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Author: Subject: Chromium from Thermit reaction?
YT2095
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Chromium from Thermit reaction?

I tend to save whatever "waste" products I can from reactions, and after a simple Ammonium DiChromate volcano I kept the Cr2O3 "waste" washed it totaly and dried it (not All of it reacts).

as this is apparently insoluble to many chems, I stored it for a thermit reaction. thats the History.

it weighed 2.2g and a stoichometric mix would be .81g of Alu, I tried that and nothing! I doubled the "Dose" of Alu and it worked, the 1st mix didnt even Look right, the second one did.
I used 10 Micron Alu powder/dust.

theres no green stuff left, only black, theres larger sparkly bits (hopefully Chromium metal) but they are Very Tiny!

Ive put this product in a beaker, added water and a little HCl to shift the Alu.

1) how do I get the Cr out?
2) Why should adding the very Dilute HCl to this be giving off H2S?
3) what is the black product? Al2O3 perhaps?

[Edited on 9-9-2006 by YT2095]

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unionised
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HCl dissolves Cr; NaOH would be beter.
Any S present as an impurity in the Al or the acid will be reduced to H2S.
YT2095
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Bloody hell YES! damn good point about the HCl/NaOH!

Ive just dumped the soln, rinsed several times and use KOH soln instead, Potassium Aluminate will be easy to remove

the only thing I can think of for the rest is like panning for Gold, let gravity and Mass do the work.
I dont want a large amount anyway, just enough for my element collection.

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Chromium itself is pretty reactive, in addition it has a higher melting point than say, iron, that's why it only forms small globules. A large (several pounds) charge may melt into a larger globule, but Brauer recommends adding sulfur.

Yes, impurities of sulfur and phosphorous make for a stinky experience.

How fast is that stuff dissolving? Fused alumina with some chromia tends to be rather resistant to most anything.

Tim

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YT2095
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well, Currently theres a steady rate of gas liberation, probably 10 small bubbles hitting the surface at any given time.

I did the "Gold Panning" when I shifted the HCl on Unionised advice, lost perhaps a third of the volume in floating stuff, there Are several visible metalic bits that dont fiz Im hoping these are my Chrome particles.

Ide be happy to get 250 to 500mg of Cr for my Vial, purity of 99,99% is not an issue for me, as long as the sample gives the general idea, these will all be left to my Daughter eventualy

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Mr. Wizard
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2.2 grams of Cr2O3 and less than a gram of Al powder is a pretty small pile. The mere heating of the material may cause problems as the heat from the reaction has more likelihood of radiating away than it does heating more thermit mix. I'm sure you could get most mixes to go with a pile that size if you carefully controlled the size of the particles, the intimacy of the mix, the density of the pile, and the intensity of the initial heat. I've made mixes out of iron slag and aluminum sawdust that would not work in that size of a pile, but work very well when put in a 12 oz soup can. There is a certain 'economy of scale' to the bigger piles.
mrjeffy321
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I preformed a 100 gram (total) Cr2O3 thermite reaction a few weeks ago.
The reaction was pretty slow compared to most other types of thermite and left with a dark blob of loosely connected smaller chunks of "product". Taking a hammer to the blob and breaking it all up while keeping an eye on the look out for something shiny, I was able to retrieve several small pieces of Chromium metal.

The Cr metal chunks are quite small, most too small even to be worth saving. On the outside they don’t look like anything special, but inside they are quite shiny.
Here is a picture I took of the insides of some of the largest pieces,
Cr metal Pictures

If you have the ability to perform [much] larger thermite reactions and the patience to do the separation manually, that will allow you to better obtain the Chromium metal.

[Edited on 9-9-2006 by mrjeffy321]
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I once burned something like 30g of such thermite (pottery Cr2O3 and magnalium), I found a few dark blue, brittle spheres which show shininess inside. I'm guessing the blue color has to do with the reducing environment producing Cr(II) in the slag melt (something like Al2O3, MgO, bits of SiO2, Cr2O3, FeO, etc...).

