# Selective precipitation of rhodium



## Inter Refiner (Sep 5, 2020)

Hello friends, 
I have a pgm powder of pt, pd, rh, I dissolved them in aqua regia, but now I need to selectively precipitate rhodium from the solution without precipitating pt or pd, is there any chemical or a process to do that.

Thank you for your time


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## Martijn (Sep 5, 2020)

Maybe this helps: 
_"The selective precipitation of rhodium was performed with (NH(4))(2)S from platinum free raffinate with a recovery of >99%."_ from:
https://www.researchgate.net/publication/225184454_Separation_of_platinum_and_rhodium_from_chloride_solutions_containing_aluminum_magnesium_and_iron_using_solvent_extraction_and_precipitation_methods

First use selective precipitants for the Pt and the Pd i would guess. You' ll have only the rhodium left and can cement that or precipitate.


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## Inter Refiner (Sep 5, 2020)

Thank you for this interesting article


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## Martijn (Sep 5, 2020)

Treating the powder with concentrated nitric acid would dissolve the Pd before going to AR.


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## orvi (Aug 2, 2021)

Martijn said:


> Treating the powder with concentrated nitric acid would dissolve the Pd before going to AR.



heating the soln of Pt Pd Rh powders (in my case Zn-cemented) in HNO3 resulted in dissolution of Pd alongside Pt, but the soln was boiled for 2 days. never gone to completion in terms of Pd in the solid residue (about 20-30% of Pd stayed in the cake), i dont know why this happened, but it was XRF measured in various stages.
separating Rh from PtPd is very tricky process, mainly if th Rh content is low.


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## Lou (Aug 2, 2021)

Some amount of the Pd was PdO which for all intents and purposes is inert to any acid that's not HBr or HI.


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## Yggdrasil (Aug 2, 2021)

Now I got a bit confused.

In an earlier post a few years back, I claimed the same and was then clearly told that PdO dissolves in HCl.
Still beeing a rookie I accepted the correction.
Now we are told the opposite, so if you don't mind Lou, 
can you elaborate a bit?


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## Lino1406 (Aug 2, 2021)

Maybe this can help: PdO loses oxygen at 750C


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## Yggdrasil (Aug 2, 2021)

Thanks Lino  
But if you read the question once more,
it does not help to explain whether or not PdO 
dissolves in HCl.

Do it dissolve, but in minute amounts 
or do it dissolve in reasonable amounts?


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## Lino1406 (Aug 3, 2021)

As Lou said, only in hot acid and in minor amount


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## zacchy (Sep 24, 2021)

Iridium and rhodium may be precipitated with diethylene triamine. These
precipitation reactions can be used to separate rhodium and iridium from each
other.


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## zacchy (Sep 24, 2021)

Yggdrasil said:


> Thanks Lino
> But if you read the question once more,
> it does not help to explain whether or not PdO
> dissolves in HCl.
> ...



Well, when there are hydroxides that are produced by borohydride in mother waters, I have seen that the bottom of this mud is not attacked by aqua regia ... I did not think it was PdO


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## Lou (Sep 26, 2021)

Usually borohydride product is completely soluble in aqua regia. It has a high enough surface area.

What are you trying to do?


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## justinhcase (Sep 26, 2021)

I can get enough Rh into solution to detect, but nowhere near the levels needed for a recovery.
Let me know if anyone works out a practical recovery method, please!


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## zacchy (Sep 26, 2021)

justinhcase and Lou, friends.
*Organic Precipitation*
Organic reagents can be used to separate metals by selective precipitation.
Palladium can be precipitated with dimethylglyoxime. This reaction is used to
remove palladium in platinum purification.
Iridium and rhodium may be precipitated with diethylene triamine. These
precipitation reactions can be used to separate rhodium and iridium from each
other.

Diethylenetriamine (DETA) I think this method is used by Heraeus


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## Lou (Sep 27, 2021)

I can't comment on other refiners using DETA or its salts but can say that it works very well in certain circumstances/solution conditions as I said in another thread. Some people prefer to make the nitrite salt instead and take that to sponge as it is easier to remove Pt(IV) from that product than from the DETA product.

The best thing that can be done with Rh is to work with very concentrated solutions.
Rh is easily solublizied with sodium peroxide or molten salt chlorination. From those solutions it can be precipitated in strongly acidic solutions and the mother liquors sent to cementation.


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## zacchy (Sep 27, 2021)

Si, se puede tener las reducciones de bastantes aguas madres, este polvo negro fundirlo con algún persulfato de sodio o potasio en un crisol de cuarzo, pero creo que queda Rh contaminado, por lo regular, la sal de Claus tambien puede usarse para esto.


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## NanoCat (Nov 2, 2021)

Lou said:


> I can't comment on other refiners using DETA or its salts but can say that it works very well in certain circumstances/solution conditions as I said in another thread. Some people prefer to make the nitrite salt instead and take that to sponge as it is easier to remove Pt(IV) from that product than from the DETA product.
> 
> The best thing that can be done with Rh is to work with very concentrated solutions.
> Rh is easily solublizied with sodium peroxide or molten salt chlorination. From those solutions it can be precipitated in strongly acidic solutions and the mother liquors sent to cementation.


