# Nitric acid in silver cell



## Drimacus (Jan 14, 2013)

I have a question concerning nitric acid in the electrolyte of a silver refining cell of Balbach-Thum design. Electrolytes seem to vary from a silver nitrate only solution to electrolytes with "some" free nitric acid. "A typical silver-refining acid bath would be 200 grams per liter of silver nitrate and 30 grams per liter of nitric acid (Recovery and Refining of Precious Metals, C.W. Ammen, 1984). 

What would be an appropriate level of nitric acid to use? In my instance I plan to refine silver of about 99% to 99.5% purity, the major impurities being copper and nickel.


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## nickvc (Jan 15, 2013)

There are loads of posts about running silver cells here on the forum, use the search button top right of your screen to browse them and gather more information. In honesty the best person to answer silver cell questions is GSP, GoldSilverPro, one of our leading lights who has refined more silver than most will ever see. I think that he has a book all about silver available from lazersteves site and well worth the money if silver is your thing. I think the general consensus for the silver levels is around 60-100 grams per litre with a little copper to allow easier removal of the crystal, this is done by placing your silver into a beaker and adding distilled water and nitric slowly, there are calculations on how much nitric you need to dissolve it, this gives you your basic electrolyte which can be diluted to achieve the ratios you need or want. The size of the cell is dependant on how much you need to refine and the power supply you have, you also need to have an anode bag. If run correctly you should be able to produce 999+ silver from your cell.


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## MysticColby (Jan 15, 2013)

Sources differ on silver cell electrolyte, and they all work well.
The reason to have free nitric acid in a silver nitrate cell is to keep it acidic. If all of the nitric is used up, oxygen from the air will eventually oxidize copper nitrate in the electrolyte, causing copper oxide contamination in your refined silver.
Some silver cell electrolyte recipes call for copper to be in it, others will accumulate copper as the cell is run.
You shouldn't need very much free nitric. For my silver cell, I have 1L of electrolyte with 100g silver dissolved in nitric, and a couple drops of nitric acid. It lasts long enough for the electrolyte to need replacing due to copper nitrate buildup, but you can add another couple drops every few days if you want.


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## Palladium (Jan 15, 2013)

I don't believe you can have free nitric in a silver cell. Extra nitric is added to dissolve more silver from the anode into the solution to increase the silver concentration.


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## justinhcase (Feb 15, 2014)

Has any one come across the Kodak patented method for replenishing electrolyte in silver cells.
It instructs you to add silver oxide to your solution to remove excess contaminants when they get to to high a level.by monitoring the solution and filtering at two PH points the first being 5.9 and the second 6.8
I tried it and after several hours could not see any PH change so put the lot into my silver chloride dump pot.two days latter I noted that the silver oxide had started to show color changes of green at it's interface with the solution and the wast pot had silver in solution again so needed toping up with sodium chloride.
Any ideas on the action and if next time I should leave it to stand longer before dumping.
I did think this would get a little  more interest ,as if it work's it would mean that a silver cell could be left running indefinitely without the need to make a new solution of silver nitrate.
My little cell tends to need replacing every week or so and i am a bit feed up with having to sacrifice some of my newly purified silver every time i need to renew it.Although as I am knocking the silver on the head for a while to concentrate on my AU it is a bit academic now.


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## justinhcase (Mar 7, 2014)

I tried a test batch of this again but still only got a slight colour change over night.
Is it meant to be that slow a reaction or am I missing something??


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## goldsilverpro (Mar 7, 2014)

Ralph,

In my books, I have maybe 20 different silver cell formulas from major silver refineries throughout the years. In all but 1 or 2 of them, they maintain from 1% to 5% free nitric in the solution. The free nitric will be consumed, mainly because it dissolves silver crystal. However, this proceeds very slowly and, the lower the nitric concentration, the slower the slower the dissolving.

A guy I worked for had 12, 30 gal Thum cells. Three of these were, what he called, "breakdown cells". They were used as the 1st step for sterling and 80% stuff. With all that copper entering the solution, the silver content went down rapidly and, the lower it got, the higher the copper content of the crystal. To combat this, he maintained the silver content with fairly frequent additions of straight nitric, which mainly dissolved crystal. When the anodes were gone, the result was about 99+% crystal and a fairly weak silver solution containing tons of copper. The silver in solution was cemented. This and the impure crystal were remelted together (probably averaged at least 99% Ag) and run through a clean cell.

While writing this, I came to the mathematical conclusion that it would take the same amount of nitric to do this as it would to directly dissolve the sterling or 80% in 50/50 nitric in a bucket. However, that would require a lot more copper (which you have to buy), to cement out the silver.

Using a breakdown cell:
PROS: A cleaner operation; Far less fumes at any given time.
CONS: Much more labor; Slower; More expensive to do.

Direct dissolving:
PROS: Much faster; Easier; Cheaper over all, even though it takes more copper;
CONS: Need extra copper; Lots of fumes;

Of these 2 choices, I would probably choose direct dissolving. It's faster and easier. I'm a big fan of easy. Wanting to make a process easier, faster, cheaper, safer, and simpler, without sacrificing efficiency, is the mother of invention.


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## Palladium (Mar 7, 2014)

I see what you're saying and now it makes sense.


