US3663416A - Silver recovery device for photo fixer solutions - Google Patents

Abstract

A PLASTIC CYLINDRICAL CONTAINER HAVING A REMOVABLE TOP LID WHICH IS ADAPTED TO RECEIVE A QUANTITY OF USED PHOTOGRAPHIC SOLUTION. AN ANODE STRIP EXTENDS IN A SPIRAL CONFIGURATION ABUTTING THE INSIDE WALLS OF THE CONTAINER. THE ANODE COMPRISES A PLURALITY OF TURNS WHICH DECREASE IN AXIAL SPACING FROM TOP TO BOTTOM ALONG THE CONTAINER WALLS. A CENTRAL CATHODE IN THE FORM OF A FLEXIBLE BLADE IS SUSPENDED FROM THE CENTER OF THE CONTAINER COVER AND

IS SPACED FROM THE ANODE. THE CATHODE EXTENDS ALONG THE FULL HEIGHT OF THE CONTAINER. A DIRECT CURRENT POWER SUPPLY IS CONNECTED TO THE ANODE THROUGH THE TOP CONTAINER WALL AND THROUGH THE COVER TO THE CATHODE. RECOVERED SILVER IS REMOVED FROM THE CATHODE BY FLEXING THE BLADE SURFACES SLIGHTLY.

Description

May 16, 1972 R. J. COOPER ET L I 3,663 416 SILVLH IH'JCJVLRY DEVICE FOR PHOTO FIXER M Filed Sept. 4, 1970 2 Sheets-Shee t l INVENTOR.

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May 16, 1972 R. J. COOPER E L 3,563,416

SILVER RECOVERY DEVICE FOR PHOTO FIXER SQQQIC'XS Filed Sept. 4, 1970 2 Sheets-5heet I 5 l 5 K 2 G+ E P! I /0 INVENTOR. /2 205527 .1 cooPEz VIC TOR. V- (AKA/ELL AT T YO.

United States Patent 3,663,416 SILVER RECOVERY DEVICE FOR PHOTO FIXER SOLUTIONS Robert J. Cooper and Victor V. Camel], Spokane, Wash., assignors to Noble Metals, Inc. Filed Sept. 4, 1970, Ser. No. 69,572 Int. Cl. B01k 3/04 US. Cl. 204-272 4 Claims ABSTRACT OF THE DISCLOSURE A plastic cylindrical container having a removable top lid which is adapted to receive a quantity of used photographic solution. An anode strip extends in a spiral configuration abutting the inside Walls of the container. The anode comprises a plurality of turns which decrease in axial spacing from top to bottom along the container walls. A central cathode in the form of a flexible blade is suspended from the center of the container cover and is spaced from the anode. The cathode extends along the full height of the container. A direct current power upply is connected to the anode through the top container wall and through the cover to the cathode. 'Recovered silver is removed from the cathode by flexing the blade surfaces slightly.

BACKGROUND OF THE INVENTION Recovery of silver from photographic solutions such as the fixing-bath used in developing film, has generally been confined to large scale users and has not been economically or practically feasible for small users, such as dentists, clinics, and others who regularly develop film for photographic or X-ray use, but on a more modest quantitative scale. Recovery of silver is desirable because it conserves a natural mineral resource of scarce supply, the recovery operation can be economically profitable, and can sometimes result in more eflicient use of the fixing-bath chemicals.

The greatest percentage of recoverable silver in a photographic process is normally found in the fixing-bath. The fixing-bath chemically converts undeveloped silver halides to a soluble compound which can be removed from the emulsion by dissolution in the fixing-bath or by washing away in the wash water. Fixing-baths generally used in black-and-white photography contain either ammonium thiosulfate or sodium thiosulfate. Ammonium thiosulfate fixers are exhausted primarily by dilution caused by solutions carried into the bath on the film and fixer ingredients being carried out of the bath on the film. Silver build-up in such fixers has relatively little etfect on their fixing rates. Removal of silver from this type of fixer does not extend its life if it is already used to the point of exhaustion. Films fixed with sodium thiosulfate fixers on the other hand, are more sensitive to silver build-up and therefore removal of silver from this type of fixer keeps the fixing time short and appreciably increases the fixer life.

There are three principal known methods of silver recovery from photographic processing solutions: metallic replacement, electrolytic plating and chemical precipitation. Metallic replacement involves the addition of solid ice metal (such as steel wool or zinc dust) to replace the silver ions in solution, the silver then being recoverable as a precipitate. Electrolytic methods of recovery involve the passage of a direct electrical current between two electrodes in the solution, whereby silver is deposited on the cathode or negative electrode in the form of nearly pure silver plate. Chemical precipitation requires the addition of compounds to the solution to precipitate the silver in the form of a sludge.

