Characterization of Individual Anode Current ... - ACS Publications

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Characterization of Individual Anode Current Signals in Aluminum Reduction Cells Cheuk-Yi Cheung, Chris Menictas, Jie Bao,* Maria Skyllas-Kazacos, and Barry J. Welch School of Chemical Engineering, The University of New South Wales, UNSW, Sydney, NSW 2052, Australia ABSTRACT: Differing from cell voltage and line current, anode current signals can provide an insight into the localized anodic dynamic behavior in an operating Hall−Héroult reduction cell, and can be used as an alternative in-depth method to study the process in the hostile industrial potline environment. This work involves further investigations on changes in the frequency response of anode current signals (such as peaks and magnitudes) with anode age and anodic reactions. Furthermore, two process abnormalities, anode effect and anode slippage, are studied. This study demonstrates that anode current signals provide an earlier indication of an approaching anode effect than the conventional cell voltage measurements. Frequency domain analysis has been found to be an additional identifier, in separating anode effects from other abnormalities, that can also cause anode current redistribution.

1. INTRODUCTION In the Hall−Héroult process, aluminum is produced via electrochemical reactions in the alumina cryolite melt, “the bath”. All anodic reactions taking place in the reduction cell generate gases, forming a nonconducting gaseous layer on the underside of anodes that contributes to additional cell resistance.1 The gaseous bubbles constituting the layer are continuously generated but periodically released, leading to periodic fluctuations in cell resistance due to changes in bubble layer thickness and coverage of the anode surface.2 This gaseous layer, commonly referred to as the bubble layer, plays an important role in cell operation and is the major driver for electrolyte mixing. Recent research has been concentrated on the study of bubble behavior and its impact on the process.3−7 Laboratory and simulation studies have demonstrated that the cell resistance fluctuation rate caused by the bubble behavior is affected by the anode current density, anode geometry and inclination, the presence of cut slots, as well as the possible surface tension.2,8 As a result, cell condition can be characterized on the basis of the observed bubble dynamics reflected by the variation in cell resistance. Studies of the bubble generation and release processes are mostly conducted in laboratory cells or by computer simulations.8,9 Most of these studies have been based on a single electrode cell arrangement, with the equivalent circuit shown in Figure 1. The response to changes in conditions for such cells depends on how the variation is achieved, and whether the cell is under current control, electrode potential control, or total cell voltage control.5,10 For instance, the impact of potential and voltage control10 at different alumina concentrations is characterized by a decrease in current when there is a change in anodic process to one that involves fluoride ion co-oxidation.11 This phenomenon is referred to as the anode effect (AE). Even at potentials below that required for direct electrochemical formation, however, fluoride gases are characteristically evolved at high voltages in industrial and laboratory cells, which are under constant current control. Figure 2 shows a schematic diagram of a multianode cell that incorporates typical spatial conditions that commonly occur © 2013 American Chemical Society

within a cell. Path resistance presented along each anode has all of the additional components illustrated in Figure 1. Accordingly, the current flowing through each anode is controlled by the actual cell voltage at that instance combined with the spatially existing conditions that affect each of the equivalent resistance components. Meanwhile, the summation of all currents through each of the anodes is controlled by the overall current supplied to the cell. Consequently, numerical studies and experimental investigations on laboratory cells have limitations on simulating population dynamics of bubbles at multiple anodes in the complicated environment of a full scale reduction cell. On the other hand, studies conducted in industrial cells have found that bubbles can behave differently at each anode as reflected by the anode current measurements.12,13 For example, Barber12 studied the change of anodic process dynamics under different cell conditions by investigating continuous current variations and intermittent frequency response of anode currents at distinct time frames. A similar study has also been conducted by Bearne and co-workers.13 Both studies showed that the bubble behavior at individual anodes varies under the influence of different factors including anode age, orientation, inclination, and slot design. These results suggested that the change of individual anode current signals occurring in both time and frequency domains may provide further insight into local cell conditions in industrial cells. An example is the anode current redistribution before the cell enters full anode effect, as illustrated in the literature.14−16 It has been suggested that the redistribution of anode currents is caused by the reduction of alumina concentration adjacent to one of the anodes. Because of the extremely high greenhouse gas warming potential of perfluorocarbons (PFC) that result during an AE,17 early detection of an approaching AE is important to enable further Received: Revised: Accepted: Published: 9632

January 25, 2013 June 17, 2013 June 18, 2013 June 18, 2013 dx.doi.org/10.1021/ie400296u | Ind. Eng. Chem. Res. 2013, 52, 9632−9644

Industrial & Engineering Chemistry Research

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Figure 1. Path resistance of a single anode.

Figure 2. Some common local cell conditions affecting anode current distribution in a multiple anode cell through work practices, design, and control.

reduction of PFC emissions and to also help identify possible low voltage PFC emissions not detectable only by the measurement of cell voltage.18 These studies show that measurement of individual anode current signals holds great

promise as an early indicator of an approaching AE as opposed to conventional detection. While individual anode current measurements have previously been explored for this purpose, however, so far only time-domain analysis has been presented. 9633

dx.doi.org/10.1021/ie400296u | Ind. Eng. Chem. Res. 2013, 52, 9632−9644


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temperature, it immediately forms a coating of frozen bath around it, which can typically extend down to the metal pad.22 Therefore, the initial rate of current pickup for a new anode is relatively slow. In addition to anode replacement, there are spatial variations associated with work practices, including varying alumina feeding, disturbances to electrolyte flow, dissolution, and mixing, which can result in individual anodes having different voltage/current response characteristics. As individual anodes in an industrial cell are under constant voltage constraint rather than constant current constraint, spatial variation present in the cell will cause anode current variation between anodes even though the line current is regulated. The following sections describe cell conditions commonly found in the process that can give rise to spatial variations. 2.2. Bubble Dynamics and Resistance. The anodically evolved gas is nonconducting, and forms a discrete gas/ electrolyte layer underneath the anode surface, which increases the resistance above that of the bulk electrolyte. It is referred to as bubble resistance, Rbubble. The average value of this resistance is dependent on the anode current, angle of inclination or shape of the anode surface, electrolyte composition, the distance or the path the gas travels, and the presence of slots.3,5,13,23 In general, the gas is partly released in surges, typically at a frequency between 0.5 and 2 Hz, causing a fluctuation in resistance and thus cell voltage fluctuations up to 0.25 V in magnitude.24,25 Because of its direct link with line current, the bubble resistance can have an impact on anode current distribution.26 Because other resistance components are relatively constant in a short-term period, bubble resistance can be observed from oscillation of anode currents that fluctuate periodically as large pockets of gas are swept away from the surface each time bubbles are released.5 Alternatively, gas release can be observed from voltage oscillations in a single electrode laboratory cell under constant current conditions.27 The magnitude of bubble resistance at each anode depends on the bubble layer thickness and gas coverage on the anode surface, which vary according to the bubble generation and release rates.2 While anode current density determines the bubble generation rate according to Faraday’s law, it also affects the bubble release frequency, along with changes in local conditions.8,13 It has also been found that bubbles can behave differently under the influence of various parameters, as reflected by the degree of fluctuation in resistance.25,28,29 When rounding of an anode occurs as a consequence of consumption at both the anode sides and under surface, a slope will form at peripheries and aid the flow and release of bubbles, leading to an increase in bubble release frequency.7,30 The presence of slots also gives a similar effect, as it significantly reduces the path traveled by bubbles.9 As anodes age, the impact of the slots will change as a consequence of a reduction in their depth, and ultimately the disappearance of the slots because they are not made of sufficient depth to be present for the full life of the anode.13 As the presence of the slots under individual anodes ceases, changes are seen in the bubble release frequency of the slotted anodes. In the case of fluorocarbon coevolution, the bubble growth and shape will also change with wetting angle, and hence the release frequency and anode current will change simultaneously.31 From the above scenarios, it is clear that a study of fluctuations in anode currents, together with bubble release frequency, can be a valuable aid in understanding and identifying local changes that occur within the cell.