Tim

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chemoleo
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I once used the product of ammonium dichromate decomposition (Cr2O3) directly for Al thermite, it worked well, but the leftover was a solid block with no discernible metal present.

However, doing the same thing with pottery supplies Cr2O3 and Al (~100g) left glittering globules of Cr metal the size of peas. I've saved them somewhere and wanted to take a pic, but can't find them.
Oh, btw the standard way to get better flux and separation of metal and oxide is to add CaF2 at the bottom of the thermite mix. I've never tried that, but that's what the books recommend.

Attached is a movie clip with this thermite, it cracked the concrete underneath

[Edited on 10-9-2006 by chemoleo]

Attachment: Cr-thermite.AVI (3.7MB)
This file has been downloaded 323 times

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mrjeffy321
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chemoleo , your Cr2O3 thermite was much more vigorous than mine. What particle size Aluminum Powder did you use?
Like you, I used pottery grade Cr2O3 in the reaction, but my reaction progressed very slowly, expanding out from the center (when the Mg ribbon was) and then died out before it got to the outer edge of the pile. Yours looked like it got much hotter too, probably helping the separation of the Cr metal from the Aluminum Oxide.

How do you suppose CaF2 helps in the separation? Where did you read that?

[Edited on 9-10-2006 by mrjeffy321]
chemoleo
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It was Al for metal fillers, I guess about 200 mesh. Not dust, but fine particles that are still too small for the eye to see. It helps pressing the thermite before ignition. And it was hard to ignite, needed several tied together sparkling candles.
Read Jander& Blasius' preparative inorganic chemistry, there it always recommends using CaF2. It is no differnt to what is used in Al2O3 electrolysis, where a related flux compound cryolite Na3AlF6 is used. Remember Al2O3 (the product in this reaction) has a supremely high MP, thus lowering it with flux agents facilitates separation. I think I mentioned this in the exotic thermites thread.

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YT2095
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Mr.Wizard, agreed, it wasnt a very large pile and the burn was incomplete (about 80% reacted), I used a sparkler to ignite it, thats where the H2S contamination came from.
I neednt have inserted the sparkler in the mix as it lit long before it got to the pile, and lit in several places (stray sparks I assume).
the 10 micron alu powder has an Min assay of 99.9%, and being so small in mesh a 3 gram pile will work reasonably well.
Currently this KOH and product mix is still producing bubbles, if I get not joy after this, Ill dissolve the Cr and try plating it out to a carbon rod, Ill not let this beat me!

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I read that adding K2CrO4 to the thermite makes it a bit more vigorous and melts the Cr produced. Adding any flux is likely to help but the heat needed to melt the flux means less heat to melt the Cr.
NaNO3 (with extra Al) might help, the Na2O formed from the reduction would convert the Al2O3 to Na aluminate- easier to melt and easier to wash off.
Obviously, with these more violent reactions you need to be more careful.
YT2095
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another contaminant, it would seem theres still traces of the Ammonium left over, even though it was washed and filtered several times.
Ammonia gas is escaping from the product+KOH.
although it shouldnt affect anything, Ill next time to be 100% sure its elliminated before Alu powder goes anywhere near it.

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woelen
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I think that you get all kinds of oxides of indeterminate stoichiometry (so-called berthollide compounds).

I have done some experimenting with (NH4)2Cr2O7 decomposition reaction and I strongly suspect that the product formed is not Cr2O3, but some indeterminate compound with formula between Cr2O3 and CrO2. I have made CrO2, which is nicely brown (it can be made by reacting dichromate with chromium (III) ions in solution over a period of weeks). I also have pottery Cr2O3, which has a fairly bright green color. The stuff I obtain from decomposition of ammonium dichromate is dark green, more brown/green than bright green. The color is somewhere between that of Cr2O3 and CrO2. This stuff is remarkably inert, as YT2095 has noticed.

