EP0580730B2 - 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 subject of the present invention is a electrode, preferably an insoluble electrode for an electrolytic cell, a coating process electrolytic, a process to remove a layer of metal and a electrolytic installation.

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

Depending on the electrolyte put in work, for example electrolytes based on sulfate or chloride, the implementation of this process generates gases at the anode, for example from oxygen or chlorine, which partially undergo unwanted bonds with the metals of coating, or which, due to their aggressiveness or their toxicity, have harmful consequences during the implementation of the coating process or for the environment. These gases that take birth at the anode mix with the electrolyte and can therefore cause reactions that are not desired and reach the environment, being given that during the electrolytic coating of metal strips, the electrolyte circuit on the tape can not be separated from the atmosphere.

We know a method of coating steel strips galvanized using compounds of iron or alloys thereof. To do this, we conducts an electrolyte based on sulfate loaded with coating metal salts between the endless circulating steel strip to be coated and insoluble anodes. Due to the processes known electrochemicals, iron precipitates under the form of iron compounds to the metal strip cathodic. Bivalent oxygen is released to 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 into trivalent iron, so that large amounts of iron oxide take birth, which soil electrolytes and which must be separated from the circuit by implementation 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 metal salts of coating, or the use of iron in corresponding dissolution facilities and the replacement of other substances used jointly trained, raise very substantially the cost of such a method of coating.

To resolve these issues, applicants use a specific electrode which is particularly useful in electrochemical coating of metal strips, but which is also suitable in other processes, such as methods for removing electrochemically coating a strip such than a steel strip.

According to the invention, the electrode is placed in an envelope defining a room and of which a wall is formed of a membrane allowing the passage of ions through it, said envelope having a first opening to supply the electrolyte chamber and a second opening to drain the electrolyte from the chamber.

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

Slats, baffles or fins direct the flow of electrolyte in the room or part of this one.

In one embodiment, the baffles or fins extend from the neighborhood from the first opening of the envelope to neighborhood of the second opening of the envelope, so as to advantageously divide the room in 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 from the electrode a current at least partially rising electrolyte. According to one particularity of this embodiment, the baffles or fins extend in one direction substantially vertical from the vicinity of the lower part of the envelope to the vicinity from the top of the envelope so that define channels bringing the electrolyte into said upper part of the envelope, this part with an opening for vacuuming out of the gas chamber and an opening for draining the electrolyte.

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

The membrane is preferably a anionic or anion exchange membrane or a cationic or cation exchange membrane. She is advantageously provided on the outside with the envelope of a layer or a veil of protection, for example made of synthetic (polymer, polyester, ....) advantageously reinforced with fibers (glass).

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

In one embodiment, the support on the opposite side to that adjacent to the membrane a layer playing the role electrode, while in another form of realization 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.

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

The present invention also has for object the use of such an electrode in an electrolytic cell.

Finally, its object is still a electrochemical strip coating process steel galvanized with metals or alloy of metals. In this process, we recycle so known an electrolyte charged with metal salts of coating between the metal strip to be coated (cathode) and the insoluble anode. According to the process according to the invention, it is preferably used as an anode insoluble an electrode according to the invention. The membrane is arranged between the anode and the strip of metal to be coated so as to form a separation between an adjacent cathode space of the strip and the anode chamber defined by the envelope of the anode. In the process according to the invention, creates a first primary electrolyte circuit in the bedroom and a second circuit secondary electrolyte in space cathodic, the membrane preventing the transfer of gases generated at the anode in the second circuit of electrolyte and the transfer of metal salts from covering of the cathode space towards the first electrolyte circuit. In this case, the gases remain in the maintained electrolyte circuit separately in the anode space and can be evacuated regularly. The circuit electrolyte anodic is not loaded with coating metals. The gas that still forms can be evacuated from this circuit in a relatively simple way. The two circuits are clearly separate one of the other, so that mixtures cannot produce.

