EP0801154A1 - Process for conditioning the copper or copper alloy external surface of a continuous casting mould comprising a nickel plating step and a nickel removing step - Google Patents

Process for conditioning the copper or copper alloy external surface of a continuous casting mould comprising a nickel plating step and a nickel removing step Download PDF

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Publication number
EP0801154A1
EP0801154A1 EP97400692A EP97400692A EP0801154A1 EP 0801154 A1 EP0801154 A1 EP 0801154A1 EP 97400692 A EP97400692 A EP 97400692A EP 97400692 A EP97400692 A EP 97400692A EP 0801154 A1 EP0801154 A1 EP 0801154A1
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EP
European Patent Office
Prior art keywords
nickel
plating
carried out
electrolyte
copper
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP97400692A
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German (de)
French (fr)
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EP0801154B1 (en
Inventor
Jean-Claude Catonne
Gérard Raisson
Christian Allely
Remy Nicolle
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Thyssen Stahl AG
USINOR SA
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Thyssen Stahl AG
USINOR Sacilor SA
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Classifications

    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/48After-treatment of electroplated surfaces
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25FPROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F5/00Electrolytic stripping of metallic layers or coatings
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B22CASTING; POWDER METALLURGY
    • B22DCASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0637Accessories therefor
    • B22D11/0648Casting surfaces
    • B22D11/0651Casting wheels
    • CCHEMISTRY; METALLURGY
    • C23COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23FNON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F1/00Etching metallic material by chemical means
    • C23F1/10Etching compositions
    • C23F1/14Aqueous compositions
    • C23F1/32Alkaline compositions
    • C23F1/34Alkaline compositions for etching copper or alloys thereof
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D3/00Electroplating: Baths therefor
    • C25D3/02Electroplating: Baths therefor from solutions
    • C25D3/12Electroplating: Baths therefor from solutions of nickel or cobalt
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25DPROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
    • C25D5/00Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
    • C25D5/34Pretreatment of metallic surfaces to be electroplated
    • CCHEMISTRY; METALLURGY
    • C25ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25FPROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F1/00Electrolytic cleaning, degreasing, pickling or descaling
    • C25F1/02Pickling; Descaling
    • C25F1/04Pickling; Descaling in solution

Definitions

  • the invention relates to the continuous casting of metals. More specifically, it relates to the conditioning of the external surface of the copper or copper alloy elements of the molds in which the solidification of metals such as steel is initiated.
  • the continuous casting of metals such as steel is carried out in bottomless ingot molds, with walls energetically cooled by an internal circulation of a cooling liquid such as water.
  • the metal in the liquid state is brought into contact with the external surfaces of these walls and initiates its solidification there.
  • These walls must be made of an excellent heat conducting material, so that they can remove enough calories from the metal in a reduced time.
  • copper or one of its alloys, for example containing chromium and zirconium, is adopted for this purpose.
  • the faces of these walls which are intended to be in contact with the liquid metal are coated with a layer of nickel whose initial thickness can generally reach 1 to 2 mm. Its role is multiple. On the one hand, it makes it possible to adjust the coefficient of thermal transfer of the walls to an optimal value (lower than if the metal was put directly in contact with copper) so that the solidification of the metal takes place under good metallurgical conditions : too rapid solidification would cause defects on the surface of the product. This adjustment is made by varying the thickness and the structure of the nickel layer. On the other hand, it constitutes a protective layer for copper which prevents it from being overheated thermally and mechanically. This nickel layer wears out over the use of the mold. It must therefore be restored periodically by removing the remaining thickness completely and then depositing a new layer, but such restoration obviously costs much less than a complete replacement of worn copper walls.
  • the complete nickel removal operation of the shell which must precede the restoration of the nickel layer is also very important. On the one hand, its successful completion largely conditions the quality of the nickel layer which will then be deposited, in particular its adhesion to the shell. On the other hand, this nickel removal operation must be carried out without very significant consumption of the copper from the ferrule which is an extremely expensive part, and the duration of use of which must be extended as much as possible. This last requirement, in particular, practically excludes the use of a purely mechanical method for this nickel-plating, since its precision would not be sufficient to guarantee both a total elimination of the nickel and a safeguard of the copper over the entire surface. of the shell.
  • the object of the invention is to propose an economical and low-pollution method providing optimum quality for conditioning copper walls or copper alloy of an ingot mold for continuously casting metals by depositing a layer of nickel, and also including a step of periodic regeneration of this layer.
  • This method should be particularly suitable in the case of the packaging of cylinder ferrules for a casting machine between cylinders or on a single cylinder.
  • the invention consists notably in carrying out the deposition of nickel as well as its elimination by electrolytic methods both employing a bath with nickel sulfamate Ni (NH 2 SO 3 ) 2 . It has been found that such baths are particularly suitable for producing nickel deposits on copper having high properties of use.
  • the possibility of regenerating the nickel-plating electrolyte by also using it as a nickel-plating electrolyte (after having possibly purified the copper which has dissolved therein) considerably limits the quantity of chemicals released by the conditioning of the ferrules, which goes in the direction of a significant reduction in the costs of using the installation and the risks of environmental pollution.
  • the nickel removed from the shell is recovered in the metallic state on the nickel cathode in the nickel-removal reactor. Said cathode can in turn be recycled in the steelworks.
  • the new ferrule is generally in the form of a hollow cylinder made of copper or a copper alloy, such as a copper-chromium alloy (1%) - zirconium (0.1%).
  • Its outside diameter is, for example, of the order of 1500 mm and its length is equal to the width of the strips which it is desired to pour, that is to say of the order of 600 to 1500 mm.
  • Its thickness may be, for information of the order of 180 mm, but varies locally depending, in particular, on the method of fixing the ferrule to the core of the cylinder which has been adopted.
  • the ferrule is crossed by channels intended to be traversed by a cooling fluid such as water, when using the casting machine.
  • the treatment stations in the nickel-plating / nickel-plating workshop each consist of a tank containing a solution suitable for carrying out a given stage of treatment, above which the said tree can be placed with its horizontal axis and put rotating around its axis.
  • the lower part of the ferrule is thus soaked in the solution, and the rotation of the shaft / ferrule assembly makes it possible to carry out the treatment of the entire ferrule (it being understood that the ferrule normally performs several turns on it - even during the same treatment, at a speed of about 10 revolutions / min, for example).
  • the naked shell preferably undergoes first mechanical preparation by polishing its surface. Then a chemical degreasing is carried out in an alkaline medium, which has the function of ridding the surface of the shell of organic materials which can pollute it. It is carried out hot, at a temperature of about 40 to 70 ° C for about fifteen minutes, and followed by rinsing with water. It can be substituted for, or even added to, an electrolytic degreasing step which would provide an even better surface quality.
  • the next step is a pickling operation in an oxidizing acid medium, which has the function of removing surface oxides, taking care to dissolve only a thickness very minimal of the shell.
  • an aqueous solution of sulfuric acid at 100 ml / l, to which 50 ml / l of a 30% solution of hydrogen peroxide or a solution of 'another compound.
  • a solution of chromic acid this compound having both acidic and oxidizing properties.
  • This pickling operation in an oxidizing acid medium has maximum efficiency when the temperature of the electrolyte is between 40 and 55 ° C. It is advantageous to maintain this temperature at the interface by circulating hot water inside the channels of the rotating shell. The operation lasts approximately 5 minutes and is followed by rinsing with water.
  • An operation to brighten the surface of the shell is then carried out, preferably with a solution of sulfamic acid at 50 g / l, in order to avoid passivation of the surface.
  • This operation takes place at room temperature and lasts approximately one minute.
  • a solution of sulfamic acid advantageously makes it possible not to pollute subsequently the nickel-plating bath, of which, as will be seen, nickel sulfamate is the main component.
  • the nickel-plating operation is preferably, but not necessarily, carried out in two stages: a so-called "pre-nickel plating" step can, in fact, precede the nickel plating operation proper during which most of the nickel deposition is carried out .
  • the purpose of this pre-nickel plating is to complete the preparation of the surface before nickel plating, so as to obtain a deposit of nickel as adherent as possible. It is particularly useful when the ferrule is not pure copper (which is relatively easy to nickel), but a copper-chromium-zirconium alloy which is more likely to passivate, passivation which would disadvantage the adhesion of nickel .
  • This pre-nickel plating operation is carried out by placing the ferrule as a cathode in an electrolysis bath consisting of an aqueous solution of nickel sulfamate (50 to 80 g / l) and sulfamic acid (150 to 200 g / l).
  • the cathodic current density is 4 to 5 A / dm 2 and the duration of the operation is 4 to 5 minutes.
  • One or more soluble (nickel) or insoluble anodes for example Ti / PtO 2 or Ti / RUO 2 ) can be used.
  • a sulfamate pre-nickel plating electrolyte with a composition close to that of nickel plating and nickel plating electrolytes, makes it possible to simplify the management of the workshop.
  • This pre-nickel plating operation makes it possible to deposit on the surface of the shell a layer of nickel a few ⁇ m thick (1 to 2 ⁇ m for example) while removing the acid deposits which could remain there.
  • the solution contains 60 to 100 g / l of nickel, which corresponds to approximately 550 to 900 g / l of nickel sulfamate solution.
  • the pH of the solution is maintained between 3 and 4.5. Above 4.5, precipitation of nickel would be observed, and below 3, the yield of the deposit would be reduced.
  • 30 to 40 g / l of boric acid can be added to the electrolyte.
  • the work in this pH range is also favorable for obtaining a nickel deposit having few internal tensions which would threaten its cohesion and its adhesion to the copper substrate.
  • soluble anode consist of pure nickel, for example in the form of beads contained in anodic titanium baskets
  • chloride anions must be introduced into the bath, essential for the electrolytic dissolution of pure nickel.
  • Magnesium chloride Mg Cl 2 , 6H 2 O at a rate of approximately 6 g / l is well suited for this purpose.
  • the bath can also contain magnesium sulphate (for example approximately 6 g / l of MgSO 4 , 7 H 2 O), which makes it possible to obtain a finer crystallization of the nickel deposit.
  • an anti-bite agent such as an anionic surfactant
  • Alkyl sulfates such as lauryl sulfate, or alkyl sulfonates are suitable for this purpose. 50 g / l of lauryl sulfate is an appropriate content.
  • a cathodic current density of the order of 3 to 5 A / dm 2 is imposed if one does not intervene in the hydrodynamics of the bath. But if stirring is practiced inside the electrolyte, this current density can be increased up to 20 A / dm 2 or even more, which makes it possible to improve the renewal of the boundary layer bordering the shell, and therefore speed up the deposition speed. From this point of view, it is also recommended to reheat the electrolyte, as it is thus possible to work at a higher current density.
  • the anode or anodes are soluble anodes constituted by one or more anodic baskets of titanium containing nickel beads . If these beads are pure nickel, we have seen that it was necessary to provide for the presence of chloride anions in the bath to allow their electrolytic dissolution. If one wishes to avoid the presence of chlorides because of their corroding power, one can use nickel "depolarized" with sulfur or phosphorus.
  • the tanks of the installation are made of a plastic material compatible with the sulfamate and, preferably, do not decompose into chlorides, or a metallic material coated with such a plastic material. In the latter case, we can recommend putting the metal part under cathodic protection. Likewise, it is preferable that the frames and other ancillary metallic infrastructures, which could be corroded by the vapors coming from the treatment baths or be the seat of stray currents, are also plasticized.
  • the operation can advantageously be carried out by continuously withdrawing a fraction of the electrolyte in use, this fraction being injected into a reactor where the precipitation of the sulfate is carried out; then, still continuously, said fraction is filtered and reinjected into the nickel-plating tank.
  • the electrolyte tends to acidify by decomposition of ammonium: NH 4 + ⁇ NH 3 ⁇ + H +
  • This progressive acidification makes it suitable for being recycled as a nickel sulfamate nickel-free electrolyte, an operation which will be seen later that it must be carried out in a more acidic medium than nickel-plating.
  • the internal tensions in the nickel coating can be advantageously minimized if one practices a so-called "alternating" electrolysis, consisting in making one succeed work phases of a few minutes and rest phases of a few seconds during which the electrical supply to the electrodes is interrupted.
  • the nickel layer undergoes attacks and mechanical wear which cause its progressive disappearance.
  • the surface of the ferrule must be cleaned, and the nickel layer can, at least from time to time, undergo a light machining intended to compensate for the possible heterogeneities of its wear which could compromise the homogeneity of the thermomechanical behavior of the shell over its entire surface. It is also important to restore the initial roughness of the shell whenever necessary.
  • a predetermined value which is generally estimated at approximately 0.5 mm
  • the use of the cylinder is interrupted, the ferrule is disassembled and undergoes a nickel-removal treatment.
  • This nickel plating can be complete, and precede the restoration of the nickel layer according to the process which has been previously described.
  • the ferrule is again mounted on the axis which supported it during the nickel-plating operations.
  • a nickel removal by purely chemical means is possible.
  • the reagent used should dissolve the nickel without significantly attacking the copper substrate.
  • a reagent consisting of a mixture of sodium dinitrobenzene sulfonate (50 g / l) and sulfuric acid (100 g / l) could be used, and already exists commercially for the nickel plating of copper substrates in general.
  • Such a procedure would have the advantage of being relatively rapid: a residual thickness of nickel of 0.5 mm could be dissolved in approximately 2 hours.
  • the reagent is chemically unstable and must be renewed frequently to maintain an advantageous rate of nickel removal. Above all, this reagent is toxic, and the effluents from the nickel removal operation must imperatively be reprocessed. They are, in particular, not recyclable in another stage of treatment or in another workshop of a steel or other factory.
  • the other possible nickel removal route is the electrolytic route, due to the appreciable differences between the normal potentials of copper and nickel (respectively 0.3 V and -0.4 V compared to the normal hydrogen electrode). It is also applicable for copper-chromium-zirconium alloys which may constitute the shell.
  • the nickel dissolves by placing the ferrule as an anode in an appropriate electrolyte.
  • this electrolyte it is known (see document FR 2535349) for the nickel removal of copper substrates in general to use an electrolyte essentially consisting of a mixture of sulfuric acid (20-60% by volume) and phosphoric acid (10-50% by volume).
  • Such an electrolyte has the advantage of causing an immediate passivation of the surface of the shell when the copper is exposed, which guarantees that the electrolytic dissolution of the nickel will take place without significant consumption of the copper in the shell.
  • a method has the drawback of requiring for its implementation a special solution, incompatible with the other operations carried out in the nickel-nickel-plating ferrule workshop.
  • this operation is accompanied by a release of hydrogen at the cathode preventing the deposition of nickel, and the formation of sludge whose elimination strikes the overall cost of the operation.
  • this electrolyte is very aggressive with respect to the infrastructure of the installation, which should therefore be carefully protected.
  • a nickel-plating bath can be reused as a nickel-plating or pre-nickel-plating bath after possible elimination of the copper which it has dissolved and a minimal adjustment of its composition, aiming in particular to compensate for the evaporation of the water and to reduce its acidity for work in the desired optimal pH range.
  • the composition proposed for the nickel-plating electrolyte is as follows: 11% nickel solution of nickel sulfamate: 550 to 900 g / l, or 60 to 100 g / l of nickel, nickel chloride: 5 at 20 g / l (to facilitate the dissolution of nickel from the ferrule into an anode and also contribute to the passivation of exposed copper), sulfamic acid: 20 to 80 g / l (preferably approximately 60 g / l) to maintain the pH at a value less than or equal to 2.
  • boric acid (30 to 40 g / l, as in the nickel-plating bath) is also acceptable.
  • the temperature is preferably maintained between 40 and 70 ° C, maintenance to which a circulation of hot water in the shell can advantageously contribute.
  • the anodic current density is generally from 1 to 20 A / dm 2 depending on whether the bath is agitated or not. It is possible, as desired, to work by imposing a determined potential difference between the ferrule as an anode and a reference electrode, or to work at an imposed current density. However, it is preferable to work at imposed potential, because under these conditions, the end of the dissolution of the nickel obviously results in a significant drop in the current density. With an imposed current density, the end of the dissolution of the nickel would be more difficult to detect, and the risk of dissolving the copper of the shell over a significant thickness would be greater.
  • the value of the imposed potential must be chosen according to the location of the reference electrode in the bath and the desired dissolution rate.
  • the duration of the operation also depends on the ratio between the intensity of the current and the volume of electrolyte used.
  • a current density of 7 to 8 A / dm 2 can correspond to a dissolution rate of nickel of approximately 150 ⁇ m / h, which is significantly higher than in a strongly acidic bath of the type that l 'we have previously cited.
  • a 50% sulfuric acid-50% phosphoric acid bath provides, under the same conditions, a nickel dissolution rate of approximately 50 ⁇ m / h.
  • the value of the potential imposed on the anode is therefore adjusted until the desired current density is obtained.
  • This operation would also have the advantage of homogenizing this thickness and removing the various surface impurities (in particular metal residues) which could locally slow down the start of dissolution. This would avoid always being the dissolution of nickel in certain areas of the shell even when in other areas the copper has already been exposed.
  • nickel removal in a nickel sulfamate bath advantageously makes it possible to recover nickel from the cathode which can be recovered, while working at a constant nickel concentration in the electrolyte.
  • the nickel thus recovered can in particular be used at the steelworks, as an addition element to liquid steel.
  • the recovery of the nickel should be carried out by treatment of the residual sludge, which would be much more expensive and complex.
  • the sulfamate bath is also much less aggressive for the installation infrastructure than a strong acid bath would be.
  • the ferrule may, as we have said, periodically remove this copper, in order to decontaminate the bath.
  • the aim is thus not to pollute the deposit of nickel on the ferrule and to obtain better recovery of the nickel deposited on the cathode.
  • the elimination of copper can be carried out in various known ways, by a chemical or electrolytic route, discontinuously or continuously.
  • a variant of the invention consists in carrying out only partial nickel removal of the shell.
  • the electrolytic dissolution of a thin layer thereof for example 10 to 20 ⁇ m, is carried out in an electrolyte of the type previously described. This removes the hardened part of the surface of the ferrule, and also obtains a passivated surface. Then, without rinsing the ferrule, it is transferred to the nickel-plating reactor as quickly as possible to avoid passivation of its surface. The desired nickel thickness is then restored by electrolytic nickel plating.
  • the content of chloride ions in the electrolyte is preferably limited to approximately 1 g / l. This content constitutes a compromise between the need to do not excessively pollute the nickel-plating electrolyte, pollution made inevitable by the absence of rinsing of the partially denickel-plated ferrule, and the desire to obtain an industrially suitable rate of dissolution of nickel.
  • the invention particularly finds its application in the conditioning of the ferrules of cylinders of installations for the continuous casting of steel between cylinders or on a single cylinder. But it goes without saying that one can envisage its transposition to the treatments of ingot molds with copper or copper alloy walls of all shapes and formats.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrochemistry (AREA)
  • Mechanical Engineering (AREA)
  • General Chemical & Material Sciences (AREA)
  • Electroplating Methods And Accessories (AREA)
  • Continuous Casting (AREA)
  • Electroplating And Plating Baths Therefor (AREA)
  • Electrolytic Production Of Metals (AREA)
  • ing And Chemical Polishing (AREA)
  • Electrical Discharge Machining, Electrochemical Machining, And Combined Machining (AREA)
  • Non-Insulated Conductors (AREA)
  • Coating With Molten Metal (AREA)
  • Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

