US3689397A - Mercury cathode electrolytic cells employing improved cathode support - Google Patents

Abstract

MERCURY CATHODE ELECTROLYTIC CELLS, SUCH AS ARE EMPLOYED IN THE MANUFACTURE OF CAUSTIC AND ELEMENTAL CHLORINE BY THE ELECTROLYTIC DECOMPOSITION OF SODIUM CHLORIDE, WHICH CAN BE AND PREFERABLY ARE CONVENTIONAL IN DESIGN AND CONSTRUCTION EXCEPT THAT THE MERCURY CATHODE IS SUPPORTED ON A SURFACE NOT WETTED BY MERCURY AND FORMED ENTIRELY OR PREDOMINANTLY OF TITANIUM, HAFNIUM, VANADIUM, ZIRCONIUM, MOLYBDENUM, AND/OR TUNGSTEN.

Description

United States Patent 3,689,397 MERCURY CATHODE ELECTROLYTIC CELLS EMPLOYING IMPROVED CATHODE SUPPORT Leslie K. Norton, St. Louis, Mo., assignor to Monsanto Enviro-Chem Systems, Inc., Chicago, Ill. N0 Drawing. Filed Oct. 8, 1970, Ser. No. 79,273 Int. Cl. B01k 3/06; C22d N04 US. Cl. 204-219 6 Claims ABSTRACT OF THE DISCLOSURE Mercury cathode electrolytic cells, such as are employed in the manufacture of caustic and elemental chlorine by the electrolytic decomposition of sodium chloride, which can be and preferably are conventional in design and construction except that the mercury cathode is supported on a surface not wetted by mercury and formed entirely or predominantly of titanium, hafnium, vanadium, zirconium, molybdenum, and/or tungsten.

BACKGROUND OF THE INVENTION (1) Field of the invention Mercury cathode electrolytic cells are widely employed in industry and have been found to be particularly useful in the manufacture of elemental chlorine and caustic soda. This type of electrolytic cell may take many physical forms well known to those skilled in the art but all such physical forms embody one or more anodes made of an electrically conductive material such as graphite or ruthenium oxide coated titanium, an electrically conductive base plate to act as a support for a flowing film of mercury, and means for introducing into the cell an aqueous solution of sodium chloride, or other electrolyte, such that it is in contact with the anode or anodes and with the mercury film which acts as a cathode. Upon the passage of a direct electrical current through the electrolyte, it is decomposed in a well known manner and, for example, in the case of sodium chloride, results in the formation of elemental chlorine which can be collected as a gas and elemental sodium which amalgamates with the mercury cathode and is subsequently recovered as caustic soda.

(2) Description of the prior art The cathode support plate in mercury cathode electrolytic cells commercially employed today in most instances comprises an uncoated steel plate although it has long been recognized that other metals which are wetted by mercury could be employed. For example, it has been suggested in the patent literature that a plate formed from aluminum, copper, or an alloy of iron, aluminum or copper can be used as a cathode support, and it has further been suggested that a nickel coated steel plate can be used as a cathode support. As far as is known, however, support plates formed from metals other than steel have never been employed commercially to any large extent and in cells constructed in the last decade, the construction of the cathode support plate has been such that in most instances an iron or steel surface, uncoated except for a layer of amalgam, has been in contact with the mercury cathode of the electrolytic cell. The preference for steel cathode support plates is due to a number of factors, including the relatively low cost and excellent physical properties of steel, but one important consideration has been the ease with which one can obtain a uniform layer of mercury upon a steel plate. This is due in large part to the fact that iron is readily amalgamated by an alkali metal-mercury amalgam so that the surface of a steel plate in an electrolytic cell is readily wetted by mercury. Once ice the surface is coated with a layer of amalgam, even a relatively small quantity of mercury will readily wet and spread rapidly over the surface. It is generally accepted that a steel surface will not perform properly unless it is wetted by the mercury cathode and care has been exercised to make certain that the cathode surfaceis clean and is wetted by the flowing cathode. Various procedures, as illustrated by that disclosed in US. Pat. No. 3,293,161, have been used or suggested for use in cleaning a steel support plate in a mercury cathode electrolytic cell so that the support plate is uniformly wetted by the mercury.

