Environmental and Safety Aspects of Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 117

Table 5.7: Toxicity Characteristic Leaching Procedure (TCLP) of pretreated and unpretreated solids
[From ref. 350 . Reprinted with permission].

Concentration in leachate (ppm)

Unpretreated Pretreated Regulatory levels

Pb 3 to 3700 < 1 5

Cd 1 to 1596 < 1to07 1

Cr 5 to 660 < 0.3 5

Se 1 to 300 < 0.0025 1

As ≤ 4190 < 1.75 5

Ni 5 to 250 < 0.5 5

Ba > 400 < 10 100

successfully applied to radioactive wastes [346-348] . For a similar purpose, Shi and
Fernandez-Jimenez [340] used alkali-activated cement comprising Ca-silicate hydrate and a
zeolite. The leachability of the material solidified with the alkali-activated cement was much
lower than that of solids, which were hardened with Portland cement.

The Maectite process, patented by Sevenson Environmental Services Inc. [349] , is capable of
converting reactive metals contained in solid wastes into non-leachable minerals in the apatite
and barite group. These minerals are resistant to acidity and degradation by geological and
chemical conditions, such as those found in landfills and natural settings. The leachability of
the unpretreated hazardous waste and that pretreated using the Maectite process is shown in
Table 5.7 [350] . It is evident that on pretreatment, the concentrations of all trace metals were
well below the regulatory levels. It is believed that the Maectite process can be also
successfully applied to spent hydroprocessing catalysts. A similar method known as Sealosafe
has been developed by Stablex Corporation for the treatment of hazardous wastes to make them
non-leachable [351] . The process involves adding a mixture of calcium containing cement
powder and an alumino-silicate powder to the waste material dispersed in water and converting
them to an impermeable solid. Cement-based solidification/stabilization/immobilization
methods have also been widely studied for treatment of hazardous wastes containing arsenic
and heavy metals [352-355] . Sun et al. [341] used Portland cement as a matrix to encapsulate
vanadium and nickel present in spent fluid catalytic cracking (FCC) catalyst and found that
Portland cement was an effective means of stabilization. In another study, these authors used
marine clay as a matrix to stabilize the heavy metals present in spent FCC catalysts and
produce value-added high strength bricks through a sintering process [355,356] similarly as
they did for the spent CoMo/Al 2 O 3 catalyst [357] . The results showed that the resulting bricks
had high compressive strength ranging between 20.0 and 92.0 N/mm 2 for marine clay-spent
FCC catalyst samples and low-water absorption values ranging between 4.8 and 18.5 wt.% as