GB1588020A

GB1588020A - Electrolytic cell

Info

Publication number: GB1588020A
Application number: GB28117/77A
Authority: GB
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1976-07-16
Filing date: 1977-07-05
Publication date: 1981-04-15
Also published as: ZA773723B; FR2358475B1; IT1077322B; FR2358475A1; AU2633577A; BE856721A; ES460768A1; LU75407A1; NL7707645A; DE2730757A1

Description

(54) ELECTROLYTIC CELL (71) We, SOLVAY & CIE, Societe Anonyme, of 33 rue de Prince Albert, B-1050, Brussels, Belgium, a Belgian Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a cell with a mercury cathode for the electrolysis of an aqueous solution of alkali metal chloride, for example a sodium chloride brine.

An unfavourable feature of mercurycathode cells lies in the formation of hypo chlorous acid in the body of the alkali metal chloride solution submitted to electrolysis.

The presence of hypochlorous acid in the dilute solution leaving mercury cells (particularly important in the case of cells equipped with metal anodes) generally necessitates the purification of this dilute solution in respect of active chlorine before any further treatment.

An excessive content of active chlorine in the dilute solutions removed from mercury cells prevents disposal of the solutions in their original state into water-courses or the sea, in order to avoid the contamination of water.

When the dilute solutions removed from mercury cells are intended for recycling to the cells, after resaturation with alkali metal chloride, it is generally equally necessary to purify them periodically in respect of hypochlorous acid, so as to avoid a progressive accumulation of hypochlorous acid and alkali metal chlorate in the solutions, which is harmful to the efficiency of the electrolysis.

Moreover, an excessive accumulation of hypo chlorous acid in the aqueous solution to be electrolysed increases its corrosive character and, more particularly, causes premature wear of the anodes when these are of graphite.

The applicant has now conceived a mercury cell for the electrolysis of aqueous solutions of alkali metal chloride, which reduces considerably the hypochlorous acid content of the dilute aqueous solutions withdrawn from the cell.

According to the present invention, therefore, there is provided a cell for the electrolysis of an aqueous solution of alkali metal chloride, comprising within an enclosure at least one anode having an effective anode surface (as hereinbefore defined) disposed opposite to a mercury cathode, the said cell also comprising, within the enclosure, a catalytic surface additional to said effective anode surface and made of a material which catalyses the decomposition of hypochlorite ion, said catalytic surface being resistant to corrosion by solutions of alkali metal chloride.

In the cell according to the invention, the catalytic surface has the effect of decomposing hypochlorous acid in the cell, with liberation of oxygen, in such proportion as this by-product material is formed. To achieve this, the catalytic surface should be situated below the normal level of the aqueous solution of alkali metal chloride in the cell, so as to be constantly submerged in the solution during operation of the cell.

The catalytic surface employed in the cell according to the invention may for example take the form of wires, plates, foraminate or corrugated sheets, disposed so as to be immersed in the solution of alkali metal chloride. It should be made of a material, e.g. a metal or a metal compound, resistant to corrosion in contact with solutions of alkali metal chloride and preferably has a low overpotential for the liberation of oxygen in an alkali metal chloride solution. Particularly suitable materials are the metals of the group consisting of iridium, osmium, palladium, rhodium, ruthenium, and the alloys and the compounds of these metals, preferably their oxides.

All other things being equal, the action of the catalytic surface on the reduction of the content of hypochlorous acid in the dilute solution withdrawn from the electrolytic cell according to the invention is increasingly effective as the area of the catalytic surface increases. In general it is advantageous if the catalytic surface has an area at least equal to about 5% of the effective anode surface of the cell, preferably at least equal to about 10% of the effective anode surface. The best results are obtained when the catalytic surface has an area at least equal to 20% of the effectiv anode surface. For economic reasons, it may b advantageous to limit the area of the catalytic surface to a maximum of 300% of the effective anode surface.

By 'effective anode surface of the cell' is meant the useful portion of the anodes, which participates effectively in the discharge of chloride ions under normal conditions of electrolysis.

Thus, in the case of a cell with a horizontal mercury cathode in which the anodes are constituted by graphite blocks, the effective anode surface is defined as being the total area of the lower face of the anode blocks, facing towards the cathode. Experience has in fact shown that other surfaces of the massive block of graphite participate to only a negligible extent, less than 2%, or even not at all, in the discharge of chloride ions.

