CA1111126A - Apparatus for regulating anode-cathode spacing in an electrolytic cell - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE An improved apparatus for adjusting the space between an adjustable anode and a cathode in an electrolytic cell wherein current measurements and voltage measurements are obtained for conductors to the anode sets and compared with predetermined standards for the same conductors and anode sets. Measurement of deviation from the predetermined standards are used to determine the direction of anode adjustment. A digital computer operably connected to motor drive means adapted to raise or lower anode sets upon appropriate electric signals from the computer is a preferred embodiment of this invention.

Description

The present invention relates to apparatus for adjusting the anode-cathode spacing in an electrolytic cell.
In particular, the invention relates to an improved apparatus for adjusting the anode-cathode spacing in electrolytic mercury cells for the electrolysis of alkali metal chlorides such as sodium chloride.
In electrolytic cells with adjustable anodes, the ~;
control of the inter-electrode distance between the anode and the cathode is economically important. The anode-cathode spacing should be narrow to maintain the voltage close to the decomposition voltage of the electrolyte. Careful control of the anode-cathode spacing reduces energy lost in the pro-duction of heat and reduces short circuiting and its accom- -panying problems which include the descruction of anode sur-faces and the contamination of the electrolytic products.
; Numerous techniques have been developed to adjust the anode-cathode gap in electrolytic cells. For example, U.S. Patent No. 3,574,073, issued April 6, 1971, to Richard W. Ralston, Jr., discloses adjustment means for anode sets in electrolytic cells. In thie patent, a ",..

C-6727 means responsive to changes in the flux of the magnetic field generated by electrical flow in a conductor sup-plying the anode sets controls the opening and closing of an electrical circuit, and activates hydraulic motors which are effective to raise or lower the anode sets. In addi-tion, a cell voltage signal and a temperature compensated amperage signal proportional to the bus bar current for the,anode set are fed as input to an analog computer which produces an output reading of resistance calculated according to the formula:
':, R = E ~Er I
where R is the resistance of one anode set, E is the cell '! .
voltage, Er is the reversible potential of the particular electrode-electroly,te system and I is the current flowing to the anode set. Each anode set has a characteristic I resistance at optimum ef~iciency to which tha~ anode set is appropriately adjusted. , ` ' U. S. Patent No. 3,558,454, which issued ,l January 26, 1971, to,Rolph Schafer et al, discloses the '20 regulation of voltage in an electrolytic cell by measuring , the cell voltage and comparing it with a reference volta~e.
The gap between electrodes is changed in accordance with deviations between the measured voltage and the reference voltage and all electrodes in the cell axe ad~usted as a unit.
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C-6727 Similarly, U.S. Patent No. 3,627,666, which issued December 14, 1971, to Rene L. Bonfils! adjusts all --electrodes in an electrolytic cell using apparatus which measures the cell voltage and current in a series of circuits which regulate the anode-cathode gap by establishing a voltage proportional to U - RI where U i5 the cell voltage, I the cell current and R the predetermined resistance of the cell.
A method of adjusting electrodes by measuring the cuxrents to individual electrodes in cyclic succession and adjusting the spacing of those anodes whose measured currents differ from a selected range of current values is disclosed in U.S. Patent No. 3,531,392, which issued September 29, 1970, to Kurt Schmeiser. All electrodes are adjusted to the same range of current values and no measurement of voltage is made.
A method of detecting incipient short circuiting is disclosed in U.S. Patent No. 3,361,654, which issued-January 2, 1968, to D. Deprez et al, by advancing an anode an unknown distance toward the cathode, measuring current a8 the anode moves and stopping movement of the anode when the current of the cell undergoes a rapid increase dispro- -portionate to the speed of anode advancement, and then , reversing the direction of anode movement a selected dis- ¦
tance. This method adjusts the electrode with respect to the cell current.

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C-6727 ~est German Patent No. 1,804,259, published May 14, 1970, and East German Patent No. 78,557, issued December 20, 1970, also describe techniques for adjusting the gap between anodes and cathodes.
While the above methods provide ways of adjusting the anode-cathode spacing in an electrolytic cell, it is well known that in a cell containing a plurality of elec-trodes, the optimum anode-cathode spacing for a particular electrode will depend on its location in the cell, and its age or length of service, among other factors. For example, in a horizontal mercury cell for electrolyzing alkali metal chlorides, the optimum anode-cathode spacing for an anode located near the entry of the cell is different from the spacing for one located near the cell exit. In addition, decomposition voltage varies throughout the cell as brine temperature and concentration change. Likewise a new anode can maintain a closer anode-cathode spacing than one which has been in the cell for a longer period of time or can operate more efficiently at the same spacing~ In addition, after an anode has been lowered it is necessary to know~whether the anode-cathode spacing is too narrow which may cause short circuiting or loss of efficiency.
There is a need at the present time for an improved method and apparatus for controlling the space between an adjustable anode and a cathode which utilizes .'~.. .

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current measurements, and/or voltage measurements or a combination thereof to effect adjustment of the electrode space of individual anode sets under the varying conditions occurring in the aforesaid electrolytic cells.
It is an object of this invention to provide an improved apparatus for adjusting anode-cathode spacing in an electrolytic cell which overcome disadvantages in previously known techniques for adjusting this spacing.
Objects of this invention are accomplished in an apparatus for adjusting the space between electrodes in an electrolytic cell, said electrodes being comprised of a ;~
plurality of anode sets,at least one conductor conveying current to each of the anode sets, and a li~uid cathode in spaced relationship with each of said anode sets, said apparatus comprising in combination:
a. digital computer means programmed with pre-determined standard signal ranges for current signals for each of said conductors, b. means for detecting a series of N current signals to each of said conductor over a pre-determined period, c. means for selecting one of said anode sets and one of said conductors to said selected anode set, and means for selecting signals generated from said selected conductor to said selected anode set, d. means for supplying said selected signals in digital form to said digital computer means, -e. means for comparing said selected signals with said predetermined standard signal ranges - ' ~, , ' . ~
' : ' for said selected conductor from said selected anode set programmed in said digital computer, f. means in said digital computer for generating activating electric signals when said selected signals in digital form are outside of said predetermined standard signal ranges, and g~ motor means operative to raise or lower said selected anode set, said motor means being energized by said activating electric signals when said selected signals are outside said standard signal ranges.
In preferred embodiments the apparatus of this invention also has in combination:
h. means for reactivating said means b. through g. immediately after said motor means is activated to lower said anode set, i. means for storing the previously detected signals obtained prior to lowering said ~0 selected anode set and means for comparing n0wly detected signals with said previously detected signals;
and may also include:-' :"~.' .:

