Electrodic apparatus for the electrodeposition of non-ferrous metals

Abstract

This invention relates to electrodic apparatus suitable for the electrodeposition of nonferrous metals, for example for the electrolytic production of copper and other nonferrous metals from solutions of ions, comprising an electrode and at least one ionpermeable screen intended for protection of the said electrode.

Claims

1. An electrode apparatus for electrodeposition of non-ferrous metals, comprising: an electrode suitable for oxygen evolution; and at least one ion-permeable screen arranged parallel to said electrode, wherein said ion-permeable screen is a structure of non-electrically conductive material comprising a multiplicity of segregated electrically conductive segments intercalated and intimately connected with the non-electrically conductive material, the electrically conductive segments imparting unidirectional electrical conductivity upon the ion-permeable screen.

2. The electrode apparatus according to claim 1, wherein said structure is porous or foraminous.

3. The electrode apparatus according to claim 2, wherein said structure is a cloth or a non-woven cloth, optionally made of not conductive polymer material.

4. The electrode apparatus according to claim 1, wherein said electrically conductive segments comprise a material selected from the group consisting of valve metals, noble metals, iron, nickel, chromium and alloys and combinations thereof, conductive carbons and graphite.

5. The electrode apparatus according to claim 1, wherein said electrically conductive segments comprise at least one element selected from the group consisting of yarns, wires, strings, strips, bands, tapes and ribbons, applied to said structure.

6. The electrode apparatus according to claim 1, wherein said at least one screen is a cloth comprising: a warp of yarns of optionally polymeric non-conductive material; a weft comprising a first predefined number of optionally polymeric non-conductive yarns intercalated with a second predefined number of conductive yarns.

7. The electrode apparatus according to claim 6, wherein said yarns of conductive material have a diameter of 0.02-0.20 mm, said first and said second predefined number being independently selected in the range 1-20.

8. The electrode apparatus according to claim 6, wherein said yarns of conductive material are arranged parallel to each other or twisted either on themselves or around at least one yarn of non-conductive material.

9. The electrode apparatus according to claim 6, wherein said cloth has a unit weight of 50-600 g/m.sup.2.

10. The electrode apparatus according to claim 6, wherein said yarns amount to 8-200 yarns per centimeter.

11. The electrode apparatus according to claim 6, wherein said cloth is equipped with a selvage wholly or partially consisting of yarns of electrically conductive material.

12. The electrode apparatus according to claim 1, wherein at least one edge of said screen is covered by a composite insulating element, optionally comprising a cover ribbon and an insert of polyacrylic material, said insert being interposed between said screen and said cover ribbon.

13. The electrode apparatus according to claim 1, wherein said screen is subdivided into at least two portions electrically insulated from each other.

14. The electrode apparatus according to claim 1, further comprising a foraminous separator of electrically insulating material interposed between said electrode and said at least one screen.

15. An electrolyser for electrowinning of non-ferrous metals comprising a multiplicity of interleaved anodes and cathodes, wherein at least one of said anodes is the electrode apparatus according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows an image of the ion-permeable screen according to one embodiment of the invention (7 magnification) obtained using a scanning electron microscope (SEM).

(2) FIG. 2 is an image of the ion-permeable screen in FIG. 1, with 35 magnification, acquired using a field emission scanning electron microscope (SEM-FEG).

DETAILED DESCRIPTION OF THE DRAWINGS

(3) FIG. 1 shows a SEM image of a textile ion-permeable screen according to one embodiment of the invention, in which the textile is manufactured using a warp comprising polyester fibre. The weft comprises the intercalation of 4 polypropylene wefts with one weft of AISI 316 stainless steel comprising a set of 8 stainless steel wires of 0.035 mm onto which a wire of 0.035 mm AISI 316 stainless steel is twisted. The image of the sample was acquired using a scanning electron microscope with an Everhart-Thornley detection system, 7 magnification (working distance 61.5 mm, accelerating voltage 500.0 V).

(4) FIG. 2 shows a SEM-FEG image of the textile ion-permeable screen in FIG. 1 with 35 magnification (working distance 25.0 mm, accelerating voltage 1.0 kV, Everhart-Thornley detection system). The polyester warp fibres (100) and the polypropylene fibres (110) intercalated with the assembly of twisted stainless steel wires (120) constituting the weft can be seen in the xy plane. The wires (120) comprise the electrically conducting segments of the screen according to the invention. This imparts upon the latter a macroscopic electrical conductivity which is substantially limited to the x direction, and therefore characterised by a specific unidirectionality in the plane of the screen.

