US3917518A - Hypochlorite production - Google Patents

Abstract

A process is described that is particularly applicable to the continuous production of hypochlorite solution over long periods of time by electrolysis of aqueous chloride solutions with an anode having a coating that is a mixture of oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum, all in certain proportions.

Description

United States Patent [191 Franks et al.

[ *Nov. 4, 1975 HYPOCHLORITE PRODUCTION [75] Inventors: Charles R. Franks, North Madison;

Barry A. Schenker, Mayfield Heights; James M. Kolb, Mentor, all of Ohio [73] Assignee: Diamond Shamrock Corporation, Cleveland, Ohio Notice: The portion of the term of this patent subsequent to Feb. 19, 1991, has been disclaimed.

[22] Filed: Apr. 19, 1973 21 Appl. No.: 352,419

[52] US. Cl. 204/95; 204/290 F [51] Int. Cl. C2513 1/24 [58] Field of Search 204/95, 290 F [56] References Cited UNITED STATES PATENTS 1,397,239 11/1921 Slater ..204/95 Primary ExaminerOscar R. Vertiz Assistant Examiner-Wayne A. Langel Attorney, Agent, or FirmTimothy E. Tinkler [57] ABSTRACT A process is described that is particularly applicable to the continuous production of hypochlorite solution over long periods of time by electrolysis of aqueous chloride solutions with an anode having a coating that is a mixture of oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum, all in certain proportions.

4 Claims, No Drawings HYPOCHLORITE PRODUCTION BACKGROUND OF THE INVENTION A variety of uses for alkali metal hypochlorite solutions, including bleaching and disinfecting, are known. One widespread use is for microbiocidal applications in sewage and water treatment facilities, where there is considerable demand for units capable of on-site 'generation of hypochlorite solutions, thereby eliminating the need for expensive shipping of dilute solutions or the sometimes hazardous handling and storage of chlorine gas. Such units must, of course, be relatively maintenance free to allow their convenient use.

To this end, compact and efficient cell units in a variety of sizes have been designed. A primary problem with such units, however, has been to achieve operation, either continuously or intermittently, over long periods of time without need for maintenance or replacement of cell components. A particular problem has been to obtain a cell and process for operating same wherein the useful lifetime of the anodes of said cell is sufficient to allow relatively maintenance free operation.

STATEMENT OF THE INVENTION Therefore, it is an. object of the present invention to provide an efficient process for the production of hypochlorite solutions by electrolysis.

It is a further object of the present invention to-provide an efficient process for the electrolytic production of hypochlorite, which process allows long-term operation without anode replacement.

These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims that follow.

There has now been found in a process for the production of hypochlorite by the electrolysis of an aqueous alkali metal chloride solution, the improvement that comprises passing an electrolyzing current through .said solution between a cathode and an opposed anode,

which anode comprises a conductive bas'e bearing on at least a portion of the surface thereof amixed oxide coating of from 30 to 90 percent stannic oxide, 1.0 to percent antimony oxide, 1.0 to 50 percent of at least one platinum group metal oxide, and 0.5 to 30 percent .,an.ode,i s considered. passivated when the voltage at which the desired electrolytic reaction occurs becomes so high as to prohibit further practical operation, e.g., in excess of 8 volts in a hypochlorite cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Basically, the invention is concerned with a process" for the production of an alkali metal hypochlorite solution by the electrolysis of an alkali metal chloride in a diaphragmless cell. The cell configuration is not critical to thepractice of the present invention except in that it must be, adapted for operation with dimensionally stable electrodes. Thus, any known cell may be used, such cells comprising an enclosureprovided withan electrolyte inlet and an outlet for electrolyte and product and containing at least one anode and one cathode opposed to each other in a substantially parallel ralationship. The cell mayibe of either the monopolar design, that is, wherein there is a separate external electrical connection for each electrode, or bipolar in configuration, wherein thecurrent passes, from a terminal anode through a plurality of bipolar electrodes disposed between said terminal anode and a terminal cathode at the opposite end of the cell. Within the cell, in the absence of a diaphragm, the products of electrolysis, i.e., chlorine and alkali metal hydroxide, immediately mix toform the desired hypochlorite solution.

