Smelting Silver Plate

[Yggdrasil]

I'm going to be testing a method with a few pieces of junky silver plate cutlery I have. I've discovered that warm concentrated sulfuric acid does a good job of attacking vulnerable metals relatively uniformly, meaning that it will dissolve the thin silver plating just as readily as the base copper or brass underneath. This was my observation after using sulfuric to process a test batch of small silver-plated e-scrap pieces. The silver went first and then the underlying base metal began to dissolve.

Given that this means only a tiny amount of metal is initially dissolving, it's then possible to pour off the acid once the plating is stripped into another batch of plated items. So for my thinking, it'd be ideal to have many batches of plated items and simply go down the line one after the other. At some point, the warm sulfuric won't hold anymore dissolved silver and it'll start cementing out, but even that it doesn't appear to be a problem because in my test batch, the cementation created little crystals of silver and not re-plating the base metal, and those crystals were easily washed loose and recovered.

This method wouldn't need a large volume of sulfuric acid, just enough to cover the small identical-size batches of plated items each time the acid is poured from batch to batch.

It might even be possible to use it for a much longer period if one can take advantage of silver sulfate's (Ag₂SO₄) rather large temperature-dependent solubility in concentrated sulfuric acid. When cold, sulfuric acid can hold 25g/L of silver sulfate. However, at just below the boiling point of water, it can hold 127g/L of silver sulfate. Since this absolute saturation occurs a few degrees below water's boiling point, it's quite convenient. Once the hot acid is saturated with silver, it could be cooled from 96C to about 10C (about 50F), which would cause about 80% of the silver sulfate to sediment out. Then the cold acid could be poured off to continue using it for new batches of plating, repeating this desaturation of the silver sulfate again and again until the base metal uses up too much sulfuric acid.

So, a single liter of sulfuric acid could potentially process many kilograms of silver plated pieces!

The one thing I don't know is how easily silver sulfate can be turned back to silver metal. Does anyone know if smelting it with an oxidizer or carbon will convert it to silver metal? It decomposes on its own eventually from heating, but only at temps close to its boiling point (1985F, 1085C), which isn't practical. That's the one reason I haven't tried the entire process, as I don't know what to do with the resulting silver sulfate.

Add NaCl to it.
The Silver will drop as Silver Chloride without adding water to your acid, keeping it concentrated.

 

[orvi]

Sulfuric/nitric is a good way to go, if you can source reasonably cheap concentrated sulfuric acid. Advantage is the speed - with 60 °C sulfuric you will go quite quickly. Dosing nitric as it is used up in the process, you can get classic plated stuff (like plated pins, circuit braker parts, or spoons/forks etc.) with 1-3 wt% Ag deplated in 10 or so minutes. Good contact with leaching solution is important, larger pieces can be advantageously put onto copper wire and dipped in.

Just pour the leaching solution, wash the deplated stuff- transfer it into big strainer and spray with water TO THE SEPARATE VESSEL. Then you can take the pregnant sulfuric (still hot) and add new material to it. This can be repeated multiple times with same sulfuric until either junk start to precipitate out or silver sulfate start to precipitate out.
Advantage is that in WARM/HOT concentrated sulfuric acid, silver sulfate is nicely soluble. But solubiility decrease with temperature and also with sulfuric concentration (this is relatively sharp).

After processing the stuff, you can recover by diluting with water and collecting the silver sulfate + crashing out the rest of the silver in solution with brine. Or as silver chloride with bulk.

As it was written by Kurt, base metal (if it is copper or brass, probably also other alloys) will be intact by the acid mixture - allowing you to resell it as is. Also, solder do not pose problem in the process since it usually passivate due to lead sulfate formation on the surface.

 

[orvi]

I'm going to be testing a method with a few pieces of junky silver plate cutlery I have. I've discovered that warm concentrated sulfuric acid does a good job of attacking vulnerable metals relatively uniformly, meaning that it will dissolve the thin silver plating just as readily as the base copper or brass underneath. This was my observation after using sulfuric to process a test batch of small silver-plated e-scrap pieces. The silver went first and then the underlying base metal began to dissolve.

