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Author: Subject: Arsenic from Skutterudite
elementcollector1
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[*] posted on 2-9-2013 at 21:08
Arsenic from Skutterudite


A few years ago, I picked up a sample of skutterudite ((Co,Ni,Fe)As3). I'd like to know if it's possible to isolate pure arsenic metal from this. I'd like to avoid the formation of arsine (AsH3), as even with ground-glass apparatus I don't feel safe from that. My guess would be to sublime arsenic oxide off in a closed reactor (capped steel pipe + blowtorch?), and then hydrolyze the arsenic (III) oxide to yield arsenous acid in solution. Could I then reduce this in solution with aluminum foil to yield arsenic powder?
On another note, is a 'thermite' of arsenic oxide possible (again, steel tube + blowtorch reagent mix As2O3 + Al powder, arsenic sublimes out due to volatility (SP = 615 C)?)
Sorry for the jumbled collection of thoughts - I really have no knowledge of arsenic chemistry.


[Edited on 3-9-2013 by elementcollector1]




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[*] posted on 2-9-2013 at 21:47


This may help:

http://www.sciencemadness.org/talk/viewthread.php?tid=12045#...




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[*] posted on 3-9-2013 at 05:51


I didn't know these contained arsenic:
<img src="http://www.reddwarf.co.uk/news/2005/12/23/the-first-skutter/dvd_the_first_skutter_231205_2.jpg" alt="skutter" />

[sorry, couldn't resist]




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[*] posted on 3-9-2013 at 06:03


Quote: Originally posted by elementcollector1  
On another note, is a 'thermite' of arsenic oxide possible (again, steel tube + blowtorch reagent mix As2O3 + Al powder, arsenic sublimes out due to volatility (SP = 615 C)?)


[Edited on 3-9-2013 by elementcollector1]


Aluminothermy of As2O3 would be a profoundly BAD idea. 'Thermite' mixtures routinely develop temperatures in excess of 2,000 C and higher (depending on target oxide and other factors) and with a sublimation point of only 615 C that's a recipe for arsenic flying all over the place, because such reactions are hard to control. I'm sure you understand that would be a very bad thing.

Like antimony it can probably be obtained from roasting its oxide with carbon at high temperature, the element then sublimes off.

My advice with regards to arsenic preparation? Unless you are well equipped and well versed in its chemistry, leave well alone! Far too toxic.


[Edited on 3-9-2013 by blogfast25]




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[*] posted on 3-9-2013 at 06:38


Skutterudite ─ a highly learned mineral . . . :D



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[*] posted on 3-9-2013 at 11:16


Quote: Originally posted by Pulverulescent  
Skutterudite ─ a highly learned mineral . . . :D


I am just not getting these references today... First a robot, and now a scholar?

Anyway, what I meant was that the mixture would be in a *closed* steel tube, so that any arsenic that sublimed off would recondense on one of the caps (similar to the decomposition of arsenic sulfide mentioned in Magpie's linked thread). Unfortunately, that thread mentions that the pressure would be too great for even a quartz ampoule, so that's out of the question. In that case, I guess the way to go would be to produce arsenic using 'wet' chemistry, and then sublime it into crystals so long-term storage is safer.

A wet-chem method I keep hearing about is SnCl2, HCl and arsenic chloride. However, I'm confused, as arsenic trichloride reacts with water to form arsenous acid (As(OH)3) and chlorine. Maybe this is what the extra HCl is for?





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[*] posted on 3-9-2013 at 22:34


AsCl3 does not give chlorine with water, but it gives HCl and arsenious acid. This is an equilibrium reaction and in very concentrated HCl the reaction is more on the side of AsCl3. That is the reason of adding extra HCl. Without that, nearly all AsCl3 would hydrolyse.

If SnCl2 is used, then another reason for using lots of HCl is to keep this and its oxidation product dissolved. SnCl2 is notoriously sensitive to hydrolysis (formation of HCl and Sn(OH)2) and solutions in water always are somewhat turbid. The oxidation product, containg tin in oxidation state +4 is even more sensitive to hydrolysis.




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[*] posted on 4-9-2013 at 08:17


If I dissolved skutterudite in HCl, would a product be arsine? It seems likely, as the same reaction occurs for sulfides (producing hydrogen sulfide). If so, would a way to avoid this be passing dry Cl2 over the powdered mineral?



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[*] posted on 4-9-2013 at 10:04


Yes! I think arsine would be evolved! You shouldn't try this!

Why don't you simply buy arsenic? It can be found in mineral stores for very low prices. You can easily make very pure arsenic from this just by subliming it in a test tube. But in most cases the "native arsenic" that you can buy is pure enough for an element collection. I heated a small piece in a test tube and a mirror of arsenic was produced with crystals of arsenic (in absence of air and in a fume hood!).


Bad photo quality, looks better in reality.