This Article presents results from studies of anode current signal responses arising under selected cell conditions that can cause short-term variability in anode current distribution. The relationship between bubble dynamics and local cell conditions is further investigated by analyzing anode current signals measured under various operating conditions in an industrial cell, in both time and frequency domains. The impact of anode age, the presence of slots, and anode replacement on the bubble behavior are also explored. In this work, deliberate disturbances were introduced to reproduce and simulate the occurrence of abnormalities (anode slippage and anode effect). The resulting signal patterns are used as an aid in characterizing likely causes of undesirable cell conditions. Possible mechanistic explanations are also presented to explain the observed phenomena based on the signal analysis. It is recognized that unique solutions of operating problems can rarely be made on one measurement alone, but it is clear that anode signal analysis offers extra diagnostics. On the basis of the analysis, it has been found that frequency response of anode current signals can be used as an additional identifier to discriminate between abnormalities, which also cause anode current redistribution, for example, an approaching AE. As the distributed nature of the anode current signals, coupled with the time and frequency domain analysis, allows characterizations of different local faults, they can also be used for future development of a fault diagnosis system that can target local process abnormalities. It will be important to the Hall−Hérout process, especially in reducing potent greenhouse gas emission, by eliminating approaching AEs through early detection and isolation of the onset of local AE. This Article is organized as follows: section 2 describes the background and the distributed nature of the Hall−Héroult process; section 3 provides information of the instrumentation setup, and a description of the analytical method carried out in this work; typical signal responses are presented in section 4; signal responses showing abnormal operating conditions are discussed in section 5, followed by conclusions in section 6.

2. RELEVANT BACKGROUND OF THE HALL−HÉROULT PROCESS 2.1. Modern Cell Design, Operations, and Control. Modern prebaked technology reduction cells have common design and operational features, including multiple prebaked anodes and several point-feeders for replenishing the alumina distributed along the center channel.19 The number of anodes and feeders can vary ranging from 18 to 48 individual blocks and from 3 to 6 feeders per cell, respectively, depending on the cell size.20 Reduction cells are interfaced in series enabling a higher DC line voltage so the cells tend to operate at constant line amperage, Icell. Individual cells are controlled to an average cell voltage, so that sufficient energy is available for the metal production reactions and simultaneously maintain heat balance.1 Practically the voltage of the individuals cells is allowed to vary within a control band to maintain the dissolved alumina concentration within acceptable limits.21 Because of the continuous anode carbon consumption, a moveable anode beam is used to maintain the voltage/heat balance and anode cathode distance (ACD) by compensating the difference between the carbon consumption rate and the metal pad growth between periods where the metal is tapped. The anode replacement is staggered with each anode being replaced at a fixed interval, typically somewhere between 24 and 30 days. Because each new anode is introduced into the cell at room 9634



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2.3. Anode Current Distribution. Figure 1 illustrates an equivalent circuit of a single anode cell. There are several resistances within the circuit, some of which are relatively constant within a limited time frame, for example, the anode resistance, Ranode, and the cathode resistance, Rcathode; yet other resistances vary according to the local cell condition and operating current density for that anode. Beside bubble resistance, for instance, the resistance of the bath, Rbath, changes a small amount as a function of alumina concentration, temperature, and electrolyte composition, and a greater amount as a function of ACD. On the other hand, the equivalent resistance representing the interfacial electrode potential gradient at the anode (Ean) is strongly dependent on the current density, electrolyte composition, and alumina concentration because of the polarization/overpotential.32 Research on the cathode reaction has demonstrated that while there is some cathodic polarization, which is also current density dependent, it is insensitive to alumina concentration, and changes very little with temperature and aluminum fluoride concentration. In a single anode cell, when the reactions take place under constant current supply, the cell voltage is strongly dependent on the resulting interfacial electrode potentials and cell condition. This is in contrast to industrial cells where anodes are connected in parallel. As the voltage at each of the parallel anode current paths is the same, the magnitude of the current flowing through each anode, anode current33 (ik), varies according to the resulting Nernst potentials, overpotentials, and localized electrolyte conditions. For instance, the sum of the Nernst potential and the overpotentials (commonly referred to as the back EMF) is strongly dependent on spatial cell conditions, such as anode current density, local alumina concentration, electrolyte composition, and temperature. The changes magnify at the high current densities and lower alumina concentration that tend to be used in modern reduction cells. Furthermore, spatial magnification of alumina concentration gradients occurs when mixing is inhibited as the freshly formed bath freeze around the new anode acts as a flow barrier giving rise to different local electrolyte conditions.3,34 Consequently, the localized back EMF becomes important in influencing anode current distribution. As a result, the response at each anode is dominated by voltage control because variations in the current are not restricted, but rather a consequence. The resulting change in anode current will lead to only a small variation in overall cell voltage except when a significant number of anodes simultaneously have a very low alumina concentration adjacent to their interface. The local conditions can become more imbalanced when an anode is replaced in the cell near a point feeder because the decrease in superheat that results can retard alumina dissolution, especially in cells with lower bath volume per anode and fewer feeders per kiloampere.14,15,35 Some examples of situations frequently cited as causing imbalanced anode current distribution within a reduction cell are depicted in Figure 2. These can arise through inaccuracies and spatial effects associated with work practices and include anode replacement (anode (b)), freeze disturbing mixing (anode (a) from a distant feeder and anode (c) adjacent to a feeder), an anode being set too high (anode (d)), or a low set anode (anode (e)). If an anode touches the metal, this will result in a direct short circuit, leading to a much higher anode current without any electrochemical processes occurring at the shorted electrode.12 The conditions described above are examples of variations that can concurrently exist in different

parts of the cell. Therefore, interfacial potential gradient changes can occur that enable different electrochemical reactions, especially in the zones with low alumina concentration. The dominant gas production then changes from carbon dioxide to carbon monoxide and carbon tetrafluoride coevolution.36 Dewetting and intermediate resistive films then form and inhibit current flow to the affected anode, thus leading to anode effect. As the total current input is regulated in the process, anode current redistributes when fluorocarbon coevolution occurs, and voltage increases.

