Sciencemadness Discussion Board - Question on pottery-grade Cr2O3.

Question on pottery-grade Cr2O3.

Velzee - 7-4-2016 at 14:06

I recently purchased a pound of Cr2O3 from a pigment store, and after reading the SM wiki and deciding to make some chromium(III) sulfate, I came across this information from the chromium(III) sulfate page:

Quote:

Chromium(III) sulfate can be more directly produced by the action of cold, dilute sulfuric acid on chromium(III) hydroxide or chromium(III) oxide. It should be noted, however, that many grades of chromium(III) oxide are not reactive enough, often as a result of being calcined, to respond to treatment with acid.

And this from the Cr2O3 page:

Quote:

Chrome green is available at pottery shops as a green pigment, sometimes with traces of calcium carbonate. This material, however, may be too unreactive to produce other chromium compounds.

And, indeed, my Cr2O3 produced no noticeable reaction upon treatment with cold H2SO4.

So, my question is, how can I increase the reactivity(if possible) of the Cr2O3, or at least enough to react with H2SO4?

blogfast25 - 7-4-2016 at 14:20

If your pottery Cr₂O₃ is anything like mine it won't even be affected by fusing with KOH!

I ended up fusing it with KNO3 which converts it (with very high yield) to K₂Cr₂O₇.

[Edited on 7-4-2016 by blogfast25]

DraconicAcid - 7-4-2016 at 14:29

I've seen a prep that called for reacting it with aluminum in a thermite reaction to get the more managable chromium metal.

blogfast25 - 7-4-2016 at 16:07

Quote: Originally posted by DraconicAcid

I've seen a prep that called for reacting it with aluminum in a thermite reaction to get the more managable chromium metal.

If you know how to run a decent aluminothermic reduction, then yes. If not, your metal yield may be quite small.

j_sum1 - 7-4-2016 at 16:13

I have found product separation with CrO3 / Al thermite to be problematic. One great fused mass.
I think the best route to something useful is KNO3 to dichromate.

blogfast25 - 7-4-2016 at 16:17

Quote: Originally posted by j_sum1

I have found product separation with CrO3 / Al thermite to be problematic. One great fused mass.

I didn't. About 73 % yield, one nice roundish regulus of metal. It does require slight heat boosting (KNO3+Al, KClO3+Al or even K2Cr2O7+Al) and slag fluidiser (CaF2).

S.C. Wack - 7-4-2016 at 18:13

Sulfuric acid will dissolve the oxide under certain conditions, such as when everything is mixed with "permanganic acid" for a second or two. Will dissolve a fair amount or none at all, because the reagents much less the conditions were not exactly nailed down. This particular permanganic acid was most reliably made from KMnO4, but MnO2 sometimes works...perhaps that depends on the exact nature of the "MnO2".

Mabus - 8-4-2016 at 08:42

Uugh the pottery grade Cr2O3 I have it's very inert: No acid I used affected it, and when I tried to fuse it with KNO3 all I got was a brown-red insoluble mass, with some deep blue spots, which also seems to be pretty inert (no reaction with bleach or other acids). I have a feeling it's not just Cr2O3 in the pigment.

Velzee - 8-4-2016 at 10:49

Quote: Originally posted by Mabus

Uugh the pottery grade Cr2O3 I have it's very inert: No acid I used affected it, and when I tried to fuse it with KNO3 all I got was a brown-red insoluble mass, with some deep blue spots, which also seems to be pretty inert (no reaction with bleach or other acids). I have a feeling it's not just Cr2O3 in the pigment.

I got my Cr2O3 from US Pigments, if that makes any difference

Velzee - 24-4-2016 at 10:01

Quote: Originally posted by blogfast25

If your pottery Cr₂O₃ is anything like mine it won't even be affected by fusing with KOH!

I ended up fusing it with KNO3 which converts it (with very high yield) to K₂Cr₂O₇.

[Edited on 7-4-2016 by blogfast25]

So, how exactly would I go about fusing them?

macckone - 24-4-2016 at 19:23

Grind the really fine, mix thoroughly and heat above the potassium nitrate decomposition point.

Velzee - 15-7-2016 at 14:50

I wound up attempting to fuse ~15 grams of Cr2O3 with a little over 15 grams of NaOH in order to see if any reaction would occur. To my surprise, although all I could see was the green Cr2O3, after washing with water, I was left with a very small amount of a yellow solution, which presumably had a lot of NaOH in it.

I attempted another go, this time with an arbitrary amount of both reactants. Here's the result:

I may heat it up again, because I think I see some NaOH left over. What do you guys thing?

[Edited on 7/15/2016 by Velzee]

blogfast25 - 15-7-2016 at 16:23

Quote: Originally posted by Velzee

What do you guys thing?

With mine fused with KOH I got no reaction whatsoever. And I mean N-O-T-H-I-N-G.

But with 20 % stoichio excess of KNO₃ the conversion to K₂Cr₂O₇ is essentially 100 %.

Bert - 16-7-2016 at 12:27

Quote: Originally posted by blogfast25

I didn't. About 73 % yield, one nice roundish regulus of metal. It does require slight heat boosting (KNO3+Al, KClO3+Al or even K2Cr2O7+Al) and slag fluidiser (CaF2).

Anyone tried the synthetic cryolite (Na3AlF6) used as a flux in Aluminum production, rather than Calcium fluoride as a flux/"slag fluidizer" for this or similar processes? I have cryolite on hand, it was used as a non hygroscopic yellow flame colorant in some pyrotechnic mixtures. No Calcium flouride, except a cute little crystaline mineral specimen I would not choose to sacrifice in an experiment.

MP of cryolite is around 1,000 C, BP not listed, said to decompose rather than vaporize. Decomposition temperature I have not yet found given anywhere-

Calcium flouride MP 1,408 C, BP 2,533 C.

blogfast25 - 16-7-2016 at 13:25

Quote: Originally posted by Bert

I've contemplated it because I have some too but I'm pretty sure it decomposes well below the minimum 2500 C needed for good slag/metal separation, acc.:

Na₃AlF₆ === > 3 NaF + AlF₃

(The pyrolysis of (NH₄)₃AlF₆ is a route to anhydrous AlF₃, see e.g. Brauer)

BP NaF = 1700 C
BP AlF₃ = not known but sublimes, almost certainly below 2500 C.

CaF₂ really just hits the sweet spot, Bert.
<hr>
One should consider also CaO as a melt fluidiser, which forms a series of low melting a aluminates with alumina. A single test with MnO₂ was very promising.

[Edited on 16-7-2016 by blogfast25]

Velzee - 24-8-2016 at 07:32

These crystals formed from the solution after a few weeks. They're probably heavily contaminated with NaCl.

blogfast25 - 24-8-2016 at 15:42

Quote: Originally posted by Velzee

These crystals formed from the solution after a few weeks. They're probably heavily contaminated with NaCl.

Which experiment does this now refer to, Velzee?