US2681884A

US2681884A - Brine electrolysis

Info

Publication number: US2681884A
Application number: US142266A
Authority: US
Inventors: Jr Clarence A Butler
Original assignee: Diamond Alkali Co
Current assignee: Diamond Shamrock Corp
Priority date: 1950-02-03
Filing date: 1950-02-03
Publication date: 1954-06-22
Anticipated expiration: 1971-06-22

Description

June 22, 1954 c. A. BUTLER, .1R

BRINEI ELECTROLYSIS Filed Feb. 3. 1950 2 Sheets-Sheet l FIG. 2

INVENTOR.

CLARENCE A. BUTLE R JR.

/4 TTOA/EY June 22, 1954 c. A. BUTLER, JR

BRINE ELECTROLYSIS 2 Sheets-Sheet 2 Filed Feb. 3, 1950 FIG. 6

Patented June 22V, 1954 UNITED STATE TENT OFFICE BRINE ELECTROLYSIS Application February 3, i959, Serial No. 142,266

(Cl. 20a-98) 5 Claims.

l This invention relates to methods for electrolysis of aqueous solutions of electrolytes. It more particularly is concerned with electrolysis of alkali metal chloride solutions to produce the corresponding alkali metal hydroxide and chlorine and has especially to do with methods for pro viding such products of electrolysis Without concomitant production of hydrogen. This is a continuation-inpart of my co-pending application Ser. No. 578,303, filed February 16, 1945, now abandoned.

Various methods have heretofore been proposed to reduce the cost of operation of electrolytic cells by reducing the amount of electrical energy necessary to operate the cells, especially those for the production of alkali metal hydroXidcs, such as caustic soda, and chlorine. The energy requirements of such cells are, in general, the sum of the potential necessary to discharge chlorine at the anode, overcome polarization of the H anode, overcome the resistance of the electrolyte, overcome polarization at the cathode, and discharge hydrogen at the cathode. Especially since hydrogen is a lay-product of the electrochemical reactions involved, and is considered to be of less value than the basic products of chlorine and, in the case of sodium chloride electrolysis, caustic soda, it has been appreciated for some time that if the main products of the reactions involved could be obtained without the production of hydrogen, a considerable saving could be eiiected.

It has heretofore been proposed to reduce the polarization of the cathode in alkali-chlorine cells by introducing oxygen into a porous cathode, I

but these means have been limited by the fact that polarization is not completely eliminated, and more particularly, that in any case hydrogen must be discharged at the cathode and the consequent potential for that purpose be available across the cell. However, a cell adapted ior practical commercial design which avoids polarization of the cathode by avoiding production of hydrogen at the cathode has not heretofore been proposed, even though the need for such a cell has been appreciated by Workers in the art.

It is an object ci the present invention to provide a method for electrolysis of aqueous solutions of electrolytes and particularly alkali metal chloride solutions, such. as sodium chloride, which method may be practiced Without production of hydrogen at the cathode.

It is a further object of the present invention to provide such a method in which the expenditure of electrical energy normally necessary to overcome polarization of the cathode and discharge hydrogen at the cathode is completely saved.

t is a further object of the present invention to provide an electrolytic method for the production of chlorine and caustic soda without the accompanying production of hydrogen and with accompanying savings in the cost of the main products of the electrolysis reaction.

As is known in the art, cells in which chlorine and caustic soda are produced generally operate at a voltage of 3 to 3.5 volts or more and have a theoretical or open circuit Voltage, e., voltage drop across the cell when no current is flowing therethrough, of about 2.3 volts. Such cells, in addition to production of caustic soda and chlorine, produce large amounts of hydrogen gas, which of course is a dangerous material to handle and 'which must be disposed of in some manner avoiding such danger. ln some cases such hydrogen gas, which may be produced in amounts as much as 10,000 cubic feet per ton of chlorine, has been put to use in various manufacturing processes but, in general, such processes cannot be satisfactorily integrated with a chlorine-caustic operation and hence, the hydrogen by-product represent-s a nuisance to the electrolytic cell operator.

