Light Metals 2003 Edited by P. Crepeau TMS (The Minerals, Metals & Materials Society, 2003
A NEW ANODE EFFECT QUENCHING PROCEDURE
Pablo Navarro1, Gustavo Gregoric1, Osvaldo Cobo1 and Alfredo Calandra1
Aluar Aluminio Argentino SAIC, Research and Development, P.O. Box 52, U9120OIA Puerto Madryn, Argentina
1
Abstract
The most common methods for the automatic quenching of anode
effects consist in tilting or pumping the anode system, or lowering
it until it touches the metal pad. These methods did not render
satisfactory results in Aluar’s pots and forced us to rely on manual
killing by green poling.
A new AE quenching procedure was developed based on the
principle that each pot technology has a characteristic anodecathode distance in which a wave in the metal-bath interface
develops very fast. In this case the wave is used to produce local
short-circuits to the anodes, allowing a fast removal of the
isolating layer and a replenishment of alumina in the interpolar
volume.
The procedure was tested in different pot technologies and
showed very low values of anode effect overvoltage and duration,
a minimum disturbance to the anode crust, and a high success
rate, providing a significant reduction on the perfluorocarbon
emissions.
Introduction
Aluar’s smelter is located in Puerto Madryn, Argentina, 1400km
south of Buenos Aires. It started operation in 1974 with two
potlines of 200 SWPB pots per line. In 1999, the construction of a
third line with 144 Aluminium Pechiney AP18 pots was
completed and commenced operations.
The original pots were 150kA Montecatini design in an end-toend configuration, side-worked, non-compensated, with 16 twoblocks anode assemblies.
During the following years they underwent several upgrades. The
most important were: an external magnetic compensation loop, a
hooding system, gas collection and treatment centers, and a point
breaking and feeding system. The pots are presently running at
172kA with a current efficiency of 94.5%.
The feeding system is made up of two alumina hoppers and one
aluminium fluoride hopper, four breakers, four alumina feeders
and one aluminium fluoride feeder. The reacted alumina is
transported to the pots by means of a dense-phase transport
system. The weight of the alumina shots is approximately 1kg.
In 1999 a new control system was installed with a proprietary
algorithm made up of one individual computer per pot and a
central supervisory system. This new control algorithm allowed
for an important reduction of the anode effect (AE) frequency to
the present value of 0.05 AE/pot day.
Previous Tests
During the last years, several strategies for AE quenching were
tested such us pumping or squelching of the anode system, or by
lowering it until the anodes are short-circuited by the metal.
Due to mechanical constraints in the anode beam movement
mechanism, tilting was not an option for this technology.
These tests gave unsatisfactory results regarding success rate,
anode crust disturbance, etc, and forced us to rely on manual AE
quenching by using green poles.
Anode Pumping
This method consists in quenching the AE by means of a
succession of loops consisting of down and up-moves of the
anode beam, separated by waiting intervals in the down and at the
up positions, progressively reducing the anode-cathode distance
(ACD).
Several tests were carried out with different anode displacements
and waiting intervals with very low success rates and high AE
energies.
As a general pattern the pot showed a strong tendency to shortcircuiting the anodes during the down-moves, reaching voltages
lower than 1.8V, but the AE reappeared when the anode was
lifted.
The low success of this strategy in this pot technology could be
attributed to the low speed of the anode beam (0.6mm/sec), that
reduces the flow rate of the bath entering and leaving the anodecathode space. Another cause could be found in the important
distortion of the metal pad that is common in end-to-end
configurations.
Anode Short-circuiting
With this method the AE is killed by causing direct contact
between the metal pad and the anodes. In this case, decreasing the
ACD between 2.0 and 2.5cm was sufficient to produce a good
short-circuit.
Different short-circuit periods were tested going from 15 to
180sec with poor results. The AE reappeared again as soon as the
anode was moved up.
This strategy also causes an important deterioration of the anode
crust and a net loss of liquid bath due to bath spillage over the
anode cover.
A New Approach
The experience acquired during the previous tests allowed us to
gain in the understanding of the AE quenching process.
A detailed revision of publications regarding the AE mechanism
and its extinguishing procedures were found in [1]. This
information was extensively revised and used in the analysis of
the data recorded during the previous tests with the object of
finding a successful quenching strategy.
