Silver cell issues

[FigiMon]

Background

I have a fair amount of 0.925 and 0.830 silver that I have accumulated over the past year and now started to refine. Living where I do, nitric acid is very hard to buy and tedious to make in the quantities required. Inspired by the "Processing Sterling Silver without nitric acid" thread I decided to try that procedure out.

Problem 1 - Regeneration of nitric from copper nitrate

With an electrolyte of Cu(NO3)2 (uncertain conc. but dark blue colour), graphite anode and copper plate as cathode the cell (1L glass beaker) was left for a few hours running at 0.5 A. Coming back the electolyte seemed unchanged apart from eroded graphite. Thinking that the deposited copper did not adhere to the cathode plate but simply fell off and dissolved, the cathode was replaced with a stainless bowl. The idea was that as the copper formed it would be protected by being in contact with the cathode. This time there was a copper deposit on the side of the bowl but I could see it being continually dissolved.

Am I running this on too low a current? I tried raising the current to 1 A but could not spot any difference.

Problem 2 - Crystal growth in silver cell stops

This is a related but separate problem. The setup is a stainless 1.5 L bowl as cathode, and a plastic anode basket with ~40 cm2 bottom surface area. No matter what I try as electrolyte (weak nitric, dilute silver nitrate, conc silver nitrate) after a few hours the silver crystals stop growing on the bowl's interior surface. Instead a mat that looks like dirty gray moss starts to grow and floats around in the electrolyte. I have tried with current from 0.2 A up to 2 A.

Does anyone have any idea where I am screwing this up?

Thanks

 

[nickvc]

You can run a silver cell using sterling and lower grade material as the feed stock but the problem you face is fouling of the electrolyte with too much copper which will co deposit, the one way to overcome that is to add more nitric to your solution to dissolve some of the refined silver allowing the cell to work properly once more as the concentration of silver increases and the copper level lowers, this is a balancing act but works with careful additions and monitoring.

 

[FrugalRefiner]

FigiMon, what voltages are you running in the two processes?

Dave

 

[FigiMon]

@nickvc

Yes, that's what I thought as well but i didn't expect that the transition would be quite so abrupt. Running with sterling feedstock I can see shiny small silver crystals in about 95% of the bowl and a narrow band at the very top of a copper layer. That would be perfect if it held but then suddenly this horrible moss starts growing instead. Is that indicative of too high copper concentration you mean?

@FrugalRefiner

Unfortunately I became so disgusted with the continued failure that I stopped taking notes but in the first few tries I had a voltage varying between 0.3 and 2.8 V. My power supply was set to current limiting at 0.5, 1.0 A etc. I think I have read somewhere not to go above 3V ideally.

 

[FrugalRefiner]

3 volts or so is good for running the silver cell. It can be pushed higher with tight control over the electrolyte, anode/cathode spacing, circulation, etc., but that's for experienced users.

When you're trying to regenerate nitric, you want to drop the silver first by cementing it with copper. Then the voltage is increased when you try to win the copper out of the solution. You have to put more "pressure" on the system with electricity to force the copper out faster than the increasing level of nitric can redissolve it.

It sounds like you've got other issues going on in your silver cell. You shouldn't be seeing silver crystals in one part of the cell and copper in another part. The build up of copper in the electrolyte should be gradual, and not an abrupt change. Circulation of the electrolyte is important. Otherwise, the silver ions can become depleted around the cathode, so the silver concentration at the cathode drops below proper levels.

Are you positive what you're running is sterling?

Dave

 

[Shark]

3 volts or so is good for running the silver cell. It can be pushed higher with tight control over the electrolyte, anode/cathode spacing, circulation, etc., but that's for experienced users.

When you're trying to regenerate nitric, you want to drop the silver first by cementing it with copper. Then the voltage is increased when you try to win the copper out of the solution. You have to put more "pressure" on the system with electricity to force the copper out faster than the increasing level of nitric can redissolve it.

It sounds like you've got other issues going on in your silver cell. You shouldn't be seeing silver crystals in one part of the cell and copper in another part. The build up of copper in the electrolyte should be gradual, and not an abrupt change. Circulation of the electrolyte is important. Otherwise, the silver ions can become depleted around the cathode, so the silver concentration at the cathode drops below proper levels.

Are you positive what you're running is sterling?

Dave

Dave, is there someplace to read up more on the process of removing the copper. Being able to regenerate my nitric would be very helpful at times.

 

[FrugalRefiner]

That's a tough question Shark. It's like asking which thread on the forum I read something, except I don't have a good search function for all the books, patents, papers, etc., I've read over the years. Start with lazersteve's post Processing Sterling Silver without nitric acid.

There is a lot of good general silver information in Silver, Economics, Metallurgy, and Use by Allison Butts and Charles D. Coxe.

I also like Electroplating & Electrorefining by Alexander Watt and Arnold Phillip. You should be able to find that one online. If not, let me know.

I don't remember if either of them covers the specific information I mentioned, but they're both good reads for a long winter night. Neither covers the subject in depth. You're lucky to get a few pages on the specific subject, but in total, you can learn a lot from both.

Dave

 

[butcher]

I would use a glass vessel, and a hard graphite anode, and graphite cathode, all of which are resistant to the nitric acid, you may be dissolving the metal cathode about as fast as you are removing metals from solution ...




My favorite way to reuse copper nitrate waste:
copper nitrate and sulfuric acid
we will have H+, NO3-. Cu+, and SO4- in solution, basically no reaction takes place at this point.

Cu(NO3)2 + H2SO4
By adding heat in a closed vessel we can develop NOx gases, bubbling these gases through the water with a little H2O2 to provide the oxygen, we can make a solution of very useful, fairly dilute nitric acid leaving us with another useful byproduct of copper sulfate...


