US3825425A - Process for the regeneration of used photographic silver bleaching baths using chlorous acid or its water-soluble salts - Google Patents

Abstract

A PROCESS FOR THE REGENERATION OF USED PHOTOGRAPHIC SILVER BLEACHING BATHS CONTAINING THE IRON-II-CYANIDE COMPLEX WHEREIN THE BATH IS TREATED WITH CHOROUS ACID OR A WATER SOLUBLE SALT THEREOF AT A PH OF 5 TO 8 OPTIONALLY IN THE PRESENCE OF A CATALYST AS HEREIN DEFINED.

Description

' 3,825,425 PROCESS FOR. THE. REGENERATION OF USED PHOTOGRAPHIC SILVER BLEACHING BATHS USING CHLOROUS ACID ORITS WATER-SOLU- BLE SALTS a 1 Max' Heilmann, Cologne, Germany, assignor to Agfa- Gevart, Aktien'gesellschaft, Leverkusen, Germany No Drawing. Filed Feb. 26], 1973, Ser. No. 335,597 ,Qlaimspriority, application Germany, Mar. 2, 1972, .1 -P 22 09 959.8

Int. Cl. G03c 5/26, 5/32 US. Cl. 96-60 R 11 Claims IABSTRACT OF THE DISCLOSURE A process for the regeneration of used photographic silver bleaching baths containing the ironII-cyanide complex wherein ,the bath is treated with chlorous acid or a water soluble salt thereof at a pH of 5 to 8 optionally in the presence of a catalyst as herein defined.

..This invention relates to a process for the regeneration of used photographic silver bleaching baths and in particular'tdazproces for oxidizing the iron-II-cyanide complex in used photographic silver bleaching baths.

The processing of color photographic materials requires the use of an oxidizing bath by which the silver formed on:developmentisconverted into a silver salt.

Particularly suitable for this purpose are baths which contain'alkali metal salts of the iron-III cyanide complex Me (FE (CN) in addition to substances which form stable compounds with silver ions e.g. chlorides and bromides.

The reactions which take place may be represented by the following equations:

In the presence of halide ions, the alkali metal silver ferrocyanide originally formed is converted into alkali metal'ferrocyanide and silver halide.

Complex'iron-II cyanide therefore accumulates in the iron- III cyanide solution, with the result that the Redox potential'drops and hence the efficiency of the solution decreases.

There is considerable interest in the possibility of reusing usedphotographic silver bleaching baths, firstly on grounds of economy and secondly because in recent times doubts? have arisen as to the advisability of simply discharging these cyanide-containing baths into the efiiuent after use. It is therefore necessary to find a process for the-regeneration of used photographic silver bleaching baths and: in particular the iron-II cyanide complex formed in the course of the process is required to be oxidized into the corresponding trivalent iron complex.

"Processes'for oxidizing the iron-II cyanide complex are known per se but such processes either have the disadvantage of being dangerous in use and therefore unsuitable for the range of consumers for which they are intended 'orfof leading to an accumulation of by-products in thebathso that their unrestricted use for photographic purposes is" impossible,"

. Oxidation. of the iron-H cyanide complex can be achieved for example,'with elementary bromine or elementary chlorine. Both of these methods would have the advantagenot only of oxidizing the iron-H complex hut at the same time replacing the halide ions (chloride of. bromide) used up in the bleaching process. Free chlorine and free bromine are .both, however, dangerous in use; and therefore unsuitable for the range of consumers envisaged-Electrolytic oxidation, which can be carried 3,825,425 Patented July 23, 1974 out on a large commercial scale, is equally unsuitable for such consumers because the installations required for this process are too complicated and because there is a risk of the formation of free hydrogen and of hydrogen cyanide.

It is also known to oxidize the iron-II cyanide complex with salts of persulfuric acid. This process, again, has the disadvantage that by-products such as potassium sulfate accumulate in the bleaching bath and reduce its efficiency.

In defensive Publication No. 752,509 there is disclosed a process for the regeneration of used bleaching baths using hydrogen peroxide, but in order that the oxidation with peroxide may proceed sufficiently rapidly, the pH of the bleaching bath must be reduced below 5 and preferably to 3 because otherwise oxidation would be achieved only in the course of several days. The necessity of reducing the pH constitutes a serious disadvantage of this process, particularly since the bleaching baths contain buffer substances and must be readjusted to a higher pH after the oxidation process. Each regeneration thereforeresults in a considerable increase in the salt concentration of the bleaching bath.

