US3455795A - Apparatus and method for the operation of cells for the igneous electrolysis of alumina - Google Patents

Apparatus and method for the operation of cells for the igneous electrolysis of alumina Download PDF

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US3455795A
US3455795A US423656A US3455795DA US3455795A US 3455795 A US3455795 A US 3455795A US 423656 A US423656 A US 423656A US 3455795D A US3455795D A US 3455795DA US 3455795 A US3455795 A US 3455795A
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cell
zone
cells
value
alumina
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Claude Boulanger
Jerome Pellissier-Tanon
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Pechiney SA
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Pechiney SA
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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

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  • the present invention relates to a new method of anticipating scorching of and systematically feeding cells for the igneous electrolysis of alumina.
  • the invention also relates to an apparatus for achieving the new method.
  • the lower limit is defined as the limit below which polarization of the anode system, called scorching, is likely to occur.
  • the upper limit is that limit above which there is observed a formation, at the bottom of the cell, of deposits of undissolved alumina which are likely to contaminate the carbon of the cathode plate, thereby seriously interfering with the operation of the cell.
  • FIGURE 1 is a circuit diagram illustrating the elec: trical system providing for the regulation of alumina feeding and providing for the anticipation of scorching; and,
  • FIGURE 2 is a graphic illustration demonstrating the procedure employed for determining the approach of scorching conditions.
  • the cell is supplied with alumina whenever the value of the internal ohmic resistance of the cell commences a regular increase, which is a sign of scorching.
  • certain specific operations are performed.
  • the approximate internal resistance, called the observed 3,455,795 Patented July 15, 1969 resistance R, of the cell is measured by calculating the ratio between the ohmic voltage drop U in the cell and the current I which passes through the latter.
  • U is determined by the difference between the voltage V existing across the terminals of the cell and an estimation a of the counterelectromotive force of the cell.
  • the mean value of R is calculated, this value being taken over a period greater than one second but less than 10 minutes, so as to obtain a mean value [R] which does not depend upon the fluctuations of the momentary internal resistance R.
  • the position of the anode system is then subjected to the value of [R] and, finally, the cell is supplied with alumina when the value of [R] commences a regular increase which is a sign of scorching.
  • the apparatus of this invention comprises a phase comparator having a saturable magnetic circuit made up of four windings.
  • the first winding is connected through a resistor to the terminals of a shunt through which the current I of the cell flows.
  • the second winding forms part of a circuit comprising, in series, a first rheostat, a unidirectional-voltage source whose voltage a represents the counterelectromotive force of the cell, and the cell to be tested, the cell and the source being connected in opposition.
  • the third winding is connected to an alternating-voltage source of frequency f (c./s.), and the fourth winding supplies a harmonic detector which provides a voltage of frequency 1 phase shifted by r2 or ,u2 in relation to the voltage of the alternating-voltage sources.
  • the apparatus also includes a biphase motor having one of its circuits connected to the alternating-voltage source and the other through an amplifier to the harmonic detector.
  • the shaft of the motor acts upon the first rheostat, and there are also provided an indicator for reading R and a second rheostat.
  • the apparatus comprises a mean-value circuit including, in series, the second rheostat, a directcurrent generator, an integrating circuit, an ammeter displaying the value of [R] and a comparison circuit which compares [R] with a reference value R and which acts upon the raising mechanism of the anode circuit of the cell.
  • FIGURE 1 illustrates three cells of the series under consideration, i.e., 1, 2 and 3, the apparatus being connected to the cell 2.
  • the calibrated shunt resistor 11, which is in series with all the cells, supplies one of the exciter windings 13 of a magnetic comparator 12 through a resistor 16. This winding is therefore traversed by a current of strength i proportional to I, which current passes through the cell: i AI.
