US2945791A - Inert lead dioxide anode and process of production - Google Patents

Description

July 19, 1960 F. D. GIBSON, JR

INERT LEAD DIOXIDE ANODE AND PROCESS OF PRODUCTION Filed March 5, 1958 3 Sheets-Sheet 1 Fee d Solulion I 1+ 4-H dro hilic Colloid Hem Eleclrolyfic Cell HN Leud Dioxide Effluent Neutrulizin Tunk n Amyl Alcohol- (Agimtged) ead Oxide Deconl Underf low Distillation Filler Cu ke To Wosle Unll Clean And Neutral Solution n-Amyl Alcohol l Hydrophilic Colloid Residue To Waste INVENTOR Fred D. Gibson Jr.

ATTORNEY Li J July 19, 1960 F. D. GIBSON, JR 2,945,791

INERT LEAD DIOXIDE ANODE AND PROCESS OF PRODUCTION Filed March 5, 1958 a Sheets-Sheet 2 w 22.1" 23 FIG. 2-

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V I0 -C- INVENTOR Fred D. Gibson,Jr.

ATTORNEY F. D. GIBSON, JR

- July 19, 1960 INERT LEAD DIOXIDE ANODE AND PROCESS OF PRODUCTION Filed March s, 1958 s sheets-sheet s m m 4/. J m %7 1m G v 11 fixfimi d l m F, II ifi: Kati: :4 9 m 3 N m. a H

ATTORNEY and;

aerator Patented July 19, 1960 INERT LEAD DIOXIDE AN ODE AND PROCESS F PRODUCTION Fred D. Gibson, In, Box 797, Henderson, Nev. Filed Mar. 5, 1958, Ser-No. 719,224

11 Claims. (Cl. 204-57) :My invention relates to the electrodeposition of lead dioxide (PbO and more particularly to the electrosolutions containing lead salts, but invariably these deposits were not suited for electrochemical use as an inert or insoluble anode because they suifered from one or more of the following defects: (a) the lead dioxide deposit was non-uniform and non-adherent to the electrode surface; (b) the lead dioxide coating was too porous and too coarse; (c) the lead dioxide deposit would not withstand the normal abuse associated with the routine handling in thecommercial plant; and (d) the active :life of the electrode was drastically shortened due to excessive anode plate corrosion particularly above the solution level at the electrical bus connection.

The consistent lack of reliability of lead dioxidecoatings and the expense involved in their preparation has compelled the industry to use platinum anodes which represent a considerable original investment, suffer from platinum losses in processing, and the platinum anodes "require a high power input and manifest a lower etficiency than that obtained by the anodes of my invention.

'Oneof the principal objects of my invention is to pro-- *vide a process for the production of tenacious, adherent,

=-unifor'm, stress-resistant lead dioxide coatings on carbon and graphite electrodes of various shapes which do not manifest the defects of prior processes.

Another object of my invention is to provide an inert and insoluble anode which can .be succmsfully used in the electrolytic production of chlorine, chlorates and per- -chlorates.from aqueous solutions without deterioration of the anode. or contamination of these electrochemical products.

It is an object of .the present invention to provide a process for the electrodeposition of lead dioxide from an aqueous solution containing lead nitrate on a base substrate material such as graphite or carbon.

larly the electrolytic production of sodium chlorate, sodium perchlorateand chlorine. v It is a still further object to provide a process for the preparation and control of aqueous solutions used in the "electrodeposition of lead dioxide.

"These and 'otherobjects of my invention are achieved by depositing leaddioxide on a prepared'graphite or car- United States Patent fiice nitrate electrolytes on base substrate materials such as platinum, platinum clad tantalum, and tantalum for use as anodes. In other instances, attempts have been made to electrodeposit lead dioxide on conductors such as graphite. Such attempts in the production of anodes have not been successful because of the lack of the adherence of the lead dioxide deposit to the graphite anode, the weak structure of the deposit, and the lack of complete coverage of the deposit over the graphite anode.

