US2870074A - Electrolysis of alkali metal chloride brine - Google Patents

Description

nite" states waft ELECTROLYSIS OF ALKALI METAL CHLORIDE BRINE No Drawing. Application June 21, 1955 Serial No. 517,090

2 Claims. (Cl. 204-98) This invention relates to improvements in electrolytic cells, and more particularly is concerned with the treatment of carbon electrodes for use in chlorine-alkali type electrolytic cells.

Various methods have heretofore been proposed to increase the durability and useful life of carbon electrodes in electrolytic cells. Typical of such a prior suggestion is the impregnation of a carbon electrode with a drying oil, which oil then is allowed to dry or harden in situ. In some instances, such treatment indeed is advantageous in providing a more durable electrolyte-resistant electrode. Occasionally, however, such treatment has led to difiiculties in the operation of an electrolytic cell when a thus-treated electrode is first placed in use. Since the drying oil in the electrode dries or hardens rather slowly and non-uniformly, far too frequently the-oil is only partially hardened or dried when the electrode is placed in service. As a result, there occurs a leaching out of the partially-cured oil and, in the case of a chlorine-alkali cell employing such an electrode as an anode, a subsequent chlorination of the oil. While this chlorination not only reduces the chlorine output of the cell and' introduces undesired by-products, an additional problem is involved, since the finely-divided chlorinated oil is deposited over the cell diaphragm, thus rapidly reducing the fluid flow therethrough and generally completely plugging the diaphragm in a short time. A further inherent problem in such chlorination is the formation of by-product HCl in the anolyte, which HCl, of course, undesirably reduces the pH of the anolyte.

Prior workers in the art have sought a method to overcome these difiiculties and various treatments have been proposed to eliminate or minimize these problems. Illustrative of prior suggestions and practice to improve the operating characteristics of impregnated carbon electrodes is the storage of freshly-impregnated anodes for a considerable period of time to allow gradual, hardening and solidification of the oil within the anode pores, or the use of a special conditioning electrolytic cell in which the oil is leached from the electrode prior to use in a commercial cell. However, so far as .is known, up to the present time no completely satisfactory solution to the problem had been discovered. v t

It is, therefore, a principal object of this invention to avoid the difiiculties heretofore encountered in the opera tion of electrolytic cells and to provide a new and improved electrode treatment.

A further object of this invention is the provision of new and improved carbon electrodes for use in electrolytic cells.

The present invention avoids the difficulties heretofore encountered in carbon electrodes by impregnating these electrodes with an alkali metal silicate. The expression alkali metal silicate, as used in the specification and claims, is intended to include various'of the alkali metal silicates, e. g., silicates of sodium, potassium, lithium, rubidium, and cesium. It will, of course,

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be recognized that of these silicates, 'sodium and potassium silicates are the most common and that of these, sodium silicate probably is the most readily available commercial silicate. Hence,'particular reference hereinafter will be made to the use of various sodium silicates, although obviously the expression alkali metal silicate is not to be so limited.

In the practice of this invention a carbon electrode, whether or not subjected to various conventional pretreatments, such as pitch impregnation and/or graphitizing, is impregnated with an alkali metal silicate, preferably in the form of a solution, and the thus-treated electrode subsequently heated to remove the solvent, diluent, or carrier for the silicate and/or to effect any desired dehydration of the silicate. It is appreciated, of course, that some alkali metal silicates are themselves liquids, and in .such cases the necessity for subsequent heat treatment is minimized, although in many cases some heat treatment prior to use is advantageous.

While details vary somewhat in commercial production of carbon electrodes, generally finely-divided coke is mixed with the desired proportions of a binder, usually a coal tar residue pitch. This mixing g nerally is carried out at an elevated temperature suificient to melt the binder. The resultant mixture is then extruded 'and' the thus-formed structures baked, typically at a temperature of about 1800 F., to remove volatile matter. Anodes produced in the foregoing manner may th n advantageously be im regnated with an alkali metal silicate in accordance with this invention.

