Biotechnological Aspects of Soil Decontamination - Biotechnologies and Biomimetics for Civil Engineering - page 373

17.2.7 Pyrometallurgical Separation
Pyrometallurgical processes use high temperature furnace to volatilize metals in
contaminated soil. Temperatures of 200-700 C are used to evaporate the con-
taminant. After volatilization, metals are then recovered or immobilized. These
methods are most applicable to mercury since it is easily converted to its metallic
form at high temperature. Other valuable metals such as gold and platinum can
also be recovered from low soil concentration. This type of treatment is usually
performed off-site due to lack of mobile units, and is applicable to highly con-
taminated soils (5-20 %) where metal recovery is profitable. Prior to pyrometal-
lurgical separation the soil must be concentrated by physical or soil washing.
17.2.8 Soil Washing
Soil decontamination can also be carried out by in situ washing of soil. It is a
physical separation technique, which consists of extraction of contaminants by
suspending them in watery solutions, i.e., by dissolution. The main principle of
soil washing is a selective classification of highly contaminated pollutants fol-
lowed by the solid or liquid phase separation of the remaining suspension (Bradl
and Xenidis 2005 ). It may consist of excavation, fragmentation, separation in
different grain sizes, washing of the different fraction, and their disposal (Castelo-
Grande et al. 2010 ). This technique is often considered as a pretreatment for the
reduction of the toxic contents of the contaminated soil, and to be treated by
another technology. The physical process of soil washing involves two principal
steps:
Mechanical/Physical sorting Physical separation may include screening fol-
lowed by density or gravity separation. Mechanical screens and hydrocyclones are
often used to separate the soils into various size fractions. The bulk oversize
material consists of clean or slightly contaminated cobbles and stones, and may
undergo a water rinse before being returned to the site as fill. The slit and clay
fraction generally contains the highest concentration of the contaminants, and is
usually treated by the solidification/stabilization techniques to immobilize the
contaminants prior to land filling. The remaining fine and coarse sands can be
further treated using density/gravity separation process to separate high density
aggregates and metal fragments. Magnetic contents are removed manually with the
help of magnets.
Wash water treatment Fresh water and cleaned process water are added to the
soil. From the suspension, cleaned soils fractions are separated and contaminants
are further proceeded for further treatment.
For the process of soil washing the knowledge of particle size dependent pol-
lutant distribution is of vital significance. It is one of the few permanent treatment