Honeywell UDA2182 manual First Stage of Cyanide Destruction, Raise pH and oxidize cyanide

Appendices

First Stage of Cyanide Destruction

Raise pH and oxidize cyanide

Sodium hydroxide (caustic) is used to raise the effluent to about 11 pH, which will promote the oxidation reaction and ensure complete treatment. The oxidizing agent is usually sodium hypochlorite, NaOCl. The reaction for the first stage is given below using the NaOCl and with cyanide expressed in ionic form (CN- ). The result is sodium cyanate (NaCNO) and chloride ion (Cl- ).

NaOCl + CN − → NaCNO + Cl−

This first-stage reaction is analyzed and controlled by independent control loops: caustic addition by pH control and oxidizing-agent addition by ORP control (redox potential or ORP, oxidation-reduction potential). Often an ON-OFF type of control using solenoid valves or metering pumps can be used. The pH controller simply calls for more caustic whenever pH falls below 11. The ORP controller calls for additional hypochlorite whenever ORP potential falls below about +450 mV. (The metal ORP electrode is positive with respect to the reference electrode.)

Titration curve

The ORP titration curve in Figure 15-4 shows the entire millivolt range if cyanide is treated as a batch. For continuous treatment, operation is maintained in the oxidized, positive region of the curve near the +450 mV setpoint. The ORP setpoint can vary between installations, depending upon pH, the oxidizing agent, the presence of various metals in solution, and the type of reference electrode used. Determine the exact setpoint empirically at that potential where all the cyanide has been oxidized without excess hypochlorite feed. This point can be verified with a sensitive colorimetric test kit or similar check for cyanide.

	+600					pH= 10.5
						pH= 11.0
	+400
(mV)
REDOX POTENTIAL	+200
	0
	-200
	-400
	0	1	2	3	4	5	6	7
				VOLUME OF HYPOCHLORITE ADDED

Figure 15-4 First Stage Cyanide Oxidation - Typical Titration Curve

January 2009 UDA2182 Universal Dual Analyzer Product Manual 205