Reducing agents for PGMs

[orvi]

Hi
I combined my personal knowledge with all I could find on the Internet but I still have a feeling that I didn´t covered it all. My main focus is now on PGMs and palladium in particular.

We all know classic cementations with base metals such zinc, aluminium, iron or copper. Work OK, but always leave residues of that metal in the precipitate, sometimes are tricky or incomplete (PGMs on copper), or rather precipitate nearly everything (iron, zinc...). Plus leave more waste in solution, which is not nice.

Then, there are inorganic reducing agents such as borohydride and hydrazine. Effective, but borohydride for PGMs is not that great, leaving many times very fine suspensions of flammable metal powders. Hydrazine is great, but hard to get (here) and hazardous - toxic and explosive.

And finally, organic reducing agents - for use in PGMs reduction mainly pronounced formic and ascorbic acid. I found formic acid to work well, altough somewhat slow - but whatever. Ascorbic acid is quicker, but vast ammounts are needed eg in processing palladium from nitrate solutions, and it also have tendency to drag copper.
I am currently in the middle of refining one stubborn material - leach from MLCCs. It is now in the stage where it has Pd as major component, alongside copper, nickel and residues of tin. Vast majority of tin, lead and barium were removed prior. I did not make it, but I accepted that I will help with the refining to the Pd button.
Problem with this feed is - it contain quite a bit of solubilized junk from ceramics (and possibly some tin residues, as tin is amphoteric). So anytime I want to approach it by some organic precipitants (which I am used to use), I need to raise the pH. Precipitate of this junk always appears - filtering is OK, I filter it crystal clear, and proceed. But then as reduction start, this junk will almost everytime appear again, creating myriad of nucleation sites and causing precipitation of ultrafine, contamined palladium powder - nightmare to deal with. Currently, due to tin/lead and barium removal step it is in nitric/nitrate enviroment (first formic acid precipitation was from nitric).

Ascorbic and formic acid by their very nature need at least low acidic pH to become reducing agents. But there is certainly more reducing agents in class of organic compounds or inorganic compounds that will reduce precious metals.
My question is, how the reduction could be done without raising the pH ? Is there any reducing agent that will work in acidic solutions for precipitation of say palladium, other than ones I mentioned ? DMG is my last resort - I know that it will save the day if it comes to palladium but DMG has it´s own problems, could be done of course, altough I feel it is unnecessary to use it here

I thought about formaldehyde, as it has stronger reducing power than formic acid.

Any input would be greatly appreciated

Thanks
orvi

 

[Platz]

Bubbling carbon monoxide or hydrogen through would work if the palladium was present as a salt. Not sure if it would if it is in solution. But you could probablly drop the palladium as ammonium chloropalladate at high acid with a good selectivity over tin, filter it then reduce it with either of those gases or burn it. Make sure you have a good hood and gas monitor if you try this.

 

[orvi]

Bubbling carbon monoxide or hydrogen through would work if the palladium was present as a salt. Not sure if it would if it is in solution. But you could probablly drop the palladium as ammonium chloropalladate at high acid with a good selectivity over tin, filter it then reduce it with either of those gases or burn it. Make sure you have a good hood and gas monitor if you try this.

I have access to hydrogen cylinder, so maybe this will be viable method. If I get it correctly, I drop the palladium salt, filter it, suspend in water or so - and bubble hydrogen through ? Is pH critical during this transformation ?
How badly palladium powder obtained by this method burn in the air (there would be good ammount of hydrogen adsorbed) ?
Thank you for the suggestions

 

[lazersteve]

Search for papers written around 1868 by Bunsen on the topic of separation of PGMs. He details the use of molten zinc as a recovery/collection method from raw ores followed by hydrogen gas reduction.
This is the same Bunsen that the famous "Bunsen Burner" is named from. Frankly I'm surprised I haven't seen his work mentioned more on the forum.

Steve

 

[kurtak]

Search for papers written around 1868 by Bunsen on the topic of separation of PGMs. He details the use of molten zinc as a recovery/collection method from raw ores followed by hydrogen gas reduction.
This is the same Bunsen that the famous "Bunsen Burner" is named from. Frankly I'm surprised I haven't seen his work mentioned more on the forum.

Steve

Hi Steve & thanks for chiming in

I did a search using - "separation of pgms from ore by bunsen " - but did not find any papers by Bunsen

Could you provide a link ?