By claims 15 to 20, we have proposed methods in accordance with the invention of coating of metal strips, preferably steel strips galvanized with iron, compounds of iron or iron-containing alloy. Depending on the nature of the electrolyte used in space or cathodic enclosure, we suggest the use, as diaphragms, of membranes cation exchangers, or exchange membranes anions known as such. By one corresponding modification or adaptation of electrolytes we can also use this which 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 the cathodic space, to ensure the only transfer of SO ₄ ^―― ions in the anode chamber as 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 membrane cation exchanger according to claims 17, 18 and of the use of a sulfuric electrolyte in cathodic space, load transport is carried out by the transfer of hydrogen ions from the anode chamber in the cathode space. The oxygen that forms here also at the anode is removed from the sulfur-free electrolyte devoid of iron from the anode circuit. Oxygen transfer in the cathode space is also prevented by this cation exchange membrane.

When using a chlorinated electrolyte, enriched with iron or zinc chloride, in space cathodic, it is also possible, in accordance with the present invention, to use membranes appropriate anion exchangers. During the implementation of this process, penetration is allowed of chlorine ions in the anode chamber 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 discharged advantageously in a controlled manner with the electrolyte circuit of the anode chamber. A transfer from chlorine in the cathode space is prevented by the appropriate exchange membrane.

When using a chlorinated electrolyte in the cathode space, it is also still possible to use cation exchange membranes of a suitable nature. In this case too, we again prevents the transfer of acids and salts from the cathode space to the anode space. The charge transport takes place by the transfer of hydrogen ions from space or anode chamber in cathodic space. The gases separated at the anode are evacuated. Separate gas transfer in space cathodic 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, form iron sludge in the electrolyte, this sludge resulting from oxidation due to oxygen released at the anode.

Since the action of atmospheric oxygen cannot be totally prevented cathodic circuit, when coating with iron operated in accordance with the present invention, it forms also a certain amount of trivalent iron in the cathode circuit. This trivalent iron stains the cathodic circuit, so this electrolyte must also still be filtered. In accordance with the present invention, it is therefore proposed to supply the cathode electrolyte circuit, for the purpose of replacement of the iron taken off during coating, in a corresponding proportion of edible iron, for example in an intermediate dissolution station. The necessary proportion of elemental iron added is enough, due to the excess, to reduce the trivalent iron to divalent iron, so that no more iron oxide sludge in the cathode electrolyte circuit.

Sulfuric acid which enriches in excess when using a sulfuric electrolyte in the circuit cathodic and when using an anion exchange membrane in the anodic circuit, is used in the dissolution station and is thus returned to the cathode circuit, where the speed of dissolution of the iron and other coating metals, for example zinc, is greatly accelerated.

Chlorine gas that forms at the anode when using a chlorinated electrolyte in the circuit cathode and an anion exchange membrane is removed by suction from the anode circuit and is burned hydrochloric acid by hydrogen gas formed in the dissolution station and used for acceleration of 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 / l, advantageously from 30 to 80 g / l 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 cells. The relative speed of the strip relative to the electrolyte is advantageously understood between 1 and 8 m / s, preferably between 3 and 5 m / s.

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

This membrane makes it possible to remedy that metals redissolved in the electrolyte such as by example of Zn and / or Ni does not form a deposit (black in the case of Zn and Ni) on the cathode, this deposit decreasing not only 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 a hydrogen release existed. To prevent hydrogen bubbles from coming together to form large bubbles, we noticed that it was useful to use the membrane as a wall of a chamber adjacent to the cathode and to maintain in said chamber a current or flow of so-called secondary electrolyte.

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 / l of Na ₂ SO 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. By 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 / l of free acid, advantageously less than 5 g / l, preferably about 1 g / l 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 peculiarities and details of the invention will emerge from the description next detailed in which it is made reference to the attached drawings.

• les figures 1 à 5 montrent diverses formes de réalisation d'électrodes appropriées pour les procédés et l'installation 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'électrodéposition,
• 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 suivant l'invention,
• les figures 13 et 14 montrent en coupe et à plus grande échelle une bande d'acier qui a été obtenue suivant l'invention, et
• les figures 15 à 18 montrent des formes de réalisation particulières d'installation suivant l'invention.

In these drawings:

• FIGS. 1 to 5 show various embodiments of electrodes suitable for the methods and the installation according to the invention,
• FIGS. 6 and 7 show electrodes similar to that shown in FIG. 2, but used in an electroplating 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 lamellae of the electrode shown in FIG. 8,
• FIG. 12 is a schematic view of an installation according to the invention,
• FIGS. 13 and 14 show in section and on a larger scale a steel strip which was obtained according to the invention, and
• Figures 15 to 18 show particular embodiments of installation according to the invention.

