United States Patent (19) [11] 3,912,514 Bulloch Et Al

United States Patent (19) [11] 3,912,514 Bulloch et al. (45) Oct. 14, 1975 54) METHOD OF REGENERATING ASPENT OTHER PUBLICATIONS PHOTOGRAPHIC SILVER BLEACH Def. Pub. T878,007, Walsh, 9-8-1970. SOLUTION Treatise on Inorg. Chem. by H. Remy. Vol. 1, 1956, (75. Inventors: David K. Bulloch, Hillsdale; Ronald pp. 810-81 l. A. Klein, Palisades Park, both of N.J. Primary Examiner-Mary F. Kelley 73 Assignee: Philip A. Hunt Chemical Attorney, Agent, or Firm-Kirschstein, Kirschstein, Corporation, Palisades Park, N.J. Ottinger & Frank 22 Filed: May 21, 1973 57 ABSTRACT 21 Appl. No.: 362,243 In a spent photographic silver bleach solution the ratio of ferrocyanide to ferricyanide has attained a value at 52) U.S. Cl................... 96/60 R; 96/50 A; 423/367 which the efficacy of bleaching is so greatly impaired I51) Int. C.’................. ... G03C 5/32; CO1C 3/12 that the solution must be either discarded or regener 58 Field of Search........... 96/60 R, 50 A; 423/367, ated. According to the present disclosure, regenera- . 423/473, 472; 252/186, 187 R tion is accomplished with an alkali metal halite that reacts with the ferrocyanide to oxidize it to a ferricya 56) References Cited nide. The reaction takes place at a pH between about UNITED STATES PATENTS 6 and about 8. An acid preferably is present to pre 2,515,930 7/1950 Seary.................................. 96/50 A vent the reaction conditions from becoming so alka 2,61 1,699 9/1952 Zappert.............................. 96/50 A line as to materially inhibit the oxidation. 2.6 t 1,700 9/1952 Brunner.............................. 96/50 A 8 Claims, No Drawings 3,770,437 1 1/1973 Brugger et al...................... 96/60 R 3,912,514 2 METHOD OF REGENERATING ASPENT cessity for regeneration long has been recognized in the PHOTOGRAPHIC SILVER BLEACH, SOLUTION photographic art and the specific regeneration of ferro cyanide to ferricyanide has been practiced for many BACKGROUND OF THE INVENTION years. 1. Field of the Invention At one time, an electrolytic method was employed Regeneration of a spent photographic silver bleach for this purpose, but because of its high maintenance solution. costs and operational problems it was abandoned. 1. Description of the Prior Art A number of chemical agents have been proposed to Bleach solutions are widely used in certain branches oxidize ferrocyanide to ferricyanide. Thus, liquid bro of photography. They are employed to remove metallic O mine is disclosed for this purpose in U.S. Pat. No. silver grains from an emulsion in which they were 2,515,930. But bromine is caustic and corrosive and, formed by development of exposed particles of silver hence, great caution must be exercised when it is em halide. Typically, the bleaching is performed in color ployed. As a variant of the bromine oxidation it has film processes to remove metallic silver grains the pres been proposed to use bromine releasing agents as, for ence of which is necessary to permit the formation of 15 example, those disclosed in U.S Pat. No. 2,61 1,700 for colored dye images, and in reversal black and white this purpose. Also, the use of hypobromides or mix processes to remove the silver grains which formed the tures of bromate ions and hydrobromic acid have been negative image prior to the re-exposure of film for ac disclosed as in U.S. Pat. No. 2,61 1,699. None of these tinically sensitizing the remaining particles of silver hal methods has found wide application, either because of ide. The purpose of bleaching is to convert the metallic 20 the caustic and corrosive nature of the compounds or silver into a silver salt which thereafter is made water the bromine released or because of their high cost or soluble by fixation (treatment with thiosulfate) and because of the involved manipulation required. subsequently removed from the film by washing with Recently ozone oxidation has come into commercial water. Although the conversion of metallic silver to the usage (see T. W. Bober and T. J. "The Regeneration of silver salt occurs in several steps the particular reaction 25 Ferricyanide Bleach Using Ozone" - Parts I and II, is not important to the discussion of this invention and, Image Technology, Vol. 14, No. 4-p. 13; No. 5-p. 19 therefore, only the overall simplified ionic equation is (1972)). The article describes large scale regeneration given below: trials on Process CRI-1 and Process K-12 bleaches using ozone, and suppliers of complete factory-built Fe(CN) + Ag+ Br - AgBr -- Fe(CNs' 30 systems for bleach regeneration are given. However, A commonly and widely used bleach solution con there is a difficulty with the ozone oxidation in that the tains as the oxidizing agent an alkali metal ferricyanide equipment used for generation of ozone has a high ini and an alkali metal bromide to effect the reaction tial capital cost and a high operation cost. Moreover, above given, and usually other ingredients such as generation of ozone is inefficient in terms of energy bleach accelerators (see, for example, British Pat. No. 35 conversion. Still further, the