Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02566831 2006-11-02
TITLE OF THE INVENTION
Method of Controlling Aluminum Reduction Cell with Prebaked Anodes
FIELD OF THE INVENTION
This invention relates to aluminum production by electrolysis of alumina. More
particularly, the invention relates to control processes for electrolytic
cells utilizing
prebaked anodes and to improving the respective control of the alumina feeding
systems.
BACKGROUND OF THE INVENTION
In the conventional operation of electrolytic cells in which alumina, A1203 is
reduced to aluminum, Al, the alumina is added to the cell on a prescribed time
schedule.
In recent years, operation of aluminum production cells and control of the
alumina
concentration in the electrolytic bath have been significantly and
progressively
automated. This automation is necessary to obtain the desired high current
efficiency
operational process flow of the cell.
An essential factor affecting process flow of an aluminum electrolytic cell,
which
produces aluminum by the electrolysis of alumina dissolved in molten cryolite,
is the rate
of introduction of the alumina into the bath. When an excess quantity of
alumina is added
over a length of time, sludge (undissolved alumina and bath deposits)
accumulates at the
bottom of the cell. Operating with high sludge accumulations eventually
results in
decreased metal production. On the other hand, operating with a deficiency of
alumina or
with too little alumina being consistently added to the bath requires extra
energy. An
alumina deficiency causes the occurrence of the 'anodic effect' or 'racing'
phenomenon,
which causes an abrupt increase in the voltage at the terminals of the cell,
which can go
from 4 to 30 or 40 volts, and which has repercussions on the entire production
process.
This occurs because of the increased anode over potential and therefore, the
metal
production in all of the cells in the potline is lowered. Thus, the control of
aluminum
feeding systems needs a rate of feed which is neither too rich nor too lean.
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Control of alumina concentration is a critical task in electrolysis control
technology
and rate of control of existing alumina feeding systems are necessary to
maintain feed
within preset limits. Such control systems must allow no deviation because
both
repetitive occurrences of anode effects and excessive sludge interfering with
the cathode
operation are technologically unacceptable.
USSR Inventor's Certificate .N21724713 discloses a prior art method for
automatically controlling an aluminum reduction cell. According to this method
cell
voltage and potline current are measured. Then, calculations determine the
"normalized"
cell voltage, the rate of change thereof, and the alumina concentration in the
bath. The
control "normalized" voltage within the preset limits of the anode movement
and with
respect to the variation of the feed rate of alumina into the cell.
Simultaneously the
frequent overfeed and rare underfeed modes are alternated. The feeding modes
are
changed depending upon variations of the normalized voltage. During this
process, either
alumina feeding into the cell is completely terminated or one or the other
feeding mode is
turned on for a certain time.
The disadvantage of the above discussed method is that the control algorithm
is too
undeveloped resulting in low quality stabilization of the thermal energy and
the
electrochemical conditions of the process. This result occurs as both in the
cathode-anode
distance and the alumina feed rate adjustments are executed based only on the
"normalized" cell voltage data.
Another method of controlling process in an aluminum reduction cell is
disclosed by
the Russian Patent RU 2,113,552. According to this method, the actual cell
voltage and
current values are measured. Then based on such measurements, the normalized
voltage,
the rate of change of the latter in time and the alumina concentration within
the cell are
determined. Further steps of this method are as follows: comparison of actual
values of
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these characteristics with their preset values, maintaining of normalized cell
voltage
within the preset limits by moving the anode and regulating the amount of
alumina fed
into the cell by alternating the overfeed and underfeed modes.
The above-discussed prior art control method is based on the known
relationship
between the cell voltage UCeii and the alumina concentration within the bath
CAI. When
other characteristics of the method are invariable, every change in the
voltage is subject
only to change in the alumina concentration of the bath. Thus, it is possible
to use the rate
of change of voltage dUCeli/dt in order to approximately evaluate CAI.
A substantial drawback of this method is that of depending upon Uce11=f(CAO,
which
is a function nonlinear in nature over (minimum) the range of CAI=3.5-4.5%. In
this
method, when in the area of high alumina concentrations (more than 4%), the
increase in
the cell voltage indicates an increase in CAi. Conversely, an increase in the
cell voltage in
the area of low alumina concentrations (less than 4%), indicates a decrease in
CAi. The
latter also predicts an upcoming anode effect. In actual industrial practice
the low
concentration range is called "left branch of the concentration curve", and
the high
concentration range is called "right branch of the concentration curve".
Operational experience suggests that the use of the above-discussed prior art
method
does not always bring positive results. In order to properly control the
process, it is
necessary to initially define the side of the concentration the curve utilized
in the current
operation of the cell. When the branch is defined incorrectly, the effect of
the controlling
action, such as the cathode-anode distance adjustment or the amount of alumina
charged
into the cell is directly opposite to the expected result.
