Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/04—Diaphragms; Spacing elements
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/02—Electrodes; Connections thereof

Definitions

• the deposition of metals such as copper, nickel, cobalt, zinc is commonly carried out in electrometallurgical plants making use of cells consisting of tanks which contain a multiplicity of intercalated planar anodes and cathodes.
• the cathodes are periodically extracted from the cell using suitable winches and replaced with new lots of thin sheets whereon the metal deposition is continued.
• Such equipments are suitable for being employed in the high capacity plants typical of the so-called primary electrometallurgy wherein metals are deposited starting from concentrated solutions obtained by an attack of ores, but are clearly not compatible with processes of secondary deposition of metals, in particular with processes of deposition from medium-low concentrated solutions such as the case of wastes of the same electrometallurgical plants, or of galvanic plants for jewellery or printed circuit manufacturing.
• the elastic sleeve is shaped as an open cylinder with a clearance (19) between the two facing edges allowing the forced insertion into the cathode body during the assemblage of the cell (1 ): in this situation the two edges practically match, thereby eliminating or strong reducing the clearance (19). Since the elastic sleeve is just put in place with no securing points, its extraction, when a predetermined thickness of the metal deposit is achieved, is very simple: it is in fact sufficient to release the nut (14) and the bolts (10), remove the closing flange (9) and extract the sleeve carrying the metal deposit. To restart the cell, the original sleeve cleaned from the metal or a new elastic sleeve must be forced again into the cathode body, the closing flange repositioned and the nut (14) and bolts (10) locked.
• AU/2003204240 discloses countersunk inserts to be installed in the upper and lower extremities of the cell of EP 845054, corresponding to element (18) in figure 1a: in this way the terminal part of the cathode is confined in a recess that is hardly accessible to the current lines and to the solution flow, both conditions hindering the growth of dendrites or similar formations.
• the device is nevertheless characterised by an efficiency progressively lessening in time due to the gas trapped in the gap between anode and insert countersink, with the generation of a new crowding of current lines and solution flow in correspondence of the liquid level, associated to a new onset of nucleation and dendrite growth phenomena.
• a further drawback of the cell of EP 845054 derives from the cost of the rod, preferably made of titanium, used for the anode construction when the cell size reaches the typical values of practical applications, for instance a height of 1200 mm, a cathode body diameter of 150 mm, an anode diameter of 50 mm.
• the rod preferably made of titanium
• the cell size reaches the typical values of practical applications, for instance a height of 1200 mm, a cathode body diameter of 150 mm, an anode diameter of 50 mm.
• thin-walled tubes typically 0.5 - 1.0 mm: such tubes however present the additional problem of current distribution which, due to the non negligible electrical resistance of titanium, turns out to be concentrated in the proximity of the threaded extremities with a substantial worsening of the phenomenon of dendrite formation.
• US 5,584,975 proposes forcing a rod of conductive material, of higher length than that of the tube, into the titanium tube: the rod mechanically in contact with the inner wall of the tube acts as a current distributor while the two extremities coming out of the tube are connected to the positive pole of the rectifier.
• the rod is preferably made of copper in view of its high electrical conductivity.
• the contact may even be scarcely effective since the beginning of the operation when the tube employed as the anode is provided with a coating for oxygen evolution by means of the known processes of thermal decomposition of catalytic material precursors: during this treatment, a thick and scarcely conductive oxide is formed on the inner surface of the titanium tube, which is very difficult to remove given the poor accessibility and which can strongly hamper the electrical conduction.
• the present invention is directed to overcome the above discussed prior art drawbacks by introducing modifications in the cylindrical cell structure of EP 845054 so as to make it economically viable and free of the short-circuiting issues originated both from the growth of dendrites or similar formations and from the deformations affecting the elastic sleeve and the relevant metal deposit.
• the invention consists of a cylindrical cell for metal deposition comprising an outer cathode body and a central coaxial anode delimiting an interelectrodic gap wherein an also cylindrical net is inserted, in its turn maintained in a coaxial position by the closing flanges.
