Environmental Engineering Reference
In-Depth Information
15.5.1.2 . Vitrification
In situ vitrification is a thermal method consisting of raising the temperature of
the ground to transform the contaminant into an inert molten material without
leachable elements. It is a method approaching the techniques of stabilization and
solidification. Figure 15.5 shows the process of in situ vitrification. Fusion is
obtained while inserting graphite electrodes in the ground to the depth necessary for
treatment. A strongly conducting material is generally placed between the electrodes
in order to start the reaction in the ground. The passage of electric current between
the graphite electrodes and conducting material allows an intense increase in the
temperature (up to 1,500ÂșC). Once the ground begins to melt it conducts electricity,
which makes it possible to continue the process for as a long as the electric current is
applied. The process is stopped by shutting off the electric current and allowing the
mass molten ground to cool (which can take up to one year). The temperatures used
allow the ground to melt and be transformed by pyrolysis into silicated glass, where
all the compounds present (organic, radioactive or other) become molten or are
vaporized. To allow the recovery of gas escaping during the reaction, a hermetic lid
and an air aspirator are placed on the surface above the ground; these gases are
treated separately. The process of vitrification is applicable to grounds polluted with
volatile products, semi-volatile organic compounds, non-volatile products and heavy
metals. In situ vitrification generally reduces the ground volume by 20-40%.
It is important to note that this method is limited by the fact that the water
content of water-logged soils can make the costs of in situ vitrification prohibitory,
because of the power consumed by water vaporization. Water-logged soils are
potentially vitrifiable if their permeability is lower than 1 X 10 -6 m/s. Moreover, the
technique is not recommended for sites where the weight of metals in the ground
exceeds 25% or when the volume of non-organic waste produces exceeds 20%
[SALT 99].
Figure 15.5. Diagram of vitrification in situ
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