Challenges in the Electrolytic Refining of Silver Influencing


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Challenges in the Electrolytic Refining of Silver –
Influencing the Co-Deposition through Parameter Control
Ann-Kathrin Pophanken1, Bernd Friedrich1, Wolfgang Koch2
1IME Process Metallurgy and Metal Recycling, RWTH Aachen University, D-52056 Aachen, Germany Corresponding email address: [email protected]
2Agosi Allgemeine Gold- und Silberscheideanstalt AG, Kanzlerstr. 17, D-75175 Pforzheim, Germany
Keywords: silver electrorefining, palladium and copper deposition, parameter influence
1 Abstract
Due to the high standards for silver in electronic uses, it is essential to control the behavior of the main impurities such as copper or palladium during the electrolytic refining. In regard to palladium, it is indispensable to control its complete transition into the anode slime. Established silver refining processes are currently operated with parameters that favor the silver cathodic deposition. Usually this requires a high precious metal stock and leads to low space-time yields. Parameter research studies show the possibility of changing these parameters to achieve a more efficient process without decreasing the silver grades. This research leads to the use of less pure electrolyte systems and more contaminated anodes. This also enables the use of increased current densities and accordingly increased production capacities. The research depicts the influence of the anode alloy, electrolyte composition, as well process parameters like current density and pH on the electrorefining. The applied electrode potential and therefore the applied current density shows great influence on the dissolution behavior of more noble metals than Ag at the anode. At the cathode, the applied current density influences the co-deposition of less noble metals, such as Cu. This phenomenon is based on the Nernst equation. The pH of the electrolyte system and thus the acid concentration influences the solubility behavior of the impurities. This leads to an influence on their concentration in the electrolyte and their co-deposition on the cathode.
2 Introduction
From an economic point of view, it becomes more and more relevant to use less pure electrolyte systems and insert more contaminated anodes into the refining process. This is based on the otherwise high necessary silver stock. Additionally, economic reasons cause an increase in current densities and accordingly an increase in production capacities and a decrease in refining duration. Therefore, the impurity behavior needs to be controlled and influenced during the electrolytic refining process.
Basic electrolysis parameter through which the process can be influenced and controlled are the current density, hence the applied potential on the electrode, the acid concentration in the electrolyte, as well as the anode and electrolyte composition.
The paragenisis of silver in copper ores as well as the use of sterling silver as main jewelry alloy leads to high Cu contents in the crude silver anodes. Moreover, the use of silver palladium alloys in electrics and electronics leads to increasing Pd contents in the crude anodes. This explains the main impact these two elements have on the electrorefining and specifies the vital importance of considering their behavior and impact in the electrorefining process. Furthermore, the electrochemical similarity of Ag and Pd can lead to further economical disadvantages. The Pd content in the electrolyte needs to be monitored precisely in order to not be deposited on the cathode and therefore be handled as a loss. This research therefore identifies possible parameter ranges which allow a high grade silver production with less economic impacts.
3 Background
Electrolysis is a process which can be handled on different complexity levels. There are many parameters that show a great influence on the product. Furthermore, the parameters even have different and/or increased impact due to interdependencies on each other. [1]
Independent of the silver origin, it is indispensable to refine silver electrochemically. This is necessary to produce high grade silver with an impurity content less than 0.01 wt.-%. These grades are needed especially for industrial use purposes. Furthermore, the silver electrolysis enables to separate the contained precious metals such as Au or PGM’s. In the electrorefining, the crude silver anodes are dissolved in a nitric acid, silver nitrate solution and the
1

Silver anode

fine silver is deposited on stainless steel cathodes. A typical cell arrangement, as

Mechanical -
scraper

+

-

Cathode sheet

shown in figure 1, is the Möbius cell1. Operating parameter such as AgNO3 concentration, amperage or electrolyte

temperature are set in a way, that ideally the

less noble metals (e.g. Cu, Pb) will stay

dissolved in the electrolyte. The more noble

metals (e.g. Au, Pd, Pt) will not dissolve and

Electrolyte

pass into the anode slime. The anode slime is collected in anode bags around the dissolving

anode. Under optimal conditions, only fine

silver (purity > 99.9 %) will deposit on the

Silver crystals

cathode. An advantageous condition for the

electrolytic deposition of Ag is the large

Anode bag Anode sludge

potential difference between Ag and Cu (0.5

V). Thus, Cu should theoretically not co-

Figure 1 Möbius Silver Electrolysis Cell [8]

deposit on the cathode. The containing Au

and PGM’s, however, are either insoluble in

the electrolyte or have a too large standard potential and hence remain in the anode slime. Pd though behaves in a

slightly different way. The higher the content of Ag in the electrolyte and the lower the current density, the purer

is the deposited fine silver. [3, 4, 5, 7, 8, 10, 11, 12]

