ES2360020T3 - INSTALLATION AND PROCEDURE FOR THE ELECTROLYTIC TINTING OF STEEL BANDS USING AN INSOLUBLE ANODE. - Google Patents

Abstract

The invention relates to equipment for the electrolytic tinning (1) of a continuously running steel strip (2) in at least one coating tank (30) fitted with at least one non-soluble anode (60) and containing an acidic electrolyte. The equipment (1) further includes a tin dissolution reactor (10) including a non-soluble cathode (120) and a soluble tin anode (160). According to the invention, at least one electrodialysis or electrolysis cationic membrane (140) is provided in the reactor (10) between the tin anode (160) and the cathode (120), thus defining an anodic area (1600) and a cathodic area (1200), and an electrolyte recirculation circuit connects the electrodeposition tank (30) to the anodic area (1600) of the reactor (10). The present invention also relates to an electrolytic tinning method with a non-soluble anode using said equipment (1).

Description

The invention generally relates to electrolytic tin plating of steel bands using an insoluble anode, and more specifically to an electrolytic tin plating process using an insoluble anode and the installation for its application.

The absence of tin toxicity and the excellent corrosion protection provided by steel have long led to the use of tinned soft steel in the field of food packaging where it is known under the name of "tin". Tinplate is generally manufactured from soft steel coils (“coils”)

or ultra soft, which are previously subjected to a hot rolling operation, followed by a cold rolling operation. At the end of these rolling operations, steel bands of a few tenths of millimeters thick are obtained. These bands are then annealed, passed after annealing to a cold laminator ("skin passées"), degreased, then stripped after tinning according to an electrolytic tinning procedure (or still "electro-tinning"). Tinning is typically followed by finishing operations such as coating refusion, passivation, and oiling.

Electrostanding is a process of electrodeposition of tin on a metal substrate, which consists of

establish the transfer of stannous ions Sn2 to the band to be coated according to the balance:

2 

Sn  2 and Sn registered

This reaction implies the availability of stannous ions in the bath. In addition to these stannous ions, the bath contains an acid intended to reduce the pH and increase the electrical conductivity. It also contains additives that contribute, among other things, to stabilize the tin ions preventing them from oxidizing, and preventing the formation of depositions of stannous oxides caused by the oxidation of these tin ions.

There are two main categories of electrostating procedures: the first category of procedures regroups procedures that use a soluble anode, or procedures called "with soluble anode," and the second category of procedures regroups procedures that use an insoluble anode, or procedures called "with insoluble anode".

Electrostating processes called "with soluble anode" are used in electrolytic tinning facilities that mostly resort to high purity tin anodes (ie anodes comprising at least 99.85% tin weight), which dissolve during electrolysis and fill the bath in tin ions

Sn2.

An example of the installation of electrostatic "with soluble anode" known to those skilled in the art is represented on figure 1. It is a vertical electrostatic installation 1, in which a strip to be coated 2 is immersed in a tank of coating 3 (or also an electrodeposition tank) winding on two conductive rollers 41, 42 and a bottom roller 5, thus forming a descending strand 21 and an ascending strand 22. Both conductive rollers 41, 42 feed the band 2 in electric current. The tin-soluble anodes 61.62 are arranged in both sides of the descending and ascending strands 22 of the steel band 2 to be coated. This steel band 2 is connected to the negative pole (represented by the symbol "-" on figure 1) of an electric current generator (not shown on figure 1), and the soluble anodes 61, 62 are connected to the positive pole (represented by the "+" symbol on figure 1) of this generator, thus constituting the anode. The anodes 61, 62 and the descending and ascending strands 21 of the steel band 2 are partially immersed in an electrolytic solution 7 (or electrolyte).

There are several electrostated "soluble anode" procedures, which differ from each other according to the electrolyte used. But, in all electro-tinning procedures "with soluble anode", the tin electrolytic coating of the steel band 2 is developed according to the following reactions:

e

 With the cathode: SnA2  2  Sn  2A

 With the anode: Sn  2A SnA2  2e

In electrostanding processes called "with insoluble anode", the tin anode is replaced by a non-soluble anode, for example a titanium anode with a coating of a metal (for example a platinum family metal) or a metal oxide In this type of procedure, the tin ions necessary for coating

they are, in this case, the result of the electrolyte bath itself in the form of a compound of the formula SnA, where A

2

an acid radical. The reactions that develop in the anode and with the cathode are obviously different:

e

 With the cathode: SnA2  2  Sn  2A

image 1 1 

 With the anode: HO 

O  2H 2e

2 22

Electrostanding processes called "with insoluble anode" are thus distinguished from those called "with soluble anode" in that they lead to the formation of acid in the electrolytic bath correlatively to its impoverishment in tin. These continuous modifications therefore require a regeneration, the bath also continues.

