile, in 1976. He received the B.S. degree in electronics engineering and the Ph.D. degree from the University of Concepción, Concepción, Chile, in 2000, 2002, and 2008, respectively. He is currently an Assistant Professor with the Department of Electrical Engineering, University of Concepción. His research interests include digital devices, electrochemical processes, power converters, and electrical stimulation.
WIECHMANN et al.: CONTACT SYSTEM DESIGN TO IMPROVE ENERGY EFFICIENCY IN CuEW PROCESSES
Guillermo A. Vidal received the B.Sc. degree in electronics engineering, the M.Sc. degree in electrical engineering, and the D.Sc. degree in metallurgical engineering from the University of Concepción, Concepción, Chile, in 1998, 2000, and 2005, respectively. He is currently a Zigbar Ltd. partner and coholder of six patents and coauthor of four IEEE T RANSACTIONS papers.
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Jorge A. Henriquez was born in Concepción, Chile, in 1986. He received the B.S. degree and the B.Eng. degree in electronics engineering from the University of Concepción, Concepción, Chile, in 2008 and 2011, respectively, where he is currently working toward the Sc.D. degree in electrical engineering. His current research interests include R&D in electrochemical processes and mining applications.