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C-6727 j. means for detecting analog type voltage signals produced by each conductor carrying current to each anode set, . means for compensating said signals for -temperature variations in said conductors to produce signals that are proportional . to the current flow in said conductor, 1. means for detecting analog type voltage signals across said anode set, m. means for selecting from said compensated signals a set of signals generated from the conductors carrylng current to a selected anode set in said electrolytic cell, n. means for amplifying said set of signals,.
o. means for transforming the thus amplified . .
set of signals at cell potential into ' proportional signals at computer potential, p. means for conditioning said proportional signals to remove rectifier-generated noise, .
q. means for converting the thus conditioned signals of the analog type to signals of the digital type, r. means for calculating the voltage coeffi-cient from said digital type signal according 1 :
to the formula: 1 :
' : Voltage coefficient = V~
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C-6727 where V is the overall voltage across said anode set in which said set o~ signals is generated, D is the decomposition voltage of the cell, and KA/M2 is the current density in kiloamperes per square meter of cathode surface below said selected anode set, s. means for comparing the thus calculated : voltage coefficients with a predetermined voltage coefficient for said anode set in ~10 said cell and determ~ning the difference between said calculated voltage coefficient and said predetermined voltage coefficient, : t. means for comparing the digital type current ;
signals with a predetermined current for.each conductor to each anode set in said cell and determining the difference between said measured current and said predetermined current, ; u. motor means operative to raise and lower `
by a predetermined amount said anode set .
fed by the conductor in which said signals are detected, said motor means being energized .
by electric signals from said computer to raise said anode set when said calculated voltage coefficient is below said predeter-, ~ : ~ mined voltage coefficient by an amount in .1 ' .
excess of-k, a predetermined limit, or said .

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C-6727 measured current is higher than said predetermined current, said differences exceed a predetermined limit, and said motor means being energized to lower said anode set when said calculated voltage coefficient is higher than said predetermined voltage - coefficient ~y more than said k, v. means for activating said means j. through q.
immediately after said motor means is activated to lower said anode set and means for comparing the new signals proportional to current flow `
in each conductor feeding said anode set with the signals proportional to current flow ~ `
to said anode set prior to lowering said anode set, `
w. means for activating said motor means to raise said anode set by a predetermined amount when the increase in current ~ollowing said .
lowering of the said anode set exceeds a predetermined amount, x. means for activating said means b. through g. ~ ;
when the increase in current is less than said predeterminad amount, but continues to increase unless said current exceeds a .' ~:; second predetermined limit, means for acti- -~
~: vating said motor means to raise said anode set by a predetermined amount when the current 1 : _ exceeds said second predetermined limit, y. means for activating said motor means to raise said anode set by a predetermined amount when said current continues to increase for longer than a predetermined period of time, and z. means for activating said motor means to raise said anode set a predetermined amount when the frequency of change in anode-cathode spacing over a predetermined period exceeds a pre-determined limit. ~

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Objects of this invention are also accomplished in the novel apparatus of this invention wherein an electro-lytic cell is used containing an electrolyte decomposable by electric current, said electrolyte being in contact with electrodes comprised of at least one adjustable anode set and a liquid cathode spaced apart a predetermined dis-tance. A voltage is applied across the cathode and anode set through at least one conductor to the anode set to develop an electric current flow from said anode set through said electrolyte to said cathode to effect decomposition of the electrolyte. In the operation of this electrolytic cell, the improved apparatus of this invention comprises:
a. motor drive means operabl~ connected to the adjustable anode set adapted to raise and lower the adjustable anode set upon receipt of electric signals from a digital computer, b. means for obtaining N current measurements of the current to each conductor to the anode set over a predetermined period, and means for conveying each current measurement by electric signal to the computer, c. means for comparing in the computer each current measurement with a preceding current measurement on the same conductor and deter-mining the difference in current, and .
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d. means for conveying an electric signal from the computer to the motor drive means to increase the space a predetermined distance when the difference in current is an increase which exceeds a predetermined limit.
In another embodiment of the invention, the im-proved apparatus of this invention also comprises:
e. means for measuring the current to each con-ductor to each anode set and conveying the current measurement by electric signal to the computer, f. means for conveying an electric signal from the computer to the motor drive means to de-crease the space between the anode set and the cathode by a predetermined distance, and after decreasing the space, g. means for obtaining N current measurements of the current to each conductor to each ; anode set over a predetermined period, and conveying each current measurement by electric signal to the computer, h. comparing in the computer, each current measurement with a preceding current measure-ment on the same conductor and determining the difference in current, and ", ;-: ~ . - . . - . . . , :

C-6727 i. means for conveying an electrical signal from the computer to the motor drive means to increase the space a predetermined distance when said di~ference in current is an increase which exceeds a predetermined limit~
The difference in current may be determined on the same conductor between any two successive current measurements or between any current measurement and a preceding current measurement during the same prede-termined period or a preceding predetermined period.
In addition, the difference in current may be determined between any current measurement for~the anode set and an average anode set current based upon the bus current for the entire cell. For example, the average conductor current or bus-bar current, is obtained by measuring the total cell current and dividing the total ~urrent by the number of conductors to the cell. If desired, the average conductor current is obtained by obtaining the sum of the individual conductor currents to the cell ;20 and dividing this sum by the number of conductors to the cell. The acceptable current to the conductor being examined may be from about 1.1 to about 1.5, and prefex-ably about 1.3 times the average cell current. Similar .1 . .
; adjustments in the space are made when the average differ-ence or the square root of the average of the squares of the differences in current measurements on the same conductor exceed predetermined limits.

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The apparatus of the present invention provides for the adjustment of the anode-cathode spacing for indivi-dual anode sets in an electrolytic cell where the optimum anode-cathode spacing may vary for all anode sets in a cell. In addition, the selection of cells and anode sets within a cell for possible ad~ustment may be made randomly or in order.
The apparatus of this invention is particularly useful in controlling commercial electrolytic cells where large numbers of cells are connected in series and each cell contains a plurality of anode sets~ .