• the cylindrical net is made of a metal selected from the group of titanium, titanium alloys and stainless steels, optionally coated with an electrically insulating material; in one alternative embodiment of the invention, the cylindrical net is made of an electrically insulating material.
• the electrically insulating material may advantageously be a plastic material selected from the group of polyethylene, polypropylene, polyvinylchloride, polytetrafluoroethylene and copolymers deriving therefrom, polyvinylidenfluoride.
• the anode preferably consists of a thin-walled titanium tube provided with a catalytic coating and of an internal rod also of titanium secured to the tube wall by means of a weld, preferably a linear roll electric resistance weld, or an arc-weld or a laser-type one; by thin wall it is hereby intended a wall of thickness comprised between 0.5 and 3 mm, preferably not higher than 1 mm.
• the two terminal surfaces of the thin-walled cylinder are preferably rectified.
• the internal rod has a length lower than that of the thin- walled tube and is provided with connecting means to the rectifier.
• the anodic catalytic coating is preferably a film suitable for oxygen evolution, for instance a film of noble (platinum group metals) and non-noble (titanium, tantalum, niobium, zirconium) transition metal oxides.
• the catalytic coating may be of new application or it may consist of a recoating film applied to an exhausted anode.
• figure 1a partial section of an electrolysis cell in accordance with the prior art, as previously described.
• figure 1 b section of the coaxial central anode of the cell of figure 1a, as previously described.
• figure 1c top view of the cell of figure 1a, as previously described.
• FIG. 2 front view of the cathodic elastic sleeve before the forced introduction into the cathode of cylindrical symmetry, as previously described.
• FIG. 3a partial section of the electrolysis cell in accordance with the invention.
• figure 3c top view of the anode of figure 3b.
• the invention consists of a substantial modification of the cylindrical cell of the prior art by installation of two novel components, consisting of a protecting net preventing the short-circuits induced by deformation of the elastic sleeve-metal deposit assembly and by an anode consisting of a thin-walled titanium tube comprising an internal rod also of titanium for current distribution.
• the protecting net (19) shaped as a cylinder, is housed in the gap existing between anode and cathodic sleeve: through a suitable dimensional selection of the net, it is possible to perturb the spiral flow induced by the tangential orientation of the electrolyte inlet and outlet nozzles just to a marginal extent.
• the net is integrally manufactured out of an electrically insulating material, optionally selected from the group of chemically resistant polymer materials such as polyethylene, polypropylene, polyvinylchloride, polytetrafluoroethylene and perfluorinated polymers or copolymers deriving therefrom, polychlorotrifluoroethylene and copolymers thereof, polyvinylidenfluoride.
• the net may be made out of wire with a diameter of 1-2 mm intertwined in square meshes with a 5-15 mm side.
• the surface void ratio (defined as ratio of void area to area filled by the material) must be at least 1 and preferably at least 2 in order not to perturb the spiral flow to a significant extent.
• the net When the net is entirely manufactured out of a polymer material, it may be forcibly put on the anode rather than housed in the interelectrodic gap between cathodic sleeve and anode.
• the net may be manufactured out of a metal, selected from the group of titanium, titanium alloys and stainless steels: the preferred metals are titanium or titanium alloys since they are easy to coat with a chemically inert non conductive oxide.
• the entirely metallic net allows obtaining a very good mechanical barrier capable of effectively counteracting the deformations which tend to displace the elastic sleeve and the relevant metal deposit towards the anode.
• the net is made of metal, besides the constructive solution based on intertwined wires it is possible to resort to expanded sheets, preferably flattened, again with the above disclosed surface void ratio required to prevent an excessive perturbation of the solution spiral upward flow.
• the net is made of metal and is provided with a coating of electrically insulating material, optionally selected from the group of chemically resistant polymer materials such as polyethylene, polypropylene, polyvinylchloride, polytetrafluoroethylene and perfluorinated polymers or copolymers deriving therefrom, polychlorotrifluoroethylene and copolymers thereof, polyvinylidenfluoride.
• the coating may be obtained by techniques known in the art, for instance by electrostatic powder application followed by sintering: a thickness around 0.1 -0.3 mm is for instance appropriate.