Kurt

 

[Lou]

I have access to hydrogen cylinder, so maybe this will be viable method. If I get it correctly, I drop the palladium salt, filter it, suspend in water or so - and bubble hydrogen through ? Is pH critical during this transformation ?
How badly palladium powder obtained by this method burn in the air (there would be good ammount of hydrogen adsorbed) ?
Thank you for the suggestions

This is your easiest route to pure ish Pd.

At a few atmospheres H2 at room temp is pretty selective for Pd over all other elements.

60 psi at room temperature will give you Pd. It is best if it is not highly acidic.

Pd, Pt, Rh, Ag, Ru, Ir in that order of ease to reduce from solution with hydrogen. Ir is practically impossible

 

[lazersteve]

Search for "On Rhodium" +Bunsen + 1868

If you can't find it, I will dig you up a copy. I believe the original version is not in English (German if my memory is correct ).
The translation was missing the drawings, but the original isn't.

Steve

 

[butcher]

Robert Bunsen german He has several chemistry books written in the German language. online book stores carry them.

 

[lazersteve]

This is a letter/article and is titled "On Rhodium", it is 3-5 pages long depending on which version you get.

Steve

 

[butcher]

https://www.abebooks.com/book-search/author/kirchhoff-gustav-robert-wilhelm-bunsen/

https://www.goodreads.com/author/show/5049533.Robert_Bunsen
https://bukrate.com/author/robert-bunsen

 

[orvi]

This is your easiest route to pure ish Pd.

At a few atmospheres H2 at room temp is pretty selective for Pd over all other elements.

60 psi at room temperature will give you Pd. It is best if it is not highly acidic.

Pd, Pt, Rh, Ag, Ru, Ir in that order of ease to reduce from solution with hydrogen. Ir is practically impossible

That is very good news. I am familiar with hydrogen, and hydrogenations in general, as I have done it for quite a while during my research. In reversed manner, of course - I was using palladium catalysts to produce active palladium species for reductions of various functional groups such as azides or alkenes

Does stainless steel stand these conditions ? I think it isn´t most suitable material for this kind of job, since it could react with liberated acids (from chlorocomplexes of PGMs reduced) and also by PGM salts themselves.
We do have nice glass Buchi small 200 mL autoclave tho, which will do the job for small batches.

But 60 psi is roughly 4 bars, room temperature, that make a nice room for creativity in terms of suitable vessels

 

[orvi]

Search for "On Rhodium" +Bunsen + 1868

If you can't find it, I will dig you up a copy. I believe the original version is not in English (German if my memory is correct ).
The translation was missing the drawings, but the original isn't.

Steve

Very interesting reading. Thank you

 

[Yggdrasil]

Very interesting reading. Thank you

Orvi, did you find it?

 

[orvi]

I believe this is the article Steve was reffering to interesting reading. I am familiar with Bunsen and his pronounced work, but I never knew he was "into" PGMs.

 

[Yggdrasil]

I believe this is the article Steve was reffering to interesting reading. I am familiar with Bunsen and his pronounced work, but I never knew he was "into" PGMs.

Thanks, I searched around a bit but did not seem to find it.

 

[lazersteve]

That's it.

 

[Yggdrasil]

Quite interesting read. And this was done in 1868 ish.
Impressive

 

[lazersteve]

It never fails to amaze me what the chemist were doing with limited equipment back then. No internet, no off the shelf supplies, and many times all alone.
The scientist of those days were nothing short of genius in my book. They did all these things in pretty much primitive lab conditions. He mentions two interesting instruments which I found fascinating: the hydrogen generator and the vacuum system. The names he uses for them are the Dobereiner's apparatus and the Sprengel's principle (aka Sprengel's pump).
A few safety notes above and beyond the standard hazards associated with working with mixed PGMs, gases, and acids are required for anyone looking to implement this information.
1) Molten zinc/sal ammoniac must be used with proper ventilation. A key point is that the zinc should be kept a just above its melting point. This mp increases as it absorbed PGMs in the reaction.
2) Barium compounds, especially solutions, are toxic. All barium solutions used should be converted to the sulfate for collection and proper disposal. Do not mix barium solutions with your normal waste, instead treat it separately.
3) Generation and or use of hydrogen gas requires additional safety and handling measures
4)Generation and handling of chlorine gas requires additional safety and handling measures.
5) Elemental Iodine is illegal in many countries today. Be especially mindful of any solution be it a mother liquor or leftover/wash solution. Keep these separated from your normal waste stream and properly dispose of them.

