GB2065705A - Electrodes for electrolysis cells - Google Patents

Abstract

An electrode with electrically- conductive current distributors and activated parts, which (at least partially) have electrocatalytic surfaces, which face the counter- electrode, for electrolysis cells, with current supplies and rectangular sections (flat sections) which may be selected according to the current loading desired. As shown, current distribution takes place uniformly via three conductor planes 1, 2, 3 in which conductors are mutually perpendicular in adjacent planes. Connection of the conductor planes with each other is by projection welding. The electrode is particularly useful as a dimensionally- stable anode in electrolysis cells with mercury cathodes. <IMAGE>

Description

SPECIFICATION Electrodes for electrolysis cells The present invention relates to electrodes for use in electrolysis cells and is particularly concerned with electrodes which are intended for use in mercury/chloralkali cells. In such cells, the current supply is provided by way of rods or pins connected to activated electrode parts of flat, rectangular cross-sectional members by means of current distributors in the form of other flat, rectangular cross-sectional members, which latter extend transversely of the first members for distributing the current.

In known metal anodes, especially dimensionally-stable anodes, an active coating is provided upon a series of round grid rods of titanium which are arranged to be horizontal and mutually parallel, the rods being held together by means of uncoated transverse ribs. Since such electrodes with round grid rods are unsatisfactory for several reasons, but especially because of their disadvantageous current distribution resulting from "current shadow formation" with respect to the counterelectrode, in mercury electrolysis cells in any case, attempts have been made to find a remedy.

Metal anodes are also known (cf. DE-AS 1818035), in which electrical conductors distribute the current in the electrode over several planes. However, since the conductor plane facing the counter-electrode consists of activated mesh material, this has the disadvantage, as in the case of round rods, that relatively large active surfaces lie in the current shadow, so that the actual surface which is accessible is relatively small in relation to the projected surface.

It has also been proposed to arrange the anode grid structure in the form of flat strips or bands or channels of U-shape or of inverse U-shape (for the latter, see GB PS 1394026). The individual channel-shaped parts are welded together backto-back, at the connecting base parts of the inverse U-sections. The latter specification describes the provision of a sufficient gap between the bands of each channel-shaped member, in order to allow acess for a spot-welding tool-head when the channel-shaped members are to be connected to a conductor by spot-welding. However, the number of individual conductor elements, which should desirably be high from the standpoint of current distribution, is thus restricted. Moreover, the base parts between the connecting webs at the top of the inverted U-shaped elements must be removed, so that relatively large amounts of titanium become wasted.Also, the problem of mass transfer, especially in mercury cells, is not met.

The basic purpose of the arrangements disclosed in DE-AS 2323497 is to promote mass transfer, particularly for improved gas escape from the undersides of the anodes, in cells which operate with current densities greater than 10 kA/m2. The solution proposed consists in providing an exceedingly large active surface, both in the zones near to the counter-electrode and also in those remote from it. It is disadvantageous, however, that the current is transported in practice only over one conductor plane with a single transversely-extending rod, which leads to markedly variable current distribution at the active electrode surface.

The principal disadvantage is that the main current distributor lies directly above the activated surface, so that the gas escape conditions and the flow conditions at the active surfaces are not uniform and thus are negatively influenced. The great height of the vertically-arranged coated titanium bands results in their operating only slightly in the remote zone, due to the relatively high electrolyte resistance, except at the expense of a higher voltage with a correspondingly higher consumption of electrical energy and therefore higher operating costs.

As the bands are interconnected at the top only by a few transverse welding seams, in this electrode structure, the bands can very easily become spread apart at their outer ends, transversely of their longitudinal direction.

Moreover, in this construction, the bands can be welded to the transverse beam only with great effort.

In DE-AS 2323497, the problem of nevertheless ensuring adequate mechanical stability or dimensional stability when using thin bands has not been taken into account, especially not in relation to bending and twisting stiffness.

These requirements must nevertheless be observed, as well as those concerning uniform current distribution and good gas kinetics and also the requirements as to lower fabrication and repair costs and also durability of the structure and the coating and good resistance to short-circuiting.

The weight of the electrodes is likewise important, not only because of manufacturing and transport costs, but also because of the use of expensive materials.

