Trans Indian Inst Met (2015) 68(3):443–451 DOI 10.1007/s12666-014-0473-9
TECHNICAL PAPER
TP 2869
Simulation Method Based on Equivalent Circuit to Investigate the Circuit Characteristics in Aluminum Reduction Cell Y. Wang • J. Tie • S. Sun • G. Tu • Z. Zhang • R. Zhao
Received: 10 July 2014 / Accepted: 28 October 2014 / Published online: 10 December 2014 Ó The Indian Institute of Metals - IIM 2014
Abstract In this paper, on the basis of simulation on the electric field of aluminum reduction cell, the simulation signals of cell voltage (CV) and anode currents were obtained through simulating the cell equivalent circuit which was established using Matlab/Simulink software. Several cell conditions, including potline current fluctuation, anode changing, low anode–cathode distance, metal pad contacting with anode surface and local anode effect, were simulated using this model. These obtained simulation data were analyzed comprehensively by time domain, frequency domain, and other statistical methods, such as skewness, and kurtosis. This study further demonstrates that the individual anode current can provide more information on local cell conditions. Different cell conditions produce different characteristics of CV and anode current in time domain or frequency domain, skewness and kurtosis are more sensitive to the change of signals. Keywords Aluminum reduction cell Equivalent circuit Simulation model Cell voltage Anode current
Y. Wang (&) S. Sun G. Tu School of Materials and Metallurgy, Northeastern University, Shenyang 110819, Liaoning, China e-mail:
[email protected] J. Tie Z. Zhang R. Zhao North China University of Technology, Beijing 100144, China
1 Introduction The primary aluminum production is characterized by high-energy consumption, which is typically 13,000 kWh per ton of aluminum [1]. In recent years, China is the largest primary aluminum producer in the world. The growing demand for energy saving in China is pushing this industry to reduce the power consumption and improve the current efficiency [2]. Increasing the management of the cell voltage (CV) and anode current distribution are the important methods to improve the current efficiency. In the actual industrial production, aluminum reduction cell is a closed system with corrosive electrolyte and high temperature (950 °C and above). The common probe will be eroded quickly after being placed in the electrolyte, so it is very difficult to directly measure the parameters in the cell. As the cell becomes larger in size, the number of anodes in the cell is more than 40, due to the various physical and chemical changes, any local changes will cause the change of cell resistance. The changes of CV and anode current distribution mean that the cell resistance changes [3]. As a result, the CV and anode current are usually used to diagnose the cell working conditions. In recent years, continuous measurement of individual anode current to predict the cell fault is now becoming more and more important with the development of technology [4–6]. In general, there are two main methods to measure the anode current: one method is by measuring the millivolt drops along anode rods, which may be affected by the anode temperature, and required to contact with the rods, so the accuracy and application of this method are both limited [6]; Another method is using the Hall effect probes to measure the magnetic field generated by the current through the anode rod. Such measurement is greatly facilitated without directly contacting with the anode rods
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and interfering with cell operations [7–9]. But the magnetic field is affected by the other current elements, such as nearby anode rods and busbars. Therefore, eliminating the interference of unwanted magnetic field is the problem needed to deal with [10]. At present, CV is still one of the main parameters used for the diagnosis of cell conditions, and lots of diagnostic methods have been introduced based on CV [11–14]. In addition, the conception of using individual anode current signal for cell control is now widely accepted with the advancement of current detection technology. Studies of the diagnosis technologies based on anode current were mostly conducted by many scholars and engineers. Jeffrey T Keniry et al. [5, 6] confirmed that anode signal analysis offers significant potential for the improvement of cell control; James W. Evans et al. [7–10] collected the individual anode current signals by sensing the magnetic fields produced by the anode current, and the signals were used to forecast anode effects (AE), unstable pots and other cell phenomena. Cheung et al. [15, 16] investigated the anode current signals by frequency response, and demonstrated that anode current signals provide an earlier indication of an approaching AE than the CV measurements. All of these studies prove that determining the CV and the anode current signals are the significant way to diagnose cell working conditions. The individual anode current signals can be measured with the improvement of technology. However, in the electrolytic plant, due to the hostile operation environment, the serious interference of complex magnetic field, and the strict production controlling conditions, it is still hard to obtain the necessary production parameters accurately. The main idea of the present work was to establish the equivalent circuit model to simulate the cell electric field with different cell conditions. The purpose of this work was to obtain the signals of CV and individual anode current under different cell conditions using the model which was established by the Matlab/Simulink software, and analyze comprehensively by time and frequency domain, and other statistical methods.