3.1 Copper Co-Deposition

Looking at the standard

potential

of

Cu

100

Copper content in the silver cathode [ppm]

(Cu/Cu2+ = 0.345 V

and

90

(6)

Cu/Cu+ = 0.522 V), it is

apparent

that

during

80

electrorefining of Ag, Cu will

70

mostly dissolve into and enrich

in the electrolyte. Partial

60

occurrence of Cu in the anode

50

slime can be detected. This is

more or less due to oxidic Cu

40

present in the crude silver

30

anodes. Theoretically, Cu

should not be co-deposited on

20

(14) (2)

GDleiircehcsttrcoumrrent

the Ag cathode, even when applying high current densities. Nevertheless, during operation, cathodic Cu co-deposition is always observed. This leads to

(2)
10 (2) (7)

0 (4)

0

20

40

60

80

100

120

140

Copper concentration in the electrolyte [g/l]

the necessity of monitoring and controlling the Cu content in the electrolyte. Studies clearly depict a direct correlation of the Cu content in the electrolyte to

Figure 2 linear correlation of cathodic Cu co-deposition on the Cu concentration in the electrolyte (basic parameter: cAg = 100 g/l, cHNO3 = 1 g/l, icathode = 550 – 600 A/m2, TElectrolyte = 25±1 °C; set in brackets: quantity of trials to calculate mean value) [17]

the Cu co-deposition as shown

in figure 2. However, it is also apparent that the electrolytic Cu concentration can rise up to 50 – 100 g/l until it

results in a significant Cu deposition (> 50 ppm) on the cathode. This co-deposition on the cathode can either be

electrochemically or due to an inclusion of Cu salt in the cathode silver. [2, 3, 13, 15–18, 21]

1 The dominant cell type in Europe is the Möbius cell, whereas in northern America and for anodes with high Au contents, the Balbach-Thum cells are preferred. Due to the cell construction, Möbius electrolysis only can handle up to 6-wt. % Au, while anodes containing 20-wt.% Au can be inserted in Balbach-Thum cells. [3, 9]
2

This shows the need to determine critical limits for Cu in the electrolyte as well as in the anodes due to anodic Cu dissolution. Ag anodes with a high Cu content also lead to high Cu impurity in the cathodes through the enrichment of Cu in the electrolyte (as discussed above). Moreover, in high Cu containing anodes (Cu > 30%, cf. [22]) cementation of Ag from the electrolyte can occur. This cemented Cu collects in the anode slime and therefore leads to Ag losses. Nevertheless, the critical Cu limits basically depend on the required purity of the cathodes. Three basic mechanism can lead to Cu impurity in the cathode based on high Cu contents in the electrolyte:
- electrochemical co-deposition, - inclusion of Cu containing electrolyte in the cathode crystals, - hydrolysis reaction2 of Cu salts and their subsequent precipitation as hydroxide during washing process
of the fine silver crystals.
These mechanisms can be applied to all contained less noble elements in the electrolyte. Further studies depict a negligible influence of other electrolyte parameter such as cathodic current density variation (250 – 3000 A/m2) or the electrolyte temperature on the Cu content in the cathode. [2, 8, 10, 13, 17, 18, 22]
3.2 Palladium Co-Deposition
Pd, however, shows a certain electrochemical similarity to Ag (standard potential 0.810 – 0.830 V). This leads to a definite Pd dissolution into the electrolyte during the electrorefining. Due to the small difference in the standard potential to Ag, small Pd concentrations lead to Pd co-deposition on the cathode. Ag rich electrolytes (min. 60 g/l Ag) as well as low acid concentrations can counteracted to this effect, as Pd (as well as Pt) preferably dissolves in electrolytes with low pH values. When looking at the dependency of the Pd concentration in the electrolyte on the content in the crude anode silver, a direct relation is visible. Plus, the Pd content in the cathode, directly connects to the Pd in the electrolyte. Both phenomena are displayed in figure 3. Research illustrates that even high Pd contents in the anodes (0.05 – 0.27%) do not necessarily lead to high Pd co-deposition. Even with this high Pd concentration, fine silver with < 3 ppm Pd and < 5 ppm Cu can be achieved. This is based on the use of slightly contaminated electrolytes (< 20 mg/l Pd) and high pH values. Studies of Cu-Ni-PGM alloys show, that in these alloys the PGMs occupy atomic places. Therefore, parts of the alloy are more noble than others. This leads to a higher dissolution potential of these parts, than the rest of the crystal. When polarizing anodically, this part accordingly is not dissolved and remains as amorphous and very fine component in the anode slimes (very fine atomic agglomerations without crystalline structure). [2, 13, 17, 24, 25, 20, 23]

