US3839043A - Process for regenerating spent photographic silver bleaching solutions - Google Patents

Process for regenerating spent photographic silver bleaching solutions Download PDF

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Publication number
US3839043A
US3839043A US00353101A US35310173A US3839043A US 3839043 A US3839043 A US 3839043A US 00353101 A US00353101 A US 00353101A US 35310173 A US35310173 A US 35310173A US 3839043 A US3839043 A US 3839043A
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Prior art keywords
bleaching
bath
bleaching bath
alkali metal
halogen
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US00353101A
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English (en)
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W Hunicke
K Schranz
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Agfa Gevaert AG
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Agfa Gevaert AG
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03C—PHOTOSENSITIVE MATERIALS FOR PHOTOGRAPHIC PURPOSES; PHOTOGRAPHIC PROCESSES, e.g. CINE, X-RAY, COLOUR, STEREO-PHOTOGRAPHIC PROCESSES; AUXILIARY PROCESSES IN PHOTOGRAPHY
    • G03C5/00—Photographic processes or agents therefor; Regeneration of such processing agents
    • G03C5/26—Processes using silver-salt-containing photosensitive materials or agents therefor
    • G03C5/395—Regeneration of photographic processing agents other than developers; Replenishers therefor
    • G03C5/3952—Chemical, mechanical or thermal methods, e.g. oxidation, precipitation, centrifugation

