US3627666A

US3627666A - Apparatus for automatically regulating the anode gap in electrolysis cells

Info

Publication number: US3627666A
Application number: US771834A
Authority: US
Inventors: Rene L Bonfils
Original assignee: Pechiney SA
Current assignee: Pechiney SA
Priority date: 1967-11-06
Filing date: 1968-10-30
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: ES359882A1; BE723451A; NL164616B; DE1807336B2; NL164616C; SE356535B; JPS527314A; JPS5637318B2; AT293738B; JPS4818710B1; GB1251519A; CH506326A; NL6815719A; NO124784B; RO54518A; FR1605433A; DE1807336A1

Abstract

An apparatus for automatically regulating the gap between anode and cathode in electrolytic cells in response to the amount and sign of voltage differential between U-RI and e in which U is the voltage at the terminals of the cell, I the intensity of the current passing through the cell, R is a predetermined value of internal resistance of a cell used as a reference while e is the counterelectromotive force of the electrolysis.

Description

United States Patent Inventor Rene L. Bonfils Saint-Jean de Maurienne, France Appl. No. 771,834 Filed Oct. 30, 1968 Patented Dec. 14, 1971 Assignee Compagnie Pechlney Paris, France Priority Nov. 6, 1967 France 127,012

APPARATUS FOR AUTOMATICALLY REGULATING THE ANODE GAP IN ELECTROLYSIS CELLS 7 Claims, 3 Drawing Figs.

US. Cl 204/225, 204/228, 204/243 Int. Cl 801k 3/00, C22d 3/12 Field of Search 204/225, 245, 228

[56] References Cited UNITED STATES PATENTS 3,573,179 3/197] Dirth et al 204/67 3,558,454 l/l97l Schafer et al. 204/228 X 3,531,392 9/l970 Schmeiser 204/225 3,396,095 8/l968 Van Diest et al. 204/225 X 3,434,945 3/1969 Schmitt et al 204/225 X 3,455,795 7/1969 Boulanger et al. 204/225 X 3,480,528 ll/l969 Clement 204/225 X Primary Examiner-John H. Mack Assislanr Examiner D. Rv Valentine Attorney-McDougall, Hersh, Scott & Ladd ABSTRACT: An apparatus for automatically regulating the gap between anode and cathode in electrolytic cells in response to the amount and sign of voltage differential between U-Rl and e in which U is the voltage at the terminals of the cell, 1 the intensity of the current passing through the cell, R is a predetermined value of internal resistance ofa cell used as a reference while e is the counterelectromotive force of the electrolysis.

APPARATUS FOR AUTOMATICALLY REGULATING THE ANODE GAP IN ELECTROLYSIS CELLS This invention relates to an apparatus for automatically regulating the anode gap in electrolysis cells.

The apparatus of this invention has particular advantage, in igneous electrolysis cells of the kind used in the production of aluminum, irrespective of whether the cells in question employ a continuous anode or a plurality of prebaked anodes, but the concepts are applicable also to other cells.

It is known that the anode gap of electrolysis cells can be automatically regulated by measuring a pseudoresistance taken as being equal to in which R is the pseudoresistance, U is the voltage at the terminals of the cell in question, e is the counterelectromotive force of electrolysis, I is the intensity of the current passing through the cell. This pseudoresistance is then compared with a reference value R0, after which a motor controlling the position of one of the electrodes, i.e., the anode or cathode, is given a pulse whose sign is governed by that of R-Ro so that the anode gap increases if R-Ro is negative and decreases if positive.

An apparatus in which this method is carried out and which is applicable to cells for the igneous electrolysis of alumina is described in French Pat. No. 1,397,946. Although this apparatus functions satisfactorily, it has the disadvantages of being complicated, expensive, and using relatively fragile components which have to be protected against the abrasive and corrosive atmosphere prevailing in an igneous electrolysis plant.

The present invention relates to an apparatus for automatically regulating the anode gap in electrolysis cells which is simple, inexpensive, and unaffected by the atmosphere prevailing inside the electrolysis plant.

