Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Reexamination Certificate
1999-12-14
2002-05-14
Dawson, Robert (Department: 1712)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
C205S104000, C205S105000, C205S263000, C205S744000, C205S753000, C205S771000, C204S228600, C204S229400, C204S647000, C430S398000, C430S399000, C430S400000
Reexamination Certificate
active
06387243
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes for recovering metals absorbed on a support of the resin type, in particular in the field of treatment and recycling of industrial effluents, in particular effluents resulting from photographic development, and concerns a process for separating and recovering metals absorbed on an ion-exchange resin and to a process and an installation for the treatment and recycling of at least some effluents of the aforementioned type, employing the aforementioned separation process.
BACKGROUND OF THE INVENTION
At present, cationic resins saturated by metal ions or a mixture of metal ions are generally regenerated by one of the following processes:
elution by an acid or a chelating agent with or without a chromatographic effect,
regeneration in an electrolytic cell
pumping of the ions by means of resin fibres and under the influence of an electrostatic field toward compartments which receive the concentrated saline solutions.
Now all the processes described hereinbefore yield concentrated metallic salt solutions which are generally mixed with one another and with other components or are optionally monometallic as the result of an expensive treatment necessitating significant investment costs.
Furthermore, the aforementioned processes are limited with regard to the obtaining of metals to those of which the normal oxidation-reduction potential is higher than or equal to −1.2 volts.
Moreover, various techniques are also currently known for treating these effluents, the most commonly used being precipitation, cementation, ion exchange, electrolysis, ultrafiltration and reverse osmosis, each of which is applied directly and independently of the other methods to the effluents to be treated, in particular to the bleaching/fixing baths loaded with silver.
The method of precipitation involves precipitating the silver ion using a sodium or potassium sulphide according to the equation:
2Ag
+
+S
−−
→Ag
2
S
This process definitively destroys the bath and produces silver sulphide which has to be recycled in silver metallurgy. This is achieved by carrying out “cupellation” during which the Ag is displaced by molten lead according to the reaction:
Ag
2
S+Pb→2Ag+PbS
The lead is recovered by an extremely laborious complex thermal process commonly known as the “Pattinson process”. This method is extremely toxic and problematic owing to the resultant environmental pollution.
For its part, cementation involves displacing the silver in solution with iron, which is electropositive, as follows
2Ag
+
+Fe→2Ag+Fe++
This process is carried out by passing the solution to be desilvered into a cartridge containing iron or steel filings and is used mainly to desilver film washing water having a low silver content. A poor quality silver mixed with iron is obtained and the treated effluent is loaded with iron ions.
The ion-exchange method involves fixing the silver dithiosulphate complex present in the baths on an ion-exchange resin of the strongly basic type by exchanging silver dithiosulphate ions for chloride ions as follows:
3[[R]+Cl
−
]+[Ag(S
2
O
3
)
2
]
−
→3[R]+[Ag(S
2
O
3
)
2
]
−
+3Cl
−
The silver is then eluted with a sodium thiosulphate or sodium chloride solution, and this eluate can be recycled in an electrolytic desilvering unit.
This process is particularly suitable for the treatment of film washing water or for finishing the desilvering of effluents already pre-desilvered by electrolysis and having low silver contents.
The main drawbacks of this process, as carried out at present are that:
the residual Ag content of the treated effluents often has a high dispersion, depending on the work regime of the development workshops, and the maximum levels of concentration in waste established by laws and regulations cannot always be guaranteed;
the effluents are loaded with chloride in proportion with the initial Ag content;
effluents originating from colour photographic processes containing ferric complexes such as iron III EDTA or ferrocyanide are also fixed and progressively poison the resin;
the anionic resins used cannot be recovered or reused;
the silver is not decomplexed relative to the thiosulphate and the treated bath never has a composition allowing it to be reused.
The electrolytic process involves depositing the Ag contained in the waste fixing or bleaching and fixing baths by direct electrolysis of these baths as follows:
Ag[(S
2
O
3
)
2
]
−
⇄Ag
−
+2S
2
O
3
−
Ag
+
+e
−
→Ag
This electrolytic process can be carried out according to different variations or applications mentioned hereinafter:
1) The electrolysis of fixing or bleaching and fixing baths with regeneration thereof involves carrying out partial desilvering to a minimum content of 1 g/l under a voltage limited to 1.3 V. Under these moderate electrolysis conditions, the deposited Ag is of good quality, the current efficiency is almost 100% and the bath is not decomposed and is recycled after addition of “regenerating” products which compensate the consumption of reagents by the irreversible reactions of the photographic process.
The volume of the bath increases as regeneration takes place, and the partially desilvered bath which is to be treated by a depolluting agent is periodically purged.
Furthermore, in the case of bleaching and fixing baths and owing to the presence of the Fe III EDTA complex in colour photographic processes, secondary reactions at the electrodes reduce the current efficiency.
In the best case, the recovery rate of the bath cannot exceed 60% as it is determined by the ratio between the average content at the inlet which is 2.5 g/l of Ag on average and that at the outlet limited by the process itself to 1 g/l.
The last evolutions of this method of direct electrolysis of the baths leads to desilvering with a minimal residual content of 0.5 g/l (still representing 20% of the initial content), produce waste in the form of graphite beads and are suitable only for the treatment of black and white photographic baths.
2) The electrolysis of fixing or bleaching baths leading to the destruction thereof ends with advanced desilvering (1 to 10 mg/l) by vigorous electrolysis at about 1.8 V. These vigorous conditions cause precipitation of Ag sulphide by decomposition of the thiosulphate. Precipitated Ag
2
S should be filtered after flocculation, and the decomposition of the thiosulphates also causes a release of toxic foul-smelling H
2
S. Furthermore, Ag
2
S has to be treated again by the above-described method of precipitation.
3) Electrolytic desilvering of the washing water involves carrying out electrolysis in special apparatuses comprising a cathode consisting either of a stainless steel foam or of a stack of vibrated graphite beads having a very large active surface area, the anode being made of titanium. These devices, which yield silver contents <1 mg/l in the treated water, are also employed by depolluters in order to finish the desilvering of fixing or bleaching and fixing baths treated as described hereinbefore (in this case, the first desilvering operation by conventional electrolysis is limited to a content of 100 mg/l for economic reasons).
This process has the drawback of producing silver-covered graphite beads which, in turn, have to be treated.
Ultrafiltration and reverse osmosis are membrane-type processes employing modules containing semipermeable membranes of which the porosity is selected according to the substances to be eliminated. They necessitate relatively high working pressures and are suitable only for sparingly loaded water such as rinsing water, for example. Furthermore, their use necessitates very large, expensive technical means.
SUMMARY OF THE INVENTION
Thus, the first object of the present invention is to design a process for the separation and recovery of metals absorbed on an ion-exchange resin, for exam
Dawson Robert
Feely Michael J
Young & Thompson
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