US3002914A - Preparation of electrodes for electrolysis of aqueous solutions by the mercury process - Google Patents

Description

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It is known that the support for the mobile cathode in horizontal mercury cells is constituted in a large number of cases by a steel or iron plate. In the case of mercury cells of the vertical type, with or without a support for the mobile cathode, iron is usedpractically exclusively. It is further known that the metal support must be perfectly amalgamated in order to ensure the best possible circulation and distribution of the mercury and in order to avoid premature corrosion of the metal support by the electrolysis. The Wetting of the metallic surface thus attains considerable importance especially when the cell is put into service. It has been proposed to cover the cathode support (German Patent No. 692,- 954), especially if it is of iron, with a very thin layer of another metal such as tin, copper, lead or the like. This metal deposit has as its object the favouring of the wetting by the mercury, the met-a1 deposited having the well known property of easy amalgamation.

This process is, however, not without disadvantages. The metals thus carried dissolve in the mercury with the result that the iron supports deteriorate rather quickly and that after the cell has been stopped it is necessary to renew the amalgamatable covering; furthermore, the metal which has passed into solution in the mercury is carried by the latter towards the decomposition apparatus for the amalgamation and it deposits on the graphite of the cathodes of this apparatus and thus rapidly reduces their activity. Moreover the couples formed by the covering metals and the iron which they cover can bring about corrosion in the latter. The object of the present invention is to avoid these disadvantages. The invention is based on the observation that the quality of the iron used as supporting metal for the mercury cathode of the electrolytic cell has a marked influence on the ease of amalgamation of this support; thus, electrolytic iron amalgamates more easily and maintains its amalgamation far longer than in the case of ordinary iron or steel.

The invention consists in a process for making supports for mobile cathodes of electrolytic cells of the vertical type which is characterised in that there is electrolytically deposited on a metallic plate, preferably of iron, a layer of pure iron, this layer being several tenths of millimetre in thickness. The added electrolytic iron of which the purity may be very high does not dissolve in the mercury as the readily amalgamated metals do. This addition to the base metal is carried out once and for all since the life of the layer is very long compared with that of the other metals referred to. Electrolytic deposition of iron, correctly carried out, provides a very finely roughened surface which, particularly in the case of the vertical mobile mercury cathode cells, retains mercury and amalgam, thus assuring efficient and regular operation of the apparatus.

To produce such an addition of iron for example with a thickness of up to 0.9 millimetre a known bath is used containing for example 300 grams of FeCl .2l-I O and 150 grams of CaCl per litre of solution, having a atent C pH value of 1.5 to 2 with or without further additions; the current density being between 4 and 4.5 amperes per square decimeter of cathode surface and the temperature being of the order to C.

The invention is not limited to the formation of a mercury cathode support with a uniformly polished surface. It comprises likewise the manufacture of grooved supports and embossed supports such as those which form the object of U.S. patent application 479,746 filed January 4, 1955, in the names of Charles Deprez and lean Clement, now Patent No. 2,849,393.

According to a further feature of the invention a network of lines is traced on the supporting plate with the aid of an insulating varnish and an electrolytic deposit of iron is obtained on the surface not covered by varnish, up to a thickness of some tenths of millimeter after which the varnish is dissolved away from the grooves thus formed.

It is clear that the varnish must be resistant to the physical and chemical influences to which it will be submitted.

In the same way one can obtain embossings of any shape constituted by the added electrolytic iron.

The supports thus formed present, apart from the advantage 'already stated, that of a lower cost of replacement than in the case of a grooved cathode prepared by mechanical means. Furthermore, it is possible to obtain a groove-design as complicated as can be desired whereas this is difficult to achieve by mechanical means. Likewise one can treat surfaces of any size or shape.

The deposits of electrolytic iron can be obtained in the electrolytic cell itself. To this end it is suflicient to replace the graphite anodes by iron anodes and to replace the electrolyte by an appropriate solution having the composition indicated above. Operating in this way permits economy in the use and with special respect to special installations which are generally undesirable.

I claim:

1. In an electrolytic cell having a mobile mercury cathode, a support for said mercury comprising a metallic base plate having upon its mercury-contacting surface an electro-deposited coating of soft iron having a thickness up to 0.9

2. The apparatus of claim 1, wherein said electrodeposited coating covers limited areas of said base plate, thereby forming a grooved pattern on said plate.

References Cited in the file of this patent UNITED STATES PATENTS with a Mercury Cathode, Field Information Agency Technical, June 17, 1946, page 7.

Claims (1)

1. IN AN ELECTROLYTIC CELL HAVING A MOBILE MERCURY CATHODE, A SUPPORT FOR SAID MERCURY COMPRISING A METALLIC BASE PLATE HAVING UPON ITS MERCURY-CONTACTING SURFACE AN ELECTRO-DEPOSITED COATING OF SOFT IRON HAVING A THICKNESS UP TO 0.9 MM.