Steve

 

[orvi]

It never fails to amaze me what the chemist were doing with limited equipment back then. No internet, no off the shelf supplies, and many times all alone.
The scientist of those days were nothing short of genius in my book. They did all these things in pretty much primitive lab conditions. He mentions two interesting instruments which I found fascinating: the hydrogen generator and the vacuum system. The names he uses for them are the Dobereiner's apparatus and the Sprengel's principle (aka Sprengel's pump).
A few safety notes above and beyond the standard hazards associated with working with mixed PGMs, gases, and acids are required for anyone looking to implement this information.
1) Molten zinc/sal ammoniac must be used with proper ventilation. A key point is that the zinc should be kept a just above its melting point. This mp increases as it absorbed PGMs in the reaction.
2) Barium compounds, especially solutions, are toxic. All barium solutions used should be converted to the sulfate for collection and proper disposal. Do not mix barium solutions with your normal waste, instead treat it separately.
3) Generation and or use of hydrogen gas requires additional safety and handling measures
4)Generation and handling of chlorine gas requires additional safety and handling measures.
5) Elemental Iodine is illegal in many countries today. Be especially mindful of any solution be it a mother liquor or leftover/wash solution. Keep these separated from your normal waste stream and properly dispose of them.

Steve

I am reading and studying research papers every day in my work. If I could decide, I always pick the old paper or letter first - reason being, it is less likely some kind of bs.
Lots of science published today is rubbish, just to make points to climb professional ladder. Irreplicable syntheses, manually corrected spectra, inflated yields, completely made-up results etc. As organic chemist I can relate to this very very much. Science has changed a lot.
Also, the experiment description is very thorough in old papers. Observations are pointed out, suggestions given, colours noted etc... Today, procedure is like - dissolve, stir, filter, wash, column chromatography...

There are exceptions. But trend is clear

---------
Designs of some old apparatus were very clever. Old folks were much more "practically skilled" in my opinion, realizing the problems and adapting their solutions with materials they had in hand. One of the most practical, and very old inventions, is aspirator vacuum pump. I use it pretty much all the time, as it allow one to suck corrosive vapors in without damaging the pump itself (glass ones are the best). They are subsequently neutralized in solution, dispersed and easily contained. I like it a lot.

-----
Lots of people do not realize how toxic barium is. Most commonly, people in refining will encounter barium as part of ceramic used for MLCCs manufacture - barium titanate. However, this is relatively safe as compound, unless you attemp to leach it with acid. Leachin MLCCs with HCL results in liqification of Ba as soluble BaCl2 - many folks avoiding pyrometallurgy will just dump the MLCCs straight to the acid and start boiling. This liberate quite a bit of barium, which then should be scavenged with sulfate afterwards. BaSO4 is practically harmless, altough I don´t know if it is still categorized as "hazardous waste".

 

[Yggdrasil]

It never fails to amaze me what the chemist were doing with limited equipment back then. No internet, no off the shelf supplies, and many times all alone.
The scientist of those days were nothing short of genius in my book. They did all these things in pretty much primitive lab conditions. He mentions two interesting instruments which I found fascinating: the hydrogen generator and the vacuum system. The names he uses for them are the Dobereiner's apparatus and the Sprengel's principle (aka Sprengel's pump).
A few safety notes above and beyond the standard hazards associated with working with mixed PGMs, gases, and acids are required for anyone looking to implement this information.
1) Molten zinc/sal ammoniac must be used with proper ventilation. A key point is that the zinc should be kept a just above its melting point. This mp increases as it absorbed PGMs in the reaction.
2) Barium compounds, especially solutions, are toxic. All barium solutions used should be converted to the sulfate for collection and proper disposal. Do not mix barium solutions with your normal waste, instead treat it separately.
3) Generation and or use of hydrogen gas requires additional safety and handling measures
4)Generation and handling of chlorine gas requires additional safety and handling measures.
5) Elemental Iodine is illegal in many countries today. Be especially mindful of any solution be it a mother liquor or leftover/wash solution. Keep these separated from your normal waste stream and properly dispose of them.

Steve

Yes they were practical in most respect because they had to.
And their theoretical chemistry had to beyond anything today with respect to ponder out ways of doing things.
So of course the practical performance.
He several times refers to the Mother liquor having or not having PGMs in it,
did they have some kind of spectrograph to test this with or how did they do it?
Of course the safety was in a different galaxy comparing to today, maybe soemtimes because of lacking data regarding long term exposures,
but also because it was more acceptable to take a certain amount of risks then.