An electrolytic cell and method for removing silver from silver-containing aqueous liquids

Abstract: The cell comprises a housing (10), an anode (20) positioned within the housing (10), a cathode (30) surrounding the anode (20) in the housing (10), an inlet (18), and an outlet (19). The outlet (19) comprises a passage (21) through the anode (20). The cell is operated under negative pressure. This construction enables a simple manufacturing of the cathode, without the need for holes therein. The liquid may be easily de-aerated leading to more uniform deposition of silver on the cathode.

Description: FIELD OF THE INVENTION This invention relates to an apparatus for the electrolytic recovery of silver from solutions containing silver, in particular used photographic solutions such as fixing and bleach-fixing solutions. BACKGROUND OF INVENTION Electrolytic silver recovery from used photographic solutions is a common way to extend the life of such solutions. An apparatus for the electrolytic recovery of silver from solutions containing silver is known from United States patent US 5 378 340 (Michiels et al. assigned to Agfa-Gevaert NV) issued 3 January 1995. The apparatus comprises an electrolytic cell including: a housing; an anode having an exposed anode portion within the housing; and a cathode having an exposed cathode portion located within the housing and encircling the anode. In use silver from the silver containing solution is deposited on the face of the cathode which is directed towards the anode. After the cell is operated for some time, the cathode is removed from the cell and replaced. In a known method of removing silver from silver-containing aqueous liquids, the liquid to be treated is pumped into the electrolytic cell and electrical power is fed to the anode and the cathode to cause silver to be deposited on the cathode. The cathode is usually removable, and after a certain amount of silver has built up thereon, the cathode is removed and replaced. US-A- 4 439 300 (inv. K. R. Houseman) discloses a vortex-type electrochemical cell for recovering silver (Ag) from an Ag-containing solution comprising a cathode and a tubular anode located within a housing, said anode extending from the base of the housing, and an outlet comprising a passage concentric with the anode. FR-A2 270 345 (inv. J. R. Fessenden) is directed to a process for the recovery of e.g. Ag from an aqueous solution in an electrochemical cell operated under a negative pressure. In the cell described in US 5 378 340, the electrical connection to the cathode is below the liquid level in the cell, so that deposits may form on this connection. This leads to unpredictability in the electrical energy fed to the cell, making control of the process difficult. Furthermore, a reference electrode is placed in a side arm of the housing and in order for this electrode to accurately reflect the condition of the bulk of the liquid in the cell, it was necessary to form the cathode with holes there-through. Such holes also contribute to good circulation of the electrolyte through the cell. Not only does the formation of these holes constitute an additional manufacturing step, the holes result in uneven deposition of silver on the cathode. Also, the presence of gas bubbles in the liquid, which bubbles may be seeded for example at the surface of the cathode, may cause non-uniform deposition of silver to occur. OBJECTS OF INVENTION It is an object of the present invention to overcome the aforesaid disadvantages. SUMMARY OF THE INVENTION We have discovered that this objective and other useful advantages may be achieved when the outlet comprises a passage through the anode and the cell is operated under negative pressure. Thus, according to a first aspect of the invention, there is provided an electrolytic cell for removing silver from silver-containing aqueous liquids, comprising a housing, a base, an anode positioned within said housing, a cathode surrounding said anode in said housing, an inlet opening, and an outlet opening through said base, characterised in that said outlet opening leads to an outlet passage through said anode, wherein said outlet opening is connected to a pump, enabling the cell to be filled, de-aerated and operated under negative pressure, and wherein the lower edge of the cathode is positioned above the base of the housing to leave a space therebetween defining a sump, from which a side arm of the housing extends, a reference electrode being positioned in said side arm and projecting into said sump. In a preferred embodiment, the housing includes a base and the anode comprises a tube extending from the base. The tube may surround and be concentric with the outlet passage. The hollow interior of the tube may constitute a circulation passage, of annular cross-section, which surrounds the outlet passage. The outlet passage opens from the interior of the cell at a level above the level at which the circulation passage opens into the cell, thereby to define a liquid level in the cell. The cathode is preferably removable from the cell and comprises an electrical connection which is positioned above the liquid level. In order to enable the cathode to be removed, a removable lid may be provided which, when secured to the housing, serves to hermetically seal the cell. Alternatively, the lid may be integral with the cathode. Preferably, the lower edge of the cathode is positioned above the base of the housing to leave a space therebetween defining a sump. The cell includes a circulation pump