silver sulfide is reversed in a process for regenerating a dissolved silver-containing spent bleach fixer solution. The process is particularly suited for treating the bleach fixer solution in a manner which enables recycling of the solution giving rise to considerable reduction in cost of bleach fixer solution and reduced environmental pollution. The process comprises introducing a hydrosulfide reagent into the bleach fixer solution, which contains silver, to precipitate silver sulfide while maintaining pH and thiosulfate levels in the bleach fixer to allow for reuse. The precipitate is isolated from the bleach fixer solution to remove silver thereby and hence enable recycling of the bleach fixer solution. It is preferred to adjust the pH and sulfite levels following silver removal to complete the regeneration process.
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1. A process for the regeneration of a dissolved silver-containing spent photographic bleach fixer solution, comprising the steps of:
mixing said dissolved silver-containing spent bleach fixer solution with an aqueous hydrosulfide reagent to produce a silver precipitate; removing said precipitate from said solution; adjusting the pH of said solution to within the operating range of a fresh bleach fixer solution; adjusting the sulfite ion concentration of said solution to within the operating concentration range of a fresh bleach fixer solution to produce a regenerated solution; whereby said regenerated solution can be used as a photographic bleach fixer solution.
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This invention relates to the use of a hydrosulfide reagent to precipitate silver sulfide (Ag2 S) from spent photographic bleach fixer solutions. The reagent provides a source of water soluble hydrosulfide ions (HS). The reaction of the HS with bleach fixer solution containing silver results in precipitation of Ag2 S, while regenerating the thiosulfate in the bleach fixer solution. This enables recycling of the bleach fixer solution. In regenerating bleach fixer, from color photo processing, it is preferred to adjust the pH and sulfite levels following silver removal to complete the regeneration process.
Silver recovery, from the spent fixer solution used to develop various types of photographic films, including black and white, color and X-ray films, is done for economic reasons and to prevent the discarding of hazardous waste containing silver or components of a bleach fixer solution. Current technologies for the recovery of silver from the bleach fixer solution include metallic replacement and electrolytic plating. These have the disadvantages of high capital and maintenance costs, inefficient recovery of silver and an inability to recycle the bleach fixer solution.
It is known how to extract silver from aqueous thiosulfate solutions using such materials as aluminum, aluminum alloys, organic phase containing a quaternary ammonium compounds and sulfide ions. In addition, publications describe the regeneration of spent photographic fixing solution using electrolysis, aeration and addition of concentrates. Some methods provide for limited bleach fixer recycling. Representative methods in this area are as follows:
1. Morana, Simon J. "Silver Recovery from Waste Film by Burning", Precious Met, Proc. Int. Precious Met. 1981 pp. 369-377.
2. Kunda, W. and Etsell T. H. "Recovery of Silver from X-Ray Film", Precious Met, (Proc. 9th Int. Precious Met. Inst. Conf.), 1985 (Pub. 1986), 289-304.
3. Kunda, W. "Processing of Photographic Spent Solution by Chemical Method", Precious Met. (Proc. 7th Int. Precious Met. Conf.) 1983, (Pub. 1984) 185-95.
4. Japan Patent 53-76027.
5. Japan Patent 63-45121.
6. Japan Patent 54-2848.
In color paper processing the silver either activates or destroys dyes incorporated into the paper and then virtually all the silver originally in the emulsion washes off into the bleach fixer solution. Discarding of the solution is a problem in that the silver is lost and discharged as pollution into the environment. Silver is currently recovered from the bleach fixer solution usually using a two stage electrolysis followed by either metallic replacement using cartridges filled with iron wires, or ion exchange to remove residual silver. Electrolysis is costly in capital outlay and in operating costs. The silver recovery is low and environmental problems remain from disposal of the effluent solution. Electrolytic plating allows for some bleach fixer recycling, but electroplating causes a lowering of the pH of the bleach fixer solution and consumption of both sulfite and thiosulfate resulting in a decrease in the fixation rate and the amount of silver that can be stripped from the film.