The present invention has the purpose of taking all the above-mentioned requirements into account and providing an electrode which meets these partly-conflicting requirements. This is achieved in accordance with this invention by providing an electrode of the type first referred to, wherein: (a) one or more conductors comprising flat or rectangular section members as activated electrode parts are arranged upright and have a ratio of width to height in the range from 1:5 to 2::3, (b) one or more current distributors comprising flat or rectangular section members, having a mutual spacing if more than one in the range from 30 to 150 mm, are welded to the one or more conductors and have a width to height ratio less than that of the members comprising the one or more conductors and (c) the ratio of the free passage area to the projected area in the region of the section members comprising the one or more conductors is in the range from 20:30 to 60:80.

Further preferred features of the electrodes of the invention are set out below and are illustrated by the subsequent description of preferred embodiments given by way of example in conjunction with the accompanying drawings.

The principal advantages of the electrode structure of the invention are: (1) favourable current distribution over three conductor planes with optimally-dimensioned flat sections (rectangular sections); (2) high dimensional stability of the electrode, as well as mechanical (torsional stiffness), especially on account of the more favourable moment of resistance of rectangular sections in comparison with round sections and square sections, but also because all flat sections (rectangular sections) of the individual planes are arranged in each case at right-angles to one another; (3) high transport safety, because the rigidity of the electrode structure is difficult to overcome even through external influences;; (4) good flatness of the planar undersides of the electrode which remains not only after fabrication and transport, but also after installation (assembly and disassembly) as well as during operation, which leads to a reduction in operating costs, because a more satisfactory and uniform spacing from the counter-electrode is maintained; (5) safety against thermal distortion during reactivation; this makes possible the torsionally resistant structure of the electrode according to the invention; (6) good mass transfer kinetics, not only by virtue of the overall-coated vertically-disposed flat sections (rectangular sections), but also by their satisfactory mutual spacing and the number of conductors per surface area; (7) nevertheless, good weldability due to the mutual association of the conductor planes; 58) reduction of the risk of short-circuiting, because flatness is maintained even after transport and installation, as well as in operation; (9) not least, a very large saving of materials relative to an electrode of equal surface of highgrade materials, such as titanium, in the illustrative embodiment up to 75%, and thus correspondingly higher economy;; (10) a further economic advantage is the simple form of the material of the conductors (flat section or rectangular section), which permits the use of standard prefabricated material at optimum purchase cost and with favourable storage requirements; (11) the good degree of energy utilization of the electrode according to the invention is also economical, especially in mercury-chlor-alkali electrolysis installations, as a result of more uniform current distribution; (12) the good parallelism of the individual conductors of the three planes is a consequence of the high torsional or twisting stiffness of the electrode structure according to the invention as well as its assembly; the average spacing between anode and cathode in the electrolyser is maintained optimally small and remains uninfluenced by slight deviations in flatness.

Preferably, the electrode comprises conductors arranged in three planes above one another, the conductors being mutually perpendicular in adjacent planes and all comprising flat or rectangular profile members or sections. In one preferred embodiment, the third plane conductors which face the counter-electrode and the overlying second plane conductors are disposed upright as current distributors and are welded perpendicularly to one another and the first plane conductors are disposed lying flat on the second plane conductors as main current distributors and are welded perpendicularly thereto, the conductors comprising the main current distributors being connected with the current supply rod or pin or protective tube thereof.In accordance with a preferred form of this embodiment, the first plane conductors are smaller in number than those of the second plane and the second plane conductors are smaller in number than those of the third plane, the first plane conductors being arranged as a main current distributor.

The invention also consists in an electrolysis cell, having as anode at least one electrode according to this invention.

Further advantages of the invention may be seen from the embodiments given by way of example. These embodiments may of course be modified in various ways, without departing from the scope of the invention, as defined in the appended claims. In particular, it is possible to provide combinations and sub-combinations of the features described and represented, if desired in combination with known features.

Various views of electrodes according to illustrative embodiments of the invention are shown in the accompanying drawings, wherein: Figure 1 shows a cross-section along the median axis through the electrode; Figure 2 shows a cross-section similar to Figure 1, but as viewed in a direction 90 round the median axis; Figure 3 shows a plan view of a form of electrode with a square base area.

As can be seen, the electrode comprises three conductor planes, all of flat profiled members or sections (rectangular sections), the conductors of the first plane being designated with 1 , the conductors of the second plane with 2 and the conductors of the third plane with 3, the latter being arranged so as to face the counter-electrode when mounted in the cell, preferably a mercury electrolysis cell with mercury flowing in a direction parallel to the conductors 3, which are then anodically connected, whilst the mercury forms the cathode.

The gap between the underside of the electrode and the counter-electrode is advantageously around 3 mm. However, it can be selected differently, because the current supply pin 4 of the electrode is so mounted or suspended above the cell that it permits uniform parallel adjustment of the gap. The electrode gap should on the one hand be as small as possible, if the current consumption is to be reduced, but must on the other hand not be too small, since the risk of short-circuiting would then be increased and side-reactions could occur, which would reduce the current efficiency.

