AN ELECTROLYTIC SILVER RECOVERY DEVICE

Abstract: An electrolytic silver recovery device for recovering silver from fixing solutions, with an electrolytic cell (28) with an anode (52) and a cathode (53), and circulation pumps (29a, 29b) for receiving solution from the cell and returning it to the cell, which device comprises a hydrocyclone (56) located in the liquid path (55) from the cell to the suction end of the pumps.

Description: BACKGROUND OF THE INVENTION Field of the invention The present invention relates to an electrolytic silver recovery device for recovering silver from photographic fixing solutions. Description of the prior art In 1931, K.Hickman, C.Sanford and W.Weyerts describe in the JSPE, Vol. 17, p. 568-590 a technique for "The electrolytic regeneration of fixing baths". This was the first practical technique to provide for high-efficiency removal of silver and reuse of the fixing bath. Passage of an electric current through exhausted fixing bath in a cell provided with a cathode and an anode leads to the deposition of silver on the cathode. Adequate control of the electrolytic process not only enables the silver to be recovered economically, but also allows the fixing solution to be used over and over again, with occasional replenishment to compensate for hypo lost on films removed from the bath. In the performance of the method, it is known that the operational lifetime of the pump or pumps that maintain a circulation of fixing solution through cell, and/or also between the cell and the fixing tank of the processor, is seriously affected by the presence of silver particles in the liquid. It is believed that these particles arise from the cathode of the silver recovery cell where they become detached from the deposited silver layer under the influence of vibration, bad adhesion, etc. These particles settle on the impeller shaft of the pumps and damage thereby the bearing sleeve or the seal of the pumps which causes leakage of the pumps. Filters in the liquid circuit towards the pumps operate not very satisfactorily because the troublesome silver particles are very fine so that they pass through the filter, whereas the coarser particles tend to prematurely clog the filter. SUMMARY OF THE INVENTION Object of the invention It is an object of the present invention to provide an electrolytic silver recovery device for recovering silver from fixing solutions used in photographic processing apparatus, which does not cause a rapid worsening of the pump means in the liquid circuit. Statement of the invention According to the invention, an electrolytic silver recovery device for recovering silver from fixing solutions used in photographic processing apparatus, which comprises an electrolytic cell with an anode and a cathode, and circulation pump means for receiving solution being processed from the cell and returning same to the cell, is characterised thereby that the device comprises at least one hydrocyclone in the liquid path from the cell to the suction end of the pump means, the feed inlet opening of the hydrocyclone being in connection with the liquid in the cell, the vortex finder (i.e. the liquid overflow pipe) of the hydrocyclone being in connection with the suction end of the pump means, and the bottom opening of the hydrocyclone leading to a collector for collecting solid particles removed from the process solution. The satisfactory operation of a hydrocyclone in the described application is surprising, since the pressure difference that can be established over the cyclone is limited because it is based on a reduction of the atmospheric pressure at the vortex finder whereas conventional hydrocyclones operate with an overpressure at the feed inlet opening. The pressure difference which is based on an underpressure is limited (theoretically 1.0 bar), whereas one that is based on the use of an overpressure may amount to many bars. Yet it is shown that a hydrocyclone in the described application forms a very efficient tool for the removal of solid particles from the fixing liquid that otherwise would considerably shorten the life of the pumps in the circuit. The hydrocyclone is a completely static component in the circuit that is maintenance-free. In a suitable embodiment of the invention, the hydrocyclone is simply hung in the electrolytic cell, its inlet opening being below the level of the liquid in the cell. According to a further suitable embodiment of the invention, the collector for collecting solid particles discharged at the discharge opening makes part of the hydrocyclone. In this way, the only connection of the hydrocyclone in the circuit may comprise a liquid conduit at its vortex finder. For emptying the collector, the conduit is disconnected from the vortex finder, and the hydrocyclone may be lifted from the recovery cell. The collector may be in the form of a small pot, liquid-tightly screwed on the underside of the hydrocyclone, and be arranged for easy closure for transport to another instance for separation and/or recovery of the collected silver. