Environmental Engineering Reference
In-Depth Information
properly treated for stability to reduce leaching of the harmful materials that could pollute the
environment. Methods such as ASTM-3987 and the TCLP are normally used to assess
leachability of the wastes dumped in landfills.
The treatments, which enhance environmental acceptance of spent catalysts prior to their
disposal in landfills, will be illustrated on several examples from the scientific and technical
literature. In every case, the aim was to minimize leachability of spent catalysts. These
concerns are reflected by a growing number of the related articles that have been appearing in
the scientific literature during the recent years. This is not surprising in view of the cost of
potential liability associated with landfilling which may exceed $200 per ton of a spent
catalyst. In this regard, several methods based on immobilization, vitrification, and
encapsulation of toxic substances have been evaluated with various degrees of success.
It has been generally observed that a significant decrease in the leachability of hazardous
solids may be achieved using thermal treatment during which the spent catalyst is fused. For
example, the process patented by Phoenix Environmental
[338]
involves heating spent catalyst
in the flow of O
2
until the solid becomes a molten bath. After solidification, the molten bath
has a spinel structure that can bond harmful metals and convert them into a non-leachable
form. As it was shown by Kox and Van der Vlist
[339]
, this method has been successfully
applied to spent hydroprocessing catalysts. It is expected that as part of this method, the
effluents formed during heating is captured because it may contain some volatile
contaminants.
Treatment methods, such as encapsulation, vitrification, and stabilization/solidification are
commonly used to immobilize the hazardous waste materials and to make them non-leachable
[340-344]
. For example, as it was indicated by Trimm
[345]
, a decrease in leachability may be
achieved by encapsulation of spent catalysts using organic substances. The method involves
thermal treatment of spent-decoked catalyst with substances, such as bitumen, paraffin wax,
and various polymers. After cooling, the catalyst material is well sealed in the thermoplastic
film. However, long-term effects of this method on leachability are unknown. Also, after
thermal treatment, some of these films may be efficient adsorbents of O
2
. It is therefore
believed that the ignitability of spent catalysts pretreated by this method may require attention.
Besides organic encapsulants, the encapsulation using various inorganic solids, such as clay,
concrete, vitreous silicates, glass composite, etc., may also be achieved. In this case, the
encapsulant converts a leachable form of metal into non-leachable, usually by
fusion.
As it was indicated, the immobilization of metals is one of the options that can decrease the
leachability of spent catalysts. In this case, catalysts are treated with other solids (e.g., silica,
clays, Portland cement, etc.) at temperatures ensuring the slugging (softening) of the mineral
matter. On cooling, the slag retains glassy properties. This prevents the release of metals in
contact with water. The immobilization by incorporating hazardous metals in glasses has been
