EP0580730B1 - Electrode for an electrolytic cell, use thereof and method using same - Google Patents

Abstract

PCT No. PCT/BE92/00022 Sec. 371 Date Feb. 23, 1994 Sec. 102(e) Date Feb. 23, 1994 PCT Filed May 27, 1992 PCT Pub. No. WO92/21794 PCT Pub. Date Dec. 10, 1992The present invention relates to an electrode preferably an insoluble electrode for an electrolytic cell. The electrode is located within an enclosure defining a chamber, a wall of said enclosure being formed by a membrane allowing ions to pass therethrough. The enclosure has an opening for feeding electrolyte, an opening for evacuating electrolyte and means conducting the upward current of electrolyte with a velocity in the vicinity of the electrode of at least 0.01 m/s. The invention relates also to plants and processes using such electrode for the plating or deplating of metal strips.

Description

The present invention relates to an electrode, preferably an insoluble electrode for an electrolytic cell.

Insoluble electrodes are commonly used in methods of electrochemically coating strips of metal, preferably strips of galvanized or galvanized steel, using metals or metal alloys, in accordance with which a electrolyte charged with coating metal salts between the cathode metal strip to be coated and the insoluble anode.

Depending on the electrolyte used, for example electrolytes based on sulphate or chloride, the implementation of this process generates gases at the anode, for example oxygen or chlorine, which undergo partially undesirable bonds with the coating metals, or which, because of their aggressiveness or their toxicity, have harmful consequences during the implementation of the coating process or for the environment. These gases which arise at the anode mix with the electrolyte and can therefore cause unwanted reactions and enter the environment, since when electroplating metal strips, the electrolyte circuit on the tape can not be separated from the atmosphere.

A process is known for coating galvanized steel strips using iron compounds or alloys thereof. To do this, a sulfate-based electrolyte charged with coating metal salts is conducted in a closed circuit between the steel strip to be coated circulating endlessly and the insoluble anodes. Due to known electrochemical processes, iron precipitates in the form of iron compounds at the cathode metal strip. Bivalent oxygen is released at the anode, oxygen which comes into contact with metal salts, more particularly due to the recycling of the electrolyte. This oxygen oxidizes part of the bivalent iron to trivalent iron, so that large quantities of iron oxide arise, which contaminate the electrolytes and which must be separated from the circuit by implementing expensive filtration processes.

On the other hand, the formation of Fe ³⁺ reduces the cathodic efficiency of the current and deteriorates the adhesion of the deposited layer.

Finally, the use of salts of the coating metal, or the use of iron in corresponding dissolution plants and the replacement of other substances used jointly entrained, very significantly increase the cost of such a coating process.

To solve these problems, the applicants have developed a particular electrode which is particularly useful in methods of electrochemically coating metal strips, but which is also suitable in other processes, such as methods for electrochemically removing a coating from a strip. such as a steel strip.

The electrode according to the invention is placed in an envelope defining a chamber and a wall of which is formed of a membrane allowing the passage of ions through it, said envelope having a first opening for supplying the chamber with a electrolyte and a second opening to evacuate from the electrolyte chamber.

The envelope is provided with strips, fins or baffles intended to ensure a speed of the electrolyte in the vicinity of the electrode of at least 0.01 m / s, and preferably greater than 0.1 m / s, in particular at 0.5 m / s.

The lamellels, baffles or fins direct the flow of electrolyte into the chamber or part of it.

In one embodiment, the baffles or fins extend from the vicinity of the first opening of the envelope to the vicinity of the second opening of the envelope, so as to advantageously divide the chamber into several separate compartments extending between the electrode and a wall of the chamber or envelope, in particular the membrane.

In another embodiment, said baffles or fins create, in the vicinity of the electrode, an at least partially ascending current of the electrolyte. According to a feature of this embodiment, the baffles or fins extend in a substantially vertical direction from the vicinity of the lower part of the envelope to the vicinity of the upper part of the envelope so as to define channels bringing the electrolyte into said upper part of the envelope, this part having an opening for sucking out of the gas chamber and an opening for the discharge of the electrolyte.

Advantageously, the baffles or fins extend at least from one edge of the electrode to the opposite edge of the latter.

The membrane is preferably an anionic or anion exchange membrane or a cationic or cation exchange membrane. It is advantageously provided on the outside of the envelope with a layer or a protective veil, for example made of synthetic material (polymer, polyes ter, ....) advantageously reinforced with fibers (glass).

Preferably, a porous support extends in the vicinity of the membrane and serves as a support for at least part of it. Such a support is, for example, a perforated element, a porous veil, a mesh advantageously made of Zr, Ti or stainless steel.

In one embodiment, the support has on the face opposite that adjacent to the membrane a layer acting as an electrode, while according to another embodiment the membrane is supported on a support playing the role of electrode, said support being provided with an insulating layer on its face adjacent to the membrane.

The membrane of an electrode according to the invention advantageously has a thickness between 50 and 150 u. In the case of an anionic membrane, it preferably has a multilayer structure, at least one layer being obtained by grafting an amino monomer or a precusor of an amino compound onto a polymer support and by crosslinking.

The present invention also relates to the use of an electrode according to the invention in an electrolytic cell.

Finally, it also relates to an electrochemical coating process for galvanized steel strips using metals or metal alloys. In this process, an electrolyte charged with coating metal salts is recycled in known manner between the metal strip to be coated (cathode) and the insoluble anode. According to the process according to the invention, an electrode according to the invention is used as the insoluble anode. The membrane is arranged between the anode and the metal strip to be coated so as to form a separation between a cathode space adjacent to the strip and the anode chamber defined by the envelope of the anode. In the process according to the invention, a first primary electrolyte circuit is created in the chamber and a second secondary electrolyte circuit in the cathode space, the membrane preventing the transfer of gases generated at the anode in the second electrolyte circuit and the transfer of coating metal salts from the cathode space to the first electrolyte circuit. In this case, the gases remain in the electrolyte circuit maintained separately in the anode space and can be evacuated regularly. The electrolyte in the anode circuit is not charged with the coating metals. The gas which still forms can be removed from this circuit in a relatively simple manner. The two circuits are clearly separated from each other, so that mixtures cannot occur.

Claims 19 to 22 have proposed methods according to the invention for coating metal strips, preferably steel strips galvanized with iron, iron compounds or an iron-containing alloy. . Depending on the nature of the electrolyte used in the space or the cathode enclosure, the use is made, as diaphragms, of cation exchange membranes, or of anion exchange membranes known as such. By a corresponding modification or adaptation of the electrolytes, it is also possible to use what are called bipolar membranes.

If there is an anion exchange membrane of a suitable nature between the anode and the metal strip to be coated, it is possible, when using a sulfuric electrolyte, enriched with iron and zinc sulfate, in cathodic space, to ensure the only transfer of ions S0 ₄ - in the anode chamber as a charge transport and to prevent the transfer of coating metal salts. The electrolyte devoid of metal and consisting of water and sulfuric acid in the anode chamber is enriched here with sulfuric acid. The oxygen which forms at the insoluble anode can be evacuated from the anode chamber. The transfer of oxygen into the cathode space is prevented by the corresponding anion exchange membrane.

When using a cation exchange membrane according to claim 21 and when using a sulfuric electrolyte in the cathode space, the charge transport takes place by the transfer of hydrogen ions from the anode chamber into cathodic space. The oxygen which also forms here at the anode is evacuated from the sulfuric electrolyte devoid of iron from the anode circuit. The transfer of oxygen into the cathode space is also prevented by this cation exchange membrane.

