US3329594A - Electrolytic production of alkali metal chlorates - Google Patents

Description

United States Patent 3,329,594 ELECTROLYTIC PRODUCTION OF ALKALI METAL CHLORATES Paul P. Anthony, Wadsworth, Ohio, and Henry W. Rahn,

Pittsburgh, Pa., assignors to Pittsburgh Plate Glass Company, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Dec. 8, 1964, Ser. No. 416,917 7 Claims. (Cl. 204-95) This application is a continuation-in-part of application Ser. No. 183,379, filed Mar. 29, 1962, and now abandoned.

This invention relates to an improvement in the electrolytic production of alkali metal chlor-a-tes. It particularly relates to the use of an air-cathode for the production of alkali metal chlorate, notably sodium chlorate.

Electrolysis of alkali metal brine is well known as a method for the production of alkali metal hydroxide and chlorine. This electrolysis is carried out in cells having the anode and cathode compartments separated in order to minimize reaction of chlorine with the alkali metal hydroxide and the resultant product loss. In chlorate manufacture by electrolysis it is not desirable to keep the 'anolyte and the catholyte separated. One the contrary, contact of the chlorine with electrolyte is necessary to form hypochlorite and chlorate. The reactions occurring in this process may be expressed by the following equations:

The overall chemical reaction may be expressed as: (4) 3Cl -l-6OH ClO -+5Cl-+3H O Operation of commercial cells is carried out in an acid environment to favor formation of HClO as indicated by reversible Equation 2. These equations represent reactions which approach a theoretical current efficiency of 100 percent. Yield reduction and loss in current efficiency are due to reduction of the hypochlorite and chlorate ions at the cathode. To prevent these hypochlorite ion and chlorate ion reductions, a small amount of dichromate, notably sodium dichroma-te, is generally added to the cell brine feed. In general, graphite anodes and steel cathodes are employed to result in a potential drop across the cell of 3.5 to 3.7 volts. Now it has been discovered that air-cathodes can be prepared which are particularly useful for providing low voltage drop and high current efliciencies in chlorate cells resulting in large power savings. Further, it has been discovered that these chlorate cells may be and preferably are operated without dichromate. Thus, improved efliciency may be obtained at low voltages while simultaneously avoiding contamination resulting from the addition of dichromate. These and other advantages will become apparent from the detailed embodiments disclosed herein.

In accordance herewith, a method has been developed for the production of alkali metal chlorates, notably sodium chlorate, which comprises electrolyzing brine containing a substantial concentration of alkali metal chloride, notably sodium chloride, at a pH of 5 to 9, preferably 6 to 7, and a preferable temperature of 20 C. to 70 C., while oxygen-bearing gas is provided at the electrolyteelectrode interface. Thus, electrolysis is typically conducted in a cell the cathode of which is made of electrically conducting oxygen-activating catalyst on chemically inert porous substrate having an interface provided with adventitious pores through which oxygen may be supplied from an oxygen-bearing atmosphere contained within the electrode interior. A positive pressure within the electrode interior is provided to cause the appearance 3,329,594 Patented July 4, 1967 of oxygen-bearing gas at the electrolyte-electrode interface. 1

An aqueous brine solution for use in the practice hereof may be provided by dissolving solid alkali metal chloride in water, or may be provided from natural brine sources, or other ways apparent to those skilled in the art. It is preferred that a brine solution is provided which is free from heavy metal impurities, such as iron, copper, or zinc, some of which poison air-cathodes and tend to deposit sludge in the cells. Corresponding brines of other alkali metal halides may also be employed in accordance with this invention, however subject to similar purity limitations. In particular, potassium chloride may be employed. Alkali metal halides other than alkali metal chlorides are not usually economical. Thus, the preferred embodiments of this invention employ sodium chloride and potassium chloride brines.

