Short-Circuit Detection for Electrolytic Processes ... - IEEE Xplore

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 45, NO. 4, JULY/AUGUST 2009

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Short-Circuit Detection for Electrolytic Processes Employing Optibar Intercell Bars Pablo E. Aqueveque, Member, IEEE, Eduardo P. Wiechmann, Senior Member, IEEE, and Rolando P. Burgos, Member, IEEE

Abstract—This paper presents a method to detect metallurgical short circuits suitable for Optibar intercell bars in copper electrowinning and electrorefining processes. One of the primary achievements of this bar is to limit short-circuit currents to a maximum of 1.5 p.u. of the actual process current. However, low-current short circuits are more difficult to detect. Thus, conventional short-circuit detection instruments like gaussmeters and infrared cameras become ineffective. To overcome this problem, the proposed method is based on detecting the voltage drop across anode–cathode pairs. The method does not affect the operation of the process and does not require modifications of the industrial plant. In order to verify the performance of this proposal, experimental measurements done over a period of four months at a copper refinery are presented. A 100% success rate was obtained. Index Terms—Copper electrorefining, electrochemical processes, process monitoring.

I. I NTRODUCTION

A

BOUT 60% of the world copper production uses either electrorefining (ER) (54%) or electrowinning (EW) (6%) processes. During the past ten years, the electrolytic production level has doubled [1]. These electrochemical processes presently use the Walker configuration to interconnect the electrolytic cell electrodes. This solution uses equipotential bars connecting the anodes of one cell with cathodes of the next cell. Therefore, multiple parallel anode cathode pairs are connected to common bars. A drawback of this approach is that, due to physical and electrochemical asymmetries in the process, the electrical resistance between different anode–cathode pairs normally differs significantly, which generates current imbalances among the cathodes across the cell with detrimental effects on the copper production [2]–[5]. Paper PID-08-21, presented at the 2007 Industry Applications Society Annual Meeting, New Orleans, LA, September 23–27, 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. Manuscript submitted for review December 15, 2007 and released for publication January 5, 2009. First published May 19, 2009; current version published July 17, 2009. This work was supported in part by the Chilean Fund for Science and Technology development (CONICYT) under Project 1060902, in part by the National Science Foundation under NSF Award EEC-9731677, and in part by the CPES Industry Partnership Program. P. E. Aqueveque and E. P. Wiechmann are with the Department of Electrical Engineering, Faculty of Engineering, University of Concepcion, Concepcion 4070386, Chile (e-mail: [email protected]; [email protected]). R. P. Burgos is with the Center for Power Electronics Systems, The Bradley Department of Electrical and Computer Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA (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.2009.2023357

Fig. 1. Electrolytic tankhouse cells and electrodes.

An alternative cell interconnection technology was presented in [4] using Optibar intercell bars. These bars connect the anode–cathode pairs of contiguous cells in series. The result is better current balance, increase in current efficiency, and, improved physical and chemical properties of the copper cathodes. Among the benefits of this technology is the reduction of the short-circuit level [4], [5]. Nowadays, processes are carried out in tankhouses (Fig. 1) with several hundreds of cells. An average tankhouse contains around 50 000 cathodes in four groups of 250 cells with 50 cathodes each [2]. Normally, 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 electrochemically deposited metal quantity, and quality, depends on the applied current. In addition, a modern plant will obtain its better performance when all the cathodes operate at 640 A (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 440 and 640 A can be encountered in the same cell. This negative effect is produced by slight parameter variations that are amplified by the electrodes electrical connection. A most damaging yet common phenomenon in EW and ER processes is the metallurgical short circuit. These short circuits produced in the electrolytic cells can drastically affect the efficiency of the process. During a short circuit, copper is

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TABLE I VOLTAGE C OMPONENTS OF A C OPPER ER C ELL

Fig. 2.

Metallurgical short circuit observed at the surface of a copper cathode.

deposited over a very small area of the cathode in question, which reduces the distance between adjacent electrodes and consequently the electrolytic resistance between them (Fig. 2). As a consequence, large amounts of energy may be dissipated during long periods of time, negatively affecting copper deposition on neighboring cathodes. Conventional equipotential bars, as the ones used in the Walker configuration, are intrinsically subject to high overcurrents during a metallurgical short circuit—up to 3 p.u. of the cathode process current—due to the equipotential voltage imposed across each cell. For short-circuit detection, handheld gaussmeters or infrared cameras are used [6], [7]. However, with the use of Optibar intercell bars, short-circuit currents do not exceed 1.5 p.u. of the process current. This represents a significant advantage due to the direct positive effect on copper production. The downside is that the resultant magnetic field and temperature increase are only slightly higher during short-circuit events rendering conventional short-circuits detection methods ineffective. Therefore, the goal of increasing the production level by 10% using Optibar intercell bars may be compromised. This paper presents a solution 100% effective and compatible with online short circuits monitoring of the tankhouse.