When using a chlorinated electrolyte, enriched with iron or zinc chloride, in the cathode space, it is also possible, in accordance with the present invention, to use appropriate anion exchange membranes. During the implementation of this process, the penetration of chlorine ions into the anode chamber is allowed as charge carriers. The transfer of metal salts into the anode space is however prevented. The electrolyte, which consists of water and hydrochloric acid, in the anode chamber is enriched with chlorine ions released in the form of gas at the anode and advantageously removed in a controlled manner with the electrolyte circuit of the anode chamber. . Transfer of chlorine to the cathode space is prevented by the appropriate exchange membrane.

When using a chlorinated electrolyte in the cathode space, it is also also possible to use cation exchange membranes of suitable nature. In this case also, the transfer of acids and salts from the cathode space to the anode space is again prevented. The charge transport takes place by the transfer of hydrogen ions from the anode space or chamber into the cathode space. The gases separated at the anode are evacuated. Transfer of the separated gases into the cathode space is prevented by the cation exchange membrane.

Thanks to this process in accordance with the present invention of coating with metal bands, the formation of trivalent iron and iron oxide is completely prevented, which, when the known process is carried out, form sludges of iron in the electrolyte, this sludge resulting from oxidation due to the oxygen released at the anode.

Since it is not possible to completely prevent the action of atmospheric oxygen in the cathode circuit, when coating with iron operated in accordance with the present invention, a certain amount of trivalent iron is also formed in the cathodic circuit. This trivalent iron contaminates the cathode circuit, so that this electrolyte must also still be filtered. In accordance with the present invention, it is therefore proposed to supply the cathode electrolyte circuit, with a view to replacing the iron removed during the coating, with a corresponding proportion of edible iron, for example in an intermediate dissolution station. Due to the excess, the necessary proportion of added elemental iron is sufficient to reduce the trivalent iron to divalent iron, so that no more iron oxide mud is formed in the circuit of the cathode electrolyte.

The sulfuric acid which is enriched in excess when a sulfuric electrolyte is used in the cathode circuit and when an anion exchange membrane is used in the anode circuit, is used in the dissolution station and is thus returned to the cathode circuit, where the rate of dissolution of iron and other coating metals, for example zinc, is considerably accelerated.

The chlorine gas that forms at the anode when using a chlorinated electrolyte in the cathode circuit and an anion exchange membrane is evacuated by suction from the anode circuit and is burned in hydrochloric acid by l Hydrogen gas formed in the dissolution station and serves to accelerate the dissolution of metals and is therefore returned to the cathode circuit via the dissolution station.

The present invention also relates to a method for removing a layer of metals or of a metal present on a metal strip such as a steel strip. This layer of metals or of metal is for example an electrolytically deposited layer such as a protective layer of Zn or of a Zn alloy. As a particular example, the layer of Zn or Zn alloy deposited as a protective layer on a face or strip has a thickness of between 0.1 and 2 microns (preferably less than 1 micron). Such a layer is preferably obtained by subjecting the strip to an electrolytic treatment in a bath containing from 15 to 100 g / I, advantageously from 30 to 80 g / I of Zn. The current density in the cells is for example between 20 and 200 A / dm ² but is preferably between 40 and 150 A / dm ² . For this deposition, it is advantageous to use the electrode according to the invention.

During this deposition, the strip and possibly the electrolyte of said cells are set in motion in the cells. The relative speed of the strip with respect to the electrolyte is advantageously between 1 and 8 m / s, preferably between 3 and 5 m / s.

In the process according to the invention for removing a layer of a metal or metal alloy from a strip, an electrolyte is recirculated between the strip acting as an anode and a insoluble cathode, an advantageously anionic membrane being arranged between the strip and the cathode, so as to form a separation between a cathode space and an adjacent anode space of the strip.

This membrane makes it possible to remedy that metals dissolved in the electrolyte such as for example Zn and / or Ni do not form a deposit (black in the case of Zn and Ni) on the cathode, this deposit not only decreasing the efficiency of the cathode, but above all the life or use thereof.

This membrane can be a porous veil (pores of a few microns, 1 to 50 μ), but is preferably an anionic membrane, that is to say a membrane which does not allow or limit the passage of cations (such as Zn ^{+ +} , Ni ++, Fe ⁺⁺ ) through it.

In the case where an acid electrolyte is used, it has been noticed that on the surface of the cathode an evolution of hydrogen existed. To prevent hydrogen bubbles from coming together to form large bubbles, it was noted that it was useful to use the membrane as the wall of a chamber adjacent to the cathode and to maintain a current in said chamber. or so-called secondary electrolyte flow.

The speed of the electrolyte in the chamber is for example greater than 0.1 m / s, but is preferably less than 1.5 m / s to ensure that the hydrogen bubbles do not come together to form large bubbles.

The electrolyte circulating in the chamber adjacent to the cathode, hereinafter called secondary electrolyte, preferably has a composition different from the primary electrolyte, that is to say the electrolyte in contact with the strip. The secondary electrolyte is advantageously an electrolyte not containing Zn and Ni but containing 50 to 100 g / I of Na ₂ S0 ₄ and the pH of which is preferably adjusted to a value of 1.5 to 2.

The upper part of the chamber is also preferably subjected to a gas suction. For example, a vacuum is created in the upper part of the chamber such that the pressure in the chamber is less than 0.75 x atmospheric pressure.

The primary electrolyte used in the deplating cell may for example be an electrolyte containing less than 50 g / I of free acid, advantageously less than 5 g / I, preferably about 1 g / I of free acid (per example of SO ₄ - free). The pH of the electrolyte is advantageously 1.5 to 2.

The current density used in the deplating cell (cell for removing a metallized layer) in which the cathode is placed in a chamber is advantageously less than 60 A / dm ² , but is preferably between 15 and 30 A / dm ² in the case of an acid electrolyte.

The temperature of the primary and secondary electrolyte is advantageously between 20 and 60 _° C, preferably between 40 and 60 _° C.

Other features and details of the invention will emerge from the following detailed description in which reference is made to the accompanying drawings.

• - les figures 1 à 5 montrent diverses formes de réalisation d'électrodes suivant l'invention,
• - les figures 6 et 7 montrent des électrodes similaires à celle représentée à la figure 2, mais utilisées dans une cellule d'électro-déposition,
• - la figure 8 est une vue en élévation avec arrachement partiel d'une forme de réalisation préférée d'une électrode suivant l'invention,
• - les figures 9 et 10 sont des vues en coupe selon les lignes IX-IX et X-X de l'électrode représentée à la figure 8,
• - la figure 11 montre en perspective et à plus grande échelle une partie des lamelles de l'électrode représentée à la figure 8,
• - la figure 12 est une vue schématique d'une installation utilisant des électrodes suivant l'invention,
• - les figures 13 et 14 montrent en coupe et à plus grande échelle une bande d'acier qui a été obtenue en utilisant des électrodes suivant l'invention, et
• - les figures 15 à 18 montrent des formes de réalisation particulières d'installation utilisant des électrodes suivant l'invention.

In these drawings:

• FIGS. 1 to 5 show various embodiments of electrodes according to the invention,
• FIGS. 6 and 7 show electrodes similar to that shown in FIG. 2, but used in an electro-deposition cell,
• FIG. 8 is an elevational view with partial cutaway of a preferred embodiment of an electrode according to the invention,
• FIGS. 9 and 10 are sectional views along lines IX-IX and XX of the electrode shown in FIG. 8,
• FIG. 11 shows in perspective and on a larger scale a part of the strips of the electrode shown in FIG. 8,
• FIG. 12 is a schematic view of an installation using electrodes according to the invention,
• FIGS. 13 and 14 show in section and on a larger scale a steel strip which has been obtained by using electrodes according to the invention, and
• - Figures 15 to 18 show particular embodiments of installation using electrodes according to the invention.