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.C-6727 Figure 1 is a block diagram showing generally the layout of the apparatus of this invention.
Figure 2 is a block diagram showing one embodi-ment of the invention including a signal isolation and signal conditioning system utilizing a transformer. ~ .
Figure 3 is a block diagram showing another em-bodiment of the invention including a signal isolation and signal conditioning system utilizing an optical isolator.
Figure 1 illustrates the apparatus of this inven-tion in block diagram form where electric signals repre-senting current measurements 1 and electric signals repre-senting voltage measurements 2 from each conductor to each anode set (not shown) for each electrolytic cell 3 are se~ected by cell selector unit 4. Anode set selector unit S in response to a signal from manual control unit 9 selects electric signals fox current measurements 1 and~voltage measurements 2 from any conductor of any desired anode set .
in electrolytic cell 3 through cell.selector unit 4. Auto-matic control unit 6 transmits signals to cell selector unit 4 to select current measu~ements 1 and voltage measure-: ments 2 from cell selector unit for desired anode sets and performs the required calculations and comparisons with predetermined limits. When these calculations and. .
comparisons chow that raising or lowering of the anode set is necessary, appropriate electric signals are conveyed to relay 7,. then to motor control unit 8 which operates upon the anode adjustment mechanism (not shown) to raise -15~
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C-6727 or lower the anode set. Motor contr~l unit 8, whic~ can be used for increasing or decreasing the anode-cathode spacing in any anode set in electrolytic cell 3, can also be controlled by manual control unit 9 through anode set selector unit 5.
Figure 2 is a block diagram showing one embodiment of the signal selection and conditioning system for two adjacent electrolytic cells 3a and 3b, respectively, in series.
Electrolytic cell 3a has a plurality of anode sets 12, 12a and 12x. Anode set 12 is comprised of at least one anode 13, for example three parallel anodes 13.
Each anode 13 is provided with at least one anode post 14, and with two anode posts 14 preferably, as shown, with the anode posts 14 arranged in two parallel rows. A conductor 15 is connected to each row of anode posts 14 in electrolytic cell 3a. Current from plant supply ~not shown) is conveyed through two conductors 15 to each row of anode posts 14 in anode set 12. Anode sets 12a and 12x are each comprised of three anodes, 13a and 13x, respectively, having two rows -of anode posts 14a and 14x, respectively, secured to con-ductors 15a and 15x, respect~vely.

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r-~727 Adjacent electrolytic cell 3b has a corresponding -number of anode sets 16, 16a, and 16x. Anode set 16 is comprised of three parallel anodes 17 having two rows of anode posts 18 in each anode set 16. Anode sets 16a and 16x each have three parallel anodes 17a and 17x with two rows of anode posts 18a and 18x.
Current from anode posts 14 of electrolytic cell 3a passes to anodes 13, through the electrolyte (not shown), the mercury amalgam lnot shown) to the bottom of electrolytic cell 3a.
Conductors 19 connect to terminals 50 and 50 at the bottom of electrolytic cell 3a at points adjacent to the nearest anode 13 and convey current to the corres-ponding rows of anode posts 18 in electrolytic cell 3b. In a similar manner, current passes from anode post 14a and 14x, respectively, to anodes 13a and 13x, respectively, through the electrolyte and the mercury cathode to the bottom of electrolytic cell 3a. The cathode terminal is shown symbolically as cathode terminal 50 at the side of ~20 electrolytic cell 3a, but it is actually positioned on the bottom of the electrolytic cell 3a, as is well known l in the art, as shown in Figure 2 of U.S. Patent No.
; 3,396,095.
Each conductor 19 conveys current from cathode terminal 50 connected to the bottom of electrolytic cell 3a below anode posts 14 to the corresponding row of anode -posts 18 in electrolytic cell 3b. Conductors l~a and l9x 1 -convey current from other cathode terminals 50a and SOx below . ~ . .
~ rows of anode posts 14a and ~4x, respectively, to anode .~ , posts 18a and 18x, respectively.

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C-6727 The voltage drop bet~een terminals 20 and 21 on conductor 15 LS measured to obtain an electrical signal which is proportional to the current flow to anode set 12.
Similarly, the voltage drop between terminals 22 and 23 on conductor 19 is measured to obtain an electric signal which is proportional to the current flow to anode set 16.
The distance between terminals 20 and 21 is the same as the distance between terminals 22 and 23. The current signals from these terminals are altered by thermistor circuits 24 and 25, respectively, where the current signals are temperature compensated. Although Figure 2 shows thermistor circuit 24 touching conductor 15, it is not in electrical contact with the conductor. Instead, the thermistor circuits are embedded in the bus bar or conductor 15 with an appropriate non-insulating shield. -Current signals from thermistor 24 are transmitted across relay circuits 27 and 28 to ampli~ier 33 and current signals from thermistor 25 are transmitted across relay circuits 30 and 31 to amplifier 33.

