Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 301

remaining on the surface of spent catalyst was decomposed and converted to CO, CO 2 , and H 2 .
Apparently, CO and H 2 produced in the process acted as reducing agents.
11.1.7 Summary of Laboratory Studies
Apparently, pretreatment prior to metal recovery, i.e., deoiling, decoking, and crushing, is
common for all methods used for metal recovery from spent hydroprocessing catalysts. The
efficiency of recovery of metals is only one of the parameters used to compare different
leaching methods. Table 11.4 shows that for leaching methods, recovery is either similar or the
reported recovery ranges are overlapping. Therefore, other parameters have to be taken into
consideration for the overall comparison. The information on cost of raw materials used for
recovery, number of steps involved, associated emissions, severity and required safety
precautions, material requirements, etc. may play an important role for identifying the most
suitable method.
Among leaching methods, the solution of ammonia or ammonium containing compounds used
for leaching can be readily prepared and leachates obtained from these methods can be
processed using established procedures. The increased requirements on materials and level of
safety have to be considered for the leaching solutions containing strong acids. In this case,
special procedures have to be applied to neutralize some by-product liquids before their safe
disposal. These problems are almost completely eliminated by using aqueous solutions of
organic acids.
A direct comparison of the leaching spent NiMo/Al 2 O 3 catalyst using the solutions of
ammonia with that of the NaOH (10%) solution was conducted by Villareal et al. [617] . The
former solution was suitable for the selective leaching of V, while leaving most of the Mo
behind. This observation differs from other studies [613,616] in which both V and Mo were
leached out. It should be, however, noted that the catalyst used by Villareal et al. [617] was
heavily deposited by V, i.e., it had the atomic V/Mo ratio of about 12. Moreover, leaching was
conducted at a nearly room temperature compared with higher temperatures used by the other
authors. The leachate was heated and cooled to recrystallise and separate NH 4 VO 3 before
being heated to 450 ◦ C to form pure V 2 O 5 . The NaOH (10%) solution leached out both
V and Mo.
Jong et al. [636] compared the two stages NaOH-H 2 SO 4 leaching process with the
chlorination process in which solvent extraction was used to recover Ni. The overall operating
cost of the leaching process was about twice that of the chlorination process. The cost of raw
materials (e.g., NaOH, H 2 SO 4 , CaCl 2 ,NH 4 OH, etc.) used for the leaching process was the
main contributor to the different operating costs. During this comparison, the environmental
issues were not addressed in details.
Bioleaching of a FCC catalyst was compared with the chemical leaching using organic acids
which are secreted during the growth of the A. niger fungus [642] . Generally, bioleaching gave