US2823177A

US2823177A - Method and apparatus for lowering the chlorate content of alkali metal hydroxides

Info

Publication number: US2823177A
Application number: US403923A
Authority: US
Inventors: Sidney G Osborne
Original assignee: Hooker Electrochemical Co
Current assignee: Occidental Chemical Corp
Priority date: 1954-01-13
Filing date: 1954-01-13
Publication date: 1958-02-11
Anticipated expiration: 1975-02-11

Description

METHOD AND APPARATUS FOR LOWERING THE CHLORATE CONTENT OF DROXIDES Sidney G. Osborne, Saint Davids, Ontario, Canada, as-

signor to Hooker Electrochemical Company, Niagara Falls, N Y., a corporation of New York ALKALI METAL HY Serial No. 403,923

9 Claims. l. 204-98) No Drawing. Application January 1954 This invention relates to methods and. apparatusfor reducing the amountof chlorates in caustic solutions produced by electrolysis in ordinary diaphragm type cells.

In the electrolytic cells of the diaphragm type, such as the type of cell described in United States Patent 1,866,065, the anode compartment is separated from the cathode compartment by a permeable diaphragm. Alkali metal chloride brine, such as lithium, sodium, or potassium chloride, is introduced. into the anode compartment, where it comes into contact with the anodes, and is caused to percolate through the diaphragm into the cathode compartment, where it comes into contact with the cathodes. When an electric current is passed between these electrodes, chlorine is liberated at the anodes and alkali metal hydroxide is formed at the cathodes with the liberation of hydrogen. In order to minimize voltage drop in the cell, the cathodes are placed as cl-ose to the A'review of the electrochemistry of deposited diav phra'gm type cells has been given by Murray, R." L.', and

Kircher, M. S. in the Transactions of the Electrochemical Society; vol. 86, pp. 83-106; 1944.

Chlorine, as such and as hypochlorous acid, is more or less soluble in brine, even at elevated temperatures, and forms hypochlorites in accordance with the followin representative equations:

' Thus, some chlorine inevitably passes through the diaphragm in solution in the percolating brine. When coming into contact with the caustic alkali in the cathode compartment, this chlorine reacts with the alkali to form alkali metal hypochlorite, in accordance with the following representative equations:

ing oxygen. However, at normal an yte P is 2,823,??? r mmed Feb. 11, 1958 about 4, hypochlorous acid may be formed by reaction with the chlorine in accordance with the following equa- Furthermore; since the hydroxyl ions carry a negative charge, which is discharged at the anode, their back migra tion represents a loss in current efiiciency.

Hypochlorite ion, which'is formed from the hydrolysis of chlorine dissolved in the anolyte, is discharged, at the anode to form chlorate ion in a manner after the followingequationz 5 1 12C1O- i.6H O- 4C lO3--[-8Cl"+ 12H++ 3O +12e (7) Further, hypochlorous acid and hypochlorite ion are unstable under the, conditions of electrolysis and tend to form, chlorate ion' and oxygen according to the following equations:', 5' ,7 .:-ClO.*+2HClO.-- ClO -+2Cl-+2I-I+ (8) The oxygen produced from hydroxyl ions discharging at the anode and from the decomposition of some of the hypochlorites in the anolyte thereby results in contaminationbf the chlorine; also, since the anodes are of graphite, some of the oxygen attacks the anodes, slowly consuming them, which results in the contamination of the chlorine with .carbon dioxide. Similarly, the oxygen produced from the decomposition of some of the hypochlorites in the catholyte results in contamination of the hydrogen with oxygen. 7 I v In the cathode compartment substantial amounts .of the hypochlorite and chlorate ions are reduced by nascent hydrogen (H formed at the cathode according to the following equations:

However, some of the hypochlorite and chlorate ions escape reduction in the catholyte and pass out of the cell and thereby contaminate the cell effiuent which is mainly spent brine having the alkali metal;hydroxide dissolved therein. g a g In the presence of an excess of alkali, the chlorate is quite stable. It therefore tends to persist in the cell effluent and to pass on through to the evaporators in which the caustic alkali is concentrated. Practically all of the chlorate survives the evaporation and remains in the final product, where it constitutes a highly objectionable contaminant,especially to the Rayon industry.

The problem of lowering chlorates has been attacked at two main points: p I a (a) The chl-orates having been formed, can be reduced in the further processing of the caustic alkali and by special treating methods. See for instance, U. S. Patents 2,622,009; 2,044,888; 2,142,670; 2,207,595; 2,258,545; 2,403,789; 2,415,798; 2,446,868; and 2,562,169; and British patents 642,946 and664,023 which show representative examples of different methods used for reducing the chlorates after they have been formed.

