CA1143330A - Means for recovering silver from photo chemicals - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The invention illustratively contemplates recovery of silver from waste photographic-fixer solution, the apparatus being so devised and controlled that high purity is achieved in the reclaimed metal without contamination of the remaining solution. As a result, the remaining solution may be recycled, and requirements for replenishment of fixer chemical are held to minimum quantities. In an automated employment of the invention, assurance is provided that electroplating action will be called for only when it can be safely performed, without impairment of quality in the reclaimed metal or in the recycled solution.

Description

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~33~13 MEANS FOR RECOVERING SILVER FROM PHOTO CHEMICALS

Back~round of the Invention This lnvention relates to recovery of precious metal from a solution conta.ining the same, and is illustratively described in connec-tion with silver-recovery apparatus for use with waste photographic-fixer solu-tion.

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11~3330 Conventional apparatus of the character indicated employs carbon anodes which are subject to gradual disintegration, thus providing a con-taminant for the solution and giving rise to a variable plating-current density, as the electrode is consumed. The solution is not reusable, and the recovery of silver is only in the order of 40 to 60 percent of that which is available. Moreover, the silver that is recovered is not of the best quality, so that further refining steps are needed. In .
general, the shortcomings of the conventional technique limit its use essentially to large commercial photo processing firms and laboratories, and the matter of anode servicing and replacement is a major maintenance factor.
Brief Statement of the Invention It is accordingly an object of the invention .
to provide improved apparatus of the character indicated.
~ ~ Another object is to provide such apparatus ~ . -whlch~will~inherently produce substantially greater yields of metallic silver, far exceeding 90 percent of that available, and at the same time yielding higher-qua1ity~silver than heretofore, namely, silver of merchantable quality.
A;further obj~ect is to meet the above objects with apparatus~that LS relative1y small, odor-free, and simple tQ maintain and Which can therefore serve institutions, such as hospitals, which only . ~ : , ~ :
; 30 incidentally~must perform their photographic processing of X-ray negatives.
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~4~30 A specific object is to provide a device of the character indicated which will permit recycled use of fixer solution from which substantially all silver has been recovered.
Another specific object is to provide a device of the character indicated with means whereby it can function in conjunction with automatic or semi-automatic photo-processing installationsO
Other objects and various further features of the invention are illustrative realized in a system for recovery of silver from waste photo-graphic-fixer solution, the apparatus being so devised and controlled that high purity is achieved in the reclaimed metal without contamination of the remaining solution. As a result, the remaining solution may be recycled, and requirements for replenishment of fixer chemical are held to minimum quantitiesO
Accordingly, the present invention provides an electrochemical device for recovering a precious metal from an electrolyte solution containing ions of said precious metal, comprising a tank with a bottom and sidewall having a predetermined upper level of liquid capacity, first and second electrodes having radially spaced cylindrical surfaces and mounted on a common upstanding axis, one of said electrodes being totally beneath 20~ ~ said predetermined level and above the bottom of said tank, said one electrode being~also open at its ends for vertical passage of solution therewithin, and impeller means operative beneath said level to develop a recirculatory flow of liquid in said tank and toroidally about said one electrode, the direction of operatlon of said impeller means being such as to induce upward flow in the~space between said electrodes.
Detailed~Descrlptlon The invention will be illustratively described in conjunction with the accompanying drawings, in which:
~ Figure 1 is a simplified diagram s~hematically indicating ; 50 components of a silver-recovery system of the invention;
Figure 2 is a perspective view of a silver-recovery tank unit, forming part of the system of Figure l;

