EP0292466A2 - Electrode for an electrolytic cell - Google Patents

Abstract

Electrode for an electrolysis cell intended for the deposition of a metal coating on a substrate. The body of the electrode has a plurality of parallel narrow slots emerging in the surface of the body of the electrode facing the substrate. A first group of slots is connected to means for feeding electrolyte, the other group of slots is connected to means for discharging the electrolyte and the slots of both groups succeed each other alternately in the abovementioned surface of the electrode body. Each slot is connected to an electrolyte feed or discharge channel respectively. <IMAGE>

Description

The present invention relates to an electrode for an electrolysis cell of the type used in particular for operating the deposition of a metallic coating, adherent or detachable, on a substrate.

For an electrolytic deposition operation, it is advantageous to use a high current density, both for the efficiency of the operation and for the quality of the deposition itself. Indeed, the current density conditions on the one hand the duration of the operation, that is to say the speed of the substrate and the size of the installation if the substrate is in motion, and on the other hand the compactness, or conversely the porosity, of the coating deposited.

It is also known that achieving high current densities, for example greater than 100 A / dm², requires high turbulence of the electrolyte in the space between the anode and the cathode, the latter being generally constituted by the substrate. Such high turbulence implies that the electrolyte is driven at a high speed when it flows between the anode and the cathode.

It is also well known that a large part of the electrical energy consumed by an electrolysis operation depends on the Joule effect losses which occur in the electrolyte, and that these losses are proportional to the distance which separates the anode. of the cathode. To minimize these losses and thus achieve as high a return as possible, it is therefore necessary to minimize the distance existing between the anode and the cathode of the electrolysis cell.

Finally, another not insignificant fraction of the electrical energy consumed during such an electrolysis operation serves to compensate for the ohmic losses in the conductive parts of the electrode, generally the anode, which provides the electrical supply of the cell. This electrode is necessarily made of a material resistant to chemical attack by the electrolyte; for this purpose, it is for example made of graphite, a Pb-Ag alloy, or titanium provided with an appropriate coating. Given the importance of the electrical intensities involved, which can exceed 10,000 A / m², it is particularly advantageous to reduce these ohmic losses by using materials having as low an electrical resistance as possible. The electrical supply of the anode will therefore preferably be provided by copper bars, which should conduct the current as close as possible to the interval between the anode and the cathode.

There are several cell arrangements as well as various methods of introducing the electrolyte seeking to achieve the three aforementioned objectives.

To fix the ideas, the description of the prior art as well as of the present invention will more particularly refer to the electrolytic deposition of a coating on a cathode constituted by a metal strip circulating in a horizontal plane. This reference has only a simple example value and it does not exclude the possibility of applying the invention in any situation compatible with the claims which will follow.

A first known solution consists in placing a plane anode parallel to the surface of the strip, and in injecting the electrolyte transversely with respect to the strip. In accordance with the first two objectives, the electrolyte introduced along one edge of the strip and collected on the other edge must therefore circulate at high speed between two flat surfaces close to each other; this arrangement causes high pressure drops and requires high supply pressures of the electrolyte. This results in numerous mechanical problems relating in particular to the sealing of the seals and the deformation of the surfaces present, and which in practice prevent operating with distances of less than 10 mm between the anode and the cathode.

A variant of this solution, consisting in injecting the electrolyte in the longitudinal direction of the strip, faces the same problems and is subject to the same limitations as in the case of transverse injection.

Another proposed solution consists in introducing the electrolyte perpendicular to the surface of the cathode, through one or more transverse slots formed in the anode, and in then collecting it on the edges of the strip. Here again, the supply pressures must be high due to the high pressure drops, which also limits the practicable distance between the anode and the strip to around 10 mm.

Patent BE-A-905,588 by the same applicant proposes a solution which makes it possible to reduce the pressure drops and therefore to avoid the use of high supply pressures. He recommends introducing the electrolyte perpendicular to the surface of the strip by small holes drilled in the anode and taking up this electrolyte by other small holes, close to the first, also drilled in the anode; the anode proper is constituted by a plate of electrically conductive material resistant to the electrolyte, such as graphite, this plate ensuring the supply of current from the lateral supply conductors up to the interval electrolysis. Such an arrangement certainly makes it possible to reduce the distance between the anode and the strip; however, it does not always ensure a perfectly homogeneous flow of the electrolyte in the interval between the anode and the strip, and the low resistance supply conductors do not yet arrive close enough to this interval to reduce losses. ohmic to a sufficient extent.

