Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 286

leaching. In contrast with spent FCC catalyst, little difference in leaching efficiency was
observed between bioleaching and leaching with the individual organic acids produced during
the growth of A. niger fungus.
In another study published by Santhiya and Ting [650] , bioleaching of spent hydroprocessing
catalysts was conducted using an adapted A. niger fungus. Adaptation experiments with single
metals revealed that the fungus could tolerate 1000, 1200, and 2000mg/L of Ni, Mo, and Al,
respectively. For the mixtures of these metals, the tolerance was decreased to 100, 200, and
600mg/L of Ni, Mo, and Al, respectively. In contrast to the adapted strain, in the presence of
catalyst, the unadapted strain exhibited no growth. For the adapted strain, leaching efficiency
for Ni, Mo, and Al approached 79, 82, and 65%, respectively over 30 days.
The mixed acidophilic culture in the presence of pyrite was used to study recovery of metals
from spent hydroprocessing catalysts [651] . This culture as initially grown in the 9K-medium
(absence of 9 g/L Fe(II)) where FeSO 4 was replaced by pyrite (FeS 2 ) and then applied for
bioleaching experiments. Bacterial action on pyrite catalyzed formation of the Fe +3 ,H + , and
SO 4 − 2 ions in the solution. This facilitated leaching of metals (Ni, Mo, and V) from the spent
catalyst. The study was conducted under a wide range of experimental conditions, i.e., reaction
time, amount of spent catalyst and pyrite, and temperature. After seven days with 30 g/L of
spent catalyst and 50 g/L of pyrite, the leaching of Ni, V, and Mo into the solution was 85, 92,
and 26%, respectively. With increasing spent catalyst loading, the extent of metal dissolution
was decreased, probably due to the enhanced removal of Fe +3 ion with the residue. Under all
conditions tested, Mo showed recovery due to its precipitation with leach residues as MoO 3 .
This was confirmed by applying spectroscopic techniques, such as EDAX and X-ray
diffraction spectroscopy (XRD).
Mishra et al. [652,653] used acidophilic bacteria, which in the presence of sulfur produced
H 2 SO 4 in the first stage. In the second stage, the acidic medium was used for leaching Ni, Mo,
and V from spent catalyst. At 50 g/L of spent catalyst and 20 g/L of elemental sulfur, the
amount of Ni, Mo, and V after seven days approached 88, 46, and 95%, respectively. In a
one-stage method, bacteria were grown in the presence of spent catalyst. In this case, the
maximum of extraction for Ni, Mo, and V approached 88, 58, and 32%, respectively.
11.1.2 Roasting with Alkali Compounds
The solubility of some metals (e.g., Mo, W, and V) in water can be significantly enhanced by
roasting spent catalysts with alkali metals containing compounds. After roasting, most of the
Ni and Co remain in the solid residue left after water extraction. A wide range of methods has
been used for the separation of metals from aqueous solutions. Both sodium and potassium
containing compounds (e.g., hydroxides and carbonates) have been used as roasting agents.