The person skilled in the art is aware of the electrostatic "with insoluble anode" processes in which a part of the electrolyte is recirculated in view of regeneration without interruption of the electrolytic bath. Thus, for example, US Patent 4, 181, 580 describes an electro-tinned installation illustrated on Figure 2, which uses non-soluble anodes 61, 62, a recirculation circuit 8 of electrolyte 7, and a fluidized bed reactor 9, in which the electrolyte 7, the tin granules 91, and a gas stream 92 rich in oxygen are introduced. This procedure nevertheless has the drawback of inducing the formation of tetravalent tin ions according to the reaction:

 4

S O  4H  Sn  2HO

n 22

2 4

2Sn  O2  4H  2Sn  2H2O

These Sn4 ions, poorly soluble in the electrolyte, precipitate in the form of depositions that need to be recovered regularly, which greatly decreases the interest of such a process.

On the other hand, US Patent 5,312,539 proposes another tinning process "with insoluble anode", which uses an anionic membrane dialysis cell and a separate tin dissolution unit in which tin is supplied in the form of oxide directly dissolved in the acid, or in the form of a tin anode, which dissolves electrolytically. Such a procedure has some drawbacks and in particular the cost of tin oxide and the need to create a strong concentration gradient of both parts of the membrane, impose the application of a concentration unit. On the other hand, even with a strong concentration gradient, the necessary membrane surface

(of the order of several thousand m2 for continuous tinning of steel bands) makes the industrial application very problematic. A variant of this procedure is proposed by the Japanese patent application JP 51-71499 which regroups the functions of tin dissolution and dialysis in the same tank equipped with two anionic membranes. The installation less complex than that of US Patent 5,314,539, therefore does not solve the problems of membrane surface or concentration gradient.

The present invention therefore has as an object an electrostating process and an installation for its application that remedies the drawbacks of the prior art, by resorting to an electrodialysis membrane

or cationic electrolysis in the tin dissolution device.

By cationic electrodialysis membrane, it is understood, within the meaning of the present invention, a cation permeable membrane and which is typically used in an electrodialysis process.

By cationic electrolysis membrane, it is understood, within the meaning of the present invention, a cation permeable membrane typically used in a membrane electrolysis process, but which can advantageously be used in the electrodialysis process according to the invention due to its robustness and its ability to withstand higher current densities than a cationic electrodialysis membrane.

The present invention aims, more particularly an installation for electrolytic tin plating of a steel strip in continuous sliding, said installation comprises:

 at least one electrodeposition tank filled with an electrolytic solution comprising an acid AH and

stannous ions Sn2 in the form of a compound SnA with A which designates an acidic function, said container of

The electrodeposition comprises an insoluble anode immersed in the electrolytic solution of the electrodeposition vessel and a cathode constituted by the continuous sliding metal band in the electrolytic solution of the electrodeposition vessel, and

 a tin dissolution reactor comprising an insoluble cathode and at least one soluble tin anode.

According to the invention, the tin anode and the insoluble cathode are separated by a cationic electrodialysis or electrolysis membrane, defining a cathodic zone that integrates the cathode and an anodic zone that integrates the tin anode, and a recirculation circuit of the electrolytic solution that joins the electrodeposition tank to the anodic zone of the tin dissolution reactor.

The presence of a cationic electrodialysis membrane in the dissolution reactor between the soluble tin anode and

H 

the insoluble cathode allows the ions of the electrolyte solution to transit through the membrane, from the zone

anodic towards the cathodic zone, while the Sn2 ions produced with the anode remain mostly in the anode zone of the reactor. The electrolyte solution contained therein is then recharged into stannous ions, and can then be directed back to the coating vessel.

Advantageously, the recirculation circuit of the electrolyte solution between the electrodeposition tank and the anodic zone of the tin dissolution reactor comprises an oxygen degassing tank, which is arranged upstream of the dissolution reactor in the direction of circulation of the electrolyte in this recirculation circuit.

