EP0138801B1 - Process for the electrolytic refining of silver - Google Patents

Description

The invention relates to an electrolytic silver refining process in which crude silver is anodically dissolved and raffinate silver is deposited cathodically, with accompanying metals being extracted selectively from the used electrolyte and separately being cathodically deposited after being converted into an aqueous phase, and the regenerated, low-accompanying-metal electrolyte is returned to silver electrolysis .

Such a method with a system is described for example in CH-PS 614 238.

In electrolytic silver refining, which is usually carried out according to the Möbius or Balbach / Thum process, the less noble accompanying metals present in the anode raw silver, in particular the copper, are a significant burden on the process, since they are dissolved and accumulate in the electrolyte. In conventional silver nitrate baths, a Cu content of around 60 g / l is considered the highest permissible value; A cathodic copper deposition can occur above this, but at least copper salts are enclosed by the raffinate silver, so that the fineness requirements are no longer met. The used electrolyte must therefore be regenerated or replaced from time to time.

The electrolyte cycle has already been used, with the used electrolyte being withdrawn from the refining cell, Cu being removed therefrom on the extraction path and the thus regenerated, low-Cu electrolyte being returned to the refining cell. The extracted Cu was transferred into an aqueous phase and cathodically separated. A large number of organic reagents have become known as selective extractants for Cu; this is how z. B. the known method described above according to the principle of solvent extraction with chelating agents such as phenonoximes or quinolines. The used electrolyte is extracted in mixer / settler systems and the Cu is stripped from the organic extract phase with an acidic stripping solution. The loaded stripping solution is electrolyzed, with Cu and optionally also Ni being able to be deposited cathodically. Depending on the chelating agent and the working 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 circulated.

As a result of the simultaneous dissolution of Cu and Ag on the anode, more Ag is deposited on the cathode than is dissolved on the anode. The electrolyte is therefore depleted of silver and it is necessary to add pure silver nitrate (as a solution) to the regenerated electrolyte. This places a burden on the process, since pure silver nitrate is an expensive product.

The object was therefore to create a process of the type mentioned at the outset in which there is a continuous complete regeneration of the electrolyte, in which the silver deficiency is also compensated for without the addition of silver nitrate solution.

This object is achieved, in particular, in that the electrolyte used is anodically enriched with silver in a common electrolysis process and accompanying metals are cathodically deposited from the aqueous phase.

The system according to the invention for carrying out this method accordingly has, in addition to at least one refining cell, at least one electrolysis cell for cathodic deposition of the accompanying metals and at least one extraction device for extracting the accompanying metals from the used electrolyte and for transferring the extracted accompanying metals into an aqueous phase, at least one common electrolysis cell for enrichment of the used electrolyte with silver and for the cathodic deposition of accompanying metals in the form of a diaphragm cell with a silver anode.

Ag is anodically dissolved in the common electrolysis cell, a crude silver anode advantageously being used, since the accompanying metal content does not have to be completely eliminated, but only has to be reduced. The current released during the cathodic deposition of the accompanying metals on the cathode supports the dissolution of the silver anodes.

It has been shown that the proposal according to CH-PS614238 to re-extract the accompanying metal-laden organic extract phase with a sulfuric acid stripping solution is technologically disadvantageous since the stripped organic solvent / extractant phase must be washed free of sulfate ions before being returned to the extraction stage in order to remove the refining electrolyte (nitric acid solution) not to contaminate.

According to the invention, it is advantageous to work with equal-ion, in particular nitric acid solutions, even when stripping, so that this problem does not occur, in particular the operation of the common electrolytic cell for cathodic accompanying metal deposition and for anionic silver enrichment is possible without problems.

The task of the anionic diaphragm is to prevent silver from getting into the cathode compartment where it would be excreted together with the accompanying metals, and to maintain the desired large concentration gradient of nitrate ions between the cathode and anode compartments.

By suitable selection of the accompanying metal extraction agents, from which the chelating agents have proven most selective, it is possible to suppress the co-extraction of silver to such an extent that the electrolytic reloading of the used refining electrolyte with silver can also be carried out before the accompanying metal extraction.