The current connector of the supply pin 4 is not shown, since it is of a type known per se. For example, the pin can be made of copper and is contained in a titanium cover-tube or sleeve 5, which in turn is connected at the lower end at 6 to the flat-section conductors of the first plane (main current distributor 1).

The pin or rod 4 advantageously comprises a contact surface 7 at the lower end, which is as large as possible, being a conical surface in the example shown in Figure 1, and this contact can be connected to the main current distributor 1 either fixedly or releasably, by welding, pressfitting, screwing, riveting or the like, a removable connection being preferred, since in this case the parts 1, 2 and 3 of the electrodes can be separately replaced and treated elsewhere, e.g. for reactivation.

The conductors of the third plane 3 advantageously comprise flat sections of rectangular cross-section made of titanium, niobium, tantalum or other electrically-conductive metals or their alloys, which are in each case resistant in the electrolysis process, as also are the conductors of the first and second planes.

The flat sections 3 are 1 to 2 mm thick, preferably about 1.5 mm thick, and have a height of 3 to 5 mm, preferably 4 to 5 mm.

The spacing between the parallel conductors 3 amounts to at least 2 mm and up to about 6 mm at the most, although the minimum range (nearer to 2 mm) is preferred.

The gap is so selected that the gas escape channels which arise in operation at the active surfaces of the conductor 3 do not come into contact with one another in the vicinity of the gap and thereby cause turbulence, but remain separate, so that the ions which are discharged at the electrode surface can pass to the active surfaces, as far as possible without hindrance from gas bubbles. When selecting the gap, the specific electrical loading per unit area also has to be considered, as well as the fact that, on the one hand, for energy reasons, a high number of flatsection conductors 3 per unit area is desirable because of the resultant greater active area, but also, on the other hand, the mass transfer and the gas kinetics must be sufficient, which is only ensured when the free passage area is adequate.

In the electrode according to the invention, the conductors of the third plane 3 consist either entirely or partly of catalytically-active material or are provided entirely or partially with a catalytically-active coating. A catalytically-active coating on the entire surface of the conductor 3 is preferred, namely also on the underside which faces the counter-electrode. The coating materials and methods are known per se. The conductor 3 as well as the conductors 1 and 2 are advantageously selected for a specific electrical loading of the electrode of about 10 kA/m2, but as far as possible in the range from 2.5 to 1 5 kA/m2.

The ratio of free passage area to projected area in the region of the conductors of the third plane 3 is in the range from 20:30 to 60:80.

The flat section conductors of the second plane 2 are welded to the conductors 1 with spacings in the range from 30 to 150 mm and the conductors 1 consist of sheet material of 3 to 7 mm in thickness and having a height of 20 to 50 mm.

The choice of the dimensions of the flat sections (rectangular sections) of the conductors of the second and third planes (2 and 3) essentially depends upon the current density desired. The conductors of the individual planes thus may be selected with various dimensions, but should always have a rectangular cross-section in accordance with the invention, in order to be able to use commercial sheet material as far as possible. This freedom to select various dimensions for the individual conductors of the different planes in fact provides an essential advantage of the invention (adaptation to the particular application'in each case).

The good current distribution of the electrode according to the invention is due primarily to the fact that, as shown particularly in Figure 3, it is arranged completely symmetrically or in mirrorimage form with respect to the median axis and so provides uniform distribution in the number of the conductors of each plane.

The conductor 1 arranged as a main current distributor preferably consists of a flat profiled section with a rectangular cross-section, which is arranged so as to lie flat and is connected at its upper side at 6 with the tube 5 of the current supply pin or rod 4 and at its lower side with the conductors 2 of the second plane, these being arranged upright and thus perpendicular to the flat section conductor 1 (see Fig. 3). The conductors of the third plane 3 are connected to the conductors of the second plane 2 by resistance welding, preferably projection welding, so that the conductors 3 are upright, i.e. vertical, and so are at right-angles to the conductors 2 (see Fig. 3). By selecting projection welding as the special resistance welding method, without welding additives, the advantage is obtained of a rapid and automatic weldability (by means of beam electrodes), whereby many conductors in one plane may be welded at one time to those of the next plane. A further advantage of projection welding is the slight heat generation during welding, whereby less overall deformation of the electrode parts occurs during fabrication.

