US3642594A

US3642594A - Electrochemical recovery of silver and regeneration of used photographic fixing solutions

Info

Publication number: US3642594A
Application number: US72571A
Authority: US
Inventors: Terry M Crellin
Original assignee: Norton Co
Current assignee: Saint Gobain Abrasives Inc
Priority date: 1970-09-16
Filing date: 1970-09-16
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15

Abstract

An improved method for recovering silver and regenerating used photographic fixing solutions electrochemically is disclosed whereby silver is electroplated from used fixing solutions using high-current densities, e.g. 300 a./ft.2, without decomposing the fixing solution. The fixing solution is then capable of being reused in the photographic process.

Description

United States Patent Crellin [54] ELECTROCHEMICAL RECOVERY OF SILVER AND REGENERATION OF USED PHOTOGRAPHIC FIXING SOLUTIONS [72] Inventor: Terry M. Crellin, Wheaton, Md. [73] Assignee: Norton Company, Troy, N.Y. [22] Filed: Sept. 16, 1970 [21] Appl. No.: 72,571

[52] [1.8. CI ..204/149, 204/ I09 [51] Int. Cl. ..B0lk 3/00, BOlk 3/04 [58] Field ofSearch ..'.....204/DIG. 10, 149, 130, 109,

[56] References Cited UNITED STATES PATENTS 1,146,942 7/1915 Landreth ..204/149 Feb. 15,1972

2,344,548 3/1944 Goetz ..204/ 149 X 2,878,171 3/1959 Ferrandm. ....204/202 X 2,997,437 8/1961 Whitaker ..204/ 222 3,577,334 5/1971 Graham et a1. ..204/222 Primary Examiner-John H. Mack Assistant Examiner-A. C. Prescott Attorney-Hugh E. Smith and Herbert L. Gatewood [5 7] ABSTRACT An improved method for recovering silver and regenerating used photographic fixing solutions electrochemically is disclosed whereby silver is electroplated from used fixing solutions using high-current densities, e.g. 300 a./ft., without decomposing the fixing solution. The fixing solution is then capable of being reused in the photographic process.

7 Claims, 6 Drawing Figures PATENTEDFEB 15 I972 Fig. 6.

IIIIIIIIIIIII Fig. 4.

inventor Terry M; Crei/in,

II I/l/I/I/I/I I/ I/I/III/I/I/I/I/ His Attorney.

ELECTROCHEMICAL RECOVERY OF SILVER AND REGENERATION OF USED PHOTOGRAPHIC FIXING SOLUTIONS FIELD OF THE INVENTION The recovery of silver from spent fixing baths in practiced by most film laboratories and the procedure is well known. The sulfide precipitation process remains the general favorite because, in spite of messiness and the uncertain control of the refining charges the requirements in skill and labor are low.

The amount of labor, or research to economize labor, which a process will carry is not a fixed quantity but increases with the growth of the operating unit. Processing laboratories have advanced to such a size that it wouldbe profitable to install elaborate recovery systems, even at the cost of higher labor charges and skilled supervision, if increased yields were available. In the search for a suitable process it has been considered essential to find ways of using the fixing bath more than once, preferably in a continuous flow cycle.

The present invention is directed to the provision of apractical and economical method for recovering silver from a used photographic fixing bath by electrolysis whereby the fixing bath is regenerated and can be reused in the photographic process.

DESCRIPTION OF PRIOR ART When an electric current is passed between two uncorrosi" ble electrodes immersed in a water solution of a metallic salt, oxygen gas is generally evolved at the anode or positive pole, and metal is deposited at the negative cathode. The metal may be powdery, crystalline, dull, or bright, loose or adherent according to the conditions. Silver deposited from silver nitrate or other simple salts is loosely attached to the electrode in a granular or microcrystalline state. Bright, adherent deposits are obtained from the double cyanide of silver and potassium, and from certain ammoniacal solutions.