The data included one-second values of the pot voltage, line
current, and anode current distribution. From this study it was
concluded that,
•
With this low anode movement speed, the anode movement
by itself and the bath flow that it produces is not sufficient to
remove the isolating layer and kill the AE,
•
Strong short-circuit of the anodes should be avoided, as it
does not allow a proper replenishment of dissolved alumina
to the anode-cathode space,
•
Any plausible strategy must be based on a limited anode
displacement to avoid deterioration of the anode crust and
bath spillage.
Measurements of the pot oscillation during alumina electrolysis
while reducing the ACD, showed four different stages:
Stage 1. At small reductions in the ACD the pot remains in
normal electrolysis, the standard deviation of the anode
current distribution increases, but no short-circuits
between the anode and the metal are registered.
Stage 2. If the reduction in the ACD continues, small shortcircuits start to appear on localized anodes.
Stage 3. If the ACD is further reduced, there exists a narrow
range in which a self-sustained wave develops very fast
in the bath-metal interface. This wave causes instant
short-circuits to the anodes and an intense bath
circulation, as it travels around the pot.
Stage 4. If the reduction in the ACD continues, the anodes
become permanently short-circuited, the pot voltage
drops to values lower than 1.8V, the pot instability
disappears, and the bath circulation ceases. However, it
is common to find that not all the anodes are in contact
with the metal and that the current flows through a
limited number of them.
The particular ACD, in which the pot remains in stage 3, would
appear to be a characteristic of each pot technology and depends
on the magnetohydrodynamic (MHD) design.
While the pot is in anode effect a high fraction of the electrical
current is conducted from the sides of the anodes, because the
isolating layer considerably hinders the flow of current through
their bottom surface. This produces an important increase in the
horizontal current densities and therefore of the MHD instability
of the pot during the AE [2].
So it could be concluded that for each pot technology there might
be an ACD lower than the normal one where:
•
If the pot is in normal electrolysis, the pot will remain in
stages 1 or 2,
•
As soon as the AE appears, the change in the MHD
conditions will move the pot to stage 3, at a higher ACD than
that corresponding to stage 3, under alumina electrolysis.
Therefore, if after the onset of the AE the anode is lowered up to
this narrow ACD range and maintained in this position, the MHD
conditions of the pot will provide the local short-circuits to the
anodes and the intense bath circulation that are necessary to
quench the AE.
As soon as the isolating layer is removed and the alumina
concentration increases, the pot will resume normal alumina
electrolysis by itself, and the oscillation will cease.
On the other hand, as described by [3] and [4], it was seen in the
previous tests that only some anodes go on AE due to low alumina
concentration, sometimes several seconds before the pot voltage
goes over the AE detection voltage threshold (usually 8V). The
rest of the anodes go on AE as they became overloaded and their
current density exceeds the critical current density at their
particular location.
This means that in the first seconds after the onset of the AE, the
average alumina content in the bath may possibly be sufficient to
reestablish the normal electrolysis, if we provide a good
distribution of this alumina in the interpolar volume by means of a
strong bath circulation.
As a result it has to be possible to quench the AE without adding
as much alumina as is normally used for manual quenching.
Experimental
In all cases, the pot voltage, line current, and anode displacement
were recorded by the control system. In some cases the anode
current distribution was recorded by a multi-channel logger
(INTAB AAC-2) with the same sampling frequency. Metal and
bath levels and bath temperature were measured before and after
the AE, and the anode cover condition was inspected.
Several tests were carried out with different reductions in the
ACD to find out the optimum range for this pot technology. The
optimum range of ACD was found to be between 1.2 and 1.5cm
below the working ACD, which is estimated to be 4.5cm for the
present conditions.
Once the optimum reduction in the ACD was found, 42 tests were
performed after setting up the main parameters of the procedure.
In most cases at least two feeders where disconnected to simulate
the most common cause of AE (failures in the feeders or breakers,
feeding holes plugged, empty hoppers, etc). Different conditions
were explored,
•
Two and four feeders disconnected,
•
Time from the last anode change ranging from 1 to 72hs,
•
Different initial bath levels, bath temperatures, and metal
levels,
•
Distorted anode current distributions by raising (+3cm) or
lowering (-2cm) one anode assembly prior to the AE.
•
AE that were left to evolve for 120sec before moving the
anode (pot voltages up to 45V) to see if the quenching
strategy was able to deal with high voltage anode effects.
Figure 1 shows the anode assemblies numbering used in the
present work.
A1
A2
A3
A4
A5
A6
A7
A8
B1
B2
B3
B4
B5
B6
B7
B8
Figure 1: Anode numbering used in the present work.