FIG. 1 shows in perspective an electrode according to the invention which is advantageously used in a deplating cell but which can also be used for the deposition of Zn, Zn-Ni, Zn-Fe, or another Zn alloy.

This electrode comprises a support 75 carrying a plate 76 intended to form the anode or the cathode. The support 75 forms an envelope having a window in which a membrane 77 is placed.

The membrane 77 forms a wall of the envelope which defines a chamber 78, 79. This membrane allows the passage of ions such as anions or cations.

The envelope has a first opening 100 for supplying the chambers 78,79 with electrolyte and a second opening 101 for removing chambers 78,79 from the electrolyte.

To ensure a minimum velocity of the electrolyte in the vicinity of the electrode 76 for discharging gases (such as oxygen when the electrode works as an anode in a process according to claim 19 or hydrogen when the electrode works in as a cathode in a deplating cell process) formed in the vicinity of the electrode, the envelope is provided with a guide wall or fin 102 extending between said first and second openings, so as to divide the 'envelope in two compartments 78, 79 adjacent, but separate from each other. The fin 102 extends between the electrode 76 and the wall of the envelope provided with the membrane.

With this fin 102, it was possible to ensure in the compartments 78,79 in the vicinity of all points of the electrode or plate 76, an electrolyte speed of at least 0.04 m / s. In the case shown in FIG. 1, the electrolyte was brought into the compartments 78, 79 with a speed of the order of 0.5 m / s. The electrode 76-membrane 77 distance was 0.5 cm.

Figure 2 shows in section another embodiment of an electrode according to the invention.

The electrode 80 made of titanium but provided with an active layer is secured to a support 81 by means of arms 82.

The support 81 forms with a membrane 83 an envelope surrounding the electrode 80. This membrane 83 is fixed on a grid or lattice 84 made of titanium and is provided on its face opposite to that adjacent to the electrode with a porous film 85 protecting the membrane. This film is resistant to acids and is reinforced with fibers. This film is for example a polyester film.

When such an electrode is used in a deplating cell, the membrane is anionic. Such a membrane is for example of multilayer structure, each of the layers being made up of a membrane obtained by the method described in FR-8900115 (application number). A membrane of this type is prepared by grafting an amino compound onto a polymeric support (film of ethylene-co-polytetrafluoroethylene) and by crosslinking thereof.

During an operation to remove unwanted Ni deposited on the first layer of Zn, Zn ⁺⁺ and Ni ++ ions leave from the face of the strip facing the cathode. These cations do not know how to cross the anionic membrane so that one avoids obtaining a rapid deposition of Zn, Ni on the cathode. This increases the lifetime or use of the electrode.

In the envelope formed by the support and the membrane, hydrogen is released in the vicinity of the cathode, while SO ₄ anions - pass through the membrane to exit the envelope.

To evacuate the gas formed in the envelope (hydrogen in the electrode of the "deplating" cell as described above), has an opening 103. Advantageously, this opening 103 allows communication of the chamber 78 with a conduit 104 on which is mounted a suction system (vacuum pump, fan, etc.) not shown.

To avoid the formation of large gas bubbles (hydrogen) which impair correct functioning of the electrode, the envelope is connected to an electrolyte circulation device in the envelope and to a gas elimination system, in particularly a system creating a vacuum in the upper part of the chamber, this vacuum being advantageously such that the pressure in the upper part of the chamber is less than 0.75 x atmospheric pressure.

The velocity of the electrolyte in the chamber was greater than 0.1 m / s, but is however preferably less than 1.5 m / s. Such a speed ensures that the gas bubbles (hydrogen in this case) do not meet to form large bubbles disturbing the correct functioning of the electrode.

Figures 6 and 7 show an electrode similar to that shown in Figure 2. It is used as a cathode to electrolytically deposit Zn and Fe on the steel strip.

In the case of FIG. 6, the membrane 83 is a cationic membrane so that the SO 4 anions formed in the vicinity of the strip 3 remain in the primary electrolyte, while the iron and the zinc are deposited on the strip. At cathode 80, oxygen is released (oxygen from the decomposition of water) and is evacuated through line 104.

In the case of FIG. 7, the membrane 83 is an anionic membrane allowing the passage of the SO ₄ anions - formed in the vicinity of the strip 3 towards the cathode 80. The oxygen released at the cathode is evacuated through the conduit 104.

In FIG. 3, the envelope surrounding the electrode 80 is always formed by a support 81 and a membrane 83. The electrode consists of a mesh or perforated plate of titanium or zirconium provided on the face facing the chamber 78 defined by the envelope of an active layer 87.

The membrane 83 is carried by the mesh and is coated with a porous protective layer 85.

The electrode shown in Figure 4 is similar to that shown in Figure 5 except that a porous insulating web 88 is placed between the mesh and the membrane.

Figure 5 shows in section another embodiment of an electrode according to the invention.

This electrode 80 is adjacent to the membrane 83 which closes the window of the envelope. This envelope defines an interior chamber 78 and has an opening or passage 100 for bringing electrolyte into the chamber 78, an opening or passage 101 for discharging electrolyte from the chamber 78 and an opening 103 for discharging formed gases. in the chamber, in particular in the vicinity of the electrode 80. These openings are located at a level higher than the electrode 80 and the membrane 83.

A fin 105 extends between two opposite walls of the envelope (front wall 106 provided with the membrane 80 and rear wall 107) so as to define a corridor 108 for bringing the electrolyte entering the chamber 78 through the passage 100 to the near the bottom 781 of the room. This corridor 108 is extended by a distribution chamber 109 adjacent to the bottom 781. This distribution chamber 109 has a wall 110 having a series of orifices 111 for distributing the electrolyte in a series of channels 112 defined between vertical fins 113. These fins 113 extend from the vicinity of the bottom 781 of the chamber or more exactly from the wall 110 to the vicinity of the upper part or more exactly to a higher level B but adjacent to the upper level A of the membrane 83 and of the electrode 80. The fins 113 which therefore extend between at least two opposite edges 120-121 of the electrode ensure an upward movement of the electrolyte along the electrode 80, such a movement (from the edge lower 120 towards the upper edge 121) favoring the evacuation of gas particles out of the electrolyte towards the upper part of the chamber advantageously subjected to a vacuum (suction gas through the passage or opening 103).

The fins in the direction of their width extend from the electrode 80 to the rear wall 107 of the envelope so as to define separate channels 112 extending from the distribution chamber or distribution of the electrolyte 109 to the top of the envelope.

Figure 6 is a partially broken away front view of an embodiment of an electrode according to the invention.

This electrode comprises a support 81 having an electrolyte channel 130 towards the lower part 131 of the electrode (arrow E) and an electrolyte discharge channel 132 (arrow S) in the upper part 133 of the electrode. The ends 135 of these channels 130, 132 form ears serving as means for fixing and placing the electrode in an electrolytic cell. The support 81 is made of a material insoluble in the electrolyte or is covered with a protective layer insoluble in the electrolyte.

This support 81 is for example made of titanium.

A first frame having two windows 137, 138 is applied against the support 81. This frame 136 has grooves in which are housed seals made of synthetic material. This frame 136 is made for example of an insulating synthetic material and resistant to the electrolyte.

The frame has along its lower 139 and upper 140 edges a series of channels 141, 142 extending between the face of the frame applied against the support 81 and the face opposite to said face applied against the support 81, so that that said channels 141, 142 communicate respectively with the supply conduit 130 and with the discharge conduit 132 via orifices 153 which have said conduits 130, 132.

The windows 137, 138 are separated from each other by a cross member 144 having on the face opposite to that facing the support 81 a bowl 145. Channels or passages 146 are dug in said cross member 144 so as to be extend between an opening 147 adjacent to an edge of the cross member 144 and an opening 148 adjacent to the opposite edge of said cross member 144, said openings 147, 148 being located on the face of the cross member opposite to that facing the support 81.

