US3535223A - Electrolysers,particularly for chlorine-gas production - Google Patents

Description

' Oct. 20, 1970 J, BAEKLUND ETAL ELECTROLYSERS, PARTICULARLY FOR CHLORINE-GAS PRODUCTION Original Filed May 5, 1964 3 Sheets-Sheet l Oct. 20, 1970 BAECKLUND EI'AL 3,535,223

ELECTROLYSERS, PARTICULARLY FOR CHLORINE-GAS PRODUCTION 3 Sheets-Sheet 2 Original Filed May 5, 1964 Fig.4

.IIHII! Oct. 20, 1970 BAECKLUND EIAL 3,535,223

ELEGTROLYSERS, PARTICULARLY FOR CHLORINE-GAS PRODUCTION 5 Sheets-Sheet 3 Original Filed May 5, 1964 n n UJJJJJJJJJ JJQ Fig.7-

United States Patent 3,535,223 ELECTROLYSERS. PARTICULARLY FOR CHLORINE-GAS PRODUCTION Johannes Baecklund and Erik Reinhold Olson, Bro,

Timra, Sweden, assignors to Avesta Jernerks Aktiebolag, Avesta, Sweden Continuation of application Ser. No. 365,129, May 5, 1964. This application Oct. 6, 1969, Ser. No. 866,096 Claims priority, application Sweden, May 6, 1963, 4,948/63 Int. Cl. B01k 3/00 US Cl. 204-275 9 Claims ABSTRACT OF THE DISCLOSURE This invention pertains to a novel construction for electrolytic cells, and in particular to a novel form of anode that has a cavity through which the electrolyte may pass into the inter-electrode space.

This is a continuation of application Ser. No. 365,129, filed May 5, 1964, now abandoned.

For the electrolysis of aqueous solutions of alkali chlorides, it is common practice today to use graphite as the anode material. In spite of the fact that graphite is resistant to chemical attacking, and although its quality has been improved more and more, the anode blocks are subject to a more or less rapid erosion in the course of the electrolysis. This depends primarily on the oxidation of both graphitised and ungraphitised carbon which takes place due to anodic side reactions. This involves the drawback that bothelectrolyte and products will be contaminated by graphite sludge and carbon dioxide and that the inter-electrode spacing will be increased.

In chlorine-alkali electrolysis using diaphragm cells, this means contamination of the chlorine gas by about 1% CO and certain organic compounds formed from, inter alia, carbon, oxygen, hydrogen and chlorine, contamination of the alkali-salt solution mixture by extremely minute graphite particles, choking of the diaphragm with sludge, increased voltage and frequent electrode replacements. All these factors involve increased expenses. The most important detrimental factor, however, is the increased inter-electrode spacing which increases the energy consumption.

In the chlorine-alkali production carried out in electrolysers operating with a horizontal mercury cathode, similar and additional drawbacks are encountered. In these cells, however, there is no diaphragm to capture the main portion of the graphite slude, so that this sludge will have to be carried away from the cell with the electrolyte. It tends to deposit within the cell and to disturb the uniformity of the flowing mercury layer. Larger graphite grains may precipitate into the amalgam layer, and in both cases the graphite residues cause the chlorine gas to be contaminated by hydrogen gas. To eliminate the increase of the inter-electrode spacing, the top covers of the mercury Cells must be constructed so as to enable the anodes to be lowered at the same rate as the anode erosion is proceeding. These arrangements and the labour involved in adjustments of the inter-electrode spacing increase the cost of the equipment and products.

The above-mentioned drawbacks involved in the use of graphite anodes have caused the technicians of these industries to desire a different anode material. In the earliest period of chlorine-alkali electrolysis, platinum was used in many cases, but rather soon its price became economically prohibitive to its continued use. Some years ago titanium was taken into use as a constructional material within the chlorine-alkali industry. Its extremely high resistance to wet chlorine and to chlorine-saturated ice salt solution very soon gave rise to the idea of using this metal as a carrier of a very thin layer of platinum and of constructing from these two materials electrodes to be used, primarily, for chlorine-alkali electrolysers.

