Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1999-12-17
2001-08-21
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S757000, C210S263000, C210S912000, C210S919000
Reexamination Certificate
active
06277290
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of the Invention
The invention relates to a process for recovering silver metal from an ionic silver-bearing spent photographic solution; and in an especially preferred embodiment exploits reduction of silver ions to silver metal with a photographic developer reagent as the reducing agent.
ii) Description of Prior Art
The commercial processing of photographic materials, industrial X-rays, medical X-rays and graphical arts produces spent solutions containing silver ions. Environmental regulations restrict the discharge of such solutions to within acceptable concentrations of silver that are much less than the concentrations of silver in the spent solutions. Various methods of removing silver from these solutions have been attempted, with varying expense and success.
The most common types of silver recovery in current use include the use of steel wool in a chemical replacement cartridge (CRC), electrolysis, ion exchange, and a precipitation method. Some CRCs are messy and the effluent therefrom can contain high concentrations of iron. Electrolysis is expensive, requires significant maintenance, and can result in ammonia gas generation. Ion exchange is costly and impractical for smaller photo processors and minilabs. Employment of sodium sulfide as a precipitating agent can result in the evolution of toxic fumes. Another approach has been use of a silver complexing agent to remove silver from photographic effluent solutions, however, complexing agents are often costly.
A problem with these approaches is that separate separation facilities and equipment are required, which can be inconvenient and expensive, particularly for small photoprocessing labs or minilabs. Furthermore, these processes do not provide a process capable of recovering silver from minilab effluent solutions such as photographic stabilizers to obtain a sufficiently low silver concentration acceptable for discharge to the environment.
U.S. Pat. Nos. 4,662,613 and 4,325,732 of Woog and U.S. Pat. No. 5,900,041 of Riviere and Ren describe recovery of silver by ion exchange between silver ions and iron metal. These processes result in an iron ion bearing discharge effluent from which the iron ions often precipitate as iron oxides.
U.S. Pat. No. 5,288,728 describes the use of a trimercapto-s-triazine complexing agent to precipitate silver in a silver complex. While the method lowers the silver concentration in the effluent, the cost of the complexing agent is significant, additionally it is a man made chemical with unknown long term effects on the environment.
No prior art is known that employs an organic reducing agent as the primary means of recovering silver from an ionic silver bearing spent photographic effluent.
SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for recovering silver metal from an ionic silver bearing spent photographic solution.
It is a particular object of this invention to provide such a process in which silver ions are reduced to silver metal with spent photographic developer solution.
In accordance with the invention there is provided a process for recovering silver from an ionic silver bearing spent photographic solution comprising: reducing silver ions in said solution to silver metal with an organic reducing agent.
DESCRIPTION OF PREFERRED EMBODIMENTS
a) Silver Rich Solution
The ionic silver-bearing spent photographic solution is referred to herein, for convenience, as a silver rich solution.
The silver rich solution may be, for example, a spent black and white photoprocessing fixer solution and which contains residual silver ions of the photographic film not employed in the development of the photographic image.
The silver rich solution may also be a spent colour photoprocessing solution.
The silver rich solution is any effluent solution or mixture of solutions, derived from photoprocessing and which contains silver ions in a recoverable amount.
Typically the silver rich solution will contain at least 10 ppm and generally at least 1000 ppm of silver ions, and the discharge solution after reduction of silver ions to silver metal will contain no more than 5 ppm of silver ions.
A completed photographic image is washed with water and the resultant wash water may contain silver ions, typically in an amount of 10 to 250 ppm. Such a wash water may be employed as a silver rich solution in accordance with the invention.
b) Organic Reducing Agent
The invention employs an organic reducing agent effective to reduce silver ions to silver metal. In the reduction reaction one silver ion receives one electron and is reduced to silver metal which exits or precipitates from the solution. At the same time, the organic reducing agent loses an electron and is thereby oxidized.
It is preferred to employ organic reducing agents which produce a harmless, non-toxic oxidation by-product.
The preferred organic reducing agent is a photographic developer reagent and for this purpose there is typically employed spent photographic developer solution. Spent photographic developer solution is developer solution which has been used in the photoprocessing procedure to develop a photographic image, and which contains residual developer reagent capable of reducing silver ions to silver metal. The solution is considered spent in that it is past optimum or satisfactory performance as a photoprocessing developer.
Typical developer reagents employed in photoprocessing developer solutions include pyrogallol, hydroquinone, catechol, p-phenylene diamine, paminophenol, methyl-p-aminophenol sulfate, amidol, pyramidal, and 4-(N-ethyl-N-2-methanesulfonylaminoethyl)-2-methylphenylenediamine sesquisulfate monohydrate; these developer reagents may be employed directly as the organic reducing agent, alone or in combination; or they may be employed as the residual developer reagent in spent developer solutions. In such case there will conveniently be spent developer solution generated at the site at which the silver rich solution is generated In this way a waste product, of the photoprocessing procedure, which itself presents a disposal problem is employed as the source of the organic reducing agent.
The developer reagents are typically phenol-type compounds, and disposal of spent developer solution containing residual phenol-type compounds is a potential problem. The present invention provides the side advantage that the residual phenol-type compounds are oxidized in the reduction of the silver ions, to non-phenolic compounds, so that the content of phenol-type compounds in the effluent is lowered resulting in a less toxic discharge effluent having a lower COD and BOD.
Other suitable organic reducing agents include sugars, glycol, glycerin, alcohols, especially lower alkanols of 1 to 6, preferably 1 to 4 carbon atoms and substituted benzenes wherein the substituent is one or more of OH; Br; I; NR
2
, where R is hydrogen or lower alkyl of 1 to 6 carbon atoms; or SR where R is as defined hereinbefore; for example, phenol, bromobenzene, iodobenzene and aniline.
c) Process Conditions
The process parameters for the reducing reaction of the invention are described hereinafter with particular reference to the preferred embodiment in which a spent developer solution is the source of the organic reducing agent.
The reducing reaction may be carried out at a temperature of 0° C. to 100° C., preferably 15° C. to 100° C.
At lower temperatures the reducing reaction may be too slow to be practicable; higher temperatures lead to a faster reaction time and a more complete and efficient reduction of silver ions to silver metal.
The reducing reaction is suitably carried out at a pH in the range of 6 to 14. In general a pH of at least 7 is preferred and especially a pH of 7 to 9. At higher pH the developer reagents in spent developer solutions are more readily oxidized and thereby the silver ions are more readily reduced to silver metal. On the other hand, at pH above 9 ammonium acetate which is a common component of fixer solutions and is thus a likely component of the silver rich solution l
Ren Jian Min
Riviere John L.
Bachman & LaPointe P.C.
Hoey Betsey Morrison
Metafix Inc.
Simmons David A.
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