# Electrowinning cell problem



## vjodhani (Dec 25, 2018)

We have recently commissioned an EMEW cell for extracting Ag from silver nitrate electrolyte.It is cylindrical cell operated at 5.6V(450 A- 550 A) with cathode as stainless steel and anode as Titanium.The problem is there is a lot of impurities deposition occurring,mainly lead an copper. Electrowinning cells powder deposition is not affected by the high amount of impurities present in it, because these cells are designed to extract silver and gold of 99.99% purity from it. I have attached a data about electrolyte composition and the powder obtained from it after electrowinning.
It would be of great help if somebody knows the science behind it and how to eliminate it.


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## ION 47 (Dec 25, 2018)

vjodhani said:


> We have recently commissioned an EMEW cell for extracting Ag from silver nitrate electrolyte.It is cylindrical cell operated at 5.6V(450 A- 550 A) with cathode as stainless steel and anode as Titanium.The problem is there is a lot of impurities deposition occurring,mainly lead an copper. Electrowinning cells powder deposition is not affected by the high amount of impurities present in it, because these cells are designed to extract silver and gold of 99.99% purity from it. I have attached a data about electrolyte composition and the powder obtained from it after electrowinning.
> It would be of great help if somebody knows the science behind it and how to eliminate it.



Hello vjodhani! From the solution, which has in its composition a lot of different impurities, it is advisable to extract silver in the form of chloride, then restore it to metal and produce electro-refining, so you get Ag 99.99. Electrolysis to extract high purity silver from such a solution is unlikely, the electrochemical potentials of silver and copper are in the neighborhood, and copper Will constantly interfere with you. I tried to restore the electrolysis of silver thiosulfate with copper impurities, when silver in the solution becomes small, copper begins to precipitate at the cathode. Try reducing the electrolysis current, it should help.But better restore AgCl, and with him work further.


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## snoman701 (Dec 25, 2018)

What is the purity of the feedstock?




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## vjodhani (Dec 25, 2018)

ION 47 said:


> vjodhani said:
> 
> 
> > We have recently commissioned an EMEW cell for extracting Ag from silver nitrate electrolyte.It is cylindrical cell operated at 5.6V(450 A- 550 A) with cathode as stainless steel and anode as Titanium.The problem is there is a lot of impurities deposition occurring,mainly lead an copper. Electrowinning cells powder deposition is not affected by the high amount of impurities present in it, because these cells are designed to extract silver and gold of 99.99% purity from it. I have attached a data about electrolyte composition and the powder obtained from it after electrowinning.
> ...



Previously we used to make AgCl which is a headache for large scale preparation.So we commissioned electrowinning cell to optimize silver gpl in my electrolyte.We are dealing with 42 m3 of electrolyte for such setups electrowinning is a must.If you have any idea pertaining to this please share.
Thank you for your response.


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## vjodhani (Dec 25, 2018)

snoman701 said:


> What is the purity of the feedstock?
> 
> 
> 
> ...



96.5+


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## snoman701 (Dec 25, 2018)

vjodhani said:


> snoman701 said:
> 
> 
> > What is the purity of the feedstock?
> ...



Why so low? 

I’m really not following your threads...they don’t make sense. On your other thread you said you get 99.5% Ag out of the converter, now you have 96.5% Ag going in to the electrolytic setup. As what form? Solid anode?



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## ION 47 (Dec 25, 2018)

vjodhani, аt what cathode current density (Ampere / m2)do you conduct the process? The process is controlled automatically or manually?


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## Palladium (Dec 25, 2018)

snoman701 said:


> vjodhani said:
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> > snoman701 said:
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I think the 96.5 is what he is getting when they try and plate out the silver from the solution they are drawing off when it's time to boost up the electrolyte.


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## snoman701 (Dec 25, 2018)

That makes more sense....thank you! 


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## vjodhani (Dec 26, 2018)

snoman701 said:


> vjodhani said:
> 
> 
> > snoman701 said:
> ...



Feed for electrorefining is solid anode of 99.5+ purity. Here in electrowinning the feed stock is electrolyte.So, I am talking about the amount of silver in electrolyte is 96.5+ (while rest are impurities as shown in data); while both processes are two different things with different feed.I hope that clears your confusion.


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## Palladium (Dec 26, 2018)

Personally i think your voltage is to high. That high a voltage is dragging the copper ions out of solution. You have mentioned in another post that the powder is of the form of cotton. It thus affects the silver powder quality. That was for your actual silver cells and i think you said you were running about 12-16 volts on them. I think you're running to high a voltage all around and this is causing you problems. Are you behind or trying to catch up production so you're pushing a higher voltage because it will push more amps and deposit more at the cathode leading to increased production? Have you always run at those voltages with no problem? In the beginning with fresh electrolyte i will push 6 volts, but once my solution starts to build copper if i continue to push that voltage it will co deposit with my silver, especially if the temp climbs. As the copper builds it takes less voltage to drag it out of solution. 

As for recovering the silver from the spent solution.....
I don't see how how you can plate the silver out of a mixed contaminated solution using an inert anode at 6 volts and expect it to be 99.99% purity. All those metals have a lower voltage potential than 6 volts. A whole lot lower!


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## ION 47 (Dec 26, 2018)

The only solution is to periodically perform voltammetric analysis of the electrolyte during electrolysis in order to determine the optimal deposition potential of silver with the exception of the deposition of other elements, and to maintain this potential at the cathode. As a power supply, use a powerful potentiostat.


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## vjodhani (Dec 26, 2018)

Palladium said:


> Personally i think your voltage is to high. That high a voltage is dragging the copper ions out of solution. You have mentioned in another post that the powder is of the form of cotton. It thus affects the silver powder quality. That was for your actual silver cells and i think you said you were running about 12-16 volts on them. I think you're running to high a voltage all around and this is causing you problems. Are you behind or trying to catch up production so you're pushing a higher voltage because it will push more amps and deposit more at the cathode leading to increased production? Have you always run at those voltages with no problem? In the beginning with fresh electrolyte i will push 6 volts, but once my solution starts to build copper if i continue to push that voltage it will co deposit with my silver, especially if the temp climbs. As the copper builds it takes less voltage to drag it out of solution.
> 
> As for recovering the silver from the spent solution.....
> I don't see how how you can plate the silver out of a mixed contaminated solution using an inert anode at 6 volts and expect it to be 99.99% purity. All those metals have a lower voltage potential than 6 volts. A whole lot lower!


FOR ELECTROREFINING:-

Are you behind or trying to catch up production so you're pushing a higher voltage because it will push more amps and deposit more at the cathode leading to increased production?
It is a big yes.  
Have you always run at those voltages with no problem?
A big yes.
As far as this is concerned "if i continue to push that voltage it will co deposit with my silver, especially if the temp climbs. As the copper builds it takes less voltage to drag it out of solution." 
I am not getting impurities deposition such as copper and lead.
I will show you my recent powder analysis.
Bi	Cu	Fe	Pb	Sb	Se	Te	Zn	Purity by diff.
3	13	5	15	4	<1	<1	8	99.9952%

FOR ELECTROWINNING:-

I would like to share an interesting result with you in my electrowinning cell the anode is inert made of tungsten.Just for the sake of curiosity i took it out from the cell scrapped the surface and look what I found in the analysis.
Bi Cu Fe Pb Sb Se Te Zn Purity by diff.
11486 9216 366	709801	638	<1	38	127 26.8328%

*impurities are in ppm
Seriously getting such interesting results.So much to identify so much to learn.


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