WO2001092604A2

WO2001092604A2 - Electrolysis cell for restoring the concentration of metal ions in processes of electroplating

Info

Publication number: WO2001092604A2
Application number: PCT/EP2001/006161
Authority: WO
Inventors: Ulderico Nevosi; Gian Nicola Martelli
Original assignee: De Nora Elettrodi SpA
Current assignee: De Nora Elettrodi SpA
Priority date: 2000-05-31
Filing date: 2001-05-30
Publication date: 2001-12-06
Anticipated expiration: 2002-11-30
Also published as: IT1318545B1; ITMI20001207A0; ITMI20001207A1; WO2001092604A3; AU2001262326A1

Abstract

The invention is relative to an electrolysis cell for the anodic dissolution of metals, in particular of metals characterised by a relatively high oxidation potential, such as copper, or metals with high hydrogen overpotential, having the purpose of restoring the concentration of metal ions in galvanic baths employed in electroplating processes with insoluble anode. The cell of the invention comprises an anodic compartment, wherein the metal to be dissolved acts as a consumable anode, and a cathodic compartment, containing a cathode for hydrogen evolution, separated by an anion-exchange membrane. The coupling of the cell of the invention with an electroplating cell allows a strong simplification of the overall process and a sensible reduction in the relative costs.

Description

ELECTROLYSIS CELL FOR RESTORING THE CONCENTRATION OF METAL IONS IN PROCESSES OF ELECTROPLATING

The processes of galvanic electroplating with insoluble anode are increasingly more widespread for the considerable simplicity of their operation with respect to the traditional processes with consumable anode, especially in view of the recent improvements obtained in the formulation of dimensionally stable anodes for oxygen evolution both in acidic and in alkaline environment. In the traditional processes of galvanic plating, the conductive surface to be coated is employed as the cathode in an electrolytic process carried out in an unseparated cell wherein the metal to be deposited is employed as the anode under different forms (plates, shavings, spheroids, and so on). The positively charged anode is thus progressively consumed, releasing cations which migrate under the action of the electric field and deposit on the negatively charged cathodic surface. Although this process is almost always advantageous in terms of energetic consumption, being characterised by a reversible potential difference close to zero, it is affected by some evident drawbacks especially when continuous layers of very uniform thickness are desired, among which the most evident is the progressive variation in the interelectrodic gap due to the anode consumption, usually compensated by means of sophisticated mechanisms. Furthermore, the anodic surface consumption invariably presents a non fully homogeneous profile, affecting the distribution of the lines of current and therefore the quality of the deposit at the cathode.

