# dissolving rhodium



## rayo13 (Jan 2, 2013)

Hello and Happy New year! Wondering what acid solutions are best to dissolve rhodium powder. Any help is greatly appreciated. Thanks


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## butcher (Jan 2, 2013)

I would use a bisulfate fusion to make a water-soluble solution.

Rhodium is very acid resistant.

Hot concentrated H2SO4 will attack it but this is very dangerous and I do not recommend it, the bisulfate fusion will work about the same way without these dangers. 

If these are fairly pure powders you may be better off just selling them to a refiner if you have enough of them.

Of course the answer would also depend on where these powders came from if they were mixed with other metals or were an alloy with another metals,other conditions and so on.


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## nickvc (Jan 2, 2013)

I have to agree with Butcher rhodium is very difficult to dissolve with acids and refining PGMs is a lot more difficult than dealing with gold or silver. As Butcher also stated it depends on exactly what you have and from where, if you have decent amounts perhaps Lou is the man to talk to, not many members can refine rhodium successfully.


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## Gratilla (Jan 2, 2013)

The pdf at SSN PDF will give you a Pourbaix Diagram for the (Eh/pH/temp) conditions under which Rhodium can be dissolved using SSN (Saturated Saline Nitric acid).

I'm sure there is also an organic solvent for Solvent Extraction to high purity.


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## freechemist (Jan 4, 2013)

@Gratilla,

The link, you are giving in your post has nothing to do with dissolution of metallic rhodium, nor is Saturated Saline Nitric Acid (SSN) it's subject. As far as I know, there exists no simple and at the same time efficient solvent extraction process for Rhodium(III). Unlike Iridium(III) it is not easily oxydized to the tetravalent state, in which form it can easily been extracted e.g. as an anionic hexahalo-complex anion into an organic phase, or been absorbed by an anion-exchanging resin.


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## Gratilla (Jan 4, 2013)

Ooops.  

Fixed - Try again.


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## Gratilla (Jan 4, 2013)

freechemist said:


> As far as I know, there exists no simple and at the same time efficient solvent extraction process for Rhodium(III). Unlike Iridium(III) it is not easily oxydized to the tetravalent state, in which form it can easily been extracted e.g. as an anionic hexahalo-complex anion into an organic phase, or been absorbed by an anion-exchanging resin.



Hmmm, perhaps. I based my comment not on personal experience but on an extrapolated ASSumption of the increasing importance being placed on SX (Solvent eXtraction), particularly for extraction of low concentration values from hydrometallurgical treatment of ores. A Google of "rhodium solvent extraction" turned up numerous hits, including this, which is a systematic study on the solvent extraction of rhodium with TBP [TriButyl Phosphate] in hydrochloric acid.


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## butcher (Jan 4, 2013)

Gratilla,
It sounds as though you may have more schooling in chemistry than I do, But my personal belief is you can find a lot of data or papers on experiments that may work in a lab (or in a particular study) and look very promising, but are really not to useful, or practical in the working environment, or in the real world, theory and practice can be two different animals, although I can be wrong.


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## Gratilla (Jan 4, 2013)

Butcher, I think you are (at least partially) right. I bumped into a lot of caveats, problems and dead ends when leaching with thiosulphate a few years back (when it was in fashion), but Solvent Extraction is different IMO; it's been around a long time - particularly in the food industry. It has a lot going for it (compared with other methods of separation/purification) and is already a well-established technique with a number of metals; for others there is a lot of ongoing research. TBP (TriButyl Phosphate) is a well-known organic solvent, although a bit wide-spectrumish, so it may pull over a number of other metals in addition to rhodium. Multiple extractions and/or tweaking the pH (from a look at the Distribution Curve), first on extraction and then on scrubbing should take care of that though.


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## Lou (Jan 5, 2013)

Doesn't work well. Easier to refine the Rh and Ir from their oxides post hydrolysis.


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## freechemist (Jan 5, 2013)

Gratilla,

Thanks for the link to the article about rhodium-extraction with tributylphosphate (TBP) in an organic solvent. I read only the abstract, and was not willing to pay 42 dollars to buy a copy of the complete paper.