For the accompanying metal extraction, a solvent extraction is usually carried out in the process according to the invention, the refining electrolyte used first being treated with a solvent / extractant phase and then stripping with an ionic (N0 ₃ ) stripping solution.

However, it has been found that, with regard to the extraction of accompanying metals other than Cu, of which in particular Pb, platinum metals such as Pd or Pt, Ni, W, Zn and Cd are possible, advantages can be achieved if liquid membrane permeation is used as the extraction method , combinations of solvent extraction and liquid membrane permeation can also be used.

The solvent / extractant phase serves as a separating membrane between the used refining electrolyte and the acidic stripping solution, which is emulsified in the separating membrane. After the transition metal or metals from the refining electrolyte has passed into the organic emulsion phase, the emulsion phase is separated off and, in a known manner, integrated into an aqueous phase (loaded stripping solution) and an organic phase (solvent / extractant). The loaded stripping solution is regenerated by subsequent accompanying metal electrolysis or separation of valuable materials without electrolysis and emulsified again in the organic phase. The organic phase only serves as a selective separation medium between the aqueous phases (used refining electrolyte and stripping solution), so that it does not have to absorb the accompanying metal as in liquid / liquid extraction. This results in a significantly reduced solvent expenditure as well as high enrichment rates, e.g. B. in the range of 1: 100.

The method according to the invention is explained in more detail below with the aid of examples with reference to the drawing, in which FIG. 1 shows a flow diagram of a system according to the invention for its implementation; Fig. 2 is a schematic elevation of a diaphragm cell in section and Fig. 3 is a corresponding plan.

The used electrolyte E 1 flows from the refining cell ₁ into the anode space 2 of a diaphragm electrolysis cell 2, 8, where it is enriched with silver by anodic dissolution of a silver anode. The silver-enriched electrolyte E2 then arrives in the collecting tank 3, where it is adjusted to a pH value suitable for the extraction by passing it over Cu / Ag scrap, by adding lye or the like. The pH-adjusted electrolyte E3. then reaches a two-stage extraction system 4.5, z. B. a mixer / setter, where it is extracted in countercurrent with a regenerated solvent / extractant phase (short solvent phase) S. The pH-adjusted electrolyte E3 gets into the mixer of the first extraction stage 4, is extracted there with the solvent phase S ₂ already loaded in the second extraction stage 5 and arrives from the settler of the first extraction stage 4 as pre-extracted electrolyte E ₄ into the mixer of the second Extraction level 5, where accompanying metal is extracted with the unloaded, regenerated solvent phase S.

The now regenerated electrolyte E _{5 is} returned from the settler of the second extraction stage ₅ to the refining cell.

The solvent phase S ₃ , which is loaded twice with accompanying metal, passes from the settler of the second extraction stage into the mixer of a washing system 6, where the silver content is reduced by washing with a washing solution W containing copper. The silver is replaced by copper in the solvent phase S ₃ and the silver-laden washing solution W ₂ is returned to the collecting tank 3. The arrangement of a washing system 6 is not absolutely necessary.

From the settler of the washing plant 6, the now low-silver solvent phase S ₄ enters the mixer of a stripping plant 7 and is freed of accompanying metal with the regenerated acid stripping solution A, by re-extraction, or the accompanying metal content is reduced to the desired extent. The now regenerated solvent phase S is returned from the settler of the stripping system 7 to the mixer of the second extraction stage 5.

The stripping solution A ₂ loaded with accompanying metal passes from the settler of the stripping system into the cathode chamber 8 of the diaphragm electrolysis cell ₂ , 8, where accompanying metal is deposited cathodically. The stripping solution A ₁ thus regenerated is returned to the mixer of the stripping system 7.

It can be seen that in the process according to the invention an electrolytic enrichment of the used electrolyte with silver and a cathodic deposition of the extracted accompanying metal (in particular copper) is carried out simultaneously in a common electrolysis cell.

A closed circuit for electrolyte, solvent phase and stripping solution is also possible.