2 Description of the Simulation Model Pre-baked aluminum reduction cell is the main equipment in the production of primary aluminum, and it is essentially a multi-anode parallel electrical circuit, as it is illustrated in Fig. 1. In the cell, several parallel anode blocks are inserted into the melt cryolite mixed with alumina, aluminum covering on the cathodes is produced via cell reactions. The bubbles are continuously generated under the carbon anode surface and released periodically from the anode bottom. The electrolytic reaction can be given by
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Trans Indian Inst Met (2015) 68(3):443–451
Fig. 1 Schematic diagram of local aluminum reduction cell and the equivalent circuit resistance
Al2 O3 ðsolution) + 1:5C(s) = 2Al(l) + 1:5CO2 ðg)
ð1Þ
The equivalent resistances of different parts of the cell can be seen in Fig. 1. The anode block is not only used as a reactant in the chemical reaction (1), but also as the current conductor, so the anode has a resistance (Ranode) which reduces gradually before the anode changing. The decomposition of Al2O3 produces a reaction resistance (Rreaction), which varies with the alumina concentration and reaction temperature. In the reaction process, anodic polarization will cause overvoltage, and the change of overvoltage resistance (Rover) depends on the current density according to Tafel equation. The gas covering on the anode bottom surface is nonconducting, and it will increase the ohmic voltage drop. The bubble resistance (Rbubble) has a close relationship with the anode current, shape of the anode, and bubbles covering conditions [17]. The electrolyte serving as the solvent will also produce resistance (RACD), which depends on bath thickness and alumina concentration. The cathode resistance (Rcathode) includes the resistances of cathode block and metal pad. Therefore, the cell circuit can be seen as several anode branches connected in parallel, and all the path currents go to the cathode. Details of the equivalent circuit model established by the Matlab/Simulink software and the reduction cell used in this work have been published in our earlier modeling work [18, 19], in order to save the space, the three-branch simulation model is shown in Fig. 2, which is only the part of the whole model. A 400 kA electrolysis cell was also chosen for this work, and this type cell contains 24 anode branches, so the whole model is a 24-branch model. In Fig. 2, in addition to the six self-designed modules mentioned above, there are several input parameter modules, ‘Al2O3 %’, ‘fc’, ‘T’, ‘ACD’ are concentration of alumina, fraction of bubble coverage, cell temperature, average ACD, separately. Moreover, ‘r’ and ‘DC’ show the resistance of anode rod and potline current, respectively. The ‘S’ module is used to display and record the passing current.
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Fig. 2 Three branches Matlab/ Simulink simulation model
In the present work, there are two points that need to noted. First, the surface area of an anode bottom is very large in actual cell, and the bath-metal interface is fluctuating, so the ACDs at different positions under the anode block are not the same. In our model, we regard the whole anode as a resistor, so the ACD of each branch in this model should be the average ACD under the anode block. Therefore, the range of ACD variation was supposed between 3.9 and 4.3 cm after lots of calculations. Second, the gas release is a periodic behavior, the frequency band of bubbles release is between 0.5 and 5 Hz [20], and the fraction of anode surface covered by bubbles is between 30 and 60 % [21]. This study assumes that the bubbles cover uniformly on the anode surface, and the periodic line makes a zigzag [22]. The simulation discrete time is 0.1 s, i.e. sampling frequency is 10 Hz, and the simulation time was set at 200 s.
