FR2864553A1 - Apparatus for the deposition of zinc or zinc alloys using a porous separator to divide the electrolyte bath into cathodic and anodic compartments - Google Patents

Abstract

Apparatus for the deposition of zinc or zinc alloys comprises: (a) a tank (14) subdivided by a separator, made of a open porous material, into cathodic and anodic compartments; (b) the component (18) is immersed in a aqueous electrolyte (16), with an alkaline pH, contained in the cathodic compartment and containing some zinc ions, and possibly some ions of a metal other than zinc,; (c) an aqueous electrolyte, with an alkaline pH, contained in the anodic compartment and surrounding an anode of an insoluble metal.

Description

The present invention relates to electrolytic installations of

  deposits of zinc or zinc alloys on a part.

  Patent Application WO 01/96631 of the inventors William Eckles and Robert Frischauf of Taskem INC. discloses a method for the electrolytic deposition of alkaline zinc-nickel using a perfluorinated cation exchange membrane. These membranes are fragile, difficult to supply and particularly expensive. Their use in industrial environments requires a great deal of care so that barrels or electrolysis fixtures do not damage the wall of these membranes.

  The technique developed in WO 01/96631 also generates the accumulation in the catholyte of undesirable anions such as sulphates and carbonates. The sulphates come from the nickel bath feed by nickel sulphate and the carbonates are caused by the soda absorption of ambient carbon dioxide.

  Above all, there is a decrease in productivity over time. Thus, in order to obtain the same thickness of the coating in a zinc nickel bath, a deposition time of 3 hours after one year of operation is required whereas the installation made this deposit at the beginning of its implementation. road in one hour and thirty minutes.

  The present invention overcomes these disadvantages. It helps to maintain better productivity over time, it gives a longer life to the installation and all this being less expensive.

  The subject of the invention is therefore a plant for depositing zinc or zinc alloys with at least one other metal selected from iron, cobalt, nickel and manganese on a part comprising: a) a vessel subdivided by a separator in a cathode compartment and in an anode compartment, b) the part being immersed in an aqueous catholyte, of alkaline pH contained in the cathode compartment and comprising zincate ions and, optionally, ions of at least one other metal than zinc and c) an aqueous anolyte, of alkaline pH, contained in the anode compartment and surrounding an anolyte insoluble metal anode, characterized in that d) the separator is of an open-pore material.

  An open-pore material such as polytetrafluoroethylene or a polyolefin, preferably polypropylene, but also polyethylene or a porous ceramic, are much less expensive water-permeable materials than much stronger ion-exchange membranes, provided that the materials preferably resistant to alkaline pH will be selected. While the separator is in a material with open pores, it does not, however, unexpectedly, a passage of the other more valuable metal than zinc and especially nickel in the anolyte, but anions including sulfates and carbonate, the sulphate from the nickel sulphate added to the catholyte and the carbonate from the carbonation of the air by the soda, which hitherto accumulated in the catholyte pass in the anolyte, that it becomes relatively easy to process since you no longer have to strive to recover the most valuable metal. Productivity is thus maintained over time.

  Preferably the anode compartment communicates with a desulfator which can be a cold trap causing crystallization of the sulphates or in which a sulphate precipitating agent such as a barium salt can be added. It is then sufficient to return the anolyte thus treated to the anode compartment. It can be considered that the anode compartment has become "the trash" of the installation.

  A separator according to the invention passes the following test: the separator to be tested is immersed in a vat containing an aqueous solution of NaOH at 120 gil so that it delimits two compartments in the vat, poured into one 20 ml / l of a solution colored in violet to 8% by weight nickel comprising by weight: - 36.2% nickel sulfate hexahydrate, - 15% tetrathylenepentamine, - the balance to 100% water.

  If, one day later, the solution in the compartment which has not received the solution colored violet, is not colored violet, the separator is suitable.

