US3464911A - Electrochemical apparatus - Google Patents

Description

. p 2, 1969. R. G.COTTAM ET AL 3,464,911

ELECTHOCHEMI CAL APPARATUS Filed July '7, 1965 United States Patent 3,464,911 ELECTROCHEMICAL APPARATUS Ronald Geoffrey Cottam, Runcorn, England, and Alan Roger Topham, Shawinigan, Quebec, Canada, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed July 7, 1965, Ser. No. 470,061 Claims priority, application Great Britain, July 29, 1964, 30,121/64 Int. Cl. (322:! 1/04; C01d 1/06; C011) 13/04 US. Cl. 204-420 Claims ABSTRACT OF THE DISCLOSURE There is provided an apparatus for decomposing amalgam withdrawn from a mercury cathode eletrolysis cell by contacting with an aqueous solution and an amalgam decomposition material, and wherein the improvement comprising an assembly for flowing the amalgam over a large surface area and in contact with an amalgam decomposition material. The assembly comprises at least one first layer of an amalgam decomposition material and at least one second layer of an intersticed metal, the said first and second layers being in physical and electrical contact. The assembly is disposed in a generally vertical plane in a suitable vessel for holding the aqueous solution and the upper surface of the said first layer has a generally horizontal trough therein for distributing and overflowing the amalgam onto the said second layer. The said second layer is so intersticed that the amalgam flows down the second layer largely as a film and through the interstices.

This invention relates to an improved apparatus and process for carrying out the decomposition of a metal amalgam.

The apparatus finds particular application in carrying out the reaction between water and an alkali-metal amalgam, whereby an aqueous solution of the metal hydroxide is formed and hydrogen is liberated.

In the production of chlorine by the electrolysis of a solution of an alkali-metal chloride using a mercury cathode the alkali metal liberated at the cathode forms an amalgam with the mercury, this amalgam being continuously withdrawn from the electrolysis cell and passed into a second compartment known as an amalgam decomposer (or denuder) wherein the amalgam reacts with water, forming a solution of the alkali-metal hydroxide and liberating hydrogen. In one conventional type of amalgam decomposer the amalgam flow through a steel trough containing graphite (which acts as an amalgam-decomposing material) in the form of blocks. In another conventional type of amalgam decomposer the amalgam flows downwards through a tower packed with pieces of graphite. A disadvantage of both types of amalgam decomposer is that very large quantities of mercury must be kept in circulation in the system.

It has also been proposed, in British Patent No. 543,- 539, to react an alkali-metal amalgam with an aqueous solution by allowing the amalgam to flow down the surfaces of a plurality of closely-spaced metal wires immersed in the aqueous solution.

According to the present invention there is provided an improved apparatus, suitable for carrying out the reaction between Water or an aqueous solution and an alkali-metal amalgam, comprising an intersticed layer of a metal wettable by but not attacked by the said amalgam in combination with a layer of amalgam-decomposing material, the apparatus being adapted to allow distribution of amal- "ice gam within the intersticed layer of metal and to provide electrical contact between the amalgam and the amalgamdecomposing material.

The intersticed layer may be built up from a plurality of perforate or reticulate sheets but it is preferred to use one or more sheets which are themselves intersticed. It is especially preferred to use one or more sheets of expanded metal. (By the term expanded metal is denoted a sheet which has been slit or perforated and stretched to form a mesh.)

The preferred amalgam-decomposing material is graphite, although other materials which promote decomposition of the amalgam may also be used, particularly materials having a low hydrogen overvoltage, for example titanium, zirconium, vanadium, chromium, molybdenum, tantalum, tungsten, or alloys containing one or more of these elements.

The metal wettable by but not attacked by amalgam may be, for example, mild steel.

The amalgam may be decomposed by reaction with water or with an aqueous solution, for example a solution of an alkali-metal hydroxide. It is convenient to immerse the apparatus in a vessel into which water or an aqueous solution is continuously fed and from which alkali-metal hydroxide solution and the decomposed or partly-decomposed amalgam are continuously removed.

