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Letter 30090
Removing copper and chrome from nitric
solution
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Our job shop provides a process called "chemical milling" to the
aircraft industry. We use a caustic based solution to selectively
reduce thickness of aluminum parts for weight reduction.
Part of the process requires deoxidation (or desmut) using a
nitric acid and sodium bichromate solution. To get away from
the chromated solution and change to nitric and Henkel Deoxalume we
would prefer to electroplate all of the excess chrome and copper from
the solution (4000 gal) and re-use the nitric vs complete disposal
and starting with fresh nitric. Any suggestions on how we might
achieve this?
Thanks.
David Cook
Chemical processing - Dallas, Texas
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"Deoxidation" is a term commonly used, but is actually a misnomer;
"desmutting" is a better term. I mention this not to be pedantic, but
because I need to note that nitric acid is a very powerful oxidizing
agent, which then begs the question of why it is called a deoxidizer
:-)
Electroplating is a reduction process, and it's difficult to
conduct in a powerful oxidizing electrolyte. I don't think you will
be able to do it., but let us know if you prove otherwise.
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Ted Mooney, P.E.
finishing.com
Brick, New Jersey
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There are several other types of repurification technologies that
could be worked to do what you want to do - which is re-use the
nitric. These may be somewhat aggressive on the different
technologies and some of the materials used, but they are quite a bit
better than shipping the waste off.
Some types of membranes will do what you want to do, especially
nanofiltration, which will remove the higher molecular weight
dichromate and copper from the background solution (the acid is
dissociated in the bath, so its pretty small ionically). This will
leave you with a greatly reduced volume of highly enriched chromium
and copper with a small amountof nitrate (and acid) background. This
can further be reduced by passing the solution through a media
column, which will remove a significant amount of the dichromate. The
copper can be electrowon from this solution, the dichromate can be
recovered and treated or if re-purified, re used.
Of course, this will need to be piloted first - which shouldn't be
too difficult, but the parameters of operation should be verified
before you consider a system purchase.
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Tom Baker
wastewater treatment specialist
Warminster, Pennsylvania
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