The invention has for its object to provide an improved electrolyser, particularly for the production of chlorine gas, in which the anodes, at least, are made of metal. Metallic electrodes, by suitable design, may be made extraordinarily rigid while using a comparatively small amount of metal, and this rigidity in it's turn will enable short inter-electrode spacings to be chosen, to thereby maintain the energy consumption at a low level. However, short inter-electrode spacings involve difficulties in respect of the possibility of ensuring a uniform supply of fresh electrolyte to the narrow space between the anode and cathode. In accordance with the invention, this problem has been solved by forming the metallic anodes with cavities for receiving fresh electrolyte and from which cavities the fresh electrolyte is passed on into the interelectrode space through suitably distributed outlet openings.

The formation of the metallic anodes, in accordance with the invention, as distributing elements for fresh electrolyte Will cause the fresh electrolyte to be distributed uniformly over the entire surface of the anode, thereby minimizing the electrical resistance of the electrolyte layer between anode and cathode, and keeping the energy losses at a low level. Further, the dwelling time of the electrolyte in the portion of the system where the electrolysis takes place will be as short as possible, thereby reducing the effects of any undesirable side reactions. During its passage through the hollow anode, which is heated by the current losses, the electrolyte will be pre-heated, which is of advantage, and at the same time any dangerous local overheating of the anode is avoided due to the cooling effect of the electrolyte.

Where electrolysers having horizontal metal anodes are concerned, the anodes are preferably made in the form of shallow boxes in which the bottom wall forms the anode plate, providing in the bottom wall a plurality of outlet openings through which the fresh electrolyte supplied to each box separately will enter the space between the electrodes. In electrolysers having vertically extending electrodes, the fresh-electrolyte receiving cavities in the metallic anodes may be constituted by vertically extending electrolyte supply ducts communicating through outlet openings with the inter-electrode space.

The invention will now be described more in detail in conjunction with the accompanying drawings, in which:

FIGS. 1, 2 and 3 illustrate in top-plan view, side elevation and in cross section along line IIIIII of FIG. 1, respectively, the invention in its application to an electrolyser having a horizontally extending mercury cathode;

FIG. 4 is a perspective view on a larger scale of one of the metallic anodes;

FIG. 5 is a cross section through a diaphragm cell for chlorine-alkali electrolysis equipped with metallic anodes formed in accordance with the invention;

FIGS. 6 and 7 show, in top-plan view with the topcover removed, and in longitudinal section along line VII-VII in FIG. 6, respectively, an electrolyser having vertically extending metallic electrodes.

In the embodiment shown in FIGS. 1 to 4, the electrolyser vessel is in the form of an elongated trough 10 having a flat bottom 11 made of iron and walls 12 and 13 of ebonite-covered steel. The longer side walls 12 have been reinforced mechanically by being formed as channel members placed on an edge. The cathode is formed of a layer of mercury 14 flowing slowly along the bottom of the trough, the mercury being supplied at one end of the trough and leaving the latter through a liquid-seal at the opposite end. The metallic bottom 11 of the trough is connected by a lead to the negative terminal of an electrical power supply.

From the top edges of the electrically insulated side walls 12 of the electrolyser vessel a plurality of metallic anodes 16 are suspended in spaced relation, these anodes being formed in accordance with the invention. Each anode consists of a shallow, rectangular box made of titanium and having flat end and side walls 17 and 18 and a corrugated bottom wall 19. The end walls 17 are formed at their top edges with bent-out lugs 20 serving to support the anode box on the trough edges. At one end wall the length of the lug is extended to form a terminal strip for connection to a bus bar 21 which, in its turn, is connected to the positive terminal of the power supply. Numetal 22 designates a close-fitting top cover for the electrolyser trough. The cover 22 is formed with an opening having secured therein a pipe socket 23 adapted to be connected to an outlet pipe for the chlorine gas formed.