The overall arrangement of the process stages can also be such that the used electrolyte is enriched with silver only after the accompanying metal extraction by anodic dissolution of a silver electrode. The used electrolyte E is first fed into the extraction stages 4, 5 and then reaches the anode space 2 of the diaphragm electrolysis cell, where it is enriched with silver and then goes back to the electrolysis cell 1. The cycle of the solvent phase and the stripping phase remains unchanged. This way of driving has the advantage that not so much Ag in the Extract S _{3 is carried} along.

If Cu and z. B. W are to be separated, for example, before being fed into the anode space 2 of the diaphragm electrolysis cell 2.8, a partial stream E ₁ 'of the used refining electrolyte E _{1 is} branched off and fed to a permeator P, where the partial stream E ₁ ' by selective mass transfer with a Emulsion of a stripping solution in an organic solvent / complexing agent phase is freed of W or the W content is reduced accordingly. The low-heat electrolyte E ₁ "is fed back into the electrolyte stream E _1. Similar arrangements can be made with regard to the other accompanying metals. In principle, extraction or permeation stages for the selective separation of different accompanying metals can also be connected in series, with or without a partial current mode of operation. The preparation of the stripping solutions loaded with accompanying metal for the recovery of valuable materials can be carried out electrolytically, by other known processing methods or by a combination of the methods specified.

FIGS. 2 and 3 schematically show the diaphragm cell with an anionic membrane M, the Cu cathode CA and the crude silver anode AN, the electrolytes, the processes in the cell and with the guidance of electrolyte E and stripping solution: A.

In the following examples, reference is made to the procedure and plant described above:

example 1

From the anode compartment 2 of the diaphragm cell 2.8, an Ag-enriched used electrolyte E2 with 37.7 g / I Cu and 67 g / l Ag reaches the storage tank 3 and is drawn off from there at 1 I / h and in the two extraction stages 4.5 with 2 l / h of an extractant S ₁ , which consists of 20 parts by volume of Acorga PT 5050 (Acorga Ltd.) as complexing agent and 80 parts by volume of Shellsol AB (aromatic solvent from Shell Intern.) As a diluent . The complexing agent Acorga PT 5050 is a mixture of 2 parts of 2-hydroxy-5-nonylbenzaldoxime and 1 part of Tridecanoi. The organic extract S ₃ contains 10.4 g / I Cu and 10 ppm Ag and is regenerated either directly in the stripper 7 with an aqueous stripping solution A ₁ (30 g / l Cu, 2 mol / l HN0 ₃ , 1 I / h) or previously washed in scrubber 6 using an aqueous 1 mol / l Cu (N0 ₃ ) ₂ solution (1 l / h) Ag-free or Ag-poor. The loaded stripping solution A ₂ contains 59.5 g / l Cu, the regenerated extractant S ₁ 3.2 g / l Cu. 7.2 g / h of Cu are deposited cathodically in the diaphragm cell 2.8, the equivalent amount of silver is dissolved anodically. The used washing solution W ₂ was, as can be seen in FIG. 1, fed to the storage tank 3 and the low-copper electrolyte E _{5 to} the refining cell 1 (Möbius).

Example 2

The procedure was as in Example 1 with the modification that the extractant S ₁ from 20 parts by volume SME 529 (2-hydroxy-5-nonylaceo-tophenonoxime) as a complexing agent and 80 parts by volume MSB 529 (aromatic solvent of the shell Intern.) Existed as a diluent. The extract S ₃ contained 7.6 g / l Cu and 750 ppm Ag and was washed in scrubber 6 with an aqueous 1.0 m Cu (NO ₃ ) ₂ solution. The Ag-arm washed extract S ₄ (12 g / I Cu, 10 ppm Ag) was treated in the stripper 7 with an aqueous stripping solution A ₁ (30 g / l Cu, 2 mol / l HNO _3, 1 I / h). The regenerated extractant S ₁ contained 1.9 g / l Cu, the loaded stripping solution A ₂ entered the cathode compartment 8 of the diaphragm cell, where 20.2 g / l Cu was deposited and the equivalent amount of Ag was dissolved anodically.