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described hereinafter by way of example with reference to the accompanying drawings, wherein : Fig. 1 is a diagrammatic illustration of one embodiment of a photographic film processor and an ecologic processing unit connected therewith, Fig. 2 is a diagrammatic representation of one embodiment of the cell for the electrolytic silver recovery of the ecologic processing unit, Fig. 3 is a lateral view, and Fig. 4 is a top view of one embodiment of a hydrocyclone used in the cell of Fig. 2, and Fig. 5 is an enlarged representation of the block 5 of Fig. 2, showing a modified construction of the electrolytic recovery cell. Referring to Fig. 1, a photographic film processor 10 which comprises a developing station 11, a fixing station 12 and a rinsing or washing station 13, is connected to an ecologic processing unit 15 which comprises a module 16 with a holder 17 for used developer and a holder 18 for used fixer, a module 19 for the recovery of silver from the fixing liquid, and a module 20 for the recovery of silver from the wash water. The processor 10 and the unit 15 have been illustrated as separate units in Fig. 1, and in practice they may be located close to each other or even be arranged in separate rooms. However, the unit 15 may also be integrated into the processor 10. The holders 17 and 18 may be in the form of small tanks, jerrycans, canisters, and the like. The developing station 11 comprises a regeneration system 21 for adding regeneration liquid to the station as developer is being consumed by the developed film and dragged into the fixing station. In a similar way, the fixing station 12 comprises a regeneration system 23 for the fixing liquid. The tanks of the processing stations may be deep tanks with film transport racks, or tray-like tanks for a nearly horizontal film transport. The cross-over between adjacent tanks may be provided with squeegee roller pairs for limiting the transfer of processing liquid into the next tank. The developing station has an overflow 24 which controls the liquid level in the station, and which leads to the holder 17. A pump 25 maintains a circulation of developing liquid through the station. A heater element 26 controls the temperature of the developer. The fixing station has an overflow 27 which conducts towards the electrolytic silver recovery cell 28 of the module 19. Pumps 29a and 29b operate in parallel for circulating the fixing liquid through the cell 28, whereas a pump 30 pumps liquid from the cell to a three-way valve 31 that conducts normally the liquid back to the fixing station, via the conduit 32, but that may also conduct the liquid through a conduit 34 to the holder 18 for used fixer. The valve 31 is controlled in response to the signal from a level sensor 35 in the cell 28, and in case the liquid level in the cell 28 exceeds a set value, as after the addition of regenerating liquid to the fixing station, the valve is switched to make the excess liquid flow to the holder 18. An adjustable restrictor 36 permits to adjust the rate of flow of fixing liquid to the fixing station. A pump 37 maintains a circulation of fixing liquid through the fixing station. A heater element 38 controls the temperature of the liquid. The rinsing station 13 has an overflow 39 which leads to the module 20. Wash water is taken from a tap water connection 40 under the control of a valve 41 which is controlled by a sensor 42 in response to the introduction of a film 43 into the processor. The module 20 is in fact an assembly of liquid tanks that contain resin cartridges with an ion-exchange resin which captures the silver that is present in the wash water. The inlet of the ion exchange tanks in the module 20 is indicated by the numeral 45, whereas 46 is the outlet of the tanks which goes to a conduit 48 and so to the sewer. The module 20 forms no part of the present invention and consequenetly no further information is given hereinafter on this subject. A detailed description of a suitable embodiment of an ion exchange silver recovery device for wash water may be found in our co-pending application entitled "A silver recovery device", filed on even day herewith. The electrolytic silver recovery device of module 19 is described hereinafter in detail with reference to Fig. 2. The cell 28 is formed by a generally rectangular tank 50 with a protruding side section 51. In the tank 50 there are provided a number of intermeshing electrodes in the form of plates 52 which form the anode and plates 53 which form the cathode of the cell. The respective plates are electrically connected in parallel, and are connected to a suitabe source of electric DC supply, not illustrated in the drawing. The liquid inlet of the cell is formed by a vertical inlet port constrictor 54 through which liquid is pumped into the cell and caused to follow a labyrinth-like path towards the outlet at position 56. The constrictor 54 forms the slotlike outlet opening of a vertical feed chamber 57. The cross section of the constrictor port is smaller than the section of the liquid flow displaced by the pumps so that a substantial increase of the speed of the liquid through the cell is obtained, what is advantageous for a high electrolytic current in the cell, and an efficient removal of silver particles that may fall off the cathode and that might short-circuit the electrode plates in case of non-removal. By the term "removal" is meant in the present case the transport of the silver particles towards the hydrocyclone where they are eliminated from the liquid. Fixing solution is pumped into the chamber at two vertically spaced positions, illustrated for the