# Silver plated brass and sodium sulfite (Na2so3) electrolysis?



## refiner123 (Jun 4, 2020)

I have collected ~150kg of silver plated brass items from RF components.
Local scrapyards would pay just regular brass scrap price for these items so I began to think whether 
there would be any way to recover the silver with some home chemistry.
I have read this forum and other sources regarding possible silver recovery methods.
I understand that many of these methods are far from being profitable, especially if you would
have to pay something for the silver plated items.

I feel slightly afraid of building sulphuric silver deplating cell due to highly reactive condensed sulphuric acid.
So far electrolysis in sodium sulfite (Na2so3) electrolyte seems most promising option.
Before I start to do anything, I would like some additional hints on the process.


1) Should I expect some toxic fumes from this cell?

2) Will some of the silver stay in the electrolyte when process is finished?
If yes, how to get it out from the solution?

3) Does sodium sulfite electrolyte get polluted by base metals from brass?
If the base metals pollute the electrolyte, it probably needs to be changed if I want to process 
the whole pile. If electrolyte needs to be changed too often, it reduces the possibility to reach
breakeven with this process.

4) How to safely dispose this electrolyte after processing is complete?
If there are loads of copper, tin, zinc in the solution, I probably should think a 
way to make it safe before I start anything.

5) About cell container material.
Based on some google searches, polypropene plastic should be pretty resistant to
sodium sulfite (Na2So3) / aqua solution.
Will the properties of electrolyte change to be incompatible with PP plastic cell container?
Or should I try to find large enough glass bowl for this purpose?

6) Tarnished silver?
Some of the items are pretty tarnished. 
Should I first drop them to aluminium + baking soda bath to make AgS back to Ag metal or will
similar reaction happen anyway within this cell? I don't want to lose some of the thin silver layer
because of the tarnish if it can be metallised easily.

7) What is the expected purity of silver got with this method?
If I get large enough amount of silver out from the process, I might want to learn to refine
it to 999 silver with nitric silver electrolysis cell. I am just wondering whether the silver from this cell
would be pure enough to be melted as anode for nitric silver cell or should some other refining done first.


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## Lino1406 (Jun 4, 2020)

Before starting, have an estimation of silver amount, according to plating thickness. Estimate the waste amount and ask for price to remove it to a confirmed location. Weigh one against the other.


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## refiner123 (Jun 5, 2020)

Hi

Thanks, very good point. 
Currently I have no idea about the amount of base metals going into electrolyte and thus how often I would have to change it. (questions 2,3,4) Also if there are other aspects that would force electrolyte change. (like in sulhpuric stripping cell, when concentrated sulphuric collects too much water, it becomes unusable for this purpose)
This affects heavily to the estimated amount of waste. Could anybody point me to source where I could learn more about chemical processes which might happen in Na2So3 cell?

On the other hand, I am not sure about the plating thickness and I don't know any easily accessible method to measure it. For this reason I have considered to do a test run and abandon the idea if yield is too low.
But before I start to do even test run, I need to figure out the waste management.


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## Lino1406 (Jun 5, 2020)

Assume 10 microns thickness, for a start and see what you get


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## refiner123 (Jun 6, 2020)

I did earlier some calculations with 10um plating thickness and landed to ~1kg of silver content, which could be worth of some amount of effort to extract.


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## Lino1406 (Jun 6, 2020)

To me 100g look more realistic


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## refiner123 (Jun 6, 2020)

It might be that my calculations are overestimating the silver content and this might not be very profitable project. But it is still interesting hobby project with possible silver bullion reward.

I found a scientific study on this method, could be worth reading to others considering using this method.

https://www.researchgate.net/publication/257861175_On_Anodic_Dissolution_of_Silver_Coatings_Deposited_on_Base_Metals

Summary: very selective oxidation of only silver happens even if other metals are present in anode as long as cell voltage is less than 0,4V.
Not all of the oxidised silver is deposited on the cathode, since 10-23% is lost somewhere (in the electrolyte or dropped in the bottom of the cell?)

Na2SO3 electrolyte will oxidise rapidly to Na2SO4 if in contact with air
--> should not expect to use the same electrolyte for multiple days.

Positive side is that Na2SO4 is not very dangerous, so used electrolyte disposal should not be large issue.
If I understand correctly sodium is more reactive than most of other metals, so there is low probablity of seeing sulphates of other metals instead of sodium sulphate in the electrolyte? (please correct me if I am wrong here)

If I understood correctly [Ag(SO3)2] complex is not infinetely stable and SO3 ions will oxidise to SO4 over time, it might be that silver left to electrolyte is automatically precipitating out as metallic from the used electrolyte within some unknown time window.

So far this method sounds pretty safe method to try.


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## snail (Jun 6, 2020)

I have been investigating this process also with a small scale cell.

I am using a rectangular 4 liter heavy wall cooking pan as cathode. The process is quite slow but creates very little fuming or bubbling.

Tried some flatware, small plated tray and a piece of silverplated pewter. It took nearly 8 hours to deplate each item.

The process Created a sludge more than a hard deposit.
My interest was the effect of the white metal and pewter to the process.

There was no extreme corrosion of the pewter but there was a continuing current unlike brass.
I am sure there is tin contamination in the sludge.

Overall I am pleased, it is not highly profitable, but does not need constant attention.

I wonder if the oxidation to sulfate could be controlled by Ph since I found processes to convert sulfate to sulfite by using NaOH.


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## Tankman (Jun 8, 2020)

I too am looking into this, but using a 10 liter stainless steel stock pot as a cathode, and a 12mm stainless steel rod suspended above it, from which hangs the pieces to be de-plated. The silver mostly ends up in the bottom of the stock pot as a sludge as well.

The question I have is, what would it take to scale it up to a 100 liter set up? In the way of a power supply.