Fig. 3.

EW chemical reaction diagram.

Fig. 4.

EW and ER equivalent circuit.

II. P ROCESS E LECTRICAL M ODEL Due to the slow dynamics of the electrochemical process a steady state model represents the voltage distribution behavior with enough accuracy [9], [10]. Table I shows the voltage distribution and Fig. 3 shows the chemical reaction during copper refining processes. It is shown that the electrolyte resistance produces the higher voltage component. This voltage can be reduced by lowering the space between electrodes. However, with less than 6 cm, the plant exhibits problems due to impaired electrolyte circulation and misalignment of the anodes. The second voltage component is the cathode polarization voltage and the necessary overpotential for the cathode current to flow. This is required for the production of the chemical reaction. The anode also requires polarization [11]. In copper ER is lower than 10 mV because a copper cathode is used. For copper EW,

the polarization voltage is approximately 1230 mV. The last voltage component is produced by the anodes and cathodes electrical contact connections. These contact connections are exposed to dust and acid. An average resistance of 60 μΩ is usually encountered. Fig. 4 shows the connections and the equivalent circuit.

AQUEVEQUE et al.: DETECTION FOR ELECTROLYTIC PROCESSES EMPLOYING OPTIBAR INTERCELL BARS

Fig. 5.

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Walker busbar configuration.

A. Walker Configuration Fig. 6. Electric model for Walker configuration.

Equipotential intercell bars used by industry base its performance in ensuring equal voltage along the intercell bar. However, contact resistance dispersion and electrode alignment reduce its effectiveness. In this solution, cathodes of one cell are connected to the anodes of the adjacent cell [3] throughout a single potential bar. Fig. 5 shows the disposition and connection of the anodes to the upper intercell bar and the cathodes connected to the lower intercell bar. The equivalent circuit developed, considers the contact resistance of the upper intercell bars with the anode, the electrolytic resistance in series with the reaction voltage (cathode and anode polarization), and the contact resistance between the cathode and the lower intercell bar. Fig. 6 shows the resultant circuit where “A” stands for anode, “Rc” for contact resistance, “Re” for electrolytic resistance, “Vr” for reaction voltage, and “C” for cathode. In the event of a short circuit, the Re gradually reduces to zero ohms. The “voltage source” parallel electrode connection reacts to this reduction with an increasing short-circuit current. In Copper ER, the cathode current reaches levels over 3 p.u. of the process current (this also means that the cathode face affected by the short circuit will confront a current densities of at least 6 p.u.). ER cathode currents measurements confirm short circuits with levels around 1400 A for a 500-A set point. B. Optibar Configuration With the Optibar intercell, a single cathode of an upper cell is connected to a single anode of a lower cell (see Figs. 7–9). This “current source” connection of the load forces the current flowing through a cathode to flow through an anode. This allows slight voltage differences among cell electrodes to compensate contact resistance and alignment dispersions.

Fig. 7. Optibar busbar configuration.

The aim of this invention was to inhibit a short-circuit formation and to equalize the current distribution between electrodes. Short circuits are formed on the cathode surface when a higher current density is observed. In this case, the metal deposit abnormally grows (see Fig. 2) while the Re gradually is reduced. The metallurgical short-circuit process progresses until it eventually reaches the anode establishing a solid electrical

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Fig. 8. Optibar segmented intercell bar in operation. Fig. 10.

Infrared method for short-circuit detection.

When a short circuit occurs, Re diminishes significantly, and so the voltage across points A and C. This voltage falls to values under 50% of the normal operating value, thus providing an easy way to detect short circuits. Moreover, this method is highly compatible with the operation of EW and ER plants since it does not involve any additional parts over the electrodes. The voltage measurement may be realized by simple circuitry placed inside the Optibar intercell bar. Therefore, does not interfere with the regular harvesting and electrode replacement operations. In addition, since the voltage sensor is fixed to the bar interconnecting the copper segments of the Optibar, there is no risk of mishandling and disconnection by operators walking (inspecting, maintaining) the cells. III. S HORT-C IRCUIT D ETECTION M ETHODS Conventional detection methods were unable to operate with the Optibar intercell bars. A. Temperature Based Using Infrared Cameras

Fig. 9. Electric model for Optibar configuration.

path between anodes and cathodes. This phenomena highly reduce the process efficiency. In the Walker arrangement, a reduction of the resistance between an anode and a cathode results in a high current flow through these electrodes. With the use of the Optibar an identical resistance variation results in a smaller current deviation. 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 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.