I have discovered that all of these difiiculties in depositing lead dioxide from lead nitrate electrolytes can be overcome by varying the current density of the anode from a high of to amperes per squarefoot for the first one to five hours of electrodeposition to a low of 20 to 60 amperes per square foot for the remaining hours of electrodeposition. The cathode current densities should be about 1.5 to 3 times the corresponding anode cur rent densities. In depositing the lead dioxide coating on graphite, it is desirable that the edges of the anodes be rounded as sharp points or angular edges tend to form trees or nodules of the coating. Before starting, it is advantageous to have present in the lead nitrate aqueous electrolyte about 4 grams per liter of free nitric acid, as this improves the throwing power or coverage of the anode. electrolyte. The graphite electrode should be soaked in water for about 24 hours before immersion in the electrolyte. A graphite electrode is normally quite porous and these pores are filled with air. By soaking in water,

preferably preceded by vacuum treatment, any contained air and gas is displaced by the water, thus inhibiting the formation of pinholes in the lead dioxide coating.

When installed in the electrolyte, all Wires are connected to the graphite substrate before the electrode is immersed in the electrolyte and the current is immediately turned on to avoid any battery action which might cause an undesired deposit on the graphite anode. The electrolyte should be agitated to wipe any adhering bubbles off of the graphite base. The voltage employed will depend on the amount of applied current and may vary from 1.9 to 3.5, using an acid lead nitrate bath as an electrolyte. The electrical connection is made directly to the graphite base plate above the solution level. By proceeding as above outlined, a lead dioxide deposit is produced having the characteristics of a fine crystalline, randomly oriented structure, hard smooth surface, high tensile strength, and strong adherence to the graphite or carbon base substrate. The lead dioxide coated graphite anode can be used directly in the electrolytic production of chlorine, chlorates or perchlorates without any further treatment.

For the purpose of the present invention, the lead nitrate bath may be prepared as follows: Anhydrous lead nitrate is dissolved in Water to produce a liter of an aqueous solution having a concentration of 200 grams of lead nitrate per liter of solution. The aqueous solution also contains the following compounds in the following concentrations: 10 grams per liter of cupric nitrate; 10 grams per liter of nickelous nitrate; 0.75 gram per liter of a surface active agent of the alkyl phenoxy polyoxy- .ethelene ethanol class; 0.50 gram per liter of sodium fluo- 5 ride; 4 grams per liter of nitric acid.

This bath will produce the desired dense type of PbO In addition, it dissolves sodium fluoride in the coated graphite electrode. The cathode may be made of a suitable metal such as copper, stainless steel, nickel, platinum or the like. It is advisable to deposit the PbO in one operation without current interruption. In addition, it has been found that moderate agitation of the electrolyte aids in the production of complete coverage of the anode and reduces the nodulation. Nodular deposits are undesirable and my process produces few, if any, nodular deposits. The PbO deposit will be found to be compact, hard, dense, smooth, tenacious, adherent to the electrode, and the crystals will be randomly oriented.

Satisfactory coatings have been used having thickness from about a thirty-second of an inch to about one-half an inch. I prefer a coating thickness of about a sixteenth to threesixteenths of an inch.

The function of the surface active agent is to raise the oxygen overvoltage at the anode, inhibit gassing and improve the throwing power of the nitrate bath, resulting thereby in a compact lead dioxide deposit. Without the surface active agent, the lead dioxide deposit tends to be spongy and porous. It functions in the same manner as natural hydrophilic colloids such as gelatine, dextrine, gum arabic, soluble starch, etc.

During the electrodeposition of the lead dioxide, the surface active agent in the electrolyte is altered as the result of electrolysis in the presence of nitric acid. These altered products interfere with electrodeposition resulting in time in a porous, non-adherent, non-uniform lead dioxide plate and ultimately the total lack of deposition. A part of this difficulty can be overcome by allowing the neutralized cell effluent to stand neutral for more than 24 hours before reuse. This allows a portion of the altered products to recombine in the original form of the surface active agent. However, this treatment is not completely eifective and over a period of time certain of the permanently altered products build up in concentration to a point where the solution is not usable and must be thrown away.

