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I'm working out the best process to chemically strip silver brazing from tungsten carbide (WC) bits. My main objective is to recover the silver as a relatively pure metal. When I use $\ce{HNO3}$ at anywhere from 20-70%, it dissolves the solder and I end up with a red solution. When I try to use cementation (putting copper metal in the solution) to precipitate the $\ce{Ag}$ as sponge, it isn't working.

Can any one help me figure out what's going on here? The brazing being used should be 50-60% $\ce{Ag}$ by weight. I'm not sure yet if there is $\ce{Pd}$ or not.

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    $\begingroup$ Well, to start, if you're trying to recover silver for its value, it's probably not economical unless you have to strip hundreds of bits—silver's not worth a whole lot. Having said that, what did you actually observe? The reaction is pretty slow; it could take several hours. Also, if the silver concentration is low and you use a lot of copper, the reduced silver deposited could be quite thin. $\endgroup$ Nov 1 '14 at 4:19
  • $\begingroup$ Also, keep in mind that copper can itself be oxidized by concentrated nitric acid so you might be better off concentrating the solution to get silver nitrate crystals, then dissolving those to use with copper. $\endgroup$ Nov 1 '14 at 4:29
  • $\begingroup$ I add 35% HNO3 to fresh WC bits and I end up with a reddish/magenta solution. If I boil off all the liquid, I get a dark purple (like potassium permanganate) thick liquid before it finally hardens into a black mass. When I add dist. H2O to the black mass and put clear copper metal, the solver does NOT precipitate. When I do this same process on a smaller scale I can get the silver to stick to the copper as sponge. $\endgroup$ Nov 2 '14 at 5:34
  • $\begingroup$ You don't really want to boil all the liquid down, just remove enough water to start precipitating silver nitrate. I'm not quite sure what the issue could be, but I found this: crscientific.com/article-silver.html which seems reasonable. $\endgroup$ Nov 2 '14 at 6:04
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    $\begingroup$ I think the red color is coming from the cobalt binder in the cemented WC (it's 6-9% Co). As the HNO3 is consumed, the Co more readily goes into solution. If I use >50% HNO3, the solution stays clear or brown until the HNO3 concentration is lower and it can more readily attack the Co. The silver braze itself has Ag (50-60%), Cu (15-33%), Sn, Zn, and may be Cd, Ni, and Mn. There should be a number of nitrate salts in solution. There is something preventing the AgNO3 from precipitating as a metal when I put copper metal in the solution. Any ideas? Thanks for all the info so far!!! $\endgroup$ Nov 3 '14 at 1:08
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Most common silver brazes are 50 to 90 % silver with copper or maybe zinc. A complication may be the WC; it is sintered with 2 to 20 % cobalt ( usually).

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No expert here, but it seems that if the copper added becomes coated with a monolayer of silver from your very complicated solution, more silver may not deposit as it does from pure solutions of silver nitrate.

One way to keep silver depositing would be to use two copper electrodes and electrolyze your solution with a relatively low voltage. The most inactive metals (Ag, Pd) will plate out first onto the negative electrode, and the positive electrode will lose an equivalent amount of copper. Finally, your solution will have been depleted of silver and enriched in copper, and the silver will be on the negative electrode. If you start with a silver wire, you will have a purer result, but that may be unimportant. If you electrolyze too fast, you will plate out copper with the silver.

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