[0001] The present invention relates to electrolytic cells and particularly to the electrolysis
of brine utilizing a cation-permselective membrane.
[0002] It is known to obtain increased current efficiency in a process for the electrolysis
of brine wherein cation-permselective membranes are utilized to separate an anode
and a cathode in a three-compartment electrolysis cell by reducing the concentration
gradient on the cell membrane as disclosed in U.S. 3,220,941 wherein current efficiency
of such a cell is improved by utilizing sodium carbonate between the two membranes
of said cell. Improved current efficiency in a diaphragm cell has also been disclosed
in U.S. 3,932,261 by means of the use of electrodes composed of supported foraminous
metal sheets.
[0003] It is also known from U.S. 3,773,634 that in cation-permselective membrane electrolysis
cells for the production of chlorine and sodium hydroxide that maximum current efficiency
is obtained by operating the cell at a critical sodium hydroxide concentration of
31-43%.
[0004] Various gas-directing electrodes are known for use in electrolytic cells; for instance,
reduced operating voltage is obtained according to U.S. 3,168,458 where perforated
electrodes are utilized which allow for the transfer of liquid from one side of the
electrode to the other. High current efficiency is obtained, according to the teaching
of U.S. 3,598,715, in an electrolytic cell for the production of sodium chlorate having
an expanded metal cathode in which the gas evolved thereon is directed away from the
interelectrode gap.
[0005] There is disclosed in German Offen. 2,419,204, an increase in efficiency of the electrodes
in a diaphragm cell for the electrolysis of brine is obtained where inclined plates
are positioned at the electrodes functioning to guide the gas evolved thereon toward
the middle of the electrode chamber for release. A similar design is disclosed in
British 1,460,357 and U.S. 3,930,151. In U.S. 3,930,981, a diaphragm cell for the
electrolysis of brine is disclosed having perforated metal anodes and baffles to direct
anode gases away from the inter-electrodic gap in order to protect the diaphragm against
erosion.
[0006] The effect of gas evolution at the electrodes on the current overpotential relation
and current distribution in diaphragm type electrolytic cells has recently been discussed
in the Journal of Applied Electrochemistry, Vol. 6, (1976) pages 171-181. The polarization
of a permeable membrane surface in a multicell electrodialysis apparatus is disclosed
in U.S. 2,948,668 wherein alternating anion- permeable and cation-permeable membranes
are separated by corrugated perforated spaces so as to cause strong turbulence in
the flow of liquid through the cell in order to overcome said polarization.
[0007] In no one of the above-discussed references is there recognition of the fact that,
in a cell for the electrolysis of brine utilizing a cation-permselective membrane,
there is a hydroxide ion concentration gradient on the catholyte side of the membrane.
At the membrane surface there is a substantially higher concentration of hydroxide
ions than in the remaining bulk of the catholyte. The higher concentration of the
hydroxide ions on said membrane leads to a lower current efficiency than would otherwise
be obtained. The electrolysis method and electrolysis cell apparatus disclosed herein
is effective in reducing this concentration gradient and thus increasing the current
efficiency of said cell as compared to those of the prior art.
[0008] While it is known from U.S. 3,6l6,444 that so called "gas blinding" of the electrodes
in an electrolytic cell for the production of sodium chlorate results in an increased
electrical resistance between the anode and cathode of said cell, it is unexpected
that the induced turbulent flow between the catholyte and the cell membrane of the
electrolytic cell disclosed herein results in increased current efficiency.
[0009] It is thus seen that the prior art teaching in the field of the electrolysis of brine
to produce chlorine and sodium hydroxide has failed to recognize both (1) the cation-permselective
membrane concentration polarization phenomena (wherein hydroxyl ions are present in
excess on the catholyte side of the membrane as compared with hydroxyl ions present
in the bulk of the catholyte) and (2) the beneficial effect on cell current efficiency
obtained by inducing turbulence in the catholyte near the surface of the cell membrane
preferably by either directing the evolved cathodic gases away from or toward the
permselective membrane of said cell.
