Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1087246
METHOD TO DETERMINE THE SUITABILITY OF
DIAPHRAGM FOR USE IN AN ELECTROLYTIC CELL
This invention pertains to diaphragms and more
in particular to a method to predetermine the suitability
of a diaphragm for use in an electrolytic cell.
It has heretofore been difficult and occasionally
impossible to predetermine whether a specific diaphragm
would be suitable for employment in an electrolytic cell.
It is known that the diaphragm construction material should
be substantially nonreactive, i.e. physically and chemically
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inert with the electroly~e within the electrolytic cell; however, a means
¦ to accurately predetermine the cffect of the configuration and surface
i characteristics of a specific foraminous diaphragm on the efficiency of a
! cell has been generally unknown. It is, therefore, highly desirable to
provide a means to determine whether a diaphragm will be effective in a
cell before insertion of such diaphragm into the electrolytic eqùipment.
The present invention provides a method to predetermine the
suitability of the metallic diaphragm for use in an electrolytic cell,
` comprising the steps of: (a) impressing a known direct current electro-
motive force between a primary anode and a primary cathode immersed in a test
cell containing an electrolyte; (b) measuring an electrical property across
a predetermined portion of said electrolyte with two measuring electrodes
positioned between said primary anode and cathode and communicating with
said predetermined portion of said electrolyte by a first salt bridge and
a second salt bridge having orifices spaced apart a predetermined distance;
i ~c~ inserting a metallic diaphragm into said solution between said
measuring electrodes, said electrolyte having a conductivity such that the
insertion of said metallic diaphragm produces a voltage change between said
primary electrodes insufficient to convert said metallic diaphragm into a
bipolar electrode; and ~d) remeasuring the electrical property across said
predetermined portion of said electrolyte as in ~b).
As used herein, the term diaphragm is defined as a porous barrier
positioned between an anode and a cathode in a electrolytic cell. Such
.
diaphragm can be,
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for example, constructed of asbestos, conductive porous
plate or screen, sintered porous material, and like materials.
Since the efficiency of a diaphragm is at
least partially dependent upon both the diaphragm's por-
osity and surface characteristics, such as roughness, aflow-through test alone (i.e. measuring the quantity of
liquid which passes through a known area of diaphragm
in a given time interval) to determine porosity is not
generally an accurate measure of future diaphragm
efficiency under operating electrolytic conditions. The
present method of determining, for example, the elec-
trical current passing through the pores in the diaphragm
has been found to be dependent upon both the physical
porosity and surface characterisitics of the diaphragm~
The voltage, resistance and/or electrical current measure-
ments obtained in accord with the described method have
surprisingly been found to provide an accurate indication of
diaphragm effectiveness. The described method can be
- used for testing, for example, the suitability of the
diaphragm for electrolytic purposes or as a means to
periodically or continuously monitor the quality control
in the production of and/or operation of such diaphragms.
BRIEF DESCRIPTION OF THE DRAWING
~ .
The single Figure shown in the drawing is a
schematic representation of one embodiment of an appa-
ratus useful in the practice of the present method.
; DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to the drawing, primary electrodes,
such as a primary anode 60 and a primary cathode 61
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are immersed in an electrolyte 62 and connected to a
power source 64. Suitable electrolytes are compatible
with the primary electrodes 60 and 61 and with a diaphragm
66, and have a sufficient electrical conductivity to
afford an accurate determination of the electrical effect
of insertion of the diaphragm 66 into electrolyte 62. The
primar~ electrodes 60 and 61 and electrolyte 62 are selected
to form a cell capable of a reversible electrolytic
reaction. Additionally, when a metallic diaphragm is
tested, the conductivity of electrolyte 62 should prefe-
rably be such that insertion of diaphragm 66 into elec-
trolyte 62 will produce an insufficient voltage change
between the primary electrodes 60 and 61 to result in the
metallic diaphragm 66 becoming a bipolar electrode.