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1~43330 Fig. 3 is an exploded view in perspective showing removal of the cover assembly from the tank unit of Fig. 2, as for servicing or inspection;
Fig. 4 is a similar view of an alternative cover assembly;
Fig. 5 is an electrical and hydraulic circuit diagram, to show means for implementing use of the system of Fig. l;
Fig. 6 is a simplified view of a silver-recovery unit, in the style of Fig. 1, to illustrate inherent recirculatory flow by reason of electro-plating action;
Fig. 7 is a fragmentary simplified diagram to lllustrate a modified silver-recovery unit and ; its adaptability to use in multiple;
~;; Fig. 8 is a diagram similar to Fig. 6 to show lnherent recirculatory flow by reason of electro-plating action in each unit of the modification 20~ of~lg. 7;~
~`; r~ Pig. 9 is another simplified diagram to show a further silver-recovery unit; and Fig. 10 is an~electrical and hydraulic circuit diagram~to~show~control means operative in the 25~modl~fication of~Fig. 7.
In Fig. 1, the invention;is shown in application to an intermittently operative or semi-automatic ; photo-development system, such as a machine 10 customarily~installed in hospitals for development ~ of X-ray negatives, promptly after exposure. ~he : - :.
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11~3330 machine is installed in the wall 11 of a dark room where means 12 provides loading access, i.e., access for insertion of each exposed negative to be developed. Within the machine is a succession of tanks, for developer solution at 13, fixer solution at 14, and first and second washing or rinsing steps at 15-16. The machine 10 will be understood to include means (not shown) for the automatic transport of the inserted negative, into the developer at 13, thence to the successive baths 14-15-16, in timed sequence appropriate to the desired processing of the negative. The machine 10 will also be under-stood to include drying means ~suggested at 17) preparatory to automatic delivery of the dried film at a delivery-access tray or door 18 outside the dark room.
The fixer-related part of machine 10 may , ~ :
;; include means such as a,pump 19 connected to a 20 ~ supply 20 of fixer solution, for make-up or replenishment of the contents of bath 14, the same being shown connected via line 21 to discharge directly into bath 14. An overflow or exhaust line 33, which may be operated by pump means 23, 25~ de~livers excess used (silver-laden) fixer solution to a plating tank 25 of the invention, and tank 25 exhausts in a line 26 to a holding tank 27 forming i part of return line 28-30, with pump means 29 for 29 recycling the use of fixer solution at bath 14.
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Basically, the plating tank 25 comprises an upwardly open cupped body 31 having a radial flange 32 for support and fastening of the flanged rim of a removable cover 33. The tank parts 31-32 present electrically insulated walls, both inside and outside, and may conveniently be of glass-fiber reinforced epoxy or other resin, molded to desired generally cylindrical internal contour. An open-ended cylindri-cal anode 34 is centrally suspended from the cover 33, one suspension point being provided with a terminal-lead connection 35; the effective cylindrical length L of anode 34 is fully submerged in fixer solution, i.e., spaced from the tank bottom and beneath the liquid level 36 established by placement of the overflow port 26' to line 26. An open-ended cylin-drical cathode 37 concentrically surrounds the anode and is shown adjacent the tank wall, spaced from the tank bottom and having an upper flange by which it seats upon the tank-body flange 32; a lead connection ~to the cathode~flange is externally accessible at 38.
Inlet liquid from means 22-23 enters the lower part of tank 25~at a port 39 beneath the cathode, said port being~shown as the bent lower~end of a vertical pipe 40 which extends above the liquid level 36 and ;Z5~ which ls preferably embedded Ln the tank body of the course of manufacture; suoh construction will be recognized~as permitting simple detachable connection of hose or o~ther~supply plumbing, without having to drain the tank~25,~ and without~weakening the anchorage "

of port-39.~ Finally, an electric motor 41 secured to , ~ :

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~143330 cover 33 includes a shaft and impeller 42 extending concentrically within the anode, to promote a gentle toroidal flow as long as plating potential is applied at 35-38; preferably, the direction of such impelled flow is as suggested by arrows, namely, downward within the anode 34, radially outward beneath the anode, upward between the electrodes 34-37, and radially inward above the anode.
The holding tank 27 may be a simple covered vessel having inlet and outlet ports 43-44 forming part of the recycling circuit already described.
However, the periodic addition of make-up solution from supply 2~0 creates a need for discharge of excess processed solution, preferably at tank 27.
This need may be met by a simple overflow drain port 45 or, if desired, automatic valve means (not shown) may be provided to permit accumulation to an upper level H1 before release via port 45, thus effecting relatively infrequent discharge of the 20~ head or difference ^H between upper and lower levels Figs. 2 and 3 show greater detail of a plating tank which differs only slightly from the plating tank descrlbed in connectlon with ~ig. l; for this 25~ reason, the ~same reference numbers are adopted for the same or~corresponding parts. The inlet pipe 40 is seen to be ~nbedded in the tank-body material and ~ ~ :

to present a standard fitting 40' for detacha~le plumbing connection~ A window 47 in cover 33 permits , ; 30 viewing of tank contents without disturbing a secured : :: :: : :

