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(11) | EP 1 207 219 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Equipment and method for covering a metallic element with a layer of copper |
| (57) Equipment and method for covering a metallic element with a layer of copper in which
the said equipment comprises an electrodeposition tank and a dissolving tank; in its
turn said dissolving tank includes a first section containing an anolyte, a second
section containing a catholyte, and an interposed element placed between said first
and said second sections and consisting of a porous diaphragm whose permeability to
copper ions is so low that the quantity of copper ions migrating from said first section
to said second section of said dissolving tank is equal to 0.01-5%/hour; and in which
the ratio between cathode current density and anode current density in said dissolving
tank is equal to at least 30 (Fig. 1). |
[0001] The present invention relates to an equipment and a method for covering a metallic element with a layer of copper.
[0002] More particularly, the present invention relates to the electrolytic deposition of a layer of metallic copper on a metallic element.
[0003] It is known that some articles made of vulcanized elastomeric material such as, for example, tyres for vehicles, conveyor belts, transmission belts and flexible tubes of natural and synthetic rubber and their mixtures, are reinforced by embedding suitable metal structures in an elastomeric matrix.
[0004] Generally, said metal structure is made of steel wires, with a carbon content between 0.6% and 0.95%, either individual or grouped in steel cords.
[0005] However, steel, which is the material of choice on account of its mechanical properties, has the disadvantage that it does not adhere sufficiently to the vulcanized elastomeric material. To obtain good adhesion to the elastomeric material, it is therefore usual to cover the steel with a layer of a suitable material, for example brass.
[0006] In the present description and in the claims, the term "brass" indicates a metallic composition, as homogeneous as possible, comprising from 10 to 50 wt.% of zinc and from 90 to 50 wt.% of copper, preferably from 20 to 40 wt.% of zinc and from 80 to 60 wt.% of copper and, even-more preferably, from 30 to 40 wt.% of zinc and from 70 to 60 wt.% of copper.
[0007] In its turn, the term "cord" indicates a cord obtained by rope-making, according to traditional techniques, from drawn steel wires covered with a layer of brass which, before drawing, is from 1 to 3 µm thick, whereas after drawing it is from 0.1 to 0.4 µm thick. Generally, the diameter of said wires is of about 1.3 mm before drawing and 0.1-0.5 mm after drawing. Typically, a cord commonly used in reinforcing structures for giant tyres consists of 7 strands, each of 4 wires with a diameter of about 0.175 mm, around which a wire is wound, the so-called filament, with a diameter of 0.15 mm.
[0008] One technique used in the past for covering a steel wire with a layer of brass consisted of the simultaneous electrodeposition of a predetermined quantity of copper ions and zinc ions to form a homogeneous layer of brass in situ. It was observed, however, that the adhesion between the elastomeric material and the layer of brass thus obtained was excellent at first, but it was not possible to guarantee that acceptable levels of adhesiveness would be maintained over time.
[0009] The currently most-used technique envisages the electrodeposition, on a steel wire, of a layer of copper and a layer of zinc in two separate stages, followed by a third stage of thermal diffusion carried out at a temperature above 450°C, preferably at a temperature from 450 to 500°C. During this stage, the aforesaid layers diffuse into one another forming a layer of brass that has excellent characteristics of drawability and adhesiveness.
[0010] Initially, the electrolytic cells used for depositing a layer of copper on steel wire were equipped with soluble anodes of copper. The steel wire was given a negative charge so that it acted as cathode and was made to pass through an electrolyte containing copper ions, preferably in the form of copper pyrophosphate. The copper ions were deposited on the wire, covering it with the desired layer. In their turn, the copper anodes dissolved and supplied the electrolyte with more copper ions. Therefore the shape of the anodes changed as the copper dissolved and this led to variations in current density at the steel cathode with corresponding non-uniformity of the copper layer deposited on the steel wire. This non-uniformity could only be contained by frequent replacement of the anodes, but in their turn these replacements were the cause of undesirable interruptions of the process and reduced its productivity. Finally there was the problem of how to recover the consumed anodes that had been replaced.
