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
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