' closure of the tank, and spaced handles 48 on the cover provide simplified manipulation of the cover and all parts subassembled thereto.
In Figs. 2 and 3 there are differences in detail (compared to Fig. 1) as to the manner of support of the anode and cathode members 34-37, but their placement and effective areas remain the same. Specifically, the cathode 37 fits closely to the inner wall surface of the tank body 31, and is positioned just below the level of flange 32, being held in place by spaced .

radial bolts 49 which are above the level 36 and which extend through the body 31; one of these bolts 49 has an extended shank 49' and serves as the cathode-lead terminal. The anode 34 is sus-pended from a spider structure comprising an upper ring 50 and spaced radially outward arms 51; ring 50 is of substantially the diameter of the anode, and spaced~straps 52 tie the inner wall of the ~anode~to the~suspension ring 50. A projection or bracket formatlon 51' on one of the arms 51 provides electrical lead-connection access to the anode 34.
Mounting holes in arm9 51 locate on upstanding studs 53, which are preferably anchored in the ~plastic body of flange 32.
;The tank assembly is;secursd by applying a suitable gssket~(not~shown) to flange 32 before re~istration of cover holes 54~with studs 53, at which time stud nuts may be applied. The application of nuts to studs 53 is found to be necessary only to ~ .

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- discourage tampering, because entirely satisfactory performance is achieved by merely placing the cover assembly over the tank-body assembly.
As to the impeller 42, good toroidal flow is achieved for a variety of axial placements.
Generally, it is preferred that the impeller be located within the anode, at least below the upper end of the anode, and it may be as low as the bottom surface of the tank. If the impeller is located near the lower end of the anode, it is desired to use the tank bottom as a stabilizing reference for impeller-shaft rotation. Thus, in Fig. 3, a baaring 55 such as a nylon or Mylar bushing is shown embedded at the center of the lS tank bottom, for guided reception of the pro-jecting end 42' of the impeller shaft.
The drawings reflect preference for the use of stainless sheet-metal electrodes 34-37 and associated suspension structure. In a highly 20 ~satisfactory empl~oyment of the invention, each electrode is of type 316 stainless steel, approxi-mately 1/16-Lnch thick, although thickness in the range of Q.015 to 0.150 inch will also be satis-factory. A;cathode diameter of 16 inches is well ; 25~accommodated in a tank bore of 16-1/8 inches, and an associated~anode of 8-inch diameter provides an effective~rel;ationship. Of course, electrode length :lS a function of tank~capacity; for five or ten-gallon capacity, anode length is approximately five or ten : ~ ~
inches, respectively, and cathode lengths are;scaled accordingly. Titanium bolts are preferred at 49-53.

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114~330 Fig. 4 illustrates a slightly modified cover assembly wherein the anode 34 and its suspension structure 50-51-52 are secured by bolt means 56 to the flange of cover 33. Mounting holes 57 in this flange register with studs 53, to secure the full tank assembly. And the shaft for impeller 42 is short enough to be fully contained within the included volume of the anode 34. Thus, upon removal of the cover assembly, it may be stood to the side of the tank body 31, resting upon the base of the anode 34.
~ In the periodic servicing of the described ! plating-tank structures, the cover assembly is removed and the anode assembly is removed. Access is then presented for removal of the cathode, the bore of which may be laden with as much as a one-inch thickness of high-quality metallic silver; in Fig. 3, inward brackets 53 at diametrically opposite `: :
locations on the cathode provide lifting access for ~removal of the cathode, while positioning feet 59 main~tain a desired spacing from the tank bottom and also provide a convenient footing when the ` cathode is removed from the tank. Metal silver is then removed from the cathode by fracturing, as by 25 ~sharp applicatlon of one or more mallet blows to the outer~surface of the cathode; alternatively, the silver may~be removed~by~melting. Both electrodes may~then be~immediately restored to service, although as a practic~al matter, a substitute cathode will p~robably be installed, to permit the loaded cathode to ; be shipped remotely, for'silver removal and then for reuse.

