CA1155792A - Air-depolarized chlor-alkali cell operation methods - Google Patents

Abstract

Abstract of the Disclosure A method is disclosed for the electrolysis of brine to chlorine and caustic in a membrane type oxygen cathode electrolytic cell having a cathode chamber and a separate oxygen compartment, wherein oxygen containing gas is fed to the oxygen compartment in a quantity substantially in excess of stoichiometry and at a positive gauge pressure.
An electric current is applied to the cell, reactants are removed, and oxygen depleted gas is removed from the oxygen compartment more or less continuously at a positive gauge pressure.

Description

AIR-DEPOLARIZED C~ILOR-ALKALI CELL OPERATION METHODS

BACKGROUl~iD OF THE INVENTION

The present invention relates generally to the operation of an oxygen electrode for use in an electrolytic cell and particularly for the production ofchlorine and caustic (sodium hydroxide) in such a manner as to significantly reduce the voltages r.ecessary for the operation of such electrolytic cells and to increase substantially the power efficiencles available from such electrolytic cells utilizing oxygen eiectrodes. More particularly, the present disclosure relates to improvedmethods of operation of oxygen electrodes which include utilizing a positive air to liquid pressure drop on the air feed side of the oxygen electrode to improve performance, control of the total flow of the gas feed stream to improve the mass transfer within the air feed side of the oxygen electrode at the reaction sites,humidification of the gas feed to the oxygen electrodes to reduce the drying out and delamination of the oxygen electrode so that it might function at a higher current density over a Ionger lifetime, and the elimination of certain gases such as carbon dioxide to increase the lifetime of the oxygen electrodes by elimination of salts lS which might be formed upon the porous structure of the oxygen electrode during the use thereof~ These rnethods of operation may be utilized singularly or preferably in combination to produce higher power efficiencies at lower voltages so as to produce a more energy-efficient oxygen electrode in an electrolytic cell especially suitable for the production of chlorine and caustic (sodium hydroxide).
Chlorine and caustic are essential large volume commodities which are basic chemicals required by all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkaline metal halides or more particularly sodium chloride with a major portion of such production coming fromdiaphragm type electrolytic cells. In the diaphragm electrolytic cell process, brine (sodium chloride soiution) is fed continuously to the anode compartment to flow through a diaphragm usually made of asbestos particles formed over a cathode structure of a foraminous nature. To minimize back migration of the hydroxide ions, ~' the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused or unreacted sodium chloride present. The hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution containing caustic soda (sodium hydroxide), S unreacted sodium chloride and other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and sodium chloride which is to be reused in electrolytic cells for further production of sodium hydroxide and chlorine. The evolution of the hydrogen gas utilizes a higher voltage so as to reduce the power efficiency possible from such an electrolytic cell thus creating an 10 energy inefficient means of producing sodium hydroxide and chlorine gas.
With the advent of technological advances such as dimensionally stable anodes and various coating compositions therefor which permit ever narrowing gaps between the electrodes, the electrolytic cell has become more efficient in that the power efficiency is greatly enhanced by the use of these dimensionally stable lS anoaes. Also, the hydraulically impermeable membrane has added a great deal to the use of tbe electrolytic cells in terms of selective migration of various lons across the membrane so as to excludc c~ntaninants frcm the resultant product thereby eliminating some of the costly purification and concentration steps of processing. Thus, with the great advancements that have tended in the past to 20 improve the efficiency of the anodic side and the membrane or separator portion of the electrolytic cells, more attention is now being directed to the cathodic side of the electrolytic cell in an effort to improve the power efficiency of the cathodes to be utilized in the electrolytic cells Shusto create a sign~ficant ener~y ~avin~s in the resultant production of chlorlne and caustic. Looking more spedfically at the 2S problem of the cathodic side of a conventional chlorine and caustic cell, it may be seen that in a cell employing a conventional anode nd a cathode and a diaphragm therebetween, the electrolytic reaction at the cathode may l~e represented as 2H20 ~ 2e yields H2 + 20H

