Uknown white material after dissolving in Nitric Acid

[NobleMetalWorks]

This has stumped me, I am hoping someone here might know what this is.

I have 38.5 lbs of gold plated discs that look like they might be battery contacts, but I am not sure what they were used for.

I dissolved a small amount in AR as a test, and precipitated with SMB. When I added SMB, Gold precipitated as an almost black powder which I assume means it dragged down another metal when it precipitated, but it also made a white cloud which looks like some type of chloride. I only used a very small amount of SMB so I do not feel that what I am seeing in the solution is excess SMB, something reacted with the SMB to make a chloride like cloud. If there was silver present, it should have made silver chloride before SMB, so at this point I wasn't suspecting silver at all. I did add a few drops of sulfuric acid to rid the solution of any lead before I added SMB. I would rather deal with a nitrates than chlorides, or anything that resembles chlorides, so I set the AR test aside and tried another method.

I took a small amount of these discs and dissolved then in Nitric Acid. It dissolved everything in a short amount of time and left the gold foils behind. The resulting solution was the blue color I have come to associate with copper nitrate. So now I know I have gold and copper and something else I have yet to identify. I am not sure what other metals are in this material so I didn't want to try Hcl/Cl in case there was Ruthenium in the mix or some other unknown metal. Sulfuric wasn't having any effect, I suspect because of the gold plating protecting the metals under the plating. Using a sulfuric cell with even a small amount of these discs makes the power source short repeatedly.

I dissolved 1 lb of these discs in a 50/50 solution of water/Nitric Acid. Waited 24 hours and was planning on filtering the gold foils. After the 24 hours the solution turned bright blue with gold foils and a white material on the bottom. If the white material was silver the only way it could have come out of solution was if the solution was so pregnant with silver nitrate, it forced it out of solution. Or if silver went into solution and was cemented out when the copper went into solution. However, the white material looks nothing like either of these. At this point I was almost certain the white material was not silver.

I filtered the solution to separate the gold foils and white material from the copper nitrate. Then washed the filtered material to separate the white material from the gold foils.

I spooned a small amount out to try to get a better picture, it's almost the consistency of yogurt, and very white.

Here is a picture of the gold foils

You can see the solution still looks milky, it hasn't been washed totally. I am having my nephew help me, I sustained an injury so I cannot really do any of this myself, and he wants to learn so it seemed like the perfect time for him to do so. When he comes over later today I am going to have him finish washing the foils. But this next picture shows some of the white material on the bottom of the beaker.

I have taken a small amount of the white material and tried to dissolve in Nitric Acid, with no reaction. I tried HCl with no reaction. I tried Sulfuric Acid with no reaction. I then made a small amount of AR and tried to dissolve the white material, with no reaction. I have not put any of the samples on heat to see if they could dissolve whatever the white material is into solution, I thought I would post here first before going any further to see if anyone else has come across this same conundrum.

I have foiled a lot of gold plated material, but I was surprised at how much gold foil just one lb of this material yields. I hate using terms like "heavy gold plate" but there seems to be a lot more gold than what I normally would have expected for this type of material. Observing the reaction, the gold foil comes off readily enough, the material under the gold turns almost black, and the solution starts turning blue. Here is another picture I took when the reaction stopped, and needed more nitric acid added to continue.

I am hoping this isn't Cadmium or some other metal I don't want to deal with, but as it's coming out as a solid, the only real hassle is in filtering it.

Does anyone have any idea what I am dealing with here, or have any suggestions on how I might proceed?

Thank you

Scott

 

[Oz]

It does not sound as though your white precipitate/residue will cause you much difficulty. From your description it is resistant to acids and nitric is putting everything else into solution but this and gold. Just digest the gold to separate it from your white residue.

 

[NobleMetalWorks]

I am planning on digesting the rest of the discs in Nitric Acid, then collecting all the filtered material Au and the mystery white material, dissolving in AR, filtering, then precipitating with hydroquinone, that way if any PGMs or other metals are present, I won't be dragging them down when I precipitate the Au.

What confuses me a bit is this. When I did the sample with AR, it seemed to dissolve everything into solution, but when I precipitated with SMB it made a cloudy layer that looked like a chloride, when allowed to settle it ends up looking exactly like the white material I am dealing with from the Nitric Acid process. Yet the white material will not dissolve in AR after Nitric Acid. I have not tried to re-dissolve the AR test in AR again, so I don't know for sure if that form will not dissolve into AR. And I cannot be sure that it really dissolved into AR to begin with, because there is very little of it.

I appreciate the suggestion, thank you.

Scott

 

[samuel-a]

sounds and seems like metastannic acid to me.

you can verify this. Take a small sample of the white material and incinirate it to red hot allowing it react with the air around.

Then try to dissolve it in hcl, if it does dissolve and produce a colorless solution, then most likely it is Tin (as chloride).
You can verify this by using it to test a standart gold solution.

 

[NobleMetalWorks]

Sam,

I think you're right...

I took a sample of the white material in AR, and put on heat. It has been on heat for a few hours, the solution has turned a creamy yellow color but still has most of the white material on the bottom that has not dissolved yet. The yellow is a different color than AR yellow. However a test with Stannous gives no result. I am going to try the method your suggested as soon as I am able, and post the results here.

Thank you so much for the reply

Scott

 

[Oz]

I’m thinking that if the contaminate was tin, you would have had a purple reaction when you did a digestion of this material in AR the first time around.

 

[NobleMetalWorks]

I’m thinking that if the contaminate was tin, you would have had a purple reaction when you did a digestion of this material in AR the first time around.

The AR turned a deep blue, after I precipitated with SMB I had a chloride cloud appear, then settle into the same type of white material, so I decided to try Nitric Acid first, and proceeded with Nitric digestion. I thought because the solution turned a deep blue, after I digested in Nitric Acid, that I was dealing with copper, but the white material had me stumped. I thought maybe there was another metal that went into solution first, then when the copper was dissolved it cemented out whatever other metal went into solution. But it was white, not like silver and not like any other PGMs. I incorrectly assumed that the Cu cemented out the white material I was seeing, and that if it did the white material would have to be lower on the reactivity scale.

I tried to digest the white material in hot AR, it did turn a different color yellow, and when everything settled out it took on the same color yellow as Au does when dissolved in AR. There are small foils in the white material so my next step is going to be to digest the foils and solids in AR, ,and filter off the metastannic acid (tin). I'm going to precipitate with Hydroquinone then check the solution with an ORP meter to make sure I reduced everything in solution.

Because the metastannic acid doesn't dissolve in AR, it will be easy to filter it off and recover the Au. I have already filtered off the copper nitrate.

I'm convinced now that what I'm dealing with is metastannic acid. So it ends up being tin/copper and gold plate. Or tin is a base metal, the copper plated on the tin, the gold plating the copper. Whatever the combination is, it makes sense to me now.

Scott

 

[goldsilverpro]

I would read this thread and follow Harold's incineration/HCl method for eliminating the metastannic acid.

http://www.goldrefiningforum.com/phpBB3/viewtopic.php?f=48&t=10678&p=103640&hilit=metastannic+incinerate#p103640