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## g_axelsson (Mar 8, 2014)

goldsilverpro said:


> A guy I worked for had 12, 30 gal Thum cells. Three of these were, what he called, "breakdown cells". They were used as the 1st step for sterling and 80% stuff. With all that copper entering the solution, the silver content went down rapidly and, the lower it got, the higher the copper content of the crystal. To combat this, he maintained the silver content with fairly frequent additions of straight nitric, which mainly dissolved the crystal. When the anodes were gone, the result was about 99+% crystal and a fairly weak silver solution containing tons of copper. The silver in solution was cemented. This and the impure crystal were remelted together (probably averaged at least 99% Ag) and run through a clean cell.
> 
> While writing this, I came to the conclusion that it would take the same amount of nitric to do this as it would to directly dissolve the sterling or 80% in 50/50 nitric in a bucket. However, that would require a lot more copper (which you have to buy), to cement out the silver.
> 
> ...


That can't be correct. There must be a lot more nitric usage in the direct dissolving case. Any nitric going into that process either ends up as fumes or as copper nitrate. The fact that the breakdown cell both requires less copper and emits less fumes should be a dead giveaway that it is more economic with the nitric acid.
A similar discussion went on in this thread http://goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=17138 or another thread at the same time.
The main difference is that in that discussion it was talked about batch wise running of sterling, not a continuous process as in the breakdown cell.

Back to the breakdown cell comparison. To make things a bit less theoretical, lets compare the numbers for the two processes preparing a kilo of coin silver (80%) as a practical example.

Break down cell :
In the breakdown cell nitric acid is only used to convert the copper into copper nitrate, the silver is just passing the cell electrolytically and doesn't affect the nitric consumption.
For every silver atom that is dissolved from the anode, one atom is deposited on the cathode. No nitric is used.
For every copper atom that is dissolved from the anode, two atoms of silver is deposited on the cathode. To replenish the silver we need to dissolve two Ag atoms for every copper atom, or for 200g of copper (Cu molar mass 63.5g, Ag molar mass 107.9g) we need to dissolve 200/63.5 moles Cu or 2*200/63.5 moles Ag = 2*(200/63.5)*107.9 g Ag = 680g silver. For that we need 1.17 ml of 70% HNO3 per gram of silver = 795 ml 70% HNO3.
In the end we get 800g silver pure enough for running in a clean cell and 200g copper converted into copper nitrate.

Direct dissolving and cementing :
This one is easier to see what's happening. We just throw it in a beaker and dissolve everything. For a kilo of coin silver we need to dissolve 800 g silver and 200 g copper to turn it into solution.
Ag : 800 * 1.17 = 936 ml 70% HNO3
Cu : 200 * 3.98 = 796 ml 70% HNO3
For a total of 1732 ml 70% HNO3
Then we have to add (800/2)*(63.5/107.9) = 235g of copper to cement the silver back into metal.
In the end we get 800g silver pure enough for running in a clean cell and 435g copper converted into copper nitrate.

We now see that the pretreatment with a breakdown cell takes approximately 795 ml of nitric and no added copper.
The pretreatment with dissolving everything in nitric first takes approximately 1732 ml of nitric and 235g of copper, 2.2 times the amount of nitric compared to the breakdown cell example.

ref: General reaction list

To me it's quite obvious that the breakdown cell route is more conservative on nitric, creating less waste, but it also takes more work to monitor the cell and adding nitric when needed and longer time to run. According to me the list should read...

Using a breakdown cell:
PROS: A cleaner operation; Far less fumes at any given time, less waste, less nitric consumption.
CONS: Much more labor; Slower; Requires an extra cell.

Direct dissolving:
PROS: Much faster; Easier; Cheaper equipment.
CONS: Need extra copper; Lots of fumes; Need extra nitric; More waste.

If one process is more expensive than the other depends on your price for equipment, nitric and labor so I haven't added it to either process. Which ever procedure that works for you depends on a lot of factors and only you can make the call on which one to use.

Göran


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## kadriver (Mar 8, 2014)

The break-down sounds like an excellent idea. It would eliminate the first nitric dissolution of the incinerated sterling/925 silver which would save much nitric acid.

Then, no copper would be required because the need for cementing with copper would be eliminated, this would eliminate much copper nitrate waste.

The PGMs and gold, if any, would get trapped nicely in the anode bag of the break-down cell, if the electrolyte pH is maintained at the right level.

BUT, the electrolyte would quickly become fouled with dissolved copper requiring frequent changes of electrolyte, hence the added labor.

I have a spare cell at my shop. I will set it up as a break-down cell as an experiment. I have experimented with this in the past but the cell became saturated with copper so quickly that I abandoned the idea.

But the benefits of eliminating all that copper looks mighty appealing.

If only there was a way to precipitate the copper out of the electrolyte while keeping the silver in solution.

How about adding sodium hydroxide to the copper saturated silver nitrate to precipitate the copper hydroxide only? Would the silver nitrate be ruin during the process? Or would the silver turn to a hydroxide also?

How about hydrolysis with KOH to remove the copper?

Is there any chemical that would remove the copper and leave the silver nitrate uncontaminated so that the silver nitrate could be used again?

If such a chemical is out there then one could rotate solutions in the break-down cell. Use one quantity of solution in the break-down cell while the other solution is being treated to remove the dissolved copper.

kadriver


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## Palladium (Mar 8, 2014)

That's sort of what the formate system that I've been playing with does but in reverse. You use NaOh to adjust the ph and drop hydroxides out leaving the silver alone. The you can drop the silver out without the fear of dropping any copper out of solution with your silver because the copper is tied up in solution and won't drop with the silver.


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