[0010] In accordance with the present invention there is disclosed a vertical electrode
electrolytic cell apparatus and a process for electrolysis, particularly the electrolysis
of an alkali metal chloride such as brine to produce sodium hydroxide, chlorine and
hydrogen wherein improved current efficiency is obtained by reducing the hydroxyl
ion -polarization on the surface of a cation-permselective membrane utilized in said
electrolytic cell. Said polarization is effectively overcome by inducing turbulence
in the catholyte liquor between the cathode and said cation-permselective membrane.
The process of the invention provides at any given weight concentration of sodium
hydroxide in the catholyte, a means of reducing the amount of anion (hydroxyl ion)
passing through the membrane by reducing the weight concentration of the hydroxyl
ion on the surface of the membrane. Since the weight concentration of sodium hydroxide
in the catholyte is directly proportional to the extent of back-migration of hydroxyl
ion through the permselective membrane and thus the electrolysis efficiency of the
cell, the process of the invention provides increased current efficiency in the cell.
Both single cell and multicell arrangements having a plurality of anodes, cathodes,
anolyte and catholyte compartments separated by a plurality of cation-permselective
membranes are contemplated. Where multicell arrangements are utilized, said cells
are preferably connected in series.
[0011] The turbulence which is induced in the catholyte at the surface of said membrane
is preferably achieved by utilizing a gas-directing cathode wherein the gas evolved
upon the cathode is directed toward or away from said membrane. Said cathode is preferably
an expanded metal sheet having an open mesh network of interconnected webs or filaments,
said webs being positioned at an angle of about 20° to about 70° to the plane of said
sheet. Other means of inducing turbulence in the catholyte include but are not limited
to recirculation of the catholyte, for instance, by pumping and agitation of the catholyte
by mechanical means, for instance, by stirring.
[0012] The invention will be more fully described by reference to an example of an embodiment
thereof shown in the accompanying diagrammatic drawings. Figure 1 is a perspective
view of a portion of one embodiment of the electrode utilized in the novel process
of this invention. Figure 2 shows a cross-section taken along line 2-2 of Figure 1.
[0013] Figure 3 is a schematic diagram of a two- compartment electrolytic cell for the electrolysis
of brine utilizing a cation permselective membrane and a gas directing cathode.
[0014] The process of the present invention is preferably practiced using an apparatus comprising
a combination of a gas-directing cathode and a cation-permselective membrane in an
electrolysis cell, particularly a chlor-alkali cell for the electrolysis of an alkali
metal chloride such as brine to produce sodium hydroxide, chlorine and hydrogen. The
process is also generally adapted for use in the electrolysis of other materials,
organic and inorganic. In such membrane cells, generally an enclosure is provided
which is divided into two compartments by said membrane. In one compartment thereof,
the catholyte compartment, there is disposed a cathode having a particular structure,
as will be described hereinafter. In the other compartment, the anolyte compartment,
there is disposed solid or flattened or unflattened expanded metal anode composed
of a conductive electrolytically active material such as graphite or more desirably
an anode known in the prior art as a dimensionally stable anode, for instance, a titanium
substrate bearing a coating of a precious metal, precious metal oxide or other electrolytically
active, corrosion resistant material. Said anode can be in the form of a gas-directing,
turbulence-inducing anode.
[0015] While any suitable membrane can be used, the present invention is preferably practiced
using a hydrolyzed 1 cation-permselective membrane made of a copolymer of tetrafluoroethylene
and a fluorosulfonated perfluorovinyl ether such as a copolymer of tetrafluoroethylene
and sulfonyl fluoride perfluorovinyl ether. Such membrane materials are sold for use
in chlor-alkali cells under the trademark "Nafion". The membranes ordinarily have
a thickness-ori the order of 0.1 to 0.4 millimeter.
[0016] In the operation of the chlor-alkali cell, a direct current is passed between the
electrodes causing the generation of chlorine at the anode and the selective transport
of hydrated sodium anions across said membrane into the catholyte compartment. These
sodium ions combine with hydroxide ions formed at the cathode by the electrolysis
of water to produce sodium hydroxide; hydrogen gas also being liberated at the cathode.
The cation-permselective membrane is not a perfect barrier to anions, and therefore,
allows a certain number of anions to pass in the opposite direction. The amount of
anion (hydroxyl ion) passing through said membrane determines the hydrolysis efficiency
or the amount of electrical energy required to produce a given quantity of chlorine
and caustic.