ln contrast to prior art procedures, the method of the present invention contemplates the production of alkali metal hydroxides, such as caustic soda, and chlorine electrolytically without the accompanying production of hydrogen. By practicing the method of the present invention, savings in electrical potential as high as 30% have been achieved. Thus, it is possible in accordance with the method of the present invention to operate electrolytic cells to produce chlorine and caustic soda at a potential of about 1.9 to 2.4 volts, as contrasted With the potential noted above of 3 to 3.5 of the prior art hydrogen-producing cells. lThe corresponding open circuit potential of cells operated in accordance with the present method is 1.6 volts or less, in contrast with the above-noted open circuit potential oi 2.3 of the prior art.

ln general, the method of the present invention contemplates the steps of passing an electric current 'through an aqueous solution containing an electrolyte, such as sodium chloride, and between an anode and an electrolyte-solution-repellent, oxygen-activating, oxygen-containing cathode, .vithdrawing catholyte from the catholyte contacting portion of the cathode and continuously replenishing the oxygen supply in said cathode.

Various means may be resorted to to withdraw catholyte from the catholyte contacting portion of the cathode, and a preferred method of the present invention is the provision of means to permit the gravity ow or" the catholyte in contact with the active face of the non-wettable cathode downwardly and out of electrolytic contact with the cathode. As noted above, the cathode itself preferably comprises an oxygen-activating material, suitably being of a substance capable of adsorbing oxygen and rendering it available in electrochemically active form. In general, for this purpose cathodes comprising ac-A tivated carbon are preferred, which carbon may be in the form of a solid block or matrix or which may be in granular from and held in position in the cell by metal screening or the like. In the case of a block of activated carbon, suitable porosity is provided to permit the passage of gases through the block of material by means f known in the art not forming a part of this invention.

In accordance with the method of the present invention, the cathode is suitably of non-wettable, electrolyte-solution-repellent material, this end being obtained by water-proong the carbonaceous matter of the cathode by various known means, such as pre-treating the cathode material with a carbon tetrachloride solution of parafn, or benzene solution of rubber, or the like. Such treatment, followed by suitable drying to remove the solvent, renders the carbon of the cathode suitably solution-repellent so that the same is substantially unwetted by the electrolyte solution, and in accordance with the method of the present invention, penetration of the catholyte into the body of the cathode thus may suitably be avoided.

Moreover, in accordance with the method of the present invention, oxygen containing gases, which may be oxygen itself, air, or other suitable mixtures having appreciable oxygen content, are suitably supplied to the cathode whereby the oxygen is activated in the cathode by the oxygen activating carbon. In accordance with a particularly preferred feature of the present invention, oxygen containing gases may be blown into the cathode, which is suitably provided with paths of egress for such gases, which paths of egress do not include the electrically active face of the cathode.

While the equations for the reactions occurring in a cell operated in accordance with the method of the present invention have not been established beyond peradventure, considerable evidence exists for the following theory of operation of such a cell, which theory is especially persuasive since it accounts for the experimentally observed reduction in potential necessary to operate such a cell. Thus, in a conventional chlorine-caustic cell employing a conventional anode, cathode, and diaphragm therebetween, the electrochemical reaction at the cathode may be represented as follows:

It is seen that hydrogen ions, whether hydrated or not, indication of hydration being omitted for simplicity, are produced at the cathode where two electrons there available, join with the hydrogen to produce gaseous hydrogen in the form of H2, which is evolved at the cathode and which, of course, causes the polarizing diiculties mentioned above. In contrast to this prior are method, the method of the present invention, which provides electrochemically activated oxygen at the cathode, combines this oxygen with water and the electrons available at the cathode in accordance with the following equation:

That this is the apparent cathode reaction is evidenced not only by the absence of the production of any hydrogen whatsoever at the cathode and the reduction in potential noted above, but also by experimentally observed high concentration of hydroxyl ions in the neighborhood of the cathode, which hydroxyl ions cannot be accounted for in any other manner than by the above equation.

