# Unexpected reaction.....



## Etfonedhome (Sep 4, 2017)

this was originally a reply to an ongoing thread, but after nearly a week with no replies, I decided to make it a new thread, and delete the previous comment. 

Okay..... well, after many hours here, and reading hokes, I decided that I was just foolish enough to try this! I ran into an interesting result, and I'm not sure where to go from here..... ive spent about 5 hours searching here, as far as i can tell, this hasn't been covered before, but common sense tells me I'm probably missing something. Here goes....

The composition of the base material was 33% Ag, 36%Cu, 29%Zn, 2%Sn, and 0.1%Si

In a 1liter open beaker, I added 25g silver solder, cleaned with 70% isopropyl alcohol, well rinsed. (It had some soldering flux plus general grime from sitting around a welding shop).

To this I added 150ml deionized water, and 150ml 70% nitric, in 25-50ml increments. The solution wasn't externally heated. All went as expected. 

When the reaction slowed, I added more nitric. The last dose of the first 150ml had no reaction. Heres where it first started to get interesting. There was still around 7 or 8 grams of undigested solder. 

I first thought my solution was saturated, so i added 200ml water, and another 150 total nitric in doses, while also pouring boiling water in a Pyrex dish around the beaker. Each add gave a small reaction but nothing dramatic. I could not get that last few grams of solder to digest! 

At this point, I realized I had WAY too much nitric, and decided to add more solder. THAT was fun. By far the most violent reaction of the day!

So I added in more solder, ( now at 200g total) when the reaction stopped completely, no fumes, no bubbling on undigested solder, I added deionized water to the 1 liter mark, to cool and dilute.

Gave it a stir, poured off 500 ml to start filtering. This is where it REALLY got interesting.....

I set my pretty bright cloudy BLUE solution aside, while i cleaned my funnel, folded filters, updated my notes etc.... 

I come back to my beaker to find a bright emerald green solution, with a white precipitate with silvery sparkles. after a few moments of utter confusion, I filtered this, dropped in a clean bar of copper, and nothing happened. The bar turned a bit grey, but nothing formed...

Out of curiosity, I dipped the bar into the original solution that still had undigested solder in it, same result. 

Im wondering if the copper in solution is somehow dropping the silver out without the need for the cementing step? 

Im on pause here, mostly because I have no clue how to proceed.... 

Any advice, or reference to information I can read myself would be well appreciated.


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## Etfonedhome (Sep 4, 2017)

Pictures of the green filtrate, the original solution, and the copper bar.


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## Lino1406 (Sep 5, 2017)

The white precipitate is your silver, in the form of chloride, mixed with tin dioxide


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## snoman701 (Sep 5, 2017)

Why is this? I have the same problem when working with a similar solder. 

I can't figure out where the chlorides are coming from.


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## Lino1406 (Sep 5, 2017)

Chlorides consist 50 - 500 ppm of tap water, but in the present case I would add more chloride to have more silver out (more worried about the tin dioxide)


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## snoman701 (Sep 6, 2017)

He used distilled though


Sent from my iPhone using Tapatalk


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## g_axelsson (Sep 6, 2017)

Take a glass of water, add some table salt to it and dissolve it so you are left with a clear solution. In this, drop a few drops of your solution of the solder. If there is any silver present you should get a white precipitate of silver chloride.
This is the easiest way to test for silver. Mercury would also give a white precipitate but that is never used in solder.

I've experienced once when cementing didn't work. An impenetrable surface of silver formed on the copper bar and cementing stopped. I don't remember what I did to get around it, probably added a dab more of nitric acid to penetrate the silver surface and start the cementation.
If the silver is there but doesn't cement on copper then another way of recover it would be to go the silver chloride path.
http://goldrefiningwiki.com/mediawiki/index.php/Silver_chloride

Göran


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## Etfonedhome (Sep 14, 2017)

But why? I used deionized water, no chance of chloride there.

Where could the chlorides be coming from?


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## g_axelsson (Sep 15, 2017)

Before worrying about chlorides in your water, do the test I suggested and see if you have any silver in solution first.

If you have silver nitrate then there is no more chlorides in your silver solution and you can save some of it as a test solution to check your water supply. A drop of silver nitrate in a glass of water will reveal any chloride present.
Maybe you will find out that your deionized water isn't as good as you thought.

But even if you used water contaminated with chloride it shouldn't rob you of all your silver. It should only create some silver chloride that eventually settles and the rest of the silver should end up as silver nitrate in solution.

If you don't test then you are only guessing.

Göran


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## nickvc (Sep 15, 2017)

Looking at your piece of copper it seems as if the reaction stopped as it got encased in silver, try reversing it and see if more silver will cement out.


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## Etfonedhome (Sep 18, 2017)

I tested before I posted, no silver in solution, no chloride in water. 

Nick, there was no reaction to stop. No silver ever cemented to begin with.

Adding a piece of low carbon steel to the green filtrate dropped out a lot of what looks like copper.


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## Etfonedhome (Sep 18, 2017)

So an update. 

After this debacle, I dumped the precipitate back into the original beaker that I first started with, I added some more nitric to dissolve the precipitate, (I was worried about metastanic from the tin) and everything went as expected. The silver cemented out on the copper normally, all was well. 

Started a new batch, same problem.

Basically I have to filter off about half the solutuon, dump the filtrate to a stock pot, then re-digest everything to get the desired reactions..... 