In an apparatus embodying the features of this invention, there is established a voltage proportional to U-RI, where U is the voltage at the terminals of the cell, I is the intensity of the current passing through it, and R is the predetermined value of the internal resistance of the cell used as reference. The value thus obtained is compared with e, the counterelectromotive force of electrolysis, after which a motor, controlling the position of one of the electrodes, anode or cathode, of the cell is energized every time the value of URl differs from e by more than a predetermined quantity so that the anode gap is reduced when R-Rl is greater than e-l-a, and increased when U-RI is smaller than e-a. The apparatus according to the invention is distinguished by the fact that it comprises a first circuit consisting of a voltage converter which supplies a voltage kU proportional to U, a second circuit comprising an intensity converter supplying a current i=k' I proportional to the voltage prevailing at the terminals of a shunt connected in series with the cell and, accordingly, to the intensity of the current flowing through this cell and a potentiometer arranged at the terminals of the second of the two aforementioned converters which supplies a voltage k'k"! proportional to k! which, through its regulation, can be made equal to kRI, where R is the predetermined value of the internal resistance of the cell, and finally a third circuit comprising at least one threshold relay which is connected in series with the first two circuits arranged in opposed relation, and which emits an electric impulse every time that k( U-RI) differs from ice by more than a predetermined quantity, the sign of this pulse being governed by that ofkek( URI).

It is an object of this invention to produce an apparatus of the type described for automatic regulation of the anode gap in the electrolytic cells in a simple and efficient manner.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, and embodiment of the invention is shown in the accompanying drawings, in which FIG. 1 is a block diagram of the regulating apparatus;

FIG. 2 is a graph illustrating the operation of the apparatus illustrated in FIG. 1; and

FIG. 3 is a modification ofa detail ofan element in FIG. 1.

We provide a regulator which can only comprise static elements that are both inexpensive and unafiected by the surrounding atmosphere, by electrically establishing the value U-Rl where R is a predetermined value adopted for the internal resistance of the cell, and by giving a regulating order or command in the form of an identical electrical impulse every time that U-Rl is outside the range between two limiting values e-a and e+a where a is half the permissible deviation which is allowed on the value e.

In this connection, illustration is made in FIG. 1 of an electrolysis cell 5 with a cathode 51 connected to a pole 54, and an anode 52 connected to a pole 53. The anode may have a vertical translatory movement imparted to it by the action of an electric motor 55, such as of the direct current or single-phase type, fed through its

terminals

56 and 57. An electrolysis current flows into the cell through the pole 53, flows through the anode 52, through the space between the electrodes and then through the cathode 51, leaving via the pole 54. The voltage between the poles 53 is designated by the letter U, and the current which flows through the cell by I. In the igneous electrolysis of alumina, electrolysis takes place at an extremely high intensity of from about 65 to kiloamperes. Accordingly, a certain number of cells are branched in series in order to increase the overall voltage. Connected in series with the cells thus branched is a shunt 6 of which one terminal 61 is connected to the pole 54 of the cell and the other terminal 62 is connected to the negative pole of the device by which the cells of the series are fed.

The regulating apparatus comprises a first circuit consisting of an input voltage converter 1, a second circuit comprising an intensity converter 2 and a potentiometer 3 and finally a third circuit comprising at least one threshold relay 4.

The input converters enable the regulating apparatus to be isolated from the potential of the cell being regulated. Each converter may comprise, for example, one modulator, one transformer and one demodulator.

The input voltage converter 1 supplies a voltage kU proportional to U. It comprises two input terminals II and 12 connected to the

poles

54 and 53, respectively, of the cell, and two

output terminals

13 and 14 between which the voltage kU appears.

The input intensity converter 2 supplies a current i=k'l proportional to I. It is connected at its input terminals 21 and 22 to the

terminals

61 and 62, respectively, of the shunt 6.

The potentiometer 3 has two input terminals 31 and 32 connected to the output terminals 23 and 24, respectively, of the input intensity converter 2. It is regulated by means of a slide 33. Thus, between its output terminals 33 and 32, it supplies a voltage proportional to its input current k'l, that is to say a voltage equal to k'k"I. The potentiometer is regulated in such a way that k'k"! is equal to kRl.

In fact, the potentiometer may be directly graduated in units of R, the predetermined value of the internal resistance of the cell 5.

The two circuits formed, the first by the input voltage converter l and the second by the combination of the input intensity converter 2 and the potentiometer 3, are connected in opposition. To this end, the terminals 14 and 32 are connected together with the result that the voltage equal to k( U-RI) appears between the terminals 13 and 33.

The threshold relay 4 has two input terminals 41 and 42 connected to the terminals 13 and 33, respectively, and two output terminals 43 and 44 connected to the

poles

57 and 56, respectively, of the motor 55 controlling the translatory movement of the anode 52. A relay 9, whose function is explained in the following, normally short-circuits its output terminals 93 and 94, so that the threshold relay 4 receives a voltage equal to k( U-RI).

