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4.3 Management of Spent Materials 349

bed has to be separated into the cation and the anion beads, which is usually done using
hydraulic separation in situ by backwashing the bed by utilizing the small differences in the
densities of the cation and anion media. After the regeneration, the media are then remixed.

4.3.3 Destruction, immobilization, and encapsulation of spent materials

Spent ion-exchange and adsorption materials represent a special type of w and pose aste,
unique problems in the selection of their treatment options. ith the evolution of environ- W
gislation,
mental le it is now required that spent materials meet specific quality require-
ments prior to disposal. In the selection of the treatment method for spent materials, their
,
physical and chemical characteristics must be considered. Basically there are two main
treatment methods: destruction and immobilization. In sedepending on the v eral occasions,
nature of the spent material, a pretreatment step is required.

Pretreatment
Available pretreatment processes include (IAEA, 2002) the follo wing.
Dewatering : Deatering the spent materials prior to immobilization impro es the o v w er- v
all volume reduction and the compatibility of the waste with the immobilization process.
Dewatering is generally accomplished by pressure, v or centrifugation. acuum f iltration,
w
Drying and heating : A complete deatering step, or drying, is sometimes required.
Steam or hot air is generally used.
Crushing and grinding : Crushing and grinding techniques are used basically for size
reduction of spent materials, which results in a more homogeneous waste for immobiliza-
tion, allo and f ic materials, al of nonspecif ws the easier remo v acilitates the further destruc-
tion of the material by thermal, chemical, or biological methods.

Destructive methods for spent materials
v
The main goal of destructie methods is to coneniently alter the physicochemical char- v
acteristics of the spent material before its fThese processes can be classif ied inal disposal.
as thermal and nonthermal processes.

Thermal pr ocesses Pyrolysis : Pyrolysis can be defined as the thermal degradation of
organic species in the absence of oxygen or other reactant gases. Ho practically , v we er ,
pyrolysis refers to any thermal degradation process in which less than the stoichiometric
quantity of oxygen is used (Freeman, 1998). Furthermore, pyrolisis of inor ganic materials
is called “calcination.”
v
Pyrolysis operates under relatiely low temperatures, in the range 500–700 °C. Upon
heating in the absence of oxygen, thermal cracking and condensation reactions take place
converting most organics into gaseous, liquid, and solid fractions (Perry and Green, 1999).
The advantages of pyrolysis are

(a) the significant reduction of the waste v and olume,
(b) the end product is biologically and geochemically stable (IAEA, 2002).