When a current of usual plating density is passed through thiosulfate solutions containing silver no oxygen is liberated at the anode, and a black cloud of silver sulfide is liberated at the cathode which soon obscures the solution. If the current is diminished a hundredfold to less than 50 milliamperes per square foot, silver is very slowly deposited in adherent metallic form, Although such slow deposition is useless commercially it shows that the plating reaction is fundamentally possible and it suggests that deposition at higher current densities is spoiled by side reactions.

The electrolytic separation ofa metal from a simple salt occurs in the following stages:

On dissolving in water the salt ionizes into two components oppositely charged electrically.

When an electric current is passed the positive silver ion moves to the negative pole, gives up its charge and deposits in the form of metallic silver on the pole piece. The nitrate ion N moves to the positive pole and, at the moment of liberation, decomposes water with the formation of nitric acid and oxygen:

The separation of metal from sodium cyanide or silver thiosulfate occurs differently. The salts yield ions thus:

It should be noted that the silver is now in the negatively charged particle or anion and is driven away from the cathode pole toward the anode. On discharge at the latter, part of the cyanide ions and all the thiosulfate ions are oxidized and the silver remains in the solution. The sodium ions or cations discharged as metallic sodium atoms at the cathode are so reactive that they decompose any ions in their vicinity yielding silver if there remain enough AgS O ions, otherwise they reduce water to hydrogen, or reduce thiosulfate to a series of compounds, most of them fatal to good silver plating.

It is not proposed to detail the very complex chemistry of the electrolysis more than to stress that the silver deposited from a used fixing bath is secondary silver liberated from com plex silver particles which are all wandering away from the only place where they can deposit. It is the problem of the recovery chemist to see that in spite of the migration there are always enough silver ions in the neighborhood of the cathode to react with the electric charge or with sodium atoms, otherwise the hypo will be attacked, yielding among other things, sodium sulfide, which will in turn precipitate some silver sul- 'fide. Even a little silver sulfide can spoil a growing silver surface.

Even with the most carefully purified silver thiosulfate solutions the silver surface becomes poisoned before it is very thick. This is because the silver is deposited, in crystalline form, with many minute individual crystals growing next to one another over the surface. The individual crystals do not all present the same crystal faces to the solution; some offer fast growing surfaces, others slow. There results a matted deposit formed from dendrites (the faster growing asperities) and valleys or voids (the slower growing asperities) which soon harbors crevices and spaces between the growing dendrites in which stagnation can occur; the stagnated solution becomes depleted in AgS Q ions, sulfide is liberated, and the surface becomes dull and powdery.

The obvious way to counteract migration is by using a bath rich in silver and by vigorous stirring which renews solution in the neighborhood of the electrodes. The quantity of current that the cathode can tolerate increases with agitation, and with really violent stirring, the current density can be increased in the order of a hundred times without generating sulfide on the cathode surface.

In U.S. Pat. No. 1,954,316, issued in 1934 to K. C. D. Hickman et al. a process is disclosed for plating of silver from thiosulphate solutions utilizing vigorous stirring of the solutions or electrolytes. Several methods of vigorously stirring the electrolyte were discussed in U.S. Pat. No. 1,953,316, namely, (1) by air, (2) by external pumps, (3) by rotating the cathodes or anodes, and (4) by mechanically agitating the solution between the anode and cathode.

Hickman et al. found air stirring, external pumping and rotating electrodes to be generally undesirable. Air stirring was found to be inefficient, external pumps were subject to detrimental corrosion by the fixing solution and rotating electrodes involved complicated systems for passing electric current to the electrode. Mechanical agitation by revolving a stirrer between the electrodes was found to permit an increase in limiting current density from 0.050 a./ft. to about 5 a./ft.

However, even with the large increase in plating rates obtained by Hickman et al. (approximately times), the electrodeposition of silver from used fixing solutions still involved too much time or excessively large equipment and, therefore, apparently never obtained commercial acceptance.

