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Cyanide treatment inconsistency

I am having some trouble controlling my cyanide concentrations on my waste water effluent. It seems that every time I have a "spike" in my effluent Chlorine concentrations I also see a "spike" in cyanide concentrations. Is this likely from iron complexes or is there a problem with my cyanide destruct system (I am using a 2 stage system that seems to be well mixed, the pH of the first stage is 10.5 - 12.5 and the second is pretty steady at pH 8.5, resident times are in the hours)

Please Help!

Thanks,

Edward Lewis
- Plymouth, Minnesota, USA
2005

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First of three simultaneous responses --

The first stage converts CN to cyanate. The second stage converts it to CO2 and N2. I do not run a destruct system, but my failing memory says that the pH of the second stage was supposed to be around 4 or the reaction is terribly slow.

James Watts
- Navarre, Florida
2005

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Second of three simultaneous responses --

How are you controlling chlorine in the first stage? Are you certain you have enough? Second stage pH seems a bit high. Heavy metal cyanide complexes can be very difficult to destroy completely with chlorination. Try lower pH in the second stage. If all else fails, you can try to negotiate for a permit modification which will require analysis of only free chlorine, and that is much easier to do. Good luck.

Jeffrey Holmes, CEF
Spartanburg, South Carolina
2005

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Third of three simultaneous responses --

Quite possibly, you have a spike of a complexed cyanide complex that is difficult to break, which the ORP system see's as cyanide demand, but which would explain the chlorine spike, and the cyanide is resistant to the destruction, which would explain the cyanide. Iron and nickel causes are usually to blame. You might raise the ORP in Stage 1 to over 700 mV and see if that helps.

Tom Baker
wastewater treatment specialist - Warminster, Pennsylvania
2005

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It is hard to understand a correlation between excess chlorine and cyanide spikes in your effluent if the influent to your cyanide treatment system is relatively constant.

Likewise, if the problem is due to complex cyanides, like ferricyanides, it would also be a consistent problem if the feed to your system is relatively constant.

However, if there are periodic dumps of a solution with a high cyanide concentration that are treated in this system, then this could be the problem. A high cyanide concentration is likely to cause overfeeding of the bleach, and may also contain complex cyanides that are not readily destroyed by chlorination, and that would not be present on a day to day basis.

I recommend that you look for a correlation between solution dumps and this problem. If you find this is the problem, then you need to treat the dumps more aggressively.

Lyle Kirman
consultant - Cleveland Heights, Ohio
2005

Thanks for the replys everyone. Here is a little more info that some of you asked for. The first stage has an ORP controler and the setpoint is currently above 700mV (I read the standard is more like 400mV but from past reports that wasn't enough). I am having a consistancy problem with the influent to the system and that is going to be looked into. Thanks for you help. Any other suggestions are more than welcome!

Edward Lewis
- Plymouth, MN, USA
2005

I don't know if anyone will be checking in on this thread but I am look for an answer regarding the use of sodium metbisulphate to reduce the excess bleach leaving my cyanide destruct system. If the cyanide is only converted to cyanate is SMB strong enough to reduce it back into cyanide?

Edward Lewis
- Plymouth, MN, USA
2005

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You cannot go back to cyanide once you have broken it down to cyanate. The reaction is not reversible by the utilization of a reducing agent.

Tom Baker
wastewater treatment specialist - Warminster, Pennsylvania
2005

Since you mention a problem controlling influent, I would suggest a surge tank giving you 15 minutes or more retention to try to eliminate spikes of influent.

Gene Packman
process supplier - Great Neck, New York
2005

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I believe that you can reduce cyanate back to cyanide by using sodium metabisulfite or any reducing agent. However, if your second stage is destroying the cyanate, i.e breaking the cyanate down to carbon dioxide and nitrogen, you should not have any cyanate to reduce after the second stage because the components of the compound (C & N) have been released as gases.

Derek Burton
- Atlanta, GA, USA
2005

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Tom Baker is right. Cyanate cannot be reduced back to cyanide after it is formed by the hydrolysis of cyanogen chloride.

If iron cyanides are present, a positive chlorine residual will ensure that they are present as the ferricyanide ion. Although both ferricyanide and ferrocyanide form insoluble precipitates with transition metals, ferricyanides are more soluble. It is possible that this inhibition of precipitative removal by the excess chlorine is the cause of your problem.

Excess chlorine also has a generally negative effect on flocculation and settling. When I do batch treatments, I always neutralize it with sodium metabisulfite before adding coagulants and flocculant. I get better results that way.

Dave Wichern
Consultant - The Bronx, New York
2005

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How are you analyzing Cyanide? There are significant interferences with titration or colorimetry after distillation that could cause the belief that iron cyanide has been destroyed when it has not.

How long after sampling is cyanide analyzed? The presence of thiocyanate in preserved samples can react upon storage to form cyanide false positives upon analysis.

William Lipps
analytical - College Station, Texas
March 23, 2008