The present invention provides an electrode intended for an electrolysis cell of the aforementioned type, the design and constitution of this anode being such that it makes it possible to largely reach the three objectives mentioned above, namely a high speed of the electrolyte in a very narrow interval between the anode and the cathode, ohmic losses reduced to a minimum, as well as a homogeneous scanning of the interval between the anode and the cathode by the electrolyte.

According to the present invention, an electrode for an electrolysis cell intended for depositing a metal coating on a substrate, which comprises an electrode body comprising means for supplying electric current and having at least one profiled surface of corresponding to the surface of said substrate, is characterized in that said electrode body has a plurality of narrow parallel slots opening into said surface of the electrode body, in that a first group of said slots are connected to means electrolyte supply, in that the other group of said slots are connected to means for discharging the electrolyte and in that the slots of one and the other of said groups alternate in succession in said surface of the electrode body.

In a manner known per se, said electrode body is preferably made of an electrically conductive material which is chemically resistant to the electrolyte, such as graphite.

According to a first additional characteristic of the invention, said slots have a width which does not exceed 3 mm.

According to another additional characteristic of the invention, the distance between two neighboring slots is between 20 mm and 50 mm.

In addition, said slots advantageously extend in the electrode body perpendicular to the surface of the electrode body into which they open.

At its end remote from said surface, each of said slots opens, according to the invention, into a channel formed in said electrode body and disposed parallel both to the plane of said surface of the electrode body and to the longitudinal direction of said slots .

Each of these channels is normally closed at one of its ends and it is connected by its other end either to means for supplying electrolyte, or to means for discharging the electrolyte, depending on the slot with which it is found. in communication. These channels are advantageously arranged head to tail, the closed end of any one of these channels being close to the open end of the adjacent channel or channels.

According to a particular advantageous embodiment, the electrode body is composed of a plurality of juxtaposed plates profiled so as to form, during their assembly, the narrow slots and the supply and discharge channels of the electrolyte required by the invention.

According to another characteristic of the invention, said electrode body is provided with means for supplying electric current constituted by bars made of a material which is a good conductor of electricity, which penetrates into said electrode body up to proximity of said slots, respectively of the channels, for supplying electrolyte.

In the case where said electrode body consists of juxtaposed plates, said bars are advantageously housed in cavities formed in at least one lateral face of said plates.

Preferably copper bars are used, the resistivity of which, clearly lower than that of the material of said electrode body, enables the electric current to be conducted, without appreciable ohmic losses, as far as immediate proximity to the electrolyte circulating in the slots, respectively the supply channels.

• Fig. 1 montre schématiquement en perspective la configuration des nap­pes d'électrolyte obtenues avec une électrode conforme à l'in­vention; la
• Fig. 2 est une vue en plan illustrant schématiquement la position re­lative des fentes d'alimentation et d'évacuation de l'élec­trolyte; la
• Fig. 3 est une vue en plan, similaire à celle de la Fig. 2, montrant schématiquement une électrode composée de plaques juxtaposées transversalement; la
• Fig.4a est une coupe transversale suivant la ligne A-A′ de la Fig. 3, montrant la disposition relative des fentes et des canaux, dans le cas d'une électrode plane; la
• Fig.4b représente un détail de la surface inférieure du corps de l'é­lectrode, vue suivant la direction de la flèche B; la
• Fig. 5 est une vue en plan, similaire à celle de la Fig. 2, montrant schématiquement une électrode composée de plaques juxtaposées en oblique; les
• Fig. 6a et 6b sont analogues aux Fig. 4a et 4b pour une électrode constituée comme le montre la Fig. 5; la
• Fig. 7 montre schématiquement, en coupe transversale, la disposition relative des fentes et des canaux dans le cas d'une électrode cylindrique; la
• Fig. 8 est une coupe transversale analogue à celle de la Fig. 4a, mon­trant un autre mode de réalisation des canaux et des fentes dans une anode conforme à l'invention; et la
• Fig. 9 illustre également en coupe transversale, une façon supplé­mentaire de réaliser les canaux et les fentes dans une anode conforme à l'invention.

