SILVER REMOVAL FROM PHOTOGRAPHIC PRODUCTS

Abstract: The invention resides in treating an image-wise exposed and colour developed photographic element with a bleaching agent and a fixing agent. The fixing agent comprises an aliphatic thiol as accelerator. The accelerator present in the fixing agent both stabilizes the fixing agent so that it can be used commercially and prevents staining resultant upon the degeneration of the thiol in the bleach fix bath. The invention also provides a means for efficient silver removal from the fixing bath thereby obviating a separate silver removal stage.

Description: SILVER REMOVAL FROM PHOTOGRAPHIC PRODUCTS DESCRIPTION The present invention relates to improvements in silver removal from photographic products, and particularly from developed photographic materials to enable successful silver removal during a bleach-fix step. It is common to use a bleach step followed by a fix step, optionally with a washing step between to remove silver from images in negative photographic products. With print materials such as colour photographic paper the bleach can be combined with the fix to give a bleach-fix composition. However the silver removing power of this bleach-fix composition is insufficient to remove all silver from camera speed materials. This is believed to be caused by inhibiting materials absorbed onto the developed silver surface. Thus one problem in the art is to remove the inhibiting absorbed materials from the developed silver surface, of the photographic product whether positive or negative. Another problem lies with silver recovery. It is common to remove silver from used photographic reagents. This is usually effected as a separate step either chemically or electrolyticall . This recovery progress is more easily effected from the fix step rather than the bleach-fix step, and further it would be most desirable if silver recovery could be effected, in a large part at least, as a function of the silver removal process from the photographic material rather than as a separate removal process. These problems have in part been addressed in US-A-3,893,858. In this disclosure an accelerator selected from certain aliphatic thiols is used to assist silver removal. These thiols may be selected from compounds containing a thio-group in combination with one or more solubilizing groups. We have now found however that where the accelerators are used per se as a prebath prior to the bleach-fix step, their useful life is not long enough to be commercially worthwhile, whereas if the accelerator is added to the bleach-fix there is a tendancy for the accelerator to cause staining as a result of the accelerator reacting with itself in the bleach-fix on standing. For these reasons the use of accelerators has not been common in the art over the last few years. We have now found that these stability problems may be addressed by adding the accelerator to the fixer which produces the bleach-fix. This has two effects; first it stabilizes the accelerator thereby to increase its active life, and second the stabilizing effect alleviates any subsequent staining problems. More importantly however silver removal may be effected at the fixer stage. Accordingly the inclusion of the accelerator in the fixer has the effect of promoting product quality while allowing effective silver removal from the fixer stage thereby, with care, allowing the silver recovery without the previously required separate silver recovery step. In a preferred embodiment in accordance with the invention the silver recovery is effected by precipitation of a silver product as a result of an intereaction of the accelerator with a silver complexed species which can be readily recovered, by usual methods of solid/liquid separation such as filtration. According therefore to a first aspect of the present invention there is provided a method for photographic processing in which an image-wise exposed and colour developed photographic element is contacted with a fixing agent, wherein said element is also contacted with an accelerator selected from a bleach accelerating amount of a thiol or a photographically effective salt thereof, said thiol being selected from monothiolglycerol, cystine, cystamine, cysteine, and aminoal ylenethiols of the formula: wherein each of R 1 and R2 is individually H, CH3 or C 2H5, and n is an integer having a value of 1 to 3; prior to contact with a bleach-fix agent to oxidize metallic silver and remove the same from the element; characterised in that the accelerator is present as part of the fixing agent composition. In a preferred form of the invention the fixing agent comprises a thiosulphate or chemically analogous compound and the bleach-fix bath contains a ferric amino polycarboxylic acid. In a further aspect of the present invention there is provided a method for the recovery of silver from a photographic process in which an image-wise exposed and colour developed silver containing photographic element is contacted with a fixing agent before contact with a bleach-fixing solution; which recovery method comprises adding to the fixing agent an accelerator selected from monothioglycerol, cystine, cystamine, cysteine, and aminoalkylenethiols of the formula in which R 1 and R2 is each H, CH3 or C2H5, and N is an integer having a value of 1 to 3; thereby to cause the silver to precipitate as a silver salt thereby to substantially remove silver from the reaction medium prior to the bleach-fix stage. Preferably the bleach-fixing agent is a thiosulphate and the silver is removed as a precipitate of silver sulphide or silver thiol. Preferred accelerating agents in accordance with the present invention are dimethylaminoe hane thiol, (DMAET) and its thiouronium derivatives, thioglycerol, cysteine and cystine. The invention will now be described, by way of illustration only, with reference to the following Example, and with reference to the accompanying drawings wherein: Figure 1 shows a graph of the sensitometry of kodacolor VR100 film processed through a standard C-41 process. This is compared to the same film processed through the example process. Figure 2 shows a graph of the sensitometry of Kodacolor VR200 film processed through a standard C-41 process. This is compared to the same film processed through the example process. Figure 3 shows a graph of the sensitometry of Kodacolor VR400 film processed through a standard C-41 process. This is compared to the same film processed through the example process. Figure 4 shows a graph of the sensitometry of Kodacolor VRIOOO film processed through a standard C-41 process. This is compared to the same film process through the example process. These figures demonstrate that the process has little or no effect on the final sensitometry i.e. a film processed through the control process would be undistinguishable from an image point of view from one processed in a standard C-41 process. EXAMPLE A fix was prepared according to the following formula using commercially available materials: Experimental Accelerating Fix. ammonium thiosulphate 32Og ethylenediamine tetracetic acid(EDTA) 2g sodium metabisulphite 8g dimethylaminoethanthiol (DMAET) 5g water to 1 litre The pH of this solution was adjusted to 6.5 by the addition of acetic acid. A bleach-fix was prepared according to the following formula using commercially available materials:- Experimental Bleach-fix. 1.56M ammonium iron(III) EDTA solution 75mls ammonium thiosulphate 60g sodium sulphite 8g water to 1 litre The pH of this solution was adjusted to 6.0 by the addition of acetic acid. A developer, bleach and fix were made up using Kodak C-41 20 litre kits according to the instructions enclosed in the kit boxes. Strips of four Kodak colour negative films, VR100, VR200, VR400, VRIOOO were exposed to simulated daylight through a 0.2 log exposure sensitometric wedge for 0.01 seconds. These strips were kept for 24 hours then processed through the following process: Similar strips of exposed and aged film were processed in a standard C-41 process as described in the C-41 manual. The processed strips were measured using a Macbeth densitometer and compared. Figures 1-4 show the results for the four different films. The levels of residual silver in all strips Dmaxs were all less than could be detected with any confidence by X-ray Fluorescence Spectroscopy i.e. 25mg/sq.m. The experiment was repeated with a number of other compounds in place of the DMAET in the experimental fix, all used at a concentration of 5g/l. Table 1 below shows the red, green and blue Dmax densities of - li ¬ the four films processed in the experimental process with the different experimental fixes, using the experimental fixes, using the experimental bleach-fix for 2'00" in all cases. - 12 - Table 1 In those films that had been processed with an additive present in the fix, no sil could be. detected by infra-red density measurements.

