GB1569127A - Electrolytic cells - Google Patents

Description

(54) ELECTROLYTIC CEI,LS (71) We, IMPERIAL CHEMICAL INDUS TRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British 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: - This invention relates to improvements in electrolytic diaphragm cells.

More particularly, it relates to electrolytic diaphragm cells having anodes made of a film-forming metal and which carry an electrocatalytically active coating. It especially relates to diaphragm cells for the electrolysis of aqueous solutions of alkali-metal halides.

A wide variety of diaphragm cells are known which consist in principle of a series of anodes and a series of cathodes disposed in a parallel alternating manner and separated from each other by a substantially vertical diaphragm. In cells of recent design, the anodes are suitably in the form of plates of a film-forming metal (usually titanium) and carry an electrocatalytically active coating (for example a platinum metal oxide); the cathodes are suitably in the form of a perforated plate or tube of metal (usually mild steel); and the diaphragms, which are usually deposited on or fitted to the surface of the cathodes, are suitably made of asbestos or a synthetic organic polymeric material, for example polytetrafluoroethylene or polyvinylidene fluoride.

In operating a diaphragm cell, it is advantageous to operate with as small a distance as possible between the anode and the cathode (the anode/cathode gap) in order to keep the ohmic losses (and hence the cell voltage) to a minimum. At the same time it is desirable to operate at an econ mic current density, for example 2 kail2.

The use of high current densities and small anode/cathode gaps, however, results in a high rate of evolution of gas (for example chlorine) during electrolysis which can in turn cause a foam of gas and electrolyte. This foam can partially fill the anode/cathode gap in the anolyte compartment, thus driving the electrolyte out of the gap and increasing the resistance to further electrolysis. This problem has been mitigated by using metal anodes provided with a plurality of vertically disposed elongated members (e.g. blades, rods, channel shaped members) to facilitate the removal of gas from the surface, for example as described in our U.K. Patent Specification No. 1,479,444. Such metal anodes, when made of a film-forming metal, for example titanium are relatively expensive to make as compared with solid-plate anodes.On the other hand, solid-plate anodes, have a further disadvantage in that the relatively low electrical conductivity of film-forming metals can lead to poor current efficiency.

In certain diaphragm cells, the current is led into the bottom of the anode, and because of the relatively low electrical conductivity of titanium, there is a considerable voltage drop from bottom to top of the anode. This voltage drop can lead to reduction in current efficiency by causing a mal-distribution of current in the anode/ cathode gap.

We have now devised an anode for use in diaphragm cells which aims to obviate or mitigate this disadvantage associated with the aforesaid anodes.

According to the present invention we provide an anode for an electrolytic diaphragm cell wherein the anode is made of a film-forming metal or an alloy thereof and carries on at least part of its surface an electrocatalytically active coating and wherein the anode is in the form of a sheet and comprises a lower solid plate portion and an upper portion comprising a plurality of parallel elongated members which extend from the solid plate portion lnd which are in electrical and mechanical contact with the solid plate portion. The solid plate portion and the portion com prising the elongated members respectively form the lower and upper portions of the anode when the anode is mounted in a diaphragm cell. When installed in such a cell the anode will generally be mounted vertically and the elongated members will extend vertically from the solid plate portion.

According to a further aspect of the present invention we provide an electrolytic cell comprising an anode, a cathode and a diaphragm separating the anode and the cathode wherein the anode is made of a film-forming metal or alloy thereof and carries on at least part of its surface an electrocatalytically active coating and wherein the anode is in the form of a sheet and comprises a lower solid plate portion and an upper portion comprising a plurality of parallel elongated members which extend from the solid plate portion and which are in electrical and mechanical contact with the solid plate portion.

We also provide a precursor for the anode of the invention which is in the form of a sheet and which comprises a structure of a film-forming metal or alloy thereof comprising a solid plate portion and a portion comprising a plurality of parallel elongated members which extend from the solid plate portion and which are in electrical and mechanical contact with the solid plate portion.

