Environmental Engineering Reference
In-Depth Information
corresponding metal oxides via metal oxosulfide intermediates. For example, the oxidation of
Ni
3
S
2
may proceed as follows:
Ni
3
S
2
+O
2
=Ni
3
SO+SO
Ni
3
SO+ 1.5O
2
=3NiO+SO
2SO+O
2
= 2SO
2
Ni
3
S
2
+ 3.5O
2
= 3NiO+ 2SO
2
The overall conversion of Ni
3
S
2
to NiSO
4
would involve the following reaction:
Ni
3
S
2
+ 4.5O
2
= 2NiSO
4
+NiO
Because of a limited stability, the SO species is expected to compete successfully with metal
sulfides or oxysulfides for O
2
and, as such, will be converted to more stable SO
2
and, if
sufficient O
2
is available, the oxidation may proceed to SO
3
.
In the presence of water, additional reactions during the exposure of spent catalysts to air may
be anticipated. For example, the reactions, such as:
SO
2
+H
2
O=H
2
SO
3
H
2
SO
3
+ 0.5O
2
=H
2
SO
4
may be part of the overall oxidation mechanism. The formation of H
2
SO
4
may be one of the
reasons for classifying spent hydroprocessing catalysts as corrosive wastes. Moreover, the
presence of H
2
SO
4
may have a dramatic effect on spent catalyst because of the potential
reaction releasing H
2
S from the unconverted metal sulfides, e.g.:
MoS
2
+H
2
SO
4
=Mo(SO
4
)
2
+H
2
S
In similar reactions, H
2
S would be also released from other transition metal sulfides, which are
part of the spent hydroprocessing catalysts. The potential release of H
2
S is one of the
hazardous characteristics of spent hydroprocessing catalysts. Information suggests that
preoxidation of waste materials, using H
2
O
2
before disposal, resulted in a significant decrease
in the content of sulfidic sulfur
[333]
. Consequently, the release of H
2
S to environment could
be minimized. Nevertheless, the formation of H
2
SO
4
would increase leachability and
corrosivity (due to lowered pH). The potential H
2
SO
4
aided release of toxic species, such as
HCN, deserves attention as well.
The study of Afonso et al.
[343]
provides some support for the above rational. Thus, these
authors placed metal boxes containing spent NiMo/Al
2
O
3
into the ground (30-50 cm depth)
