# Reducing use of Nitric during Silver processing?



## HPete (Mar 5, 2018)

After a lot of reading and experimenting I've come to a question I cannot find the answer to.

I've carried out test runs on Sterling, using both the precipitation by Copper method, and the Silver chloride way and Hoke's Zinc method to reduce it to Ag metal. The latter seems to work best for me as filtration and cleaning of the chloride is easier, especially when using large amounts of hot water as Hoke recommends ( Karo syrup is not available here - so that way is ruled out ). The downside is the cost of Zinc, and the large volumes of diluted solutions which cannot be re-used.

I've now got a fair quantity of "qauternary metal" ( 50%Ag, 40%Cu, 5% Zn & Ni ea ). The problem is the amount of Nitric needed to dissolve the metal ( Goldsilverpro's 1.22ml/g of Ag & 4.3ml/g of base metal is spot on ) - about 2.75l per kg of the metal.

I see two methods to proceed:

1. Precipitation by Cu, and then using electrowinning of the Cu to restore some of the Nitric, a bit like this thread by lazersteve;
https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=2868

I would then use the "restored" nitric to dissolve more metal and repeat until the Nitric is "used up". The Cu removed from the Nitric could be re-used to drop more Ag, that way any Silver that gets electro-deposited with the copper would return to the process and get removed with the rest of the Ag.

2. Precipitation of Silver Chloride using HCl ( only 90-95% as mentioned here - https://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=26660 )
The copper would also have to be removed electrochemically as it builds up as in the previous method. ( Loss of some Silver seems inevitable here if any of it deposits along with the Copper when it is removed, as it will be mixed with large amounts of Cu ).

The problem I see with both processes is that the Nitric acid eventually becomes "used up" through the loss of Nitrogen as NOx and NO2 when the source metal is dissolved.
Other than regeneration of Nitric acid by bubbling the NOx and NO2 through water with a wash bottle is there another way to regenerate the HNO3?

Some research seems to suggest that the addition of Hydrogen Peroxide to the solutions might help, as per the following:
2NO + 3H2O2 - 2HNO3 + 2H2O and,
2NO2 + H2O2 - 2HNO3
See dangers here:
http://hydrogen-peroxide.us/chemical-catalyst-decomposition/Solvay-Hydrogen-Peroxide-Nitric-Acid-Hazards-2005.pdf

I would believe the time to add the H2O2 would be when the gasses are created ( ie. when dissolving the metal ), and when regenerating the Nitric during electrochemical removal of the Cu, as well as when dropping AgCl with HCl.

So, eventually I get to the questions....

1. Will addition of some dilute H2O2 to the Nitric acid help in making the acid "go further" by regenerating some Nitric acid as per the above 2 equations while dissolving the source metal?

2. Will addition of some dilute H2O2 to the solution when adding HCl ( to drop AgCl ) help in recapturing some of the NO3?

3. Will addition of some dilute H2O2 to the solution when electrolytically removing Cu from Copper Nitrate help in regenerating the HNO3?

Anything I'm missing? Has anyone tried adding Peroxide at any time during Silver processing? Any comments or refinements to the processes would be appreciated.
I also apologise in advance for the length of the post....
Pete.


----------



## nickvc (Mar 5, 2018)

Welcome to the forum Pete.
First point is you can use table sugar instead of corn syrup or use old steel nails and a little sulphuric to convert your chloride.
A question for you, what form is this silver copper etc mix in?
I ask because if it’s a powder you could possibly use AP to dissolve much of the base metals away.
Another possibility if you intend to refine more sterling is using this mix to cement any silver nitrate solutions you produce, this will remove the base metals and recover the silver for free.


----------



## HPete (Mar 5, 2018)

Thanks Nick,

Yes, I did see the alternate methods ( sugar, - iron or zinc with sulphuric etc ) for converting AgCl, however I plan to run it all through a cell as a second step, so copper precipitation will probably be easier ( although if I can get 3N or slightly better as some claim with the the chloride methods I might be tempted to leave it at that - I'm still to have my samples tested for purity ).