Abstract

The external surface of a copper or copper alloy mould, which may comprise one or two rollers and is used for continuous casting of metal, is degreased, pickled in acid, polished and electrolytically coated with nickel, using an aqueous electrolyte solution of nickel sulphamate containing 60-100 gms. of nickel per litre with the mould acting as cathode. The mould is then used for a period, after which the coating is partly or wholly removed electrolytically, the mould acting as anode in an electrolyte comprising the same amount of nickel sulphamate plus 20-80 gms/litre of sulphamic acid giving it a pH of up to 2, and then a new nickel coating is applied.

Description

L'invention concerne la coulée continue des métaux. Plus précisément, elle concerne le conditionnement de la surface externe des éléments en cuivre ou en alliage de cuivre des lingotières dans lesquelles on initie la solidification des métaux tels que l'acier.The invention relates to the continuous casting of metals. More specifically, it relates to the conditioning of the external surface of the copper or copper alloy elements of the molds in which the solidification of metals such as steel is initiated.

La coulée continue de métaux tels que l'acier est effectuée dans des lingotières sans fond, aux parois énergiquement refroidies par une circulation interne d'un liquide refroidissant tel que de l'eau. Le métal à l'état liquide est amené au contact des surfaces externes de ces parois et y amorce sa solidification. Ces parois doivent être réalisées en un matériau excellent conducteur de la chaleur, afin qu'elles puissent évacuer suffisamment de calories du métal en un temps réduit. Généralement, on adopte à cet effet le cuivre ou un de ses alliages, contenant par exemple du chrome et du zirconium.The continuous casting of metals such as steel is carried out in bottomless ingot molds, with walls energetically cooled by an internal circulation of a cooling liquid such as water. The metal in the liquid state is brought into contact with the external surfaces of these walls and initiates its solidification there. These walls must be made of an excellent heat conducting material, so that they can remove enough calories from the metal in a reduced time. Generally, copper or one of its alloys, for example containing chromium and zirconium, is adopted for this purpose.

Les faces de ces parois qui sont destinées à être au contact du métal liquide sont revêtues d'une couche de nickel dont l'épaisseur initiale peut atteindre en général 1 à 2 mm. Son rôle est multiple. D'une part, elle permet d'ajuster le coefficient de transfert thermique des parois à une valeur optimale (plus faible que si le métal était mis directement au contact du cuivre) pour que la solidification du métal s'effectue dans de bonnes conditions métallurgiques: une solidification trop rapide provoquerait des défauts à la surface du produit. Cet ajustement est effectué en jouant sur l'épaisseur et la structure de la couche de nickel. D'autre part, elle constitue pour le cuivre une couche protectrice qui lui évite d'être trop sollicité thermiquement et mécaniquement. Cette couche de nickel s'use au fil de l'utilisation de la lingotière. Elle doit donc être restaurée périodiquement par enlèvement complet de l'épaisseur subsistante puis dépôt d'une nouvelle couche, mais une telle restauration coûte évidemment beaucoup moins cher qu'un remplacement complet de parois de cuivre usées.The faces of these walls which are intended to be in contact with the liquid metal are coated with a layer of nickel whose initial thickness can generally reach 1 to 2 mm. Its role is multiple. On the one hand, it makes it possible to adjust the coefficient of thermal transfer of the walls to an optimal value (lower than if the metal was put directly in contact with copper) so that the solidification of the metal takes place under good metallurgical conditions : too rapid solidification would cause defects on the surface of the product. This adjustment is made by varying the thickness and the structure of the nickel layer. On the other hand, it constitutes a protective layer for copper which prevents it from being overheated thermally and mechanically. This nickel layer wears out over the use of the mold. It must therefore be restored periodically by removing the remaining thickness completely and then depositing a new layer, but such restoration obviously costs much less than a complete replacement of worn copper walls.