(b) The productionof chlorates during the electrolysis can be lowered by adding a reagent to the brine feed which reacts preferentially withzthe back'migrating hydroxyl ions from thelcathode compartment of the cell making their way through the diaphragm into the anode compartment, and'by such a reaction prevents the formation of some of the hypochlorites in the manner shown by Equation 6 and thus additionally preventing these hypochlorites from further. reacting to form chlorates in the manner shown by Equations -7, 8, and 9. Reagents such as hydrochloric 'acid shown in -U. S. Patent 583,330, and sulfur in an 9xidiza l form, su has sodium tetrasulfide, shown in 3 U. S. Patent 2,569,329 are illustrative of methods which have been used to attackthe problem of chlorates in caustic by removing the back migrating hydroxyl ions before they can react to form chlorates.

It is an object of this invention to provide a new point of attack for the lowering of chlorates in caustic alkali solutions produced in cells of the diaphragm type. Another object is to provide a method for the prevention of chlorate formation during the electrolytic decomposition of alkali metal chlorides in diaphragm type cells wherein the hypochlorites are reduced before they can form chlorates. A third object of this invention is to provide a method for the prevention of the formation of chlorates during the electrolysis of alkali metal chlorides which is economical to adapt into the electrolysis process and does not introduce into the cell effiuent any undesirable impurities. A fourth object of this invention is to provide an improved diaphragm for the electrolysis of alkali metal chlorides. A fifth object is to provide a method for preventing the formation of chlorates during the electrolysis of alkali metal chlorides in cells of the diaphragm type, which method can be used in conjunction with other known methods for lowering chlorates to give an improved result thereover, and without any harmful effects therewith.

I have discovered that the problem of lowering chlorates can be attacked at another point. My discovery is that production of chlorates during electrolysis of alkali metal chlorides in diaphragm type cells can be prevented by attacking the overall reaction at the point of hypochlorite formation, that is, destroying the hypochlorite before it can react to form chlorate.

I have now found that if small amounts of a finely divided material selected from the group consisting of nickel, cobalt, their brine soluble salts, their hydroxides and mixtures thereof are dispersed into the diaphragm, and the resulting diaphragm containing the finely divided compounds used in the electrolysis of alkali metal chlorides, the chlorate content of the resultant cell effiuent can be lowered substantially from what the chlorate content would have been had such a compound not been added; likewise, the-chlorate content of the alkali metal hydroxide produced therefrom is proportionately lowcred.

Although I do not wish to be limited to any-theory, I believe the reduction of the hypochlorites might be effected in a manner after the following illustrative equa- .tions:

The reduction of the hypochlorites might also be effected in a manner after Equation above to a greater extent than if the finely divided compound of my invention had not been added to the diaphragm. I

The compounds to be used in this invention are powdered or otherwise finely divided nickel and cobalt, and their brine soluble salts of nickel and cobalt which upon exposure to dilute caustic solutions such as are found in diaphragm cells during the electrolysis of alkali metal chlorides, react to form insoluble hydroxide salts of nickel and cobalt and are thereby retained in the diaphragm and not dissolved away during continuous use of the diaphragm in the electrolysis of alkali metal chlorides. Mixtures of nickel, cobalt, their brine soluble salts and their hydroxides thereof may also be used herein. For instance, during electrolysis the catalytic materials dispersed into the diaphragm may be both finely divided nickel and finely divided hydroxides of nickel; or, the materials may be powdered cobalt and finely divided hydroxides of cobalt; or they may be powdered nickel and cobalt; or, they may be powdered nickel and powdered cobalt and the finely divided hydroxides of nickel and cobalt; or, the'catalytic material may be the finely divided '4 hydroxides of nickel and cobalt. I prefer to use nickelin a finely divided state, such as powdered nickel commonly avail-able in saleable form, or a soluble salt of nickel such as a chloride of nickel which is also commonly available. However, finely divided cobalt and soluble salts thereof such as cobalt chloride, are also satisfactory. Other brine soluble salts of nickel and cobalt or mixtures thereof which may be used to disperse the catalyst into the diaphragm are their acetates, ammonium chlorides, ammonium sulfates, ammonium nitrates, bromides, ammonium bromides, chlorates, fluorides, formates, hypophosphite, iodides, nitrates, perchlorates, sulfates and others. The particular brine soluble salt used is not important, because the catalyst is converted over into an insoluble hydroxide form during'electrolysis.

