# Black residue from nitric digest of silver



## jsargent (Aug 8, 2009)

Been doing some experimenting with using three nines pure silver as collector metal on various ore concentrates. After prolonged reaction in a scorifying dish using a simple borax/soda ash flux and finely powdered concentrate, I pour the silver to an iron mold, let it cool, clean it and dissolve it in 20% nitric acid. I nearly always get large amounts (2-4grams per ounce of silver) of heavy, black residue that is totally insoluble in any concentration of nitric acid at any temperature. If I heat the residue to about 600C, it sometimes turns gold but sometimes it goes to a brilliant silvery metal. Now the gold is obviously gold, but what about the silvery metal? 

Is there ANY metal other than gold and platinum that this residue can be?


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## butcher (Aug 8, 2009)

only a guess here,if the ore is from sulfide rock and sulfides were not roasted free from it,driving off the sulfuric, you may be seeing silver sulfide (black tarnish on silver is silver sulfide). :?:


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## jsargent (Aug 9, 2009)

butcher said:


> only a guess here,if the ore is from sulfide rock and sulfides were not roasted free from it,driving off the sulfuric, you may be seeing silver sulfide (black tarnish on silver is silver sulfide). :?:



Would it be possible for silver sulfides to survive being dissolved into molten silver then being parted out of the soldified silver with nitric acid?


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## Juan Manuel Arcos Frank (Aug 9, 2009)

Jsargent:

Indeed!!!!!!!....silver sulfide melts without change so you will never see metalic silver.However,
we have to learn how to deal with silver sulfide because it is the most important ore to produce metalic silver and all silver compounds can be converted to silver sulfide...Here are three tips from my experience:

1.- Melt the silver sulfide with potassium nitrate in 1:3 weight proportion until you get a nice silver nugget.

2.- Convert the silver sulfide to silver chloride,just add AR and boil for a while (do it outside because toxic fumes will be released),white clumps of silver chloride will form and you can get metalic silver with zinc/sulphuric acid method.Ferric chloride will work too,instead of AR.

3.-Silver sulfide is extremely insoluble...but you can dissolve it with a strange complex salt.The complex salt consists in sodium thiosulfate(common photo fixer) with a pinch of copper sulfate and tap water...so add the silver sulfide,mix for a few minutes and the silver sulfide will be dissolved then you can recover the silver with the common methods.

Probably the black residue comes from soda ash flux...change it for potassium nitrate.First add the borax and then melt the metal,when it is liquid then add some potassium nitrate

Have a nice day.

Manuel


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## butcher (Aug 9, 2009)

AHH Manuel to the rescue not only one recipee but 3. 
 
thanks.


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## jsargent (Aug 10, 2009)

Juan Manuel Arcos Frank said:


> Jsargent:
> 
> Indeed!!!!!!!....silver sulfide melts without change so you will never see metalic silver.However,
> we have to learn how to deal with silver sulfide because it is the most important ore to produce metalic silver and all silver compounds can be converted to silver sulfide...Here are three tips from my experience:
> ...



Good thoughts all, Manuel. I wonder though, if it was silver sulfide, passing from the ore concentrate, thru the silver collector metal unchanged at 1100C, and emerging as black residue upon nitric digestion of the silver, would it not remain as a black residue when collected and heated to only 850C at the end of the process? Maybe I'm missing something here?


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## Juan Manuel Arcos Frank (Aug 11, 2009)

Jsargent:

Good point...let us find out if that black residue is silver sulphide.Put some into AR,boil the solution for a while,let settle down,pour off the liquid,filter the precipitate,wash it three times with tap water.Add some sodium thiosulfate or ammonia,dip a piece of copper(i.e.,a common copper wire),if it gets plated then the black residue is silver sulphide.

Regards

Manuel


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