US3712857A - Method for controlling a reduction cell - Google Patents

Method for controlling a reduction cell Download PDF

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Publication number
US3712857A
US3712857A US00730408A US3712857DA US3712857A US 3712857 A US3712857 A US 3712857A US 00730408 A US00730408 A US 00730408A US 3712857D A US3712857D A US 3712857DA US 3712857 A US3712857 A US 3712857A
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resistance
cell
cells
feeding
anode
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US00730408A
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R Piller
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Reynolds Metals Co
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Reynolds Metals Co
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/20—Automatic control or regulation of cells

Definitions

  • the cells resistance is continuously monitored, the feeding of alumina is initiated by a feed control signal responsive to cell resistance changes, and, after each feeding operation the cell is required to undergo a dwell period during which no feeding occurs even though said signal otherwise indicates that further feeding should be initiated.
  • the method of the invention might require the cell to be fed upon the occurrence of a selected number of successive resistance increases in a predetermined time period; but at another time in the cells operation, an even higher resistance might not require the cell to be fed.
  • the cells AC distance is first adjusted to obtain a previously determined cell resistance (or voltage).
  • a relatively wide set of limits are established within which the cells resistance is permitted to fluctuate without any resulting changes in the AC spacing. So long as the cells resistance value remains within these limits the position of the cells anode is not changed.
  • the cells resistance is continually monitored, a determination is made of its minimum or track resistance, which is used to determine when the cell is to be fed. That is, the resistance during the upward trend must be above the cells most recently determined track resistance.
  • FIG. 1 is a schematic illustration of an aluminum reduction cell
  • FIG. 2 is a graph illustrating the variations of atypical reduction cells resistance as its alumina content varies
  • FIG. 3 is a schematic illustration of a feeding and anode control system that is suitable for use inthe practice of the method of the invention.
  • FIG. 4 is a graph of the variation of a typical cells resistance during the course of several typical feed cycles; and is used to illustrate the method of the invention.
  • FIG. 1 represents one important type of aluminum reduction cell.
  • This type of cell is known in the industry as a prebake cell and is to be particularly distinguished from a Soderberg type of cell which is equally well known.
  • the illustrated prebake cells anode is comprised of a plurality of carbon blocks 12.
  • the anode of a Soderberg cell is comprised of a single large mass of carbon that is baked in situ during the cells operation.
  • the instant invention is applicable to either type of cell, it will be primarily described in connection with a prebake cell.
  • each of the carbon blocks 12 is connected to a positive anode bus 14 by means of a suitable adjustable connection not shown.
  • the pot portion of the cell which is referred to generally as 15, is comprised of a steel shell 16 which is lined with both an insulating layer 18 and a carbonaceous conductive lining 20. Iron rods 22 are embedded in the lining 20 and connected to a cathode bus 24.
  • the lining 20 contains a pool of molten aluminum 26 and a bath 28 of alumina (A1 0 dissolved in an electrolyte comprised of molten cryolyte.
  • a suitable voltage forces an electrolyzing current to flow from the positive bus 14, through the carbon blocks 12, the bath 28, the molten aluminum 26, and the carbon cathode 20.
  • the current then flows to the various current collecting iron rods 22 from which it is delivered to a subsequent cell by means of the cathode bus 24.
  • the molten electrolyte 28 is covered with a crust 30 which consists essentially of frozen cryolite constitutents and additional alumina. As alumina is consumed, more may be fed into the bath by either breaking in a portion of the crust 30 or using a mechanical alumina-feeder 31 in FIG. 3. As the electrolytic process progresses, molten aluminum accumulates in the pool 26 and the aluminas oxygen is combined with the anodes carbon. Conse quently, the accumulated aluminum must be periodically siphoned or tapped from the molten pool 26 and the carbon blocks must be replaced. In the prebake cells entire new carbon blocks are periodically inserted, while in Soderberg cells fresh carbon paste is added to the top of the single anode.
  • An ammeter such as 32 is used to measure the current through the cells; and a volt-ohm meter such as 34 is used to measure the voltage and/or resistance across each cell.
  • the method of the instant invention contemplates periodic adjustment of the cells anode at various times and for various reasons during the course of the cells operation and feeding cycles.
  • the instant invention requires that the cell be operated so as to provide feeding cycles of one or another predetermined type wherein each such feeding cycle is followed by a mandatory dwell period during which the cell is not permitted to be fed.