Given that this means only a tiny amount of metal is initially dissolving, it's then possible to pour off the acid once the plating is stripped into another batch of plated items. So for my thinking, it'd be ideal to have many batches of plated items and simply go down the line one after the other. At some point, the warm sulfuric won't hold anymore dissolved silver and it'll start cementing out, but even that it doesn't appear to be a problem because in my test batch, the cementation created little crystals of silver and not re-plating the base metal, and those crystals were easily washed loose and recovered.

This method wouldn't need a large volume of sulfuric acid, just enough to cover the small identical-size batches of plated items each time the acid is poured from batch to batch.

It might even be possible to use it for a much longer period if one can take advantage of silver sulfate's (Ag₂SO₄) rather large temperature-dependent solubility in concentrated sulfuric acid. When cold, sulfuric acid can hold 25g/L of silver sulfate. However, at just below the boiling point of water, it can hold 127g/L of silver sulfate. Since this absolute saturation occurs a few degrees below water's boiling point, it's quite convenient. Once the hot acid is saturated with silver, it could be cooled from 96C to about 10C (about 50F), which would cause about 80% of the silver sulfate to sediment out. Then the cold acid could be poured off to continue using it for new batches of plating, repeating this desaturation of the silver sulfate again and again until the base metal uses up too much sulfuric acid.

So, a single liter of sulfuric acid could potentially process many kilograms of silver plated pieces!

The one thing I don't know is how easily silver sulfate can be turned back to silver metal. Does anyone know if smelting it with an oxidizer or carbon will convert it to silver metal? It decomposes on its own eventually from heating, but only at temps close to its boiling point (1985F, 1085C), which isn't practical. That's the one reason I haven't tried the entire process, as I don't know what to do with the resulting silver sulfate.

Nah, you beat me by 25 minutes

Practically everything you write is done & also I have done. With exception that plain sulfuric does attack copper, but it is rather slow and product mostly isn´t soluble copper sulfate at first, but copper oxide. It is far more useful to dose in nitric acid and stir the contents. NOx evolution is good indicator. And of course the colour of the items, as the change in appearance is very obvious if it is regular copper or regular brass. High zinc brass alloys can be problematic in sulfuric, but you can always test it first.

With crystallization of silver sulfate... It can be done, but from practical standpoint you probably never reach saturation point even at 60 °C. But that is OK - you will just let the pregnant solution cool, covered with plastic wrap - and when you decide to continue, just decant the leachant into other beaker/barrel leaving eventual crystals on the bottom.

As you mention, base metals also dissolve into the solution and there is always some junk present with your material so the solution will eventually get cloudy and coloured. Not a huge deal, but you will carry contaminants along the way. With prolonged use, you will also take quite a bit of water from the air into the acid - what will cause worse etching of base metal and lower silver holdup.

One option can be addition of fresh sulfuric into existing leaching solution. But after 4-5 or so rounds of stripping, I usually called it done and processed it. Sulfuric isn´t that expensive, and my material (plated pins/plated casings of RF equipment/plated whatever from breakers) was always on the more "dirty" side, not allowing to recycle leachant too far.

 

[Alondro]

Add NaCl to it.
The Silver will drop as Silver Chloride without adding water to your acid, keeping it concentrated.

The sodium won't affect it either? Does the concentration of sodium eventually lead to excess salt loading (forming sodium sulfate?) I suppose if that doesn't happen until many rounds of leaching, it doesn't matter.

 

[Alondro]

Nah, you beat me by 25 minutes

Practically everything you write is done & also I have done. With exception that plain sulfuric does attack copper, but it is rather slow and product mostly isn´t soluble copper sulfate at first, but copper oxide. It is far more useful to dose in nitric acid and stir the contents. NOx evolution is good indicator. And of course the colour of the items, as the change in appearance is very obvious if it is regular copper or regular brass. High zinc brass alloys can be problematic in sulfuric, but you can always test it first.

With crystallization of silver sulfate... It can be done, but from practical standpoint you probably never reach saturation point even at 60 °C. But that is OK - you will just let the pregnant solution cool, covered with plastic wrap - and when you decide to continue, just decant the leachant into other beaker/barrel leaving eventual crystals on the bottom.