[0011] To overcome this drawback, it was proposed to use insoluble anodes and to continuously restore the concentration of copper ions in the electrolytic solution by adding suitable copper compounds.
[0012] For example, the patent US-A-5 516 414 describes a method and an equipment in which the concentration of copper ions is restored by supplying cupric hydroxide to the electrolytic solution of pyrophosphates.
[0013] However, the copper compounds used in such processes for restoring the concentration of copper ions in the electrolytic solution have the disadvantage that they are more expensive than metallic copper.
[0014] Methods were then developed that envisage the use of two tanks: one for electrodeposition and the other for dissolution. There are two known methods of this type. The first is known as the diaphragm method and the second as the oxygen method.
[0015] The diaphragm method is described, for example, in the patent EP-B-0 508 212 and comprises the following stages:
a) application of a negative charge to the steel wire and continuous passage of the steel wire through an electrodeposition cell in which the negatively charged steel wire is in contact with an aqueous copper pyrophosphate solution and in which the aqueous copper pyrophosphate solution is in contact with a positively charged inert anode;
b) residence of the negatively charged steel wire in the pyrophosphate solution for a sufficient time to cover the steel wire with a layer of copper of the desired thickness;
c) restoration of the concentration of copper in the copper pyrophosphate solution in the deposition cell by circulating the copper pyrophosphate solution in the deposition cell with a copper pyrophosphate solution supplied with copper ions from a supply cell, in which the copper pyrophosphate solution supplied in the supply cell is in contact with at least one positively charged copper anode and in which the copper pyrophosphate solution supplied is in contact with a conductive membrane, such as a copolymer of tetrafluoroethylene and perfluoro-3,5-dioxa-4-methyl-7-octenesulphonic acid, which separates the copper pyrophosphate solution supplied from a potassium hydroxide solution in which the potassium hydroxide solution is in contact with a negatively charged cathode;
d) transfer of a sufficient quantity of the potassium hydroxide solution, which is in contact with the negatively charged cathode that produces hydroxide ions, to the copper pyrophosphate solution to supply the hydroxide ions of the copper pyrophosphate solution that have been consumed at the inert anode in the copper pyrophosphate solution in the deposition cell;
e) addition of a sufficient quantity of water to the potassium hydroxide solution to replace the potassium hydroxide transferred to the copper pyrophosphate solution and the water lost by reduction and evaporation.
[0016] Briefly, in the diaphragm method, the dissolving tank is divided into two sections by a diaphragm consisting of a conductive porous membrane in which the pore size is smaller than that of the Cu2+ ions but larger than that of KOH, H2O and OH- ions. In the first section there is a copper anode, and in the second there is an insoluble cathode. The copper anode dissolves and the Cu2+ ions move towards the cathode but cannot reach it because they are held back by the membrane. The concentration of Cu2+ ions therefore increases in the first section. In the second section, on the other hand, at the cathode, the electric charges (e-) split the water into gaseous H2 and OH- ions. The OH- ions pass through the membrane into the first section. The KOH and the H2O that are consumed by each electrode are replaced via the membrane by a process of reverse osmosis.
[0017] The solution thus restored passes to the deposition tank and that of the deposition tank, depleted of Cu2+ ions, passes to the dissolving tank to be restored again.
[0018] A first drawback of the aforementioned method is that, to effect the desired selectivity with respect to the species present in the solutions of the two sections of the dissolving tank, the size of the pores of the aforesaid conductive membrane (Nafion™, column 7, lines 37-58) must be accurately calibrated. It is therefore very expensive.
[0019] A second drawback is that the aforesaid method requires the use of an equipment provided with several groups of pumps and pipe (EP-B-0 508 212, see the numerical references 22, 29, 31 and 34 in Fig. 1) involving complex management and programming for successfully keeping the compositions and the pH of the solutions in the two electrolytic cells within the desired values.
[0020] A third drawback arises from the fact that proper operation of the whole plant is dependent on the ability of the aforementioned conductive membrane to remain efficient over time, maintaining the aforesaid high degree of selectivity.