  According to one embodiment of the installation according to the invention, the anode compartment communicates with an alkali metal persulfate reservoir. By adding the anolyte of sodium or potassium persulfate, it is possible to destroy the cyanides that may be formed when the catholyte comprises a complexing agent for a metal, in particular in the form of a polyalkylamine.

  Preferably the alkaline pH is greater than 12, although in some cases it may be in particular between 8 and 12.

  According to a very preferred mode of preference, the separator is made of a water-repellent material such as polytetrafluoroethylene. In this case, it is recommended, when putting the separator in the tank, to impregnate it at the beginning of an alcohol solution to prime.

  The invention relates to an installation for the electrolytic treatment of zinc and zinc alloy on a piece of steel or iron alloy. The system can be applied with alkaline electrolytes having a pH greater than 12. The deposition bath may contain zinc ions and organic additives. The steel or iron alloy part is placed in a cathode. Anodic Mounting is in contact with the bath. The Anodic Assembly may include an anolyte, an anode insoluble metal anode, and a container whose wall in contact with the catholyte consists of a porous vessel or a porous membrane. Organic additives may be able to be destroyed by electrolytic oxidation in contact with the anode. Anodic Mounting can avoid the electrolytic decomposition of organic additives. The Anodic Assembly must contain a compartment called anolyte. The Anodic Assembly is composed outside of a porous vase or a porous membrane. The anolyte is then contained in the volume released by the porous vessel or the porous membrane. The insoluble metal used in the anode can be immersed in the anolyte. On the other hand, the electrolytic bath may comprise different metal ions that can be electrolytically deposited on the steel part with the zinc ions. For example, the ions of the additional metals may include nickel, manganese, iron, cobalt ions and all combinations of these metals. On the other hand, organic additives can be potentially oxidized in contact with the anode. The decomposition of organic additives forms byproducts factors of decrease of cathodic efficiency. The anode can be any metal or metalloid that can serve as an anode in a caustic solution. The anolyte may be soda or potash in solution.

  The tank may be made of any organic plastic resistant to electrolytic bath and anolyte, for example polypropylene, and non-electrically conductive.

  The porous vase or the porous membrane of the Anodic Assembly can be any porous membrane of mineral or organic composition provided that it chemically resists the alkalinity of the two anode compartments and cathodes, and this for temperatures up to 60 C Its texture must be permeable to water. The methods for obtaining its porous walls are numerous. Non-exhaustive examples are: woven, sintered or expanded membranes. The pore diameter is between 10 nm and 5 μm.

  The porosity of the material will be chosen so as to allow good electrical conductance of the assembly, to prevent this barrier between anode and cathode generates an overvoltage greater than 5V.

  The thickness of the porous membrane or separator depends on the anode assembly used. In the case of FIGS. 4 and 5 below, the thickness is fine, between 0.1 and 2 mm thanks to the mechanical reinforcement of the supports. In the case of Figures 1, 2 and 3 below, the thickness will be very variable from 0.1 mm to 1 cm because the porous membrane may have no support.

  The porous membranes can be hydrophobic or even lipophobic, which reduces the diffusion of the species without pressure through the porous membrane.

  - Porous mineral membranes: These are ceramics containing oxides of elements such as silicon, aluminum, titanium, zirconium or other components contained in natural clays. These porous membranes are obtained by baking at high temperature. Porous membranes can also be made by sintering.

  They can be parallelepipedal or tubular as in the case of Figure 5. It is also possible to use cup-shaped materials, as shown in Figure 3 or flat elements, as Figure 1 and 2.

  - Organic porous membranes: Plastics are numerous to resist in alkaline medium. For example PolyTetraFluoroEthylene (PTFE or Teflon Dupont), polyethylenes or polypropylenes, among the most common. The choice will be guided by the mechanical resistance and its porosity.

  One example is the use of a porous Teflon membrane with a thickness of 0.43 mm, a weight of 510 g / m2 and an air permeability of 5 L / dm2 / min. 196 Pa or 30 m3 / h / m2 to 196 Pa Mortelecque society under the reference TC 110. This porous membrane is cut and assembled on the walls with a seal as shown in Figures 2 and 4. This same fabric can also be welded around a cylindrical support as shown in figure 5.