, I The conductive substrate is preferably titanium, al-

thoughtantalum, niobium, and zirconium may also be employed. In addition, a covering of one of the aforementioned metals over a more conductive material,

such as copper or aluminum, may be employed. Further, layers on the substrate intermediate the base metal and the coating, such as those described in US.

within the range of from 30 to 90 percent by weight of I 1 the total coating composition.

. The antimony oxide component enters into the tin oxide crystal lattice, rendering same more electrically conductive. Although the antimony is present in an indeterminate oxide form owing to its entrance into the stannic oxide crystal lattice, it is expressed for convenience sake as Sb O Thus, on this basis, the antimony oxide is present within the range from 1.0 to 10, prefer- V ably 4.0 to 10, percentby weight.

The foregoingranges of tin and antimony oxides are further qualified bythe proviso that they be present, respectively, in the range, on a. mole ratio basis, of 95:5 to :15, especially :10. In this fashion, there is obtained the desired doping effect of the antimony on the tin oxide without the presence of an excess separate.

phase of antimony. oxide.

The third component of the mixed coating is at least one platinum group metal oxide, by which term it is intended .to include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, especially those of ruthenium and iridium. These platinum group metal oxides are present for the most part in their most highly oxidized state and within the range of from 1.0

to 5.0 percent by weight. An especially preferred anode is one the coating of which contains a combination of RuO Ir0 or rhodium oxide.

The final component is a valve metal oxide selected from the group consisting of titanium and tantalum oxides. Whereas the titanium is present in the form of TiO- and is essentially crystalline (rutile) in nature, when tantalum is employed, a generally amorphous tantalum oxide results. Therefore, although it is expressed as Ta O it is understood that mixtures of tantalum oxides may in fact be present. The'amounts of a the preferred method of preparing the multicomponent" coating composition on the titanium substrate is by deposition from a solution of the appropriate thermochemically decomposable salts. For example, it is desirable to paint or brush an acidified alcoholic solution of said salts onto the substrate followed by drying at ll40 C for from 3 to 10, especially 5, minutes and finally by baking in an oxidizing atmosphere, e.g., air, at 450 to 520 C, especially 500 C, for from 5 to 10, especially about 7, minutes. This procedure may then be repeated any number'of times until the desired coating thickness is obtained, for example, 6 to 10 coats. The preferred solvents for the thermally decomposable salts are the lower alkanols such as ethanol, propanol, amyl alcohol, and especially n-butyl alcohol, although other solvents including water may be employed. To these solvents there is generally added from 0 to 50 percent by volume of an acid, such as hydrochloric acid. The concentration of the metals'in the solution from 6hich the coating composition is to be formed ranges between about 50 and 200 grams per liter. The

' trichloride or tribromide, orthobutyl titanate, antimony trichloride or pentachloride, and stannic chloride or dibutyl tin dichloride.

It will be understood by those skilled in the "an that it is possible to use a number of combinations of preformed oxides of the various component metals and salts of the remaining materials, although it is generally believed that preformed valve metal oxides should not be employed nor should separately preformed tin and antimony oxides be used. Further, if thermal decomposition is incomplete, small amounts of salts may remain without detrimental effect in the coating, for example, small amounts of chloride in the primarily oxide coating. 7 I

The cathode, again, is not critical to the practice of the present invention, any of those conventionally employed being satisfactory. A preferred material is titanium, although steel, stainless steel, or titanium bearing an electrocatalytically active, electrically conductive coating, such as a mixed titanium dioxide-ruthenium dioxide coating, may be employed. Configuration is not critical to the process but rather is a consideration of the cell design. Generally a continuous sheet is employed. Of course, in the instance of a bipolar electrode, the substrate for the anode (e.g., titanium) may metals may be employed.