Electrolytic recovery has been primarily confined to large scale operations because of the relatively high price of conventional equipment of this type. The process is relatively clean and yields silver of a high degree of purity. It has the advantage of increasing the life of so dium thiosulfate fixing-baths. Prior equipment has required frequent monitoring and agitation of the solution and has posed mechanical difiiculties in the stripping of silver from the cathodes.

Metallic replacement recovery methods have been adapted to small scale operations. The devices require little attention. However, the fixing-bath has no further photographic use after silver recovery because of the metal values added to it.

Chemical precipitation, while economical and efficient, requires long settling times and also renders the fixing- .bath unsuitable for reuse in photographic processes.

The present apparatus arose in an eflort to develo a practical electrolytic system for recovering the silver from spent photographic solutions at the users location, specifically being directed to medical and dental ofiices and clinics where considerable quantities of X-ray and photographic film are used and developed. The present apparatus is adapted to be used with a minimum of manual effort and to substantially eliminate the need for agitation of the solution, which is prevalent in prior electrolytic arrangements of this type. The apparatus is further defined to facilitate the removal of deposited silver without requiring special tools or diflicult techniques.

Several examples of prior devices for recovering silver from photographic solutions are disclosed in the following US. Patents:

Aukerman, 1,425,935; Gilten, 1,629,212; Garbutt et al., 1,866,701; Doran, 2,158,410; Willier, 2,615,839; Aukerman, 2,619,456; Mandroian et al., 3,072,557; Cothran, 3,450,622.

The devices disclosed in the above patents are generally complicated in structure and cannot be economically used on the scale necessary for application to individual medical or dental oflices. Most of these devices require constant monitoring and many specify the need for agitation of the solution during use. The electrode elements disclosed in the patents are not readily access ble for cleaning or replacement purposes. While some might be practical for use in a specialized firm devoted to the business of recovering silver, none would be practically adaptable to use by technicians or professionals working in medical or dental offices and recovering silver as a by-product of normal photographic and X-ray operations in such offices.

The present apparatus provides a relatively simple device which requires no constant monitoring and little agitation of the solution. Recovery of silver by use of the apparatus is substantially automatic and removal of the deposited silver is rendered exceptionally simple. It can be designed to handle quantities of solution down to one gallon on an economical scale and can be increased in size to meet various capacity requirements for different user applications. All of the elements of the apparatus are readily accessible for cleaning, maintenance and replacement when necessary. It is adapted to be powered from a conventional alternating current wall socket by a power supply requiring no adjustment or monitoring.

SUMMARY OF THE INVENTION The invention comprises the combination of an upright cylindrical container of electrically insulated material capable of holding the photographic solution, together with an upright central plate cathode of relatively thin material and a surrounding spiral anode abutting the container interior walls. A power supply is operatively connected to the cathode and anode to supply direct current and thereby eifect the depositing of metallic silver on the surfaces of the cathode.

One object of the invention is to provide an economical design for an electrolytic apparatus that will lend itself to use in user locations Where film is developed, such as in medical and dental ofiices or clinics.

Another object of the invention is to provide such an apparatus which does not require frequent monitoring or manual observation during use.

Another object of the invention is to provide such an apparatus which facilitates the removal or stripping of silver from the cathode.

These and further objects will be evident from the following disclosure, taken along with the accompanying drawings which illustrate a preferred form of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the silver recovery device;

(FIG. 2 is a vertical cross-sectional view taken through the center of the device;

FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. '2;

FIG. 4 is an enlarged cross-sectional view taken through the cathode along line 44 in FIG. 2;

'FIG. 5 is a view similar to FIG. 2, showing a second form of cathode; and

FIG. 6 is a cross-sectional view taken along line 66 in FIG. 5

DESCRIPTION OF THE PREFERRED EMBODIMENT Two forms of the invention are disclosed in the drawings. The tfirst is illustrated in detail in FIGS. 1-4 The modified portions of the second embodiment are illustrated in FIGS. 5 and 6. The exterior components of the device are identical in both embodiments and common numerals are applied to these elements in all of the figures. The embodiment shown in FIGS. 1-4 has been specifically designed for larger applications, such as a five-gallon capacity container. The embodiment illustrated in FIGS. 5 and 6 has been designed for smaller applications, such as for use as a one-gallon capacity container.

Referring to FIG. 1, the apparatus is housed within a liquid-tight container 10 preferably molded of a suitable plastic resin impervious to the chemical solutions treated therein. Container 10 includes cylindrical side walls 11 and an integral bottom wall 12. In use, the bottom wall 12 is horizontal and the center axis of container 10 is upright and vertical. The open top of container 10 is selectively sealed by a removable cover 13, preferably of the same material as the container 10 itself. A flexible peripheral seal of any conventional design may be used to removably secure the cover 13 to the container 10. Other attaching devices may be substituted. Container 10 provides an enclosed working environment for the photographic solutions received therein, and is essentially sealed to minimize evaporation and the evolution of the gases. However, slight ventilation is desirable to prevent the build-up of gases within the container.