This is by far the easiest and safest way to get some arsenic (no toxic wastes, no difficult/dangerous procedure)

BTW: arsenic trioxide can also be made from arsenic and it shows a remarkable property: crystalloluminescence , the strongest known. And also very strong white triboluminescence (same link)! (translation with google if required)

[Edited on 4-9-2013 by Pok]
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[*] posted on 4-9-2013 at 11:35


Fascinating; As dissolves in sulfuric acid, reducing some of the acid to SO2? Looks like a great yield (of As2O3 crystals). Cool!

As I recall, arsenic can be reduced from solution, similar to Se and Te? Or is it just those...

I would also suppose, once you isolate As2O3, a ground-up mixture with charcoal should distill (well, sublimate) elemental As under similar conditions to the picture above, and accordingly, in high purity. Probably under inert conditions (CO2, N2 or etc.), but not vacuum. The active ingredient in carbon reduction is often CO; the same reaction might be done under a stream of CO, with more expensive equipment of course (CO generator, reaction tube, connectors). The reduction could also be done with hydrogen, with the obvious danger of AsH3, but that decomposes over 200C so it should be a fine method as well. I think you'd want to stick with nonmetals and gasses for reduction, as metallothermic reduction will produce arsenides as impurities (compare to Mg + SiO2 thermite making silicide), besides the drawback of the low boiling point.

That said, metallothermic reduction in a bomb might be interesting. Like the sodium reaction, it should produce a lump of fused arsenic, which would be an unusual sample for the collector.

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[*] posted on 4-9-2013 at 13:00


Quote: Originally posted by 12AX7  
Probably under inert conditions (CO2, N2 or etc.), but not vacuum. The active ingredient in carbon reduction is often CO; the same reaction might be done under a stream of CO, with more expensive equipment of course (CO generator, reaction tube, connectors).
You want a reduction with CO under CO2 cover with extra N2? That's what you get from a forced air fire with insufficient oxygen, in other words, a reducing flame. I'll admit the reaction vessel etc. is indeed extra gear, but the CO generator isn't difficult at all.
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[*] posted on 4-9-2013 at 13:42


Quote: Originally posted by Pok  
BTW: arsenic trioxide can also be made from arsenic and it shows a remarkable property: crystalloluminescence , the strongest known. And also very strong white triboluminescence (same link)! (translation with google if required)
Interesting. I had never heard of <a href="https://en.wikipedia.org/wiki/Crystalloluminescence" target="_blank">crystalloluminescence</a> <img src="../scipics/_wiki.png" /> before now. Thanks for sharing that link.



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[*] posted on 4-9-2013 at 15:15


Quote: Originally posted by Pok  
Yes! I think arsine would be evolved! You shouldn't try this!

Why don't you simply buy arsenic? It can be found in mineral stores for very low prices. You can easily make very pure arsenic from this just by subliming it in a test tube. But in most cases the "native arsenic" that you can buy is pure enough for an element collection. I heated a small piece in a test tube and a mirror of arsenic was produced with crystals of arsenic (in absence of air and in a fume hood!).


Bad photo quality, looks better in reality.

[Edited on 4-9-2013 by Pok]


That should belong in Pretty Pictures...

I've yet to find native arsenic, try though I may... You're right that this is dangerous, but my budget is very low.

What about burning the powdered skutterudite in a closed area - this should produce Fe2O3, NiO, and CoO as well as sublimed As2O3, correct? This would avoid the hazardous arsine, and if the container were well sealed, be almost safe.

[Edited on 4-9-2013 by elementcollector1]




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[*] posted on 4-9-2013 at 15:56


Quote: Originally posted by elementcollector1  
A few years ago, I picked up a sample of skutterudite ((Co,Ni,Fe)As3).
A sample of powdered ore or a mineral specimen?



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[*] posted on 4-9-2013 at 16:16


Quote: Originally posted by bfesser  
Quote: Originally posted by elementcollector1  
A few years ago, I picked up a sample of skutterudite ((Co,Ni,Fe)As3).
A sample of powdered ore or a mineral specimen?


Mineral specimen - about a 2"- by- 3"- by- 2" lump of metallic substance.




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[*] posted on 5-9-2013 at 06:04


EC1:

It could be worth grinding a pinch of it down and trying to sublime it in a test tube. The As might just sublime off, who knows? It definitely sounds more like an alloy than a compound...




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[*] posted on 5-9-2013 at 08:06


Quote: Originally posted by blogfast25  
EC1:

It could be worth grinding a pinch of it down and trying to sublime it in a test tube. The As might just sublime off, who knows? It definitely sounds more like an alloy than a compound...


According to the Wiki, it has a definite structure and arsenides are well known. But I'll give it a shot, see what I come up with. The test tube will be sealed first, obviously - I'll crack it open after the experiment.




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[*] posted on 5-9-2013 at 10:46


I don't have phase diagrams for most arsenides, but the phosphides and antimonides that I do have seem to suggest somewhere between compound and intermetallic behavior with the Fe-Ni-Co crowd. I think they'll be reasonably stable. At the very least, there will be high (solid or liquid state) affinity or solubility of As, so the partial pressure will be small. It is certainly possible to liberate P, As or Sb from any of these compounds with enough heating, vacuum and time, but that doesn't mean you want to bother.