This method is based on the surface temperature of the cells. Outukumpu developed this technique for the Walker configuration [7]. In the absence of short circuits, the thermal image is homogeneous. In the event of short circuits, the infrared image clearly shows the electrodes compromised (Fig. 10). However, Optibar intercell bars reduce the short-circuit currents to 1.5 p.u. Therefore, only a slight increase in the temperature is produced and cannot be detected by thermal images. Furthermore, if a short circuit is produced using Optibar, it will affect adjacent upper and lower electrodes because of the series connection. This is shown in Table II, cathode 21 of the cell 4 and 5. The method is not applied because a canvas is usually placed covering the cells to reduce electrolyte temperature losses (see Fig. 8). B. Magnetic Field Detection The Gaussmeter-based method usually employed is also ineffective for the Optibar. The instrument measures the intensity of

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TABLE II C ATHODE C URRENTS IN THE V ICINITY OF A S HORT C IRCUIT

Fig. 12. Voltmeter connection for anode–cathode voltage measurement. The point for measurement is exactly over the “S”-shaped Optibar segment.

Fig. 11. Gaussmeter used at the industrial facility for short-circuit detection.

the magnetic field which is proportional to the cathode current [8]. In the conventional case, the field intensity is about 3 p.u. Fig. 11 shows a Gaussmeter for short-circuit detection. There are two reasons to discard the use of the Gaussmeter on the Optibar intercell bars. 1) The Gaussmeter is not sensitive to slight current deviations. On site, the reading of the instrument did not detect short circuits. If a more sensitive Gaussmeter were used, the resultant readings will depend on the instrument distance from the measured electrodes making its human based use impractical. 2) Even if a Gaussmeter successfully detects the overcurrent, it will also detect adjacent electrodes overcurrents. This will impair the operator decision judgment. C. Proposed Method: Voltage-Based Short-Circuit Detection The method proposed in this paper is based on the measurement of the anode–cathode voltage. This measurement is implemented directly on the Optibar connection segments (no moving parts of the process). Once more, the duality between voltage and current holds. With the voltage-sourced Walker intercell bars, short circuits are detected by measuring currents. While with the current-sourced Optibar intercell, short circuits are to be detected by measuring voltages. In fact, the voltage in

short circuit compromised electrodes varies quite significantly making the detection straightforward. Table I shows the voltage components in a industrial ER cell. The main voltage component is the electrolyte voltage. In a short-circuit event, Re is reduced along with the formation of the copper dendrite (Fig. 2). This produces a voltage reduction to a 50% of the cell voltage. The voltage is measured between a cathode connecting segment and the two upper anode connecting segments. This shows whether a short circuit exists on any of the cathode faces. Fig. 12 shows the procedure. The method is also useful in detecting open circuits that represent the second more relevant phenomena after short circuits in ER and EW processes. The system must be implemented inside the intercell in order to avoid interference with the process operation. This measurement is implemented directly on the Optibar connection segments (no moving parts of the process). Short circuits are detected at preliminary stages of formation. The short-circuit status of the tankhouse can be refreshed every 10 s. A schematics diagram of the automated proposed method is shown in Fig. 13. IV. E XPERIMENTAL V ERIFICATION AT I NDUSTRIAL S ITE The proposed method has been verified at a copper refinery facility in Chile. The voltage across the copper segments of an Optibar intercell bar were measured, as shown in Fig. 12, three times at day during a period of four months, providing the information of all individual anode–cathode pairs (more than 3500 measurements). As an example, Table III shows voltages measured at five different anode–cathode pairs on contiguous cells. As seen, cathode 21 of cell 4 presents a distinctive voltage reduction (159.6 mV) between itself and the adjacent anode, the first one being nearly half of the value of the second one and of the remaining anode–cathode voltage readings. When

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TABLE IV C OMPARATIVE R ESULTS M EASURED OVER 12 ER C ATHODIC C YCLES (F OUR M ONTHS ). R ESULTS W ERE O BTAINED F ROM O NE S ECTION W ITH T EN WALKER I NTERCONNECTED C ELLS AND T EN O PTIBAR I NTERCONNECTED C ELLS

determine Optibar short-circuit formation in an early stage of progress. V. C ONCLUSION

Fig. 13. Schematics diagram of the automated proposed method. TABLE III VOLTAGES ACROSS E LECTRODES PER C ELL

physically examining the corresponding cathode, the existence of a metallurgical short circuit was effectively verified. During the four-month period the proposed scheme proved to be a 100% effective in the detection of short circuits. On the experimental industrial site, it was required the use of a Fluke 189 multimeter and a Fluke i1010 current clamp because the Optibar intercell being tested were not manufactured with voltage and current sensing capabilities. As voltage reference point, the negative terminal of the instrument was connected to the negative section bar header. Each section comprises 20 cells. The voltages being measured were stored with the aid of a Notebook. A suitable algorithm was written for short- and open-circuit detection. For comparison purposes, the industrial test was done in a section with ten cells interconnected with Optibar and ten cells connected with the conventional Walker system (see Table IV). The method based on voltage drop was 100% successful to