Other features and advantages of the invention will appear on reading the description, given below by way of example, of a preferred embodiment of an electrode according to the invention. In this detailed description, reference is made to the accompanying drawings, in which the

• Fig. 1 schematically shows in perspective the configuration of the electrolyte layers obtained with an electrode according to the invention; the
• Fig. 2 is a plan view schematically illustrating the relative position of the electrolyte supply and discharge slots; the
• Fig. 3 is a plan view, similar to that of FIG. 2, schematically showing an electrode composed of plates juxtaposed transversely; the
• Fig.4a is a cross section along line AA ′ of Fig. 3, showing the relative arrangement of the slots and the channels, in the case of a planar electrode; the
• Fig.4b shows a detail of the lower surface of the body of the electrode, seen in the direction of arrow B; the
• Fig. 5 is a plan view, similar to that of FIG. 2, schematically showing an electrode composed of plates juxtaposed obliquely; the
• Fig. 6a and 6b are similar to Figs. 4a and 4b for an electrode constituted as shown in FIG. 5; the
• Fig. 7 schematically shows, in cross section, the relative arrangement of the slots and channels in the case of a cylindrical electrode; the
• Fig. 8 is a cross section similar to that of FIG. 4a, showing another embodiment of the channels and slots in an anode according to the invention; and the
• Fig. 9 also illustrates in cross section, an additional way of producing the channels and the slots in an anode according to the invention.

In all the figures, identical or analogous elements are designated by the same reference numerals. The direction of flow of the electrolyte and the electric current is indicated by appropriate arrows. Finally, the elements which are not directly necessary for understanding the invention have not been shown, so as not to unnecessarily overload the drawings.

The figures refer to an electrolysis cell intended to deposit a coating on a continuous metallic substrate in movement, such as a steel strip, which constitutes the cathode of the cell. The electrode according to the invention then constitutes the anode of the cell. In the envisaged application, this anode is arranged in such a way that the electrolyte supply and discharge slots are oriented transversely with respect to the direction of movement of the cathode. It goes without saying that this cathode could be an endless strip on which a detachable coating is deposited, in particular a strip of titanium or of titanium alloy, possibly provided with a protective coating. Furthermore, the anode could be arranged in such a way that said slots are oriented in any way, for example obliquely, relative to the direction of movement of the cathode.

Fig. 1 schematically shows, in perspective, the configuration of the electrolyte plies circulating on the one hand in the slots of the anode and on the other hand in the space between the anode and the cathode. The anode proper is not shown and the cathode consists of a metal strip 1 moving in the direction of arrow 2.

The electrolyte is supplied in thin vertical sheets 3, flowing from top to bottom (arrows 4) in the supply slots of the anode (not shown); in the space between the anode and the cathode, it is subdivided into two horizontal layers such as 5 and 6, of small thickness, which separate from one another; then, concurrent horizontal plies such as 6 and 7 are taken up in vertical plies 8 flowing from bottom to top (arrows 9) in the evacuation slots of the anode (not shown).

The supply 4 and discharge 8 plies are produced by corresponding slots connected respectively to means for supplying and discharging the electrolyte, according to an arrangement which is schematically illustrated in FIG. 2.

This Fig. 2 is a plan view of an anode according to the invention, which comprises a body 10 disposed transversely with respect to the cathode 1. The body 10 of the anode has an inlet (11) and an outlet (12) for the electrolyte, respectively connected to external supply and discharge means, not shown. The body 10 of the anode comprises a distribution chamber connected to the inlet 11 and connected to supply branches 13 in which the supply slots 14 are formed, drawn in solid lines, which open into the space included between cathode 1 and body 10 of the anode. These branches and supply slots are traversed by the vertical layers of electrolyte symbolized by the descending arrows 4. After a short path parallel to the cathode, the electrolyte is taken up by the discharge slots 15, drawn in broken lines which , by evacuation branches 16 and a collection chamber, return it towards the exit 12. The slots and the evacuation branches are traversed by the vertical layers of electrolyte symbolized by the ascending arrows 8. In this schematic illustration, the supply branches 13 are separated from the discharge branches 16 by a labyrinth wall 17.

The advantage of such an arrangement is that the variable pressure losses along a supply branch are compensated by the pressure losses varying in opposite directions along a discharge branch. This results in a homogeneous flow of the electrolyte over the width of the strip.

In Fig. 3, there is shown schematically an advantageous implementation of an anode of the type illustrated in FIG. 2, implementation in which the body 10 of the anode is composed of a plurality of transverse plates 18, 19, 20, 21, arranged perpendicularly to the plane of the cathode 1.