[0021] As already mentioned, the second method that makes use of a deposition tank and a dissolving tank is the so-called oxygen method. This method is described, for example, in an article by M.Kikuchi et al. (Proceedings of the 66th Annual Conference of the Wire Association International 1996; Charlotte, NC, USA, pages 30-35).
[0022] In the oxygen method, the dissolving tank consists of a pressure reactor containing copper plates, an electrolyte, pressurized gaseous oxygen and an injector. The method envisages constant monitoring of the pH of the electrolyte in the dissolving tank because its value is indicative of the concentration of Cu2+. When the pH value is below a predetermined value, the system commands the injector to supply gaseous oxygen and thus to increase the amount of oxygen dissolved in the electrolyte and this, in its turn, promotes the dissolution of the copper according to the following reaction scheme:
[0023] The Cu2+ concentration can thus be maintained within predetermined limits by adjusting the pressure of the gaseous oxygen. According to the authors of the aforesaid article, using pure gaseous oxygen the pressure required by the system is of about 1 kg/cm2. On the other hand, if air is used the pressure required by the system is of about 1 kg/cm2.
[0024] The inventors realized that it is not necessary to use a conductive membrane possessing the high degree of selectivity indicated above but it can be advantageous to use a porous diaphragm that is scarcely permeable to copper ions provided that:
- in the dissolving tank the ratio between cathode current density and anode current density is greater than 30 and, preferably, greater than 50, and
- the cathode consists of an insoluble material with low hydrogen overpotential; typically a material that has a hydrogen overpotential less than 0.05 V in 2N sulphuric acid with current density of 10E-3 A/dm2.
[0025] A first aspect of the present invention relates to an equipment for covering a metallic element with a layer of copper, said equipment including an electrodeposition tank and a dissolving tank, in which
a) said electrodeposition tank contains an aqueous solution of copper ions in the form of a copper salt of an acid and of at least one basic compound suitable for adjusting the pH of said solution, in which a cathode, consisting of the aforesaid metallic element, and an insoluble anode are immersed, said cathode and said anode being connected electrically to a first source of direct current, and
b) said dissolving tank includes
i) a first section containing an anolyte consisting of an aqueous solution, containing said copper salt and said at least one basic compound, in which an insoluble anode in the form of a container containing small pieces of metallic copper is immersed,
ii) a second section containing a catholyte consisting of a dilute aqueous solution, containing mainly said at least one basic compound, in which an insoluble cathode is immersed, said cathode and said anode of said dissolving tank being connected electrically to a second source of direct current, and
iii)an interposed element placed between said first and said second sections,
c) said electrodeposition and dissolving tanks are connected together hydraulically
for feeding said anolyte and said catholyte from said dissolving tank to said electrodeposition
tank and for sending said solution of said electrodeposition tank to the first section
of said dissolving tank,
characterized in that
d) in said dissolving tank, the ratio between cathode current density and anode current density is equal to at least 30, and
e) said interposed element consists of a porous diaphragm that is completely permeable to electric current, to water and to said at least one basic compound whereas its permeability to copper ions is so low that the quantity of copper ions migrating from said first section to said second section of said dissolving tank is equal to 0.01-5%/hour.
[0026] Advantageously, the aqueous solution of the electrodeposition tank is obtained from copper pyrophosphate and potassium pyrophosphate.
[0027] Preferably, the quantity of copper pyrophosphate in said solution is between 80 and 120 g/l. Even more preferably it is equal to about 100 g/l.
[0028] In its turn, the quantity of potassium pyrophosphate trihydrate in said solution is preferably between 350 and 450 g/l and, even more preferably, it is equal to about 400 g/l.
[0029] Said insoluble anode of said electrodeposition tank consists advantageously of any metallic compound that does not oxidize in the operating conditions of the electrodeposition tank. For example, it can consist of titanium coated with oxides of noble metals such as iridium, tantalum and the like; or it can consist of platinized titanium.
[0030] When the metallic element is a steel wire, said anode preferably has an elongated shape and is positioned parallel to the steel wire to ensure good current distribution.