connected between the circulation passage and the interior of the housing to circulate liquid being treated through the cell. It is particularly beneficial if this circulation pump injects recirculating liquid tangentially into the sump of the housing, since this arrangement results in efficient mixing of the liquid. According to a second aspect of the invention, there is provided a method of removing silver from silver-containing aqueous liquids in an hermetically sealed electrolytic cell, comprising the steps of filling the cell with a liquid through an inlet opening which opens into a chamber of the cell between an anode and a cathode, maintaining a reference electrode in the liquid, measuring a voltage potential between the cathode and the reference electrode, and providing a current between the cathode and the anode in response to said voltage potential, circulating the liquid through the chamber by applying negative pressure to an outlet passage which extends through the anode, thereby de-aerating the liquid in the cell by generating a vortex above the outlet passage. A volumetric pump is connected to the outlet of the cell. Where the cell is hermetically sealed, operation of the volumetric pump can be used to fill the cell with liquid through the inlet, by creating a negative pressure in the cell. The use of this arrangement enables the cell to work under negative pressure and also ensures that the liquid in the cell is de-aerated. This leads to more uniform deposition of silver at the cathode. It is desirable to stop the circulation pump when too much air passes through the outlet. To achieve this, an optical sensor, capable of distinguishing between fluid and air in the outlet, may be positioned between the cell and the volumetric pump, but above the latter. In this way deaeration of the cell can be achieved very quickly. Due to the action of the centrifugal pump a vortex is formed above the outlet. The air in the vortex is sucked in by the volumetric pump. When too much air is sensed in the outlet, the circulation pump is caused to stop, while the volumetric pump continues to operate. When the circulation pump stops, the vortex remains for about one second, circulation pump stops, the vortex remains for about one second, allowing even more air to leave the cell. Once the optical sensor detects fluid, the centrifugal pump starts again, but with less air in the cell. After a few such deaeration cycles, only a small air bubble is left. This bubble is too small to create a vortex and does not therefore enter the pumps. The method according to the invention further includes filling the cell with liquid to be treated therein through an inlet passage which opens into the cell between the anode and the cathode. For optimum performance of the cell, it is important that the potential between the cathode and the reference electrode is accurately controlled. Usually the electrolytic cell further comprises a reference electrode for this purpose. The reference electrode is positioned in a side arm of the housing, projecting into the sump. Where, for example, an Ag/AgCl reference electrode is used, the potential between the cathode and the reference electrode is about 400 mV. When the unit is to perform optimally, meaning employing the maximum current without causing side reactions to occur, the potential should be measured with an accuracy of some millivolts. The reference electrode may be a calomel type electrode or an Ag/AgCl type electrode. A suitable electrode has been disclosed in application EP 0 598 144 (Agfa Gevaert NV) filed 11 November 1992 entitled "pH Sensitive Reference Electrode in Electrolytic Desilvering". In a preferred embodiment of the invention, the top of the exposed anode portion lies below the top of the exposed cathode portion. This is easily achieved where the anode is supported within the housing from the base thereof. Thus, the housing is preferably formed of electrically non-conductive material, and comprises a base wall and side walls, the anode being supported by the base wall and the cathode being positioned adjacent the side walls. The housing may be of any suitable shape, but it is preferred to be generally cylindrical, the anode being in the form of a tube positioned axially within the housing. In any case, the anode is encircled by the cathode. The cathode is preferably in sheet form and ideally has a frusto-conical cross-section, with its larger radius end uppermost, that is towards the circular upper opening of the electrolyte cell. This configuration enables easy removal of the cathode even after a silver deposit has built up there-on after use. Usable cathode materials include stainless steel, silver and silver alloys, and other conductive materials, the non-silver containing materials being preferred from the point of view of costs, while the silver containing materials cause fewer starting-up problems. A cylindrical shape to the housing enables the cathode to be positioned near to the wall of the cell. By arranging for the lower edge of the cathode to be spaced from the base of the housing, it is possible for the reference electrode to be located in a side arm of the housing, the side arm opening into the housing below the level of the cathode. The material used for the anode is less critical, although platinated titanium is usually used. The "solutions containing silver" which can be desilvered using the apparatus according to the present invention include any solution containing silver complexing agents, e.g. thiosulphate