Previous work has shown that compounds, such as Na2 S and H2 S, which dissociate into sulfide ions, are effective reagents for removing silver from solution in a chemical process. Problems with this process involve formation or use of poisonous hydrogen sulfide (H2 S) gas and the degradation of thiosulfate to elemental sulfur.
It is known in the prior art that the silver product can then be converted to metallic silver by heat treatment in an air atmosphere at 600° C. or by dissolution in nitric acid.
A chemical process, which could selectively remove silver from spent bleach fixer solution without destruction of the thiosulfate in solution and which would allow the bleach fixer solution to be reused, would have the advantages of lower costs, ease of operation, and a reduced environmental hazard.
According to an aspect of the invention, a process is provided for recovering silver as silver sulfide from spent photographic bleach fixer solutions. The process comprises introducing a hydrosulfide reagent, which provides a source of hydrosulfide ions, into the spent bleach fixer solution to precipitate silver sulfide as a insoluble particle. The resultant precipitate is isolated from the spent bleach fixer solution. Sufficient silver is thereby removed from the spent bleach fixer solution and sufficient thiosulfate is regenerated to enable recycling of the bleach fixer solution. In regenerating bleach fixer, from color photo processing, it is preferred to adjust the pH and sulfite levels following silver removal to complete the regeneration process.
According to another aspect of the invention, the process is particularly suited for treating spent photographic bleach fixer solutions which contain thiosulfate and in particular sodium or ammonium thiosulfate. The hydrosulfide reagent is preferably either sodium, potassium or ammonium hydrosulfide. The hydrosulfide reagent precipitates silver in the form of silver sulfide while regenerating thiosulfate levels in the bleach fixer. When treating bleach fixer, from color photo processing, pH and sulfite levels are adjusted, by adding an acid solution and sulfite ions (SO3), added as a sulfite salt, following the regeneration process. A sufficient amount of an acidic solution and sulfite ions are added to the treated bleach fixer to restore the pH and sulfite ion concentration to within the operating ranges of the original bleach fix solution.
According to another aspect of the invention, sufficient acid solution is added to maintain the pH of the regenerated bleach fixer from about 5.0 to about 5.5. Furthermore, sufficient sulfite ions are added to maintain sulfite levels from about 4 g/L to about 10 g/L.
FIG. 1 charts the daily densitometer quality control readings for maximum and minimum density, red, green, blue and black during the 10 use and regeneration cycles.
FIG. 2 shows the concentration of sulfur compounds in the bleach fixer through 10 use and regeneration cycles.
FIG. 3 shows pH of the bleach fixer through 10 use and regeneration cycles.
FIG. 4 shows the concentration of silver and iron in the bleach fixer through 10 use and regeneration cycles.
FIG. 5 shows the molar ratio of moles of the hydrosulfide reagent used per moles of silver removed through 10 use and regeneration cycles.
FIG. 6 shows the molar comparison of the moles of hydrosulfide reagent used and the moles of silver removed from the spent bleach fixer through 10 use and regeneration cycles.
FIG. 7 shows the concentration of sodium and potassium through 10 use and regeneration cycles.
FIG. 8 shows the ammonia concentration in the bleach fixer through 10 use and regeneration cycles.
In the Figures, "B" is used to represent the sample taken before the regeneration cycle and "A" is used to represent the sample taken after the regeneration cycle. The numeral following "A" or "B" denotes the cycle number. "F" represents fresh bleach fixer.
The photographic processing of color prints involves the development of an image by decomposition of silver halide crystals to metallic silver and the removal of the unused silver halides from the paper. The most common form of photographic bleach fixer solution used in the printing of color photographs contain a reducing agent, usually a form of ferric EDTA, and an ammonium thiosulfate component to act as a silver solvent. After use, therefore they include silver thiosulfate complexes which, in accordance with this invention, may be precipitated from the spent bleach fixer solution by use of the hydrosulfide reagent. By "spent bleach fixer solution" it is meant bleach fixer solution which has been used for fixing a color image on paper.
