# Please help: palladium precipitation goes wrong



## pisitj (Mar 5, 2012)

Hi all,

Fist of all I need to thank you all the active participants and the forum for the great knowledge that is shared here. I'm a newby learning this science (and art) for refining my small jewelry shop's scrap - starting from refining silver and droppeding with copper, to inquartating gold and now is trying to learn to separate silver and palladium. Our scrap is quite unconventional with very high palladium proportion compared to normal gold (ie up to 50% palladium portion in the melted scrap.)

In the past I would try to separate gold from the rest using inquartation and drop silver and palladium together using copper sheet. In the last batch, I tried to separate palladium from silver by following Hoke's method of precipitation with sodium chlorate. The problem is at the evaporating stage (I'm sure I overuse nitric - just learnt in the last few days reading the forum after the sin has been committed in the last batch. I tried to evaporate by heating the nitrate solution of palladium and copper by putting pyrex beaker on correlle dish. I tried to make sure there's no boiling (only once happened early in the evaporating but I put in HCL and the boiling calmed and stopped). I continue to heat - turning the heat up and down so I see only very small bubble. Liquid was down by 70-80% when I noticed some crust started to form on the surface and then later the side of the beaker turned blue color from dark brown color that it has been for a while. Around this time when I add a bit HCL there's till brown fume. And when I stirred the liquid I realized that crusted has form at the bottom and the liquid has turned deep blue.

Is this a separation of pd (and Ag? may be) nitrate into crust form and left this concentrate Copper nitrate solution? Or do I totally lost my Pd somehow in the process? If Pd is still there how should precipitate this Pd?

One additional information, before this evaporation I tried to separate Agcl out but I'm not really sure it is completed (adding Nacl didn't seem to result in additional white colloid) or if I put in too much Nacl (I added a bit more even if I can see white colloid of Agcl anymore). 

Again thank you for this great forum. And I just learnt about silver cell in the past two days so in the future I'll probably inquart gold, drop Ag and Pd using copper dilute Pd with Ag and run this result through silver cell.


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## Lou (Mar 6, 2012)

It looks like you have crystals of chlorate or some salt that are forming on the bottom.

First, cement everything out with zinc. Remove the zinc with dilute HCl and then soluble everything with sodium chlorate and HCl at about 60*C. Then...
Best thing to do is to get it all soluble again, filter any particulates, and then basify with ammonia to 10-10.5. If the palladium content is high, you'll see a bright pink transient palladium compound form, but it'll dissolve. Then floc it and filter. Acidify to 7, filter again, and then take it down to 2 to get the Pd.


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## butcher (Mar 6, 2012)

follow Lou's advice. 

I just have a question, did you use homemade nitric? To me the crystals look a lot like copper sulfate.


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## pisitj (Mar 7, 2012)

Thank you very much Lou and Butcher.

Butcher: I used commercial nitric. (Nitric is not restricted here.)

Lou and Butcher: I have a question, I was going to follow Lou's advice when I looked at the content in the beaker again. There's so much more blue needle-like crystals on top of the precipitation. One adjustment from Lou's advice come to mind - that the blue liquid/crystal is predominantly copper salt and may be I should separate it from the brown palladium salt underneath. So I decant the blue liquid, put additional water to melt the blue crystal (room temperature water)- most of the blue crystal melted. Then I put in hot water to wash the brown salt, but most of the brown salt is now melt into thick liquid. Should I skip precipitation with zinc and go ahead with sodium chlorate?


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## awbrew (May 15, 2012)

Lou, Sorry to be so ignorant. But what does Floc it mean ?


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## rusty (May 15, 2012)

http://en.wikipedia.org/wiki/Flocculation


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## freechemist (May 17, 2012)

> Cited from pisitj: Our scrap is quite unconventional with very high palladium proportion compared to normal gold (ie up to 50% palladium portion in the melted scrap.)



Hi pisitj,

What else, beside silver, is contained in the molten scrap? If it is not more, than 20-25% gold, inquartation is not necessary, and it should be possible to dissolve the material in HNO3, giving a combined solution of silver and palladium and a solid residue of metallic gold. If the gold-content is higher, and the silver higher than 15%, then you could add more gold, thus lowering the silver to maximal 15% of the materials total weight. This newly formed alloy could be dissolved in AR, giving a combined solution of gold and palladium, and a solid residue of silver chloride.

As far, as I can see from your pictures, your molten material contains also a lot of copper, which means, that over all you may have an alloy with no more than 15% Ag, provided, the palladium content is as high, as cited above. This alloy could be dissolved in AR and also in HNO3, provided it's total Au-content is not higher than 20-25%.

Whatever method you choose, you have to deal with the following problems:

1.) Effective separation and recovery of Pd and Ag from a fairly concentrated HNO3-solution.
2.) Effective separation and recovery of Pd and Au from a fairly concentrated AR-solution
3.) Effective purification of freshly precipitated AgCl and recovery of metallic Ag from it.

Problem 1.) can be solved by slowly adding the combined HNO3-solution to a great excess of hot and continously stirred aqueous HCl-solution; at least 6 moles of HCl per gram-equivalent of Pd and 2 moles of HCl per gram-equivalent of Ag are used. Be careful: a lot of brown NOx-fumes may be evolved in this procedure. Ag is recovered as solid AgCl, which can be further treated by known methods. Pd can be recovered from the filtered solution by known methods.

Problem 2.) can be solved by first dropping metallic Au from the AR-solution with sodium-meta-bisulfite (SMB) or sodium sulfite and then working up the filtered Pd-solution by known methods. Note: Au can not be precipitated selectively with oxalic acid, because Cu present in the solution interferes by precipitation of sparingly soluble copper(II)-oxalate.

To solve problem 3.) known and well established procedures can be used.


Good luck, and be careful, freechemist


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## pisitj (Jun 19, 2012)

Thank you very much Freechemist. The refining choices you suggested are what I've been trying to decide all along. I've tried the first method unsuccessfully (and the process of driving off excess nitric is really poisonous like you suggested). 

It seems the second method is a better way to go in the future.

Thank you again.


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