~ he potential of this reaction versus a standard H2 electrode is ~.8~
30 volts. The desired reaction under ideal circumstances to be prornoted at the cathode would be 2H20 ~ 2 + 4e yields 40H
The potential for this reaction is +0.40 volts which would result in a theoretical volt-age savings of 1.23 volts. The electrical energy necessarily consumed to produce S the hydrogen gas which is an undesirable reaction of the cathode of the conventional electrolytic cells has not been counter balanced efficiently in the industry by the utilization of the resultant hydrogen since it is basical~y an undesired product of the reaction. While some uses have been made of the excess hydrogen gas those uses have not made up the difference in the expenditure of electrical energy necessary to 10 evolve the hydrogen thus if the eYolution of a hydrogen could be eliminated it would save electrical energy and thus make production of chlorine and caustic a more energy efficient reaction.
The oxygen electrode presents one possibility of elimination of this reaction since it consumes electrochemically activated oxygen to combine with 15 water and the electrons available at the cathode in accordance with the follou.ing e~uation 2H20 + 2 + 4e yields 40H
It is readily apparent that this reaction is more energy efficient by the very absence of the production of any hydrogen at the cathode, and the reduction in potential as 20 shown above. This is accomplished by feeding an oxygen rich fluid such as air or oxygen to an oxygen side of an oxygen electrode where ~he oxygen has ready access to the electrolytic surface so as to be consumed in the fashion according to the equation above. This does, however, require a slightly different structure for the electrolytic cel~ itself so as to provide for an oxygen compartment on the cathodic 35 side of the cathode so that the oxygen rich substance may be fed thereto.
lhe oxygen electrode i~self is well known in the art since the many NASA proJects utilized to promote space travel during the 1960s also provided funds for the development of a fuel cell utilizing an oxygen electrode and a hydrogen anode such that the gas feeding of hydrogen and oxygen would produce an electrical 30 current for utilization in a space craft. While this major government-financed 115~792 research effort produced many fuel cell components including an oxygen electrode the circumstances and the environment in which the oxygen electrode was to function were quite different from that which would be experienced in a chlor-alkali cell. Thus while much of the technology gained during the NASA projects is of value 5 in the chlor-alkali industry with regard to development of an oxygen electrode, much further development is necessary to adapt the oxygen electrode to the chlor-alkali cell environment.
Some attention has been given to the use of an oxygen electrode in a chlor-alkali cell so as to increase the efficiency in the manner described to be 10 theoretically feasible, but thus far the oxygen electrode has failed to receive significant interest so as to produce a commercially effective or economically viable electrode for use in an electrolytic cell to produce chlorine and caustic. While it is recognized that a proper oxygen electrode will be necessary to realize the theoretical efficiencies to be derived therefrom, the chlor-alkali cell will require lS operational methodology significantly different from that of a fuel cell since an electrical potential will be applied to the chlor-alkali cell for the production of chlorine and caustic in addition to the supply of an oxygen-rich fluid to enhance the electrochemical reaction to be promoted. lherefore, it would be advantageous to develop the methodology for the operation of an oxygen electrode directed 20 specifically toward the maximization of the theoretical electrical efficiencies possible with such an oxygen electrode in a chlor-alkali electrolytic cell for the production of chlorine and caustic.

SUMMA~Y OF T~E INYENTION

It is therefore an object of the present invention to provide a methodology of operation of an oxygen electrode which will enhance and maximize 25 the energy efficiencies to be derived from an oxygen electrode within the environment of a chlor-alkali electrolytic cell.
It is another object of the present invention to provide an adjusted pressure of the gas feed ~o the oxygen electrode to promote this maximization.

It is ano.her object of the present invention to control the total flow of the gas feed to the oxygen electrode to maximize its efficiencies.
It is still another object of the present invention to provide a humidified gas feed to the oxygen electrode to maximize its efficiencies. and lifetimes.
It is a further object of the present invention to eliminate contaminating substances such as CO2 from the gas feed to maximize the lifetime and efficiencyof the oxygen electrodes.
These and other objects that present invention, together with the avantages thereof over existing and prior art forms which will become apparent to those skilled in the art from the detailed disclosure of the present invention as set forth herein and below, are accomplished by the improvements herein shown, descri~ed and claimed.
It has been found that a chlor-alkali electrolytic cell having an anode compartment, a cathode compartment divided from the anode compartment by a lS separator and an oxygen compartment divided from the cathode compartment by an oxygen electrode can be operated by a method comprising the steps of: feeding analkali metal halide solution to the interior of the anode compartment; feeding an aqueous solution to the interior of the cathode compartment; feeding a molecularoxygen containing fluid to the interior of the oxygen compartment at a positive gauge pressure; so as to accomplish a total flowrate in excess of the theoretical stoichiometric amount of oxygen necessary for the reaction; applying an electrical potential between the cathode and anode of the electrolytic cell; removing halogen gas from the anode compartment; removing alkali metal hydroxide from the cathodecompartment; and removing an oxygen depeleted fluid from the oxygen compart-2S ment while maintaining the positive gauge pressure upon the interior of the oxygen ccmpartment.
lt has also been found that a chlor-alkali electrolytic cell having an anode compartment, a cathode compartment divided from the anode compartment by a separator and an oxygen compartment divided from the cathode compartment 3~ by an oxygen electrode can ~e operated by a method comprising the steps of:

feedin~ an alkali metal halide solution to the interior of the anode compartment;
feeding an aqueous solution to the interior of the cathode compartment; feeding a molecular oxygen containing carbon dioxide depleted gas having a high humidity content to the interior of the oxygen compartment; applying an electrical potential S between the cathode and anode of the electrolytic cell; removing the halogen gas from the anode compartment; removing the alkali metal hydroxide from the cathodecompart:nent; and removing the oxygen depleted humidified gas from ~he oxygen compartment.
It has also been found that a chlor-alkali electrolytic cell for the produc-tion of chlorine and alkali metal hydroxide comprising: an anode compartment adapted to contain an anolyte containing an alkali metal chloride; a cathode compartment adapted to contain a catholyte containing an alkali metal hydroxide and divided from said anode compartment by a separator; a separator; an oxygen compartment adapted to receive an oxygen eontaining fluid, free of carbon dioxide, humidified, at a posit.ve gauge pressure, and at a positive total flow of from 1.' to 10 times the stoichiometric amount of oxygen; an oxygen electrode dividing said cathode compartment from said oxygen compartment; means for controlling the moisture content of the oxygen containing substance; means for controlling the pressure of the oxygen containing fluid within said oxygen compartment; means for controlling the total flowrate of the oxygen containing fluid within said oxygencompartment; means for removing chlorine from said anode compartment; means for removing alkali metal hydroxide from said cathode compartment; means for supplying alkali metal chloride to said anode compartment; and means for supplying an electroly-ing electrical energy to said anode and said cathode.
2S The preferred embodiments of the subject invention are shown and described by way of example in this disclosure without a~tempting to show all of the various forms and modifications in which the subject invention might be embodied;
the invention being measured by the appended claims and not by the details of this disclosure.