[0017] The concentration of hydroxide ion in the cathode compartment of the cell which is
related to the extent of back-migration of the hydroxyl ions through the cation-permselective
membrane, results in a certain number of hydroxyl ions passing through the membrane
thus allowing the formation
Lof oxygen and other less valuable products in the anolyte, thereby reducing the current
efficiency of the cell.
[0018] The tendency of hydroxyl ions to back-migrate through the membrane can be diminished
(and the current efficiency increased) by feeding additional water to the catholyte
compartment. A weaker caustic results thereby, for instance, one having as little
as 25 to 50 grams per liter concentration. The production of so weak a caustic in
the catholyte of the cell is generally too great a price to pay for the improved current
efficiency obtained since the dilute caustic produced must eventually be concentrated
by evaporation prior to marketing and use.
[0019] Referring now to Figure 1 of the drawing, there is illustrated one embodiment of
a gas-directing cathode 10 utilized in the process of this invention which comprises
an unflattened, expanded-metal. Where an electrode is used as a cathode 10 it is generally
made of nickel coated steel or nickel (coating not shown in the drawing) wherein the
nickel coating exists at least on the face or front of the cathode 10 which is directed
toward the cation-permselective membrane 24. The coated face of each cathode 10 is
mounted opposite an anode 23 in the electrolysis cell as illustrated in Figure 3.
The anode 23 can be either solid or flattened or unflattened expanded metal and is
generally made of a corrosion resistant metal such as titanium having a coating of
a precious metal oxide. The cathode 10 shown is provided with diamond-shaped openings
11 in which the top half section consisting of sides 12 and 13 of each diamond is
pushed forwardly of the vertical center plane of the cathode 10 and the bottom half
section consisting of sides 14 and 15 of each diamond-shaped opening 11 is pushed
rearwardly of the vertical center plane of the cathode 10. The corners of each diamond-shaped
opening situated between sides 12 and 13, and the corresponding sides 14 and 15 lie
approximately in the vertical plane of the cathode. The bottom half section, consisting
of sides 14 and 15, of each diamond-shaped opening 11 is tilted or pushed toward the
back of the cathode 10 (the side not facing the cation-permselective membrane 24)
while the top half section, consisting of sides 12 and 13, of each diamond-shaped
opening 11 is tilted or pushed toward the front of the cathode 10 so that gas which
is released on both halves of the diamond-shaped opening 11 pass through said opening
to the back of the cathode 10 and are deflected rearwardly by the tilted top half
of the cathode 10 and into the electrolyte space between the cathode 10 and the cell
wall 20 as indicated in Figure 3.
[0020] The shaded portions of the cathode 10, which is depicted in schematic form in Figures
2 and 3 in a cross-sectional view, are the top (sides 12 and 13) and the bottom (sides
14 and 15) half sections of the expanded metal cathode 10 surrounding the openings
11 in the cathode 10.
[0021] Referring again to Figure 3 of the drawing, there is illustrated in schematic form
a cross-sectional view of one embodiment of an electrolytic cell of the invention
comprising a cell wall 20, a flattened, expanded metal anode 23, a gas-directing expanded
metal cathode 10 and a cation-
Lpermselective membrane 24. Conductive means for connecting the anode and cathode to
sources of positive and negative electrical potentials, respectively, are not shown.'
An aqueous solution of alkali metal chloride, preferably acidic, is fed through line
22 and exits from line 21. Water is fed in through line 25 and sodium hydroxide solution
is obtained through line 26. During electrolysis, chlorine gas is removed through
line 28 and hydrogen gas is correspondingly removed through line 27. The electrolysis
is conducted at high caustic current efficiency by maintaining the gas-directing,
expanded metal cathode 10 in relation to the' cation-permselective membrane 24 such
that the hydrogen gas evolved on the cathode 10 is directed rearwardly (as shown)
or forwardly toward the cation-permselective membrane 24 so as to induce turbulent
flow between said membrane 24 and said cathode 10. Thus a high concentration sodium
hydroxide solution can be obtained through line 26 while at the same time maintaining
high caustic current efficiency.