  The anolyte in the anode compartment may comprise a conductive salt or a base in solution such as an aqueous alkaline solution of potassium hydroxide or sodium hydroxide. These alkaline solutions may have varying concentrations in the range of one mole up to 20 moles / L of hydroxide, with a preferred concentration of between 1 to 10 moles / L of hydroxide. The preferred anolyte may be a sodium hydroxide solution of between 50 and 760 g / l, and more preferably between 50 and 250 g / l.

  The anode of the anode assembly may be made of a metal or metalloid capable of functioning as an anode in an electrolytic bath and must be stable in a caustic solution. Stable in a caustic solution means that the anode must not decompose, deteriorate or erode in solution. Examples of metals may be used among nickel, cobalt, iron, chromium and their alloys. Such as for example steel and ferrous alloys. Other metals or metalloids can also be used provided that they are able to function as an anode and be stable in a caustic solution.

  The anode may be a solid metal or metalloid or a metal electrodeposited on a substrate. For example, the anode may be nickel, nickel alloy or nickel deposited on a substrate. The substrate may be a metal such as steel, copper or aluminum or a plastic. An example of a nickel alloy is hastelloy which is composed of 55% nickel and 45% chromium. Nickel and nickel alloys can be electrodeposited on a substrate, using a Watts electroplating bath for nickel or alloys or deposited from a chemical nickel.

  Similarly, the anode may be cobalt or cobalt electrodeposited on a substrate such as these alloys. The anode can also be mild steel, alloy steel, iron alloy or an iron-chromium alloy such as stainless steel. The construction material of the anode is not restrictive. For example, electrolytic or electrochemical deposits can be used on the anode. Electrodeposited noble metals can be used on a metal substrate, for example platinum or iridescent titanium. Practical considerations such as cost and caustic solution stability will dictate the choice of equipment to be used for the anode.

  The salicylic acid bath may be an alkaline aqueous solution of which the hydroxide salt or alkali metal salt content is between 50 and 250 g / l. Examples of hydroxides or alkali metal salts include sodium hydroxide, potassium hydroxide, sodium carbonate and potassium carbonate. The bath is prepared by dissolving the alkaline compound in the aqueous solution.

  The electroplating bath may also include a controlled amount of zinc ions. The source of the zinc ions for the electroplating bath can be any soluble zinc salt in an alkaline medium. For example, there may be mentioned zinc compounds capable of being added to the electroplating bath: zinc oxide or a soluble salt of zinc, such as zinc sulphate, zinc carbonate, zinc sulphamate and zinc acetate. The concentration of zinc ions in the electroplating bath may be from 1 to 100 g / L (1000 ppm to 100,000 ppm), preferably from 4 to 50 g / L (4000 to 50,000 ppm). At pH 14, the dominant zinc species in the bath is the zincate ion. The bath may also contain a controlled amount of metal ions that are not zinc ions. In accordance with the invention, these additional metals may be any metal ion that can be effectively electrodeposited with zinc on a cathode part in an alkaline electroplating bath. Examples of such metal ions include transition metal ions such as nickel, manganese, iron, cobalt ions and combinations of these ions. Other non-listed metal ions that can be deposited with the zinc ions on the cathode in an alkaline electroplating bath can also be used and are included in the spirit of the invention.

  The additional metal ions may also be only nickel ions, the source for the nickel ions in the electroplating bath may be any nickel compound capable of being solubilized in aqueous alkaline solution. Examples of nickel compounds are organic and inorganic nickel salts such as nickel sulphate, nickel carbonate, nickel acetate, nickel sulphamate and nickel formate. The concentration of nickel ions (or other metal) in the electroplating bath can be 0.1 to 50 g / l preferably in the range of 0.1 to 3 g / l.

