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operative range). It can be seen that reaction is still
effi cient at higher inlet concentrations (1 ppm) and
concentrations lower than required concentration
(0.05 ppm) could be guaranteed Fig. 7.
Moreover, while with a rection of 10 minutes + 10
minutes was not possible to guarantee an outlet
CN concentration as per required specifi cation,
with a reaction of 30 minutes + 30 minutes was
possible to reach a fi nal CN concentration lower
than 0.05 ppm. For this reason, at a large-scale
project was considered a reaction time for the de-
sign equal to 30 minutes + 30 minutes.

The high costs associated with chemical con-
sumption for pH regulation (namely caustic soda
and Sulphuric acid) induced the hypothesis to carry
out the reaction at the natural pH of the solution
(8.05). Moreover, it was investigated the effi ciency
of cyanides removal without addition of acid and Figure 5. Alkaline chlorination on solution B (initial cyanides concentration: 0.5 ppm; hypo-
caustic soda: natural pH was not very far from the chlorite concentration: 10 ppm +10 ppm; reaction times: 10 min+10 min).
optimal pH range. Despite this, the results showed
lower cyanide reduction effi ciency than reaction at
optimal pH with chemical dosages. In fact, after 30
minutes a residual cyanides concentration of 0.29
ppm is reached after reaction at natural pH (at op-
timized pH was 0.033 ppm) Fig. 8.

As a result of our laboratory investigation, the opti-
mized dosages and reaction parameters were ap-
plied also at large scale in the refi nery plant. Alka-
line chlorination was performed by adopting two
concrete reaction basins in series with a volume
allowing a reaction time of at least 30 min per tank
(was considered 40 min as safety factor), each agi-
tated by two dedicated mixers, and both equipped
with hypochlorite dosages. In addition, the increa-
se of pH in the fi rst reaction was made by caustic
soda while its reduction to optimal values of the
second reaction was carried out by Sulphuric acid
addition.
Figure 6. Alkaline chlorination on solution B (initial cyanides concentration: 0.5 ppm; hypo-
Conclusions chlorite concentration: 10 ppm +10 ppm; reaction times: 30 min+30 min).
Alkaline chlorination is very promising for cyanides
reduction in wastewater. Cannon Artes carried out
an experimental campaign to optimize process
effi ciency while containing CAPEX and OPEX that
provided the following results:
1) Process performs better effi ciency at pH 10.5
than at pH 9. Despite that, is preferable to apply
the standard scheme of second reaction (fi rst at pH
10.5 and second at pH 9) to avoid formation of un-
desirable products;
2) As expected, effi ciency of cyanides removal is
strongly reduced in presence of other competitive
compounds;
3) Reaction of 10 minutes fi rst reaction and 10 mi-
nutes second reaction is not suffi cient to obtain
the performances required. Reaction of 30 minu-
tes fi rst reaction and 30 minutes second reaction
is suffi cient to reach fi nal cyanides concentration
of 0.05 ppm;
4) Alkaline chlorination shows high effi ciency, if Figure 7. Alkaline chlorination on solution B (initial cyanides concentration: 1 ppm; hypoch-
optimized in terms of chemicals consumption and lorite concentration: 10 ppm +10 ppm; reaction times: 30 min+30 min).

Impiantistica Italiana - Novembre-Dicembre 2022 63