In the utilization of the cathodes of this invention an aqueous solution of alkali metal chloride brine is electrolyzed with direct current. Chlorine gas is liberated at the anode and allowed to mix with the electrolyte. The interaction of the chlorine and the electrolyte may be aided by agitation or by circulating electrolyte. As in the conventional chlorate cell, hypochlorite is formed by the reaction of chlorine with both hydroxide ion and water. Additional oxidation of the hypochlorite to chlorate by electrolysis is minimized by the stirring action produced either naturally by escaping gases liberated at the electrodes or by mechanical means which removes hypochlorite ion from the vicinity of the anode. Likewise, the desired chemical action is aided by maintaining the pH between 5 and 9, and preferably between 6 and 7, to reduce the concentration of actual hypochlorite ion present.

'Electrolyte consisting of brine of the appropriate metal halide is passed through the cell at a rate of from 3 to 20 liters per hour per square foot of anode surface. When the alkali metal halide is sodium chloride, a typical concentration is grams per liter of solution; how, ever, this may be varied considerably. By way of illustration, after electrolysis has begun and some sodium chlorate has been produced, it is often desirable to cool the electrolyzed brine to deposit the product, viz. sodium chlorate. The concentration of the depleted brine may then be restored by the addition of sodium chloride to it and it may then be recycled to the cell for further electrolysis. This can be done continuously such that an amount of sodium chloride is added commensurate with that amount of product deposited. A suitable recycled brine for production of sodium chlorate may contain 300 grams per liter of sodium chloride and 500 grams per liter of sodium chlorate.

Electrodes for use as air-cathodes in the instant invention are fabricated from porous material to permit an application of a gas pressure to the interior of the electrode. A suitable gas pressure which is just sufiicient to cause the appearance of oxygen-bearing gas at the electrolyte-electrode interface is preferably employed and is generally 0.01 to 1.0 atmosphere above the external pressure. Material for fabrication of the cathode is typified by porous carbon. Such porous carbon may be drilled to provide a hollow interior and may also be machined to provide means of attachment for fittings by which the oxygen-bearing internal atmosphere such as air or oxygen gas may be supplied. Such a porous carbon support may be converted into a useful electrode by a variety of procedures. By way of illustration, a porous carbon electrode supplied with an internal atmosphere of air may be electroplated with oxygen activating metallic catalysts, notably platinum. Platinum or other platinum group metals such as rhodium, ruthenium, palladium, osmium and iridium is deposited in the pores of the porous carbon from the metal chloride solution. Specifically, platinum may be deposited from a platinum chloride solution containing grams per liter platinum at a current density of 40 millamperes per square centimeter. While the metal is being deposited, the internal pressure is alternately raised and lowered so that the solution can be intermittently drawn into and expelled from the interior of the electrode thereby preventing the pores from becoming completely blocked or plated over and permitting the oxygen-activating catalyst to be deposited within the pores.

Other air-cathodes, comprising a noble metal on which is deposited a film of a transition metal oxide are prepared by plating noble metal on the porous support followed by a treatment to deposit a thin film of selected metal oxide thereon. Deposition of the metal oxide may be accomplished by electrolysis of a solution containing a halide of the metal selected, by way of example, chromic chloride. Alternately, a low concentration of metal halide, such as chromic chloride, may be added to brine of an alkali metal, such as sodium chloride, from which the film of metal oxide is deposited by electroplating on the cathode.

Another type of air-cathode may be prepared by deposition of a metal oxide. In this embodiment the porous carbon cathodes are soaked in a solution of a salt such as chromium nitrate from which active cromium oxide cathodes result by simply heating to 250 C. These cathodes may be made especially active by depositing a platinum group metal, such as platinum, thereon.

Air cathodes may also be fabricated with spinel deposition. An aqueous solution of mixed soluble salts is utilized to deposit a coating of the salts on a porous carbon cathode when it is dipped therein. The treated electrode is then heated red hot to result in the formation of mixed metal oxides having the characteristic crystalline structure of spinels. By way of illustrtaion, an impregnating solution of ferric nitrate and magnesium nitrate is prepared. A porous carbon cathode is dipped into this solution and vacuum is applied on the hollow interior. After a suitable length of time the cathode is removed and heated to about 110 C., preferably in an oven to prevent hot spots. When the deposited layer has dried, this procedure may be repeated a few times to build up the deposit. The duration and frequency of the soak depends upon the particular deposit desired. Commonly, the cathode is soaked a total of a few hours and dried between each succesive application; however, a soak of as short as about ten minutes will suffice as long as the cathode is completely saturated therefrom. Alternatively, the cathode may be placed in the soak solution and a vacuum applied over the solution containing the cathode.

