Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention rela-tes to a method of controlling an
aluminum electrolyti.c cell, and more par-ticularly a method of
stably operating an aluminum electrolytic cell while maintaining
the temperature thereof at a constant value~
As is well known in the art, aluminum is prepared by
electrolytically reducing alumina in an electrolytic bath
consisting mainly of cryolite. Since the alumina dissolved in
the electrolytic bath is consumed as a result of an electrolytic
reaction it is necessary to continuously, or at a definite
interval, feed alumina into the electrolytic bath. When the
concentration of alumina in the electrolytic bath decreases below
a certain critical limit, a so-called anode effect phenomenon
appears in which the voltage of the electrolytic cell rapidly
rises to 30 to 50V. While the anode effect persists, since the
normal electrolyti.c reaction is impaired, it is necessary to
supply alumina to eliminate the anode effect. Though the anode
effect can be taken as an index for supplying alumina, it leads
to power loss, increase in work, and causes dispersion and
evaporation of fluorides into the surrounding atmosphere. For
this reason, it has been the common practice to prevent the
occurrence of the anode effect by supplying alumina continuously,
or at a predetermined interval, or by supplying alumina in
anticipation of the occurrence of the anode effect so as to
limit the frequency of occurrence of the anode effect to a
permissible number.
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It is advantageous to operate an electrolytic eell
at a constant temperature because overheatin~ thereof resu~ts
in a decrease in the current e~ficiency~ Moreover, as the
thickness of a so-called self-lining layer formed on the wall.
of the electrolytie cell in contact wit~ the electrolytic bath
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`.j due to solidification thereof deereases or~ c~p~ the
molten eleetrolytie bath eomes into direct eontact with the
wall surfaee of the cell corrodin~ the wall surfaee and
shortening the operating life of the electrolytic eell.
Conversely, too low temperature of the electroLytic
bath inereases the thickness of the self-lining thus disturbing
the supply of alumina and removal of the formed aluminum. This
also decreases the solubillty of alumina so that the alumina
supplied sinks and deposits on the bottom of the electrolvtie
cell without being dissolved. Consequently, ~he eoncentration
of the alumina dissolved in the electrolytie bath deereases
thereby eausing fre~uent anode effeet. As above deseribed,
too high and too low eleetrolytic cell temperatures result in
problems.
The temperature of the electrolytie eell during the
operation is determined by the eleetrie energy supplied thereto.
On the other hand, sinee the eurren~ of respeetive eleetrolytie
ee:l.:Ls be.Longing to the same pot-line is -the same and
substantiallv eo~stant, the tempera-tures of respeetive eells
are determined by respeetive eell voltayes. Each eell voltaye
varies depending upon the distanee between the bot-tom surfaee
of the anode electrocle and the upper surfaee of molten aluminum
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in each elec-troly-tic cell, that is the in-terpolar distance~
and the cell voltage increases wit~ the interpolar distance.
Consequently, it is possible to maintain the bath temperatu~e
at a substantially constant value by measuring the bath
temperature and by raising the anode electrode to increase the
interpolar distance when the bath temperature decreases below
a standard value and vice versa.
However, measurement of the bath temperature utili2ed
in such control is relatively difficult. In an aluminum
electrolytic factory, since several tens or several hundreds of
electrolytic cells are operated simultaneously, it is not only
expensive to install independent temperature measuring devices
for all cells but there is also increase in the cost and labor
of maintenance and inspection of such large number of
temperature measuring devices.
We have made investigations to find a novel method
of maintaining the temperature of an electrolytic bath at a
constant value by adjusting the quantity of electric power
supplied to the cell based on a readily obtainable index without
directly measuring the temperature of the electrolytic cell
and find that the temperature of the bath can be maintained
at a constant value and the operation of the cell can be
stabili~ed when the electric power supplied to the electrolytic
cell is adjusted in accordance with the variation in the sum
of the quantity oE alumina present in the bath and the
quantity oE alumina present in the metal in the solid state.