The voltage drop across conductor 15 of anode set 12 in electrolytic cell 3a is measured between terminal 20 on conductor 15 and terminal 22 on conductor 19, which is the corresponding terminal for the corresponding anode set of the adjacent electrolytic cell 3b. Simllarly, the voltage drop across conductor 19 in anode set 18 in electrolytic cell 3b is measured between terminal 22 on on conductor 19 and terminal 26 on conductor 51, which is : ~ . . ' ' ' -. ,.~.- -. :, :,: , .. .
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C-6727 the corresponding terminal for the corresponding anode set of the next adjacent electrolytic cell. Thus, the "vol-.
tage drop across an anode set", such as anode set 12, is based upon the flow of current from a given point 20 on conductor 15 through anode posts 14 to anodes 13, through - the electrolyte, mercury cathode and cathode terminal 50 to terminal 22 on conductor 19. A second voltage drop across anode set 12 is obtained in the same way between the other conductors lS and 19 communicating with the other row .
~ of anode posts 14. These voltage drops for each conductor 15 of anode set 12 are averaged to determined the voltage - :
drop across anode set 12. :~
Current signals are obtained for the other conductor 15 to anode set 12 as well as all of the other conductors 15a, 15x, 19, l9a and l9x in the same manner as described above and as shown in Figure 2 for conductor 15.
Voltage signals based upon voltage drop across the anode set are obtained for the other row of anode posts 14 of anode set 12 as well as for each of the other rows of anode posts for anode sets 12a, 12x, 16a and 16x in the same .
manner as described above and as shown in Figure 2. .
Current is conveyed from the mercury cathode of electrolytic cell 3b through cathode terminals 52, 52a and 52x po~itioned beneath rows o anode posts 18, 18a and 18x, respectively, to conductors 51, 51a and 51x, respec-tively.
: ' _lg_ , , ~ ' ' , . ~ . -C-6727 Thus, for an electrolytic cell containing ten anode sets, each anode set having two rows of anode posts connected to the anodes in the set, there are twenty con-ductors, each providing through relay circuits 27-32, the first level multiplexing means, a current signal to one of twenty separate amplifiers 33 and a voltage signal to one of twenty separate amplifiers 34.
Relay circuits 27 and 28 are activated through power supply 53 when switch 54 is moved to a closed position.
Relay circuits 30 and 31 are also activated through power supply 53 when switch 55 is moved to a closed posi- ;
tion.
Temperature compensated current signals are amplified in amplifier 33 and conveyed to chopper 35 in ~-~
signal isolation and conditioning system 48 where they are converted from direct current signals to alternating current signals. These signals are then transmitted at cell potential to transformer 36 having one terminal of the primary winding connected to cell potential and one terminal of the secondary winding connected to earth po-tential. The current signals are isolated in transformer 36 and leave at earth potential in order to be compatible with automatic control unit 6. The current signals are -transmitted from transformer 36 to detector 37 where the . .

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C-6727 isolated current signals are converted from alternating current signals to direct current signals, and the resul~ing direct current signals are transmitted to a gated integrator 38 where rejection of electrical noise, particularly that generated by the rectifier which supplies current to electrolytic cells 3a and 3b. Noise conditioned current signals are transmitted to hold unit 39 (capacitor) and stored until selected by selector 40, the second level multiplexing means.
In a similar manner, the voltage signals are amplified in amplifier 34 and conveyed to a chopper 42, then at cell potential are conveyed to a transformer 43, where - the voltage signals are isolated and leave at earth potential.
These signals are converted from alternating to direct current in detector 44 and then to gated integrator 45 where rejection of electrical noise is also effected. The re-sulting voltage signals are transmitted to hold unit 46, (capacitor) where they are stored until selected by selec-tor 40 in the same manner as current signals stored in ~;20 hold unit 39. In response to a programmed electric signal from automatic contr~l unit 6, (or if desired, an electric signal initiated manually from manual control unit 9 of Figure 1), current signals and voltage signals from selector 40 for any conductor of any desired anode set such as conductor 15 of anode set 12 or conductor 19 of anode set 16 are selected and transmitted to convertor 41 whera ; they are converted from analog form to binary form and .~ .

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C-6727 then transmitted to automatic control unit 6 for processing.
In automatic control unit 6, the selected signals are compared with predetermined values for the same conductor and anode set, and when necessary, the selected anode set is raised or lowered by an appropriate electric signal from , ,.. . :
automatic control unit 6 through relay 7 to motor drive 8, which operates to raise.or lower the selected anode set.
Generally only one selector 40 is needed as a ~ .-.
second le~el multiplexing means for the entire cell seriesj.
but additional selectors 40 may be employed, if desired.
Figure 3 shows another embodiment of the inven- .
tion utilizing an optical isolator. In Figure 3, temperature compensated current signals from amplifier 33 in Figure 2 are conveyed to gated integrator 38 where rejection of electrical noise, particularly that generated by the ; rectifier which supplies current to electrolytic cells 3a and 3b, is effected. Noise conditioned current signals .
are transmitted to hold unit 39 and stored until selected , by selector 40.
~20 In a similar manner, voltage signals from ampli-fier 34 of Figure 2 are conveyed in Figure 3 to a gated integrator 45 where rejection of electrical noise is also effected. The resulting voltage signals are transmitted to hold unit 46, where they are stored until selected by selector 40 in the same manner as current signals stored in hold unit 39. In response to a programmed electric signal from automatic control unit 6, or , if desired, :

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a manually initiated electrical signal, current signals and voltage signals ~rom selector 40 for any desired anode set are selected, the signals are transmitted to convertor 41 where they are converted from analog form to binary form and then transmitted to optical isolator 47.
Signals enter optical isolator 47 at cell poten-tial, are~isolated and transmitted at earth potential to automatic control unit 6, where the selected signals are compared with predetermined values, and when necessary the selected anode set is raised or lowered in the same manner as described for Figure 2.
The apparatus of the present invention may be used on a variety of electrolytic cell types used for different electrolytes and electrolysis systems. The invention is particularly useful in the electrolysis of alkali metal chlorides to produce chlorine and alkali metal hydroxides. More particularly, the invention is es-pecially suitable for use in combination with the anode adjusting mechanisms driven by an electric motor or the like operating on adjustable anodes positioned in horizontal electrolytic cells having a li~uid metal cathode such as mercury, as disclosed, for example in U.S. Patents 3,390,070 and 3~5741073~ which are hereby incorporated by reference in their entirety.