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The circuit diagram of Fig. 5 shows two control arrangements, either or both of which may be used in the automatic operation of the described system. One of these control techniques, shown available for the "up" position of a selector switch 71, reli-es upon a timer 60, which may be set to deliver a control pulse once for each given selected interval, for example one pulse every four hours. This pulse is shown connected to a bistable flip-flop relay 61, the connection being such that a starting signal is imparted via delay means 62 to another flip-flop relay 63; it being understood that passage of a predetermined -delay at 62 is a condition precedent to establishing a starting condition of relay 63. Once relay 63 ~ operates, excitation controls are established for ;~ the impeller motor 41 and for the plating supply ~ ~ 64 to the electrode circuit 35-38. Included in one ~:
arm of this clrcuit is current-responsive means 65, set by selectively adjustable means 66, to produce an;~output signal in a control~line~67 should the detected plating-circuit current be less than a predetermined level. Such a signal in line 67 is used, by connection to the relays 61-63, to disable 25~;~the starting mechanism, thereby avoiding a start up should the current level be of pre-selected insufficient ;amplltude. It will be understood that that delay at ; ;62 should be~at least sufficient to assure termination of~ the timer pulse at 69, before excitation of the plating cirouit. By this means, one is assured that, :
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if inadequate plating-circuit current flow is detected, then the timer 60 will be ineffective to attempt another plating start-up, until passage of the selected timeinterval for the next pulse, here assumed to be another four hours.
The other control technique, which may be concurrently operative with timer 60, but which through selector-switch operation may be the sole operating control for the plating circuit, relies upon operation of the developing machine 10. As shown, this machine includes a limit switch 68 having a probe arm poised to respond to a film ; insertion at 12. Such a switch 68 or its equivalent is to be found in most photo-developing machines and is relied upon, via suitable control means 69, to operate valves or to drive displacement pumps for the predetermined incremental supply o replenishment developer and fixer, from make-up supplies of stock solution; the present situation is concerned solely 20 ~ with adaptation to the fixer solution and its supply 20, which w~ be understood to be briefly drawn, to the extent of a predetermined volume Vl by operation of pump 19 under the control of means 69. Concurrently, control means 69~is connected to pump means 29 to 25 ~deliver into bath 14 a predetermined volume V2 of recycling fixer solution, fror.l the holding tank 27;
and c~ntrol means 69 is further connected to provide ~ a~starting~input~to the delay means 62. It will be `~ understood that adjustments are made to assure a correct proportionlng of the Lndicated volumes, such :

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~ -13-, 11~3330 that the minimum necessary fresh chemicals are drawn from the supply 20. At the same time, means 70 responsive to fixer level in bath 14 is operatively connected to pump means 23 to draw off used, silver-laden fixer from tank 14, in a quantity to maintain the desired fixer-bath level.
The used fixer is of course supplied directly to tank 25, and it may contain enough silver enrichment to the~contents of tank 25 to enable a plating operation. This condition is tested by the current-responsive means 65, already descrlbed. If there is an inadequate current, the plating circuit is restored to shut-down condition, but i the current is at an adequate level, the described toroidal 10w and plating action proceed, until ::
;~ means 65 functions;to shut down the plating through having detected the minimum acceptable current level.
It will be seen that the invention meets all ,~
stated objects with a relatively simple and inherently~clean plating-tank structure. The arrangement, aontrol and choice of materials are such that~electrodes are repeatedly reusable; more than 98 percent of the available silver is reclaimable;
; the processed~fixer is recycled; no gases~are given 25~ off to the surroundlng space (whlch in the case of a hospital`dark room is very confined); and no solution : LS ~"burned" by excessive current at high voltage. For example, ~or the indicated ~anode and cathode dimensions, and for a relatively high plating potential of 5 volts, :
ourrent can range up to 15 amperes without bbrning the ,: ; , 11~3330 silver, but preferably the current is held to a level in the range 2 to 7.5 amperes; the maximum current being determined by assuring sufficiently frequent attempts at plating, as by shortening the pulse interval at timer 60 should the fixer bath be called upon to carry heavy and continuous use.
While the invention has been described in detail for the preferred forms shown, it will be understood that modifications may be made without departing from the invention. For example, by reversing the polarity of plating potential applied to the electrodes 34-37, the outer electrode becomes the anode and the inner electrode becomes the cathode, in which case metallic silver is plated upon the outer surface of the inner electrode. Use ; ~ ~ of the expressions "cathode" and "anode" in applica-tion to the electrodes 34-37 will thus be understood , .
to be illustrative, ràther than limiting, in the 20 ~ present context.
Devélopment~of the described toroidal flow is~inherent Ln the platlng action of the described ele~ctroplating tank unit 25, the function of impeller 42 being merely to~produce flow in the same direction 25~ and~therefore~to~provlde a means for enhancing such flow. And it is~helpful to briefly discuss the inherent flow-inducement platlng function of tank 25 in~conj;unction with Fig. 6, as;an introduction to description~of the further embodiments of Figs. 7 30~ to 10.~

