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Author: Subject: Arsenic from Skutterudite
plante1999
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[*] posted on 21-12-2013 at 14:11


Gas all the equipment with H2S, and filter paper, washes etc is put into the ground. After all, realgar and orpiment are sulphides...



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[*] posted on 21-12-2013 at 17:04


Makes sense. What about the reaction products from the leach with NaOH? I can't find any balanced equation for either sodium arsenite (NaAsO2 or Na3AsO3). I would assume the products would be the arsenite, some oxide or hydroxide of cobalt, and possibly water or hydrogen.

EDIT: I have found one balanceable reaction.
11 O2 + 36 NaOH + 4 CoAs3 -> 12 Na3AsO3 + 18 H2O + 4 CoO

Does this seem at all feasible? It would take some time for the oxygen in the air to react with the fused mixture of the ore and the sodium hydroxide, and I wouldn't be able to tell when it was done (except that the ore would have turned into a black powder?)

[Edited on 12-22-2013 by elementcollector1]




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[*] posted on 22-12-2013 at 05:19


I have tried the reaction between As2O3 (dissolved in excess HCl) with SnCl2. This reaction is fairly facile, but it is not clean. You get very impure arsenic (dirty brown stuff). I also tried making As with NaH2PO2 instead of SnCl2, but this is not working very well. You get very pure fine crystals of As, but the yield is pathetic. From 1.5 grams of As2O3 I only get 100 mg or so of arsenic after hours and hours of heating, it simply won't react easily.

I did not yet find an aqueous method of making As from its compounds (I have some As2O3 and I have some orpiment from eBay sellers). All well-known methods require heating and deposition of vapors. This is something I certainly will not do at home, I do not want to poison myself with As-vapors.




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[*] posted on 22-12-2013 at 06:17


There's an old forensic test which consists of plating out the As on copper wire, from an AsCl3 solution. See Reinsch test:

http://en.wikipedia.org/wiki/Reinsch_test

[Edited on 22-12-2013 by blogfast25]




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[*] posted on 22-12-2013 at 10:13


Quote: Originally posted by blogfast25  
There's an old forensic test which consists of plating out the As on copper wire, from an AsCl3 solution. See Reinsch test:

http://en.wikipedia.org/wiki/Reinsch_test

[Edited on 22-12-2013 by blogfast25]


In the much higher concentrations expected of this experiment, do you think it would still be a 'plating' or more akin to the copper 'tree' from CuSO4 and Fe?

@woelen: I don't mind pathetic yields in this case, as I only have to make a pure, visible sample - even 100mg is acceptable (I ampouled this amount of palladium, and it looked fine). I agree that I'd really rather not handle any vaporous compound of arsenic, hence the query above about leaching with molten NaOH and air.




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[*] posted on 22-12-2013 at 14:16


Quote: Originally posted by elementcollector1  

In the much higher concentrations expected of this experiment, do you think it would still be a 'plating' or more akin to the copper 'tree' from CuSO4 and Fe?



I've no idea, it's conditions dependent, I would think...




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[*] posted on 22-12-2013 at 15:42


Hmm. Something to try, but I should probably go with the hypophosphite method (which will unfortunately involve distilling white phosphorus and reacting with NaOH, due to lack of availability).

EDIT: Found in an earlier thread that apparently tartaric acid can take care of the tin contamination, but I need more clarification on this:

Quote:
Other people seem to be happy with reduction with As+3. Here they use warm very conc. HCl with their SnCl2, using tartaric acid to produce a precipitate free of tin. The actual analysis using iodine titration seems typical.
http://books.google.com/books?id=40QPAAAAIAAJ&pg=PA316
But a couple different authors say that As+5 is not so fast to be reduced.
Really there are very many articles over the years using either hypophosphite or SnCl2 in some way for quantitative analysis of arsenic, but this does not necessarily apply here.

Quote is from S.C.Wack, in this thread.

[Edited on 12-23-2013 by elementcollector1]




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[*] posted on 23-12-2013 at 10:36


Found what I think is the reaction mechanisms for the full process I intend to follow, based on balanceable reactions and all products being water-soluble except for As at the end.

First, CoAs3 is powdered and fused with NaOH and air for a long period of time:
4 CoAs3 + 36 NaOH + 11 O2 -> 12 Na3AsO3 + 18 H2O + 4CoO

The sodium arsenite is dissolved in water, and the insoluble cobalt oxide is filtered off.

