Environmental Engineering Reference
In-Depth Information
was postulated that HNO
3
participated in leaching reactions when metal nitrates were used as
oxidation agents with oxalic acid
[476]
. Apparently, the acid was released according to the
following reaction:
2Fe(NO
3
)
3
+
3(COOH)
2
=
Fe
2
{(COO)
2
}
3
+
6HNO
3
In similar manner, HNO
3
can also be released from Al(NO
3
)
3
. This suggests that the control of
pH of the leaching solution may be necessary to avoid excessive release of HNO
3
. Otherwise,
a desirable selectivity for leaching contaminant metals could not be maintained.
The Fe(NO
3
)
3
+ oxalic acid system was evaluated in the fixed bed and ebullated bed pilot
plants
[485]
. The latter system was specially designed for leaching experiments. The amount
of V leached out was significantly greater in the ebullated bed. Moreover, three modes of the
Fe(NO
3
)
3
addition to oxalic acid, i.e., continuous, successive, and batch additions, were tested
[477]
. During these experiments, the mixture of oxalic acid (0.66M) and Fe(NO
3
)
3
(0.66M)
was pumped continuously through the fixed bed of spent catalyst (
Table 7.1
) upwards. The
liquid was collected in the reagent vessel and recirculated through the bed. The results in
Fig. 7.7
show that with progress of leaching, continuous addition gained in leaching efficiency
relative to batch and successive additions. This was complemented by the largest recovery in
pore volume and surface area as well as the removal of V.
Figure 7.8
compares the HDS
activity of the catalysts rejuvenated via three different modes. Almost 85% recovery in HDS
Figure 7.7: Vanadium leached versus leaching time for different modes of promoter addition
[From ref.
477
. Reprinted with permission].
