Production process of metallic foil by electrolysis

Abstract: A process of producing a metallic foil by electrolysis is disclosed, which comprises using an insoluble electrode having an electrode active coating composed of a composite oxide of iridium and tantalum on an oxygen impermeable coating formed on a thin film-forming metal substrate as an anode and depositing impurities in the electrolytic bath on the anode at an anodic potential higher than the electric potential at which the impurities in the electrolytic bath form insoluble oxides on the anode.

Description: The present invention relates to a continuous production process of a metallic foil by electrolysis and more particularly to a production process of a copper foil by electrolysis. There are various production processes of metallic foils according to the quality of the metal and the use of the metallic foil but a process of producing by rolling and a process of producing by electrolysis are typical. In the process of producing a metallic foil by rolling, the metal is rolled by passing through rolling rollers while pressing the metal with the rollers. In the case of rolling, for producing a thinner metallic foil, the rolling technique is very complicated and there occur the problems in the manner of applying a tension, the control of the distance of rollers, etc. Also, the uniformity of the thickness in the width direction of a metallic foil being formed is said to be not always constant from the restriction caused by the forms of the rollers. Also, the process by electrolysis has recently been widely used for the production of a copper foil, in particular, a copper foil for a copper laminated plate being used for a printed circuit board. This is the process as follows. That is, as shown in Figure as the cross sectional view of example of a copper foil producing apparatus 1 by electrolysis, an electric current is passed between a large cathode roller 4 the lower portion of which is immersed in a copper electrolyte 3 contained in an electrolytic tank 2 as a cathode and an insoluble anode 5 as a counter electrode, while supplying the electrolyte through an electrolyte supplying slit 6 of the anode to continuously plate copper on the surface of the roller 4 and metallic copper 7 deposited is continuously scrapped from the surface of the roller 4. This process has the feature that the average thickness of the copper foil 8 obtained can be easily controlled by controlling the amount of the supplying electric current and also a thin copper foil can be easily obtained. Since for a copper foil being used for a printed circuit board, copper having a very high purity is required, in the case of producing the copper foil by rolling, it is necessary to roll copper having a high purity as the raw material but the process of producing an electrolytic copper foil has the feature that scrap copper, etc., having a possibility of containing impurities can be used as the raw material. That is, the foregoing feature of the production of the electrolytic copper foil is based on the characters that in the production of the electrolytic copper foil, a copper sulfate bath is generally used and since the deposition potential of copper in the copper sulfate bath is nobler than the deposition potentials of other metals, even when other metal component(s) are contained in the copper sulfate bath, these other metal components are not deposited from the copper sulfate bath and thus the electrolytic foil producing apparatus accomplishes the role of purification. However, copper generally contains a slight amount of lead and many lead is mixed in a copper sulfate bath from scrap copper. As the result thereof, the content of lead in the copper sulfate bath is gradually increased with the progress of the electrolysis and finally lead is deposited as lead sulfate in the copper sulfate bath. Lead sulfate thus deposited is in the state of being dispersed as particles thereof in the copper sulfate bath and it sometimes happens that the dispersed lead sulfate particles are deposited in the copper foil depositing on the cathode together with copper. In the case of a copper foil having a thick thickness, the copper foil can be used even when the particles of lead sulfate exist in the copper foil, but the copper foil containing the particles of lead sulfate can not be used for forming a printed circuit having a thickness of about 10 µm or thinner as a recent minute printed circuit board and thus there is a problem in the electrolytic production of copper foil that a thin copper foil having a thickness of not thicker than 20 µm can not actually be produced. Thus, as a thinner copper foil having a thickness of 20 µm or less, a rolled copper foil has always been used. Also, since in the production of a copper foil by electrolysis, a lead alloy is conventionally used as the anode, the deposition of lead sulfate and metallic lead in the copper foil obtained is