DE2209959A1 - PROCESS FOR THE REGENERATION OF USED PHOTOGRAPHICAL BLEACHING SILVER BATHS WITH CHLORINE ACID OR ITS WATER-SOLUBLE SALT - Google Patents

Description

AGFA-GEVAERTAG

PATENTAEiTElLUNa

LEVERKUSEN

Ln / HG -

Process for the regeneration of used photographic silver bleaching baths with chlorous acid or its water-soluble ones Salt

The invention relates to a method of regenerating used photographic silver bleach baths, and in particular a process for the oxidation of the iron (II) cyanide complex in used photographic silver bleaching baths.

When processing color photographic materials, a Oxidation bath required, which converts the silver formed during development into a silver salt.

Baths with the alkali salts of the iron-III-cyanide complex Me ₃ (Fe ^{I: EI} (CN) ₆ ), which also contain substances that give stable compounds with silver ions, such as chlorides and bromides, are particularly suitable.

The following reactions take place:

K ₃ / Pe ~ ^II]: (CN) ₆ _7 Ag +> K ₃ Ag FFE ¹¹ (CN) ₆ J

K ₃ Ag / "Fe ^ ¹ (CN) ₆ + J KBr) K ^ ffe ^lI (CK) ₆ J + AgBr

In the presence of halide ions, the alkali silver ferrocyanide formed primarily converts to alkali metal ferrocyanide and Silver halide.

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There is therefore an accumulation of complex iron (II) cyanide in the iron (III) cyanide solution, which reduces its redox potential and therefore its effectiveness .

There is considerable interest in the reuse of used photographic silver bleaching baths, both for reasons of profitability and because of recent concerns that these cyanide-containing baths are simply added to the wastewater after use. What is needed, therefore, is a method of regenerating used photographic silver bleach baths. In particular, the iron (II) cyanide complex formed during use must be oxidized to the corresponding trivalent iron complex.

Processes for the oxidation of the iron (II) cyanide complex are on known. But they have the disadvantage that they are questionable for safety reasons and therefore for the intended group of consumers are out of the question, or that they lead to an accumulation of substances foreign to the bathroom, which the unlimited usability for photographic Purposes.

The oxidation of the iron (II) cyanide complex can be carried out, for example, with elemental bromine or with elemental chlorine. Both processes have the same advantage that the halide ions (chloride or bromide) consumed in the bleaching process are supplemented at the same time as the oxidation of the iron (II) complex. Working with free chlorine or bromine is equally dangerous and therefore cannot be expected of the intended group of consumers. The electrolytic oxidation, which can be carried out industrially, is also out of the question for the designed circuit, as required for this system is too complicated and there is a risk of the formation of fi-eiem hydrogen and hydrogen cyanide.

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The oxidation of the iron (II) cyanide complex is also known with salts of persulphuric acid. However, this method has the disadvantage that substances foreign to the bath, e.g. potassium sulfate, accumulate in the bleach bath and reduce its effectiveness.

In Defensive Publication No. 752 509 there is a procedure for Regeneration of used bleach baths with the help of hydrogen peroxide is described. So that the oxidation with the peroxide takes place quickly enough, however, the pH of the bleaching bath must be reduced below 5, advantageously to 3. Otherwise it takes place the oxidation only takes a few days. The lowering of the pH is a major disadvantage for this process especially since the bleaching baths contain buffer substances and are set to a higher pH value again after the oxidation process Need to become. With each regeneration there is therefore a considerable increase in the salt concentration in the bleach bath.

A process has now been found for the oxidation of the iron (II) cyanide complex in used photographic silver bleaching baths, which is characterized in that the baths have a pH of 5-8, preferably between 5 and 7, optionally in the presence of iron , Ruthenium or osmium, or compounds of these metals are treated with chlorous acid or its water-soluble salts. The process enables used photographic silver bleach baths to be regenerated in a simple manner in terms of apparatus, without the need for a change in the pH value and without the accumulation of substances foreign to the bath.

The oxidizing agents here are the chlorides of chlorous acid or its salts, which are preferably used in the form of an approximately 30% aqueous solution of the sodium salt of chlorous acid. The 30 % aqueous

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The solution may contain small amounts of sodium chloride, soda and caustic soda. Suitable in the same way The oxidizing agent is commercially available sodium chlorite in solid form, which is generally in the form of an 80% product for bleaching purposes for the textile industry or for drinking water treatment is commercially available, whereby sodium chloride, soda and small amounts of caustic soda have been added to increase the resistance.