3. PROCESS MEASUREMENTS As anode currents provide spatial information of the cell, they have been used in different areas, including cell monitoring and estimation of energy distribution.14,33,37−39 In this work, individual anode current signal responses associated with variations in local conditions are investigated. 3.1. Measurements Setup. In setting up a data acquisition system for collecting individual anode current signals, a number of considerations need to be taken into account, including signal pick-up and transmission. In the experiments presented in this Article, the individual anode currents were obtained by measuring the voltage drop along the anode rods over a set distance in the location between the bottom of the anode beam and above the cell hood. This distance was constrained to less than 3 cm due to cell design. As such, the expected voltage drop required signal amplification to have an adequate signal-tonoise ratio in the measurements reaching the data acquisition system. Individual signal amplifiers were specifically designed and fabricated for mounting near the signal source close to the anode beam. They were powered by individual batteries with all components capable of operating at temperatures above 70 °C. Wires used to carry the anode current signals from the signal source to the data acquisition system were capable of withstanding the high temperature and corrosive environment. The measuring device also consisted of a type K Teflon coated thermocouple wire used to monitor the change of anode rod temperature. As the anode rod resistivity varies with temperature, the temperature of the rod at the vicinity of the measuring point was measured for the purpose of calibrating resistance values for use in converting the voltage drop signals into anode current signals. The data acquisition system was developed using a National Instruments NI compact Rio controller, with built-in solid state memory, together with analogue input and thermocouple input modules. The data acquisition system was powered by isolated DC battery packs, and exhibited reliable and stable operation in the harsh environment of aluminum reduction cells, with high temperatures and magnetic fields. 3.2. Data Acquisition of Cell and Anode Behavior Patterns. To understand the general behavior of the process, acquisition of anode current signals was conducted in an operating reduction cell at a sampling rate of 10 Hz on a continuous basis for over 1 year. The sampling rate was well above the Nyquist frequency of the dynamics related to bubble releases as reported in the literatures.8 The signals were continuously recorded under normal operation and compared to those acquired under “abnormal conditions”. In this study, experiments were conducted on the same cell to generate signal responses in “abnormal conditions” caused by anode effect and anode slippage. The anode current signals were analyzed using windowed Fourier transform to study variations in the 9635



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Figure 3. Anode numbering layout and feeder locations in the experimental reduction cell.

Figure 4. Anode current profile of a newly replaced anode.

practices, including metal tapping and alumina feeding, have less effect on the distribution. The recorded anode current profile of a new anode (anode 9) after being set in the bath is presented in Figure 4. Note that anodes are replaced on an individual basis in this experimental cell. The anode current profile therefore only shows the current pick-up caused by the corresponding newly replaced anode. While there will be a current redistribution, there is a potential to impact and redistribute over 19 other anodes, leading to small current changes per anode. Three distinct regions are identified from the profile shown in Figure 4. Each region shows various rates of current uptake and different degrees of current fluctuation. The current signal reflects the anode condition at different stages during the heating process in its early life. When a new anode is set in the bath, a large amount of heat is extracted from the bath and transferred into heating the new cold anode, freezing the surrounding bath.1,41 In comparison to the freeze formed at the bottom of the new anode, freeze formed at the sides has better access to the thermal energy produced by the strong vertical bath movement induced by bubble released at neighboring anodes. As a result, melting and cracking of the freeze first occurs at the sides of the new anode. This leads to bubble generation taking place at the vertically oriented surface, and the rounding of the periphery of the anode due to carbon consumption caused by the sideways current.12 The current therefore flows from the anode sides to the bath, as reflected by the initial fast update of current shown in region I. Current fluctuation is minor in comparison to that in other regions, because bubbles are not retained on the surfaces. This low level of noise might also suggest that there was negligible bubble formation taking place underneath the anode. A slowdown of the initial current uptake rate observed in region II suggests that the surface area for current passage from

frequency responses at different time points. The time window size was chosen to be 1 min (containing 600 data points), to ensure sufficient resolution of spectral analysis while keeping the time window small enough to allow early detection of abnormal conditions.40 Analyses of signal responses for each case are presented in the following sections.

4. TYPICAL OPERATING RESPONSE The normal operation in this Article is defined as when the process is not under the influence of any unintentional disturbances other than those introduced as part of routine work practices, cell control, and operations. Signals responses, therefore, reflect bubble dynamics under the influence of local cell conditions imposed only by the process itself, such as the anodic process and the anode configuration. The layout of the 20 anodes for a point feeder prebaked anode industrial cell is shown in Figure 3, annotated with anode and feeder numbers. While it was intended to capture signals from all anodes continuously, some signals were not captured during certain time intervals due to the limited power of the amplifier batteries and the harsh potroom environment. Analysis was carried out on the available data measured over a period of 1 year. In this Article, a set of data obtained over a 28 day period, which encompasses all routine work practices, is presented. The following sections discuss factors that influence anode current responses due to anode changing and aging of slotted anodes. 4.1. Anode Currents under Normal Operation. Anode replacement is the major routine practice for the prebaked anode technology.22 When an anode reaches the end of its service life, the spent anode butt is removed and replaced with a new cold anode. This operation involves disturbing part of the parallel circuit formed with other operating parallel anodes, thus changing the anode current distribution, as has been shown in the literature.14 On the other hand, other routine 9636



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Figure 5. Slot consumption at different stages during the anode aging process.

Figure 6. Frequency response of anode current during anode aging process. Age of the anode: (a) 5%, (b) 60%, (c) 62%, and (d) 64% of its service life.