Definitions

  • the free halogen is fed into the bleaching bath under [56] References and the surface.
  • Other baths for processing the UNITED STATES PATENTS photographic material are regenerated by variably 7 2,515,930 7/1950 Seary 96/50 A controlling the input of the respective regenerating 2,611,699 9/1952 Zappert substances into the respective baths by means of the 3,615,507 Bard BF redox potential 3,770,437 11/1973 Brugger et al.
  • This invention relates to a process for the regeneration of spent bleaching baths which contain mainly a soluble alkali metal ferricyanide and an alkali metal halide.
  • Bleaching baths are used in photographic processes for converting the silver formed on development into a salt which is soluble in the fixing bath.
  • the bleaching baths generally used contain an alkali metal ferricyanide, an alkali metal halide and other additives such as buffers to adjust the pH, anti-corrosives, watersofteners and bleaching accelerators.
  • ferricyanide is reduced to ferrocyanide and the halide ions are bound by the oxidized silver as follows:
  • the bleaching bath is reoxidized by the addition of an alkali metal hypobromite or a mixture of an alkali metal bromate and hydrobromic acid or a mixture of alkali metal bromates and alkali metal bromides with the addition of alkali metal bisulfates, sulfamic acid or sulfuric acid.
  • the alkali metal bromate or mixture of alkali metal bromate and alkali metal bromide used in this process is dissolved in the bleaching bath and hydrobromic acid or sulfuric acid is then added. If the mixture which liberates bromine is mixed with acid before addition to the bleaching bath, bromine is liberated spontaneously, which causes serious inconvenience.
  • the process has the disadvantage that considerable dilution takes place when the reactant which liberates bromine is dissolved in the bleaching bath, with the result that the reaction velocity is very low owing to the low concentractions.
  • the pH of the bleaching bath is greatly reduced by hydrobromic acid or sulfuric acid which has not yet reacted, and although the reoxidation velocity is increased, the decomposition velocity of the bleaching bath is also greatly increased.
  • Quantitative reaction of the reactants added to the bleaching bath is not achieved within an economical time since the concentrations continue to decrease while the pH rises as the reaction progresses. Repeated adjustment of the pH, which has been suggested for sodium hypobromide, is complicated and time-consuming and results in an undesirable increase in the salt concentrations.
  • This invention relates to a process for the regeneration of spent photographic silver bleaching baths which contain alkali metal ferricyanide and alkali metal halide by reoxidizing the ferrocyanides formed in the process with bromine, characterised in that a quantity of bromine equivalent to the concentration of ferrocyanide at any given moment is continuously produced in aqueous solution in a closed vessel, controlled by the redox potential of the bleaching bath, and flows in under the surface of the bleaching bath. In this way, bromine is only surface of the bleaching bath. In this way, bromine is only produced in small quantities at a time in a closed vessel and therefore cannot escape into the environment.
  • Equations (III) to (VI) are suitable for obtaining aqueous bromine solutions in accordance with the present invention.
  • the reaction depends on the presence of acid which is used up. If the reaction is fully completed, neutral solutions are obtained if the reactants are mixed in the molar ratios indicated by the equations.
  • bromide ions are oxidised in an acid solution; in equations (III) to (V) bromate is used as oxidising agent and the reaction is therefore a synproportionating reaction.
  • FIG. 1 shows a reactor for producing the aqueous bromine solution by the process according to the invention
  • FIG. 2 shows a practical example of reoxidation apparatus for the process according to the invention
  • FIG. 3 is a schematic representation of the control and dosing pump assembly for the process according to the invention.
  • FIG. 4 is a diagram representing the dependence of the redox potential on the ratio of ferricyanide to ferrocyanide concentration in a bleaching bath.
  • Redox potential is an abbreviation for reductionoxidation potential and is understood hereinafter to mean the electric potential between a platinum electrode dipped into the solution under consideration and a second reference electrode.
  • This second reference electrode is for the purpose of defining the redox potential a socalled standard hydrogen electrode, which is represented by a platinum electrode charged with hydrogen at one atmosphere and dipping into a hydrogen ion solution having a hydrogen ion activity of 1. More convenient are other reference electrodes such as the calomel electrode or the Ag/AgCl electrode.
  • the redox potential is an indication of the oxidizing power and therefor indirectly of the concentrations of the oxidized and reduced forms in a redox system.
  • the starting materials which are easy to handle in the form of aqueous solutions, are mixed in a small glass apparatus represented in FIG. 1.
  • oxidation of the halide ions takes place instantly in acid solution with formation of the free halogen.
  • high concentrations of starting materials such high reaction velocities are usually obtained at room temperature that a high capacity is achieved even with a small apparatus.
  • the volumes required are so small when concentrated solutions are used that in the process according to the invention practically no increase in volume occurs in the processing tank. It is therefore possible to reduce the amount of effluent water by more than percent.
  • the reactor represented in FIG. 1 substantially comprises two inlet tubes 101 and 102, a reaction chamber 103 and an outlet tube 104.
  • the reaction chamber may be filled with small filling bodies 15 such as Raschig rings, glass spirals or glass pellets.
  • a solution of the acid for example hydrohalic acid, is introduced through the inlet tube 101 and a solution of oxidizing agent through the other inlet tube 102.
  • the two solutions meet at 106 where vigorous mixing occurs.
  • the reaction which sets in rapidly, takes place mainly in the reaction chamber 103.
