US3287250A - Alkali-chlorine cell containing improved anode - Google Patents

Description

N 1966 T. 'r. BROUN, JR., ETAL 3,237,250

ALKALI-CHLORINE CELL CONTAINING IMPROVED ANODE Original Filed May 28, 1962 P401. P INT/ION) ATI'OAC/VEY United States Patent 3,287,250 ALKALI-CHLORINE CELL CONTAINING IMPROVED ANODE Thorowgood Taylor Broun, Jr., Corpus Christi, T ex., and Howard H. Hoekje, Akron, and Aleksandrs Martinsons and Paul P. Anthony, Wadsworth, Ohio, assignors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania Original application May 28, 1962, Ser. No. 198,216, new Patent No. 3,250,691. Divided and this application Jan. 20, 1964, Ser. No. 345,551

4 Claims. (Cl. 204-263) This application is a division of our copending application Serial No. 198,216, filed May 28, 1962, now US. Patent No. 3,250,691.

This invention relates to the electrolysis of an alkali metal chloride solution in an electrolytic cell resulting in the production of ch-orine and alkali metal products. More particularly, this invention relates to the electrolysis of an aqueous solution at alkali metal chloride in an electrolytic cell wherein the anode has a specially treated platinum surface. In addition, this invention relates to the treated anode and the environment in which it is employed.

In the manufacture of chlorine and alkali metal hydroxide or other alkali metal products by the electrolytic decomposition of alkali metal chloride in an electrolytic cell, one of the most important considerations from an economic standpoint is the power cost necessary to achieve reasonable rates of production of chlorine and alkali metal products. Normally, in the production of, for example, caustic soda and chlorine, power costs represent 75 percent or more of the total expenditures in producing the products obtained. Practices which increase power consumption in the manufacture of these products are obviously economically unattractive, while a small savings in this area represents a tremendous overall economic advantage.

Increased power consumption in an electrolytic cell for the electrolytic decomposition of an alkali metal chloride results, in part, from the dissolution or erosion of the anode within the cell. As the size of the anode is thereby reduced, the distance between the anode and cathode is increased. This results in increased internal resistance within the cell requiring consequent voltage increase to maintain the applied current level.

In addition, anode dissolution or erosion eventually requires costly cell shutdown for anode replacement. In the case of the well-known graphite anode, replacement is usually required every six to twelve months.

As a result thereof, various metallic anodes have been tried to determine if they could withstand the stringent conditions of electrolysis. Such efforts have not been found altogether fruitful. For example, platinum surface anodes were envisioned inert enough to electrolysis to minimize the dissolution problem. However, on use in the electrolysis of alkali metal chloride brine solutions, platinum losses were found to be very costly. Moreover, there typically was found on using these anodes 3,287,250 Patented Nov. 22, 1966 "ice that polarization resulted in causing enough voltage increase that no significant advantage in power consumption was realized.

It has been discovered that long continuous periods of electrolysis, typically longer thanone year without anode replacement, can be achieved without appreciable increase in power cost and in many instances resulting in power cost reductions. This may be accomplished by electrolytically decomposing an acidic aqueous alkali metal chloride solution (preferably sodium chloride) in an electrolytic cell having a cation treated platinum surface anode.

This discovery may be effected by treating a platinum surface anode in an electrolytic cell as a cathode therein, which cell contains a treating electrolyte. A current is passed across the anode and cathode of the cell, causing electrolytic decomposition of the electrolyte. After this treatment, the treated platinum surface anode is employed as an anode in an electrolytic cell containing product alkali metal chloride in aqueous solution in contact with said anode. Electrolysis of the solution is effected while the solution has a pH below about 5, preferably below 3.5, and typically above a pH of 1.

The process of this invention may be effected in any electrolytic cell capable of discharging chlorine at the anode and alkali metal product (e.-g., sodium or potassium hydroxide and/ or carbonate) at the cathode. Typical cells in which the process and treated anode of this invention may be employed are disclosed in US. Patents 2,542,523, 2,858,263 and 2,409,912.