SUMMARY The present invention is directed towards an improved process for regenerating used photographic fixing solution by electroplating dissolved silver therefrom. The silver is recovered as a bright pure electrodeposit and the regenerated fixing solution is fully capable of being reused in the photographic process. The improved process is capable of operating at current densities in the range of 100-300 a./ft. without degrading the used fixing solution nor producing sulfide sludge at the cathode.

The maximum operable currentdensity for conventional processes such as Hickmans is 5 a./ft. With current densities in excess of 5 a./ft. the conventional processes degrade the thiosulfate fixing solution and form sulfide sludges at the cathode rather than silver. The large, unexpected increase in plating rates of the present invention decreases the time required in processing the used fixing solution. The equipment required to process large amounts of used fixing solution by the present process is simplified and greatly reduced in size compared to the equipment suggested by the prior art.

The unexpected increase in operable current densities of the present process is obtained by mechanically activating the surface of the electrode and subsequently of the electrodeposit formed on the electrode. Activating" the surface of the electrodeposit within the meaning of the present invention means so treating the surface as to create at such surface a high tendency to utilize the current to deposit the metal in sound. adherent form. In addition, the action of the activating medium, hereinafter explained, results in the removal of the stagnant layer overlying the electrodeposit. By removing the stagnant layer, a high concentration of silver ions is maintained in the neighborhood of the cathode. The activating" effect and the removal of the stagnant layer overlying the elec trode produce a synergistic effect whereby high quality silver can be plated from the used fixing bath at greatly increased depositing rates (increases operable current density) with simultaneous regeneration ofthe fixing solution.

The mechanical activation" is achieved by repetitively contacting the surface of the electrodeposit with what is termed herein as dynamically hard particles. By this term is meant that the combination of the hardness of the particles, the contact pressure ofthe particles on the surface of the electrodeposit and the speed at which such particles are moving relative to the electrodeposit surface is such as to produce an action on such surface sufficient to mechanically activate the surface.

The mechanism of activation is believed to be rather complex, consisting of several actions taking place essentially simultaneously. Among others, there appears to be new surface defect site generation resulting from distortion of the crystal lattice structure. This provides for uniform formation of many more active sites for depositing silver than is the case absent this mechanical distortion. Additionally, formation of any dominant asperities or dendrites is prevented by the cutting and crushing effect of the dynamically hard particle contact. This action results in substantial elimination of the current robbing which takes place at the asperities formed in normal plating and, in addition, eliminates the crevices and areas between the asperities where stagnation of the fixing solution can occur. The elimination of the asperities is believed to he one ofthe major contributing factors to the ability to maintain high current densities for substantial periods of time while obtaining acceptable deposits without formation of sulfide precipitates.

The process requires the use of a surface disturbing or activating medium having the characteristics of providing a plurality of small, dynamically hard, relatively inflexible particles held in substantially fixed, spaced relationship to one another and generally vertical to the surface receiving the deposit by a preferably porous matrix which also provides a plurality of liquid entrapping or sweeping members extending parallel with and closely adjacent to the surface being plated. The process further requires relative motion during the deposition operation between the surface receiving the deposit and the activating medium. In addition, sufficient pressure is applied to said activating medium in a direction normal to the electrodeposit surface to cause mechanical distortion of the crystal lattice structure of the metal deposited thereon. The spacing of the particles and the speed of relative movement is such that the deposited metal surface above any given point one the cathode surface is contacted or influenced by a particle at extremely short time intervals, e.g., intervals in the range of 6.l l to 3.8 l0 seconds. Fresh electrolyte is supplied to the zones of activated metal deposit at the same rate through entrapment by the liquid entrapping or sweeping member of the activating medium (which members may be the edges ofthe particles) parallel with the electrodeposit surface. These members sweep fresh electrolyte along with them,

the electrolyte reaching such members due to the porosity of the supporting matrix of the activating medium or through proper disposition of the electrolyte supply adjacent the contact area between the activating medium and the electrodeposit surface.

Accordingly, the principal object of the present invention is the provision of a high speed silver electrodeposition process from a used photographic fixing solution whereby the silver is obtained in a pure, bright deposit on the cathode and the fixing solution is regenerated for reuse in the photographic process.

