Spent Catalysts from Dewaxing Operations - Spent Hydroprocessing Catalysts - page 264

respect to the HYD equilibrium of aromatics, such conditions are favorable for maintaining a
high rate of hydrodearomatization.

The regeneration of noble metals containing spent catalysts can rely on the experience gained
during regeneration of spent reforming catalysts, which usually contain Pt on -Al 2 O 3 [15] .In
some cases, regeneration is an integral part of the overall reforming operation. For
regeneration, spent catalyst is withdrawn from reactor and transferred to the regenerator where
coke is burnt off and catalyst reactivated. It has been noted that a small portion of catalyst, so
called non-flowing catalyst, remains on the walls and at the bottom of reactor. The coke on this
catalyst may continuously build up to approach 50 wt.%. For such catalyst, metal recovery
may be the only option. However, it was reported by Blashka et al. [595] , that a light fraction
could still be recovered from the heavily deactivated catalyst by density separation. The
separated light fraction can then be reactivated and returned to the operation.

10.2.5 Metal Reclamation

Compared with the conventional spent hydroprocessing catalysts, entirely different approach
is used for the non-regenerable catalysts containing noble metals. Thus, because of the high
value of the noble metals, refineries must pay special attention to the inventory and handling of
such catalysts. This involves an accurate chemical analysis determining the content of noble
metal, which has to be performed before shipping spent catalysts for metal reclamation. This
was confirmed by Rosso et al. [596,597] . For example, in the case of Pt, a tolerance of 0.5%

may represent a significant monetary value. The accuracy of the analysis can be significantly
improved by the removing contaminants (e.g., coke, sulfur, moisture, etc.) using preburning
[589-591] . Both on-site and off-site preburning has been practiced, although with respect to
the final settlement with the metal reclaiming company, the former appears to be more
favorable for the refinery.

±

There are a number of processes operating on the commercial scale for the recovery of precious
metals from various types of spent catalysts [15] . These methods can be applied towards spent
dewaxing catalysts after minor modifications. One of the methods is based on the dissolution
of the -Al 2 O 3 support, while leaving precious metals in a solid form. The dissolution can be
achieved using either NaOH or H 2 SO 4 [591,597,598] . For NaOH, decoked catalyst is treated
at 200 ◦ C and under pressure of 10 bar [598] . During this treatment, the support is converted to
soluble sodium aluminate, which is separated from Pt by filtration. Then, the Pt can be
dissolved using the treatment with HCl and Cl 2 . Once in the solution, Pt higher than 99.9%
purity can be obtained. In similar manners, Ir can be isolate from spent catalysts [598] .

On the other hand, the dissolution of metals, while leaving the support undissolved, can be
achieved by the mixture of HCl and an oxidant, i.e., HNO 3 ,H 2 O 2 , NaClO 3 , NaOCl, etc.
[599,600] . The process can operate either in a batch mode or a continuous mode. In the study