The arrow indicates the line current direction.
a)
400%
Anode load of nominal [%]
300%
200%
0%
40
0.5
30
0.0
20
-0.5
10
-1.0
0
-60
0
60
120
180
240
300
360
-1.5
420
Time [sec]
b)
c)
400%
40%
Fraction of time [%]
Anode load of nominal [%]
500%
300%
200%
100%
0%
30%
20%
10%
0%
A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 B8
Anodes
A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 B8
Anodes
Discharged
Short-circuited
Figure 2: First example. a) Upper graph: current load for the 16 anode assemblies relative to their value at high alumina
concentrations. Lower graph: pot voltage (black triangles) and anode beam displacement (gray line). b) Box and whiskers plot of
the current load for the 16 anode assemblies during the quenching period (15-120sec). c) Fraction of the time during the
quenching period that each anode assembly remained discharged (<20%) and short-circuited (>300%).
Beam Displacement [cm]
Pot Voltage [V]
100%
a)
400%
Anode load of nominal [%]
300%
200%
0%
40
0.5
30
0.0
20
-0.5
10
-1.0
0
-60
0
60
120
180
240
300
360
-1.5
420
Time [sec]
b)
c)
400%
40%
Fraction of time [%]
Anode load of nominal [%]
500%
300%
200%
100%
0%
30%
20%
10%
0%
A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 B8
A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 B8
Anodes
Anodes
Discharged
Figure 3: Second Example: Idem figure 2
Short-circuited
Beam Displacement [cm]
Pot Voltage [V]
100%
First Example
A first example of anode effect quenching strategy is presented in
figure 2. In figure 2.a the load of the 16 anodes relative to their
respective value at high alumina concentration, as well as the pot
voltage, and the anode beam displacement are presented. The time
t=0 corresponds to the onset of the AE.
It is clear from figure 2.a that during the 60 seconds previous to
the onset of the anode effect (defined from the sudden increase of
the pot voltage at t=0) some anodes go on AE, their current
decreases, and is transferred to other anodes with lower
overvoltage.
The voltage increases as the isolating layer develops and, after 15
seconds of the onset of the AE the anode was moved downward
1.2cm (this movement took 20 sec). At the same time the pot
voltage started to decrease as the metal wave developed and
partially short-circuited the anodes while moving around the pot.
In this period, between t=40 and t=120sec the pot voltage
oscillated between 2.0 and 3.5V (Stage 3).
At t≅120sec the pot returned to normal alumina electrolysis and
the voltage remained in the 3.5-3.9V range (Stage 2). Small shortcircuits on localized anodes appeared.
The pot was maintained in this condition until enough alumina
was added, dissolved, and moved to the interpolar region, to
ensure that the critical current density is not exceeded while
raising the anodes to the normal ACD.
In this case the AE duration was only 40sec and, 120sec after the
onset of the AE, the pot resumed normal electrolysis. The
characteristic values for this anode effect are the following:
Initial Bath Temperature
Initial Bath Level
Maximum Pot Voltage
Anode Effect Time
Total Time with Pot Voltage > 8 V
Anode Effect Energy
Anode Effect Overvoltage
959
17
33
40
40
38
0.18
ºC
cm
V
sec
sec
kWh
Vh
Figure 2.b presents a box and whiskers plot of the anode load for
each anode during the quenching period, in this case between t=15
and t=120sec. It is clear from this figure that almost all the anodes
present a certain degree of short-circuit.
Figure 2.c shows the fraction of the quenching period in which
every anode remained discharged (defined as anode loads lower
than 20% of its nominal) and strongly short-circuited (defined as
anode loads higher than 300%).
It is clear from figure 3.a how the pot goes back and forth from
stages1-2 to stage 3 while the pot goes from normal electrolysis to
anode effect and back to normal electrolysis again.
As the individual anodes come and go from anode effect to
electrolysis, sudden changes in the anode current distribution
appear that foster an increase in the MHD instability of the metal
pad.
The AE is definitely quenched only when enough alumina is
dissolved to maintain the reaction all around the pot.
The characteristic values for this anode effect are presented
below:
Initial Bath Temperature
Initial Bath Level
Maximum Pot Voltage
Anode Effect Time
Total Time with Pot Voltage > 8 V
Anode Effect Energy
Anode Effect Overvoltage
963
17
38
119
70
71
0.35
ºC
cm
V
sec
sec
kWh
Vh
Discussion
Figures 3.b and 3.c show that only a reduced number of anodes
were strongly short-circuited during the quenching period, but all
of them remained discharged and/or short-circuited during at least
some seconds.