formulations for ozone 1,150,466), buffers (see, for example, U.S. Pat. No. generation are rapidly being changed so that often gen 3,342,598), corrosion inhibitors (see, for example, erating equipment is obsoleted before its initial cost British Pat. 872,275), anti-sludge agents, and biocides can be fully amortized. (see, for example, British Pat. 1,177,048). The particu Another approach to the regeneration of ferrocya lar constitution of the bleach solution is of no particular 40 nides has been by the use of peroxides and per-acids, importance to the present invention as long as it con but in such cases the bleach must be made very acidic tains ferricyanide which is in part reduced to ferrocya for the reaction to proceed rapidly and to completion. nide during the bleaching process. As just implied, the The high acidity results in undesirable side reactions ferricyanide oxidizes metallic silver to silver in its ionic and unduly complicates the regeneration of ferricya form which thereupon reacts with the bromide ion to 45 nide. form the sparsely soluble silver bromide. As the ferricy At the present time the most widely practiced anide oxidizes the metallic silver, the ferricyanide itself method for converting ferrocyanide to ferricyanide uti is reduced to ferrocyanide. lizes a water-soluble persulfate as the regenerating Ferrocyanide accumulates in the bleach solution. agent. The latter method has several advantages. Capi The decrease in ferricyanide and the corresponding in 50 tal outlay is low, the method is relatively safe and it is crease in ferrocyanide gradually impairs the efficacy of simple to use. However, it suffers several drawbacks, the bleach until finally a point is reached where the so the most serious of which is the formation of sulfate lution no longer is commercially effective and either ions as a by-product of regeneration. These ions reduce must be discarded or it must be regenerated. the efficacy of the bleaching action of the ferricyanide At this point there still is a preponderance of ferricy 55 on metallic silver grains, the reduction being so serious anide. It is only the ferrocyanide which must be regen that after four or five regeneration cycles the bleach erated by converting the same back to ferricyanide. has to be discarded. It is not currently feasible to inex The actual ratio between the two cyanides has no bear pensively remove the sulfate ions so as to restore the ing upon the use of the present invention and, indeed, usefulness of the bleach. As the sulfate ion concentra the limiting ratio beyond which the solution should not 60 tion increases, saturation eventually is approached and be used will depend upon various other factors such, in some portions of the system, e.g. a tank, or in some for example, as the solution temperature and the of the lines, crystallization occurs. To avoid this, users bleach accelerators employed. Mention has been made of persulfate for silver bleach regeneration frequently of the spending of ferricyanide simply in order to un 65 discard a portion of each batch of recycled bleach and derstand the manner in which the ferrocyanide came replace it with an equal volume of sulfate-free bleach. into being and the necessity of restoring ferrocyanide Typically, the loss incurred in 10% or more of an aver to ferricyanide when the occasion requires. Such ne age recycled volume of bleach. Perhaps a more impor 3,912,514 3 4 tant consequence of a high sulfate concentration in the the spent photographic silver bleach solution being re bleach is the necessity for a higher equilibrium concen generated. To prevent these two alkalizing effects, it is tration of ferricyanide than would be necessary if sul desirable to compensate for the rise in pH by including fate ions were not present. A cursory comparison of an acid in the regenerating reaction. Any acid is useful sulfate-free with sulfate-containing bleaches indicates 5 for example, a 10% water solution of an orthophos that the ferricyanide concentration for a sulfate-ion phoric acid. A preferred acid is hydrobromic acid be free bleach for an equivalent rate of bleaching is about cause it does not add any extraneous ions to the system 30% less than for a bleach containing a saturation and, indeed, provides a bromide ion which is useful in quantity of sulfate ions. the system. Also involved in connection with ferricyanide silver O It is of passing interest to observe that the same reac bleaches and in which the spent bleaches contain ferro tion, which essentially is an oxidation of the ferrocya cyanide as well, and which, for example, with the per nide constituent of the spent photographic silver sulfate regenerating process require discarding of bleaching can be used for the manufacture of an alkali bleach fractions to keep the sulfate ion concentration metal ferricyanide starting from an alkali metal ferro at an acceptable level, is that economic and conve 15 cyanide, or as a method of preparing a fresh ferricya nience factors are not the only governing consider nide bleach starting with a composition including an ations as they once were.

Load more