'In spite of the numerous advantages of utilizing a support plate having a steel surface for mercury contact in an electrolytic cell, there are at least three severe disadvantages. A first disadvantage is that a sizable quantity of mercury is required to amalgamate the steel surface prior to operation of the cell and since this mercury is non-functional as far as cathodic operation is concerned, this represents a considerable addition to the cost of using a steel support plate for the mercury cathode support. A second and very severe disadvantage of utilizing a support plate having a steel surface in contact with the mercury cathode is that a considerable portion of the mercury required to amalgamate the cathode support plate surface is lost to the atmosphere or otherwise each time the mercury cell is cleaned, and this represents an undesirable pollution problem. Those skilled in the art are acutely aware of the seriousness of mercury pollution problems at the present time. A third disadvantage of steel surfaced cathode support plates is that iron removed from the plate by the action of mercury-alkali metal amalgam causes the formation of mercury butter which seriously interferes with economical operation of the electrolytic cell. Mercury butter is a very viscous material which will not flow in the form of a thin film on the cathode support plate surface, and if the collection of mercury butter is sufficiently excessive, it can result in shorting of the electrolytic cell by contact with the anode surfaces above the cathodic surface. At best, the formation of mercury butter forces one to repeatedly increase the distance between the anode and cathode surfaces and this reduces the efficiency of the cell.

SUMMARY OF THE INVENTION In accordance with applicants invention, the problems associated with the use of a steel cathode support surface in a mercury cathode electrolytic celll are overcome by the use of a metallic surface entirely or predominantly of titanium, hafnium, vanadium, zirconium, molybdenum, and/ or tungsten. That one can satisfactorily employ a cathode support plate surface formed from such a metal is indeed surprising because these metals are not wet by mercury and if a quantity of mercury in the absence of an electric field is placed upon asurface formed of a metal or combination of metals from the above list, the mercury will tend to collect in small globules and will not spread and uniformly cover the metallic surface. As is well known, if the surface of the cathode support plate is not completely covered by the mercury cathode, operation of the cell results in the generation of hydrogen in addition to chlorine so that explosive conditions can be created.

While applicant does not wish to be bound by any particular theory, it is believed that surprisingly under the influence of an electric field, the mercury forming the cathode, without actually wetting the metal surface of a cathode support plate formed from one or more of the metals listed above, is caused to act as if the surface is wetted because applicant has found that such non-wettable surfaces can be employed contrary to prior art belief and experience that only mercury wettable surfaces are satisfactory.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS A cathode support plate in accordance with the present invention can be employed quite satisfactorily with any type of mercury cathode electrolytic cell now in commercial operation and ones preference for a particular style of cell must be based on considerations other than the use of the improvement herein described. For example, a cathode support plate in accordance with this invention can be satisfactorily employed with what is commonly referred to as a horizontal mercury cell such as described in U.S. Pat. No. 2,544,138 or it can be employed with mercury cells designed for a much greater inclination with respect to the horizontal such as are described in U.S. Pat. No. 3,308,044. Similarly, the improvement herein described can be quite satisfactorily employed in single tier mercury cells or it can be employed advantageously in multiple tier mercury cells such as are described in U.S. Pat. No. 3,308,047. Further, the physical shape of the cathode support plate is not of great importance insofar as this invention is concerned and the improved cathode support plates of this invention can be employed with cells such as described in the above-mentioned patents or it can be employed with mercury cells requiring a disc shaped support plate such as are described in U.S. Pat. No. 3,445,374. In summary, the improvement of this invention may be embodied in any electrolytic cell having one electrically conductive anode, an electrically conductive base plate disposed in underlying spaced relationship to the anode and inclined at an angle to the horizontal, means for introducing mercury onto the upper surface of the base plate in an area remote from the lowermost area of the upper surface so that the mercury flows downwardly over the upper surface of the base plate and in electrical contact therewith, means for operatively introducing an electrolyte between and in electrical contact with the anode and mercury being supported on the base plate and means for applying an electromotive force between the anode and the base plate such that the base plate operatively serves as a cathode support and the mercury in contact therewith serves as a cathode. As far as is known, all electrolytic mercury cells in use today embody all of these elements.

