Sciencemadness Discussion Board

Platinum Group Purification

elementcollector1 - 8-10-2020 at 17:57

Recently got a sample of natural placer platinum from a Canada gold paydirt company. I have reason to believe it contains significant amounts of other platinum group metals and tellurides, and want to isolate the platinum for some future work (and as many of the others as possible for potential element samples). Total mass is a few milligrams, so I'm not expecting much, but I've been developing a plan for extraction of each PGM and want some opinions.

(I'm not sure if I need to finely powder the sample for this to go smoothly, since the largest particle from visual inspection is no more than 1mm in any dimension.)

1) Heat metal in concentrated (white fuming, vacuum distilled) nitric acid to dissolve Re, Pd while not affecting others. Set aside as Solution A - this should have a yellow tinge from perrhenic acid and/or palladium nitrate in solution

2) Heat solid residue from 1) with aqua regia to dissolve Pt, Pd (if left) while not affecting Os, Ir, or Ru. Set aside as Solution B - this should have a deep red tinge from hexachloroplatinic acid (generally, as the reaction products are somewhat complex).

3) Heat solid residue from 2) with bleach (NaOCl) to dissolve Ru. This should result in an orange-red tinge. Set aside as Solution C.

4) Collect remaining, undissolved solid as Os and Ir.

I'm not sure where to go from here - I essentially need separation methods for Re/Pd and Os/Ir in solution, neither of which I understand are easy, and the problem only gets more difficult at this microscale for Os/Ir. Any thoughts?

RogueRose - 8-10-2020 at 18:31

I recently came across some method of refining PGM's and also gold & silver, by using lead to dissolve the metals and then using a pourous clay "crucible" of some type that allows the lead (oxidized I think) to soak into the clay body and it leaves the precious metal behind.

Ahh, I found the process. It's called "Cupellation".
https://en.wikipedia.org/wiki/Cupellation

IDK if this can be used but it may be worth using if you get a larger sample size.

elementcollector1 - 8-10-2020 at 18:43

Seen that in quite a few CodysLab videos, and I think it only works for separation from base metals? This is all native platinum (well, native PGM, really), so I don't think it would do much here. Unless there's a variant for PGMs specifically somewhere?

Panache - 8-10-2020 at 20:28

Theres a good method in this two volume analytical ch3mistry text from the 1930, im seperated from my library presently so i cannot even remember the name of the books. Someone else here has the two books also. God how us3eless am i, im going to my storage this weekend ill dig out the books and upload the cbapters. Its a very exhaustive treatment and is pre- instrumenrtation era

draculic acid69 - 9-10-2020 at 06:58

Quote: Originally posted by elementcollector1  
Recently got a sample of natural placer platinum from a Canada gold paydirt company. I have reason to believe it contains significant amounts of other platinum group metals and tellurides, and want to isolate the platinum for some future work (and as many of the others as possible for potential element samples). Total mass is a few milligrams, so I'm not expecting much, but I've been developing a plan for extraction of each PGM and want some opinions.

(I'm not sure if I need to finely powder the sample for this to go smoothly, since the largest particle from visual inspection is no more than 1mm in any dimension.)

1) Heat metal in concentrated (white fuming, vacuum distilled) nitric acid to dissolve Re, Pd while not affecting others. Set aside as Solution A - this should have a yellow tinge from perrhenic acid and/or palladium nitrate in solution

2) Heat solid residue from 1) with aqua regia to dissolve Pt, Pd (if left) while not affecting Os, Ir, or Ru. Set aside as Solution B - this should have a deep red tinge from hexachloroplatinic acid (generally, as the reaction products are somewhat complex).

3) Heat solid residue from 2) with bleach (NaOCl) to dissolve Ru. This should result in an orange-red tinge. Set aside as Solution C.

4) Collect remaining, undissolved solid as Os and Ir.

I'm not sure where to go from here - I essentially need separation methods for Re/Pd and Os/Ir in solution, neither of which I understand are easy, and the problem only gets more difficult at this microscale for Os/Ir. Any thoughts?


I remember reading a catalytic converter recovery paper that seperated a few different ones and if expecting rhodium or osmium (maybe) U add sulfuric acid to the first step.

Texium - 9-10-2020 at 07:03

With such a small sample, this sounds like an exercise in futility. Let’s assume for a minute that the ratio of elements in your ore is roughly the same as the average crustal abundance of the elements. You’d have roughly equal amounts of platinum and palladium, which together would probably comprise at least half of the sample. The rest of the elements (Os, Ru, Ir, Rh, Re) are even less abundant, some potentially tenfold less than platinum. If some are present as tellurides, then Te would be taking up a considerable amount of mass there too. If you only have “a few milligrams” then at most you could expect “a couple milligrams” of Pd and Pt, and sub-mg, barely visible amounts of anything else if you’re even able to separate it on this scale in a home environment without losing it all to mechanical losses.