When it is reduced by excess aluminium, I expect formation of Cr, but also compounds of the form CrOx, with x anywhere between 0 and 1. These sub-oxide compounds have a very dark color. Many metals form such sub-oxide compounds. They may be solutions of oxygen in the metal, but they may also be ions of metal, embedded in a metal lattice, together with oxygen ions in the lattice. Try to determine the electrical conductivity of the black stuff you obtain. If its resistance is high (more than a few ohms), then it almost certainly is some sub-oxide material with ill-defined stoichiometry. The sad thing is that many metals form such sub-oxides and that makes purification quite hard. If also other impurities are present, such as ammonium-remnants, then you also may have nitride contamination.

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YT2095
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well what I Have noticed is that the material has a Very strange smell, something Ive never encountered before and very hard to describe, its Sort of weak phenolic meets metalic, like some of the very old circuit boards.
anyway the fizzing stopped now and its had a good wash, its been Panned to keep the denser particles and Ive decided on Nitric acid to dissolve it as opposed to HCl, according to my solubility chart, the Nitrate is soluble, whereas the Chloride is only Sparingly soluble.
Im only using a very weak nitric soln (38%), and all being well, given time I should notice a distinct color change

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ethan_c
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 Quote: Originally posted by YT2095 another contaminant, it would seem theres still traces of the Ammonium left over, even though it was washed and filtered several times. Ammonia gas is escaping from the product+KOH. although it shouldnt affect anything, Ill next time to be 100% sure it`s elliminated before Alu powder goes anywhere near it.

I found the exact same thing! Using both pottery-grade Cr2O3 and homemade Cr2O3 from lab-grade ammonium dichromate, I did thermit-type reactions with slightly more than stoichiometric amount of aluminum, and in both cases, the masses (both ~300g) were quite difficult to ignite- it took several minutes of a MAPP gas pencil torch -and when ignited, seemed much different than iron-type thermit. It did, however, react throughout the entire thing and produced an absolutely extraordinary amount of heat (7 feet away felt like a campfire; part of the ceramic flowerpot actually melted!).
It took about an hour to cool, and the final result was a huge mass of…black/white/grayish crap. After tearing it apart with a hammer, little shiny flecks were apparent. When submerged (still warm) in plain water and dilute hydrochloric acid, a very acrid ammonia-like smell evolved, very small gas bubbles. I was thoroughly confused at the time, asked a few of my more knowledgeable friends, and shelved it.

Very different from what I had heard! For example, found on the United Nuclear site:
Chromium Oxide can be made into pure elemental Chromium metal through a thermite reaction by mixing it with twice its weight in granular Aluminum
and igniting. It requires a very high
temperature (over 3,000 °F) to start the reaction.
A small amount of burning Thermite, or other high temperature pyrotechnic mixture will generally succeed in getting it started.

frustrating
chemoleo
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Just to show you that it isn't impossible to extract pure chromium from normal (no CaF2-containing) Cr2O3 (pottery grade) -Al thermite, have a look at the chromium pellets below. They are about 1 cm in size (40 g in total), crush relatively easy (it has a crystalline touch to it, probably because it had time to cool slowly), and the breaks have a shiny lovely metallic lustre, even after a year of being exposed to air.

So please, just use large enough amounts and you'll get chromium pellets just like me

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Cr2O3 undergoes a transformation to a fully crystalline form when heated, if made by a wet method - heating Cr(OH)3 - the oxide will suddenly glow hotly as it changes from amorphic to crystalline form. There is also another change whet it is heated to higher temperatures, this seems to be a fusing of the surface to give a lower surface area and less reactive oxide.

Cr2O3 heated in air or oxygen is slowing oxidised to higher oxides. This is a hazard in processes that use Cr2O3 as an catalyst for gas phase oxidations, the Cr(5+) and Cr(6+) oxides form fine dusts that are health hazards. The temperature of the ammonium dicromate reaction is high enough that traces of the higher oxides are formed, which seem to dissolve in the bulk Cr2O3 giving a darker, brownish oxide.