Distribution of Pd in eletrolyte [%]
Pd in fine silver [ppm]

60 experimental

50

operational

point 40

30

20

10

0 0.0

0.5 1.0 1.5 2.0 Pd in anodes [%]

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0

10

20

30

Pd in the electrolyte [mg/l]

Figure 3 Dependence of the Pd transition into the electrolyte on the Pd content in the anodes (left) and Ag quality depending on the Pd concentration in the electrolyte (right); both [18]
3.3 Process Consideration and Research Strategy
The ultimate goal of the process improvement is to reduce the necessary metal stock while ensuring the required product purity. In silver refining electrolysis, Pd is a challenge which shall not be underestimated as the standard

2 Hydrolysis can occur during the washing process of fine silver, and therefore be accordingly eliminated by washing with acidified water followed by washing with a neutral hot water. [17]
3

potential of Pd (E0 = 0.987 V) is very close to Ag (E0 = 0.799 V). Cu accumulates in the electrolyte and is also a challenge due to a co-deposition at the cathode when exceeding certain values that are too high in the electrolyte. Consequently, the electrolyte is continuously checked on the Cu content. When reaching certain limits, the electrolyte is exchanged. During the electrolysis process, the Ag concentration decreases while Cu increases. This offers two alternatives for handling the electrolyte; an entire removal of the electrolyte as soon as the Cu content in the cathode exceeds the specifications or a continuous exchange of part of the electrolyte. The latter results in a more uniform operational management and is therefore the preferable process handling. Cu in the electrolyte has indeed disadvantageous effects on the cathode product purity, but the presence of Cu ions increases the conductivity of the electrolyte. Hence, a certain presence of Cu ions during the process is desirable. This compensates for the loss of conductivity due to the decrease of the silver content (based on the created misbalance when anodically dissolving Cu2+-ions while depositing Ag+-ions on the cathode). The nitric acid content in the electrolyte should remain constant and the HNO3 content of the electrolyte should not be too high (approximate value pH of 1.5 to 2.5). Otherwise this can lead to increased dissolution of Pd. [2, 3, 9, 13]
The following table 1 displays an overview on published processing data of industrial silver electrolysis. Based on this table 1, the investigated parameter in this research were defined. The main focus lay on investigating high Cu (up to 100 g/l) and Pd (up to 1.4 g/l) contents in the electrolyte (and for Pd also in the anode) in order to still ensure a high grade silver production with moderate current densities and pH values whilst decreasing the Ag concentration in the electrolyte.
Table 1 Overview on published processing data for industrial silver electrolysis

Reference [8] [13] [6] [12] [14] [15] [16] [9] [3] [17] [18]

[19]

anod

400-

<

i

e

400 500 1000 1000

[A/m2] catho

460-

300

de

500

400 – 500

360 500 430 335

Ag concentration 50 >60 electrolyte [g/l]

100 – 150 60 –

150

160

80 – 50 100

100 60

40 130

Cu

max.

concentration

50 –

electrolyte [g/l]

80

max. 60 max 5 – 1

45

60 0

15 – 3 - 5 35 10 50

Pd electrolyte

20

mg/l

Ag:

Ag:

final

50

30 –

electrolyte

Cu:

40

[g/l]

100

Cu:

70

anode mass [kg]

15 3.9 – 45 4.2

21.77 (48 lbs)

Ag 98

95 - 9 86 – 92

>99

[%]

9

%

Cu 1 %

0,5 – 1

anodes Pd

0.16 – 0.18%

Au 0.50 [%]

8 – 9

Temperature 45

<55 32

45 –

electrolyte [°C]

55

pH/ cHNO3

1 – 1.5 1.5 – 2.5

1 – 2 10 g/l 1 g/l 5 g/l 3.8 2

0 g/l

g/l

Anode rest

5 –

[%]

