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Cyanide destruct ORP problems

I run a 2 stage cyanide destruct system for a client. The system uses NaOH and NaOCl to accomplish the destruction and is set up to control the pH and ORP automatically. Our pH(s) are 11.0 at stage 1 and 10.0 at stage 2. Our ORP values are 600 mv at stage 1 and 600 mv at stage 2. We have noticed recently that when the mv readings are high above 650 additional NaOCl will drive the ORP down. If the system does not have an excess of NaOCl the ORP will not respond this way. In both cases the pH is at 10 or above. We have contacted the probe manufacturer and they have indicated that this is unusual. At the manufacturer's suggestion we have replaced parts - i.e. ORP sensor, transmitter - and we have checked and rechecked the calibration. We checked the system for free chlorine using a DPD reagent and there was an excess present. Has anyone seen this type of thing before?

King Turner
Project Manager - Hamden Connecticut
2007

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2007

It sounds like a pH effect to me. Sodium hypochlorite is pretty alkaline.

Try taking samples of the waste from the treatment tank in question, dosing in hypochlorite at several levels, and measuring the ORP. Now, adjust the pH of all the waste portions to the same value - in a fume hood. Now, try measuring the ORP values again. I think then you will see the expected relationship.

It's not commonly recognized that just about everything affects an ORP reading - TDS, pH, temperature, even the rate of agitation in the area of the sensor.

Are you not getting full cyanide destruct? Or do you feel you're using too much chlorine?

Dave Wichern
Consultant - The Bronx, New York