From figures 2.c and 3.c it can be concluded that the present
quenching strategy produces an effect in the pot that is similar in
nature to the manual quenching by green poling that was
described by [5].
That is, some anodes are strongly short-circuited and the isolating
layer is mechanically removed from their surface. At the same
time, as the line current flows through the short-circuited anodes,
the current density in the rest of them decreases considerably, and
the electrochemical formation of fluorocarbon compounds on
their bottom surface is reduced.
The increase in the anode immersion when the ACD is decreased
by 1.2 to 1.5cm is in the order of 6 to 8cm for this particular pot
technology, as shown in figure 4.
This increase in the anode immersion, in combination with the
intense bath circulation produced by the metal wave causes the
liquid bath to come into contact with the anode cover. As the
lower part of the anode cover is composed by alumina and acidic
bath [6], this represents an alumina source to the liquid bath that is
dissolved all around the pot, and not only in localized positions in
the central channel.
Second Example
This example is shown in figure 3. In this case some anodes go on
AE nearly 50sec before the sudden increase in the pot voltage.
Another source of alumina could be some sludge dissolution that
is removed by the metal wave from the cathode surface.
Even as the anode effect is killed by the wave at t=42sec, and the
pot started to electrolyze at t=53sec, with the consequent decrease
of the instability level, the lack of enough dissolved alumina in the
pot produced the reappearance of the AE at t=70sec. This
phenomenon appeared again some seconds later.
These mechanisms, and the good distribution of the alumina
produced by the bath stirring effect produced by the metal wave,
could explain the fact that in all the tests where two or even all the
alumina feeders were disconnected, the procedure was able to
successfully quench the AE.
concentration below the anodes, assures that if the AE reappears,
the oscillation will kill it immediately, but does not provide a
reduction in the anode current density.
10
6
4
2
0
-2.0
-1.5
-1.0
-0.5
-2
0.0
0.5
1.0
1.5
2.0
The comparison of figures 2 and 3 shows that the pot in the
second example is more stable at the reduced ACD in both AE
and normal electrolysis conditions. When this happens, a second
small reduction in the ACD will help to extinguish the AE. The
final algorithm to be implemented will include this extra
movement if the AE is not quenched within a certain period of
time. An example of the final algorithm is shown in figure 6.
-4
50
0.5
40
0.0
30
-0.5
20
-1.0
10
-1.5
-6
Pot Voltage [V]
-8
-10
Beam movement [cm]
Figure 4: Change in the anode immersion as a function
of the beam movement.
Figure 5 shows the histogram of the difference between the bath
levels before and after the AE. As can be seen in this figure, there
is not an appreciable change in the bath height during the anode
effect.
0
-60
0
60
120
180
240
300
360
420
Beam movement [cm]
Change in anode immersion [cm]
8
-2.0
480
Time [sec]
Figure 6: Final algorithm including a second downmove of the anode.
50%
This anode effect quenching procedure was tested on the
Aluminium Pechiney AP18 pots in our line C, to see if this
strategy could also be used in side-by-side configurations, and to
compare the new procedure with the pumping procedure used by
the AP technologies. An example is given in figure 7.
40%
30%
30
0.5
25
0.0
20
-0.5
15
-1.0
10
-1.5
5
-2.0
0%
-2
-1
0
1
2
Beam movement [cm]
10%
Pseudo-resistance [ µΩ]
20%
Bath Level (Final - Initial) [cm]
Figure 5: Histogram of the change in bath level during
the anode effect.
Consequently, the increase in the anode immersion is not enough
to produce a significant loss of liquid bath due to bath spillage
over the anode cover, or this loss is compensated by some cover
material that is dissolved when in contact with the liquid bath.
The total anode area calculated using the fanning equations
proposed by [7] indicates that there is no significant difference in
the effective anode area between the normal and the reduced ACD
conditions. The reduction in the ACD compensates the increase in
the anode immersion and in both cases an effective anode area of
~25.9m2 is obtained.
Therefore, maintaining the reduced ACD for some minutes after
the AE is quenched, waiting for an increase in the alumina
0
-60
0
60
120
180
240
300
360
-2.5
420
Time [sec]
Figure 7: Anode effect quenching in an AP18 pot.
Even though more tests are necessary to optimize the parameters
of the algorithm for this pot technology, the procedure also
appears to be very successful in side-by-side configurations.