In solution, the sodium arsenite is acidified. This precipitates As2O3.
12 Na3AsO3 + 36 HCl -> 6 As2O3 + 36 NaCl + 18 H2O

The supernatant solution is decanted, and the arsenic oxide is dried. It is then dissolved in an excess of fresh HCl.
6 As2O3 + 36 HCl -> 12 AsCl3 + 18 H2O

Meanwhile, sodium hypophosphite is reacted with hot, concentrated HCl.
NaH2PO2 + HCl -> NaCl + H3PO2

The phosphinic acid is added to the arsenic trichloride, and with much heating, some small amount of arsenic precipitates.
12 AsCl3 + 12 H3PO2 -> 12 As + 12 POCl3 + 12 H2O + 6 H2

The POCl3 immediately reacts with the water.
POCl3 + 3 H2O -> 3 HCl + H3PO4

There are a few caveats to this reaction pathway:
-As must not be reactive with HCl. I cannot find a reference that says it does react, nor can I find one that says it doesn't.
-As must also not be reactive with phosphoric acid.
-H2 must be released - but no one has commented on any gas evolution?

For 100g of CoAs3 at the beginning, the theoretical yield of pure arsenic is 80g. Given woelen's comment on an earlier arsenic thread about a yield of less than 7% using the hypophosphite method, the expected yield is about 5.6g As, which is almost exactly enough to fill a 1mL ampoule.




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[*] posted on 24-12-2013 at 05:18


Quote: Originally posted by elementcollector1  
-As must not be reactive with HCl. I cannot find a reference that says it does react, nor can I find one that says it doesn't.
-As must also not be reactive with phosphoric acid.
-H2 must be released - but no one has commented on any gas evolution?



As won't react with acids to any appreciable degree, I think. The element is still too non-metallic to react directly with non-oxidising acids. It probably responds to HNO3.

That Reinsch test is also carried out in acid conditions.

[Edited on 24-12-2013 by blogfast25]




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[*] posted on 24-12-2013 at 09:04


Ah. That works out perfectly then.



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[*] posted on 24-12-2013 at 17:47


Hee is a comment from Atomistry.com on isolating Arsenic (link: http://arsenic.atomistry.com/production.html ):

"Metallic arsenic is sometimes obtained from arsenious oxide. Thus in Altenberg (Silesia) the oxide is heated at 650° to 700° C. with charcoal in an earthenware crucible covered with a conical iron cap which acts as receiver, while in Chicago retorts composed of steel pipes large enough to take a charge of 450 lbs. were at one time used for the same purpose, the arsenic being collected in water-cooled pipe condensers. This reduction method is not satisfactory, however, as the product is largely amorphous and is not so desirable as the crystalline form, being suitable only for making arsenic compounds. Moreover it always contains arsenious oxide. A Japanese method consists in fixing the vapour of arsenious oxide by means of ferric oxide or alumina at a temperature above 218° C. and then reducing the product with water-gas, Mond gas or producer gas above 100° C.; the arsenic thus freed is then sublimed. "

The Japanese method sounds interestingly.
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[*] posted on 25-12-2013 at 09:12


It sounds interesting, but high-temperature reduction is not something I'd like to try if it can be avoided.



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[*] posted on 28-12-2013 at 11:15


Attempting the first step (ore and molten sodium hydroxide). Pics forthcoming.



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[*] posted on 28-12-2013 at 12:01


Quote: Originally posted by elementcollector1  
Attempting the first step (ore and molten sodium hydroxide). Pics forthcoming.


Looking forward to the results!
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[*] posted on 28-12-2013 at 12:24


Aqueous analysis is not started yet, but here are some photos of the reaction!
First, a pic of the mineral itself. It weighs 120g, and looks extremely metallic in nature. For something that behaves and looks like a metal or alloy, this stuff is remarkably fragile - likely due to the high arsenic content? This was from one hit with a hammer.



Next, the mix: 2.5g of skutterudite and 3.5g of NaOH (adjusted upward from stoichiometry due to hydration).



The setup (the crucible on a hot plate, turned to max):



During the reaction, noticeable gas evolution was observed. According to the reaction schema I stated in a previous post, this should be H2O boiling off, but could also be As2O3 sublimating - this is discussed farther down. Additionally, significant amounts of skutterudite were still present after the hot plate was turned off - with more heating, these may have disappeared.

And finally, after pretty much one hour of heating, the post-reaction mix (some more NaOH was added because I thought it was somehow boiling to dryness):



I think that brown stuff is CoO? I'm not sure.

A new worry I thought of is that the produced Na3AsO3 might have just decomposed, and the As2O3 sublimated away. Well, I guess we'll see if there's any soluble arsenic in the aqueous extraction. If not, there's really no point to the molten procedure - it would be the same in terms of what happens to the arsenic when the ore is heated in air.
In that case, would an extraction with concentrated aqueous NaOH and somewhat (~15%) concentrated H2O2 do the trick?




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[*] posted on 28-12-2013 at 13:07


If As2O3 is sublimating away it can't be a good thing. Try to catch at least some of this stuff by having something chilled on top of the crucible.
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[*] posted on 28-12-2013 at 13:42


EC1:

Was the ore powdered prior to fusion?

Usually these fusions prescribe twice the weight of alkali to ore, sometimes more.

Using finely ground powders, I've 'cracked' materials like Beryl, Zircon and food grade TiO2 in a matter of about 20 minutes, by fusion with KOH. Once steam evolution is over, the reaction is essentially over, heating further won't do much if anything.

