Sciencemadness Discussion Board

does anyone know how to recover Cr metal from K2Cr2O7?

akok - 3-8-2016 at 12:42

Hello Guys!
I have a fairly large quantity of "fossilized" K2Cr2O7. Possibly hasn't been used for over a decade.
Is there a way to recover the Cr? I have read some Electrochemistry methods and is mentioned that can be deposited over the electrode. But not a full experimentation.
Have anyone try that?
Any suggestion to use it for something else? #recycling :P

zwt - 3-8-2016 at 14:05

Chromium - From Wikipedia, the free encyclopedia:
Quote:
The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.<ref name="IndMin"/>
Na<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> + 2 C → Cr<sub>2</sub>O<sub>3</sub> + Na<sub>2</sub>CO<sub>3</sub> + CO
Cr<sub>2</sub>O<sub>3</sub> + 2 Al → Al<sub>2</sub>O<sub>3</sub> + 2 Cr

For electrowinning, it's probably best to convert the dichromate to chromium(III) sulfate first. Use carbon electrodes and a low current density (5-10mA/cm2).

[Edited on 3-8-2016 by zwt]

blogfast25 - 3-8-2016 at 15:25

Reduce the dichromate with methanol, ethanol, hydrogen peroxide or other reducers in acid conditions, to Cr(+3)(aq). Neutralise solution carefully with NaOH to close to pH = 7. Cr(OH)3.nH2O will precipitate, filter and wash carefully.

Dry and calcine to get green Cr2O3.

Reduce with Al powder in thermite style reaction.


[Edited on 3-8-2016 by blogfast25]

akok - 3-8-2016 at 22:00

Thanks for the suggestions!
But is there a reference paper or book with specific experimental?

Sulaiman - 4-8-2016 at 00:20

it seems wasteful to use potassium dichromate as a source of elemental chromium,
(like using hydrogen peroxide as a source of water)
e.g. UK eBay 100g 99.4% chromium = £5 (with p&p from China),
an equivalent molar mass of chromium as potassium dichromate is £13.5 (with p&p from Ukraine),
I'd keep the useful dichromate and just buy a lump of chromium,
and save on the cost of recovering chromium from potassium dichromate.
EDIT: have a look at dicromate uses (and hazards) https://en.wikipedia.org/wiki/Potassium_dichromate

blogfast25 ... hydrogen peroxide as a reducer ?

[Edited on 4-8-2016 by Sulaiman]

woelen - 4-8-2016 at 02:15

Yep, hydrogen peroxide as reductor. It first forms a deep blue complex, CrO(O2)2, which in turn decomposes to Cr(3+), oxygen and water in the acidic environment in which this compound is formed.

Hydrogen peroxide sometimes can act as reductor, often through some special pathway, involving peroxo complex species.

Sulaiman - 4-8-2016 at 02:26

Aaargh !

just when I think I'm learning .....

I just found a microscale experiment for potassium dichromate so I'll use that as a starting point
http://media.rsc.org/Microscale%20chemistry/Microscale%2016....



[Edited on 4-8-2016 by Sulaiman]

woelen - 4-8-2016 at 04:59

The blue complex is quite interesting. If you have ether, amyl alcohol or ethyl acetate, then you can extract the blue CrO5 from the aqueous layer into the organic layer. It is a purely covalent compound.
If you work with chilled solution and not too high concentrations, then the CrO5 remains present for quite some time and then you can do nice experiments with it, either in its aqueous layer, or extracted in an organic layer.

If youalso have pyridine, then you can stabilize the CrO5, so that it does not decompose anymore. Pyridine forms a stable complex with CrO5, CrO(O2)2(py), which is insoluble in water and can be kept around for some time as a blue solid. I have done this experiment and it is really remarkable:

http://woelen.homescience.net/science/chem/exps/cr_peroxo_py...

If you want to explore the peroxo-chemistry of dichromate, then you also like this one:

http://woelen.homescience.net/science/chem/exps/raw_material...

Finally, you can also synthesize peroxo complexes of chromium, which are stable and can be kept around for years:

http://woelen.homescience.net/science/chem/exps/K3CrO8_synth...

Finally, here is info about another peroxo complex of chromium. This one is much less stable than the previous one and it should not be kept around in more than mg-quantities:

http://woelen.homescience.net/science/chem/exps/Cr_ammine_pe...


PHILOU Zrealone - 4-8-2016 at 07:58

Maybe a stupid proposition but mixing a Cr(3+) salt like sulfate and K2Cr2O7 would lead to a precipitate of Chromium (III) dichromate, right?
Cr2(SO4)3 + 3 K2Cr2O7 --> Cr2(Cr2O7)3 + 3 K2SO4

Cr2(Cr2O7)3 = Cr8O21

Otherwise mix K2Cr2O7 with 2 equivalents of NH4Cl and burn to make maybe like the green volcano ...Cr2O3 and KCl...

woelen - 4-8-2016 at 22:41

The first thing you mentioned I of course tried. It was something I wondered about years ago :)
But, I can tell you, no precipitate is formed. Apparently chromium(III) dichromate is soluble in water.
However, on long standing (weeks) a dark brown precipitate is formed. Dichromate and chromium(III) do react, but in a very slow redox reaction. The chromium (III) is oxidized to chromium(IV) and the chromium(VI) is reduced to chromium(IV) and the result is a precipitate of CrO2. When I was a teenager, CrO2 was more common, it was used in casette tapes for audio recording and it had better quality than the iron oxide based tapes.

------------------------------

Making the volcano with K2Cr2O7 and NH4Cl is not possible. Even very very well grinding and mixing does not yield a mix which works. Real (NH4)2Cr2O7 barely is sufficiently energetic to make the reaction self-sustaining. The mix of K2Cr2O7 and NH4Cl is just below the energetic requirement for a self-sustaining reaction. When kept in a flame, it smoulders and it does react, but when removed from a flame, the reaction slows down and ceases.

[Edited on 5-8-16 by woelen]

akok - 5-8-2016 at 07:16

thank again for the suggestions!!
One more think I have to ask? I there a specific topic about "re-purposing old fossilized chemicals"?I have a few more that i would like to ask.
I decided that best way to repurpose K2Cr2O7 is converting it to KCrO4. All eliminating some fossilized KOH that I also have.
According to this:

"K2CrO4 can also be prepared from K2Cr2O7. To the water solution of K2Cr2O7 calculated amount of KOH or K2CO3 is added. On evaporating the resulting solution lemon-yellow crystals of anhydrous potassium chromate are obtained.
K2Cr2O7 + 2KOH --> 2K2CrO4 + H2O
K2Cr2O7 + K2CO3 --> 2K2CrO4 + CO2
It forms lemon-yellow crystals (m.p = 968°C) which is isomorphous with K2SO4."

No, what about FeCl3.3H2O? Is in a watery sludge state... Any suggestions?