The use of Optibar intercell bars for electrolytic processes enables the detection of metallurgical short circuits with minimum impact on the process operation. In fact, the proposed detection scheme does not require any type of modification of the intercell bar or the mounting of any equipment on the electrodes. Short circuits are detected by measuring the voltage across the Optibar segments which correspond to the cell anode–cathode voltages. In this way, the harvesting and maintenance of the electrolytic process are not affected. The proposed detection scheme has been verified experimentally by monitoring a copper refinery in Chile during a period of four months with a 100% success rate. R EFERENCES [1] Chilean Copper Commision, Yearbook: Statistics of Copper and Other Minerals 1986–2005, Santiago, Chile, 2007. [Online]. Available: www. cochilco.cl [2] 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. [3] A. Walker, “Plant for the electrodepositación of metals,” U.S. Patent 687 800, Dec. 3, 1901. [4] E. Wiechmann, G. Vidal, and 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. [5] E. P. Wiechmann, G. A. Vidal, J. A. Pagliero, and J. A. Gonzalez, “Copper electrowinning using segmented intercell bars for improved current distribution,” Can. Metall. Q., vol. 41, no. 4, pp. 425–432, 2001. [6] S. B. Borg, “Infrared detection of invisible shorts in electrolytic copper refining tanks,” IEEE Trans. Ind. Electron. Control Instrum., vol. IECI-18, no. 2, pp. 36–37, May 1971. [7] E. Makipaa, J. T. Tanttu, and H. Virtanen, “IR-based system for shortcircuit detection during copper electrorefining process,” in Proc. SPIE— Machine Vision Applications in Industrial Inspection VII, Mar. 1999, vol. 3652, pp. 2–9. [8] R. Bittner, L. Salazar, M. Valenzuela, and A. Pagliero, “Modeling the electric field and potential of an electrowinning cell with bipolar floating electrodes,” in Proc. 24th Annu. IEEE IECON, Aug. 31–Sep. 4, 1998, vol. 1, pp. 365–370. [9] H. Aminian, C. Bazin, D. Hodouin, and C. Jacob, “Simulation of a SXEW pilot plant,” Hydrometallurgy, vol. 56, no. 1, pp. 13–31, May 2000. [10] G. Barton and A. Scott, “Industrial applications of a mathematical model for the zinc electrowinning process,” J. Appl. Electrochem., vol. 24, no. 5, pp. 377–383, May 1994. [11] J. A. Gonzalez, “Zinc electrowinning: Anode conditioning and current distribution studies,” in Proc. Electrometallurgy, 2001, pp. 147–162.

AQUEVEQUE et al.: DETECTION FOR ELECTROLYTIC PROCESSES EMPLOYING OPTIBAR INTERCELL BARS

Pablo E. Aqueveque (S’05–M’08) was born in Santiago, Chile, in 1976. He received the B.S., Electronics Engineering, and Ph.D. degrees from the University of Concepcion, Concepcion, Chile, in 2000, 2002, and 2008, respectively. He is currently an Assistant Professor in the Department of Electrical Engineering, University of Concepcion. His research interests include modern digital devices, high-current rectifiers, electrochemical processes, and power converters.

Eduardo P. Wiechmann (S’81–M’86–SM’94) received the Electronics Engineering degree from Santa Maria University, Valparaiso, Chile, in 1975, and the Ph.D. degree from Concordia University, Montreal, QC, Canada, in 1985. Since 1976, has been with the Department of Electrical Engineering, University of Concepcion, Concepcion, Chile, where he is currently a Professor. His research interests are power converters, highcurrent rectifiers, ac drives, uninterruptible power systems, harmonics, and power factor control in industrial power distribution systems. His industrial experience includes more than 6000 hours in engineering projects and consulting. He has published numerous technical papers and has coauthored technical books. Dr. Wiechmann was the recipient of the year 2000 Concepcion City Award for Outstanding Achievements in Applied Research.

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Rolando P. Burgos (S’96–M’03) received the B.S., Electronics Engineering, M.S., and Ph.D. degrees from the University of Concepción, Concepción, Chile, in 1995, 1997, 1999, and 2002, respectively. In 2002, he was a Postdoctoral Fellow with the Center for Power Electronics Systems, Virginia Polytechnic Institute and State University, Blacksburg, where he is currently Research Assistant Professor. His research interests include multiphase power conversion, control theory, hierarchical modeling, and the synthesis of power electronics conversion systems for sea, air, and land vehicular applications.