  • the length of the flow path of the reactants is considerably increased by the filling bodies 105 and unwanted circulation of the solution due to the thermal flow and the injection effect of the solution entering from 106 are effectively suppressed.
  • the liquid is discharged through the outlet tube 104.
  • the outlet tube 104 is dipped into the spent bleaching bath, and the halogen which is discharged from it in the form of an aqueous solution is greedily taken up by the spent bleaching bath.
  • the ferrocyanide is thereby converted into the required ferricyanide by the reaction represented in equation II and at the same time the halide required for the reaction according to equation 1 is formed from the halogen.
  • FIG. 2 shows one embodiment of the reoxidation apparatus according to the invention. It consists of the closed vessel 207 with inlet tube 208 for spent bleaching bath and outlet tube 209 for reoxidized bleaching bath.
  • the reactor (FIG. 1) is fixed inside the sealed vessel 207.
  • the reactor is equipped with non-return valves 210, 211 and 212 which prevent entry of the bleaching bath from the closed vessel 207 into the reaction chamber 203 and prevent escape of free halogen from the reaction chamber 203 into the surroundings by way of the inlet tubes 201 and 202.
  • a fast stream of spent bleaching bath is directed through the inlet tube 208 to the tip of the glass tube 204 (outlet tube of the reactor) so that aqueous solution of halogen leaving the outlet tube is immediately taken up by the bleaching bath.
  • the bleaching bath passes over the darn 213 which has a safety aperture 237 to the outlet tube 209 from where it can be conducted into the processing tank. Delivery of the bleaching bath may be carried out with the aid of any liquid pump which must, however, be resistant to ferricyanide.
  • the state of oxidation of the bleaching bath is determined whenever required by means of a redox potential measuring chain which may consist, for example, of a platinum electrode 214 as measuring electrode and a silver/silver chloride electrode 215 as reference electrode. Whereas the platinum electrode 214 dips directly into the bleaching bath, the silver chloride electrode 215 is placed in a potassium chloride solution 216 inside the inner tube 222 which is surrounded by yet another protective tube 218 filled with potassium chloride solution 217, and electrically connected to the bleaching bath by way of the diaphragms 220 and 221.
  • the function of the protective tube 218 is to prevent contamination of the silver/silver chloride electrode by bleaching bath entering through diaphragm 220.
  • the tube 218 is filled with potassium chloride solution 216 right up to the storage bulb 219 so that there is a slight flow of potassium chloride 216 from the inner tube 222 into the protective tube 218 due to the hydrostatic pressure on the diaphragm 220.
  • Any bleaching bath penetrating diaphragm 221 to enter the protective tube 218 can be removed through the overflow 223.
  • a small safety aperture 237 in the dam 213 prevents an overdose of free halogen inside the housing 207 in the event of failure of the delivery of bleaching bath.
  • the voltage applied to the electrode leads 224 and 225 is measured by an electric control instrument which forms part of a control and dosing pump assembly.
  • FIG. 3 is a schematic representation of such an assembly consisting of control instrument 326 and dosing unit 331.
  • the control instrument 326 consists substantially of a direct current amplifier 327 and an mV- measuring instrument 328 with a lower and upper limit voltage contact 329 and 330.
  • a dosing unit 331 which contains a common drive motor 332 for the two dosing pumps 333 and 334.
  • the pumps 333 and 334 are connected at the input end to the storage tank for hydrohalic acid 335 and storage tank for oxidising agent 336 by way of flexible tubes while the pump output ends are connected to the inlet tubes 301 and 302 of the reoxidation apparatus.
  • the threshold contact 330 is adjusted to the redox potential which corresponds to the required ratio of potassium ferricyanide concentration to potassium ferrocyanide concentration.
  • the dosing apparatus continues to deliver hydrohalic acid and oxidizing agent until the potential on the platinum electrode 214 reaches the adjusted redox potential, and the motor 332 is then switched off at the contact 330.
  • ferricyanide is reduced to ferrocyanide and the redox potential drops.
  • the motor 332 is switched on again.
  • the best way to achieve this is to connect the reoxidation apparatus to the pumping cycle of the bleaching bath tank so that the ferrocyanide concentration of the processing tank is controlled virtually continuously and kept constant.
  • the threshold contact 329 can be used most simply for signalling when the potassium ferrocyanide concentration exceeds a previously adjusted maximum level.
  • a relay may be connected to the threshold contact 329 to act as cable break safety fuse for the lead to the detecting element so that the dosing apparatus 331 will be switched off as soon as the redox potential drops below the value to which contact 329 is adjusted.
  • FIG. 4 is a diagram showing the relationship between the redox potential and the ratio of potassium ferricyanide concentration to potassium ferrocyanide concentration in a bleaching bath (formulation see example 1). It will be seen that the rate of change of redox potential increases with decreasing proportion of ferrocyanide. In the region of low ferrocyanide concentrations, the control circuit has a high sensitivity to changes in ferrocyanide concentration. Since in practice it is desirable to have only low ferrocyanide concentrations in bleaching baths, the corresponding redox potential range is eminently suitable for controlling a reoxidation system.
  • the process according to the invention may advantageously be used as a through-flow process for continuously regenerating spent bleaching baths by reoxidation of the ferrocyanide with bromine.
  • Local accumulation of bromine which, as is well known, causes decomposition phenomena, is effectively prevented by the fact that the outlet tube 204 is situated in a region of vigorous flow so that any bromine discharged from it is immediately diluted or used up in oxidizing the ferrocyanide.
  • the redox potential measured by electrometric measuring chains in the bleaching bath can be used not only for controlling the regeneration of the bleaching bath but also for indirectly controlling the regeneration of other processing baths, e.g. a developing, fixing, buffering, hardening or stabilizing bath.