Against this frame 136 are applied two titanium plates 149 with interposition of seals 150 housed in grooves that have said plates. These plates are provided along its edges with a protuberance 151 thereby forming a basin. These plates 149 are perforated along the protuberance in the vicinity of the upper and lower edges 152, 153 so as to form channels 154 situated in the extension of the channels 141 and the openings 147, 148 of the frame 136.

Each plate 149 also has two holes 155 intended to allow passage to a cylindrical element 156 made of titanium or another electrically conductive material but resistant to the electrolyte. This cylindrical element is provided with a head 157, a wall of which is intended to bear against the plate 136 with the interposition of circular seals 158 housed in grooves in the element 156 or more exactly the head 157 and the plate 149 and of a seal 159 forming a sleeve partially covering the cylindrical element 156 and the face of the head 157 facing the plate 149.

The cylindrical element 156 has, near its end opposite to that carrying the head 157, a tapped hole 160 intended to work with the threaded rod 161 of a bolt 162. For each bolt, the support 81 has an orifice 163 intended to deliver passage to the rod 161. One end of the orifice 163 opens into a recess 164 intended to receive the head 165 of the bolt 162, while the other end of the orifice opens into a recess 166 which has the support 81, said support hollow used for the correct placement of the cylindrical element relative to the support 81.

When the bolt 162 is tightened, the cylindrical element 156 is pressed against the support 81 so as to seal between the support 81, the frame 136, the plate 149 and the head 157 of the element 156.

The plate 149 carries a layer 167 of synthetic material insulating from electricity and whose thickness is such that the face 168 of the free end of the head 157 and the face 169 of the layer 167 opposite to that resting on the plate 149 extend substantially in the same plane. The latter corresponds to the plane along which the titanium electrode 170 extends. Thanks to the insulating layer 167 and the insulating sleeve 159, it is possible to insulate the plate 149 with respect to the current supplied by the bolt 162 and the cylindrical element 156 to the electrode 170.

This electrode 170 consists of a series of vertical titanium blades 171 connected to each other by rods or other load-bearing elements (plate) 173 conductors of electricity. These strips are advantageously parallel to each other. However, these slats could have been slightly inclined with respect to each other. In this case, the longitudinal edges (172) of the strips should advantageously not touch each other.

These strips 171 and carrying elements 173 are advantageously provided with an electrically conductive layer.

The strips advantageously have a height h of 5 to 10 mm and are advantageously separated from each other by a distance of between 5 and 10 mm.

The lamellae therefore form between them a series of vertical channels 11 intended to direct the electrolyte in the vicinity of the electrode and in particular to ensure a minimum ascending speed of the electrolyte relative to the electrode (see FIG. 11).

The strips 171 or preferably the supporting elements 173 are welded to the head 157 to ensure electrical contact between the electrode and the conductive bolt 162. It goes without saying that other methods of fixing the strips relative to the head 157 allowing electrical contact are possible.

The lamellae or fins 171 of an electrode extend in the vertical direction from the level N of the channels 154 adjacent to the lower edge 152 of the plate 149 to the level M of the channels 154 adjacent to the upper edge 153 of the plate 149. The length I of the strips corresponds substantially to the width L of the insulating layer 167.

Above the longitudinal edges 172 of the lamellae, edges opposite to the longitudinal edges turned towards the head 157 extends a porous insulating protective sheet 88 which is covered with a membrane 83. This membrane 83 is, for example, an anionic membrane or cationic when the electrode is used to deposit a metal or a metal alloy on a strip, but is preferably an anionic membrane when the electrode is used to remove a deposit of a metal or metal alloy from a strip metals.

This veil 88 and this membrane 83 are stretched between the protuberances 151 so as to form a chamber 78 in which the electrode extends. A secondary electrolyte e2 can pass through said chamber 78, this electrolyte e2 advantageously being different from the primary electrolyte e1 adjacent to the strip 3 to be coated or treated to remove a layer of metal or metal alloy therefrom.

The free edges of the web 88 and of the membrane 83 are applied against a face 174 of the protuberance 151, this face advantageously forming the external lateral face of the protuberance 151 of the plate 149.

Along their edges, the membrane 83 and the web are pressed between, on the one hand, a frame 175 of L-shaped cross section and, on the other hand, the protuberance 151 and a seal 176 mounted in a groove which has the protrusion 151. To hold the frame 175 against the protrusion 151, U-shaped clamping profiles 177 are used.

A wing 178 of the profile bears on the face of the protrusion 151 facing the support 81, while the other wing 179 of the profile bears against the frame 175, so that the latter 175, the web 88 and the membrane 83 are enclosed between the protuberance 151 and the wing 179.

Four profiles 177 are advantageously used per plate 149, so as to enclose and fix the web 88 and the membrane 83 substantially all along the protuberance 151 of the plate 149. In a particular embodiment, the profiles 177 have ends in tab so that the four profiles of a plate substantially form a continuous frame extending along the protuberance 151 of the plate 149. The bowl 145 of the cross member 144 of the frame 136 allows, for the protuberances 151 adjacent said cross member, the fitting of the clamping profiles 177. For these protuberances, the wing 178 extends between the face of the protuberance facing the support 81 and the bottom of the bowl. A seal 180 in synthetic material is inserted between the clamping profiles 177, so as to prevent the primary electrolyte e1 from entering the cup 145, but above all so as to form a bead 181 extending beyond the vertical plane in which extend the membranes 83 and the vertical plane in which extend the wings 179 of the profiles 177. Such a bead makes it possible to reduce, or even completely avoid any risk of contact of the strip to be treated with a membrane. This increases the service life of a membrane.

The membrane attachment system shown (profiles 177) allows rapid installation or replacement of the membrane and also allows, if necessary, easy maintenance of the electrode.

It goes without saying that other membrane attachment systems could have been used.

The circuit of the secondary electrolyte e2 in the electrode shown in FIG. 6 will be described below: The electrolyte e2 enters through the opening 100 and is brought by the conduit 130 in the vicinity of the bottom of the electrode (arrow E). The electrolyte e2 then passes via the channels 141 and 154 into the chamber 78 in which it flows vertically, from bottom to top, between the lamellae 171 of the electrode.

The electrolyte leaves this chamber 78 through the channels 154 and 141 adjacent to the upper edge 153 of the plate 149 to be brought, via the channel 146 drilled in the cross member 144 and the channels 154 and 141 adjacent to the lower edge of the plate 149 closing the window 137, in the chamber 79. The electrolyte then passes through the passages formed between the lamellae 171 of the electrode to finally exit from the chamber 79 by the channels 154 and 142 adjacent to the upper edge 153 of the plate 149. This electrolyte is finally discharged from the electrode through line 132.

FIG. 12 schematically shows an installation using electrodes according to the invention.

• - une série de cellules d'électrolyse 1 pour déposer une couche de Ni-Zn sur la face 2 d'une bande d'acier 3, ces cellules comprennent des anodes suivant l'invention ;
• - des moyens 5 pour munir les faces 2, 4 de la bande d'acier d'une première couche de Zn avant d'introduire ou plonger la bande d'acier dans les cellules 1, de manière à pouvoir retirer chimiquement ou électrochimiquement le Ni déposé sur la première couche de Zn de la face 4, et
• - une installation 21 pour éliminer du nickel déposé sur la première couche de Zn de la face 4, ainsi que pour éliminer au moins partiellement ladite première couche.

