strange precipitate while deNOXing.

[au-artifax]

Hello. While evaporating/deNOXing a gold AR solution, which had minor impurities enough to make the solution slightly green ( it was yellow against white background until condensed got darker and took on the green color more).

The question is in regards to a light tan (looked yellowish while it sat on the bottom of solution) precipitate that formed in the solution after the nitric was driven off and the solution cooled down. Suspected metals in the solution other than gold are rhodium, and some nickel (from small piece of stainless that I found hanging around the bottom of the flask after dissolving.

Is there the possibility that this precipitate is composed of values I should process, or has it been someone else's experience that this occurs and the precipitate is just junk. It did not redissolve upon the additional of HCl while deNOXing, so the solubility issue threw me for a loop as well.
Thank you.

Oh, and PS: the solution was well filtered, perfectly clear in the beginning and had no haze or particulate matter, and I used a clean container whole deNOXing.

 

[maynman1751]

First of all........more information! What materials did you have in the AR? If you diluted your solution, it could be silver chloride that has settled out. Will this liquid go through a filter NOW or does it clog? I myself have run into strange precipitates and wasn't sure of their composition.

 

[au-artifax]

First of all........more information! What materials did you have in the AR? If you diluted your solution, it could be silver chloride that has settled out. Will this liquid go through a filter NOW or does it clog? I myself have run into strange precipitates and wasn't sure of their composition.

I thought I mentioned what was in the AR. Gold, and possibly rhodium and a slight bit of nickel from a piece of stainless that I missed because it was plated. Anyways, the precipitate formed as the boiled down, deNOXed AR solution was cooling. So no, I would hazard to say it wasn't diluted at all. I have filtered it out of an earlier batch of the same solution and it does not clog the filter and is trapped by the filter also. This precipitate settles fairly quickly and looks particulate in nature as a powdery substance that does not stay in suspension.

I added some HCl ( not enough to consider it a dilution), after to see if the precipitate would dissolve, and it did not.

I think maybe it still could be AgCl, but besides that all I can guess is that it was something soluable in the free nitric that precipitated as the nitric was driven off. If it is nickel then the answer is clear, trow it away, but if it could be rhodium in some form then I may want to save it for experimenting with. There is not enough otherwise to bother with. I just want to know what it is or could be.

Is it too late to throw in some H2SO4 to see if anything else drops? Or should I just proceed in dropping my Au (after filtering, diluting with H2O, then filtering again if any more precipitates form)?

 

[g_axelsson]

You can always add a bit H₂SO₄ to see if some lead sulphate is dropping.

There are many salts that are a lot harder to dissolve when it have crystallized. I don't know if that covers any precious metal salts too. I usually get some undissolvable salts too when evaporating to denox a solution. (Please, do not use boiling when meaning evaporating, boiling will lead to losses.) It has never concerned me that much as I always save any filter I have filtered dissolved values in, it goes straight to my burn pile.

Göran

 

[Anonymous]

Artifax I may have missed where you mentioned it, but what raw materials were you using?

 

[au-artifax]

You can always add a bit H₂SO₄ to see if some lead sulphate is dropping.

There are many salts that are a lot harder to dissolve when it have crystallized. I don't know if that covers any precious metal salts too. I usually get some undissolvable salts too when evaporating to denox a solution. (Please, do not use boiling when meaning evaporating, boiling will lead to losses.) It has never concerned me that much as I always save any filter I have filtered dissolved values in, it goes straight to my burn pile.

Göran

Thanks. I stayed with Holkes' recommendation to keep the temp low and use air to blow over the evaporation dish, as she had mentioned about how it splaters when it gets thicker. I kept it at a temp where you could see the liquid swirling and rolling but with no bubbles to speak of. As far as my filters, I have always had a question about safe storage of saved paper filters. Is there a hazard of spontaneous combustion with dried paper filters that have been saturated with nitric or some other kind of oxidizer? I do not have a metal waste container on site, like the kind they use for oily rags.

 

[au-artifax]

Artifax I may have missed where you mentioned it, but what raw materials were you using?

Thank you for responding spaceships. My raw materials were all gold filled jewelry, and I started removing base metals with HNO3 until no more dissolved, (treated the solution to get all my silver out), then dissolved the rest in HCl. I knew there was some stainless in there and was sure the only thing left was the gold (karat of course, and powder from the karat that dissolved), and all the rhodium flakes from any plating on the white gold. I did not find any palladium to speak of in my nitric solution so that is not a concern.

It wasn't until after dissolving the gold in AR that I found a small piece of stainless undissolved and passivated at the bottom of the flask. It was inside a small gold plated pin that was part of a tie clasp.

The only other thing unaccounted for is whatever material was alloyed with the karat gold (some being white gold also with possible nickel in it). But my filters didn't clog the way I have seen happen with nickel in nitric acid solutions.

 

[maynman1751]

I thought I mentioned what was in the AR. Gold, and possibly rhodium and a slight bit of nickel from a piece of stainless that I missed because it was plated.

I was asking the same thing as spaceships. What was the raw material? I guess I didn't make that clear!

 

[au-artifax]

I thought I mentioned what was in the AR. Gold, and possibly rhodium and a slight bit of nickel from a piece of stainless that I missed because it was plated.

I was asking the same thing as spaceships. What was the raw material? I guess I didn't make that clear!

See previous post.

 

[g_axelsson]

I wonder if your precipitate could be lead chloride? Lead dissolves in nitric acid and if there were some dissolved in the first step it could easily been carried along to the AR step. When concentrated the little lead chloride would fall out as white precipitate and unless you heat your solution it can be hard to dissolve it again.

Göran

 

[maynman1751]

I wonder if your precipitate could be lead chloride? Lead dissolves in nitric acid and if there were some dissolved in the first step it could easily been carried along to the AR step. When concentrated the little lead chloride would fall out as white precipitate and unless you heat your solution it can be hard to dissolve it again.

Göran

That is why when I process gold filled I do a 50/50 H2SO4/H2O wash, decant, H2O hot rinse, after nitric and before AR.