In most of the cases, the anode must be replaced once a consumption of 70-80% is reached; then, a new shortcoming becomes evident, as it is almost always necessary to shut-down the process to allow the replacement of the anode, especially in the case, very frequent indeed, that the latter is hardly accessible. All of this implies higher costs for maintenance and for productivity decrease, particularly for the continuous cycle manufactures (such as coating of wires, tapes, rods, bars and so on). For the above reasons, in most of the cases it would be desirable to rely on an electroplating cell wherein the metal to be deposited is entirely supplied in ionic form into the electrolyte, and the anode is of the insoluble type with adjustable geometry, so that the preferred interelectrodic gap may be fixed thereby granting a suitable deposit for the most critical applications in terms of quality and homogeneity as well as the possibility of continuous operation. For this purpose, as the vast majority of the galvanic applications is carried out in an aqueous solution, the use of an electrode suitable to withstand, as the anodic half-reaction, the evolution of oxygen from the support electrolyte, is preferred. The most commonly employed anodes are constituted of valve metals coated with an electrocatalytic layer (for instance noble metal oxide coated titanium), as is the case of the DSA^® commercialised by De Nora Elettrodi S.p.A. To maintain a constant concentration of the ion to be deposited in the electrolytic bath, it is necessary however to continuously supply a solution of the same to the electroplating cell, accurately monitoring its concentration; obtaining the solution of the metal ion may however be a problem in some cases. In particular, for the majority of the galvanic applications, the product has a too small added value to allow the use of oxides or salts of adequate purity, and economic considerations demand to directly dissolve the metal to be deposited in an acidic solution. The direct chemical dissolution of a metal is not always a feasible or easy operation: in some cases of industrial relevance, for instance in the case of copper, simple thermodynamic considerations indicate that direct dissolution in an acid with concurrent evolution of hydrogen is not possible, as the reversible potential of the couple Cu(0)/Cu(ll) is more noble (+ 0.153 V) than the one of the couple H₂/H⁺; for this reason, the copper plating baths are often prepared by dissolution of copper oxide, that nevertheless has a prohibitive cost for the majority of the applications of industrial relevance. In other cases, conversely, a kinetic type obstacle is responsible of rendering the direct chemical dissolution problematic; in the case of zinc, for example, even if the reversible potential of the couple Zn(0)/Zn(ll) (-0.76 V) is significantly lower than the one of the couple H₂/H⁺, the kinetic penalty of the hydrogen evolution reaction on the metal surface (hydrogen overpotential) is high enough to inhibit its dissolution, or in any case to make it proceed at unacceptable velocity for applications of industrial relevance. A similar consideration holds true also for tin and lead. This kind of problem may be dodged acting externally on the electric potential of the metal to be dissolved, namely carrying out the dissolution in a separate electrolytic cell (dissolution or enrichment cell) wherein said metal is polarised anodically thus being released in the solution in ionic form, with concurrent evolution of hydrogen at the cathode. The compartment of such cell must be evidently divided by a suitable separator, to avoid that the cations released by the metal migrate towards the cathode depositing again on its surface under the effect of the electric field. A solution of this kind is described in the European Patent 0 508 212, relative to a process of copper plating of a steel wire in alkaline environment with insoluble anode, wherein the electrolyte, based on potassium pyrophosphate forming an anionic complex with copper, is recirculated through the anodic compartment of an enrichment cell, separated from the relative cathodic compartment by means of a cation-exchange membrane. Such device allows to continuously restore the concentration of copper in the electrolytic bath, exhibiting however some apparent, serious drawbacks which make it scarcely practical. The claimed cation-exchange membrane is of the perfluorinated type, equivalent to the product commercialised as Nafion^® by DuPont de Nemours (U.S.A.), or as Aciplex^® by Asahi Chemicals (Japan); this choice is imposed by the need of having a high selectivity (cupric anionic complex rejection) guaranteed only by this kind of rather expensive membranes. Moreover, the copper released into the solution in the enrichment cell is only partially engaged in the pyrophosphate complex, and thus in anionic form. The fraction of copper present in cationic form tends to migrate towards the cathode, partly crossing the cation-exchange membrane and depositing on its surface or, even worse, binding to the functional groups of the same making its ionic conductivity decrease accordingly. Finally, in EP 0 508 212 an unwanted process complication is evident, as the electroplating cell tends to be depleted of hydroxyl ions (consumed at the anodic compartment), which must be re-established through the addition of potassium hydroxide formed in the catholyte of the enrichment cell. It is apparent that such re-establishment of the alkalinity requires a continuous monitoring, implying an increase in the costs of the system and of its management.

The present invention is aimed at providing an integrated system of galvanic electroplating cell of the insoluble anode type in hydraulic connection with a dissolution or enrichment cell, overcoming the drawbacks of the prior art.

In particular, the present invention is directed to an integrated system of galvanic electroplating cell of the insoluble anode type hydraulically connected to an enrichment cell, which may be operated both with acidic and alkaline electrolytes, characterised in that the balance of all the chemical species is self-regulating, and that no auxiliary supply of material is required except the possible addition of water.