This installation includes:

• - A series of electrolysis cells 1 for depositing a layer of Ni-Zn on the face 2 of a steel strip 3, these cells include anodes according to the invention;
• - Means 5 for providing the faces 2, 4 of the steel strip with a first layer of Zn before introducing or immersing the steel strip in the cells 1, so as to be able to chemically or electrochemically remove the Ni deposited on the first layer of Zn on face 4, and
• an installation 21 for eliminating nickel deposited on the first layer of Zn on face 4, as well as for at least partially eliminating said first layer.

The electrolysis cells 1 for depositing a layer of Zn-Ni are for example of the type described in DE-A-3510592, but comprising anodes according to the invention.

These cells 1 are connected to a reservoir 54 by means of pumps, a supply pipe 58 and a discharge pipe 59 so as to ensure a substantially constant concentration of Ni and Zn in the electrolyte. The concentration of Ni and Zn in the electrolyte is for example that given in BE-A-881635 and BE-A-882525. The electrolyte can also contain additives such as polymers, ZrS0 ₄ , ...

The reservoir 54 is connected to a device for enriching the electrolyte with Zn and / or with Ni, so as to maintain the Zn and Ni concentration of the electrolyte at a substantially constant value.

The means 5 for providing a first layer of Zn on the faces 2, 4 of the steel strip 3 preferably comprise electrolytic cells 11 into which the steel strip 3 is introduced. These cells are also advantageously of the type described in DE-A-3510592, but comprising anodes according to the invention.

The steel strip moves in the installation by resting on rollers 13, 14 and on idler rollers 15.

The cells 11 contain an electrolyte (a solution of ZnS0 ₄ ) and are connected to a reservoir 16 by means of pumps 17, 18, a supply line 19 and a discharge line 20 to ensure a concentration of Zn more or less constant of the electrolyte. This reservoir is connected to a Zn enrichment reactor (not shown) of the electrolyte.

The installation 21 for removing the Ni possibly deposited on the layer of Zn and for at least partially eliminating said layer of Zn consists, in the embodiment shown, in a displacing cell 50 advantageously comprising a cathode according to the invention.

After this flattening operation (elimination of a metal layer), the strip 3 is subjected to rinsing by means of the rinsing device 51, to brushing in a brushing installation 52 to ensure that all the Ni deposited on the first layer of Zn on face 4 has been eliminated and advantageously on polishing in unit 91.

The installation also advantageously comprises a series of reservoirs 54, 55, 56, 57. The first reservoir 54 contains the electrolyte intended to be brought to the cells 1 by conduits 58 provided with pumps, while the second reservoir 55 is intended to collect the electrolyte leaving the electrolytic cells 1 by conduits 59. The third reservoir 56 contains the electrolyte intended to be brought to the "deplating" cell 50 by the conduit 60, while the fourth reservoir 57 is intended collecting the electrolyte leaving the deplating cell 50 through the conduit 61. On the conduit 63 is mounted a filter 72 to recover Ni in the form of powder which has been removed from the steel strip. This Ni powder must be removed from the electrolyte since it is in a poorly soluble form.

Part of the electrolyte in the second reservoir 55 and the electrolyte in the fourth reservoir 57 are sent via conduits 62, 63 to an installation 64 for regenerating or enriching the electrolyte, the enriched electrolyte then being sent by a conduit 65 towards the reservoir 54 intended for supplying the cells 1.

Another part of the electrolyte from the second reservoir 55 is sent through a conduit 66 to the reservoir 56 intended to supply the "deplating" cell 50.

The installation further comprises a storage unit and / or preparation 67 of secondary electrolyte; this electrolyte poor in Zn and Ni being sent into the envelope in which the cathode 53 is placed. This unit 67 comprises a storage tank 68 connected by a conduit 69 intended to bring electrolyte into the envelope 53 and by a conduit 70 intended for the evacuation of electrolyte out of the envelope and to return it to the tank 68. Water and sulfuric acid are brought to this unit to compensate for the losses in H ₂ 0 and H ₂ SO ₄ (SO ₄ -) in the electrode chambers.

Electrolyte poor in Zn and Ni could possibly be sent to the reservoir 56 by a conduit.

In this installation, the steel strip was provided with a first layer of Zn with a thickness of 1 micron. To obtain such a layer on the faces 2,4 of the strip, the strip was immersed in an electrolytic cell 11, the electrolyte of which contained 60 g / l of Zn. The current density between the cathode (the steel strip) and the anode 26 was 100 A / dm ^2. The relative speed of the strip with respect to the electrolyte was 1.5 m / s.

Once the strip was provided with the layer of Zn, the strip was brought into electrolytic cells 1 to deposit on the face 2 of the strip a layer of Zn-Ni.

In a particular form of use of the installation shown in FIG. 12, a thin layer of Zn-Ni has been deposited in the cells 11 on both sides of the steel strip 3. The thickness of said layer was 0.5 u (grammage: ± 3.5 g / m ² ), while the Ni content of said layer was of the order of 10%. To effect this deposition, the electrolyte used was the electrolyte used in cells 1.

The electrolyte which was used in cells 1 contained 25 g / I Zn ⁺⁺ , 50 g / I Ni ⁺⁺ and 75 g / I Na ₂ SO ₄ . The pH of this electrolyte was 1.65 to 57.5 ° C. The anode-steel strip distance was approximately 15 mm.

The primary electrolyte used in the deplating cell had in the tests which were carried out the same composition as the electrolyte of cells 1. However, one could have used an electrolyte containing less Zn ⁺⁺ and Ni +.

The secondary electrolyte sent into the envelope contained 75 g / I Na ₂ SO ₄ (pH about 1.7).

The cathode-strip distance in the deplating cell was 16 mm. The speed of the secondary electrolyte in the envelope was 0.04 m / s, while the speed of the primary electrolyte was 1.5 m / s.

Tests were carried out with the "deplating" cell to remove a layer of Zn or Zn-Ni deposited electrolytically.

In these tests, the envelope of the cathode presented an anionic membrane of 150 μ, of thickness sold by MORGANE (FRANCE), while the current density in the "deplating" cell varied between 0 and 50 A / dm ² .

When the current density was zero, no elimination of Ni was observed. Then, the current density was increased and an increasingly complete removal of Ni and Zn was observed, as shown in the following table.

The passage time of the strip opposite the cathodes was 4 seconds. It goes without saying that by using a longer passage time, it is possible using a density of 20-25 A / dm ² to obtain a Ni + Zn / Fe ratio close to 0 or equal to zero.

The installation shown which allows partial or total elimination of Ni deposited on a layer of Zn is an installation which makes it possible to reduce electrolyte losses as much as possible thanks to a recirculation system. This also makes it possible to reduce the total consumption of Zn and Ni of the installation and to reduce the operating and investment costs of waste purification installations.

It goes without saying that the rinsing device can be provided with a unit (not shown) for recovering electrolyte, Zn and Ni.

In order to further reduce the losses of electrolyte and to simplify the operation of the installation, a thin layer (0.5 μ) of Zn-Ni is advantageously deposited in the cells 11. To make such a deposit, the same electrolyte is advantageously used. than that used in cells 1. In this case, the same electrolyte can be used in cells 1, 11 and "deplating" cells (cells to remove Ni and / or Zn and / or a Zn alloy) .

Likewise, to reduce the number of electrodes of different types used in the installation, both in the "deplating" cells and in the cells used to deposit a layer of Zn or of a Zn alloy, a membrane electrode.

In the case of deplating cells, the current density is advantageously less than 60 A / dm ² . However, for cells to deposit a layer of Zn, Zn-Ni or other Zn alloy, this density can be greater than 60 A / dm ² , for example 100 A / dm ² .

Finally, FIGS. 13 and 14 show in section and on a larger scale respectively a steel strip which was obtained in an installation of the type shown in FIG. 12 and a steel strip of which one face has been subjected to over-stripping. or polishing.