Results
In the 42 tests made with the final algorithm, in only one case did
it fail to quench the AE and green poles were used. This case had
the particularity that the test was performed on a pot whose
neighbor down-stream pot was stopped and its anode beam had
been removed. A possible MHD imbalance could be the
explanation for this failure. In two successive tests under the same
conditions the procedure was successful.
In the following figures some characteristic values of the different
AE quenching procedures are compared as cumulative
distributions.
100%
75%
50%
25%
0%
0.0
Figure 8 shows the cumulative AE energy distribution. The
energy associated with the new procedure is 25% of the manual
quenching and 55% of the energy associated with anode pumping
in AP18 pots. This results in lower perfluorocarbon (PFC)
emissions and less disturbance to the thermal balance of the pots.
The lower energy associated with the new procedure implies that
there is not enough energy available to heat and dissolve large
amounts of alumina. Therefore, it is necessary to reduce the
alumina feeding rate during the anode effect treatment and then let
the alumina content regulation procedure to adjust the alumina
concentration.
0.5
1.0
1.5
2.0
Overvoltage [Vh]
New Procedure (avg = 0.24)
Pumping AP18 (avg = 0.57)
Figure 9: Cumulative AE overvoltage distribution.
100%
75%
100%
50%
75%
25%
50%
0%
0
25%
60 120 180 240 300 360 420 480 540 600
Time [sec]
New Procedure (V > 8V) (avg = 60)
New Procedure (avg = 114)
Manual (avg = 297)
0%
0
50
100
150
200
250
300
350
400
Energy [kWh]
New Procedure (avg = 48)
Manual (avg = 194)
Pumping AP18 (avg = 87)
Figure 10: Cumulative AE time distribution.
540
480
The AE overvoltage per AE, shown in figure 9, is 42% of that
obtained by anode pumping in AP18 pots, which implies lower
perfluorocarbon emissions.
420
The total AE time (figure 10) for this new procedure represents
38% of the resulting value for manual quenching, as the time to
respond to the AE is lower. From the total duration of the anode
effect with the new procedure (avg=114sec), the time with pot
voltage higher than 8V only averages 60sec.
Therefore, if we plot the time with V > 8V against the total AE
time (figure 11), it is clear that the strategy appears to be very safe
in terms of limiting the total PFC emissions per AE and the
disturbance to the thermal balance and side ledge.
Time with V > 8V [sec]
Figure 8: Cumulative AE energy distribution.
360
300
240
180
120
60
0
0
60
120 180 240 300 360 420 480 540
Time [sec]
Figure 11: Time with pot voltage higher than 8V vs.
total AE time.
Conclusions
A new anode effect quenching procedure was developed that
relies on the MHD characteristics of the pot to kill the anode
effect.
A narrow ACD range was found where the anode effect becomes
inherently unstable and the pot returns to normal electrolysis.
If the ACD is reduced to this characteristic range while in anode
effect, the pot develops a wave in the metal-bath interface that
provides the local short-circuits to the anodes and the intense bath
circulation that are necessary to quench the AE,
This procedure was successfully tested on both end-to-end and
side-by-side pot configurations.
The procedure shows very low values of anode effect energy,
overvoltage, and duration, and a very high success rate.
References
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the Hall-Héroult Process, (Düsseldorf, Germany, AluminiumVerlag, 3rd edition, 2001), 186-215.
[2] J. Marks, A. Tabereaux, D. Pape, V. Bakshi, and E. Dolin,
“Factor Affecting PFC Emissions From Commercial Aluminium
Reduction Cells”, Light Metals, 2001, 295-302
[3] K. Rye, M. Konigsson and I. Solberg, “Current Redistribution
Among Individual Anode Carbons in a Hall-Heroult Prebake Cell
at Low Alumina Concentrations”, Light Metals, 1998, 241-246
[4] A. Panaitescu, A. Moraru, N. Panait, G. Dobra, N. Munteanu,
and M. Cilianu, “Experimental Studies on Anode Effects by the
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[5] A. Calandra, C. Castellano, C. Ferro, and O. Cobo,
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Industrial Cells”, Light Metals, 1982, 345-358
[6] O. Cobo, P. Navarro, A. Calandra, “Anode Cover and Bath
Level Control”, (Paper presented at the 1st International Congress
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November 2000).
[7] W. Haupin, “Interpreting the Components of the Cell
Voltage”, Light Metals, 1998, 531-537