  • a minimum cell resistance corresponds to a particular alumina content, such as, for example, 5% in FIG. 2.
  • alumina content increases, its resistance also increases at a negative slope, i.e., sloping upwardly and to the left in FIG. 2.
  • the cells resistance also increases as its alumina content decreases. Eventually, a point is reached where the cell enters a condition which is commonly referred to as an anode effect. This resistance increase,
  • the method of the invention can be practiced by means of apparatus such as that which is schematically illustrated in FIG. 3.
  • the carbon anode blocks are moved upwardly or downwardly by means of an anode positioning mechanism 40 which is operative in response to signals from a system controller 42, whose sophistication can range from that of a mere switch panel to that of a computer. In fact, in some cases, it may even be satisfactory to position the anode manually.
  • a conventional alumina feeder 31 is controlled by a feed controller 44 which, in turn, is responsive to the system controller 42.
  • the cells amperage and voltage are sampled and the cells resistance is determined.
  • the cells resistance was determined about every three minutes.
  • four rapid resistance measurements were made about every thirty seconds; their values averaged; and the three minute resistance determinations were the result of the average resistance values after they had been subjected to an exponential smoothing operation.
  • a primary aspect of the invention relates to an objective method of cell control wherein the initiation of a selected feed cycle is determined by the trend of the cells resistance; and where the various feed cycles are separated by a predetermined minimum dwell period.
  • Reduction cells have been conventionally controlled by both periodically breaking in the crust to introduce alumina into the bath, and by periodically adjusting the AC spacing so as to obtain a set voltage or set resistance. Both of these control steps have the effect of changing the cells resistance.
  • AC spacing changes vary the electrical resistance path through the bath between the anode and cathode; and the crust breaks vary the electrical resistance baths dissolved alumina content.
  • a cells set resistance is based upon a host of factors such as the cells age; the type of alloy being produced; the cells temperature history; and so on. These factors are well known in the art and will not be further discussed at this time.
  • suifice it to say that a cell is initially placed in operation by adjustin its anode so as to obtain a particularly determined set resistance. This value is illustrated by the set resistance line 50 (R in FIG. 4.
  • the set resistance line 50 (at 41.50 micro-ohms) is bounded by an upper limit boundary line 52 at 43.50 micro-ohms and a lower limit boundary line 54 at 41.00 micro-ohm.
  • the method of this invention includes a close monitoring of the cells resistance; and feeding the cell at a time when its resistance is determined to be in a positively sloping upward trend; but requiring each feeding step to be followed by a dwell period during which no further feeding is permitted.
  • This type of operation is illustrated in FIG. 4, wherein the line 59 represents the variations of the cells resistance at various times during a given cells continuous operation in accordance with the method of the invention.
  • the circles 60 represent the cells track resistance (R or R for corresponding periods of time (T).
  • the cells track resistance represents an effort to detect or track the cells minimum resistance point such as that represented by the point 38 in FIG. 2.
  • a new minimum resistance (track resistance) is taken as being established any time that two successive resistance values on line 59 are less than the existing track resistance, irrespective of the relative magnitudes of those two succeeding resistance values.
  • a new R (R is established if:
  • R R or; if preferred, the lesser of R or R
  • the illustrated cells set resistance (line 50) was 41.50 micro-ohms; its track resistance was about 42.00 micro-ohms; and its actual resistance was about 42.50 micro-ohms.
  • the cell was being fed; and the feeding cycle did not terminate until time 3(T This was followed by a dwell period from T to and T and according to the method of the invention, the cell could not be fed during a dwell period.
  • track resistance provides a frame of reference for determining when a feed cycle should be initiated.
  • feed cycles or special events such as tapping and anode effects
  • the track resistance becomes somewhat meaningless and can be terminated.
  • the AC spacing is changed as required to bring the actual resistance (R into within about 1% or so of agreement with the set resistance (R )
  • the cells track resistance is redetermined at such times to account for this change in the cells resistance profile.
  • the track resistance is not monitored during feeding it is a simple and preferable matter to redetermine track resistance after each feed cycle whether or not the AC spacing is changed.
