US3248311A - Manufacture of sodium - Google Patents

Description

April 26, 1966 J. M. WOOD MANUFACTURE OF SODIUM Filed March 29, 1962 FIG.

FIG. 2

United States Patent 3,248,311 MANUFACTURE OF SODIUM James M. Wood, Baton Rouge, La., assignor to Ethyl Corporation, New York, N .Y., a corporation of Virginia Filed Mar. 29, 1962, Ser. No. 183,444 2 Claims. (ill. 204-68) This invention relates to the art of producing metallic sodium and in particular to improvements in electrolytic cells, especially incells of the Downs type.

Metallic sodium is produced generally from amolten mixture of the chlorides of calcium and sodium in electrolytic cells of the Downs type (U.S. 1,501,756) or in modifications of these cells. These cells are characterized by having one or more bottom mounted vertically aligned cylindrical graphite anodes each of which is projected upwardly into a separate cylindrical opening within a unitary cathode assembly. The diameters of the anodes are smaller than the diameters of the cylindrical cathode openings so that the'combination of the surfaces of the graphite anodes and those of the cathode cylindrical openings produce annular openings which serve as electrolysis zones. Within each of these zones is provided a foraminous metal diaphragm located generally equidistant from the anode surface and the cathode surface and its purpose is to effectively separate the anode and cathode surfaces into compartments through which chlorine and metallic sodium are removed. The sodium is produced at the cathode surfaces and the chlorine at the anode surfaces and because of the difference in densities between these products, and that of the molten bath the products rise to the surface of the bath and are then collected.

To collect the products, chlorine and sodium, a collector is mounted above, though partially submerged within, the surface of the molten bath. The collector is provided with an outlet and outlet port above the anodes for removal of the rising chlorine gas and also with a manifold above the cathode surfaces for collection of the sodium, which is gradually removed to the cell exterior.

Unfortunately, cells of this type rarely ever achieve current efficiencies greater than about 83-86 percent and tremendous effort has been exerted over past years to achieve greater current efliciency. The reasons for these low efiiciencies however are not altogether known. Though not desiring to be bound by theory, the present invention is directed toward the virtual elimination of certain types of parasitic currents which are believed by the instant inventor to exist. His theory on the origination of certain types of parasitic currents is as follows.

The cell products, sodium and chlorine, though substantially physically separated are nevertheless in electrical contact both through the metal walls of the collector and through the cell bath. Thus, the metal walls from an electronic conductor and the cell b ath provide an ionic conductor. This arrangement constitutes at least in effect a battery which consumes sodium from the collector by anodic oxidation and simultaneously reduces chlorine on the opposite side of the wall of the collector, and also on the anode side of the diaphragms. But, moreover, the driving force for the reionization of sodium and chlorine is not only due to the nature of the chemical products and their electrical proximity but is even intensified by the electric field which exists between the anode and cathode itself. Thus, an additional driving force is provided by the series of equipotential lines which extend outwardly in a non-parallel direction from the anode and cathode. The net effect of these forces is that a certain amount of the electrical energy which is externally applied to the electrodes for decomposition of the sodium chloride is completely wasted. This type of parasitic current produced by this chemical battery arrangement is most unde- 3,248,311 Patented Apr. 26, 1966 ice sirable in that it lessens current efficiency, decreases production, and generally reduces cell performance.

Now, the direct object of this invention is to overcome the above and other disadvantages particularly as regards these types of parasitic currents within the present commercial process, or present commercial cell, and to provide for the more eflicient production of metallic sodium. Also, it is an object to provide a method for the reduction of galvanic corrosion of the metal parts of an electrolytic cell which separate sodium and chlorine. Further, it is an object to provide method and apparatus which will lengthen diaphragm life, improve cell performance by, inter alia, increasing production, by decreasing undesirable heating at the anode-cathode gap, and by lessening of the corrosion of metal parts. Yet further, it is an object to provide a new and improved diaphragm structure.

