# My silver powder won't dissolve in nitric acid



## marethyu (Jun 9, 2018)

I have recently refined a batch of silver powder by dissolving 300 grams of sterling silver chains in nitric acid
adding sodium chloride to precipitate silver chloride
adding sodium hydroxide to convert the silver chloride to silver oxide 
adding sugar to reduce the silver oxide back to silver metal
washing with hydrochloric acid to remove copper and nickel hydroxides 
vacuum filtering and washing with copious amounts of water
and then oven drying the precipitate
After oven drying the silver powder for 4 hours at 500 degrees I was left with lumps of what seemed to be dry silver but the weight came out to 325 grams which I found very odd considering there was 300 grams of metal initially and there should only have been 277 grams of silver metal. I just assumed there must have been a bunch of water trapped in the silver that was struggling to boil off due to the extremely small particle size of the powder that made up the huge lumps of silver after it was oven dried.
Today I attempted to redissolve the silver lumps with nitric acid to make silver nitrate. I used an excess of azeotropic nitric acid that I believed to be contaminated with a small but still significant amount of ammonium nitrate. Very little silver would go into solution so I decided to filter off the acid and rinse the remaining silver powder and start again. I now put the silver powder into a beaker with some very pure nitric acid with about an 85% concentration but even after hours of refluxing the powder in the acid it a large amount of silver remains.
Help!?


----------



## FrugalRefiner (Jun 9, 2018)

Did you add any distilled water to your nitric when you tried to redissolve the silver?

Dave


----------



## Topher_osAUrus (Jun 9, 2018)

Maybe turned some silver back into silver chloride with the HCl rinse.
Or did not convert it all to silver in the first place.

Or Dave is right

Maybe even all of the above.


----------



## Geo (Jun 10, 2018)

Try to melt a sample. If it melts clean, you can run it through a parting cell. If it turns into a black syrupy mess, there's silver chloride in the mix.


----------



## marethyu (Jun 10, 2018)

I tried melting some and was left with the black syrupy liquid you described. I guess I'll have to start from scratch. How do I convert the black silver compound back to a dissolvable form?


----------



## archeonist (Jun 10, 2018)

If you still have AgCl just repeat the process of adding lye and suger. This time be sure you convert all of your AgCl.


----------



## 4metals (Jun 10, 2018)

As much as I love the caustic / sugar process to convert silver chlorides, the single biggest problem comes from insufficient mixing. I do large batches of silver chloride in poly cement mixers and I constantly reach in with a long sleeve rubber glove to break up clumps with my hand when I feel them. This is especially true if the chlorides have been sitting around exposed to light or have been allowed to dry out a bit. If you mix it up well and provide the proper ratio's of caustic and sugar this method can provide you with some high purity silver.


----------



## baruch6034 (Jun 10, 2018)

We are producing large amounts of silver this way every week.
From my experience you did not complete the process turning the Silver Chloride to silver that means you did not have the right amount of lye and sugar.
You can not dry Silver Chloride and then use this to use Lye and sugar to complete the process.
You need to try and brake it as much as possible and melt it with borax and soda and this needs to keep at 500 degrees in order to let the chemical process to happen then increase the temperature in order to turn the Silver Chloride and the Silver metal in silver and pure it to a bar you will have big amounts of fluxes on top.


Sent from my iPhone using Tapatalk


----------



## kernels (Jun 10, 2018)

As per FrugalRefiner's post, did you dilute your HNO3 ? you only want your HNO3 to be at 35% or so to dissolve Silver.


----------



## FrugalRefiner (Jun 10, 2018)

I'll take a guess that you may have a couple of problems.

First, based on the weight of your starting metal, the silver you expected, and your final dried precipitate weight, you have obviously introduced some contaminant. Most likely, it is unconverted silver chloride. So your original conversion probably gave you a mixture of metallic silver and unconverted silver chloride.

When you tried to dissolve your precipitate, you said


> Today I attempted to redissolve the silver lumps with nitric acid to make silver nitrate. I used an excess of azeotropic nitric acid that I believed to be contaminated with a small but still significant amount of ammonium nitrate. Very little silver would go into solution so I decided to filter off the acid and rinse the remaining silver powder and start again. I now put the silver powder into a beaker with some very pure nitric acid with about an 85% concentration but even after hours of refluxing the powder in the acid it a large amount of silver remains.


I asked if you added any water, but you didn't answer. Concentrated nitric acid, either azeotropic (~68%) or 85%, is too concentrated. Not much silver will dissolve because the water is already pretty saturated with HNO3. 

If you didn't add water, try another leach with a 50/50 mix of azeotropic nitric and distilled water. Anything left is probably silver chloride.

Dave


----------



## nickton (Jul 12, 2018)

passivation?


----------



## Geo (Jul 12, 2018)

When you are dissolving a metal or mix of metals that is either oxidized so quickly that it forms an oxidized layer, coating the metal and preventing further attack of the metal by the acid or, a metal is present that is not attacked by the acid you are using. If the non-reactive metal is present in enough quantity, it builds up in a layer, coating the metal and preventing further attack by the acid. Either of these is called passivation. As an example, nitric acid and aluminium. The nitric acid oxidizes the aluminium surface creating a layer of aluminium oxide preventing the nitric acid from attack the metal further. Highly concentrated nitric acid may not have much effect on bare copper metal for this reason.


----------