In order to maintain the plating solution so that it will consistently produce close textured crystalline smooth PbO coats completely covering the graphite or carbon anode, I have found that the solution can be regenerated as it leaves the cell as effluent by treating it with a small amount of n-amyl alcohol, i.e., about one liter of alcohol for four liters of solution and adding sufficient lead oxide to neutralize the contained nitric acid. The alcohol removes substantially all of the residual surface active agent and its altered products and permits unlimited reuse of the solution. When the solution is allowed to stand, the surface active agent and its altered products being immiscible with the solution, stratify out and can be removed by decantation. In starting electrodeposition of the lead dioxide with a freshly prepared solution, and on return of the regenerated solution to the cell, a fresh amount, as above specified, of the surface active agent is added to the solution. The solubility of the amyl alcohol in the nitrate bath is about 1% by weight.

Reference is made to the drawings for a more complete disclosure of the invention in which 1 is a flow sheet of a complete cyclic operation;

Fig. 2 is a diagrammatic elevation of a plant arrangement;

Fig. 3 is a vertical section of a cell for depositing lead dioxide;

Fig. 4 is a diagrammatic top plan view of a cell of the type showing in Fig. 3;

Fig. 5 is a diagrammatic vertical section at right angles to Fig. 3, showing the cathode in fragmentary form.

Referring to Figs. 3 and 4, 1 is the graphite anode and 2 are the cathodes. For controlling the temperature of the electrolyte 7 there are submerged 110 volt electric resistance heaters 3. Reference numeral 8 indicates the level of the electrolyte 7 in the cell tank 4. Suitable electrical connections 9 and 10 lead to a power source for the supply current to the anode 1 and cathodes 2, respectively. For agitating the electrolyte there is a glass enclosed magnet 5 operated by a volt magnet motor 6.

Referring to Fig. 2, the cell arrangement is the same as shown in Fig. 3. For feeding nitrate solution to the cell tank 4 there is a feed solution tank 12 with a valved pipe 13 discharging solution into the cell tank 4. For discharging solution from the cell tank 4 to a neutralizing tank 14, there is a siphon 15 which discharges into a pipe 16 and then into the tank 14. An agitator 17 is operated by the motor 18. The amyl alcohol 19 separates out and is decanted through the siphon 20 to a still for reuse and for removal to waste of the surface active agent residue. The underflow from tank 14 is returned to the system through a pipe 21 to a filter 23 by a force pump 22. The filtered neutral solution is returned to the feed tank 12 for delivery to the cell tank 4.

The flow sheet Fig. 1 with legends represents the sequence and character of operations as just described with respect to Fig.2.

Referring to Figs. 3 and 5, there is shown one form of cathode in which annealed copper wire 2a is wound across a plexiglass supporting plate 2b. Other forms of well known similar cathodes may be used and are readily available. Apertures 2c in the plate 2b and 1a in the graphite anode receive rods for supporting the anode and cathode in proper position in the cell tank 4. The angular edges of the graphite anode are rounded at 1b and the bottom end 1c is cut in a half circular shape.

The following Examples I and II illustrate my process for production of a lead dioxide anode.

EXAMPLE I A liter of aqueous electrolyte was prepared containing the following compounds in the concentration shown for each:

Grams per liter This solution was contained in a cell tank and heating and agitation of the solution was started. On reaching a temperature of 72 C., a graphite anode and copper cathodes were installed and electrodeposition started. The anode was prepared for electrodeposition by smoothing and cleaning with sandpaper, followed by soaking in dis tilled water for 24 hours. The edges of the graphite base were rounded and the bottom end cut in a half-circular shape. Electrical connection was made to both electrodes before immersion in the electrolyte.

Feed solution was started at the same instant electrodeposition was initiated and was continuous thereafter at a rate sufiicient to maintain an acid concentration of approximately 4 grams per liter and a lead nitrate concentration of approximately grams per liter. The feed solution was neutralized efiiuent from a previous run, free of surface active agent and containing approximately 1.15% by weight of n-amyl alcohol. Along with the neutralized feed solution sodium fluoride and surface active agent were added at the rate of .5 gram and .75 gram respectively per liter of feed solution.

