RU_KLO
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Chromate/dichromate from stainless steel via oxalic acid route
Finally after a long research time got potassium chromate / dichromate from stainless steel (SS) in a more or less pure form.
I tried the SS + HCl, hydroxide(or carbonate), oxidize, cristallization, but got no good resutls, because a lot of another salt always was
interfering/cristallizing. (mainly from the HCl + Hydroxide or oxidation with Na/K hypochlorite.)
Tried different oxidizers, H2O2, potassium persulfate, Na/K hypochlorite.
So in order to minimize adding other cations and removing other metals from the solution , the best route I found was to use oxalic acid.
Chromium oxalate is soluble in oxalic acid as long as Ph is less than 2., which by using a 1M oxalic acid this requirement is met.
Fe (II) oxalate is almost insoluble (0.097 g/100ml (25 °C))
Fe (III) oxalate is soluble, but as oxalic acid is a reducing agent, any Fe (III) is reduced to Fe(II)
Ni oxalate is also insoluble (if SS contained nickel).
this is the procedure:
1) MAKING CHROMIUM OXALATE:
Dissolve SS in 1M Oxalic acid (this procees takes long time)
I used 1 or 2 molar oxalic acid, but any concentration which gives you Ph 1 or less is ok. The stronger the oxalic acid the faster the dissolution.
Use heat (I used boiling reflux) for speeding things.
Use stirrer (because Fe(II) oxalate deposites in the SS) but if the SS is magnetic, the stirrer is of no use, but using the stirring magnet slow, the
slow magnetic field shakes the SS)
as an example: 3 SS sponges (aprox 30gr) took 500 ml 2 M oxalic acid and 6 hrs to fully dissolve. (it was an big excess, but I wanted to reduce the
time as much as possible.)
Excess oxalic acid is bad for the second part (removing the oxalate)
Filter by decantation (because filtering Fe(II) oxalate is very difficult- or at least for me. the longer the decantation the better (I use 24 hs)
You will obtain a strong violet solution (Cr oxalate) and a precipitate of the other metals of the SS - mainly Fe(II) oxalate.
2) REMOVING THE OXALATE - MAKING CHROMIUM HYDROXIDE:
To the Cr oxalate solution add Ca(OH)2 by drops till the Ph of the solution reaches 5-6. (try not to add an excess because it will start precipitating
the Chromium hydroxide, which starts to precipitate at ph 5-6)
You can more or less eyeballin when adding the Ca(OH)2 does not produce a lot of precipitate.
Also some leftover Iron that was in solution will precipitate as Iron hydroxide once you reach ph 4+, thus cleaning your new Cr(OH)3 solution.
Filter by decantation the Cr(OH)3 from the Ca oxalate.
(you will be left with a green/blue solution -when looked with a light behind or blue/red by reflecting light)
If you use an excess of oxalic acid, you will need a lot of Calcium hydroxide.
3) MAKING THE "SALT" CHROMATE BY ADDING HYDROXIDE + OXIDIZING THE CHROMIUM HYDROXIDE
Add "salt" hydroxide to get the ph of the chromium hydroxide solution to ph 10+ (dark blue ph paper). By "salt" I mean , KOH, NaOH, CaOH, depending
which chromate do you want. (maybe ammonia could also be used if you need NH4 chromate - did not tried this, but why not.)
Oxidize by adding H2O2. Do not add to much, heat slowly, for example till 50C, and it has an induction time of aprox 10 minutes. (This is very
important, because you think nothing is happening, and you add an excess, and the suddenly you have overflowing bubbling chromate solution
everywhere....)
I add a few drops (3-4) 10% H2O2 to 50 mls Chromium hydroxide solution.
After aprox 8-10 minutes, the solution goes from green/blue to yellow.
Boil it to remove excess H2O2.
Let it decant, for removing, if any Iron was left, because it precipitates as a red flocculent precipitate.- very minor.
In the end you will be left with a strong yellow solution of your chromate that could be reduced (the liquid) by heat till cristalls appear or till
dryness.
(depending on the purity you want)
If you want the dichromate, add H2SO4 to acidify. (tried HCl, but H2SO4 is better)
Test:
Add lead acetate (or any soluble lead solution) to a solution of the chromate -> yellow lead chromate precipitates
Add acid (H2SO4 better) to chromate -> change from yellow to orange red
Go SAFE, because stupidity and bad Luck exist.
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Keras
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Registered: 20-8-2018
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This is a good way, but I think it’s a bit of an overkill. Dissolving stainless steel in HCl, then turning the solution alkaline will precipitate
hydroxides of chromium, iron and nickel. Adding an oxidant, such as hydrogen peroxide or bleach, will oxidise all those ions into Cr VI, Fe III (Ni
won’t oxidise, I think), the only one soluble is Cr VI. Further operations of rinsing and turning the pH to neutral will get you rid of the iron.
This is quite detailed in this video (in French) where the guy obtains quite pure calcium chromate from stainless steel cutlery (his stainless steel is 18/0, no nickel)
and egg shells.
[Edited on 22-11-2024 by Keras]
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bnull
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Registered: 15-1-2024
Location: South of the border, wherever the border is.
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Mood: Happy New Year!
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The chromium salt with undefined color is an oxalate complex. I have some in a test tube (I'm still thinking what to do with it and what color is
that). The bubbles were mostly carbon dioxide from oxidation of oxalate.
I would prefer to precipitate oxalate as a calcium or copper salt (calcium seems better, thanks for testing), decant and then add a base and oxidiser
but, as you used peroxide, the only residues are water and carbon dioxide. And you solved two problems (disposal and removal of the anion) at the same
time.
Well done.
Quod scripsi, scripsi.
B. N. Ull
P.S.: Did you know that we have a Library?
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