These and other objects are achieved in accordance with the present invention which advances the art by providing for the reduction of parasitic galvanic action by the use in cells of protective coatings applied upon metal surfaces or barriers which separate sodium from chlorine.

Pursuant to this invention, it has been found that parasitic currents of the type described, generally reducing current efficiency by about 2 percent or more, can be greatly reduced even as much as 99 percent by coating the metal surfaces separating the sodium and chlorine with a refractory insulating material. Materials specially suitable for this purpose are those refractory materials having a melting point above about 700 C. and properly classi- =fied as electrical insulators.

The most preferred refractory insulating materials for the practice of this invention are those refractory oxides (ceramics) such as beryllium oxide, aluminum oxide, magnesium oxide, zirconium oxide, mixtures of these oxides, and also such oxide combinations including in addition silicates and aluminates.

A particularly outstanding oxide, because of its very high chemical resistance to attack by sodium because of its high shock resistance and its high melting point, inter alia, is aluminum oxide.

A major portion of the current efiiciency loss due to parasitic currents results from the parasitic currents at the diaphragms themselves and therefore an especially preferred embodiment of this invention relates to an improved foraminous metal diaphragm for location within an annular electrolysis zone. The diaphragm comprises a cylindrical iron or steel wire structure, the individual gauze wires of which are impregnated or coated with a refractory electrically insulating material of the character described. The most particularly preferred coating however is also a refractory electrically insulating material selected from the group of compounds consisting of oxides of aluminum, beryllium, magnesium, zirconium, and mixtures thereof and also such compounds which include in addition aluminates and silicates.

These coatings when applied to diaphragm surfaces are particularly effective not only in reducing parasitic currents but also in reducing the current density of these members so that less metallic sodium is actually formed on the anode side of the diaphragm and less chlorine is formed on the cathode side of the diaphragm. This additional desirable effect is also of major importance and contributes to further increase the overall current efficiency of the electrolytic cell within which these coated diaphragm members are used.

The thickness of the refractory-insulating materials is preferably from about 0.005 to about 0.1 inch and even more preferably from about 0.01 to about 0.07 inch. A thickness of from about 0.01 to about 0.03 inch is found to be particularly useful.

The nature of the invention and.the problems solved by its use will be readily understood and appreciated by the following detailed description given with reference to the figures wherein FIGURE 1 is a cross-sectional elevation view of a sodium cell of the Downs type.

FIGURE 2 is a foraminous metal diaphragm of the type used in such cells.

Referring to FIGURE 1 is shown the principal parts of a cell installation. Thus, in this figure is shown a generally cylindrically shaped iron cell shell 10, the inside of which is lined with an insulating material 11. Within the confines of the shell is mounted a unitary cathode assembly 20. Extended upwardly from the floor of the cell is a graphite anode 30. The peripheral surface of the anode 30 is concentrically aligned with respect to the inner peripheral surface of the cathode assembly 20. Thus, the outer anode surface and the inner surface of the cathode assembly form the annular opening or electrolysis zone 21.

The unitary cathode assembly 20 is mounted within the cell shell 10 by projections 22 22 which are bus bars electrically insulated, through the insulator blocks 17, from the cell shell 10. An external current is applied to the cathode through these bus bars 22 22 and current is also applied to the anode 30 through bus bar 31.

Above the anode 30 and cathode assembly 20 is mounted a collector-diaphragm assembly, a portion of which assembly is submerged within the cell bath 46. This assembly is constituted by attachment of diaphragm 50 (shown in FIGURE 1 'by dotted lines) upon ports (details not shown) located upon the lower portion of an inverted dome-shaped collector 40. Within the collector is contained an inverted trough or manifold 41 within which sodium is collected for transmission through the vaned riser pipe 42 to the cell exterior. Within the dome portion 43 of the collector is passed chlorine which is sent via line 44 to the cell exterior for collection.

The foraminous diaphragm 50 is shown in more detail in FIGURE 2. The diaphragm consists generally of a ferrous metal gauze which is stapled or otherwise fastened together in cylindrical shape. Upon the top of the diaphragm is secured a metal hoop 51 by means of which the diaphragm can be attached to the collector 40.

