Environmental Engineering Reference
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Figure 11.9: Effect of H 2 C 2 O 4 concentration on metal leaching efficiency at 50 Cin3.0MH 2 O 2
solution after 4 h leaching [From ref. 632 . Reprinted with permission].
mixed with H 2 O 2 . The effects of oxalic acid and hydrogen peroxide concentrations and the
stirring speed on the rate of metal leaching were studied. The results revealed that addition of
hydrogen peroxide to oxalic acid up to 3.0M H 2 O 2 concentration enhanced leaching of metals
remarkably, and thereafter remained relatively constant. Change in the concentration of oxalic
acid solution within the range 0.25-0.7M caused a gradual increase in the extraction of all
metals up to 0.5M and then a slight decrease with further increase to 0.7M ( Fig. 11.9 ). The
highest extraction of metals from the spent catalyst (at 50 C with a solution of 0.5M H 2 C 2 O 4
with 3.0M H 2 O 2 ) was found to be 90% Mo, 94% V, 65% Ni, and 33% Al in four hours
leaching.
Table 11.2 summarizes the studies on recovery of metals from spent hydroprocessing catalysts
using either acids or mixtures of acids with different reagents. In most studies, Mo was the
metal of primary interest. For this metal, more than 90% recovery was achieved regardless of
the type of reagents. Similarly, a high recovery of V and Co should be noted.
11.1.1.3 Alkali Leaching
Leaching of spent catalyst with aqueous alkaline solution (NaOH or Na 2 CO 3 ) has been
reported in a few studies. For example, Rokukawa [633] described a process in which an
aqueous alkaline solution of sodium carbonate mixed with hydrogen peroxide as an oxidizing
agent was used to extract Mo and V selectively from unroasted spent hydroprocessing
catalysts. More than 99% of Mo and about 85% of V were recovered in the process while the
extraction of Ni, Co, and Al 2 O 3 were minimum. This work was recently advanced by Park
et al. [634,635] for selective extraction of high purity MoO 3 from spent HDS catalyst using a
 
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