Figure 1 shows in perspective a electrode suitable for the following processes and installation the invention which is advantageously used in a "deplating" but which can also be used for depositing Zn, Zn-Ni, Zn-Fe, or another Zn alloy.

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

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 presents a first opening 100 to supply the rooms 78.79 with electrolyte and a second opening 101 for evacuate chambers 78,79 of the electrolyte.

To ensure a minimum speed of the electrolyte in the vicinity of electrode 76 for evacuate gases (such as oxygen when the electrode works as an anode in a process according to claim 15 or hydrogen when the electrode works 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 into two compartments 78, 79 adjacent, but separate one of the other. The fin 102 extends between the electrode 76 and the wall of the envelope provided with the membrane.

Thanks to this fin 102, it was possible to insure in compartments 78.79 at 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 Figure 1, the electrolyte was brought into the compartments 78, 79 with a speed of the order of 0.5 m / s. The distance electrode 76-membrane 77 was 0.5 cm.

Figure 2 shows in section another embodiment of an electrode suitable for the following processes and installation the invention.

The electrode 80 made of titanium but provided with an active layer is integral with 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 trellis 84 made of titanium and has on its opposite side to that adjacent to the electrode of a porous film 85 protecting the membrane. This film is resistant with acids and is armed with fibers. This film is by example a polyester film.

When such an electrode is used in a deplating cell, the membrane is anionic. Such a membrane is for example at multilayer structure, each of the layers being consisting of a membrane obtained by the process described in FR-8900115 (request nr.). A membrane of this type is prepared by grafting a amino compound on a polymer support (film ethylene-co-polytetrafluoroethylene) and by crosslinking of it.

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 allows to increase 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 cell electrode) "deplating" as described above), has an opening 103. Advantageously, this opening 103 allows communication between chamber 78 and a conduit 104 on which a system is mounted suction (vacuum pump, fan, etc.) not shown.

To avoid the formation of large gas bubbles (hydrogen) which affect the correct functioning of the electrode, the envelope is connected to an electrolyte circulation device in the envelope and to a gas removal system, especially a system creating a vacuum at the top 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 speed 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 the case present) do not come together to form large bubbles disrupting the proper functioning of the electrode.

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

In the case of FIG. 6, the membrane 83 is a cationic membrane so that the anions SO ₄ ⁼ 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 anions SO ₄ ⁼ 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 Figure 3, the surrounding envelope the electrode 80 is always formed by a support 81 and a membrane 83. The electrode consists a titanium mesh or perforated plate or zirconium on the side 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 layer of protection 85.

The electrode shown in Figure 4 is similar to that shown in Figure 5 if it is only a porous insulating veil 88 is placed between the trellis and the membrane.

Figure 5 shows in section a another embodiment of an electrode suitable for the following processes and installation 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 bring into the chamber 78 of the electrolyte, a opening or passage 101 to evacuate from the electrolyte out of the chamber 78 and an opening 103 for discharging gases formed in the chamber, in particular at vicinity of electrode 80. These openings are located at a level higher than electrode 80 and at 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 supplying the incoming electrolyte in room 78 through passage 100 near the bottom 781 of the room. This corridor 108 is extends 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 precisely up 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 lower edge 120 to the upper edge 121) promoting the evacuation of gas particles from 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 width direction 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 upper part of the envelope.

Figure 6 is a partially broken away front view of an embodiment of a next electrode 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 of fixation and placement of the electrode in an electrolytic cell. The support 81 is made in one 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 plastic seals. This frame 136 is produced for example in an insulating and electrolyte resistant synthetic material.

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 applied face against the support 81, so that said channels 141, 142 communicate respectively with the supply conduit 130 and with the evacuation conduit 132 via orifices 153 which these said have 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 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 sealing 150 housed in grooves that have said plates. These plates are fitted along from its edges a protuberance 151 thereby forming a basin. These plates 149 are perforated along of the protrusion in the vicinity of the upper and lower edges 152, 153 so as to form channels 154 located 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 an element cylindrical 156 made of titanium or some other 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 interposition of circular seals 158 housed in grooves of element 156 or more exactly of the head 157 and of the plate 149 and of a seal 159 forming a covering sleeve partially the cylindrical element 156 and the face of the head 157 facing the plate 149.