Under another aspect, the present invention is directed to an enrichment cell for a galvanic electroplating system of the insoluble anode type comprising a separator insensitive to the pollution from cations, and in particular from metallic cations.

The invention consists in a system of electroplating cell of the insoluble anode type integrated with an enrichment cell comprising an anodic compartment, wherein the anodic dissolution of the metal to be deposited in the electroplating cell is carried out, a cathodic compartment, comprising a hydrogen evolving cathode and a support catholyte, and a separator, dividing the anodic compartment form the cathodic compartment, comprising at least one anion-exchange membrane.

The anion-exchange membranes are separators of limited cost; polystyrene-based anionic membranes are available on the market, such as those commercialised by Asahi Glass Corporation (Japan) as Selemion^®, or by Tokuyama Soda (Japan) as Neosepta^®, but there are also polyphenilsulphide-based ones, such as Ryton^®, commercialised by TBA (United Kingdom). In both cases, the polymeric backbone is functionalised with positively charged quaternary ammonium groups, able to form bonds with the anions transporting the same throughout the membrane thickness provided their steric size is compatible, but above all to constitute an effective barrier to the cation transport.

Even though their backbone has scarce chemical properties, some of these membranes have proven surprisingly resistant to the process conditions required by the system which is the object of the invention.

Figure 1 shows the general scheme of the relevant process; referring to figure 1 , the continuous electroplating cell of the insoluble anode type is indicated as (1 ), and the enrichment cell in hydraulic connection therewith is indicated as (2). The electroplating treatment is illustrated for a conductive matrix (3) suited to be coated by continuous metal plating, for instance a tape or wire; nevertheless, as it will be apparent from the description, the same considerations apply for the operation on pieces to be treated in batch. The matrix (3) is in electric contact with a cathode (4) having negative polarity. The counter-electrode is an insoluble anode (5) having positive polarity. The anode (5) may be made, for instance, of a platinum group metal oxide coated titanium matrix, or more generally by a conductive matrix non corrodible by the electrolytic bath in the process conditions, coated with a material exhibiting electrocatalytic activity for the oxygen evolution half-reaction. The enrichment cell (2), having the function of supplying the metal ions consumed in the electroplating cell (1 ), is divided by an anion-exchange membrane (6) into a cathodic compartment (9) provided with a cathode (7) made of a material which is not corrodible in the process conditions adopted, for instance stainless steel or nickel, and an anodic compartment (10), provided with a soluble anode (8) made of the metal which has to be deposited on the matrix to be coated (3). The anode (8) can be a planar sheet or another continuous element, but more commonly it can be made of an assembly of shavings, spheroids or other small pieces, in electric contact with a permeable conductive confining wall having positive polarity, for instance a web of non-corrodible material.

The anodic compartment (10) is fed with the solution to be enriched coming from the electroplating cell (1) through the inlet duct (11); the enriched solution is in its turn recirculated from the anodic compartment (10) of the enrichment cell (2) to the electroplating cell (1) through the outlet duct (12). In the case of electroplating of metal M from cation M^z+ in acidic environment, the process occurs according to the following scheme:

- at the conductive matrix (3):

M^z++ z e^" → M

- at the insoluble anode(5): z/2 H₂0 → z/4 0₂ + z H⁺ + z e^"

The solution depleted of metal ions M^z+ and enriched in acidity (for the anodic production of z H⁺), as previously said, is circulated through the duct (11) in the anodic compartment (10) of the enrichment cell (2), wherein a soluble anode (8) made of positively charged M metal, is oxidised according to

M → M^z++ z e-

and the excess acidity is neutralised through the transport, shown in figure 1 , of hydroxyl ions from the cathodic compartment (9), filled with an alkaline electrolyte (e.g. sodium or potassium hydroxide), to the anodic compartment (10) of the enrichment cell (2).