  • the second exciter winding 14 of the comparator 12 is supplied through a first rheostat 19 of resistance r with a voltage obtained by providing a source 18 of voltage a in opposition to the potential difference V existing across the terminals of the cell 2. This winding 14 is therefore traversed by a current equal to:
  • the saturable magnetic circuit 17 of the comparator 12 comprising a third winding 15 supplied by an alternating-voltage source 21 of any frequency f, which is here equal to 400 c./s.
  • a fourth winding 16 supplies the harmonic detector 20. The latter selects the harmonic 2 of the alternating current supplied by the generator 21 and then converts this harmonic current into a sinusoidal current of frequency which is phase-shifted by in relation to the current supplied by 21.
  • the biphase motor 23 has its fixed phase winding 24 connected to the terminals of the generator 21, while its control-phase winding 25 is connected through the amplifier 22 to the output of the detector 20.
  • the motor 23 drives the slider of the first rheostat 19 as well as a direct reading indicator 26 optionally provided with a device for recording the momentary internal resistance; and the slider of a second rheostat 27.
  • the rheostat 27 forms part of a circuit through which there flows a current I proportional to the mean value [R] of R.
  • the circuit comprises, in series, a directcurrent source 28, an integrating device 29 and a comparator device 30 which compares the value [R] with a refer ence value R and sends through the time limit device 31 a regulating order to the motor for shifting the anode system of the cell 2.
  • the current I actuates a measuring device 32 graduated in [R], and the measuring device may be provided with a recording device.
  • Terminals 41 to 44 are provided so that the device may be connected to any one of the cells.
  • This deformed current acts upon the detector 20, which selects the harmonic 2 of frequency 2 and then converts it into a current of frequency f phase-shifted in relation to the current flowing through the winding 15 by depending upon the sign of the unidirectional magnetic field which saturates the magnetic circuit 17 and upon which the polarity of the harmonic 2 depends.
  • This signal passes through the amplifier 22 and acts upon the winding 25 of the motor 23, of which the other Winding 24 is directly supplied by the generator 21.
  • the direction of rotation of this motor depends upon the sign of the phase shift of the current supplied by the detector 20' and therefore upon the sign of the unidirectional magnetic field in the circuit 17, which in turn depends upon the relative magnitude of i and 1'.
  • the rapid variations of the internal resistance constitute one of the great difliculties which have been responsible for the failures encountered in the application of automatic regulation. It is therefore desirable to eliminate or lessen these variations by basing the regulation, not on the momentary value R of the resistance, but on a mean value [R] taken over a time longer than one second and shorter than 10 minutes, but generally between 15 seconds and two minutes.
  • the motor 23 drives a second rehostat 27 which is supplied by a generator 28 supplying constant unidirectional voltage.
  • the voltage is supplied through an integrator 29 which may simply consist of a resistance-capacitance system, or preferably of an equilibrium circuit of the Wheatstone bridge type, of which the time constant is substantially equal to the time over which it is desired to take the average.
  • This integration may, however, be carried out by any other mechanical or electrical device.
  • FIGURE 2 illustrates an embodiment of such a regulation.
  • the time t before the scorching is plot-ted along the abscissae and the variations MR] of the observed internal resistance [R] as read or recorded at 32 are plotted along the ordinates.
  • the circuit 30 distinguishes, for the variations MR] the following zones:
  • the circuit 30 gives in accordance with the position of MR] the following orders:
  • Ignition of a supply signal which may simply be a luminous signal without adjustment if MR] is in the zone +2.
  • the circuit 30 permits variations of the zone widths up to 0.5 o, that is to say 0.05 volt for 100,000 amperes, and the values of p, N and r.
  • a mean-adjustment button is also provided for varying the reference value R that is to say, for shifting all the zones for each of the cells being regulated.
  • the regulation takes place only at fixed intervals of time assumed to be equal to 30 minutes in FIGURE 2, in which the times at which the regulation can occur are represented by the arrows 61 to 64.
  • the unbroken curve 65 shows how MR] changes before scorching, in the absence of adjustment of the interpolar distance or manual adjustment, while the broken line curve 66 shows the same curve with the regulator in operation. It will be seen that at the instant 105 minutes corresponding to the arrow 61 the value of MR] is in the zone 0. Therefore, the circuit 30 does not send any automatic regulation signal. On the other hand, at the instant -75 minutes, arrow 62, the value of MR] is in the zone +1, and the regulator acts and restores the value of MR] to the zone 0. The same action takes place at the instant +45 minutes, arrow 63.