This degassing vessel allows the gaseous oxygen formed in the insoluble anode of the coating vessel to be removed efficiently.

Advantageously, the installation according to the invention also comprises a degassing circuit of the electrolytic solution contained in the cathodic zone of the tin dissolution reactor, this degassing circuit integrates a hydrogen degassing vessel.

The acid AH is advantageously chosen among the sulfonic acids.

As usable sulfonic acids according to the present invention, we can particularly mention methanesulfonic acid and phenol sulfonic acid.

The preferred sulfonic acid is methane sulfonic acid.

If a sulfonic acid is used, and, in particular, an acid chosen from methanesulfonic acid and phenol acid

sulfonic, the compound SnA will therefore advantageously be a tin sulfonate corresponding to the acids

2

Preferred sulfonic acids according to the invention: tin phenol sulphonate or tin methane sulphonate.

The present invention also aims at an electrolytic tinning process of a steel band in continuous sliding at least in an electrodeposition vessel filled with an electrolytic solution that

comprises an acid AH and stannous ions Sn2 in the form of a compound SnA with A designating a function

2

acidic, said tinning process applies an unsolvable anode and the metal band constituting a cathode that is

immersed in the electrolyte solution and between which a potential difference is applied, and the compound SnA that

2 comes from a tin dissolution reactor, which comprises an insoluble cathode and a tin anode, between which a potential difference is applied.

According to the invention, the concentration in acid AH in the electrolytic solution of the coating tank is kept constant by performing the following steps:

a) a cationic electrodialysis or electrolysis membrane is disposed in the tin dissolution reactor between the tin anode and the insoluble cathode, thus defining a cathodic zone that integrates the insoluble cathode and an anode zone; Y

b) a part of the electrolyte solution enters into circulation between the electrodeposition tank and the anodic zone of the tin dissolution reactor.

Other advantageous features of the invention will appear in the following description of some embodiments given by way of simple example and represented on the attached drawings:

-

 Figure 1 is a diagram of the principle in section of an example of installation of electrostanding with soluble anode according to the state of the art,

-

 Figure 2 is a diagram of the principle in section of an example of the installation of electrostanding with insoluble anode according to the state of the art,

 -

 Figure 3 is a diagram of the principle in section of an example of an electrostatic installation according to the invention,

-

 Figure 4 represents a diagram of the principle in section of an example of a dissolution reactor of an electrostatic installation according to the invention,

-

 Figure 5 is a bottom view depicting a top view of another example of a dissolution reactor of an electrostatic installation according to the invention.

The electrostatic installation (or electrolytic tinning installation) depicted on Figure 1 is an electro tin plating installation 1 with soluble anode according to the state of the art, which was described above in reference to the prior state of the art.

The electrostatic installation (or electrolytic tinning installation) depicted on Figure 2 is an electrostatic installation 1 with insoluble anode according to the state of the art, which was described above in reference to the prior state of the art.

On figure 3, a schematic diagram of an installation example according to the invention is shown, in which the strip to be coated 20 and an insoluble anode 60 are partially submerged in an electrodeposition tank

30 (or coating vessel) containing an electrolytic solution (or electrolyte) containing stannous ions Sn2

in the form of a compound SnA and an acid AH, with A being a radial acid. The SnA compound comes from a reactor

22 of tin solution 10, comprising an insoluble cathode 120 and a soluble tin anode 160, which are submerged in a reservoir 130 which also contains the same electrolytic solution as the coating vessel 30. A cationic electrodialysis or electrolysis membrane 140 it is disposed between the electrodes 120, 160 of the reactor 10, so that the reservoir 130 of the reactor 10 is divided into a cathodic zone 1200 containing the insoluble cathode 120 and an anodic zone 1600 containing the soluble anode 160.

In the embodiment of the tin plating system 1 according to the invention, shown on figure 3, the anode 160 of the reactor 10 is constituted by tin granules 161 contained in a basket 162 (called "tin dissolving basket"). This basket 162 full of granules 161 is connected to the positive pole (represented by the symbol "+" on figure 3) of an electric current source (not shown on figure 3), the tin granules 161 play the role of anode soluble. The insoluble cathode 120 of the tin dissolution reactor 10 is connected to the negative pole (represented by the symbol "-" on Figure 3) of the same source of electric current.