The thiosulfate-based bleach fixer solutions, for example, sodium thiosulfate and ammonium thiosulfate are commonly used in photographic bleach fixer solutions. The time required to dissolve residual silver halides is related to the concentration of available thiosulfate in the bleach fixer solution. The silver halide, as removed from the paper forms silver thiosulfate complexes such as AgNaS2 O3 or AgNH3 S2 O3, the build up of which delays the removal of residual silver from the paper. It is therefore important to remove silver from the bleach fixer solution to ensure sufficient concentration of the uncomplexed thissulfate to expediently clean the paper during the fixing process.
In accordance with this invention, a hydrosulfide reagent, which provides a source of hydrosulfide ions reacts with the various silver thiosulfate complexes to produce silver sulfide without significantly degrading the thiosulfate component of the bleach fixer solution. Examples of such hydrosulfide reagents include: sodium hydrosulfide, potassium hydrosulfide or ammonium hydrosulfide.
During the processing of photographic prints using bleach fix there is a consumption of sulfite ions. To address this change in bleach fix composition it is preferred to adjust the sulfite level by adding a source of soluble sulfite in sufficient quantity to restore the concentration of the sulfite ions to within the normal operating range of the original bleach fix solution. For example with photo processing chemicals manufactured by Kodak it is preferred to adjust the sulfite level by adding a source of soluble sulfite in sufficient quantity to deliver between 4 and 10 grams per liter of bleach fixer treated. The operating range of sulfite ions in individual manufacture's bleach fix solutions may vary and can easily be determined by semi-quantitative analyses.
Sulfite is easily adjusted through the addition of 5 to 10 grams of a soluble sulfite powder, in the form of a soluble sulfite salt, per liter of bleach fixer to the processor replenishment tank while providing sufficient agitation to ensure that the powder dissolves in the solution. Suitable sources of soluble sulfite salt include for example: sodium sulfite, ammonium sulfite, potassium sulfite, sodium bisulfite, ammonium bisulfite, sodium meta bisulfite, and potassium meta bisulfite.
It may be possible to reuse the regenerated bleach fixer a few times without adjusting sulfite levels, provided that the concentration of sulfite ions does not fall below about 4gm/L. This lower optimum level was determined using Kodak photo processing reagents and may vary somewhat depending on the individual chemical make-up of the reagents. The optimum levels of sulfite ions can easily be determined by semi-quantitative analyses. To maximize the number of regeneration cycles, it is preferred to adjust the sulfite concentration following each regeneration cycle. It is preferred that the sulfite concentration of the bleach fixer solution should be adjusted after silver sulfide removal, usually in the photo processor replenishment tank while agitating the bleach fix.
During the processing of photographic prints using bleach fix there is a rise in the pH of the bleach fix solution. To address this change in bleach fix composition it is preferred to adjust the pH by adding a sufficient amount of an acidic solution to return the pH of the bleach fix solution to within the normal operating pH range for that specific bleach fix. For example, with the particular photo processing chemicals used in Example 1, it is preferred to adjust the pH by adding a sufficient quantity of an acidic solution to adjust the pH to a range from about 5.0 to about 5.5. The pH operating range of individual manufacture's bleach fix solutions may vary and can easily be determined by semi-quantitative analyses.
The pH is easily adjusted by the addition of small quantities of an acidic solution. The concentration of the acidic solution is not critical. Any suitable solution can be used provided that the addition of the acid solution does not substantially effect the overall volume of the treated fixer. Suitable acidic solutions which can be used to adjust the pH include for example a 50% (vol/vol) solution of acetic acid. This should be added in an amount of approximately 1% by volume of the amount of bleach fixer being regenerated. Although acetic acid is the acid of choice for use in the photo processing industry, sulfuric may also be used. Other acidic solutions may be suitable provided that they do not interfere with the photo processing procedure.