  • the time lag between the bleaching of a certain silver image and thepreceding development of the same image results in a corresponding idle time in the control circuit as regards regeneration of the color developer bath. Since, however, the buffering capacity of the developer for color developer substance is relatively high and moreover the average quantity of silver per unit surface area of the photographic material is substantially constant, this idling time does not in practice give rise to any trouble.
  • the quantity of regenerating solution required for regeneration is proportional to the quantity of photographic material passing through the baths and hence to the total quantity of silver halide whereas in the color developer bath and the following bleaching bath the consumption and regeneration depend only on the quantity of exposed silver halide. It has been found, however, that on an average the quantity of silver halide which has been exposed and can be developed by color development bears an almost constant ratio to the total quantity of silver halide and that this ratio is approximately 1:3. It follows from this that the other processing baths can be regenerated in the same manner, controlled by the electrometric measurement of the redox potential in the silver bleaching bath. This applies particularly to baths in which the concentration of active substance need not be observed strictly accurately, e.g. the final fixing baths, buffering baths, hardening baths and stabilizing baths. For these baths, regeneration controlled by the redox potential in the bleaching bath constitutes a simple and economical methods.
  • EXAMPLE 1 200 metres of a color negative film 35 mm in width which had been exposed imagewise and then color developed were bleached in 10 litres of a ferricyanide bleaching bath of the following composition:
  • the threshold contact 330 (FIG.
  • Loss of concentration due to material being carried along with the film passing through the apparatus was made up by the addition of 2 g/l of potassium ferricyanide and 0.5 g/l of potassium bromide.
  • Photographic tests showed that the bleaching effect of the reoxidised and regenerated bleaching bath was equal to that of the fresh solution.
  • EXAMPLE 2 10 litres of an exhausted bleaching bath have the same composition as in example 1 were reoxidized and regenerated.
  • 72 ml ofa 5.6 percent potassium bromate solution and 72 ml of a solution containing 200 g/l of potassium bromide and 204 g/l of a 36 percent hydrochloric acid were pumped as reoxidation solutions into the inlet tubes 201 and 202 of the reoxidation apparatus (FIG. 2).
  • the redox potential rose to 335 mV and the supply of reoxidation solutions stopped.
  • the bleaching effect of the reoxidised and regenerated bleaching bath was excellent.
  • EXAMPLE 3 10 litres of an exhausted bleaching bath of the same composition as in example 1 were reoxidized and regenerated. 72.5 ml of a 5.6 percent potassium bromate solution and 72.5 ml of a solution containing 410 g/l potassium bromide and 230 g/l of an percent orthophosphoric acid were pumped as reoxidation solutions into the inlet tubes 201 and 202 of the reoxidation apparatus according to FIG. 2. The redox potential rose to 335 mV and supply of the solutions stopped.
  • the bleaching effect of the reoxidized and regenerated bleaching'bath was excellent.
  • EXAMPLE 4 10 litres of an exhausted bleaching bath having the same composition as in example 1 were reoxidized and regenerated. 24 ml of a 10 percent hydrogen peroxide solution and 16 ml of a 48 percent hydrobromic acid solution were pumped as reoxidation solutions into the inlet tubes 201 and 202 of the reoxidation apparatus according to FIG. 2.
  • Potassium ferricyanide l potassium ferrocyanide potassium bromide disodium hydrogen phosphate made up with water to pH adjusted with acetic acid to Analysis of the bath after bleaching showed that the ferricyanide concentration had dropped to 86.9 g/l and the potassium bromide concentration to 15.0 g/l while the ferrocyanide concentration had risen to 12.5 g/l.
  • the pH of the bath was found to be 5.2.
  • the spent bleaching bath was reoxidized and regenerated in example l.
  • the threshold contact 330 was adjusted to a redox potential of 350 mV (redox potential of unused bath). 168 ml of a 5.6 percent potassium bromate solution and 168 ml of a 14.5 percent hydrobromic acid were pumped into the inlet tubes 201 and 202 of the reoxidation apparatus within a few minutes. The redox potential had then risen to 350 mV and the dosing pumps were automatically switched off. Analysis of the bath indicated 95.1 g/] of ferricyanide, 1.3 g/l of ferrocyanide, 18.9 g/l of KBr and a pH of 5.2.
  • a method of regenerating a used photographic silver bleaching bath containing an alkali metal ferricyanide, an alkali metal ferrocyanide and an alkali metal halide which comprises continually measuring the redox potential of the bleaching bath, thereby sensing the concentration of ferrocyanide formed in the bleaching bath by reduction of ferricyanide simultaneously with the bleaching of silver in the photographic material, producing a halogen by oxidation of halide ions in an aqueous solution in a closed vessel upon reaction between hydrohalic acid and an oxidizing agent to completely decompose the acid and form the halogen, immediately delivering the free halogen from the closed vessel into the bleaching bath under the surface of the bath, variably controlling the input of the hydrohalic acid and the oxidizing agent into said reaction in the closed vessel in response to the redox potential in the bleaching bath, whereby the quantity of halogen produced in the vessel is equivalent to the concentration of ferrocyanidc in the bleaching bath and reacting the halogen with the ferro
  • halogen is bromine formed in acid aqueous solution by oxidation of bromide ions.
  • bromine is formed by reaction of an alkali metal bromate with an alkali metal bromide and hydrochloric acid.
  • bromine is formed by reaction of an alkali metal bromate with an alkali metal bromide and orthophosphoric acid.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Silver Salt Photography Or Processing Solution Therefor (AREA)
  • Water Treatment By Electricity Or Magnetism (AREA)
US00353101A 1972-04-27 1973-04-20 Process for regenerating spent photographic silver bleaching solutions Expired - Lifetime US3839043A (en)