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## refiner123 (Jun 9, 2020)

What kind of concentration you have used for the electrolyte?
And what kind of cell voltage you have used?
How long you can operate with the same electrolyte?
Have you done any effort to minimise the oxidation of the sulfite ions to improve the lifetime of electrolyte?
Do you have any estimates how much silver stays in the electrolyte or will it eventually drop to the bottom of cell without extra effort? (or should we expect some amount of silver sulfate as leftover?)

I found a diagram from the science paper mentioned eariler that when they used 0,43M solution (55g / l ?) current was something like 90mA / cm^2 with cell voltage ~0,5V. If this would match also larger scale, you could calculate power supply requirements based on the expected surface area.


The same paper compares different Na2SO3 concentrations and the higher concentration increases current density. I wonder would it improve the process to use higher than 0,5M concentration.


Stainless steel bowl as cathode seems to be a good idea. I will try to find a suitable one from some second hand store.


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## Tankman (Jun 10, 2020)

What kind of concentration you have used for the electrolyte?
The electrolyte used is between 50 and 58grams/liter. 

And what kind of cell voltage you have used?
For the 10 liter set up, it is between 1.54v and 1.78v I find that 1.76v works the best

How long you can operate with the same electrolyte?
8 hours usually, but sometimes less. The amps stop dropping once the solution is depleted.

Have you done any effort to minimise the oxidation of the sulfite ions to improve the lifetime of electrolyte?
No, not yet.

Do you have any estimates how much silver stays in the electrolyte or will it eventually drop to the bottom of cell without extra effort? (or should we expect some amount of silver sulfate as leftover?)
One experiment that I tried was, to de-plate 18 soup spoons with an estimated 0.9 grams of silver on each. The solution was 10 liters with a sodium sulfite concentration of 56 grams/liter. It ran for a little over 7.5 hours with the amps starting at 0.66 and dropping to 0.14 at the end. The quantity of non refined silver recovered from the sludge was 15.4 grams. This is not taking into account the extremely thin layer on the inside of the stock pot.

One would expect there to be some silver sulfite in solution.

These are the silver buttons I have recovered using this method so far. Just over 4.4oz


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## kurtak (Jun 11, 2020)

You can strip silver plating with nothing other then TAP water & power supply (as in a stripping cell - wherein only tap water is needed for the electrolyte)

Read this thread :arrow: Processing SilverPlate With H2O Cell

it is a "long" thread (8 pages) --- about half way into it solar_plasma picked it up & ran with it posting a great deal of info

I suggest reading the entire thread - LOTS of good info & a FULL discussion 

solar-plasma played with other stripping cells & his conclusion is that the "tap water cell" is "the way to go" concerning a stripping cell

the BEST way to strip silver plating is cyanide leaching - the NEXT best is the tap water cell - IMO

Kurt


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## refiner123 (Jul 12, 2020)

I did a small trial batch with this method.
Stainless steel pot as cathode, silver plated brass pieces hanging on a copper wire as anode.
~50g/l Na2SO3 as electrolyte. With ~1,5V cell voltage I got quite clean brass items and black sludge on bottom of the cathode pot.
With my lot the current was only 0,2 - 0,5A, but that probably depends on the surface area of items.

If proceeding, I need to enhance the mechanics in order to make it easier to change the items and roll them slightly when necessary to avoid not reachable spots.


The next question is how have you refined the resulting sludge?
As far as I have understood, electrolysis with Na2SO3 won't be able to make tarnished silver back to metallic, so the tarnished part of the silver is going to drop in the sludge as Ag2S?

SInce the base metal is brass and brass is a mixture of metals, it might contain also some other metals like Sn, Pb in addition to Cu, Zn.

With these assumptions, the sludge at the bottom of pot is mixture of Silver metal, tarnished silver (Ag2S), base metals (Cu, Zn, Sn, Pb)

Silver refining requires processing in nitric acid, but Nitric acid and tin is a bad idea. So, it might be wise to try removing some metals first (Sn, Zn, Pb) with HCl wash (boiling?). SnCl2, ZnCl2 are water soluble, PbCl2 is soluble in hot water, but not in cold water. (1)

But what will happen to Ag and Ag2S in such wash?
In theory silver won't react with HCl, but there are several texts mentioning that very concentrated HCl will dissolve some amount of silver as AgCL. And since the silver is in very fine particles, there is a lot of surface area where HCl can react with silver metal.
If I have understood correctly, Ag2S will react with HCl and create AgCl.

And there are some texts mentioning that in some cases AgCl will react with Cl- ions and forming AgCl2- complexes. AgCl2- is water soluble?

Again, based on my limited understanding, washing (or boiling) the sludge with HCl might result in sludge with Ag, AgCl, Cu after filtering. Would I lose some amount of AgCl as water soluble AgCl2- ?


After dissolving this metal mixture with nitric acid, filtering will separate AgCl (and some other insoluble impurities) from silver and copper nitrate solution. Silver can be cemented out from the solution by placing a copper piece in. (2)

And then I would have some amount of AgCl and some other impurities to process. (3)


From waste management viewpoint, I would have 
(1) solution with HCl, ZnCl2, SnCl2, PbCl2
(2) copper nitrate solution
(3) Silver Cholride + some other impurities

Any comments on the intended refining process?


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## snail (Jul 13, 2020)

I’m at work so don’t have much time.
First I never had black sludge deposit on cathode, always cement silver color. 
I just water rinse and use dilute nitric acid. Never had tin issues, just a little brown sludge That did not dissolve. I assume It is a tin compound. I definitely had tin in the sludge from some pewter I deplated.
Used HCl to drop silver chloride then iron and dilute sulfuric to convert to silver metal.
I haven’t melted any but it looks good.


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