3 Analysis Methods Several time-series signals will be obtained by using Matlab/Simulink simulation software to model the cell equivalent circuit. Various methods have been applied to analyze these signals, including spectral response analysis and statistical analysis, such as standard deviation, kurtosis and skewness. Spectral response analysis can convert time domain into frequency domain. Fourier transformation is usually used to obtain the frequency component and frequency distribution range in the dynamic signal, which can compute the amplitude distribution and energy distribution of each frequency component, and get the frequency value of main amplitude and energy distribution.
Standard deviation (r) is used to measure the degree that the data spreads around an average value. It is defined as follows: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n 1 X r¼ ð2Þ ðxi xÞ2 n 1 i¼1 The average value is n 1X xi ; x ¼ n i¼1
ð3Þ
where xi is amplitude of the time-series signal, n is length of the time-series signal. Skewness (S) is a measure of dissymmetry of series data, which can be defined as the third central moment: n P ðxi xÞ3 S ¼ i¼1 : ð4Þ ðn 1Þr3 When the data distribution is symmetric, skewness is zero. When the skewness is negative, the left tail of the distribution curve is longer. When the skewness is positive, the right tail is longer. Kurtosis (K) can be used to measure the concentration degree of data distribution, this is whether the data distribution is peaked or flat compared with a normal distribution, which can be defined as: n P ðxi xÞ4 K ¼ i¼1 3: ð5Þ ðn 1Þr4 The kurtosis of standard normal distribution is usually zero. If the data with positive kurtosis tends to have a peak state, and data drops faster. If the data has the negative kurtosis, the distribution tends to be in a flat state.
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1000
140
800
0
50
100
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200
I7 Current (A)
t (s) 1.8
x 10
4
CV PSD (W)
4.06 4.04
I7 PSD (W)
4.08
120 100 80
600 400 200
60
1.7 40 0
1.6
1
2
3
4
0 0
5
0
50
100
150
1
2
3
4
5
f (Hz)
f (Hz) 200
t (s)
Fig. 3 Simulation cell voltage and I7 current during normal operation
4 Results and Discussion 4.1 Normal Operation
Fig. 4 PSDs of cell voltage (CV) and I7 current during normal operation
Cell voltage (V)
Cell voltage (V)
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4.1 4.05 0
50
100
150
200
150
200
The normal operation in this paper is the ideal state, which is defined as the production process is not disturbed by any interferences, the cell is in the perfect state. It is actually the actual production process in a very short period of time. Of course, the actual production is affected by the routine operations, such as the feeding, metal tapping. The input potline current is constant at 400 kA without any fluctuation. The simulation CV and No. 7 (I7) anode current were obtained after running the model for 200 s. The CV and I7 current are shown in Fig. 3. In Fig. 3, the fluctuation range of CV is between 4.04 and 4.08 V, while it can be seen clearly that the curve of I7 current has a sinusoidal pattern characteristics which is according to the characteristics of assuming ACD change under the No. 7 anode block. Figure. 4 shows the power spectrum distribution (PSD) of CV and I7 anode current using frequency response method. The spectrum of CV shows a peak at about 0.02 Hz, which is the assuming frequency of the metal pad, and the spectrum power exceeds 150 W. There are peaks present at the frequency range between 0.02 and 5 Hz. Two larger peaks shown in the current spectrogram are at 0.0195 and 0.28 Hz, which is according with the fluctuation frequency of metal pad and release frequency of bubbles. Therefore, the CV and anode current can provide the message of cell working conditions, especially the individual anode current which is closely related to the local cell conditions. The statistical method was further used to analyze the simulation data, and the results are presented in Table 1. The fluctuation range of CV is between 4.0327 and 4.0854 V, and its standard deviation is only 7.5 mV. Meanwhile, the skewness and kurtosis of the CV are both close to zero, which suggest that the data distribution of CV is very close to standard normal distribution. The average value of I7 current is 16,791.12 A, and the standard
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I7 Current (A)
t (s) 1.8
x 10
4
1.7 1.6 0
50
100
t (s)
Fig. 5 Simulation cell voltage and I7 current during potline current fluctuation
