ABSTRACT
The aim of this study was to evaluate the possible decontamination methods for dredged
sludge containing heavy métal pollutants and détermine the destiny of the material after
Processing. The work, which is divided into three parts, is based on estuary and canal
sédiments sampled from varions sites in France.
The physical, Chemical and minerai characterization of the samples revealed the presence of
complex pollutants, sometimes very marked. A succession of physical séparations, binocular
observations and SEM and microprobe analyses has made it possible to détermine the main
Chemical types with which the metals are associated for each sédiment. This has shown that
contamination of the fine fraction (0/10 pm) is not only due to metal/organic matter or
metal/clay affinities, but also to the presence of micron size metallic sulphides. In addition, the
high pollutant content of the coarse fraction of certain sédiments is commonly related to phases
on the particle surface; thus, précipitation of metals as carbonate and metallic adsorption on
iron and manganèse oxide grains of a few hundred microns were observed. Furthermore, manmade éléments, such as vitreous slag, perfectly spherical metallic particles, and sulphur-bearing
ore fragments were commonly noted.
Processing tests were then carried out aimed at concentrating pollutants into a smaller weight
fraction using physical and physico-chemical séparation techniques selected from the
characterization results. For most of the sédiments, the contaminated fine fraction was isolated
by hydrocyclone before processing the coarse fraction with other techniques, such as flotation
which provided very promising results despite the high initial contamination. However, the
efficiency of this sorting technique is particularly sensitive to the type of Chemical collecter
used, which can only be selected after optimization tests. Depending on the set
decontamination objectives, it may be possible to refine processing through complementary
gravimétrie and densimetric séparation.
Finally, the mechanical behaviour of the processed materials was studied. The end resuit of
processing is on the one hand a material with little or no contamination that is potentially
upgradable, and on the other hand a clayey fine product rich in organic matter that
concentrâtes the pollutants. One possibility for the latter is dumping at confinement sites,
which seems to be the best answer in view of the environmental and économie constraints. An
understanding of the behaviour of the material during consolidation is vital for both the shortterm (storage capacity) and long-term (réhabilitation) management of these confinement sites.
For this, macrogravity tests by centrifugation were carried out to simulate the évolution of
consolidation beneath the natural weight of raw and processed sédiments; the time factor for
the tested 50 and 70 g macrogravities was multiplied by 2500 and 4900 respectively. The
degree of consolidation after testing (90% after a time representing more than 900 days) has
demonstrated the slowness of the consolidation process Mechanical strength remains very low
and the estimated compaction of the fine fraction after consolidation represents almost 70% of
the initial column height.
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The aim of this study was to evaluate the possible decontamination methods for dredged
sludge containing heavy métal pollutants and détermine the destiny of the material after
Processing. The work, which is divided into three parts, is based on estuary and canal
sédiments sampled from varions sites in France.
The physical, Chemical and minerai characterization of the samples revealed the presence of
complex pollutants, sometimes very marked. A succession of physical séparations, binocular
observations and SEM and microprobe analyses has made it possible to détermine the main
Chemical types with which the metals are associated for each sédiment. This has shown that
contamination of the fine fraction (0/10 pm) is not only due to metal/organic matter or
metal/clay affinities, but also to the presence of micron size metallic sulphides. In addition, the
high pollutant content of the coarse fraction of certain sédiments is commonly related to phases
on the particle surface; thus, précipitation of metals as carbonate and metallic adsorption on
iron and manganèse oxide grains of a few hundred microns were observed. Furthermore, manmade éléments, such as vitreous slag, perfectly spherical metallic particles, and sulphur-bearing
ore fragments were commonly noted.
Processing tests were then carried out aimed at concentrating pollutants into a smaller weight
fraction using physical and physico-chemical séparation techniques selected from the
characterization results. For most of the sédiments, the contaminated fine fraction was isolated
by hydrocyclone before processing the coarse fraction with other techniques, such as flotation
which provided very promising results despite the high initial contamination. However, the
efficiency of this sorting technique is particularly sensitive to the type of Chemical collecter
used, which can only be selected after optimization tests. Depending on the set
decontamination objectives, it may be possible to refine processing through complementary
gravimétrie and densimetric séparation.
Finally, the mechanical behaviour of the processed materials was studied. The end resuit of
processing is on the one hand a material with little or no contamination that is potentially
upgradable, and on the other hand a clayey fine product rich in organic matter that
concentrâtes the pollutants. One possibility for the latter is dumping at confinement sites,
which seems to be the best answer in view of the environmental and économie constraints. An
understanding of the behaviour of the material during consolidation is vital for both the shortterm (storage capacity) and long-term (réhabilitation) management of these confinement sites.
For this, macrogravity tests by centrifugation were carried out to simulate the évolution of
consolidation beneath the natural weight of raw and processed sédiments; the time factor for
the tested 50 and 70 g macrogravities was multiplied by 2500 and 4900 respectively. The
degree of consolidation after testing (90% after a time representing more than 900 days) has
demonstrated the slowness of the consolidation process Mechanical strength remains very low
and the estimated compaction of the fine fraction after consolidation represents almost 70% of
the initial column height.
9