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11~3330 In Fig. 6, the mechanical impeller 42 has been omitted, in order to limit discussion to the toroidally recirculating flow-inducement function inherent in application of plating potential to the leads 35-38 for the concentric electrodes 34-37 of the tank unit 25 of Fig. 1.
The flow arrows of Fig. 6 delineate the direction of toroidal flow, namely, downward in an inner zone within the inner electrode 34, and upward in the annular zone between electrodes 34-37, radially outward crossover between these zones being possible in the bottom clearance beneath the lower end of inner electrode 34, and radially inward crossover between these zones being possible in the upper region where the upper end of electrode 34 is beneath the tank-capacity level of spillway 26'. Before application of plating potential, the : ~ : waste photo-chemicals which fill the tank provide a relatively dense electrolyte solution containing silver ions, and this density is locally reduced in the annular~reglon between electrodes 34-37 by reason of plate-out to the cathode surface. ~aving thus:locally reduced solution density, the heavier ;solution within the inner electrode 34 gravitationally 25~ seeks:equilibrium~with remaining solution, inherently di~splacing less-dense solution upwardly in and from :the plating zone, to then spill over into the inner :zone within electrode 34~ For the relatively low current densities involved in the described plating operation, only some of the silver ions are pIated-out `
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' ' ' ' - ' , 1~43330 for each pass upward through the plating ~one.
As a result, the toroldal flow indicated by arrows proceeds continuously until substantial exhaustion of the silver content of the solution, and if new waste solution is introduced via inlet 40, the toroidal-flow process continues.
In the arrangement of Fig. 7, a plurality of plating-electrode pairs A, B, etc. are supported in laterally spaced relation, from each other and from wall structure 70. The inner electrode 34' of each pair may be as described at 34 in connection with Fig. 1, but for purposes of initial discussion it will be assumed to be closed internally but neverthe-less presenting an operative outer cylindrical surface for establishment of plating potential with respect to the concentrically disposed cylindrical open-ended outer electrode 37'. In contr~ast to Fig.;l, it is the outer electrode 37' which is of length L and fully submerged a dlstance ~ below the capaclty level 2~0 ~71 and~a dLstance '~ H~2 above the tank bottom.
Upon~applic~ation of plating potentlal, as via bus bars 72-73 to corresponding electrodes of each pair A~, B, etc., it is again the annular plating zone (between electrodes~of each pair) which develops 25~ ~reduced~denslty through~plate-out action, and s~ince ; solution external~to the outer~electrode 37' is more dense,~toroidal flow develops around electrode 37' by reason of~the density differential and by~reason of the freedom ~or~solution to crossover radially inward bene~ath electrode 37', with radially outward ~, : . : : : :