It may be possible to reuse the regenerated bleach fixer solution several times without adjusting pH, provided that the pH of the regenerated solution does not exceed a pH of about 6.5. This maximum pH value was determined using Kodak photo processing reagents and may vary somewhat depending on the individual chemical make-up of the reagents. As noted above the optimum working pH of the bleach fix solutions can easily be determined by semi-quantitative analyses. To maximize the number of regeneration cycles, it is preferred to adjust the pH following each regeneration cycle. It is preferred that the pH be adjusted after silver sulfide removal and sulfite addition, usually in the photo processor replenishment tank while agitating the bleach fix.
Hence, the bleach fixer solution, as treated in accordance with this invention, may be recycled for use as a bleach fixer solution in the photographic development process. This significantly cuts down on the cost of the bleach fixer solution as well as reducing pollution in the environment. In the prior systems for recovering silver, the spent bleach fixer solution with a portion of the silver removed therefrom had to be discarded because, in the process of recovering the silver, the thiosulfate component was degraded into compounds which could not be recycled and the ferric EDTA was not returned to a form which could be reused as a reducing agent.
In accordance with this invention, the hydrosulfide reagent as used to treat the bleach fixer solution is readily available as a concentrate or in its dehydrated form. For example, sodium hydrosulfide is available commercially as a 47.5% concentrate solution which can be readily diluted to the desired concentration, for use, such as a two molar solution. Sodium hydrosulfide may also be commercially obtained as a hydrated salt in the form of NaHS·0.89 H2 O. This salt may be dissolved in water to provide the desired concentration of solution such as a two molar solution. Alternatively, the sodium hydrosulfide may be prepared by reacting H2 S gas with sodium hydroxide. Similarly potassium hydrosulfide and ammonium hydrosulfide may be prepared by reacting a solution of potassium hydroxide (KOH) or ammonium hydroxide, respectively with hydrogen sulfide (H2 S) gas. In aqueous solution the hydrosulfide reagents will dissociate to provide water soluble hydrosulfide ions.
Although the silver is complexed with the thiosulfate, it is believed that the hydrosulfide ions, in one manner or another, react with the various silver thiosulfate complexes to produce silver sulfide. In this solution, the silver sulfide is not soluble; hence it precipitates and forms a mass which may be easily filtered or readily settles out of solution.
Although the invention is not intended to be limited by any particular theory, it is suggested that an equation which represents the chemical reaction is as follows:
2AgNaS2 O3 +NaHS→Ag2 S+Na2 S2 O3 +NaHS2 O3
The hydrosulfide reagent in this particular equation, represented as sodium hydrosulfide, preferentially reacts with the silver thiosulfate complex (here shown as sodium thiosulfate) and does not react in any significant way with other components of the bleach fixer solution. Although sodium hydrosulfide will also react with sodium thiosulfate, this reaction is minimized because of the preferential reaction of sodium hydrosulfide with the silver sodium thiosulfate. It has been found that excessive amounts of sodium hydrosulfide, if introduced into the spent bleach fixer solution, will react with the sodium thiosulfate to degrade the thiosulfate to elemental sulfur, which is not desirable.
It is preferred to maintain the concentration of the thiosulfate in the bleach fixer solution, particularly for purposes of recycling, and therefore it is important to control the quantity of hydrosulfide reagent used in precipitating silver. In accordance with this invention, for the regeneration of bleach fixer for color photo processing, up to 2 moles of sodium hydrosulfide per one mole of silver in the bleach fixer solution may be used. The optimum amount of reagent, resulting in relatively little degradation of the thiosulfate, can be determined by pretreatment analysis with commercially available semi-quantitative analysis strips. Generally, amounts of sodium hydrosulfide reagent in excess of 2 moles per mole of silver in the bleach fixer solution results in increasing degradation of the thiosulfate and sulfite ions. The preferred range of hydrosulfide reagent added to the spent bleach fixer solution is approximately 1.0 moles to 2.0 moles per mole of silver in the bleach fixer solution.
