Environmental and Safety Aspects of Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 118

well as low metal leaching rates with metal concentrations ranging between 0.1 and 6.6mg/L
for vanadium and between 9.5 and 52.8 ug/L for nickel. Both chemical fixation and
encapsulation mechanisms were proposed by these authors for stabilization of V and Ni. In the
chemical fixation, the heavy metals form chemical bonds with the surrounding matrix to
become part of its crystal structure, while in the encapsulation, a physical barrier is formed
around the heavy metals preventing them from leaching into the environment.
The effect of spinel based construction ceramics (e.g. -Al 2 O 3 , hematite, kaolinite) in the
stabilization of nickel-containing waste sludge was evaluated by Shih et al. [358] . The study
demonstrated the feasibility of transforming nickel-laden sludge into spinel phases with the
use of readily available and inexpensive ceramic raw materials and successful reduction of
metal mobility under acidic environments. Solidification and stabilization of toxic cadmium
ions in sand-cement clay mixture were achieved recently by Shawabkeh [359] . Clay-based
stabilization of the heavy metals (V, Mo, and Ni), present in spent hydroprocessing catalysts,
was also reported by Stanislaus et al. [360] . In this study, the spent catalyst in the form of a
fine powder was mixed with clay, gatch (containing SiO 2 -Al 2 O 3 and CaO), sand, and water
and heated at high temperatures in the range 1150-1130 ◦ C. Leaching of the heavy metals
from the resulting material was very low ( < 1.0mg/L).
A combined process for the recovery of metals of interest from spent hydroprocessing
catalysts and immobilization of the unleached metals remaining in the residue by stabilization
has been reported by Sun et al. [361] . The spent catalyst that contained 16 wt.% C, 7.3 wt.% S,
10.9 wt.% Mo, 4.0 wt.% Co, and 4.6 wt.% V was first calcined in air at 500 ◦ Ctoremove
carbon and sulfur and to convert the metal sulfides to metal oxides. The metals were then
leached in two stages using concentrated NH 4 OH in the first stage and 10% H 2 SO 4 in the
second stage to recover 87% of Mo and 77% of Co. The residue containing the unleached
metals was mixed with marine clay and fired at temperatures around 1000 ◦ C to produce
commercial value bricks. The leachability of the heavy metals from the produced bricks was
very low. It is believed that the SiO 2 in the clay together with the -Al 2 O 3 in the residue
formed an impermeable ceramic material to prevent leaching.
In another related study, Sun et al. [362] used ladle furnace slag (LFS) rich in CaO and SiO 2 to
stabilize small amounts of the heavy metals (V, Mo, and Co) remaining in the -Al 2 O 3 residue
after recovering a major portion of the metals by leaching. The combined waste material
containing mainly Al 2 O 3 , SiO 2 , CaO, and small quantities of heavy metals (V, Mo, and Co)
was melted in a high temperature electric furnace at 1130 o C for 3 h to form a glass-ceramic
product. The TCLP leaching values of the metals from the glass-ceramic material were lower
than the allowable level, which indicates immobilization of the metals in the glass-ceramic
matrix.
A combined leaching, recovering, and residue immobilization approach is applied by
Hydromet Corporation Ltd. [363] in Australia for treating metals-containing wastes. When the