10

15

25

Current

95

98 –

efficiency [%]

99

* calculated value from amperage and number anodes under the assumption that 60% of the surface are active

[20] 1435 * 645*
150 – 200
12 99.30 0.4 – 0.6 68± 19 ppm 0.04 – 0.07 35 – 50 2.5 g/l

4

All experiments were conducted in an electrolytic refining laboratory cell, which is displayed in figure 4. The basic parameters are presented in table 2. During the experiments, the acidic concentration, the anodic current density and the Ag-, Cu- and Pdconcentration in the electrolyte as well as the Pd- concentration in the anode was varied. The main idea was, to increase the maximum Cu and Pd content in the electrolyte, as well as the maximum Pd content in the crude silver anodes. The main goal was to still produce high grade silver (> 99.9%) even with the stated unfavorable conditions.
Table 2 Basic parameter

4 Experimental Validation
Figure 4 Laboratory Ag electrorefining cells (3 cells in total) at IME Aachen

Parameter

Value

Electrolyte composition (varied values)

Anodic composition

Anodes/cathodes per cell Velectrolyte per cell electric charge per cell & trial Cathodic current density Other Parameter

1 / 2 ~ 27 l ~ 580 Ah 335 – 535 A/m2

Ag Cu Pd HNO3

65 – 120 g/l 45 – 98,6 g/l 0 – 1 g/l 0.6 – 10 g/l

Ag 96.5%

Cu 3%

Pd

0.05% (varied 0.01-0.1%)

Au 0.01%

Anodic current density
End of trials approx. iA Cathode active surface Cathode material

500 – 800 A/m2
660 – 1060 A/m2 15.6 cm x 30.5 cm Stainless steel

electrode distance anodecathode
Active anode surface*
Electrolyte temp.

70 mm
25.7x 10 x 2 cm 35 °C

5 Results and Discussion

Taking into consideration the studied impurities Cu and Pd in Ag anodes, ideally Cu should dissolve and enrich in the electrolyte whereas Pd should be insoluble and pass completely into the anode slime.

Figure 5 presents the Pd concentration in the anode slime depending on different electrolyte compositions (with unvaried Pd impurity in the used anodes). The Pd content in the anode slime shows a significant dependence on its concentration in the electrolyte. Whereas the Cu content in the anode slime also shows a dependency on the Pd concentration in the electrolyte. With increasing Pd concentration in the electrolyte, the Cu content in the anode sludge decreases. Consequently, one can assume that palladium is anodically dissolved parallel to the Ag during electrolysis. Subsequently it reacts through cementation with the metallic Cu (or even Ag) present in the anode sludge according to the equation below. The structure of the anode sludge supports this assumption, since Cu only appeared as metal in the anode sludge.

Pde2l+ectrolyte+Cua0node sludge→Pda0node sludge+Cue2l+ectrolyte

5

Pd in anode sludge [%]

Cu in anode sludge [%]

15

8

10

6

5

100 4

0 80 2 0.81.2

0

60

0.4 0.8 1.2 40

40

60

80

0.4 0

100

Figure 5 Illustration of Cu (left) and Pd (right) content in the anode slime as a function of Cu and Pd electrolyte concentration (Ag influence on Cu and Pd anode sludge concertation was negligible at this point)

The subsequent figure 7 shows this dependence of the Pd and Cu content in the anode sludge on the Pd concentration in the electrolyte, without taking into account the concentrations of Cu and Ag in the electrolyte. In this diagram, the discussed influence becomes even more apparent.

Pd in anode sludge [%]

8

18

Cu in anode sludge [%]

7

16

6

14

12 5
10 4
8
3 6

2

4

1

2

0

0

0.00

0.50

1.00

1.50

0.00

0.50

1.00

1.50

Pd concentration in the electrolyte [g/l]

Pd concentration in the electrolyte [g/l]

Figure 7 Influence of Pd concentration in the electrolyte on the Pd (left) and Cu (right) content in the anode slime

With an increasing Ag concentration in the electrolyte, it basically can be observed, that the Cu co-deposition slightly decreases. An increasing Cu concentration in the electrolyte on the other hand leads to a slight increase in Cu co-deposition. If only the dependence of the Cu concentration in the produced fine silver on the Cu content in the electrolyte is applied, the figure 6 can be generated. With increasing Cu concentration in the electrolyte, a tendency of increasing Cu concentration in the cathode can be seen, regardless of the Ag electrolyte concentration. It is also apparent, that this is not a distinct linear dependency, rather a co-influence with the Ag electrolyte concentration. Whether the Cu concentration in the cathode is based on electrochemical deposition or rather based on electrolyte inclusions, cannot be clarified. Investigations of fine silver crystals via microprobe and SEM analysis were not able to identify whether Cu electrochemically alloyed in the silver matrix or was present as electrolyte inclusion. This is due to the detection limits of the used analysis and the low Cu contents reached in the fine silver (< 100 ppm).