[0022] In general, any cation-permselective membrane, which is electrolytically conductive
in the hydrated state, which exists under electrolytic cell conditions can be utilized
in the process of the invention. As previously noted, the preferred membrane material
is sold under the trademark "Nafion". Said material is a copolymer having structural
units of the formula:
[0023] This copolymer generally has an equivalent weight of from about 900 to about 1600,
preferably from about 1100 to about 1500. Such copolymers are prepared as disclosed
in U.S. Patent No. 3,282,875, incorporated herein by reference, by reacting at a temperature
below about 110°C. a perfluorovinyl ether with tetrafluoroethylene in an aqueous liquid
phase, preferably at a pH below 8 in the presence of a free radical initiator such
as ammonium persulfate. Subsequently, the acyl fluoride groups of the copolymer are
hydrolyzed to the free acid or salt form using conventional means. Other ion exchange
membranes can be used which are resistant to the heat and corrosive conditions exhibited
in such cells. Generally these membranes are utilized in the form of a thin film which
can be deposited on an inert support such-as a cloth woven of polytetrafluoroethylene,
or the like or can have a thickness which can be varied over a considerable range,
generally thicknesses of from about 0.1 to about 0.4 millimeter being typical. Preferably,
the membrane is a composite structure composed of a 0.038 millimeter coating of said
copolymer having an equivalent weight of about 1500 on one side of said woven polytetrafluoroethylene
cloth and a 0.1 millimeter to 0.13 millimeter coating of said copolymer having an
equivalent weight of about 1100 on the opposite side of said woven cloth. The membrane
can be fabricated in any desired shape. The copolymer sold under the trademark "Nafion"
is preferably fabricated to the desired dimension in the form of the sulfonyl fluoride.
In this non-acid form, the copolymer is soft and pliable and can be heat- sealed to
form strong bonds. Following shaping or forming to the desired configuration, the
material is hydrolyzed. The sulfonyl fluoride groups are converted to free sulfonic
acid or sodium sulfonate groups. Hydrolysis can be effected by boiling the membrane
in water or alternatively in caustic alkali solution.
[0024] After the hydrolysis step described above, the cell membrane is desirably subjected
to a heat treatment at 100°C. to 275°C. for a period of several hours to 4 minutes
so as to provide improved selectivity and higher current efficiency, i.e., lower energy
consumption per unit of product obtained from the chlor-alkali cell. In addition,
the aqueous alkali metal hydroxide solution is obtained having a lower salt concentration
when the membrane is treated in this manner. The treatment can consist of placing
the membrane between electrically heated flat plates or in an oven where said membrane
is suitably protected by placing slightly larger thin sheets of polytetrafluoroethylene,
for instance, on either side of the membrane. Satisfactory results have been obtained
in the treatment where no pressure has been exerted on the membrane during the heat
treatment but it is desirable to use a small pressure on the.membrane during the heat
treatment step. The duration of the heat treatment is dependent upon the temperature
used for the treatment and can be as short a time as 4 to 5 minutes where a temperature
of 275°C. is utilized. Further details of the heat treatment of the membranes used
in the practice of the present invention are disclosed in copending, commonly assigned
applications, Serial No. 619,606, filed October 6, 1975 and Serial No. 729,201, filed
October 4, 1976 and incorporated herein by reference.
[0025] The anodes can be solid, flattened expanded metal or gas-directing anodes such as
unflattened, expanded metal anodes. They can be made of materials having surface coatings
of noble metal, noble metal alloys or noble metal oxides, for instance, ruthenium
oxide and mixtures thereof with titanium dioxide on a substrate which is conductive
such as titanium. Platinum is an especially useful coating on a titanium anode. Preferably,
dimensionally stable anodes are utilized as exemplified by a ruthenium oxide- titanium
dioxide coating on a titanium substrate.
[0026] Bipolar electrodes can also be employed. Those having skill in the art will know
the variations in structure that will be made to accommodate bipolar rather than monopolar
electrodes in such cells and, therefore, these changes in structure need not be described
in detail. Of course, as is known in the art, pluralities of individual cells can
be employed in multicell units having common feed and product manifolds and being
housed in unitary structures. Such constructions are also known in the art and need
not be discussed herein.
