Something went wrong with silver/palladuim solution

[rtm]

Hi! My first post here. Excuse my bad English.

I`m trying to recover silver and Pd for first time.
So.. i process this in picture 1 with nitric acid.



Those are from old Bulgarian and Russian contactors and other silver plated conectors( they may contain some palladium).

Base metals are tin, copper, bras and maybe iron(rusty contactors).

1. I separate the liquid from undissolved particles. There was some sludge.
2. Neutralization with urea (dissolved in tab water)
3. I started to filter the solution. It was slow, so i left it over the night.

When i get back this morning there was shiny particles in the filtered and in the not filtered solution.
My first thought was there is too much urea, so i add some hot water, but there was no effect.
Then i pour out the filtered solution and on the bottom i see some silver sludge.
Is this really silver and what was wrong with the solution so it start the precipitation of the silver.
And what should i do next?
















My idea WAS to drop Pd with dimethylglyoxime (CH₃CCCH₃), after this to drop silver with copper. And after i get some amount to refine the silver and Pd again for best quality.
But in this case i think to cement(drop) the silver and the palladium with copper and then refine again.
Any ideas?

 

[butcher]

Did you DE-solder the contact points from the silver plated bus bars and dissolve just the contacts, or did you put everything in nitric?
The large amount of base metals in then would tend to use large volumes of nitric acid, and would tend to cement values out of solution as they dissolve, some of the base metal like the ferrous may not dissolve well in nitric (depending on its concentration).
There is no reason to use urea here.

 

[rtm]

I`m separating only gold plated stuff because i don`t have enough time for everything . And here the nitric is cheap, only 2 euro for 1L 58%.
So, yes i put everything in nitric (what appears to be mistake).

Yes, not all ferrous was dissolved, but they were separated 2 days before i started to filter.

12 hours after neutralization the nitric it started to cement.
Ok, too much base metals and they started premature cementing.

Why there is no reason to use urea? If i cement silver with copper it will "eat" it back or i`m wrong?

 

[butcher]

Even if nitric is cheap dissolving the buss bar makes no sense, it wastes acid and can put metals into solution that can give you several different problems.
There is not enough silver on the plated buss bar to worry about in this process (if you want to recover silver from the copper buss bar save them up and run them in a copper cell).

The steel you pulled out of the process will most likely hold some of your silver.
The filtering problem could have been tin if bronze was involved.
Brass will have zinc which is also a very reactive metal.

If you do not overuse nitric, and do not have an excess of nitric acid in solution after all metals that will, are dissolved, you basically make a nitrate metal salt solution with very little excess nitric acid in solution,.
For example copper nitrate and silver nitrate in solution, by cementing with copper the silver is pushed out of solution (cemented), and the copper goes into solution.
The copper displaces the silver, silver ions in solution gain electrons from the copper metal to make silver atoms that join to make silver metal powders, the copper loosing its electrons goes into solution as copper ions (or a copper nitrate salt solution), without excess nitric you are just trading copper for silver, and no excess copper dissolves.
Using urea would defeat what your trying to do, you want the solution slightly acidic.
Urea also can make a dangerous solution of urea nitrate.
Urea is good for fertilizer, for recovery or refining of metals is too dangerous and basically useless in most all cases.

 

[rtm]

Thank you for your detailed answer.

 

[Lino1406]

Nitric acid should cover tin with tin dioxide and dissolve all other excluding iron.
Check Pd by DMG
Then add salt to precipitate Ag