In its simplest form, the apparatus employs a unit made up of a single layer of the amalgam-decomposing material in conjunction with a single layer of the metal wettable by the amalgam. Arrangements employing more layers may also be used, however. Thus a single layer of amalgam-decomposing material may be used between two layers of the metal wettable by amalgam, or a single layer of the metal may be used between two layers of the amalgam-decomposing material, to constitute a unit. A plurality of units, which may be the same or different, may be combined in a single apparatus for convenience of operation. Such multiple units may be arranged to work in parallel in order to increase throughput capacity and/or in series to increase the degree of decomposition achieved. Whatever arrangement is adopted, space is left to allow escape of hydrogen from the vicinity of the amalgam-decomposing material.

Electrical contact between the amalgam and the amal gam-decomposing material may be achieved in a variety of ways. Thus, for example, electrically-conducting elements may connect the layer of metal and the amalgamdecomposing material in such a way that the amalgam flows over the conducting elements. Alternatively, electrically-conducting elements may be embedded in the amalgam-decomposing material and project into the space through which the amalgam flows. In another embodiment the layer of metal and the layer of amalgam-decomposing material are separated by an electrically-conducting material in the form, for example, of wires, strips or a mesh.

In preferred embodiments, however, the layer of metal is formed in such a way that it makes contact at one or more points with the amalgam-decomposing material while amalgam is free to flow through the interstices of the metal layer.

It is especially advantageous to use a sheet of expanded metal. When such a sheet is placed in contact with the surface of the amalgam-decomposing material, electrical contact is made at a plurality of points; an amalgam which wets the metal flows, largely as a film,'between the amalgam-decomposing material and the three-dimensional surface of the expanded metal sheet. The structure of the expanded metal sheet ensures even distribution of the amalgam and allows the aqueous solution to percolate through the sheet. It is especially preferred to orient the expanded metal sheet so that the hydrogen evolved is directed away from the vicinity of the amalgam-decomposing material and escapes via the interstices of the expanded metal sheet.

The operation of the apparatus may be carried out at any desired temperature by adjustment of the temperature of the feeds of water and of amalgam and by applying heating or cooling means as appropriate. The temperature of operation, as is well known, is a factor in determining the concentration of aqueous solution of caustic alkali which can be produced from the amalgam. It is preferred to operate at a temperature in the range 80 C.150 C., although higher or lower temperatures may be used.

The invention is illustrated, but not limited, by the embodiment which will now be described with reference to the accompanying drawing, which is a diagrammatic representation of an embodiment comprising a single graphite plate in conjunction with a single sheet of expanded mild steel.

The graphite plate 1 /2 inch thick and having faces 2 inches by 2 /2 inches) is bolted between a sheet of expanded mild steel 2 and a mild steel plate 3 which serves as a support for the unit. Sheet 2 is a sheet of expanded metal (Expamet No. 400, made of 22 gauge mild steel, parallel strand mesh, with 94 apertures per foot in the horizontal plane and 23 apertures per foot in the vertical plane; Expamet is a trade name). The top surface of the graphite plate 1 contains a trough in which is placed a mild steel rod 4.

The assembly is immersed in an aqueous solution contained in the outer vessel 5, which is provided with an inlet 6 for water, an outlet 7 for the aqueous solution and an outlet 8 for amalgam. The aqueous solution is agitated by a stirrer (not shown in the diagram).

Amalgam is fed in through the tube 9 and overflows from the trough in the plate 1, the evenness of distribution being assisted by rod 4 and flows downwards by way of the expanded mild steel sheet 2 and flows out through tube 8. Hydrogen evolved is removed by way of outlet 10.

The following examples illustrate the use of this embodiment of the apparatus.

