Remediation Techniques for Hg-Contaminated Sites
117
about 150 DC and heats the gas for the heating drum. Subsequently, the gas passes
through a dust filter and a spray tower to reduce dust and S02 emissions.
A thermal process, optimized for the remediation of Hg-contaminated soil,
was developed and constructed by Fortmann and Jahns (1993). The semimobile
plant is able to clean approximately 6 t/h. Soil and demolition material is crushed
in a special grinding device, which is under air control. The crushed and sieved
material enters a rotary kiln at a temperature of 1000 DC, where it rests for a
maximum of 3 h. Heating is performed by natural gas. After leaving the kiln, the
soil is cooled to a temperature of 80 DC. The waste gas enters an after-burner and
subsequently a heat exchanger where the gas is cooled down to 200 DC. After
passing through a dust filter, the gas still contains most of the Hg. This is
removed by a gas-washing system, which consists of a venturi washer and a twostep spray tower in which the gas is cooled to 40 DC. Hg is removed by chemical
oxidation at low pH and subsequent chemical sorption. The gas contains less
than 0.05 mg Hg/Nm-' at the end of the pipe.
Since mechanical treatment is cheaper, most of the remediation companies
combine wet classification with subsequent thermal treatment. It is advantageous
to enlarge the fraction which can be cleaned by simple wet classification and
therefore to minimize the fraction which needs to be treated by thermal
techniques.
Renner (1995) separated a fraction with a grain size <1 mm, which is dewatered
and subsequently introduced into a steam distillation chamber. The water steam
is produced by an external steam generator and is introduced into the chamber
by nozzles, which are located above and below a sieve. The soil fraction is moved
above the sieve and the steam streams through the contaminated material. A
duration time of 0.5 to 1 h at temperatures of 250 to 540 DC are used for different
kinds of material, prior to entering a cooling chamber. The steam, which is
enriched with pollutant, is pumped out of the chamber and passes through a hotgas dust filter which prevents a recontamination of the dust. Subsequently, the
steam is condensed and the resulting water is treated in a wastewater facility.
Before the gas is exhausted to the atmosphere, it passes through a charcoal filter.
The technique has been implemented in a pilot plant which has a capacity of
0.5 t/h. Various materials (soil, rubble material, charcoal, wood and oil sludge)
with contamination levels between 71 and 13 000 mg Hg/kg were tested. Final
concentrations of Hg in the cleaned soil of 0.2 to 11 mg Hg/kg were obtained.
In 1992, Harbauer GmbH installed a large soil-washing and vacuum distillation
plant for cleaning mercury-contaminated soil and demolition rubble from the
former Marktredwitz chemical works (Hennig 1993). The wet classification plant
itself has a capacity of 25 tlh, while the thermal part of the plant has a capacity of
6 t1h. After a crushing and sorting step, which is in-house, the soil is separated by
wet classification. An Hg-enriched soil fraction is fed into a vacuum distillation
plant and cleaned. In a second stage the fine-grained suspension «100 ~Im) is
prethickened with the help of flocculating agents to form compressible sludge.
After dewatering in a chamber filter press, the sludge is predried. Subsequently,
the material enters the vacuum distillation module where it is treated at a pressure
of 100 hPa and an average temperature of 310 DC. The waste gases are condensed
so that the mercury can be recovered for reuse. The condensed water is treated in
117
about 150 DC and heats the gas for the heating drum. Subsequently, the gas passes
through a dust filter and a spray tower to reduce dust and S02 emissions.
A thermal process, optimized for the remediation of Hg-contaminated soil,
was developed and constructed by Fortmann and Jahns (1993). The semimobile
plant is able to clean approximately 6 t/h. Soil and demolition material is crushed
in a special grinding device, which is under air control. The crushed and sieved
material enters a rotary kiln at a temperature of 1000 DC, where it rests for a
maximum of 3 h. Heating is performed by natural gas. After leaving the kiln, the
soil is cooled to a temperature of 80 DC. The waste gas enters an after-burner and
subsequently a heat exchanger where the gas is cooled down to 200 DC. After
passing through a dust filter, the gas still contains most of the Hg. This is
removed by a gas-washing system, which consists of a venturi washer and a twostep spray tower in which the gas is cooled to 40 DC. Hg is removed by chemical
oxidation at low pH and subsequent chemical sorption. The gas contains less
than 0.05 mg Hg/Nm-' at the end of the pipe.
Since mechanical treatment is cheaper, most of the remediation companies
combine wet classification with subsequent thermal treatment. It is advantageous
to enlarge the fraction which can be cleaned by simple wet classification and
therefore to minimize the fraction which needs to be treated by thermal
techniques.
Renner (1995) separated a fraction with a grain size <1 mm, which is dewatered
and subsequently introduced into a steam distillation chamber. The water steam
is produced by an external steam generator and is introduced into the chamber
by nozzles, which are located above and below a sieve. The soil fraction is moved
above the sieve and the steam streams through the contaminated material. A
duration time of 0.5 to 1 h at temperatures of 250 to 540 DC are used for different
kinds of material, prior to entering a cooling chamber. The steam, which is
enriched with pollutant, is pumped out of the chamber and passes through a hotgas dust filter which prevents a recontamination of the dust. Subsequently, the
steam is condensed and the resulting water is treated in a wastewater facility.
Before the gas is exhausted to the atmosphere, it passes through a charcoal filter.
The technique has been implemented in a pilot plant which has a capacity of
0.5 t/h. Various materials (soil, rubble material, charcoal, wood and oil sludge)
with contamination levels between 71 and 13 000 mg Hg/kg were tested. Final
concentrations of Hg in the cleaned soil of 0.2 to 11 mg Hg/kg were obtained.
In 1992, Harbauer GmbH installed a large soil-washing and vacuum distillation
plant for cleaning mercury-contaminated soil and demolition rubble from the
former Marktredwitz chemical works (Hennig 1993). The wet classification plant
itself has a capacity of 25 tlh, while the thermal part of the plant has a capacity of
6 t1h. After a crushing and sorting step, which is in-house, the soil is separated by
wet classification. An Hg-enriched soil fraction is fed into a vacuum distillation
plant and cleaned. In a second stage the fine-grained suspension «100 ~Im) is
prethickened with the help of flocculating agents to form compressible sludge.
After dewatering in a chamber filter press, the sludge is predried. Subsequently,
the material enters the vacuum distillation module where it is treated at a pressure
of 100 hPa and an average temperature of 310 DC. The waste gases are condensed
so that the mercury can be recovered for reuse. The condensed water is treated in