Le dépôt de cette couche de nickel sur les parois de la lingotière est donc une étape fondamentale dans la préparation de la machine de coulée, et il est important d'en optimiser à la fois le coût, les propriétés d'emploi et les qualités d'adhérence. C'est, en particulier, le cas sur les machines destinées à couler des produits sidérurgiques sous forme de bandes de quelques mm d'épaisseur qui n'ont pas besoin d'être ensuite laminées à chaud. Ces machines, dont la mise au point est actuellement en cours, comportent une lingotière constituée par deux cylindres tournant en sens contraires autour de leurs axes maintenus horizontaux, et de deux plaques latérales en réfractaire plaquées contre les chants des cylindres. Ces cylindres ont un diamètre pouvant atteindre 1500 mm, et une largeur qui, sur les installations expérimentales actuelles, est d'environ 600 à 800 mm. Mais à terme, cette largeur devra atteindre 1300 à 1500 mm pour satisfaire les impératifs de productivité d'une installation industrielle. Ces cylindres sont constitués par un noyau en acier autour duquel est fixée une virole en cuivre ou alliage de cuivre, refroidie par une circulation d'eau entre le noyau et la virole ou, plus généralement, par une circulation d'eau interne à la virole. C'est la face externe de cette virole qui doit être recouverte de nickel, et on imagine aisément que, du fait de la forme et des dimensions de cette virole, son conditionnement soit plus complexe que celui des lingotières de coulée continue classiques qui sont formées d'un assemblage de plaques planes, ou d'éléments tubulaires, et qui sont de dimensions beaucoup plus réduites. L'optimisation du mode de dépôt du nickel est d'autant plus importante dans le cas des viroles pour cylindres de coulée que:

  • du fait de l'absence de laminage à chaud ultérieur, les défauts de surface de la bande qui résulteraient d'une médiocre qualité du revêtement de nickel risquent davantage de s'avérer rédhibitoires pour la qualité du produit final;
  • comme les quantités de nickel à déposer sur les viroles avant leur utilisation et à enlever au début de l'opération de régénération de la couche sont relativement importantes, on doit manipuler des volumes de produits chimiques conséquents qu'il faut optimiser pour minimiser le coût de l'opération; se pose également le problème de la quantité et de la toxicité des sous-produits non recyclables liquides et solides résultant des différentes étapes du traitement.
The deposition of this layer of nickel on the walls of the mold is therefore a fundamental step in the preparation of the casting machine, and it is important to optimize both the cost, the properties of use and the qualities of adhesion. This is, in particular, the case on machines intended for casting steel products in the form of strips a few mm thick which do not need to be then hot rolled. These machines, the development of which is currently in progress, comprise an ingot mold constituted by two cylinders rotating in opposite directions around their axes which are kept horizontal, and two refractory side plates pressed against the edges of the cylinders. These cylinders have a diameter that can reach 1500 mm, and a width which, on current experimental installations, is approximately 600 to 800 mm. But ultimately, this width will have to reach 1300 to 1500 mm to meet the productivity requirements of an industrial installation. These cylinders are constituted by a steel core around which is fixed a ferrule of copper or copper alloy, cooled by a circulation of water between the core and the ferrule or, more generally, by a circulation of water internal to the ferrule . It is the external face of this ferrule which must be covered with nickel, and it is easy to imagine that, because of the shape and dimensions of this ferrule, its packaging is more complex than that of the conventional continuous casting molds which are formed. an assembly of flat plates, or tubular elements, and which are of much smaller dimensions. Optimizing the nickel deposition mode is all the more important in the case of ferrules for casting rolls since:
  • due to the absence of subsequent hot rolling, the surface defects of the strip which would result from poor quality of the nickel coating are more likely to prove to be unacceptable for the quality of the final product;
  • as the quantities of nickel to be deposited on the ferrules before their use and to be removed at the start of the layer regeneration operation are relatively large, large volumes of chemicals must be handled which must be optimized to minimize the cost of the operation; There is also the problem of the quantity and the toxicity of non-recyclable liquid and solid by-products resulting from the different stages of treatment.

L'opération de dénickelage complet de la virole qui doit précéder la restauration de la couche de nickel est également très importante. D'une part, son bon achèvement conditionne en grande partie la qualité de la couche de nickel qui va être ensuite déposée, notamment son adhérence sur la virole. D'autre part, cette opération de dénickelage doit être effectuée sans consommation très significative du cuivre de la virole qui est une pièce extrêmement onéreuse, et dont la durée d'utilisation doit être prolongée autant que possible. Cette dernière exigence, notamment, exclut pratiquement l'emploi d'une méthode purement mécanique pour ce dénickelage, car sa précision ne serait pas suffisante pour garantir à la fois une élimination totale du nickel et une sauvegarde du cuivre sur l'ensemble de la surface de la virole.The complete nickel removal operation of the shell which must precede the restoration of the nickel layer is also very important. On the one hand, its successful completion largely conditions the quality of the nickel layer which will then be deposited, in particular its adhesion to the shell. On the other hand, this nickel removal operation must be carried out without very significant consumption of the copper from the ferrule which is an extremely expensive part, and the duration of use of which must be extended as much as possible. This last requirement, in particular, practically excludes the use of a purely mechanical method for this nickel-plating, since its precision would not be sufficient to guarantee both a total elimination of the nickel and a safeguard of the copper over the entire surface. of the shell.

D'autres procédés de coulée visent à couler des bandes métalliques encore plus minces par dépôt du métal liquide sur la périphérie d'un cylindre unique en rotation, qui peut également être constitué d'un noyau en acier et d'une virole refroidie en cuivre. Les problèmes de conditionnement de la surface de la virole qui viennent d'être décrits leur sont aisément transposables.Other casting methods aim at casting even thinner metal strips by depositing the molten metal on the periphery of a single rotating cylinder, which can also consist of a steel core and a cooled copper ferrule. . The problems of conditioning the surface of the shell which have just been described can easily be transposed to them.

Le but de l'invention est de proposer une méthode économique et peu polluante procurant une qualité optimale de conditionnement des parois en cuivre ou alliage de cuivre d'une lingotière de coulée continue des métaux par dépôt d'une couche de nickel, et incluant également une étape de régénération périodique de cette couche. Cette méthode devrait être particulièrement adaptée au cas du conditionnement des viroles de cylindres pour machine de coulée entre cylindres ou sur un cylindre unique.The object of the invention is to propose an economical and low-pollution method providing optimum quality for conditioning copper walls or copper alloy of an ingot mold for continuously casting metals by depositing a layer of nickel, and also including a step of periodic regeneration of this layer. This method should be particularly suitable in the case of the packaging of cylinder ferrules for a casting machine between cylinders or on a single cylinder.

A cet effet, l'invention a pour objet un procédé de conditionnement de la surface externe en cuivre ou alliage de cuivre d'un élément d'une lingotière de coulée continue des métaux, du type comportant une étape de nickelage et une étape de dénickelage de ladite surface, caractérisé en ce que:

  • on réalise une préparation de ladite surface comportant successivement une opération de dégraissage de ladite surface nue, une opération de décapage en milieu acide oxydant de ladite surface nue, et une opération d'avivage de ladite surface nue;
  • puis on réalise une opération de nickelage de ladite surface nue par dépôt électrolytique en plaçant ledit élément en cathode dans un électrolyte constitué par une solution aqueuse de sulfamate de nickel contenant de 60 à 100 g/l de nickel;
  • puis, après utilisation dudit élément, on réalise une opération de dénickelage électrolytique partiel ou complet de ladite surface en plaçant ledit élément en anode dans un électrolyte constitué par une solution aqueuse de sulfamate de nickel contenant de 60 à 100 g/l de nickel et de l'acide sulfamique à raison de 20 à 80 g/l, et dont le pH est inférieur ou égal à 2;
  • puis on réalise un nouveau nickelage de ladite surface, éventuellement précédé d'une préparation de la surface du cuivre mise à nu comme exposé précédemment.
To this end, the subject of the invention is a process for conditioning the external surface of copper or copper alloy of an element of a mold for the continuous casting of metals, of the type comprising a nickel-plating step and a nickel-plating step of said surface, characterized in that:
  • a preparation of said surface is carried out successively comprising a degreasing operation of said bare surface, an etching operation in an oxidizing acid medium of said bare surface, and a brightening operation of said bare surface;
  • then a nickel-plating operation is carried out on said bare surface by electrolytic deposition by placing said cathode element in an electrolyte consisting of an aqueous solution of nickel sulfamate containing from 60 to 100 g / l of nickel;
  • then, after using said element, a partial or complete electrolytic nickel-plating operation is carried out on said surface by placing said element as an anode in an electrolyte consisting of an aqueous solution of nickel sulfamate containing from 60 to 100 g / l of nickel and sulfamic acid in an amount of 20 to 80 g / l, and whose pH is less than or equal to 2;
  • then a new nickel plating of said surface is carried out, possibly preceded by a preparation of the surface of the exposed copper as described above.

Comme on l'aura compris, l'invention consiste notamment à réaliser le dépôt de nickel aussi bien que son élimination par des méthodes électrolytiques employant toutes deux un bain au sulfamate de nickel Ni(NH2SO3)2. Il s'est avéré que de tels bains sont particulièrement aptes à la réalisation de dépôts de nickel sur du cuivre présentant des propriétés d'usage élevées. De plus, la possibilité de régénérer l'électrolyte de dénickelage en l'utilisant également comme électrolyte de nickelage (après l'avoir éventuellement épuré du cuivre qui s'y est dissous) limite considérablement la quantité de produits chimiques rejetés par l'atelier de conditionnement des viroles, ce qui va dans le sens d'une diminution sensible des coûts d'utilisation de l'installation et des risques de pollution de l'environnement. De plus, le nickel éliminé de la virole est récupéré à l'état métallique sur la cathode de nickel dans le réacteur de dénickelage. Ladite cathode peut à son tour être recyclée dans l'aciérie.As will be understood, the invention consists notably in carrying out the deposition of nickel as well as its elimination by electrolytic methods both employing a bath with nickel sulfamate Ni (NH 2 SO 3 ) 2 . It has been found that such baths are particularly suitable for producing nickel deposits on copper having high properties of use. In addition, the possibility of regenerating the nickel-plating electrolyte by also using it as a nickel-plating electrolyte (after having possibly purified the copper which has dissolved therein) considerably limits the quantity of chemicals released by the conditioning of the ferrules, which goes in the direction of a significant reduction in the costs of using the installation and the risks of environmental pollution. In addition, the nickel removed from the shell is recovered in the metallic state on the nickel cathode in the nickel-removal reactor. Said cathode can in turn be recycled in the steelworks.

L'invention va à présent être décrite en détail dans l'une de ses formes de réalisation, appliquée au conditionnement d'une virole en cuivre ou alliage de cuivre de cylindre pour machine de coulée continue de l'acier entre deux cylindres. Mais il est clair que l'exemple décrit pourra aisément être adapté aux cas d'autres types de lingotières à parois en cuivre ou alliage de cuivre.The invention will now be described in detail in one of its embodiments, applied to the packaging of a copper ferrule or copper alloy of a cylinder for a machine for continuously casting steel between two cylinders. But he is It is clear that the example described could easily be adapted to the cases of other types of ingot molds with copper or copper alloy walls.