Effluent from the cell was neutralized with lead oxide and thoroughly mixed with n-amyl alcohol. On settling, the alcohol containing substantially all of the surface active agent and altered products separated from the nitrate solution. This alcohol layer was decanted and then distilled to recover the alcohol. The cleaned neutral solutions containing approximately 1% by weight of alcohol was filtered and returned to the electrolytic cell as feed solution and the above cycle repeated.

"Feed solution:

Table I I OPERAT'IN G CONDITIONS OF EXAMPLE -I Electrolyte:

.Surface active agent g.p.l- 0.75 NaF. g n l 0. 50 HNOQ Q n l 4 Anode: 1" x 6" x 80" untreated graphite, surface cleaned, edges rounded.

Immersed in solution 21" giving available plating area of-approximately 300 sq. inches. V p g g Cathode: Two sections each containing 064 dia; soft aiinealedc'opper wire wound across 7.5" x 24" Plexiglas plates so as to give an immersed area of approximately 142 sq. in.

Cell: 12" dia. x 24" high Pyrex iar Spacing: 2" between cathode surface and anode surface Temperature: 72'82 C (Noah pil 200 2 p.ll g.p.l 10 1 Additives:

Surface active agent of alkyl phenoxy polyoxyethelene ethanol olass e-n.- .75. NaF cpl.. .5

Current density 1st 60 min. Next 90 min Next 180 min. Anode 101 Amps] 50.5 Amps] 24 Amps/ sq. ft. sq. ft. sq. ft.

Cathode 213 Amps] 106.5 Amps/ 50.6 Am s] sq. rs. sq. it. I sq. ft. Applied current 210 Amps. 105 Amps. 50 Amps. Volta e 3.5 2.38. 1.9. Feed rate. 108 cc./min 54 cc.-/min 27 cc./min.

Current efliciency: 98% Time: 5.5 hours At the end of /2 hours elect-rodeposition was stopped.

.'Ihe anode was removed from the cell and thoroughly washed with water to remove the drag-out. That portion of the graphite immersed in the plating solution was found to be completely covered with a very smooth, compact, finely crystalline, firmly adherent layer of lead dioxide free of cracks and nodules.

EXAMPLE II I proceeded as in Example I, above, except that neither the electrolyte nor the feed solution contained n-amyl alcohol. The anodewas a 3" diameter by 14" long rod of'untreated graphite with the lower end rounded in a' spherical shape.

Table II OPERATING CONDITIONS OF EXAMPLE II Electrolyte:

Anode: 3" dia. x 14" long graphite rod, 9.5 immersed. Edective area approximately 90 sq. in. g I

Cathode: 273 x .064 dia. soft annealed copper wire strung vertrcally between two 7 dia. Plexiglas plates (bird cage efiect) to give an effective area of approximately 55 sq. in.

Cell: 12", dia. x 12 high Pyrex jar Spacing: 2" between cathode surfaceand anode surface Temperature: 73-92 C.

Current efficiency: 95% Time: 6 hours At the end of electrodeposition the anode was removed from the cell and thoroughly washed with water to re move the drag-out. That portion of the graphite immersed in the electrolyte was found to be completely "6 covered with asmooth, compact, firmly adherent layer of lead dioxide tree of cracks and pin holes.

The use of my PbO graphite anodes in the electrochemical production of chlorine, chlorates and perchlo rates is illustrated in the following Examples III, IV and V. V

EXAMPLE III The lead dioxide coatedgraphite anodeprepared as above, was used as-an *an'ode in the electrolysis of an aqueous sodium chloratesolution to produce a sodium perchlorate solution. Subsequent to the electrodeposition of the lead dioxide and before use in this operation no further treatments of any kind were made to the anode. The same electrical connection to the uplated top portion of the graphite base was utilized in both the electrodeposition of the lead dioxide and the electrolysis of the sodium chlorate solution.