When a sutiable deposit has been applied, the cathode may be heated for a short time in an oxygen-illuminated gas flame or other suitable heating arrangement to a temperature of approximately 1200 C. A mixed magnesium iron oxide deposit results which has the characteristic crystalline structure of magnesium iron spinel. Similar results may be obtained by application of an oxide paste or aqueous slurry prior to ignition.

Porous supports other than porous carbon may be utilized when the cathode contains an oxygen-activating catalyst such as a spinel. These are prepared by sintering powdered metal at suificiently high temperatures to cause the metal particles to adhere to each other without actually melting. Alternately, a fine screen of a metal wire may be used to fashion a hollow porous cathode. This cathode may then be dipped in an aqueous solution of appropriate metal salts and ignited to about 1200 C. to result in the spinel coating.

Any spinel may be employed in the practice of this invention. The spinel group of minerals encompasses a vast number of possibilities. Although one skilled in the art could readily perceive of many spinels, the following lists some of them. It is to be understood that the 4 invention is not to be limited to embodiments employing only these specific spinels but that the scope of the in.- vention includes all possible spinel catalysts.

Spinels are, in essence, fused mixtures of diand trivalent metal oxides. There exists three main spinel types: (1) A, vB designated 2:3, (2) A, B, 4:2; and (3) A, .B 6:1 where A and B are metal cations. The most stable and, hence, most common structure is the 2:3 type or the binary-metal oxide of the general type AB O where A and B represent the metal cation, some of which are given in the hereinbefore contained list.

In the practice of this invention it is common to employ an additive in the cell liquor when the oxygen-activating catalyst is a spinel. These additives increase the overall efficiency of the cell. This additive is generally a metal salt with the metal corresponding to one of the metals comprising the spinel. By way of example, if a magnesium iron spinel is used as the oxygen-activating catalyst, magnesium halide may be used as the additive. Alternately, mixed magnesium and iron halides may be used to advantage as additive. Similarly, metal salts corresponding to any of the utilized spinels may be employed.

Additives include any salt of the corresponding spinel metals, however it is advantageous to employ the metal halides. Mixed additives of salts of both spinel metals may be employed. These additives may be in any concentration but preferably 0.5 to 5.0 grams per liter of electrolyte. Lower concentrations may prove inadequate for extended use of the cell. Higher concentration may project an economic disadvantage.

It is necessary that cathodes for use in the instant invention be prepared from porous substances which are electrical conductors. However, non-conducting coarsely ground organic resins may be sintered to prepare a porous cathode which may subsequently be immersed in solutions which permit the lining of the pores with conducting substances and render the whole cathode conduct ing. An example of this involves dipping a sintered polystyrene support in a dilute solution of formaldehyde followed by a second dipping in a dilute solution containing silver nitrate. Such a cathode may then be coated with a deposit of a selected oxygen-activating catalyst, such as a spinel.

Cathodes may be employed which embody oxygenactivating catalysts selected from metals of Group VIII of the Periodic Table, metals of Group I-B, IIB, VI-B and VII-B of the Periodic Table and mixtures of oxides of iron, cobalt and nickel; spinels, perovskites and Group VIII metals coated with oxides of trivalent metals.

The hereinbefore described cathodes are used in an electrolytic cell along with anodes which are fabricated of any suitable anode material, notably graphite. Anodes as an embodiment in this invention may be fabricated from any inert material such as platinum, graphite, platinum-plated copper, platinum-plated titanium or other platinum metals or base metals coated with platinum metal by electroplating or otherwise depositing thereon.

An oxygen-bearing gas, notably air, is supplied to the interior of the cathode with sufiicient pressure to cause the appearance of gas bubbles on the surface of the cathode when emersed in electrolyte. The pressure required varies with pore size and brine concentration; however, under virtually all conditions of temperature, pressure, concentration and pore size, an excess pressure of from 0.01 to 1.0 atmosphere is preferred. I The distance between the anode and the cathode is maintained as small as possible to permit liquid circulation without current interruption resulting from large gas pockets. A convenient distance is between 0.25 inch and 1.0 inch. Anode current density is maintained from 5 to 1000 and preferably 20 to 100 amperes per square foot. The cathode current density ranges from 5 to 100, preferably to 50 amperes per square foot.