As clearly visible particularly in FIG. 4, the corrugations formed in the bottom wall 19 of each anode box 16 extend in the longitudinal direction of the electrolysis trough 10 and consist of depending ridges 24 spaced by relatively narrow channels 25. The bottom surface of each ridge 24 is slightly curved downwards and is coated by a thin layer 26 of platinum. The platinum plating is extended for a short distance upwards over the vertically extending bottom surfaces defining the channels. Extending along the top of the row of anode boxes 16 is an open channel 27 for supplying fresh electrolyte. This channel is formed directly above each anode box with a bottom opening 28 through which fresh electrolyte is continually supplied into the box, and in the bottom wall of the box a row of openings 29 is provided near one of the side walls, one opening in each ridge, and through said openings the electrolyte is passed into the electrolysis trough 10 in a uniform pattern of distribution throughout its transverse width, whereby the electrolyte will be uniformly distributed over the anode surface. The electrolyte supplied, after leaving the space between the electrodes, will flow slowly in the longitudinal direction of the trough through the passageways formed adjacent each side wall, and will ultimately reach an overflow at one end wall of the trough.

The respective anode boxes may be made in any desired size. In an actual case their dimensions are as follows: length 70 cm., width 24 cm., and depth 10 cm.

The electrolyte consists of an aqueous solution of alkali chloride. When applying a suitable voltage between the anodes and the cathode, the ions of alkali metal will migrate through the electrolyte to the mercury cathode 14 where they will amalgamate with the mercury. The chlorine ions will migrate upwards to the active anode surfaces, i.e. to the platinum-plated bottom surfaces of the ridges 24, where they will form minute bubbles of chlorine gas. Due to the static pressure of the electrolyte and the curvature of these surfaces, the gas bubbles will move rapidly over the surfaces and upwards into the channels where they will form a more or less continuous gas cushion in the upper portion of the respective channels and will flow along the channels to the sides of the anode box where the chlorine gas will bubble up through the electrolyte and leave the free upper liquid surface and enter the gas collecting space beneath the top cover 22. From this space the gas will be passed out through the outlet 23 to be collected and disposed of in any suitable manner.

By corrugating the bottom wall 19 of the anode boxes 16 in the manner shown, a rigid horizontal metal anode has been provided, where only a very small portion of its total surface area is not electrically active, and where, in spite of this fact, no large and continuous gas bubbles will form on the bottom surface of the electrode and detract from the effectivity of the electrolyser. This has been effected by creating the gas collecting spaces uniformly distributed throughout the electrode surface and represented by the impressed bottom channels 25. It is readily possible to form the corrugations so as to cause 70 to of the horizontally extending anode surface area to be electrolytically active.

In dimensioning the electrode boxes 16, it should be ensured, in the first place, that the box will be rigid with the active anode surfaces disposed accurately in one common plane, thereby maintaining equal distance between all parts of the anode surface and the plane mercury cathode level, and thus a substantially uniform current density throughout the anode surface. It is of particular importance to impart to the side walls 18 of the box a sufficient cross sectional area to cause the voltage drops to be negligible and to enable substantially the same potential to be maintained in every point of the anode surface.

It is of extremely great importance to the useful service life of the anodes that the platinum layer be applied to the carrier metal, i.e. titanium, in a technically correct manner so as to cause the platinum layer to adhere satisfactorily to the carrier metal, or backing, without any scaling-off tendency. It should be noted, however, that if, for some reason or other, the platinum layer should be subject to injury, an electrically insulating oxide skin will form very rapidly on the exposed titanium surface, and that such oxide skin will prevent any further erosion of the titanium metal. Such a protective oxide skin, of course, will also form on all surfaces which are initially uncoated with platinum.