A nearly-saturated aqueous solution of sodium chlorate was prepared from technical grade sodium chlorate. The solution was purified of chromate by precipitation with barium chloride and filtered. Approximately .5 g.p.l. of NaF was added to inhibit cathode reduction.

The electrolysis was carried out in a 12 dia. x 24" high Pyrex jar in which the anode and two type 316 stainless steel cathodes were immersed. The electrolyte was cooled by water flowing thru glass coils placed within the cell. The electrolyte was agitated by means of magnetic stirrer. During the first 6 hours of electrolysis hydrochloric acid was added in order to maintain a pH of 5.0 to 6.8. No further acid additions were required. The electrolysis was stable and continuous and continued until theelectrolyte was nearly free of chlorate. The lead dioxide coated graphite was untouched and unattacked and no contaminants were transferred to the sodium perchlorate therefrom. The external electrical con;

nection was preserved and no difiiculty was indicated atthat point.

The cumulative current efiiciency of 75.5% was greater than the 60 to usually encounterediin commercial sized platinum anode perchlorate cells. The average voltage of 4.75 was also considerably lower than the usual 6.5 to 6.9 recorded in platinum anode perchlorate cells.

Table III OPERATING CONDITIONS OF EXAMPLE III Electrolyte:

Initially- NaClOr c n.l 500 NaOl g p 1 3 NaF g.p l 5 Anode: Lead dioxide coated graphite produced in Example I-20"- immersed. Efiective area approximately 285 square inches. Cathodes: Two type 316 stainless steel sheets each 8" x 22 immersed.

Efiective area approximately 166 square inches each. Spacing: 1/2 inch between cathode surface and anode surface Cell: 12" dia. 24 high Pyrex jar Voltage: 4.75 Applied current: 285 amperes Current density:

Anode amp ,lsq. 111.. 1 (mhmin a p sq. in.. .86 Cumulative current:

Efliciency ercent" 75. 5 Time hmlrs 41 .pH: 5.0-6.8 Temperature: 38-45 C.

EXAMPLE IV rnersion heaters.

' tion. The cell and auxiliary equipment used in the previous example were also utilized for this operation. The

The elecmoving sodium chlorate from the bath by concentration by evaporation and selective crystallization, and returning the depleted solution for electrolysis. An amount of salt equivalent to the sodium chlorate removed was added to the recirculated solution. A pH of 6.0 to 6.8'

was maintained by the periodic addition of HCl.

Table IV OPERATING CONDITIONS OF EXAMPLE IV Feed solution:

N aClO- Anode: Same as in Example III.

Dathode: Same as in Example III.

Spacing: Same as in Example III.

Cell: Same as in Example III.

Voltage: 4.0.

Applied current: 285 amperes Current density: Same as in Example III.

pH: 6 to 6.8.

Temperature: 38 to 45 0.

Current efficiency: 90%+.

No perceptible attack or loss of lead dioxide was observed after prolonged use. In this operation the anode current density was five times as great as that obtainable with a graphite anode and in addition there was no destruction and hence no graphite or lead compounds were transferred to the electrochemical sodium dioxide product.

EXAMPLE V The anode prepared as above was used as an anode in the electrochemical production of chlorine from a sodium chloride solution. A saturated solution of sodium chloride was used as an electrolyte. The anode and cathode compartments were separated by an asbestos diaphragm and each compartment was sealed from the air by Plexiglas covers. Chlorine formed at the anode was drawn ofi and absorbed in caustic soda. Hydrogen and caustic were formed at the cathode. The caustic soda was continuously removed as a 14% solution. This effiuent from the cathode chamber was replaced by an equivalent volume of salt solution which percolated thru the diaphragm from the anode chamber. This salt solution was in turn replaced by fresh feed to the anode chamber. A temperature of approximately 95 C. was maintained in the cell by means of two quartz type im- Periodic inspection of the lead dioxide anode showed both the plate and the graphite to be unattacked and unchanged.

Table V OPERATING CONDITIONS OF EXAMPLE V Electrolyte: Saturated sodium chloride solution Anode: Same as in Example IV Cathode: 2 sections A thick 6" x 24" iron sheets immersed 20". Effective area 120 sq. inches each.

