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Figure 1 - Alkaline chlorination semi-reactions operative properties

sodium hypochlorite (NaOCl) at alkaline conditions. wed a preliminary step for suspended solids and
The process can be summarized with the following oil removal constituted by API separator and DAF
two reactions Fig. 1. (dissolved air flotation) and a biological treatment
devoted to organic load degradation. Downstream,
The wastewater containing the project included sand filters for further suspen-
cyanides shows a very high ded solids removal and activated carbon filters for
phenols adsorption.
“toxicity and environmental
impact. Increasingly stringent In order to optimize the reaction efficiency, minimi-
zing the reaction tanks (APEX) and the quantities
limits must be respected before of caustic soda, Sulphuric acid and sodium hypo-
such discharges, requiring the chlorite (CAPEX), an experimental campaign was
optimization of the degradation carried out Fig. 2.
It was evaluated the effect of water composition
processes (cyanides concentration, presence of other com-
pounds) and process parameters (reaction time,
pH) on efficiency of cyanides degradation.
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In addition, Alkaline chlorination shows some key In details, the effect of different initial CN concen-
advantages: trations (0.5 ppm and 1 ppm), reaction times (10
• Utilization of a strong oxidant and fast kinetics; min, 10 min+10 min e 30 min +30 min) and pH (9,
• Copper catalyst not required (as required in 10.5, 10.5 + 9) on CN removal were investigated.
-
AOP processes); A total sodium hypochlorite concentration of 20
• Possibility to oxidize thiocyanate and CN-WAD ppm was dosed (10 ppm in each tank). Sulphuric
(Weak acid dissociable cyanides). acid and caustic soda were used for pH regulation.
Cyanides concentration was measured by dedica-
ted low range cyanides kit compatible with spec-
Despite the potential to represent a high efficient trophotometric analysis. pH and ORP values were
solution for cyanides remediation, the process re- recorded during reactions. Figure 2 - Schematic repre-
quires different chemical dosages of reagents and Two different solutions were tested: the first one sentation of chemical dosa-
pH regulation with consequent relevant impact on containing only cyanides dissolved in distilled water ges in Refinery project.
costs. In fact, chemical consumption costs could
represent a very high percentage of the global
costs of the project, thus eliminating the advan-
tage to adopt alkaline chlorination as solution for
cyanides degradation. In addition, the technology
requires an accurate monitoring of operating pa-
rameters during both reaction steps. For these
reasons, an adequate optimization of the process
parameters and of chemical dosages is required in
order to reduce costs while maintaining high effi-
ciency process.
For a refinery water treatment plant with installation
in Iraq and treating an inlet flow rate of 274 m3/h, it
was envisaged a cyanides treatment consisting of
a biological treatment plus an additional treatment
of alkaline chlorination (to keep down the cyanides
concentration from an initial concentration of 0.5
ppm to a final concentration of 0.05 ppm, as per
project specification). The chlorination phase follo-

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