EXAMPLE 1 A sodium amalgam containing 0.31% by weight of sodium was fed into the apparatus shown in FIG. 1 at the rate of 153 ml. per minute. Water was fed in through tube 6 at such a rate that the aqueous solution overflowing through tube 7 contained 50% by weight of sodium hydroxide. The aqueous solution in vessel 5 was maintained at 120 C. The amalgam flowing out through tube 8 contained 0.26% by weight of sodium.

EXAMPLE 2 A sodium amalgam containing 0.06% by Weight of sodium was fed into the apparatus shown in FIG. 1 at the rate of 149 ml. per minute. Water was fed in through tube 6 at such a rate that the aqueous solution overflowing through tube 7 contained 40% by weight of sodium hydroxide. The aqueous solution in vessel 5 was maintained at 100 C. The amalgam flowing through tube 8 contained 0.03% by weight of sodium.

What we claim is:

1. In an apparatus for the manufacture of an aqueous solution of alkali metal hydroxide by reaction of an alkali metal amalgam with an aqueous medium at a metal surface, comprising a metal structure that is wettable by the amalgam but is not attacked thereby and an amalgam decomposing material for the liberation of hydrogen, which is electrically connected to the said metal structure within a container provided with inlets for amalgam and an aqueous medium and with outlets for amalgam, alkali metal hydroxide solution and hydrogen, the improvement being characterized in that the metal structure wettable by amalgam is in the form of an intersticed sheet of the metal and the decomposing material is a sheet of graphite or one of the metals titanium, zirconium, vanadium, chromium, molybdenum, tantalum, tungsten, or an alloy of at least one of these metals, and is in contact at a plurality of points with the intersticed metal sheet.

2. An apparatus according to claim 1 wherein the intersticed metal sheet is a sheet of expanded metal.

3. An apparatus according to claim 1 wherein the intersticed metal sheet is made of steel.

4. An apparatus according to claim 1 wherein the intersticed metal sheet is a composite of a plurality of metal sheets clamped together.

5. In an apparatus for decomposing amalgam that has been withdrawn from a mercury cathode electrolysis cell for producing chlorine, and wherein the amalgam is contacted with an aqueous solution and an amalgam decomposition material in a vessel for holding the aqueous solution and said vessel being provided with entry and exit means for the hydrogen produced during decomposition of the amalgam, the improvement comprising an assembly disposed in said vessel having at least one first layer of an amalgam-decomposing material and at least one second layer of an intersticed metal wettable by amalgam, the said first and second layers being in physical and electrical contact and said first and second layers being disposed in a generally vertical plane, said first layer having a generally horizontal trough in the upper surface thereof for distributing and overflowing the amalgam onto the said second layer, an amalgam entry means in the said vessel disposed above said trough and an amalgam exit means in the said vessel disposed below the said assembly, and the interstices of the said second layer being such that the amalgam flows down the second layer largely as a film and is free to flow through the interstices of the second layer and in contact with the said first layer.

6. An apparatus as claimed in claim 5 wherein the intersticed layer of metal comprises one or more sheets which are themselves intersticed.

7. An apparatus as claimed in claim 5 wherein the intersticed layer of metal comprises one or more sheets of expanded metal.

8. An apparatus as claimed in claim 7 wherein the intersticed layer of metal is a layer of steel.

9. An apparatus of claim 8 wherein the amalgam decomposing material is selected from one or more metals of the group consisting of titanium, vanadium, chromium, molybdenum, tantalum and tungsten and from graphite.

10. An apparatus as claimed in claim 9 wherein the amalgam-decomposing material is graphite.

References Cited UNITED STATES PATENTS 709,971 9/1902 Edser et al. 204--248 790,922 5/1905 Roepper et al. 204-220 918,370 4/1909 Rink 204220 1,336,281 4/ 1920 Cataldi 204-248 JOHN H. MACK, Primary Examiner H. M. FLOURNOY, Assistant Examiner US. Cl. X.R. 20498, 129

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