85    RESIDUAL CHLORINE BASED AUTOMATIC PROCESS
CONTROL FOR THE ALKALINE CHLORINATION
PROCESS TREATING CYANIDE WASTE
Abbas Zaidi, Process Development Engineer
Larry Whittle, Process Technologist
Wastewater Technology Centre
Environment Canada
Burlington, Ontario L7R 4A6
Mike Jovanovic, Project Manager
McClaren Engineering, Inc.
Toronto, Ontario M5E 1E7
INTRODUCTION
The traditional treatment approach for destruction of cyanide contained in metal finishing and gold
milling wastewaters has been the alkaline chlorination process and automatic chlorine addition based
on the measurement of effluent redox (reduction-oxidation) potential has been widely used for
controlling the process [1]. Environment Canada's Wastewater Technology Center (WTC) has conducted extensive evaluations of full scale alkaline chlorination systems treating cyanide bearing wastewaters from gold mining and milling operations [2]. These evaluations and in-house bench scale
studies proved that good chlorine dosage control was difficult to achieve using redox potential
feedback because of the complex relationship which exists between chlorine dosage and redox potential. The bench scale tests also showed that the use of chlorine residual measurement would provide a
superior feedback signal for process control and could result in a reduction in chlorine consumption.
To demonstrate this residual chlorine control strategy, a mobile alkaline chlorination pilot plant
incorporating the necessary instrumentation and control equipment was constructed and tested at a
metal finishing plant where cyanide was used in the process.
This paper presents the results of the pilot plant investigations.
BACKGROUND
Conventional Redox Potential Control
Redox potential is a measure of the ability of a solution to reduce or oxidize another chemical that
may be added to it. It is measured in millivolts (mV) and is usually referenced to a common base such
as the potential of a Standard Hydrogen Electrode (S.H.E.) [3].
During alkaline chlorination (a typical system is shown in Figure 1) of a cyanide solution, the
addition of chlorine results in the oxidation of cyanide to cyanate (Equations 1 & 2). When all of the
cyanide has been oxidized to cyanate, the redox potential of the solution rises rapidly to a new level
known as the cyanate potential as shown in Figure 2. If the pH of the solution is adjusted at this point
and chlorination is continued, the redox level of the solution rises again when all the cyanate has been
destroyed (Equation 3).
CN" + H20 + CIO" - CNCI + 20H" (1)
CNC1 + 20FT - CNO" + CI   + H20 (2)
6C1" + 60H" + 2CNO" - 2HCO3 + N2 +6C1" + 2H20 (3)
In ideal situations involving chlorination of pure cyanide solutions, the straightforward relationship
between redox potential and chlorine dosage results in a simple control strategy. However, in wastewater treatment situations, the relationship between redox potential and chlorine dosage is affected by
variations in temperature and pH and by the presence of complex metal cyanides typically present in
gold milling and metal finishing wastewaters.
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