As you mention, base metals also dissolve into the solution and there is always some junk present with your material so the solution will eventually get cloudy and coloured. Not a huge deal, but you will carry contaminants along the way. With prolonged use, you will also take quite a bit of water from the air into the acid - what will cause worse etching of base metal and lower silver holdup.

One option can be addition of fresh sulfuric into existing leaching solution. But after 4-5 or so rounds of stripping, I usually called it done and processed it. Sulfuric isn´t that expensive, and my material (plated pins/plated casings of RF equipment/plated whatever from breakers) was always on the more "dirty" side, not allowing to recycle leachant too far.

What about converting the silver sulfate that DOES precipitate out? My first test did definitely leave me with a fair percentage of it. What process works best for reducing it to silver?

 

[snail]

There was thread near the beginning of the forum about that process. He stripped in heated Sulfuric and nitric then let it cool overnight.

He poured off the liquid leaving the crystals behind. Made a small nitric addition as needed and reused the acid.

When he was done he used copper to cement the silver out of the container of crystals.

I still prefer electrolysis with sodium sulfite solution. No strong sulfuric to deal with. Not fast but not near as dangerous. Gives you a loose silver sludge that plates out on stainless and settles to the bottom of the container.

 

[snoman701]

There was thread near the beginning of the forum about that process. He stripped in heated Sulfuric and nitric then let it cool overnight.

He poured off the liquid leaving the crystals behind. Made a small nitric addition as needed and reused the acid.

When he was done he used copper to cement the silver out of the container of crystals.

I still prefer electrolysis with sodium sulfite solution. No strong sulfuric to deal with. Not fast but not near as dangerous. Gives you a loose silver sludge that plates out on stainless and settles to the bottom of the container.

But don't you feel like the electrolysis doesn't pull it uniformly?

When I tried it I would end up with entire areas that were still plated, while I also had other areas that seemed to sit long enough that I"m pretty sure it was eroding the base metal.

I always thought it would work really well with a circulating pump and bag filter.

 

[snail]

The process is not perfect. It works better when the substrate is copper. When the solution is getting oxidized to sulfate it starts to attack brass. I have over time received poor quality sodium sulfite that oxidized much sooner than batches from other suppliers.

I processed a few military surplus bus bar and contractor parts and they stripped very clean with no attack on the substrate. With time all the corners and shadowed areas stripped.

You are right that it is not always removing everything. It does occasionally miss spots but removes the vast majority of the silver. My local scrap dealers have never complained about the condition when completed other than the white metal trim and handles which I usually remove.

I have always used the scrap value of the base metals to make my price decisions. That way worst case the scrap could be sold as is without loss.

I am working at a hobby level. For me it is the best option. I tried concentrated Sulfuric and Nitric 15 years ago. It worked, but safety and disposal concerns made it not worth continuing for the small return in my situation.

I was glad to discover the sodium sulfite method to help me clear my 15 year accumulation of estate and thrift store finds.

I have noticed a large variation it the silver content. Some newer items strip for me in an hour or two. Some older and higher quality items have taken 36 hours. That has made me selective in my purchases.

 

[anarxi]

In solutions involving dissolving the coating without dissolving the base, it is very important that the thickness of the dissolving layer is more or less the same on all parts....

 

[Dreamer]

I attempted to electrowin my impregnated solution using a graphite rod as an anode and a stainless cathode thinking the silver would crystallize on the cathode. RESULTS: Graphite rod dissolved and I have a conductive chocolate slurry. I'm planning on just using SS for both next time, but will I need a bag or will the cathode hold the crystals?

Any suggestions would be appreciated. Thanks!

 

[Dreamer]

And yeah, I'll just recover the values in my failed experiment by conversion to AgCl but what about the AgS ? Never delt with it before. Thanks!

 

[anarxi]

just pour hot water, definitely not chlorinated, into the container where the silver sulfite is located.
sulfite dissolves in more hot water than in cold water
(you can find the table on the Internet).
Then pour the water with the sulfite dissolved in it into salt water.
chloride is obtained....
this is the easiest way.
but it uses a lot of water.