The use of chlorous acid and its salts as oxidizing agents enables a process to regenerate used K, Fe (CN) g-containing silver bleach baths that is extraordinary is advantageous.

1) The chlorous acid or especially its sodium salt in solid or liquid form can be handled easily and safely.

2) Chlorite ions provide four oxidation equivalents and are reduced to chloride ions during the reaction, which advantageously absorb the bath with the formation of poorly soluble Can partially replace halide ions withdrawn from silver salts. The bathroom is therefore not distracting Reaction products polluted.

In general, after determining the amount of Fe-II-cyano complex, the required equivalents are added to the silver bleach bath Amount of chlorite ions added. When using equimolar amounts of oxidation equivalents, the oxidation proceeds above pH 5, relatively slowly. The rate of reaction is considerably accelerated by increasing the oxidation equivalents of chlorite ions. When applied of 1.25 oxidation equivalents, an autocatalytic reaction occurs after approx. 13 hours.

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halt) a few minutes vigorously terminated the oxidation reaction; occurs when using 1.5 or more oxidation equivalents this reaction occurs after about 8 hours.

Both the slow-moving and the autocatalytic Oxidation can be greatly accelerated by adding iron, ruthenium or osmium and their compounds.

The catalyst can be added to the bath to be oxidized in the form of water-soluble compounds, e.g. in salt-like form will. In particular, ruthenium chloride RuCl or osmium tetroxide OsOa are suitable for this purpose. Since these substances' cause no photographic adverse effect when using the silver bleaching baths, can be achieved by separating the Catalyst after oxidation of the iron (II) cyanide complex be disregarded. The catalyst is simply left in the bath and it is subject to oxidation after using the In the silver bleaching bath, only the addition of chlorite ions is required. If iron salts are used as a catalyst come, in order to prevent the formation of Berlin blue, one will limit oneself to those that are complexed with very low dissociation constant exist, for example on the iron complexes of nitrilotriacetic acid, ethylenediaminetetraacetic acid or other complex-forming amino polycarboxylic acids.

In another embodiment, the catalyst is used in a water-insoluble form, for example a metallic one Ruthenium on one of the usual carrier materials, such as activated carbon, silica or pumice stone. With this catalyst material the bath to be reoxidized, mixed with sodium chlorite, is brought into close contact, for example by suspending the catalyst or by passing it through a with the Catalyst-filled column is allowed to run through. The oxidation can be carried out in batches or also in continuous

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Licher working method ρ where one looks at the processing tank, which contains the bleaching bath, a regeneration unit "closes, in which the bleach is reoxidized in the flow.

The degree of depletion of the silver bleaching bath, and thus that of the Reoxidation needed for chlorite ion, can be done in simpler Way can be determined by determining the redox potential.

A used bleach bath has the following composition:

Potassium ferricyanide 39.5 g / l

Potassium ferrocyanide 6th g / 2 Potassium bromide 15th g / 1 Monopotassium o sphCL t 19th g / 1 Dlnatr iuiapho «phat 7.5 g / 1 IRt 7 g / 1 pH 6.0-6.1.

Bas Bad nat a redox potential, measured with a Pt-Blektrod® against »in® calomel electrode, of + 275 mV. I'm fresh Silberbitichbad should be about «ir. Redox potential of + 320 mV to have"

The amount of sodium chlorite used for the oxidation can vary within wide limits, in general it is sufficient to use the amount of oxidation equivalents sufficient for the oxidation, in this case about 0.4 g / l of 80 tf-1 g ^ n NaCl ' _? ~ Solution.

The silver bleaching baths to be used according to the invention contain, in addition to the Fe- * Cyaiio complexes mentioned and, if appropriate, the catalysts mentioned, the customary known additive "", for example sulfites, carbonates of sodium and potassium, phosphate and borates, and sodium or potassium bromide.

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example 1

One liter of a used bleaching bath that contains 39.5 g of potassium ferricyanide and 6 g of potassium ferrocyanide and its pH is 6.05, has a redox potential measured on a

Pt electrode against a calomel electrode of + 275 mV. The bath is divided into four samples and different amounts of an 80% NaClO ₂ solution as an oxidizing agent and samples 2 and 4 as a catalyst, ammonium vanadate are added to the various samples.