The members may suitably be in the form of blades, rods, or channel members of U-shape or hemicylindrical shape. It is preferred, however, that the elongated members are in the form of louvres. Such louvres are conveniently produced from a sheet of film-forming metal by pressing with a slitting and forming tool. The louvre slats so obtained may suitably be turned at right angles to the original plane of the film-forming metal sheet, but they may be inclined to the plane at an acute angle or obtuse angle if desired, or thev may be rolled round to form a series of aDDroximately hemicylindrical members which alternate with the slots from which the metal forming them has been pressed out.

The elongated members may be mechanically and electrically connected to the solid-olate portion by anv convenient method, for example by welding. Alternativelv, the elongated members may be mounted on one side or both sides of a suitable suonort means, for example a sheet or backing frame of a film-forming metal, and the support means mechanically and electrically connected to the solid-plate portion, for example by welding. A preferred anode according to the invention, however, comprises a unitary structure in which the lower solid-plate portion and the upper portion comprising the elongated members are formed from a single sheet of a film-forming metal by pressing out louvres from one portion of the sheet only.

The dimensions of the portion of the anode comprising the elongated members, along the line of the said members, suitably constitutes at least 20%, and preferably 30% to 50%, of the dimension of the anode in this line, the remainder of the anode being constituted by the solid-plate portion.

In the electrolytic cell the solid-plate portion of the anode may be connected to a source of electrical current by any convenient means. For example, the lower end of the solid-plate portion may be provided with one or more projections of a film-forming metal or alloy thereof which may in turn be connected to a current leadin means, e.g. by bolting. Such projection or projections are preferably of the same film-forming metal or alloy constituting the anode and may be attached by any suitable means, e.g. welding, or may form an extension or extensions of the solid plate portion.

In this specification, by "a film-forming metal" we mean one of the metals titanium, zirconium, niobium, tantalum or tungsten or an alloy consisting principally of one of these metals and having anodic polarisation properties which are comparable to those of the pure metal. It is preferred to use titanium alone or an alloy based on titanium and having polarisation properties comparable to those of titanium.

Examples of such alloys are titaniumzirconium alloys containing up to 14% of zirconium, alloys of titanium with up to 5% of a platinum group metal such as platinum, rhodium or iridium and alloys of titanium with niobium or tantalum containing up to 10% of the alloying constituent.

The electrocatalyticaly active coating is a conductive coating which is resistant to electrochemical attack but is active in transferring electrons between electrolyte and the anode.

The electrocatalytically active material may suitably consist- of one or more platinum group metals, i.e. platinum, rhodium, iridium, ruthenium, osmium and palladium, and alloys of the said metals, and/or the oxides thereof, or another metal or a compound which will function as an anode and which is resistant to electrochemical dissolution in the cell, for instance rhenium, rhenium trioxide, magnetite, titanium nitride and the borides, phosphides and sili ci des of the platinum group metals. The coating may consist of one or more of the said platinum group metals and/or oxides thereof in admixture with one or more non noble metal oxides. Alternatively, it may consist of one or more non-noble metal oxides alone or a mixture of one or more non-noble metal oxides and a non-nob'e metal chlorine discharge catalyst.Suitable non-noble metal oxides are, for example, oxides of the film-forming metals (titanium, zirconium, niobium, tantalum or tungsten), tin dioxide, germanium dioxide and oxides of antimony. Suitable chlorine-discharge catalysts include the difluorides of manganese, iron, cobalt, nickel and mixtures thereof. Especially suitable electrocatalytically active coatings according to the invention include platinum itself and those based on ruthenium dioxide/titanium dioxide and ruthenium dioxide/tin dioxide titanium dioxide.

Other suitable coatings include those described in our U.K. Patent Specifications Nos. 1,402,414 and 1,484,015 in which a nonconducting particulate or fibrous refractory material is embedded in a matrix of electrocatalyticaly active material (of the type described above). Suitable nonconducting particulate of fibrous materials include oxides, fluorides, nitrides and sulphides. Suitable oxides (including complex oxides) include zirconia, alumina, silica, thorium oxide, titanium dioxide, ceric oxide, hafnium oxide, ditantalum pent oxide, magnesium aluminate (e.g. spinel MgO.