a large problem and it has been practiced to co-precipitate the lead components by adding strontium carbonate, etc., and remove them by filtration. Recently, in the electrolytic production of a copper foil, an insoluble metallic electrode composed of a thin film-forming metal substrate called DSE (Dimensionally Stable Electrode) having formed thereon coating containing the oxide of a platinum group metal is used as described in U.S. Patent 4,318,794 and hence it has been avoided that a copper sulfate bath is contaminated by the intermixing of lead dissolved from a conventional anode but as described above, lead contained in the copper raw material is accumulated in the electrolyte to precipitate lead sulfate although the amount thereof is slight, which gives bad influences onto the copper foil although the frequency is less than the aforesaid case. Furthermore, since the amount of lead is slight as compared with the case of using the lead alloy electrode, the desired purpose can not be sufficiently attained even by the co-precipitation method which is carried out in the case of using the lead electrode, and also since very fine flock only of lead sulfate are formed, it is difficult to sufficiently remove the flock by using a means such as a filtration, etc. The present invention has been made for solving the foregoing problems in the conventional techniques and an object of the present invention is to provide a production process of a metallic foil by electrolysis, wherein it is prevented to deposit lead components intermixing from a metal raw material dissolved in a sulfate bath in a metallic foil formed to deteriorate the characteristics of the metallic foil. Other object of the present invention is to provide a production process of metallic foil by electrolysis capable of substantially omitting the work of removing lead from the electrolytic bath. In particular, still other object of the present invention is to provide a process of continuously producing a metallic foil having an excellent quality. The objects described above can be attained by the present invention as described hereinafter. That is, according to the present invention, there is provided an electrolytic production process of a metallic foil, which comprises using an insoluble electrode having an electrode active coating composed of a composite oxide of iridium and tantalum on an oxygen impermeable coating formed on a thin film-forming metal substrate as an anode and depositing impurities on the anode at an anodic potential higher than the electric potential at which the impurities in the electrolyte bath form insoluble oxides on the anode. The figure is a cross sectional view showing an example of an apparatus for producing a copper foil by electrolysis. Then, the present invention is described in detail. In the electrolytic production of metallic foil typified by a copper foil, it is an important theme to remove impurities contained in the metallic foil produced. In metal components as the impurities, the removal of lead components having a low solubility contained in a sulfuric acid acidic electrolytic bath is a most important theme and various processes have been proposed but effective processes have not yet been developed. In the present invention, it has been discovered that by keeping the anodic potential in the sulfuric acid acidic electrolytic bath higher than the standard hydrogen electrode potential by 1.6 volts or more, the lead components in the electrolytic bath are deposited as lead dioxide on the surface of the anode. Also, it has been discovered that excessively accumulated lead in the electrolytic bath is deposited on the surface of the anode as lead sulfate but when the electric potential of the anode is higher than the standard hydrogen electrode by 1.6 volts or more, lead sulfate is oxidized to lead dioxide on the surface of the anode and lead dioxide is deposited on the surface of the anode as a relatively strong deposit which grows with the progress of the electrolysis. On the other hand, if the electric potential of the anode is lower than 1.6 volts to the standard hydrogen electrode, Pb²⁺ becomes stabler than Pb⁴⁺, a dissolution occurs or lead covers the surface of the anode as lead sulfate, which is undesirable for the electrolysis. Furthermore, it has also been discovered that lead dioxide thus deposited has an electric conductivity and can be used as a part of the electrode, and in the case of lead dioxide deposited on the surface of an oxide anode, the deposited lead dioxide does not show a high electric potential specific to lead dioxide during the production of the electrolytic metallic foil, the electric potential of the anode is scarcely changed, and the electrolytic voltage is scarcely increased. The inventors have succeeded in accomplishing the present invention based on the discoveries described above. In the present invention, for depositing and removing lead contained in