  • a feed signal would be generated at T in FIG. 4. But because of the instant inventions requirement for a mandatory dwell period, such a feeding cycle was not initiated during the operation of the illustrated cell. That is, a dwell override signal from the controller, in effect, cancelled the feed signal generated at T Inasmuch as the required dwell period did not terminate until T therefore, a feed cycle could not be initiated until that time. In this respect it should be noted that the actual resistance point 68 also meets the above noted criterion for the initiation of a feed cycle.
  • point 68 is greater than both the cells track resistance (R and its preceding actual resistance (R as well as being less than its next succeeding actual resistance value at point 70
  • point 68s satisfaction of that criterion could not have been determined until after point 70 was established.
  • a feed cycle was not initiated until T -a time after which the controllers dwell override signal had terminated.
  • the duration and composition of a given feed cycle depends upon many factors, not the least of which is an and analysis of the historical peculiarities of a particular cell. For example, older cells have a tendency to take alumina into solution somewhat faster than others.
  • the feed cycle extending between T and T might have been comprised of seven punches by the feeder mechanism 31, at 100-second time intervals; followed by eleven punches at l-second intervals; in which event the entire feed cycle would have extended over a 45 minute period.
  • Another factor to be considered in connection with the format of a particular feed cycle is the rate at which the cells actual resistance is increasing. For example, if the cells actual resistance value at T had been at point 70', it might have been more desirable to feed the cell both more rapidly and for a longer period. In that case, the feeder 31 might have been controlled so as to provide eleven punches at 70-second intervals followed by perhaps thirteen punches at 180-second intervals. In this manner, the cell would have received 6 additional charges in the course of a 52 minute period. Having had the benefit of the above described illustrations of various feeding-cycle formats, it is believed that additional suitable feedingcycle formats can be similarly established by men skilled in the art. Consequently, other types of feeding formats will not be further discussed.
  • Equations 2a and 2b were satisfied and met the requirements for the initiation of a feed cycle at T Again, however, the feed signal occurred during a required dwell period so the initiation of a new feed cycle was not permitted.
  • the actual resistance value was again below the most recently established R, value; and at T the actual resistance remained below the established level of R ('R Consequently, even though R was above R a new R, was established because R was followed by two consecutively lower values (even though not progressively lower).
  • the cells resistance at point 82 (41.87 micro-ohms) was considerably less than the cells resistance at point 70 (43.00 micro-ohms). This is significant because feed cycles were begun at both of those resistance levels even though there were no ob served intervening changes in the cells AC distance.
  • the feeding cycle which began at T terminated at T at which time the cell entered another mandatory dwell period.
  • the track resistance was arbitrarily reestablished to coincide with the actual resistance at point 83, but by T another new track resistance was established at point 84; and this was followed by still lower track resistance values at points 85 and 86 (T and T respectively)
  • the mandatory dwell period ended at T and a new feed cycle was initiated at T even though the requirements of Equations 2 were not satisfied. This was because a second set of feed criteria were met. That is In other words, the increase in the cells actual resistance was so great in such a short period of time that the cell was probably heading for an anode effect if it were not fed immediately. Although sometimes helpful to prevent anode effects, these later criteria have been eliminated in some cases without particularly detrimental effects upon the cells operation.
  • the method of the invention contemplates the establishment of normal R boundary limits for control during normal cell operation, the R boundary limits can be suitably altered in the event of crust breaks, anode effects, metal taps or the like.
  • said cell resistance range extends from a value not exceeding about 10% higher than said set resistance value to a value not exceeding about 7% lower than said set resistance value.
  • both the cell resistance during said given interval and the cell resistance during the next preceding interval are less than the value of cell resistance corresponding to the then existing track resistance reference signal.
  • the rate of change of cell resistance over a particular time interval is greater than the rate of change over the next preceding time interval.
  • said feeding includes a series of separate feeding steps separated by a selected feed step time interval, each of said steps providing a selected amount of alumina.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
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US00730408A 1968-05-20 1968-05-20 Method for controlling a reduction cell Expired - Lifetime US3712857A (en)