If, instead of only heating the arsenides to drive off arsenic, they were slowly oxidized (let some oxygen leak into the vacuum?), it would push the equilibrium significantly. The result would probably be a combination of metal oxides and As (otherwise, As2O3 and metals). Of course, slow oxidation of the whole thing would give separation of the oxides. (You'd also want to check on the stability of the arsenites and arsenates, but it's likely these decompose with heating, just as the sulfates do.)

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[*] posted on 5-9-2013 at 15:50


As2O3 boils at just 465 degrees C, so this could well separate the oxide from the rest. What to do with the oxide, however, is another thing entirely.



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[*] posted on 5-9-2013 at 17:29


Has anyone bothered to do a lit. search on possible use of this mineral as an ore?



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[*] posted on 6-9-2013 at 07:11


Quote: Originally posted by bfesser  
Has anyone bothered to do a lit. search on possible use of this mineral as an ore?


All I found from a cursory search was that it finds a use as a cobalt and sometimes-nickel ore, with arsenic as the byproduct. I did find a source for purification by sublimation of arsenic:

Quote:
Wikipedia:
On roasting in air of arsenopyrite, arsenic sublimes as arsenic(III) oxide leaving iron oxides,[18] while roasting without air results in the production of metallic arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum or in a hydrogen atmosphere or by distillation from molten lead-arsenic mixture




[Edited on 6-9-2013 by elementcollector1]




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[*] posted on 20-12-2013 at 21:42


Quote: Originally posted by 12AX7  
Fascinating; As dissolves in sulfuric acid, reducing some of the acid to SO2? Looks like a great yield (of As2O3 crystals). Cool!

As I recall, arsenic can be reduced from solution, similar to Se and Te? Or is it just those...

I would also suppose, once you isolate As2O3, a ground-up mixture with charcoal should distill (well, sublimate) elemental As under similar conditions to the picture above, and accordingly, in high purity. Probably under inert conditions (CO2, N2 or etc.), but not vacuum. The active ingredient in carbon reduction is often CO; the same reaction might be done under a stream of CO, with more expensive equipment of course (CO generator, reaction tube, connectors). The reduction could also be done with hydrogen, with the obvious danger of AsH3, but that decomposes over 200C so it should be a fine method as well. I think you'd want to stick with nonmetals and gasses for reduction, as metallothermic reduction will produce arsenides as impurities (compare to Mg + SiO2 thermite making silicide), besides the drawback of the low boiling point.

That said, metallothermic reduction in a bomb might be interesting. Like the sodium reaction, it should produce a lump of fused arsenic, which would be an unusual sample for the collector.

Tim


Bringing this back because I missed this post.
Does this apply to arsenides? I feel like it wouldn't, it would just make arsine due to the H+ ions and the negative arsenide ions.

In that case, is there any wet way to produce arsenic oxide from an arsenide? I think it might be possible to roast the mineral with molten sodium hydroxide to form sodium arsenate, and acidify this to precipitate arsenic (hydr)oxide. This would not affect the cobalt, iron or nickel to my knowledge.

I still have yet to try sublimating this material in a closed test tube (nerves more than anything), or do any wet chemistry (due to that arsine hazard).

It appears at this point that isolating As2O3 will be the tricky part of this. The reduction with HCl and SnCl2 seems fairly straightforward, but I have one question for this step: Presuming I filter and wash the arsenic precipitate, as well as all arsenic-containing wastes from earlier, how do I dispose of them safely? As far as I know, there is no 'safe' form of arsenic.




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[*] posted on 21-12-2013 at 06:58


Safest way is probably to recover all As as some solid, insoluble compound. Then mix with cement and after hardening dispose at your local tip as 'non-recyclable'.



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[*] posted on 21-12-2013 at 13:18


Sounds good. What about all the filters and such that will be undoubtedly contaminated?
Found that when precipitated arsenic oxide is mixed with sodium hydroxide in solution and boiled, it goes back into solution as sodium arsenite. This would then be filtered off from the precipitated metal hydroxides (mostly cobalt), since none are amphoteric in this case. The solution would then be acidified, and this would precipitate arsenic trioxide again, correct? If so, this is a convenient and simple wet-chemistry route to arsenic trioxide.

However, the question remains as to precipitating arsenic trioxide in the first place, while avoiding arsine. Acids cannot be used, as arsenides react with any strong acid to give arsine. Bases might be possible due to the arsenite double salt. In this case, what would be the products of the reaction?

Additional note: Formula of skutterudite is actually CoAs3, with occasional substitutions of Ni and Fe.

??CoAs3 + ??NaOH -> ??Na3AsO3 + ???(Co)



[Edited on 12-21-2013 by elementcollector1]




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[*] posted on 21-12-2013 at 14:09


Just include any kind of solid waste too.

It may not be ideal but it's better than to get the 'smurf suited people' in at high cost and with suspicion falling on yourself ('what's a non chemist private person messing about with a mega poison for like this?') and the search for bodies to start! :D




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