Such migration of hydroxyl ions is made possible by the fact that the separator (6) selected to divide the compartments (9) and (10) is an anionic membrane; its driving force is the electric field, with the additional contributions of osmotic pressure and diffusion. In stationary conditions, a simple setting of the ratio between the current density in the enrichment cell (2) and in the electroplating cell (1 ) allows the passage of one mole of hydroxyl ions through the anionic membrane (6) per mole of H⁺ ions generated at the anode (5) to take place, thereby achieving a perfect balance of the acidity of the system; in this way, the concentration of M^z\ is automatically restored as the balance of reaction shows, because the passage of z moles of electrons corresponds to the release of one mole of M^z+ in the anodic compartment (10) together with the deposition of one mole of M on the conductive matrix (3). The double regulation may be possibly facilitated by complexing the metal ion to be deposited with a suitable ligand, stable in the reaction environment, contributing to the buffering of acidity and M^z+ ion concentration in the circulating electrolytic bath. The cathodic compartment of the enrichment cell (2), which contains an alkaline electrolyte, is interested to the hydrogen discharge reaction on the surface of the cathode (7), according to z H₂0 + z e^' → z/2 H₂ + z OH^"

An immediate check of the mass and charge balance in this compartment shows how, by means of this half-reaction, the exact restoring of the z moles of hydroxyl ions transported through the anionic membrane (6) per each mole of metal M deposited in the cell (1) occurs. Hence, the afore described is a self-regulating process whose overall balance of matter solely implies, besides the desired reaction of deposition of the metal M on the conductive matrix (3), the consumption of water, electrolysed into hydrogen and oxygen for a total of one mole per each two moles of electrons, whose level can be restored with utmost simplicity through a plain topping-up, for instance in the electroplating cell (1). In any case, this water topping-up does not involve any further complication of the process, as it would be normal, in any electroplating process whether of the consumable or of the insoluble anode type, that extensive phenomena of evaporation lead by themselves to the need of keeping the water level under control by continuous replenishments. The disclosed general scheme can be further implemented with other expedients known to the experts of the field, for instance by delivering the oxygen which evolves at the anode (5) in the cathodic compartment (10) of the enrichment cell (2), to keep in the oxidised state the possible M^z+ cations diffusing, in spite of the barrier opposed by the positively charged functional groups, through the anionic membrane (6) and avoiding their deposition on the cathode (7) as metals. A largely similar process can be realised, according to the present invention, for the enrichment of alkaline electrolytic baths. In this case the conductive matrix (3) is still the site of the electroplating half-reaction

M^z++ z e-→ M

while oxygen evolution takes place at the anode (5), according to

z OH- → z/4 0₂ + z 2 H₂0 + z e^"

The solution depleted in metal ions and alkalinity (for the anodic consumption of z OH^") is circulated through the duct (11) in the anodic compartment (10) of the enrichment cell (2), wherein a soluble anode (8) made of positively charged metal M, is oxidised according to

M -> M^z++ z e-

and the alkalinity in defect is restored through the transport, indicated in figure 1 , of hydroxyl ions from the cathodic compartment (9), filled with an alkaline electrolyte (for instance sodium or potassium hydroxide), to the anodic compartment (10) of the enrichment cell (2), via the same mechanism of the previous case. Once more in stationary conditions, with a simple setting of the ratio between the current density of the enrichment cell (2) and of the electroplating cell (1), the passage of one mole of hydroxyl ions through the anionic membrane (6) per mole of OH^" ions consumed at the anode (5) takes place, thereby perfectly balancing the alkalinity of the system; in this way, the concentration of M^z+ ions is automatically restored as shown by the balance of reaction, the passage of z moles of electrons corresponding to the release of one mole of M^z+ in the anodic compartment (10) together with the deposition of one mole of M on the conductive matrix (3). Also in this case the double regulation may be possibly facilitated by complexing the metal ion to be deposited with a suitable ligand, stable in the reaction environment, which contributes to buffer the alkalinity and stabilise the M^z+ concentration in the recirculated electrolytic bath.