The steel strip 200 according to the invention is provided with a layer of Ni-Zn on one side. On the other side of the strip, the concentration of remaining Zn is less than 50 ug / m ² (in particular 10 µg / m ² ). This Zn remaining on this face is distributed in a regular and homogeneous manner.

Such a distribution combined with the presence of a very small amount of Zn and Ni (advantageously less than 25 μg / m ² and preferably less than 10 μg / m ² ) makes it possible to obtain good phosphating.

A strip according to the invention is therefore a strip having a face covered with a layer of Zn-Ni and the other side of which is provided with Zn and / or Ni distributed in a regular and / or homogeneous manner, the grammage in Zn and / or Ni of said other face being greater than 0.1 µg / m ² but less than 25, preferably 10 ug / m ² . A grammage of 0.1 µg / m ² is a grammage demonstrating the absence of over-stripping and therefore of the attack on one face of the steel strip.

A strip which it is possible to obtain by a process according to the invention has a face not covered with Zn and Ni, the roughness of which is substantially equal to that which the steel strip had before its treatment (deposit of a layer of Zn-Ni).

Thus, it is possible to obtain a steel strip 200 having an upper face 201 and an upper face 202 of substantially equal roughness, one (201) of said faces being covered with a layer of Zn-Ni 203.

When the strip was subjected to an over-stripping or a polishing, the face 205 not covered with the Zn-Ni layer underwent an attack modifying the roughness of the steel strip. In addition, an over-stripping will cause a decrease in the thickness of the layer of Zn-Ni 204, while during polishing the claws will be formed in the steel strip.

The steel strip according to the invention can then be subjected to a phosphating and be covered with one or more layers of paint on the face 105 not covered with the layer of Zn-Ni. It was noted that it was possible to obtain better adhesion of the paint layers or at least an adhesion equivalent to that of a steel strip not provided with a layer of Zn-Ni.

To electrochemically coat a strip with a layer of metal, an electrode with both an anionic membrane and a cationic membrane can be used. However, since for the elimination of a layer of a metal, an electrode with an anionic membrane is preferably used, it may be advantageous to use the same electrodes with an anionic membrane both for the electrolytic deposition as for the electrolytic removal of a metal layer, so that an electrode can be used once for electrolytic deposition and once for electrolytic removal from a layer.

FIG. 15 shows, schematically, an installation comprising, on the one hand, a cell 1 for depositing on the face 2 of a galvanized strip 3 a Zn-Ni layer, and on the other hand, a cell 50 for remove from the face 4 the galvanized layer of the strip 3. In this installation, electrodes according to the invention are used as electrodes provided with anionic membranes.

The electrolyte which leaves the chamber of the electrode 501 of the cell 50 is depleted in SO ₄ =. This electrolyte is sent through line 502 into the tank 503. From the electrolyte leaving this tank 503 is sent through line 504 and the pump 505 into the anode chamber 401 of cell 1. During its passage through the room anodes, the electrolyte is enriched in H ₂ SO ₄ . This enriched electrolyte is brought via line 506 into a tank 507.

The tanks 503 and 507 are advantageously associated with a unit 530 to compensate for the water and / or SO ₄ losses from the secondary electrolyte circuit in the electrodes. Such a unit comprises a tank 531 for mixing electrolyte coming through the pipe 532 from the tank 507 and water and / or H ₂ SO ₄ coming from a pipe 510.

In this case, case shown in FIG. 14, the electrolyte from the tank 531 is brought into the chamber of the cathode 501 by the conduit 508 on which the pump 509 is mounted.

• - un réservoir 511 pour récolter l'électrolyte sortant de la cellule 50 ;
• - un réservoir 512 pour récolter l'électrolyte sortant de la cellule 1, électrolyte pauvre en Zn-Ni ;
• - un réservoir 513 pour alimenter la cellule 50 en un électrolyte pauvre en Zn-Ni, et
• - un réservoir 514 pour alimenter la cellule 1 en un électrolyte riche en Zn-Ni.

The installation also includes

• - a reservoir 511 for collecting the electrolyte leaving the cell 50;
• a reservoir 512 for collecting the electrolyte leaving cell 1, an electrolyte poor in Zn-Ni;
• a reservoir 513 for supplying the cell 50 with an electrolyte poor in Zn-Ni, and
• a reservoir 514 for supplying cell 1 with an electrolyte rich in Zn-Ni.

The reservoir 514 receives via the conduits 515 and 516 of the electrolyte from the reservoirs 511 and 512 and possibly via the conduit 517 of the electrolyte coming from the tank 507. This conduit 517 optionally makes it possible to purge the secondary circuit. The electrolyte is enriched in the reservoir 514 by adding Zn-Ni metal powders and optionally H ₂ S0 ₄ acid.

The enriched electrolyte is sent to cell 1 through line 518 and pump 519.

The reservoir 513 which supplies the cell 50 with poor Zn-Ni electrolyte is supplied with electrolyte coming from the reservoir 512 and advantageously from the reservoir 507 (conduit 520, pump 522 and conduit 521, pump 523).

Such an installation makes it possible to significantly reduce the losses of Zn-Ni and allows better use of the electrolytes.

Finally, FIGS. 16 to 18 show, schematically, embodiments of installation similar to that shown in FIG. 12.

In the form shown in FIG. 16, electrodes provided with an anionic membrane are used as an anode in cells 1 for the electrolytic deposition of Zn or Zn-Ni on the face 2 of strip 3 and as cathode in the cells 50 to remove a layer of Fe-Zn, Zn or Zn-Ni optionally coated with Ni or Ni-Zn, layer present on the face 4 of the strip 3.

The secondary electrolyte sent into the electrodes comes from the tank 68 of an electrolyte preparation unit, which is supplied with water to obtain a correct dosage of the secondary electrolyte.

Secondary electrolyte can be sent via line 71 to reservoirs 55 and 56 intended to collect primary electrolyte coming from cells 1 and 50 respectively.

Part of the electrolyte from tank 57, after filtration (filter 72) is returned to tank 56 supplying cell 50 (conduit 90).

The other elements, conduits and parts of the installation shown in FIG. 16 are similar to the elements, conduits or parts of the installation shown in FIG. 12. These same elements, conduits and parts are designated by the same reference numbers.

In this embodiment, it is possible both to ensure a balance in the material balance of cells 1 (electrolytic deposition) and of cell 50 (electrolytic elimination) thanks to the transfer of electrolyte from tank 57 to tank 56 via line 90, advantageously after filtration (filter 72).

The installations shown in FIGS. 17 and 18 relate to installations for the deposition of a first layer of Zn, ZnNi or other alloys of Fe and of a second layer of Fe, Zn-Fe or iron alloy.

These installations allow, among other things, the deposition of Zn or Zn-Ni on one side 2 of the strip 3 and the deposition of Fe or Fe-Zn or other Fe alloy on the side 4 of the strip 3, this side 4 being opposite to side 2. The deposit of Fe or other Fe alloy (Fe-Zn) is intended to cover the Zn or Zn-Ni which would have deposited on side 4. This deposit of Fe or Fe alloy allows phosphating of face 4 as well as good adhesion of paint layer.

The installation of FIG. 17 comprises cells 600 and 601 with anodes having an anionic membrane according to the invention. These anodes are intended for depositing on the face 2 of the strip 3 a layer of metal. It goes without saying that the cells could have included anodes arranged on the two sides of the strip so as to provide the two faces of the strip with a layer of metal.