Suspended from the center of cover 13 is a central cathode 14 in the form of flexible metal plates. Cathode 14 extends the full length of container 10 between the cover 13 and bottom wall 12. It is surrounded by a spiral anode 15 that abuts the inside wall surfaces of container 10. The anode 15 also extends substantially the full height of container 10. Thus, all of the solution within container 10, which is normally filled to the dashed line indicated at 23 in FIG. 2, lies within the area between the outer anode 15 and cathode 14, providing a most efiicient electrical path for electrolysis purposes.

Direct current electrical power is provided to the electrode elements within container 10 from a power supply 16. Power supply 16 is adapted to be inserted into a conventional alternating current electrical outlet. It includes a step-down transformer and an interposed rectifier to reduce the normal residential or ofiice current and provide full or half-wave rectification. The power supply 16 delivers direct current to power leads 17, 18 at a low voltage of the order of 1.0 to 1.5 volts. The leads 17, 18 are releasably connected to cathode 14 and anode 15 through the center of cover 13 and the side walls 11 respectively! The details of the cathode construction are best understood from FIGS. 2 and 4. As shown, the cathode is constructed from two identical bent blades 20 having relatively large upright areas that project radically at about from one another. The blades 20 are centrally connected along a vertical center bar 21 of conductive material. The upper end of bar 21 protrudes beyond the top edges of blades 20 and projects through a complementary slot at the center of cover 13 to permit connection to the power lead 17 (FIG. 1). The blades 20 are joined along the bar 21 by connecting bolts 22.

The blades 20 and the spiral strip that forms anode 15 are preferably of stainless steel, having a relatively thin cross-section which permits the cathode blades 20 to be slightly flexed by manual manipulation Flexing of blades 20 is desirable to facilitate the removal of the deposited silver therefrom. i

The design of cathode 14 is such that its exposed surface are-a exceeds the surface area of anode 15 by 50% to This avoids the build-up of high current density on the cathode and helps to prevent the deposition of sulfides rather than the desired metallic silver.

As can be seen in FIG. 2, the axial spacing of the multiple turns in the spiral anode 15 decreases from top to bottom. Thus, a greater concentration of anode surface area is progressively developed towards the bottom of container 10. The side walls 11 of the container 10 should be substantially cylindrical to maintain the lateral separation between the cathode and anode at a constant distance throughout the height of container 10. The greater concentration of anode area toward the bottom of container 10 compensates for the higher concentration of silver formed toward the lower portions of fixer solution.

The design of the cathode and anode is such that the current density is no greater than 1.5 milliamperes per square inch of cathode surface on the blades 20. A current density above this amount results in a tendency to deposit silver sulfide rather than the desirable metallic silver.

As seen in FIG. 2, both the cathode 14 and anode 15 are completely submerged in the liquid fixer-bath solution. The projecting portions of bar 21 are preferably plasticcoated to minimize corrosion at the solution line.

The modification shown in FIGS. 5 and 6 relates to a more simple cathode construction designed for small quantity use where the volume of fixer-bath solution being treated for silver recovery might be on the order of one gallon. The only modification of substance differing from the first embodiment relates to the cathode 14, which is indicated to be in the form of a single elongated strip 24 bent in an inverted U-shaped configuration with two parallel spaced legs extending the full height of the container 10. The upper end of strip 24 has a protruding conductor 25 connected to it and received through the center of the container cover 13. Again, the design of strip 24 is such that the area of strip 24 beneath the solution level Within container is approximately 50% greater than the surface area of the spiral anode 15. The strip 24 is also preferably of flexible stainless steel to permit manual release of deposited silver therefrom.

To use either embodiment of the invention, the spent photographic fixer-bath solution is placed within container 10, which is filled to the solution line indicated at 23 in FIG. 2. The anode should be completely covered by the solution. The cover 13 is then placed on container 10 and the releasable leads 17, 18 are connected to the cathode 14 and anode 15 respectively. The power supply 16 can then be inserted into a conventional alternating current outlet and the assembly then may be left unattended for a 24-hour period.

The unit is designed to be monitored once every 24 hours. All that is required is the removal of cover 13, to permit visual inspection of the container contents. The cathode 14 can be used as a stirring rod to mix the solution thoroughly. The solution can be tested for silver content, using conventional analytical techniques, such as sensitive paper used in photographic processes to monitor the silver concentration in a fixer-bath.