4.2. Anode Aging. The continual electrochemical consumption of carbon on the surface of the anodes causes changes in their shape and height as the anodes age. This leads to a gradual reduction of the anode height, as well as the slot height, H̃ s, if the anode is slotted. The impacts of slots and anode aging on bubbles, and thus the induced resistance, were reported in the literature for both nonslotted and slotted technologies in different industrial cells.12,13 Those studies compared frequency responses of anode current signals in distinct time frames, without investigating the change of responses due to varying conditions. In this work, the frequency response of the anode current signals is studied at multiple time points corresponding to different anode ages, to examine the change in bubble dynamics under the influence of local cell conditions. The experimental industrial cell used in this study implemented anodes with a pair of longitudinal slots grooved on the anode underside. The slots are machined in a manner that one side is shorter than the other side, that is, H̃ 1s (t) < H̃ 2s (t), as shown in Figure 5. The pair of slots is located at an equal distance from each longer side of the anode, and is identical in terms of their height and gradient. In the smelting process, most of the carbon is consumed from the bottom surface of the anode, where the oxidation reactions mainly take place. During the aging process, the slots are consumed due to the anode height reduction. Once the slots have disappeared, the surface becomes flat and functions like a nonslotted anode. The continual reduction process reduces the anode height and the shape as shown in Figure 5, until the spent anode butt is replaced and the cycle starts over. During the anode service life, it has been observed that bubble dynamics exhibit different behavior in these stages as the bubbling process, especially bubble release, changes with the anode surface condition. Figure 6 shows the frequency responses of anode current signals captured at different time points of the service life from the same anode.

the side of the anode might have reached saturation, and no side freeze remains. As the horizontal driving force of the bath is less aggressive than the bubble-induced flow,3 the melting process of the freeze underneath the anode is therefore comparatively slow. The slow melting process restrains the rate of increase of underside area exposed to the bath for current passage. Thus, the current uptake rate slows. During the melting process, freeze underneath the anode gradually melts from the edges toward the center in a horizontal direction.41 The area available for current passage underneath the anode is therefore limited, resulting in a small amount of bubble generation at the underside. In addition, the bubbling process at this stage mostly takes place at the outer part of the anode bottom. As a result, bubbles have less distance to travel, and less chance for coalescence and growth before they escape from the anode. Resistance contributed by the bubble layer is therefore of smaller magnitude and variation, as reflected by the small current fluctuation shown in region II. The current pick-up steadily increases, as seen in region III, and slowly approaches the full current carrying capacity. The anode at this stage operates like the other older anodes in the cell. The majority of the current flows to the bath from the anode bottom as all of the freeze has melted. The anode underside provides a large electro-active area for current conduction and the reactions. Large amounts of bubbles are generated, coalesced, and released at the underside, leading to a greater fluctuation of the anode current.6 The current profiles obtained from different anodes have shown a similar overall trend of current pick-up by a new anode regardless of its location, although there are small differences in the uptake rates of the different stages. The average time at which an anode reaches region III is approximately 6 h, with a standard deviation of 2.7 h. Anode current uptakes time is dependent on a number of factors, including different localized heat transfer conditions and anode location.42 9637



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bubble dynamics is illustrated by the peak shifting observed in a series of power spectra generated at different time points. One of those power spectra is shown in Figure 6c, which corresponds to approximately 62% of the anode life. The series of power spectra shows that the peak, starting from 2.5 Hz as shown in Figure 6b, shifts to a low frequency range with a gradual increase in amplitude. The peak then settles when the anode reaches approximately 64% of its service life. It merges with the small peak at the frequency of 1.2 Hz as seen in Figure 6d. Note that peak shifting was not reported in Bearne et al.13 The evolution of the peak is expected to be caused by the reduction of the remaining slots due to the continual consumption of the anode. As the slots become shallower, bubble release becomes more intensive and slower as described previously. The change of intensity and frequency of bubble release continues until the anode has reached stage 3, when the slots have totally disappeared. Note that the small peak found at the frequency of around 1 Hz has been present since t = stage 1. The merging of the peaks and the large amplitude of the resulting peak strongly suggest that all bubbles are released to the bath at the anode edges as the surface becomes totally flat. There is no significant change observed in the frequency response after the anode has passed stage 3. This might be because bubble dynamics has became steady when the surface condition stabilizes after the disappearance of the slots. From the overall observations, it is found that the frequency response generally becomes noisier as the anode ages, and as a consequence of the slot disappearance. Frequency analysis of other anodes has showed a similar trend, with only one exception, which will be discussed later in the section. To verify the explanation, the observed times at which the spectra show the responses corresponding to t = stage 2 and t = stage 3, respectively, are compared to the required times calculated on the basis of Faraday’s law and measured anode currents, ik(t). As carbon consumption from the bottom of an anode is negligible during its initial heating process (regions I and II in Figure 4), measurements before the anode reaches a stabilized current uptake, that is, region III, were not included in the calculation. The difference between the observed and calculated times of the same anodes was then computed. The same calculation procedure was applied to all measured anodes for which complete data during the transition stage were available (i.e., five different anodes). It was found that the time differences of each anode reaching stage 2 and stage 3 are both small, both with standard deviations of around 3.8%, thus verifying the explanation proposed above. Note that the calculations were only concerned with carbon consumption from the bottom of an anode due to the electrochemical reactions; other causes of anode consumption or reactions such as the Boudouard reaction were not considered.1 The results, especially the peak shifting, show that frequency response can reflect changing surface conditions through the local bubble dynamics. In contrast, it is harder to draw such interpretations on the change of local cell conditions from a time domain analysis other than the noise levels. As such, frequency response analysis of anode current signals can provide a better characterization of different local cell conditions. Identification of a fault may therefore be achieved if the response deviates from the typical response obtained under normal condition. The spectra presented in Figure 6 illustrate the typical frequency response of anode currents obtained from the particular cell technology. Among the data analyzed, it was