• - un réservoir 602 pour alimenter les cellules 600 par le conduit 603 en électrolyte riche en Zn, Zn-Ni ou autre alliage ;
• - un réservoir 604 pour collecter l'électrolyte appauvri sortant des cellules 600 par le conduit 605 ;
• - une unité 606 pour enrichir de l'électrolyte provenant par le conduit 607 du réservoir 604, cet électrolyte enrichi étant envoyé par le conduit 608 vers le réservoir 602 ;
• - un réservoir 618 pour alimenter la cellule 601 par le conduit 609 en électrolyte riche en Zn Fe, ou autre alliage ;
• - un réservoir 610 pour recevoir l'électrolyte appauvri sortant de la cellule 601 par le conduit 611 ;
• - une unité 612 pour enrichir de l'électrolyte provenant par le conduit 613 du réservoir 610, cet électrolyte enrichi étant renvoyé par le conduit 614 dans le réservoir 618, et
• - une unité de stockage et préparation 67 d'électrolyte destiné à circuler dans les chambres des anodes.

The installation includes:

• a reservoir 602 for supplying the cells 600 via the conduit 603 with an electrolyte rich in Zn, Zn-Ni or other alloy;
• - a reservoir 604 for collecting the depleted electrolyte leaving the cells 600 via the conduit 605;
• a unit 606 for enriching the electrolyte coming through the line 607 from the tank 604, this enriched electrolyte being sent through the line 608 to the tank 602;
• a reservoir 618 for supplying the cell 601 via the conduit 609 with an electrolyte rich in Zn Fe, or other alloy;
• - a reservoir 610 for receiving the depleted electrolyte leaving the cell 601 through the conduit 611;
• a unit 612 for enriching the electrolyte coming through the line 613 from the tank 610, this enriched electrolyte being returned by the line 614 into the tank 618, and
• - An electrolyte storage and preparation unit 67 intended to circulate in the anode chambers.

This unit 67 comprises a tank 68 connected by conduits 69 and 70 to the anodes for supplying secondary electrolyte and for bringing the secondary electrolyte to tank 68 after passing through the anodes.

This unit 67 includes a water supply 615 to compensate for the water losses from the electrolyte or the increase in its H ₂ SO _{4 content} . The surplus of H ₂ SO ₄ in the electrolyte due to the passage of the latter through the anodes is advantageously sent via line 616 into reservoirs 604 and 610 to receive the depleted primary electrolytes leaving cells 600 and 601.

Advantageously, only part of the electrolyte from tanks 604 and 610 is sent to the enrichment units 606 and 612. In this case, conduits 630 and 631 allow the electrolyte to be sent directly from the tanks 604, 610 to tanks 602 and 618.

FIG. 18 represents an installation similar to that represented in FIG. 16 except that the cells 600, 601 included anodes provided with a cationic membrane and that, consequently, the secondary electrolyte after passage through the chambers of the anodes are not sent to tanks 604 and 610.

In FIGS. 17 and 18, the same reference signs represent identical elements.

The reservoirs for supplying the cells with electrolyte rich for example in Zn, Ni, the reservoirs for receiving the depleted electrolyte leaving the cells and the electrolyte enrichment units are advantageously of the type described in application EP-A-0388386 .

As can be seen from FIGS. 15 to 18, the chamber of an electrode of a first cell (1,600) and the chamber of an electrode of a second cell (50, 601) are mounted in the same circuit.

In one embodiment, in particular when the membranes used are anionic, the electrolyte circuit is such that electrolyte leaving the chamber of an electrode of a first cell (1) is sent, possibly after treatment ( addition of water, H ₂ S0 ₄ , ...) into the chamber of an electrode of a second cell (50) and that electrolyte leaving the chamber of an electrode of a second cell is sent in the chamber of an electrode of the first cell, possibly after treatment (addition of water, ...).

Claims (36)

1. Electrode for electrolytic cell, said electrode being located within an enclosure defining a chamber (78), a wall of said enclosure consists of a membrane (83) allowing the passage of ions therethrough, said enclosure having a first opening (100) for feeding the chamber with an electrolyte and a second opening (101) for removing electrolyte from the chamber so as to create an upward current of electrolyte, characterized in that the enclosure is provided with thin strips, fins or baffles (113, 102, 171) conducting the flow of electrolyte in the chamber (78), so as to ensure a velocity of the electrolyte in the vicinity of the electrode of at least 0.01 m/s, preferably of 0.1 m/s.

2. Electrode according to claim 1, characterized in that the electrode consists of a set of blades or fins (171) defining therebetween channels (112) intended to direct the flow of electrolyte.

3. Electrode according to claim 2, characterized in that the baffles or fins divide the chamber into a number of separate compartments (78, 79) which extend between the electrode and the membrane (77) or one wall of the enclosure.

4. Electrode according to anyone of the claims 1 to 3, characterized in that the enclosure has a third opening (103) so as to remove and/or suck gases outside the chamber.

5. Electrode according to claim 4, characterized in that the baffles or thin strips or fins (102, 112, 171) extend substantially vertically from the vicinity of the lower part (781) of the enclosure as far as to the vicinity of the upper part of the enclosure so as to define channels (113) conducting the electrolyte into said upper part of the enclosure, this part having an opening for sucking gases outside the chamber and an opening for discharging the electrolyte.

6. Electrode according to claim 5, characterized in that the baffles or fins (112, 171) extend at least from one edge of the electrode (80, 170) as far as the opposite edge thereof.

7. Electrode according to anyone of the claims 1 to 6, characterized in that the membrane (77, 83) is an anionic membrane or a cationic membrane.

8. Electrode according to anyone of the claims 1 to 7, characterized in that the membrane (83) is provided of a protecting layer (88) on the outer or inner side of the enclosure.

9. Electrode according to claim 1, characterized in that a porous support (845) extends in the vicinity of the membrane and acts as supporting means for at least one part thereof.

10. Electrode according to claim 9, characterized in that the support (84) is a perforated component, a porous web, a treillis, preferably made of Zr, Ti or stainless steel.

11. Electrode according to claim 9, characterized in that the support has a layer (87) acting as electrode on the face opposite that adjacent to the membrane.

12. Electrode according to claim 9, characterized in that the membrane (83) rests on a support (84) acting as an electrode and in that said support (84) is provided with an insulating layer (88) on its face adjacent to the membrane.

13. Use of an electrode according to anyone of the preceding claims in an electrolytic cell.

14. Process for the electrochemical plating of metal strips, preferably of galvanized steel strips with metals or metal alloys, in which an electrolyte containing salt(s) of plating metal(s) is recycled between the cathodic metal strip to be plated and the insoluble anode, in which an electrode according to anyone of the claims 1 to 12 is used as anode so that the membrane is arranged between the anode and the metal strip to be plated, the said membrane forming separation between the cathodic space of the cell and the anode chamber defined by the enclosure of the anode, and in which a first electrolyte circuit in the chamber and a second circuit of electrolyte in the cathodic space are created, the membrane preventing the passage of gases formed at the anode into the second circuit of electrolyte and the passage of salt(s) of plating metal(s) from the cathodic space into the first circuit of electrolyte.

15. Process for the electrochemical plating of metal strips according to claim 14 with iron, iron compounds or alloys containing iron, characterized in that an anion exchange membrane is arranged between the anode and the metal strip to be plated, membrane allowing, when using a sulfuric electrolyte enriched in iron and zinc sulfate in the cathodic space, the transfer of charge only by the transfer of SO4- ions into the anode chamber and preventing the passage of the metal salt(s), so that the electrolyte devoid of metal(s) and consisting of water and sulfuric acid or the anode chamber is supplementary enriched in sulfuric acid, while the oxygen formed at the insoluble anode is discharged from the anode chamber and the passage of oxygen into the cathodic space is prevented by means of the anion exchange membrane.