Though the process of this invention is desirably used for the electrolysis of saturated aqueous sodium chloride brine solution, more dilute or more concentrated aqueous solutions of sodium chloride may also be employed, e.g., from 5 percent by weight of salt saturation to above percent by weight of salt saturation. Other product electrolytes (alkali metal chloride) included within the terms of this invention are aqueous solutions of lithium chloride and potassium chloride.

The platinum surface anode of this invention comprises any electrode having a platinum surface. Thus, the anode may be solid, unitary platinum, a platinum clad electrode, or a platinum coated electrode. Of these different types, the platinum coated electrode is found the most favorable because of extended life when employed as an anode during the electrolysis of aqueous alkali metal chloride solutions. Furthermore, a platinum coated anode typically requires only one cationic treatment to operate under conditions of continuous electrolysis for one year or more with low power consumption. Unitary platinum and platinum clad electrodes generally require more periodic cationic treatment, for example, once every month during a continuously operated electrolytic process.

The coated platinum anode includes vapor deposited, chemically deposited or electrolytically deposited platinum coating on a metal base. The vapor deposition conventionally involves, for example, plasma jet spraying of platinum metal vapor on a metal base. Chemical deposition can be effected by coating platinum salt on '3 J? a metal base followed by firing of the coating. Electrolytic deposition involves plating a metal electrode with platinum by the electrolytic reduction of a platinum salt. The metal base may take the form of a plate, bar, screen or cylinder. The shape of the base is not critical to the invention except insofar as its specific employment with a particular type of electrolytic cell operated under certain controlled conditions. This feature of the base is within the province of the skilled worker in the application of the invention herein disclosed.

Preferably, the base is made of a metal such as titanium and tantalum, which represent the preferred base materials in the operation of this invention. Other metals may be employed with significant efiicien-cy, but typically their overall utility does not measure up to titanium and tantalum. Included within this class are platinum, tungsten, aluminum, vanadium, niobium (columbium) and'paladium. With regard to the latter metals, it is desirable that the platinum coating thereon be at least 100 microinehes thick. When titanium or tantalum is the base material, the platinum coating as a rule should be at least 3 microinches thick. Because of operational benefits, platinum coated titanium is found to be the most desirable anode employable in the operation of the process of this invention.

The platinum surface anode is electrolytically treated under an electrolytically induced cationic environment prior to its use in the decomposition of the aforementioned chloride solution. The type of cations which effect this treatment can result from the electrolytic decomposition of any inorganic or organic electrolyte which is, inert to the platinum surface, that is, does not decompose or erode it or deposit material thereon during the treatment, i.e., electrolysis. The treating electrolyte may be, for example, an aqueous solution of alkali metal chloride (such as NaCl, KCl or LiCl), sodium dichromate, potassiumchromate, calcium sulphate, potassium permanganate, sodium acetate or N-trimethyl-benzyl ammonium chloride.

The cationic treatment of the platinum surfaced anode involves, in effect, reversing the direction of the current to the anode from that employed when it is an anode in the electrolytic decomposition of the product alkali metal chloride. In the invention as herein described, the language cation discharge at the platinum surface anode is meant to encompass the utilization of the platinum surface anode as a cathode in an electrolytic cell during electrolysis of an electrolyte. Thus, the platinum surface anode is employed as a cathode in an electrolytic cell containing the aforementioned treating electrolytes so that positive ions (cations) are discharged at the platinum surface electrode (anode). This cationic treated anode is inserted in the cell provided for the electrolysis of product acidic aqueous alkali metal chloride solution so that negative ions (anions) are discharged at its surface. This is the equivalent of using the anode first as a cathode and then later as an anode during electrolytic decomposition.