While the intere'lectrode gap may be within the range of 0.04-0.125 inch, the lower gaps are generally preferred for efficient operation.

The solution tobeelectrolyzed is an aqueous alkali metal chloride, especially a solution containing from about 28-35 grams per liter of alkali metal chloride. When the solution is made by dissolving relatively pure alkali metal chlorides in water, the use of solutions having a, for example, sodium chloride content in excess of 35 gpl, is uneconomic. On the other hand, concentrations substantially less than about 28 gpl sodium chloride provide solutions in which excessive amounts of oxygen are generated at the anode, thereby accelerating passivation. Rather than electrolyze an artificial alkali metal chloride solution, it is quite possible and mainly to'potassium and especially sodium.

' The temperature at which electrolysis is conducted should be within the range of from 5 to 50 C, especially l530 C. At temperatures in excess of this range, the chemical conversion of hypochlorite to chlorate is favored, thus reducing efficiency. On the other hand, at low temperatures, the relative overvoltages of oxygen and chlorine are displaced with the result that larger quantities of oxygen are evolved, again attributing to more rapid anode passivation.

"The pH at which the hypochlorite-forming reaction proceeds is within the range of from about 7 to 10. At a lower pH, the chlorate-forming reaction is preferred while higher ranges must be artifically induced and serve no economic purpose. Generally, it will be found that there is no need to adjust the electrolyte to obtain the desired pH since a salt solution of the appropriate concentration will on establishment of electrolysis eq'uilibrate at a pH within the range of 9.0-9.5.

desirable to control the flow rate and-other reaction conditions so that a concentration of hypochlorite within-therailge'of from8 to 10 grams per liter is achieved, thus avoiding a waste of the relatively expensive alkali metal chloride. On the other hand, where sea water is the electrolyte, cost of-the alkali metal chloride is not a consideration and concentrations within the range of 0.5 to 2.0 gpl are economical; To this end,

while recirculation may be employed, it is more feasible-to dictate the hypochlorite concentration by controlling the flow rate through the cell. Since the amount of available chlorine required by the average sewage treatment plant is only on the order of 10 ppm and only a 2 ppm concentration is required, in the waters of cooling towers, it will be apparent that any of the concentrations obtained by the invention are useful.

Typically, the process is operated at a relatively low current density, e.g., within the range of from about 0.5 to 1.0 ampere per square inch. While at higher current densities, the production rate is increased somewhat, this is offset'by the decrease in useful anode life. On the other hand, a low current density generally results in a low current efficiency. Hence, an increase in electrode area must be provided to achieve a like production.

Thus, a balance is struck within the above-stated range.

ln summary, a preferred process for the production of solutions having a sodium hypochlorite concentration within the range of 0.5 to gpl comprises passing an electrolyzing current of between 0.5 and 1.0 asi through an aqueous solution having a pH of greater than 7 and containing 28 to 35 gp] sodium chloride at a temperature from 5 to 50 C between a cathode and an opposed anode, which anode comprises a titanium substrate bearing on at least a portion of the surface thereof a coating of the mixed oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group consisting of titanium and tantalum.

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

EXAMPLE 1 An anode coating solution is prepared by dissolving 25 grams SnCl -5H O, 1.8 grams SbCl 5.3 grams RuCl -xH O (38% Ru), and 0.86 gram orthobutyl titanate in 25 ml butanol and 12.5 ml HCl (36%). A titanium sheet is coated by brushing with this solution followed by drying at 110 C for 3 minutes and baking in air at 500 C for 7 minutes. The procedure is repeated until a coating containing 0.64 gram per square foot of ruthenium is attained. The resultant electrode has the following composition, calculated on the indicated oxide weight basis: 73.1% SnO 7.8% Sb O 17.5% RuO and 1.6% TiO The electrode is disposed as the anode in an electrolytic cell opposite and spaced 0.04 inch apart from a titanium metal cathode. Electrolysis of a 28 to 30 gram per liter aqueous sodium chloride solution having a temperature of 5 C is conducted for 235 hours at an anode current density of 1 asi before the voltage increases to in excess of 8, indicating passivation.

