Environmental Engineering Reference
In-Depth Information
LIX 51 agents were used to extract Ni and Co from the solution after Mo and V extraction
[612]
.
In another related study, Marcantonio
[613]
used an aqueous solution of ammonia, ammonium
salt, and hydrogen peroxide to extract metals (V, Mo, Ni, and Co) from spent hydroprocessing
catalysts. The ammonium salts used in the study included (NH
4
)
2
CO
3
and (NH
4
)
2
SO
4
. The
initial pH and the concentrations of ammonia, ammonium salt, and H
2
O
2
in the mixed reagent
strongly influenced the extent of extraction of metals. The V and Co extractions were
remarkably enhanced by H
2
O
2
in addition to the ammonia + ammonium salt reagent, while
little effect of H
2
O
2
on the Mo and Ni extractions was noted. About 93% of Mo, 88% of V,
80% of Ni, and 78% of Co present in the spent catalyst were extracted by leaching with a
reagent having an initial pH of 10.4, and ammonia, ammonium salt and H
2
O
2
concentrations,
2M, 0.5M and 0.14M, respectively.
The selective recovery of Mo from the unsupported spent MoS
2
catalyst contaminated with Ni
and V, by reducing the oxidized catalyst with a reducing agent, such as hydrazine, before
extraction with ammonia, is described in another patent assigned to Chevron
[614,615]
. The
spent catalyst was first oxidized to convert the metal sulfides to oxides and then treated with
hydrazine to reduce the high valence state of vanadium to a lower valence state. It was then
leached with aqueous ammonia. The use of a reducing agent before the ammonia extraction
step resulted in superior separation of Mo from V and Ni. The recovered Mo can be sulfided to
produce an active MoS
2
catalyst for recycle.
In another study patented recently, Marcantonio
[616]
used ammonia pressure leaching
technique in the presence of oxygen to produce ammonium molybdate, ammonium
metavanadate (AMV), and nickel ammonium sulfate from an unsupported spent
hydroprocessing catalyst that contained sulfides of Mo, V, and Ni. The reactions that occur in
the process are shown below:
MoS
2
+ 4.5O
2
+ 6NH
3
+3H
2
O
⇒
(NH
4
)
2
MoO
4
+ 2(NH
4
)
2
SO
4
V
2
S
3
+7O
2
+ 8NH
3
+4H
2
O
⇒
2NH
4
VO
3
+ 3(NH
4
)
2
SO
4
NiS+2O
2
+ 6NH
3
⇒
Ni(NH
3
)
6
SO
4
The ammonium molybdate and the nickel ammonium sulfate remained in solution while the
AMV precipitated out as a solid in the leach slurry which was separated by filtration and
converted to V
2
O
5
by calcination. The solution containing Mo and Ni was subjected to further
solvent extraction steps for separation of these metals. Vanadium pentoxide, ammonium
molybdate, and nickel sulfate were the final marketable products in this process.
Villarreal et al.
[617]
were able to recover selectively 98% of vanadium as AMV (NH
4
VO
3
)
from the spent hydrodemetallization (HDM) catalyst, that contained 27.3 wt.% V
2
O
5
,