  • the observed internal resistance depends upon the dissolved alumina content of the electrolytic bath. Since this content continually falls between two successive operations for the supply of alumina to the cell, a slow and continuous rise of this resistance is observed, whether it be a question of R or of [R]. This rise is also corrected by the automatic device. When the dissolved alumina content falls below a value such that the instant at which the polarization of the anode, or scorching occurs, i.e., the zero instant of FIGURE 2 is close, an accelerated rate of rise is observed.
  • This rise does not at first exceed the regulation zone 52 and it is corrected by the regulator, but it constantly accelerates to the point where it represents more than one zone between two actions of the regulator, as occurs at the tim 45 minutes, corresponding to the arrow 64.
  • the regulator is then placed out of circuit and the cell is supplied 1 with alumina at the appealance of the aforesaid luminous signal.
  • the alumina content of the bath must constantly remain within two limits.
  • the lower limit is the level which is slightly higher than the content which produces scorching, and the upper limit corresponds to the point of saturation of the electrolytic bath with dissolved alumina. It is very important that this limit should not be exceeded, because the alumina, which has fallen to the bottom of the cell, would remain there and contaminate the cathode. This would seriously interfere with the operation of the cell, resulting in a gradual decrease of the output and ultimately completely preventing electrolysis from occurring.
  • the quantity of alumina supplied at each sup ply operation is empirically determined so as to remain distinctly below the limit content corresponding to saturation of the bath.
  • the zone +2 is omitted, so as to allow the regulator to act in all circumstances as soon as the curve 66 rises above the ordinate 56.
  • the supply of alumina is controlled by the number of actions of the regulator in zone +1.
  • this number of actions is between 1 and 10, and generally between 1 and 5.
  • the anticipation time is short and the supply must take place almost immediately, scorching often taking place before there has been time to effect the supply. This number of actions is greatly influenced by the relative speeds at which the anode consumption and the rise of the metal take place.
  • the number of actions chosen is three or four when it is desired to have available a fairly long period, for example 2 hours, before effecting the supply, which makes it possible to combine the operations for a certain number of cells, and five or even six actions are desirable when it is preferred to supply fairly quickly, for example in a period of the order of half an hour after signalling.
  • One of the factors determining the choice of the number of actions is the volume of the bath, the other factors remaining unchanged. In cells having a very small bath volume and in the case of cells having prebaked anodes, the supply may be necessary at the very first action of the regulator, while in the case of cells having a very large bath volume, the supply may take place only after a number of actions of up to 10.
  • the method is applied to a series of 100-kiloampere cells having Soederberg anodes wherein the regulation is effected with the aid of the apparatus described.
  • the extent of the anode displacement is evaluated at a number of revolutions of the shaft by which the displacements are effected, each revolution being equivalent to a displacement of the anode through one-third of a millimeter, i.e., to a voltage variation of 0.01 volt or to an internal resistance variation of 0.1 microhm. It may also be evaluated by the duration of the pulse imparted to the motor by which the anode system is displaced.
  • the regulator is successively connected to the cells of the series under consideration, each cell being adjusted every 30 minutes.
  • the measuring time is one minute 15 seconds in order to permit integration over a sufficient period of time
  • the regulation proper lasts 10 seconds
  • the change-over to the succeeding cell by the operation of the contactors 42, 43 and 45 lasts seconds.
  • the apparatus is therefore stopped for 1 minute 30 seconds per cell.
  • An apparatus may thus serve 20 cells.
  • a value of 1.6 volts is chosen for w.
  • the orders transmitted by the regulator are the following.
  • a single cell was regulated, and over a period of four months the mean Faraday output of the regulated cell was 89.0 percent against 87.7 percent in the case of the manually adjusted reference cells, which were as similar as possible.