  The metal ions can also include any metal ion. The electroplating bath may comprise, for example, a mixture of zinc ion and iron ion, a mixture of zinc ion, nickel ion and iron ion.

  The sources for these additional metal ions for the electroplating bath may be any metal compound that may be soluble in an aqueous alkaline solution. The concentration of the metal ions in the electroplating bath can be from about 0.1 to about 10 g / L (about 100 to 10,000 ppm), preferably in the range of 0.1 to 3 g / L (100 to 3000 ppm). ppm).

  The electroplating bath may also contain in addition to zinc and additional metal ions, at least one additive commonly used in electroplating baths of zinc and zinc alloy, which improve the appearance of the deposit. These organic additives can be used to improve the physical properties of the electroplating and the complexing properties of the bath.

  The additive may be any type of organic product that may have an effect on the electrodeposition of zinc or its alloys. The additive potentially capable of decomposing electrolytically at the anode and producing a degradation product that will affect the electrolytic process. These degradation products can seriously affect the electrolytic process for example by inhibiting the deposition rate, producing a void or dull deposit, increasing the toxicity of the electrolytic bath or by precipitating insoluble species.

  Thus organic additives in accordance with the invention, and potentially capable of being decomposed at the anode, will not be decomposed with the use of the Anodic Assembly. The Anodic Mounting of the invention prevents electrolytic decomposition by avoiding or minimizing contact of the organic additives at the anode. One of the organic additives potentially capable of being decomposed at the anode may include an amine. The amines are potentially oxidizable to cyanide in the vicinity of the anode. Examples of oxidizable cyanide amines include aliphatic amines derived from the polymerization of alkylenimines, such as polyethylenimines. having molecular weights between 100 and 100,000 and soluble in the bath. For example, polyethylenimines having a molecular weight of from 150 to 2,000 may be used. These polyethylenimines are available under the trade name Lugalvan G15, G20, G35 from BASF.

  Other aliphatic amines may also be used, such as monoethanolamine, diethanolamine, triethanolamine, ethylenediamine, tetraethylenepentamine (TEPA), pentaethylenhexamine (PEHA), and hepatethyleneoctamine. One function of these aliphatic polyalkylamines is to complex the metal ions in the alkaline zinc bath. Their concentration can vary from 2 to 70 g / L.

  Another type of organic additives capable of being electrolytically decomposed at the anode is the reaction product of imidazole and an electrophilic monomer such as epichlorohydrin.

  Among the other types of organic additives that can be electrolytically decomposed at the anode are quaternary ammonium polymers, such as polymers derived from the synthesis of dichlorethyl ether on urea condensates on dimethylaminopropylamine, marketed under the name Mirapol WT by Rhodia. These molecules, in contact with the anode can degrade, decrease the rate of deposition and cause veiled deposits.

  Another type of additive capable of being electrolytically decomposed at the anode is the complexing agent such as gluconate or tartrate. These organic additives can be oxidized at the anode and produce oxalate. It is obvious that one skilled in the art can use other organic additives in the electrolytic bath such as the brighteners and the complexing agents of metals which may or may not be electrolytically decomposed at the anode. A known complexing agent is, for example, the Quadrol from BASF. Quadrol is N, N, N ', N', tetrakis (2-hydroxypropyl) ethylenediamine.

  The invention is essentially characterized by the continuous purification of the electrolysis bath. The concentration of anions such as sulphates or carbonates increases in zinc or zinc alloy baths because of the carbonation of the alkaline medium but also by the salt maintenance of the metals to be electrolyzed. It is customary to consider as inconvenient and penalizing for the yield and the appearance, the total salt concentrations expressed above 60 g / L of their sodium salts (for example Na2CO3 and Na2SO4).

  Thanks to the invention, the anions are attracted to the anode and concentrated in the anode compartment. Their extraction can then be done continuously or by bath fraction. Examples of extraction include cold precipitation between 3 C and 6 C or precipitation by addition of an insolubilizing cation such as barium. Those skilled in the art can also appeal to subcontractors specialized in the recycling of industrial waste.

