Environmental Engineering Reference
In-Depth Information
respectively, were achieved by Rastas et al.
[627]
using the mixture of H
2
SO
4
+Al
2
(SO
4
)
3
and
H
2
SO
4
+ (NH
4
)
2
SO
4
. Rabah et al.
[628]
used a mixture of the concentrated sulfuric and nitric
acids (3:1) to extract Mo and Co from the spent HDS catalysts. After leaching, the metals were
separated as hydroxides which were roasted in oxygen atmosphere to obtain the respective
oxides. Reduction of Mo and Co oxides in hydrogen at 1100
◦
C and 900
◦
C, respectively,
produced metal powders with high purity. The maximum recovery efficiency was claimed to
be 96%.
The two mixtures of strong acids, i.e., one comprising 50mL of 70% HNO
3
+50mLof96%
H
2
SO
4
(mixture A) and the other 50mL of 70% HNO
3
+25mLof96%H
2
SO
4
+25mL37%
HCl (mixture B), were used in the process described by Lai et al.
[629]
. The spent-decoked
NiMo/Al
2
O
3
·
SiO
2
catalyst was treated at 70
◦
C. The leachates were filtered to remove solid
residue and then diluted with 0.5M HNO
3
. The isolated solid was leached again in the second
leaching stage and then in the third leaching stage. To recover metals from the filtrate,
electrolysis experiments were conducted using a moving bed electrolytic cell installed with
non-conductive glass beads. Fluidization of the bed was maintained by recirculating the
medium between the storage tank and the electrolytic cell. Before the electrolytic experiments,
the pH of filtrate was adjusted to 2-4 by 10 N NaOH. A parallel set of experiments was carried
out using a static bed. The schematics of both static bed and fluidized bed systems used to
carry out electrorefining are shown in
Fig. 11.6 [629]
. The electrolytic recoveries of Mo, Ni,
and V achieved in this study as well as potentials of the cell versus time correlations are shown
in
Fig. 11.7
.
The use of aqueous solutions of SO
2
for leaching metals from the spent CoMo/Al
2
O
3
HDS
catalyst was studied by Raisoni and Dixit
[630]
. Dissolution of the metals was strongly
influenced by the pH of the solution as shown in
Fig. 11.8
. Both Co and Mo were readily
dissolved from the calcined spent catalyst by aqueous SO
2
having pH of 1 and 100%
extraction of both metals was obtained in 8-10min at ambient temperature. The spent catalyst
used in this study contained Co
3
O
4
6.12%, MoO
3
10.8%, Al
2
O
3
65.2%, SiO
2
17.7%, S 0.10%,
and C 0.35%. The following reactions of Co and Mo with SO
2
solution were reported to occur
in the dissolution process.
CoSO
4
+ CoS
2
O
6
+8H
+
2Co + 3SO
2
+4H
2
O
⇒
2Mo(SO
4
)
2
+ 2Mo(S
2
O
6
)
2
+ 16H
+
4Mo + 12SO
2
+ 16H
2
O
⇒
The extent of dissolution of Al
2
O
3
and the separation of the metals from the mixed metal
solution were not reported in this study.
11.1.1.2.2 Organic acids
The leaching of metals present in spent hydroprocessing catalysts with organic acids has been
studied by many researchers. Beuther and Flinn
[479]
compared the efficiencies of many
