protem
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recycling hexavalent chromium
Reading another post prompted me to ask this question - The disposal of hexavalent chromium arose in post http://www.sciencemadness.org/talk/viewthread.php?tid=11517 which begs the question "why dispose of it?" - can't chromium VI salts be regenerated
from chromium III forms with H2O2 in alkaline solution, heating (to drive off excess H2O2) and crystalisation in concentrated sulphuric acid or
glacial acetic acid? That way the chromium can be stored in a safer form and reused. Not wanting to ask this in the above thread (off topic) but is
this feasible and does anybody have practical experience with proportions, hazards, contaminants etc?
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woelen
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The cost ad labor, associated with working up some chromium-waste as hexavalent chromium salts, is way too high to make this process economically
feasible. Even on a small scale in a lab it is not worth the effort. Think of the cost of all chemicals you need to recover the chromium.
I would say that this is not interesting at all. If you want chromium(VI) compounds and you can't buy them, then start off from pure chromium(III)
chemicals or even chromium metal. Precipitate with ammonia and quickly separate the precipitate from the liquid. Rinse with water and then add the
precipitate to a solution of (preferrably) KOH, giving you a clear green solution of potassium chromite. Add H2O2 and heat. From this you can
crystallize K2CrO4.
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chloric1
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Yes recycling Cr VI would consume alot of reagents. It is best to reduce to Cr III and use as such or dispose.
Now, after visiting websites on staining concrete with acidic solutions of metal salts, there may be a way to make even the moderately toxic Cr III
benign. I would think that reducing chromate waste to CrIII and adding this to Portland cement would create a nice emerald green cement for garden
decoration etc. I want to test this to be sure. I want to see if this and other heavy metal values can be leached from cured cement.
Fellow molecular manipulator
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JohnWW
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Cr(III) salts are also used for tanning leather. Insoluble ones, e.g. the phosphate, can be used as a green paint pigment.
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protem
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My original idea was that nothing really bad ever happens to the chromic acid - say it is used to oxidise an alcohol to aldehyde what intractable
waste results? The idea was that knocking the chromium up (to VI) and crystalising it with a seed dangling from the lid of the bottle (in the cold
over time etc) would retrieve at least some chromium VI oxide or a bichromate clean enough to be usable. But I guess reclamation of material from
dirty messes requires energy on a planetary scale in the end.
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panziandi
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Because most people likely buy 500g + of Cr6+ compounds and use likely 1-10g per reaction.To recycle such a small quantity of Cr3+ to Cr6+ is a bit
pointless when you have such a huge supply of Cr6+.
On the other hand however. For those people who can not get Cr6+ compounds and who have to oxidise Cr3+ to Cr6+, re-oxidising to Cr6+ after a reaction
is more feasible since it reduces the amount of starting Cr3+ you have to re-invest in.
Cr6+ is not a nice reagent to use in any form unless you have some sense and knowledge about them. Certainly if you are not going to re-oxidise to 6+
then you should add all chromium waste to a reducing agent and preferably a complexing agent to ensure no free 6+ ( and minimal 3+ if you are flushing
substatial quantities at a time i.e. more than a few grams)
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