EXAMPLE 2 Example 1 is repeated, the anode coating in this instance being 47.2% SnO 5.3% Sb O 27.3% RuO and 20.2% Ta O and an anode lifetime of 153 hours is obtained.

We claim:

1. In a process for the production of hypochlorite by the electrolysis of an aqueous alkali metal chloride solution, the improvement that comprises passing an electrolyzing current through said solution between a cathode and an opposed anode, which anode comprises a conductive base bearing on at least a portion of the surface thereof a coating consisting essentially of from 30 to 90 percent by weight stannic oxide, from 1.0 to 10 percent antimony oxide, calculated as Sb O from 1.0 to 50 percent of at least one platinum group metal oxide and from 0.5 to 30 percent ofa valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of SnO :Sb O is between 95:5 and :15.

2. A process for the production of a solution containing from 0.5 to 10 grams per liter of hypochlorite, which process comprises passing an electrolyzing current through an aqueous solution of alkali metal chloride, at a pH of greater than 7 and a temperature between 5 and 50 C, between a cathode and an opposed anode, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 30 to percent stannic oxide, from 1.0 to 10 percent antimony oxide, calculated as Sb O from 1.0 to 50 percent of at least one platinum group metal oxide and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of SnO :Sb O is between :5 and 85:15.

3. A process as in claim 2 wherein the electrolyzing current is within the range of 0.5 to 1.0 ampere per square inch.

4. A process as in claim 2 wherein the alkali metal liter.

Claims (4)

1. IN A PROCESS FOR THE PRODUCTION OF HYPOCHLORITEBY THE ELECTROLYSIS OF AN AQUEOUS ALKALI METAL CHLORIDE SOLUTION, THE IMPROVEMENT THAT COMPRISES PASSINGAN ELECTROLYZING CURRENT THROUGH SAID SOLUTION BETWEEN A CATHODE AND AN OPPOSED ANODE, WHICH ANODE COMPRISES A CONDUCTIVE BASE BEARING ON AT LEAST APORTION OF THE SURFACE THEREOF A COATING CONSISTING ESSENTIALLY OF FROM 30 TO 90 PERCENT BY WEIGHT STANNIC OXIDE, FROM 1.0 TO 10 PERCENT ANTIMONY OXILE, CALCULATED AS SB2O3, FROM 1.0 TO 50 PERCENT OF AT LEAST ONE PLATINUM GROUP METAL OXIDE AND FROM 0.5 TO 30 PERCENT OF A VALVE METAL OXIDE SELECTED FROM TE GROUP CONSISTING OF TITANIUM AND TATALUM OXIDES, WITH THE PROVISO THAT THE MOLE RATIO OF SNO2:SB2O3 IS BETWEEN 95:5 AND 85:15.

2. A process for the production of a solution containing from 0.5 to 10 grams per liter of hypochlorite, which process comprises passing an electrolyzing current through an aqueous solution of alkali metal chloride, at a pH of greater than 7 and a temperature between 5* and 50* C, between a cathode and an opposed anode, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 30 to 90 percent stannic oxide, from 1.0 to 10 percent antimony oxide, calculated as Sb2O3, from 1.0 to 50 percent of at least one platinum group metal oxide and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of SnO2:Sb2O3 is between 95:5 and 85:15.

3. A process as in claim 2 wherein the electrolyzing current is within the range of 0.5 to 1.0 ampere per square inch.

4. A process as in claim 2 wherein the alkali metal chloride concentratiOn is between 28 and 35 grams per liter.