It now has been found a process for the oxidation of the iron-II cyanide complex in an used photographic silver bleaching bath in which the bath is treated with chlorous acid or a water-soluble salt of chlorous acid at a pH of 5 to 8, preferably 5.5 to 7, optionally in the presence of iron, vanadium ruthenium or osmium or compounds of these metals. This process enables the regeneration of used photographic silver bleaching baths to be carried out in simple apparatus without requiring any change in pH and without resulting in any accumulation of by-products in the bath.

The oxidizing agents used for this process are of chlorous acid or its salts, preferably in the form of an approximately 30% aqueous solution of the sodium salt of chlorous acid. The 30% aqueous solution may still contain small quantities of sodium chloride, sodium carbonate and caustic soda. An equally suitable oxidizing agent is solid commercial sodium chloride which is generally available as a bleaching agent for the textile industry or for the purification of drinking water as an product to which sodium chloride, sodium carbonate and small quantities of sodium hydroxide solution have been added to increase its stability.

The use of chlorous acid and its salts as oxidizing agent provides an extremely advantageous process for the regeneration of used silver bleaching baths which contain K Fe(CN) for the following reasons:

In general, after determination of the quantity of Fe-II cyanide complex present, the required equivalent quantity of chlorite ions is added to the silver bleaching bath. If equimolar quantities of oxidizing equivalents are used, oxidation proceeds relatively slowly above pH 5. The reaction velocity is considerably increased by increasing the oxidation equivalents of chlorite ions. When using 1.25 oxidation equivalents, an autocatalytic reaction sets in after about 13 hours so that vigorous oxidation then takes place which is completed within a few minutes. If 1.5 or more oxidation equivalents are used, this reaction sets in after about 8 hours.

Both the slow oxidation and the autocatalytic oxidation can be considerably accelerated by the addition of iron, vanadium, ruthenium or osmium or compounds of these metals.

The catalyst may be added to the bath which is to be oxidized in the form of water-soluble compounds, e.g. in the form of a salt, in particular ruthenium-III-chloride, ammonium-vanadate or osmium-IV-tetroxide. As these substances have no deleterious photographic elfect on the silver bleaching baths, they need not be removed after oxidation of the iron-II complex. The catalyst is simply left in the bath and all that is necessary for oxidation after use of the silver bleaching bath is simply the addition of chlorite ions by using chlorous acid or its salts. When 4 dition to 39.5 g. of potassium ferricyanidehas a Redoxpotential of +275 mv. measured at a platinum electrode against a calomel electrode. The bath is divided into four samples. Different quantities of an 80% NaClO solution are ad ded as oxidizing agent to the various samples and ammonium vanadate is added as'catalyst to samples 2and4. v 5 i f *3 The results of the experiment are summar I Table 1 in which column2 shows the oxidation equivalents, of the oxidizing agent added to's amples 1 16 column '3 shows thequantity of catalyst in g. requiredjn each case and columns-4, 5 and 6 ,showthetimes reqniredlonthe various samples to reacha'potential value of 300, 320 and 340 mv.

TABLE 1 Period after Oxlda- Time (in hours) taken to r'which autotion Catalyst reach redox potential catalysisequivquantity it any? Sample alents in g. 300 mv 3 20 mv. 340 mv. set in I 1 0.0 -8 14 e 16 3 None.

1 .0 A 2 0.0 7 75/ 8-'8%hours. 2 0.05 at t I a la'ihWI-f using iron salts as catalysts, the formation of Prussian Blue is prevented by restricting the choice of iron salts to those whose complexes have a very low dissociation constant, for example the iron complex of nitrilotriacetic acid, ethylene diaminotetraacetic acid or other complex forming aminopolycarboxylic acids.

According to another embodiment of this process, the catalyst is used in a water insoluble form, e.g. metallic ruthenium on a conventional carrier material such as active charcoal, silica or pumice stone. The used bath to which sodium chlorite has been added is brought into close contact with this catalyst material, for example by forming a suspension of the catalyst. Oxidation may be carried out either discontinuously or continuously, in which case a regenerating unit may be connected to the processing tank containing the bleaching bath, the bleaching bath being then reoxidized by flowing continuously through this unit.

The degree of exhaustion of the silver bleaching bath and hence the quantity of chlorite ions required for reoxidation can easily be determined by determining the Redox potential.

A used bleaching bath may have the following composition:

G./l. Potassium ferricyanide 39.5 Potassium ferrocyanide 6 Potassium bromide Monopotassium phosphate 19 Disodium phosphate 7.5 Sodium hexametaphosphate 7 pH 6.0-6.1.

The Redox potential of the bath, measured with a platinum electrode against calomel electrode, is +275 mv. A fresh silver bleaching bath should have a Redox potential of +320 mv.

The quantity of sodium chlorite used for oxidation may vary within wide limits. It is generally sufficient to use the quantity of oxidation equivalents required for oxidation, in this case about 0.4 g./l. of an 80% NaClO solution.