Also in this case, the cathodic compartment of the enrichment cell (2), containing an alkaline electrolyte, is interested to the hydrogen discharge reaction on the surface of the cathode (7), according to

and once more the alkalinity of the system results self-regulated, the only intervention to be effected for a full balance of the system being a water replenishment, to the extent of one mole for every two moles of electrons. Also in this case, the disclosed general scheme may be implemented with further expedients known to experts of the field. For instance, in case copper is deposited from alkaline electrolytes, it is convenient to introduce in the catholyte of the enrichment cell (2), a certain amount of ammonia, which is capable of complexing and hence of maintaining in solution the small amount of cupric ions diffusing through the anionic membrane (6).

In the following examples a copper plating process has been carried out on a steel wire, analogously to what described in EP 0 508 212, employing an enrichment cell provided with an anionic membrane.

EXAMPLE 1

In this experiment, a steel wire has been submitted to a copper plating process in an electroplating cell containing a bath of sulphuric acid (150 g/l) and cupric sulphate (50 g/l) added of corrosion inhibitors according to the prior art, employing as the anode a titanium sheet having positive polarity, coated with iridium and tantalum oxides, deputed to the oxygen evolution half-reaction. An enrichment cell, fed at the anodic compartment with the exhaust electrolytic bath coming from the electroplating cell, has been equipped with an AISI 316 stainless steel cathode and a consumable anode of copper shavings, confined by means of a titanium mesh having positive polarity. As the catholyte a potassium hydroxide solution (0.5% by weight), in which the oxygen produced at the electroplating cell anode was bubbled, has been employed. The catholyte and the anolyte of the enrichment cell have been divided by means of a Neosepta^® anionic membrane, produced by Tokuyama Soda. Utilising a current density of 2.6 kA/m² in the enrichment cell, a continuous copper plating of the steel wire could be carried out for an overall duration of 29 hours, with a copper dissolution efficiency in the enrichment cell greater than 99.99%, without any intervention besides the progressive water replenishment in the electroplating cell, monitored through a level control.

EXAMPLE 2

The test of the previous example has been repeated employing an alkaline bath containing cupric pyrophosphate at pH 8, with a pyrophosphate concentration of 180 g/l; a Selemion^® anionic membrane produced by Asahi Glass as the separator in the enrichment cell, and a solution containing 1 mol/l of sodium hydroxide, 1 mol/l of NH₃ and 0.1 mol/l of sodium sulphate as the catholyte have been employed. The continuous copper plating of the steel wire has thus been carried out for 23 hours at a current density of 1 kA m² in the enrichment cell, during which an efficiency of dissolution of the copper anode higher than 99.99% was observed.

Also in this case, the sole external intervention required has been the restoring of the electrolyte level in the electroplating cell, obtained through water addition. Although the invention has been described with reference to specific embodiments, the latter are not intended to limit the invention, whose extent is defined in the appended claims.

Throughout the description and claims of the specification the word "comprise" and variations of the word, such as "comprising" and "comprises" is not intended to exclude other additives, components, integers or steps.

Claims

1. Cell for enrichment by anodic dissolution of metal comprising an anodic compartment containing the metal to be dissolved and a cathodic compartment, characterised in that said cathodic compartment and said anodic compartment are divided by at least one anion-exchange membrane.

2. The cell of claim 1 characterised in that the anion-exchange membrane is deputed to the transport of hydroxyl ions from the cathodic compartment to the anodic compartment.

3. The cell of claim 2 characterised in that the cathodic compartment contains a cathode whereat the hydrogen evolution reaction takes place.

4. The cell of claim 2 characterised in that the metal for the anodic dissolution in the anodic compartment has positive polarity.

5. The cell of claim 4 characterised in that said metal for the anodic dissolution in the anodic compartment has an oxidation potential more positive than that of hydrogen.

