CA1193573A - Method of stably operating aluminum electrolytic cell - Google Patents
Method of stably operating aluminum electrolytic cellInfo
- Publication number
- CA1193573A CA1193573A CA000390254A CA390254A CA1193573A CA 1193573 A CA1193573 A CA 1193573A CA 000390254 A CA000390254 A CA 000390254A CA 390254 A CA390254 A CA 390254A CA 1193573 A CA1193573 A CA 1193573A
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- alumina
- electric power
- bath
- electrolytic cell
- anode effect
- Prior art date
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Abstract
ABSTRACT OF THE DISCLOSURE
Increase or decrease in a quantity of alumina present in a bath or a metal in an aluminum electro-lytic cell is detected and a quantity of electric power supplied to the cell is increased or decreased in accordance with the result of detection. The detec-tion is made by calculating a difference between a quan-tity of alumina supplied to the cell during an interval between an instant at which a previous anode effect 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. According to this invention it is possible to stably operate the cell while maintaining the cell temperature at a constant value.
Increase or decrease in a quantity of alumina present in a bath or a metal in an aluminum electro-lytic cell is detected and a quantity of electric power supplied to the cell is increased or decreased in accordance with the result of detection. The detec-tion is made by calculating a difference between a quan-tity of alumina supplied to the cell during an interval between an instant at which a previous anode effect 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. According to this invention it is possible to stably operate the cell while maintaining the cell temperature at a constant value.
Description
3~73 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.
.? ~, ".,~,1 cr/~
3S~3 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 ~,,, `.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 cr/"-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.
.4 cr/~
~3~ 3 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 cr/,~
3~
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, ~ 5 --cr/j~
.
3~
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 . ,~, .
``` durin~ this interval, ~h~ one would determine that the diEference between these calculated values represents an increase cr/,~
35~3 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 , . ~
cr/~
5~73 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 ., .
cr/)~
~3~i~3 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.
O i ~
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 ~ ~n~l,or~ y 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 _ g _ cr/~
a;3~'~3 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 cr/~
3~3 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.
cr/\c~
3~7~
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 cr/`~
3L~'5~3~
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 er/`., 35'~3 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.
~;j, cr/,`~
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.
.? ~, ".,~,1 cr/~
3S~3 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 ~,,, `.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 cr/"-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.
.4 cr/~
~3~ 3 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 cr/,~
3~
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, ~ 5 --cr/j~
.
3~
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 . ,~, .
``` durin~ this interval, ~h~ one would determine that the diEference between these calculated values represents an increase cr/,~
35~3 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 , . ~
cr/~
5~73 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 ., .
cr/)~
~3~i~3 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.
O i ~
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 ~ ~n~l,or~ y 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 _ g _ cr/~
a;3~'~3 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 cr/~
3~3 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.
cr/\c~
3~7~
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 cr/`~
3L~'5~3~
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 er/`., 35'~3 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.
~;j, cr/,`~
Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. 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 said electrolytic cell in solid state by calculating the difference between the quantity of alumina supplied to said electrolytic cell during an interval between an instant at which a previous anode effect has occurred or was anticipated to occur and an instant at which a further anode effect occurs or is anticipated to occur, and the quantity of alumina consumed during said interval, and increasing or decreasing the quantity of electric power supplied to said electrolytic cell corresponding to the result of the detected change.
2. The method according to Claim 1, wherein a deficiency in electric power supplied to said electrolytic cell during the interval between the previous anode effect and a further anode effect, which is detected as an increase in the sum of the quantity of alumina present in the bath and the quantity of alumina present in the metal in said electrolytic cell in the solid sate, is compensated for by supplying a surplus electric power added to a standard quantity of power before occurrence of a next anode effect.
3. The method according to Claim 2, wherein subsequent to extinguishment of said further anode effect, said surplus electric power which is larger than a quotient obtained by dividing said deficiency of electric power by an average time between two consecutive anode effects is added to said standard quantity of power, and when the integrated total quantity of the supply of said surplus electric power is equal to said deficiency of electric power, the quantity of the electric power is returned to said standard quantity.
4. The method according to Claim 3, wherein said surplus electric power is maintained constant with time.
5. The method according to Claim 3, wherein said surplus electric power is made high initially and then is allowed to decrease gradually with time.
6. The method according to Claim 1, wherein an increment or decrement per unit time in the sum of the quantity of alumina present in the bath and the quantity of alumina present in the metal in the solid state is determined at a first predetermined interval, and the sum of the quantity of electric power corresponding to the increment or decrement thus determined and the standard quantity of power is supplied during a next predetermined interval.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000390254A CA1193573A (en) | 1981-11-17 | 1981-11-17 | Method of stably operating aluminum electrolytic cell |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000390254A CA1193573A (en) | 1981-11-17 | 1981-11-17 | Method of stably operating aluminum electrolytic cell |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1193573A true CA1193573A (en) | 1985-09-17 |
Family
ID=4121430
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000390254A Expired CA1193573A (en) | 1981-11-17 | 1981-11-17 | Method of stably operating aluminum electrolytic cell |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1193573A (en) |
-
1981
- 1981-11-17 CA CA000390254A patent/CA1193573A/en not_active Expired
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