Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 280

mixture of Na 2 CO 3 and H 2 O 2 . The results indicated that the recovery of molybdenum was
largely dependent on the concentrations of Na 2 CO 3 and H 2 O 2 in the reaction medium, which
controls the acidity (pH) of the leach liquor and carry over of impurities, such as Al, Ni, P/Si,
and V. In this case, the leaching process was exothermic and leaching efficiency of
molybdenum decreased with increasing solid to liquid ratio. For large scale leaching of spent
catalyst, under optimum conditions (20% pulp density, 85 g/L Na 2 CO 3 , 10 vol.% H 2 O 2 , and
1 h reaction), it was possible to recover 84% Mo. The obtained leach liquor contained (g/L):
Mo: 22.0, Ni: 0.015 and Al: 0.82, P: 1.1, Si: 0.094 and minor quantities of V: 8mg/L, As and
Co < 1mg/L. Purification of the leach liquor by carbon adsorption at pH around 0.75 and
desorption of the adsorbed molybdate with 15% NH 4 OH produced high purity ammonium
molybdate from which MoO 3 with 99.4% purity was obtained by calcination. The process
details are schematically shown in Fig. 11.10 [635] .
Villareal et al. [617] compared two solutions of NaOH, i.e., one containing 10% and the other
40% of NaOH. For the former, the amount of Mo and V leached out approached 92 and 89%,
respectively, whereas for the concentrated solution, 79 and 72%, respectively. For both
solutions, very small amounts of Ni and Al could be leached out.
11.1.1.4 Two-Stage Leaching
The process tested by Jong et al. [636] comprised two stages. The first stage involved leaching
spent NiMo/Al 2 O 3 catalyst with NaOH solution to extract Mo and some Al, while the rest of
Al and Ni remained in the residue. The Mo was isolated from the NaOH leach by precipitation
with CaCl 2 . In the second stage, the residue from the first step was leached with sulfuric acid
to extract Ni. The leachate from this step was treated with NH 4 OH and (NH 4 ) 2 SO 4 to
precipitate Al and other metal impurities. At about pH 10, Ni was recovered by solvent
extraction. The NaOH leaching step extracted from 81 to 91 wt.% of the Mo. The H 2 SO 4
leaching step extracted from 92 to 98 wt.% of the Ni. In the final recovery step, over 84 wt.%
of the Mo and 99% of Ni were recovered.
Two-stage leaching processes involving leaching with an alkali solution in the first stage and
an acid or ammonia in the second stage or the other way around have also been reported in
many other studies. Angelids et al. [637] used NaOH in the first stage and H 2 SO 4 in the second
stage to leach Mo, Co, and Ni from spent HDS catalysts. The process is based on the
difference of solubility of MoO 3 and CoO or NiO in acidic and basic media. Both
CoMo/Al 2 O 3 and NiMo/Al 2 O 3 type spent catalysts were first roasted in air using controlled
conditions at temperatures around 450 ◦ C for 2 h to remove the coke and sulfur present in the
catalyst. The calcined CoMo/Al 2 O 3 catalyst contained 10.3% Mo, 3.5% Co, and 39.7% Al.
The spent NiMo/Al 2 O 3 catalyst contained 10% Mo, 2.4% Ni, and 31.7% Al. The effects of
reagent concentration, leaching time, and temperature were investigated for both first stage
leaching with NaOH and second stage leaching with H 2 SO 4 . Leaching temperature
significantly influenced both Mo as well as Co and Ni extractions in the first and second