There has to a better way....


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## g_axelsson (Sep 18, 2017)

So what you are saying is that at first your copper get covered by something that stops the cementing action?

I've run into this once. I don't know why or remember exactly what I did to get around it.

There might be something in the solution that creates a smooth silver surface, that would shut down the cementing action and limit the silver surface to quite a thin layer. Like ENIG is self regulating in thickness.

You could try to dilute your solution or add a bit more nitric acid to get the pH lower, that might help the cementing on it's way.

If you find a solution or a reason for your problems, please let us know.

Göran


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## butcher (Sep 18, 2017)

Combination of concentration of the salt solution, and no free nitric acid, might have been a problem?
There was not any free nitric, the copper atoms on the surface gave electrons to silver ions, the silver ions reduce to silver metal (coating the copper), the silver begins to coat the copper shielding the copper from fresh silver ions to come into contact with the copper atoms.

No free nitric, basically low acidity, (mainly a silver ionic salt solution) the reaction may have stopped, where if there was some free nitric it could also attack some of the surface copper, forming some copper ions from the acid in solution at the same time silver ions are also taking electrons from the copper (possibly keeping the silver ions from forming a protective coating on the exposed copper bar.

Cementation works better when the solution is acidic or when some free acid is involved, dilute solutions seem to do better than concentrated salt solutions.


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## Etfonedhome (Sep 19, 2017)

g_axelsson said:


> So what you are saying is that at first your copper get covered by something that stops the cementing action?



No, I'm saying, there was no silver in the solution when i added copper. 

When I added a copper bar, absolutely nothing happened, when i removed the bar it was still bright shiny copper. The grey in the picture appeared after the bar sat on my desk and dried.

When nothing happened with the copper, I tried testing with salt water, again, nothing happened. 

The silver precipitated out of solution all by its self. The problem is, this happened before I could filter out the tin.

When I added steel, copper cemented, so i filtered, discarded the copper laden filtrate to a stock pot, then digested the precipitate in fresh water and acid, then everything behaved normally.

Same exact process in the 2nd batch. 

Is it possible the copper content in solution is somehow cementing the silver even though it is in solution, rather than a solid bar?


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## Etfonedhome (Sep 19, 2017)

P.S. 
I appreciate your time and responses, I'm not trying to be argumentative, rather I feel I'm not explaining well what actually happened. 

the best way I can explain what happened is this, as soon as the acid digestion stopped, the silver dropped out of solution immediately, and without any action taken.

I've tested my water, (deionized, purchased from a very reputable local lab supply shop), no chlorides are present. 

The only thing I can think of is my base material is 33% silver, and 36% copper, and somehow that copper is reacting, is that possible/does it even make sense? 

If I have to digest everything twice, this is a losing scenario, is there another way to separate the metastanic acid from the silver precipitate?


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## g_axelsson (Sep 19, 2017)

It sounds like you used too little nitric acid to fully dissolve everything. Copper in solution can't cement any silver, it is already oxidized.
When the nitric acid ran out the silver in solution cemented back onto the undissolved metal so in the end all the silver came out of solution while base metals like tin and copper went into solution.

When I have copper and silver mixed (laminated contact points) I usually do a similar trick. When the reaction is over but there is still metals left I check if there is any silver in solution. If there isn't any silver then I can just discard the solution, the rest of the metals gets another dose of nitric acid treatment and when it's time to cement it is from a much cleaner solution.

The same trick can be used when dissolving ceramic CPU:s, add too little AR to dissolve all the metals, then the gold cements onto base metals and a barren liquid can be decanted off. In the end only a small amount of base metals are present when the gold is precipitated, resulting in a cleaner drop.

Göran


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## g_axelsson (Sep 19, 2017)

By the way, a gram of copper takes a lot more acid to dissolve than a gram of silver, close to three times as much if I remember correctly.

There is a number of different ways to refine the silver, but dissolving it in nitric acid and cementing on copper is the easiest way.

You can smelt it with oxygen sparging to remove the tin, then mix in some more copper and run it in a copper cell, The silver will end up in the anode slime.
This is what I would recommend for large scale refining but not for an amateur level refiner.

In the end it all boils down to what you have to pay for chemicals.

Göran


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## kurtak (Sep 20, 2017)

g_axelsson said:


> By the way, a gram of copper takes a lot more acid to dissolve than a gram of silver, close to three times as much if I remember correctly.



Goran 

Its actually more like 4 times more silver then copper --- a gallon (3.78 liter) of 67 - 70% nitric will dissolve "about" 8 pounds of silver compared to only "about" 2 pounds of copper

I underlined "about" because just how much metal a given amount nitric will dissolve depends somewhat on the conditions of the reaction when dissolving the metal

Kurt


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## Etfonedhome (Oct 4, 2017)

Ok, thanks for the info and tips guys, looks like I'm just gonna need to find a cheaper nitric supply. Any advice there? I've been using Hach, but at $100/ liter, there's no way I can digest everything twice and come anywhere near break even, which is my absolute minimum goal. 

Obviously even a small profit would be nice, but I'm ok with break even and the accomplishment of DIY and learning something new. 

Besides, at breakeven with today's spot price I can always sit on my PM for a while and wait for it to go up!


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## Topher_osAUrus (Oct 4, 2017)

Wow.
Pricey

Dudadiesel.com
10L $250


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