The amount of catalyst to be dispersed into the diaphragm may be varied within wide ranges without depart- .ing from the scope of this invention. In general, a catalytic. amount is all that is necessary. I have found that catalyst concentrations in the diaphragm as low as 0.5 percent by weight and as high as 10 percent by weight of the diaphragm are effective. Concentrations as low as 0.25 percent and as high as 20 percent may also be used,

but catalyst concentrations above 20 percent are not necessary and may be uneconomical to use. I prefer to use a catalyst concentration of between one percent and 5 percent by weight of the diaphragm.

The diaphragms of this invention may be fabricated by employing permeable diaphragm material in any of the usual'forms. Asbestos is the most commonly used material for preparing permeable diaphragms and any of the commercially available forms may be used, such as asbestos paper, asbestos cloth, finely divided asbestos and other forms of asbestos may all be included within the scope of this invention as being adaptable to fabrideposited diaphragm having dispersed thereon a minor amount of catalyst which promotes the reduction of the hypochlorites as they come into association therewith. Similarly, the catalysts may be dispersed into other types of diaphragms, such as asbestos paper and asbestos cloth type diaphragms, by dispersing minor amounts of the materials into finely divided or porous materials. For example, where possible I prefer to disperse the catalytic material into the diaphragm while the diaphragm is being fabricated, in order to obtain a more uniform distribution of catalyst throughout the porous surfaces of the resulting improveddiaphragm.

After fabricating the improved diaphragms of this invention and while under the influence of the electrochemical conditions existingrwithin diaphragm type cells during the electrolysis of alkali'metal chlorides, the brine soluble salts of cobalt and nickel are converted over to abrine insoluble hydroxide form; Thus, during electrolysis the improved diaphragms of this invention have dispersed therein a catalytic amount of a material selected from the group consisting of nickel, cobalt, their hydroxides, their brine soluble salts, and mixtures thereof.

My invention is more fully described by the following examples but I do not wish to be limited thereto except as defined inthe appended claims.

EXAMPLEI Part1 Adeposited diaphragm was fabricated a slurry of asbestos suspended in brine through a steel screen of the type used as cathodes in cells for the electrolysis of alkali metal chlorides. After most of the brine had drained through the screen, the mat of asbestos remaining on the screen was drawn down onto the screen more firmly by means of a mild vacuum being placed on the back side of the screen. This deposited diaphragm was assembled into a horizontal cell, constructed essentially of a steam-heated Biichner funnel into which was placed the steel cathode screen having the asbestos diaphragm deposited thereon and above the diaphragm was suspended a horizontal type graphite anode. Brine was fed in from the anode side and the cell brought to about 84 degrees centigrade by passing steam into the jacket of the Biichner funnel, electrical connections were made between the graphite anode and steel cathode screen, and the current turned on and regulated to about 8.0 amperes. The cell was operated in this manner with the brine feeding into the anode compartment and exiting from the cathode compartment out of the bottom of the Btichner funnel at the rate of about 1.7 cubic centimeters of flow per minute for a period of about two hours at which time it was observed that equilibrium operating conditions had been reached. The sample of cell effluent taken at this time analyzed 144 grams per liter of sodium hydroxide and 1.89 grams of sodium chlorate per 1000 grams of sodium hydroxide.

Part 2 A second deposited diaphragm was fabricated as in Example I, Part 1, above, except that the slurry of asbestos suspended in brine also contained nickel powder in an amount such that the asbestos deposited on the steel screen contained percent by weight nickel dispersed therein. The diaphragm thus formed was placed in the same cell of Example I, Part 1, and operated in substantially the same manner. The sample of the cell efiluent taken after equilibrium operating conditions had been reached, analyzed 148 grams per liter of sodium hydroxide and 0.17 gram of sodium chlorate per 1000 grams of sodium hydroxide.

A comparison of the results of Part 1 with the results of Part 2 of this example shows that the chlorate content of the cell effluent was reduced 91 percent by dispersing 10 percent powdered nickel into the deposited diaphragm of the cell.

EXAMPLE II The following table gives the results obtained in the operation of commercial cells of the type illustrated in U. S. Patent No. 1,866,065, with brine saturated at 60 degrees centigrade and containing about 310 grams of sodium chloride per liter and into the deposited diaphragms of which have been dispersed varying amounts of powdered nickel. The percentages of nickel given are based on the combined dry weight of asbestos and nickel deposited. The chlorate contents are the averages of several months operation in each of the individual No harmful or abnormal effects were observed during the period that each of the above cells was run.