DRAWINGS FIG. 1 illustrates schematically one preferred embodiment of apparatus for performing the process of the present inventron.

FIG. 2 is a schematic illustration of another preferred embodiment of apparatus for performing the present invention.

FIG. 3 illustrates schematically a third embodiment of the present invention.

FIG. 4 illustrates schematically a fourth variation in the embodiment shown in FIG. 3.

FIG. 5 illustrates diagrammatically a portion of a cross section of one type of porous activating means useful in the present invention.

FIG. 6 illustrates schematically another embodiment for performing the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS The process of the present invention requires the controlled application under pressure to the surface of the electrodeposit of a supporting, preferably porous, matrix which supports in closely spaced relationship a plurality of small, relatively inflexible particles. These particles are so positioned in the matrix as to contact the deposit forming on the cathode surface and relative motion between such particles and the deposit must be maintained during electrodeposition. The cathode surface, itself, is normally covered during electroplating with a relatively stagnant layer of electrolyte which may be identified as the diffusion or polarization layer. The thickness of this layer, even at high flow rates of electrolyte or turbulent agitation of a plating bath, is at least 0.00l centimeters. It is this layer, as explained above, which becomes depleted in silver ions. The sodium ions being produced at the cathode then react with the thiosulfate solution yielding the unwanted sodium sulfide which diffuses through the diffusion layer and reacts with silver ions to precipitate silver sulfide. Under application of the supporting matrix and associated particles according to the present invention, this polarization layer is repetitively removed. As described above, the process aetivates the electrodeposit surface by apparently multiplying many times the number of nucleation sites on such surface and generating a controlled growth of a tremendous number of very short asperities which are repetitively restricted in vertical growth throughout the deposition cycle. The metal deposit reflects this action since photomicrographs of the cross sections ofsuch deposits illustrate a structure in which the growth axis of the crystals appears substantially parallel to the substrate rather than showing the normal columnar vertical orientation ofconventional electrodeposits This technique has been found to increase the limiting current density many times beyond that possible with conventional methods, resulting in much more rapid sound metal deposition without sulfide precipitation than is possible with the conventional methods. The present method has further been found to produce a hard, dense, smooth silver deposit.

To insure adequate activation" of the surface it is necessary to apply sufficient pressure to produce a light scratch pattern in the silver deposit. Thus the dynamic hardness of the particles may be substantially greater than the actual hardness, e.g., a resin particle may produce a scratch in the harder silver deposit. This scratch pattern may be visible to the naked eye but, in any case, will be seen under a magnification of 10,000 power or less. While the scratches may be produced by minute metal removal, preferably the dynamic hardness is so controlled that a displacement of metal atoms on the surface rather than actual removal is the basis for the scratch formation.

By using small, relatively inflexible, nonconductive particles as the activating tool, no spot on the deposit surface is covered for any appreciable length of time by the activating particle. These particles are generally randomly distributed over at least the cathode surface-contacting side of the matrix and are preferably spaced in fixed relation to one another over very short spans, e.g. 1.25 l 0 to 565x inches. lfdesired, accurate and non-random. distribution of the particles on the supporting matrix can be resorted to although this is generally an unnecessary complication. By the term particle" as is used herein is meant not only completely separate and discrete three dimensional bodies, but also larger bodies with a plurality of points, tips, projections or the like thereon as for instance a relatively hard resinous coating on a fiber wherein the coating contains multiple irregular spaced projections and is generally uneven in nature. The particles, as described herein, contact or at least influence essentially all of the surface of the electrodeposit and are believed to knock down or cut off, if they form, most of the dominant asperities on such surface. The particles themselves may vary widely in size from 1X10" 5 inches to l.25 l0 inches (average diameter) for example,

but should generally be in the size range of from 9X10 inches to 2Xl0 inches for best results. The particles can generally be defined as hard, i.e., having a Knoop hardness in excess of 10.0, but the degree of hardness per se is not critical except that control should be exercised not to use a product which is too abrasive for the silver being deposited. The degree of pressure applied must also be considered with respect to the hardness of the particles and generally with the softer range of particles more pressure normal to the cathode surface is required than with the harder range of particles.