The test results are compiled in Table 1, with Column 2 indicates the oxidation equivalents of added oxidizing agents of samples 1 to 4, column 3 the respective indicates the required amount of catalyst in g and columns 4, 5 and 6 indicate the times at which the individual ProTbea the Have reached a potential value of 300, 320 or 340 mV «,

Table 1

Φ -p cd
cd > 2 lys CU 300 8th 3/4 7th 1/6 Redox potential mV 320 1 mV Hours, 1 340 1 mV Hours. Autocatalysis P ΐ) Η CD β Hours. 14th 1 Hours. 6th Hours. after U ig.σ ¹ & Hours. 1 1 / 2 Hours. 3 1 / 2 Hours. no 1 - 1 O Hours. 7th / 2 Hours. 8th Hours. no 2 1 O, 05 Hours. / 4 / 3 8- 1/2 hours 3 2 O, 0 1/2 hour 4th O, 05

Samples 1 and 3 characterize the course of oxidation ₉ which is observed when no catalyst is involved in the reaction.

Samples 2 and 4 contain the catalyst, which shortens the reaction time of the samples by 84 and 96% compared to samples 1 and 3.

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In contrast, the shortening of the reaction time when 2 oxidation equivalents / reduced Fe-II-cyano complex are added is only about 50 %.

No autocatalysis is observed when using an oxidation equivalent / reduced Fe-II-cyano compound, while using 2 oxidation equivalents / reduced Fe-II-cyano-complex an autocatalysis can be observed.

In parallel tests, traces of osmium tetroxide are used as a catalyst applied.

With the addition of 1.25 oxidation equivalents / reduced Fe-II-cyano complex, the autocatalytic reaction is observed after just 10 minutes.

Example 2 :

50 ml of a used bleach bath as described in Experiment 1 is divided into 2 samples and with different amounts of 80% NaClO ^ .- solution added as an oxidizing agent, their Oxidation equivalents are given in Table 2 below.

The two samples 5 and 6 are each treated with a catalyst which consists of 1 g of activated carbon loaded with 5 % metallic ruthenium, with

a) the catalyst is suspended in the bleach solution to be oxidized will and

b) the bleach bath solution runs through a column which is connected to the Catalyst filled ίετ,

The two testers a) and b) observed results were identical and w * rdsr. reproduced in Table Z.

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Table 2 ;

Sample jOxydat. ! Equivalent,

Redox potential

300 mV

320 mV

340 mV

Autocatalysis

1 1.5

1 H.

1/2 hour

2 hours 1 hour

2 1/2 hours

none after 11/2 hours

Example 3 ;

One liter of a used bleach bath that contains 39.5 g of potassium ferricyanide. still contains 6 g of potassium ferrocyanide and its pH value is 6.05, 0.4 g of 80% igera NaClO _{2 is} added, the mixture is stirred and left to stand for 16 hours. After this time, the bath can be reused as such or it can be supplemented with a bleach bath regenerator by adding the substances that are still missing.

Example 4 ;

One liter of the used bleach bath is made with 0.45 g of 80% NaClOg and mixed with approx. 1 mg osmium tetroxide. After a half an hour, the bath can be reused, as example 3 indicates.

Example 5:

The procedure is as in Example 4, except that osmium tetroxide is used instead Add 0.1 g of ammon vanadate. The bath can be reused after 3 hours as indicated in Example 3.

Example 6:

250 ml of the used bleach bath mentioned in Example 3

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are mixed with 0.15 g of NaClO ₂ or with 0.5 ml of 30 # NaC10 ₂ solution and with 50 g of a ruthenium catalyst, which consists of idCv balls (diameter 3-4 mm) with 0.5 % metallic ruthenium, left to stand for 1/2 hour.

After the catalyst has been poured off, the bath can be reused.

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Claims (2)

Patent claims: Process for the oxidation of the iron (II) cyanide complex in used photographic silver bleaching baths, characterized in that the baths are at a pH of
5 - S are treated with chlorous acid or with its water-soluble salts. 2) Method according to claim 1, characterized in that the oxidation in the presence of ruthenium or osmium, or
Compounds of these metals is carried out « AG 949 - 11 ORlGlMAL INSPECTED