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C-6727 As indicated in U.S. Patent No. 3,574,073, issued April 6, 1971, to Richard W. Ralston, Jr., horizontal mercury cells usually consist of a covered elongated trough sloping slightly towards one end. The cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows along the bottom of the cell toward the lower end. The anodes are generally composed of slotted rectangular blocks of graphite or metal distributors having an anodic surface comprised of titanium rods or mesh coated with a metal oxide secured to the bottom of the distributor.
Anode sets of different materials of construction may be employed in the same cell, if desired. The anodes are suspended from at least one anode post such as a graphite rod or a protected copper tube or rod. Generally, each rectangular anode has two anode posts, but only one, or more than two, may be used, if desired. The anodes in each anode set are placed parallel to each other, the anode posts forming parallel rows across the cell. The bottoms of the anodes are spaced a short distance above the ~20 flowing mercury cathode. The electrolyte, which is usually salt brine, flows above the mercury cathode and also contacts the anode. Each anode post in one row of an anode set is secured to a first conductor, and the other row of anode post- i9 secured to a second conductor. Each conduo-,,, .:
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C-6727 tor is adjustably secured at each end to a supporting post secured to the top of the cell. Each supporting post is provided with a drive means such as a sprocket which is driven through a belt or chain or directly by a motor such as an electric motor, hydraulic motor or other motor capable of responding to electric signals from automatic signal device 6.
Although the invention is particularly useful in the operation of horizontal mercury cells used in the electrolysis of brine, it is generally useful for any liquid cathode type electrolytic cell where adjustment of the anode-cathode space is necessary for efficient operation.
The number of electrolytic cells controlled by ` ~ the (~e~h~d-~d~apparatus of this invention is not critical.
Although a single elec~rolytic cell can be controlled, commercial operations conta;ning more than 100 cells can be successfully controlled.
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Each electrolytic cell may contain a single anode, but is preferred to apply the apparatus of this invention to electrolytic cells containing a multiplicity of anodes.
Thus the number of anodes per cell may range from 1 to about 200 anodes, preferably from about 2 to about 100 anodes.
It is preferred, particularly on a commercial scale to adjust anode sets when adjusting the space between the anodes and cathode of electrolytic cells. An anode set may contain a single anode, but it is preferred to include from 2 to about 20 anodes, and preferably from about 3 to about 12 anodes per anode set. Voltage and current measurements are obtained for each conductor for each row of anode posts of each anode set in each cell.
When each anode set, such as anode set 12, is initially connected in an electrolytic cell 3a, which is operated by the apparatus of this invention, anode set 12 is lowered to a point where the bottoms of anodes 13 are about 3 millimeters above the mercury cathode. In addition, a set point for the standard voltage coefficient, S, for each conductor 15 is entered into the program of automatic control unit 6. This set point voltage coefficient and subse~uent measurements of voltage coefficients, Vc, are calculated according to the formula:

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Vc = V-D _ ~;
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. ~ . . ,: . . . ~ : , , C-6727 where V is the measured voltage across ~ -an anode set, D-is the.decomposition voltage for the electrolysis being conducted, and KA/M2 is the current density in kiloamperes per square meter of cathode surface below each anode set.
In the electrolysis of sodium chloride in a mercury cell fox producing chlorine, the value for D is about 3.1.
Standard or set-point voltage efficient, S, may vary with a number of factors such as the material of construction of the anode (graphite or metal), the form and condition of the anodes (blocks of graphite which are . slotted or drilled, metal mesh or rods coated with a noble metal or oxide) and the location of the anode set in the cell, among other factors. As indicated in "Intensifica-tion of Electrolysis in Chlorine Baths with a Mercury ..
Cathode", The Soviet Chemical Industry, No. 11, November, 1970, pp. 69-70, the standard voltage coefficient (K or S~ : .
was found to vary as follows: . :
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, C-6727 K, standard voltage coefficient, V/kA Condition ~
0.55 no device for regulating . anode position 0.3 . use of device for lowering anode 0.2 intensive perforation of the anodes 0.14 increased perforation of the anodes 0.09 .use of titanium anodes with ruthenium dioxide coating 0.022 anodes specially placed in the amalgam When the anode set is comprised of metal anodes }O having a titanium distributor with an anodic surface formed of small parallel spaced-apart titanium rods coated ..
with an oxide of a platinum metal secured to the bottom : of the distributor, a standard voltage coefficient ranging :
from about 0.09 to about 0.13 is entered as the set-point into the program of automatic control unit 6. A deviation, k, which i5 the permissable range of deviation from S, is :
also entered into the program. Generally, k varies from about 0.1 to about lO, and preferably from about 2 to about ~
8 percent of S. ~
After positioning anode set 12 as described above : and entering the values for S and k into the program anode :~
~;: set 12 is lowered a small predetermined distance, from about 0. as to about 0.5, and preferably from about 0.15 .: .
to about 0.35.mm. Then two electrical signals are generated .: :
~ -28- -~.: : .. ; , C-6727 and measured for each conductor 15 o~ anode set 12. One electric signal corresponds to the current flow in conductor 15 for anode set 12, and may be obtained by measurins the vol~age drop between a plurality of terminals, preferably two (20 and 21) spaced a suitable distance apart along the conductor. The spacing between terminals may vary from about 3 to about 100 inches, but a space of about 30 inches is generally used. The space between terminals should be the same distance for all conductors.
It is desirable that the terminals be located laterally in the middle of the conductor, in a straight segment of conductor of uniform dimensions. This straight segment of conductor serves as a shunt to provide a signal for the measurement of current through the conductor.
Current measurements may also be obtained using other well known methods such as by the Hall effect or other magnetic detection devices.
The current signal i9 compensated for temperature changes in the conductor by thermal resistor 24 and other thermal resistors of the system which are coated with glass or other insulating material and then embedded or otherwise attached to the section of conductor or bus bar being used as the source of the current signal.
The other electric signal is the voltage drop which is measured between corresponding terminals across the anode set. When a multiplicity of cells are controlled ' -by the method and apparatus of this invention, the .

-2~-C-6727 terminals are on the conductors for the corresponding anode sets of two adjacent cells, such as terminal 20 on conductor 15 and terminal 22 on conductor 19.
The current signals and the voltage signals for each conductor 15 to anode set 12 are transmitted to automatic control unit 6 as described above in the dls-cussion of Figure 2. It is preferred to obtain the average of a series of N current measurements and the - average of a series of N voltage measurements for each conductor lS for a predetermined period. For example, automatic control unit 6 is programmed to obtain current measurements and ~oltage measurements at the rate of from about 10 to about 120, and preferably from about 20 to 60 measurements per second. These measurements are obtained for a period of time ranging from about 1 to about 10, and preferably fr~m about 2 to about 5 seconds.
The maximum difference in the current measurements in the series at this position i.e., a gap of at least about ; 3 mm between the anode and cathode, is determined and utilized as described below in the second current analysis.

The average current measurement and average voltage measurement is obtained in the computer for each series of measurements for each conductor 15. The average total current measurement for anode set 12 is obtained from the sum of the average currents to each conductor. The average voltage measurement is obtained for each anode set 12 by averaging the average voltage measu~rements for each conductor 1~. These average values are then ' . ' ' ' ' :~

~ -30--. ' ' '.