After the initial heating process described in the previous section, a stabilized current uptake (region III, Figure 4) indicates that the new anode has reached a state where steady consumption of carbon takes place. This stage is represented by stage 1 in Figure 5. At this stage, the presence of the two slots divides the anode surface to three separate working area. Each area functions like a smaller anode. This effectively reduces the surface area to retain bubbles at the underside. Bubble growth is therefore abated because bubbles have less chance to coalesce as the distance to travel before bubble release is shortened. In addition, the inclination of the slots accelerates bubble evacuation from the anode surface.25 This will result in smaller bubbles being released at a higher rate. With the presence of the slots, variations in the current signals caused by bubble release are therefore less visible. This is reflected by the power spectrum shown in Figure 6a. The anode at this stage was at approximately 5% of its entire service life. It can be seen that the level of noise is low across the frequency band, suggesting fluctuation of the bubble coverage is small in the bubbling process. The spectrum also shows the presence of a peak at the frequency range between 0.9 and 1.1 Hz, which matches the finding from a nonslotted anode reported in the literature.13,33 It implies that the releasing behavior of some bubbles is similar to bubbles on a nonslotted anode, meaning not all of the bubbles are evacuated through the slots. They might have released to the bath from the anode edges as larger pockets of gas, in comparison to those released from the slots, and hence resulting in the peak at the low frequency of around 1 Hz. As the height is reduced further, the anode reaches stage 2 where the shorter side of the slots starts to disappear. In the case of this particular anode, it reached stage 2 at approximately 60% of its service life. One side of the anode surface therefore becomes flattened, while the opposite side is still slotted. The uneven surface might have changed the bubble release pattern, reflected by the two peaks shown in Figure 6b. The small peak at the lower frequency (∼1 Hz) is the same peak observed in Figure 6a with a reduced amplitude, indicating bubble release that takes place at the edges occurs less intensively than before. This might be caused by rounding of the edges. Anodes with rounded edges exert less physical resistance to bubble evacuating from the underside surface. Thus, the residence time of the bubbles is less as compared to those generated at new anodes with sharp edged peripheries, leading to the release of bubbles of a finer size. Meanwhile, the remaining slots do not evacuate bubbles as effectively as on a new anode because the depth was significantly reduced. If uniform consumption occurs across the anode surface, the deepest part of the remaining slot is only ΔH̃ s = H̃ 2s (t) − H̃ 1s (t), that is, 2 cm in this cell technology, as defined in Figure 5. The slot would be expected to be shallower if the surface is not level. Therefore, bubbles generated further away from the rounded edges can have a longer residence time on the anode surface and accumulate in the shallow slots. As a result, bubble coverage varies significantly during the bubble process because the size of releasing bubbles increases, leading to a greater fluctuation in anode current signals. This is reflected by the formation of the second peak at the frequency of 2.4−2.5 Hz. This peak has a relative higher frequency than the first peak, suggesting that the shallow slots might still provide better gas release than edges from a rounded anode. Beyond stage 2, the anode enters a transition state where the underside changes from a partially slotted surface to a flat surface. The impact of this changing surface condition on the 9638



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Figure 7. Frequency response of anode current showing anode abnormality.

higher than those found in the typical response (∼1 Hz). It also implies the avenue might not be the slots as the peaks occur in a relatively low frequency. One possible reason for the power spectra showing such response can be attributed to an unusual anode condition such as a thermal crack or nonuniform anode surface. Both of these would provide additional avenues to facilitate and accelerate bubble release, leading to the resulting peak appearing at a frequency higher than 1 Hz. Because the distance and time for bubbles to travel before release could vary, possibly as a result of the influences of an abnormal anode condition, bubble release frequencies are not as uniform as at a flat anode surface, leading to the broadening of the peak at the low frequency range in Figure 7. Although further investigation is required to identify the cause of such response, identification of such deviation from typical frequency response highlights the opportunity to identify and isolate anode problems based on anode current signal characterization.

found that the frequency response of current signals of one anode showed an exceptional behavior. Such frequency response was also found very uncommon in the data measured at other times from the experimental cell. They are illustrated in Figure 7, which captures three different instances during the transition stage. Two distinct peaks are observed in the frequency range (>1 Hz), higher than in the typical response. Those peaks appear at t = t1, approximately 14 h after the anode has reached stage 2 (Figure 7a). Similar to the peak dynamics observed in the typical response, the amplitudes of both of the peaks increase gradually as they shift to lower frequencies. The peaks settle at t = t3 as one of them (which has always a relatively low frequency) reaches the frequency at 0.6−1.1 Hz, as shown in Figure 7c. At this stage, the peak at the low frequency remains stable, while the amplitude of another peak (∼2.1 Hz) oscillates intermittently as the anode passed stage 3. Occasionally, small peaks at higher frequencies appear and disappear rapidly. An example of such a peak is the peak observed at a frequency of approximately 3.5−3.6 Hz. As discussed above, the frequency at which a peak occurs indicates the particular avenue, for example, the slots or anode edges by which the majority of the bubbles are released. The formation of the two distinct peaks during the transition stage shown in the power spectra (Figure 7) therefore suggests two dominant releasing avenues, which change with time. Neither of the peaks is found to occur at the frequency corresponding to the bubble release at the edges from a flat surface, that is, around 1 Hz, until the anode has reached stage 3. This indicates that the local bubble dynamics at the respective anode deviates from the typical condition as defined previously, suggesting a different anode or surface condition. To find out how the condition changes the bubble release pattern, the frequencies and the shifting of the peaks are investigated. As seen from the pervious results, bubble release through the slots is much more effective, and therefore gives rise to the peak at higher frequencies. That suggests the peaks observed at the higher frequencies, that is, ∼3 Hz (t = t1), ∼2.5 Hz (t = t2), and ∼2.1 Hz (t = t3), in each power spectrum shown in Figure 7 correspond to bubble release from the slots. On the other hand, the frequencies of the prominent peaks at each time frame, that is, ∼1.5 Hz (t = t1), ∼1.2 Hz (t = t2), and ∼0.5−1.2 Hz (t = t3), suggest some bubbles might release from an avenue, which is more efficient than the rounded edge, as the frequencies are

5. ABNORMAL OPERATING RESPONSE Abnormality in this work is defined as a variation associated with mechanical defects, cell conditions, or control practices that bring about either a measurable spatial or temporal anode current signal response that differs from what is generally reported in the literature and the typical response obtained from this particular cell technology, such as the response presented in the previous section. Anode slippage and anode effect were studied in this work. Both of these are process abnormalities that result in anode current redistribution upon their occurrences. Their influence on bubble dynamics as reflected in anode current signals is presented and discussed in the following sections. 5.1. Abnormal Condition − Anode Slippage. When an anode unintentionally slides down in a direction perpendicular to the beam into the bath, it is known as anode slippage. This could be caused by a loose anode clamp or could occur accidentally during beam raising. Consequently, if an anode has slipped, it is set lower than the other anodes in the cell. This results in a relatively smaller local ACD, causing redistribution of the individual anodes currents within the cell. In this work, the impact of anode slippage was simulated by deliberately lowering combination of anodes by approximately 2 cm. The anode current signals acquired before and after the anode lowering were analyzed in both the time and the frequency 9639



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Figure 8. Anode current profiles, simulating anode slippage.