Cathode support plates in accordance with the present invention can be employed in cells utilizing any type of anode which is comercially available today and, for example, their use is quite advantageous with conventional carbon anodes of all types and styles. It is an advantage of the invention, however, that the use of a cathode support plate in accordance with this invention can be employed with the so-called dimensionally-stable" anodes such as described in British Pat. No. 1,147,442 which are coming into widespread use at the present time and, in fact, the use of a cathode support plate in accordance with this invention makes the use of dimensionally stable anodes even more advantageous. Dimensionally stable anodes have the theoretical advantages that they can be placed much closer to the cathode surface and can be employed with much higher current densities than carbon anodes thus resulting in greatly improved etliciency. In actual practice, however, it has frequently been found that when one endeavors to use the high current densities which are theoretically usable with dimensionaly stable anodes, the formation of mercury butter is greatly accelerated so that the cathode-anode separation must be increased, thus resulting in decreased cell efficiency. Further, in some instances, the accumulation of mercury butter using dimensionally stable anodes is so excessive that the frequency of cleaning the cell must be greatly increased thus increasing cell downtime and mercury losses. The new cathode support plates of the present invention materially reduce the formation of mercury butter which would otherwise result from iron impurities being introduced into the system by mentioned, this greatly reduces the mercury loss associated with operation of the cell, thus alleviating the problem of environmental mercury contamination. Mercury losses are sustained during cell shut-down in all instances as is well known in the art by losses to the brine and certain mechanical losses. In addition to these losses during shut-down, there are also substantial losses when utilizing a conventional steel surfaced cathode support plate when the cell is open to the atmosphere and the amalgamated cell bottom is exposed. Utilizing a cathode support plate in accordance with this invention, there is substantially no mercury retained on the surface of the support plate during cell shut-down so that mercury loss by evaporation from the exposed surface of the cathode support plateis materially reduced.

A cathode support plate in accordance with the present invention can comprise a solid sheet of one or a combination of metals of the group consisting of titanium, hafnium, vanadium, zirconium, molybdenum and tungsten, but it is an advantage of the invention that the cathode support plate can comprise a base plate formed from a relatively inexpensive electrically conductive metal having its upper surface coated with a metal or combination of metals from the above group. Examples of electrically conductive metals from which the base plate may suitably be formed include ferrous metals such as iron and steel, and copper alloys such as brass. It is not even essential that the coating be of the entire upper surface or that it be pinhole free as long as the base plate is of a metal conventionally employed as a cathode support plate because if the base plate is not completely covered, one will still obtain in accordance with this invention an improvemerit proportional to the percentage of the upper surface area of the base plate which is coated. The coating can be of any suitable thickness and, for example, can range from 1 or 2 microns of thickness to a thickness such that the over-all thickness of the cathode support plate becomes objectionable from a weight and cost point of wiew. The preferred thickness is from about 5 to 15 mils.

In instances where a coated support plate is to be employed, the coating can be applied in any suitable manner and, for example, can be applied by adhering to a suitable base plate a metal foil formed from one or more metals of the group consisting of titanium, hafnium, vanadium, zirconium, molybdenum, and tungsten. More conveniently, it has been found that a suitable coating can be applied with a plasma spray gun or, in the case of the lower melting of the metals of the above group, with a flame gun such as are commercially available from numerous manufacturers. Spray guns which have actually been tested and found to be satisfactory include Avco Plasma Gun No. 901065 and Plasmadyne Plasma Gun Model SG1B and for the lower melting metals, Metco Wire Flame Gun Model 4E and Metco Flame Gun Model ST.