the electrolytic bath, it is important to deposit lead in the electrolytic bath on the anode as lead dioxide and stably dispose as lead dioxide on the anode as described above and for the purpose, it is necessary to carry out the electrolysis by keeping the electric potential of the anode at an electric potential of higher than the standard hydrogen electrode by at least 1.6 volts. However, in a conventional electrolytic production of a metallic foil, it is noticed to lower the electrolytic voltage. As to the anode, in the case of using an anode coated with a binary composite oxide composed of iridium oxide and tantalum oxide, for lowering the electrolytic voltage, it is practiced to increase the molar ratio of iridium/tantalum or provide iridium oxide on the surface of the anode. On the other hand, in the anode being used for the electrolytic production of a metallic foil of the present invention, the amount of tantalum in the composite oxide is increased to stabilize the electrode and at the same time the electric potential of the electrode (anode) is raised to the electric potential of higher than the standard hydrogen electrode potential by 1.6 volts or more, which is an electric potential at which lead is deposited as lead dioxide and stably exists as lead dioxide on the anode, and for the purpose, the metal molar ratio of iridium and tantalum is Ir ≤ Ta. Also, if the amount of tantalum is increased over the ratio of Ir : Ta = 3 : 7, the life of the anode is quickly shortened and hence it is undesirable that the amount of tantalum is increased over the ratio. Also, when the electric potential of the anode is increased, a passive oxide is liable to be formed at the interface between the active coating and the thin film-forming metal substrate to substantially shorten the life as the electrode, and hence it is necessary to form an oxygen impermeable coating layer between the electrode active coating and the substrate. The oxygen impermeable coating layer may be same as a conventionally known one, but since the electric potential is high and, with the formation of the lead dioxide layer, the electric potential is gradually raised even though it is slight, it is preferred that the the coating layer has a high oxygen impermeable function. As the oxygen impermeable layer, various materials can be used but, in particular, a semiconductive composite oxide with titanium and tantalum is preferred. The composite oxide of titanium and tantalum is a composite oxide of tetravalent titanium and pentavalent tantalum. An ordinary semiconductive titanium oxide utilizes the defect structure of oxygen, that is the non-stoichiometric property but by adding thereto tantalum, the oxygen impermeable layer composed of the semiconductor composite oxide of titanium and tantalum has the feature that even when titanium oxide is changed to the oxide having no defect structure by the migration of oxygen, since penta-valent tantalum co-exists and enters the same oxygen lattices, the electric conductivity is maintained. Furthermore, it is an effective means that by adding thereto platinum, the passivation is reluctant to occur by utilizing the electric conductivity of platinum. The composite oxide of titanium and tantalum, or the oxide of titanium or tantalum whose oxygen amount is intentionally controlled to stabilize the oxide, is also used as the oxygen impermeable layer, and in this case, it is not one produced by coating a coating liquid containing titanium or tantalum on the surface of a substrate composed of a thin film-forming metal followed by sintering as conventional but one prepared by being sintered by adjusting the atmosphere as in the case of forming ceramics. For coating of such a sintered material, a method of adjusting the atmosphere to a weak reductive atmosphere, such as a plasma flame-coating method, a flame-coating method, a reactive PVD (Physical Vapour Deposition) method, etc., can be used. Coating of the electrode active material on the anode can be formed by coating a solution containing iridium and tantalum and thereafter, sintering the coating in an oxidizing atmosphere. For example, the coating liquid is prepared by dissolving under heating a definite amount of iridium chloride in an aqueous hydrochloric acid solution having dissolved therein from 5 to 20% by weight tantalum chloride or the coating liquid is prepared by dissolving an organotantalum compound such as 5-butyl tantalate in a solvent such as butyl alcohol, etc., adding thereto diluted hydrochloric acid of about 10% by weight as a stabilizer, and further dissolving therein a definite amount of iridium chloride with heating. The coating liquid thus prepared is coated on the surface of a substrate having formed thereon the oxygen impermeable layer and sintered by an ordinary thermal decomposition method. Coating is carried out by a brush coating method, a roller coating method, or a spray coating method. There is no