I have also found that my method for preventing chlorate formation during the electrolysis of alkali metal chlorides in deposited diaphragm type cells can be used in conjunction with other known methods for reducing chlorates duringelectrolysis such as the method given in U. S. Patent 2,569,329, which includes adding a reagent to the brine feed which reacts preferentially with the back migrating hydroxyl ions, and that no harmful effects are found due to this joint process. Further, I have found that these two methods have an additive effect on the actual overall reduction in chlorate content of the cell efiluent. These findings are illustrated in Example III.

EXAMPLE III A commercial cell of the type illustrated in U. S. Patent No. 1,866,065 having a deposited diaphragm in which was dispersed therein finely divided nickel chloride in the amount of 2.4 percent by weight was operated in the same manner as those cells shown in Example II except that the brine feed contained 0.03 weight percent sodium tetrasulfide. The average chlorate content over a period of several months operations was 0.28 pound of sodium chlorate per 1000 pounds of sodium hydroxide. No harmful or unusual efiects were observed during this period.

Thus in a manner after Example III the method of this invention may be used in conjunction with the prior art methods of adding an acid reagent such as hydrochloric acid or a reagent of the type described in U. S. Patent No. 2,569,329, that is, a sulfur compound in an oxidizable state, to the brine feed to help lower the chlorate formation even more than it would have been lowered had my method not been used with one of these known methods.

I claim:

1. In the method of decomposing alkali metal chlorides in electrolytic alkali chlorine cells of the diaphragm type, the improvement which comprises effecting said electrolysis in the presence of a finely divided catalytic material selected from the group consisting of nickel, cobalt, their hydroxides and mixtures thereof dispersed in said diaphragm wherein said finely divided material dispersed in the diaphragm is between about 0.25 percent and about 10 percent by weight of the diaphragm.

2. The method of claim 1 wherein the amount of material dispersed in the diaphragm is between about 1.0 percent and about 5.0 percent by weight of the diaphragm.

3. The method of claim 1 wherein the material is powdered nickel.

4. The method of claim 1 wherein the material is an hydroxide of nickel.

5. In a method for decomposing sodium chloride brine in electrolytic alkali chlorine cells of the diaphragm type, the improvement which comprises: effecting said electrolysis in the presence of a finely divided hydroxide of nickel dispersed in said diaphragm in an amount between about 0.5 percent and 10 percent by Weight of the diaphragm.

6. A diaphragm for use in electrolytic cells for the electrolytic decomposition of alkali metal chlorides comprising a permeable asbestos diaphragm having dispersed in finely divided form therein a catalytic material selected from the group consisting of nickel, cobalt, their hydroxides and mixtures thereof wherein said finely divided material dispersed in the diaphragm is between about 0.25 percent and about l0 percent by weight of the diaphragm.

7. A diaphragm according to claim 6 wherein the asbestos is deposited on a steel screen.

8. The diaphragm of claim 7 wherein the catalytic material is powdered nickel.

9. The diaphragm of claim 7 wherein the catalytic material is an hydroxide of nickel.

(References on following page) 7 References Cited in the file of this patent 2,031,844 UNITED STATES PATENTS 368,608 Peyrusson Aug. 23, 1887 389,186 Askew et a1. Sept. 11, 1888 669,441 Frasch Mar; 5, 1901 797,547 1,826,724 BOOSS Oct. 13, 1931 {16,614

' 8 Moore, Feb. 25, 1936 Ove'r dick et a1 June 23, 1936 wwofodbridge Dec. 7, 1937 FOREIGN PATENTS Fral lce Apr. 28, 1936 Norway June 24, 1929

Claims

1. IN THE METHOD OF DECOMPOSING ALKALI METAL CHLORIDES IN ELECTROLYTIC ALKALI CHLORINE CELLS OF THE DIAPHRAGM TYPE, THE IMPROVEMENT WHICH COMPRISES EFFECTING SAID ELECTROLYSIS IN THE PRESENCE OF A FINELY DIVIDED CATALYTIC MATERIAL SELECTED FROM THE GROUP CONSISTING OF NICKEL, COBALT, THEIR HYDROXIDES AND MIXTURES THEREOF DISPERSED IN SAID DIAPHGRAM WHEREIN SAID FINELY DIVIDED MATERIAL DISPERSED IN THE DIAPHRAGM IS BETWEEN ABOUT 0.25 PERCENT AND ABOUT 10 PERCENT BY WEIGHT OF THE DIAPHRAGM.