Examples of generally satisfactory electrolytes include
inorganic, aqueous salt or acid electrolytic ;solutions,
suc~ as the chlorates, chlorides, nitrates and sulfates of
metals. Suitable metals are, for example, alkali, alka-
line earth and transition metals and preferably the alkali
and alkaline earth metals, such as Li, Na, K, Rb, Cs, Be,
Mg, Ca, Sr, and Ba. The materials employed as primary
electrode material are those generally known in the art to
be useful as electrodes, for example, graphite, Ru, Rh,
Pd, Ag, Os, Ir, Pt and Au. Silver-silver chloride
electrodes have proven to be especially suitable for use
; as primary electrodes and are preferred.
The primary electrodes 60 and 61 are suitably
positioned within substantially electrically nonconductive
retaining members 68 and 69 to space surface 65 of elec-
trode 60 a predetermined distance, for example one inch apart,
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10~7246
from surface 67 of electrode 61. The retaining members68 and 69 can be constructed from, for example, a methyl
acrylate plastic and adapted to direct substantially all
of the electrical current passing between the elec-
5 v trodes 60 and 61 through the diaphragm 66 when suchdiaphragm is abuttingly detachably attached to the retain-
ing members.
Two auxiliary calomel measuring electrodes
70 and 72 are connected to the electrolyte 62 by a first
salt bridge 74 and a second salt bridge 76. Orifices 78
and 80 and salt bridges 74 and 76, respectively, pass
through the retaining members 68 and 69 at a predetermined
position between the primary electrodes 60 and 61 and
communicate with the electrolyte 62. The orifices 78 and
lS 80 are positioned apart to define a predetermined dis-
tance, for examper 3/4 inch, of electrolyte 62 between the
center of such orifices as represented by center lines B
and C.
The measuring or auxiliary electrodes 70 and 72
suitable for use in the present invention are well-known.
For example, calomel, cadmium, hydrogen, mercury electrodes
and the like can be used as measuring electrodes.
In the practice of the present invention, a
direct current electromotive force is impressed between
the primary anode 60 and the primary cathode 61 to produce
a constant current flow between such primary electrodes.
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The electromotive force impressed between
the primary electrodes 60 and 61 should produce a volt~ge
across the electrolyte which is less than the
potential needed to decompose the electrolyte 62.
For example, when an aqueous solution of NaCl is the
electrolyte, the voltage across the electrolyte should be
at least less than the decomposition potential of H2O.
An electrical property, preferably the voltage,
across the predetermined distance between the salt bridge
orifices 78 and 80 is measured by the measuring electrodes
70 and 72. The resistance of the electrolyte 62 is
determined by dividing the measured voltage between the
measuring electrodes 70 and 72 by the known current flow.
The diaphragm 66 is placed in electrolyte
62 between the primary electrodes 60 and 61 and the salt
bridge orifices 78 and 80 to thereby alter the elec-
trical resistance between the measuring electrodes across
the predetermined distance between salt bridge orifices
78 and 80. As aforementioned, the diaphragm 66 is placed
in contact with the retaining member 68 in a manner suited
to maximize the flow of current through the area of the
diaphragm defined by the retaining member 68 and to mini-
mize the passage of current through any openings at the
interface between the surface of the retaining member
68 and the diaphragm 66 or around the edges of diaphragm
66.
The diaphragm 66 is positioned in electrolyte
62 between the primary electrodes 60 and 61 and the salt
bridge orifices 78 and 80 to the measuring electrodes 70
.
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and 72 to thereby alter the electrical resistance between
the measuring electrodes. At a constant known current,
the change in voltage across the predetermined portion
of electrolyte 62, as measured by the measuring electrodes
70 and 72, is an amount characteristic of the porosity and
surface characteristics or effectiveness of the diaphragm
in an electrolytic diaphragm cell. Such diaphragm cells
are suitable for electrolytically producing, for example,
chlorine from a sodium chloride brine or, more preferably,
a multivalent metal, such as titanium from titanium
tetrachloride.