The cylindrical element 156 has in the vicinity of its end opposite to that carrying the head 157 a threaded 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 allow 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 the support 81 presents, said recess being used for placement correct of the cylindrical element with respect to the support 81.

When tightening the bolt 162, the cylindrical element 156 is pressed against the support 81 so as to sealing 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 bearing on the plate 149 extend substantially in the same plane. The latter corresponds to 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 from the current supplied by the bolt 162 and the cylindrical element 156 at 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 slats are advantageously parallel to each other. However, these slats could have been slightly inclined to each other. In this case, the longitudinal edges (172) of the slats do not should advantageously not touch each other.

These strips 171 and carrier elements 173 are advantageously provided with a conductive layer of electricity.

The slats advantageously have a height h of 5 to 10 mm and are advantageously separated each other from 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 by relative to the electrode (see Figure 11).

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

The lamellae or fins 171 of an electrode extend in the vertical direction from level N 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 lamellae corresponds substantially to the width L of the insulating layer 167.

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

This veil 88 and this membrane 83 are stretched between the protrusions 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 coating or treating to remove a layer of metal or metal alloy.

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

Along their edges, the membrane 83 and the veil are pressed between, on the one hand, a frame 175 of cross section in L and, on the other hand, the protuberance 151 and a seal 176 mounted in a groove that has the protrusion 151. To maintain the frame 175 against the protrusion 151, profiles of tightening in U 177 are used.

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

Four profiles 177 are advantageously used per plate 149, so as to grip and fix the veil 88 and the membrane 83 substantially all along the protuberance 151 of the plate 149. In a particular embodiment, the profiles 177 have mitered ends 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 to 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 180 joint in synthetic material is inserted between the clamping profiles 177, so as to prevent electrolyte primary e1 to enter the bowl 145, but above all so as to form a bead 181 extending beyond the vertical plane in which the membranes 83 extend 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 lifespan of a membrane.

The membrane fastening system shown (profiles 177) allows installation or replacement 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 through the conduit 130 in the vicinity of the bottom of the electrode (arrow E). The electrolyte e2 then passes via 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 via the channels 154 and 141 adjacent to the upper edge 153 of the plate 149 to be brought in, via channel 146 drilled in cross member 144 and 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 emerge from the chamber 79 via the adjacent channels 154 and 142 of the upper edge 153 of the plate 149. This electrolyte is finally discharged from the electrode through the conduit 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 comprise 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 deposited Ni on the first layer of Zn on face 4, and
• an installation 21 for removing 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, ZrSO ₄ , ...

The reservoir 54 is connected to a device for enriching the electrolyte with Zn and / or Ni, so 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 the faces 2, 4 of the steel strip 3 preferably comprise electrolytic cells 11 in 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 deflection rollers 15.

The cells 11 contain an electrolyte (a solution of ZnSO 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 eliminating at least partially said layer of Zn consists, in the embodiment shown, of a displacing 50 advantageously comprising a cathode according to the invention.

After this flattening operation (removal of a layer of metal), the strip 3 is subjected to a rinsing thanks to the rinsing device 51, to a 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 at a 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 cells 1 by conduits 58 provided with pumps, while the second reservoir 55 is intended to collect the electrolyte leaving the cells electrolytic 1 by conduits 59. The third reservoir 56 contains the electrolyte intended to be brought to the "deplating" cell 50 through the conduit 60, while the fourth reservoir 57 is intended to collect the electrolyte leaving the deplating cell 50 through the conduit 61. On the conduit 63 is mounted a filter 72 to recover Ni in powder form which has been removed from the steel strip. This Ni powder must be removed from the electrolyte since it is in a sparingly soluble form.

Part of the electrolyte in the second tank 55 and the electrolyte in the fourth tank 57 are sent via conduits 62, 63 to an installation 64 for regeneration or enrichment of the electrolyte, the enriched electrolyte then being sent via a conduit 65 to the reservoir 54 intended for cell feeding 1.