I must warn you again of the dangers of arsenic, especially as you now seem not to know what's become of it! :(

Arsenic oxide's BP is 465 C, that sounds a little high for a hot plate.

[Edited on 28-12-2013 by blogfast25]




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[*] posted on 28-12-2013 at 15:00


Quote: Originally posted by blogfast25  
EC1:

Was the ore powdered prior to fusion?

Usually these fusions prescribe twice the weight of alkali to ore, sometimes more.

Using finely ground powders, I've 'cracked' materials like Beryl, Zircon and food grade TiO2 in a matter of about 20 minutes, by fusion with KOH. Once steam evolution is over, the reaction is essentially over, heating further won't do much if anything.

I must warn you again of the dangers of arsenic, especially as you now seem not to know what's become of it! :(

Arsenic oxide's BP is 465 C, that sounds a little high for a hot plate.

[Edited on 28-12-2013 by blogfast25]


The ore was in the form of 3-5mm irregular granules (which are now somehow welded onto the crucible...). Not the best for fusion, but my original intent was to see if I could measure an appreciable loss of mass, and given the expected CoO powder, granules would be better separated from the end mixture. Judging from the granules present, there was no appreciable loss of mass - not even half a gram. This indicates either a longer melt was needed, or the reaction does not proceed under these conditions. Noticeably, the ore granules were significantly darkened afterwards, indicating surface oxidation to black CoO.

Steam evolution did seem to occur, though I was unsure what gas was being produced at the time. But considering that the heating element was red-hot (faintest of glows in daylight, much stronger in shadow) and this thing barely reaches 200 C with a sandbath, I doubt that it can sublimate arsenic under any conditions.

Well, at least I know what's become of most of it. It's probably worth testing the aqueous extraction for any arsenic, but any oxide precipitation will be on the order of milligrams at most, meaning the solution needs to be heavily concentrated.

The thing is, in your cases, the metal/element in question was a cation. Here, arsenic is an anion - so how does that play differently?

EDIT: I took a look at the stoichiometry, and according to the reaction, if it were to proceed to completion, about 0.88 liters of steam should have been driven off (about 0.7 mL of liquid). There was nowhere near this much steam, and given that some reaction clearly occurred (the brown powder, blackening of the ore particles, some gas evolution) I would say that the reaction is indeed happening. Just very slowly, and very inefficiently due to the particle size. Perhaps another test with very fine powder is in order...

EDIT.2: The pertinent reaction for an aqueous extraction would be
11 H2O2 + 18 NaOH + 2 CoAs3 -> 20 H2O + 2 CoO + 6 Na3AsO3
I'm imagining that this might proceed, but the molten process would be more efficient.

[Edited on 12-29-2013 by elementcollector1]




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[*] posted on 30-12-2013 at 10:36


Quote: Originally posted by elementcollector1  
The thing is, in your cases, the metal/element in question was a cation. Here, arsenic is an anion - so how does that play differently?



Firstly I'm somewhat surprised your ore didn't appreciably react with molten NaOH. Pure arsenic should do that quite vigorously, I should think.

But the arsenic isn't necessarily present as an anion: the ore appears to be an alloy or intermetallic compound (even if As doesn't really qualify as a metal). Have a look at its structure:

http://en.wikipedia.org/wiki/Skutterudite

I wonder if this stuff reacts appreciably with hot, conc. sulphuric acid, which might get a handle on the cobalt.

Aqua Regia (even the 'poor man's version') might also dissolve it through the combined oxidising action of the nitrosyl chloride and the solvating action of the hydrogen chloride (AsCl<sub>4</sub><sup>-</sup> complex anions exist).

Re. hot plates, I think many household types are calibrated for about 200 C surface temperature at high setting, which is more than enough for basic cooking requirements. Yours obviously ran hotter because it melted NaOH.

[Edited on 30-12-2013 by blogfast25]




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[*] posted on 31-12-2013 at 11:57


I did wonder about the structure.
I heard elemental arsenic reacts with concentrated sulfuric acid to make arsenic oxide and SO2. If this occurs even for skutterudite, this would be very convenient!
In that case, another test with sulfuric acid is in order.




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[*] posted on 31-12-2013 at 14:28


Nitric acid and sulphuric acid should in theory be able to attack the powdered ore.



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[*] posted on 31-12-2013 at 19:07


Wait... wouldn't this produce arsine?



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[*] posted on 31-12-2013 at 19:40


The oxidizing condition should oxidize it if formed at all. Arsine is a very strong reducing agent.



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[*] posted on 31-12-2013 at 19:46


Numbers, please.



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[*] posted on 31-12-2013 at 21:30


Quote: Originally posted by bfesser  
Numbers, please.

For the reaction?

EDIT: Picture of the skutterudite powder:



Stuff got a lot darker when in powder form - as a lump it was very bright and shiny, like polished steel. I should have about 120g here.

[Edited on 1-1-2014 by elementcollector1]




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