(5) The examples below are included to demonstrate particular embodiments of the invention, the implementability of which has been abundantly checked throughout the range of values claimed. Those skilled in the art will appreciate that the compositions and techniques described in Example 1 represent compositions and techniques which the inventors have found to work well in practical embodiments of the invention; however, in the light of this description those skilled in the art must appreciate that many changes may be made to the specific embodiments disclosed while still obtaining a similar or analogous result without going beyond the scope of the invention.

EXAMPLES

(6) In the examples and comparison examples described below laboratory tests have been performed in an experimental cell for the electrolytic extraction of copper having an overall transverse cross-section of 170 mm170 mm and a height of 1500 mm, containing two cathodes separated by an anode. The cathodes and the anode were located parallel to each other and faced each other vertically at a distance of 40 mm apart between the outer surfaces. A sheet of 3 mm thick, 120 mm wide and 1000 mm high AISI 316 stainless steel was used for the cathodes; the anode comprised a stretched grid of titanium of thickness 1 mm, width 120 mm and height 1000 mm, activated with a coating of mixed iridium and tantalum oxides.

(7) The cell was provided with a programmable logic control system governing the process parameters (temperature, throughput, voltage and electrical current), with excess temperature and excess current alarms. The cell was also provided with a data acquisition and recording system for the analysis of process parameters measured over time.

(8) The cell operated using an electrolyte containing approximately 61 g/l of copper as Cu.sub.2SO.sub.4 and 210 g/l of H.sub.2SO.sub.4 and was fed with a constant potential difference of 1,800 V corresponding to an expected current density of approximately 455 A/m.sup.2 (110 A). Oxygen was evolved at the anode and copper was deposited at the cathode.

(9) A dendrite was artificially produced by inserting a screw, as a centre for nucleation, into one of the two cathodes, perpendicularly thereto and in the direction of the anode. The tip of the screw was positioned 10 mm from the anode.

Example 1

(10) A textile ion-permeable screen according to an embodiment of the invention comprising a warp of polyether sulfone (PES) fibres and a weft comprising a sequence of 4 PES fibres intercalated with 8 AISI 316 stainless steel wires of diameter 0.05 mm was placed in the cell described above at a distance of 5 mm from each surface of the anode and parallel thereto. The conducting elements were assembled by twisting one of the steel wires over the remaining 7 wires arranged in parallel to each other. The fabric was characterised by a yarn per cm number of 20 and a unit weight of 220 g/m.sup.2.

(11) A polyethylene separator 4 mm thick, provided with square holes of size 1.5 cm orientated at 45 with respect to the vertical, was placed between the screen and the anode.

(12) The cell was operated under the electrolysis conditions described above and in the course of operation it was possible to establish, by observing the growth of gas bubbles, that the anode reaction was taking place selectively on the anode surface and not on the screen in front of it.

(13) It was also observed that the dendrite growing in the direction of the anode came into contact with the screen after approximately 6 hours. After 21 hours from this primary contact the data acquisition system recorded a current peak of 250 A lasting a few seconds, indicating a secondary short circuit caused by contact between a secondary dendrite and the anode. A further peak of 500 A lasting a few seconds was observed after 10 minutes, followed by an alternation of current peaks of between 170 and 190 A during the subsequent 10 minutes. This behaviour of the current was repeated for the subsequent 40 minutes, as recorded by the data acquisition system.

(14) At the end of the test the cathodes were removed from the experimental cell and the primary dendrite was detached from the protective screen without damaging it.

(15) The experimental cell was then dismantled and from a visual inspection it could be observed that: 1) the screen was structurally intact, 2) diffusion of copper onto the screen was confined to a small set of adjacent metal wires. Globular growth of copper of limited size, with the exception of two secondary dendritic points of diameter 2 and 3 mm respectively which touched the anode at 2 points, were also observed on the anode side of the screen, corresponding to conductive wires in contact with the primary dendrite (and those immediately adjacent thereto). At the contact points the anode showed extremely localised damage (less than 1 and 1.5 cm.sup.2) which was not prejudicial to its subsequent functioning.