The cationic treatment of the platinum surface electrode (anode) may be effected prior to the utilization of the electrode in the electrolytic decomposition of the product alkali metal chloride or may be effected during the decomposition thereof. If the anode is treated prior to the electrolysis of product alkali metal chloride, then such is preferably done in an electrolytic cell other than that employed for producing chlorine and alkali metal products. After treatment, the electrode may be inserted in the brine cell as an anode for the manufacture of chlorine and alkali metal products.

If treatment of the anode is effected in the alkalichlorine cell where it is employed as the anode, then electrolysis. When the voltage in the cell shows a 1 percent increase during electrolysis of sodium chloride in aqueous solution, while employing an untreated or previously cationic treated platinum surface anode, current reversal may be effected in the above-described manner.

Polarity reversal in the cell containing the platinum surface anode as the cathode therein should be maintained for a period of at least /2 to 1 second. When current reversal is effected in the cell employed for the electrolytic decomposition of, for example, sodiurnchloride brine solution, reversal as a rule should not exceed a period of about 10 seconds. A substantially longer period. of reversal tends to damage the anode therein and also creates a hazardous and explosive environment resulting from excessive hydrogen discharge at the anode. When current reversal is provided in a cell other than the cell employed for the production of product chlorine and alkali metal products, then the reversal treatment period may be maintained up to 1 hour or more.

After the current across the platinum surface anode.

has been reversed, the anode may be established as such in the cell for the electrolytic decomposition of the product alkali metal chloride. Surprisingly enough, a platinum surface anode to which current reversal has been effected may be stored for an apparently unlimited time before again used in the aforementioned electrolytic decomposition process. It is found that the treated anode acquires a memory of the effect of current reversal which appears to last indefinitely. For example, when the current is reversed across a platinum coated anode and it is thereafter stored for a long time, for example, 6 months,

on reuse of the anode in the electrolysis of an acidic.

aqueous alkali metal chloride solution, the cell starts operating at a low voltage. Thus, the treated anodes of this invention constitute a saleable item per se.

Acidification of the aqueous alkali metal chloride solution to the pH values described above, before or con-. current with the start of electrolysis in the cell, serves to maintain the reduced voltage obtained by treating the anode. To obtain the benefits of this process, it is important that electrolysis be effected only when the aforementioned chloride solution is at the desired pH. Thus, the solution should bemaintained acidic throughout the electrolysis reaction. If the pH of the solution is allowed to rise above a pH of 5, electrolysis must be stopped or else platinum loss from the treated anode will be excessive.

Acidification may be accomplished by adding acid to alkali metal chloride solution in the cell or by reconstituting the salt solution in the cell by replacing it with acidified aqueous alkali metal chloride. sired pH is achieved by the continuous addition of acidic aqueous alkali metal chloride solution to the cell at the start of and during electrolysis. Any one or combination of the above methods for acidification may be effectively employed.

Usable acids contemplated for acidifying the aforementioned product electrolyte are the mineral acids, such as hydrochloric acid, sulfuric acid or hydrofluoric acid, or the known strong organic acids. Preferably, acidification is effected with hydrochloric acid (dilute or concentrated) as described above.

It has been found that current reversal of the anode. alone is not capable of maintaining low voltages for a period of time suitable for commercial usage. Acidification without current reversal fails to significantly reduce. voltage.

Preferably, the del Each of these techniques alone and not in accord with this invention, fail to reduce dissolution of the platinum surface anode. Yet when employed together according to the above-described technique, anode dissolution is substantially minimized.

The temperature of the product alkali metal chloride solution during electrolysis may range from about 20 C. to the boiling point of the solution. Preferably, the temperature of the product solution during electrolysis is above 50" C., and for the most effective results, maintained above 85 C. to the boiling point of the solution.