If the cell is perfectly regulated, this voltage is equal to ke where e is the electromotive force of electrolysis. The relay has a threshold ka, in other words it only emits a current impulse that is always identical between its output terminals 43 and 44 if k(URI) is outside the range between k(ea) andk(e+), that is to say if U-RI is outside the range between (e-wz) and (e-l-a). if this is in fact the case, the relay '3 transmits an impulse which, though of the same intensity, has a polarity governed by the sign of the difference U-RI-e. if URI is greater than e+a, the polarity is such that the motor 55, once it has been started, reduces the anode gap, whereas if U-R1 is smaller than e-a, the polarity is inverse, and the motor 55 increases the anode gap.

Accordingly, the effect of the process described thus far is to regulate a value which is sufficiently encountered in practice, rather than the pseudo resistance R=Ue/I In this respect, the intensity I has been plotted as the abscissa in FIG. 2 and the voltage U as the ordinate. The ordinate dots (e-a), e, and (e-l-a) are plotted on the axis of the U s, the first and the last of these values representing the limiting values ensured by regulation at the counter electromotive force of electrolysis. if it be assumed that the intensity normally equal to I may vary between two limits I, and 1,, the following equation lines are drawn:

U=e+(R-r)l cutting the vertical line of the abscissa I at A, A, D and D, respectively, and the vertical line of the abscissa l, at B, B, C and C, respectively, r represents the regulation threshold of the pseudoresistance R.

The equation line:

U=e+RI has also been drawn, cutting the abscissa vertical 1 at 0.

in the theoretical of the pseudoresistance, the cell would not receive any regulation orders while its point of operation remained inside the trapezoid ABCD defined by the straight lines:

U=e+( Rr)l, defining the regulation tolerance.

The principle adopted in the apparatus described thus far replaces the trapezoid ABCD by the parallelogram A'BC'D' defined by the straight lines:

U=ea+RI The difference in regulation, which is zero at normal intensity I,, increases with the variation in intensity.

The significance of this difference is illustrated by way of example in the following: in the case of cells where [=80 lta. regulated to U=3.9 v. for nominal intensity with a permissible deviation of 10.04 v. (a=0.04 v.), the variation in the regulation thresholds is 5 mv. for a fall in current of ka. (l,=70 ka.). This value is perfectly acceptable in practice, if we consider the number of modern cells that are also regulated in terms of intensity, from which it follows that the variations in I are very small and that the majority of regulations are exactly carried out at nominal intensity. it is also pointed out that this difference leads to an enlargement of the permissible deviation when the drop in intensity in the preceding case has passed from 0.04 v. to 0.045 v., and that accordingly there is no danger of a regulating order reaching a cell whose operating point would be at the limit of the permissible deviation for normal intensity. The same does not apply in the case of an increase in intensity where, by contrast, the fork is reduced, although this situation hardly ever arises in an intensity-regulated series.

It is of advantage to be able quickly to remove the anode 52 in the event of an emergency. For this purpose, there is connected in parallel with the threshold relay whose threshold is adjusted to k-a, a second relay 7 which is branched in exactly the same way, i.e., whose input terminals 71 and 72 are connected to the corresponding terminals 41 and 42, respectively, of the relay 4 and whose output terminals 73 and 74 are connected to the corresponding terminals $3 and 44, respectively. This relay only functions when U-RI is smaller than e-a', where a is a predetermined quantity, very much greater than a (for example five times greater in the case of an actual reduction to practice). This relay 7 transmits an electric impulse to the motor 55 which is either more powerful or of longer duration than that delivered by the relay 4.

it is of advantage to add a third relay 3 which, in the event of a lack of stability in the operation of the cell, causes the anode to ascend quickly. This relay is of the same type as the relay 7 except that one of its input terminals, for example the terminal 81, is connected to the corresponding terminal 41 of the relay 3 through a capacitor which only allows unstable oscillations through. The other input terminal 82 is connected to the terminal 62 while the output terminals 83 and 84 are connected to the corresponding terminals 43 and 44, respectively.

Finally, a fourth relay 9 may be provided for the purpose of interrupting the regulation of a cell too far outside the normal regulating tolerances. The

input terminals

91 and 92 of this relay are connected respectively to the terminals 41 and 42 while the output terminals 93 and 94 are incorporated in one of the connecting lines, for example the line 43-57 connecting the regulator to the motor 55. This relay 9 is identical with the relay 4 except that its threshold is regulated to ka" where a" is greater than a.

The operation of the apparatus is as follows: after adjustment of the potentiometer 3, a voltage equal to k( U-Rl) is developed as already explained between the terminal 13 and the slide 33 of the potentiometer 3. This voltage is applied directly to the input of the

relays

4, 7, and 9 and across the capacitor 85 to the input of the relay 8.