Awl203) aluminosilicates (e.g. mullite (Al2O,)3 (SiO2)2), zirconium silicate, glass, calcium silicate (e.g. bellite (CaO),SiO2), calcium aluminate, calcium titanate (e.g.

perovskite CaTiO,), attapulgite, kaolinite, asbestos, mica, codierite and bentonite; suitable sulphides include dicerium trisulphide, suitable nitrides include boron nitride and silicon nitride; and suitable fluorides include calcium fluoride. A preferred non-conducting refractory material is a mixture of zirconium silicate and zirconia, for example zirconium silicate particles and zirconia fibres.

The anodes of the invention may be prepared by the painting and firing technique, wherein a coating of metal and/or metal oxide is formed on the anode surface by applying a layer of a paint composition comprising thermally-decomposable compounds of each of the metals that are to feature in the finished coating in a liquid vehicle to the surface of the anode, drying the paint layer by evaporating the liquid vehicle and then firing the paint layer by heating the coated anode, suitably at 2500C to 8000 C, to decompose the metal compounds of the paint and form the desired coating.When refractory particles or fibres are to be embedded in the metal and/or metal oxide of the coating, the refractory particles or fibres may be mixed into the aforesaid paint composition before it is applied to the anode. Alternatively, the refractory particles or fibres may be applied on to a layer of the aforesaid paint composition while this is still in the fluid state on the surface of the anode, the paint layer then being dried by evaporation of the liquid vehicle and fired in the usual manner.

The coated electrodes are preferably built up by applying a plurality of paint layers on the anode, each layer being dried and fired before applying the next year.

The cathode may suitably be in the form of a perforated metal sheet or tube, for example in the form of a gauze or may comprise a plurality of parallel elongated members mounted on support means, for example a plurality of blades, rods or channel shaped members mounted on support means, as described in the aforesaid U.K. Patent Specification No. 1,479,444.

The cathode is preferably of mild steel.

The anode according to the invention may be used in conjunction with any con ventional diaphragm. Suitable diaphragms include those made of asbestos or a synthetic organic polymeric material, for example polytetrafluoroethylene of polyvinylidene fluoride.

When used in a diaphragm cell, the anode/cathode gap in the vicinity of the solid-plate portion of the anode, is suitably in the range 8 to 15 mm, for example 10 to 12 mm.

The anode/cathode gap in the vicinity of the elongated members (the upper portion of the anode) is reduced by an amount depending on the depth of the elongated members and on their inclination to the plate; suitably, the anode/cathode gap is 1 to 6 mm, for example 24 mm, less than the anode/cathode gap in the vicinity of the solid plate portion. It is an advantage of the anode according to the invention that the smaller anode/cathode gap reduces the electrolytic resistance in the vicinity of the elongated members and so compensates to a large extent for the increase in resistance to current which occurs as the distance from the current source increases.

This leads to a more even distribution of current up the anode and an improved current efficiency. In addition, the small reduction in average anode/cathode gap leads to some reduction in cell voltage.

The invention is especially applicable to diaphragm cells used for the manufacture of chlorine and alkali metal hydroxides by electrolysis of aqueous alkali metal chloride solutions, for example in diaphragm cells manufacturing chlorine and sodium hydroxide from sodium chloride solutions.

By way of example, an embodiment of the invention will now be described with reference to Figs. 1 and 2 in which: Figure 1 is a perspective view of an anode according to the invention, and Figure 2 is a vertical cross-section along the section line A-A of Fig. 1.

Referring to Fig. 1, the anode 1 is made of a film-forming metal, for example titanium, and consists of a lower solid-plate portion 2 and an upper louvred portion 3.

The individual louvres 4 are formed from a single sheet of the film-forming metal (of a size corresponding to the overall dimensions of the anode) and pressing out the louvres with a slitting and forming tool.

The anode 1 is coated on both sides with an electrocatalytically active material, for example a coating comprising ruthenium dioxide and titanium dioxide.

The anode is suitably provided with projecting portions or bosses of a film-forming metal (not shown) for connecting the anode to a source of electrical current. The projecting portions or bosses may be extensions of the solid-plate portion of the anode or may be separate elements which are mechanically and electrically connected to the anode, for example by welding.

The invention is further illustrated by the following Example.

