Environmental Engineering Reference
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3-20% higher metal extraction efficiency than the chemical leaching. Apparently, bioleaching
may not be competitive with the other methods used for leaching. A significantly longer
leaching time required during bioleaching to achieve extraction efficiencies, which are
comparable to those observed using other leaching methods may be the main drawback of
bioleaching. Environmental issues may play a decisive role in evaluating and/or comparing
commercial processes for metal reclamation. For example, the roasting and chlorination
processes generate gaseous effluent which requires treatment. No waste gases are generated in
the case that leaching step is conducted as the first step instead of roasting and chlorination.
This may be an important advantage of the leaching methods compared with other two
methods. The separation of metals from leachates generates liquid streams, which require
purification before disposal. It should be noted that decoking of spent catalysts generates
gaseous emissions, such as SO X ,NO X ,NH 3 , etc., that have to be removed from the waste gas.
This step is common for all processes used for metal reclamation. Apparently, the information
on environmental aspects of the commercial processes used for metal reclamation from spent
catalysts is the scarce. Therefore, a speculative discussion of this issue can only be
afforded.
In recent years, attempts have been made to use rather novel techniques for metal recovery
from spent catalysts. This included carbothermic reduction of metal oxides to metals [672] .
Electrolytic cells have been employed as well [673,674] . Energy intensive method, such as
thermal plasma received attention as well [676] . There is little information indicating
commercial use of these methods for metal recovery from spent hydroprocessing catalysts.
11.2 Separation of Metals from Solution
In the preceding sub-chapters, frequent reference has been made to the isolation of metals
and/or metal compounds in a pure form from leaching solutions, without providing details. In
this regard, extensive information on the separation of metals from solutions can be found in
the literature dealing with various aspects of hydrometallurgy. This experience can be applied
to spent hydroprocessing catalysts. In this case, more than one metal are usually present in the
solution after leaching and/or dissolution of spent catalysts. Additional treatments are required
to isolate metal compounds in their pure form. For these purposes, numerous extraction agents
of organic origin, with a high selectivity for a metal of interest have been available and used in
commercial applications. The structure of some commercial agents used for the extraction of
metals from various solutions is shown in Fig. 11.1 [622] . The selectivity can be further
enhanced by optimizing the conditions applied during extraction.
As the final step, a pure metal compound can be obtained from the solution by precipitation
under controlled conditions (e.g., pH) using a suitable precipitant. In some cases, only minor
modifications to the methods used commercially in hydrometallurgical industry are required
before they can be applied for metal reclamation from the spent hydroprocessing catalysts.
 
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