creative solution needed: seperate silver from palladium

[pyromaniac_guy]

Hello all,

Long time no log on...

I have a small favor to ask of the forum since there are so many uber creative folk here.

I have about 50 troy ounces of 77% palladium bal silver alloy. This is 4 9's material so If I can find an easy way or removing the silver I'll be left with fine palladium scrap to sell for a fair chunk of change.

What I would ideally like is a process such as quartering used to refine karat gold (dilute gold to about 25% purity in copper or silver then dissolve in nitric acid leaving metallic gold behind with very little of the gold in solution).

The material is in the form of very thin ( 1 or 2 mil) tubing so there is a huge surface area to weight ratio. If I could avoid dilution in a base metal (to avoid masking) that would be great. I dont care how long the process takes, I'd just prefer something A) easy (ideally - but not the most important priority) b) low loss - we are after all talking about a precious metal here and C) give a high purity finished product.

The best I have come up with thus far is dissolve in aqua regia, let the silver percipitate out as chloride, percipitate the palladium with ammonia then reduce.

If anyone can come up with anything better I'd be forever in your debt.

TIA!

[Man Down Under]

I know gold can be purified from base metals by melting it and passing chlorine gas through the molten mix, vaporizing off the base metals.

Try heating your metal red-hot in a tube furnace with chlorine gas passing through it.

The silver should volatilize off as silver chloride, leaving palladium powder behind.

Presumably you will collect the silver chloride vapor for purifying as silver.

[pyromaniac_guy]

Hmmm...

Good idea, but wont work. Palladium is less nobel than gold and even with gold you can only use this process to get maybe 99.5% purity because eventually you will start producing chlorides of everything in your melt, including the gold, loosing it out the exhaust.

I didn't mention this previously as I didn't think it was important, but I guess it is. I'm HOPING for at least 4 nines purity so that wherever I dispose of this material it wont need to be refined again - saving me on the markup.

Thanks for the suggestion though...

[jimmyboy1]

Silver is much more reactive than palladium - one of the halogens should be able to separate them.

[Man Down Under]

You should have mentioned the 99.99% requirement when asking the question, saving me the time of replying with a method that I know wouldn't match it.

[mil&co]

Palladium reacts quite slow with hydrochloric acid, but silver is quickly dissolved. That might be an idea to work out, lot less work then with aqua regia, which should work ok.

[pyromaniac_guy]

You should have mentioned the 99.99% requirement when asking the question, saving me the time of replying with a method that I know wouldn't match it.
in retrospect i realize this now - however keep in mind that the palladium is already 4 9's pure. it just happens to be alloyed with 4 9's silver as well. This isn't really your average refining problem, where you have a bunch of junk mixed in with a precious metal and you have to get everything but that one metal out. I have a mixture of two very pure metals and need to simply unmix them :D

your suggestion would have worked if it weren't for the fact that even the nobel metals aren't really THAT nobel.

[megalomania]

You may want to look at electrochemical methods. There are some interesting patents released over the last 20 years concerning recycling of mixed high value metals.

I was going to suggest the silver chloride precipitation route, until I saw you want something better than that.

[pyromaniac_guy]

Thanks for the suggestion on possible electrochemcial routes. I found a reference specifically dealing with isolating silver from palladium (in 100's of ppm Ag in Pd concentrations) for radiochemistry. I need to dig up the actual paper but if controlled potential plating of the silver will work for this that will save me a heck of alot of effort!

[meyer25]

Dissolve the alloy in 65 - 68% nitric acid taking approx. 8 - 10 mL acid per gram of the metal; be careful, NOx evolves (quite toxic by inhalation, so do it in a proper fume hood or outdoors). Maybe a gentle heating would be necessary to dissolve all rests of the metal.
What you get is basicaly solution of palladium(II) and silver nitrates with some nitric acid and NOx in water.
Let all eventual dirt (insoluble in HNO3) settle and decant the solution. I would omit filtering unless you have a proper frit filter (no paper or cellulose).

Now add excess saturated NaCl solution, until precipitation stops. The basic principle is simple: silver chloride is practically insoluble in water whereas PdCl2 and [PdCl4]2- are soluble.

Now decant the solution from the heavy precipitate, filter (I'd use a glass frit, but analytical (ashless) filtpaper can be used, if not too long in contact with the solution) it and wash the precipitate back in jar with distilled water acidified with HCl to about pH 1.
Filter this also.
The filtrate contains essentialy 99% pure ionized palladium in form of dichloride and tetrachloropalladate, along with plenty of nitrate, chloride and sodium ions.
You could reduce the volume of this solution by boiling it down until some particulate matter begins to form (saturation).
Now adjust pH by carefully adding 4% (1M) NaOH soln drop by drop to about 6 - 7 (not higher, if possible); add excess sodium acetate, and reduce the palladium out by formaldehyde solution.
To do this, one should place the solution in a RB flask under a reflux cooler, heat until it boils/refluxes, than adding few mLs of standard 35 - 40% formalin. The solution turns dark brownish-black, one should reflux it for at least 2 hours, adding some more formalin eventualy.

What you now got is a suspension of crude palladium black, after cooling the reaction mixture, filter it out on the finest filtpaper you can afford (again, ashless, preferably, with high density), wash the flask with copious amouths of distilled water and wash the Pd black on the filter with it. wash again several times. Than, wash it with pure acetone twice; it helps to get it dry faster.

Leave it to dry completely and...voici, you should have crude Pd black. That is, pulverous Pd metal with quite high area pro mass...

If you want it as a metal (bar, rod etc.), you'd have to melt it and pour in a conveniant form. However, it should be molten preferably under nitrogen and requires heat source of at least 1,600 - 1,700°C, which is rather tricky to afford, if you're not working in a well-equiped facility.


Forget the "molten red-hot metal-chlorine gas" crap....the wet chloride method is the way to go.