Referring now to the electrolyser shown in FIG. 5, the trough 30 containing the electrolyte (alkali chloride in aqueous solution) is assumed to be made of metal throughout. Mounted on the trough bottom are a plurality of iron cathodes 31 disposed in side-by-side relation and spaced from each other by a certain clearance, and suspended above each cathode from the top edges of the trough and with insulating strips 32 interposed therebetween is an anode box 33 of the same design as that shown in FIG. 4. A diaphragm 34, consisting, for instance, of Teflon foil, is supported directly on the active top surfaces of the cathodes 31. In the course of the electrolysis the chlorine ions migrate to the bottom surface of the bottom walls of the anode boxes 33 and the chlorine gas is passed through the channels in the box bottoms to the space above the free upper surface of the electrolyte, from which space the chlorine gas is passed on to any suitable point of collection or disposal. The presence of the gas collecting channels impressed in the bottom walls of the anode boxes, as before, will prevent any deleterious collection of chlorine gas beneath the horizontally extending active surfaces. The ions of the alkali metal migrate through the diaphragm 34 to the top faces of the cathodes 31 where the alkali metal immediately reacts with water while forming a hydrate which is dissolved in the electrolyte, and hydrogen gas. The hydrogen gas is collected in a downwardly open channel 35 on the underside of the diaphragm 34 and is carried away through this channel. In order to prevent any collection of hydrogen gas on the bottom surface of the diaphragm 34, it is preferable to dispose the anodes and cathodes with their active surfaces sloping from one side wall of the electrolysis trough towards the opposite side wall, as indicated in FIG. 5. Fresh electrolyte is continuously supplied in a manner similar to that of the electrolyser shown in FIGS. 1 to 4, and at the same time an equal, amount of electrolyte contaminated with alkali hydrate is discharged from the space below the diaphragm.

FIGS. 6 and 7 diagrammatically show an electrolyser of a modified design. The electrolysis trough 36 is made entirely of iron, and projecting upwards from the trough bottom are vertically disposed plate-shaped iron cathodes 37 in uniformly spaced parallel relation and distributed along the length of the trough. The anodes, being made of titanium, are in the form of vertically extending, flattened tubes 38 which are closed at their bottom ends where they terminate at a certain height above the bottom of the electrolysis trough. As shown in the drawing, between any two adjacent cathode plates 37 a plurality of such tubes are disposed in laterally closely spaced interrelation, whereby the tubes together form a substantially continuous plate-shaped anode. As an alternative, the anode may be formed by one single tube or by a smaller number of tubes. On their surfaces facing the cathodes 37 the anode tubes 38 are coated with a layer 39 of platinum, and are also formed with suitably spaced side openings. Fresh electrolyte is continually supplied to the trough through the tubular anodes 38 provided with outlet openings, and spent electrolyte is continually discharged over an overflow, not shown, at one end wall of the trough. Owing to the vertical arrangement of anodes and cathodes, the chlorine gas will show no tendency of collecting at the platinum-plated active surfaces of the anodes. The box-sectioned construction of the anodes and the utilization of the interior of the box for supplying the electrolyte through openings made in the box walls will ensure a uniform supply of fresh electrolyte to all active electrode surfaces.

If it should be deemed to be desirable, the anodes may be fixed relative to the adjacent cathodes by means of electrically insulating and chemically resistant spacers made of polytetrafluoroethylene, for example. This would enable the assembly of a greater or smaller number of electrode groups into compact packs or elements, thereby reducing the floor area required for the installation.

The concept of supplying the electrolyte through vertically disposed anodes in the form of perforated tubes or boxes, of course, may be applied not only to electrolysers having plate-shaped electrodes, but also to electrolysers of the type Where the electrodes are constituted by concentrically nested cylinders. In this case each anode may be formed with one single passageway, or with a plurality of passageways, for receiving and supplying the electrolyte.

Obviously, the invention may be applied to electrolysers having anodes made of other chlorine-resistant metals than titanium, and having coatings of a metal other than platinum. Such carrier or backing metal", however, similarly to titanium, should be one adapted to exert an anodic barrier effect, for instance by the formation of an electrically insulating surface skin as a result of the electrolysis. Such metals are, for example, niobium (columbiurn), tantalum, tungsten and zirconium. As the elec trically conductive surface coating, besides platinum, also rhodium, iridium and palladium may come into question. However, at the present marketing prices, it would be preferable, as a rule, to use titanium as the carrier or backing metal and platinum as the electrically conductive surface coating.

