CYLINDRICAL ELECTRODE

BACKGROUND OF THE INVENTION

The invention is relative to an electrode of cylindrical geometry for electrochemical processes, particularly a cylindrical electrode consisting of a central metallic conductor with a superficial catalytic activation.

STATE OF THE ART

The utilisation of electrodes of cylindrical geometry is known in several sectors of electrochemistry; cylindrical electrodes, in the majority of cases concentrically disposed inside hollow cylindrical counter-electrodes, are currently employed in electrodialysis (US 20030010639), water electrolysis (DE 196 36 171), ozone production (J P2001198574) and other applications. The most important processes making use of cylindrical-type, mostly coaxial, electrodic geometries are nevertheless the recovery of metals from aqueous solutions (see for instance US 6,451,183 or DE 197 49 445) and the treatment of waste water (industrial wastes, civil waters and others), for the potabilisation or purification thereof from various contaminants (for instance US 5,635,040). The cylindrical geometries, especially on small-size electrochemical cells, offer substantial advantages particularly in terms of fluid distribution. Depending on the process under consideration, the cylindrical electrodes can be both anodes or cathodes; in the majority of cases, such electrodes are suitable for gas evolution reactions, for instance hydrogen cathodic evolution or oxygen, ozone or chlorine anodic evolution. The gas-evolving reactions, in particular the anodic ones, must be catalysed in order to take place with a sufficient efficiency; the cylindrical electrodes of the prior art therefore consist of a metal cylindrical conductive support (usually titanium or other valve metal, in the case of anodes) coated with catalysts usually based on metal oxides, depending on the type of the gas to be evolved and of the required potential, as widely known. The application process of the catalytic coating to the cylindrical support provides painting the latter with a precursor, and the subsequent conversion of the precursor by means of a high temperature thermal treatment (350-700 ⁰ C). The painting of metallic electrodes with precursor solutions is

preferably carried out by electrostatic spraying processes; the cylindrical geometry is in this case less favourable than the planar one in terms of homogeneity of application. Furthermore, the catalytic coatings have a limited operative lifetime (indicatively 1 to 5 years depending on the applications). Once the original coating is exhausted, it must be completely removed by mechanical means and restored; the coating removal operation is particularly onerous for cylindrical geometries, especially for those of small size. In any case, the prolonged times required for restoring the catalytic activity of the electrodes lead to undesirable limitations to the plant operation, alternatively imposing a temporary interruption of the production, an oversize of the whole plant to allow a planned cyclic electrode reactivation or the need of storing a remarkable amount of replacement electrodes, which is a very onerous solution from the investment cost standpoint. It is therefore an object of the present invention to provide a cylindrical electrode for electrochemical processes overcoming the limitations of the prior art. Under another aspect, it is an object of the present invention to provide a cylindrical electrode allowing an increased easiness of application or of restoring of the catalytic coating. Under a further aspect, it is an object of the present invention to provide an improved method for the catalytic reactivation of a cylindrical electrode in terms of process management efficiency. These and other objects will be clarified by the following description, which is illustrative of the invention and not limiting the scope thereof.

THE INVENTION

Under a first aspect, the invention is relative to an electrode for gas evolution comprising a conductive core acting as current collector, whereto a detachable component is externally secured, for instance a mesh or a solid, perforated or expanded sheet, made of conductive material. In a highly preferred embodiment, the detachable component is provided with a catalytic coating and constitutes the active element of the electrode. The electrodic geometry of the invention is particularly suited to the construction of anodes for oxygen, ozone or chlorine evolution in electrometallurgical or water-treatment processes. In this case, the

conductive cylindrical core is advantageously made of a valve metal, in the most typical case titanium. The cylindrical core may have any size, the most typical diameter being comprised between 1 and 25 cm, depending on the application. The detachable component is preferably a metallic mesh or sheet, which may be advantageously made of the same material of the conductive core, preferably having thickness comprised between 0.3 and 0.8 mm even though other thicknesses are equally possible.

Under a second aspect, the invention is relative to a method for the reactivation of a cylindrical anode provided with an exhausted catalytic coating, comprising the application of a detachable element provided with a catalytic coating externally thereto.

The invention will be better comprised making reference to the following figures, which have a merely exemplifying purpose:

BRIEF DESCRIPTION OF THE FIGURES

Figs. 1 A-C show a first embodiment of the electrode of the invention; in figure 1 A are shown the cylindrical conductive core and the metallic sheet or mesh as separate components, in figure 1B and 1C are shown the same components assembled, respectively in a plan-view and in a side-view.

Figs. 2 A-B show a second embodiment of the electrode of the invention; in figure 2A is shown the metallic sheet or mesh welded along a generatrix of the conductive core, and in figure 2B the same metallic sheet or mesh rolled around the conductive core and fixed with weld spots.

