Extraction of Chromium from Stainless Steel via Oxalic acid:

In my quest to refine the process to get Potassium chromate, starting from Chromium (III) salt (hydroxide for example) is better than starting from Stainless steel.

So I have 3 processes of extracting to test, being:
1) Via Oxalic acid
2) Via high alkalinity (ph 14+) – as Chromium is amphoteric
3) Via phosphate route.


Today, via Oxalic acid – in progress

To take into account: (most information was taken from iron oxide – oxalic acid reaction – there is a lot of information - papers)

Fe (II) and Ni(II) oxalates are insoluble. FeC2O4 (0.097 g/100ml (25 °C)) NiC2O4 (could not find it, but says: insoluble in water; “soluble in acid – regarding this, I think is not soluble in excess oxalic acid)

Fe(III) oxalate IS soluble. So we want to avoid oxidation of Fe(II)
The good news is that oxalic acid is reducing, so as long as there is an excess, we are good.

Light (more sun light) reduces Fe3+ to Fe2+ and destroys oxalate ion. But this process is rather slow.
(Photochemical reduction. The photochemistry of Fe(III) oxalate to be converted into Fe(II) oxalate has been studied since the 1950s in relation to its absorption characteristics. Its strong ligand-tometal charge transfers are responsible for its unique photochemical properties upon excitation within
the tropospheric solar UV-visible region (290–570 nm))

Iron (Fe) reduces Fe3+ to Fe2+ in this setup. As we are processing SS, there is a lot off Fe to keep the reducing atmosphere.

Chromium 3+ starts to precipitate at ph 4+. (actual oxalate solution is ph 1 or less)
The best pH for leaching iron is 2.6 (or 2.5-3). I have not done it (I use only oxalic acid – ph 1, the best “ph raiser” is ammonium hydroxide.
(* Ammonia was always used to prepare the hydroxide, as the substance, while precipitated by sodium or potassium hydroxide, invariably retains alkali in spite of repeated washing).

Leaching proceed better with higher temperature, staring at 50C. best: boiling –(I use this with reflux)

The higher the concentration of Oxalic Acid the better the leaching.


This is how I proceed:
SS sponge (aprox 16gr) (it was ferromagnetic) was put in an Erlenmeyer with 100ml Oxalic acid 1M solution (12.6gr oxalic acid dihydrate) – heat was necessary to dissolve it completely.

(A stirrer was added, but as SS was ferromagnetic, it was of no great help)

Heating till boiling (in a reflux setup) in some time (10 minutes) the solution became green-brown slurry (I thing a mix of precipitated yellow FeC2O4 and green NiC2O4)

Another 100ml 1M oxalic acid was added. And 1 hour more refluxing. The SS sponge was not completely dissolved. (did not weight it, but l think 50% was dissolved.
It was left to cool to proceed next day.

Next day 200ml 1M oxalic acid was added (and 250ml erlenmayer changed to 500 ml vacuum Erlenmeyer flask)

Here catastrophe arouse, because of thermal shock, the bottom of the flask broke….. some was recovered, because the hotplate was on a plastic container…. But the hotplate broke too…


Currently what was recovered is leaching at normal temperature. Its very slow.

Here is a picture of the setup as find the next morning, you could see the bottom insoluble oxalate and the violet solution of chromium (III).

(I think is Chromium, because: A related complex with weak-field ligands, the [Cr(H2O)6]3+ ion, absorbs lower-energy photons corresponding to the yellow-green portion of the visible spectrum, giving it a deep violet color.
24.7: Color and the Colors of Complexes - Chemistry LibreTexts)

Next, I will take the violet solution, and try to get chromium hydroxide (o maybe sulfate and then hydroxide) or carbonate-

From here: Oxalates could be changed to sulfates with H2SO4 9M (or more - freeing oxalic acid- or Oxalates could be transformed to carbonate via reflux with sodium carbonate (read from a book)
or heating beyond 400C to decompose the oxalate.

From the Ni/Fe oxalate precipitate, probably will try to separate them or maybe try to get Ni/Fe nanotubes or a mixture of Ni/Fe nanoparticles.



Some references I took:

The New Technique on Separation of Cr and Fe as Well as Ni–Co–Mn Impurity in Leaching Sulfate Solution of Ferrochrome Alloy
Jijun Liu, Guorong Hu*, Ke Du, Zhongdong Peng, Weigang Wang, and Yanbing Cao
Sekine, Isao; Yuasa, Makoto; Yajima, Hideharu . (1997). Corrosion Behavior of Various Stainless Steels in Oxalic Acid Solutions and Their Corrosion Resistance. Zairyo-to-Kankyo, 46(6), 367–372. doi:10.3323/jcorr1991.46.367

Kinetics of thermal decomposition of nickel oxalate dihydrate in air
Dan Zhana, Changjie Conga, Kahirou Diakitea, Youtian Taoa, Keli Zhang

patent US20090194001A1

JOURNAL 2022 Volume 62 Issue 12 Pages 2466-2475
DOI https://doi.org/10.2355/isijinternational.ISIJINT-2020-726

patent US1899674A

Leaching kinetics of iron from low grade kaolin by oxalic acid solutions
A. Martínez-Luévanos a, ⁎, M.G. Rodríguez-Delgado a, A. Uribe-Salas b, F.R. Carrillo-Pedroza c, J.G. Osuna-Alarcón a

Optimization and kinetics of oxalic acid treatment of feldspar for removing the iron oxide impurities
K. Pariyan,M. R. HosseiniORCID Icon,A. AhmadiORCID Icon &A. Zahiri

https://doi.org/10.1016/j.hydromet.2015.08.021 (Recovery of iron from red mud by selective leach with oxalic acid)

https://doi.org/10.1016/S0304-386X(97)00049-2 (photochemical reduction)

Quote: Originally posted by RU_KLO

When there was little H2SO4 the chromate salt became brown, but on cooling changed to apple green. This salt does not dissolve in water (thats why I think is oxide, instead of sulfate. It also does not dissolve in concentrated boiling H2SO4 (or maybe the H2SO4 was saturated with chromium)