Electrodes according to the invention can be manufactured according to this method with a plane parallelism (at the underside of the conductors 3) of 0.25 mm. Also the possibility of repair or reactivation is considerably improved with electrodes welded in this manner.

Improvement of the flatness, in practical operation of an electrolysis cell, leads to more uniform local current distribution on the surface of the electrode facing the counter-electrode and so to a better current efficiency during operation of the cell and, in addition, to a longer service life of the coatings (increase in durability).

As can be seen especially from Figure 3, a rectangular base area of the electrode is preferred (surface of the conductor 3). However, this is not imperative. Also, the number of the conductors 3 per surface area may be varied, as long as the limits given in the appended claims with regard to the ratio of free area to projected area in the region of the conductors of the third plane are maintained.

In an electrolysis installation, several electrodes may of course be connected electrically and/or mechanically via bus bars in any desired manner for common operation.

Instead of a conductor of the first plane (main current distributor), as shown, several may also be arranged, e.g. in the manner of a cross of flat sections, with the rod or bolt 4 as the point of intersection.

The number, shape and arrangement of the conductors of the second plane (current distributor of flat section) can be suited to the particular application in each case, as long as the conditions mentioned in the description and in the appended

Claims (14)

claims are met. CLAIMS

1. An electrode for an electrolysis cell, comprising a current supply rod or pin connected to activated electrode parts comprising flat or rectangular section members via one or more current distributors in the form of flat or rectangular section members extending transversely thereof wherein: (a) one or more conductors comprising flat or rectangular section members as activated electrode parts are arranged upright and have a ratio of width to height in the range from 1:5 to 2::3, (b) one or more current distributors comprising flat or rectangular section members, having a mutual spacing if more than one in the range 30-150 mm, are welded to the one or more conductors and have a width to height ratio less than that of the members comprising the one or more conductors and (c) the ratio of the free passage area to the projected area in the region of the first section members comprising the one or more conductors is in the range from 20:30 to 60:80.

2. An electrode according to claim 1, which comprises conductors arranged in three planes above one another, the conductors being mutually perpendicular in adjacent planes and all comprising flat or rectangular profiled members or sections.

3. An electrode according to claim 1 or 2, wherein the third plane conductors which face the counter-electrode and the overlying second plane conductors are disposed upright as current distributors and are welded perpendicularly to one another and the first plane conductors are disposed lying flat on the second plane conductors as main current distributors and are welded perpendicularly thereto, the conductors comprising the main current distributors being connected with the current supply rod or pin or protective tube thereof.

4. An electrode according to claim 2 or 3, wherein the first plane conductors are smaller in number than those of the second plane and the second plane conductors are smaller in number than those of the third plane, the first plane conductors being arranged as a main current distributor.

5. An electrode according to claim 4, wherein the first plane conductors comprise a main current distributor in the form of a rod or beam of rectangular section of greater width than height, which extends parallel to the third plane conductors facing the counter-electrode.

6. An electrode according to any of claims 2 to 5, wherein the second plane conductors have a width of 3 to 7 mm and a height of 20 to 50 mm.

7. An electrode according to any of claims 2 to 6, wherein the third plane conductors are 1 to 2 mm in thickness and have a height of 3 to 5 mm.

8. An electrode according to any of claims 2 to 7, wherein gaps are provided between the third plane conductors which measure at least 2 mm in width.

9. An electrode according to any of claims 2 to 8, wherein the conductors of all three conductor planes are arranged for a current density in the range from 2.5 to 15 kA/m2.

10. An electrode according to claim 9, arranged for current density of 10 kA/m2.

11. An electrode according to any preceding claim, wherein the conductors comprise titanium, niobium, tantalum or other stable electricallyconducting metal or their alloys.

12. An electrode according to any of claims 2 to 10 or claim 11 as dependent thereon, wherein the third plane conductors consist wholly or partly of catalytically-active material or have their surface partly or wholly coated therewith.

13. An electrode according to any preceding claim, wherein the conductor sections are connected together by projection welding.

14. An electrode according to claim 1, substantially as described with reference to the accompanying drawings.

1 5. An electrolysis cell, having as anode at least one electrode according to any preceding claim.

1 6. An electrolysis cell according to claim 15, wherein the counter-electrode is a mercury cathode, which in operation is arranged to flow in the direction in which the conductors extend, the anode being substantially flat on the lower side thereof and being mounted in the cell in such a manner that the spacing between anode and cathode is adjustable.

1 7. An electrolysis cell according to claim 16, wherein the gap between the anode and the cathode is 3 mm.

1 8. An electrolysis cell according to claim 15, substantially as hereinbefore described.