16. Process for the electrochemical plating of metal strips according to claim 14 with iron, iron compounds or alloys containing iron, characterized in that an anion exchange membrane is each time arranged between the anode and the metal strip to be plated, membrane which, when using a chloride electrolyte enriched in iron or zinc chloride in the cathodic space, allows the passage of chlorine in the anode chamber but prevents the passage of the metal salt(s), so that the electrolyte which consists of water and chlorhydric acid in the anode chamber is not enriched in metal salt(s), while the chloride transformed into the first electrolyte flow devoid of metal of the anode chamber is removed and the passage of chlorine in the cathodic space is prevented by the anion exchange membrane.

17. Process for the electrochemical plating of metal strips according to claim 14 with iron, iron compounds or alloys containing iron, characterized in that a cation exchange membrane is each time arranged between the anode and the metal strip to be plated, membrane which, when using a sulfuric electrolyte enriched in iron and zinc sulfate in the cathodic space, prevents the transfer of acids from the cathodic space into the anode chamber and allows the transfer of charge by the transfer of hydrogen ions from the anode chamber into the cathodic space, while the oxygen formed at the anode is removed from the sulfuric electrolyte devoid of iron of the anode chamber and the passage of oxygen in the cathodic space is prevented by the cation exchange membrane.

18. Process for the electrolytic plating of metal strips according to claim 14 with iron, iron compounds or alloys containing iron, characterized in that a cation exchange membrane is each time arranged between the anode and the metal strip to be plated, membrane which, when using a chloride electrolyte enriched in iron and zinc chloride, in the cathodic space, prevents the passage of acids and salts from the cathodic space into the anode chamber and allows the transfer of charge by the transfer of hydrogen ions from the anode chamber into the cathodic space, while the gases formed at the anode are removed from the anode chamber with the electrolyte devoid of iron containing chlorhydric acid and the passage of gases formed in the cathodic space is prevented by the cation exchange membrane.

19. Process according to anyone of the claims 15 to 18, characterized in that in order to replace the iron deposited on the metal strip, an amount of elemental iron corresponding to the deposited amount is added to the electrolyte which flows through the cathodic space.

20. Process according to claim 15 or 17, characterized in that the part of electrolyte in excess which is formed, the nature of which is similar to that of the anodic circuit, is conveyed to the cathodic electrolyte circuit through a dissolving station.

21. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode according to anyone of the claims 1 to 12, for the working of a process according to anyone of the claims 14 to 20.

22. Process for the electrolytic removal of a layer of metal or alloy of metals located on a strip, especially on a steel strip, such as a galvanized steel strip,

^* in which an electrolyte is recycled between an insoluble cathode and the anodic metal strip,

in which an electrode according to anyone of the claims 1 to 12 is used as cathode so that the membrane, especially an anionic membrane, is arranged between the cathode and the strip, the said membrane forming a separation between the anodic space of the cell and the cathodic chamber defined by the enclosure of the cathode, and

in which a first circuit of the electrolyte (22) in the chamber and a second circuit of electrolyte (21) in the anodic space are created, the membrane preventing the passage of gases formed at the cathode into the second circuit of electrolyte (C1) and the passage of salt(s) of metal(s) of the layer from the anodic space towards the first circuit of electrolyte (C2).

23. Process according to claim 22, characterized in that an anionic membrane is used, the said membrane allowing, when using a sulfuric electrolyte for the first circuit of electrolyte (C2), the transfer of charge only by the passage of SO4- ions out of the chamber, and preventing the passage of metal salt(s), the hydrogen formed at the cathode is removed from the chamber, the membrane preventing the passage of said hydrogen towards the anodic space of the cell.

24. Process according to claim 23, characterized in that the electrolyte of the first circuit (C2), i.e. the electrolyte flowing in the cathodic chamber, contains from 50 to 100 g/I Na₂SO₄ and has a pH comprised between 1.5 and 2.

25. Process according to claim 22, characterized in that a velocity of the secondary electrolyte of at least 0.1 m/s is ensured.

26. Process according to anyone of the claims 22 to 25, characterized in that the upper part of the chamber is submitted to a gas suction.

27. Process according to claim 26, characterized in that a vacuum is created at the upper part of the chamber, said vacuum being such that the pressure at the upper part of the chamber is lower to 0.75 x the atmospheric pressure.

28. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode according to anyone of the claims 1 to 12, for the working of a process according to anyone of the claims 22 to 27.

29. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode according to anyone of the claims 1 to 12, for the working of a process according to anyone of the claims 14 to 20 and 22 to 27 which comprises two electrolyte cells (1,50 ; 600,601) provided with an electrode according to anyone of the claims 1 to 12, the chambers of said electrodes being mounted in one and same circuit for the flow of secondary electrolyte.

30. Plant according to claim 29, characterized in that the electrolyte flow is such that the electrolyte flowing out of the chamber of an electrode of a first cell (1) is conveyed, possibly after treatment (531), into the chamber of an electrode of a second cell (50) and in that the electrolyte flowing out of an electrode of the second cell (50) is conveyed, possibly after the treatment, in the chamber of an electrode of the first cell.

31. Plant according to anyone of the claims 29 to 30, characterized in that the electrode (53) is placed in an enclosure, the wall of which directed to the strip (3) is a membrane, the said enclosure being connected to a device for conveying electrolyte in the enclosure and to a suction system for removing gases formed in the enclosure.

32. Plant according to claim 29 for the preparation in continu of a steel strip provided with a layer electrodeposited, said plant comprising successively a first unit, preferably an electrolytic cell (11), for depositing on both faces of the steel strip a first layer of Zn or Zn alloy, a second electrolytic cell (1) for depositing on one face (2) of the steel strip (3) a Zn-Ni layer and a unit for the electrolytic removal of Ni possibly deposited on the first layer of Zn or Zn alloy of the other face (4) of the strip (3), in accordance to a process according to anyone of the claims 22 to 27, in which the unit for removing the Ni possibly deposited on the first layer of Zn or Zn alloy is an electrolytic cell (50) comprising an electrode (53) placed in an enclosure, a wall of which is made of the membrane (77).

33. Plant according to claim 32, characterized in that it comprises a first tank (54) for the supply of electrolyte to the electrolytic cell (1) for the deposit of a Zn-Ni layer, a second tank (55) for collecting the electrolyte flowing out of the electrolytic cell (1) for the deposit of a Zn-Ni layer, a third tank (36) for the supply of electrolyte to the cell (50) for removing the Ni possibly deposited on the first layer of Zn or Zn alloy and a fourth tank (57) for collecting the electrolyte flowing out of the cell (50) for the removal of the Ni possibly deposited on the first layer of Zn or Zn alloy, the fourth tank (57) is linked to the first tank (54) so that the enriched electrolyte flowing out of the cell (50) for the removal of Ni possibly deposited on the first layer of Zn or Zn alloy is conveyed to the electrolysis cell (1).

34. Plant according to claim 33, characterized in that a filter (72) is mounted between the cell (50) for the removal of the Ni possibly deposited on the first layer of Zn or Zn alloy and the fourth tank (57).

35. Plant according to claim 33 or 34, characterized in that the second tank (55) and/or the fourth tank (57) is linked to a plant (64) for the enrichment of the electrolyte in Zn and Ni, this plant conveying the enriched electrolyte to the first tank (54).

36. Plant according to claim 34, characterized in that it comprises a unit (67) for stocking and/or preparing secondary electrolyte which is lean or devoid of Zn and Ni, this unit (67) comprising a tank (68) for secondary electrolyte connected to the enclosure surrounding the cathode (53) by means of a supply pipe and a pipe for removing electrolyte, this tank (8) being also connected to the third tank (56) for possibly supplying it with fresh electrolyte.

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