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DE19722220667 DE2220667A1 (de) 1972-04-27 1972-04-27 Verfahren zum regenerieren von gebrauchten fotografischen silberbleichloesungen

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US (1) US3839043A (sv)
JP (1) JPS4942347A (sv)
BE (1) BE798381A (sv)
CH (1) CH577697A5 (sv)
DE (1) DE2220667A1 (sv)
FR (1) FR2182174B3 (sv)
GB (1) GB1408523A (sv)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120070365A1 (en) * 2009-02-09 2012-03-22 Mira Bergstein Freiberg Process and apparatus for preparing molecular bromine

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS556269Y2 (sv) * 1974-06-07 1980-02-13

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2515930A (en) * 1946-05-04 1950-07-18 Pavelle Color Inc Regeneration of photographic silver bleach solution
US2611699A (en) * 1949-12-30 1952-09-23 Gen Aniline & Film Corp Regeneration of exhausted silver bleaching solutions
UST878007I4 (en) * 1970-01-29 1970-09-08 Defensive publication
US3615507A (en) * 1968-12-13 1971-10-26 Eastman Kodak Co Photographic bleach-fix solutions
US3770437A (en) * 1972-04-06 1973-11-06 D Brugger Photographic bleach compositions

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2515930A (en) * 1946-05-04 1950-07-18 Pavelle Color Inc Regeneration of photographic silver bleach solution
US2611699A (en) * 1949-12-30 1952-09-23 Gen Aniline & Film Corp Regeneration of exhausted silver bleaching solutions
US3615507A (en) * 1968-12-13 1971-10-26 Eastman Kodak Co Photographic bleach-fix solutions
UST878007I4 (en) * 1970-01-29 1970-09-08 Defensive publication
US3770437A (en) * 1972-04-06 1973-11-06 D Brugger Photographic bleach compositions

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120070365A1 (en) * 2009-02-09 2012-03-22 Mira Bergstein Freiberg Process and apparatus for preparing molecular bromine

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FR2182174B3 (sv) 1976-04-16
JPS4942347A (sv) 1974-04-20
FR2182174A1 (sv) 1973-12-07
GB1408523A (en) 1975-10-01
CH577697A5 (sv) 1976-07-15
DE2220667A1 (de) 1973-11-08
BE798381A (nl) 1973-10-18

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