[0031] The preferred operating conditions in said electrodeposition tank are: pH = 8.6-8.9, preferably 8.7 (adjusted with pyrophosphoric acid); cathode current density = 5-16 A/dm2; temperature = 50 ± 5°C.
[0033] Therefore deposition of metallic copper on said metallic element reduces the concentration of copper ions in solution, whereas the evolution of oxygen at the anode reduces the pH of the solution. The initial conditions are continually restored by adding the anolyte, which is gradually enriched with copper ions in the first section of the dissolving tank, and the catholyte which is gradually enriched with OH- ions in the second section of the dissolving tank.
[0034] The solution sent from the electrodeposition tank to the said first section of the said dissolving tank has, on average, the following characteristics:
| copper pyrophosphate trihydrate | 98.00-99.9 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.4-8.7 |
| copper pyrophosphate trihydrate | 105-115 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.9-9.2 |
[0036] The insoluble anode is preferably in the shape of a basket. Typically, it is a basket of titanium or of any other metallic compound that does not oxidize in the operating conditions of said first section of said dissolving tank.
[0037] The current density of said anode varies with variation of the quantity and shape of the small pieces of copper that it contains. Advantageously, it is maintained within the range from 1 to 5 A/dm2.
[0038] The solution (catholyte) contained in said second section of said dissolving tank has, on average, the following characteristics:
| copper pyrophosphate trihydrate | 0.0-5 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.7-9.2 |
[0039] The insoluble cathode immersed in this solution is, advantageously, formed of wires or strips of platinum or of some other material possessing a hydrogen overpotential which, in 2N sulphuric acid and at current density of 10E-3 A/dm2, is preferably between 0.3 and 0.001 V and, even more preferably, between 0.05 and 0.02 V. Typically, in these conditions, platinum has a hydrogen overpotential of 0.024 V.
[0040] In a preferred embodiment, the total area of said cathode is such that the cathode current density is equal to at least 100 A/dm2.
[0041] The porous diaphragm interposed between said first and second sections of said dissolving tank can consist of a vitreous, ceramic or polymeric material, for example a polyester.
[0042] Depending on the quantity of copper that has to be dissolved, the dissolving tank will contain one or more first and second sections separated by respective interposed elements possessing the aforesaid characteristics and properties.
[0044] Discharge of hydrogen at the cathode is favoured relative to discharge of copper at the anode for the following reasons:
- high cathode current density,
- presence of the interposed element that dramatically reduces the quantity of copper ions passing from the said first to the said second section,
- low hydrogen overpotential at the cathode.
[0045] The water lost by evaporation and in the reaction of reduction is replaced continuously by supplying fresh water to the electrodeposition tank via a suitable pipe (not shown).
[0046] Initially, an aqueous potassium pyrophosphate solution or even a solution possessing the same composition as that of deposition tank 1 is placed in section 8. In the latter case, the quantity of copper in the catholyte solution 12 decreases until it reaches, in normal operation, a level ≤ 5 g/l.
[0047] A person skilled in the art will readily appreciate that the equipment of the present invention is very simple and is easy to maintain, in particular with regard to cleaning of the cell or cells and, even more particularly, with regard to cleaning of the cathode of the dissolving cell.
[0048] A second aspect of the present invention relates to a method for covering a metallic element with a layer of metallic copper, in which
a) said metallic element is connected electrically to a negative pole of a first source of direct current and is immersed in an aqueous electrolytic solution containing from 30 to 60 g/l of copper ions in form of a copper salt of an acid and at least one basic compound capable of controlling the pH of said solution between 8.5 and 8.9;
b) an insoluble anode, connected electrically to the positive pole of said first source of direct current, is also immersed in said electrolytic solution;
c) the quantity of copper ions and the pH level, that tend to decrease during electrodeposition of metallic copper on said metallic element, are restored continuously;
d) said restoration is obtained electrolytically by continuous circulation of said
electrolytic solution in a dissolving tank equipped with an insoluble anode and an
insoluble cathode, connected electrically to a second source of direct current and
separated from one another by an interposed element, in which said anode is in the
shape of a container and contains small pieces of metallic copper;
characterized in that
e) in said dissolving tank, the ratio between the cathode current density and the anode current density is equal to at least 30, and
f) said interposed element consists of a porous diaphragm that is completely permeable to electric current, to water and to said at least one basic compound whereas its permeability to copper ions is so low that the percentage of copper ions migrating from said first section to said second section of said dissolving tank is equal to 0.01-5%/hour.
