Electrolytic silver refining process

An electrolytic silver refining process in which crude silver is anodically dissolved and refined silver is cathodically deposited and at the same time accompanying metals are selectively extracted from the spent electrolyte and separately cathodically deposited after having been transferred into an aqueous phase and the regenerated electrolyte stripped of accompanying metals is recycled to the refining process and in which further the spent electrolyte is anodically enriched in silver and accompanying metals are cathodically deposited from the aqueous phase in a joint electrolysis step. The invention resides in that the joint electrolysis step is carried out in a diaphragm cell in which a diffusion zone is provided between one each cathode and one each anode and separated from the anode zone by an anionic separating membrane and from the cathode zone by a cationic separating membrane and that the diaphragm cell is charged via the diffusion zone with accompanying metal extract, of preferably controlled acid content, and that the apparatus for carrying out said process is preferably provided with an acid trap through which the accompanying metal extract flowsk in particular a diffusion dialyzer, which is arranged before the inlet of the accompanying metal extract into the diaphragm cell.

The invention relates to an electrolytic silver refining process in which 
crude silver is anodically dissolved and refined silver is cathodically 
deposited and at the same time accompanying metals are selectively 
extracted from the spent electrolyte and separately cathodically deposited 
after having been transferred into an aqueous phase, and the regenerated 
electrolyte stripped of accompanying metals is recycled to the refining 
process and in which further the spent electrolyte is anodically enriched 
in silver and accompanying metals are cathodically deposited from the 
aqueous phase in a joint electrolysis operation, the extraction step(s) 
optionally being provided in the form of a liquid membrane permeation, 
preferably in combination with solvent extraction. 
An electrolytic silver refining process in which crude silver is anodically 
dissolved and refined silver is cathodically deposited and at the same 
time accompanying metals are selectively extracted from the spent 
electrolyte and separately cathodically deposited after having been 
transferred into an aqueous phase and the regenerated electrolyte stripped 
of accompanying metals is recycled to the silver electrolysis is known and 
a process of this type with an installation is described, for instance, in 
Swiss Patent Specification No. 614,238. 
In electrolytic silver refining, which is normally carried out according to 
the Moebius or Balbach/Thum processes, the more common accompanying metals 
present in the crude anodic silver, such as copper, constitute an 
essential burden to the process because they are dissolved and accumulate 
in the electrolyte. In conventional silver nitrate baths, Cu content of 
about 60 g/1 is considered as the maximum admissible value; above this, a 
cathodic copper deposition may occur, but at least the refined silver 
occludes Cu salts so that the requirements as to fineness are no longer 
complied with. The spent electrolyte must thus periodically be regenerated 
or replaced from time to time. 
Electrolyte recycling was already employed in this respect, the spent 
electrolyte was withdrawn from the refining cell, Cu was extracted 
therefrom and the thus regenerated electrolyte depleted in Cu was recycled 
to the refining cell. The extracted Cu was transferred into an aqueous 
phase and cathodically deposited separately. A large number of organic 
reactants are known as selective extractants for Cu; so, for instance, the 
known process described above works on the principle of solvent extraction 
with chelating agents such as phenone oximes or quinolines. The spent 
electrolyte is extracted in mixer/settler installations and the Cu is 
stripped from the organic extract phase by means of an acidic stripping 
solution. The enriched stripping solution is electrolyzed, Cu and 
optionally Ni as well may be cathodically deposited. Depending on the 
chelating agent and the operating conditions, it is possible to extract 
accompanying metals more or less selectively. The organic phase ( 
solvent+extractant) is washed. The organic phase, the stripping solution 
regenerated by electrolysis and the regenerated electrolyte are recycled. 
Due to the simultaneous dissolution of Cu and Ag at the anode, more Ag is 
deposited at the cathode than dissolved at the anode. For this reason, the 
electrolyte is depleted in silver and it is necessary to add pure silver 
nitrate (as a solution) to the regenerated electrolyte as compensation. 
This constitutes a burden to the process because pure silver nitrate is an 
expensive product. 