~--6727 used by automatic control unit 6 to calculate the voltage coefficient for anode set 12 in accordance with the above formula for Vc.
In making the calculation for Vc for each anode set, the area of cathode surface below each anode set may be obtained by utilizins the individual conductor voltages and measuring the area of each anode set. If desired, the current density, KA/M2 may be calculated by -assuming that the current in one conductor 15 passes through hal~ of the anode set area and current in the other conductor passes through the other half of the anode set. A formula utilized for Vc in an anode set having conductor 1 and conductor 2 is as follows:

~Vl f V2 ) Vc- ~ 2 ~ - D
KAl + KA2 where Vl is the average voltage drop in volts across conductor 1.
V2 is the average voltage drop in volts across conductor 2.
KAl is the average current in kiloamperes through conductor 1.
KA2 is the average current in kiloamperes through conductor 2.
M2 is the area of the cathode under the anode set, , in square meters.
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C-6727 When the anode set 12 is initally installed it is generally positioned with a large gap, tabout 3 ~m. or more) between the bottom of the anodes and the cathode, As a result, the first measured voltage coefficient Vc usually exceeds S by more than deviation k. After this comparison :
is completed, an electrical signal is transmitted from automatic control unit 6 to motor drive unit 8 to lower anode set 12 a small distance within the ranges described above.
A new voltage coefficient, Vc, is calculated for :
the new position of the anode set by the same procedure ~ .
and the resulting voltage coefficient is compared with S.
If the new voltage coefficient, Vc exceeds S by more than de~iation, k, the adjustment procedure is repeated until an anode set position is obtained where voltage coefficient Vc does not vary from S by more than the value of deviation k. After anode set 12 is in a position where the voltage coefficient falls within the d~viation k.of value S, the current measurements of conductor 15 for anode set 12 are ~20 also analyzed to determine whether the anode is too close to the cathode.
Following each decrease in the anode-cathode spacing, a series of N current measurements for each ~ conductor 15 to anode set 12 are taken for a predetermined : ~ period within the above defined ranges. Each current measurement is compared with the preceding current measure-~ ment to determine the amount of current increase, and -:; .
~ , , ' -::

t , ~ ' ' : ' .
' ~ . ' . ' , ' ~ ., `' . ' . ~ ' C-6727 where the current increase exceeds one of several pre-determined limits the anode-cathode spacing is immediately increased a predetermined distance, In the first analysis, if the increase in current between the current measurements made immediately before and immediately -after the ~ecrease in anode-cathode spacing is greater than a predetermined limit, the anode-cathode spacing is immediately increased. For example, if the anode set is lowered a distance within the above-defined ranges, or example about 0.3 mm, and an increase in current on `
either conductor 15 in excess of a predetermined limit occurs, for example, an increase of more than about 5 percent above the previous current measurement, auto-- matic control unit 6 is programmed to transmit an electric signal to motor drive means 8 to cause the -anode-cathode spacing to be immediately increased a distance within the above-defined ranges. If the de-crease in anode-cathode spacing i9 smaller than 0.3 mm, ; a proportionately smaller increase in current differences is used as a limit to effect raising of the anodes.
~ . ~
In a second current analysis, if anode set 12 has ' not been raised in the first current analysis, a series of N current measurements are taken for each conductors ;~ 15 for a predetermined period in the ranges described ;
above to determine the magnitude of current fluctuations.
The second current analysis is made based upon the average magnitude of the current fluctuations or differences as ' ~ ~ ~33~ ~

C-6727 determined by any convenient method prior to comparing with a predetermined average difference limit, This average difference limit is determined, for example, by doubling the average difference in the current measurements made in the series N for each conductor 15 when the anode set was initially installed at a large gap between the anode and cathode of at least about 3 mm. -The average difference in current in the series of measure-ments obtained at the initial position generally ranges from about 0.2 to about 0.4 percent of the current to each conductor the anode set in that series and thus the predetermined limit for average current difference in a series N ranges from about 0.4 to about 1.6 percent.
The term "average difference" when used in the description and claims to define the magnitude of the current fluctuations is intended to include any known method of averaging differences. For example, in a preferred embodiment a calculation is made ~ ~ 2/N, where L~ is the difference in current between each successive reading in the series and N is the total number of current measurements taken. If this average difference is greater than the predetermined average difference limit, the anode-cathode spacing is immediately increased a predetermined distance. As an alternate, the average difference may be obtained by the calculation ~ or any other similar statistical technique.
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C-6727 A third current analysis determined from the series N of current measurements is whether the current continues to increase for each measurement during series N during a predetermined time period described above, If the current continues to increase for each measurement, the anode-cathode spacing is immediately increased, ~or example, to the previous position. The number of measurements and the predetermined time period used in this analysis are wLthin the ranges described above, but are more preferably 1~ about 180 measurements in four seconds.
The fourth analysis of the current measurements determines whether an increase in current for any two measurements during series N, is greater than a pre-determined limit, ~or example, an increase of about 6-8.
. percent. I so, the anode-cathode spacing is immediately - :
; increased by an appropriate electric signal from automatic control unit 6 to motor drive unit 8.
A fifth current analysis compares each current : measurement in the series with the previous current measurement, and if the difference between two successive . : :
current measurements exceeds a predetermined limit, the distance between the anode and cathode is increased by transmitting an appropriate electrical signal from automatic control unit 6 to motor drive unit 8. When one current measuxement is exceeded by the next succes-sive current measurement in an amount from about 0.5 to about 3 percent, and preferably from about 1 to about 1.5 .~ ' ' ' ' ' ' ~-percent of the prior current measurement, the distance between the anode and cathode is increased as described above.
In a sixth current analysis, particularly in a simultaneous scan of all conductors, if any current measurement of a conductor exceeds the average bus current or average conductor current for the entire electrolytic cell by a difference ranging from about 10 to about 50 percent, and preferably from about 20 to about 40 percent of the average cell current for the entire electrolytic cell, then the anode set to which this conductor supplies current is raised a predetermined distanceO