Another part of the electrolyte from the second reservoir 55 is sent via a conduit 66 to the tank 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 of H ₂ O 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 drove.

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 Zn layer, the strip was brought into cells electrolytic 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 μ (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 / l Zn ⁺⁺ , 50 g / l Ni ⁺⁺ and 75 g / l Na ₂ SO ₄ . The pH of this electrolyte was 1.65 to 57.5 ° C. The anode-steel strip distance was about 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 / l Na ₂ SO ₄ (pH about 1.7).

The cathode-strip distance in the deplating cell was 16 mm. The speed of the electrolyte secondary 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 electrolytically deposited.

In these tests, the envelope of the cathode had an anionic membrane 150 μm thick 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. Weight before passage in the deplating cell g / m ² Current density in the deplating cell A / dm ² Zn + Ni / Fe or Zn / Fe ratio of the face after brushing Zn + Ni deposit Zn flash repository 3 0 30-42 3 15 16-17 3 20 0-8 3 25 0-5 3 50 0-4 3 20 0

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 minimize electrolyte losses thanks to a recirculation system. This also makes it possible to reduce the total consumption of Zn and Ni by installation and reduce the operating and investment costs of water purification plants rejections.

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

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

Similarly, to reduce the number of different type electrodes used in the installation, we use both in deplating cells and in cells for depositing 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, Figures 13 and 14 show in section and on a larger scale respectively a strip steel which was obtained in an installation of the type shown in Figure 12 and a steel strip which one side was 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 µg / 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 µg / 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 of the steel strip before its treatment (deposition of a Zn-Ni layer).

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

When the strip has been subjected to over-stripping or polishing, the face 205 not covered with the Zn-Ni layer has undergone an attack modifying the roughness of the steel strip. In addition, an over-stripping will cause a decrease in the thickness of the Zn-Ni 204 layer, while when 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 one or more layers of paint on the side 105 not covered with the Zn-Ni layer. We have noticed that it was possible to obtain better adhesion of the paint layers or at least a 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 can be used with both an anionic membrane and a cationic membrane. However, since for removing a layer of a metal, preferably use an electrode with a membrane anionic, it may be advantageous to use the same electrodes with anionic membrane for both electrolytic deposition only for the electrolytic removal of a metal layer, so that use an electrode once for electroplating and once for electroplating a layer.

FIG. 15 schematically shows an installation comprising, on the one hand, a cell 1 for deposit on the face 2 of a galvanized strip 3 a Zn-Ni layer, and on the other hand, a cell 50 for remove from face 4 the galvanized layer of strip 3. In this installation, as electrodes, electrodes according to the invention 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 losses of water and / or SO ₄ ⁼ of 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 the cathode 501 through 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 optionally 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 SO 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 a poor Zn-Ni electrolyte is supplied by the electrolyte from reservoir 512 and advantageously from reservoir 507 (conduit 520, pump 522 and conduit 521, pump 523).

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

Finally, Figures 16 to 18 show, schematically, installation embodiments similar to that shown in Figure 12.

In the form shown in Figure 16, electrodes with an anionic membrane are used as an anode in cells 1 for the electrolytic deposition of Zn or Zn-Ni on side 2 of the strip 3 and as a cathode in 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 a preparation unit electrolyte, which is supplied with water to obtain a correct dosage of the secondary electrolyte.

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

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

The other elements, conduits and parts of the installation shown in Figure 16 are similar to elements, conduits or parts of the installation shown in Figure 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 of the material balance of cells 1 (electrolytic deposition) and cell 50 (electrolytic elimination) thanks to the transfer of electrolyte from the tank 57 to the tank 56 via the pipe 90, advantageously after filtration (filter 72).

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

These installations allow, among other things, the deposition of Zn or Zn-Ni on one face 2 of the strip 3 and the deposition of Fe or Fe-Zn or other Fe alloy on the face 4 of the strip 3, this face 4 being opposite to the 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 face 4. This deposit of Fe or Fe alloy allows phosphating of face 4 as well as good paint layer adhesion.

