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Cyanide Destruction ORP

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 Ferris 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,

Donelle Ann Mickevicius
EHS Engineer (Wastewater Management) - Tucson, Arizona, USA
2007

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Your ORP readings are probably wrong. More importantly, is free chlorine present (by PAO titration or KI-starch indicator paper)? See Letters #12531 & 40394.

Also, is your hypochlorite dosing sufficient, e.g., 7.5 lbs of NaOCl per lb of CN? https://www.nmfrc.org/bluebook/sec623.htm

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007

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First of two simultaneous responses --

First step - calibrate the ORP probe.

A little quinhydrone in pH 4 buffer ought to give you 264 mV; in pH 7 buffer, 84 mV.

What's your pH in that first cyanide destruct chamber? You could also try calibrating that sensor, and double checking the measurement with a handheld. The high pH conditions in a cyanide destruct unit aren't kindly to probes at all.

Dave Wichern
Consultant - The Bronx, New York
2007

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Second of two simultaneous responses --

The ORP probes are reading the standards correctly, and the KI paper is changing to a blue.
Is there a good way to make sure the probes are working, because these probes are new. The old probes were acting the same way.

Donelle Ann Mickevicius
EHS Engineer (Wastewater Management) - Tucson, Arizona, USA
2007