  The anolyte can accidentally be loaded with organic compounds.

  These compounds can oxidize at the anode and accumulate in the anolyte. Their in situ treatment can be done by regular maintenance of an oxidizing solution such as sodium persulfate. In the case of oxidation to cyanide, persulfate destroys the cyanide generated at the anode.

  Other salts and oxidants may be suitable for the invention and the examples cited are not limiting.

  The present invention and its advantages will become more apparent with the accompanying drawings. In which: Figure 1 is a schematic illustration of an electrolytic zinc nickel bath with an installation according to the invention.

  Figure 1 illustrates an electrolytic equipment in accordance with one aspect of the invention. The electrolytic equipment comprises a tank 14. The tank 14 contains an electrolytic bath 16 and a cathode piece 18. The tank 14 also comprises an Anodic Assembly.

  Figure 2 is a schematic illustration of an anode assembled in its Anodic Assembly.

  In Fig. 2 the Anodic Assembly represents a container 22 which defines a compartment for the anode compartment 24. The anode compartment 24 may be closed by the container 22 from all sides and in the bottom. At least one wall 26 of the container 22 may be a porous membrane or a porous vase. The anode compartment 24 contains an anolyte 28. An anode 30 can be immersed in the anolyte 28. FIG. 1 shows that the container 22 isolates the anode 30 from contact with the electrolytic bath 16. The porous vase or the porous membrane 26 can be used to face the cathode part 18. This allows the passage of an electric current between the anode 30 and the cathode part 18, by the application of an electrical potential between the anode 30 and the cathode part 18. The amount of induced residual electrolytic current causes electrolytic deposition on the cathode part 18.

  Figure 3 is a schematic illustration of Anodic Mounting in an electrolytic treatment tank.

  It is obvious to those skilled in the art that the container 22 and the anode compartment 24 of Figure 2 may have several different configurations. For example, Figure 3 shows that a container 42 may be a foldable bag 44 suspended in the catholyte 46 of the vessel 48. At least a portion of the bag 44 or preferably all of the bag 44 itself may be a porous membrane. A cathode piece 52 is disposed in the catholyte 46. A metal anode 54 is disposed in the anolyte 56 which is contained in the bag 44.

  Figure 4 is a schematic illustration of an Anodic Mounting in accordance with another aspect of the invention.

  Another aspect of the invention has been shown in FIG. 4. A container 60 may comprise a wall 62 common to the tank 64 and consist of a porous membrane 70. A cathode piece 72 is disposed in the catholyte 74 and a metal anode 76 is disposed in the anolyte 78.

  Figs. 5A and 5B are a schematic illustration of an Anodic Mounting in accordance with yet another aspect of the invention.

  Another aspect of the invention has been described in FIG. 5. A container 80 may comprise a cylinder 82. The cylinder 82 may be composed of a cap 84 and a cylinder body 86. The cylinder 82 may be positioned in the catholyte 88 contained in the tank 90. At least a portion of the cylinder 86 and preferably the entire cylinder body may be a porous vessel or a porous membrane. The plug 84 may include a connection to the anolyte 94 for the inlet and a connection of the anolyte 96 to the outlet. The connections 94 and 96 allow the anolyte to circulate in the container 80.An anode 98 is disposed in the container 80 and is connected to the outside. A cathode piece 100 is disposed in the catholyte 88.

  Other containers and compartment configuration may be used by those skilled in the art. In the invention, the cathode part can be any piece of steel or ferrous alloy, typically used in electroplating. In the example of FIGS. 1, 3, 4 and 5, a steel plate has been used.