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SUMMARY OF THE I_VE TION
Accordingly, it is an object of this invention to
provide an improved method of controlling an aluminum
electrolytic cell capable of stable operation while maintaining
the cell temperature at a constant value.
Another object of this invention is to provide a
novel method of controlling an aluminum electrolytic cell
capable of maintaining the cell temperature at a relatively low
value thus increasing the yleld of aluminum per unit electric
power b~ accurately and readily determining the temperature
condition of the cell.
According to this invention there is provided a
method for controlling an aluminum electrolytic cell
comprising the steps of detecting an increase or decrease in
the sum of the quantity of alumina present in the bath and the
quantity of alumina present in the metal in the electrolytic
cell in solid state by calcula-ting the difference between the
quantity of alumina supplied to the electrolytic cell during an
interval between an instant at which a previous anode e~fect
has occurred or was anticipated to occur and an instant at which
a present anode effect occurs or is anticipated to occur, and
the quantity of alumina consumed during the interval, and
increasing or decreasing the quantity of electric power supplied
to the electrolytic cell corresponding to the result of the
detection.
DESCRIPTION OF T~IE PREFERRED EMBODIMENTS
A pre:Eerred embodiment of this invention will now be
described in detail. As above described, during the operation
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of an aluminum electrolytic cell, alumina of a quantity of
little less than that consumed by electrolysis is usually
supplied continuously or intermittently to the cell so as to
limit the interval of occurrence of the anode effect to a
predetermined value. The anode effect occurs when the concen-
tration of alumina in the electrolytic bath decreases below a
certain critical limit. Accordingly, when the total quantity of
alumina fed into the cell dissolves immediately in the
electrolytic bath, the interval of occurrence of the anode effect
could be made to be constant. Actually, however, it is almost
impossible to control the interval of occurrence of the anode
effect to a constant value. Since, the specific gravity of
alumina is larger than that of the electrolytic bath or of molten
aluminum a portion of the supplied alumina precipitates and
deposits on the bottom of the electrolytic cell without being
dissolved. Furthermore supply of alumina causes a decrease in
the temperature of the electrolytic bath so that a self-
lining containing a large quantity of alumina would grow on the
wall surface of the electrolytic cell. These phenomena
become remarkable especially when the electrolytic cell is
operated at a relatively low bath temperature for the purpose
of increasin~ the current efficiency because low bath temperature
decreases the solubility of alumina into the electrolytic bath.
For this reason, even when the quantity of alumina supplied is
commensurate with or equal to that of the alumina consumed at a
certain t.Lme, when the temperature of the bath is low.the quantity
o;E a:Lumina in a sol.id sta-te in the electrolytic cell increases,
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thus decreasin~ the concentrati.on of the alumina in the
electrolytic bath and causing an anode effect. On the other
hand, when the tempera-ture of the bath increases the alumina
present in the cell in the solid state dissolves therein to
increase the concentration of alumina in the bath which is
effective to decrease the number o~ occurrences of the anode
effects. This invention is based on the unique utilization
of this phenomenon. More particularly, according to this
invention an increase or decrease in the quantity o~ alumina
present in the cell in the solid state is utilized as an index
to adjust the quantity of electric power supplied to the bath.
5ince increase in the alumina quantity means a low temperature
of the electrolytic bath, the anode electrode i5 raised to
increase the power supply. Conversely, since decrease in the
alumina quantity means a high bath temperature, ~he anode
electrode is lowered to decrease power supply.
Variation in the alumina ~uantity present in the
electrolytic cell in the solid state can be readily calculated
by utilizing the fact that the anode effect occurs when the
alumina eoncentration in the bath reaches a certain eritieal
value. More particularly, by calculatin~ the quantity of
alumina supplied to the electrolytic cell between a time at
which a previous anode ef~ect has occurred or was anticipated
to occur and a time at which present anode effec~ occurs or is
anticipated to occur and the quantity of alumina electrolyzed
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``` durin~ this interval, ~h~ one would determine that the
diEference between these calculated values represents an increase
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in the alumina quantity present in the solid state.