To initiate operation, a mixture of salts including sodium chloride is charged into the electrolysis cell. Sodium chloride is, of course, an essential part of this mixture and a second salt, or salts, is added for lowering of the melting point. In present commercial practice, a mixture of calcium chloride and sodium chloride is used. Such mixtures are charged into the cell and electrolysis of this mixture initiated. Operating temperatures range generally from about 525 C. to about 700 C.; temperatures of from about 525 to about 625 C. being preferred.

As electrolysis proceeds, sodium is deposited upon the cathode surface and chlorine is deposited upon the anode surface of the electrolysis zone 21. Ideally, the deposited sodium will remain on the cathode side of the diaphragm 50 and will rise through the molten bath to be collected within manifold 41. On the other hand, chlorine will remain on the anode side of the diaphragm 50 of zone 21 and will rise through the bath to the dome 43. Even in this ideal state however, according to the inventors theory, parasitic currents are produced because of the metallic barriers which separate sodium and chlorine. Thus, the sodium metal within the manifold 41 and the chlorine within the dome 43 are separated by the metal walls 45 of the collector 40. Moreover, the metallic diaphragm gauze 50 located within the annular electrolysis zones 21 separates sodium from chlorine. These metallic separators, though substantially physically separating the sodium and chlorine, are nevertheless in electrical connection and give rise to parasitic currents. It is this parasitic galvanic action which the present invention almost substantially eliminates.

The following non-limiting examples are illustrative of the present invention.

Examples A series of l0 runs are made wherein both sides of ferrous metal diaphragms are coated with aluminum oxide of a thickness of 0.03 inch. These diaphragms in groups of 4 are simultaneously attached to a collector and inserted within an electrolysis cell of the type described except that 4 anodes are mounted within the cell to form 4 separate electrolysis zones. The electrolysis cell is operated at a temperature of 575 C. and contains a fused melt consisting essentially of 64 Weight percent calcium chloride and 36 weight percent sodium chloride. After 30 days of continuous operation, it is found that the average current efliciency is improved by 1.96 percent as contrasted with similar runs wherein the diaphragms were uncoated.

Current efi iciency is further improved when the chlorine side of the collector wall separating sodium and chlorine is spray coated with the aluminum oxide, a coat thickness of 0.07 inch being provided.

When the foregoing is repeated except that beryllium oxide, magnesium oxide and Zirconium oxide are used in lieu of aluminum oxide, current efficiency is also-significantly improved.

Barium aluminate, calcium zirconate and potassium aluminum silicate are also found to improve current efficiency when these materials are spray coated upon the metallic diaphragms and collector surfaces in thicknesses of 0.01, 0.05 and 0.1 inch, respectively.

It is apparent that the present invention is susceptible to considerable variations without departing from its spirit and scope. Thus, in essence, the invention relates to the combinations of using an electrically insulating refractory material as a coating for metallic surfaces to separate sodium and chlorine; such combinations reducing parasitic galvanic action. These combinations are applicable to any and all types of cells wherein sodium and chlorine are produced and wherein the sodium and chlorine are in electrical proximity.

As stated, the coating materials for the metal surfaces must be an insulator in the traditional sense and must be capable of withstanding high temperatures. Such materials are those having a conductance generally on the order of 10 ohmcm.- or less. The melting point of the material must be as high as about 700 C. but should preferably be greater, preferably on the order of 1000 C. and above. A most preferred and highly unique class of materials for this purpose are those refractory oxides and ceramics such as the oxides of beryllium, aluminum, magnesium, zirconium, and mixtures of these compounds and also including oxide combinations of these materials wherein silicates and aluminates are also present.

Other refractory insulating materials include, for example, the silicates and aluminates of alkali and alkaline earth metals to which have been added refractory oxides. Exemplary of such compounds are barium aluminate, strontium zirconate, magnesium zirconate, magnesium silicate, magnesium zirconium silicate and sodium aluminum silicate. Other simple and complex refractory materials are also suitable for the practice of this invention, especially those having melting points above about 700 C. which are properly classifiable as electrical insulators and are suificiently resistant to chemical attack from sodium and chlorine.