The formation of hydroxyl ions at the cathode in this manner insures that sodium ions migrating to the cathode and which would in a conventional cell become sodium atoms, react with water and thus produce hydrogen, will rather react with the hydroxyl ions without any hydrogen formation.

It will be appreciated that the method of the present invention thus includes the combined steps of providing electrochemically activated oxygen at the electrochemically active face of the cathode while preventing the penetration of the cathode by the electrolyte solution, which would prevent the activation of the oxygen.

The method of the present invention may be practiced in a variety of apparatus which will suggest themselves to those skilled in the art, an especially suitable apparatus being that disclosed and claimed in my co-pending application Serial No. 142,267 led of even date herewith. In order that the method of the present invention may suitably be elucidated for those skilled in the art, that structure is described herein. The description, moreover, includes accompanying drawings, in which Fig. 1 is a top plan view of a cell embodying the method of the present invention;

Fig. 2 is a section taken on line 2 2 of Fig. 1, looking in the direction of the arrows;

Fig. 3 is a section on line 3 3 of Fig. 1, looking in the direction of the arrows;

Fig. 4 is a section on line 4 4 of Fig. 2, looking in the direction of the arrows;

Fig. 5 is a perspective View of a means for making electrical connection to the cathode;

Fig. 6 is a perspective View with parts broken away for clarity of the entire cell; and

Fig. 7 is a section similar to Fig. 4 of a modified form of the cell.

Referring particularly to Figs. l-6, the cell as shown comprises anode assembly A, cathode assembly B, which are held in assembled position by clamping means shown generally at C.

The anode assembly A includes the casing or box-like member I, which has an open side as may be seen from Figs. 2 and 6. The box member is provided at its upper end with a feeder and separator chamber 2 communicating with the anode chamber interior through opening pas` sages 3 and having a liquid inlet pipe 4 and a gas outlet 5. Box or anode container l may suitably be fashioned of a material which is not affected by the liquid or gaseous products to which it is exposed and is further preferably electrolytically non-conductive and impermeable to the gaseous and liquid products of electrolysis. As shown, concrete is a suitable material for this purpose.

Anode 6 may suitably be a rectangular block of carbon or the like of such dimension to permit its being positioned, as particularly shown in Figs. 2 and 3, within the box l and out of contact with the interior surfaces of said box and with sufcient space between the anode 6 and the walls of the box to permit the accommodation of an adequate quantity of electrolyte solution. Anode 6 may be supported in box I by any suitable means, such as conductors 'l and conductor sheath 3, which extend from outside of box I, where they may be attached to a suitable source of potential, into and through the walls of box I and, as particularly shown in Fig. 3, into the anode 6. The sheathing 3 may suitably be of some sealing material to prevent the escape of electrolyte solution from the box I and may, moreover, be of electrically insulating material, whereby the conductors 'l are protected from the electrolyte solution. A suitable material for this purpose is any asphaltic sealing material. The recesses in the anode into which the conductor 'l extends may suitably be lined with conductive material such as antimony-lead, or the like, in order to insure good electrical contact between the anode and the conductors The cathode assembly B, as shown in Figs. l-G, comprises, referring particularly to Fig. 6, electrolyte-solution-permeable diaphragm ill, asbestos cords Ill, cathode II, and conductor I2. As shown in Fig. 6 and also in Fig. e, diaphragm i8 may suitably be slightly larger in area than the open space at the back of box l, whereby this space is completely covered by the diaphragm in the course of assembling the cell. rlhe diaphragm may comprise any suitable material, such as asbestos paper or the like, which material is preferably inert to the electrolyte solution and products of electrolysis.