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11~3330 spillover above electrode 37', all as indicated by directional arrows for the plural electrode pairs A, B, etc.
Fig. 8 illustrates the nature of double-toroidal S flow, induced by plating action when both electrodes 34'-37' of a given electrode pair (or of each electrode pair in Fig. 7) are open-ended, fully immersed ~Hl below the spillway level 71 and spaced ~H2 above the tank bottom. In this case, the first toroidal path may be as described for Fig. 6, namely about the inner electrode 34' as the core of the first toroid, and the second toroidal path may be as described for Fig. 8, namely about the outer electrode 37' as the core of the second toroid, it being noted that the upward flow in the intermediate annulus or plating zone between electrodes 34'-37' is a flow that is shared by the two toroidal-flow : :
paths. In all cases (Figs. 6, 7 and 8), it matters not~whether the inner electrode is an anode or a ~ cathode, but preference is indicated for the outer électrode to be~the cathode, in view of the larger plate-out area it affords.
It will be understood that for enhancement of the toroidal-flow action in an electrode con-25~ figura~ion in~which~the inner electrode 34' is open-ended~and fully immersed~and spaced from the tank bottom, as for~the case of Fig. 8, the centra1 impeller~42~may~be~provided in the manner~discussed for inner~electrode 3 4 in the tank unit 2S of Fig. 1.
30: ~ In such event, it iS preferred that the impelier-drive :

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11~3330 means be located above the capacity level 71.
However, for the case of a closed-end inner electrode, there is no reliance upon flow inside the inner electrode and therefore the upper end S of such inner electrode may be above the capacity level 71, to permit non-corrosive electric-cable access to an impeller motor contained within the inner electrode. Such an arrange~ent is depicted in Fig. 9, wherein the inner electrode 34" is of length L' greater than the length L of the fully immersed outer electrode 37'. ~he two electrodes are spaced ~H2 from the tank bottom (as ln Fig. 8), but the greater length L' of electrode 37" is enough to place the upper closed end thereof above level 15 71. A motor 75 within electrode 37" will be under-stood to incorporate adequate gear reduction so that only the most gentle and slow enhancement of flow is attributable to the action of its impeller blades 76.
In situations in which multiple pairs of 20~electrodes~A,~ B, etc. are arrayed in laterally spaced relation, as in the tank 70 of Fig. 7, it is desirable~that each plating zone be given substantially equal shares of new electrolyte solution via inlet 40.~ To provide 9uch sharing, the inlet 40 in Fig. 7 25~ is~shown serving plural individual inlet-discharge poitlts, each at the outlet of a check valve, such as the valve 77a servLng the electrcde pair A and the valve 77b serving the electrode pair B. It will be understood that by appropriate design and/or adjustment, ; 30 equal-volume injections of new silver-laden fixer :: , :~ :

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solution will occur at 77a, 77b, etc. for each operation of the pump 23 which serves inlet 40.
In spite of efforts made, as just described, to assure equal sharing of plating load at each electrode pair, it is possible that one or more electrode pairs may locally exhaust their supply of silver-laden fixer solution before one or more other electrode pairs are able to achieve the same yield. This circumstance can be noted by having separate current-responsive means 65 associated with the plating circuit to each electrode pair, thus enabling each such means 65 to locally terminate plating action when the predetermined low current level is locally achieved.
Alternatively, and as illustrated in Fig. lO, ; current-responsive means 80 associated with each plating circuit may monitor the instantaneous level of current to develop~a suitable current-indicating output ;sign~aL,~and further means 81 may be connected 20~ for;response~to a plurallty of such output signals to sense whether current consumption in any~parti-cular~circuit exceeds a predetermined amount less than the current~consumption in one or more other circuits~. ~For~the case shown, in which two electrode 25~ pairs A-B are served,~the means~81 is a polarity-sensitive difference~detector, having separate control autpu~t connectlons tc~ first and~second operatlng solenoi~ds of~a directional valve 82~in the~fixer supply line~40~to~hath 7~0; and~depend1ng on which solenoid is excited, fixer solution will be ~ :: : , ; -20-:
: : . ~ ' ~' , ' , preferentially delivered via line 83 to the electrode pair A or via line 34 to the electrode pair B, it being understood that if neither solenoid is excited, any operation of pump 23 will deliver substantially equal division of the new silver-laden electrolyte solution, in both lines 83 and 84. Thus, if the current differential is observed by means 81 to be in the polarity sense indicative of lower current consumption by electrode pair A (than the current consumption of pair s), and if the magnitude of this differential exceeds the predetermined amount, then the correct solenoid of valve 82 will be actuated to divert the next discharge by pump 23 into the line 83, or at least to a greater extent into line 83 than into line 84;
and when the current consumed at A is seen to rise such that the predetermined differential at 81 no : longer exists, then the solenoid of valve 82 will be de-energized to allow valve 82 to return to its 20: mid-position wherein lines 83 and 84 share equally in the flow from pump 23.
It will be understood that the principle of :Flg. 10, where1n fresh silver-laden solution is preferentially~routed to the electrode pair doing 2~5 ~the least work, can be applied to contexts wherein : more than two electrode pairs are operating con-currently. In such case, for example, automatic electrical sensing means exist whereby any electrode pair which is~detected to be drawing, to a predetermined ~: 30 extent, less current than the pair drawing the greatest ~ . :
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current, will be automatically served with a greater share of silver-laden solution next-delivered by pump 23. The Fig. 10 arrangement thus merely serves to illustrate the multiple-pair situation wherein the number of electrodepairs is two.
The invention has been particularly described for the specific case of recovering silver from waste photographic fixer solution. But the invention will be seen to be broadly applicable to the recovery of elementary metals such as precious metals from any electrolytic solution which contains ions of the metal to be recovered. Electrode surfaces may be of materials other than described, but stainless steel is preferred for durability, for non-contamination o~ high-quality plating accumulation, and for ease of removal of the plating accumulation.

