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citric acid Passivation - Disposal and Reclamation of Effluent

I am a metallurgist with Iowa State University. My program provides technical assistance to manufacturers throughout the state of Iowa. A company who manufactures architectural stainless steel components has contacted me with a problem.

Recently they have gone to citric acid passivation for their products. The problem they are having relates to disposal of the used citric acid solution. The Cr levels in the bath accumulate and lead to costly disposal.

First question. From the information I've read on this website, should the bath be accumulating chromium? And is the effluent actually hazardous? As I understand it, the advantage of citric acid passivation is that the citric acid forms chelates with the Fe and Ni and leaves the Cr alone.

Second question. If the effluent is in fact hazardous, is there a way to reclaim the citric acid solution? Can the chromium be removed or precipitated?

Thanks for your help.

Paul Berge
Iowa State University - Ames, Iowa

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Cation exchange can be used to remove trivalent chromium, iron, nickel and aluminum from citric acid , much like it is used for the purification of chromic acid plating bathes. This will not, however, remove hexavalent chromium.

Lyle Kirman
consultant - Cleveland Heights, Ohio

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Paul:

The amounts of chromium that a correctly formulated citric acid product will remove from the surface is very small compared to nitric acid systems. Typically the amount is small enough that any rinse water used to rinse the parts will not be high enough to produce "hazardous waste" per EPA guidelines. However, we see in our Lab that the main bath can build up chromium in levels that will surpass the EPA guidelines. With our customers we have them set up a system that prevents the need to dispose of the bath, but you can also use ion exchange systems that will remove all of the ions from the solution and produce clean water for use in rinsing. We typically recommend carbon bed/anionic bed/mixed bed combination - in that order. If you need further information, please contact us through our web page.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

In this day and age there is no reason to dispose of a citric acid passivation bath (or almost any other contaminated acid). Chemistrys are available that change the organics into inorganics and the metals into an easily filterable silicate like forms. Keep on recycling (not dumping).

David French
- Charlotte, North Carolina

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If possible, it's always better to recycle the bath, or purify it for reuse. But, if your inquirer is comitted to disposing it, you might have them try this. Bring the pH to below 4 with sulfuric acid, and add about 50 lbs/100 gallons magnesium sulphate. Then elevate the pH to about 11 with sodium hydroxide. This may remove the chromium down to the required level. You will get a lot of sludge, sorry to say. Try a bench test first, of course.

Dave Wichern
Consultant - The Bronx, New York