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SUMMARY OF THE INVENTION
The method of the invention can be utilized to control processes in the
aluminum
reduction cells with prebaked anodes, so as to improve the quality of
controlling the
alumina feeding systems. The method consists of the steps of measuring present
electric
current, comparing the value of the present electrical current with the
predetermined
values thereof, and maintaining such present values within preset limits by
regulating the
amount of alumina charged into the cell. Further, in the method of the
invention, an area
of reduced alumina concentration is identified in the cell by measuring the
existing,
present electrical current values. The area of reduced alumina concentration
can be also
identified by reviewing the rate of increase of the back EMF value at each
anode
independently of other anodes in the cell. Then, a control a signal is
generated by the
control system to a specific feeder of the alumina feeding system to raise
alumina
concentration in the area of reduced alumina concentration to predetermined
levels.
One object of the invention is to automatically control the level of alumina
concentration Cai within the cell and to stabilize the same at a preset level
within the cell
volume.
It is a further object of the invention to eliminate unplanned anode effects
and
alumina aggregation of the cathode bottom and to ultimately improve
performance of the
aluminum reduction process.
DESCRIPTION OF THE DRAWINGS
Figure 1 is front elevational view showing partially in a cross-section an
aluminum
reduction cell with a measuring system of the invention;
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Figure 2 is a diagram of anode current load prior to and during the occurrence
of
the anode effect; and
Figure 3 is a diagram of algorithm illustrating operation of the control
system of
the reduction cell, wherein Block 1 is the diagram reflecting operation of the
control
system according to prior art, and Block 2 is the diagram illustrating
operation of the
control system of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1 showing a partial cross-sectional view of an
aluminum
reduction cell with a measuring system of the invention. A plurality of
prebaked anodes
1 are disposed within the cell and are combined with a system for measuring
electric
current. Each anode I is provided with electric load measuring transducer 2
for detecting
and directing signals to a control and data acquisition unit 3. In this
manner, the current at
each individual anode is measured and evaluated independently of other anodes
in the
system. In order to determine within an aluminum reduction cell an area having
the
reduced level of alumina concentration, information about the electrical
current load at
each individual anode is collected, accumulated and analyzed. These data form
the basis
for the dynamic analysis of the electrical current load change at each anode.
Each anode of the cell is assigned to a predetermined area. The concentration
of
alumina in this area is maintained by a predetermined feeder of the alumina
feeding
system. Reduction of the electrical current load in a single anode or in the
group of the
anodes (with the current load in other anodes being invariable) is considered
as the local
reduction of alumina concentration. Referring now to Fig. 2 which is a diagram
reflecting
the anode current load prior to and during the occurrence of the anode effect.
The diagram
illustrates considerable reduction of the electrical load in individual anodes
relative to
other anodes of the cell (see for example, anode N24 and anode N212). This
load reduction
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is indicative of isolation of the anode working surface by a gas film. As a
consequence,
the alumina concentration decreases in this bath area in the vicinity of the
respective
anodes. As the alumina concentration in the bath decreases the wetting of the
anode
by the bath declines, and as a result, gas bubbles on the anode surface
enlarge causing at
least partial isolation of the anode surface from the bath. As the isolation
increases, so
does the current density on the free surface of the anode. The data reflecting
this
condition is predicative of the anode effect, as further deterioration of this
process
generally results in just such an occurrence. In the diagram of Fig. 2 this
undesirable
development takes place at the time spot 12:52. To obviate this problem, a
responsive
signal is generated by the control system of the invention for the control of
the feeder
associated with the area of the cell adapted to accommodate the monitored
anodes.
Operational mode of the respective feeder is chosen on the basis of the
existing, present
mode of the cell operation. However, this feeder functions with increased
operational
frequency compare to other feeders of the system. The increased rate of
alumina feeding
is carried out for a predetermined time interval or until the electrical load
in the respective
anodes starts to increase.
The method of controlling an aluminum reduction cell with prebaked anodes of
the
invention consists of the steps of measuring the present values of the
current, comparing
the present current values with preset values thereof and maintaining such
values within
preset limits by regulating the amount of alumina charged into the cell.
According to the
invention an area of reduced alumina concentration within the cell is defined
by
measuring the existing, present values of the current in each anode of the
cell
independently of other anodes. During this procedure a specific automatic
alumina feeder
is associated with the group of neighboring anodes. The electric current
reduction in an
individual anode or a group of anodes is detected and measured. Such current
reduction is
being evaluated comparative to the electric current in other anodes or a group
of anodes
of the cell. Upon detection of the reduced level of electric current in the
individual anode
or group of anodes, the rate of feeding of alumina is increased into the bath
area in the
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vicinity of the reduced alumina concentration. The rate of feeding of alumina
is increased
relative to the typical feeding rate of alumina into the bath.