When operating properly, the metallic silver plated on cathode 14 has a cream-white appearance. As the silver content of the solution becomes exhausted, the appearance of the plated cathode area will turn dark brown to black, indicating the presence of sulfiding. At this time, the power supply should be disconnected and the solution within container 10 either discarded or returned for additional photographic use.

The deposited silver is readily removed from the cathode blade by simply scraping off the metal with a straight-edged knife. If the plates 20 have been allowed to dry prior to silver removal, the metal will flake off following simple flexing of the stainless steel. The removed silver, which is of extremely high purity, can then be stored for further refining by conventional smelting and refining techniques.

In larger units, it may be advisible to reduce the current to the electrodes after initial deposition of silver on cathode 14. This can be readily accomplished by inserting a suitable resistance in the power connection to anode 15 to reduce the current density as desired.

One advantage of this arrangement is the full envelopment of the solution between the cathode and anode and the relatively large area of the cathode surfaces within the solution volume. We have found that with this arrange ment, it is not necessary to require constant agitation of the solution and that very infrequent mixing of the solution will suffice to assure efiicient recovery of silver therefrom.

Obviously, the apparatus can be designed in varying sizes to meet the requirements of particular users. The one gallon and five gallon sizes discussed in detail above are particularly adaptable to use in dental ofiices and small clinics. The size of the container 10 should preferably match the size of the fixer tanks in a particular photographic apparatus.

While we have found that the best anode and cathode material appears to be stainless steel, the disclosure is not to be limited to any particular constructive material. It is particularly desirable that the cathode blades be flexible to assist in easily removing the silver. This is important in the design of an apparatus to be used in professional ofiices for the recovery of silver as a by-product of photographic processes, since the use of the apparatus must be as simple as possible and not require unusual manipulation or the use of complicated tools or machinery.

The apparatus disclosed is designed for batch operation, but is not to be limited only to such use. A flow-through process might also be designed in a similar container, providing recirculation or constant flow through the container for electrolytic removal of silver, particularly from fixers solutions Which are reusable after silver removal. In a flow-through unit, silver will be collected first at the top of the cathode 14. Solution can be drained at the top of container 10 and introduced at its lower portion. A series of similar containers 10 might be used to provide an elongated working path for a constant flowing solution. If necessary, air may be added to the containers for agitation, although this has not been found to be essential for use of the apparatus for small scale batch operation.

Minor modifications might be made in the details of the device without deviating from the basic concepts discussed herein. For these reasons, only the following claims are intended as limiting definitions of the invention disclosed.

Having thus described our invention, we claim:

1. An apparatus for recovery of silver from used photographic solutions, comprising:

an upright cylindrical container of electrically insulating material impervious to the solution;

an upright cathode suspended within the container along the center thereof, said cathode comprising vertically disposed plates of an electrically conductive material;

a spiral anode in the form of a continuous strip of conductive material positioned against the inner cylindrical walls of said container;

said anode including a plurality of full turns along the container walls, the axial spacing of the turns being decreased along the height of the container from top to bottom; and

power supply means operatively connecting the cathode and anode respectively to the negative and positive terminals of a direct current source of electrical power.

2. An apparatus for recovery of silver from used photographic solution, comprising:

an upright cylindrical container formed of a non-conductive plastic resin impervious to the solution and having an upper lid removably sealed thereto and formed of like material, the cylindrical walls of the container being of substantially constant cross-section along the full height of the container;

an upright cathode suspended within the container at the center of said lid, said cathode comprising stainless steel plates extending the full height of the container, the plates being capable of flexure to assist in removal of metal deposited thereon;

a spiral anode fixed to the interior walls of said container, said anode comprising a strip of stainless steel abutting the interior walls and spaced outward from said cathode, said anode being in the form of multiple turns extending the full height of the container and progressively decreasing in axial separation from top to bottom;

and a direct current power supply connected to the cathode through said lid and to the anode through the walls of said cylindrical container.

3. An apparatus for recovery of silver from used photographic solutions, comprising:

an upright cylindrical container of electrically insulating material impervious to the solution;

an upright cathode suspended within a container along the center thereof, said cathode comprising vertically disposed planer strips of stainless steel capable of being flexed so as to assist in removal of metal deposited thereon;

a spiral anode in the form of a continuous strip of con- References Cited dnetive material positioned against the inner cylin- FOREIGN PATENTS drical walls of sa1d container;

and power supply means operatively connecting the 623,332 10/1962 Belglum 204-272 cathode and anode respectively to the negative and 5 c positive terminals of a direct current source of elec- HOWARD WILLIAMS Primary Examiner trical power. W. I. SOLOMON, Assistant Examiner 4. An apparatus as set out in claim 3 wherein the strips of stainless steel extend the full length of the container, 'U.S. Cl. X.R.

said strips being joined to one another along the center 10 204 1O9 271 axis of the container and being projected radically from said axis toward the container walls.

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