I~here the quantity of the electrolytic bath varies
in the interval between the previous time and the present
time, the quantity of alumina in the bath in a dissolved state
at both -times also vary, thus influencing the quantity of
alumina present in the solid sta-te. Consequently, to attain
accurate control it is necessary to take into consideration
also the variation in the quantity of the electrolytic bath
at the time of calcula-ting the variation in the alumina quantity
present in the soli.d state. However, since the alumina concentra-
tion in the hath at a time a-t which an anode effect actually
occurs or is anticipated to occur is low, the effec-t of the
variation in the bath quantity upon the calculation is
relatively slight so that even when the method of -this i.nvention
is worked ou-t by neglecting such a small influence, the
opera-tion oE the cell can be stabilized and the variation in
the bath quantity can be decreased
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meaning lesser influence upon the calculation caused thereby.
In other words, the method of control of this invention can
be worked out without considering a variation in the bath
quantity.
The quantity of alumina supplied can be readily
calculated by weighing it by a weighing device installed in
an alumina supply station, while the quantity of alumina
consumption can be calculated according to the following
equation.
QalumtintaYcon = electric quantity (amp.hour) x 3600x102xlO
sumption (Ky) x 6500
current efficiency (%)
X 1~0
The electric quantity can be calculated by integrating current.
Under a normal operating condition, since variation in the
current value is small, it is possible to use the product of an
average current value and the time elapsed as the electric
quantity. Usually, variation in the current efficîency is also
small so that its average value can be advantageously used.
Consequently, the quantity of alumina consumption can be
conveniently determined by multiplying a time elapsed with a
predetermined constant.
When it is noted that the sum of of the quantity
of alumina present in the bath and the quantity of alumina
present in the metal in the electrolyte cell in solid state
has lncreased, the anode electrodes are
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raised to increase the interpolar distance thereby increasing
the electric power supply. The amount of increase thereoE is
determined by the heat quantity necessary to heat the increased
quantity of detected alumina present in the solid state from
the temperature of the alumina supplied to the temperature of
the bath and to dissolve the supplied alumina.
The required minimum quantity of the increase in
the power supply corresponds to the heat quantity necessary to
heat up the increased quantity of the alumina from its supply
temperature to the bath temperature. When the quantity of
the electric power is less than the minimum quantity it is
impossible to perfectly compensate for the heat quantity
absorbed by the increased quantity of the solid state alumina.
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Consequently, it is impossible to sufficiently recovertthe
decrease in temperature of the electrolytic bath which causes
a vicious cycle oE the occurrence of the anode effect due to
poor solubility of the alumina, caused by a low bath temperature
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as well as temperature lowerinq of the bath caused by a ~e~o~a-I
supply of alumina added for the purpose of eliminatiny the
anode effect.
The maximum quantity of the increase in the electric
power supply corresponds to the heat quantity necessary
to heat up the increased quantity of the alumina to the bath
temperature and to dissolve it in the bath. Supply of electric
power exceed:ing the maximum quantity is
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dangerous because it overheats the bath.
The quantity of increase of electric power supplied
is determined to a suitable value between the maximum and
minimum values described above. The heat quantity necessary
to heat 1 Kg of alumina from room temperature to the bath
temperature amounts to about 300 kilocalories, while the heat
quantity necessary to dissolve 1 Kg of alumina in the bath
amounts to about 500 kilocalories. It is desirable to quickly
increase or decrease the quantity of electric power. Thus,
it is most desirable to calculate increase or decrese in
the sum of the quantity of alumina present in the bath and
the quantity of alumina present in the metal in the electrolytic
cell in solid state immediately after occurrence of the
anode effect, such variation in the quantity of alumina
corresponding to the difference between the quantity of
alumina supplied after occurrence of a previous anode effect
and the quantity of alumina consumed during this interval,
and to increase or decrease the electric power supply
based on the calculated value. In this case, increase in
the electric power should be completed before an instant
at which the next anode effect occurs or is anticipated to
occur. In other words, according to the method of this
invention, as a basic concept, a deficiency of the electric
power during an interval between the previous and present
anode effects is compensated for or supplernented before
the next anode eEect occurs. More particularly, according
to the method of this invention, when an anode effect is
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eliminatedr electrlc power larger than a quotient obtained
by dividing the increased quantity of the power supply by
an average time between two consecutive anode power is added
to a standard quantity of power supply, and when supply of
such surplus power is completed, the quantity of the electric
power supply i5 returned to the standard value. The surplus
power added to the standard power may be constant with time
or may be large initially and then decrease gradually with
time. Alternatively, the following simplified method may be
used although the efficiency is slightly lower than the
preferred method described above. More particularly, at
definite intervals, for example 24 hours, the sum of the
quantity of alumina present in the bath and the quantity
of alumina present in the metal in the electrolytic cell in
solid state at such point is determined and the quantity of
the electric power is varied based on the result of
determination.
