Metal finishing Q&As since 1989
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Cyanide-iron complex treatment
Q. I would like to ask if there is some process to treat wastewater with iron-cyanide complexes. I wrote some old discussion forum (2003) where mention there isn't some easy way to do it. Someone knows if there is some useful technology developed at this days?
Thank you,
- México D.F.
2006
A. There are some methods using high temperature and pressure
(thermal hydrolysis) that will work, but the end products are formaldehyde and ammonia which may also be a problem.
Other than that, ferricyanides can also be precipitated with iron under the right conditions to form Prussian Blue, a dye that used to be commonly used.
Some forms of advanced oxidation may also work.
All of these are fairly expensive.
Ion exchange can also be used to selectively remove iron cyanide complexes in the presence of high concentrations of other ions, but good prefiltration is required for this method of removal. This method works because IX resins prefer ions with higher valences and the iron cyanide complexes have a charge of -3 (ferricyanide) or -4
(ferrocyanide). Very few other anions ions have a charge that is this high.
consultant - Cleveland Heights, Ohio
2006
A. Try Photochemical oxidation with UV reactor and Oxidising agent for removing this from the effluent. I think this should work.
Aravind A Narayan- Coimbatore, Tamilnadu, India
2006
A. Iron cyanides can be removed from water with precipitative techniques. Iron ferrocyanides (Prussian Blue) can be precipitated at a pH of about 2; zinc and manganese ferrocyanides at pH 6. Of course, all of this must be done following alkaline chlorination for "free" cyanide.
It is necessary to reduce ferricyanides to ferrocyanides before the precipitation step. This can be accomplished by the addition of metabisulfite at a pH of about 6.
Dave Wichern
Consultant - The Bronx, New York
2006
A. I agree with Lyle Kirman's approach, but I just want to correct one point. In thermal hydrolysis, the breakdown products are formate and ammonia, not formaldehyde and ammonia. In considering the technology, that is an important distinction as formate is not likely to be a regulated pollutant, while formaldehyde is.
Jon Barrows, MSF, EHSSC
GOAD Company
Independence, Missouri
2006
Q. I'm running a 2 stage alkaline chlorination and cannot meet discharge standards for amenable cyanide. pH control and ORP's are by the book running a standard operating conditions.
Will adding ferrous sulphate ferrous sulphate
[affil links] after the chlorination process complex with amenable cyanide and reduce it's concentration?
- Grafton, Wisconsin, USA
September 7, 2013
A. One other thing - make sure the lab is doing the test right. When one tests for amenable cyanide, two sample aliquots are prepared. One is chlorinated for an hour, the excess chlorine neutralized, then it is analyzed for total cyanide. A second aliquot is tested as is.
A common mistake made by labs is to fail to neutralize the chlorine completely. They will just stick a KI/Starch paper in there, have it come up negative, then distill. This yields a false negative. Now, when the CN(t) aliquot is tested and the arithmetic done, it appears that there is amenable cyanide when, in fact, there isn't.
The method specifies adding a phosphate buffer to the chlorinated waste water. That prevents the problem. This step is NOT optional.
What are your numbers like?
Dave Wichern
Consultant - The Bronx, New York
September 9, 2013
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