# Precipitate tin from aqua regia



## radhia87 (Nov 28, 2018)

How to precipitate tin from aqua regia without precipitate other metal(gold, palladium, copper, etc)


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## Lino1406 (Nov 29, 2018)

Give time and tin dioxide will settle out


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## Marcel (Mar 6, 2019)

radhia87 said:


> How to precipitate tin from aqua regia without precipitate other metal(gold, palladium, copper, etc)



1. You should always remove tin,lead,indium first before applying Aqua Regia, if you want to succeed in refining.
2. Working along the galvanic series (or electropotential series) as we do here there is no way. There might be some chemical procedure but I doubt that you will find the chemicals or the equipment to do it in an economical way. Because ususally refiners.. ( read point 1.)....
3. To improvise, I would use copper to precipitate all metals below copper. Then heat the powder to around 230°C which is above the melting point of tin an lead and can be easily achieved on a normal hot plate. Stirr the hot powder and the tin balls should join togehter without adhering to the rest of the powder. Then let it cool down and remove the tin. You can then dissolve the tin ball using acids and whatever precious metal particles have been enclosed (which is very unlike due to the high surface tension of tin) will be left over.

@radhia87: I do not understand how tin dioxide will form in Aqua Regia from tin. What is the reaction here?


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## g_axelsson (Mar 6, 2019)

Nitric acid oxidizes tin into metastannic acid... tin chemistry is actually quite messy so the story is a lot more complicated than that.



Marcel said:


> 1. You should always remove tin,lead,indium first before applying Aqua Regia, if you want to succeed in refining.


Tin can form metastannic acid and can make filtering a nightmare.
Lead is detrimental to workability of gold. Even minute amounts of lead can make a gold alloy crack when worked by a goldsmith or drawn into wire.

But what problem does indium cause? Please elaborate.



Marcel said:


> 2. Working along the galvanic series (or electropotential series) as we do here there is no way. There might be some chemical procedure but I doubt that you will find the chemicals or the equipment to do it in an economical way. Because ususally refiners.. ( read point 1.)....


Tin in aqua regia will form metastannic acid and can be filtered off (slowly)... see above.



Marcel said:


> 3. To improvise, I would use copper to precipitate all metals below copper. Then heat the powder to around 230°C which is above the melting point of tin an lead and can be easily achieved on a normal hot plate. Stirr the hot powder and the tin balls should join togehter without adhering to the rest of the powder. Then let it cool down and remove the tin. You can then dissolve the tin ball using acids and whatever precious metal particles have been enclosed (which is very unlike due to the high surface tension of tin) will be left over.


Are you talking from experience? I would call this a fancy and totally wrong theory unless you can show this will happen in practice.

Tin dissolved with HCl forms stannous chloride. When there is gold or other precious metals present it will oxidize while the gold reduces back into metallic gold in the form of a colloid. If there is any aqua regia present the gold will go into solution again but the stannous chloride is used up and turned into various tin compounds, probably metastannic acid and other tin hydroxides and oxides. (Sn2+ to SN4+)

And even if you could cement tin and precious metals as a powder, heating it without a protective atmosphere would oxidize the tin. And how a high surface tension would protect it against dissolving precious metals I don't understand. Tin is very good at wetting precious metals, if you have ever compared soldering a tinned circuit board with an ENIG board then you would know that tin easily wets any gold.

"The True Story of Purple of Cassius"
https://link.springer.com/content/pdf/10.1007%2FBF03215423.pdf

Tin chemistry summary
http://www.sciencemadness.org/talk/files.php?pid=118256&aid=4828

Göran


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## Marcel (Mar 8, 2019)

dear Göran,
I agree with what you, but this case seems to be a non-standard one, because tin should not be present in the Aqua Regia in the first place, because you should always remove the basemetals first.
And yes , I "played around" with some of the methods described and had the result tested with an XRF.
Most of the melted ( < 250°C) material contained tin, lead and some indium. Just some vid, where I collected tin from electronic components as collector metal. Later I had these bars analyzed. There was barely any Au, Ag or Pd collected not even Cu, Zn etc., just soft base metals:
https://www.youtube.com/watch?v=YEiMavFolq0
And yes I know what PCBs are. I have worked in that industry for several decades. 
I think sometimes it is all clear in theory, but you have to test and confirm it in practical test to see if a theory really works out.
And gold needs flux to adhere to tin. That is why there is flux in every soldering tin. And one result from my research is that the many of the involved metals very easly form oxides while heating them, f.e. copper.
This oxide layer will prevent the metal/copper to mix with the molten tin. It is difficult to solder tin to other metalic surfaces not easy! It needs flux and only certain metals work flawless. Just check some info on RoHs, leadless tin, the introduction of Ag in solder paste and the problems the assembly companies got with this.
But some here is theory and some is just partial practical experience. I think the best would have been to not let the tin come that far. He should have worked cleaner and better in the first place. 
I will check if I can find the white paper and some photos from those experiments back then.
As for Indium: It does not disturb me, I am just working on indium recovery and yields and I find it to precious and easy to handle ( price is almost like silver) to have it discarded. Unfortunatly the amounts in many components are rather small.


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