(16) On completion of the visual inspection the cathodes were reinserted in their seats and the cell was again placed in operation for a period of 4 hours. During this period of time it was observed that copper dissolved from the protective screen primarily on the side facing the cathode. The copper deposited on the screen in the direction of the anode partly dissolved. The residual copper became detached, and deposited on the base of the cell in fragments of small size.

Comparative Example 1

(17) In the cell described above, a textile ion-permeable screen made using a warp and a weft of polyester fibre was positioned in the cell described above at a distance of 5 mm from each surface of the anode and parallel thereto. The fabric was characterised by a number of yarns per cm of 18 and a unit weight of 150 g/m.sup.2.

(18) A polyethylene separator of 4 mm thickness provided with square openings of size 1.5 cm orientated at 45 with respect to the vertical was placed between the screen and the anode.

(19) The cell was placed in operation under the electrolysis conditions described above and during operation it was possible to verify by observing the growth of gas bubbles that the anode reaction was taking place selectively at the surface of the anode and not on the screen in front of it.

(20) It was also observed that the dendrite grew in the direction of the anode and came into contact with the screen after approximately 6 hours. After approximately one hour the data acquisition system recorded a current peak of over 500 A, which was repeated at intervals of a few seconds for the next 10 minutes.

(21) At the end of the test the cathodes were removed from the experimental cell and the primary dendrite was detached from the protective screen without damaging it.

(22) The experimental cell was then dismantled and from a visual inspection it was possible to observe that 1) the screen was structurally intact, 2) diffusion of copper on the screen was limited to a small area corresponding to the contact, 3) only one secondary dendritic formation of diameter approximately 10 mm had grown at the point of contact between the primary dendrite and the screen and had reached the anode causing extensive damage to it. The damage to the anode surface affected an area of approximately 4 cm6 cm, prejudicing subsequent use of the electrode.

Comparative Example 2

(23) A screen comprising a grid of titanium comprising wires of 1 mm diameter was positioned in the cell described above at a distance of 5 mm from each surface of the anode and parallel thereto.

(24) A polyethylene separator of 4 mm thickness provided with square openings of side 1.5 cm orientated at 45 with respect to the vertical was placed between the screen and the anode.

(25) The cell was placed in operation under the electrolysis conditions described above and during operation it was possible to verify by observing the growth of gas bubbles that the anodic reaction was taking place selectively on the surface of the anode and not on the screen in front of it.

(26) It was also observed that the dendrite grew in the direction of the anode and came into contact with the screen after approximately 6 hours. 10 hours after this primary contact the data acquisition system recorded a current peak of 300 A, followed by a peak of 500 A which was recorded at intervals a few seconds apart for the next 5 minutes.

(27) At the end of the test the cathodes were removed from the experimental cell and the primary dendrite was removed from the protective screen without damaging it.

(28) The experimental cell was then dismantled and from a visual inspection it was possible to observe that 1) the screen was structurally intact and completely covered with copper, on both the cathode side and the anode side. The growth of a dendritic point of mean diameter 12 mm which touched the anode at 1 point was also observed on the anode side of the screen, at the contact with the primary dendrite. At the point of contact the anode suffered damage of area 6 cm8 cm which prejudiced its subsequent functioning.

(29) At the end of the visual inspection the cathodes were reinserted into their seats and the cell was again placed in operation for a period of 4 hours, after the damaged anode had been replaced. During this period of time it was observed that copper dissolved from the protective screen first on the side facing the cathode. The copper deposited on the screen in the direction of the anode partly dissolved and partly detached in fragments of different size, some of more than 1 cm.sup.2. Some fragments remained embedded between the screen and the anode, creating a direct electrical contact between them and compromising the protective function of the screen in the event of subsequent contact with dendritic formations originating from the cathode.

(30) The above description is not intended to limit the invention, which may be used in various embodiments without thereby departing from its objects and its scope is unequivocally defined by the appended claims.

(31) In the description and claims of this application the word comprises and its variations such as comprising and comprise do not rule out the presence of other additional elements, components or process stages.

(32) The discussion of documents, actions, materials, apparatus, articles and the like is included in the text solely for the purpose of providing a context for this invention; it should not however be understood that this material or part thereof constitutes general knowledge in the field relating to the invention prior to the priority date of each of the claims appended to this application.