Add a little bit of cut-up gold plated copper to this mix, and the copper will help to drive the reaction forward, dissolving the copper and base metals, leaving you with gold foils and copper sulfate, dissolving the copper sulfate crystals in water, leaving us with gold foils, nitric acid, and copper sulfate...

No waste
like Irons always said "everything but the squeal"

 

[FigiMon]

Based on the suggestions above I redesigned the HNO3 regeneration experiment. Graphite anode and a strip of thin Cu plate as cathode placed in a glass beaker. The electrolysis ran at a voltage of 4.5 V and current of 3.2 A. After three hours the cathode (which i forgot to weigh before the start) was nicely plated with bright red copper and significantly heavier. If my calculations are correct I should have removed a maximum of 11.4 g of Cu in this time.

There was some gas given off from the cathode (hydrogen?) though during the electrolysis and shutting off the power supply also lead to many small bubbles (nitrous oxide) forming as copper redissolved in the nitric that formed. I expect that depleting the majority of the copper will take a pretty long time as these competing reactions pick up the more concentrated the nitric becomes.

One problem is erosion of the graphite electrode (placed in anode bag to catch eroded material). Could a lead dioxide electrode be used possibly?

 

[FrugalRefiner]

Based on the suggestions above I redesigned the HNO3 regeneration experiment. Graphite anode and a strip of thin Cu plate as cathode placed in a glass beaker. The electrolysis ran at a voltage of 4.5 V and current of 3.2 A. After three hours the cathode (which i forgot to weigh before the start) was nicely plated with bright red copper and significantly heavier. If my calculations are correct I should have removed a maximum of 11.4 g of Cu in this time.

That sounds better!

There was some gas given off from the cathode (hydrogen?) though during the electrolysis and shutting off the power supply also lead to many small bubbles (nitrous oxide) forming as copper redissolved in the nitric that formed. I expect that depleting the majority of the copper will take a pretty long time as these competing reactions pick up the more concentrated the nitric becomes.

Exactly right. You can never completely win the copper out of solution, and you're going to lose hydrogen (thus, concentration of the nitric acid being formed). But given the low price of electricity compared to nitric acid for most folks, it can still be worth the effort.

One problem is erosion of the graphite electrode (placed in anode bag to catch eroded material). Could a lead dioxide electrode be used possibly?

You might also look into MMO and/or platinized anodes.

Dave

 

[rickzeien]

Based on the suggestions above I redesigned the HNO3 regeneration experiment. Graphite anode and a strip of thin Cu plate as cathode placed in a glass beaker. The electrolysis ran at a voltage of 4.5 V and current of 3.2 A. After three hours the cathode (which i forgot to weigh before the start) was nicely plated with bright red copper and significantly heavier. If my calculations are correct I should have removed a maximum of 11.4 g of Cu in this time.

That sounds better!

There was some gas given off from the cathode (hydrogen?) though during the electrolysis and shutting off the power supply also lead to many small bubbles (nitrous oxide) forming as copper redissolved in the nitric that formed. I expect that depleting the majority of the copper will take a pretty long time as these competing reactions pick up the more concentrated the nitric becomes.

Exactly right. You can never completely win the copper out of solution, and you're going to lose hydrogen (thus, concentration of the nitric acid being formed). But given the low price of electricity compared to nitric acid for most folks, it can still be worth the effort.

One problem is erosion of the graphite electrode (placed in anode bag to catch eroded material). Could a lead dioxide electrode be used possibly?

You might also look into MMO and/or platinized anodes.

Dave

MMO?

Sent from my SM-G950U using Tapatalk

 

[FigiMon]

https://en.wikipedia.org/wiki/Mixed_metal_oxide_electrode

 

[rickzeien]

https://en.wikipedia.org/wiki/Mixed_metal_oxide_electrode

Thanks

Sent from my SM-G950U using Tapatalk

 

[FrugalRefiner]

When you're trying to regenerate nitric, you want to drop the silver first by cementing it with copper. Then the voltage is increased when you try to win the copper out of the solution. You have to put more "pressure" on the system with electricity to force the copper out faster than the increasing level of nitric can redissolve it.

Dave, is there someplace to read up more on the process of removing the copper. Being able to regenerate my nitric would be very helpful at times.

I had time to look for this today while waiting for my wife at an eye appointment. While I'm confident I've seen similar information elsewhere, I found the following in The Electroplating & Electrorefining Of Metals by Alexander Watt and Arnold Philip:

page 571

The anode precipitate contained in the linen bags round the anodes
contains all the gold, platinum, lead (as lead peroxide), and antimony,
and frequently some silver as peroxide. The copper, zinc, and iron
accumulate in the electrolyte, and when the copper becomes too con-
centrated it is removed from the solution by passing, firstly a feeble
current with carbon anodes, by means of which the silver is removed
from the solution, and then the silver cathode is replaced by a copper
one, and the whole of the copper is deposited from the solution as
a powdery crystalline precipitate upon the cathode by means of a
current of high density. The liquid thus nearly or completely freed
from copper is then used once more, either for making up new
electrolyte or for replacing liquid lost by evaporation in the course
of the process.

pages 575-576

The official abstract of Mobius' first patent (English Patent 16,554,
1884) is as follows:
"{snip}
The excess of copper is removed from
the exciting liquid in the battery by first suspending copper plates,
etc., in the solution (whereby the silver remaining is deposited) and
then precipitating the copper by an electric current."

Lazersteve also described this process in Processing Sterling Silver without nitric acid.

Dave

 

[Shark]

Thanks Dave, it took me a few minutes but I have a copy on hand. Now to find time to read some of it.

 

[FrugalRefiner]

There's not much more on the subject than what I posted, but the whole book is still a very interesting read.

Dave