~ 155792 In accordance with the present teachings, a method is provided for operating a chlor-alkali electrolytic cell which has an anode compartment, a cathode compartment divided from the anode compartment by a separator and an oxygen compartment divided from the cathode compartment by an oxygen electrode. The method comprises the steps of feeding an alkali metal halide solution to the interior or the anode compartment; feeding an aqueous solu-tion to the interior of the cathode compartment; removing sub-stantially all carbon dioxide from air, thereafter saturating the air with water at a temperature in the range of 40-70C. and feeding the air at a higher temperature in the range of 40 to 90C
to the interior of the oxygen compartment at a positive gauge pressure in the range of 0.25 to 250 grams per square centimeter ~0.1 to 100 inches of H2O); providing a total flow rate in the range of 1.5 to 5 times the theoretical stoichiometric amount of oxygen necessary for the reaction; applying an electrical potential between the cathode and anode of the electrolytic cell;
removing halogen gas from the anode compartment; removing alkali metal hydroxide from the cathode co~partment; and removing an oxygen depleted air from the oxygen compartment while maintaining the positive gauge pressure upon the interior of the oxygen compartment.

-7a-~' I~RIEF DESCRIPTION OF THE DRAWING

Figure 1 is a schematic view of an electrolytic cell for the production of halogen gas and alkali metal hydroxides according to the concepts of the present in-~rention.
Figure 2 is a graphical representation of the relationships between total S flow, pressure differential, and measured potential of the cathode.

DESCRIPTION OF THE PREFERREI) EMBODIMENTS

~eferring to Figure 1, numeral 12 refers to a monopolar divided electro-lytic cell whlch is suitable for use according to the concepts of the present invention. The applicants recognize that various other designs for electrolytic cells could incorporate the methods according to the concepts of the present invention, but that for illustration purposes the applicants choose the present schematic to more amply describe the details of the applicants' invention. Electrolytic cell 12, as shown in Figure ~, would generally have some environmental supporting structure or fountation to maintain each electrolytic cell 12 in correct alignment so as to build a bank of electrolytic cells for production purposes. The details of this environmental lS structure have not been shown for ease of illustrating the concepts of the present invention. The cell itself could be manufactured from various materials either metallic or plastic in nature as long as these materials resist the caustic surrountings of the chlorine environment, and temperature characteristics duringthe operation of the basic chlor-alkali cell which are well known in the ~rt. Such materials generally include but are not limited to metallic materials such as steel, nickel, titanium and other valve metals in addition to plastics such as polyvinylchlo-ride, polyethylene, polypropyltene, fiberglass and o~chers too nur~ous to n~ntioa~.
The ~alve metals include aluminum, molybdenum, niobium, titanium, tungsten, zirconium and alloys thereof.
2S it can be observed from the drawing that the electrolytic cell 12 shown has an anode 14, a separator 16, and a cathode 18 such that three individual .,, , . .
,,, ~ , ~ 155792 compartments are formed within the electrolytic cell being mainly the anode compartment 20, the cathode compartment 22, and the oxygen compartment 24.
The anode 14 will generally be constructed of a metallic substance, although graphitic carbon could be used as in the old electrodes which have lar~ely S been discarded by the industry presently. These anodes, particularly if they are to be used in a chlor-alkali cell 12, would generally be active material resistant to the anolyte such as a valve metal. A preferred val~e metal based upon cost, availability and electrical chemical properties is titanium. There are a number of forms a titanium substrate may take in the manufacture of an electrode, including for exarnple, solid metal sheet material, expanded metal mesh material with a large percentage open area, and a porous titanium with a density of 30 to 70 percent pure titanium which can be produced by cold compacting titanium powder. Porous titanium seems to be the preferred substance presently for its long life character-istics along with its relative structural integrity. If desired, the porous titanium can lS be reinforced ~ith titanium mesh in the case of large electrodes.
Usually ~hese substrate materials will have a surface coating to protect the material against passivation such as to make same what is generally known inthe ar~ as a dimensionally stable anode. Most of these coatings contain a noble metal, a noble metal oxide either alone or in combination with a valve metal oxide or other electro~talytically acti-~ ~rr~si~tan~ materials. ffle~e 80 called dimensionally stable anodes are well-lcnown and are widely used in the industry. One type of coatin~ for instance would be a Beer-type coating which can be seen fromU.S. Paten~ Numbers: 3,236,7S6; 3,623,498; 3,711,38S; 3,751,~96; and 3,g33,616.
Another type of coating utilized is one which tin, titanium and ruthenium oxides are used for surface coating as can be seen in U.S. Patent Numbers 3,776,834 and 3,85~,092. Two other examples of surface coatings include a tin, antimony with titanium and ruthenium oxides as found in U.S. Patent Number 3,875,043 and a tantal um iridium oxide coatin~ as found in U.S. Patent Number 3,878,083. There are, of course, other coatings which are available to those skilled in the art for use in chlor-alkali cells as well as other types of applications in which electrodes would ~e necessary for elec~lytic reactions.
~ here are a nurnber of materials which may be utilized for the separator 16 as s~ in the drawing. ane ~pe of material such as Naficn, atrademarkforaperfl~rocarbonmaterial, of o~urse, anticipates t~e use of s~nething substantially hydraulically in~enreable or a cation exchange m~[ibrane as it is known in the art. ~ne type of hydralll;ca11y erneable cation exchan~e ~rane, which can be used in the apparatus of the present i~tion, is a 'chin film of fluorinated cc~o1y~ having p~a~nt su1fonic acid gra~s. The f1ut~rinat~ c~oly~[~ is derived fmn m;~rs of the formul~:
1) CF2 = CF-( R ~ nS02F