In order to provide a process of the type initially mentioned in which a 
continuous complete regeneration of the electrolyte is effected and in 
which the silver deficiency is compensated without the addition of silver 
nitrate solution, it was proposed to anodically enrich the spent 
electrolyte in silver and cathodically deposit accompanying metals from 
the aqueous phase by a joint electrolysis operation, with a liquid 
membrane permeation, preferably combined with solvent extraction, 
optionally provided as extraction step(s). 
For this purpose, the apparatus for carrying out the process is provided 
with at least one refining cell, with at least one eletrolysis cell for 
the cathodic deposition of the accompanying metals and at least one 
extraction means for extracting the accompanying metals from the spent 
electrolyte and for transferring the extracted accompanying metals into an 
aqueous phase and with at least one joint electrolysis cell, preferably a 
diaphragm cell with an anionic diaphragm, for enriching the spent 
electrolyte in silver and cathodically depositing the accompanying metals. 
In the joint electrolysis cell, Ag is anodically dissolved, preferably by 
means of a crude silver anode, as the content in accompanying metals only 
needs to be reduced and not completely eliminated. The electric current 
liberated at the cathode during the cathodic deposition of the 
accompanying metals enhances the dissolution of the silver anodes. 
It is the function of the anionic diaphragm to prevent silver from entering 
the cathode zone where it would be re-deposited together with the 
accompanying metals and to convey the N0.sub.3 ions required for the 
formation of AgN0.sub.3 into the anode zone. 
By an appropriate selection of the extractants for the accompanying metals, 
of which chelating agents have proved to be most selective, it is possible 
to prevent the co-extraction of silver to the extent that the electrolytic 
re-enriching of the spent refining electrolyte with silver can be carried 
out even before the extraction of the accompanying metals . 
The extraction of the accompanying metals is normally a solvent extraction 
wherein first the spent refining electrolyte is treated with a 
solvent/extractant phase which is subsequently stripped by means of an 
(N0.sub.3.sup.-) stripping solution of ion parity. 
It was found, however, that advantages can be obtained in the extraction of 
accompanying metals other than Cu, particularly of Pb, platinum metals 
such as Pd or Pt, Ni, W, Zn and Cd, if the extraction process employed is 
liquid membrane permeation, possibly a combination of solvent extraction 
and liquid membrane permeation. 
In this, the solvent/extractant phase serves as a separating membrane 
between the spent refining electrolyte and an acidic stripping solution 
which is emulsified in the separating membrane. After the transition 
(permeation) of the accompanying metal(s) from the refining electrolyte 
into the organic emulsion phase, the emulsion phase is separated and 
disintegrated in a manner known per se to form an aqueous phase (enriched 
stripping solution) and an organic phase (solvent/extractant). The 
enriched stripping solution is again regenerated by means of subsequent 
electrolysis of accompanying metals or separation of valuable materials 
without electrolysis and emulsified in the organic phase again. In this 
case, the organic phase merely serves as a selective separating medium 
between the aqueous phases (spent refining electrolyte and stripping 
solution) so that it does not have to absorb the accompanying metal(s) 
such as in liquid/liquid extraction. This results in a clearly reduced 
solvent consumption and high enrichment rates such as in the range of e.g. 
1:100. 
It was found that the proposal according to Swiss Patent Specification No. 
614,238 to re-extract the organic extract phase enriched in accompanying 
metals by means of a sulfuric acid stripping solution technologically 
inconvenient because the stripped organic solvent/extractant phase must be 
washed free of sulfate ions prior to its recycling into the extraction 
step in order not to contaminate the refining electrolyte (nitric acid 
solution). 
As a result, it was proposed to conveniently work exclusively with 
solutions of ion parity, in particular nitric acid solutions, in stripping 
as well, so that this problem would not arise, in particular the operation 
of the joint electrolysis cell for the cathodic deposition of accompanying 
metals and for the anodic enrichment in silver would remain troublefree. 
On the other hand, however, this meant putting up with the fact that since 
copper, for instance, is in practice electrolytically depositable in 
favorable form virtually from sulfuric acid solutions only, the deposition 
of copper from the nitric acid medium is not optimal. 
