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

"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.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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.

Tom Baker       wastewater treatment specialist Warminster, Pennsylvania

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