lO in which the pendant -SO2F groups are converted to -S03H groups, and monomers of the formula

(2) CF2 = CXXl Rl wherein R represents the group -CF- CF2 ~- ~CFY-CF2 ~ ~)m in which the R is fluorine or fluoroalkyl of 1 thru lO carbon atoms; Y is fluorine or trifluoromethyl; m lS is 1, 2 or 3; n lS O or l; X is fluorine, chlorine or trifluoromethyl; and Xl is X or CF3 ~CF2~ a~ wherein a is O or an integer from 1 to 5.
This results in copolymers having the repeating structural units

(3) -CF2 -CF-,)n and (4) -C~2-CXXl-In the copolymer there should be sufficient repeating units, according to formula t3) above, to provide an -SO3H equivalent weight of about 800 to l600.
Materials having a water absorption of about 2S percent or greater are preferredsince higher cell voltages at any given current density are required for materials having less water absorption. Similarly, materials having a film thickness 2S ~unlaminated) of about 8 mils or more, require higher cell voltages resulting in a lower power efficiency.
Typically, because of large surface areas of the membrane in commercial - 10- .

,... .

cells, the substrate film material will be laminated to and impregnated onto a hydraulically permeable, electrically non-conductive, inert, reinforcing member such as a woven or non-woven fabric made of fibers of asbestos, glass, TEFLON, or thelike. In film/fabric composite materials, it is preferred that the laminating produce S an unbroken surface of the film resin on at least one side of the fabric to prevent leakage through the substrate film material.
The materials of this type are further described in the following patents U.S. Patent Numbers 3,û41,317;
3,282,875; 3,624,053; 3~784,~99 and aritish Patent Number 1,184,321. Substrate materials as aforedescribed are available from E. 1. duPont deNemours and Co.
wlder the trademark NAFION.
Polymeric materials, according to formulas 3 and 4, can also be made wherein the ion exchange group instead of being a sulfonic acid exchange group could be many other types of structures. One particular type of structure is a carboxyl group ending in either an acid, and ester or a salt to form an ion exchange group similar to that of the sulfonic acid. In such a group instead of having SO2F
one would find COOR2 in its place wherein R2 may be selected from the group of hydrogen, an alkali metal ion or an organic radical. These polymeric materials are a~ailable presently from E. I. duPont deNemours ~ Co. Furthermore, it has been foùnd that a substrate material such as NAFION having any ion exchange group or function group capable of being converted into an ion exchange group or a function group in which an ion exchange group can easily be introduced would include suchgroups as oxy acids, salts, or esters of carbon, nitrogen, silicon, phosphorus sul~ur chlorine, arsenic, selenium, or tellurium.
2S A second type of substrate material has a backbone chain of copolymers of tetrafluoroethylene and hexafluoropropylene and, grafted onto this backbone, a fifty-fifty mixture of styrene and alpha-methyl styrene. Subsequently, these grafts may be sulfonated or carbonated to achieve the ion exchange characteristic. Thistype of substrate while having different pendant groups has a fluorinated backbone chain so that the chemical resistivit es are reasonably high.

- 11 - .
., .