Claims: CLAIMS 1. A method for photographic processing in which an image-wise exposed and colour developed photographic element is contacted with a fixing agent, wherein said element is also contacted with an accelerator selected from a bleach accelerating amount of a thiol or a photographically effective salt thereof, said thiol being selected from monothiolglycerol, cystine, cystamine, cysteine, and an aminoal-ylenethiol of the formula N - (CH 2 )n-SH R 2 wherein each of R 1 and R2 is individually H, CH3 or C 2H5, and n is an integer having a value of 1 to 3; prior to contact with a bleach-fix agent to oxidize metallic silver and remove the same from the element; characterised in that the accelerator is present as part of the fixing agent composition. 2. A method according to claim 1 characterised in that the fixing agent contains a thiolsulphate and the bleach-fix agent contains a ferric amino polycarboxylic acid. 3. A method according to either of claims 1 or 2 characterised in that the accelerator is selected from DMAET or its thiouronium derivative, thioglycerol cysteine or cystine. 4. A method according to either of claims 2 or 3 characterised in that the ratio by weight of accelerator to the thiosulphate in the fixer bath is 1 to 2:100. 5. A method for the recovery of silver from a photographic process in which an image-wise exposed and colour developed silver containing element is contacted with a fixing agent before contact with a bleach-fixing solution; which recovery method comprises adding to the fixing agent an accelerator selected from thioglycerol cystine, cystamine, cysteine, and an amnioalkylenethio of the formula: R J wherein each of R1 and R2 is individually H, CH3 or C 2H5, and n is an integer having a value of 1 to 3; thereby to cause the silver to precipitate as a silver salt to substantially remove silver from the reaction medium prior to the bleach-fix stage. 6. A method according to claim 5 characterised in that the fixing agent contains a thiosulphate and the bleach-fix stage contains a ferric amino carboxylic acid. 7. A method according to either of claims 5 or 6 characterised in that the accelerator is selected from DMAET or its thiouronium derivative, thioglycerol cysteine or cystine. 8. A method according to either of claims 6 or 7, characterised in that the weight ratio of accelerator to the thiosulphate in the fixer bath is 1 to 2:100. 9. A method according to claim 1 and substantially as hereinbefore set forth with reference to, and/or as illustrated in. Figures 1 to 4 of the accompanying drawings. 10. A method for the recovery of silver substantially as hereinbefore set forth with reference to, and/or as illustrated in. Figures 1 to 4 of the accompanying drawings.

Inventors: FYSON JOHN RICHARD, FYSON, JOHN, RICHARD
Applicants: EASTMAN KODAK CO, KODAK LTD, EASTMAN KODAK COMPANY, KODAK LIMITED
Application year: 1991
Main classification: G03C5
IPC codes: G03C5, G03C7