To me the abstract shows clearly, that the described extraction process may be only of academicinterest (if ever), and is definitely not feasible for rhodium recovery/refining in practice. Thus, butcher in his reply is completely right. The rhodium-concentration for optimum-extraction is about 1.3 g Rh per liter. Extraction of Rh is done after addition of 4 moles of stannous chloride (SnCl2) per mole of Rh, which means that the solution contains in addition 5.9 g tin per liter, and finally, the solution to be treated is 3M in HCl, corresponding to an acid-concentration of 109 g HCl per liter. According to the abstract, optimum-extraction of rhodium occurs, if equal volumes of aqueous solution and organic phase are used. So, recovery of a bit more, than 1 gram of rhodium means an enormous expenditure, not even taken into account additional outlay for getting rid of all the tin involved in the process. And as a conclusion: Lou, too, is completely right, stating that recovery/refining of Rh is much easier by e.g. precipitating rhodium hydroxide (Rh(OH)3) after it's solubilisation in a strongly acidic salt-melt.


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## Gratilla (Jan 6, 2013)

And so by extension, freechemist, I guess I am completely wrong! <he he :mrgreen:>

The link was just an example from many thousands turned up by a quick google. I don't know what a/the useable organic solvent for rhodium is, but I do know that there is (at least) one. Some months ago I saw on an online source fairly detailed flowcharts of extraction/refining systems for three major mining companies. The final refining for virtually all the PGMs (for all 3 cos) was done by Solvent Extraction (SX) - and each company generally used different solvents. Unfortunately, I've just had to reinstall my Dimdows XP and although I backed up virtually all my files, my Chrome Bookmarks file wasn't one of them. <Arghhh!>

SX solvents and extraction/scrubbing systems (particularly for PMs) are a moving target, with new research being done virtually daily. For example, the best known organic solvent for AuCl3 is Butyl Diglyme, but it has been superceded as the best by another one - now being used in the Mintek Minataur(TM) licensed gold refining system.

What is it? And why? Excellent questions. <he he :mrgreen:>


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## freechemist (Jan 6, 2013)

Gratilla,

From your reply I get the impression, that you miss two very important points.

1.) The most crucial factor in the solvent-extraction process of our dispute is not the nature/species of the organic solvent used. It is the rhodium-concentration in both phases. A general concentration of only about 1 g Rh/liter in a certain Rh-recovery/refining-process is simply too small to be useful in the production of multigram quantities of pure rhodium, even on a reasonable lab scale, and certainly much too small for industrial production of pure rhodium. Especially nowadays, when rhodium-pricing is deep in the cellar, compared to it's value 10 to 20 years ago.

2.) You're right, a lot of research may be done on solvent-extraction-processes, especially for PM's. - But what reason for? - Let me do a guess: because there exists not yet one industrially well established PM-recovery/refining-process, exempt, may be, the extraction of gold with Butyl Diglyme.


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## butcher (Jan 6, 2013)

When working with ore and its complications of the chemistry involved, it can be much different than working with more pure metals or scrap in recovery, most every mine with their ore, the chemistry involved in the ore, and the chemistry involved in recovering values from the ore can be different, here some of the solvent extraction's or organic recovery methods may prove to work better than other conventional methods, although I suspect there is not much conventional about recovery of values from ore, it is normally an experiment and see what works best operation.

Freechemist, I just wanted to say Thank You, I have been learning some very good tidbits from your posts.


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## Gratilla (Jan 6, 2013)

freechemist, I wasn't aware that our friendly discussion had escalated to dispute status. More seriously:

1a.) I think it is clear from my previous posts that I'm selling neither TBP nor a specific rhodium SX methodology; I was merely trying to be helpful in pointing out that many SX methods and organic solvents clearly exist. And an SX option is worth at least being aware of.