Another type of substrate film material would be polymeric substances having pendant carboxylic or sulfonic acid groups wherein the polymeric backbone is derived from the polymerization of a polyvinyl aromatic component with a mono-vinyl aromatic component in an inorganic solvent under conditions which prevent solvent evaporation and result in a generally copolymeric substance although a 100 percent polyvinyl aromatic compound may be prepared which is satisfactory.
The polyvinyl aromatic component may be chosen from the group including: divinyl benzenes, divinyl toluenes, divinyl n~hchalenesdivinyl diphenyls, divinyl-phenyl vinyl ethers, the substituted alkyl derivatives thereof such as dimethyl divinyl benzenes and similar polymerizable aromatic compounds which arepolyfunctional with respect to vinyl groups.
The monovinyl aromatic component which will generally be the im-purity present in commercial grades of polyvinyl aromatic compounds include:
styrene, isomeric vinyl toluenes, vinylr~aphthalenes, vinyl ethyl benzenes, vinyl lS chlorobenzenes, vinyl xylenes, and alpha substituted alkyl derivates thereof, such as alpha methyl vinyl benzene. In cases where highpurity polyvinyl aromatic compounds are used, it may be desirable to add monovinyl aromatic compounds so that the polyvinyl aromatic compound will constitute 30 to 80 mole percent of polymerizable material.
Suitable solvents in which the polymerizable material may be dissolved prior to polymerization should be inert to the polymerization (in that they do not react chemically with the monomers or polymer), should also possess a boiling point greater than 60C, and should be miscible with the sulfonation medium.
Polymerization is effected by any of the well known expedients, for in-2S stance, heat, pressure, and ~atalytic accelerators, and is continued until an insoluble, infusible gel is formed substantially throughout the volume of solution.
The resulting gel structures are then sulfonated in a solvated condition and to such an extent that there are not more than four equivalents of sulfonic acid groups formed for each mole of polyvinyl aromatic compound in the polymer and not less than one equivalent of sulfonic acid groups formed for each tenm~Les of poly and monovinyl aromatic compound in the polymer. As with the NAFION type material these materials may require reinforcing of similar materials.
Substrate film materials of this type are further described in the following patents which are hereby incorporated by reference: U.S. Patent Numbers S 2,731,408; 2,731,411 and 3,887,499. These materials are available from Ionics, Inc.
under the trademark IONICS CR6.
Various means of improving these substrate materials have been sought, one of the most effective of which is the surface chemical treatment of the substrate itself. Generally these treatments consist of reacting the pendant ~roups with substances which will yield less polar bonding and thereby absorb fewer water molecules by hydrogen bonding. This has a ~endency to narrow the pore openin~s through which the cations travel so that less water of hydration is transmitted with the cations through the membrane. An example of this would be to react the ethylene diamine with the pendant ~roups to tie two of the pendant groups to~ether lS ~y two nitrogen atoms in the ethylene diamine. Generally, in a film thickness of 7 mils, the surface treatment will be done to a depth of approximately 2 mils on one side of the film by controlling the time of reaction. This will result in good electrical conductivity and cation transmission with less hydroxide ion and associated water reverse migration.
The separator 16 could also be a porous diaphragm which may be made of any material compatible with the cell liquor environment, the proper bubble pressure and electrical conductivity characteristics. One example of such a material is asbestos which can be used either in paper sheet form or be vacuum-deposited fibers. A further modifkation can be effected by adding polymeric substances, 23 generally fluorinated, to the slurry from which the diaphragm is deposited. A~so polymeric materials themselves can be made porous to the extent that they show operational characteristics of a diaphragm. Those skilled in the art will readily recognize the wide variety of materials that are presently available for use as separators ih chlor-alkali cells.
The third major component of these subject cells to be utilized according .o the mcthods of the present invention is a cathode 18 as seen in the drawing. The cathode 18, in order to be utilized according to the methods of the present invention, will necessarily be an oxygen cathode. An oxygen electrode or oxygen cathode may be defined as an electrode which is supplied with a molecular oxygen 5 containing fluid to lower the voltage below that necessary for the evolution of hydrog~n. The basic support for an oxygen cathode will generally include a current collector which could be constructed of a base metal although carbon black might also be used. The expression base metal is used herein to refer to inexpensive m~tals which are commercially available for common construction purposes. Base I0 metals are characterized by low cost, ready availability and adequate resistances to chemical corrosion when utllized as a cathode in electrolytic cells. Base metals would include, for instance, iron, nickel, lead and tin. Base metals also include alloys such as mild steels, stainless steel, bronze, ~nel and cast ir~n. me h~ce metal preferablyisd~rdcal~resistant to the catholyte and has a high electrical 15 conductivity. Furthermore, this material will generally be a sligl tly porous material such as a mesh when used in the construction of an oxygen cathode. A preferred metal, based upon cost, resistance to the cathclyte and voltages available, is nickel.
Other current collectors would include: tantalum, titanium, silver, gold, and plated base metals. Upon one side of this basic support material will be a coating of a 20 porous material either compacted in such a fashion as to adhere to the nickel support or held together with some kind of binding substance so as to produce a porous substrate material. A preferrcd porous material based upon cost is carbon.
Anchored within the porous portion of the oxygen cathode is a catalyst to catalyze the reaction wherein molecular oxygen combines with water molecules to produce 25 hydroxide groups. These catalysts are generally based upon a silver or a platinum group metal such as palladium, ruthenium, gold, iridium, rhodium, osmium, or rhenium but also may be based upon semiprecious or nonprecious metal, alloys, metal oxides or organometal complexes. Generally such electrodes will contain a hydrophobic material to wetproof the electrode structure. Of course, those skilled 30 in the art will realize that the porosity of the carbon material, the amount and the *Trad~oark fc~r a nic~ c~ allc~y -- ~