Lou, I was thinking about using NaOH/KOH fusion on the suspected Rh product. you have some details for this? sounds cleaner direct to Chloride: "molten salt chlorination"


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## Lou (Nov 2, 2021)

You'll need an oxidant. No point in mixing either but I'd keep the KOH out of it.

It's six of one half dozen of the other. NaOH/NaNO3, Na2O2...etc.


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## NanoCat (Nov 2, 2021)

thnx Lou, I found some Na2O2 in the chem cupboard. what ratio of each to the amount of insoluble metal powder? Temp? And NaCl in the fusion to get it to a Chloride?


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## Deane (Nov 2, 2021)

I thought I knew all the terms in chemistry but several people here use "cementation". What is that? Same as precipitation?


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## FrugalRefiner (Nov 2, 2021)

See When In Doubt, Cement It Out.

Dave


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## NanoCat (Nov 4, 2021)

I used a self forged Ti boat (that I had previously used for Pt fusion) , 500 mg blacks + 2g NaOH to get a good mix. Heated from bottom with MAP torch on very low with orange heat on bottom. As the melt proceeded, I would get orange mini fireballs in the melt and orange flames at edge of Ti boat. Then OOPS, the Ti boat melted hole in bottom. I was able to retrieve almost all of the melt. Added HCl to cover boat immersed in the beaker. Got the boat cleaned, pulled it then added H2O2 and got it hot for about an hour. Vac filtered and solution is yellow/reddish brown, depending on the lighting: more reddish from bottom against incandescent light. I'm boiling it down to remove the H2O2 then go for NH4Cl and see what I get. Best guess Pd/Rh? That's what the XRF says is predominant in the starting material, some Pt and negligible Ir (error). Whaddaya think? I have probably 150g of these blacks that are highly reflective white metal under scope light.


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## Yggdrasil (Nov 4, 2021)

NanoCat said:


> I used a self forged Ti boat (that I had previously used for Pt fusion) , 500 mg blacks + 2g NaOH to get a good mix. Heated from bottom with MAP torch on very low with orange heat on bottom. As the melt proceeded, I would get orange mini fireballs in the melt and orange flames at edge of Ti boat. Then OOPS, the Ti boat melted hole in bottom. I was able to retrieve almost all of the melt. Added HCl to cover boat immersed in the beaker. Got the boat cleaned, pulled it then added H2O2 and got it hot for about an hour. Vac filtered and solution is yellow/reddish brown, depending on the lighting: more reddish from bottom against incandescent light. I'm boiling it down to remove the H2O2 then go for NH4Cl and see what I get. Best guess Pd/Rh? That's what the XRF says is predominant in the starting material, some Pt and negligible Ir (error). Whaddaya think? I have probably 150g of these blacks that are highly reflective white metal under scope light.


??????
Very confusing story.
What did you start with from where?
0.5 grams (500mg) of black and so on.
If it is sponge, dissolve in HCl and peroxide. Then drop again.
It is not enough to separate in decent amounts I think.


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## NanoCat (Nov 5, 2021)

the Blacks won't go into solution with HCl and KClO3, and HCl/H2O2 doesn't put a dent in it either. that why the attempted fusion to Chloride solution


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## Yggdrasil (Nov 5, 2021)

NanoCat said:


> the Blacks won't go into solution with HCl and KClO3, and HCl/H2O2 doesn't put a dent in it either. that why the attempted fusion to Chloride solution


You still has not answered the questions.
Where did it come from?
How was it treated?

The whole story

During the melt in the Ti boat it probably fused with Ti as well.

You could try Sodium peroxide or oxone, but I have no idea if it will work with your mix.


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## upcyclist (Nov 5, 2021)

Deane said:


> I thought I knew all the terms in chemistry but several people here use "cementation". What is that? Same as precipitation?





FrugalRefiner said:


> See When In Doubt, Cement It Out.



Deane--it's more of a refiner's term than a chemist's. As Dave explains in the linked article, it's the precipitant from a replacement reaction. 

I'm not enough of a chemist to know if the term "sponge" is in the same boat or not.


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## FrugalRefiner (Nov 5, 2021)

If you run into other terms you don't recognize, take a look in A Glossary of Common Terms.

Dave


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## NanoCat (Nov 5, 2021)

Yggdrasil said:


> You still has not answered the questions.
> Where did it come from?
> How was it treated?
> 
> ...


it is from a mining concentrate. I'm just trying to get the handle on what it is. I need a good XRF on it. I tried it out as a catalyst today in an organic dehydrogenation and it works perfect. So PGM canttellyarite. But can't find that on the periodic table Group 8.
And looking for someone to take it to a refiner.


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## Yggdrasil (Nov 5, 2021)

What kind of mining?
Where and so on?

"canttellyarite" You are kidding...
Right???
Read that out loud.


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## NanoCat (Nov 6, 2021)

Anyone have a reliable XRF with standards? BTW, I did use the product in an organic dehydrogenation yesterday and it works as any good PGM should. I'm still thinking mostly Pd.


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