It was further found that in the process proposed, a certain silver slip 
occurs in the diaphragm cell so that the cathodically deposited copper is 
doped with silver. 
It was thus the object of the invention to eliminate these disadvantages by 
carrying out the joint electrolysis operation in which the spent 
electrolyte is anodically enriched in silver, on the one hand, and the 
re-extracted accompanying metals, in particular copper, are cathodically 
deposited from the aqueous phase, on the other hand, in such a way that 
the cathodic operations are completely independent of the anodic 
operations. 
This object is achieved according to the invention in that the joint 
electrolysis operation is carried out in a diaphragm cell wherein a 
diffusion zone is provided between one each cathode and anode, which 
diffusion zone is separated from the anode zone by an anionic separating 
membrane and from the cathode zone by a cationic separating membrane, with 
the diaphragm cell being charged with accompanying metal extract, 
preferably of controlled acid content, via the diffusion zone. 
This allows, a.o., to provide a sulfuric acid medium instead of a nitric 
acid medium in the cathode zone and thus optimally deposit the copper and 
to prevent any silver slip to the cathode so that the copper deposited is 
free of silver. 
In this, the re-extracted, strongly nitric acidic copper solution i.e. the 
stripping solution enriched in Cu, is fed into the diffusion zone of the 
diaphragm cell which is called membrane cell or membrane electrolysis cell 
in the following. From the diffusion zone, the copper cations migrate into 
the cathode zone through the cation exchanger membrane, on the one hand, 
and the nitrate anions into the anode zone through the anion exchanger 
membrane, on the other hand, so that the copper is deposited on the 
cathode from sulfuric acid solution and silver is dissolved as silver 
nitrate on the anode. The anion exchanger membrane prevents the diffusion 
of the silver ions and the cation exchanger membrane prevents the 
diffusion of the sulfate ions into the diffusion zone. 
The stripping solution depleted in copper is withdrawn from the diffusion 
zone of the membrane cell and recycled to the re-extractor (stripper) 
after adjustment of its acid content. 
Depending on the plotted operation of the membrane cell, high acid 
concentration differences between anode and cathode zone can have a more 
or less negative effect. In order to prevent these disadvantages, it is 
further proposed according to the invention to control the acid content of 
the solution of accompanying metals fed into the membrane cell. This is 
conveniently effected by interposing an acid trap, in particular a 
diffusion dialyzer, between stripper and membrane cell. Diffusion 
dialyzers are known systems which by means of commercially available 
exchanger membranes concertedly release acid from acidic salines into 
water flowing in counter-current. In this way, fluctuations in the system 
can be compensated and the stripping operation may be done at very high 
acid concentrations; the membrane cell is operated in the manner most 
convenient for the respective case.

From the refining cell 1, the electrolyte E.sub.1 which is spent, i.e. 
deficient in silver and enriched in copper, passes into a two-step 
extraction mixer 2, 3 where it is extracted in counter-current by means of 
a Cu-selective extractant by liquid/liquid extraction (solvent 
extraction). 
Suitable solvent extractants for copper are e.g. cation exchangers such as 
versatic acid (synthetic carboxylic acid mixture of highly branched 
isomers of C.sub.10 monocarboxylic acids of mainly tertiary structure; 
commercial product e.g. Versatic 911) and HDEHP 
(bis-(-2-ethylhexyl)-phosphoric acid) as well as chelating agents such as 
oximes of the formula 
##STR1## 
the chelating agents being more selective for Cu, while Ag is always 
co-extracted with the cation exchangers mentioned. 
The following commercial products are mentioned as examples for chelating 
agents containing oximes of the above formula: 
SME 529 (contains oxime with R=CH.sub.3) 
LIX 64N (contains oxime with R=C.sub.6 H.sub.5 and oxime of the formula 
##STR2## 
ACORGA Pt 5050 (contains oxime with R=H, tridecanol and kerosene) 
The spent electrolyte and the once-enriched solvent extractant S.sub.2 are 
introduced into the mixer of the first extraction unit 2, the regenerated 
solvent extractant S.sub.1 and the electrolyte E.sub.2 once depleted in 
copper are charged into the mixer of the second extraction unit 3. 
