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NEW TECHNOLOGIES

Solution A Solution B
Compound Concentration Concentration
Potassium cyanide 0.5 ppm 0.5 ppm
COD - 830 ppm
Table 1 - Composition of so- Total Phosphorus - 25 ppm
lution A and solution B used Ammonia - 48 ppm
for experimental campaign
Chlorides - 193 ppm
Sulfates - 140 ppm

(solution A) and the second one synthetic solution concentration is very high: starting from an initial
with cyanides and other compounds (solution B) concentration of 0.5 ppm it is possible to achieve a
with composition very close to specific project inlet final concentration of 0.01 ppm after only 10 min.
water characteristics (as reported in Table 1). This shows a high cyanides abatement efficiency
Fig. 3.
Preliminary reactions of cyanides with sodium
hypochlorite were carried out in solution of distilled Before proceeding with the complete reaction, it
water containing only cyanides (Solution A). At was investigated the effect of the pH 9 and 10.5 on
both pHs (9 and 10.5) the reduction in cyanides each single reaction of cyanides with sodium hypo-
chlorite at different times (0, 10, 30, 60, 90 min).
Better results are observed at pH 10.5 than at pH
9. For this reason, it is important that the complex
reaction occurs at pH 10.5 Fig. 4.

The real process efficiency in cyanides removal can
be estimated only by considering the presence of
the other compounds as for the real wastewater to
be treated.
This effect was simulated by considering, for the
reaction, a synthetic solution as reported in Table 1
(Solution B). As for the results, it could be clearly
observed the negative impact of the presence of
other compounds in solution: at the same reagents
dosages, the efficiency in cyanides concentration
reduction is much lower (i.e. 0.1 ppm after 10 min-
utes).
After the first reaction (10 min) residual cyanides
concentration is still higher than project require-
Figure 3 - Concentration of untreated and treated cyanides with sodium hypochlorite (20 ment (0.17 ppm>0.05 ppm), while after second
ppm) after 10 min at pH 9 and 10.5 reaction it is possible to comply with outlet required
process performances (0.067 ppm). The presence
of other compounds in solution strongly reduces
the overall process efficiency by reducing the re-
sidual sodium hypochlorite available for cyanides
oxidation Fig. 5.
Same reactions were done also at longer time (30
minutes the first reaction and 30 minutes second
reaction); in this case after the first reaction, it is still
reached a residual cyanides concentration equal
to 0.01 ppm (in line with project required perfor-
mances). Despite with the first reaction is still pos-
sible to reach the required outlet concentration, it
is strongly recommended to carry out also the se-
cond reaction to allow the degradation of the par-
tial undesired chlorides sub-products that could be
formed during first reaction Fig. 6.
The same reaction was also studied at a doubled
inlet cyanides concentration (1 ppm) to assess if
Figure 4 - Reaction of cyanides oxidation by NaOCl in solution A at pH 9 and pH 10.5 vs the process was efficient even at higher inlet cyani-
time (NaOCl concentration: 20 ppm) des concentration (in case of max inlet cyanides of

62 Impiantistica Italiana - Novembre-Dicembre 2022