Classiquement, la virole neuve se présente globalement sous la forme d'un cylindre creux en cuivre ou en alliage de cuivre, tel qu'un alliage cuivre-chrome (1%)-zirconium (0,1%). Son diamètre extérieur est, par exemple, de l'ordre de 1500 mm et sa longueur est égale à la largeur des bandes que l'on désire couler, soit de l'ordre de 600 à 1500 mm. Son épaisseur peut être, à titre indicatif de l'ordre de 180 mm, mais varie localement en fonction, notamment, du mode de fixation de la virole sur le noyau du cylindre qui a été adopté. La virole est traversée par des canaux destinés à être parcourus par un fluide refroidissant tel que de l'eau, lors de l'utilisation de la machine de coulée.Conventionally, the new ferrule is generally in the form of a hollow cylinder made of copper or a copper alloy, such as a copper-chromium alloy (1%) - zirconium (0.1%). Its outside diameter is, for example, of the order of 1500 mm and its length is equal to the width of the strips which it is desired to pour, that is to say of the order of 600 to 1500 mm. Its thickness may be, for information of the order of 180 mm, but varies locally depending, in particular, on the method of fixing the ferrule to the core of the cylinder which has been adopted. The ferrule is crossed by channels intended to be traversed by a cooling fluid such as water, when using the casting machine.

Pour faciliter les manipulations de la virole lors des opérations qui vont être décrites, celle-ci est d'abord montée sur un arbre, et c'est ainsi qu'elle sera transportée d'un poste de traitement à l'autre avant son montage sur le noyau du cylindre. Les postes de traitement de l'atelier de nickelage/dénickelage sont chacun constitués par un bac contenant une solution adaptée à l'exécution d'une étape donnée du traitement, au-dessus duquel on peut placer ledit arbre avec son axe horizontal et le mettre en rotation autour de son axe. On fait ainsi tremper la partie inférieure de la virole dans la solution, et la mise en rotation de l'ensemble arbre/virole permet de réaliser le traitement de l'ensemble de la virole (étant entendu que la virole effectue normalement plusieurs tours sur elle-même au cours d'un même traitement, à une vitesse d'environ 10 tours/mn, par exemple). Sur ces postes de traitement, il peut également être utile, afin d'éviter une pollution ou une passivation par l'atmosphère ambiante de la partie émergée de la virole, de prévoir un dispositif d'arrosage de cette partie émergée avec la solution de traitement. On peut également, à cet effet, envisager d'inerter l'atmosphère ambiante au moyen d'un gaz neutre tel que de l'argon, et/ou installer un système de protection cathodique du cylindre. Toutefois, si cela est possible, on peut prévoir que ces bacs permettent une immersion totale de la virole, ce qui rend un tel arrosage ou inertage sans objet.To facilitate handling of the shell during the operations which will be described, it is first mounted on a shaft, and this is how it will be transported from one treatment station to the other before it is assembled. on the cylinder core. The treatment stations in the nickel-plating / nickel-plating workshop each consist of a tank containing a solution suitable for carrying out a given stage of treatment, above which the said tree can be placed with its horizontal axis and put rotating around its axis. The lower part of the ferrule is thus soaked in the solution, and the rotation of the shaft / ferrule assembly makes it possible to carry out the treatment of the entire ferrule (it being understood that the ferrule normally performs several turns on it - even during the same treatment, at a speed of about 10 revolutions / min, for example). On these treatment stations, it may also be useful, in order to avoid pollution or passivation by the ambient atmosphere of the emerged part of the shell, to provide a device for sprinkling this emerged part with the treatment solution. . One can also, for this purpose, consider inerting the ambient atmosphere by means of a neutral gas such as argon, and / or installing a cathodic protection system of the cylinder. However, if this is possible, provision can be made for these tanks to allow total immersion of the shell, which makes such watering or inerting not applicable.

La virole nue subit d'abord, de préférence, une préparation mécanique par polissage de sa surface. Puis on pratique un dégraissage chimique en milieu alcalin, qui a pour fonction de débarrasser la surface de la virole des matières organiques qui peuvent la polluer. Il est réalisé à chaud, à une température d'environ 40 à 70°C pendant une quinzaine de minutes, et suivi d'un rinçage à l'eau. On peut lui substituer, voire lui ajouter, une étape de dégraissage électrolytique qui procurerait une qualité de surface encore meilleure.The naked shell preferably undergoes first mechanical preparation by polishing its surface. Then a chemical degreasing is carried out in an alkaline medium, which has the function of ridding the surface of the shell of organic materials which can pollute it. It is carried out hot, at a temperature of about 40 to 70 ° C for about fifteen minutes, and followed by rinsing with water. It can be substituted for, or even added to, an electrolytic degreasing step which would provide an even better surface quality.

L'étape suivante est une opération de décapage en milieu acide oxydant, qui a pour fonction d'ôter les oxydes de surface, en veillant à ne dissoudre qu'une épaisseur très minime de la virole. On utilise à cet effet, par exemple, une solution aqueuse d'acide sulfurique à 100 ml/l, à laquelle on ajoute avant chaque opération 50 ml/l d'une solution à 30% d'eau oxygénée ou d'une solution d'un autre percomposé. On peut également utiliser une solution d'acide chromique, ce composé présentant à la fois des propriétés acides et oxydantes. Cette opération de décapage en milieu acide oxydant présente une efficacité maximale lorsque la température de l'électrolyte est comprise entre 40 et 55°C. Il est avantageux de maintenir cette température à l'interface par une circulation d'eau chaude à l'intérieur des canaux de la virole en rotation. L'opération dure environ 5 minutes et est suivie d'un rinçage à l'eau.The next step is a pickling operation in an oxidizing acid medium, which has the function of removing surface oxides, taking care to dissolve only a thickness very minimal of the shell. For this purpose, for example, use an aqueous solution of sulfuric acid at 100 ml / l, to which 50 ml / l of a 30% solution of hydrogen peroxide or a solution of 'another compound. It is also possible to use a solution of chromic acid, this compound having both acidic and oxidizing properties. This pickling operation in an oxidizing acid medium has maximum efficiency when the temperature of the electrolyte is between 40 and 55 ° C. It is advantageous to maintain this temperature at the interface by circulating hot water inside the channels of the rotating shell. The operation lasts approximately 5 minutes and is followed by rinsing with water.

On réalise ensuite une opération d'avivage de la surface de la virole, de préférence avec une solution d'acide sulfamique à 50 g/l, dans le but d'éviter une passivation de la surface. Cette opération a lieu à température ambiante et dure environ une minute. Le fait d'utiliser, pour cet avivage, une solution d'acide sulfamique permet avantageusement de ne pas polluer par la suite le bain de nickelage, dont, comme on le verra, le sulfamate de nickel est le composant principal.An operation to brighten the surface of the shell is then carried out, preferably with a solution of sulfamic acid at 50 g / l, in order to avoid passivation of the surface. This operation takes place at room temperature and lasts approximately one minute. The fact of using, for this brightening, a solution of sulfamic acid advantageously makes it possible not to pollute subsequently the nickel-plating bath, of which, as will be seen, nickel sulfamate is the main component.

L'ensemble des opérations préparatoires au nickelage que l'on vient de décrire a une durée totale qui, en principe, n'excède pas 30 minutes. La virole est ensuite transférée aussi rapidement que possible à la station de nickelage sans subir de rinçage, afin de profiter de la présence à sa surface, après l'avivage, d'un film de sulfamate qui la protège de la passivation.All the operations for nickel plating which have just been described have a total duration which, in principle, does not exceed 30 minutes. The ferrule is then transferred as quickly as possible to the nickel-plating station without undergoing rinsing, in order to take advantage of the presence on its surface, after brightening, of a film of sulfamate which protects it from passivation.

L'opération de nickelage est, de préférence mais pas obligatoirement, effectuée en deux temps: une étape dite de "prénickelage" peut, en effet, précéder l'opération de nickelage proprement dite lors de laquelle l'essentiel du dépôt de nickel est effectué. Le but de ce prénickelage est de parachever la préparation de la surface avant le nickelage, de manière à obtenir un dépôt de nickel aussi adhérent que possible. Il s'avère particulièrement utile lorsque la virole n'est pas en cuivre pur (qui est relativement facile à nickeler), mais en un alliage cuivre-chrome-zirconium qui est davantage susceptible de se passiver, passivation qui défavoriserait l'adhérence du nickel. Cette opération de prénickelage est réalisée en plaçant la virole en cathode dans un bain d'électrolyse consistant en une solution aqueuse de sulfamate de nickel (50 à 80 g/l) et d'acide sulfamique (150 à 200 g/l). La densité de courant cathodique est de 4 à 5 A/dm2 et la durée de l'opération est de 4 à 5 minutes. On peut utiliser une ou des anodes solubles (en nickel) ou insolubles (par exemple en Ti/PtO2 ou Ti/RUO2). Dans le cas d'utilisation d'anodes insolubles, il est préférable de travailler avec une faible densité de courant anodique, de 0,5 à 1 A/dm2, pour limiter la réaction d'hydrolyse de l'acide sulfamique, et donc le besoin de régénérer périodiquement le bain de prénickelage. Il est également envisageable d'utiliser comme électrolyte de prénickelage le bain connu sous le nom de "bain de Wood", qui est un mélange de chlorure de nickel et d'acide chlorhydrique. Il permet de travailler à une densité de courant cathodique de l'ordre de 10 A/dm2, voire davantage. Toutefois, l'utilisation d'un électrolyte de prénickelage au sulfamate, de composition proche de celle des électrolytes de nickelage et de dénickelage, permet de simplifier la gestion de l'atelier. Cette opération de prénickelage permet de déposer sur la surface de la virole une couche de nickel de quelques µm d'épaisseur (1 à 2 µm par exemple) tout en enlevant les dépôts acides qui pourraient y subsister.The nickel-plating operation is preferably, but not necessarily, carried out in two stages: a so-called "pre-nickel plating" step can, in fact, precede the nickel plating operation proper during which most of the nickel deposition is carried out . The purpose of this pre-nickel plating is to complete the preparation of the surface before nickel plating, so as to obtain a deposit of nickel as adherent as possible. It is particularly useful when the ferrule is not pure copper (which is relatively easy to nickel), but a copper-chromium-zirconium alloy which is more likely to passivate, passivation which would disadvantage the adhesion of nickel . This pre-nickel plating operation is carried out by placing the ferrule as a cathode in an electrolysis bath consisting of an aqueous solution of nickel sulfamate (50 to 80 g / l) and sulfamic acid (150 to 200 g / l). The cathodic current density is 4 to 5 A / dm 2 and the duration of the operation is 4 to 5 minutes. One or more soluble (nickel) or insoluble anodes (for example Ti / PtO 2 or Ti / RUO 2 ) can be used. When using insoluble anodes, it is preferable to work with a low anodic current density, of 0.5 to 1 A / dm 2 , to limit the hydrolysis reaction of sulfamic acid, and therefore the need to periodically regenerate the pre-nickel plating bath. It is also conceivable to use as a pre-nickel electrolyte the bath known under the name of "Wood bath", which is a mixture of nickel chloride and hydrochloric acid. It makes it possible to work at a cathodic current density of the order of 10 A / dm 2 , or even more. However, the use of a sulfamate pre-nickel plating electrolyte, with a composition close to that of nickel plating and nickel plating electrolytes, makes it possible to simplify the management of the workshop. This pre-nickel plating operation makes it possible to deposit on the surface of the shell a layer of nickel a few μm thick (1 to 2 μm for example) while removing the acid deposits which could remain there.