Example 3

The procedure was as in Example 1 with the modification that the extractant S ₁ consisted of 20 parts by volume of HS-LIX 64 as a complexing agent and 80 parts by volume of MSB 529 (aromatic solvent from Shell Intern.) As a diluent. The complexing agent HS-LIX 64 contains 20 parts of 3-hydroxy-5-nonylbenzophenonoxime and 1 part of 5,8-diethyl-7-hydroxy-6-dodecano-noxime as active ingredients. The extract S ₃ contained 6.9 g / l Cu and 1.5 g / l Ag and was in the scrubber 6 with an aqueous 2 mol / l Cu (NO ₃ ) ₂ solution to a content of 10.5 g / l Cu and washed with 230 ppm Ag. The washed extract S _{4 was} then stripped in the stripper 7 in the phase ratio extract S ₄ : stripping solution A ₁ of 2: with an aqueous stripping solution which contained 30 g / l Cu and 2 mol / l HN0 ₃ . From the loaded stripping solution A ₂ 2.8 19.2 g / l Cu were deposited in the cathode compartment 8 of the diaphragm cell 2.8 and the equivalent amount of Ag was dissolved in the anode compartment.

Example 4

An electrolyte E2, 90 g / l Ag, 30 g / l Cu, 5 g / l Ni, 0.6 g / l Cd, 0.4 g / IW, 0 was withdrawn from the anode compartment 2 of the diaphragm cell 2.8 , 25 g / l Pd and 0.05 g / l Pt, and how. treated further in Example 1 with the modification that in the extractant S ₁ the complexing agent Acorga PT 5050 was not diluted with Shellsol AB but with Shell MSB 210 (predominantly aliphatic solvent from Shell Intern.). The extract S ₃ contained 11.0 g / I Cu, 2 ppm Ag and 300 ppm Pd and was stripped in the stripper 7 with an aqueous stripping solution A ₁ (35 g / l Cu, 2 mol / l HN0 ₃ ). The regenerated extractant S ₁ contained 3.2 g / l of Cu and 300 ppm Pd and was not entirely returned to extraction stage 5, but a partial stream was branched off in an amount of 1/10 of its total volume and in a separate extraction stage with the same volume 6 n HCl extracted and completely freed of Cu and Pd. After on closing washing with water to remove the chlorine ions, the regenerated partial stream was added to the extractant S ₁ again; the Pd was precipitated as Pd (NH ₃ ) ₂ Cl ₂ from the hydrochloric acid stripping solution.

Example 5

• a) Der Elektrolyt wurde durch übliche Metallsalzextraktion von Cu und durch Fällung (AgCI) von Ag befreit und enthielt anschließend 5 g/l Ni und 0,6 g/I Cd. Der Elektrolyt wurde anschließend mit dem gleichen Volumen eines Extraktionsmittels, bestehend aus 20 Vol.-Teilen Bis-2-äthylhexylthiophosphorsäure und 5 Vol.-Teilen Tributylphosphat als Komplexbildner und 75 Vol.-Teilen Shellsol T (aliphatisches Lösungsmittel der Shell Intern.) als Verdünnungsmittel, extrahiert, wobei Ni und Cd in den Extrakt gehen, und das wässerige Raffinat zur anodischen Ag-Anreicherung dem Hauptstrom des Elektrolyten wieder zugemischt.

A used Möbius cell electrolyte was decoupled in the system described at the beginning according to one of Examples 1 to 4, the procedure being such that the Ag loading in the anode space 2 of the diaphragm cell 2.8 is provided as the last regeneration step, and a partial stream before entering the anode space 2 deducted and processed as follows:

• a) The electrolyte was freed of Cu by conventional metal salt extraction and Ag (precipitation) and then contained 5 g / l Ni and 0.6 g / l Cd. The electrolyte was then treated with the same volume of an extractant consisting of 20 parts by volume of bis-2-ethylhexylthiophosphoric acid and 5 parts by volume of tributyl phosphate as a complexing agent and 75 parts by volume of Shellsol T (aliphatic solvent from Shell Intern.) As a diluent , extracted, with Ni and Cd going into the extract, and the aqueous raffinate for anodic Ag enrichment mixed into the main stream of the electrolyte again.