or thiocyanate, sulphite ions as an anti-oxidant and free and complexed silver as a result of the fixing process. The apparatus can also be used with concentrated or diluted used fixing solutions, or solutions containing carried-over developer or rinsing water. Apart from the essential ingredients, such solutions will often also contain wetting agents, buffering agents, sequestering agents and pH adjusting agents. The apparatus of the present invention can also be used for desilvering bleach-fixing solutions which may additionally contain bleaching agents such as complexes of iron(III) and polyaminocarboxylic acids. The desilvering process can be carried out batch-wise or continuously, the apparatus being connected to the fixing solution forming part of a continuous processing sequence. DETAILED DESCRIPTION OF THE INVENTION The invention will be described by the following illustrative embodiments with reference to the accompanying drawings without the intention to limit the invention thereto, and in which: Figure 1 shows a cross section of an electrolytic cell according to the invention; Figure 2 shows schematically the liquid and electrical connections to the cell. As shown in the Figures, the apparatus comprises a generally cylindrical bucket-shaped electrolytic cell housing 10, formed of electrically non-conductive material such as PVC, and comprising a base 15, sides 16 and an upper portion 17. The upper diameter of the housing 10 is marginally larger than the lower diameter by a factor of 1.05. Positioned within the cell are a tubular anode 20 and a cylindrical cathode 30. A liquid inlet 18 leads through the base 15 of the cell and opens into the cell between the anode tube 20 and the cathode 30. An outlet 19 opens from the base 15 of the cell and leads to a relatively narrow PVC tube defining an outlet passage 21. An annular circulation passage 23 is thereby defined, which surrounds the outlet passage 21 and is concentric therewith. The outlet passage 21 opens from the interior of the cell at a level 25 above the level 26 at which the circulation passage 23 opens into the cell, thereby to define a liquid level in the cell. An annular PVC cap 37 sits on top of the anode tube 20 and includes a U-shaped cross-section channel 38 opening downwards at one end into the circulation passage 23 and at the other end into the interior of the cell. The cathode 30, formed for example of stainless steel covered with a thin layer of silver, is located in the cell 10 with its faces spaced from the sides 16. The lower edge 12 of the cathode is spaced above the base of the housing so as to leave a sump 13 from which a side arm 24 of the housing leads. The anode 20, in the form of a platinised titanium tube, is secured to the base 15 of the cell by means of a contact piece (not shown in detail) integral with the housing of the cell, which contact piece acts as an electrical connector for the anode. The anode tube 20 lies along the axis of the housing 10. A centrifugal circulation pump 50, together with an associated pump motor 52, is connected to the base of the cell and serves to circulate the liquid in the cell by removing liquid from the circulation passage 23 and injecting it tangentially into the sump 13 of the housing 10, as indicated by the arrows in Figure 1. The reference electrode 45 is positioned in the side arm 24 of the housing and protrudes into the sump 13 of the cell. A suitable reference electrode is a pH sensitive glass electrode such as a YOKOGAWA SM21/AG2 or an INGOLD HA265-58/120 glass electrode. The upper part 17 of the cell is in the form of a neck portion having an opening defined by a stainless steel ring 22. The stainless steel ring 22 is permanently fixed to one end of a bolt 31 which extends through the wall of the cell and provides a connector for the cathode 30. Positioned in the neck of the cell, below the level of the annular ring 22, is a sealing ring 14. The apparatus further comprises a lid 40 so shaped as to fit into the neck portion of the cell. The lid 40 is formed of electrically non-conductive material such as PVC. The cathode 30, formed for example of stainless steel sheet having a thickness of 100 µm, is wrapped around into a cylindrical configuration. The cathode 30 is provided with a deformable upper edge portion, formed by the provision of slots (not shown), the sheet material of which the cathode is formed being sufficiently resilient to allow the upper edge portion to bend outwardly in response to outwardly directed force. As the lid is screwed into place, a contact surface on the lid bears against the upper edge portion of the cathode 30, causing the upper edge portions to bend outwardly against the annular surface of the ring 22. Tightening of the lid causes the upper edge portion to be clamped firmly by the lid against the ring 22, thereby establishing good electrical contact there-between. In the closed position of the lid, the sealing ring 14 bears against the lower edge of the lid 40, thereby forming a tight seal. The liquid and electrical connections to the cell are shown schematically in Figure 2. Fixer or other silver-containing liquid enters along an inlet line 27 having an internal diameter of say 10 mm. When the cell is initially empty, but the lid 40 is attached hermetically sealing the cell, operation of a volumetric pump 29 extracts air from the cell and pulls liquid from the inlet line 27 into the cell through the inlet 18. Treated liquid from the cell is pumped by the pump 29 along an exit line 32, of say 10 