As indicated above, the controlling factor is the dynamic hardness of the particles, i.e., the apparent hardness resulting from a combination of the actual Knoop hardness, the pressure applied and the speed with which the particles are moved across the electrodeposit. A visible indication that the dynamic hardness is sufficiently high is the presence in the deposit of the scratches visible under 10,000X magnification.

The most graphic effect of the present process is the increase in practical limiting current density achieved without the formation of silver sulfide in the regenerated fixing solution. As is well known, the current density is directly related to the speed of metal deposit. The limiting current density is achieved when the application of increased voltage ceases to result in further electrodeposition of silver and instead produces the unwanted silver sulfide precipitate.

The matrix used to support the activating particles is preferably electrolyte-permeable, having a through porosity in the order of at least 6.5 Sheffield units (as measured by a Sheffield porosimeter using a 2%-inch ring). Preferably, this matrix is also at least somewhat compressible and deformable so that it can be conformed to irregular surfaced cathodes and associated deposits where necessary. As indicated above, the matrix preferably has a plurality of thin-walled members extending between the activating particles to act as electrolyte sweeps. While these members may be the edges of the particles themselves, in the preferred embodiments these thinwalled members are formed by the porous matrix and define small compartments or pores of either regular or irregular shape which function much like a bucket conveyor in carrying small quantities of electrolyte over the activated elec trodeposit surface. Many variations of porous supporting matrices have been used, e.g., open mesh screens with activating particles adhered to the mesh; nonwoven abrasive articles, both compressed and uncompressed; open cell foam sheets with the activating particles incorporated in or on the foam cell walls; sponge materials containing the required particles shown by the arrows.

and the like. Examples of products which can be used in the present invention as activating media are illustrated in U.S. Pat. No. Re. 21,852 to Anderson which shows an open mesh product having abrasive grains adhered thereto; in U.S. Pat. No. 3,020,139 to Camp et al. which illustrates nonwoven webs having a plurality of hard particles adhered to and along the web fibers; in US. Pat. No. 3,256,075 to Kirk et al. which illustrates a sponge containing hard resin-impregnated sponge particles; and in U.S. Pat. No. 3,334,041 to Dyer et al. which illustrates a coated abrasive product having perforations through which electrolyte can flow. In this latter instance, the product must be modified for the present process by making it nonconducting, i.e., it essentially becomes a standard coated abrasive product with electrolyte-passing holes therethrough.

In some instances, such as when it is desired to reduce the anode-cathode spacing to a minimum, a nonporous matrix may be used. A suitable nonporous product is illustrated in U.S. Pat. No. 3,377,264 to Duke et al. wherein a coated abrasive sheet is provided with a front conductive layer of metal through which protrude the tips of nonconductive abrasive grains. This product for use in the present process must have as the conductive layer of metal an inert metal such as lead or antimony or alloys thereof. The tips of the abrasive particles cooperate with the metal layer therebetween to form compartments which serve as electrolyte sweeps to move the electrolyte to the face of the electrodeposit. Similarly, the product ofaforementioned Dyer et al. patent, U.S. Pat. No. 3,334,041, can be used with rivets. or similar conductive paths provided from the back of the product. Additionally, the matrix may be in particulate form, e.g., spheres, with a plurality of dynamically hard particles adhered to or protruding from the surface thereof in spaced relationship to one another.

Referring now to FIG. 1 of the drawings, the minimal elements of the apparatus of the present invention are disclosed and identified by legends. As shown, a particle supporting means A (here shown in porous, endless belt form) is provided in combination with an electrolyte B and electrodes C and Dv Relative motion between the porous means A and the cathode surface (or the plate depositing thereon) and, if desired, between such porous means A and the anode surface D (shown in dotted lines as D for this variation) is provided as This relative motion continues throughout that portion of the plating cycle when high speed deposition is required.