Although, this doesn't solve the problem of nitric usage, because I have to get through about 10kg of this 50% silver mix first ( old 50% Ag coins that are too bashed up for the numismatic trade ). Regardless of whether I use the copper precipitation or chloride route, I'll still need just under 30l of nitric to process it all, if I can't get any "recycling" of nitric done. And then I'll have 30l of copper nitrate to clean before I can dispose of it.

I don't think the AP method will work, but I'll give it a try anyway - I'm guessing it will just leave a passivating layer of AgCl. In production the coin blanks used to be "whitened" in an acid leach, probably weak sulphuric, leaving a layer of purer silver on the surface. ( The natural colour of the alloy is slightly yellowish - nasty looking for coins, especially considering this alloy replaced 925 coins, so would not have been well received by the public in 1946 ).

I'm tempted to run a test, perhaps 100g, convert to AgCl using HCl, filter and then electrowin copper from the solution. Then use that "cleaned" nitric to see how much of another 100g batch it will dissolve.

I'm thinking it won't be too critical how much HCl is used as long as it's quite close. Any small excess in the cleaned Nitric should just drop out as AgCl as soon as the next batch of metal is dissolved, and if not enough HCl the remaining silver nitrate will just be converted with the next drop to AgCl.

Thanks anyway, Pete.


----------



## nickvc (Mar 5, 2018)

The coins were debased in 1920 and became cupronickel in 1946.
If you are going to process more sterling then try the cementation route as it saves on acid,try a small batch to see how it works, as to disposing of the copper nitrate solution simply add steel and cement the copper out.
As an aside have you tried to get terms to sell as is?


----------



## HPete (Mar 5, 2018)

Not these ones. Sterling to .500 in 1946, and then cupro-nickel in 1965 along with decimalisation. They're rejects from numismatic trade, so no sale value other than melt.

Not sure why you say cementation with copper will save Nitric - still need the same amount to dissolve the metal. HCl is not an issue if that's what you meant?

I tried electro-winning Copper from the Copper Nitrate solution after dropping all the Silver with Copper ( confirmed by weighing the Silver ). Using Graphite stirring rods, a grey sludge dropped from the cathode, and the anode partially eroded into a fine black powder leaving the solution black with suspended graphite powder. When the current was cut off, the grey sludge redissolved, confirming that Nitic was being "regenerated".

Now I just need to find something better for the electrodes, thinking of using stainless mesh for the cathode to better catch the copper/sludge. Perhaps encase the anode in something to catch the graphite powder ( cloth bag etc ). Addition of a small amount of Peroxide only seemed to increase the "fizzing" at the anode. (I have since found a reference somewhere on the forum - by butcher if I remember correctly - that a small amount of 3% peroxide will make the Nitric go further during the first dissolving stage).

I'll use HCl to drop the next test batch. I realize now that it will be important to get the amount of HCl correct - less than 100% Ag drop to prevent formation of CuCl ( I'm presuming the Cl ion will go to Ag first before forming CuCl? )

I'll update later.


----------



## FrugalRefiner (Mar 5, 2018)

HPete said:


> Not sure why you say cementation with copper will save Nitric - still need the same amount to dissolve the metal. HCl is not an issue if that's what you meant?


If you're referring to Nick's post, he's talking about using the coins as a source of copper (and other metals) to cement silver from other material you've dissolved in nitric. Let's say you've dissolved some sterling in nitric. Rather than using copper buss bars to cement the silver, you can use the coins. The copper and other alloys will go into solution and the silver from the sterling will cement out, while the silver content of the coins will combine with the silver from the sterling. It's a good way to use low quality scrap to kill two birds with one stone as Butcher would say.

Dave


----------



## HPete (Mar 5, 2018)

Thanks Dave,

Unfortunately I don't have anything other than the coin metal at this stage so that's not an option.

Am I correct in assuming that addition of HCl to a silver nitrate /copper nitrate mixture will first produce AgCl with the excess HCl going to CuCl? Or will both AgCl and CuCl be produced in some ratio dependent on their valencies ( or some other property )?