Other parameters, which can influence the effectiveness of the process, are the rate of introduction of the hydrosulfide reagent to the bleach fixer solution and the degree of agitation of the solution. It is preferred that the rate of introduction of the hydrosulfide reagent to the spent bleach fixer solution be at a rate which is slow enough, depending upon the conditions, to avoid evolution of H2 S gas. Hence the rate of introduction of the hydrosulfide reagent may vary considerably depending upon the degree of agitation of the bleach fixer solution. These parameters are related. Normally if the rate of introduction is increased, then correspondingly the degree of agitation of the bleach fixer solution must also be increased. Providing the degree of agitation is high enough to avoid formation of pockets of hydrosulfide reagent, the rate of introduction thereof may be correspondingly higher.
It has also been found that to minimize the degradation of the thiosulfate, the bleach fixer solution should preferably be agitated. The degree of agitation contemplated is that produced by a mechanical ultrasonic or gas bubbling means. Mechanical stirrers, recirculating pumps, ultrasonic stirrers, ultrasonic vibration devices or gas bubbles through the system create sufficient agitation to ensure good mixing of the introduced hydrosulfide with the bleach fixer solution to eliminate any pockets of high concentration of the hydrosulfide reagent and thereby ensure that the preferential reaction with the silver complex proceeds.
To avoid the formation of pockets of high concentration of hydrosulfide reagent as already noted, the rate of introduction of the reagent is selected to be in step with the degree of agitation. For a high degree of agitation, higher rates of introduction of the hydrosulfide reagent are permissible where the desired degree of agitation can be established by experimentation with a particular bleach fixer solution. For example, in treating a quantity of bleach fixer solution in a reaction vessel which holds approximately 50 liters, the bleach fixer solution may be circulated through the reaction vessel at a rate of approximately 8 to 10 liters per minute. This establishes a substantial degree of agitation in the 50 liter reaction vessel. The circulation rate of the bleach fixer solution may be achieved by a suitable pump, such as a diaphragm pump which is not corroded by the bleach fixer solution. The solution may be withdrawn from the bottom of the tank and returned to the side of the tank through a suitable nozzle. The solution may be circulated at a pressure of 40 to 60 psi. The nozzle may be approximately one quarter inch in diameter to provide for a high speed injection of the bleach fixer solution into the tank. For this high speed injection of the bleach fixer solution back into the tank, the rate of injection of the hydrosulfide reagent may be in the range of 2 to 3 ml per second. Sufficient hydrosulfide reagent is injected into the tank at this rate until a desired molar ratio of hydrosulfide reagent to silver in the spent bleach fixer solution is in the range of 1.0 to 2∅ Preferably, the hydrosulfide reagent is injected adjacent the nozzle so that the reagent is swept immediately into the high speed stream of the bleach fixer solution to provide for immediate dispersal of reagent and to avoid the formation of pockets of high concentration of the reagent. The nozzle may be angled within the vessel to encourage a swirl flow of the bleach fixer solution in the vessel. After injection of the hydrosulfide reagent is completed, the pump continues to run to provide agitation to the solution and ensure a full reaction.
Although the above comments have been directed specifically towards the use of sodium hydrosulfide to precipitate silver from a silver sodium or ammonium thiosulfate complex, as found in the bleach fix formulations, the invention contemplates the use of other hydrosulfide reagents i.e. compounds which provide hydrosulfide ion to the bleach fixer solution. Thus for example potassium hydrosulfide or ammonium hydrosulfide may also be used in converting silver as it exists in spent bleach fix solutions as used in the color print processing industry
It has been found that the temperature of the bleach fixer solution does not have a direct bearing on the precipitation of silver from the bleach fixer solution. Therefore, for convenience, the process may be carried out at room temperature. The silver precipitate in the form of Ag2 S salt forms readily and has very good settling and filtration characteristics. Hence on a commercial scale, the process is very effective in treating bleach fixer solutions and the use of the hydrosulfide reagent does not degrade, to any appreciable extent, the thiosulfate in the solution.