In the case of metal anodes of the type described in Belgian Patent 752433 in the name of Imperial Chemical Industries Limited, formed of a foraminate metal plate carrying an active coating on its face turned towards the cathode, the effective anode surface of the anode is equal to the area of the active coating applied to the face of the anode plate which is turned towards the cathode.

In the case where the anode are formed of parallel vanes, disposed opposite the cathode as described in Belgian Patent 811155 in the name of Imperial Chemical Industries Limited, the effective anode surface corresponds to the area of the longitudinal strip of the vanes which faces the cathode, in some cases added to by the area of the edge zone of the faces of the vanes, next to the said strip. In general, the width of the edge zone, perpendicular to the strip of the vanes, is at most equal to the anode-cathode distance normally provided in the cell. Experience has in fact shown that the remaining zone of the lateral faces of the vanes participates to a negligible extent, generally less than 2%, in the discharge of chloride ions during electrolysis.

In the electrolytic cell according to the invention, the catalytic surface may be nonpolarised or, on the other hand, it may be connected to the positive pole of a source of direct current. In particular the catalytic surface may be connected to the anodes of the cell.

The invention has the feature of considerably reducing, generally to a negligible value, the content of hypochlorous acid in the dilute solutions leaving electrolytic cells with mercury cathodes. It thus provides the appreciable advantage of abolishing the need for a subsequent treatment of these dilute solutions and it allows them to be put to any purpose whatever, for example for them to be disposed of in the sea or in a water-course, or for them to be treated in a plant for resaturation with alkali metal chloride before being recycled to an electrolytic cell.

In the cell according to the invention, the decomposition of the hypochlorous acid in contact with the catalytic surface has the effect of acidifying the solution circulating in the cell and thus of reducing the tendency for later information of hypochlorous acid.

The invention thereby provides the appreciable advantage of no longer requiring pre-acidification of the aqueous solutions of alkali metal chloride prepared for mercury cells, by bringing about this acidification automatically and in optimum manner within the cells themselves, during electrolysis. From this follows the supplementary advantage of an appreciable improvement in the current efficiency of mercury cells and a reduction in the tendency to formation of hydrogen at the cathode.

Special features and details of the invention will become apparent from the following description of the single figure of the appended drawing, which represents schematically in longitudinal vertical section one particular embodiment of the electrolytic cell according to the invention.

In the appended figure there is shown a cell with a substantially horizontal flowing mercury cathode, for the electrolysis of a sodium chloride brine. The cell comprises, in known manner, an enclosure 1 formed by a steel casing 2 closed by a cover 5. Through the cover, in fluid-tight manner, pass rods 6 supporting substantially horizontal anode plates 7 disposed very close to a mercury cathode 4 flowing over the baseplate 3 of the casing 2.

The side walls of the casing 2 and the cover 5 are covered with a layer (not shown) protecting them against corrosion by chlorine and brine, generally made of ebonite.

Each anode 7 is formed by a flat foraminate plate made of titanium, the lower face of which, turned towards the cathode, carries a coating which is active for the discharge of chloride ions during electrolysis of the brine. By way of example, this active coating may comprise a metal of the group consisting of platinum, iridium, osmium, palladium, rhodium, ruthenium or an alloy or a compound, for example an oxide, of these metals. The coating consists advantageously of one or other of the coatings described and claimed in Belgian Patents 769677, 769680, 784255 and 785605, all four in the name of the present applicant.

According to the aforesaid definition, the effective anode surface of the electrolytic cell is equal to the total area of the coating of the lower face (turned towards the cathode 4) of the assembly of anodes 7 of the cell.

The rods 6 supporting the anode plates 7 serve also as current leads to the anodes and are for this purpose connected to a current distributor (not shown).

The enclosure 1 of the cell is connected at its up-stream end to a pipe 8 for admission of a sodium chloride brine to be electrolysed and, at its down-stream, to a pipe 9 for removal of the dilute solution after electrolysis in the cell 1.

The cover 5 is provided with a port 10 for withdrawing chlorine produced at the anodes 7 during electrolysis, and the baseplate 3 is connected at its up-stream end to a distributor 11 for admission of mercury and, at its downstream end, to a conduit 12 for removing sodium amalgam produced during electrolysis.

According to the invention, the enclosure 1 of the cell contains a horizontal foraminate plate 13 made of a material capable of catalysing the decomposition of hypochlorite ions insofar as they are formed in the brine during electrolysis. The plate 13 has the anode rods 6 passing through it and is suspended from the cover 5 by supporting rods 14, so as to be disposed below the normal level 15 of the brine in the cell during electrolysis.