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Claims (24)

WHAT IS CLAIMED IS:

1. An electrochemical device for recovering a precious metal from an electrolyte solution containing ions of said precious metal, comprising a tank with a bottom and sidewall having a predetermined upper level of liquid capacity, first and second electrodes having radially spaced cylindrical surfaces and mounted on a common upstanding axis, one of said electrodes being totally beneath said predetermined level and above the bottom of said tank, said one electrode being also open at its ends for vertical passage of solution therewithin, and impeller means operative beneath said level to develop a recirculatory flow of liquid in said tank and toroidally about said one electrode, the direction of operation of said impeller means being such as to induce upward flow in the space between said electrodes.

2. The electrochemical device of claim 1, in which said impeller means comprises means for applying polarized plating potential to said electrodes, whereby when filled with solution to said level, the precious metal will plate out of solution in the region between said electrodes, thereby locally reducing the specific gravity of the solution and allowing more-dense solution from beneath said one electrode to flow radially below said one electrode to replenish and upwardly displace plated-out solution in said region between electrodes.

upwardly displaced solution spilling radially as less-dense solution over the top of said one electrode while the more-dense solution radially offset from the space between electrodes enters the space between said electrodes via the space beneath said one electrode.

3. The electrochemical device of claim 1, in which said impeller means includes blades mounted for driven rotation beneath said level.

4. The electrochemical device of claim 1, in which said one electrode is the inner electrode, whereby the toroidal flow is upward between said electrodes and at least in part downward within the inner electrode.

5. The electrochemical device of claim 1, in which said one electrode is the outer electrode and in which said outer electrode is at least in part in lateral clearance with said sidewall, whereby the toroidal flow is upward between said electrodes and at least in part downward external to the outer electrode.

6. The electrochemical device of claim 1, in which both said electrodes are totally beneath said pre-determined level and above the botom of said tank, and in which said outer electrode is at least in part in lateral clearance with ssid sidewall, whereby the toroidal flow is upward between said electrodes and at least in part downward both within the inner electrode and externally of the outer electrode.

7. The electrochemical device of claim 1, in which said electrodes constitute one pair of a plurality of similar pairs of electrodes similarly mounted in laterally spaced relation in said tank.

8. The elec-trochemical device of claim 7, in which said electrode pairs are interconnected for electrical excitation with plating potential across the electrodes of each pair.

9. The electrochemical device of claim 8, in which said electrode pairs are interconnected in parallel.

10. The electrochemical device of claim 3, in which said inner electrode is closed at its ends and contains motor means Eor imparting blade rotation about said axis.