Cu in fine silver [ppm]

160 cAg < 70 g/l 140 70 g/l < cAg < 100 g/l
cAg > 100 g/l
120

100

80

60

40

20 40

50

60

70

80

Cu in electrolyte [g/l]

90

100

Figure 6 Influence of the Cu concentration in the electrolyte on the Cu content in the fine Ag in electrolytes containing 60 – 100 g/l of Ag (parameter: cHNO3 = ~ 3 g/l, ia = 700 A/m2)

6

The effect of the Pd content in the anode on the Pd and Cu concentration in the produced fine Ag as well as the

electrolyte concentration is presented in figure 8. It is noticeable that elevated Pd contents in the anode lead to both

Pd and Cu in fine silver [ppm]

Pd in electrolyte [g/l]

increased Pd and Cu concentrations in the

produced fine silver. This is based on the

0.6

300

effect that an elevated palladium content

250

in the anode should increase the anode

0.5

potential. This causes a higher cathode

200

potential and thus an increased deposition

0.4

150

of Cu and Pd. Moreover, Pd contents

> 1 g/kg in the anode (green in figure 8) 0.3 100

lead to higher Pd contents in the

50

electrolyte (>0.5 g/l) and therefore also in the fine silver due to the dependency of the activity on the concentration of Pd in the electrolyte.

0.2 Pd in anode [g/kg] 0

0

2

4

6

8

10

Pd in electrolyte [g/l] varying the Pd content in the anode

Pd in fine Ag [ppm]

Figure 9 shows the generated results of

Cu in fine Ag [ppm]

the experiments dealing with the Figure 8 Fine silver composition as well as palladium

influence of the current density and the concentration in electrolyte depending on the Pd content in the

acid concentration on the Pd co- anode

Pd in fine silver [ppm]

60

40

20

800

0

700

0

600

5 10 500

Figure 9 Pd co-deposition dependent on the anodic current density and acid concentration

deposition. It can be seen that the acid concentration in the electrolyte is the main influence factor. Consequently, a higher acidity results in an increased Pd codeposition on the cathode. This is based on the strong dependence of the Pd solubility in the electrolyte on the p. The higher the acid content is, the greater concentrations of palladium in the electrolyte are reached. It can be concluded accordingly that the acidity is more or less an indirectly relevant parameter, whereas the palladium concentration in the electrolyte is crucial for Pd co-deposition. The anodic current density on the other hand tends to show a minor influence. Based on these results in order to not exceed critical Pd contents in the fine silver, the acid concentration generally should not extent 7 g/l.

6 Conclusion

Concluding all test parameters, with respect to their influence on the copper content in produced the fine silver, the following figure 10 can be obtained. It is distinct that, although the copper concentration in the electrolyte is a non-negligible factor, an elevated palladium content in the anodes and a high acidity as well as high current densities also adversely affect the copper co-deposition. At least when the copper and silver concentrations in the electrolyte are almost at an equal level. This leads to the general conclusion, that in order to ensure low copper levels in the produced fine silver, the palladium content in the crude anodes should not be too high. Moderate current densities (ia up to 600 A/m2) and acid concentrations enable to operate electrolytic refining of silver with copper contaminated electrolyte without jeopardizing the silver quality.

Overall, the experimental results demonstrate, that the pH of the electrolyte system shows a great influence on the behavior of Pd. At least in a range of anodic current densities from 500 – 800 A/m2 and a nitric acid concentration from 1 – 10 g/l. Consequently, it is indispensable to monitor the pH for an electrolyte refinery if the production rate and thus the current density shall be increased. However, monitoring and controlling the pH is a complex topic especially on a production site. One possibility is to add a manual acidic concentration titration in the electrolyte analysis. In order to continuously monitor the pH, an electrolyte resistant pH electrode can be included into the electrolyte flow streams.
7

Cu in fine silver [ppm]

Cu in fine silver [ppm]