• a particularly preferred combination consists of a titanium net coated with a perfluorinated polymer.
• the second innovative element of the invention is given by the anode (4) whose internal structure is sketched in figure 3b: the anode consists of a thin- walled titanium tube (22) enclosing a current distributing rod (23) also of titanium provided with a lower connection (24) and an upper connection (25), both with threaded terminal sections, (20) and (21 ) respectively, which in co-operation with nuts (12) and (14) allow securing the anode in the cell with simultaneous compression of the upper gasket (15) and of the lower gasket (not shown).
• the two terminal surfaces of the tube are preferably rectified.
• titanium as construction material also for the internal rod is of particular interest because the so-obtained anodic structure may be subjected to the required thermal treatments for the application of the catalytic film, typically carried out in the temperature range comprised between 400 and 600 0 C, with no harmful distortions taking place. It is known in fact that such distortions occur when the anodic structures subjected to thermal treatments comprise distinct construction materials characterised by different thermal expansion coefficients. It has been found that the use of titanium, particularly advantageous under an economic standpoint with respect to bimetals and generally disregarded by the prior art due to its remarkable electrical resistivity, is on the contrary viable when internal rods of adequate thickness (for instance of 10 to 20 mm depending on the anode diameter and length and on the operative current density) are used.
• Figure 3c shows a section of a top-view of the anode of figure 3b, wherein the two longitudinal welds (26) securing the internal rod (23) to the tube wall (22) are evidenced: it was found that such welds, of the linear type, can be easily made by roll electric resistance, electric arc or laser technique, the latter being preferred for speed of execution.
• the thin-walled tube besides being cheap, also presents the advantage of an easy alignment, through the use of a suitable template, of the internal rod with the internal surface of the tube, which is a necessary condition for obtaining flawless welds.
• the preferred thickness is of 0.5 to 1 mm.
• the internal rod has a length at least equal to that of the tube: however in the course of metal deposition it was noticed that this design, although perfectly suitable under a purely electrical standpoint to achieve a homogeneous longitudinal current distribution, allows the formation of dendrites whose point of nucleation is generally located in correspondence of the upper edge of the cathodic elastic sleeve: the real current distribution for the metal deposition in fact not only depends on the cathode and anode electrical features but also on certain features of the current lines in the electrolytic solution.
• the anode provided with an internal rod of at least equal length allows accomplishing one of the scopes of the invention, namely the manufacturing of cheap anodes, while leaving the problem of anomalous metal growth in single spots of the cell at least partially unsolved.
• the metal deposition on the cathodic elastic sleeve conversely turns out to be effectively uniform, with complete absence of dendrites or similar local formations, when the internal rod of the anode of the invention has an inferior length than the tube: it is believed that this positive result is due to the ohmic drop that must be overcome for distributing the current along the portion of the tube not contacting the rod.
• a suitable selection of both the length of the portion of tube not contacting the rod and the thickness of the wall determines an ohmic drop level capable of sensibly lessening the current density in the two peripheral lower and upper sections of the cell, effectively counteracting the anomalous growth of metal in correspondence of the relevant edges.
• One of the anodes used in the tests in particular the one with the internal rod of length lower than the tube, whose portions not contacting the rod had an extension of 300 mm, was first sandblasted on the external surface until completely removing the catalytic coating, then subjected to a new application of the same coating, with the purpose of simulating the reactivation cycle periodically applied to the oxygen- evolving anodes which are subject, as known to those skilled in the art, to a progressive deactivation during functioning.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)

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Cited By (7)

Citations (3)

• 2005
  • 2005-12-20 IT ITMI20052420 patent/ITMI20052420A1/it unknown
• 2006
  • 2006-11-29 TW TW095144069A patent/TW200724721A/zh unknown
  • 2006-12-20 WO PCT/EP2006/069982 patent/WO2007071713A1/en active Application Filing
  • 2006-12-21 AR ARP060105695A patent/AR058717A1/es not_active Application Discontinuation

Patent Citations (3)

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