particular restriction on the sintering temperature but for improving the corrosion resistance and keeping a high electric potential, the sintering temperature is preferably from 450 to 550°C. The atmosphere is preferably an oxidizing atmosphere such as air, etc. In addition, the steps of coating and sintering are repeatedly carried out to form the coating of the desired coating amount. The molar ratio of iridium and tantalum (Ir : Ta) in the electrode active coating is preferably in the range of from 1 : 1 to 3 : 7. If the proportion of iridium is larger so that the molar ratio of Ir : Ta does not fall within the above range, there is a tendency of lowering the electric potential, the faculty of depositing lead in the electrolyte bath is lowered and at the same time, the thickness of the coating is thickened, and there occurs a problem in the strength of the electrode. Also, if the proportion of tantalum is larger so that the molar ratio of Ir : Ta does not fall within the above range, there is a tendency of shortening the life of the electrode. In the process of the present invention, an insoluble electrode having the electrode active coating composed of the composite oxide of iridium and tantalum on the oxygen impermeable coating formed on the thin film-forming metal substrate is used as an anode, an electrolysis is carried out at the anodic potential of depositing lead in the electrolytic bath on the surface of the anode as insoluble lead dioxide, and a metallic foil is electrolytically deposited on the cathode. In the invention, the metallic foil obtained does not contain lead contained in the electrolytic bath, it is unnecessary to use a specific means for removing lead components in the electrolytic bath, and a metallic foil having an excellent quality can be electrolytically produced. Then, the present invention is described in detail by the following examples. Example 1 Titanium plate was used as a substrate to prepare an anode. First, after roughening the surface of titanium by blasting, the titanium substrate was pickled in an aqueous solution of 20% by weight sulfuric acid at 85°C for 3 hours to carry out the activation of the surface thereof. As an oxygen impermeable layer, a coating liquid prepared by dissolving titanium tetrachloride in an aqueous hydrochloric acid solution of 10% by weight tantalum chloride such that the ratio of titanium to tantalum became 9 : 1 by molar ratio was coated on the substrate by a brush and thereafter, sintered in flowing air for 10 minutes at 550°C. By repeating the foregoing step 4 times, an oxide coating composed of 0.02 mol/m² of the metals was formed. It was confirmed that the oxide coating formed had a sufficient electric conductivity. Then, a coating liquid prepared by dissolving iridium trichloride in an aqueous hydrochloric acid solution of 10% by weight tantalum chloride such that the ratio of iridium to tantalum became 40 : 60 by mole was coated on the foregoing oxide coating by thermal decomposition. That is, the coating liquid was coated on the substrate coated with the electric conductive oxide as described above with a brush and after drying at 60°C, sintered in flowing air for 10 minutes at 540°C. By repeating the foregoing step 20 times, an electrode having the coating containing 15 g/m² of iridium was obtained. When the single electrode potential of the electrode thus obtained as an anode at 20 amperes/dm² in sulfuric acid of 150 g/liter at 60°C was measured, the electric potential was 1.65 volts to the standard hydrogen electrode. Then, when an electrolysis was carried out in an electrolyte composed of 150 g/liter of copper sulfate containing 1 ppm of lead and 100 g/liter of sulfuric acid using the foregoing electrode as the anode and titanium as the cathode at an electrolyte temperature of 60°C and a current density of 60 amperes/dm² while removing copper deposited on the cathode, the deposition of brown lead dioxide was observed on the surface of the anode. Also, with the progress of the electrolytic time, the formed amount of lead dioxide was increased. Furthermore, in the electrolysis in a sulfuric acid acidic electrolytic bath, the electrolysis could be carried out for 2,300 hours at a current density of 300 amperes/dm², whereby it was confirmed that the life thereof was sufficiently long, while the electric potential of the electrode was slightly higher. On the other hand, in the case of the electrode prepared by the same condition as above except that the molar ratio of iridium to tantalum was changed to 70 : 30, the single electrode potential of the electrode was 1.58 volts and when the electrolysis was carried out under the same condition as above, the formation of lead dioxide was not observed on the surface of the electrode. Example 2 By following the same procedure as Example 1 except that the oxygen impermeable oxides in Example 1 were further mixed with platinum such that the molar ratio of the sum of