type of catalytic material used will affect the voltages and current efficiencies of the resultant electrolytic cell as well as their lifetimes. A preferred cathode 18 may be constructed according to U.S. Patent No. 3,423,247.
As seen in the drawing, utilizing an anode 14, a separator 16 and oxygen cathode 18 as described above will result in an electrolytic cell 12 having three compartments, basically an anode compartment 20, a cathode compartment 22 and an oxygen compartment 24. A brine of an alkali metal halide solution is introduced into the anode compartment 20 using inlet 26. The alkali metal halide solution preferably would be one which would evolve chlorine gas, such as sodium chloride or potassium chloride. An aqueous solution is introduced into the cathode compartment 22 through an inlet 28. The aqueous solution must contain sufficient water molecules to be broken down to form the required hydroxide groups necessary for the reaction. A gaseous fluid containing oxygen is introduced into oxygen ~ompartment 24 using,an;o~ygen~inlet 30, the quantity of oxygen in t~e fluid and th~ volume of the fluid being sufficie~t to sustain the chemical reaction occurring at the oxygen cathode, and preferably being in a substantial stoichiometric excess for such reaction. Such a substance would generally be a gas and most preferably would be air with carbon dioxide removed and humidified or pure molecular oxygen which had been humidified. The reaction products such as chlorine gas would be removed from the anode compartment 20 through the halogen outlet 32 and aqueous NaOH or KOH would be removed from the cathode compartment 22 through the alkali metal hydroxide outlet 34 and an oxygen depleted fluid either in the form of residual pure oxygen or air most preferably would be removed from the depleted fluid outlet 36.

In the three compartment cells 12 according to the above description, an oxidizing gas depolarized chlor-alkali cell, a pressure differential is applied across the porous cathode 18 so that the pressure in the oxygen compartment 24 is higher than that in the cathode compartment 22. The pressure of gaseous fluid introduced into the oxygen compartment can be zero gauge and thereby just sufficient to trigger some bubbling through the gases into the oxygen cathode, or may be greater to compensate for the hydraulic head o~ liquid catholyte present in the cathode compartment on the opposite side of the oxygen cathode from the oxygen compartment.

-15a-Elevated pressure assists in mass transfer of the axi~izing gas such as o~ygen into the cathode 18 thereby preventing oxyqen depletion in the reaction zone within the cathode 18 ar~ leading to a longer cathode 1ifetime.
lhis pressure differential it shc>uld be rem~bbered is based upon the partial S pressure of the axygen present if less than 100% axygen is used.
Increasing the total flow of the depolarizing gas in the oxygen compartment 24 also enhances the mass transfer of oxy~en into the reaction siteswithin the cathode 18. This is particularly important where less than 100% pure 2 is used. Molecular oxygen is consumed by the reaction taking place at the catalytic 10 sites within the porous material of the oxygen cathode 18. As oxygen is consumed, additional quantities must be available continously and must, therefore, be fed on a continuous basis into the oxygen compartment 24. The preferred total flows are between 0 and 10 times the theoretica~ stoichiometric amount of oxy~en necessaryfor the reaction with a flow of about 2.5 times being the best.
lS Pure oxygen gas may be supplied to the oxygen compartment 24, however air may also be used since it contains approximately 23% free molecular oxy~ell by weight. In the case of air though, carbon dioxide must be removed from the air before it is delivered to the oxygen compartment 24. It has been found that car~on dioxide will promote a formation of certain carbonate deposits upon the cathode 20 which sharply reduces its liietime and power efficiency while increasing the volta~e. By eliminating the major portion of carbon dioxide this problem was also largely eliminated.
llle applicants have noticed that the presence of nitrogen in the air creates problems since it acts as a diluent so as to there}~y decrease the 2S concentration of the o~ygen present within the oocygen canpartment 24 of the electrolytic cell 12. me nitrogen m)1ecules enter the pores of the cathode 18 and r~st be diffused back out of the pores since they are not used in the reaction. If not removed they would cause a lack of activity within the porous catalytic areas of the oxygen c~thode 18 su~h as to reduce the pc~er 30 efficiency possible and increase the ~701tage necessary for the operation of such a oell. The applicants have fur~er found that this may be redu~d 1 ~55792 .o a minimum by increasing the total flow so as to provide ample oxygen supply to the oxygen compartment 24, thus reducing to a minimum the voltage necessary to operate the cell while increasing to a maximum the possible power efficiency from such an electrolytic cell 12.
S Applicants have furthermore discovered that evaporation and mass transfer pose a problem with oxygen cathodes 18 as shown in the electrolytic cells 12. This problem they found could be eliminated by increasing the relative humidity of the oxygen or air to be supplied to the oxygen compartment 24 by bubbling thegas through water at a temperature of 40 to 70C. so as to produce a relative lû humidity in the range of 85 percent. This in turn reduces the evaporation and reduces the drying out of the cathodes which can cause delamination of the porous material from the solid support material of the oxygen cathode 18 and further enhances the mass transfer across the porous surfaces. The gas temperature as itwas actually fed to the oxygen compartment 24 was generally in the range of 40 to 90C. and therefore saturated. Furthermore, the humidification seems to have another effect. Mainly, the evaporative driving force, which causes the mass transfer of the water from the cathode compartment 22 into the cathode struc.ure18, causes the crystallization of electrolyte to form solids which reduce seriously the lifetime of a given cathode 18 because the solids plug up the pores. ay the humidification of the feed gas to the oxygen compartment 24 this is drastically reduced by eliminating the evaporative driving force involved in transferring the liquid electrolyte from the cathode compartment 22.
It has been noted though that if the dew point of the gas feed is higher than the cathode skin temperature, condensation occurs on the cathode surface.
2S When this happens, sites of oxygen mass transfer are occluded so as to decrease seriously the performance of a given oxygen cathode 18. Therefore, a gas stream dew point was adjusted to balance the two deleterious effects described above, specifica~ly to maintain the dew point a few degrees below the cathode skin temperature while maintaining the relative humidity wlthin a range to eliminate the evaporative driving force in~olved. It should be noted that higher operation temperatures lower the voltage of the cell but may shorten the life of the cathode 18. A temperature in the range of 60 to 85C. is ~onsidered optlmum.