The advantages of the process of this invention are significantly apparent when aqueous sodium chloride brine solution is electrolyzed in a compartmental cell. This type of cell has an anode-containing compartment (anolyte compartment) and a cathode-containing compartment (catholyte compartment) separated by a barrier. These barriers are either permionic or non-permionic. A typical cell containing a permionic barrier (or membrane) is descibed in copending application, Serial No. 29,559, filed May 17, 1960, now abandoned. Atypical non-permionic barrier is an asbestos diaphragm. A cell used in such a diaphragm is described in US. Patent No. 2,409,912. Of course, this does not preclude the desirable benefits accruing when the process of this invention and the treated anodes herein disclosexl are employed in well-known mercury cells.

Reference is made to the drawing, which diagrammatically illustrates a cross-section view of an electrolytic cell whereby production of chlorine and alkali metal products according to the process of this invention may be effected.

In the drawing, cylindrical glass tanks 1 and 3 are openly connected through cylinder 2. Near the base of the side wall of tank 1 is tank opening 15, where aqueous alkali metal chloride solution is introduced. Provided at the bottom of tank 1 is platinum coated anode 4. Connected to anode 4 is platinum rod 5 encased in glass tube 6.

The rod is connected to anode collar 7, to which is attached lead 7 connected to a power source. Immersed in the aqueous alkali metal chloride solution introduced to tank 1 through opening is heater 13, employed for regulating the temperature within the anode compartment of the cell.

Immersed in catholyte 17 contained in tank 3 is cathode 8 which may be made of conventional cathodic material such as platinum and steel. Cathode 8 is connected at the top thereof to anode collar 10, which in turn is connected to lead 11 attached to the corresponding power source. Immersed below the surface of catholyte 17 is glass tube 14 serving to remove catholyte from the cell as desired.

Centrally positioned in cylinder 2 is diaphragm 12 which may be a permionic membrane such as an asbestos diaphragm impregnated with a polymer of maleic acid or anhydride and divinyl benzene and/or styrene. Diaphragm 12, on the other hand, may be a simple diaphragm such as asbestos or sintered glass.

Current reversal across the platinum coated anode 4 may be readily effected during electrolysis of product electrolyte in the aforementioned cell by reversing the leads to the electrode. That is, the lead 7 may be attached to collar 10 and lead 11 attached to collar 9. Other methods for current reversal will be readily apparent to the artisan.

In additions, the aforementioned cells may also be employed for treating the platinum surface anode. Thus, the electrodes, as defined in the drawing, can be reversed. For example, cathode 8 can be immersed in tank 1 and Pt surface anode 4 can be immersed in tank 3. The electrolytes employed under these conditions may be any one of the treating electrolytes described above and within the contemplation of this invention. In this arrangement, the treating electrolyte is fed to tank 1 through opening 15. This cell arrangement may be permanently employed for treating one or several of the anodes and the platinum surface anodes can be removed therefrom after treatment for employment in the production of alkali products and chlorine in an alkali chlorine cell.

In a large scale operation, reversing the polarity of the bus bars by a double pole, double throw switch will serve to reverse the polarity within the caustic chlorine cell. Such a technique can be operated by inserting a voltmeter into the circuit which on showing, for example, a 1 percent voltage increase, automatically signals and initiates polarity reversal within the cell.

The following examples serve to illustrate specific embodiments of the invention described above, but to which the invention is not limited.

Example I Referring to FIGURE 1, cylindrical glass tank 1 has a 2-inch inside diameter and cylindrical glass tank 3 has a l /z-inch inside diameter. Cylinder 2 connecting tanks 1 and 3 has a 1-inch internal diameter and is 2 inches in length. Centered intermediate the length of cylinder 2 is sintered glass diaphragm 12. Centered within tank 1 is platinum wire 5 encased in glass tubing 6, both of which connect directly with platinum surface anode 4. Platinum surface anode 4 is a plate, one inch by one inch by 0.0625 inch in thickness. The anode is made of titanium metal, onto which is electrolytically deposited platinum. The platinum coating on the titanium metal base is 30 microinches thick. Immersed in tank 1 is an electrical heater 13 for temperature control of the bath. Anode 4 is connected through platinum Wire 5 to lead 7. In tank 3 is immersed cathode 8 which is a platinum wire having a 0.025 inch diameter. Cathode 8 is connected to lead 11. Both leads 7 and 11 are connected to appropriate power sources. Also immersed in tank 3 is tube 14 for catholyte withdrawal.

