Selenium contaminated Ag & Pd

[vjodhani]

Hello everyone
I have been trying to process this metal to get rid of selenium but I think everything went in vain.
We tried with various fluxes such as sodium nitrate, potassium permanganate, wood chunks, caustic soda independently. Direct Nitric acid dissolution followed by AgCl precipitation and cementation of palladium by Zn. But the product always contains the same amount selenium always.
It is at this juncture I feel that I will not be able to proceed further without help.
I want to extract fine Pd from it.
Please guide me on how I can do it. :?:
Attached is the image of the bar (I have 5 such bars of 500gm each) with the analysis.

 

[g_axelsson]

First of all, be very careful when melting this metal. Selenium can be poisonous and can form volatile products, so the smoke coming off can be deadly. It compares to arsenic.

I don't think you could refine the metal by simply melting it with a flux. At the least you need oxygen or chlorine sparging to remove the last part of base metals.

If it was me I would refine it by using standard wet chemistry methods, still with great care so I don't get exposed to selenium compounds.
There are a lot of different ways you could go depending on what equipment you could build or which chemicals you could get for a decent price.

You wrote that dissolving it in nitric acid, separating silver chloride and cementing with zinc gave the same selenium bar in the end? What exactly was the result, surely you managed to remove silver and gold by that treatment? Did some of the selenium follow the silver or did it all end up with the palladium? It feels like there is a part missing here.

What is your final goal? Just make something you can sell or produce pure metals? Do you get a penalty for selenium content? Do you need to separate silver from palladium or is it okay to sell as an alloy?
The answers to these questions could guide you in which method is best for you.

Göran

 

[Lino1406]

Give the details of liquid/s before cementing/dropping

 

[vjodhani]

First of all, be very careful when melting this metal. Selenium can be poisonous and can form volatile products, so the smoke coming off can be deadly. It compares to arsenic.

I don't think you could refine the metal by simply melting it with a flux. At the least you need oxygen or chlorine sparging to remove the last part of base metals.

If it was me I would refine it by using standard wet chemistry methods, still with great care so I don't get exposed to selenium compounds.
There are a lot of different ways you could go depending on what equipment you could build or which chemicals you could get for a decent price.

You wrote that dissolving it in nitric acid, separating silver chloride and cementing with zinc gave the same selenium bar in the end? What exactly was the result, surely you managed to remove silver and gold by that treatment? Did some of the selenium follow the silver or did it all end up with the palladium? It feels like there is a part missing here.

What is your final goal? Just make something you can sell or produce pure metals? Do you get a penalty for selenium content? Do you need to separate silver from palladium or is it okay to sell as an alloy?
The answers to these questions could guide you in which method is best for you.

Göran

Thanks for your response Goran, I assure you that all the safety measures are being taken while handling this material and also I'm well aware of their hazards. I intend to purify my crude Pd-Ag alloy into 999 Pd, However, the above mentioned process is only a small trial (300 gm) of my bigger lot of 3.2 Kg. Hence to process the remaining lot I need help. I have attached a diagrammatic representation of the trial where I lost (unaccounted weight) Pd during various stages of treatment . This diagram will help you understand it better. I intend to go for a method which will yield to reach my desired goal of fine Pd.

 

[vjodhani]

Give the details of liquid/s before cementing/dropping

Thanks for your response Lino, please refer the chart where I have described in reply to Goran in this thread.
Please reach out for any other query.

 

[Lino1406]

AgCl should be washed by HNO3 to remove Se and Pd
Liquid before Zn addition, precipitate with ascorbic acid which has priority for Se and Au , then remove most Se by decantation, wash Au with nitric acid

 

[g_axelsson]

Thanks for the diagram, that makes it easier to see what is going on.

One thing that jumps out to me is that you started with a bar without any lead and ended up with almost 5% lead. That contamination must have come from some chemicals you used along the path.

If you have an XRF on site, you could use it to test liquid samples through a plastic bottle or cup. It might help you hunting down the lead contamination.

I haven't refined any PGM:s myself beyond dropping it as a black powder with zinc and washing it with HCl afterwards to remove any remaining zinc. I think Lino's advice to get rid of selenium before adding zinc is a good way to do it.
The palladium black you get after zinc treatment is a good source for re-refining the palladium. It is hard to get a clean separation between palladium and other metals and if your goal is 999/1000 then you have to go to other methods than using zinc.

A classical way to refine palladium is to precipitate it with ammonium chloride and chlorine. You might have to redo the refining process to reach the fineness you are targeting.
As a start you can study the tutorial that Sam wrote. It is accompanied with two YouTube videos too.
https://www.goldnscrap.com/post/palladium-refining-tutorial-part-1

Any waste with low amounts of palladium can be collected in a stock pot and concentrated to a level where you can recover the palladium effectively.

Melting pure palladium is a chapter in it self. It is even worse than pure silver to absorb oxygen when molten and exspell it when it solidifies.

Göran

 

[vjodhani]

AgCl should be washed by HNO3 to remove Se and Pd
Liquid before Zn addition, precipitate with ascorbic acid which has priority for Se and Au , then remove most Se by decantation, wash Au with nitric acid

Thanks Goran & Lino for your replies.

