116
M. Hempel and ,. Thoeming
Table 2. Chemical and physical properties of some mercury compounds (Hempel 1993)
Mercury
Formula Density Melting Boiling Vapour- Vapor pressure at Solubility
compound
(glcm') point
point
saturated
in H2O
("C)
("C)
(mg/m.!) (mm Hg)
(uC) (gil)
Mercury (element) Hg"
13.6
-39
356.58
14.36
1.22 x 10 .! 20
6 x 10 '
Hg(l)-chloride
Hg2CI2
7.15
400
0.002
Hg(Il)-chloride
HgCI2
4.44
276
302
8.34 x 10 ' 25
69
Hg(lll)-suifide
HgS
584
I x 10 '
Methyl-Hg-chloride CH,HgCI
4.06
170
66
4.7 x 10
20
5
Phenyl-Hg-chloride CoH,HgCI
251
0.06
3.6
35
0.03
Dimethyl-Hg
(CH.!)2Hg 3.07
93
58.6
23.7
For remediation purposes it is an advantage that various Hg compounds, as
well as elemental Hg, are volatile at relatively low temperatures (see Table 2). To
remove Hg from excavated soil by thermal treatment, Hg needs to be
transformed from a solid phase of the soil to the gas phase by heating. The
temperature required for removing Hg and its compounds is theoretically below
600°C; if HgO is absent, 250°C should be sufficient. Most thermal soil plants
are, however, designed to remove all kinds of pollutants from the soil, e.g. volatile
hydrocarbons (petrol, oil, PAH, etc.) and halogenated organic compounds (HCH,
PCB, dioxine, furane). Treatment plants work at much higher temperatures (up
to 1300 DC) than needed to remove Hg and its compounds from the soil. Most of
these all-purpose thermal soil plants use a direct heating system with afterburner, but pyrolysis techniques are also used. A typical soil treatment process of
an all-purpose soil treatment plant is shown schematically in Fig. 2. After
entering the first oven, the soil is warmed to a temperature of 100°C in a drying
drum; subsequently, the soil enters a heating drum at a temperature of
approximately 600°C and the pollutants are released. Before leaving the plant as
a clean matrix, the soil passes through a cooling drum where it is cooled and
moistened. The waste gas from the drying and the heating drum enters an afterburner (ca. 800-900 DC). A heat exchanger cools the waste gas to temperature of
\ :
POliuted
material
r----_
heating drum
heat exchanger
,
dust filter
spray tower
r--_~
Cooling drum
I ,
clean soil
Fig. 2. Simplified process scheme of an all-purpose soil treatment plant
M. Hempel and ,. Thoeming
Table 2. Chemical and physical properties of some mercury compounds (Hempel 1993)
Mercury
Formula Density Melting Boiling Vapour- Vapor pressure at Solubility
compound
(glcm') point
point
saturated
in H2O
("C)
("C)
(mg/m.!) (mm Hg)
(uC) (gil)
Mercury (element) Hg"
13.6
-39
356.58
14.36
1.22 x 10 .! 20
6 x 10 '
Hg(l)-chloride
Hg2CI2
7.15
400
0.002
Hg(Il)-chloride
HgCI2
4.44
276
302
8.34 x 10 ' 25
69
Hg(lll)-suifide
HgS
584
I x 10 '
Methyl-Hg-chloride CH,HgCI
4.06
170
66
4.7 x 10
20
5
Phenyl-Hg-chloride CoH,HgCI
251
0.06
3.6
35
0.03
Dimethyl-Hg
(CH.!)2Hg 3.07
93
58.6
23.7
For remediation purposes it is an advantage that various Hg compounds, as
well as elemental Hg, are volatile at relatively low temperatures (see Table 2). To
remove Hg from excavated soil by thermal treatment, Hg needs to be
transformed from a solid phase of the soil to the gas phase by heating. The
temperature required for removing Hg and its compounds is theoretically below
600°C; if HgO is absent, 250°C should be sufficient. Most thermal soil plants
are, however, designed to remove all kinds of pollutants from the soil, e.g. volatile
hydrocarbons (petrol, oil, PAH, etc.) and halogenated organic compounds (HCH,
PCB, dioxine, furane). Treatment plants work at much higher temperatures (up
to 1300 DC) than needed to remove Hg and its compounds from the soil. Most of
these all-purpose thermal soil plants use a direct heating system with afterburner, but pyrolysis techniques are also used. A typical soil treatment process of
an all-purpose soil treatment plant is shown schematically in Fig. 2. After
entering the first oven, the soil is warmed to a temperature of 100°C in a drying
drum; subsequently, the soil enters a heating drum at a temperature of
approximately 600°C and the pollutants are released. Before leaving the plant as
a clean matrix, the soil passes through a cooling drum where it is cooled and
moistened. The waste gas from the drying and the heating drum enters an afterburner (ca. 800-900 DC). A heat exchanger cools the waste gas to temperature of
\ :
POliuted
material
r----_
heating drum
heat exchanger
,
dust filter
spray tower
r--_~
Cooling drum
I ,
clean soil
Fig. 2. Simplified process scheme of an all-purpose soil treatment plant