[0027] The expanded metal, gas-directing cathodes generally can be made of any electrically
conductive material which will resist the attack of the contents of the cell.
J Such materials are, for instance, nickel, steel and iron. 1 Titanium or noble metal
coatings on steel or other conductive substrate as well as metals such as platinum,
iridium, ruthenium or rhodium are especially useful as coatings. Nickel and the noble
metals can be deposited as surface coatings by plasma or flame spraying, electrodeposition
or electroless coating on suitable conductive substrates, for instance, copper, silver,
steel and iron.
[0028] The cathodes are preferably nickel coated, steel cathodes which can be prepared in
accordance with procedures known to those skilled in the art or with procedures disclosed
in copending, commonly assigned application Serial No. 658,538, filed February 17,
1976 in the U.S. Patent Office and incorporated herein by reference. By the process
of this application, a steel cathode can be coated with a dense, non-porous, electroless
nickel coating by immersing said steel cathode in a bath at a suitable temperature;
the bath containing a suitable nickel salt, water, a complexing agent and a reducing
agent. Considerable savings in power in the electrolysis of brine in a chlor-alkali
cell are achieved by the use of such electrodes.
[0029] The preferred nickel coated, steel cathodes can also be prepared in accordance with
copending, commonly assigned application Serial No. 611,030, filed September 8, 1975
in the U.S. Patent Office and incorporated herein by reference. By the process of
this application, a steel cathode can be coated with nickel by either flame-spraying
or plasma-spraying the powdered metal onto the steel cathode surface.
[0030] The expanded metal, gas-directing cathodes. include means for directing the gas evolved
from the cathode during electrolysis toward or away from the cation-permselective
membrane of the cell. Utilizing these cathodes, the evolved gas is deflected from
its natural upward path between the cathode and the cation-permselective membrane
thus causing turbulent flow of the catholyte to occur in the area between the cathode
and said membrane. The expanded metal, gas-directing cathode is formed of a sheet
of metal which is characterized as a continuous fabric mesh having an open mesh network
of interconnected webs or filaments enclosing openings of diamond shape, although
oval or other shaped openings can be used. The webs are, in general, flat in cross-section
and are positioned at an angle of about 20° to about 70°, preferably about 35° to
about 55° to the plane of the original sheet from which they are formed. Such sheets
of expanded metal are known to those skilled in the art for use as electrodes in chlor-alkali
electrolysis cell technology and are shown in Figures 1, 2 and 3 of the drawings herein
and further described in the prior art, for instance, in U.S. 3,598,715 and U.S. 3,930,981,
which are hereby incorporated by reference. The size of the openings in said cathodes
can be, for instance, from about 3/16 inch x 1/2 inch to about 3/8 inch x 1-1/4 inch
(height and width of the opening, respectively).
[0031] The electrolysis process of the invention is generally adapted for use in the electrolysis
of organic as well as inorganic materials. Preferably the electrolysis 1 solution
contains chloride ions and is a water solution of at least one alkali metal chloride
such as sodium chloride. Other soluble or partially soluble salts or hydroxides are
useful in aqueous solution. For instance, sodium sulfates, sulfites or phosphates
can also be utilized, at least in part. In water electrolysis, sodium hydroxide and
potassium hydroxide can be used. Sodium chloride is the preferred alkali metal chloride
for the production of chlorine and caustic since sodium chloride, as well as potassium
chloride, do not form insoluble salts or precipitates and produce stable hydroxides.
The concentration of sodium chloride in a brine which is charged to the anolyte compartment
of the cell will usually be as high as feasible, generally at least about 200 to about
340 grams per liter of sodium chloride and about 200 to about 360 grams per liter
of potassium chloride with intermediate figures for mixtures. The anolyte can be neutral
or acidified to a pH in the range of about 1 to about 6, preferably about 2 to about
4, and most preferably about 3 to about 4, acidification normally being effected by
utilizing a suitable acid such as hydrochloric acid. The anolyte is desirably acid
to effect neutralization of any hydroxyl ions entering the anode compartment from
the catholyte thus preventing the formation of oxygen.