The present method can be employed to determine
the uniformity of a diaphragm by using primary electrodes
of such size and shape that the direct current produced
therebetween passes only through a known area of the dia-
phragm. An electrical property, such as voltage, resis-
tance or current flow, between the primary electrodes can
be measured across a predetermined portion of the elec-
trolyte before and after the insertion of the diaphragm
between such primary electrodes. After each measurement,
the diaphragm is moved with respect to the primary elec-
trodes so that the subsequent measurement relates to a
different portion of the diaphragm. Comparison of the
results of two or more measurements will then reflect the
uniformity or lack thereof in diaphragm permeability and
surface characeristics. The tests can be carried out
at any temperature or pressure, provided that they are
held constant.
The hereinbefore method has been found to be
acceptable for porous metallic screen, plate, or grid
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diaphragms and especially suitable for porous woven metal
screen with a metal plating thereon.
The following examples illustrate the method
of the present invention.
Example 1
Employing an apparatus substantially as shown
in the Figure, the suitability of a two inch diameter by
ten inch long cylindrical nickel plated, woven nickel
screen for use as an electrolytic cell diaphragm was
determined using a 0.1 molar sodium chloride aqueous
electrolyte (reagent grade sodium chloride with a purity
of 99.5 weight percent was dissolved in distilled water),
two 1-1j4 inch by 1/2 inch by 1/16 inch thick rectangular
silver-silver chloride primary electrodes spaced one inch
apart, and two standard calomel electrodes suitably
physically connected between the primary electrod~s by
salt bridges to afford measurement of a voltage impressed
across a 3/4 inch distance of sodium chloride solution.
The silver-silver chloride electrodes were suitably
mounted in methyl acrylate plastic frame adapted to permit
insertion of the screen diaphragm between the electrodes.
A direct current electromotive force of suf-
ficient voltage was impressed across the primary electrodes
to produce a 2 milliampere (ma) current flow between the
primary electrodes. The voltage and direct current across
the measuring electrodes was determined before and after
positioning the screen diaphragm between the electrodes.
The tests were carried out at constant room temperature
(about 20C) and one atmosphere pressure.
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The voltage of the sodium chloride electrolyte was determined
to be 68 millivolts (mv) and the current was verified at
2 milliamperes (ma) before insertion of the diaphragm. The
voltage across the measuring electrodes increased to 93
millivolts after the diaphragm was inserted into the test
cell; the current was maintained at a constant 2 milli-
amperes ~ma).
The increase in voltage of 25 millivolts was
calculated by standard method to be equivalent to an
increase in test cell resistance of 12.5 ohms or 0.276
inch of sodium chloride electrolyte between the electrodes.
Several diaphragms of identical material were
tested as above described and employed in an electrolytic
cell for producing titanium metal from titanium tetra-
chloride. The diaphragm coefficient (Cd) or diaphragmelectrolyte equivalent (inches) was determined by the
following formula and compared for the satisfactory and
unsatisfactory diaphragms. A satisfactory diaphragm
coefficient (Cd) range was thereby determined.
The diaphragm coefficient is represented by the
formula:
Cd = Vd+s/Id+s ~ Vs/Is X D where:
..
' ' VS/I S
Vd+s is measured voltage (millivolts) across a pre-
determined portion of an electrolyte
as determined by measuring electrodes com-
municating with the electrolyte by salt
bridges with orifices to such salt bridges
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spaced apart by a predetermined distance (D), a
diaphragm being positioned between said
salt bridge orifices during operation
Id+S is the measured electrical current
(milliamperes) between the primary electrodes
in the electrolyte with a diaphragm posi-
tioned as for V
d+s
~s is the measured voltage (millivolts) deter-
mined under identical conditions as for
Vd+s, but without the diaphragm
s is the measured electrical current (milliamperes)
between the primary electrodes in the
electrolyte as determined for Id+S, but with-
out the diaphragm
D is the predetermined distance between the salt
bridge orifices
Examples 2-4
In a manner substantially in accordance with
that described in Example 1, the coefficients (Cd) of
other metal screen diaphragms were determined. The
testing conditions and results are reported in Table I.
Additional metal screen diaphragms were evaluated
by the present process using (a) 0.01 molar H2SO4 as the
electrolyte and graphite as the primary electrodes and (b)
0.01 molar NaCl as the electrolyte and silver-silver
chloride as the primary electrodes. Satisfactory results
were obtained.
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