If this is something you’re really keen on doing, I suggest you invest in at least a gram of material to work with. It would also be good to find a friend at a university with access to an AAS if you want to actually know what’s in your sample and whether you were successful at separating any individual elements, because you can actually use sub-mg amounts with that unlike any old fashioned qualitative tests that would be likely to consume your entire sample.

Panache - 12-10-2020 at 10:55

Quote: Originally posted by Texium (zts16)  
With such a small sample, this sounds like an exercise in futility. Let’s assume for a minute that the ratio of elements in your ore is roughly the same as the average crustal abundance of the elements. You’d have roughly equal amounts of platinum and palladium, which together would probably comprise at least half of the sample. The rest of the elements (Os, Ru, Ir, Rh, Re) are even less abundant, some potentially tenfold less than platinum. If some are present as tellurides, then Te would be taking up a considerable amount of mass there too. If you only have “a few milligrams” then at most you could expect “a couple milligrams” of Pd and Pt, and sub-mg, barely visible amounts of anything else if you’re even able to separate it on this scale in a home environment without losing it all to mechanical losses.

If this is something you’re really keen on doing, I suggest you invest in at least a gram of material to work with. It would also be good to find a friend at a university with access to an AAS if you want to actually know what’s in your sample and whether you were successful at separating any individual elements, because you can actually use sub-mg amounts with that unlike any old fashioned qualitative tests that would be likely to consume your entire sample.


jesus! you read the initial post/question fully, why didn't i think of that!! You are of course so absolutely correct and give good advice.
This is difficult chemistry both as your matrix is well pretty much rocks but you are attempting to remove and separate from them the most chemically resistant elements in the periodic table.
the aa will definitely get you the answers required however...just shitloads of work, which means money but then you will know what your expectations are. doing the analysis yourself from experience is not sensible....texium is spot on.

unionised - 12-10-2020 at 11:11

Quote: Originally posted by Texium (zts16)  


because you can actually use sub-mg amounts with that unlike any old fashioned qualitative tests that would be likely to consume your entire sample.


Quote: Originally posted by Panache  
....texium is spot on.


Well...

My copy of Vogel gives the following detection limits.

Gold 100 ng
Platinum 500 ng
It doesn't give a mass limit for palladium, but it will detect it in a solution at 10 ppm. That's something of the order of 500 ng.

And, being an old-school chemist, I'd expect to be able to achieve limits like that.

I'd imagine that the OP, even allowing for a lack of experience, might be able to do a hundred times worse and that's still sub- milligram.

So the first question is; why can't you?
The second question is why shouldn't he give it a go?


Panache - 12-10-2020 at 11:33

Ok i will rephrase/elaborate.

Absolutely give it a try, go slowly and thoroughly. That was my problem when doing this chemistry on a less complicated matrix with pgm's at home, impatience. It can be frustrating. There is a reason why the mix of metals costs far less than their separated counter parts. The AA thing i advocate because i eventually, after several months' pulled a favour to get the analysis just so i could understand if i was even in the ballpark and i wished i had begun with it.
Also hubris goes a long way to informing ones bias, lol. for some ridiculous reason i figured it should be a breeze, but im not an inorganic chemist. Had i acknowledged this immediately i would have read more beforehand.
So give it a try, without the taint of these discouraging replies...that being said i still think texiums advice will eventually bear out, the detection limits unionised quoted from vogel are not going to be achievable on your first seciond or tenth go, but you will get there if you stick to it...
ill still post the pages from...almost had the name of the book (its the super famous analytical methods text from the mid 30's, two compendiums, one with the elements thse other with sub section on natural products, coa and fuels, poisions in a corpse etc

elementcollector1 - 12-10-2020 at 14:18

Er... I think everyone could use a little clarification here.

I'm working with a milligram-scale sample of native metal (I expect maybe 1g as the upper bound on how much there is in the vial). This metal is primarily platinum, but according to surveys contains significant quantities of other PGMs (a survey conducted in 1886 on native platinum in this area found it contained 72.07% Pt, 1.14% Ir, 2.57% Rh, 0.19% Pd, balance Te, Fe, Cu), and hope to extract as many PGMs as possible from the mix (not necessarily in solid bulk form - a dilute but colored aqueous solution would be good enough).

This of course means that some of the proposed steps will do absolutely nothing - but if those quantities are off by one percent, they might well extract something visible.

zed - 16-10-2020 at 20:35

Perhaps. Have you weighed your sample?

Pt, sometimes confounds our estimating sense. I have a coin hereabouts. It weighs about twice what I would guess it to weigh.