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US (1) US3712857A (enrdf_load_stackoverflow)
JP (1) JPS5025410B1 (enrdf_load_stackoverflow)
CH (1) CH545856A (enrdf_load_stackoverflow)
DE (1) DE1925201B2 (enrdf_load_stackoverflow)
FR (1) FR2008951A1 (enrdf_load_stackoverflow)
GB (1) GB1274892A (enrdf_load_stackoverflow)
NO (1) NO129257B (enrdf_load_stackoverflow)

Cited By (13)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3847761A (en) * 1972-04-06 1974-11-12 Aluminum Co Of America Bath control
US3850768A (en) * 1972-07-18 1974-11-26 Alusuisse Method of controlling the supply of al{11 o{11 {0 during the operation of a cell for electrolytic recovery of aluminum
US3878070A (en) * 1972-10-18 1975-04-15 Southwire Co Apparatus for and method of producing metal
US3888747A (en) * 1972-10-18 1975-06-10 Nat Southwire Aluminum Method of and apparatus for producing metal
US4008142A (en) * 1973-07-25 1977-02-15 Siemens Aktiengesellschaft Apparatus for operating the furnaces of an electrolysis plant
US4098651A (en) * 1973-12-20 1978-07-04 Swiss Aluminium Ltd. Continuous measurement of electrolyte parameters in a cell for the electrolysis of a molten charge
US4425201A (en) 1982-01-27 1984-01-10 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells
US4654130A (en) * 1986-05-15 1987-03-31 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells employing point feeders
US5089093A (en) * 1989-02-24 1992-02-18 Comalco Aluminum Ltd. Process for controlling aluminum smelting cells
RU2149223C1 (ru) * 1999-01-05 2000-05-20 Крюковский Василий Андреевич Способ управления процессом электролитического получения алюминия
US6126809A (en) * 1998-03-23 2000-10-03 Norsk Hydro Asa Method for controlling the feed of alumina to electrolysis cells for production of aluminum
US20030173210A1 (en) * 2000-08-15 2003-09-18 Parker Hannifin Ab Pneumatic actuator system
US20100065435A1 (en) * 2006-12-19 2010-03-18 Michael Schneller Aluminum production process control

Families Citing this family (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
ZA74950B (en) * 1973-02-21 1975-01-29 Nat Southwire Aluminum Method and apparatus for producing metal
FR2487386A1 (fr) * 1980-07-23 1982-01-29 Pechiney Aluminium Procede et appareillage pour reguler de facon precise la cadence d'introduction et la teneur en alumine d'une cuve d'electrolyse ignee, et application a la production d'aluminium
FR2581660B1 (fr) * 1985-05-07 1987-06-05 Pechiney Aluminium Procede de regulation precise d'une faible teneur en alumine dans une cuve d'electrolyse ignee pour la production d'aluminium

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3847761A (en) * 1972-04-06 1974-11-12 Aluminum Co Of America Bath control
US3850768A (en) * 1972-07-18 1974-11-26 Alusuisse Method of controlling the supply of al{11 o{11 {0 during the operation of a cell for electrolytic recovery of aluminum
US3878070A (en) * 1972-10-18 1975-04-15 Southwire Co Apparatus for and method of producing metal
US3888747A (en) * 1972-10-18 1975-06-10 Nat Southwire Aluminum Method of and apparatus for producing metal
US4008142A (en) * 1973-07-25 1977-02-15 Siemens Aktiengesellschaft Apparatus for operating the furnaces of an electrolysis plant
US4098651A (en) * 1973-12-20 1978-07-04 Swiss Aluminium Ltd. Continuous measurement of electrolyte parameters in a cell for the electrolysis of a molten charge
US4425201A (en) 1982-01-27 1984-01-10 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells
US4654130A (en) * 1986-05-15 1987-03-31 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells employing point feeders
US5089093A (en) * 1989-02-24 1992-02-18 Comalco Aluminum Ltd. Process for controlling aluminum smelting cells
US6126809A (en) * 1998-03-23 2000-10-03 Norsk Hydro Asa Method for controlling the feed of alumina to electrolysis cells for production of aluminum
RU2149223C1 (ru) * 1999-01-05 2000-05-20 Крюковский Василий Андреевич Способ управления процессом электролитического получения алюминия
US20030173210A1 (en) * 2000-08-15 2003-09-18 Parker Hannifin Ab Pneumatic actuator system
US6776081B2 (en) * 2000-08-15 2004-08-17 Parker Hannifin Ab Pneumatic actuator system
US20100065435A1 (en) * 2006-12-19 2010-03-18 Michael Schneller Aluminum production process control
US8052859B2 (en) 2006-12-19 2011-11-08 Michael Schneller Aluminum production process control

Also Published As

Publication number Publication date
GB1274892A (en) 1972-05-17
JPS5025410B1 (enrdf_load_stackoverflow) 1975-08-23
DE1925201B2 (de) 1980-01-17
DE1925201A1 (de) 1969-11-27
CH545856A (enrdf_load_stackoverflow) 1974-02-15
FR2008951A1 (enrdf_load_stackoverflow) 1970-01-30
NO129257B (enrdf_load_stackoverflow) 1974-03-18

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