The extract was stripped first with m HN0 ₃ and then with 2.5 n HN0 ₃ ; the extractant then contained 30 ppm of Cd.

The first strip solution contained only Ni, the second strip solution Cd and Ni in the ratio Cd / Ni = 20/1. The strip solutions were worked up in the usual way.

b) The electrolyte was decoupled and desilvered as in Example 5a); Pd was removed as Pd (NH ₃ ) ₂ C1 ₂ . The electrolyte then contained 50 ppm Pt and was extracted at room temperature with the same volume of an extractant consisting of 30 parts by volume of trioctylphosphine oxide as a complexing agent in 70 parts by volume of Shellsol T (aliphatic solvent from Shell Intern.) As a diluent, a Extract with 45 ppm Pt was obtained. The aqueous raffinate with 4 ppm Pt was added to the main stream of the electrolyte for anodic Ag enrichment.

The extract was extracted in an extraction column with half the volume of water at 80 ° C., 3 ppm Pt remaining in the extract and the water phase being worked up in the usual manner with 91 ppm Pt as nitrate.

c) The electrolyte was freed of Cu, Ag and Pd as in Example 5b). The electrolyte then contained 0.5 g / IW and was subjected to liquid membrane permeation. To form the liquid membrane, a solvent / extractant phase containing an aliphatic solvent, 5% M tertiary amine, namely Alamin 336 or Hostarex 327, as a complexing agent and 3% M ECA 4360 (surfactant from Esso), in a volume ratio of 2: 1 with a aqueous stripping solution, consisting of saturated ammonium nitrate, emulsified. Alamin 336 (Henkel) is a mixture of tertiary n-alkylamines with C ₈ -C ₁₀ alkyl chains, where C8 chains predominate. Hostarex 327 (Hoechst) is a 1: 1 mixture of tri-n-octylamine and tri-n-decylamine.

With a permeator throughput of 100 l / h electrolyte and 3 l / h of the above emulsion, a W content of 49 g / l could be achieved in the aqueous stripping phase after emulsion splitting. The electrolyte, which had a residual W content of 10 ppm, was recombined with the main stream of the electrolyte for the purpose of Ag enrichment. The W-enriched solution is worked up by customary methods. Here, for. B. crystallized ammonium tungstate, thermally decomposed and the resulting W-oxide reduced to metallic W. Ammonium tungstate as such can also be obtained.

Example 6

A used Möbius electrolyte is regenerated in a system as described above, in which the extraction stage 4, 5 is designed as a conventional liquid membrane permeation unit: In order to form a liquid membrane, an extractant phase consisting of kerosene as solvent, 3 vol.% Acorga P 5100 (1 Part 2-hydroxy-5-nonylbenzaldoxim + 1 part nonylphenol) as complexing agent and 3% ECA 4360 (surfactant based on polyamine), in a volume ratio of 2: 1 emulsified with an aqueous stripping solution containing 30 g / I Cu and 2 mol / 1 HN0 ₃ contained.

The permeator was operated with a throughput of 10 1 / h Ag-enriched used electrolyte E2 or used electrolyte E ₁ and 3 I / h liquid membrane. In both cases, a Cu content of about 60 g / l was achieved in the aqueous stripping solution obtained after the emulsion had been cleaved. Further processing can be carried out analogously to Examples 1-5.

In Examples 1 to 6, a diaphragm cell was used with a plastic-based anion exchange membrane with the trade name MA3148 from IONAC Chemical Co., New Jersey.