In the operation of this cell, a saturated aqueous sodium chloride brine solution is fed through opening 15 into tank 1 to a level above the anode and brought to a temperature of C. by heater 13. Current is passed through the anode at a current density of amperes per square foot and a voltage of 3.6 is provided in the cell. Voltage is measured by a voltmeter attached to the terminals of the anode and the cathode within the cell. After 40 days of electrolysis, the voltage increases to 3.8 volts. At this time, the leads connecting the anode and the cathode are reversed and current is then re-established at the same level. At this point anode 4 becomes the cathode and cathode 8 becomes the anode. Hydrogen is discharged at platinized anode 4 and chlorine is discharged at platinum wire 8. This reversal lasts for 10 seconds. After reversal is stopped, the leads are re-adjusted so that chlorine is formed around the platinized anode 4 and hydrogen is discharged at the platinum wire cathode 8, i.e., anode 4 is re-employed as an anode and cathode 8 is re-employed as a cathode. Before current to the cell is so re-established, the pH of the sodium chloride brine solution in the cell is adjusted to 3 by the addition of HCl to the anode tank 1. All brine added thereafter to the cell has a pH no greater than 5. Current is then re-established at 100 amperes per square foot and the voltage drop across the cell is found to be 3.6 volts.

7 Example II The following table shows voltage measurements for 579 days of operation under conditions as described above with a cell having two platinized anodes both containing the same thickness of platinum and operated under the same acidified conditions of electrolysis except that one of the anodes did not receive current reversal treatment.

TABLE pH of Volt-ageAcross Voltage Across Days from Reversal Anolyte Cell of Cell of Non- Reversed Anode reversed Anode -1 (before 1st reversal) 3.90 3.65 3.70 1 (after 1st reversal) 3. 90 3.35 3. 69 3. 90 3. 51 3. 85 3. 90 3. 54 3. 85 3. 90 3. 54 3.85 3. 90 O 3.50 3. 81

1 2 3. 9O 3. 58 3. 81 255 (Before 2d reversal 3. 90 3. 68 3. 95 255 (After 2d reversal) 3. 90 3. 54 3. 93 3. 90 3. 66 4. 02 3. 90 3. 4. 27 356 3. 90 3. 4. 56 366 (Shut down cell for anode inspection) 367 (Started up cell) 3. 3. 29 415 3. 90 3. 64 453 3. 90 3. 50 465 3. 90 3. 54 465 (Cell shut down for anode inspection)-.. 466 (Start-up) 3 90 3. 20 515 3. 90 3. 59 555 3. 90 3. 71 578 3. 75 578 (Shut down for anode inspection) 57 3. 90 3. 70

Removed this anode as Example III The same cell and conditions of operation described above are employed except that the platinized titanium anode is fitted into tank 3 for use as a cathode therein and a similar anode is fitted in the anode compartment. Thus, the cell employed in Example I has the same platinum coated anode in tanks 1 and 3 To the cell was added an aqueous solution of sodium hydroxide containing 4 percent by weight of solution. Current is provided to the cell by making the anode in tank 1 the anode in terms of electrolysis and the anode in tank 3 the cathode in terms of electrolysis. After electrolyzing said solu-' tion at a temperature of 25 C. using a voltage of 5.0 and a current density of amperes per square foot for 5 minutes, the platinized titanium anode in tank 3 is removed. The pH of 'theaqueous solution, on addition to the cell, is in excess of 14.

The platinized anode removed from tank 3 is laid on a laboratory supply shelf for 3 months and then re-employed in the cell described in Example I as the anode portion thereof.