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C-6727 Although it is possible to compare conductor current with average conductor current hased upon the total cell current, it is preferred to compare conductor current with a prior current reading for the same conductor.
When two or more conductors feed a single anode set, there may be a small amount of current crossing over one anode in the set to another anode in the same set due to changes in anode characteristics.
However, the bulk of the current, generally at least about 90% of the current, travels directly to the électrolyte for decomposition, through the liquid cathode to the cell bottom. At the cell bottom, the current is redistributed to the conductors carrying current to the next cell. Each of these conductors will generally have a different current from the corresponding conductor on the preceeding cell, even though the total current to each cell is equal. Measuring the change of current in the conductor based upon prior current measurements for the sa~e conductor in accordance with this invention gives a more realistic basis for adjustin~ the anode than previously known techni~ues. `~
Under unusual circumstances, the current measurement o~ one conductor may indicate a need to lower the anode set while the measurement for another conductor to the same anode set may indicate a need to raise the anode set. In this situation, the anode set is raised. As ` indicated ~elow, when the frequency of change of anode-- .~ .
cathode spacing exceeds a predetermined limit, the : ~
.. :: . :
~ 37-anode set is raised and removed from automatic control.
If any of the current analyses require raising of the anode set a predetermined distance, a new series of current and voltage measurements are obtained and a new voltage coefficient, Vc, is calculated. If the calculated voltage coefficient is below S by more than deviation, k, an electricalsignal is transmitted from automatic control unit 6 to motor drive unit 8 to raise anode set 12 a small ; distance within the ranges described above. If the cal-culated voltage coefficient is above S by more than deviation k, the anode set is lowered a predetermined distance. If the new voltage coefficient is within the limits k, then the current analyses are repeated. ;~
After a position is found for anode set 12 where the voltage coefficient is within the above-defined ~ predetermined range and none of the above defined current ;~ analysis requires raising anode set 12, it may be retained 'j in this position until subsequent automatic scanning, which is defined more fully below, shows the need for further movement of the anode.
i All anode sets in a selected cell may be simul-taneously adjusted using the above method. The method of ~,~ the second current analysis can also be employed to locate in a series of adjacent cells, the cell having the highest ~~ amount of current fluctuation.
i ~ In a further embodiment of the apparatus of the present invention, means are provided;for scanning periodically ~ ;
all anode sets for all oells in operation ::
~ 30 , .t', ~ ~ , by the automatic control unit 6 and for comparing the current and volta~e readings for each anode set compared with their predetermined value ranges. Where the current reading exceeds the above defined predetermined limits, the anode-cathode spacing is increased. This periodic scan detects current overloads to any anode set on a continuing basis.
The automatic control unit requires about three seconds to scan the current and voltage measurements for a group of 58 cells containing about 580 anode sets. Any suitable interval between scans may be selected, for example, intervals of about one minute~ If during a scan, the anode-cathode spacing for an anode set isincreased, the scan is repeated for all anode sets for all operative cells.
A further embodiment of the apparatus of the present invention comprises means for counting the frequency of change in the anode-cathode spacing for a particular anode set during a predetermined time period, and where this -frequency exceeds a predetermined number, means for raising the anode set to remove it from automatic control. For example, if the anode-cathode spacing for any anode set in the system is adjusted from about 20 to about 80, and preferably from about 50 to about 70 times over a 24-hour period, the anode set is raised and removed from automatic controlO When this predetermined number of adjustments is exceeded, an appropriate signal such as sounding of an alarm, activating a light on a control . : .- ,.

.

.. . . . ~ : . . .

panel or causing a message to be printed out on a reader-printer unit associated with a computer is effected, in order that the operator will examine the set to determine what the problem is and correct it.
If the current analyses indicates that the distance between the anode and cathode must be increased at several successive positions, the anode set is raised to the original starting position and a new standard voltage coefficient, S, is placed in the program of the automatic control unit 6. The new standard voltage coefficient, S
is increased a predetermined amount above the initial standard voltage coefficient S. Generally the increase is from about 5 to about 20, and preferably from about 10 to about 15 percent of the initial standard voltage coefficient. The above defined procedure for positioning the anode set based upon voltage coefficient is then ;~ repeated until a position is found where the voltage coefficient is within the above defined predetermined ` range.
Automatic control unit 6, when scanning shows voltage coefficient and current measurements to be out-side predetermined limits, is means which may also provide appropriate electric signals to motor drive unit 8, to lower anode set 12 a predetermined distance, r, obtain another set of measurements of current and voltage coefficient and continue lowering anode set incrementally a predetermined distance until the voltage coefficient or current analyses . .

., .

.

: .

, : ~ -. . . : .-C-6727 indicates that the anode set should be raised a pre-determined distance, r~ Automatic control unit 6 then provides signals to lower anode set 12 a fraction of r, for example 1/2 r, and a new set of measurements are obtained. If measurements do not require moving anode set 12 from this position, it is retained here until subsequent scanning shows the need for further adjustment.
The following examples are present to define the invention more fully without any intention of being limited thereof.

~ ' ..... .

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A horizontal mercury cathode cell for electro-lyzing aqueous sodium chloride to produce chlorine con-taining 12 anode sets of 8 graphite anodes per set was equipped with the anode control system of Figure 2. Current and voltage signals for all 12 anode sets were transmittPd simultaneously to automatic control unit 6, a digital computer, ~or about 5 seconds until about 180 readings of current and of voltage were received for each anode set.
The average voltage, current, and the difference between each current reading and the previous current reading was determined by the digital computer for the series of readings. The voltage coefficient was calculated for each anode set according to the formula:
, V -3 1 Vc = -- ~
` KA/M2 Anode set 2, with a cathode surface area of 2.4 square meters, was found to have a Vc o 0.128, based on an average 'I voltage of 4.38 and an average current reading of 12.0 kiloamperes. When Vc was compared with its standard co-efficient S of O.llS, was found to have a value above the deviation range k, where k was + 0.006. When the coefflcient i~ comparison determined the value of Vc was above S by a value greater than k, a signal from the computer activated a relay which energized a hydraulic motor to lower anode , set 2 to decrease the anode-cathode spacing by 0.3 mm. Fol-:~

;:

~: : .. ........ . .. . . . .