Referring again to the drawings, FIG. 2 is a schematic plan view of one form of apparatus for performing the process of the present invention. The used photographic fixing solution 1 1,in container 10, has positioned therein electrode means 12 and 13 comprising an anode 12 and a cathode 13 connected to means 22 for supplying current thereto. The cathode I3 is the member to be plated and that portion thereof which it is desired to plate is suspended in the electrolyte bath 10. Ad-

jacent to the face 14 of the cathode 13 to be plated is the ac tivating member. As illustrated, this is a drum or cylinder 15 of porous material such as nonwoven fibers 16 having a plurality of small, hard particles 17 adhered to the fibers 16 by a suitable'adhesive. Drum 15 is mounted for rotation on a shaft 18 drived by a suitable motor 19. lfdesired, the drum 15 may also be oscillated up and down as illustrated by the arrows 20 as well as rotated. Motor 19 and the associated shaft and drum assembly can be moved laterally on support 21 to vary the pressure applied to the cathode 13 by the activating drum l5. Rotation of the drum 15 initially against the face 14 of cathode 13 and thereafter against the electrodeposit 14' causes the previously described activation of the electrodeposited layer 14 which builds up on the face 14. This rotation, which continues for the time the electrodeposit 14 is forming, also causes fresh electrolyte to be pumped across the face 14' of the deposit by the entrapping action of the fibers forming the porous cylinder 15. The portion of cathode 13 which does not come into contact with drum I5 is preferably coated with a nonconducting material such as plastic or rubber.

FIG. 3 schematically illustrates another embodiment of apparatus useful in the process of the present invention. The apparatus is positioned in a tank 30 containing the used photographic fixing solution 31. A rotating inert, perforated backup disc 37 is mounted on drive shaft 33. Adhered to the backup disc 37 is a porous media 34 containing spaced particles as described herein. This activating medium 34 is in contact with the electrodeposit 35 of silver forming on the face of cathode 36 which is connected to power supply 38. As the electrodeposit grows, the shaft 33 may be moved away from the cathode 36, keeping a constant pressure between the activating medium 34 and the electrodeposit 35, ifdesired.

FIG. 4 illustrates apparatus similar to that shown in FIG. 3 wherein the backup disc 41 is made of an inert conducting material such as lead. The backup disc is then connected to the source of power and acts as the anode.

FIG. 6 illustrates still another embodiment of apparatus useful for the present invention. The apparatus is positioned in tank 60 containing used photographic fixing solution 61. Anode 62 is moveably mounted within the electrolyte in contact with a porous activating medium in the form ofa continuous belt 64. Power supply 69 connects to cathode 63 and a layer of pure silver 68 is deposited on cathode 63. Belt 64 rotates on

idler rollers

65 and 66 and driver roll 67. The belt and associated rollers are capable of adjustment away from the surface ofcathode 63 as the deposit 68 builds up. In operation, the activating belt wipes the electrodeposit surface for the reasons herein elsewhere described in detail and as illustrated in FIG. 6 also wipes the surface of the anode. However, the anode surface can be moved out of contact with the activating belt.

FIG. shows a highly enlarged and idealized portion of one type of activating media suitable for use in the present invention and illustrates the hard particle-connecting matrix relationship. Reference numeral 55 represents fibers of a nonwoven web (nonconducting fibers such as poly[ethylene terephthalate] or the like) which are anchored one to the other at their points of intersection by an adhesive binder 56. A plurality of small, hard, discrete particles 57 are positioned on the fibers S5 and in the present illustration are held to such fibers by the adhesive 56. At least some of the fibers 55 extend relatively parallel to the cathode face 59 as shown at 58 to form the thin-walled cells or electrolyte sweeping members referred to above. (For purposes of illustration, the activating particles 57 are here shown at some distance from the cathode face 59 and associated electrodeposit 51 although in operation of the present apparatus they would be in contact therewith.)