The foraminate plate 13 may be a perforated sheet or an expanded sheet of filmforming metal coated with a material catalysing the decomposition of hypochlorite ions in brine.

The film-forming metal of the plate 13 may advantageously be selected from titanium, tantalum, niobium, tungsten, zirconium and alloys of these metals.

The material catalysing the decomposition of hypochlorite ions may advantageously be selected from the metals iridium, osmium, palladium, rhodium and ruthenium, their alloys and their oxides, the oxygen-overpotential of which is low in an aqueous solution of sodium chloride. The material preferably consists of a mixture of an oxide of a metal of the group consisting of iridium, osmium, palladium, rhodium and ruthenium and an oxide of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten and zirconium. Advantageously it consists of a mixture of 50% by weight of ruthenium oxide and 50% by weight of titanium dioxide.

The dimensions of the plate 13 are chosen so that the total area of its catalytic coating is at least equal to 5%, preferably at least equal to 10%, of the effective anode surface of the cell as defined hereinbefore.

According to an advantageous modification of the embodiment of the cell shown in the figure, the catalytic plate 13 consists of a lattice made of a synthetic polymer resistant to corrosion by chlorine and brine, and covered with the catalytic material. The polymer forming the lattice may be a fluorinated polymer, for example polytetrafluoroethylene, polyvinylidene fluoride or polychlorotrifluoroethylene, such as that known by the name Kel-F (Kellog Co.). The rigidity of the lattice may be increased if desired by means of cross-braces.

In a modified embodiment (not shown) of the cell depicted in the figure, the foraminate plate 13 is replaced by rods, small plates or vanes made of film-forming metal, fixed at right angles to the upper surface of the anodes 7 and carrying a coating catalysing the decomposition of hypochlorite ions, such as described above.

Although the preceding description has been applied to an electrolytic cell with substantially a horizontal mercury cathode, clearly the invention is also applicable to cells with oblique or vertical mercury cathodes.

WHAT WE CLAIM IS: 1. A cell for the electrolysis of an aqueous solution of alkali metal chloride, comprising, within an enclosure, at least one anode having an effective anode surface (as hereinbefore defined) disposed opposite to a mercury cathode, the said cell also comprising, within the enclosure, a catalytic surface additional to said effective anode surface and made of a material which catalyses the decomposition of hypochlorite ion, said catalytic surface being resistant to corrosion by solutions of alkali metal chloride.

2. A cell according to Claim 1, wherein the material of the catalytic surface is a metal or a metal compound having a low overpotential for the liberation of oxygen in an aqueous solution of alkali metal chloride.

3. A cell according to Claim 2, wherein the material of the catalytic surface is selected from the group consisting of iridium, osmium, palladium, rhodium, ruthenium, alloys and compounds of these metals.

4. A cell according to Claim 2, wherein the material of the catalytic surface comprises a mixture of an oxide of a metal of the group consisting of iridium, osmium, palladium, rhodium and ruthenium, and an oxide of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten and zirconium.

5. A cell according to Claim 4, wherein the catalytic material is constituted of a mixture of 50% by weight of ruthenium oxide and 50% by weight of titanium dioxide.

6. A cell according to any one of Claims 1-5, wherein the area of the catalytic surface is at least equal to 5% of the effective anode surface of the cell.

7. A cell according to Claim 6, wherein the area of the catalytic surface is at least equal to 10% of the effective anode surface of the cell.

8. A cell according to Claim 7, wherein the area of the catalytic surface is substantially between 20 and 300% of the effective anode surface of the cell.

9. A cell according to any one of Claims 1-8, wherein the catalytic surface comprises a substrate made of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten, zirconium and their alloys, on which the catalytic material is applied.

10. A cell according to any one of Claims 1-8, wherein the catalytic surface comprises a substrate made of a synthetic polymer resistant to corrosion in the anolyte, on which the catalytic material is applied.

11. A cell according to Claim 10, wherein the synthetic polymer is a fluorinated polymer.

12. A cell according to any one of Claims 1-11, wherein the tatalytic surface is in the form of wires and/or plates and/or foraminate

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

11 for admission of mercury and, at its downstream end, to a conduit 12 for removing sodium amalgam produced during electrolysis.