Figure 9. Frequency response of anodes 1 and 2, before and after anode 2 was lowered.

domains. Current profiles of selected anodes (anodes 1, 2, and 5) are shown in Figure 8. The currents drawn by anodes 2 and 5 are seen to increase after they were lowered during the experiment. As the increase of anode surface area immersed in the bath (∼15%) was not great enough to even out the increase in anode current of the lowered anodes (35% and 41%, respectively), the current density of both of the lowered anodes increased. Because the line current was regulated during the process, the anode current redistributed and eventually led to a current reduction in other anodes, where it is most obvious on anode 1. The change in bubble dynamics of anodes 1 and 2 due to the anode current variation is reflected by the frequency responses, as illustrated in Figure 9. Anodes 1 and 2 were aged 24 and 21 days, respectively, at the time when the experiment was conducted, hence the presence of a prominent peak at the frequency of 1 Hz in all of the power spectra. After anode 2 was lowered, a higher level of noise is observed in the anode 2 power spectrum as shown in Figure 9b. While the noise contributed by the metal pad might also be accentuated, it is believed that the noise in the spectrum is less likely caused by the metal wave, because the frequency shown is much higher than that from metal pad movements. Instead, it corresponds more closely to that of an increased rate of anode gas release and evolution due to the increase in anode current density. The increased noise might also indicate an increase in size

distribution of gases generated underneath the lowered anode. The higher rate of bubble evolution affects the bubble residence time under the anode, hence the bubble-induced resistance as well as the rate of bubble release. This therefore leads to a broadening of the frequency range, which corresponds to the bubble dynamics and an increase of the signal amplitude across these frequencies. A similar type of response is also observed from spectra obtained for anode 5. The frequency responses of the anode current for anode 1 as illustrated in Figure 9c and d show that the amplitude of the prominent peak as well as the noise level are reduced, which is consistent with what can be expected according to Faraday’s law. Fewer bubbles are produced as the anode carries less current. This leads to the reduction of bubble-induced resistance as a result of a decrease in bubble generation. The experiment was repeated by lowering one single anode and two anodes, which were located next to each other. The trend and behavior observed were similar to those presented earlier in this section. It is shown that a decrease in anode current can be due to an anode having slipped in a different part of the cell. However, anode current reduction can also be caused by other abnormalities, such as an approaching anode effect, which will cause an increase in resistance in the current path, and lead to a reduction in the current flow. In the case where time domain 9640



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Figure 10. Anode current and cell voltage profiles as the cell is approaching AE.

Figure 11. Frequency responses of anodes 14 and 15 as the cell is approaching AE (corresponding to Figure 10).

in the figure, before the sudden increase in the cell voltage and the aggressive oscillation of the anode currents. It is therefore clear that anode current signals respond to an approaching AE earlier than the cell voltage measurements. Such anode current reduction, prior to the onset of an anode effect, has been reported in the literature,14,16 suggesting that anode currents provide an early sign of an approaching AE. However, a similar anode current redistribution can also be caused by a slipped anode, which is due to a completely different mechanism as discussed in the previous section. An additional identifier is needed, to pinpoint whether a reduction in anode current is due to either a localized/partial AE or other abnormalities (such as anode slippage). The frequency responses for anodes 14 and 15, aged 22 and 19 days, respectively, at different stages (as annotated in Figure 10) are shown in Figure 11. As anode 14 was located immediately adjacent to anode 15, it is chosen as a reference to compare with the signals of anode 15 in the frequency domain. In stage A, both power spectra of anodes 14 and 15 (Figure 11a and d, respectively) show significant peaks formed in the frequency range of 0.8−1 Hz, similar to the typical response

analysis is not sufficient to identify faults that have a similar impact on the time series data, frequency domain analysis can be used as an additional identifier to discriminate different abnormalities, as demonstrated in the next section. 5.2. Abnormal Condition − Anode Effect (AE). To minimize the impact of the aluminum smelting process on global warming, extensive work has been carried out to reduce the emission of PFC greenhouse gases at the anode through better cell operating strategies that allow early detection and elimination of anode effects. In this study, an AE was initiated during the measurement campaign by blocking feeder 2 (see Figure 3), and the current signals were recorded before and after the event. The changes in the cell voltage and the current profiles of the anodes located in the vicinity of the blocked feeder as the cell approached AE are shown in Figure 10. Note that only the anode current of anode 15 shows a variation before the onset of the AE. Although a slight increase in cell voltage (4.75 V) is also observed, it only occurred less than 1 min prior to the onset of the AE (taken when the cell voltage has reached 21.77 V). On the other hand, a current reduction at anode 15 is observed at almost 2.5 min, as marked by the arrow 9641



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extreme condition. The combination of the adhering fluoride gas layer formation at the anode surface and the shift in current to the sides of the anode causes the change in frequency of bubble release as well as its intensity. As a result, the AE mechanism gives rise to a different response of anode 15 in comparison to anode 14 and other anodes in the cell, which also differs from the case where current reduction is merely due to a change in anode cathode distance. The decrease in anode current as well as the decrease in the bubble peak are a consequence of the approaching anode effect where a passivating gas film develops on the electroactive surface of the anode. The small decrease in anode current indicates that the current can still flow through the anode/bath interface, as the gas film is developing. Therefore, the cell voltage only sightly increased at this stage. Another experiment was conducted to trigger an anode effect by blocking a feeder at the duct end of the cell. Similar characteristics in the obtained signals were observed, showing the frequency response of the anode current signal provides an additional identifier that allows discrimination of process abnormalities between anode slippage and an approaching AE. As explained above, initiation of AE affects the bubble dynamics on the anode when fluoride coevolution begins. The onset of anode effect can be detected at a localized level by the reduction of the individual anode current, coupled with the changes in frequency response as discussed earlier. As opposed to conventional AE detection based on the cell voltage, anode current signal analysis thus offers great promise for advanced and early AE detection, facilitating improved cell control for more effective anode effect prevention.

depicted in Figure 6d. As the anode current of anode 15 is reduced in stage B, the peak in the frequency range of 0.8−1 Hz is seen to reduce significantly, as shown in Figure 11e. However, the peak in the spectrum of anode 14 in Figure 11b is founded at a frequency and amplitude similar to that in stage A. Figure 11c and f shows both responses in stage C before the cell entered anode effect. The peak in the spectrum of anode 15 further reduces, while the peak of anode 14 remains. The difference and variation in the responses of anodes 14 and 15 suggest that the bubble dynamics at the anodes have changed as the cell enters AE. Besides the significant reduction in the peak intensity, the frequency response of anode 15 exhibits less noise, when compared to anode 1 in the anode slippage experiment shown in Figures 8 and 9, where both anodes carried less current. The reduction in noise and the disappearance of the peak, along with the decrease in anode current of anode 15, can be explained by the anode effect mechanism. During normal operation, the selective evolution of carbon oxides in the bath takes place when the anode polarization remains approximately 0.5 V, referenced against pure carbon dioxide evolution in an alumina saturated solution. Coevolution of COF2 with the carbon oxides begins when the anode potential increases above that limit. Its formation results in the generation of CO and CF4 gases according to ΔG° = −45.77 kJ at 960 °C:43 2COF2(g) + C → 2CO(g) + CF4(g)

(1)