sake of clarity next to each other in Fig. 2, by the pumps 29a and 29b. The purpose of the two vertically spaced feed openings, in conjunction with the constrictor 54, is to produce a uniform flow rate along the height of the liquid channel through the electrodes. The cell may be emptied by a valve 47. The outlet of the valve discharges into to the sewer, and so does the conduit 49 of a dripping pan 58 provided in the module 19. The outlet of the cell illustrated at 56 is in fact a hydrocyclone which is immersed into the liquid in the cell. This hydrocyclone is shown diagrammatically in Figs. 3 and 4. The hydrocyclone consists of a tubular body 59 with a tangential feed inlet 60, a vortex finder 61, and at the bottom an annular gap 62 which is formed between the cyclindrical wall and a cone 63 which stands on a stud 64 in a cyclindrical collector pot 65 which is screwed on the hydrocyclone as indicated diagrammatically by the broken line 67. The hydrocyclone is not fixedly mounted in the electrolytic cell, but is immersed therein in the same way an immersion pump is placed in a body of liquid. A bracket or the like may hold the hydrocyclone in position in the corner of the cell section 51. The manifolding of the hydrocyclone comprises only the connection of the vortex finder 61 to the outlet pipework 55 which leads to the suction end of the pumps 29a, 29b and 30, see Fig. 2. Apart from the hydrocyclone 56, there are provided in the section 51 of the cell the level sensor 35 and a reference electrode 44 for measuring the silver content of the fixing bath. Said electrode may be a so-called Calomel electrode, but for reasons of ecologic disposal the use of a silver chloride electrode is preferred. In the operation of the fixer circuit of the described processor, the pumps 29a and 29b draw fixing liquid from the recovery cell 28 by establishing via the conduit 55 an underpressure at the vortex finder 61 whereby a reduced pressure is caused at the feed inlet opening 60 that is located below the level of the fixing liquid in the cell. Liquid enters the hydrocyclone through the feed opening and moves in an outer helical flow towards the bottom of the cyclone where it begins to feed across the centre thereby reversing its vertical direction and going up via an inner helical flow and out through the vortex finder 61. Silver, and also other particles that are present in the liquid are captured by the cone 63, and are deflected towards the annular gap 62 where gravity makes them sink into the collector pot 65. The pot 65 is filled with fixing liquid which, however, is in an almost stationary condition with almost no vertical component of motion so that the deposited particles remain within the pot. The pot is fitted by a liquid-tight screwfitting to the hydrocyclone as mentioned already, and has a capacity such that uninterrupted operation during many months is possible. The hydrocyclone operates as a very effective filter to remove solids from the fixing solution, and forms thereby a highly efficient protection for the pumps 29a, 29b and 30. In an indirect way the hydrocyclone forms also a protection for the pump 37, since this pump also makes part of the overall liquid circulation in the fixing station. The reference electrode 44 continuously monitors the silver concentration of the fixing solution, and controls the DC power supply to establish an electrolyse current which produces the best results for the removal of silver from the used fixing solution and for the deposition of silver on the cathode. When the pot 65 must be emptied, the hydrocyclone is withdrawn from the cell 28, and the pot 65 is screwed off the hydrocyclone and a cover is screwed onto it so that it may readily be handled for transport. The following data illustrate by way of example the device described hereinbefore : The present invention is not limited to the embodiment described hereinbefore. The hydrocyclone say have other shapes than the illustrated cyclindrical one. Thus, the hydrocyclone may have a cono-cylindrical body as known in the art, the opening in the apex of the cone serving to carry off the underflow which carries the solids. It is clear that in such case there must be a conduit from the apex opening to a suitable receptacle for the removed solids. The operational pressure difference over the hydrocyclone may have other values than the value mentioned in the example. Suitable values are in practice situated between 0.1 and 0.6 bar. The hydrocyclone say be in the form of an assembly which houses several small hydrocyclones. The assembly provides common feed and product collection of the cyclones. The electrolytic cell 28 may be arranged for obtaining high electrolyse currents by reducing the distance between the anode and cathode plates to a small value, e.g. down to 10 mm. However, such reduction of distance increases the risk for shortcircuiting the electrodes as silver fragments become detached from the cathode and become so located that, occasionally in combination with other fragments, physical contact is made between the cathode and the anode. Also, the resistance to the liquid flow increases as the section of the passage decreases. Therefore, according to an alternation embodiment of the invention, the effective section for the liquid flow between the cathode and anode is reduced by the provision of a frame-like