If it's the former I should be able to calculate the correct amount of HCl to add, to only convert the Ag in a mixture and not the Cu. Assuming I knew the strength/molarity of the HCl?

Thanks again, Pete.


----------



## butcher (Mar 5, 2018)

With an acidic silver/copper solution, the HCl provides chloride ions, the hydrogen can add to acidity or even convert a salt to an acid (nitrate to nitric), the silver ions will pull chloride ions out of solution.

Free chlorides in solution could also form copper chlorides CuCl2, but not in this case(because of the nitrate or nitric involved).
we cannot form insoluble CuCl, or soluble CuCl2, there is no product, besides the silver chloride.

The copper and other metals stay in solution as ions (positive Cations), and the nitrates or chlorides stay in solution as ions (negative Anions). just a soup of ions.
We can not actually call it a copper chloride and copper nitrate and other metal hobo soup...

This also forms an aqua regia solution, which can hold metals in solution or even put more metal into solution depending on free acid (H3O).

Adding HCl, as long as a chloride ion is in solution the silver will pull it out, once all of the silver ions are gone, precipitated as silver chloride, adding chloride ions, just adds them to the ionic soup, at this point they do not form anything unless we change the reaction or add to it chemically, or even physically like evaporation, to convert the copper ion to another form...

Silver nitrate (silver ions) can be used to identify and determine how much chloride ions are in solution...
A chloride salt or acid like HCl source of chlorides can be used to identify or tell how much silver is in solution...

Example:
Silver nitrate can be used in impure nitric acid to pull out (or check for any) of the chloride ions.


----------



## FrugalRefiner (Mar 5, 2018)

Pete, as butcher has described, adding HCl adds both H+ and Cl- ions to the solution. As you assume, if you add _just not quite enough_ HCl to the solution, all the Cl- anions will combine preferentially with the Ag+ cations and precipitate as AgCl. There will be no Cl- anions left in the solution and you regenerate a weaker solution of nitric acid with some copper, zinc, nickel, and a bit of silver dissolved in it.

If you add too much HCl, you'll have excess Cl- anions in solution, which will interfere with the dissolution of additional coins as an impervious coating of AgCl can form on the coins. Because of the high levels of base metals, and the dilution factor of HCl, you won't get much mileage out of regenerating the solution.

Dave


----------



## HPete (Mar 5, 2018)

Thanks for clarifying Butcher!

So in my case there are Ag, Cu, Zn & Ni ions in solution with the NO3 ions. Theoretically speaking with the addition of HCl ( if the correct amount of Cl ions ) all the Cl ions will go to precipitate as AGCl? Leaving only Cu, Zn, and Ni ions in solution. The added H ions will also remain in the solution (effectively replacing the Ag ions and "rejuvenating" the nitric acid).

Any excess HCl, will result in Cl ions remaining in solution, essentially producing AR? Assuming an excess of HCl is avoided, will addition of a small dilute amount of Peroxide be beneficial in any way (during the electrolytic removal of the Cu, Zn & Ni) in the "rejuvenation" of the acid - to provide excess H ions to replace the metal ions removed in the electrolysis?

Pete.

Edit:
Thanks Dave - didn't see your reply until after my post.
It seems the secret to restoring the nitric will be the removal of the base metals, assuming not too much HCl is added. The problem will be getting the quantities correct to assure removing most but not all of the Silver, as any Ag not converted to AgCl will be removed along with the base metals, and end up as a very small portion of alloy that will be too poor in silver to economically process. As pointed out earlier, it will have to be used to precipitate other Silver processed later on.
Thanks for your help Guys. At least I've got some direction to go now!


----------



## HPete (Mar 5, 2018)

As another route, working on Nick's suggestion as a pretreatment to digestion with Nitric:

Would pouring the metal to shot and treating with dilute sulphuric to remove some of the base metal be effective, or would it only be worth while with metal in powder form? I'm thinking it would be a waste of time and effort, unless I could get incredibly fine shot?

Comments?


----------

