# Silver dissolution issues



## inspector071 (Sep 28, 2014)

I have a little over 700g of a roughly 80/20 Ag/Cu alloy that I'm trying to recover the silver from. The alloy seems to have been part of a little metallurgy experiment, as much of the metal was inside quartz test tubes and quartz crucibles. The tubes were labeled with the corresponding alloy, so I'm assuming copper and silver are the only two metals present. In a 2L beaker I added the metal to roughly a liter of 2:1 70% nitric acid to distilled water. The reaction commenced, forming plenty of nitrogen dioxide. Unfortunately, I let it get a little too hot and a little of it boiled over. As the reaction calmed down, the solution began to turn opaque white, forming a dense and thick precipitate that settled slowly. The precipitate would best be described as a sludge. It is unfilterable with Whatman GF/C and hardened 50 filters. After settling for a week, the precipitate fills up about half of the 2 L beaker. I had expected this to be an easy process: dissolve the silver and copper, be left with a clear blue solution, then cement out the silver with copper. I siphoned off some of the clear, light blue solution after letting it settle and attempted to cement out any of the silver using copper. Indeed, the solution contains silver nitrate as silver cements out, however, there seems to be excess acid as the silver redissolves as soon as I remove the copper. I took a sample of the sludge and attempted to dissolve it in hot distilled water. It did not seem to dissolve at all. While all of this sounds similar to silver chloride, no source of chloride has been used anywhere near this. And certainly not enough chloride to produce this much silver chloride. Any thoughts on what this sludge is and why it formed? Also, what is the best way to remove excess nitric acid from a silver nitrate solution?


----------



## solar_plasma (Sep 28, 2014)

Obviously there was at least a third metal in that alloy. Your description let me believe it could be tin.

Refining of silver is at least as special as the refining of gold, so be sure to have read all posts about it. You say HNO3 to H2O was 2:1, do you mean, you used 2 parts HNO3 and 1 part H2O? If so, it has been too concentrated and you got much more fumes than necessary. Also here, you would use only as much HNO3 as needed. The obtained nitrate solution should only have a very slight excess nitric before cementation. Obviously you had a large excess of nitric, since the silver redissolved.


----------



## nickvc (Sep 28, 2014)

Could be tin.


----------



## justinhcase (Sep 28, 2014)

Are you sure you used Distiled water??Some shop's have been known to refill with tap water. 
It sounds to me like you had some chloide in your solution and have turned your silver to AgCl ,
you get a lot for only a little Cl which is why it is such a good test.
Even if you scrub a flask very well there is still a large reaction with any trace's ,so I keep two sets of glass one for silver only..


----------



## solar_plasma (Sep 28, 2014)

For the dissolution process read:

About the different equations when using different concentrations of nitric:
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=50&t=18598#p187350

More important notes:
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=37&t=7021
http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=895

@justinhcase
very good point! most probable!


----------



## kurtak (Sep 28, 2014)

The OP said the precipitate is going through filters - that puts tin in the running as a suspect

if its AgCL a little time in the sun will photo change the color to a purple/black

Kurt


----------



## solar_plasma (Sep 28, 2014)

True. I take the


> "most probable"


 back.


----------



## kurtak (Sep 28, 2014)

Here is a pic of AgCl exposed to the sun for a couple minutes - Its the jar I have been rinsing my beakers into during my latest silver project - I just rinsed another beaker into it so even the suspended AgCl photo changed in color

Kurt


----------



## justinhcase (Sep 28, 2014)

I thought that with out Cl present tin would form SnO.
and SnO had two forms, a stable blue-black form and a metastable red form.
Do I take it the red form is not red and can be a white sludge as discribed??


----------



## solar_plasma (Sep 28, 2014)

HNO3 can give white metastannic acid with tin.


----------



## kurtak (Sep 28, 2014)

justinhcase said:


> I thought that with out Cl present tin would form SnO.
> and SnO had two forms, a stable blue-black form and a metastable red form.
> Do I take it the red form is not red and can be a white sludge as discribed??



From Hokes - page 70 --- Second: Try the effect of hot strong nitric acid, first on a scrap of tin, then (in a clean test tube or beaker) on lead. . . The effect is quite different. In both cases the acid goes to work at once, giving off the usual brown fumes. . . The tin goes to pieces, and in its place appears a white pasty or slimy substance. We call this tin paste. The chemists call it meta stannic acid. If you add some water the tin paste will thin out, like jelly, but it will not dissolve. This tin paste, which forms when nitric acid acts on tin (and at other times as well) is the main reason why tin is troublesome in refining.

Note (as underlined) tin + nitric acid = "white" precipitate

tin in the meta stannic form tends to be "very" fine & therefore tends to go though filters - it also tends to "stay" suspended in solutions - often the cause of a cloudy solution that wont settle &/or filter clear/clean

Tin = the royal pain in the refiners butt

Kurt


----------



## justinhcase (Sep 28, 2014)

I have not had to deal with any tin yet and forgot it is difrent when hydrated.


----------



## Palladium (Sep 28, 2014)

Its tin and the reason it would turn purple is because any trapped silver nitrate will cause it to do so. Its a devil to wash. Takes four or five hot water washes with decanting, not filtering steps in between.


----------



## inspector071 (Sep 30, 2014)

Thanks for the help, guys. From your descriptions, it definitely appears to be tin, and a seeming lot of it. Bummer! How is tin typically separated from silver?

By unfilterable, I meant that the precipitate immediately clogged both types of my filters. It did not pass through. 

I'll try successive hot water rinses until I don't detect any silver. Then I'll evaporate down the solution to concentrate the silver nitrate and drive off any excess acid.


----------



## justinhcase (Sep 30, 2014)

It is just a long shot and I would try it with a small batch first.
But as Tin Oxide is Farly insoluble and silver nitrate is very,I would try drying it out by reduction.
Then Place it in a filter and try flushing.
As the tin oxide will have formed larger partials in it's anhydrous form it should not block the filter so badly for some time and the silver nitrate should flush out.
But always try a new technique on a small sample first.
It may not work on tin but I have used it on other hard to filter mediums.


----------



## butcher (Sep 30, 2014)

The tin in nitric will not really dissolve, the oxidized tin gel does not settle or precipitate, so decanting is out of the question, filtering is almost impossible if much tin is involved as it clogs filters almost solid.

Evaporate the solution to dryness, the tin and silver will form a salt, heating this salt to gas off NOx gas, incinerate the crushed powders red hot with stirring and good exposure to air or oxygen, to form silver and tin oxide powders, cool and give powders a boil in HCl, the silver will become a fluffy (slow to settle) white silver chloride that can be separated from the tin chloride, note adding water to a tin chloride solution will make it hydrolyze and begin to form a milky gel , keeping it fairly concentrated and acidic and even warm, will make the separation go easier...

I do not like making or dealing with silver chloride, I also try to avoid tin in solutions as much as possible.


----------