deviation is 410.73 A. The skewness and kurtosis of I7 current are both below zero. 4.2 Potline Current Fluctuation In the actual production, the alternating current from the power plant should be converted into direct current using the transformer rectifier. The line current is so large that it is difficult to keep it constant. Meanwhile, there are hundreds of cells in one plant, and the changes of each cell condition will cause the fluctuation of line current. So the fluctuation range of line current is usually several or even dozens of kiloampere in actual production. In this paper, the potline current was assumed between 395 and 405 kA. The simulation was conducted using the above model, and the simulation results are presented in Fig. 5. Compared with the results of the above normal operations, the overall change characteristic of the CV is very similar, but the fluctuation range is greater in Fig. 5; With regard to I7 current, the curve of current looks more bold. The frequency responses for CV and I7 current are shown in Fig. 6. The voltage noise is greater due to the fluctuation of line current when compared with normal operation, while a slight increase of noise occurs in the PSD of I7 current. This is because the 10 kA fluctuation range of potline current divided into 24 parts is only hundreds of ampere, which is very little compared to the 16 kA current.
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Table 1 The statistical results of cell voltage and I7 anode current in normal operation Average value Cell voltage
Maximum
4.0617 V
I7 current
Minimum
4.0854 V
16,791.12 A
4.0327 V
17,647.19 A
15,870.34 A
1000 140
I7 PSD (W)
CV PSD (W)
800 120 100 80
40
600 400 200
60
0
1
2
3
4
5
f (Hz)
0
0
1
2
3
r
4
5
f (Hz)
Fig. 6 PSDs of cell voltage and I7 current during potline current fluctuation
So the noise of the I7 current increases a little. A similar result is illustrated by Jin Xiao et al. [23], showing that the potline current fluctuation will increase the cell resistance, but have a very small effect on the main frequency of the cell resistance. The results of statistical analysis are shown in Table 2. The average values of CV in two conditions are almost the same, but the standard deviation increases from 7.5 mV of normal operation to 11.8 mV, which increases probably by 55 %. This indicates that the fluctuation of CV increases due to the fluctuation of potline current. The skewness and kurtosis of voltage are more deviated from zero, so the data distribution is less close to normal distribution. With regard to I7 current, the statistical results have little difference. In a word, the fluctuation of potline current increases the cell noise. 4.3 Anode Changing The carbon anode is continuously consumed in the aluminum production, so the anode changing is necessary. In anode changing process, the anode stub hood up to the carbon block is locally removed from the cell. The current path is cut down this moment, and redistribution of anode current occurs. The new anode is subsequently installed. At
S
K
7.5 mV
0.0266
-0.0687
410.73 A
-0.0311
-0.9172
first, it is almost nonconductive when the new cold anode is set in the bath [16]. The anode comes to normal conductivity as the time goes. This process will also lead to the redistribution of anode current, which may cause violently physical and chemical changes. As has been shown in the literature [6, 9], when the anode is removed from the cell, the CV and the current through the other anode will increase. And some abnormal phenomena like AE are easier to follow this process. So the anode changing operation is very important for the cell production. In this paper, the I8 anode path is cut down, and it is assumed that only current redistribution occurs during this process. The simulation CV and I7 current are obtained, as shown in Fig. 7. Figure 7 shows the values of both CV and I7 current increase, and the fluctuation ranges also increase, compared with results of normal operation, which is the same as the literature [6] shown. But the shapes of the waveform in Fig. 7 are very similar to the curves in Fig. 3. The frequency responses of CV and I7 current during anode changing are illustrated in Fig. 8. From the CV PSD in Fig. 8, the power with peak at about 0.02 Hz compared with normal operation is almost the same, but the powers at high frequency peaks have a little increase, such as the frequency at 1.25 Hz, the power increases from 127 to 134 W. While the PSD of I7 current has almost no different with the one under normal operation. The statistical results of CV and I7 current during anode changing are given in Table 3. Compared with the normal operation, the average value and standard deviation of the CV increase respectively by about 90 and 0.7 mV, which illustrate the fluctuation of CV is more violent. However in the actual production, the values are larger. The reason for this may be that the anode currents except I8 will increase after cutting down the I8 path, as I7 current increases by about 732.76 A. So the anode bubbles will be produced more quickly due to the accelerating of anode reaction. There may be more bubbles covering on the anode bottom surface, which may increase the cell resistance and result in the increasing of CV.