As long as k( U -RI) remains between l (e+a) and k(ea in which operation remains stable, nothing happens, the motor 55 is not fed and the anode 5w remains immobile. If k( URI) becomes greater than k(e+a) or smaller than k(e-a) without however reachingk(ea'), only the relay 4 is activated transmitting a current impulse to the motor 55, resulting in movement of the anode 52. R( UR!) then reassurnes a value approaching ke and, in any case, within the range from k(e+a) to k(eaa).

if, following an incident, the internal resistance of the cell suddenly drops to an extent sufficient for k( URl) to become smaller than k(ea'), the relay '7 is activated and transmits to the motor 55 a current impulse of either greater power or longer duration which starts the motor moving at a faster speed: the anode thus rises quickly until k( U-Rl) is again similar to ke.

if for any reason the cell becomes unstable in operation, k(U-Rl) undergoes rapid fluctuations which, filtered by the capacitor 85, affect the relay 8 which then functions like the relay 7 and ensures rapid ascent of the anode.

Finally, if k( U-Rl) varies suddenly and is reduced to a value smaller than k(ea'), the relay 9 breaks the contact between the terminals 93 and 94 so that regulation is interrupted.

Since regulation has only to be carried out on one and the same cell at fairly long intervals, the same apparatus may be used for several cells.

For this purpose, the connections 11-54, 12-53 and 94-57 merely have to be switched providing that the same reference value R can be used for all the cells connected to one and the same regulator. if it is desired to be able independently to fix this value for each of the cells, the connecting lines 33-82 and 23-31 will also have to be switched and separate potentiome ter provided for each cell.

This additional switching and the expense which it involves may be avoided by providing an additional potentiometer 10 in the voltage circuit as shown in FIG. 3. As this circuit has already been switched, there is no point in any additional switching. The

terminals

101 and 102 of the ends of the winding of this potentiometer 10 are connected to the

poles

53 and 54 respectively of the cell while the slide and the terminal 102 are connected to the input terminals 11 and 12 respectively of the converter 1 through the cell changeover switch.

it is possible by means of this potentiometer 10 to deliver to the regulator a fraction U, rather than all, of the cell voltage. in this case, the regulator keeps the cell at a higher voltage so that U is equal to U,,, the predetermined value for normal intensity.

It is possible through the following calculation to evaluate the error attributable to this method of individually regulating cells.

Let us assume we are dealing with a cell whose potentiometer is at maximum regulation, that is to say with its slide 103 at 101: the regulator regulates it for normal intensity 1,, to a predetermined value U, so that U=Rl+e.

If the potentiometer is displaced, the regulator receives a voltage U'=hU where it is assumed that h remains for example between 0.8 and 1.

For normal intensity, the cell is restored to a regulation where U'=U, and the corresponding resistances R is defined by:

For a variation in intensity which brings the intensity of the cell to a value of 1,, the voltage across the terminals of the cell becomes:

U,=R'1,+e The regulator then receives the following voltage:

= R h-F? e(1 h) +he In the regulator this voltage is compared with the voltage U,,,of a cell regulated to the predetermined value with the potentiometer at its maximum (h=l nl Ul 1 The error attributable to the use of the potentiometer 10 is equal to the difference:

AU is cancelled where I,=I, and where h=l, and increases with the variation in intensity and with the extent of displacement.

By way of example, in the case of a cell fed at 80 ka. and regulated to 4 v., and for a variation in intensity of 10 ka., the error amounts to 0.01 v. for h=0.95, that is to say for a correction of +0.2 v. It reaches 0.02 v. for h=0.90, that is to say for a correction of +0.4 v. which is hardly ever reached in practice.

Since the corrections are almost always made by increasing the anode-cathode gap, the position which corresponds to h=l that is to say with the slide 103 at 101, may be adopted as the normal position of the potentiometer 10.

There are several advantages in transmitting a constantly identical impulse through the relay 4. It is possible by means of a pulse counter to determine their repetition frequency at any given moment and thus to detect the approach of an anode effect by a progressive increase in this frequency. In this event, the cell may be fed with alumina which prevents the anode effect from arriving unexpectedly.

It will be apparent that -I have provided a simple and efficient means for controlling the anode gap in electrolytic cells whereby positive controls with rapid adjustments in the event of resistance are capable of being achieved.