Situated behind diaphragm iii and mounted in spaced parallel vertical relationship are a plurality of flow elements comprising asbestos cords Ill, preferably comprising long bered asbestos threading oriented along the longitudinal axis of the cords, which cords have the function with respect to diaphragm It or" support thereof and which cords also act as spacers to provide a physical spacing between the diaphragm It and the actual cathode il. The cords iii may suitably extend from the top of the diaphragm, which itself is suitably coextensive with the top of the cathode Il, to a point below the lower extremity of both the diaphragm. and the cathode II, as particularly shown in Fig. 4. While the spaces between cords Id provide channels for downward escape of liquids coming through the diaphragm, the cords themselves comprise downwardly directed -capillary channels or passageways for liquids passing through the diaphragm. Moreover, the oord-s assure that the liquid which comes through the diaphragm bridges the gap between the diaphragm iii and cathode i I and comes into electrochemical contact therewith, whereby the electrolysis of the solution is completed. The diaphragm itself, however, will be observed particularly in Fig. 6 to be entirely out of physical contact with the cathode II. Accordingly, the hydrostatic head of the solution in the cell does not bear directly against the active face of the cathode, whereby seepage of such solution into the oxygen lled cathode is avoided. A particular advantage of this arrangement therefore is that the waterproofing of the carbon of the cathode need not be of highest efiiciency, even in a large cell as in any case, hydrostatic head against the cathode is precluded.

Cathode il may suitably be a single block-like Cil piece of gas-permeable activated carbon which has been rendered repellent to the electrolyte solution, whereby it is not wetted by the solution. The inner face of cathode Il bears, as noted above, against the cords I4 which space it from the diaphragm It, and the outer face of the cathode is in electrical contact with conductor I 2, which is more particularly illustrated in Fig. 5. AS shown in Fig. 5, conductor I2 may suitably resemble an inverted F, in which both of the arms I5 in the F are provided with windows it.

Clamping assembly C comprises solid rectangular supports 2i?, EI, which may suitably be held in place as by bolts 22. The bolting serves to secure or clamp the cathode assembly firmly against the open side of box I and thus maintains the parts in assembled position. Any suitably rigid material of adequate strength and electrical neutrality may be employed for the supports 2i?, 2 I, examples of such materials being wood, either natural or plywood, and various plastic materials. Piping assembly I3 suitably extends through support 2l and terminates in the windows it of conductor I2, whereby gaseous material may be blown through pipe I3 into contact with cathode li. Moreover, as noted above, the cathode ii of porous material, i. e., porous carbon, so that it is possible to pump air or oxygen-containing gases through pipe I3 and into cathode Il. lowever, as may be observed especially from Fig. 2, suitable passages through the porous carbon are provided in all cases, shorter than the passage from the entrance point of gases into the cathode I I to the active electrode face in contact with the cords Id. Bly this means, the passage of gases through the porous carbon cathode is assured and equally assured is the removal of the gases either from the periphery of the cathode il or other portions thereof not including the electri cally active face, so that no oxygen-containing gases are blown into the area of electrical activity.

Caustic soda formed in the course of the elem trolysis flows downwardly largely through the cords ld and is collected in trough 23 and may be drawn from the cell through collecting pipe The modification shown in Fig. 7 will be recognized as substantially the same cell as Figs. 1 6, differing only in the construction of cathode. Thus, in this embodiment of the pracu tice of the method of the present invention, the cathode comprises a quantity of finely divided electrolyte-solution-repellent activated carbon held in place juxtaposed to the cords ill and diaphragm lll by frame 2l, which may suitably com-- prise metallic screen or the like. The extension 29 of the screen may be employed for the purn pose of securing or attaching the screen to a source of potential. The screen may suitably present a flat surface to the electrolytically active portion of the cell and as shown, may have the irregular surface 25 at the rear thereof, whereby the space between the support il@ and the irregular surface of the screen provides a Zone for introduction of oxygen-containing gas through pipe 2% or the like.