We took your advice of using ascorbic acid to drop down Selenium with our other effluent having similar chemical composition as that of the bar.



If it succeeded with this we wanted to go with the bars. So, I would like to share the results what we got after Lino’s suggestion.




We are still getting Selenium in all our successive stages of filtration even Pd losses after ascorbic treatment was significant.



We encountered from the link shared by Goran that NOx interferes with the Selenium reduction. So, we treated our solution with urea to de-NOx, then add ascorbic acid. Despite which Pd contaminated ppt of Selenium was obtained. Post DMG addition to the filtered solution Selenium contamination in both filtered solution as well as Pd-DMG ppt.

We understand that the solution we worked contained higher Selenium compared to the original bar that was digested in nitric acid which was discussed earlier. But the solution to both the problems lies in the selectivity of the process.

Case 1 (Pd > Se bars) and Case 2 (Se >> Pd), should they both be approached in the same manner? Is it because Case2 has higher selenium that we are not getting desired results as described by you? Please share your thoughts and guide further.

V!sh

 

[vjodhani]

Thanks for the diagram, that makes it easier to see what is going on.

One thing that jumps out to me is that you started with a bar without any lead and ended up with almost 5% lead. That contamination must have come from some chemicals you used along the path.

If you have an XRF on site, you could use it to test liquid samples through a plastic bottle or cup. It might help you hunting down the lead contamination.

I haven't refined any PGM:s myself beyond dropping it as a black powder with zinc and washing it with HCl afterwards to remove any remaining zinc. I think Lino's advice to get rid of selenium before adding zinc is a good way to do it.
The palladium black you get after zinc treatment is a good source for re-refining the palladium. It is hard to get a clean separation between palladium and other metals and if your goal is 999/1000 then you have to go to other methods than using zinc.

A classical way to refine palladium is to precipitate it with ammonium chloride and chlorine. You might have to redo the refining process to reach the fineness you are targeting.
As a start you can study the tutorial that Sam wrote. It is accompanied with two YouTube videos too.
https://www.goldnscrap.com/post/palladium-refining-tutorial-part-1

Any waste with low amounts of palladium can be collected in a stock pot and concentrated to a level where you can recover the palladium effectively.

Melting pure palladium is a chapter in it self. It is even worse than pure silver to absorb oxygen when molten and exspell it when it solidifies.

Göran

Please refer above to my replies to Lino.

 

[Lino1406]

One point, (although it was said) ascorbic acid should be used after de-noxing, not as de-noxer, as seen in photos

 

[nickvc]

Ascorbic will precipitate both gold and some palladium.

 

[Lino1406]

True, but most palladiun remains in solution by equilibrium considerations

 

[g_axelsson]

This is a really interesting thread and I have a hart time to keep out of it. Just keep in mind that I have no practical experience in PGM refining.

The details you give in your report is exemplary, I wish more people could give as good details as you do, it makes it so much easier to help when you don't have to guess.

I was wondering if cementing gold and palladium on copper powder would separate the values from the selenium. Then in a later stage it would be easier to separate gold and palladium from the copper.

Another idea I got came to me after studying some selenium chemistry. Selenium dissolved in nitric acid forms selenous acid, H₂SeO₃. If it dries out it will form selenium dioxide SeO₂. If we raise the temperature it will sublime at a temperature of 350 C.
So what will happen with copper, gold and palladium nitrate? It looks like all will decompose around 100 C, forming both oxides and pure metals of the three metals.

Sounds too good to be true, just evaporate the nitrate solution and then heat it up to above 350 C. The toxic selenium dust that is formed when the smoke cools down is then removed in a gas filter or in a scrubber.

It can't be this simple, I'm probably missing something, so what am I missing? :?

Selenium dioxide is a toxic compound that is turned into selenous acid in contact with moisture, so don't inhale the smoke. It is chemically related to arsenic so it reacs in a similar way. In copper smelters they remove arsenic when they roast the ore, or via oxygen sparging the molten copper and then extraxt it as dust in huge filter houses. So maybe my idea isn't too far out.

No guarantees that it will work though.

Göran

 

[Lou]

Maybe consider making the nitric solution basic and separating it off as sodium selenate which can then be reprecipitated with sulfite (under acidic conditions) to reddish brown Se metal(loid)?

If it helps, even precipitate the AgCl, leave the Pd in with the selenic acid, then try and precipitate the Pd like done in bromate hydrolysis.

Pd should stay as hydroxide. Boiling maybe needed to get good filtration.

 

[nickvc]

Maybe consider making basic and separating it off as sodium selenate which can then be repricipated with sulfite (under acidic conditions) to Se metal?

If Lou suggests something listen very carefully and follow his advice.

 

[Lino1406]

The following information shows that choosing the reduction path involves optimization (of times, concentrations, temperature...) whether you choose ascorbic acid, FeSO4 because some redox potentials are close
Redox potentials relevant to selenium/palladium/gold precipitation:
AuCl4- /Au 1.00v
HSeO3-/Se 0.901v
FeSO4/Fe+++ 0.771v
Ascorbate ion/ascorbyl radical 0.701v
PdCl4--/Pd 0.622v, sensitive to pH, concentrations

 

[sena]

Cuppelation would get rid of lower melting point metals like tin lead selenium