Vient ensuite l'opération de nickelage proprement dite. Elle est conduite dans un électrolyte à base essentiellement d'une solution aqueuse de sulfamate de nickel à 11% de nickel. La solution contient de 60 à 100 g/l de nickel, ce qui correspond à environ 550 à 900 g/l de solution de sulfamate de nickel. Préférentiellement, on maintient le pH de la solution entre 3 et 4,5. Au-desssus de 4,5 on observerait une précipitation du nickel, et en-dessous de 3 on diminuerait le rendement du dépôt. A cet effet, on peut ajouter à l'électrolyte de 30 à 40 g/l d'acide borique. Le travail dans cette gamme de pH est, en outre, favorable à l'obtention d'un dépôt de nickel présentant peu de tensions internes qui menaceraient sa cohésion et son adhérence au substrat en cuivre. Lorsque la ou les anodes solubles sont constituées par du nickel pur, par exemple sous forme de billes contenues dans des paniers anodiques en titane on doit introduire dans le bain des anions chlorure, indispensables à la dissolution électrolytique du nickel pur. Du chlorure de magnésium Mg Cl2, 6H2O à raison d'environ 6 g/l est bien adapté à cet effet. Le bain peut également contenir du sulfate de magnésium (par exemple environ 6 g/l de MgSO4, 7 H2O), qui permet d'obtenir une cristallisation plus fine du dépôt de nickel. Il est aussi conseillé d'ajouter au bain un agent anti-piqûres, tel qu'un tensio-actif anionique. Les alkyl-sulfates, comme le lauryl-sulfate, ou les alkyl-sulfonates conviennent à cet effet. 50 g/l de lauryl-sulfate est une teneur appropriée. On impose une densité de courant cathodique de l'ordre de 3 à 5 A/dm2 si on n'intervient pas dans l'hydrodynamique du bain. Mais si on pratique une agitatation à l'intérieur de l'électrolyte, cette densité de courant peut être augmentée jusqu'à 20 A/dm2 voire davantage, ce qui permet d'améliorer le renouvellement de la couche limite avoisinant la virole, et donc d'accélérer la vitesse de dépôt. De ce point de vue, il est également recommandé de réchauffer l'électrolyte, car on peut ainsi travailler à une densité de courant plus importante. Il est cependant préférable de ne pas dépasser une température de 50 °C, car au-delà l'hydrolyse du sulfamate en sulfate d'ammonium est sensiblement accélérée, et la qualité du dépôt est détériorée: on constate une augmentation de sa dureté et de ses tensions internes. Simultanément, il est conseillé de chauffer la virole elle-même à une température voisine de celle du bain, par exemple en y faisant circuler de l'eau chaude. L'expérience montre qu'en procédant ainsi, on parvient à optimiser les propriétés d'emploi du revêtement de nickel et sa structure cristalline.Next comes the actual nickel-plating operation. It is carried out in an electrolyte essentially based on an aqueous solution of nickel sulfamate with 11% nickel. The solution contains 60 to 100 g / l of nickel, which corresponds to approximately 550 to 900 g / l of nickel sulfamate solution. Preferably, the pH of the solution is maintained between 3 and 4.5. Above 4.5, precipitation of nickel would be observed, and below 3, the yield of the deposit would be reduced. For this purpose, 30 to 40 g / l of boric acid can be added to the electrolyte. The work in this pH range is also favorable for obtaining a nickel deposit having few internal tensions which would threaten its cohesion and its adhesion to the copper substrate. When the soluble anode (s) consist of pure nickel, for example in the form of beads contained in anodic titanium baskets, chloride anions must be introduced into the bath, essential for the electrolytic dissolution of pure nickel. Magnesium chloride Mg Cl 2 , 6H 2 O at a rate of approximately 6 g / l is well suited for this purpose. The bath can also contain magnesium sulphate (for example approximately 6 g / l of MgSO 4 , 7 H 2 O), which makes it possible to obtain a finer crystallization of the nickel deposit. It is also advisable to add an anti-bite agent, such as an anionic surfactant, to the bath. Alkyl sulfates, such as lauryl sulfate, or alkyl sulfonates are suitable for this purpose. 50 g / l of lauryl sulfate is an appropriate content. A cathodic current density of the order of 3 to 5 A / dm 2 is imposed if one does not intervene in the hydrodynamics of the bath. But if stirring is practiced inside the electrolyte, this current density can be increased up to 20 A / dm 2 or even more, which makes it possible to improve the renewal of the boundary layer bordering the shell, and therefore speed up the deposition speed. From this point of view, it is also recommended to reheat the electrolyte, as it is thus possible to work at a higher current density. It is however preferable not to exceed a temperature of 50 ° C., since beyond the hydrolysis of the sulfamate to ammonium sulfate is appreciably accelerated, and the quality of the deposit is deteriorated: there is an increase in its hardness and its internal tensions. At the same time, it is advisable to heat the shell itself to a temperature close to that of the bath, for example by circulating hot water there. Experience shows that by proceeding thus, it is possible to optimize the properties of use of the nickel coating and its crystal structure.

Comme on l'a dit, dans l'exemple décrit (qui, de ce point de vue, n'est pas limitatif), la ou les anodes sont des anodes solubles constituées par un ou des paniers anodiques en titane contenant des billes de nickel. Si ces billes sont du nickel pur, on a vu qu'il était nécessaire de prévoir la présence d'anions chlorure dans le bain pour permettre leur dissolution électrolytique. Si on désire éviter la présence de chlorures à cause de leur pouvoir corrodant, on peut utiliser du nickel "dépolarisé" au soufre ou au phosphore.As has been said, in the example described (which, from this point of view, is not limiting), the anode or anodes are soluble anodes constituted by one or more anodic baskets of titanium containing nickel beads . If these beads are pure nickel, we have seen that it was necessary to provide for the presence of chloride anions in the bath to allow their electrolytic dissolution. If one wishes to avoid the presence of chlorides because of their corroding power, one can use nickel "depolarized" with sulfur or phosphorus.

Les cuves de l'installation sont en une matière plastique compatible avec le sulfamate et, de préférence, ne se décomposant pas en chlorures, ou en un matériau métallique revêtu d'une telle matière plastique. Dans ce dernier cas, on peut recommander de mettre la partie métallique sous protection cathodique. De même, il est préférable que les bâtis et autres infrastructures métalliques annexes, qui pourraient être corrodés par les vapeurs issues des bains de traitement ou être le siège de courants vagabonds, soient également plastifiés.The tanks of the installation are made of a plastic material compatible with the sulfamate and, preferably, do not decompose into chlorides, or a metallic material coated with such a plastic material. In the latter case, we can recommend putting the metal part under cathodic protection. Likewise, it is preferable that the frames and other ancillary metallic infrastructures, which could be corroded by the vapors coming from the treatment baths or be the seat of stray currents, are also plasticized.

On a déjà parlé du phénomène d'hydrolyse du sulfamate en sulfate d'ammonium, selon la réaction: NH 2 SO 3 - + H 2 O → SO 4 2- + NH 4 +

Figure imgb0001
We have already talked about the phenomenon of hydrolysis of sulfamate to ammonium sulfate, depending on the reaction: NH 2 SO 3 - + H 2 O → N / A 4 2- + NH 4 +
Figure imgb0001

Elle conduit à une accumulation de sulfate dans le bain, ce qui, au-delà d'une concentration d'une dizaine de grammes par litre, contribue à augmenter les tensions internes dans le dépôt de nickel. Il faut donc surveiller la concentration en sulfate de l'électrolyte, et procéder à son élimination lorsqu'elle est nécessaire. Celle-ci est réalisée par précipitation d'un sel de sulfate, tel que du sulfate de baryum dont la solubilité est particulièrement faible. Les ions baryum peuvent être introduits grâce à une addition d'oxyde de baryum, ou de sulfamate de baryum. Les précipités de sulfate de baryum peuvent être éliminés par filtration, et la solution filtrée est réintroduite dans le bac de nickelage. L'opération peut, avantageusement, être réalisée par prélèvement en continu d'une fraction de l'électrolyte en cours d'utilisation, cette fraction étant injectée dans un réacteur où est effectuée la précipitation du sulfate; puis, toujours en continu, ladite fraction est filtrée et réinjectée dans la cuve de nickelage.It leads to an accumulation of sulphate in the bath, which, beyond a concentration of ten grams per liter, contributes to increasing the internal tensions in the nickel deposit. It is therefore necessary to monitor the sulfate concentration of the electrolyte, and proceed with its elimination when necessary. This is carried out by precipitation of a sulfate salt, such as barium sulfate, the solubility of which is particularly low. Barium ions can be introduced through the addition of barium oxide, or barium sulfamate. The barium sulfate precipitates can be removed by filtration, and the filtered solution is reintroduced into the nickel-plating tank. The operation can advantageously be carried out by continuously withdrawing a fraction of the electrolyte in use, this fraction being injected into a reactor where the precipitation of the sulfate is carried out; then, still continuously, said fraction is filtered and reinjected into the nickel-plating tank.

Par ailleurs, l'électrolyte tend à s'acidifier par décomposition de l'ammonium: NH 4 + ⇔ NH 3 ↑ + H +

Figure imgb0002
In addition, the electrolyte tends to acidify by decomposition of ammonium: NH 4 + ⇔ NH 3 ↑ + H +
Figure imgb0002

Cette acidification progressive le rend apte à être recyclé comme électrolyte de dénickelage au sulfamate de nickel, opération dont on verra plus tard qu'elle doit être exécutée dans un milieu plus acide que le nickelage.This progressive acidification makes it suitable for being recycled as a nickel sulfamate nickel-free electrolyte, an operation which will be seen later that it must be carried out in a more acidic medium than nickel-plating.

Les tensions internes au revêtement de nickel peuvent être avantageusement minimisées si on pratique une électrolyse dite "alternée", consistant à faire se succéder des phases de travail de quelques minutes et des phases de repos de quelques secondes pendant lesquelles l'alimentation électrique des électrodes est interrompue.The internal tensions in the nickel coating can be advantageously minimized if one practices a so-called "alternating" electrolysis, consisting in making one succeed work phases of a few minutes and rest phases of a few seconds during which the electrical supply to the electrodes is interrupted.

A moins qu'il ne soit possible de réaliser l'immersion totale de la virole dans l'électrolyte, il est très conseillé de réaliser un arrosage permanent de la surface de la partie non immergée de la virole par ce même électrolyte, ou un inertage de cette même partie par un gaz neutre. On évite ainsi les risques de passivation de la surface fraîchement nickelée, passivation qui serait préjudiciable à la bonne adhérence et à la bonne cohésion du revêtement. Pour cette même raison, un arrosage de la virole ou un inertage de sa surface lors de son transfert entre le poste de prénickelage et le poste de nickelage est également recommandé. Une mise sous protection cathodique de la virole est également envisageable. Ce transfert doit, de toute façon, être effectué le plus rapidement possible.Unless it is possible to completely immerse the shell in the electrolyte, it is highly advisable to permanently spray the surface of the non-submerged part of the shell with this same electrolyte, or inerting of this same part by a neutral gas. This avoids the risks of passivation of the freshly nickel-plated surface, passivation which would be detrimental to the good adhesion and the good cohesion of the coating. For this same reason, watering the ferrule or inerting its surface during its transfer between the pre-nickel plating station and the nickel plating station is also recommended. Cathodic protection of the ferrule is also possible. In any case, this transfer must be carried out as quickly as possible.