6. The cell of claim 5 characterised in that said metal is copper.

7. The cell of claim 4 characterised in that said metal is a metal with high hydrogen overpotential.

8. The cell of claim 7 characterised in that said metal with high hydrogen overpotential is selected from the group comprising zinc, tin and lead.

9. The cell of claim 4 characterised in that said metal is a continuous element.

10. The cell of claim 9 characterised in that said continuous element is a planar sheet.

11. The cell of claim 4 characterised in that said metal is constituted by an assembly of pieces having small dimensions, in electric contact with a conductive and permeable confining wall, polarised positively.

12. The cell of claim 11 characterised in that said confining wall is a mesh or a perforated sheet.

13. The cell of claim 11 characterised in that said assembly of pieces having small dimensions comprises shavings, chips or spheroids.

14. The cell of claim 3 characterised in that said cathode comprises at least a metallic material selected from the group comprising nickel and steels.

15. The cell of claim 2 characterised in that said anion-exchange membrane comprises a base structure containing at least one polymer and functional groups comprising quaternary ammonium groups.

16. An apparatus for metal electroplating comprising at least a metal electroplating cell and at least a cell for the anodic dissolution of metal of the previous claims, comprising an anodic compartment and a cathodic compartment divided by at least one anion-exchange membrane.

17. The apparatus of claim 16 characterised in that said electroplating cell comprises an electrolytic bath, a negatively polarised conductive matrix and a positively polarised insoluble anode.

18. The apparatus of claim 17 characterised in that said insoluble anode comprises a metal coated with a catalyst for oxygen evolution.

19. The apparatus of claim 18 characterised in that said catalyst comprises noble metal oxides.

20. The apparatus of claim 17 characterised in that said electroplating cell and the anodic compartment of said enrichment cell are in mutual hydraulic connection.

21. The apparatus of claim 20 characterised in that said electrolytic bath is circulated so as to cross said anodic compartment of said enrichment cell by means of said hydraulic connection.

22. The apparatus of claim 21 characterised in that at least said metal which is dissolved at the anodic compartment of said enrichment cell is deposited in said electroplating cell.

23. The apparatus of claim 22 characterised in that said electrolytic bath comprises at least one compound which forms a complex with said metal which is dissolved.

24. The apparatus of claim 23 characterised in that said complex is an anionic complex.

25. The apparatus of claim 17 characterised in that said conductive matrix is suited for a continuous cycle manufacture.

26. A process for the electroplating of at least one metal on a negatively polarised conductive matrix by means of an electroplating cell comprising an insoluble oxygen-evolving anode and an electrolytic bath containing the ions of said metal, wherein the concentration of said ions in the electrolytic bath is restored by means of an enrichment cell, comprising an anodic compartment and a cathodic compartment separated by an anion- exchange membrane, characterised in that said enrichment cell is a cell of the claims 1 to 4.

27. The process of claim 26 characterised in that said electrolytic bath is acidic.

28. The process of claim 26 characterised in that said electrolytic bath is alkaline.

29. The process of claim 26 characterised in that said electrolytic bath contains a chemical compound capable of forming a complex with said metal.

30. The process of claim 29 characterised in that said complex is anionic.

31. The process of claim 26 characterised in that said oxygen evolved at the insoluble anode of the electroplating cell is bubbled in the cathodic compartment of said enrichment cell.

32. The process of claim 29 characterised in that said metal of the complex formed by said chemical compound is copper.

33. The process of claim 32 characterised in that said chemical compound comprises the pyrophosphate ion.

34. The process of claim 32 characterised in that the cathodic compartment of the enrichment cell comprises an alkaline hydroxide and ammonia.

35. The process of claims 26 to 34 characterised in that only the water consumed by electrolysis or by evaporation is replenished, and that the balance of matter of all the remaining chemical species is self-regulating.

36. An apparatus for metal electroplating substantially as hereinbefore described with reference to any of the examples or drawings.