EXAMPLE 1 A porous activating device was made up by first forming on a Rando-Web machine (as described in US. Pat. No. 3,020,319 to Camp et al.) a nonwoven web from 3-denier polyester fibers of two inch fiber length. The web was needlepunched (multiple equispaced, barbed needles were pushed through normal to the plane of the web and forced fibers caught by the barbs through the web to mechanically interlock the fibers. The web had an average weight of 175 g./yd. and was approximately 0.1 inches thick. The needle-punched web was then roll-impregnated with a curable urethane emulsion containing 600-grit silicon carbide abrasive particles (40 percent solids). This slurry consisted of 70 percent urethane emulsion and 30 percent abrasive grain by weight. The deposited weight of impregnant was 14 pounds per sandpaper ream. The impregnated web was air dried at 50 percent relative humidity and 70 F. for one hour and then air cured at 250 F. for 5 hours.

Using the equipment arrangement as is illustrated in H6. 3, silver was electroplated from a used fixing solution of 300 parts hypo (sodium thiosulfate), parts Na SO 10 parts acedic acid, 10 parts of chrome alum, approximately 4 parts of silver and 1,000 parts of water.

The plating tank 30 was filled with the used fixing solution described above. A 5-inch diameter disc of the abovedescribed activating means 34 was adhered firmly to the surface of a 5 inch diameter perforated plastic backup plate 37 and mounted on drive shaft 33. A graphite cathode 36 was situated beneath the surface of the fixing solution so that it could be moved into contact with the surface of the activating means 34 adhered to the backup plate 47. The contact area of cathode 36 with activating means 34 was 0.133 in.*. The contact pressure between the cathode 36 and the activating means was 421 g./in. The cathode 36 was so mounted that it could also be withdrawn one-eighth inch away from activating means 34. The activating means 34 was set in motion such that the average velocity ofthe surface in contact with the cathode 36 with respect to the cathode was 86 ft./min. in all runs whether contact between the cathode 36 and activating means 34 was made or not. Anode 32 was placed as shown in FIG. 3, on the opposite side of the activating means with respect to the cathode 36. 4

The results of several samples run at varying current densities and with or without abrasive contact between the activating means 34 and cathode 36 are shown in Table 1.

TABLE 1 Contact pressure between cathode and activating means None 5 5 Cur- rent density, a.s.f.

Appearance of solution at end of run Time, min.

Cathode deposit appearance 0. do Thick sludge deposit, no silver.

(a) ego! OOl MOI Run 1 corresponds as closely as possible with the apparatus of the present invention, to the conditions found operable by Hickman in U.S. Pat. No. 1,954,316. It is noted that a small amount of dull silver was deposited on the cathode and no clouding or forming of precipitates in the fixing solution was observed. When the current density is increased to 50 a./ft. with only violent agitation occurring and with no contact between the cathode 36 and the activating means 34 (run 2), no silver was deposited and a slight clouding of the fixing solution appeared after only 5 minutes of operation. When the current density was increased to 100 a./ft. with only violent agitation occuring and with no contact between the cathode 36 and activating means 34 (runs 3, 5), no silver was deposited and the fixing solution appeared cloudy after only 5 minutes of operation. After 30 minutes of operation at I00 a./min. with no contact between the cathode 36 and activating means 34 (run 8) a thick sludge deposit containing no silver was formed on the cathode. These results are again consistent with the teaching of Hickman in U.S. Pat. No. 1,954,316.

Runs 2, 4, 6 and 7 show clearly the synergistic effect of the present invention. When abrasive contact was made between the cathode 36 and rotating activating means 34 at varying current densities up to 300 a./ft. a bright silver deposit formed on the cathode and the fixing solutions remained clear.

The amount of silver removed from the fixing solution per unit time is directly proportional to the current density. Thus, it can be seen that the present process can regenerate up to 60 times as much used fixing solution than the prior art methods using similarly sized equipment. Tests of film clearing time (time required to remove the silver halide in the emulsion layer of fresh film) for the various solutions resulting from runs 2, 4, 6 and 7 showed, as to be expected, that as the silver content of the solution decreases, the clearing time also decreases.