  FIG. 5B further shows that an outlet pipe 102 of the anode compartment leads to a bottom buffer tank 104 from which a conduit 106 flows towards a cryogenic desulfator 108 from the top of which a top of a conduit 110 leads to the vessel 104 buffer. From the bottom of the buffer tank there is a conduit 112 provided with a pump 114 which returns anolyte which has been treated in the desulfator 108 to an inlet manifold 116 in the anode compartment. A reservoir 118 of sodium persulfate makes it possible, by an automatic feed 120, to add sodium persulfate to the buffer tank 104, it being understood that this sodium persulfate could also be added directly to the anode compartment.

  The invention will be illustrated by the following examples. These examples show the advantages of using Porous Membrane Anodic Mounting in alkaline zinc and zinc alloy baths. These examples are given as an illustration and are not limiting for the invention.

  Example 1: The vessel is made of polypropylene. An aqueous 15L zinc nickel bath containing 120 g / L of sodium hydroxide (alkaline) containing 10 g / L of zinc, 1.5 g / L of nickel, 20 g / L of tetraethylenepentamine (TEPA) and 2 g / L of Quadrol. An Anodic Mounting (depicted in FIG. 1) having a porous membrane on one side containing 500 ml of a 150 g / L solution of sodium hydroxide was placed in the zinc nickel bath. The porous membrane used in Teflon is marketed by Mortelecque under the name TC 110. A metal anode was immersed in the anode compartment. The anodic metal consisted of a chemical nickel deposit containing 10% phosphorus on steel. A current of 5 A was passed into the 15 L cell for 75 h. With regular maintenance of nickel, zinc, sodium hydroxide and organic additives consumed by electrolysis, the cathodic efficiency remained at more than 80% at 1.2 A / dm 2. There was no erosion of the chemical nickel deposit at the anode. Without Anodic Mounting, the cathodic efficiency would have fallen to 63% as indicated below: Zinc-nickel cathodic deposition yield obtained with porous membrane after 25 Ah / L: ddc (A / dm 2) Yield (%) 0.7 84 0, 8 86 0.9 85 1 80 1.180 1.2 84 Zinc-nickel cathodic deposition yield obtained without porous membrane after 25 Ah / L: Ddc (A / dm 2) Yield (%) 0.7 75 0.8 77 0.9 71 1 68 1.1 67 1.2 63 Example 2: In this example, the Anodic Assembly was filled with a solution of 150 gIL of sodium hydroxide in water. The anodic metal is composed of metallic nickel. A cell identical to that of Example 1 worked at 5 A for 6 hours. The nickel anode had a thin conductive layer of nickel oxide and nickel hydroxide which did not interfere with the electrolytic process. There was no weight loss at the anode. The cathodic efficiency of the bath was maintained at 80% at 1 A / dm 2. Example 3: The Anodic Assembly of Example 1 was filled with a 20% solution by weight of 50% liquid caustic soda. The metal anode consisted of an electrolytically nickel-plated steel plate from a Watts type bath, electrolysis at 5A for 90 hours was carried out at 7 V. With regular maintenance of nickel, zinc, sodium hydroxide and in organic additives consumed by electrolysis, the cathodic efficiency remained stable at 80% at 1 A / dm 2. There was no weight loss at the anode.

  Example 4 (Comparative): A 2 L zinc nickel bath containing 30 g / L polyethylenimine (TEPA) was electrolyzed for 160 Ah, with a nickel anode directly in the bath. The analysis determined 508 ppm decyanide as a degradation product and the cathodic efficiency decreased to less than 50% for 2 A / dm 2.

  Example 5: The Anodic Assembly of Example 1 was filled with a solution of 150 g / l of potash, the anode metal in the anolyte was mild steel. The bath which was used as in Example 1 was electrolyzed at 5A for 6 hours. There was a slight loss of weight of the anode. After the analysis, no cyanide trace was detected.

  Example 6: The Anodic Assembly of Example 1 is filled with a solution of 150 gil of sodium hydroxide. The metal of the anode is cobalt. The zinc nickel alkali bath contains 20 g of polyethylenimine and is electrolyzed for 30 Ah. After 30 Ah, the cathodic efficiency remained stable at 60% at 2 A / dm 2 and there was no loss of weight at the anode.

