Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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
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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
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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~
., .
: , . .
~. ~
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
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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
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-12-
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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~ .
14-
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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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-16-
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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.
; -17-
, . . .
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
-18- -
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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.
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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.
' .
''~ ' ..
~. , .
. '~. ' ' ' 1 - .
. .
.~ . .
. ~ . .
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:
.
Vc = V-D _ ~;
KA;M2 ~. . '.
,',";..
: . '
:
; 30
~;
.:
-26-
. . . :
. ~ . . ,: . . . ~ : , ,
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: . :
~', ~ ' ,. "'
, . ' '
,' ' , '' '
,~. ' ' ~ ' ' '
~ ~ .
27
,
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.
.1 . - .
: : -
~ ~ -31-
~':
,
~ $
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 -
:; .
~ , , ' -::
3 2
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.
' "':
.
~ _34_ -
.
~ ~ . . . .
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
.~ ' ' '
' ' '
~-
-35-
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
I'
~ ~ ".''.
~: ' , .
-36-
~ .
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
. : .- ,.
-39-
.
.. . . . ~ : . . .
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
. .
., .
.
40-
: .
, : ~ -. . . : .-
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.
~ ' ..... .
.~ '
, .
-41-
- . .
C-6727 EXAMPLE 1
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.
EXAMPLE 2
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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~ 45-
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m
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r~ 0 0
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o
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H ::J ~ ~ ~r ~ ~ ~~r ~ ~' '1' :2
~ U el~ ~r ~ ~ ~ ~ ~ ~ l~
m ~ ~
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- . : . ~ . : - : , . . . , -
.
:
C-6727 EXAMPLE 3
~ .
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.
. .
.
.
~ -47- ~
:
~ u~ ~ ~ 'o~ OIncr~ ~1 O ~
u~ .
I ~ Uq~ 1 0 u~
,~ ~ ~~ c~ co o
~q
h
.~ o u~ I ~ o
ooa~~ o a~
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H :~01a~ a~a~Cl~ ~ ~ cn , ,
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a) u ~ ~ o :
a
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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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~ -49-
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