Table 2 The statistical results of cell voltage and I7 current during potline current fluctuation Average value Cell voltage I7 current
4.0616 V 16,790.38 A
Maximum 4.0997 V 17,758.90 A
Minimum 4.0234 V 15,830.21 A
r
S
K
11.8 mV
0.1096
-0.0714
415.35 A
-0.0319
-0.8498
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4.18 4.16 4.14 4.12 0
1.9
x 10
50
100 t (s)
150
200
Cell voltage (V)
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3.88 3.86 3.84
0
4
1.8 1.7 1.6 0
50
50
100
150
200
150
200
t (s)
100
150
200
I7 Current (A)
I7 Current (A)
Cell voltage (V)
448
5.5
x 10
4
5.4 5.3
0
50
100
t (s)
t (s)
Fig. 7 Simulation cell voltage and I7 current during anode changing 1000
Fig. 9 Simulation cell voltage and I7 current when metal pad contacts with anode bottom surface
4.4 Metal Pad Contacting with Anode Bottom Surface
140 800
I7 PSD (W)
CV PSD (W)
120 100 80
400 200
60 40
600
0
1
2
3
4
5
0 0
1
2
3
4
5
f (Hz)
f (Hz)
Fig. 8 PSDs of cell voltage and I7 current during anode changing
Table 3 shows the standard deviation increases, the skewness deviates more obviously from zero. Both of them prove that the fluctuation of CV is more violent. But the kurtosis is closer to zero. This shows that the concentration degree of CV data distribution is more like a normal distribution. That may be because the anodic superficial area increases due to the increase number of anode blocks, which will reduce the fluctuation of CV, but the instability will be increased. With regard to I7 current, the current increases obviously, but the standard deviation, skewness and kurtosis are a little different with the parameters in normal operations. However in the actual anode changing process, the fluctuation of I7 current nearby the I8 anode is larger than the anode current far from the I8 anode. So this may not be caused by the change of electric field. The reasons may be that the bath near the I8 anode will be disturbed when I8 anode block is removed from the bath, which will cause the change of ACD more violently under a nearby anode block bottom. But the disturbance is very little far from the I8 anode.