  Example 7: An alkaline zinc bath without cyanide was prepared with 10 g / l zinc, 130 gil sodium hydroxide, 8 ml / l brightener and about 5 g / l sodium tartrate. The electrolysis was conducted without anodic assembly with a simple mild steel anode. After 100 Ah per liter, a white precipitate formed in the bath. The precipitate is sodium oxalate produced by anodic oxidation. The oxalate precipitate interferes with the brightener and causes a void and rough zinc deposit. The use of an anode assembly with a nickel anode avoids the oxidation of tartrate oxalate and thus eliminates the lack of appearance of the deposit caused by the precipitation of oxalates.

  Example 8: A zinc-iron alloy bath containing 20 g / l of zinc, 300 ppm of iron and 130 g / l of sodium hydroxide and 50 g / l of triethanolamine (TEA) in order to complex the iron was electrolyzed during a period of 100 Ah / L. The anodic oxidation of TEA causes degradation products that can interfere with the treatment of the rejects of the bath. The use of an Anodic Mounting with a pure nickel anode avoids the oxidation of the TEA.

  As illustrated in the preceding examples and in accordance with the invention, an installation and a method are provided with which zinc and zinc alloy can be safely deposited on a substrate using an electrolytic bath containing in particular polyalkanimins. This can be accomplished without anodic corrosion and without generating anodic decomposition products in the cathodic side of the electrolytic treatment bath. An electrolytic bath comprising organic additives additional to the organic additives described above may be used.

Claims (13)

  1. Deposition plant for zinc or zinc alloys with at least one other metal selected from iron, cobalt, nickel and manganese on a part comprising: a) a tank (64) subdivided by a separator (70) ) in a cathode compartment and in an anode compartment, b) the part being immersed in an aqueous catholyte (16) of alkaline pH contained in the cathode compartment and comprising zincate ions and, optionally, ions of the other metal that zinc, and c) an aqueous anolyte (28) of alkaline pH, contained in the anode compartment and surrounding an anode insoluble metal anode (30) (28), characterized in that d) the separator (26) ) is of open-pore material.   2. Installation according to claim 1, characterized in that the separator passes the following test: is immersed in a tank containing an aqueous solution of NaOH 120 g / L the separator to be tested so that it delimits in the tank two compartments are poured into one 20 mL / L of a solution colored in violet to 8% by weight nickel comprising by weight: - 362% of nickel sulfate hexahydrate, - 15% of tetrathylenepentamine, - the balance to 100% water, one day later, the solution, in the compartment not having received the colored solution, is not colored in purple.   3. Installation according to claim 1 or 2, characterized in that the pores have a size between 10 nm and 50 m.   4. Installation according to claim 1, 2 or 3, characterized in that the separator is polytetrafluoroethylene or polyolefin such as polypropylene or polyethylene.   5. Installation according to claim 1, 2 or 3, characterized in that the separator is a porous ceramic.   6. Installation according to claim 1, 2 or 3, characterized in that the separator is a water-repellent material.   7. Installation according to one of the preceding claims, characterized in that the anode compartment communicates with a desulfator (108).   8. Installation according to one of the preceding claims, characterized in that the anode compartment or a tank communicating with it communicates with a reservoir (118) of alkali metal persulfate.   9. Installation according to one of the preceding claims, characterized in that the alkaline pH is greater than 12.   10. Installation according to one of the preceding claims characterized in that the catholyte comprises a complexing agent of a metal.   11. Installation according to one of the preceding claims, characterized in that the catholyte and / or the anolyte contain from 50 to 250 g / L of alkali metal hydroxide.   12. Installation according to one of the preceding claims, characterized in that the catholyte contains Zn, expressed as zinc metal, in a content of 4 to 50 gIL.   13. Installation according to one of the preceding claims, characterized in that the catholyte contains the other metal in a content between 0.1 g / L and 50 g / L.