# Silver/Palladium solution congealing



## inspector071 (Oct 4, 2014)

I'm processing some solder paste containing silver, platinum, palladium, aluminum, and bismuth. The solder paste should be mostly silver, followed by platinum, then palladium. All of the organics have been burned off. I started off by getting the solids into a fine powder, then covering with dilute nitric and heating. The acid reacted a while and eventually the solution turned orange/brown, likely indicating dissolved palladium. A significant amount of fine, difficult to filter, black powder was left behind. This is presumably the platinum and any other insoluble components of the solder paste. There may still be some undissolved silver and palladium in that powder. I filtered the mix over a bed of Celite 545 which worked like a charm. After filtering I was left with a clear orange solution that soon turned sparkly and began to form congealing blobs of gel. Here's a photo of the blobs of gel.







What's going on here? My plan was to drop the silver out as silver chloride using hydrochloric acid, filtering, then boiling down the palladium solution, adding ammonium chloride, then bubbling chlorine. The gel is difficult to filter, but when it gets washed with distilled water, it reveals itself to be clear.


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## FrugalRefiner (Oct 4, 2014)

Could be the aluminum. I haven't experienced it, but I've read that aluminum can create a difficult to filter gel.

Dave


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## nickvc (Oct 4, 2014)

The mix of elements seems odd what was the application the solder was used for?
If the alloy is as explained your PGMs will be in solution with your silver especially if it's mainly silver.


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## MarcoP (Oct 4, 2014)

Celite 545 is SiO2 and after searching the web for SiO2 + HNO3 + gel I've found and attached a document which hopefully explain your issue. Just skip to the bottom paragraph.


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## solar_plasma (Oct 4, 2014)

That's an A+
Good job, MarcoP!

I can't say, if this is the reason for the gel, but it is obviously a reaction, that is taking place in the OP's setup.


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## MarcoP (Oct 5, 2014)

Thank you solar! However my luck was driven by curiosity and will to help. Nothing came from my own knowledge but certainly I had to understand what I was reading to find the supposedly right answer.


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## inspector071 (Oct 6, 2014)

nickvc said:


> The mix of elements seems odd what was the application the solder was used for?
> If the alloy is as explained your PGMs will be in solution with your silver especially if it's mainly silver.



Companies like Heraeus and DuPont make specialty solder pastes for the microelectronics industry. The solder paste is made up of extremely fine metal particles suspended in organics such as pine resins, which serve as fluxing agents. One of the other pastes I've worked with is composed of 80/20 Au/Sn. This is a eutectic alloy that has a rather low melting point. It doesn't even look like gold, even with its high gold content. 

Only the silver, palladium, and other base metals should be going into solution with my dilute nitric dissolution. I'll then add HCl, heat, and try and convert all of the palladium nitrate into palladium chloride. Then I'll add ammonium chloride and bubble chlorine to precipitate the ammonium hexachloropalladate. I'm looking through some of my inorganic texts to see if there's a quicker route to reduce palladium nitrate to an insoluble, calcineable salt, or the metal.


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## FrugalRefiner (Oct 6, 2014)

inspector071 said:


> Only the silver, palladium, and other base metals should be going into solution with my dilute nitric dissolution. I'll then add HCl, heat, and try and convert all of the palladium nitrate into palladium chloride. Then I'll add ammonium chloride and bubble chlorine to precipitate the ammonium hexachloropalladate. I'm looking through some of my inorganic texts to see if there's a quicker route to reduce palladium nitrate to an insoluble, calcineable salt, or the metal.


If you add HCl, the silver will precipitate as AgCl. Then you can cement the palladium with copper. It's one of the few times I would choose to create AgCl.

Dave


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