According to the invention, the enclosure 1 of the cell contains a horizontal foraminate plate 13 made of a material capable of catalysing the decomposition of hypochlorite ions insofar as they are formed in the brine during electrolysis. The plate 13 has the anode rods 6 passing through it and is suspended from the cover 5 by supporting rods 14, so as to be disposed below the normal level 15 of the brine in the cell during electrolysis.

The foraminate plate 13 may be a perforated sheet or an expanded sheet of filmforming metal coated with a material catalysing the decomposition of hypochlorite ions in brine.

The film-forming metal of the plate 13 may advantageously be selected from titanium, tantalum, niobium, tungsten, zirconium and alloys of these metals.

The material catalysing the decomposition of hypochlorite ions may advantageously be selected from the metals iridium, osmium, palladium, rhodium and ruthenium, their alloys and their oxides, the oxygen-overpotential of which is low in an aqueous solution of sodium chloride. The material preferably consists of a mixture of an oxide of a metal of the group consisting of iridium, osmium, palladium, rhodium and ruthenium and an oxide of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten and zirconium. Advantageously it consists of a mixture of 50% by weight of ruthenium oxide and 50% by weight of titanium dioxide.

The dimensions of the plate 13 are chosen so that the total area of its catalytic coating is at least equal to 5%, preferably at least equal to 10%, of the effective anode surface of the cell as defined hereinbefore.

According to an advantageous modification of the embodiment of the cell shown in the figure, the catalytic plate 13 consists of a lattice made of a synthetic polymer resistant to corrosion by chlorine and brine, and covered with the catalytic material. The polymer forming the lattice may be a fluorinated polymer, for example polytetrafluoroethylene, polyvinylidene fluoride or polychlorotrifluoroethylene, such as that known by the name Kel-F (Kellog Co.). The rigidity of the lattice may be increased if desired by means of cross-braces.

In a modified embodiment (not shown) of the cell depicted in the figure, the foraminate plate 13 is replaced by rods, small plates or vanes made of film-forming metal, fixed at right angles to the upper surface of the anodes 7 and carrying a coating catalysing the decomposition of hypochlorite ions, such as described above.

Although the preceding description has been applied to an electrolytic cell with substantially a horizontal mercury cathode, clearly the invention is also applicable to cells with oblique or vertical mercury cathodes.

WHAT WE CLAIM IS: 1. A cell for the electrolysis of an aqueous solution of alkali metal chloride, comprising, within an enclosure, at least one anode having an effective anode surface (as hereinbefore defined) disposed opposite to a mercury cathode, the said cell also comprising, within the enclosure, a catalytic surface additional to said effective anode surface and made of a material which catalyses the decomposition of hypochlorite ion, said catalytic surface being resistant to corrosion by solutions of alkali metal chloride.
2. A cell according to Claim 1, wherein the material of the catalytic surface is a metal or a metal compound having a low overpotential for the liberation of oxygen in an aqueous solution of alkali metal chloride.
3. A cell according to Claim 2, wherein the material of the catalytic surface is selected from the group consisting of iridium, osmium, palladium, rhodium, ruthenium, alloys and compounds of these metals.
4. A cell according to Claim 2, wherein the material of the catalytic surface comprises a mixture of an oxide of a metal of the group consisting of iridium, osmium, palladium, rhodium and ruthenium, and an oxide of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten and zirconium.
5. A cell according to Claim 4, wherein the catalytic material is constituted of a mixture of 50% by weight of ruthenium oxide and 50% by weight of titanium dioxide.
6. A cell according to any one of Claims 1-5, wherein the area of the catalytic surface is at least equal to 5% of the effective anode surface of the cell.
7. A cell according to Claim 6, wherein the area of the catalytic surface is at least equal to 10% of the effective anode surface of the cell.
8. A cell according to Claim 7, wherein the area of the catalytic surface is substantially between 20 and 300% of the effective anode surface of the cell.
9. A cell according to any one of Claims 1-8, wherein the catalytic surface comprises a substrate made of a film-forming metal of the group consisting of titanium, tantalum, niobium, tungsten, zirconium and their alloys, on which the catalytic material is applied.
10. A cell according to any one of Claims 1-8, wherein the catalytic surface comprises a substrate made of a synthetic polymer resistant to corrosion in the anolyte, on which the catalytic material is applied.
11. A cell according to Claim 10, wherein the synthetic polymer is a fluorinated polymer.
12. A cell according to any one of Claims 1-11, wherein the tatalytic surface is in the form of wires and/or plates and/or foraminate

or currugated sheets.
13. A cell for the electrolysis of an aqueous solution of alkali metal chloride, according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.