~ li ~

In order that those skilled in the art may more readily understand the present invention and certain preferred aspects by which it may be carried into effect, the following specific examples are afforded.

~ n oxy~en cathode according to U. S. I'atent No. 3,423,245, was installed into a nE~rbrane type electrolytic oell so that the ~rbon side faced the o~ygen canparbnent and the nickel side faced the cathode arpar~t in which an electrolyte was placed. A dimensionally stable anode, having a catalytic layer ca~posed of tantalum and iridiun o~ides, was installed apprc~cimately 7 centimeters away and parallel to the oxygen cathode. A
flaw of carbon dioxide free air was passed into the axygen c~ar~t of the oell at a fl~ rate of ap~rodumately 790 cubic centimeters per minutc which is approximately 21 times the theoretical stoichiometric amount needed when the cell is operated at 1 ampere per square inch current density. lhe pressure in the oxygen compartment was adjusted to approximately 110 grams per square centimeter (44 inches of water) above atmospheric pressure by restricting the lS flow exiting from the outlet 36. The pressure was maintained at that level during the test. Electrolyte consisting of approximately 400 grams NaOH per liter was then added to the cathode compartment 22 and agitated continuously by magnetic stirring aparatus.
The cathode was then conditioned by operating tl-e cell at 60C. and a 20 current density of approximately 0.0S amperes per square centimeter (one-third ampere per square inch) for about one day. After conditioning was completed, the current density was increased to approximately 0.1S amperes per square centimeter (one ampere per square inch~. The air flow, pressure, temperature and current der~ity were held constant during the rcmainder of the test. It should be noted that 2S these tests were calducted with sodiun hydra~ide electrol~te only h~ever the results from these tests should correlate closely with those th~
e ~ta~ned ~y using c~ ional ~hlor-aL'cali oells s~ce the type of ar~ode or the spacing of the anode to the cathode are not critical factors, although the anode must be stable in sodium hydroxide solution.
The cathodes were conditioned at the reduced current density because it was thought that the catalytic platinum layer becomes partially oxidized, during the period when the cathodes are stored before use. The conditioning process restores 5 the catalytic layer to high activity without causing deterioration in the quality of the cathode. Slow break-in for less noble catalysts however may be harmful.
The electrical connection was made on the nickel side of the cathode be-cause it was easier to make a good electrical contact on nickel rather than on the carbon. The cathode reference voltage measured versus a mercury/mercuric oxide reference electrode cell, changed from -0.31 on day number 1 to -1.03 on day number 98 when the .est was considered completed. The lifetime of this particular cathode under these test conditions was 2350 hours.

A cathode test was done as described in Example 1, except that the air flow rate was reduced from 7~0 cubic centimeters per minute to 220 cubic lS centimeters per minute (approximately 6 times the theoretical stoichiometric amount necessary for reactionj. The reference vol~age changed from -0.43 on day number I to -2.27 on day number 52. The cathode lifetime was 1240 hours in this test as compared to 2350 when increased air flow was used in Example 1. This shows that when the total flow increases, the potentials are lower and the lifetimes are 20 ex~ended.

A cathode test was done as describ~ed in Example 1, except that an oxy~en flowrate of 150 cubic centimeters per minute was used instead of an air flowrate of 790 cubic centimeters per minute. This oxygen flow rate was about 19 times the theoretical stoichiometric flow of oxygen required at one ampere per 25 square inch current density for operation of a cell. For this test, the electrical connection was made on the ca. bon side of the cathode. The cathode was conditioned by running the cell at a current density of 0.0S amperes per square centimeter (1/3 ampere per square inch) for about 24~ hours, increasing it to 0.1 ampere per square centimeter (2/3 ampere per square inch) for about 24 hours, and finally increasing it to about 0.15 ampere per square centimeter (1 ampere per S square inch) at which level it was held for the remainder of the test. The reference voltage changed from -0.38 on day number 2 to-0.42 on day number 293. ~he test was discontinued on day number 293 because delamination of the cathode lamination occurred. The cathode lifetime was a~out 7030 hours.
This again shows that when the total flow is increased in terms of stoichiometric 10 amounts of available oxygen, the life is extended at lower potentials.

An oxygen cathode test was done according to Example 1 with an operating temperature of approximately 85C at a current density of approximately 2 amperes per square inch and a 300 grams per liter NaOH solution. Furthermore, the membrane utilized in the subject tes- was a standard NAFION. This lS experimental cell was operated using various types of cathodes. Comparative cell voltages for the different cathodes were obtained as follows:

Avg. Volt.
Saving Over Hydrogen Evol.
Average In Volts Cathode Tvpe Cell Volt. - (% Difference) Steel Mesh 4.33S

Oxygen Electrode with platinium catalyst 3.039 1.296 (30%) using pure oxygen Oxygen Electrode with silver catalyst3.306 1.02g (2496) using pure oxygen Oxygen Electrode with silver catalyst3.536 0.799 (18%) using air feed .,, ~. .