The most advantageous metal or metals for use in forming the upper surface of the cathode support plate in any particular instance will depend upon a number of factors, including price, the nature of the base plate in instances where a coated support plate is to be used, and the availability of suitable equipment for applying a coating of the particular metal or metals selected. Based on present experience, a steel cathode support plate having its upper surface coated with a film of molybdenum from 5 to 10 mils thick is optimum, but excellent results can also be obtained by using similar coatings of zirconium and vanadium on a steel base plate. Other suitable combinations include molybdenum on brass, a 5050 mixture of molybdenum and zirconium on steel, and titanium or hafnium on steel or iron.

Although the cathode support plate in accordance with this invention should have its upper surface formed primarily or entirely from titanium, zirconium, hafnium, molybdenum, vanadium, tungsten or a combination thereof, it is permissible for the surface to obtain small amounts, for example, up to about 20% by weight of another metal or metals either in metallic form or in the form of metal oxides. In most instances, the presence of metals other than those listed is not advantageous, but in some instances, the presence of a small amount of an activator material to increase the initial conductivity of the support plate surface is advantageous and, for example, for this purpose one can add a small amount, for example, from 1% to 20% and preferably from 2% to of a metal or metal oxide of the palladium or platinum families. Specific examples include 5% of metallic iridium and 10% ruthenium oxide.

Operation of mercury cathode cells embodying a cathode support plate in accordance with the present invention can be and preferably is conventional in all respects except that it is usually advantageous to set the cathodeanode spacing somewhat less than would otherwise be normal with the same type of cell and, for example, one can in most instances operate with the distance between the anodes and cathode from 5 to 25 percent less than would be normal with the same cell under the same operating conditions utilizing a conventional cathode support plate. This is particularly true in instances where problems with mercury butter have been encountered using conventional cathode support plates. Similarly, the current density can usually be advantageously somewhat higher than with the same type of cell utilizing a conventional cathode support plate and again, this is particularly true when the use of relatively low current densities has been required in the past because of problems with mercury butter formation. Both of these adjustments result in a more economical operation of the cell. There are no special precautions which must be exercised when using a cell embodying a cathode support plate in accordance with this invention except that care should be exercised to introduce the mercury utilized as a cathode onto the upper surface of the cathode support plate in as uniform a film as possible and to avoid excessive contact of the brine solution with the surface of the cathode support plate.

Having thus described my invention and several preferred embodiments thereof, What I desire to claim and secure by Letters Patent is:

1. In an electrolytic cell comprising at least one electrically conductive anode, an electrically conductive base plate disposed in underlying spaced relationship to said anode and inclined at an angle to the horizontal, means for introducing mercury onto the upper surface of said base plate in an area remote from the lowermost segment of said surface such that said mercury flows downwardly over said surface and in electrical contact therewith, means for operatively introducing an electrolyte between and in electrical contact with said anode and mercury being supported on said base plate, and means for applying an electromotive force between said anode and said base plate such that said base plate operatively serves as a cathode support and said mercury in electrical contact therewith serves as a cathode, the improvement which comprises a base plate having an upper surface formed at least predominantly of one or more metals selected from the group consisting of titanium, hafnium, vanadium, zirconium, molybdenum, and tungsten; said upper surface not being wet by mercury.

2. An electrolytic cell in accordance with claim 1 wherein said base plate comprises a ferrous metal plate having an upper surface coating consisting at least by weight of one or more metals selected from the group consisting of titanium, hafnium, vanadium, zirconium, molybdenum, and tungsten.

3. An electrolytic cell in accordance with claim 2 wherein said base plate comprises a steel plate having an upper surface coating of molybdenum.

4. An electrolytic cell in accordance with claim 2 wherein said base plate comprises a steel plate having an upper surface coating of zirconium.

5. An electrolytic cell in accordance with claim 2 wherein said base plate comprises a steel plate having an upper surface coating of titanium.

6. An electrolytic cell in accordance with claim 2 wherein said base plate comprises a steel plate having an upper surface coating of vanadium.

References Cited UNITED STATES PATENTS 3,464,911 9/1969 Cottam et al. 204-22O 3,400,067 9/1968 Shibata 204-250 X 2,597,545 5/1952 Taylor 204-250X 3,499,829 3/1970 Messner et al. 204250X HOWARD S. WILLIAMS, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R.