Environmental Engineering Reference
In-Depth Information
potential of ignition. It is again emphasized that these facts have to be taken into consideration
even during a temporary storage of spent catalysts.
5.2.1.2 Reactions of Air with Catalyst
The composition of fresh, fresh-sulfided, and spent hydroprocessing catalysts were discussed
earlier. On exposure to air under dry conditions, the mineral matter of spent catalyst undergoes
oxidation even at ambient temperatures. Apparently, during the storage, this process is very
slow. However, in view of the unlimited liability, even a slow oxidation deserves attention in
the case that it may lead to the release of hazardous constituents. Thus, the oxidation of
inorganic sulfur in solid waste (e.g., coal rejects) has been noted after more than 50 years
exposure to air. Consequently, leaching of metals and decrease in pH of the ground water in
proximity were noted. It is believed that a brief discussion of these events may be
necessary.
During the exposure to air, the sulfided form of metals is gradually converted to an oxidic
form. This process may be very slow and may involve a gradual replacement of sulfur atoms
by oxygen atoms to form oxysulfide intermediates before a complete oxidation was achieved.
For MoS
2
, this may be depicted using the following set of reactions:
MoS
2
+O
2
=MeSO+SO
MoSO+O
2
=MoO
2
+SO
2SO+O
2
= 2SO
2
MoO
2
+ 0.5O
2
=MoO
3
The sum of all these reactions provides a reaction for the overall conversion of MoS
2
to MoO
3
,
i.e.:
MoS
2
+ 3.5O
2
=MoO
3
+ 2SO
2
However, the probability of a complete oxidation of MoS
2
during storage (e.g., in landfill) is
rather low. The conversion of MoS
2
to Mo(SO
4
)
2
, e.g.:
MoS
2
+2O
2
=Mo(SO
4
)
2
cannot be ruled out, although a slow diffusion of O
2
through and/or consumption by coke layer
would not be favorable for this reaction to proceed, suggesting that under typical exposure of
spent catalysts to air this reaction may be kinetically limited. Other metal sulfides, which are
part of the spent hydroprocessing catalysts, may undergo a similar gradual transformation to