The pH of the brine is conveniently adjusted between 6 and 7 by the addition of hydrochloric acid. The acid required may be conveniently added during the operation of the cell. In order to minimize oxidation of hypochlorite ion at the anode, it is necessary to maintain the pH slightly below 7. If the pH falls much below 6 the reaction of chloride ion with hypochlorous acid to liberate chlorine gas in encouraged. Thus, while the invention is operative in the range of pH 5 to 9, it is preferred that the pH be maintained between 6 and 7 for optimum results. With the pH in this range chlorate formation appears to be highly favored. i The rate of conversion of hypochlorite to chlorate increases with temperature. It is desirable to operate the cell at as high a temperature as is consistent with anode attrition. Anodeattrition becomes severe at temperatures above 70 C. Consequently, temperatures below 70 C. are preferable, although temperatures above 70- C. may be employed if anode attrition can be economically tolerated. At temperatures below 20 C. sodium chlorate solubility decreases greatly. At 20? C. the solubility of sodium chlorate is about 300 grams per liter of solution, hence, at temperatures below 20 C. some crystallization of the sodium chlorate may occur. Thus, it is preferabl to operate the cell above 20 C. and below 70 C.

The invention may be better understood by reference to the following examples but it is not intended that the invention be limited thereto.

Example I 1 An air-cathode was prepared using a porous carbon support from Stackpole Carbon Company, Stackpole 139 carbon tube. This tube was drilled to provide a hollow interior and machined to provide electrical connections. The carbon section was placedinto an aqueous solution containing 1.5 percent by weight magnesium nitrate and 4.8 percent by weight ferric nitrate and vacuum was applied overthe solution for two hours. The carbon electrode was then removed, atmospheric pressure was restored, and the cathode was next placed in an oven at 110 C. for 22 hours. The carbon electrode was again placed into the solution and vacuum was applied for 5 minutes. The vacuum was turned off and the electrode was allowed to remain in the solution for 25 hours. At the end of this time the vacuum was again appliedfor 10 minutes. After this treatment, the carbon cathode was dried for hours in a 110 C. oven. An additional treatment .of 2 hours under vacuum in the solution was given to the carbon cathode after which time it wasagain dried at 110 C. for 2 hours. After these treatments, the carbon section was heated in an oxygen-illuminating gas flame to 1 6.8 by dropwise addition of l-normal hydrochloric acid throughout the operation of the cell. The temperature was adjusted to 45 C. and 2 grams magnesium chloride per liter of electrolyte solution was added. An electric current was applied and adjusted to 22 amperes per square foot of cathode surface.

Under the same conditions other cathodes were substituted for the air cathode. Table I indicates the observed cell and cathode voltages.

An air cathode was prepared as in Example I. The following data was obtained after 192 hours of operation at 65 C.

Current density, amperes per square foot of cathode surface 22 Air pressure, atmospheres 0.50.7 Brine analysis, fed:

Sodium chloride, grams per liter of electrolyte 171 Magnesium chloride, grams per liter of electrolyte 2 Sodium chlorate, grams per liter of electrolyte 398 Cathode potential, volts 0.54-0.27 Cell potential, volts 2.35-2.05 Brine volume, liters:

Start 2.0 End 2.8 Brine analysis, withdrawn:

Sodium chloride, grams per ilter of electrolyte Sodium chlorate 333 Sodium hypochlorite 1.1 Current efiiciency on chlorate produced, percent 83.6

Example III An air cathode was prepared as in Example I except a zinc chromium spinel was deposited by alternately raising and lowering the pressure within the porous carbon support while the carbon support was dipped in an aqueous solution 10 percent by weight chromium nitrate and 4 percent by weight zinc nitrate. A final ignition to about -0 C. for one minute formed the mixed oxide spinel.