As a soluble anode 160, it is also possible to use, in the dissolution reactor of the tin 10 of the installation according to the invention, an anode in solid form (not shown on figures 3 to 5).

On the other hand, the electrostating installation shown on Figure 3 contains, in addition to the coating tank 30 and the tin dissolution reactor 10, an oxygen degassing tank 210 and a hydrogen degassing tank 310.

The oxygen degassing vessel 210 is part of a recirculation circuit 200 of the electrolyte that connects the coating vessel 30 and the anodic zone 1600 of the dissolution reactor 10, the degassing vessel 210 is disposed upstream of the dissolution reactor (10 ) in the direction of recirculation of the electrolyte in circuit 200.

On the other hand, the hydrogen degassing vessel 310 forms part of a recirculation circuit 300 of the electrolyte contained in the cathodic zone 1200 of the dissolution reactor 10, in which the electrolyte is subjected to hydrogen degassing in a degassing vessel of hydrogen 310.

In operation, when a potential difference is applied between electrodes 20, 60 submerged in the container

coating 30, the stannous ions Sn2 present in the electrolyte in the form of the compound SnA are deposited

2

on the band to be coated 20 according to the reaction:

SnA2  2e Sn  2A

The following reaction is observed in parallel to the anode:

2H2O  O2  4H  4e

Therefore, an electrolyte depleted in stannous ions is obtained, the part of which is taken in the coating vessel 30, then subjected to a degassing of the gaseous oxygen in the degassing vessel 210 before being introduced into the anode zone 1600 of the dissolution reactor 10.

As in the coating vessel 30, a potential difference is applied simultaneously between the electrodes 120, 160 of the tin dissolution reactor 10, which leads to the electrolytic dissolution of the soluble tin anode 160 according to the reaction:

S  2A SA  2e

nn 2

In parallel, the following reaction is observed at the cathode of reactor 10:

A H  AH

The electrolytic dissolution of the tin granules 161 guarantees the production of stannous ions Sn2, which thanks to the selective permeability of the cationic membrane 140 remain mostly in the vicinity of the anode.

As a cationic membrane that can be used according to the invention, the membrane marketed by the company TOKUYAMA SODA is recommended under the trade name CMX-S.

H 

The cationic membrane 140 has a selective permeability that allows the transfer of ions to

the cathode 120 and the maintenance of the Sn2 ions in the vicinity of the anode 160.

The electrolyte of the anodic zone 1600 thus recharged in stannous ions can then be recovered and directed from

H 

again towards the coating vessel 30. On the contrary, the ions present in the anodic zone 1600 pass through the cationic membrane 140 due to the electric field created by the potential difference between the anodic zones

H 

1600 and cathode 1200 of reactor 10. The ions recovered in cathode zone 1200 of reactor 10 are

then recombine with the anions A, in order to reform the acid AH.

On FIG. 4, an example of a dissolution reactor 10 according to the invention is shown, which contains a reservoir 130 filled with electrolytes, which can be cylindrical in shape and which is separated in two by a cationic electrodialysis membrane 140, which can also be cylindrical in shape, thus defining a central anodic zone 1600 containing soluble anode 160, and an external cathode zone 1200 containing cathode 120.

The cylindrical shape of the tank 130 and the cationic membrane 140 are given here by way of example. But, the reservoir 130 and the cationic membrane 140 can also be parallelepipedic.

The cathode 120 is connected to the negative pole of an electric current source (represented by the symbol "-" on figure 4) and the anode 160 is connected, in its upper part, to the positive pole (represented by the symbol "+" on figure 4) of the same source of electric current.

Figure 4 shows that the soluble tin anode 160 comprises a solution basket 162 comprising the tin granules 161. This basket 162 is divided into three distinct overlapping parts:

- a lower zone 1621 submerged in the electrolyte contained in the tank 130;

-a middle zone 1622 for the recovery of the electrolyte, which is located above the lower zone 1621 that is contiguous to it and that is not immersed in the electrolyte contained in the tank 130, but is wetted by the electrolyte solution when placed in circulation in circuit 200, and

-a superior dry zone 1623 for feeding in dry tin granules 161 and the transmission of the dissolving electric current.

The lower areas 1621 and middle 1622 of the dissolution basket 162 of the anode 160 are both made of a non-conductive material of electricity.

