Sciencemadness Discussion Board - Elemental Chromium from Paint

Elemental Chromium from Paint

Chemistry Alchemist - 19-7-2012 at 09:32

Chromium(III) Oxide is a common green pigment, it is literally easy to get and i was wondering if the following reaction can be done.

Cr2O3 + 6HCl = 2CrCl3 + 3H2O

CrCl3 + Al = Cr + AlCl3

Could all that take place at room temperature? and does a solution containing Chromium(III) Chloride react with Aluminum or any other metal more reactive then it?

Also How high of temprature would you have to reach for the following reaction to take place

Cr2O3 + 3C = 2Cr + 3CO

MrHomeScientist - 19-7-2012 at 10:33

Your first scheme of an aqueous reaction of the chloride with aluminum would work, to yield fine Cr powder. Assuming you can dissolve the oxide in acid, that is. That can be difficult depending on how the oxide was prepared. I don't know enough to comment on the reduction with carbon, but I'm sure it's possible.

To make lumps of Cr metal, you can use the thermite reaction with powdered Al:
Cr2O3 + 2Al --> Al2O3 + 2Cr

I've done this many times. It's difficult to ignite if your powders aren't very fine and well-mixed.

blogfast25 - 19-7-2012 at 13:03

Correct, MrHomeScientist.

To make Cr thermites run better add a small amount of finely ground K2Cr2O7 to the mix: about 0.05 mol per mol of the oxide (adjust the amount of Al accordingly). This makes the thermite run hotter, smoother and should increase Cr yield.

But reduction with carbon would require blast furnace type conditions...

[Edited on 19-7-2012 by blogfast25]

Chemistry Alchemist - 19-7-2012 at 21:08

Ive got a Oxytorch which reaches really high temperatures, if i focus the energy to a single point, is that enough to smelt the chromium oxide?

woelen - 19-7-2012 at 22:29

Cr2O3 from potteries cannot be dissolved in acids. It is calcined oxide and such oxides are VERY inert and even boiling in conc. HCl or 50% H2SO4 does not dissolve any of it. You need molten NaOH or molten NaHSO4 to dissolve this oxide and these are not pretty at all to deal with.

I think that the Cr2O3/C mix can be used to create Cr, but I have no personal experience with that at all. Probably you need very high temperatures in a vessel in which air is excluded. Probably the Cr produced in that way is impure and may contain dissolved carbon.

Chemistry Alchemist - 19-7-2012 at 23:43

Would there be a way to purify the metal afterwards? Maybe if your yield is high enough, could you add the chromium metal back into HCl and dissolve it to remove carbon, and then add Aluminum to it afterwards to get back the chromium and then melt it afterwards again?

blogfast25 - 20-7-2012 at 05:12

If you have purity concerns about the oxide, why not synth it yourself?

Reduce potassium dichromate in acid conditions with methylated spirits, now you have basically a Cr (III) salt with some ethanoic and methanoic acid. Neutralise carefully (Cr (III) is amphoteric!) to precipitate blueish/green Cr(OH)3.nH2O. Wash with copious amounts of hot water to remove salts. Softly calcine till dryness, now you have Cr2O3. Good purity Cr2O3 will give good quality metal in thermite

MrHomeScientist - 20-7-2012 at 06:13

Quote: Originally posted by blogfast25

Correct, MrHomeScientist.

To make Cr thermites run better add a small amount of finely ground K2Cr2O7 to the mix: about 0.05 mol per mol of the oxide (adjust the amount of Al accordingly). This makes the thermite run hotter, smoother and should increase Cr yield.

But reduction with carbon would require blast furnace type conditions...

[Edited on 19-7-2012 by blogfast25]

Interesting, thanks for the tip! I think I recently ran out of the oxide, but I'll certainly give the dichromate addition a try next time I run this type of thermite. Someday soon I'm going to make a thermite montage video that has lots of different types, and perhaps a thermite 'guide' that has tips like yours.

I also finally found some ground fluorite on eBay, so I scooped that up and I'll be trying your titanium thermite formulation as well. Exciting times ahead!

francis - 20-7-2012 at 06:41

What sort of paint do you have, as in, what does it say it's used for? And is it solvent based or water based? Out of curiosity, what brand is it?
Cheers

SM2 - 20-7-2012 at 10:23

Why not go to junk yard, and reverse electro-plate off any of Detroit's finest. J/K. I'd be leery messing w/ the hexavalent, but the thermite process looks tried and true..
careful, heavy metals.

blogfast25 - 21-7-2012 at 05:56

Quote: Originally posted by Fennel Ass Ih Tone

[...] but the thermite process looks tried and true..
careful, heavy metals.

It doesn't 'look' tried and true: it IS true. Both theory (thermodynamics) and practice confirm that Cr metal is easy to produce with aluminothermy, provided you know what you are doing. I've got the Cr nuggets to prove it.