[0032] The temperature of the electrolyte is generally maintained at less than 105°C., preferably
between about 20°C. to about 95°C., and most preferably about 65°C. to 'about 95°C.
The temperature of the electrolyte can be increased by recirculation of portions thereof
and by the proper regulation of the proportion of feed to the anolyte. Alternatively,
cooling of the electrolyte can be effected by exposure of the anolyte liquid to ambient
conditions before entry or re-entry of such liquid into the anolyte of the cell. The
weight percent of salt conversion in the anolyte of the cell is determined by dividing
the weight concentration of the alkali metal chloride in the anolyte effluent by the
weight concentration of the alkali metal chloride in the solution which is continuously
added to the anolyte, correcting for the water of hydration transported across the
membrane and multiplying by 100. Generally, the weight percent salt conversion is
about 50% to about 85%, preferably about 65% to about 75%. Preferably, an alkali metal
chloride brine containing about 300 to about 340 grams per liter is continuously added
to the anode compartment of the cell and the depleted brine removed.
[0033] The concentration by weight of the caustic solution made in the cell is from about
10% to about 40% and is free of chloride or essentially free thereof, often containing
as low as 0.1 to 10 grams per liter of chloride and usually about 1 gram or less per
liter. As is known to those skilled in the art, the caustic concentration can be further
increased by evaporation of water and because of the unusually high concentration
of caustic obtained directly from the cell very little additional energy in the form
of heat is required to raise the concentration to a desirable, marketable concentration
of about 50% by weight.
[0034] The electrical operating conditions of the cell can vary over a wide range. Cell
voltages are generally about 2.9 to about 5 volts, and current density is generally
about 0.75 to about 3 amperes per square inch. Theoretically, it has been determined
that in prior art electrolytic cells utilizing cation-permselective membranes, a reduction
in current efficiency occurs as a result of the increase in hydroxide concentration
on the surface of the membrane which amounts to a 2.5% to 7.5% reduction in current
efficiency. By the process disclosed herein comprising the use of an expanded metal,
gas-directing cathode to induce turbulence between the cathode and the cation-permselective
membrane of the cell, increased current efficiency results as the hydroxide ion concentration
at the surface of the membrane is reduced from about 1 to about 3 moles per liter
less than the theoretically calculated increased ionic concentration present at the
surface of said membrane which is in contact with the catholyte.
[0035] The walls of the electrolytic cells utilized in the process disclosed herein can
be formed of any suitable electrically non-conductive material having resistance to
chlorine, hydrochloric acid and sodium hydroxide at the temperatures at which the
cell is operated. Suitable materials have been found to be coated metals, chlorinated
polyvinyl chloride, polypropylene containing up to 40% of an inert, fibrous filler
such as asbestos or talc, chlorendic acid-based polyester resins, phenol-formaldehyde
resins and the like. Preferably, the materials of construction have sufficient rigidity
to be self-supporting.
[0036] The following examples illustrate the various aspects of the invention but are not
intended to be limiting. Where not otherwise specified throughout the specification
and claims, temperatures are given in degrees centigrade and parts, proportions and
percentages are by weight.
EXAMPLE 1
[0037] This example illustrates the use of the electrolytic cell of the invention in the
electrochemical conversion of an aqueous solution of sodium chloride to sodium hydroxide
and chlorine. An electrolytic cell body was constructed of chlorinated polyvinyl chloride
plastic containing 20 percent by weight of asbestos based upon the total weight of
said filled plastic. The cell is schematically shown in Figure 3 and contained a cathode
assembly as schematically shown in Figure 2. The cell contained a flattened expanded
metal anode made of ruthenium oxide-coated titanium and a cathode made of nickel coated
steel. The electrodes communicate with current sources by means of steel members.
The cathode was shaped into a turbulence inducing form by expanding a metal sheet
by stamping out openings between the remaining webs or filaments of the mesh which
measure 3/8 inch high by 1-1/4 inches wide; the remaining metal filaments being about
2 millimeters in thickness. The electrodes were mounted in the cell on either side
of a cation-permselective membrane so as to provide an electrode spading of 0.1 inch
with the cathode installed so as to direct cathodic gases away from the membrane.
