Radiation imagery chemistry: process – composition – or product th – Regenerating image processing composition
Patent
1990-11-05
1993-09-14
Bowers, Jr., Charles L.
Radiation imagery chemistry: process, composition, or product th
Regenerating image processing composition
430400, 430455, G03C 538
Patent
active
052447774
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method of recovering silver from photographic processing solutions and, in particular to recovering silver from sulphite fixer solutions.
In processing both black-and-white and colour photographic silver halide materials it is necessary to remove unwanted silver halide from the developed material. This is customarily accomplished by using a fixer solution which contains a silver halide solvent, usually a thiosulphate.
Methods are known for recovering silver from spent thiosulphate fixer solutions. It is not, however, possible to use any of these methods "in-line" to extend the working life of the solutions without continuous solution monitoring and solution adjustment, nor does removal of silver leave a solution for disposal (i.e. a solution containing thiosulphate) which is fully environmentally acceptable.
In our cofiled application Ser. No. 07/613,515 to Fyson, filed Nov. 5, 1990, now U.S. Pat. No. 5,171,658, entitled "METHOD OF PHOTOGRAPHIC PROCESSING" there is described and claimed a method for removing unwanted silver chloride from photographic materials using a fixer comprising an alkali metal sulphite as sole silver halide solvent.
According to the present invention there is provided a method of recovering silver from used photographic fixer solutions characterised in that the solution contains an alkali metal sulphite as the sole silver halide solvent.
The advantages of the present invention are that treated fixers can be discharged into sewer systems in areas where limits on silver discharge are low; valuable silver is recovered and available for refining and reuse; and in-line silver recovery increases the life of the fixer and therefore reduces chemical cost to the user.
A number of methods of recovering silver from thiosulphate or thiosulphate plus sulphite fixers are known and similar methods may also be used in the present case. Such methods include:
Electrochemical deposition of silver,
Metal exchange with a less noble metal and
Addition of a powerful reducing agent to reduce the silver sulphite complex to silver metal and the free ligand.
There are additional methods which are not efficient or practical for thiosulphate-containing fixers but are practical for sulphite-only fixers and these are:
Precipitation of silver sulphide by the addition of a water soluble sulphide,
Precipitation of silver sulphide by contacting the silver laden fixer with an insoluble sulphide either added as the free solid or suspended in a matrix,
Addition of a cationic or anionic ion exchange resin, or
Destruction of the silver sulphite complex by the addition of a powerful oxidant or a mineral acid.
Electrochemical silver recovery is carried out by passing current through seasoned fixer between two electrodes made of a suitable material which may be essentially metallic or of one of many forms of carbon. Silver is deposited on the cathode. The electrodes may or may not be rotated to improve plating quality and efficiency.
Metal exchange is a process by which a metal that is more electronegative than silver is added to the fixer in one of many forms, e.g. wire, powder or billets. The electronegative metal displaces the silver from the silver complex precipitating it on the metal surface. Suitable metals include First Row Transition Metals.
Silver can also be precipitated by addition of a powerful reducing agent to the fixer. Complexed silver is reduced and precipitates. Suitable agents would include alkali metal borohydrides and dithionites, alkali metal hydrides and mixed metal hydrides, e.g. lithium aluminium hydride.
Silver can be recovered as silver sulphide from the fixers by addition of an aqueous solution of water soluble sulphides. These include the alkali metal, alkaline earth metal and ammonium sulphides and polysulphides.
Silver can be recovered as silver sulphide by contacting the seasoned fixer with sulphides that are essentially water insoluble. These are all the metal sulphides excluding those of alkali metals, alkaline earth metals and silver sulphide.
Ion-exchange resin
REFERENCES:
patent: 4029510 (1977-06-01), Speers
patent: 4139431 (1979-02-01), Scheidegger
patent: 4445935 (1984-05-01), Posey
patent: 5171658 (1992-12-01), Fyson
James, The Theory of the Photographic Process, Chapter 15, "Complementary Processes", pp. 437-443 (4th ed. (1977).
SMPTE Journal, vol. 86, No. 2, Feb. 1977, D. J. Degenkolb et al.; "Silver Recovery from Photographic Wash Waters by Ion Exchange".
Bowers Jr. Charles L.
Eastman Kodak Company
Huff Mark F.
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