These coatings can be applied to metals in various ways though a highly preferred method is by spray coating. Pursuant to this method, the refractory oxide, a particularly unique material in this regard, is heated above its melting point, is volatilized, and then sprayed from a suitable container upon the metal surface to be coated. Upon coming in contact with this surface, the volatilized oxide condenses and adheres to the metal surface. Such refractory materials can also be applied by forming oxide slurries and dipping the metal to be coated therein. Upon baking, a suitably shock resistant coating is adhered thereto.

The preferred thickness of the coatings ranges from about 0.005 to about 0.1 inch but thinner or thicker coatings can be employed to provide some protection. Thus, a coating thinner than 0.005 inch can be applied though this is generally undesirable because of chemical attack, especially by metallic sodium. Thicker coats are generally undesirable also because they are less adherent than the thinner coats and leave some surfaces exposed and also shock resistance is decreased to some extent.

Thicker coats are also generally undesirable. For one reason, the thickness of the coating is also somewhat dependent on the mesh size of the wire gauze Where a diaphragm is to be coated. Thus, the thickness of the coating must be compensated for by the size of the openings desired. Thus, a suitable gauze size is generally from about 20 to about 30 (American standard) and if the coating becomes too thick, then obviously the gauze openings must be increased in area to compensate for this thickness.

Other obvious modifications can also be made.

Having described the invention, what is claimed is:

1. In a process for the preparation of sodium by the electrolytic decomposition of a fused sodium chloride bath conducted in a cell comprising anode means and cathode means positioned relative to each other to form an electrolysis zone wherein said means are separated by a foraminous diaphragm member to form an anode compartment in which chlorine is liberated and a cathode com- 6 partment in which sodium is liberated, the improvement comprising employing as said member a ferrous metal diaphragm member having an electrically insulating refractory coating covering the entire surface area of said diaphragm exposed to said fused bath whereby parastic electric currents are essentially eliminated.

2. The process of claim 1 further characterized by said electrically insulating coating being aluminum oxide.

References (Iited by the Examiner UNITED STATES PATENTS 1,907,984 5/1933 Kraner 204-181 2,150,289 3/1939 Moltkehansen 204-68 2,928,783 3/1960 Bacon 204129 2,965,552 12/1960 Gruber 204- 3,011,964 12/1961 Guillot 204295 3,022,244 2/1962 Le Blane et a1 204-295 3,098,802 7/1963 Beer 204295 3,102,085 8/1963 Edwards et al. 204295 FOREIGN PATENTS 305,022 4/ 1930 Great Britain.

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, WINSTON A. DOUGLAS,

Examiners.

B. JOHNSON, H. S. WILLIAMS, Assistant Examiners.

Claims (1)

1. IN A PROCESS FOR THE PREPARATION OF SODIUM BY THE ELECTROLYTIC DECOMPOSITION OF A FUSED SODIUM CHLORIDE BATH CONDUCTED IN A CELL COMPRISING ANODE MEANS AND CATHODE MEANS POSITIONED RELATIVE TO EACH OTHER TO FORM AN ELECTROLYSIS ZONE WHEREIN SAID MEANS ARE SEPARATED BY A FORAMINOUS DIAPHRAGM MEMBER TO FORM AN ANODE COMPARTMENT IN WHICH CHLORINE IS LIBERATED AND A CATHODE COMPARTMENT IN WHICH SODIUM IS LIBERATED, THE IMPROVEMENT COMPRISING EMPLOYING AS SAID MEMBR A FERROUS METAL DIAPHRAGM MEMBER HAVING AN ELECTRICALLY INSULATING REFRACTORY COATING COVERING THE ENTIRE SURFACE AREA OF SAID DIAPHRAGM EXPOSED TO SAID FUSED BATH WHEREBY PARASTIC ELECTRIC CURRENTS ARE ESSENTIALLY ELIMINATED.

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