The type of movement of the activating media over the surface of the cathode may be varied widely, i.e., it may be linear as well as rotative; it may be a combination of movements, e.g. a rotating device which is also oscillated as it rotates, etc. Likewise, this relative movement can obviously be achieved with a moving cathode and stationary activating media or a combination of movements ofboth.

The activating media described herein may likewise be varied widely, both in shape or configuration and in composition. The requirements of the supporting members and associated dynamically hard particulate materials have been discussed in detail above. Any nonconductive fibrous material capable of resisting erosion by the fixing solution and capable of producing the described supporting matrix may be used for the porous matrix as well as nonfibrous material such as sponge, foam, or the like. The nonconductive particulate activating materials likewise are noncritical in that many materials such as resin particles, abrasive grain, ceramic particles, glass particles, walnut shells or the like can be utilized.

The pressure of the activating medium on the electrodeposit surface, which as indicated above is variable depending on the particular activating particle used and the system in which it is used, may either be held relatively constant throughout any given deposition operation or varied, as desired, during the operation within the limits set by the requirement of the development of the aforementioned scratch pattern and the practical limit set by removal of undue amounts of metal. Pressures from as low as 0.035 p.s.i. up to 25 psi. have been used. Generally, pressures in the range of from 0.1 to 0.5 p.s.i. are sufficient to produce the desired scratch formation and are usually preferred.

What I claim is:

1. In a process of electrolyzing a used photographic fixing solution to recover silver therefrom and regenerate said fixing solution for further use in the photographic process comprising impressing an electrical potential across an anode and a cathode in said solution whereby an electrodeposit of silver is obtained on said cathode, said anode being of a substance other than silver, the improvement comprising:

a. interposing a supporting medium having a plurality of hard particles supported thereon between said anode and said cathode;

b. establishing contact under pressure between said supporting means, said hard particles and at least a surface of said cathode;

c. establishing relative motion between (i) said supporting medium and said hard particles and (ii) said anode and cathode thereby simultaneously agitating said used fixing solution, sweeping a portion of said used fixing solution across said surface of said cathode and (iii) mechanically activating the surface of said cathode; and

d. continuing said relative motion and contact during the period of impressing said electrical potential across said anode and cathode whereby (i) the surface of said silver electrodeposit forming on said cathode surface is continuously activated by the repetitive contact at extremely short time intervals of said hard particles with said surface of said silver deposit, (ii) said fixing solution is continuously agitated and (iii) fresh portions of said fixing solution are continuously swept across the surface of said silver deposit on said cathode at extremely short time intervals.

2. A process as in claim 1 wherein said supporting medium comprises a permeable matrix having a plurality of small particles adhered thereto in fixed spaced relationship one to the other.

3. A process as in claim 2 wherein said nonwoven web.

4. A process as in claim 2 wherein said matrix is an openweave fabric.

5. A process as in claim 1 wherein said supporting medium is continuously moved relative to said cathode surface.

6. A process as in claim 1 wherein said particles have a dynamic hardness sufficient to produce a scratch in said electrodeposit visible under magnification of 10,000 power.

7. A process as in claim 1 wherein said particles comprise abrasive grain.

matrix is a porous

Claims

2. A process as in claim 1 wherein said supporting medium comprises a permeable matrix having a plurality of small particles adhered thereto in fixed spaced relationship one to the other.
3. A process as in claim 2 wherein said matrix is a porous nonwoven web.
4. A process as in claim 2 wherein said matrix is an open-weave fabric.
5. A process as in claim 1 wherein said supporting medium is continuously moved relative to said cathode surface.
6. A process as in claim 1 wherein said particles have a dynamic hardness sufficient to produce a scratch in said electrodeposit visible under magnification of 10,000 power.
7. A process as in claim 1 wherein said particles comprise abrasive grain.