Screen 25 is so designed to permit the oxygen'- containing gas either as delivered by pipe or entering from the atmosphere to permeate the granular carbon material 25 and to escape from the cathode, without passing into the electrolyte, through areas of screen 2t at portions not exposed to gas introducing means, thus preventing the passage of gases through the diaphragm or past the electrolytically active face of the cathode.

As inferred in the paragraph above, the use of subsidiary means, such as piping i3 in Figs. 1 6, and piping 28 in Fig. 7, to introduce oxygen-containing gas into the cathode may be dispensed with, especially in the case of operation or" a single or a small number Vof cells. Particularly, however, in large batteries of cells where a considerable quantity of oxygen is required andinsuicient oxygen may be present in the atmosphere fully to supply the needs ol the cathodes and present suihcient oxygen to the electrolytically active face of the cathode in electrochemically active form,V piping i3 and 2t, plus introduction of oxygen-containing gases, is preferred.

The operation of the cells described in Figs. l-7 in accordance with the method of the present invention is substantially self-apparent in view of the description above but includes the introduction of electrolytes, such as sodium chloride solution, into the chamber 2 through inlet t until the liquid level stands above opening 3 and preierably until the chamber 2 is substantially more than half full, whereby some additional brine is present to allow for use thereof in the electroiytic chamber proper. Upon lling the cell, and if means for introducing additional air or other oxygen-containing gas to the cathode are to be employed, the air is turned on in contact with the cathode as, for example, illustrated in Fig. 2, and. passes through the porous electrolyte-solutionrepellent cathode and escapes therefrom without crossing the electrolytically active face of the cathode. Thereupon, the electric circuit may be closed and as the electrolysis proceeds, chlorine is collected in chamber 2 and drawn off through outlet pipe 5 and catholyte runs down the cords i-i and therebetween and drips ofi the bottom of the cords, the cords being, as noted above, specifically provided as slightly longer than the elements with which they are in contact, electrolysis continuing while the solution is continuing downwardly in the cords i4 by virtue of electrolytic contact with the electrolytically active face ci" the cathode containing oxygen in electrochemically active form. The catholyte dripping off the bottom of the cords M is collected in trough 23 and may be withdrawn from the cell through pipe 24.

Electrolyte may be replenished as needed through the inlet il in chamber 2, the electrolyte preferably being added at a temperature of above 70 C. in order to maintain the temperature the cell between about 85 and 90 C. Such addition of further electrolyte may, of course, be automatically adjusted to the needs of the cell and. will normally not require the attention of an operator.

in accordance with the method of the present invention and as may be observed from the exemplary cells illustrated in the drawings, the air or oxygen-lled cathode is not under any hydrostatic pressure whatsoever during the operation of the cell. The seepage through the diaphragm i0, referring, for example, to Fig. 6, is picked up by the cords l l which draw the liquid by capillary action downwardly and out and away from the cell. This flowing liquid, however, is in electrolytic contact with the active face of the cathode but no hydrostatic pressure exists thereagainst. Moreover, the pressure of air or oxygen-containing gas in the cathode obviously is not relied upon to withhold the seepage o1" solution into the cathode both for the reasons pointed out above and for the reason that in accordancewith the method of the present invention, the cell in which the method is to be practiced is suitably designed to permit the passage of the oxygen-containing gas through the cathode and out of the'cathode at some point other than the electrolytically active face thereof. This may suitably be accomplished by providing paths of least resistance which do not include this active face.

While there have been described various embodiments.` of the invention, the methods described are not intended to be understood as' limiting the scope of the invention as itis realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the'invention broadly in whatever form its principle may be utilized.