  • the regulation was applied to a group of five cells under the same conditions, but the zone +2 was omitted, the luminous supply signal being actuated by a storage relay when five actions in zone +1 had been observed after one supply.
  • a complete series of 20 cells was maintained in operation for several months with an adjustment every 30 minutes, so that a single measuring and regulating device is suflicient to ensure regulation of the cells of the series.
  • the operation is very satisfactory and a reduction of the electrical consumption and above all of the fluorine consumption has been observed.
  • the described device is subject to many variations.
  • the integrating device may be simplified if there is used for the measurement of theinternal resistance a device possessing a higher time constant and, perhaps, lower sensitivity. A much simpler damping device is then sufficient.
  • the resistance variations are smaller in the case of cells having prebaked anodes and the described integrating device is then unnecessarily complicated.
  • it is suflicient to provide in the main circuit comprising the winding 16 of the comparator and the motor 23', an electrical or mechanical time constant, which permits omission of the integrating circuit 27 to 29.
  • the motor 23 which, through a comparator performing the function of 30, controls the motor which effects the displacement of the anode system of the cell.
  • An apparatus for operation of a cell employed for the igneous electrolysis of alumina said apparatus being utilized for measuring the internal ohmic resistance of the cell for detecting a regular pattern of increasing resistance which is an indication of scorching, said measuring being accomplished by measuring an approximate internal resistance R of the cell by calculating the ratio between the ohmic voltage drop U in the cell and the current I flowing through the latter, wherein U comprises the difference between the voltage V existing across the terminals of the cell and the estimated counterelectromotive force of the cell; calculating a mean value of R, which is taken over a period longer than one second but less than 10 minutes, so as to obtain a mean value [R] which does not depend upon the fluctuations of the momentary internal resistance R; and supplying the cell with alumina when the value of [R] commences a regular pattern of increase which is a sign of scorching, said apparatus comprising a phase comparator whose satura'ble magnetic circuit comprises four windings of which the first is connected through a resistor to the
  • the second forms part of a circuit comprising, in series, a first rheostat, a unidirectional-voltage source of which the voltage represents the counterelectromotive force of the cell, and the cell to be measured, the cell and the source being connected in opposition;
  • the third is connected to :an alternating-voltage source of frequency f in c./s.;
  • a harmonic detector which provides a voltage of frequency f phase-shifted by in relation to the voltage of the alternating-voltage source
  • An apparatus in accordance with claim 1 in whic the circuit comprising the second winding of the phase comparator and the regulating circuit comprises contactors by means of which it can be successively connected to various cells supplied in series from a common current source.
  • An apparatus in accordance with claim 1 wherein the circuit provided for introducing a response time includes a damping device.
  • zone +1 located between the upper limit of the zone 0 and a higher positive value, to which there corresponds a descending order applied to the mechanism actuating the anode system
  • zone 1 comprised between the lower limit of the zone 0 and a lower negative value, to which there corresponds a rising order applied to the mechanism actuating the anode system;
  • zone +2 situated above the upper limit of the zone +1, to which there corresponds the despatch of an alumina supply signal, the anode system not being displaced;
  • zone --2 situated below the lower limit of the zone -1, to which there corresponds a rising order applied to the mechanism actuating the anode system, the rising rate exceeding that of zone -1.
  • Zone 0 centered on the zero value of A[R], to which there corresponds no order, the anode system not having to be displaced;
  • zone -1 located between the lower limit of the zone 0 and a lower negative value, to which there corresponds a rising order applied to the mechanism actuating the anode system
  • zone 2 situated below the lower limit of the zone 1, to which there corresponds a rising order applied to the mechanism actuating the anode system;
  • the alumina supply signal being sent when the order corresponding to the zone +1 has been given between one and 10 times.

