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-----Cyanide Destruction ORP
Quickstart:
Wastewater cannot be discharged with cyanide in it. Treatment is usually alkaline oxidation using caustic soda plus bleach, which converts the cyanide to the less toxic cyanate.
Many jurisdictions, however, require a secondary treatment of reducing the pH, then further oxidizing the cyanate to carbon dioxide and nitrogen. This requires great care to make sure all of the cyanide has been converted to cyanate because acidifying cyanide releases leathal hydrogen cyanide gas.
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Q. My facility has been having trouble destroying Cyanide in our two stage system. We have been keeping the first stage at a pH between 10.5 and 11.0, but have not been able to get the ORP above +200 mV. There are a lot of metals in the influent that the Cyanide can complex with in the solution. There is cadmium and chromium for certain and ferrous sulphate is used in our processes as well.
The second stage is at a pH of 8.5 and we are having trouble controlling the ORP there as well.
Is there another way, other than addition of bleach, that will help us control our ORP readings better. We have been fighting with this Cyanide for some time now.
Thanks for the help,
EHS Engineer (Wastewater Management) - Tucson, Arizona, USA
2007
A. Your ORP readings are probably wrong. More importantly, is free chlorine present (by PAO titration or KI-starch indicator paper)? See threads #125/31 &
403/94.
Also, is your hypochlorite dosing sufficient, e.g., 7.5 lbs of NaOCl per lb of CN? https://www.nmfrc.org/bluebook/sec623.htm ⇩
- Goleta, California
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A. First step - calibrate the ORP probe.  Dave Wichern Consultant - The Bronx, New York Q. The ORP probes are reading the standards correctly, and the KI paper is changing to a blue. EHS Engineer (Wastewater Management) - Tucson, Arizona, USA |
A. Other materials in the waste stream can affect the ORP. The important thing is that the cyanide and cyanate are destroyed. If the starch paper is purplish, indicating an excess of chlorine, and analysis shows effective destruction at the ORP you are getting, the ORP does not necessarily have to be an exact book value.
Luck & Regards,
Ted Mooney, P.E.
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