structure on the anode, the open side of which is covered by a net structure that is pervious to the fixing liquid but yet has a non-neglectable flow resistance. Referring to Fig. 5 which is an enlarged view of the section 5 of Fig. 2, the surface of the anode 52 which faces the cathode 53 is provided with a rectangular frame 70 of an electrically insulating material, two opposed walls of which are illustrated as 68 and 69, and the open side of which is covered by a net 71 of nylon, polyester or like threads. The opposed, open side of the frame is in contact with the anode plate 52. This latter contact must not be liquid-tight, and this makes the frame to be held in place by simple locating means, for instance the frame may have projecting resilient fingers or studs that take a support on the opposed cathode, so that the frame may be freely vertically slid between opposed electrode plates and held in this position by resilient biasing forces. The net structure allows the passage of fixer through its pores and thus the volume determined by the frame 70 becomes filled with liquid. However, the net structure presents a substantial resistance to the liquid flow through it, and in this way it causes a substantial increase of the liquid liquid velocity in the space between the net 71 and the adjacent cathode 53. In other words, the net 71 has in fact reduced the original section of the opening between the electrode plates, which is proportional to the width a , to a section which is almost proportional to the distance b between the net and the cathode. The increased liquid velocity allows the use of a higher recovery current while yet the original distance between the electrodes remains unchanged so that there is no increased risk for shortcircuiting the electrodes, as would occur if the distance between the plates would have been physically reduced. The fixing solution processed in the silver recovery cell 28 may be used for the regeneration of the ion exchange resin used in the module 20. To that end the conduit 32 may be interrupted and enter into and then leave the module 20 so that processed fixing solution is passed through resin kept in separate tanks for its regeneration. The latter arrangement is described in detail in our co-pending application entitled "Silver recovery device" filed on even day herewith.

Claims: An electrolytic silver recovery device for recovering silver from fixing solutions used in photographic processing apparatus, which comprises an electrolytic cell (28) with an anode (52) and a cathode (53), and circulation pump means (29a, 29b) for receiving solution which is processed from the cell and for returning same to the cell, characterised in that it comprises at least one hydrocyclone (56) in the liquid path (55) from the cell to the suction end of the circulation pump means, the feed inlet (60) of the hydrocyclone being in connection with the liquid in the cell, the vortex finder (61) of the hydrocyclone being in connection with the suction end of the pump means, and the bottom opening (66) of the hydrocyclone leading to a collector (65) for collecting solid particles removed from the processed solution. A device according to claim 1, wherein the pressure difference between the feed inlet and the vortex finder of the hydrocyclone (56) is smaller than 0.6 bar. A device according to claim 1, wherein the hydrocyclone (56) is mounted within the electrolytic cell (28), its feed inlet (60) being below the level of the liquid in the cell. A device according to claim 1,2 or 3, wherein the collector (65) for collecting solid particles makes part of the hydrocyclone. A device according to claim 4, wherein the hydrocyclone (56) and the collector (65) fitted thereto, are arranged for immersion in the liquid of the electrolytic cell. A device according to claim 3, wherein the collector for collecting solid particles is separated from the hydrocyclone and can be emptied without necessity for removing the hydrocyclone from the device. A device according to any of claims 1 to 6, wherein the anode (52) and the cathode (53) of the cell are in the form of parallel plates forming a labyrinth-like passageway for the liquid in the cell (28), and wherein the inlet opening of the cell is in the form of a vertical slot (54) in a side wall through which the fixing solution is fed between the anode and cathode. A device according to claim 7, wherein said slot (54) is in the form of a constrictor opening which forms the outlet of a liquid feed chamber (57) communicating with the cell. A device according to claim 8, wherein liquid is fed to said liquid feed chamber through at least two vertically spaced openings in a side wall of the chamber, each such opening being connected to the outlet of a pump (29a, 29b). A device according to claim 7, wherein the passage for the free liquid flow between the electrodes of the cell is restricted by net means (71) which is fitted to frame means (70) which determines on the anode side of the gap between the plates a zone of a very small liquid velocity, and conversely a zone of substantially increased liquid velocity between the net and the cathode.

Inventors: DE NIEL, MARC ALFONS, ENGELS, HERMAN MARIA, VERLINDEN, BARTHOLOMEUS JULIANUS
Applicants: AGFA-GEVAERT NAAMLOZE VENNOOTSCHAP
Application year: 1989
Main classification: C25C7
IPC codes: C25C1, C25C7