The constitution of this anode will be more easily understood by referring to FIG. 4, in particular in FIG. 4a, which partly represents a section along the line AA ′ in FIG. 3. The body of the anode 10 is composed of a series of plates such as 18, 19, 20, 21 juxta laid and assembled by appropriate means, for example bolts 22. The configuration of the anode assembly being repetitive, it will suffice to describe the relative arrangement of four plates such as 18, 19, 20 and 21.

These plates are arranged vertically, and their lower face is located at a very short distance from the upper surface of the cathode 1 to be coated. They are made of graphite or an electrically conductive and electrolyte resistant alloy. In the right face of the odd plates (19, 21), a half-channel 23 and a half-slot 24 have been machined intended for the evacuation of the electrolyte, while in the left face of these same odd plates (19 , 21), a half-channel 25 and a half-slot 26 have been machined intended for the supply of electrolyte. Conversely, on the left side of the pay plates (18, 20), on the one hand, a half-channel 27 and a half-slot 28 have been machined intended for the evacuation of the electrolyte, while on the right side of these same pair plates (20) a half-channel 29 and a half-slot 30 have been machined intended for the supply of electrolyte. The assembly of the plates makes it possible to form the channels and the slots for supplying and discharging the electrolyte. Of course, the end plates such as 18 are not machined on their outer face. The body 10 of the anode also includes copper bars 31 which are housed in grooves machined in the plates 19, 20 and which penetrate as close as possible to the supply channels 25, 29.

To seal the assembly, the contact surfaces between the plates are lined with seals such as 32, 33. Furthermore, the evacuation slots 24, 28 are advantageously discontinuous in the transverse direction of the strip, as shown in Fig. 4b. The slit sections are separated by short contact zones which practically do not disturb the discharge flow of the electrolyte, but which ensure mechanical contact between the plates 18 and 19 in the event of an increase in the pressure in the feed slots 26, 30. The plates 18, 19 cannot flex due to this contact. It is therefore possible to increase the supply pressure of the electrolyte and consequently improve the performance of the cell without run the risk of causing deformation of the entire anode.

As shown in Fig. 3 and in accordance with the arrangement shown diagrammatically in FIG. 2, the channels 24, 29 and the supply slots 26, 30 communicate with the inlet 11 of the electrolyte, while the channels 23, 27 and the discharge slots 24, 28 communicate with the outlet 12 of the 'electrolyte.

This arrangement is particularly advantageous, since the slots and the channels can be produced by simple machining operations of the various plates.

In certain circumstances, however, it is not possible to manufacture sufficiently strong transverse plates. It is then possible to use another variant of the electrode according to the invention.

This variant is illustrated schematically in FIGS. 5 and Figs. 6a and 6b which respectively show a plan view, a partial section along the line AA ′ and a bottom view along the arrow B. The body 10 of the anode is composed of a plurality of modular elements 34 whose width is determined by the existing machining possibilities. These elements are arranged obliquely to the direction of movement of the strip 1. FIGS. 6a and 6b show the channels 25 and the supply slots 26 as well as the channels 27 and the discontinuous slots 28 for discharging the electrolyte.

This oblique arrangement makes it possible to mask the discontinuity effect caused by the joints between the various modular elements, by distributing these joints over the entire width of the strip.

Fig. 7 illustrates the configuration of an electrode according to the invention profiled so as to correspond to a cylindrical substrate. In this variant, the plates such as 20, 21 have a trapezoidal section. The various channels and the various slots are also produced by simple machining operations. The pla they must be assembled by appropriate means which guarantee compliance with the curvature of the assembly.

One can of course imagine other embodiments of the channels and slots for supplying and discharging the electrolyte, without departing from the scope of the present invention.

Fig. 8 shows, by a cross section similar to that of FIG. 4a, the constitution of an anode in which the channels are formed by a suitably folded metal sheet.

The sheet 17, for example made of titanium, is folded back so as to form successive folds which alternately constitute channels or branches 13 for supply and discharge 16 of the electrolyte. To ensure the overall rigidity of the anode, the folds of the sheet 17 have the shape of alternate trapezoids, the large bases of which are welded respectively to an upper plate 35 and to a lower plate 36, both parallel to the lower face. of the body of the anode 10. By the very nature of the folding, only the large bases of the trapezoids are formed by the sheet 17, while the small bases of these trapezoids are closed only by said plates 35 and 36.