The method according to the present invention, thus permits the treated bleach fixer solution to be recycled readily for reuse in the photographic development process. This prevents significant problems presently encountered with regards to disposal of the spent bleach fixer solution, avoiding pollution problems normally occurring in disposal of spent bleach fixer solutions to municipal treatment systems.
The silver may be recovered from the precipitate by a number of methods known in the art. For example the precipitate may be calcined at temperatures ranging from 300°C to 600°C to convert silver sulfide to silver. Alternatively, the silver can be recovered by dissolving the precipitate in nitric acid, or by using conventional smelting and refining techniques.
In addition to adding sulfite ion and adjusting pH, it may be necessary to add a small portion of fresh bleach fixer solution, in the form of a bleach fixer solution prepared ready for use or as a bleach fixer concentrate, to replenish the volume of bleach fixer, after repeated regeneration cycles. The addition of bleach fixer concentrate or prepared bleach fixer solution serves to adjust the sulfite level and pH in addition to providing ferric EDTA to make up for losses due to carry-over of bleach fixer into the wash water during processing.
The following examples illustrate the best modes contemplated for carrying out this invention, but are not to be construed as limiting.
Regeneration and reuse of the regenerated bleach fixer was repeated for ten cycles. The testing was conducted using 150 liters of fresh bleach fixer solution. The testing was done under actual operating conditions. The testing was conducted during normal operations using a Hope model RA4226V Print Processor. The bleach fixer used was Kodak Ektacolour RA Bleach Fix.
The fresh bleach fixer was prepared according to the manufacturer instructions and introduced to the replenishment tank. Following use in the processor, the first regeneration cycle took place approximately one week later. Regeneration thereafter took place every week to ten days as needed by volume use.
The bleach fixer solution was loaded with silver under normal operating conditions, using Kodak Ektacolor Supra Type N paper, until it reached approximately 4 grams per liter and was discharged via the processor overflow to a receiving vessel. The bleach fixer solution was then transferred to a TTI CF-50 Rejuvenator and the silver was converted to silver sulfide by the addition of a hydrosulfide reagent and removed in two 40 liter batches. The filtered bleach fix was transferred to the processor replenishment tank where sulfite and pH adjustment was conducted.
A sample of the fresh bleach fixer solution was taken before any film was processed. Additionally, samples were taken before and after each regeneration. Kodak Ektacolor RA densitometer test strips were run with the fresh solution and after each regeneration in order to maintain quality control and determine the developer and bleach fix stability and efficiency. The densitometer readings were taken using a Macbeth TR944 Densitometer and Kodak quality control strips. The densitometer quality control chart shows that throughout the regeneration cycles all parameters remained well within allowable variation limits (FIG. 1).
Total sulfur was measured by oxidizing the sulfur species to sulfate using hydrogen peroxide and measuring sulfur by inductively coupled plasma. The thiosulfate was titrated with iodine after sulfide was precipitated and bisulfite was masked with formaldehyde using the method described by Kolthoff and Belcher in "Volumetric Analysis III", 1957, page 294. Total sulfur rose through the ten regeneration cycles, largely due to the addition of fresh bleach fixer and sulfite (FIG. 2). Sulfide decreased roughly by half between regenerations with no discernable quality change to the print image. Thiosulfate showed less than five percent loss before the addition of fresh bleach fix necessary to make up for evaporation loss (FIG. 2).
