hodges
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Chromium Recovery Question
Is it possible to recover the chromium left over after using a dichromate salt in an aqueous oxidation reaction? I had some chromium sulfate produced
by such a reaction, and was going to recover it and try experiments with it such as electroplating. So I added NaOH to precipitate (presumably)
Cr(0H)3. This appeared to work - precipitate was very voluminous and appeared jelly-like. However, after letting it settle and then washing it
several times, it appears to have turned into the oxide. It will not react with HCl. Is there a better way?
Hodges
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Picric-A
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try dissolving the oxide in sodium hypochlorite...
If not boil down the chromium sulphate to form Cr2(SO4)3 crystals, recrystalise to purify then use those for electroplating ect.
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DJF90
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If you know how much chromium sulfate is in your solution then you could add potassium sulfate to create the double salt; chrome alum. This is
supposedly extremely soluble in water, even at room temp, so the solution can be concentrated and any precipitate removed. This could possibly remove
most other salts in there (if any). The the solution can be evaporated to dryness to collect crystals of potassium chromium (III) sulfate.
I dont know what sort of scale you're working on but generally I wouldnt consider it worth the time/hassle/energy.
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Picric-A
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KCrSO4 isnt terribly usefull but yes you can do that, work out how much sulphate there is in there by working out how much dichromate you added.
Ask woeelen about dissolving chrome oxides, he seems to be the most experienced in that field of chemistry, ie. transition metal chemistry
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DJF90
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I know its not useful, its just one method of "recovering" the chromium content. You could always perform a thermite reaction with the chromium (III)
oxide to get chromium metal.
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kclo4
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The chromium oxide may react with sulfur dioxide to form the soluble, and original chromium sulfate. This works with other oxides such as MnO2, which
often have difficulties reacting with acids. Did you try using conc. HCl with heating? It seems like it should react, but I've never dealt with
chromium compounds.
This could be one tedious method of extracting the chromium.
Perhaps use a carbonate next time?
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Picric-A
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MnO2 reacts with aqeuous SO2, aka sulphurous acid, because SO2 is a good reducing agent and it reduces the dioxide to the sulphite. I doubt chromium
oxide precipitated form the sulphate would act similarly becuase it is already in a low oxidation state however its worth a shot...
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protem
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CrIII recovery
Remember that K2CO3 is insoluble in ethanol, while KOH is soluble (40g/100ml). The carbonate can be converted to the hydroxide by boiling with slaked
lime and removed from the waste with ethanol. Filter the rest of the orange liquid from precipitates and you get fairly clean CrVI - just add a bit
of sulphuric acid and youre ready for some more oxidations. This process can be repeated indefinitely, much cheaper than buying new CrVI crystals,
and far far kinder on the old environment.
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not_important
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Quote: Originally posted by hodges  | | ... So I added NaOH to precipitate (presumably) Cr(0H)3. This appeared to work - precipitate was very voluminous and appeared jelly-like. However,
after letting it settle and then washing it several times, it appears to have turned into the oxide. It will not react with HCl.
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I don't think it turned into the oxide. Cr(OH)3 when left in contact for awhile with dilute alkali becomes less soluble in acids and bases.
Things you might try are
A) Filter it more quickly. Add paper pulp, from cheap paper towels torn up and mix with water in a blender, to make it easier to filter the gel.
You'll still likely need a Buchner and vacuum, but it should be quicker to filter.
B) Use Na2CO3 instead of NaOH. I don't know if this would be slower in he change over of the Cr(OH)3, but it would be worth a try.
C) Bubble CO2 through the solution after precipitation, or add crushed dry ice. Really carbonate it, to the point of fizzyness This converts the
alkali to NaHCO3, the excess CO2 leaves the solution slightly acid.
D) Combine B+C by using NaHCO3 to neutralise the cold solution.
I've always used Na2CO3 for neutralising solutions for metals recovery, it's much cheaper than NaOH. This is possibly why my Cr(OH)3 would dissolve
in acids.
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12AX7
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If you're certain you have oxide now (a forest green, rather insoluble compound), you can fuse it with potassium chlorate (perchlorate probably also
works, and maybe nitrate) to recover dichromate.
Tim
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hodges
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The reactions are on a small scale. I was mainly interested just in getting a Cr(3) salt isolated to mess around with since the only chromium salts I
have presently are dichromates. Was also a bit surprised when the Cr(OH)3 would not dissolve again.
Next time I will try making the carbonate instead of the hydroxide. I have to separate the chromium salt in some way. If I just evaporate the
solution, it will contain potassium as well as chromium, plus potentially leftover organic product.
Hodges
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JohnWW
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You could use it to do your own chromium-plating with. Or, if you have a furnace, you could melt it with iron and nickel to make your own stainless
steel castings.
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