In addition, prior to impregnation with an alkali metal silicate the anode may be graphitized', i. e., the carbon crystal structure converted to the g'raphitic crystal structure by heating one or more times to a temperature in the ran e from 4l505000 F. At times, either a baked anode or graohitized anode can be further treated with itch when it is desired further to improve the anode density and strength. impregnation of a carbon electrode with an alkali metal silicate can be accomplished in any conventional way, as by placing the electrode under vac.- uum and immersing the anodes while under vacuum into the impregnant.

So that those skilled in the art may have a more complete understanding of the Present invention, the following specific examples are offered:

EXAMPLE I Part A into an autoclave is introduced an untreated carbon block anode and a vacuum of 28" mercury is applied for 15 minutes to remove contained air. There is 'then intrcduced into the autoclave at a temperature of 160 F. sodium silicate (Na SiO having a specific g'ravity'at 160 F. of 1.38 (1.40 (rh 78 F.). After all of thesilicate is introduced, 60 p. s. i. pressure is applied for one hour whi e the autoclave temperature is maintained at aoproxir ately F. The pressure is then released and the silicate-impregnated block is removed and the surfacecoat'ing removed.

Part B 3 EXAMPLE II Another advantage of carbon electrodes of this invention used in chlorine-alkali cells is shown by the following data obtained by the method described in Example I and comparing a linseed oil-impregnated carbon anode with an anode of the present invention:

As the foregoing data indicates, a substantial reduction in iron removal from a carbon electrode is accomplished by impregnating a carbon'electrode in accordance with this invention. Since iron contamination of electrolytic cell liquor is a significant problem in the art, the above data reflects a highly desired beneficial result.

It will be appreciated that the practice of this invention may include the use of various types of alkali metal silicate compositions. Hence, in certain applications various liquid alkali metal silicates can be used, although it is generally desirable to employ the less viscous liquid silicates. For example, liquid sodium silicate having a specific gravity within the range from about 1.381 to 1.73 or 40.0 to 61.0 B. These liquid products .generally have an alkali-silica ratio of less thanone, a typical alkalizsilica ratio being 1Na O:2.6-3.22 silica.

At times, it may be desirable to employ a dry or gran ular alkali metal silicate, e. g., materials such as sodium metasilicate in either anhydrous or hydrated form, such as the pentahydrate, supersilicate, an orthosilicate or mixtures thereof, which can be dissolved or dispersed in water or other liquid diluent or carrier.

The amount of the alkali metal silicate to be retained by the carbon electrode can be varied. However, it generally is desirable to provide about 5% to 15% by weight of the silicate in the carbon electrode pores, a preferred amount ranging from about 8-14%, e. 3., 13.1%.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. In the operation of chlorine-alkali electrolytic cells, the improvement which comprises the use as anodes of carbon electrodes impregnated with an alkali metal silicate.

2. The method of electrolyzing an alkali metal chloride brine which comprises passing an electric current between an anode and a cathode through an alkali metal chloride brine solution, said anode comprising a carbon member impregnated with an alkali metal silicate.

References Cited in the file of this patent UNITED STATES PATENTS 680,441 Rhodin Aug. 13, 1901 882,169 Ruthenberg Mar. 17, 1908 1,000,761 Snyder Aug. 15, 1911 1,729,343 Haas Sept. 24, 1929 2,074,885 Bender Mar. 23, 1937 2,680,081 Probcrt June 1, 1954 2,756,201 Muller July 24, 1956

Claims (1)

1. 2. THE METHOD OF ELECTROLYZING AN ALKALI METAL CHLORIDE BRINE WHICH COMPRISES PASSING AN ELECTRIC CURRENT BETWEEN AN ANODE AND A CATHODE THROUGH AN ALKALI METAL CHLORIDE BRINE SOLUTION, SAID ANODE COMPRISING A CARBON MEMBER IMPREGNATED WITH AN ALKALI METAL SILICATE.