Cell: Same as in Example IV with the addition of an asbestos diaphragm located between the anode and cathodes.

Spacing: Same as in Example IV Applied current: 143 amperes Voltage: 3.0

Current density: 0.5 ampere per square inch Current efliciency: 98%

Temperature: 95 C.

of the anode varies from about 75 to 150 amperes per Having thus described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In the process of electro-depositing a lead dioxide coating on a graphite anode, theelectrolyte consisting essentially of an aqueous acid solution of lead nitrate in a conceneration, and at a temperature that will deposit lead. dioxide on the graphite anode. and a surface active agent, the improvement comprising varying the anode current density for at least two successive periods of electro-deposition, the anode current density varying from about to 150 amperes per square foot for the prior period of electro-deposition to about 20 to 60 amperes per square foot for the succeeding period of electrodeposition, to completely cover the graphite anode with a compact, hard, dense, smooth coating having randomly oriented crystals firmly bound to the graphite base.

2. The process of claim 1, in which the cathode current density is about 1.5 to 3 times the corresponding anode current density.

3. The process of claim 1, which produces a lead dioxide coating having a thickness from about a thirtysecond of an inch to about one-half an inch.

4. The process of claim 1, in which the lengths of the periods of time of electro-deposition vary, the length of the prior period of time being substantially shorter than the length of the succeeding period of time.

5. The process of claim 1, in which the lengths of -the periods of time of electro-deposition are about the same.

6. The process of claim 1 in which the current density square foot for the first one to five hours of electrodeposition to a low of about 20 to 60 amperes per square foot for the remaining hours.

7. The process of claim 1 in which the surface active agent is of the alkyl phenoxy polyoxyethylene ethanol class.

8. The process of claim 1 in which the spent solution is regenerated by dissolving therein about 1% by weight of n-amyl alcohol.

9. The process of claim 4, in which the anode current density of the prior period of electrodeposition is about twice that of the succeeding period of electro-deposition.

10. The process of claim 5, in which the current density of the prior period of electro-deposition is about one and a half times that of the succeeding period of electro-deposition.

11. An anode for use with corrosive electrolytes having a graphite base, and having electrolytically deposited thereon a massive coating of lead dioxide completely covering the immersible active portion of the anode, the coating being smooth, compact, hard, firmly bound to the base, with randomly oriented crystals and produced by the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,846,378 Hoffman Aug. 8, 1958 2,872,405 Miller et a1. Feb. 3, 1959 FOREIGN PATENTS 456,082 Great Britain Nov. 3, 1936 OTHER REFERENCES Journal of the Electrochemical Society, vol. 105 (February 1958), pages -101; and vol. 104 (July 1957), pages 448-449.

Claims (1)

1. IN THE PROCESS OF ELECTRO-DEPOSITING A LEAD DIOXIDE COATING ON A GRAPHITE ANODE, THE ELECTROLYTE CONSISTING ESSENTIALLY OF AN AQUEOUS ACID SOLUTION OF LEAD NITRATE IN A CONCENTRATION, AND AT A TEMPERATURE THAT WILL DEPOSIT LEAD DIOXIDE ON THE GRAPHITE ANODE AND A SURFACE ACTIVE AGENT, THE IMPROVEMENT COMPRISING VARYING THE ANODE CURRENT DENSITY FOR AT LEAST TWO SUCCESSIVE PERIODS OF ELECTRO-DEPOSITION, THE ANODE CURRENT DENSITY VARYING FROM ABOUT 75 TO 150 AMPERES PER SQUARE FOOT FOR THE PRIOR PERIOD OF ELECTRO-DEPOSITION TO ABOUT 20 TO 60 AMPERES PER SQUARE FOOT FOR THE SUCCEEDING PERIOD OF ELECTRODEPOSITION, TO COMPLETELY COVER THE GRAPHITE ANODE WITH A COMPACT, HARD, DENSE, SMOOTH COATING HAVING RANDOMLY ORIENTED CRYSTALS FIRMLY BOUND TO TYHE GRAPHITE BASE.

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