In the aluminum reduction cell, the ACD is only 3–5 cm, and the bath-metal interface is fluctuating. So the metal pad is easy to contact with the anode bottom surface due to the anode slippage and some other reasons. This situation can be regarded as short circuit. In this paper, it is assumed that the metal pad only contacts with the I7 anode bottom surface. The electrical resistivity of bath or bubble is very large compared with metal pad, so both of their resistances are assumed as zero in this situation. The simulation results are presented in Fig. 9 after running the model. According to the assumption that the resistances of bath and bubble are zero, the total resistance of the cell decreases, so the CV decreases on the basis of Ohm law. Figure 9 shows the same results. In addition, the waveform shape of CV is nearly the same with the curve in normal operation. On the contrary, the waveform shape of I7 current is quite different. The fluctuation of I7 current is more likely affected by the change of cell total resistance without considering the resistances of bath and bubble which are assumed as zero. Meanwhile, the feeding cycle (89 s) is also clearly seen in the I7 current curve. The frequency responses of CV and I7 current when metal pad contacts with anode surface are illustrated in Fig. 10. Figure 10 shows that the power of CV PSD at 0.02 Hz decreases by about 30 W compared with normal operation, and the powers of other peaks are almost the same. The power of I7 PSD at 0.02 Hz decreases, and the noise increases a lot. Table 4 presents the statistical results when the metal pad contacts with anode surface. Compared with normal operation, the average CV is only 3.8619 V which decreases by about 200 mV, the fluctuation range is
Table 3 The statistical results of cell voltage and I7 current during anode changing Average value Cell voltage I7 current
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4.1511 V 17,523.88 A
Maximum 4.1799 V 18,413.53 A
Minimum 4.1212 V 16,572.60 A
r
S
K
8.2 mV
0.0413
-0.0026
424.18 A
-0.0279
-0.9097
800
600
I7 PSD (W)
CV PSD (W)
120 100 80
0
1
2
3
4
f (Hz)
5
4.02 4 0
50
100
150
200
150
200
t (s)
0
40
4.04
400
200
60
4.06
0
1
2
3
4
5
I7 Current (A)
140
449 Cell voltage (V)
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f (Hz)
x 10
2.6
4
2.4 2.2
0
50
100
t (s)
140
1000
120
800
I7 PSD (W)
between 3.8415 and 3.8798 V, the standard deviation and skewness decrease. These illustrate that the CV is more stable. The reason is that the assumptions were made without considering the resistances of bath and bubble covering on the I7 anode surface. This can be regarded as that two interferences are removed from the model. The average I7 current is 54,057.51 A which increases two times more than normal operation.
Fig. 11 Simulation cell voltage and I7 current during low ACD condition
CV PSD (W)
Fig. 10 PSDs of cell voltage and I7 current when metal pad contacts with anode surface
100 80 60
Low ACD is a very common phenomenon that the anodes are closer to metal pad in aluminum production. So the metal pad is easier to contact with anode surface, particularly when the cell is unstable. In this paper, it is assumed that the ACD under I7 anode is between 1.8 and 2.2 cm. The simulation results are shown in Fig. 11. Compared with normal operation, both the CV curve and the I7 current have a more obvious characteristic of sinusoidal pattern, which implies that the metal pad wave has a more important influence on the CV and anode current. The literatures [6] and [16] shown the same results. The results of frequency response are shown in Fig. 12. The power of CV PSD with peak at around 0.02 Hz decreases about 15 W. That is because the resistance of I7 branch will decrease with the decreasing of average ACD, so the power provided by this path decreases. Table 5 shows that the average CV decreases by about 38 mV, while the fluctuation range and standard deviation are nearly the same as normal operation. But the skewness and kurtosis of CV are more deviated from zero, which illustrate that the fluctuation of CV is more violent. The average current of I7 increases by about 43 %, and the standard deviation is almost double the value in normal operation. The change of individual anode current is more obvious than the CV. 4.6 Local Anode Effect In the actual production, AE will reduce current efficiency, and has an adverse effect on the production, which is an
400 200 0
40
4.5 Low ACD
600
0
1
2
3
f (Hz)
4
5
0
1
2
3
4
5
f (Hz)
Fig. 12 PSDs of cell voltage and I7 current during low ACD condition