400 400

350 300 250

increased Pd content in anodes
350deoxidized anodes
varied iA and HNO3
300varied Ag and Cu in the electrolyte
varied Ag, Cu and Pd in the electrolyte
250

increased Pd content in an deoxidized anodes varied iA and HNO3 varied Ag and Cu in the el varied Ag, Cu and Pd in th

200

200

150

150

100

100

Pd in fine silver [ppm]
Cu in fine silver [ppm]

50

50

400

0

0

40

50

60

70

80

90

100

0.4

0.6

0.8

1.0

1.2

1.4

1.6

350 Cu concentration in the elecitnrcorelyatseed[gP/dl]content in anodes

ratio Cu:Ag in the electrolyte

deoxidized anodes

3in0c0reased Pd content in anodes
deoxidized anodes

varied i and HNO

250 A

3

varied Ag and Cu in the electrolyte

varied iA and HNO3 varied Ag and Cu in the electrolyte varied Ag, Cu and Pd in the electrolyte

Figureva1r0iedInAfgl,uCeunacnedoPfd tinhethceoeplepcterrolyctoencentration in the electrolyte (left) and of the copper to silver ratio in the

electro2ly0t0e (right) on the copper content in the fine silver as a function of the varied experimental parameters

80 150

As exemplified above, one of the main influence parameter on a palladium co-deposition is the

70
100

palladium content in the crude anode. Concluding

60

destohxiedievrtaerAiendodepnarameter, figure 11 is prepared. This

iA unfidgHuNrOe3 vdaorieiesrt not consider an elevated palladium

50 50

Ag,aCnuoudnde PdcoimnEcleeknttroralyttieonnvariiinert order to identify other

Pd FeinAg Durschnitt letzte Werte

40

Pd FineifnlAugeDnucrisnchgnitt lpetazrteamWeertteer. Therefore, figure 11

0

prevails dependencies of the palladium content in

30

40

50

60

70

80

9th0e produ1c0e0d fine silver as a function of the

Cu concentration in the electrolyte [pga/lll]adium concentration in the electrolyte and

20

80

50

60

70

80

90

100

therefore also the pH.

Pd in fine silver [ppm]

Cu concen80tra1t0ion in the e70lectrolyte [g/l]

Pd in fine silver [ppm]

60 Pd concentration in the electrolyte [g/l] These results furthermore indicate that monitoring

70 0

deoxidized anodes

the palladium content in the anode and the

60

0.0

0.2

500.4

0.6 varie0d.8i and1.H0 NO 1.2

1.4

A

3

electrolyte, as well as the acidity are essential for

deoxidized anodes

50

40
varied i and HNO

varied Ag, Cu and Pd in the electrolyte

controlling the behavior of palladium. Ensuring

A

3 Pd FeinAg Durschnitt letzte Werte

40

varied Ag, C3u0 and Pd in the electrolyte

low acid concentrations and thus higher pH during

Figure 1Pd1FIeninfAlgueDnucrsechonintt ltehtzPetdepFWaeeilnlrAtaedgiDuumrscchonintttelentztteinWethrtee fine the refining, process lead to low palladium co-

30

20

silver dePpdeFnediniAnggDounrscthhneitpt laeltlzatedWiuemrtecontent in the electrolyte deposition. If, however, heavily palladium

20as well as the aci1d0 concentration

contaminated anode shall be inserted in the

Pd concentration in the electrolyte [g/l]

10

0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

process, the use of separate anode and cathode

c0hambers as aPdscuogngceenstrtaitoionncinatnhebeelerctercoloytme [mg/el]nded. To avoid the contact of palladium-rich electrolyte and the cathode

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

altogether, an intermediate palladium precipitation from the anolyte and a fine filtration is suggested. Afterwards

this purified electrolyte can be supplied as a catholyte in the cathode chamber.

This research study is part of the authors dissertation thesis “Behavior of Copper and Palladium in the Electrolytic Refining of Recycling Silver”, to reach the academic degree Dr.-Ing.

7 Acknowledgments
The author would like to thank the Agosi, Allgemeine Gold- und Silberscheideanstalt, for enabling and supporting the entire collaborative research project.

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8

[2] Tafel, V; Lehrbuch der Metallhüttenkunde Band I, 2nd ed., S. Hirzel Verlagsbuchhandlung, Leipzig. [3] Renner, H. et al; Silver, Silver Compounds, and Silver Alloys, in Ullmann's Encyclopedia of Industrial
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Challenges in the Electrolytic Refining of Silver Influencing