Although the invention has been described hereinbefore in its application to electrolysers for processing aqueous solutions of alkali chlorides, it could, of course, to equal advantage be applied to other electrolytical processes which can be carried out by the use of metallic electrodes. As an example of such processes, the electrolytical production of perborates may be mentioned, in which case no gas development is aimed at, but where a uniform supply of fresh electrolyte according to the present invention could, to advantage, be employed in the electroplating field, as well. The particular electrode design concerned is also of value in applications where the electrolyte or the resulting products are non-erosive so that no special requirements as to chemical resistance need be placed on the electrode metal.

What is claimed is:

1. An electrolytic cell which includes at least one anode, and

at least one cathode the improvement which comprises:

(a) at least one of said metallic anodes being a nonporous metallic anode,

(b) each metallic non-porous anode having bottom and side walls that are shaped to form a cavity that will create at least a limited reservoir of a liquid upon the introduction of fresh electrolyte,

(c) each metallic non-porous anode having a liquid inlet opening into said cavity for the introduction of fresh electrolyte, to the electrolytic cell, and

(d) each metallic non-porous anode being provided with at least one liquid outlet for the electrolyte that is introduced into said cavity, said outlet communicating with the inter-electrode space between said anode and said cathode.

2. An electrolytic cell as set forth in claim 1 having a plurality of horizontally extending metal anodes, said metal anodes being constructed in the form of shallow boxes, each box having a bottom wall that is corrugated and forms the anode plate, the bottom wall of each box also being provided with a plurality of outlet openings through which the electrolyte received in the box will enter the space between the electrodes. 3. An electrolytic cell as set forth in claim 2, in which the bottom wall of each box is in the form of a corrugated anode plate having its corrugations extending longitudinally of the electrolytic cell, and said outlet openings are made in the ridge portions of the corrugated anode plate that are closest to the cathode and suitably close to one of the side walls of the box extending transversely to the corrugations.

4. An electrolytic cell according to claim 2 wherein a plurality of cathodes are disposed in side-by-side relation beneath said metallic anodes, said cathodes being spaced from each other by a certain clearance.

5. An electrolytic cell according to claim 4 wherein a diaphragm is supported directly on the active top surfaces of said plurality of cathodes.

6. An electrolytic cell according to claim 5 wherein said diaphragm comprises polytetrafluoroethylene foil.

7. An electrolytic cell according to claim 5 wherein said diaphragm is sloping and a channel is provided adjacent one side of the diaphragm for collection of gas which forms below said diaphragm.

8. An electrolytic cell according to claim 1 wherein said metallic anodes are generally tubular in configuration, each metallic anode is disposed between two cathodes, and the tubular walls of the anodes contain outlets which permit fresh liquid electrolyte introduced into the cavity of each anode to pass through the anode into the interelectrode space.

9. An electrolytic cell according to claim 8 wherein there are a plurality of anodes arranged vertically in sideby-side relationship and wherein each metallic anode has a generally rectangular cross section.

References Cited UNITED STATES PATENTS 1,003,456 9/1911 'Hazard-Flamand 20426O 1,074,549 9/1913 Henkel et al 204260 1,575,627 3/1926 Heinze 204-278 2,000,815 5/1935 Berl 204--9 2,273,795 2/1942 Heise et a1. 2041.06 XR 2,643,223 6/1953 Notvest 204 3,103,473 9/1963 Juda 2041.06 XR 3,310,482 3/1967 Bon et al. 204219 FOREIGN PATENTS 106,717 10/ 1898 Germany. 708,023 4/ 1954 Great Britain.

HOWARD S. WILLIAMS, Primary Examiner A. C. PRESCOTT, Assistant Examiner US. Cl. X.R.

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