titanium and tantalum to platinum became 75 : 25 and the molar ratio of iridium to tantalum of the electrode active coated layer was changed to 30 : 70, an electrode (anode) was produced. The single electrode electric potential of the electrode measured as in Example 1 was 1.68 volts to the standard hydrogen electrode, which was higher than the electrode in Example 1. When the electrode was used as an anode for the electrolysis as in Example 1, the faculty of forming lead dioxide on the surface of the electrode was same as that in Example 1 and as the result of carrying out the electrolysis, the electrolysis could be continuously carried out for 2,540 hours at 300 amperes/dm², which showed the sufficient life of the electrode. Example 3 A titanium substrate was treated as in Example 1 and a powder of from 10 to 50 µm obtained by sintering a 10 : 1 mixture of titanium oxide and sponge titanium in an argon atmosphere for 3 hours at 1,350°C was sprayed onto the substrate by a plasma spray coating method to form thereon the coating of the oxygen impermeable oxide having a thickness of 50 µm. As the result of the X ray diffraction analysis, it was confirmed that the coating formed Magneli phase titanium oxide. On the surface thereof was formed an electrode active coating composed of the composite oxide of iridium and tantalum as in Example 1. As the composition thereof, the molar ratio of iridium to tantalum in the electrode active coating was 32 : 68. The single electrode electric potential (at 20 amperes/dm²) in sulfuric acid of 150 g/liter at 60°C was 1.69 volts to the standard hydrogen electrode and in a lead-containing solution, the brown coating of lead dioxide was formed on the surface of the anode and the formation of lead dioxide was increased with the progress of electrolysis. On the other hand, in the case of the electrode wherein the ratio of iridium and tantalum in the electrode active coating was 29 : 72, the single electrode electric potential was 1.73 volts to the standard hydrogen electrode under the same condition as above and thus the sufficient faculty of depositing lead dioxide on the surface of the anode was sufficiently confirmed, but when the electrolysis was carried out in a sulfuric acid bath as in Example 1 at a current density of 300 amperes/dm², the life of the anode was 1,300 hours, which was greatly lowered. As described above, in the present invention, since in the process of electrolytically producing a metallic foil using an insoluble electrode having an electrode active coating composed of a composite oxide of iridium and tantalum on an oxygen impermeable coating formed on a thin film-forming metal substrate as an anode, the anodic potential is controlled to be an electric potential for depositing lead contained in the electrolytic bath on the anode as insoluble lead dioxide by controlling the composition of the active coating of the electrode, and the metallic foil is electrolytically deposited on a cathode, there occurs no problem of shortening the life of the anode, the lead components contained in the electrolytic bath are deposited on the surface of the anode as lead dioxide without depositing in the metallic foil, whereby the metallic foil having an excellent quality can be electrolytically produced without using an additional means for removing lead. While the invention has been described in detailed with reference to specific embodiments, it will be apparent to one skilled in the art that various changes and modifications can be made to the invention without departing from its scope as defined in the claims.

Claims: A process of producing a metallic foil by electrolysis, which comprises using an insoluble electrode having an electrode active coating composed of a composite oxide of iridium and tantalum on an oxygen impermeable coating formed on a thin film-forming metal substrate as an anode and depositing impurities in the electrolytic bath on the anode at an anodic potential higher than the electric potential at which the impurities in the electrolytic bath form insoluble oxides on the anode. A process of producing a metallic foil by electrolysis as claimed in claim 1, wherein a copper foil is electrolytically produced from a sulfuric acid acidic bath of copper sulfate containing lead as an impurity. A process of producing a metallic foil by electrolysis as claimed in claim 1, wherein the electrode active coating has a composition that the molar ratio of iridium to tantalum is from 1 : 1 to 3 : 7. A process of producing a metallic foil by electrolysis as claimed in claim 1, wherein the oxygen impermeable coating of the anode has a semiconductive composite oxide of titanium and tantalum or further containing platinum.

Inventors: HIRAO KAZUHIRO, NAKAJIMA YASUO, SHIMAMUNE TAKAYUKI, HIRAO, KAZUHIRO, NAKAJIMA, YASUO, SHIMAMUNE, TAKAYUKI
Applicants: PERMELEC ELECTRODE LTD
Application year: 1993
Main classification: C25D1
IPC codes: C25D17, C25D1