On peut travailler soit à tension imposée, soit à densité de courant imposée. Lorsque l'électrolyse est effectuée sous une tension de l'ordre de 10 V avec une densité de courant d'environ 4 A/dm2, une durée de 5 à 8 jours environ (qui dépend aussi de la profondeur d'immersion de la virole dans le bain) permet d'obtenir un dépôt de nickel atteignant 2 mm d'épaisseur. La virole est ensuite désolidarisée de son axe support, et est prête à être assemblée au noyau pour former un cylindre qui va être utilisé sur la machine de coulée, après un éventuel ultime conditionnement de la surface de la couche de nickel, tel que l'impression d'une rugosité déterminée par un grenaillage, un usinage laser ou un autre procédé. Comme il est connu, un tel conditionnement vise à optimiser les conditions de transfert thermique entre la virole et le métal en cours de solidification.We can work either at an imposed voltage or at an imposed current density. When the electrolysis is carried out at a voltage of the order of 10 V with a current density of approximately 4 A / dm 2 , a duration of approximately 5 to 8 days (which also depends on the depth of immersion of the ferrule in the bath) provides a nickel deposit up to 2 mm thick. The ferrule is then detached from its support axis, and is ready to be assembled with the core to form a cylinder which will be used on the casting machine, after a possible final conditioning of the surface of the nickel layer, such as the impression of a roughness determined by shot blasting, laser machining or another process. As is known, such packaging aims to optimize the conditions of heat transfer between the shell and the metal being solidified.

Au cours de cette utilisation, la couche de nickel subit des attaques et une usure mécanique qui entraînent sa disparition progressive. Entre deux coulées, la surface de la virole doit être nettoyée, et la couche de nickel peut, au moins de temps en temps, subir un léger usinage destiné à compenser les éventuelles hétérogénéités de son usure qui pourraient compromettre l'homogénéité du comportement thermomécanique de la virole sur l'ensemble de sa surface. Il est également important de restaurer la rugosité initiale de la virole chaque fois que cela est nécessaire. Lorsque l'épaisseur moyenne de la couche de nickel de la virole atteint une valeur prédéterminée, que l'on estime généralement à environ 0,5 mm, l'utilisation du cylindre est interrompue, la virole est démontée et subit un traitement de dénickelage.During this use, the nickel layer undergoes attacks and mechanical wear which cause its progressive disappearance. Between two castings, the surface of the ferrule must be cleaned, and the nickel layer can, at least from time to time, undergo a light machining intended to compensate for the possible heterogeneities of its wear which could compromise the homogeneity of the thermomechanical behavior of the shell over its entire surface. It is also important to restore the initial roughness of the shell whenever necessary. When the average thickness of the nickel layer of the ferrule reaches a predetermined value, which is generally estimated at approximately 0.5 mm, the use of the cylinder is interrupted, the ferrule is disassembled and undergoes a nickel-removal treatment.

Ce dénickelage peut être complet, et précéder la restauration de la couche de nickel selon le procédé qui a été précédemment décrit. A cet effet, la virole est à nouveau montée sur l'axe qui la supportait lors des opérations de nickelage.This nickel plating can be complete, and precede the restoration of the nickel layer according to the process which has been previously described. For this purpose, the ferrule is again mounted on the axis which supported it during the nickel-plating operations.

Plusieurs possibilités s'offrent à l'utilisateur pour réaliser ce dénickelage. Un dénickelage par voie purement chimique est envisageable. Le réactif utilisé devrait dissoudre le nickel sans attaquer significativement le substrat en cuivre. A cet effet, un réactif constitué par un mélange de dinitrobenzène-sulfonate de sodium (50 g/l) et d'acide sulfurique (100 g/l) pourrait être employé, et existe déjà dans le commerce pour le dénickelage des substrats en cuivre en général. Un tel mode opératoire aurait l'avantage d'être relativement rapide: une épaisseur résiduelle de nickel de 0,5 mm pourrait être dissoute en 2 heures environ. Mais le réactif est chimiquement instable et doit être fréquemment renouvelé pour garder une vitesse de dénickelage avantageuse. Surtout, ce réactif est toxique, et les effluents de l'opération de dénickelage doivent impérativement être retraités. Ils ne sont, en particulier, pas recyclables dans une autre étape du traitement ou un autre atelier d'une usine sidérurgique ou autre.Several possibilities are available to the user to achieve this nickel plating. A nickel removal by purely chemical means is possible. The reagent used should dissolve the nickel without significantly attacking the copper substrate. For this purpose, a reagent consisting of a mixture of sodium dinitrobenzene sulfonate (50 g / l) and sulfuric acid (100 g / l) could be used, and already exists commercially for the nickel plating of copper substrates in general. Such a procedure would have the advantage of being relatively rapid: a residual thickness of nickel of 0.5 mm could be dissolved in approximately 2 hours. However, the reagent is chemically unstable and must be renewed frequently to maintain an advantageous rate of nickel removal. Above all, this reagent is toxic, and the effluents from the nickel removal operation must imperatively be reprocessed. They are, in particular, not recyclable in another stage of treatment or in another workshop of a steel or other factory.

L'autre voie de dénickelage envisageable est la voie électrolytique, du fait des différences sensibles entre les potentiels normaux du cuivre et du nickel (respectivement 0,3 V et -0,4 V par rapport à l'électrode normale à hydrogène). Elle est aussi applicable pour les alliages cuivre-chrome-zirconium pouvant constituer la virole. Dans ce cas, la dissolution du nickel se fait en plaçant la virole en anode dans un électrolyte approprié. Concenant le choix de cet électrolyte, il est connu (voir le document FR 2535349) pour le dénickelage des substrats en cuivre en général d'utiliser un électrolyte constitué essentiellement d'un mélange d'acide sulfurique (20-60 % en volume) et d'acide phosphorique (10-50 % en volume). Un tel électrolyte présente l'avantage de provoquer une passivation immédiate de la surface de la virole lorsque le cuivre est mis à nu, ce qui garantit que la dissolution électrolytique du nickel se fera sans consommation significative du cuivre de la virole. Toutefois, là encore, une telle méthode présente l'inconvénient de nécessiter pour sa mise en oeuvre une solution spéciale, incompatible avec les autres opérations effectuées dans l'atelier de nickelage-dénickelage des viroles. De plus, cette opération s'accompagne d'un dégagement d'hydrogène à la cathode empêchant le dépôt du nickel, et de la formation de boues dont l'élimination grève le coût global de l'opération. Enfin cet électrolyte est très agressif vis-à-vis de l'infrastructure de l'installation, qu'il faudrait donc protéger soigneusement.The other possible nickel removal route is the electrolytic route, due to the appreciable differences between the normal potentials of copper and nickel (respectively 0.3 V and -0.4 V compared to the normal hydrogen electrode). It is also applicable for copper-chromium-zirconium alloys which may constitute the shell. In this case, the nickel dissolves by placing the ferrule as an anode in an appropriate electrolyte. Regarding the choice of this electrolyte, it is known (see document FR 2535349) for the nickel removal of copper substrates in general to use an electrolyte essentially consisting of a mixture of sulfuric acid (20-60% by volume) and phosphoric acid (10-50% by volume). Such an electrolyte has the advantage of causing an immediate passivation of the surface of the shell when the copper is exposed, which guarantees that the electrolytic dissolution of the nickel will take place without significant consumption of the copper in the shell. However, here again, such a method has the drawback of requiring for its implementation a special solution, incompatible with the other operations carried out in the nickel-nickel-plating ferrule workshop. In addition, this operation is accompanied by a release of hydrogen at the cathode preventing the deposition of nickel, and the formation of sludge whose elimination strikes the overall cost of the operation. Finally, this electrolyte is very aggressive with respect to the infrastructure of the installation, which should therefore be carefully protected.

Les inventeurs ont donc imaginé, pour la réalisation de cette étape de dénickelage de la virole, d'utiliser un électrolyte à base d'acide sulfamique et de sulfamate de nickel, donc d'une composition voisine de celle des électrolytes de nickelage et de prénickelage. Cela simplifie considérablement la gestion des matières de l'atelier de conditionnement des viroles. Un bain de dénickelage peut être réutilisé comme bain de nickelage ou de prénickelage après une éventuelle élimination du cuivre qu'il a dissout et un ajustement minime de sa composition, visant notamment à compenser l'évaporation de l'eau et à réduire son acidité pour travailler dans la gamme de pH optimale désirée. De plus, lorsqu'un bain de nickelage est usé et doit voir sa composition réajustée, il peut être recyclé à l'intérieur même de'l'atelier en bain de dénickelage auquel il faudra simplement ajouter de l'acide sulfamique, et dont la teneur en nickel va pouvoir être augmentée lors de l'opération de dénickelage. Le résultat est que l'atelier de nickelage-dénickelage des viroles ne génère aucun effluent à retraiter à l'extérieur en quantité significative. Cela conduit à des économies de matières importantes et à un impact minime sur l'environnement, alors même que, avec des flux de matières mal gérés, un tel atelier serait susceptible de présenter d'importants risques de pollution du fait de la nature des produits qu'il utilise et des sous-produits qu'il serait susceptible de générer.The inventors have therefore imagined, for carrying out this step of nickel plating of the shell, to use an electrolyte based on sulfamic acid and nickel sulfamate, therefore of a composition close to that of the nickel plating and pre-nickel plating electrolytes . This considerably simplifies the management of materials in the ferrule packaging workshop. A nickel-plating bath can be reused as a nickel-plating or pre-nickel-plating bath after possible elimination of the copper which it has dissolved and a minimal adjustment of its composition, aiming in particular to compensate for the evaporation of the water and to reduce its acidity for work in the desired optimal pH range. In addition, when a nickel plating bath is worn and must see its readjusted composition, it can be recycled within the workshop itself in the nickel removal bath to which it will simply be necessary to add sulfamic acid, and the nickel content of which will be able to be increased during the nickel removal operation. The result is that the nickel-nickel nickel removal workshop on the ferrules does not generate any effluent to be reprocessed outside in significant quantities. This leads to significant material savings and minimal impact on the environment, even though, with poorly managed material flows, such a workshop would be likely to present significant pollution risks due to the nature of the products. that it uses and by-products that it could generate.

