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Non-amenable cyanides in wastewater


Waterwater is treated with NaOCl in my plant to destruct cyanide (ppm level). However, it seems there are some non-amenable cyanides that could not be destructed with NaOCl. Anyone has experience with non-amenable cyanides such as iron-cyanide complexes? Are there simple methods to treat non-amenable cyanides in ppm level?


Jim Ji
- Rochester, New York, The United States

The short answer is no, there are no simple methods. Further, there are no good tests to even determine ppm levles of non-amenable cyanides. This has been one of the major issues the plating industry has been fighting for decades.

But minimizing their formation by avoiding the use of steel tanks and anode baskets, and avoiding the use of common rinses is important.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

There is no simple way to destroy iron cyanide complexes. However, it is possible to remove them with ion exchange. This works because the complexes are -3 or -4 valence, and IX strongly prefers more highly charged ions. The resins are also regenerable with strong brine or caustic, but this would still leave a waste solution with iron cyanide complexes. The only two treatments that I know of would be either advanced oxidation, or chemical precipitation as Prussian blue.

Lyle Kirman
consultant - Cleveland Heights, Ohio


I found the page at wastewater/cyanide.html to be helpful, at least page 1 of it. Steryldimethylbenzylammonium chloride mentioned on p. 2, as it turned out for me, didn't work, and I'm sure this was due to all the anionic trash in our wastewater competing for the cationic sites on this quat chemical. Also, I wouldn't expect peroxymonosulfuric acid or persulphates to work, either, as mentioned on p. 2 of the above. I am presently experimenting with ultraviolet light and peroxide (as detailed on p. 1 of the above), and I'm encouraged. A beaker of wastewater having 12 ppm ferrocyanide was mixed with a little hydrogen peroxide, and set out in the bright sunlight, and 9 hours later, all was destroyed as near as I can tell (this is presently being analyzed by our outside lab). I have had mixed results using the zinc white method:

It seems to not be reproducible, with respect to the results, and perhaps the reason is that I am trying to remove just traces of ferrocyanide rather than larger amounts (which would have a better opportunity to form a decent body, or amount, of precipitate?). One must keep in mind with this, that ferricyanide will not form an insoluble zinc precipitate, so sodium metabisulfite must be used to reduce any ferricyanide to ferrocyanide, prior to adding zinc sulphate [on eBay or Amazon affil link] . In a week I'll have a little more info on UV-peroxide, and I'll post again to share. Also, I hadn't thought to use a weak or strong base resin, since we have so much sulphate that would tie up all the resin sites.

However, Lyle, it seems to me, is suggesting that a resin saturated with sulphate would still pick up the iron cyanides, due to the strong affinity for them. What concerns me about forming a Prussian blue precipitate is that soluble Prussian blue (yes there is such a thing) is formed whenever ferricyanide is reacted with a ferrous salt, and in my case with the metabisulfite we use, it seems possible that ferrous ions would form, and spoil this approach. Also I tried US patent 4,312,760 (available for free to download at US Patent Office), which teaches the use of ferrous sulphate ferrous sulphate [affil link] to get rid of iron cyanides, but I couldn't get it to work. It claims wastewater can be brought down to 0.02 ppm total cyanides, when the contaminant is iron cyanides. I would like to learn why it didn't work. I am hopeful more participants will add to this discussion!

David H. Duncan
- Ponderay, Idaho USA

We have had excellent success in getting ferrous cyanides down to less than 0.15 ppm using advanced oxidation technology. It does require some segregation of waste, assuming you already have that done, and also will precipitate metals in the waste water without the vast requirement of other oxidizers and large amounts of caustic soda [affil link] . I am not surprised too much by the experimentation results of the peroxide and UV, however in practical application, they are rarely an effective treatment as far as cost, maintenance, and do not respond to variability in effluent concentrations very well. In a beaker - yes, on a 5 GPM waste stream - not so much. One this that you do need to be cognizant of is that the metals in a stream will be consumed or reacted first, then the cyanides. Mass factors for the treatment need to take that into account.

tom baker
Tom Baker
wastewater treatment specialist - Warminster, Pennsylvania

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