The installation of FIG. 17 comprises cells 600 and 601 with anodes having a anionic membrane according to the invention. These anodes are intended for deposition on the face 2 of the strip 3 of a layer of metal. It goes without saying that the cells could have included anodes arranged two sides of the strip so as to provide the two sides 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 conduit 607 from the reservoir 604, this enriched electrolyte being sent by the conduit 608 to the reservoir 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 to bring the secondary electrolyte and to bring the secondary electrolyte back to the 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 make it possible to send directly from the electrolyte in the tanks 604, 610 to the tanks 602 and 618.

Figure 18 shows an installation similar to that shown in Figure 16 except that the cells 600, 601 included anodes provided with a cationic membrane and that, therefore, the electrolyte secondary after passage through the anode chambers is not sent to the tanks 604 and 610.

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

The reservoirs to supply the cells with electrolyte rich for example in Zn, Ni, the reservoirs for receive 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 Figures 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 SO ₄ , etc.) 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, said enclosure being provided with means defining vertical channels (112) intended to direct the upward flow of electrolyte with a velocity of at least 0.01 m/s, characterised in that the enclosure is provided with a series of vertical fins (111) extending between a lower end and an upper end, whereby the enclosure comprises an opening for feeding the chamber defined by the enclosure with electrolyte at the lower end of the fins (111), and an opening adjacent the upper end of the fins for removing electrolyte from the chamber, and whereby the fins act as electrodes and form distinct channels (112) between them, one wall of said channels being formed by a treillis or porous web (84) bearing the ion-exchange membrane (83), and whereby the fins (111) have a height (h) between 5 and 10 mm, while the distance separating two adjacent fins is between 5 and 10 mm.
2. Electrode according to claim 1, characterised in that the fins (111) are parallel, the height (h) of said fins being greater than the distance separating two adjacent fins.
3. Electrode according to claim 1 or 2, characterised in that it comprises an electrolyte delivery conduit and an evacuation conduit, whereby for each channel (112) defined between two adjacent fins (111), a distinct electrically insulated channel (154, 141) extends from the delivery conduit (130) up to the said channel (112) defined between the said two fins (111), and whereby for each channel (112) defined between two adjacent fins (111), a distinct, electrically insulated channel extends from the said channel defined between the said two fins up to the evacuation conduit.
4. Electrode according to anyone of claims 1 to 3, characterised in that the porous web or treillis presents, on its face adjacent to the membrane (84), a layer (87) also acting as an electrode.
5. Electrode according to anyone of claims 1 to 4, characterised in that said porous web or treillis (84) is provided with an insulating layer (88) on its face adjacent to the membrane.
6. Electrode according to anyone of claims 1 to 5 for the electrolytic treatment of a steel strip in movement with respect to the electrode, characterised in that the membrane (83) rests on a treillis and is covered by a porous protective layer (85), the treillis being directed towards the chamber defined by the enclosure, whereas the porous layer is directed towards the steel strip in movement.
7. Electrode according to claim 3, characterised in that the channel which extends from the delivery channel up to a channel defined between two fins (111) is arranged so as to direct the electrolyte towards the membrane (83) before passing into the said channel (112) defined between the two fins (111).
8. Electrode according to claim 1, characterised in that it comprises :

   (a) a first series of vertical fins (111) presenting a lower end and an upper end, said first series of fins being situated in an enclosure defining a first chamber (137), a vertical wall of said chamber being formed by a membrane (83) through which ions pass, said enclosure presenting an opening for feeding the chamber at the lower end of the fins (111) and an opening for removing electrolyte from the chamber at the upper end of the fins (111), said fins acting as electrodes and defining between them distinct channels (112), a wall of said channels being formed by a porous web or a treillis (84) bearing the membrane, said fins (111) having a height (h) between 5 and 10 mm and being spaced one from the other by a distance between 5 and 10 mm ;

   (b) a second series of vertical fins (111) presenting a lower end and an upper end, said second series of fins being situated in an enclosure defining a second chamber (138), a vertical wall of said chamber being formed by a membrane (83) through which ions pass, said enclosure presenting an opening for feeding the chamber at the lower end of the fins (111) and an opening for removing electrolyte from the chamber at the upper end of the fins (111), said fins acting as electrodes and defining between them distinct channels (112), a wall of said channels being formed by a porous web or a treillis (84) bearing the membrane, said fins (111) having a height (h) between 5 and 10 mm and being spaced one from the other by a distance between 5 and 10 mm ;