Dans ces conditions, la composition proposée pour l'électrolyte de dénickelage est la suivante: solution à 11% de nickel de sulfamate de nickel: 550 à 900 g/l, soit 60 à 100 g/l de nickel, chlorure de nickel: 5 à 20 g/l (pour faciliter la dissolution du nickel de la virole en anode et également contribuer à la passivation du cuivre mis à nu), acide sulfamique: 20 à 80 g/l (préférentiellement 60 g/l environ) pour maintenir le pH à une valeur inférieure ou égale à 2. La présence d'acide borique (30 à 40 g/l, comme dans le bain de nickelage) est également acceptable. La température est de préférence maintenue entre 40 et 70 °C, maintien auquel une circulation d'eau chaude dans la virole peut avantageusement contribuer. La densité de courant anodique est généralement de 1 à 20 A/dm2 selon que le bain est agité ou non. On peut, au choix, travailler en imposant une différence de potentiel déterminée entre la virole en anode et une électrode de référence, ou travailler à densité de courant imposée. Toutefois, il est préférable de travailler à potentiel imposé, car dans ces conditions, la fin de la dissolution du nickel se traduit de manière évidente par une baisse significative de la densité de courant. Avec une densité de courant imposée, la fin de la dissolution du nickel serait plus difficile à détecter, et le risque de dissoudre le cuivre de la virole sur une épaisseur significative serait plus important. La valeur du potentiel imposé doit être choisie en fonction de l'emplacement de l'électrode de référence dans le bain et de la vitesse de dissolution souhaitée. La durée de l'opération dépend également du rapport entre l'intensité du courant et le volume d'électrolyte utilisé. A titre indicatif une densité de courant de 7 à 8 A/dm2 peut correspondre à une vitesse de dissolution du nickel d'environ 150 µm/h, ce qui est sensiblement plus élevé que dans un bain fortement acide du type de ceux que l'on a précédemment cités. Par exemple, un bain acide sulfurique 50%-acide phosphorique 50% procure dans les mêmes conditions une vitesse de dissolution du nickel d'environ 50 µm/h. On règle donc la valeur du potentiel imposé à l'anode jusqu'à l'obtention de la densité de courant désirée. Lorsque la valeur mesurée de la densité de courant chute significativement, cela signifie que la dissolution du nickel est achevée et que l'atttaque du cuivre de la virole a débuté (une densité de courant de 2 A/dm2 correspond à une dissolution du cuivre d'environ 25 µm/h). Il faut donc arrêter l'électrolyse pour éviter une dissolution trop sensible de la virole. Dans les conditions qu'on a citées, une dissolution d'une couche résiduelle de 0,5 mm de nickel dure environ 3 heures, ce qui est peu, et on peut envisager de tolérer des vitesses de dissolution moins élevées qui permettraient d'utiliser des bains d'électrolyte de capacité réduite. Un autre moyen de raccourcir l'opération de dénickelage consisterait à la faire précéder d'une opération d'enlèvement mécanique du nickel qui viserait à diminuer son épaisseur résiduelle sans toutefois atteindre le cuivre. Cette opération aurait également l'avantage d'homogénéiser cette épaisseur et d'ôter les impuretés superficielles diverses (notamment les résidus métalliques) qui pourraient ralentir localement le début de la dissolution. On éviterait ainsi d'en être toujours à la dissolution du nickel en certaines zones de la virole alors même que dans d'autres zones le cuivre aurait déjà été mis à nu.Under these conditions, the composition proposed for the nickel-plating electrolyte is as follows: 11% nickel solution of nickel sulfamate: 550 to 900 g / l, or 60 to 100 g / l of nickel, nickel chloride: 5 at 20 g / l (to facilitate the dissolution of nickel from the ferrule into an anode and also contribute to the passivation of exposed copper), sulfamic acid: 20 to 80 g / l (preferably approximately 60 g / l) to maintain the pH at a value less than or equal to 2. The presence of boric acid (30 to 40 g / l, as in the nickel-plating bath) is also acceptable. The temperature is preferably maintained between 40 and 70 ° C, maintenance to which a circulation of hot water in the shell can advantageously contribute. The anodic current density is generally from 1 to 20 A / dm 2 depending on whether the bath is agitated or not. It is possible, as desired, to work by imposing a determined potential difference between the ferrule as an anode and a reference electrode, or to work at an imposed current density. However, it is preferable to work at imposed potential, because under these conditions, the end of the dissolution of the nickel obviously results in a significant drop in the current density. With an imposed current density, the end of the dissolution of the nickel would be more difficult to detect, and the risk of dissolving the copper of the shell over a significant thickness would be greater. The value of the imposed potential must be chosen according to the location of the reference electrode in the bath and the desired dissolution rate. The duration of the operation also depends on the ratio between the intensity of the current and the volume of electrolyte used. As an indication, a current density of 7 to 8 A / dm 2 can correspond to a dissolution rate of nickel of approximately 150 µm / h, which is significantly higher than in a strongly acidic bath of the type that l 'we have previously cited. For example, a 50% sulfuric acid-50% phosphoric acid bath provides, under the same conditions, a nickel dissolution rate of approximately 50 μm / h. The value of the potential imposed on the anode is therefore adjusted until the desired current density is obtained. When the measured value of the current density drops significantly, this means that the dissolution of the nickel is complete and that the attack on the copper of the ferrule has started (a current density of 2 A / dm 2 corresponds to a dissolution of the copper about 25 µm / h). It is necessary therefore stop electrolysis to avoid too sensitive dissolution of the shell. Under the conditions that have been mentioned, dissolution of a residual layer of 0.5 mm of nickel lasts for about 3 hours, which is not very long, and it is possible to tolerate lower dissolution rates which would make it possible to use electrolyte baths of reduced capacity. Another way to shorten the nickel removal operation would be to precede it with a mechanical removal operation of the nickel which would aim to reduce its residual thickness without however reaching the copper. This operation would also have the advantage of homogenizing this thickness and removing the various surface impurities (in particular metal residues) which could locally slow down the start of dissolution. This would avoid always being the dissolution of nickel in certain areas of the shell even when in other areas the copper has already been exposed.

De plus, le dénickelage dans un bain au sulfamate de nickel permet avantageusement de récupérer sur la cathode du nickel que l'on peut valoriser, tout en travaillant à une concentration en nickel constante dans l'électrolyte. Le nickel ainsi récupéré peut notamment être utilisé à l'aciérie, comme élément d'addition à l'acier liquide. Dans le cas d'un dénickelage électrolytique en milieu acide fort tel que celui qui a été évoqué précedemment, la récupération du nickel devrait être effectuée par un traitement des boues résiduelles, ce qui serait beaucoup plus coûteux et complexe. Le bain au sulfamate est également beaucoup moins agressif pour l'infrastructure de l'installation que ne le serait un bain d'acides forts.In addition, nickel removal in a nickel sulfamate bath advantageously makes it possible to recover nickel from the cathode which can be recovered, while working at a constant nickel concentration in the electrolyte. The nickel thus recovered can in particular be used at the steelworks, as an addition element to liquid steel. In the case of electrolytic nickel plating in a strong acid medium such as that which has been mentioned previously, the recovery of the nickel should be carried out by treatment of the residual sludge, which would be much more expensive and complex. The sulfamate bath is also much less aggressive for the installation infrastructure than a strong acid bath would be.

En fonction de la quantité de cuivre issu de la virole, voire également des éléments de connexion électrique de l'appareillage, et passant dans le bain de dénickelage, il peut, comme on l'a dit, être nécessaire de réaliser périodiquement une élimination de ce cuivre, afin de décontaminer le bain. On vise ainsi à ne pas polluer le dépôt de nickel sur la virole et à obtenir une meilleure valorisation du nickel déposé sur la cathode. L'élimination du cuivre peut être réalisée de différentes manières connues, par une voie chimique ou électrolytique, de manière discontinue ou continue.Depending on the amount of copper from the ferrule, or even also of the electrical connection elements of the apparatus, and passing through the nickel-removal bath, it may, as we have said, periodically remove this copper, in order to decontaminate the bath. The aim is thus not to pollute the deposit of nickel on the ferrule and to obtain better recovery of the nickel deposited on the cathode. The elimination of copper can be carried out in various known ways, by a chemical or electrolytic route, discontinuously or continuously.

Une variante de l'invention consiste à ne réaliser qu'un dénickelage partiel de la virole. A cet effet, de préférence après une opération d'enlèvement mécanique par usinage et ponçage d'une partie de la couche de nickel, on procède à la dissolution électrolytique d'une faible épaisseur de celle-ci, par exemple 10 à 20µm, dans un électrolyte du type précédemment décrit. On retire ainsi la partie écrouie de la surface de la virole, et on obtient également une surface dépassivée. Ensuite, sans rincer la virole, on la transfère dans le réacteur de nickelage, le plus rapidement possible pour éviter une passivation de sa surface. On restaure ensuite par nickelage électrolytique l'épaisseur de nickel souhaitée. Dans le cas où on souhaite que l'électrolyte de nickelage soit exempt de chlorures, on limite de préférence la teneur en ions chlorure de l'électrolyte à 1g/l environ. Cette teneur constitue un compromis entre la nécessité de ne pas trop polluer l'électrolyte de nickelage, pollution rendue inévitable par l'absence de rinçage de la virole partiellement dénickelée, et le désir d'obtenir une vitesse de dissolution du nickel industriellement convenable. A titre indicatif, lorsqu'on utilise un bain de dénickelage à 45°C contenant 60 à 75 g/l de sulfamate de nickel, 30 à 40 g/l d'acide borique, 60 g/l d'acide sulfamique, 1 g/l d'ions chlorure apportés par du chlorure de nickel, une durée d'électrolyse de 190 minutes est nécessaire pour retirer 15 µm de nickel d'une virole immergée sur le tiers de sa hauteur et soumise à une densité de courant de 1 A/dm3. Pour une densité de courant de 5 A/dm3, cette durée est de 38 minutes. Puisqu'en procédant ainsi on écourte très sensiblement l'opération de nickelage et qu'on supprime toutes les opérations de préparation de la surface de cuivre de la virole, la durée du reconditionnement de la surface d'une virole usagée est considérablement réduite par rapport au mode opératoire précédemment décrit.A variant of the invention consists in carrying out only partial nickel removal of the shell. To this end, preferably after a mechanical removal operation by machining and sanding a part of the nickel layer, the electrolytic dissolution of a thin layer thereof, for example 10 to 20 μm, is carried out in an electrolyte of the type previously described. This removes the hardened part of the surface of the ferrule, and also obtains a passivated surface. Then, without rinsing the ferrule, it is transferred to the nickel-plating reactor as quickly as possible to avoid passivation of its surface. The desired nickel thickness is then restored by electrolytic nickel plating. In the case where it is desired that the nickel-plating electrolyte is free of chlorides, the content of chloride ions in the electrolyte is preferably limited to approximately 1 g / l. This content constitutes a compromise between the need to do not excessively pollute the nickel-plating electrolyte, pollution made inevitable by the absence of rinsing of the partially denickel-plated ferrule, and the desire to obtain an industrially suitable rate of dissolution of nickel. For information, when using a 45 ° C nickel removal bath containing 60 to 75 g / l of nickel sulfamate, 30 to 40 g / l of boric acid, 60 g / l of sulfamic acid, 1 g / l of chloride ions provided by nickel chloride, an electrolysis time of 190 minutes is necessary to remove 15 μm of nickel from a ferrule immersed over a third of its height and subjected to a current density of 1 A / dm 3 . For a current density of 5 A / dm 3 , this duration is 38 minutes. Since by doing so, the nickel-plating operation is very shortened and all the operations for preparing the copper surface of the ferrule are eliminated, the time required to recondition the surface of a used ferrule is considerably reduced compared to in the procedure described above.

L'invention trouve particulièrement son application au conditionnement des viroles de cylindres d'installations de coulée continue de l'acier entre cylindres ou sur un cylindre unique. Mais il va de soi qu'on peut envisager sa transposition aux traitements de lingotières de coulée à parois en cuivre ou alliage de cuivre de toutes formes et formats.The invention particularly finds its application in the conditioning of the ferrules of cylinders of installations for the continuous casting of steel between cylinders or on a single cylinder. But it goes without saying that one can envisage its transposition to the treatments of ingot molds with copper or copper alloy walls of all shapes and formats.