C-6727 lowing the decrease in anode-cathode spacing, the following ~-sequence of operations were performed:
1) A second set of about lS measurements of current was taken for each conductor 15 to anode set 12 only and the difference between each measurement in each set was determined.
2) The first analysis compared the initial increase in current after decreasing the anode-cathode spacing with the maximum increase prior to the adjustment and was found to be within the predetermined limits.
3) A second set of about 15 current readings was taken and the second analysis for current ' fluctuation determined using the formula ~ ~ /N.
The fluctuation was found to fall within the predetermined limit of 0.5 percent.

4) A third analysis showed that the time since :, .
lowering the anode had not exceeded a ~ixed limit.
~ ; 5) A fourth analysis revealed that the total ,20 increase in current did not exceed a predetermined ,. . .
; limit of 7 percent.
6) The last reading was found to be larger than the previous reading and steps 3 to 5 were re-peated with the same result. The latest reading ~: , was then ~ound to be smaller than the previous reading indicating that the current to the anode . ~ . .

~ 43- ~

C-6727 set has stopped increasing. Readinys were then taken for all anode sets on the cell and the - -Vc calculated for each was found to have a value within 5 percent of the stored value S. No further adjustments were made and the next cell to be adjusted was selected.

A group of horizontal mercury cathode cells for the electrolysis o~ sodium chloride were employed in this L0 Example, each cell containing 10 anode sets, and each anode , set contained 5 anodes. The anodes were constructed of ~ titanium metal and partially coated with a noble metal ,~ compound,. Each anode set was supplied with current by two conductors. The anode adjustment system of Figure 2 was installed on the cells. Upon selection of one cell :, fox possible ad~ustment of the anode-cathode spacing, a series of 180 readings were taken ~imultaneously for all anode set~ ?n the cell over a period of about 5 seconds.
The current measurement was obtained by measuring the ¦
voltage drop between,two terminals spaced 30 inches apart on each conductor and the voltage measurement was obtained l' between two corresponding terminals on each conductor sup-plying current to the aorresponding anode set for the next~adj w ent cell. Thus, a group of 180 current measure-meAts and'l80,voltage measurements were obtained for ea~h ', .LZ6 C-6727 of the two conductors supplying an anode set and for all ten sets in the cell. Each group of measurements were signal conditioned and converted from analog to digital form and supplied to automatic control unit 6, a digital computer, where the average total current and voltage measurements were calculated and ave~age total noise deter-mined by summing the square of the difference between ;-successive readings to each conductor and then averaging the 20 values for the cell. The voltage coefficient was .0 calculated from the average total current and voltage readings obtained and then compared with a predetermined standard individually selected for each of the anode sets.
Measurements of current and voltage taken for each set of anodes along with the calculated Vc and the predetermined standard Vc are given in Table I. ~rom these results J it can be seen that none of the anode sets fell outside of the limits of k and therefore no adjustment of the anode-cathode spacing was required.
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Example 2 was repeated using a horizontal mer-cury cathode cell having graphite anodes. Table II shows the current and voltage measurements and the calculated Vc and standard S voltage coefficients. Deviation range k was + 0.010. These results show no adjustment of the anode spacing for any of the 10 anode sets was required.

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fi C-6727 . In Example 3, as well as Example 2, electric motors were used as the motox drive means which received . -electric signals from the digital computer to adjust the anodes when necessary.

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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for adjusting the space between elec-trodes in an electrolytic cell, said electrodes being comprised of a plurality of anode sets, at least one conductor feeding current to each of said anode sets, and a liquid cathode in spaced relationship with each of said anode sets, said apparatus comprising in combination:
a. digital computer means programmed with pre-determined standard signal ranges for current signals for each of said conductors, b. means for detecting a series of N current signals to each of said conductor over a pre-determined period, c. means for selecting one of said anode sets and one of said conductors to said selected anode set, and means for selecting signals generated from said selected conductor to said selected anode set, d. means for supplying said selected signals in digital form to said digital computer means, e. means for comparing said selected signals with said predetermined standard signal ranges for said selected conductor from said selected anode set programmed in said digital computer, f. means in said digital computer for generating activating electric signals when said selected signals in digital form are outside of said predetermined standard signal ranges, and g. motor means operative to raise or lower said selected anode set, said motor means being energized by said activating electric signals when said selected signals are outside said standard signal ranges.

2. The apparatus of claim 1 wherein said electrodes are comprised of a plurality of adjustable anode sets.

3. The apparatus of claim 2 having in combination:
a. means for reactivating said means b. through f. immediately after said motor means is acti-vated to lower said anode set, and b. means for storing the previously detected signals obtained prior to lowering said selected anode set and means for comparing newly selected signals with said previously selected signals.

4. The apparatus of claim 3 wherein said digital computer means is provided with means for comparing each of said selected current signal with the previous current signal in said series and raising said anode when the difference in current is an increase which exceeds a predetermined limit.

5. The apparatus of claim 3 wherein said digital computer means is provided with means for obtaining the average difference in said current measurements in said series of N
current signals, means for comparing said average difference with a predetermined average difference limit and means for raising said anode when said average difference exceeds said predetermined average difference limit.

6. The apparatus of claim 5 wherein said means for obtaining said average difference obtains the difference be-tween each successive current measurement in said N current measurements, squares each difference to obtain a product, adds each resulting product and divides the resulting sum by N to obtain said average difference.

7. The apparatus of claim 3 wherein said digital computer means is provided with means for increasing said anode-cathode spacing when the difference in current increases in each successive measurement in said N current signals throughout said predetermined period.

8. The apparatus of claim 3 wherein said digital computer means is provided with a means for increasing said anode-cathode spacing when the difference between any two cur-rent signals in said N series exceeds a predetermined limit during said predetermined period.

9. The apparatus of claim 3 wherein said digital computer means is provided with means for counting the fre-quency of change in each anode-cathode spacing for each anode set for a predetermined period and when said frequency exceeds a predetermined number, means for raising the anode set and removing it from automatic control.

10. The apparatus of claim 9 wherein said frequency of change is from about 20 to about 80 changes over a 24 hour period.

11. The apparatus of claim l wherein temperature compensating means is secured to each of said conductors to adjust said current signals for temperature variations.

12. The apparatus of claim 11 wherein said tempera-ture compensation means is a thermistor circuit.