There are a number of operating conditions that can cause the increase of the anode potential, including an increase in current density or aluminum fluoride concentration, decreased stirring and agitation, and depletion of the alumina concentration.44 Once fluoride ion codeposition is initiated on a carbon surface, the above reaction is enabled, leading to the formation of a resistive intermediate surface layer. This is most likely due to the kinetics of the product desorption reactions. Depending on the cell voltage and other cell conditions, it can lead to partial or total passivation of the surface of the anode.45,46 Under galvanostatic conditions, the formation of the resistive layer leads to an increase in cell voltage. Arcing occurs at the electrode as the insulating gases are evolved. In this reaction, the dominant products are CO and CF4 gases. The evolution of the latter product is characterized as the abnormal condition normally referred to as an anode effect. A change in the anode/bath interfacial tension and wetting angle occurs due to the codeposition of fluoride ions. These events in combination cause the larger bubbles to form on the anode underside. These large bubbles adhere and spread over the anode surface and further inhibit the current flow in that area.47 The generation of the gases can shift to the vertically oriented and worn sides of the anode where gas release is easier, especially with the conditions of higher temperatures generated by the arcing. In an operating cell, the line current fed to the cell is regulated. Therefore, as an anode initiates the codeposition of fluoride and forms various products, the current on that particular anode is expected to reduce due to a higher resistance, as demonstrated by anode current reduction of anode 15. This results in additional current flowing to other anodes, leading to an uneven current distribution.48,49 An increase in anode current in other anodes can accelerate the depletion rate of the alumina concentration in the vicinity of those anodes, thus driving local conditions further to the

6. CONCLUSION This Article presents a method to observe local cell conditions in aluminum reduction cells based on time and frequency domain analysis of individual anode current signals through the study of bubble dynamics. Selected cell conditions, including anode aging, anode changing, anode slippage, and anode effect were studied, and the observed signal patterns arising in those conditions are presented in this work. Possible mechanistic explanations are given to explain the resulting signals by relating the bubble behavior based on related findings reported elsewhere in the literature. The results demonstrate that bubble dynamics are closely related to local cell conditions, and shortterm variations can be reflected in the frequency response of the individual anode current signals. It is found that anode current signals with similar characteristics in time domain (e.g., anode current reduction) may not necessarily share similar patterns in frequency domain. As such, additional information provided by the frequency response analysis allows interpretations of the anode surface condition and allows discrimination of abnormalities that would have similar effects on the signals in time domain. The use of anode current signals therefore allows early detection and isolation of the local AE, which conventional measurements alone are difficult to achieve. On the basis of the results, this study highlights the opportunity and the potential gains of using the distributed anode current signals, along with the time and frequency domain analysis to characterize short-term variations that can lead to different local faults. Although the current study only covers a few selected cell conditions, the results presented in this work are a starting point of research that can ultimately lead to the development of a fault diagnosis system to target abnormalities 9642



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(16) Evans, J. W.; Urata, N. Wireless and non-contacting measurement of individual anode currents in Hall Héroult pots: Experience and benefits. Proc. TMS Light Metals, Orlando, FL, 2012; pp 939−942. (17) Marks, J.; Bayliss, C. 2008 global anode effect survey results. Proc. TMS Light Metals, Seattle, WA, 2010; pp 259−263. (18) Zarouni, A.; Reverdy, M.; Zarouni, A. A.; Venkatasubramaniam, K. A study of low voltage PFC emissions at Dubal. Proc. TMS Light Metals, San Antonio, TX, 2013; pp 857−863. (19) Tarcy, G. P.; Kvande, H.; Tabereaux, A. Advancing the industrial aluminum process - 20th century breakthrough inventions and developments. JOM 2011, 63, 101−108. (20) Tabereaux, A. Prebake cell technology: A global review. JOM 2000, 52, 23−29. (21) Bearne, G. The development of aluminum reduction cell process control. JOM 1999, 51, 16−22. (22) Grjotheim, K.; Kvande, H. Introduction to Aluminium Electrolysis: Understanding the Hall-Héroult Process, 2nd ed.; Aluminium-Verlag: Dusseldorf, 1993. (23) Das, S.; Morsi, Y.; Brooks, G.; Yang, W.; Chen, J. J. J. The principle characteristics of the detachment and sliding mechanism of gas bubbles under an inclined anode. Proc. 10th Aust. Aluminium Smelting Tech. Conference, Launceston, TAS, 2011. (24) Wang, Z.; Gao, B.; Li, H.; Shi, Z.; Lu, X.; Qiu, Z. Study on Bubble Behavior on Anode in Aluminum Electrolysis - Part I. Proc. TMS Light Metals, San Antonio, TX, 2006; pp 463−466. (25) Cassayre, L.; Plascencia, G.; Marin, T.; Fan, S.; Utigard, T. Gas evolution on graphite and oxygen-evolving anodes during aluminium electrolysis. Proc. TMS Light Metals, San Antonio, TX, 2006; pp 379− 394. (26) Cooksey, M. A.; Taylor, M. P.; Chen, J. J. J. Resistance due to gas bubbles in aluminum reduction cells. JOM 2008, 60, 51−57. (27) Aaberg, R. J.; Ranum, V.; Williamson, K.; Welch, B. J. The gas under anodes in aluminium smelting cells Part II: Gas volume and bubble layer characteristics. Proc. TMS Light Metals, Orlando, FL, 1997; pp 341−346. (28) Kasherman, D.; Skyllas-Kazacos, M. Effect of anode material and bath composition on bubble layer resistivity and gaseous volume fraction in an aluminium electrolysis cell with sloping anode and cathode. J . Appl. Electrochem. 1991, 21, 716−720. (29) Einarsrud, K. E.; Johansen, S. T.; Eick, I. Anodic bubble behavior in Hall-Héroult cells. Proc. TMS Light Metals, Orlando, FL, 2012; pp 875−880. (30) Cooksey, M. A.; Yang, W. PIV measurements on physical models of aluminium reduction cells. Proc. TMS Light Metals, San Antonio, 2006; pp 359−365. (31) Gao, B.; Li, H.; Wang, Z.; Qiu, Z. A new study on bubble behavior on carbon anode in aluminum electrolysis. Proc. TMS Light Metals, San Francisco, CA, 2005. (32) Welch, B. J. Technology of electrolytic reduction of alumina by the Hall-Heroult process: I. A voltage analysis under conditions of varying alumina concentration. Proc. Aust. I.M. & M, Carlton South, VIC, 1965; pp 1−19. (33) Keniry, J. T.; Barber, G. C.; Taylor, M. P.; Welch, B. J. Digital processing of anode current signals: an opportunity for improved cell diagnosis and control. Proc. TMS Light Metals, New Orleans, LA, 2001; pp 1225−1232. (34) Whitfield, D.; Skyllas-Kazacos, M.; Welch, B.; McFadden, F. S. Aspects of alumina control in aluminium reduction cells. Proc. TMS Light Metals, Charlotte, NC, 2004; pp 249−255. (35) Viumdal, H.; Mylvaganam, S. Beyond the dip stick: Level measurements in aluminum electrolysis. JOM 2010, 62, 18−25. (36) Tabereaux, A.; Richards, N.; Satchel, C. Composition of reduction cell anode gas during normal conditions and anode effects. Proc. TMS Light Metals, San Francisco, CA, 1995; pp 325−333. (37) Urata, N.; Evans, J. W. The determination of pot current distribution by measuring magnetic fields. Proc. TMS Light Metals, Seattle, WA, 2010; pp 473−478.

arising at a localized level in aluminum reduction cells, allowing for improved cell operation and control.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is financially supported by the CSIRO Cluster on Breakthrough Technologies for Aluminium Production, in collaboration with Dubai Aluminium Company Ltd. In particular, we acknowledge the kind support and assistance with the experiment from Dr. Maryam Mohamed Al-Jallaf, Dr. Daniel Whitfield, Dr. Adam Sherrif, and Mr. Ali Jassim, as well as assistance from Mr. Vincent Lau and Mr. Yuchen Yao.

