Cyanide Destruction Monitoring with pH and ORP0 pages

Application Data Sheet

Metals and Minerals Industry

ADS 3300-01/rev.B
January 2008

Cyanide Destruction Monitoring
with pH and ORP
BACKGROUND
Mining and electroplating processes generate waste
materials containing alkaline, rare earth metals, and
other heavy metals such as iron, nickel, zinc, cadmium, copper, silver, and gold. The wastes often contain cyanide, a deadly poison. Federal and State
regulatory agencies require that cyanide discharge
be below 0.5 ppm, and further tightening may be
forthcoming. Therefore, the best policy is to totally
destroy the cyanide before discharging the waste.
OVERVIEW OF THE CYANIDE DESTRUCTION
PROCESS (Figure 1)
One of the most effective means to treat these waste
materials is to destroy the cyanide with chlorine.
This process is accomplished in two stages:
1.

Cyanate is oxidized to nitrogen and carbon
dioxide, harmless gases, that can be safely discharged into the environment. This stage also
uses pH and ORP control.

In the second reaction tank, the pH of the waste is
measured, and acid is injected to lower pH to 7-8.
This process takes 2 to 5 minutes. In the third reaction tank, the ORP of the waste is measured and
chlorine gas (Cl2) is automatically injected to raise
the ORP to 600 mV or higher (Meanwhile the pH
controller maintains the set-point at 7-8, correcting
for any acidity created by the addition of the chlorine gas).
The following reaction occurs, taking 10 to 15 minutes:
2NaCNO + 3Cl2 + 4NaOH

6NaCl + 2CO2 + N2 + 2H2O

The cyanide is eventually converted to harmless
materials by the above reaction and the waste can
be discharged.

Cyanide is oxidized to cyanate, using pH and
ORP control.

2.

DESTRUCTION OF CYANATE TO NITROGEN
AND CARBON DIOXIDE

OXIDATION OF CYANIDE TO CYANATE
In the first reaction tank, the pH of the waste is
measured and caustic (NaOH at 50% strength) is
automatically injected to raise the pH to 10 or higher.

INSTRUMENTATION:
pH and ORP levels in this area can be simultaneously monitored with the dual sensor model 1056
Analyzer. This reliable multi-parameter instrument
includes two isolated current outputs (and optional
alarm relay board) that can be used to adjust the
feed of chlorine and caustic as needed. The model
1056 Analyzer performs continuous sensor diagnostics to notify the user when the sensor is coated or
the glass electrode has broken.
®

The oxidation reduction potential (ORP) of the waste
is measured, and chlorine gas (Cl2) is automatically
injected to raise the ORP to 400 mV or higher.
The following reaction occurs, taking from 5 to 10
minutes:
NaCN + Cl2 + 2NaOH

NaCNO + 2NaCl + H2O

The PERpH-X model 3500 sensor with forward and
reverse facing threads is convenient to install and
®
use in the reaction tanks. The PERpH-X design
includes a replaceable reference junction and several
customized SOLUTIONS are available for long life in
difficult environments. A separate sensor is required
for ORP measurement, but both sensor cables can
be connected to a single model 1056 analyzer. The
poison resistant model 3500PR sensor is recommended for this application as some levels of
cyanide may be present at startup or under upset
conditions.
PERpH-X® is a registered trademark of Rosemount Analytical.