Sulfite was the only species which proved to be a critical component. It was observed that sulfite was dramatically consumed during the bleach fixer residence time in the processor. As the sulfite level dropped to under 200 ppm there was a corresponding fluctuation in the minimum density readings indicating that the bleaching function was being jeopardized. As a result, 5 gm of sodium sulfite per liter of treated bleach fixer was added after each regeneration cycle. This was done by adding the sodium sulfite powder into the replenishment tank while agitating with a hand held laboratory stirring propellor. The addition of sodium sulfite immediately resulted in bleaching function returning to normal. For the bleach fixer used in this example, the optimum sulfite level was found to be between 4.0 g/L and 10.0 g/L.
The pH was determined for all samples taken throughout the 10 regeneration cycles (FIG. 3). The pH of the solution increased during its residence in the processor. It was very easily controlled by the addition of small quantities of 50% acetic acid. The addition of a 1% volume based on the total regenerated fix volume was sufficient to maintain proper pH control from cycle 3 to the end of the test. The addition of 50% acetic acid was done in the replenishment tank after the addition of sulfite by manually pouring in the determined amount while agitating with a hand held laboratory stirring propellor. The pH control proved to be an essential feature for the continued regeneration and reuse of the bleach fix.
The concentrations of silver, sodium, potassium, aluminum, and iron were analyzed using an Applied Research Laboratories Model 34000 inductively coupled plasma atomic emission spectrometer (ICP).
Silver removal was efficient throughout as shown by the molar ratio for removal remaining almost constant at 1.2, despite removing a wide range of total silver (FIG. 4, 5 and 6).
Sodium and potassium show regular and steady increase over the 10 cycles (FIG. 7). Sodium increased due to the addition of reagent during regeneration and due to the addition of sodium sulfite to maintain sulfite levels. Potassium increased during residence in the processor due either to developer carryover or dissolution from the print paper. However these ions have a high solubility in bleach fixer and the increasing concentrations did not show any negative effect on image fixation at these levels.
Iron proved to be very stable and rose somewhat due to the addition of fresh bleach fix (FIG. 4). It was found that the TTI method of regeneration also restored iron to the proper valence to reduce silver to ion in the subsequent processor cycle.
Ammonium was analyzed using a Lachat Quick-Chem flow injection analyzer using Lachat Method #10-107-06-0-B (FEPA 350.1 equivalent). The ammonia levels stayed relatively constant through the ten regeneration cycles as shown in FIG. 8. Some of the variability in the ammonia concentration was in part due to the large dilution required to bring the concentration within the operating range of the flow injection analyzer. Repeated analysis on several samples showed a variability of about ±8%. In any case the relative speed of fixation remained constant indicating no breakdown of the ammonium species.
As noted above, pH and sulfite concentration control is required for the regeneration of bleach fixer. During the multiple regeneration and reuse cycles, optimum pH and sulfite ion concentration was accomplished by the addition of acetic acid and sodium sulfite.
It was necessary to add fresh bleach fix during the test in order to make up for evaporative loss. Specifically, in the present Example, five liters each of part A & B of fresh bleach fix concentrate was added after cycle three. A second addition of 40 liters of mixed bleach fixer solution (mixed according to manufacturer's specification) was made after five regenerations in order to top up solution volume due to processor evaporation, carry-over with wash water, filter changing, and sampling loss.
Based on the results of this 10 regeneration test it would appear that repeated regeneration and reuse of bleach fixer is possible when pH and sulfite additions are incorporated after silver removal with the TTI system. More than ten regenerations are also possible, and twenty regenerations have been done with good results. Once pH and sulfite control were initiated all operational parameters were found to be essentially the same as those found in freshly mixed bleach fix. The chemical analysis conclusions are borne out by the ongoing quality control data from the densitometer strips.
Although preferred embodiments of the invention are described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the spirit of the invention or the scope of the appended claims.
Dutka, Michael E., Whitney, Paul H., Malcolm, Keith E.
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Aug 09 1991 | Technology Transfer Barbados Inc. | (assignment on the face of the patent) | / |
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