(BAT) Reference Document for the Production of Chlor-alkali ...

Chapter 4
4.5.4.3 Soil washing
Description
This technique consists in mixing contaminated soil with water and subsequently separating the
fine particle fraction to which the mercury is attached. Elemental mercury is also separated from
the bulk of the soil.
Technical description
A summary description is provided in Table 4.24. Prior to washing, the soil is typically screened
to remove oversized particles and then homogenised. The soil is then mixed with a wash
solution of water or water enhanced with chemical additives such as leaching agents,
surfactants, acids, or chelating agents to help remove organic compounds and heavy metals.
Particles are separated by size (cyclone or gravity separation, depending on the type of
contaminants in the soil and the particle size), concentrating the contaminants in the fine particle
fraction which requires further treatment. The coarser-grained soil is relatively clean, usually
requiring no additional treatment. Wash water from the process is treated and either reused in
the process or disposed of (Figure 4.11) [ 266, US EPA 2007 ].
Source: [ 266, US EPA 2007 ]
Figure 4.11: Flow diagram of a soil washing system
Achieved environmental benefits
The achieved environmental benefit of this technique is the decontamination of soil.
Environmental performance and operational data
At the Orica chlor-alkali plant in Sydney (Australia), soil washing trials were carried out in
2008 and a full-scale system was put into operation in May 2011. The washing included five
steps:
WORKING DRAFT IN PROGRESS
1. Screening to remove coarse material (diameter > 10 cm);
2. Washing with water which removes particles and elemental mercury droplets with
diameters > 6 mm;
3. Processing of the aqueous fraction from step 2 in a rotary separator with removal of the
elemental mercury;
4. Screening of the remaining material through two sieves ('sand' 0.18 – 6 mm and 'silt'
0.04 – 0.18 mm);
5. Dewatering of slurry from step 4 in a centrifuge.
264 December 2011 TB/EIPPCB/CAK_Draft_1