What is claimed is:

l. ylhe method of electrolyzing an aqueous alkali metal chloride solution, which includes the steps of passing .an electric current through such solution between an anode, a diaphragm, a diaphragm-cathode spacer providing a catholyte flow passage, and an electrolyte-solution-repellent oxygen-activating, oxygen-containing, unsubmerged cathode, thereby reacting oxygen activated in said cathode with water at said cathode to produce hydroxyl ions at said cathode, continuously flowing catholyte downwardly along the catholyte contacting portion of said cathode through said spacer, and continuously replenishing the oxygen supply in said cathode.

2. The method of electrolyzing an aqueous alkali metal chloride solution, which includes the steps of passing an electric current through such solution between an anode, a diaphragm, and the electrolyte-solution-repellent electrically active face of an oxygen-activating, oxygen-containing, unsubmerged cathode,providing solution permeable supports at spaced points between said diaphragm and said active face of said cathode to maintain said active face of said cathode out of immediate physical contact with said diaphragm, continuously flowing catholyte downwardly in electrolytic contact with said active face of Asaid cathode and through said supports, separately collecting said catholyte, and continuously rc-r plenishing the oxygen supply in said cathode.

3. The method of electrolyzing an aqueous alkali metal chloride solution, which includes the steps of passing an electric current through such solution between an anode, a diaphragm, and the electrolyte-solution-repellent electrically active face of .an oxygen-activating, oxygen-containing, unsubmerged cathode, continuously flowing catholyte downwardly in Contact with the said active face of said cathode through nonconductive flow elements which space said diaphragm from said cathode and in contact with said cathode and said diaphragm, separately collecting said catholyte, and continuously replenishing the oxygen supply in said cathode.

4. The method of electrolyzing brine without formation of hydrogen at the cathode, which includes passing an electric current through said brine between an anode, a diaphragm, and the electrolyte repellent, electrically active face or" an oxygen-containing, oxygen-activating, porous, unsubmerged cathode, maintaining said cathode out of immediate physical contact with said diaphragm, flowing catholyte downwardly between trolyte repellent, electrically active face of an oxygen-containing, oxygen-activating, porous, unsubmerged cathode, maintaining said cathode out of immediate physical contact With said cliaphragni by said spacer means, draining catholyte downwardly While in electrical Contact with said face, through said spacer means, and continuously replenishing the oxygen supply in said oxygen activating cathode.

10 References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 652,511 Hargreaves June 26, 1900 2,000,815 Berl May 7, 1935 2,011,171 Baker Aug. 13, 1935 2,273,795 Heise et al Feb. 17, 1942 2,390,591 Jones Dec. 11, 1945 OTHER REFERENCES Allmand, Principles of Applied Electrochemistry (2nd edition), 1924, pp. 435-445.

Heise, Transactions of the Electrochemical Society, Vol. 75 (1939), pp. 147-151, 155, 156, 158, 162, 163.

Claims

1. THE METHOD OF ELECTROLYZING AN AQUEOUS ALKALI METAL CHLORIDE SOLUTION, WHICH INCLUDES THE STEPS OF PASSING AN ELECTRIC CURRENT THROUGH SUCH SOLUTION BETWEEN AN ANODE, A DIAPHRAGM, A DIAPHRAGM-CATHODE SPACER PROVIDING A CATHOLYTE FLOW PASSAGE, AND AN ELECTROLYTE-SOLUTION-REPELLENT OXYGEN-ACTIVATING, OXYGEN-CONTAINING, UNSUBMERGED CATHODE, THEREBY REACTING OXYGEN ACTIVATED IN SAID CATHODE WITH WATER AT SAID CATHODE TO PRODUCE HYDROXYL IONS AT SAID CATHODE, CONTINUOUSLY FLOWING CATHOLYTE DOWNWARDLY ALONG THE CATHOLYTE CONTACTING PORTION OF SAID CATHODE THROUGH SAID SPACER, AND CONTINUOUSLY REPLENISHING THE OXYGEN SUPPLY IN SAID CATHODE.