[0049] The present invention will now be further described with reference to embodiments and to the drawings, in which
Fig. 1 is a schematic diagram of an equipment for covering a metallic element with a layer of metallic copper according to the present invention;
Fig. 2 shows a first variant of the equipment of Fig. 1;
and
Fig. 3 shows a second variant of the equipment of Fig. 1.
[0050] As shown in the aforementioned figures, the equipment includes an electrodeposition tank 1 and a dissolving tank 2.
[0051] Tank 1 contains an aqueous solution 3 of (i) copper ions in the form of a copper salt of an acid and of (ii) at least one basic compound suitable for adjusting the pH of the solution 3. Insoluble anodes 4 and a cathode consisting of a steel wire 5 are immersed in said solution 3. Deposition of metallic copper on said steel wire 5 tends to lower the concentration of copper ions in the solution 3, while the evolution of oxygen at the anodes 4 tends to lower the pH of said solution.
[0052] The insoluble anodes 4 and the cathode 5 are connected electrically to a first source of direct current (not shown).
[0054] The first section 7 contains an anolyte 9 consisting of the electrolytic solution obtained from the electrodeposition tank 1. An insoluble anode 10 in the shape of a basket containing granules 11 of metallic copper is immersed in said anolyte 9.
[0055] The second section 8 contains a catholyte 12 consisting of components of said electrolysis solution 3 being able of passing through said porous glass diaphragm 6 that has a permeability to copper ions of about 1%/hour. A cathode 13 is immersed in said catholyte 12. Said cathode 13 and anode 10 are connected electrically to a second source of direct current (not shown).
[0056] Said tanks 1 and 2 are connected by a pipe 15 for sending the electrolyte 3 to the first section 7 of tank 2.
[0057] In the case of the equipment in Fig. 1, said tanks 1 and 2 are also connected together by a pipe 14 for feeding the anolyte 9 and the catholyte 12 from tank 2 to tank 1.
[0058] In the variant in Fig. 2, the anolyte 9 and the catholyte 12 are fed from tank 2 to tank 1 via two separate pipes 14 and 16 respectively.
[0059] In the variant in Fig. 3, there are two sections 8 containing the catholyte 12 which communicate together by a pipe 17 which can also be connected, by means of suitable valves, to a water line.
Example 1
[0061] A steel wire 5 with diameter of 1.6 mm had previously been pickled electrolytically in sulphuric acid and was then covered with a layer of copper in an equipment like that shown in Fig. 1.
[0064] The solution 3 in tank 1 had the following initial characteristics:
| copper pyrophosphate trihydrate | 100 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.7 |
| copper pyrophosphate trihydrate | 99.62 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.6 |
[0065] Pellets of electrolytic copper with diameter of about 2.5 cm were placed in the anode basket 10 and a current of 35 A was supplied. As already mentioned, the surface area of this anode varies over time with variation of the quantity of copper present in the basket and of the diameter of the pellets as they dissolve.
[0066] However, it was estimated that the average value of this area was 30 dm2, equivalent to an average value of anode current density of approx. 1.16 A/dm2.
[0067] The total surface area of the cathodes 13 was 0.25 dm2 and the cathode current density was therefore 140 A/dm2.
[0068] The solution 3 leaving the electrodeposition tank 1 passed to the first section 7 of the dissolving tank 2 and from there it passed to section 8 through porous diaphragms 6 of glass grade 4 ISO 4793 produced by the company Schott Mainz that had a permeability to copper ions of only about 1%/hour.