Added to the brine before electrolysis was mixed additives of chromic chloride and zinc chloride. A current density of 30 amperes per square foot of cathode surface for 48 hours resulted in a cell potential of 2.4 volts compared to 3.2 volts when a steel cathode was used.

Example IV An air cathode was prepared as in Example II and it was employed in an identical cell except that the' temperature was maintained at 45 C. and the current density at 30 amperes per square foot of cathode surface. One gram magnesium chloride per liter of electrolyte was used in the cell. After 75 hours at a cell potential of 2.07 to 7 2.02 volts and a cathode potential of 0.40 to 0.36 volt a 74 percent current efiiciency based on chlorate produced was realized compared with respective voltages of 3.36 to 3.2 1 and 1.27 to 1.19 and a current efiiciency, based on chlorate produced, of 59 percent with a carbon cathode embodiment.

In accordance with the present invention best economy may be realized when the concentration of sodium chloride is kept reasonably high, i.e., 125 grams sodium chloride per liter of brine. Below this level electrolysis to hypochlorite is satisfactory but cell potential increases and the risk of forming perchlorate becomes substantial at concentrations of less than 50 grams sodium chloride per liter of brine. Above 125 grams sodium chloride per liter of brine, the risk of discharging free chlorine increases. Thus from 50 to 200 grams of sodium chloride per liter of brine is the useful range and 90 to 170 grams sodium chloride per liter of brine is preferred.

In embodiments involving the recycling of brine, it is not practical to remove all of the sodium chlorate before recycle. Thus, recycled brine contains substantial concentrations of both sodium chloride and sodium chlorate. For maximum current efliciency it is best to convert only about 20 percent of the chloride content to chlorate. Thus, by way of example, a brine containing 125 grams of sodium chloride per liter is electrolyzed to reduce the concentration of sodium chloride to 100 grams per liter while 25 grams of chloride is converted to 46.5 grams of sodium chlorate. Where it is desired to recover the chlorate product by cooling the electrolyzed brine to crystallize the product therefrom, a substantial concentration 'of sodium chlorate must be present in the brine to raise the level of chlorate to a concentration just short of saturation at the particular cell operation temperature. If this practice is followed, the recycling brine need only be cooled enough to deposit the newly formed chlorate, viz. 46.5 grams in the hereinabove mentioned example. Salt to replace the 25 grams of sodium chloride converted to sodium chlorate may then be dissolved in the brine which may be further electrolyzed and recycled. While one preferred practice is described thusly, many other practices which fall within the state of the art can be described. However, the invention may be best practiced by electrolyzing brine containing 50 to 31 grams of sodium chloride per liter and sodium chlorate from nil to 800 grams Per liter.

Similar considerations apply in the electrolysis of potassium chloride to potassium chlorate; however, different solubility relationships exist which would have to be accounted for on approximately the same molar basis as those involving sodium chlorate production.

While the invention has been described with respect to certain details of specific embodiments, it is not intended that the invention be limited thereto except insofar as they appear in the following appended claims.

We claim:

1. A method for production of alkali metal chlorate in an acid environment which comprises electrolyzing brine containing 50 to 310 grams per liter alkali metal chloride and up to 800 grams per liter alkali metal chlorate in an electrolytic cell containing chemically inert anodes and cathodes, the cell cathodes comprising electrically conducting oxygen-activating metal oxide catalyst on a chemically inert porous carbon substrate, said brine containing metal salt additive of the metal of said oxide, said additive being present in a quantity of between 0.5 to 5.0 grams per liter of electrolyte, and supplying to the interface of said cathodes and said brine through adventitious pores in the cathodes oxygen from an oxygen-bearing atmosphere, and crystallizing a portion of the thus formed alkali metal chlorate by cooling the electrolyzed brine.

