# Rhodium when leaching CATs



## kurt (Nov 4, 2012)

How effective is the Cl on the rhodium when using HCL/Cl to leach CATs ?

I ask because in talking with another forum member I was told that the HCL/Cl method does not get all of the rhodium & to get it all you need to use an AR leach (hot) that has an addition of sulfuric acid in the AR.

What was said is that the HCL/Cl leach only dissolves the Rh that has been oxidized by the use of the CAT

Another why to put my question --- is the Cl in the HCL/Cl leach an "effective" oxidizer for the dissolving of Rh --- would HCL/Cl dissolve the Rh in the PGM black once it is recovered with zinc as opposed to AR dissolving the Pt & Pd but leaving the Rh behind

Will AR with the addition of sulfuric dissolve all three?

Just trying to figure out the chemistry here

Kurt


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## Lino1406 (Nov 4, 2012)

Concentrated sulfuric will heat the AR, temporarily.
So it may help with verification of Pt presence.
Normally, extended time is needed to get Pt.
Rhodium is extracted by either Bisulfate
or H2SO4 + NaCl (stinks of Chlorine). Anyhow
it takes MANY hours of heating.


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## kurt (Nov 5, 2012)

Lino 1406 thanks for the reply & yes I understand your answer when it comes to the "refining" process.

The question I am asking concerns the leaching process - when I use the HCL/Cl method Rh goes into solution - we know that because when zinc is used to recover the PGMs from solution Rh comes down with the Pt & Pd.

According to the person I have talked with the claim is that HCL/Cl leaching is not effective enough in getting all of the Rh to go into solution & that therefore you need to use AR (hot) with sulfuric added to get an effective & complete dissolving (& therefore complete recovery) of the Rh

He also claims that HCL has "no" effect on the alumina - however all of my research says it does

Kurt


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## freechemist (Nov 23, 2012)

@kurt:

Please excuse my late reply. - In your first post you wrote:

_"How effective is the Cl on the rhodium when using HCL/Cl to leach CATs ?"_

Chlorine (Cl2) is indeed effective as an oxidant for Rh-metal, - in an alkali chloride-melt. It may be effective in an aqueous system like HCl/Cl, especially at high temperature, under pressure.

_"What was said is that the HCL/Cl leach only dissolves the Rh that has been oxidized by the use of the CAT"_

This may well be the case. Rhodium oxide, presumably Rh2O3, dissolves in hot, fairly concentrated HCl, together with some alumina, to form soluble Rh-chloro-complexes and some aluminum chloride.

_"I ask because in talking with another forum member I was told that the HCL/Cl method does not get all of the rhodium & to get it all you need to use an AR leach (hot) that has an addition of sulfuric acid in the AR."_

Hot aqua regia may be effective, too, - without addition of sulfuric acid. Finely divided Rh-metal is said to dissolve slowly in molten NaHSO4 and/or in very hot, concentrated sulfuric acid. Aqua regia, with some sulfuric acid added, may be regarded as a quite dilute solution of H2SO4 in AR, in place of water, and therefor, IMHO is not more effective, than AR alone.

_"Another why to put my question --- is the Cl in the HCL/Cl leach an "effective" oxidizer for the dissolving of Rh --- would HCL/Cl dissolve the Rh in the PGM black once it is recovered with zinc as opposed to AR dissolving the Pt & Pd but leaving the Rh behind"_

To dissolve Pt/Rh-blacks, obtained by cementation with zinc in my practice the best method was to use 48% HBr (azeotropic mixture), oxidizing with 35% H2O2, and then refluxing the mixture for a prolonged time. In this case, the oxidant formed is mainly elemental bromine (Br2), which is much less volatile, than Cl2 (it condenses in the reflux condenser and flows back into the hot reaction mixture), and apparently also more effective.

_"Will AR with the addition of sulfuric dissolve all three?"_

I don't think so: see above.

In your second post you wrote:

_"He also claims that HCL has "no" effect on the alumina - however all of my research says it does"_
According to my practical experiences, I can only confirm your research.


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