It will be understood that changes may be made in the details of construction, arrangement and operation, without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. An apparatus for automatically regulating the gap between anode and cathode in electrolytic cells in which there is established a voltage pro ortional to U-RI, in which U is the voltage at the terminal 0 the cell, 1 the intensity of the current passing through the cell and R is a predetermined value of the internal resistance of a cell used as a reference, which includes means for comparing the value thus obtained with e, the counterelectromotive force of the electrolysis, means mounting the anode and cathode for movement of one in the direction towards and away from the outer, a motor operatively connected to one of the electrodes of the cell, said motor being energized when the value of U-RI differs from e by more than a predetermined quantity to reduce the gap when U-RI is greater than e+a and to increase the gap when U-Rl is smaller than e-a, where a is half the permissible deviation which is allowed on the value e, a first circuit consisting of a voltage converter supplying a voltage kU proportional to U, a second circuit comprising an intensity converter supplying a current i k'I proportional to the voltage across the terminals of a shunt connected in series with the cell and to the intensity of the current passing through the cell and a potentiometer arranged at the terminals of the second converter which supplies a voltage klc"I, means for adjusting the potentiometer to kRl in which R is the predetermined value of the internal resistance of the cell, a third circuit comprising at least one threshold relay connected in series with the first and second circuits and in which the first and second circuits are connected in opposition, and which emits an identical electric impulse whenever k( U-RI) differs from ke by more than a predetermined quantity.

2. An apparatus as claimed in claim 1 in which the sign of the pulse is governed by kek(U-Rl).

3. An apparatus as claimed in claim 1 in which the regulator serves a plurality of cells in series, and which includes a switch between the poles of the cell and the voltage converter, and between the threshold relay and the motor.

4. An apparatus as claimed in claim 3 which includes a second potentiometer between the poles of the cell and the voltage converted, said second potentiometer enabling the reference value selected for the internal resistance to the individually regulated for the cells, the regulation of the first potentiometer remaining the same for all of the cells.

5. An apparatus as claimed in claim 1 in which the third circuit comprises a second threshold relay branching parallel from the first which is only actuated when U-Rl is smaller than e-a', and in which a is a predetermined quantity considerably larger than a, said second relay being connected to the control motor for delivery of an electrical impulse more powerful than that delivered by the first relay to enable the electrode being controlled rapidly to rise in the event of resistance.

6. An apparatus as claimed in claim 1 which includes a third relay having its input connected in parallel with the input of the first relay through a capacitor which only allows unstable oscillations to pass therethrough, the output of said third relay being connected in parallel with the output of the first relay, said third relay causing the controlled electrode quickly to rise in the event of any instability in the cell.

7. An apparatus as claimed in claim 1 which includes a fourth relay having its input connected in parallel with the input of the first relay and its output connected in series with the circuit feeding the control motor, said fourth relay corresponding to the first relay except that its threshold is regulated to a value ka", in which a", is greater than a, said fourth relay enabling regulation to be interrupted whenever a cell is beyond the normal regulating force.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,627,666 bated December 14, 1971 Inventor(s) Rene Bonfils It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

column 1, line 46, change "R-Rl" to "U-Rl";

column 3, I line 1, at the beginning of the line,

before II (ea) insert "k";

column 4, line 36, change "k(e-aa) "to "k(e-a) column 6, line 12, change "outer" to "other" Signed and sealed this 15th day of May 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM uscoMM-oc scans-P69 U.. GOVERNMENT PRlNTlNG OFFICE I 1969 O355-334

Claims

2. An apparatus as claimed in claim 1 in which the sign of the pulse is governed by ke-k(U-RI).
3. An apparatus as claimed in claim 1 in which the regulator serves a plurality of cells in series, and which includes a switch between the poles of the cell and the voltage converter, and between the threshold relay and the motor.
4. An apparatus as claimed in claim 3 which includes a second potentiometer between the poles of the cell and the voltage converted, said second potentiometer enabling the reference value selected for the internal resistance to the individually regulated for the cells, the regulation of the first potentiometer remaining the same for all of the cells.
5. An apparatus as claimed in claim 1 in which the third circuit comprises a second threshold relay branching parallel from the first which is only actuated when U-RI is smaller than e-a'', and in which a'' is a predetermined quantity considerably larger than a, said second relay being connected to the control motor for delivery of an electrical impulse more powerful than that delivered by the first relay to enable the electrode being controlled rapidly to rise in the event of resistance.
6. An apparatus as claimed in claim 1 which includes a third relay having its input connected in parallel with the input of the first relay through a capacitor which only allows unstable oscillations to pass therethrough, the output of said third relay being connected in parallel with the output of the first relay, said third relay causing the controlled electrode quickly to rise in the event of any instability in the cell.
7. An apparatus as claimed in claim 1 which includes a fourth relay having its input connected in parallel with the input of the first relay and its output connected in series with the circuit feeding the control motor, said fourth relay corresponding to the first relay except that its threshold is regulated to a value ka'''', in which a'''' is greater than a'', said fourth relay enabling regulation to be interrupted whenever a cell is beyond the normal regulating force.