As a non-conductive material of electricity that can be used according to the invention to make the lower areas 1621 and middle 1622 of the basket 162 of the soluble anode 160, we recommend plastic materials, and compounds such as reinforced polyester resins and polymer coated steels .

In contrast, the upper feed zone 1623 in tin granules 161 is made of an electrically conductive material.

As title of electricity conductive material usable according to the invention to make the basket 162 of the soluble anode 160, we can mention particularly the stainless steel.

The lower area 1621 submerged in the electrolyte contains a lattice 163 comprising a mesh plastic net adapted to the retention of the tin granules, be between 0.05 and 0.50 mm, and preferably between 0.1 and 0, 30 mm This network is supported by the envelope of the basket that has openings for contacting the electrolyte, which are at least 50 times wider than the meshes of the network of the openings (dotted on Figure 4) that are formed in the envelope of the basket 162.

The middle zone 1622 contains a recovery channel 164 of the regenerated electrolyte, this channel is fed through a lattice 165 (identical to 163 of the lower zone 1621) and openings (dotted on Figure 4) formed in the envelope of basket 162 (identical to those in lower zone 1621).

In operation, the middle zone 1622 is wetted by the electrolytic solution of the tank 130 in circulation in the circuit

200.

The upper zone 1623 contains a feed hopper 166 of tin granules 161, which is connected to the positive pole of the power supply in electric current and that ensures the transmission of the dissolving electric current in the bed of tin granules, through the contact surfaces between the tin granules and the hopper in electrically conductive material.

The lower area 1621 of the basket 162, which is immersed in the electrolyte, is surrounded by a cationic membrane 140 circular in shape. This cationic membrane 140 is advantageously supported by at least one network of plastic material, which ensures the rigidity of the membrane 140.

10 The electrolyte to be treated is introduced into the lower zone 1621 of the basket through the intake ducts 201 at a sufficient pressure to allow its overflow in the recovery pit 164 of the middle zone 1622. During the course of the granule path of tin 161 through basket 162, the electric current ensures dissolution

of said granules 161 and the acid is charged in Sn ions that remain close to anode 160. The electrolyte thus recharged in tin is recovered at the level of bucket 164, before returning to the liner vessel 30 by means

15 of the return ducts 202.

On FIG. 5, another example of a dissolution reactor 10 according to the invention is shown in top view, comprising a plurality of soluble anodes 160 each containing a basket 162 filled with tin granules 161, each basket 162 is surrounded by a cationic membrane 140 circular.

A feeding device 400 in granules 161 communicates the hoppers 166 of all the baskets 162 of the reactor

20 solution 10. This device 400 may be a treadmill or vibrating belt, or non-conductive pipes of electricity. The device 400 acts intermittently according to a signal given by a device for detecting the level of granules in the hoppers 166, to maintain a constant level of granules 161 in the basket 162.

Claims (15)

 REINVINDICATIONS 1. Installation (1) for electrolytic tin plating of a steel strip (2) in continuous sliding, said installation (1) comprises:  at least one electrodeposition tank (30) filled with an electrolytic solution that includes an acid AH and stannous ions Sn2 in the form of a compound SnA with A that designates an acidic function, said tank of 2 electrodeposition (30) includes an insoluble anode (60) immersed in the electrolytic solution of the electrodeposition tank (30) and a cathode (20) constituted by the band (2) in continuous sliding in the electrolytic solution of the electrodeposition tank ( 30),  at least one tin dissolution reactor (10) comprising an insoluble cathode (120) and at least one soluble tin anode (160), characterized because, the tin anode (160) and the insoluble cathode (120) are separated by a cationic electrodialysis or electrolysis membrane (140) defining a cathodic zone (1200) that integrates the cathode (120) and an anode zone (1600) which integrates the tin anode (160), and also characterized because, said tin installation (1) on the other hand comprises a recirculation circuit (200) of the electrolytic solution between the electrodeposition vessel (30) and the anodic zone (1600) of the tin dissolution reactor (10). 2. Installation (1) according to claim 1, characterized because, said recirculation circuit (200) of the electrolytic solution between the electrodeposition tank (30) and the anodic zone (1600) of the tin dissolution reactor (10) consists of an oxygen degassing vessel (210) arranged waters above the dissolution reactor (10) in the direction of electrolyte circulation in said recirculation circuit (200). 3. Installation (1) according to claim 1 or 2, characterized because, It also comprises a degassing circuit (300) of the electrolytic solution contained in the cathodic zone (1200) of the tin dissolution reactor (10), the degassing circuit (300) that integrates a hydrogen degassing vessel (310).