• 1. Die Kosten der Herstellung großer Mengen an Silbernitratlösung aus gereinigtem Wasser und Silbernitrat entfallen.
• 2. Die Kosten der Beseitigung des verschmutzten Elektrolyten entfallen.
• 3. Kupfer wird in einer Form gewonnen, die für die Weiterverarbeitung ausgesprochen günstig ist.
• 4. Die Aufarbeitung von Silberniederschlägen oder zementiertem Silber wird vermieden.
• 5. Die Raffinationszelle kann bei einem optimalen Kupfergehalt betrieben werden, so daß Silber größter Feinheit gewonnen werden kann.
• 6. Begleitmetalle, insbesondere Kupfer, können aus dem Silberelektrolyten nach Bedarf in größeren Mengen und Geschwindigkeiten abgeführt werden, so daß sehr unreines Ausgangsmaterial ohne vorhergeschaltete Reinigungsschritte eingesetzt werden kann. Anoden mit Silbergehalten unter 95 % können in der Diaphragmazelle aufgelöst werden, wobei gleichzeitig Kupfer kathodisch gewonnen wird.
• 7. Der wässerige Elektrolyt ist eine gleichionige, vorzugsweise salpetersaure Lösung. Es kommt zu keinen Verschmutzungen durch Fremdionen wie z. B. Sulfat oder Chlorid.

The method according to the invention offers the following advantages:

• 1. The cost of producing large amounts of silver nitrate solution from purified water and silver nitrate is eliminated.
• 2. The costs of removing the contaminated electrolyte are eliminated.
• 3. Copper is extracted in a form that is extremely cheap for further processing.
• 4. The processing of silver deposits or cemented silver is avoided.
• 5. The refining cell can be operated at an optimal copper content, so that the finest silver can be obtained.
• 6. accompanying metals, in particular copper, can be removed from the silver electrolyte in larger quantities and at speeds as required, so that very impure starting material can be used without prior cleaning steps. Anodes with silver contents below 95% can be dissolved in the diaphragm cell, copper being obtained cathodically at the same time.
• 7. The aqueous electrolyte is an ionic, preferably nitric acid solution. There is no contamination by foreign ions such as e.g. B. sulfate or chloride.

Claims (11)

1. An electrolytic silver refining process in which crude silver is anodically dissolved and refined silver is cathodically deposited and at the same time contaminating metals are selectively extracted from the used (spent) electrolyte and separately cathodically deposited after transferring to an aqueous phase and the regenerated electrolyte stripped of contaminating metals is recycled to the refining process, characterized in that in a common electrolysis operation, the spent electrolyte is anodically enriched in silver and contaminating metals are cathodically deposited from the aqueous phase.

2. The process according to claim 1, wherein the electrolysis operations are carried out by means of solutions with ion parity, in particular nitric acid solutions.

3. The process according to claim 1 or 2, wherein Cu is extracted from the spent electrolyte and cathodically deposited simultaneously with the anodic enrichment in silver.

4. The process according to claim 3, wherein Cu is first extracted from the electrolyte and the enrichment in silver is deferred until prior to recycling into the refining process.

5. The process according to claim 4, wherein further contaminating metals, in particular Cd, Ni, Pd, Pt and/or W are extracted from the spent electrolyte after Cu depositing, but prior to enrichment in silver.

6. The process according to any one of the claims 1 to 5, wherein the extraction of the contaminating metals is carried out as a liquid membrane permeation, preferably in combination with a solvent extraction.

7. The process according to claim 5, wherein the further contaminating metals are processed non-electrolytically after their selective extraction.

8. An apparatus for carrying out the process according to any one of the claims 1 to 6, having at least one refining cell, at least one extraction means for extracting contaminating metals from the spent electrolyte and for transferring the extracted contaminating metals into an aqueous phase, and at least one electrolysis cell for the cathodic deposition of contaminating metals from the aqueous phase, wherein at least one common electrolysis cell for the enrichment of the spent electrolyte in silver and the cathodic deposition of contaminating metals is provided in the form of a diaphragm cell with silver anode.

9. The apparatus according to claim 8, wherein an anionic diaphragm, in particular in the form of an anion exchange membrane on plastics material basis, is provided.

10. The apparatus according to claim 8 or 9, wherein the diaphragm cell is provided with a crude silver anode.

11. The apparatus according to any one of the claims 8 to 10, wherein the extraction means is a means for liquid membrane permeation.

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