undesirable phenomenon. The reduction of alumina concentration will inhibit the electrolysis reaction and gas escaping, so the fraction of bubble covering on anode surface increases, which will cause the increasing in CV and arc discharge [24, 25]. With the increasing size of the reduction cell, the concentration gradient of alumina in the cell is greater. When the alumina concentration decreases adjacent to one of the anodes, AE may occur locally, resulting in a local effect. The local effect may gradually propagate to the other anode, until the entire cell AE occurs. In the actual production, the cell can still produce normally without AE when one or two anodes are removed from the pot during anode changing. So the local AE occurring on one or two anode blocks is entirely possible. Although several methods based on CV have been developed to predict the AE, the predicted minimum value of CV is about 5.19 V [26]. Therefore, it is still difficult to determine whether the AE is local AE or not. In this paper, it is assumed that the alumina concentration under I7 anode is very low to cause AE, and the fraction of bubble coverage increases from 20 to 100 %. The I7 path is not electrically conductive when the bubble coverage is 100 %, and the redistribution of anode current occurs only in this process. Of course, this assuming process does not exist in actual production. The obtained data
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Table 4 The statistical results of cell voltage and I7 current when metal pad contacts with anode surface Average value Cell voltage
Maximum
3.8619 V
I7 current
Minimum
3.8798 V
54,057.51 A
r
3.8415 V
54,884.44 A
53,240.14 A
S
K
5.9 mV
0.0011
-0.1237
304.92 A
0.0332
-0.4895
Table 5 The statistical results of cell voltage and I7 anode current during low ACD condition Average value Cell voltage
Maximum
4.0228 V
I7 current
Minimum
4.0484 V
24,068.53 A
3.9918 V
25,847.23 A
22,283.10 A
Average cell voltage (V)
4.25
4.2 Two anode AE
4.15
4.1
One anode AE
4.05 0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fraction of bubble coverage
Fig. 13 Changes of average cell voltage with increasing coverage of bubbles during local AE
Average current (A)
2
x 10
4
r
S
K
7.7 mV
-0.0378
0.1536
835.98 A
0.0067
-0.9544
the CV is already over 4.1 V while the fraction reaches 65 %, and the CV is 4.25 V when the fraction is 100 %. This shows that the AE has a close relationship with the bubbles under the anode surface. In literature [15, 16], it is suggested that the bubble dynamics have changed as the cell enters AE, and fewer bubbles release from the anode and tend to adhere on the surface. Figure 14 shows the decreasing current of I7 with the increasing coverage of bubbles. The curve of one anode with AE almost coincides with the curve of two anodes with AE. The change of current is more obvious compared with the change of CV. When the bubble coverage reaches 80 %, the current is only 8,420 A, which is just the half of normal value. This suggests that individual anode current is more useful to predict the local AE.
1.5
5 Conclusions 1
0.5
0 0.2
0.3
0.4
0.5 0.6 0.7 0.8 Fraction of bubble coverage
0.9
1
Fig. 14 Changes of I7 current with increasing coverage of bubbles during local AE
were processed by linear regression method, and the results are shown in Fig. 13. The simulation result of two anodes with AE is also shown in Fig. 13. Figure 13 illustrates that CV increases with the increasing fraction of bubble covering on the I7 anode surface. The average CV is about 4.09 V as the bubble coverage reaches 70 %, and the CV exceeds the range of normal operation. When the fraction reaches 80 %, the average CV is over 4.1 V, which exceeds the range when the potline current fluctuates. When two anodes with AE,
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The electric field simulation of aluminum reduction cell was used to investigate the circuit characteristics in aluminum reduction cell. Different cell conditions based on assumption, including potline current fluctuation, anode changing, metal pad contacting with anode surface, low ACD and local AE, were investigated by this simulation. The results show that different cell conditions can result in different response characteristics of CV and current distribution. The fluctuation of potline current will increase the cell noise. The CV and anode current will change obviously during anode changing, low ACD or metal pad contacting with anode surface. When the bubble covering on one anode reaches 80 %, the CV may exceed normal condition. The local AE is difficult to be predicted using prior computer algorithm. The individual anode current is more useful to diagnose the local cell condition. The simulation results indicate that different cell conditions have different characteristics in time and frequency domain, but the characteristics are not always obvious. The
Trans Indian Inst Met (2015) 68(3):443–451
skewness and kurtosis are more sensitive to the change of cell signals, and both of them can be used to explain the cell conditions. Acknowledgments The authors wish to thank the financial support by the National Key Technology R&D Program of China (2012BAE08B09).
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