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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)
US423656A 1964-01-14 1965-01-06 Apparatus and method for the operation of cells for the igneous electrolysis of alumina Expired - Lifetime US3455795A (en)

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FR960247A FR1397946A (fr) 1964-01-14 1964-01-14 Procédé pour la prévision des brûlures, l'alimentation systématique et la régulation automatique du système anodique des cuves pour l'électrolyse ignée de l'alumine

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AT (1) AT259887B (id)
BE (1) BE658220A (id)
BG (1) BG16458A3 (id)
CH (1) CH463797A (id)
DE (1) DE1260156B (id)
ES (1) ES308069A1 (id)
FR (1) FR1397946A (id)
GB (1) GB1088636A (id)
IS (1) IS865B6 (id)
LU (1) LU47777A1 (id)
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Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3539461A (en) * 1967-10-19 1970-11-10 Kaiser Aluminium Chem Corp Anode effect termination
US3625842A (en) * 1968-05-24 1971-12-07 Kaiser Aluminium Chem Corp Alumina feed control
US3627666A (en) * 1967-11-06 1971-12-14 Pechiney Apparatus for automatically regulating the anode gap in electrolysis cells
US3629079A (en) * 1968-02-23 1971-12-21 Kaiser Aluminium Chem Corp Alumina feed control
US3660256A (en) * 1967-12-07 1972-05-02 Gen Electric Method and apparatus for aluminum potline control
US3674674A (en) * 1968-12-27 1972-07-04 Delfzijl Aluminium Apparatus for controlling electrode adjustment during aluminum oxide reduction
US3888747A (en) * 1972-10-18 1975-06-10 Nat Southwire Aluminum Method of and apparatus for producing metal
US4654130A (en) * 1986-05-15 1987-03-31 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells employing point feeders

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2545413A (en) * 1946-10-29 1951-03-13 Alais & Froges & Camarque Cie Apparatus for automatic regulation of electrolytic cells
US2918421A (en) * 1957-06-14 1959-12-22 Anaconda Aluminum Co Measuring means for the bath resistance of aluminum reduction cells
US2933440A (en) * 1956-10-09 1960-04-19 Kaiser Aluminium Chem Corp Method and apparatus for detection of anode effect
US3317413A (en) * 1963-09-23 1967-05-02 Pechiney Cie De Produits Control of alumina content during igneous electrolysis
US3329592A (en) * 1963-08-30 1967-07-04 Reynolds Metals Co Method of and apparatus for controlling aluminum reduction pots

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2545413A (en) * 1946-10-29 1951-03-13 Alais & Froges & Camarque Cie Apparatus for automatic regulation of electrolytic cells
US2933440A (en) * 1956-10-09 1960-04-19 Kaiser Aluminium Chem Corp Method and apparatus for detection of anode effect
US2918421A (en) * 1957-06-14 1959-12-22 Anaconda Aluminum Co Measuring means for the bath resistance of aluminum reduction cells
US3329592A (en) * 1963-08-30 1967-07-04 Reynolds Metals Co Method of and apparatus for controlling aluminum reduction pots
US3317413A (en) * 1963-09-23 1967-05-02 Pechiney Cie De Produits Control of alumina content during igneous electrolysis

Cited By (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3539461A (en) * 1967-10-19 1970-11-10 Kaiser Aluminium Chem Corp Anode effect termination
US3627666A (en) * 1967-11-06 1971-12-14 Pechiney Apparatus for automatically regulating the anode gap in electrolysis cells
US3660256A (en) * 1967-12-07 1972-05-02 Gen Electric Method and apparatus for aluminum potline control
US3629079A (en) * 1968-02-23 1971-12-21 Kaiser Aluminium Chem Corp Alumina feed control
US3625842A (en) * 1968-05-24 1971-12-07 Kaiser Aluminium Chem Corp Alumina feed control
US3674674A (en) * 1968-12-27 1972-07-04 Delfzijl Aluminium Apparatus for controlling electrode adjustment during aluminum oxide reduction
US3888747A (en) * 1972-10-18 1975-06-10 Nat Southwire Aluminum Method of and apparatus for producing metal
US4654130A (en) * 1986-05-15 1987-03-31 Reynolds Metals Company Method for improved alumina control in aluminum electrolytic cells employing point feeders

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IS865B6 (is) 1974-06-28
OA01117A (fr) 1968-08-07
AT259887B (de) 1968-02-12
GB1088636A (en) 1967-10-25
IS1449A7 (is) 1965-07-15
LU47777A1 (id) 1965-07-13
NL6500357A (id) 1965-07-15
ES308069A1 (es) 1965-06-16
DE1260156B (de) 1968-02-01
FR1397946A (fr) 1965-05-07
BG16458A3 (bg) 1972-11-20
BE658220A (id) 1965-07-13
CH463797A (fr) 1968-10-15

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