[0069] The solutions leaving section 7 (anolyte) and section 8 (catholyte) returned to tank 1 via the pipe 14 and had, on average, the characteristics shown below.
| Anolyte: | |
| copper pyrophosphate trihydrate | 111.4 g/l |
| potassium pyrophosphate | 400 g/l |
| temperature | 50°C |
| pH | 8.7 |
| Catholyte: | |
| copper pyrophosphate trihydrate | 1 g/l |
| potassium pyrophosphate | 4 g/l |
| temperature | 50°C |
| pH | 8.9 |
[0070] For the entire duration of the experiment
- the solution was made to circulate continuously from tank 1 to tank 2 and vice versa, through the lines 14 and 15, at a rate of 600 l/hour,
- the rate of dissolution of the copper pellets proved to be approx. 41 g/h and remained substantially constant,
- in tank 1, the pH values of the solution 3 remained in the range from 8.5 to 8.9,
- in tank 1, the levels of copper ions in the solution 3 remained in the range from 40 to 44 g/l,
- the anolyte/catholyte volume ratio at outlet from sections 7 and 8 of the dissolving tank was about 1 : 0.11,
- the thickness of the layer of copper deposited on the steel wire 5 was 0.5 µm and remained substantially constant,
- the water lost by evaporation and in the reaction of reduction was replaced continuously by feeding fresh water to the electrodeposition tank 1.
a) said electrodeposition tank contains an aqueous solution of copper ions in the form of a copper salt of an acid and of at least one basic compound suitable for adjusting the pH of said solution, in which a cathode, consisting of the afore said metallic element, and an insoluble anode are immersed, said cathode and said anode being connected electrically to a first source of direct current, and
b) said dissolving tank includes
i) a first section containing an anolyte consisting of an aqueous solution, containing the said copper salt and said at least one basic compound, in which an insoluble anode in form of a container containing small pieces of metallic copper is immersed,
ii) a second section containing a catholyte consisting of a dilute aqueous solution, containing mainly said at least one basic compound, in which an insoluble cathode is immersed, said cathode and said anode of the said dissolving tank being connected electrically to a second source of direct current, and
iii) an interposed element, placed between said first and said second sections,
c) said electrodeposition and dissolving tanks are connected together hydraulically
for feeding said anolyte and said catholyte from said dissolving tank to said electrodeposition
tank and for sending said solution of said electrodeposition tank to said first section
of said dissolving tank,
characterized in that
d) in said dissolving tank, the ratio between cathode current density and anode current density is equal to at least 30, and
e) said interposed element consists of a porous diaphragm that is completely permeable to electric current, to water and to said at least one basic compound whereas its permeability to copper ions is so low that the quantity of copper ions migrating from said first section to said second section of the said dissolving tank is equal to 0.01-5%/hour.
characterized in that said insoluble anode of said electrodeposition tank consists of titanium coated with oxides of noble metals selected from the group comprising iridium and tantalum.
characterized in that said insoluble anode of said electrodeposition tank consists of platinized titanium.
a) said metallic element is connected electrically to a negative pole of a first source of direct current and is immersed in an aqueous electrolytic solution containing from 30 to 60 g/l of copper ions in form of a copper salt of an acid and at least one basic compound capable of controlling the pH of said solution between 8.5 and 8.9;
b) an insoluble anode, connected electrically to the positive pole of said first source of direct current, is also immersed in said electrolytic solution;
c) the quantity of copper ions and the pH level, that tend to decrease during electrodeposition of metallic copper on said metallic element, are restored continuously;
d) said restoration is obtained electrolytically by continuous circulation of said
electrolysis solution in a dissolving tank equipped with an insoluble anode and an
insoluble cathode, connected electrically to a second source of direct current and
separated from one another by an interposed element, in which said anode is in form
of a container and contains small pieces of metallic copper;
characterized in that
e) in said dissolving tank, the ratio between the cathode current density and the anode current density is equal to at least 30, and
f) said interposed element consists of a porous diaphragm that is completely permeable to electric current, to water and to said at least one basic compound whereas its permeability to copper ions is so low that the percentage of copper ions migrating from said first section to said second section of the said dissolving tank is equal to 0.01-5%/hour.