Four.

Installation (1) according to any one of the preceding claims, characterized in that, The soluble tin anode (160) is in the form of tin granules (161) contained in a basket (162).

5.

Installation (1) according to claim 4, characterized because, The basket (162) comprises three distinct overlapping parts:

-

 a lower zone (1621) that is immersed in the electrolyte solution contained in the tank (130) of the dissolution reactor (10),

-

 a middle electrolyte recovery zone (1622), which is located above said contiguous lower zone (1621), said middle zone (1622) is not immersed in the electrolyte solution contained in the reactor tank (130) of solution (10), but it gets wet by the electrolytic solution when it is put into circulation in the circuit (200),

-

 a dry upper zone (1623) for feeding in tin granules (161) and the transmission of the dissolving electric current, said upper zone (1623) is located above said middle zone (1622) which is adjacent to it.

6.

Installation (1) according to claim 5, characterized because, the lower (1621) and middle (1622) areas of the basket (162) are made of a non-conductive material of electricity.

7.

Installation (1) according to claim 6,

characterized in that, The non-conductive electricity material of the lower (1621) and middle (1622) areas of the basket (162) is a plastic material or a composite material chosen in the group consisting of reinforced polyester resins or polymer coated steels.

8.

Installation (1) according to one of claims 5 to 7, characterized in that, the upper area (1623) of the basket (162) is made of an electrically conductive material

9.

Installation (1) according to any of claims 5 to 8, characterized in that, the lower area (1621) of the basket (162) contains:

-

 a trellis (163) comprising a plastic net whose mesh is between 0.05 mm. and 0.5 mm., and

-

 an envelope for holding said lattice (163) and containing one or more covers to bring the granules (161) into contact with the electrolyte solution.

10. Installation (1) according to one of claims 5 to 9, characterized in that the middle area (1622) of the basket (162) contains:

-

 a trellis (165) comprising a plastic net whose mesh is between 0.05mm. and 0.5 mm., and

-

 a recovery channel (164) of the electrolytic solution, said channel (164) is fed in electrolytic solution by means of the lattice (165).

eleven.

Installation (1) according to one of the preceding claims, characterized because, The dissolution reactor (10) contains a plurality of soluble anodes (160), each of these anodes (160) consists

of a hopper (166) and is surrounded by a cationic electrodialysis or electrolysis membrane (140).

12.

Installation (1) according to claim 11, characterized in that,

It contains a feeding device (400) in granules that intermittently communicates the hoppers (166) of the anodes (160).

13.

Installation (1) according to claim 12, characterized because, the feeding device (400) in granules (161) is a vibrating or rolling carpet, or non-conductive pipes of

electricity.

14.

Electrolytic tin plating process of a steel band (20) in continuous sliding at least in an electrodeposition tank (30) filled with an electrolytic solution consisting of an acid AH and stannous ions

Sn2 in the form of an SnA compound with A designating an acid anion, said tinning process puts 2 execution of at least one non-soluble anode (60) and the metal band (20) constituting a cathode that it is immersed in the electrolyte solution and between which a potential difference is applied, and the compound SnA that comes from a tin dissolution reactor (10), comprising an insoluble cathode (120) and a tin anode (1602) , between which a potential difference is applied, characterized in that, The concentration of OH acid in the tub electrolyte solution (30) is kept constant by performing the following steps: a) a cationic electrodialysis membrane (140) is placed in the tin dissolution reactor (140) between the tin anode (160) and the insoluble cathode (120), thus defining a cathodic zone (1200) that integrates the insoluble cathode (120) and an anodic zone (1600); Y b) we circulate a part of the electrolyte solution between the electrodeposition tank (30) and the anodic zone 5 (1600) of the tin dissolution reactor (10). 15. Method according to claim 14, characterized in that the electrolytic solution taken in the coating vessel (30) is subjected to oxygen degassing before being injected into the anode zone (1600) of the reactor. Method according to claim 14 or 15, characterized in that, a part of the electrolytic solution contained in the cathodic zone of the dissolution reactor (10) is placed in recirculation and undergoes hydrogen degassing.