EXAMPLE A diaphragm cell was provided with a titanium anode according to the invention, a mild steel gauze cathode and a polytetrafluoroethylene diaphragm. The top one third of the area of the anode was louvred and the remaining two thirds was in the form of a solid plate. The anode/cathode gap was 9 mm in the vicinity of the louvred portion and 12 mm in the vicinity of the solid plate portion of the anode. The poly tetrafluoroethylene diaphragm was prepared by calendering a mixture of an aqueous polytetrafiuoroethylene dispersion, titanium dioxide and starch, and subsequently removing the starch by electrolytic extraction in situ in the cell.

The cell was fed with sodium chloride brine (at a concentration of 310g/litre). A current of 520 amp was passed through the cell, which corresponded to a current density of 25kA/m2 when compared with the effective area of the diaphragm. The cell operating voltage was 3 55 volts. The chlorine produced contained 982% by weight of chlorine and less than 1-5% by weight of oxygen. The aqueous sodium hydroxide produced contained 11% by weight of NaOH. The cell operated at a current efficiency of 955%.

WHAT WE CLAIM IS: 1. An anode for an electrolytic diaphragm cell wherein the anode is made of a film-forming metal (as herein before defined) or an alloy thereof and carries on at least part of its surface an electrocatalytically active coating and where the anode is in the form of a sheet and comprises a lower solid plate portion and an upper portion comprising a plurality of parallel elongated members which extend from the solid plate portion and which are in electrical and mechanical contact with the solid plate-portion.

2. An anode as claimed in Claim 1 wherein the dimensions of the portion of the anode comprising the elongated members, along the line of the said members, constitutes at least 20% of the total dimension of the anode in the said line, the remainder of the anode being constituted by the solid-plate portion.

3. An anode as claimed in Claim 2 wherein the dimensions of the portion of the anode comprising the elongated members, along the line of the said members, constitutes 30% to 50% of the total dimension of the anode in the said line.

4. An anode as claimed in any one of the preceding claims wherein the elongated members are in the form of louvres.

5. An anode as claimed in Claim 4 whenever formed as a unitary structure from a single sheet of a film forming metal or alloy thereof by pressing louvres from one portion of the sheet only.

6. An anode as claimed in any one of the preceding claims wherein the filmforming metal is titanium.

7. An anode as claimed in any one of the preceding claims whenever coated with a mixture of a platinum group metal oxide and a film-forming metal oxide.

8. An anode as claimed in Claim 7 whenever coated with a mixture of ruthenium oxide and titanium dioxide.

9. An anode substantially as hereinbefore described with reference to Figures 1 and 2 of the accompanying drawings and with reference to the Example.

10. A precursor for the anode as claimed in any one of the preceding claims which is in the form of a sheet and which comprises a structure of a film-forming metal or alloy thereof comprising a solidplate portion and a portion comprising a plurality of parallel elongated members which extend from the solid plate portion and which are in electrical and mechanical contact with the solid plate portion.

11. An electrolytic cell comprising an anode, a cathode and a diaphragm separating the anode and the cathode, and wherein the anode is as claimed in any one of

Claims (1)

1. Claims 1 to 9.

   12. An electrolytic cell as claimed in Claim 11 in which the anode/cathode gap in the vicinity of the solid-plate portion of the anode is 8 to 15 mm and the anodel cathode gap in the vicinity of the elongated members is 1 to 6 mm less than the anode/cathode gap in the vicinity of the solid-plate portion.

   13. An electrolytic cell as claimed in Claim 12 wherein the anode/cathode gap in the vicinity of the elongated members is 2 to 4 mm less than the anode/cathode gap in the vicinity of the solid plate portion.

   14. An electrolytic cell as claimed in any one of Claims 11 to 13 wherein the cathode is of mild steel.

   15. An electrolytic cell as claimed in any one of Claims 11 to 14 wherein the diaphragm comprises polytetrafluoroethylene.

   16. An electrolytic cell as claimed in any one of Claims 11 to 15 whenever used for the electrolysis of an aqueous alkali metal chloride solution.

   17. Chlorine whenever produced in an electrolytic cell according to Claim 16.

   18. Aqueous alkali metal hydroxide solution whenever produced in an electrolytic cell according to Claim 16.