The Al + Cr2O3 reaction is less energetic than that of iron oxides with aluminium. Several of the older inorganic prepration books have this method in them, and mixing some amount of dichromate in with the Cr2O3 is typical. The aluminate formed from the dichromate works as a flux and helps the chromium to form large lumps; sulfur as a flux is another way, or using some dehydrated borax as flux. I believe that cryolite also has been used to flux the reaction.

A flux is a good idea for another reason. Chromium is fairly reactive when hot, it reacts with nitrogen giving several different nitrides. While on of those is rather inert to acids, another could be reacting with the acid to give ammonia. Having a flux that floats on top of the thermit reaction would help seal out air, increasing the yield as well as helping the chromium form compact masses.

As chemoleo shows, larger batches can generate enough heat to make compact chromium metal; however a flux still helps.

The opposite problem exists when trying to make manganese, the dioxide reacts too energetically, the mixed oxide Mn3O4 is better.
mrjeffy321
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Is it better to put the flux on top, underneath, or mixed in with the thermite?

I plan to (hopefully) try a Cr2O3 thermite reaction this weekend using CaF2 and Na3AlF6 flux to see if I can get larger chunks of Chromium metal.
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I would heat the mixture in a crucible and then ignite it. Reduce activation energy a bit for one.

Fluorides should help a lot, I'd say at a rate of 10%.

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chemoleo
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Jander&Blasius always says to place 10 g of CaF2 at the bottom, this in a roughly 100 g mixture (same for Mn3O4).

For Cr production it recommends this:

70 g Cr2O3 (heated to red heat), 25 g K2Cr2O7 (melted and powdered) and 32 g Al (Griess, i.e. the size of semolina particles). 10 g CaF2 at the bottom of the crucible.

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I tried a small (25 g) Cr2O3 thermite reaction today specifically for the purpose of testing out the effects of using CaF2 as a flux to help the Cr metal separate better from the Al2O3.
Using chemoleo's advice, a placed about 2.7 grams of CaF2 at the bottom of the thermite reaction container and then packed down 25 grams worth of Cr2O3 thermite on top.
I sort of spread the CaF2 around evenly on the bottom of the container to ensure more contact area between the powers when the reaction started.
The thermite was pretty easy to light and was more vigorous than my previous Cr2O3 thermite attempts (although not quite as vigorous as chemoleos' video shows).
In the end, I did not notice any significant improvement in the yield of Cr metal; the CaF2 instead just seemed to cake up and form a layer on the bottom chunk of Al2O3-Cr which was formed.
Perhaps a larger, hotter, reaction is needed to see any benefits of the use of CaF2 or to notice a better Cr metal separation.
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25 grams sounds a bit small for a Cr2O3/Al run, to me; I'd expect it to run 'cold'. The melting point of CaF2 is around 1350 C, Cr is 1900, Al2O3 is 2050.

If you're up to it, try a similar run with 20 gr Cr2O3 and 5-10 gr of a dichromate. Or, as you said, a larger run.

I sometimes used a capping layer of cryolite, maybe a cm thick. It melts a bit over 1000 C, and is a decent solvent of Al2O3. The idea was to quickly get a liquid layer covering the reaction mix, blocking out air and dissolving and rendering less viscous the upper region of the Al2O3/metal mix. The flux under the thermit would also melt, and rise through the reaction mix doing a similar job from the bottom up..
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So far, this thread has been completely centered on using Cr2O3 with potassium dichromate to add energy to the reaction. How about the prospect of using CrO3? I know that its toxic, carcinogenic, corrosive…but it's also an extremely powerful oxidizer, which might mean that no dichromate is needed. Does anyone have any information on this? I could try when I go home for Thanksgiving…
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