In the area of the bottom plate 36 which closes the small base of the supply channels 13 are perforated supply orifices 37 distributed over the width of the anode. Likewise, in the large bases of the evacuation channels 16 welded to this same bottom plate 36, evacuation orifices 38 are drilled distributed over the width of the anode. The supply ports 37 and the discharge ports 38 communicate respectively with supply slots 14 and discharge slots 15 comprised between transverse blocks 39. These blocks 39 are removable and can be easily replaced to modify the width slots or restore the desired width of a slot in case of excessive widening, for example due to the wear of a block.

The relative arrangement of the bottom plate 36, the orifices 37, 38 and the blocks 39 with the slots 14, 15 is illustrated in perspective in the box of FIG. 8.

The blocks 39, the lower 36 and upper 35 plates as well as the folded sheet 17 form the circuit for supplying the anode with electric current; all these elements are therefore preferably made of titanium, as are the upper sections, marked with a broken arrow, which symbolize the connection of the anode to the source of electric current.

Fig. 9 illustrates, in vertical section A-A ′ and in horizontal section C-C ′, another way of producing the supply channels 13 and discharge 16 of the electrolyte, by means of parallel vertical separation walls 17. These walls are provided with cylindrical bulges 40 in which are pierced passages through which tie rods 41. These tie rods 41 serve on the one hand to fix the walls 17 to upper 35 and lower 36 horizontal plates and on the other hand to bring the electric current to the blocks 39. The feed channels 13 are connected to the feed slots 14 by feed holes 37 drilled in the bottom plate 36, likewise, the discharge slots 15 are connected to the discharge channels 16 by evacuation orifices 38 drilled in the bottom plate 36.

The walls 17 and the upper 35 and lower 36 plates can be made of a composite material resistant to the electrolyte, for example of epoxy resin reinforced with glass fibers.

The electric current is supplied by the tie rods 41, one end of which is for example screwed into the blocks 39 and the other end of which has a head which, by a copper plate 32 and a spring 43, bears on the plate upper 35. The copper plate 42 is connected to a source of electric current by known means.

The electrode according to the invention can be placed at a very short distance from the substrate to be coated, since the electrolyte travels only a very short path between the anode and the cathode; it therefore does not undergo high pressure drops and a moderate pressure ensures a speed and a turbulence very favorable to the electrolytic deposition. The electrically active parts cover the entire surface of the electrode, with the exception of the small portion corresponding to the electrolyte supply and discharge slots. The electrode therefore ensures a homogeneous distribution of the electric current with low ohmic losses, which leads to an excellent efficiency of the operation. Finally, the entire surface of the substrate is subjected to a homogeneous sweep by the electrolyte, which also promotes the homogeneity and the yield of the coating deposition.

Claims (8)

1. Electrode for an electrolysis cell intended for depositing a metal coating on a substrate, which comprises an electrode body comprising means for supplying electric current and having at least one surface profiled corresponding to the surface of said substrate, characterized in that said electrode body has a plurality of parallel narrow slots opening into said surface of the electrode body, in that a first group of said slots are connected to means for supplying electrolyte, in that the other group of said slots are connected to means for discharging the electrolyte and that the slots of one and the other group alternate in succession in said surface of the electrode body. 2. Electrode according to claim 1, characterized in that the width of said slots does not exceed 3 mm. 3. Electrode according to either of Claims 1 and 2, characterized in that the distance between two adjacent slots is between 20 mm and 50 mm. 4. Electrode according to either of claims 1 to 3, characterized in that said slots extend in the electrode body perpendicular to said surface into which they open. 5. An electrode according to any one of claims 1 to 4, characterized in that at its end remote from said surface, each of said slots opens into a channel parallel to said surface of the electrode body, said channel being closed at one of its ends and being connected by its other end to said supply means, respectively for discharging the electrolyte. 6. Electrode according to either of Claims 1 to 5, characterized in that the said electrode body is composed of more reality of juxtaposed plates, and in that said plates are machined so as to form, during assembly, the slots, respectively the channels which constitute the circuit of the electrolyte. 7. An electrode according to either of claims 5 and 6, characterized in that said channels are arranged head to tail, the closed end of any of the channels being close to the open end of the adjacent canals. 8. Electrode according to either of Claims 1 to 7, characterized in that the said means for supplying electric current penetrate into the said body of the electrode as close as possible to the slots, respectively to the supply channels. in electrolyte.

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