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REFERENCES

(1) Grjotheim, K.; Welch, B. J. Aluminium Smelter Technology - A Pure and Applied Approach; Aluminium-Verlag: Dusseldorf, 1980. (2) Kiss, L. I.; Poncsák, S.; Antille, J. Simulation of the bubble layer in aluminum electrolysis cells. Proc. TMS Light Metals, San Francisco, CA, 2005; pp 559−564. (3) Purdie, J. M.; Bilek, M.; Taylor, M. P.; Zhang, W. D.; Welch, B. J.; Chen, J. J. J. Impact of anode gas evolution on electrolyte flow and mixing in aluminium electrowinning cells. Proc. TMS Light Metals, Denver, CO, 1993; pp 355−360. (4) Xue, J.; Oye, H. A. Bubble behaviour - cell voltage oscillation during aluminium electrolysis and the effects of sound and ultrasound. Proc. TMS Light Metals, San Francisco, CA, 1995; pp 265−271. (5) Wang, X.; Tabereaux, A. T. Anodic phenomena - observations of anode overvoltage and gas bubbling during aluminum electrolysis. Proc. TMS Light Metals, San Diego, CA, 2000; pp 1−9. (6) Kiss, L. I.; Poncsák, S. Effect of the bubble growth mechanism on the spectrum of voltage fluctuations in the reduction cell. Proc. TMS Light Metals, Seattle, WA, 2002; pp 217−223. (7) Eick, I.; Klaveness, A.; Rosenkilde, C.; Segatz, M. Voltage and bubble release behaviour in a laboratory cell at low anode-cathode distance. Proc. 10th Australasian Aluminium Smelting Technology Conference, Launceston, TAS, 2011. (8) Poncsák, S.; Kiss, L.; Toulouse, D.; Perron, A.; Perron, S. Size distribution of the bubbles in the Hall-Héroult cells. Proc. TMS Light Metals, San Antonio, TX, 2006; pp 457−461. (9) Richards, N.; Gudbrandsen, H.; Rolseth, S.; Thonstad, J. Characterization of the fluctuation in anode current density and “bubble events” in industrial reduction cells. Proc. TMS Light Metals, San Diego, CA, 2003; pp 315−322 (10) Haverkamp, R.; Rolseth, S.; Thonstad, J.; Gudbrandsen, H.. A voltammetric study of the anode effect in NaF-AlF3-CaF2-Al2O3 mixtures. Proc. Electrochem Soc. 2000, 207−220. (11) Dorreen, M. M. R. Cell performance and anodic processes in aluminium smelting studied by product gas analysis. Ph.D. thesis, The University of Auckland, 2000. (12) Barber, G. C. The impact of anode-related process dynamics on cell behaviour during aluminium electrolysis. Ph.D. thesis, Department of Chemical and Materials Engineering, The University of Auckland, 1992. (13) Bearne, G.; Gadd, D.; Lix, S. The impact of slots on reduction cell individual anode current variation. Proc. TMS Light Metals, Orlando, FL, 2007; pp 305−310. (14) Keniry, J.; Shaidulin, E. Anode signal analysis- the next generation in reduction cell control. Proc. TMS Light Metals, New Orleans, LA, 2008; pp 287−292. (15) Hvidsten, R. M.; Rye, K. Practical applications of the continuous measurement of individual anode currents in Hall-Héroult cells. Proc. TMS Light Metals, New Orleans, LA, 2008; pp 329−331. 9643



Article

(38) Evans, J. W.; Urata, N. Technical and operational benefits of individual anode current monitor. Proc. 10th Australian Aluminium Smelting Technology Conference, Launceston, TAS, 2011. (39) Cheung, C.-Y.; Menictas, C.; Bao, J.; Skyllas-Kazacos, M.; Welch, B. J. Spatial temperature profiles in an aluminum reduction cell under different current distribution. AIChE J. 2012, 59, 1544−1556. (40) Jenkins, G. M.; Watts, D. G. Spectral Analysis and Its Applications; Holden-Day: San Francisco, CA, 1968. (41) Thonstad, J.; Fellner, P.; Haarberg, G. M.; Hives, J.; Kvande, H.; Sterten, A. Aluminium Electrolysis: Fundamentals of the Hall-Héroult Process, 3rd ed.; Aluminium-Verlag: Dusseldorf, 2003. (42) Ødegård, R.; Solheim, A.; Thovsen, K. Current pickup and temperature distribution in newly set prebaked Hall Héroult anodes. Proc. TMS Light Metals, San Diego, CA, 1992; pp 457−463. (43) Dorreen, M. M. R.; Chin, D.; Lee, J.; Hyland, M. M.; Welch, B. J. Sulfur and fluorine containing anode gases produced during normal electrolysis and approaching an anode effect. Proc. TMS Light Metals, San Antonio, TX, 1998; pp 311−316. (44) Richards, N. E.; Welch, B. J. Anodic overpotentials and mechanisms of the anode process on carbon in cryolite-alumina electrolytes. Proc. 1st Australian Conference on Electrochemistry, Sydney, 1964; pp 901−922. (45) Frazer, E. J.; Welch, B. J. Reactions occurring at the anode during aluminium electrowinning. Proc.-Australas. Inst. Min. Metall. 1976, 17−22. (46) Haverkamp, R. G. Surface studies and dissolution studies of fluorinated alumina. Ph.D. thesis, University of Auckland, 1992. (47) Meunier, P. Electrochemical study of the anode effect in aluminium reduction cells. Ph.D. thesis, University of New South Wales, 2004. (48) Thonstad, J.; Utigard, T. A.; Vogt, H. On the anode effect in aluminum electrolysis. Proc. TMS Light Metals, Nashville, TN, 2000; pp 249−256. (49) Thonstad, J.; Vogt, H. Terminating anode effects by lowering and raising the anodes - A closer look at the mechanism. Proc. TMS Light Metals, Seattle, WA, 2010; pp 461−466.

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