Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 282

and second stages respectively from a spent NiMo/Al 2 O 3 catalyst using ammonia in the first
stage and H 2 SO 4 in the second stage [638] .
In the patented process, Veal et al. [639] used caustic leach to extract Mo and V in the first
stage and aqueous ammonia/ammonium carbonate in the second stage to extract Ni from a
de-oiled but not decoked spent HDS catalyst that contained the metals (Mo, V, and Ni) in a
sulfided form. The caustic leach was conducted in two steps: first step at atmospheric pressure
in the presence of caustic and air at a temperature less than 60 ◦ C and a pH range of 10-13.
About 50-70% of V and Mo sulfides were converted to soluble thiosulfate and sulfate species
while Ni and Co were not reactive under these conditions. After filtration, the solids from the
atmospheric caustic leach were subjected to pressure leach with caustic at a higher oxygen
pressure and at a higher temperature to solubilize the remaining V and Mo sulfides. About
97% of the Mo, 92% of the V, and 98% of the sulfur were solubilized in the atmospheric and
pressure leaching steps of caustic leaching. The soluble metals (V and Mo) were then
separated by solid/liquid separation. The final recovery of the soluble V and Mo from the
liquid stream was accomplished by solvent extraction. The residual solid that contained nickel
was leached with an aqueous ammonia/ammonium carbonate solution at a pH range of about
10.5-12 and at temperatures in the range of 40-80 ◦ C. The ammonia leach solution containing
the solubilized nickel amine complex was then stripped and heated to remove ammonia. As the
ammonia was removed, the pH of the solution decreased to about 10 and a basic nickel
carbonate was precipitated. Nickel was finally recovered as nickel carbonate with high purity.
11.1.1.5 Bioleaching
Bioleaching offers a novel approach for metal recovery from various solids. It is based on the
ability of some microorganisms to transform solid compounds to extractable entities [640] .In
this case, the microorganisms can secret either organic or inorganic acids which are
participating during the metal dissolutions. Bioleaching was used industrially for recovery of
various metals from low-grade mineral resources [641] . In addition, the article published by
Santhiya and Ting [525] lists the waste materials, such as fly ash, sewage sludge, spent
batteries, and electronic scrap materials, as well as the fluid catalytic cracking (FCC) and
hydroprocessing catalysts as potential solids which can be processed by bioleaching. The most
common microorganisms, which are capable of the metal solubilization, include bacteria, such
as Thiobacillus ferrooxidans and Thiobacillus thiooxidans , as well as the fungi, such as
Aspergillus and Penicillium genera [642] .
Earlier studies on potential application of bioleaching for metal recovery from spent refinery
catalysts were reviewed by Furimsky [27] . It was evident that the first attempt to apply the
bioleaching method to spent refinery catalysts was made in early 1990s. For example,
Blaustein et al. [643,644] used T. ferrooxidans and L. ferrooxidans for leaching Mo from a coal
liquefaction catalyst. Bioleaching experiments were conducted in an autoclave at about 30 ◦ C
with shaking at 170 to 200 rpm for about six weeks. The amount of the solubilized Mo