mm diameter at say 1 litre/min. An optical level sensor 39 is provided in a cavity adjacent the exit line 32 at a position above the level of the volumetric pump 29. This sensor stops the circulation pump 50 each time too much air passes through the cavity. The volumetric pump 29 continues to operate however. By this arrangement de-aeration of the cell proceeds quickly. Due to the action of the circulation pump 50 a vortex is formed above the outlet passage 21. The air of the vortex is sucked in by the volumetric pump 29. This air is sensed by the sensor 39 which causes the circulation pump 50 to stop. The vortex remains for about one second, allowing even more air to leave the cell. Once the sensor 39 detects liquid, the circulation pump 50 is caused to re-start. Further pumping not only continues to fill the cell, but also de-aerates the liquid in the cell. After 2 to 4 de-aeration cycles, in a span of less than a minute, only a small air bubble is left above the outlet passage 21. This bubble is too small to create a vortex and no further air enters the outlet passage 21. The liquid is circulated through the cell by the circulation pump 50 at say 20 litres/min. The cell is then operated under usual conditions, during which a silver deposit builds up on the cathode 30, primarily on the inside surface thereof. Electronic circuitry 36 controls the de-silvering process in a known manner. After a period of time determined by the required amount of deposited silver, the operator unscrews the lid 40 and lifts the cathode 30 out of the cell. Due to the frusto-conical cross-section of the housing 10, the sides of the cathode will not foul against the ring 22, even when some small amount of silver deposit has built up on the outside surface thereof. The silver deposit is then removed from the cathode, which may then be re-used as desired or replaced by another cathode of similar construction for the de-silvering of a further batch of electrolyte. The cell may be drained via a drain valve 34 and drain line 35. Reference Number List housing 10 lower edge 12 sump 13 sealing ring 14 base 15 sides 16 upper portion 17 outlet 18 inlet 19 anode tube 20 outlet passage 21 ring 22 circulation passage 23 side arm 24 liquid level 25 circulation level 26 inlet line 27 vol pump 29 cathode 30 bolt 31 outlet line 32 drain valve 34 drain line 35 controller 36 cap 37 U-channel 38 sensor 39 lid 40 reference electrode 45 circ pump 50 pump motor 52

Claims: An electrolytic cell for removing silver from silver-containing aqueous liquids, comprising a housing (10), a base (15), an anode (20) positioned within said housing (10), a cathode (30) surrounding said anode (20) in said housing (10), an inlet opening (18), and an outlet opening (19) through said base, characterised in that said outlet opening (18) leads to an outlet passage (21) through said anode, wherein said outlet opening is connected to a pump (50), enabling the cell to be filled, de-aerated and operated under negative pressure, and wherein the lower edge (12) of the cathode (30) is positioned above the base (15) of the housing (10) to leave a space therebetween defining a sump (13), from which a side arm (24) of the housing extends, a reference electrode (45) being positioned in said side arm and projecting into said sump. An electrolytic cell according to claim 1, wherein a circulation passage (23) surrounds said outlet passage (21). An electrolytic cell according to claim 2, wherein said circulation pump (50) connected between said circulation passage (23) and the interior of said housing (10). An electrolytic cell according to any preceding claim, wherein said outlet passage (21) is concentric with said anode (20). An electrolytic cell according to claim 2, wherein said outlet passage (21) opens from the interior of said cell at a level (25) above the level (26) at which said circulation passage (23) opens into said cell, thereby to define a liquid level in said cell. An electrolytic cell according to claim 5, wherein said cathode (30) is removable from said cell. An electrolytic cell according to any preceding claim, further comprising a removable lid (40) which, when secured to said housing (10), serves to hermetically seal said cell. A method of removing silver from silver-containing aqueous liquids in an hermetically sealed electrolytic cell, comprising the steps of filling the cell with a liquid through an inlet opening which opens into a chamber of the cell between an anode and a cathode, maintaining a reference electrode in the liquid, measuring a voltage potential between the cathode and the reference electrode, and providing a current between the cathode and the anode in response to said voltage potential, circulating the liquid through the chamber by applying negative pressure to an outlet passage which extends through the anode, thereby de-aerating the liquid in the cell by generating a vortex above the outlet passage. A method according to claim 8, further comprising the steps of detecting the presence of a critical level of water in the outlet passage, continuing the circulation of the fluid upon the detection of a threshold level of water or air in the outlet passage, and discontinuing the circulation of the liquid upon the detection of a threshold level of water or air in the outlet passage.

Inventors: VAN DEN BERGEN PATRICK, VAN DEN BERGEN, PATRICK, VAN DEN BERGEN, PATRICK, C/O AGFA-GEVAERT N.V.
Applicants: AGFA GEVAERT NV, AGFA-GEVAERT N.V.
Application year: 1997
Main classification: C25C7
IPC codes: C25C7