DETAILED DESCRIPTION OF THE FIGURES

In figs. 1 A-C a first embodiment of the electrode of the invention is shown; in particular, in fig. 1A is shown a plan-view of a cylindrical conductor (100), which may consist of a metal rod or hollow cylinder, for instance of titanium or in the case of a cathode of stainless steel. The cylindrical conductor (100), which will act as the conductive cylindrical core of the electrode of the invention once assembled,

may also be a cylindrical electrode of the prior art, for instance an electrode having an exhausted catalytic coating.

(200) indicates the plan-view of an undulated metal sheet rolled up, closed on itself and welded so as to form a corrugated cylinder, provided with a superficial catalytic coating at least on the external surface. In the figure, the corrugations of the undulated sheet are not represented in real scale, but largely exaggerated for a better understanding of the drawing. In a less preferred embodiment, the sheet (200) can be replaced by an undulated mesh of equivalent geometry. The diameter of the sheet (200) rolled in a cylinder is slightly lower than that of the conductive core (100), but the undulated geometry is such as to impart a certain flexibility thereto, so that it may be forcedly inserted on the core itself. The flexibility is higher for undulated sheets of reduced thickness (typically 0.5 mm, in any case comprised between 0.3 and 1 mm). Fig. 2B shows the two pieces in a plan-view after the assemblage: after inserting the undulated sheet (200), the two pieces may be further secured by means of weld spots (not shown). Fig. 1C shows the same assembly as a side-view. As it can be seen particularly from fig. 1B, compared to the catalytic activation obtainable directly on a cylindrical conductor, the use of an undulated sheet has the clear advantage of sensibly increasing the active surface. Moreover, once exhausted the catalytic coating, the sheet (200) may be easily removed and replaced with another previously activated one directly on site, with minimum shut-down times and with the only need of keeping the activated sheets stored on site, rather than a series of complete electrodes. In this way, the handling costs of the material in the case of a reactivation are also reduced.

In figs. 2 A-B a second preferred embodiment of the invention is shown; in particular, fig. 2A shows a cylindrical conductor (101), which also in this case may consist of a metal rod or hollow cylinder, for instance of titanium or other valve metal in the case of an anode, or of nickel or stainless steel in the case of a cathode. Also in this case the cylindrical conductor (101) may be a cylindrical electrode of the prior art, for instance an electrode with an exhausted catalytic coating.

(201) indicates a mesh provided with catalytic coating, flag-welded along the generatrix of the cylindrical conductor (101); the figure shows the junction of the two pieces as a continuous weld (300), for instance obtainable via laser, but also different types of welding such as spot welding are possible. The mesh (201) may also be replaced by an expanded or perforated sheet or in a less preferred embodiment by a solid sheet. Figure 2B shows as in a subsequent step the mesh (201) is folded back so as to enclose the cylindrical core and welded on itself by means of the spot welds (301) after overlapping the opposed edges. In a less preferred embodiment, the mesh (201) may have non overlapping edges, both welded to the conductive core (101); the spot welding (301) may also be replaced by another type of fixing, for example a continuous welding. Also in this case, an appropriate geometry of the mesh (201) may allow a substantial increase of the active surface of the obtained electrode with respect to the direct activation of the cylindrical conductor; furthermore the metallic mesh (201) externally secured to the cylindrical core (101) may be easily replaced once exhausted the catalytic activation, and replaced with a new one.

EXAMPLE

A titanium cylindrical anode for a cell employed in copper electrodeposition tubular cells disclosed in US 6,451,183, consisting of a 15 cm thick titanium hollow cylinder, was activated with a titanium and tantalum oxide-based coating (16 g/m ² overall) over a titanium and tantalum oxide-based intermediate layer, as known in the art. The anode was subjected to a standard oxygen evolution test in 5% sulphuric solution at a temperature of 25°C and at a current density of 500 A/m ² . In the course of eight hours of test a stable voltage of 3.30 V was detected (electrode according to the prior art).

An electrode according to the invention was prepared making use of an identical titanium rod free of catalytic activation, whereto a 0.5 mm thick titanium mesh activated with the same composition of the previous sample according to the prior art was secured, utilising the configuration shown in figure 2; the activated mesh

was therefore firstly secured along a generatrix of the cylinder by continuous welding, then rolled around itself and fixed by means of three weld spots.

The electrode was subjected to the same oxygen evolution test of the previous sample, at identical process conditions. In the course of eight hours of test, a stable voltage of 2.90 V was detected (electrode of the invention).

It was thereby shown how the electrode of the invention, besides solving the inconveniences of the prior art mainly associated with the reactivation of exhausted cylindrical electrodes in a very practical fashion, is also capable, probably due to the greater exposed surface, to operate at a higher energy efficiency (lower voltage) corresponding to a higher expected lifetime.

As it is readily apparent to one skilled in the art, the invention may be practised making other variations or modifications with respect to the above mentioned examples.

The previous description is not intended to limit the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is univocally defined by the appended claims.

Throughout the description and claims of the present application, the term

"comprise" and variations thereof such as "comprising" and "comprises" are not intended to exclude the presence of other elements or additives.