As can be seen from the results above each of these subject cathodes when compared to a standard hydrogen evoluting steel mesh cathode shows superior performance utilizing the methods of the present invention.

An oxygen cathode test was done accordin~ to Example 1 wherein the run 5 was made on air which was not scrubbed of carbon dioxide. The cathode was broken in on oxygen and then switched to air, and failure occurred less than 48 hours after switchlng to air. This performance was typical of cathodes supplied with air which contained carbon dioxide, thus, showing the necessity of removing carbon dioxide for the lifetime of an oxygen cathode. The basic conditiors were as those contained in 10 runs according to Examples 1 - 3 and the table below shows the cell voltages and reference voltage along with remarks.

SEP vs Hgtll~O

Reference Cell VoltageVoltage Remarks 1.168-.255 at 1/3 asion oxygen for break in 1.044 -.124 0.995 -. 1 1 2 1.790-.222 at 1 asi 1.768 -.212 1.944 -.354 switch to compressed air 1 .940 -.347 2.034 -.4~1 2.120 -.490 2.745 -1.066 cathode failed EXAMPLES 6 to 12 _ _ Oxygen cathode tests were done according to Example 1 wherein the total air flow rate was varied according to Figure 2 of the drawings and with the pressure also varied. As seen in Figure 2, the cathode potential decreased with increasing flowrates and also decreased with increasing pressures. In each case the air supplied 1 1557~2 was free of c~rbon dioxide and humidified.
The cathode tests as illustrated by the examples above were ended in each case when the reference voltage reached -1.00 or when delamination of the various layers of the cathode occurred. The air (or oxygen) flow was recorded on the cathode 5 life cell test data sheets as the height (in millimeters) of the steel ball on the Matheson number 601 flow meter (except for Example 3 for a which a Matheson number 602 flow meter was used). These readings were then converted into cubic centimeters per minute by referring to the appropriate calibration curves. The ex-amples give results of the cathode tests where the pressure differentials in the range of 100 grams per square centimeter (40 inches of water). The term pressure differential means the net pressure exerted between the two sides of the cathode. In this case the difference between the pressure and the oxidizing gas compartment (100 ~srams per square centimeter above atmospheric pressure) and the average hydrostatic pressure exerted by the electrolyte on the other side of the cathode (10 grams per 15 square meter) is approximately 100 grams per square centimeter. The hydrostatic pressure is calculated by multiplying the density of the electrolyte (1.33) by average height above the cathode which averaged 3 inches. According to an estimate, the useful range of pressure differential probably is in the range of 0.25 through 500 grams per square centimeter (0.1 to 20~ inches of water column). It is expected that those 20 cathodes utilizing atmospheric pressure or where the gas compartment pressure is not allowed to exceed atmospheric pressure would be iess than 1240 hours of lifetime obtained in Example 2 for instance. It should be noted, however, that all the examples give the results of tests using NaOH electrolyte~only,)chlor-alkali cells were not used since no porous cathodes are in commercial use to date. All the above described tests ~5 were conducted at 1 ampere per square inch which was selected for test purposes only for standardization and should not be considered a maximum possible value. It is expected that current densities of the range of 2 amperes per s~uare inch or higher could be used. The tests as illustrated by Example 1 - 3 were conducted at 60C, this temperature being chosen simply as a convenient temperature for ~vhich standardiza-30 tion can be established.

~ 155792 Thus, it shouid be apparent from the foregoing description of the preferredembodiments that the methods for operation of an oxygen air cathode in an elec-trolytic cell herein shown and described accomplish the objects of the invention and solve the problems attendant to such methodology for use in a production chlor-alkali S electrolytic cell utilizing an oxygen cathode.

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Claims (2)

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

1. A method for operating a chlor-alkali electro-lytic cell having an anode compartment, a cathode compartment divided from the anode compartment by a separator and an oxygen compartment divided from the cathode compartment by an oxygen electrode comprising the steps of: feeding an alkali metal halide solution to the interior of the anode compartment; feed-ing an aqueous solution to the interior of the cathode compart-ment; removing substantially all carbon dioxide from air, thereafter saturating said air with water at a temperature in the range of 40°-70°C. and feeding said air at a higher tem-perature in the range of 40°-90°C. to the interior of the oxygen compartment at a positive gauge pressure in the range of 0.25 to 250 grams per square centimeter (0.1 to 100 inches of H2O);
providing a total flow rate in the range of 1.5 to 5 times the theoretical stoichiometric amount of oxygen necessary for the reaction; applying an electrical potential between the cathode and anode of the electrolytic cell; removing halogen gas from the anode compartment; removing alkali metal hydroxide from the cathode compartment; and removing an oxygen depleted air from the oxygen compartment while maintaining the positive gauge pressure upon the interior of the oxygen compartment.

2. A method according to claim 1 wherein the positive gauge pressure is in the most preferred range of 100 to 200 grams per square centimeter (40 to 80 inches of H20) .