1b.) As you seem keen to make specific debating points (some people like a good debate), I revisited your previous two posts and found both your logic and conclusion(s) to be flawed.

i. _"The rhodium-concentration for optimum-extraction is about 1.3 g Rh per liter."_

The fact that this is optimum doesn't preclude a range of other rhodium concentrations in the aqueous fraction (and one of the reasons for banks of counter-current SX reactors in commercial systems). For a small extraction, splitting the organic solvent into two (or more) fractions and repeating the extraction with each will decrease the Rh in the aqueous fraction significantly.

ii. _"Extraction of Rh is done after addition of 4 moles of stannous chloride (SnCl2) per mole of Rh, which means that the solution contains in addition 5.9 g tin per liter, and finally, the solution to be treated is 3M in HCl, corresponding to an acid-concentration of 109 g HCl per liter."_

Minor expenses compared to the value of the rhodium extracted, particularly as the reagents can be reused.

iii. _"According to the abstract, optimum-extraction of rhodium occurs, if equal volumes of aqueous solution and organic phase are used. So, recovery of a bit more, than 1 gram of rhodium means an enormous expenditure, not even taken into account additional outlay for getting rid of all the tin involved in the process."_

If you'd have looked at the graphs more carefully, you'd have seen that one shows max Rh/solvent loading of at least 10gms per L before it shoots off the top of the chart.

iv._ "The most crucial factor in the solvent-extraction process of our dispute is not the nature/species of the organic solvent used. It is the rhodium-concentration in both phases."_

Ahh, you mean the Distribution Coefficient [DC]. However, if you look at the Extraction Equilibrium graph, you'll see a DC factor of around 50! More than sufficient for practical extractions.

For a commercial level system the Rh SX link appears quite reasonable. Even for the relatively trivial (in the logical sense) case of around 1 gm Rh and a 500 ml separatory funnel, the cost both relative and otherwise is hardly an "enormous expenditure".

2.) Right, research is being done because there is research to be done ... and Gold/Butly Diglyme [DBG] is no exception. BDG is only relevant for acid-side chloride extractions (and as I've already mentioned, there is a better organic solvent already being used commercially). More important volume-wise is alkali-side cyanide extractions, for which a number of practical organic systems have been identified. Why has there not been a greater uptake by industry? The main reason is the capital cost in change-over from charcoal, zinc cementation, electrowinning, etc. But it's happening. And I think you'll find that new refineries are increasingly turning to SX.


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## Lou (Jan 6, 2013)

SX is old tech...MRT (although expensive), is, in some instances, the best.


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## freechemist (Jan 8, 2013)

*@Gratilla:*

You're right. I didn't read all the graphs too carefully. I focussed only on what is cited in the abstract, and on some graphs, related to the rhodium/tin-chemistry involved. So, it may well be, that I missed something.


*@Lou:*

Although I have some practice in PM-chemistry, I am no expert in abbreviations, and have absolutely no idea, what *MRT* means. Please enlighten my humble mind, explaining to me what for *the shorthand MRT* is standing.

Thanks in advance and regards,

freechemist


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## Lou (Jan 8, 2013)

Molecular Recognition Technology.

If it were more cost-competitive, it'd be the wave of the future.


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## butcher (Jan 9, 2013)

Would this be Akin to filtering a concentrated PGM solution through filter membrane packed with NH4Cl?


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## HAuCl4 (Jan 9, 2013)

Rhodium has only 4 basic difficulties:

1-Difficulty to dissolve completely.
2-Difficulty to separate from impurities completely.
3-Difficulty to precipitate completely, and...
4-Difficulty to melt and form.

Otherwise it is a very simple metal to work with... if you find any, that is... :lol:


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## freechemist (Jan 13, 2013)

*@Lou:*

Thanks for having enlightend me. If my memory is functioning rightly, I heard the first time of MRT, attending an IPMI seminary about precious metal recovery and refining in 1989. So, this method is not very new, and may also be referred to as old technique, like solvent-extraction. Personally, I don't believe it to become more cost-competitive, at least in near future, and doubt it's role as a forthcoming wave. 

freechemist


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## Lou (Jan 13, 2013)

It's the cost differential.


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