11. The electrochemical device of claim 8, in which said tank includes provision for independent supply of new electrolyte solution to the toroidal flow path unique to each electrode pair, means associated with said supply for varying the proportion of new electrolyte flow supplied to one as compared to another of said electrode-pair flow paths, means comparatively monitoring platiny current flow to the respective electrode pairs, and a control connection from said monitoring means to said proportion varying means, the directional sense o said control connection being to increase new-electrolyte flow to the electrode-pair flow path exhibiting lesser electric current consumption.

12. The electrochemical device of claim 8, in which said tank includes provision for independent supply of new electrolyte solution to the toroidal flow path unique to each electrode pair, means associated with said supply for varying the proportion of new electrolyte flow supplied to one as compared to another of said electrode-pair flow paths, means comparatively monitoring plating current flow to the respective electrode pairs, and a control connection from said monitoring means to said proportion varying means, the directional sense of said control connection being to decrease new-electrolyte flow to the electrode-pair flow path exhibiting greater electric current consumption.

13. The electrochemical device of claim 1, in which said tank has an upper spillway port at said upper level, and an inlet port communicating with the bottom region within the tank.

14. The electrochemical device of claim 1, in whcih said electrodes are of stainless steel.

15. The electrochemical device of claim 4, in which said tank has a removable cover, and in which said flow-impelling means comprises a motor mounted on said cover and an impeller shaft driven by said motor and extending coaxially within said first electrode, and an impeller on said shaft in the region beneath said first electrode.

16. Automatic means for recovering silver from waste photographic-fixer solution, comprising the controlled electrochemical device of claim 1, fixer-bath means including exhaust outlet means and replenisher inlet means, a supply connection from said outlet means to said tank, said tank including an upper-level discharge outlet for exhausting processed liquid, automatic plating potential supply means connected to said electrodes and including current-sensitive control means connected to said electrodes for supplying sustained plating potential to said electrodes only in the event of a detected current exceeding a predetermined level, a recycling connection from said discharge outlet to said replenisher inlet means, and means coordinated by flow in said recycling connection and including a fresh-fixer supply connection for introducing fresh fixer at said replenisher inlet means in a predetermined relation to the flow in said recycling connection.

17. Automatic means according to claim 16, in which said coordinating means includes motor-driven pumps in the respective recycling and fresh-fixer connections to siad replenisher inlet means, and control means for driving said pumps to effect said predetermined flow relation.

18. Automatic means according to claim 17, in which said recycling connection includes a holding tank.

19. Automatic means according to claim 16, in which said fixer-bath means is part of an automatic intermittently operated photo-processing system wherein a fresh-fixer supply is actuated to introduce a pre-determined quantity of fresh fixer solution at said inlet means upon introduction of photographic work piece calling for development, and means coordinated with such actuation for initiating the supply of plating potential to said electrodes.

20. Automatic means according to claim 16, in which said fixer-bath means is part of an automatic intermittently operated photo-processing system wherein a fresh-fixer supply is actuated to introduce a pre-determined quantity of fresh fixer solution at said inlet means upon introduction of a photographic work piece calling for development, and means coordinated with such actuation for initiating flow in said recycling connection.

21. Automatic means for recovering silver from waste photographic-fixer solution, comprising the controlled electrochemical device of claim 1, control means connected to said electrodes for supplying said electrodes with a plating-excitation potential, said control means comprising current-responsive means operative to disable the supply connection upon detection of an excitation current of less than a predetermined minimum value, said predetermined minimum being that applicable to a substantially silver-exhausted condition of waste-fixer liquid in said tank, and intermittently operative means for periodically exciting said electrodes at plating potential, whereby silver plating will only be allowed to proceed if a current can be passed in excess of said predetermined minimum value.

22. Automatic means according to claim 21, wherein said intermittently operative means includes a timer preset to apply plating-excitation potential to said electrodes at predetermined intervals.

23. Automatic means according to claim 21, wherein said intermittently operative means includes a photo-developing machine having a fixer bath with a discharge connection to said tank, and a control connection from said machine to said control means.

24. Automatic means according to claim 23, in which said machine includes a fixer stock-solution supply and means governing the periodic admission of a predetermined quantity of fixer stock-solution from said supply to said bath, said control connection including means responsive to an operation of said governing means.