As to Fig. 3, it reflects a combined diagram of algorithm illustrating
operation of
the control system of the reduction cell, wherein Block 1 is the diagram
showing
operation of the control system according to prior art, and Block 2 is the
diagram
illustrating operation of the control system of the invention. These diagrams
will be
discussed in detail with respect to the embodiment of Example 1.
The area of reduced alumina concentration is identified by verifying the rate
of
increase of back EMF value in each anode or group of anodes of the cell. The
value of
the back EMF is determined by reviewing changes of the total electric current
passing
through all anodes of the cell. Utilization of the back EMF value for the
purposes of the
invention are discussed in detail with respect to the embodiment of Example 2.
According to the prior art method to maintain the alumina concentration within
preset limits cell voltage and line current are measured. Furthermore,
calculations are
conducted to define the present normalized voltage values (Unorm) and the rate
of change
in time (dUnorm/dt) so as to compare them with preset values and form cycles
consisting
of the sequence of basic feeding mode, underfeed mode and overfeed mode of the
cell.
As to the method of the invention, the area of reduced alumina concentration
within the
cell is defined by measuring present electrical current values in all anodes
of the cell
independently of each other. Upon detection of such area, the alumina
concentration in
this area is increased until a predetermined level of concentration is
reached. To carry out
this task a signal is generated by the control system of the cell to
respective feeder of the
alumina feeding system. As a result the rate of alumina feeding is increased
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Example 1.
Frequent and unplanned anode effects are taking place in the prior art
aluminum
reduction cells utilizing control systems where the level of alumina
concentration in the
bath melt is controlled by evaluation of changes in the values of Unorm and
dUnorm/dt.
Such anode effects are typically caused by the lag in response time of the
feeding system
to reduction in the alumina concentration. This results when the review of the
normalized
voltage is too close in time to the occurrence of an anode effect and the
corrective action
taken cannot forestall the anode effect. In other words, such a method does
not have the
sensitivity to detect an approaching anode effect. By the method of the
invention,
however, it is possible to predict the occurrence of an anode effect and to
anticipate the
situation by providing earlier warning signals of the need to change the
alumina
concentration. Furthermore, the method of the invention is reflected in the
improved
alumina feeding control algorithm. An example of such algorithm is illustrated
in the
Block 2 of the diagram illustrated in Fig. 3. According to the Block 2
diagram, the
control system of the invention operates as follows. The existing current load
at a specific
anode of the cell is measured and compared to the previous value thereof.
Then the difference AIi (where i is the number of the respective anode)
between the
existing and previous current load values is calculated. When the difference
between the
loads exceeds a preset value, a signal is generated by the control system, so
as to regulate
the alumina concentration in the cell. In this manner, the increased rate of
alumina
feeding into the cell by one of the feeders of the feeding system is
initiated. This feeder is
directly associated with the area of the cell having reduced alumina
concentration.
Example 2
A method of evaluation of the reverse EMF value of the cell based on analysis
of the
changes in the line current is known in the art. According to this method at
the time of
considerable changes in the cell current, the values of the cell current and
voltage are
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monitored before and after such changes are taking place. Then, the reverse
EMF of the
cell is calculated by the following formula:
Ulcell -4hack rai ) _ U2cefl 14 hack (after) ; where
Ilu l 12veu
UlCeli, Iare the cell voltage and cell current values before to the change of
amperage;
U2Celi, I2ceii are the cell voltage and cell current values after the change
of amperage.
According to the method of the invention the area with the reduced level of
alumina concentration is evaluated according to the change of the reverse EMF
which is
calculated for each anode of the cell. The reverse EMF values for each
individual anode
are calculated by the same method as for the entire cell. However, in this
instance the
electrical current values of each anode are determined by the following
formula:
U1i"c11 -"hack /a,i ) _ U2iceIr -Aback (after), where
Il ian~ide l 12ianrxle
UliCeli, Ilicei1 are the values of cell voltage, and i'h is the anode current
value before the
change of amperage;
U2ic,,ii, I2iCe11 are the values of cell voltage, and i'h is the anode current
value after the
change of amperage.
The values of the reverse EMF of the anodes are compared to each other and the
area with reduced alumina concentration is determined. A respective signal is
generated
for the feed control system of the invention to regulate to the level of
alumina
concentration in the cell. To accomplish this task the alumina feeding rate by
one of the
feeders is increased.
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Utilization of the method of the invention results in the improved
distribution of
alumina concentration in the cell bath volume. Evaluation of the reverse EMF
values
provides reliable and earlier diagnostic of the forthcoming anode effect. All
of the above
result on the improved quality control and stability of the cell operation and
enhances the
anticipatory aspects of the feeding system control functions.