From the settler of the second extraction unit 3, the electrolyte E.sub.3 
twice depleted in copper is fed into the anode zone 4 of a membrane 
electrolysis cell 4, 8, 10 where it is enriched in AgN0.sub.3 by the 
electrolytic dissolution of a silver anode in nitric acid solution and 
then recycled to the refining cell 1 as a regenerated electrolyte E.sub.4. 
From the settler of the first extraction unit 2, the doubly enriched 
solvent extractant S.sub.3 passes into the mixer of a scrubber-extractor 5 
where the co-extracted silver is eliminated by means of washing with 
copper nitrate solution W.sub.1. The washing solution W.sub.2 enriched in 
silver is withdrawn from the settler of the scrubber-extractor 5 and 
further processed in the usual manner; the solvent extractant S.sub.4 
depleted in silver is re-extracted in a two-step re-extraction mixer 6, 7, 
i.e. washed (stripped) free of copper in the counter-current by means of 
aqueous nitric acid. The solvent extractant S.sub.4 to be stripped and the 
stripping acid A.sub.2 once enriched in copper are fed into the mixer of 
the first reextraction unit 6 and the fresh (regenerated) stripping acid 
A.sub.1 and the solvent extractant S.sub.5 once depleted in copper are fed 
into the mixer of the second re-extraction unit 7. 
From the settler of the second re-extraction unit 7, the solvent extractant 
S.sub.1 twice depleted in copper and thus regenerated is recycled into the 
mixer of the first extraction unit 3 for closing the cycle; from the 
settler of the first re-extraction unit 6, the stripping solution A.sub.3 
doubly enriched in copper passes into the diffusion zone 8 of the membrane 
electrolysis cell 4, 8, 10 in which silver is anodically dissolved and 
copper is cathodically deposited, as a strongly acid copper nitrate 
solution. Nitrate anions are released from the stripping solution A.sub.3 
into the anode zone 4 via the anion exchanger membrane ATM and 
correspondingly copper cations are released into the cathode zone 10 where 
a sulfuric acid medium dominates, via the cation exchanger membrane KTM. 
The stripping solution A.sub.4 depleted in copper and nitrate ions in this 
way is withdrawn into a mixing container 9 where the acid content is 
completed with nitric acid and then recycled as a regenerated (fresh) 
stripping acid A.sub.1 to the mixer of the second re-extraction unit 7 for 
closing the cycle. 
The apparatus can be provided with a diffusion dialyzer 11 arranged between 
the re-extractor (stripper) group 6,7 and the membrane electrolysis cell 
4, 8, 10. The dialyzer 11 uses water to wash out nitric acid, which is 
passed directly into the mixing tank 9, from the stripping solution 
A.sub.3. This permits the adjustment of the acid concentration in the 
stripping solution A.sub.3 on entering into the membrane electrolysis cell 
to the respective optimal value. 
As already proposed, it is further possible to extract further accompanying 
metals, in particular Cd, Ni, Pd, Pt, Bi and/or W, from the spent 
electrolyte after the Cu-separation, but prior to the enrichment in 
silver. For this purpose, the drawing schematically shows a further 
extraction unit 12 between the settler of the second extraction unit 3 and 
the anode zone of the membrane electrolysis cell 4, 8, 10 in the path of 
the electrolyte E.sub.3 doubly depleted in copper. 
It is also possible to extract accompanying metals together with the copper 
and then separate them by means of selective stripping. 
As already proposed, it is also possible to enrich the spent electrolyte in 
silver first and then deplete it in copper instead of depleting it in 
copper first. This way of proceeding presupposes the use of a chelating 
agent as the solvent extractant in order to prevent the useless carrying 
along of silver. 
The anion and cation exchanger membranes used in the membrane electrolysis 
cell 4, 8, 1O and the diffusion dialyzer are commercially available and 
are offered by various firms with graded performance spectrum.