The silver bleaching baths used according to the invention contain in addition to the Fe-cyande complex mentioned above and optionally the catalysts mentioned above usual known additives such as sulfates, carbonates of sodium and of potassium, phosphates and borates as well as sodium or potassium bromide.

Example 1 One litre of a used bleaching bath which is at pH 6.05 and contains 6 g. of potassium ferrocyanide in ad- Samples 1 and 3 demonstrate the course of the oxidation when no catalyst is present to accelerate the reaction.

Samples 2 and 4 contain catalyst which reduces the reaction time of the samples by 84 and'96% as compared with samples 1 and 3 respectively. I,

By comparison, the reduction in reaction time'achieved with the addition of two oxidation equivalents'per reduced Fe-II cyanide complex is only about 50%."

No autocatalysis is observed when using one-oxidation equivalent per reduced Fe-II cyanide compound bu't auto catalysis is observed when using two oxidation equivalents per reduced Fe-II cyanide complex. I

In parallel tests, traces of osmium-tetroxide-a're used as catalyst. 7

When 1.25 oxidation equivalents per reduced Fe-II cyanide complex are added, the autocatalytic reaction is observed after only 10 minutes.

Examplez 50 ml. of a used bleaching bath as described in Experiment 1 are divided into two samples to which are added different quantities of a NaOlO solution of oxidizing agent, the oxidation equivalents of which are indicated in Table 2 below.

'(a) Each of the two samples 5 and -6 are treated with a catalyst consisting of 1 g. of active'charcoalcharged with 5% of metallic ruthenium; wherein the catalyst is suspended in the solution of bleaching bath which-is to be oxidized and 1 (b) In parallel tests each of the two samples 5 and 6 are treated as mentioned-above, wherein the solution of bleaching bath runs through a column which-is" filledwith the catalyst. Y? The results observed in experiments ,(a) and (b) are identical and are showninTable 2:1,, v

One litre of aused'bleaching bath whichhas sin 61: 6.05 and still contains 6 g, of potassium frrocyanidein addition to 39.5 g. of potassium ferricyanideji's treated with 0.4 g. of NaClo 'stirredl'andileft to' stand 16 hours. At the end of this'tirn'e,'the bath niaybithet be used again as such or worked up to a bleaching"batli' regenerator solution by addition of the still missing substances.

Example 4 0.45 of 80% NaClO and approximately 1 mg. of osmium tetroxide are added to 1 litre of the used bleaching bath as described in Example 3. After /2 hour, the bath may be used again as indicated in Example 3.

Example 5 The procedure is the same as in Example 4 except that 0.1 g. of ammonium vanadate is added instead of osmium tetroxide. The bath may be used again as indicated in Example 3 after 3 hours.

Example 6 0.15 g. of 80% Na'ClO or 0.5 ml. of 30% NaClO solution are added to 250 ml. of used bleaching bath mentioned in Example 3 and the bath is left to stand for /2 hour with 50 g. of a ruthenium catalyst consisting of SiO pellets (diameter 3-4 mm.) charged with 0.5% of metallic ruthenium.

After pouring otf the catalyst, the bath can be used again.

Patent claims! 1. A process for the oxidation of the iron-II cyanide complex in a used photographic silver bleaching bath in which the bath is treated with chlorous acid or a watersoluble salt of chlorous acid at a pH of from S to 8.

2. A process as claimed in Claim 1, in which the bath is treated at a pH of from 5.5 to 7.

. 3. A process as claimed in Claim 1, in which the bath is treated with a 30% aqueous solution of the sodium salt of chlorous acid. w i

4. A process as claimed in Claim 1, in which the bath is treated with solid commercial soldium chlorite.

5. A process as claimed in Claim 1 which is carried out in the presence of iron, vanadium, ruthenium or osmiumf or a compound of one of these metals as catalyst for the oxidation.

6. A process as claimed in Claim 1 in which a quantity of chlorite ions equivalent to the quantity of iron-H cyanide complex present is added to the used photographic silver bleaching bath.

7. A process as claimed in Claim 1, in which more than the equivalent quantity of chlorite ions is added to the used bath.

8. A process as claimed in Claim 6, in which ruthenium chloride ammonium vanadate or osmium te'troxide is added to the bath.

9. A process as claimed in Claim 5, in which the catalyst is used in a water-insoluble form.

10. A photographic silver bleaching bath which has been regenerated by a process as claimed in Claim 1.

11. A photographic silver bleaching bath which has been regenerated by a process as claimed in Claim 5.

References Cited UNITED STATES PATENTS 2,515,930 7/1950 Seary 9660R NORMAN G. TORCHIN, Primary Examiner M. F. KELLEY, Assistant Examiner