Claims (32)

Procédé de conditionnement de la surface externe en cuivre ou alliage de cuivre d'un élément d'une lingotière de coulée continue des métaux, du type comportant une étape de nickelage et une étape de dénickelage de ladite surface, caractérisé en ce que: - on réalise une préparation de ladite surface comportant successivement une opération de dégraissage de ladite surface nue, une opération de décapage en milieu acide oxydant de ladite surface nue, et une opération d'avivage de ladite surface nue; - puis on réalise une opération de nickelage de ladite surface nue par dépôt électrolytique en plaçant ledit élément en cathode dans un électrolyte constitué par une solution aqueuse de sulfamate de nickel contenant de 60 à 100 g/l de nickel; - puis, après utilisation dudit élément, on réalise une opération de dénickelage électrolytique partiel ou complet de ladite surface en plaçant ledit élément en anode dans un électrolyte constitué par une solution aqueuse de sulfamate de nickel contenant de 60 à 100 g/l de nickel et de l'acide sulfamique à raison de 20 à 80g/l, et dont le pH est inférieur ou égal à 2; - puis on réalise un nouveau nickelage de ladite surface, éventuellement précédé d'une préparation de la surface du cuivre mise à nu comme exposé précédemment. Method for conditioning the external surface of copper or copper alloy of an element of an ingot mold for the continuous casting of metals, of the type comprising a nickel-plating step and a step of nickel-plating of said surface, characterized in that: - A preparation of said surface is carried out successively comprising a degreasing operation of said bare surface, a pickling operation in an oxidizing acid medium of said bare surface, and a brightening operation of said bare surface; - Then a nickel-plating operation of said bare surface is carried out by electrolytic deposition by placing said cathode element in an electrolyte consisting of an aqueous solution of nickel sulfamate containing 60 to 100 g / l of nickel; - Then, after using said element, a partial or complete electrolytic nickel-plating operation is carried out on said surface by placing said element as an anode in an electrolyte consisting of an aqueous solution of nickel sulfamate containing from 60 to 100 g / l of nickel and sulfamic acid in an amount of 20 to 80 g / l, and whose pH is less than or equal to 2; - Then a new nickel plating of said surface is carried out, possibly preceded by a preparation of the surface of the exposed copper as explained above. Procédé selon la revendication 1, caractérisé en ce que l'électrolyte de nickelage est maintenu à un pH compris entre 3 et 4,5.Process according to Claim 1, characterized in that the nickel-plating electrolyte is maintained at a pH of between 3 and 4.5. Procédé selon la revendication 1 ou 2, caractérisé en ce que l'électrolyte de nickelage contient également 30 à 40 g/l d'acide borique.Method according to claim 1 or 2, characterized in that the nickel-plating electrolyte also contains 30 to 40 g / l of boric acid. Procédé selon l'une des revendications 1 à 3, caractérisé en ce que l'opération de nickelage est effectuée en utilisant au moins une anode soluble en nickel pur, et en ce que l'électrolyte de nickelage contient des ions chlorure.Method according to one of claims 1 to 3, characterized in that the nickel-plating operation is carried out using at least one soluble anode in pure nickel, and in that the nickel-plating electrolyte contains chloride ions. Procédé selon l'une des revendications 1 à 4, caractérisé en ce que l'électrolyte de nickelage contient du sulfate de magnésium.Method according to one of claims 1 to 4, characterized in that the nickel electrolyte contains magnesium sulfate. Procédé selon l'une des revendications 1 à 5, caractérisé en ce que l'électrolyte de nickelage contient également un agent anti-piqûres.Method according to one of claims 1 to 5, characterized in that the nickel-plating electrolyte also contains an anti-pitting agent. Procédé selon la revendication 6, caractérisé en ce que ledit agent anti-piqûres est un tensio-actif anionique, tel qu'un alkyl-sulfate ou un alkyl-sulfonate.A method according to claim 6, characterized in that said anti-bite agent is an anionic surfactant, such as an alkyl sulfate or an alkyl sulfonate. Procédé selon l'une des revendications 1 à 7, caractérisé en ce que l'opération de nickelage est conduite avec une densité de courant cathodique comprise entre 3 et 20 A/dm2.Method according to one of claims 1 to 7, characterized in that the nickel-plating operation is carried out with a cathodic current density of between 3 and 20 A / dm 2 . Procédé selon l'une des revendications 1 à 8, caractérisé en ce qu'on réchauffe l'électrolyte de nickelage.Method according to one of claims 1 to 8, characterized in that the nickel electrolyte is heated. Procédé selon la revendication 9, caractérisé en ce que ledit élément de lingotière est également réchauffé, à une température proche de celle de l'électrolyte de nickelage.Method according to Claim 9, characterized in that the said ingot mold element is also heated to a temperature close to that of the nickel-plating electrolyte. Procédé selon l'une des revendications 1 à 10, caractérisé en ce qu'on réalise, de manière périodique ou continue, une élimination des sulfates formés au sein de l'électrolyte de nickelage.Method according to one of claims 1 to 10, characterized in that one carries out, periodically or continuously, an elimination of the sulfates formed within the nickel-plating electrolyte. Procédé selon l'une des revendications 1 à 11, caractérisé en ce que, lors de l'opération de nickelage, on fait se succéder des phases de travail de quelques minutes et des phases de repos de quelques secondes.Method according to one of claims 1 to 11, characterized in that, during the nickel-plating operation, there are successive working phases of a few minutes and resting phases of a few seconds. Procédé selon l'une des revendications 1 à 12, caractérisé en ce que l'opération de nickelage est précédée par une opération de prénickelage par voie électrolytique, destinée à déposer une couche de nickel de quelques µm d'épaisseur sur ledit élément de lingotière placé en cathode.Method according to one of claims 1 to 12, characterized in that the nickel-plating operation is preceded by an electrolytic pre-nickel-plating operation, intended to deposit a layer of nickel a few µm thick on said ingot mold element in cathode. Procédé selon la revendication 13, caractérisé en ce que ladite opération de prénickelage est réalisée dans un électrolyte consistant en une solution aqueuse à base de sulfamate de nickel et d'acide sulfamique.A method according to claim 13, characterized in that said pre-nickeling operation is carried out in an electrolyte consisting of an aqueous solution based on nickel sulfamate and sulfamic acid. Procédé selon la revendication 14, caractérisé en ce que ladite opération de prénickelage est réalisée sous une densité de courant cathodique de 4 à 5 A/dm2.A method according to claim 14, characterized in that said pre-nickel plating operation is carried out under a cathodic current density of 4 to 5 A / dm 2 . Procédé selon la revendication 13, caractérisé en ce que ladite opération de prénickelage est réalisée dans un électrolyte à base de chlorure de nickel et d'acide chlorhydrique dit "bain de Wood".Process according to Claim 13, characterized in that the said pre-nickel-plating operation is carried out in an electrolyte based on nickel chloride and hydrochloric acid known as the "Wood bath". Procédé selon l'une des revendications 1 à 16, caractérisé en ce que l'opération de dégraissage est précédée par une opération de polissage de la surface dudit élément de lingotière.Method according to one of claims 1 to 16, characterized in that the degreasing operation is preceded by an operation of polishing the surface of said ingot mold element. Procédé selon l'une des revendications 1 à 17, caractérisé en ce que ladite opération de dégraissage est un dégraissage chimique en milieu alcalin et/ou un dégraissage électrolytique.Method according to one of claims 1 to 17, characterized in that said degreasing operation is a chemical degreasing in an alkaline medium and / or an electrolytic degreasing. Procédé selon l'une des revendications 1 à 18, caractérisé en ce que l'opération de décapage est réalisée dans une solution aqueuse d'acide sulfurique et d'eau oxygénée.Method according to one of claims 1 to 18, characterized in that the pickling operation is carried out in an aqueous solution of sulfuric acid and hydrogen peroxide. Procédé selon l'une des revendications 1 à 18, caractérisé en ce que l'opération de décapage est réalisée dans une solution d'acide chromique.Method according to one of claims 1 to 18, characterized in that the pickling operation is carried out in a chromic acid solution. Procédé selon l'une des revendications 1 à 20, caractérisé en ce que l'opération d'avivage est réalisée dans une solution d'acide sulfamique.Method according to one of claims 1 to 20, characterized in that the brightening operation is carried out in a solution of sulfamic acid. Procédé selon l'une des revendications 1 à 21, caractérisé en ce que l'électrolyte de dénickelage contient au moins 1 g/l d'ions chlorure.Method according to one of claims 1 to 21, characterized in that the nickel-plating electrolyte contains at least 1 g / l of chloride ions. Procédé selon la revendication 22, caractérisé en ce que l'électrolyte de dénickelage contient 5 à 20 g/l de chlorure de nickel, et en ce qu'on réalise une élimination complète du nickel de ladite surface.Process according to claim 22, characterized in that the nickel-plating electrolyte contains 5 to 20 g / l of nickel chloride, and in that complete removal of the nickel from said surface is carried out. Procédé selon l'une des revendications 1 à 23, caractérisé en ce que ledit électrolyte de dénickelage contient 30 à 40 g/l d'acide borique.Method according to one of Claims 1 to 23, characterized in that the said nickel-plating electrolyte contains 30 to 40 g / l of boric acid. Procédé selon l'une des revendications 1 à 24, caractérisé en ce que l'opération de dénickelage est réalisée sous une densité de courant anodique de 1 à 20 A/dm2.Method according to one of claims 1 to 24, characterized in that the nickel-removal operation is carried out under an anode current density of 1 to 20 A / dm 2 . Procédé selon l'une des revendications 1 à 25, caractérisé en ce que l'opération de dénickelage est réalisée à potentiel imposé.Method according to one of claims 1 to 25, characterized in that the nickel-removal operation is carried out at imposed potential. Procédé selon l'une des revendications 1 à 26, caractérisé en ce que l'opération de dénickelage est précédée par une opération mécanique d'enlèvement partiel de la couche de nickel résiduelle.Method according to one of claims 1 to 26, characterized in that the nickel removal operation is preceded by a mechanical operation of partial removal of the residual nickel layer. Procédé selon l'une des revendications 1 à 27, caractérisé en ce qu'on réalise de manière discontinue ou continue une élimination du cuivre contenu dans l'électrolyte de dénickelage.Method according to one of claims 1 to 27, characterized in that the copper contained in the nickel-plating electrolyte is produced discontinuously or continuously. Procédé selon l'une des revendications 1 à 28, caractérisé en ce que ledit élément de lingotière est une virole de cylindre de coulée continue entre deux cylindres ou sur un cylindre.Method according to one of claims 1 to 28, characterized in that said ingot mold element is a ferrule of a continuous casting cylinder between two cylinders or on a cylinder. Procédé selon la revendication 29, caractérisé en ce que, lors d'au moins certaines desdites opérations, ladite virole est montée sur un arbre placé en position horizontale au-dessus d'un bac contenant la solution de traitement, de manière à immerger une portion de ladite virole dans ladite solution, et en ce que ledit arbre est mis en rotation au cours de ladite opération.Method according to claim 29, characterized in that, during at least some of the said operations, the said ferrule is mounted on a shaft placed in a horizontal position above a tank containing the treatment solution, so as to immerse a portion of said ferrule in said solution, and in that said shaft is rotated during said operation. Procédé selon la revendication 30, caractérisé en ce qu'on arrose la partie non immergée de ladite virole avec ladite solution de traitement.A method according to claim 30, characterized in that the non-submerged part of said ferrule is sprayed with said treatment solution. Procédé selon la revendication 30, caractérisé en ce qu'on réalise un inertage par un gaz neutre de l'atmosphère environnant la partie non immergée de ladite virole.Method according to claim 30, characterized in that inerting is carried out by a neutral gas of the atmosphere surrounding the non-submerged part of said shell.
EP97400692A 1996-04-12 1997-03-27 Process for conditioning the copper or copper alloy external surface of a continuous casting mould comprising a nickel plating step and a nickel removing step Expired - Lifetime EP0801154B1 (en)

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FR9604562A FR2747400B1 (en) 1996-04-12 1996-04-12 PROCESS FOR CONDITIONING THE EXTERNAL COPPER OR COPPER ALLOY SURFACE OF AN ELEMENT OF A CONTINUOUS METAL CASTING LINGOTIER, OF THE TYPE INCLUDING A NICKELING STEP AND A DENICKELING STEP

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