   (c) an intermediate cross-member (144) situated between said chambers, said cross-member presenting channels or passages (146) for delivering the electrolyte leaving a channel defined between two fins of the first series into a channel defined between two fins of the second series ; and

   (d)a beading (181) in synthetic material extending between the said chambers, beyond the vertical plane in which the membranes (83) lie and vertically beyond the enclosures.
9. Process for the electrolytic 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) or of plating metal alloys 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 8 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.
10. 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) or of plating metal alloys is recycled between the cathodic metal strip to be plated and the insoluble anode, in which an electrode is located within an enclosure defining a chamber, a wall of said enclosure consisting 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, the enclosure being provided with thin strips or 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, said electrode being 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.
11. Process for the electrolytic plating or metal strips according to claim 9 or 10, 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 be the transfer of S₄ ^- 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.
12. Process for the electrolytic plating of metal strips according to claim 9 or 10 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.
13. Process for the electrolytic plating of metal strips according to claim 9 or 10 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 sulfuric electrolyte enriched in iron or 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.
14. Process for the electrolytic plating of metal strips according to claim 9 or 10 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, 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 or iron containing chlorhydric acid and the passage of gases formed in the cathodic space is prevented by the cation exchange membrane.
15. Process according to anyone of the claims 11 and 14, 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.
16. Process according to claim 11 or 13, characterised 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.
17. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode according to anyone of the claim 1 to 8, for the working of a process according to claim 9 or according to claim 9 in combination with anyone of claims 11 to 16.
18. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode located within an enclosure defining a chamber, a wall of said enclosure consisting 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, the enclosure being provided with thin strips or 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, for the working of a process according to claim 10 or according to claim 10 in combination with anyone of claims 11 to 16.
19. 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 8 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 toward the first circuit of electrolyte (C2).
20. 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 located within an enclosure defining a chamber, 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, the enclosure being provided with thin strips or 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, said electrode being 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 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)
21. Process according to claim 19 or 20, characterised 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 S₄ ^- 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.
22. Process according to claim 21, characterised in that the electrolyte of the first circuit (C2), i.e. the electrolyte flowing in the cathodic chamber, contains from 50 to 100 g/l Na₂SO₄ and has a pH comprised between 1.5 and 2.
23. Process according to claim 19 or 20, characterised in that a velocity of the secondary electrolyte of at least 0.1 m/s is ensured.
24. Process according to anyone of the claims 21 to 23, characterised in that the upper part of the chamber is submitted to a gas suction.
25. Process according to claim 24, characterised 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 atmosphere pressure.
26. 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 8, for the working of a process according to claim 19 or according to claim 19 in combination to anyone of the claims 21 to 25.
27. Plant for treating in continu a steel strip in an electrolytic cell, said cell comprising at least one electrode within an enclosure located within an enclosure defining a chamber, 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, the enclosure being provided with thin strips or 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, for the working of a process according to claim 20 or according to claim 20 in combination with anyone of claims 21 to 25.
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 8, for the working of a process according to claim 9 or according to claim 9 in combination with anyone of claims 11 to 16 or according to claim 19 or according to claim 19 in combination with anyone of claims 21 to 24, the plant comprising two electrolyte cells (1,50;600.601) provided with an electrode according to anyone of the claims 1 to 8, the chambers of said electrodes being mounted in one and same circuit for the flow of secondary electrolyte.
29. Plant for treating in continu a steel strip in an electrolytic cell for the working of a process according to claim 10 or according to claim 10 in combination with anyone of claims 11 to 16 or according to claim 20 or according to claim 20 in combination with anyone of claims 21 to 25, said plant comprising two electrolyte cells (1,50;600,601) each provided with an electrode located within an enclosure defining a chamber, 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, the enclosure being provided with thin strips or 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, the chambers of said electrodes being mounted in one and same circuit for the flow of secondary electrolyte.
30. Plant according to claim 28 or 29, characterised 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, characterised 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 deposition 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, characterised 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, characterised in that a filter (72) is mounted between the cell (50) for the removal of the Ni possible deposited on the first layer of Zn or Zn alloy and the fourth tank (57).
35. Plant according to claim 33 or 34, characterised 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, characterised 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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