[0038] The membrane was manufactured by E. I. du Pont de Nemours & Company, Inc., and sold
under the trademark "Nafion", type 313. The membrane was joined to a backing or supporting
layer network of polytetrafluoroethylene filaments woven into a cloth having an area
percentage of openings i rtherein of about 22% by volume. The membranes which were
initially flat are fused onto the polytetrafluoroethylene cloth under conditions of
high temperature and pressure with some of the membrane'portions actually being caused
to flow around the filaments of the cloth during the fusing so that the membrane and
cloth become an integral unit. Before being sold, the membrane was hydrolyzed by boiling
in water. It has been found that heating the membrane at about 200°C for about 2 hours
is required to allow the attainment of a desirable base level of current efficiency
after installation of the membrane in the electrolytic cell. The cation-permselective
membrane utilized was in two layers each bonded together and consisting of a hydrolyzed
copolymer of a perfluorinated hydrocarbon and a fluorosulfonated perfluorovinyl ether;
the outer layer being 2 mil in thickness and having an equivalent weight of about
1350 and the inner layer being 4 mil in thickness and having an equivalent weight
of about 1100.
[0039] Ruthenium oxide coated titanium anodes were used which were prepared by coating a
titanium mesh having about 2 millimeter thickness filaments with about 50% by volume
open area with ruthenium oxide to a thickness of about 10
-3 millimeters.
[0040] The cell was operated under the following conditions:
[0041] During the operation of the cell, saturated brine was fed to the anode compartment
at a rate to consume 60% by weight of the brine with no recycling of the brine used.
Water was fed to the cathode compartment at a rate to produce approximately 5 normal
sodium hydroxide and caustic concentration was determined accurately within ±0.5%
by weight by repeatedly accumulating known volumes of catholyte in the amount of 0.22
liter over a time interval of about 2 hours. Concurrent with the collection of these
known volumes, an integrated sample was accumulated using a metering pump and subsequently
titrated to determine the normality of the sodium hydroxide solution within -0.5%,
the caustic current efficiency was calculated utilizing the following equation:
[0042] An overall accuracy of ±1.2% was obtained in the calculation of the current efficiency.
[0043] The cell was operated continuously for a period of 10 days.
[0044] Results obtained show a current efficiency average of 82% for the cell.
EXAMPLE 2
[0045] Example 1 is repeated except that said turbulence inducing cathode is positioned
so as to direct cathodic gases toward said membrane. An average value for current
efficiency of the cell is comparable to results obtained in ,the cell of Example 1.
EXAMPLES 3 and 4 (COMPARATIVE EXAMPLES)
[0046] A cell forming no part of this invention was operated in accordance with the above
procedure with the exception that the expanded-metal cathode utilized was a flattened,
expanded-metal cathode so that turbulence on the surface of the cation-permselective
membrane is not induced by directing the evolving cathodic gases toward or away from
said membrane. A flattened, expanded-metal cathode having openings measuring 3/16
inch in height by 1/2 inch in width . was utilized in addition to a flattened, expanded
metal electrode having larger 3/8 inch high by 1-1/4 inch wide openings. Current efficiencies
obtained utilizing said cells having flattened, expanded-metal electrodes present
as cathodes indicate a current efficiency average of 77%. These comparative examples
indicate that in the absence of turbulence-inducing, expanded-metal cathodes in the
cell, the current efficiency of the cells is decreased.
EXAMPLES 5 and 6
[0047] Examples 1 and 2 are repeated except that the anode used is an expanded metal anode
shaped into a turbulence inducing form by expanding a metal sheet of ruthenium oxide-coated
titanium by stamping out openings between the remaining webs of filaments of the mesh
which measure 3/8 inch high by 1-1/4 inches wide. The anode is positioned in the cell
so as to direct evolving gases away from the cell membrane. Average values for current
efficiency is
Lcomparable to the results obtained in the cell of Example 1.
[0048] The invention has been described with working examples and other illustrative embodiments
but it is not intended that the invention be limited to these embodiments since it
is evident that one of ordinary skill in the art will be able to utilize substitutes
and equivalents without departing from the spirit and scope of the invention.
