Sciencemadness Discussion Board

Purefy Silver with Acetic Acid

Takron - 21-9-2011 at 22:23

I have some silver that I was refining and when I put in my bits of copper to precipitate my silver out of solution, I forgot to neutralize my solution and it erupted all over as the nitric reacted with the copper bits. As a result I have some heavy copper contamination in my silver and to help keep the already rising cost of this refining down. I would like to try and dissolve the copper contamination with hydrogen peroxide and acetic acid. I figure I can purify the silver and get useful copper acetate out of it, but my concern is will that mixture dissolve any of the silver, as I know acetic acid is used in making silver acetate.

ldanielrosa - 21-9-2011 at 23:56

Copper is a common alloying element with silver, and a common impurity. Your first idea is a good one, you just have excess nitric acid. There aren't many elements between silver and copper on the activity series, so I see a few options from that standpoint:

Add more junk silver until the RXN stops, then begin the precipitation with copper.

Cook off the excess nitric acid- dangerous fumes! Then begin precipitation with copper.

Add copper until RXN stops, then add more copper to start precipitation.

You would have that "heavy copper contamination" during the precipitation process anyway, as copper displaces silver from the solution.


As for the acetate idea, that will add ions to the soup and may give you more grief. Granted silver acetate has poorer solubility than copper or silver nitrate so it will likely come out first, but as always you should examine all possible products before you commit to this.

Changing boats midstream often involves complications that may have been avoided with a single path. I think I see that you want to recover the silver for the value, and don't want to waste potentially expensive copper- but remember that you can precipitate copper from the waste with another metal like lead or zinc. Also copper uses up the excess nitric acid much faster gram per mole than silver.

AndersHoveland - 22-9-2011 at 00:06

Yes, white vinegar mixed with hydrogen peroxide will slowly dissolve silver.
The reaction is not very efficient, because the silver ions that form in solution catalyse the decomposition of much of the hydrogen peroxide into oxygen.

If you have a solution that contains silver nitrate, copper nitrate, and nitric acid, you can simply add a small quantity of zinc. Silver will be the first to begin to form on the zinc. The zinc should be removed just before copper starts to form. Remove the remaining piece of zinc, which should have crystals of silver adhering to it, and dissolve in dilute hydrochloric acid, to dissolve away the remaining zinc and leave silver. If you want to dissolve away the copper, without dissolving the silver, since both these elements are now separated from their original mixed alloy, you can use Fe(NO3)3.

some relevent reduction potentials:

Ag+ + e- ---> Ag............. +0.80v
Fe3+ + e- ---> Fe2+....... +0.77v
Cu2+ + 2e- ---> Cu........ +0.34v

(presence of chloride ions changes the reduction potentials)

Or perhaps just add some baking soda, to neutralise the nitric acid, then add more copper.

You might also do some reading in this forum:
http://goldrefiningforum.com/

[Edited on 22-9-2011 by AndersHoveland]

barley81 - 22-9-2011 at 09:41

You can neutralize the solution until about pH 2 or so with baking soda/washing soda/lye. Then precipitate the silver with excess chloride-containing salts (table salt works). Boil it for a bit to coagulate the silver chloride/increase crystal size and let it cool. Then decant off the supernatant, rinse with water, dry, and heat/melt the powder to get metallic silver (chlorine is evolved).

unionised - 22-9-2011 at 10:58

Silver chloride boils at about 1550C.
Just heating it won't get you silver (chlorine is not evolved), though you can recover the silver by heating the chloride with borax or caustic soda.

Silver acetate is not very soluble.
You can precipitate a lot of the silver from a solution of, for example, the nitrate, by adding sodium acetate.
You can cautiously heat that acetate to get silver or you can treat it with sodium chloride solution and get silver chloride which you can heat with alkali to get metallic silver.

barley81 - 22-9-2011 at 11:45

Quote: Originally posted by unionised  
Silver chloride boils at about 1550C.
Just heating it won't get you silver (chlorine is not evolved), though you can recover the silver by heating the chloride with borax or caustic soda.

D'oh. I thought it would. Sorry for misinforming.

not_important - 22-9-2011 at 12:32

It's been sad many times before in other threads (UTFSE), either

A) ppt AgCl, wash, reduce to the metal with glucose+NaOH

or

B) take the impure AgNO3 solution, evaporate it to dryness then heat it past its melting point with stirring to decompose copper nitrate, cool, dissolve in water, filter. Repeat as needed to fully eliminate copper.


Details in those earlier threads.

Takron - 22-9-2011 at 13:57

Yeah, I know about the refining of the silver and whatnot. I was trying to avoid that refining the silver step and just eliminate some of the copper powder that is contaminating my silver powder instead of having to use more nitric acid to re-precipitate the silver. I was just trying to think of a way to just take out some of the copper and leave the silver behind. I appreciate all the great ideas though.:)

not_important - 22-9-2011 at 14:53

Formic acid__may__ do the job, it' s used to remove the surface film of copper formed when old corroded silver or silver-coated objects are restored. But it's tricky, Cu and Ag are similar enough that most things that dissolve one will do so for the other.

The alternative is to melt the metal and cast it into an electrode, then do electrolytic refining like the big boys.

Takron - 22-9-2011 at 15:44

Quote: Originally posted by not_important  


The alternative is to melt the metal and cast it into an electrode, then do electrolytic refining like the big boys.



I was thinking of doing that but I haven't done electrolyzing yet and I heard that too much copper in your product over contaminates your electrolyte and can cause problems.

not_important - 22-9-2011 at 16:29

If the metals are finely divided you could try heating them in air at a temperature above the decomposition point of silver oxide. A thin layer with good access to air, and stirring occasionally, should oxidise metallic copper. After some time let cool, and extract with acetic acid, which will attack and dissolve CuO. After that then go for electrorefining the silver to polish it up.

Takron - 22-9-2011 at 16:45

Quote: Originally posted by not_important  
If the metals are finely divided you could try heating them in air at a temperature above the decomposition point of silver oxide. A thin layer with good access to air, and stirring occasionally, should oxidise metallic copper. After some time let cool, and extract with acetic acid, which will attack and dissolve CuO. After that then go for electrorefining the silver to polish it up.



I like that idea. I will definitely try that.