For example, it is possible to calculate, for each
24 hours, the increment per unit time of the quantity of
alumina present in the bath in the solid state based on an anode
effect which occurred most recently and an anode effect which
occurred just prior to that anode effect (where the anode
effect occurs more than 3 times during the 24 hours the
calculation may be made starting from much earlier anode effect)
and a quantity of electric power corresponding to the
increment is added to the standard quantity of electric power
per Ullit time thereby supplying the total quantity required
for the next 24 hours. According to this method, it becomes
possible to adjust the electric power every 24 hours.
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While the foregoing description concerns a case
wherein the sum of the quantity of alumina present in the
bath and the quantity of alumina present in the metal in
the electrolytic cell in solid state has increased, it follows
that where the quantity of alumina decreases, the power supply
is decreased in the same manner as above described. In the
latter case, however, it is advantageous to make constant
(with time) the quantity of electric power to be decreased.
Increase or decrease in the electric power is effected
by increasing or decreasing the interpolar distance. However,
increase in the interpolar distance should be effected within
a limit in which the anode electrode is not completely drawn
out of the electrolytic bath. On the other hand, excessive
decrease in the interpolar distance rapidly decreases the
current efficiency. In this case, even though the interpolar
distance has been decreased and the power supply has also been
decreased, heat absorption caused by the reaction decreases
at a higher rate to raise the bath temperature. For this
reason, a decrease in the interpolar distance, for those cases
in which there is margin for adjustment of the interpolar
distance, is limited and even in such a case, the decrease
in -the interpolar distance should be performed gradually.
Since the calculation of the increase and decrease in
the quantity of alumina present in the solid state
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is not exact, it is advantageous -to adjtlst the interpolar
distance onl~ when the variation in the quantity of alurnina
exceeds a predetermined value.
One exarnple of an ade~uate method of con~rol is a
method wherein the control is effected b~ taking as a standard
a state in which the power supply is a little d~icient
as a result of the decrease in the interpolar distance. With
this method of control, the quantity of alumina present
in the bath or the me-tal in the solid sta-te alw~ys tends to
increase Consequen-tly, after extinguishing an anode e~fect
the electrolytic cell is operated with an increased interpolar
distance reiated to the de-tec-ted value of an increase in the
alumina quantity and the operation is continue~ by resuming
the original interpolar distance when supply of surplus
electric power is completed. With this rnethod o~ con-trol,
since the control always tends to increase the interpolar
distance, there is no fear oE decreasin~ the interpolar
distance beyond a limit thereby ensu.ring a stable operation.
As a~ove described, according to the method o~
eontrol of this invention, the operation of the electrolytic
cell ean be stabilized by adjusting the quantity of
electric power supplied to the cell in accordance with
the result of the detection oE the tempera-ture c~ndition
of an electrolytic cell from
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the quantity of alumina present in the bath or the metal
in the solid state. Also according to this invention, as
it is possible to accurately and readily determine the
temperature condition of an electrolytic cell it becomes
possible to maintain the bath temperature at a low value, thus
increasing yield oE alumi.num per unit power consumption.
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