1. An electrode and cation-permselective membrane combination capable of providing
high current efficiency in an electrolytic cell comprising a cathode and an anode
and catholyte and anolyte compartments divided by said permselective membrane wherein
said cathode is a gas-directing expanded metal cathode having an open mesh network
of interconnected webs, a portion of said webs being positioned at an angle to the
plane of said electrode sheet and adapted to induce turbulence in the catholyte of
said electrolytic cell at the surface of said permselective membrane.
2. The combination of claim 1 wherein said electrolytic cell is adapted for the electrolysis
of an alkali metal chloride.
3. The combination of claim 2 wherein said gas-directing cathode is adapted to direct
gases evolving thereon during said electrolysis away from said cation permselective
membrane and said alkali metal chloride is sodium chloride.
4. The combination of claim 2 wherein said cathode is adapted to direct the gases
evolved on said cathode Luring said electrolysis toward said cation permselective
membrane and said alkali metal chloride is sodium chloride.
5. The combination of claim 3 wherein a plurality of said electrode-permselective
membrane combinations are utilized in a multicell arangement and wherein said cells
are connected in series.
6. A vertical cation-permselective membrane electrolytic cell of high current efficiency
comprising
(a) an anode and a cathode,
(b) an anolyte and a catholyte compartment separated by an electrolytically conductive,
hydraulically impervious cation-permselective membrane
wherein said cathode is a gas-directing, expanded metal cathode sheet having an open
mesh network of interconnected webs with a portion of said webs being positioned at
an angle to the plane of said cathode sheet and adapted to induce turbulence in the
catholyte at the surface of said cation-permselective membrane.
7. The cell of claim 6 wherein said cell is adapted for use in the electrolysis of
an alkali metal chloride solution and wherein said anode is a flattened expanded metal
sheet or an unflattened expanded metal sheet.
8. The cell of claim 7 wherein said cathode is adapted to direct gas evolved at said
cathode away from said cation permselective membrane and said alkali metal chloride
is sodium chloride.
9. The cell of claim 7 wherein said cathode is adapted to direct gas evolved at said
cathode toward said cation permselective membrane and said alkali metal chloride is
sodium chloride.
10. - The cell of claim 8 wherein said cell contains a plurality of anodes, cathodes,
anolyte and catholyte compartments separated by a plurality of cation-perselective
membranes and said anodes and cathodes are connected in series.
11. In a process for electrolysis in a vertical electrode electrolytic cell having
as electrodes an anode, a cathode and anolyte and catholyte compartments separated
by an electrolytically-conductive, hydraulically-impervious, cation-permselective
membrane, the improvement comprising inducing turbulence by turbulence inducing means
in the catholyte of said cell at the surface of said permselective membrane resulting
in an increase in the current efficiency of said cell.
12. The process of claim 11 wherein said anode is a flattened expanded metal and said
process is adapted for the electrolysis of an alkali metal chloride solution and said
turbulence is induced by providing as the cathode of said cell, a gas-directing, expanded
metal electrode having an open mesh network of interconnected webs, a portion of said
webs being positioned at an angle to the plane of said sheet and adapted to induce
turbulence at the surface of said cation-permselective membrane resulting in an increase
in the current efficiency of said cell.
13. The process of claim 12 wherein said gas-directing cathode is adapted to direct
gas evolved on said cathode' toward said cation-permselective membrane and wherein
said alkali metal chloride is sodium chloride.
14. The process of claim 12 wherein said gas directing cathode is adapted to direct
gas evolved at said cathode away from said cation-permselective membrane and wherein
said alkali metal, chloride is sodium chloride.
15. The process of claim 14 wherein said cation-permselective membrane is a hydrolyzed
copolymer of tetrafluoroethylene and fluorosulfonated perfluorovinyl ether having
an equivalent weight of about 1100 to about 1500 and wherein said alkali metal is
sodium chloride.
16. The process of claim 15 wherein said expanded metal cathode is selected from the
group consisting of a plasma sprayed nickel coated steel cathode, a steel cathode,
an electroless-nickel plated steel cathode or a solid nickel cathode.
17. The process of claim 16 wherein said cathode has openings therein having a diamond
shape and said webs are positioned at an angle to the plane of said sheet of about
20° to about 70°.