The operating conditions of electrolysis are the same as those employed in Example I using an acidified saturated sodium chloride brine solution having a pH of 3. The electrolysis is started only after the pH of the anolyte has this value. The cathode in tank 3 is the platinum wire described in Example I.

Upon commencing electrolysis, it is noted that the voltage drop in the cell as measured across the electrodes is 3.6. After 20 days, the voltage is not found to increase by more than 1 percent of the voltage drop value and many times the voltage drop is found to fall below 3.6. After a continuous run of 50 days, the platinized titanium anode is removed from the cell and examined for platinum loss. It is found that platinum loss is not detectable.

When the same run is effected employing a non-acidified brine solution, the voltage of the cell immediately increases more than 5 percent of the initial voltage of the cell within hours from the start of electrolysis.

Typically, the current density at the anode during current reversal treatment thereof is above 20 percent of the current density prior to reversal, usually not more unfit [or further than percent of the current density prior to reversal; If reversal is effected in a cell separate from the product chlorine-alkali cell, then the current density employed for current reversal treatment is typically above 20 per cent ofthe cur-rent density to be employed in the chlorine-alkali cell and usually not above 150 percent of this current density value.

Usually, when current reversal is effected in the product chlorine-alkali cell, current density is not changed.

the production of sodium carbonate, sodium bicarbonate and sodium sesquicarbonate. A preferred method of achieving thisis disclosed in copending application, Serial.

No. 136, 312, filed September 6, 1961, now Patent No. 3,179,579.

A further and more specific embodiment of this invention involves employing a platinum surface anode having a low chlorine over-voltage. that the aforementioned beneficial and long-lasting effects resulting from electrolysis of acidified aqueous alkali metal chloride (e.g. NaCl) brine solution is obtained when the platinum surface anode has alow chlorine over-voltage, typically in the range of from about 0.03 to 0.15 volt. One way of obtaining a platinum surface anode having a low chlorine over-voltage is to effect the aforementioned cationic treatment by discharging cations at the platinum surface anode as discussed above and disclosed in the above examples. An anode treated in this fashion maintains the aforementioned 10w ohlorine over-voltage when used in the electrolysis of acidic brine solution having a pH below about 5. However, if electrolysis at any time is effected wherein the anolyte has a pH exceeding 5, the anode reverts to a high chlorine over-voltage, typically about 0.5 to 0.6 volt. In ad dition, deterioration of the anode, as previously discussed, is found to occur.

Though the invention herein disclosed describes specific embodiments, the invention is not limited thereto unless these specific embodiments are set forth in the claims.

It has been noted We claim:

1. In an electrolytic alkali-chlorine cell having an anode and a cathode connected to a power source the improvement wherein said anode has a platinum surface, said platinum surface having been previously exposed as a cathode to an electrolyte inert to the platinum surface.

2. The cell of claim 1 wherein said anode comprises a platinum coating over a metal base.

3. The cell of claim 2 wherein said metal base is from the class consisting of titanium and tantalum.

4. The cell of claim 2 wherein said anode has a plati num coating at least 3 microinches thick.

References Cited by the Examiner UNITED STATES PATENTS 9/1962 Ruff 204290 X 9/ 1963 Messner 204290 FOREIGN PATENTS 9/ 1961 Great Britain.

10 JOHN H. MACK, Primary Examiner.

D. R. JORDAN, Assistant Examiner.

Claims (1)

1. IN AN ELECTROLYTIC ALKALI-CHLORINE CELL HAVING AN ANODE AND A CATHODE CONNECTED TO A POWER SOURCE THE IMPROVEMENT WHEREIN SAID ANODE HAS A PLATINUM SURFACE, SAID PLATINUM SURFACE HAVING BEEN PREVIOUSLY EXPOSED AS A CATHODE TO AN ELECTROLYTE INERT TO THE PLATINUM SURFACE.

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