2. A method for the production of alkali metal chlorate in an acid environment which comprises electrolyzing brine containing 50 to 310 grams per liter alkali metal chloride and up to 800 grams per liter alkali metal chlorate in an electrolytic cell containing chemically inert anodes and cathodes, the cell cathodes comprising electrically conducting oxygen-activating spinel on a chemically inert porous substrate, said brine containing a member of the group consisting of (a) metal salt additive and (b) mixed metal salt additives, a metal of said member corresponding to at least one of the metals comprising the spinel in a quantity of between 0.5 to 5.0 grams per liter of electrolyte, and supplying to the interface of said cathodes and said brine through adventitious pores in the cathodes oxygen from an oxygen-bearing atmosphere, and crystallizing a portion of the thus formed alkali metal chlorate by cooling the electrolyzed brine.

3. The method of claim 2 wherein the said spinel is a magnesium iron spinel and the said additive is a magnesium halide.

4. The method of claim 2 wherein the said spinel is a zinc chromium spinel and the said additive is a mixture of zinc and chromium halides.

5. The method of claim 2 wherein the said spinel is a nickel chromium spinel and the said additive is a mixture of nickel and chromium halides.

6. A method for the production of sodium chlorate in an acid environment which comprises electrolyzing brine containing 50 to 310 grams per liter sodium chloride and up to 800 grams per liter sodium chorate in an electrolytic cell containing chemically inert anodes and cathodes, the cell cathodes comprising electrically conducting oxy gen-activating spinel on porous carbon, said brine containing metal salt an additive corresponding to one of the metals of said spinel in a quantity of between 0.5 to 5.0 grams per liter of electrolyte, and supplying to the interface of said cathodes and said brine through adventitious pores in the cathodes oxygen from an oxygen bearing atmosphere, and crystallizing a portion of the thus formed sodium chlorate by cooling the electrolyzed brine.

7. A method of producing sodium chlorate comprising electrolyzing brine in an acid environment in an electrolytic cell containing chemically inert anodes and cathodes, said cathodes comprising a porous substrate having an oxygen activating spinel thereon, said brine containing a metal salt corresponding to one of the metals of said spinel in a quantity of between 0.5 to 5.0 grams per liter of electrolyte, while supplying to the interface of said cathodes and said brine through said cathodes oxygen during the electrolysis of said brine.

References Cited UNITED STATES PATENTS 1,431,301 10/1922 Grunstein et al. 204- 2,384,463 9/ 1945* Gunn et al. l36--86 2,669,598 2/1954 Marko et al. 136-12O 2,860,175 11/1958 Justi 1368'6 X 3,043,757- 7/1962 Holmes 204 3,147,203 9/1964 Klass 20'480.

FOREIGN PATENTS 832,196 4/ 1960 Great Britain.

OTHER REFERENCES Fuel Cells, edited by Will Mitchell, Jr., Academic Press, New York, 1963, pages 113, 114 and 407-409.

JOHN H. MACK, Primary Examiner.

HOWARD S. WILLIAMS, Examiner.

H. M. FLOURNOY, Assistant Examiner.

Claims (1)

1. A METHOD FOR PRODUCTION OF ALKALI METAL CHLORATE IN AN ACID ENVIRONMENT WHICH COMPRISES ELECTROLYZING BRINE CONTAINING 50 TO 310 GRAMS PER LITER ALKALI METAL CHLORIDE AND UP TO 800 GRAMS PER LITER ALKALI METAL CHLORATE IN AN ELECTROLYTIC CELL CONTAINING CHEMICALLY INERT ANODES AND CATHODES, THE CELL CATHODES COMPRISING ELECTRICALLY CONDUCTING OXYGEN-IACTIVATING METAL OXIDE CATALYST ON A CHEMICALLY INERT POROUS CARBON SUBSTRATE, SAID BRINE CONTAINING METAL SALT ADDITIVE OF THE METAL OF SAID OXIDE, SAID ADDITIVE BEING PRESENT IN A QUANTITY OF BETWEEN 0.5 TO 5.0 GRAMS PER LITER OF ELECTROLYTE, AND SUPPLYING TO THE INTERFACE OF SAID CATHODES AND SAID BRINE THROUGH ADVENTITIOUS PORES IN THE CATHODES OXYGEN FROM AN OXYGGEN-BEARING ATMOSPHERE, AND CRYSTALLIZING A PORTION OF THE THUS FORMED ALKALI METAL CHLORATE BY COOLING THE ELCTROLYZED BRINE.