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M. Hempel and j. Thoeming
a wastewater treatment facility and all air involved in the process is passed
through a waste gas-cleaning device. The plant is able to clean soil with an average
contamination of 3000 mg Hg/kg to an average concentration of 2 mg Hg/kg.
Another thermal technique combined with vacuum is described in a patent
(Bernecker and Ratzel 1988) where mercury-bearing materials are cleaned by
distillation. The material is introduced as a suspension or solution by a thin layer
combuster under vacuum/low pressure.
Chemical Waste Management Inc. (1992) desorbs Hg from soil at temperatures
between 93 and 482°C in an externally fired rotary dryer. The processed solids
are cooled with treated condensed water to eliminate dusting. A nitrogen gas
transports the volatilized pollutants out of the dryer to the gas treatment system.
The gas first passes a high-energy scrubber to remove particles and part of the
organic contaminants. Afterwards, it passes through two condensers in series,
where it is cooled to less than 10 0c. The plant can process 120 to 180 tlday.
Mercury levels have been reduced from 5lO0 to 1.3 mg Hg/kg.
A batch system which combines thermal and vacuum techniques is described
by Gronholz, (Gronholz 1992; 1995). The main steps of the technique are a
thermal pretreatment at approx. 200°C and 200 hPa with a duration time of 5 to
30 min, followed by the main thermal process, taking place in an externally
heated reactor under a vacuum of 0.5 to 1 hPa at temperatures of 250 to 400°C.
The technique was applied to Hg-contaminated soil, drilling sludge and wood,
and showed final concentrations of 0.23 to 1.8 mg Hg/kg d.w. from an input
concentration of 0.41 to 150 000 mg Hg/kg d.w.
A thermal technique with a chemical pretreatment is described by Weyand
et al. (1994). The soil is mixed with chemical additives that facilitate the
decomposition of mercury compounds and the formation of elemental mercury.
The pretreated soil is heated in two stages, firstly to vaporize the low-temperature
volatiles such as water and thereafter at a higher temperature to vaporize/
decompose the elemental mercury and mercury compounds present in the soil
mixture. The technique has been applied on a pilot plant with an approximate
capacity of 113 kg.
Although different principles, techniques and designs are implemented in the
above thermal soil treatment plants, some problems and limitations often occur:
1. Rotating parts in the plant, connected with heating devices or in the areas with
temperatures differences, require special material, maintenance, stoppages
and costs.
2. Material which is entering the heated zone of the plant may calcinate or
agglomerate to burned gravels. Evaporation of the Hg captured in these
gravels requires a much longer duration time in the oven.
3. In plants with high volumes of gas, Hg separation out of the waste gas could be
a problem, or at least expensive. Besides, the separation of the fine dust from
gas is a technical challenge. In some plants a recondensation of the treated
dust could occur.
4. Contamination of the plant devices itself should be noted. High concentrations
of Hg in the surfaces of the devices, for example the oven, could limit the final
concentrations of the processed material.
M. Hempel and j. Thoeming
a wastewater treatment facility and all air involved in the process is passed
through a waste gas-cleaning device. The plant is able to clean soil with an average
contamination of 3000 mg Hg/kg to an average concentration of 2 mg Hg/kg.
Another thermal technique combined with vacuum is described in a patent
(Bernecker and Ratzel 1988) where mercury-bearing materials are cleaned by
distillation. The material is introduced as a suspension or solution by a thin layer
combuster under vacuum/low pressure.
Chemical Waste Management Inc. (1992) desorbs Hg from soil at temperatures
between 93 and 482°C in an externally fired rotary dryer. The processed solids
are cooled with treated condensed water to eliminate dusting. A nitrogen gas
transports the volatilized pollutants out of the dryer to the gas treatment system.
The gas first passes a high-energy scrubber to remove particles and part of the
organic contaminants. Afterwards, it passes through two condensers in series,
where it is cooled to less than 10 0c. The plant can process 120 to 180 tlday.
Mercury levels have been reduced from 5lO0 to 1.3 mg Hg/kg.
A batch system which combines thermal and vacuum techniques is described
by Gronholz, (Gronholz 1992; 1995). The main steps of the technique are a
thermal pretreatment at approx. 200°C and 200 hPa with a duration time of 5 to
30 min, followed by the main thermal process, taking place in an externally
heated reactor under a vacuum of 0.5 to 1 hPa at temperatures of 250 to 400°C.
The technique was applied to Hg-contaminated soil, drilling sludge and wood,
and showed final concentrations of 0.23 to 1.8 mg Hg/kg d.w. from an input
concentration of 0.41 to 150 000 mg Hg/kg d.w.
A thermal technique with a chemical pretreatment is described by Weyand
et al. (1994). The soil is mixed with chemical additives that facilitate the
decomposition of mercury compounds and the formation of elemental mercury.
The pretreated soil is heated in two stages, firstly to vaporize the low-temperature
volatiles such as water and thereafter at a higher temperature to vaporize/
decompose the elemental mercury and mercury compounds present in the soil
mixture. The technique has been applied on a pilot plant with an approximate
capacity of 113 kg.
Although different principles, techniques and designs are implemented in the
above thermal soil treatment plants, some problems and limitations often occur:
1. Rotating parts in the plant, connected with heating devices or in the areas with
temperatures differences, require special material, maintenance, stoppages
and costs.
2. Material which is entering the heated zone of the plant may calcinate or
agglomerate to burned gravels. Evaporation of the Hg captured in these
gravels requires a much longer duration time in the oven.
3. In plants with high volumes of gas, Hg separation out of the waste gas could be
a problem, or at least expensive. Besides, the separation of the fine dust from
gas is a technical challenge. In some plants a recondensation of the treated
dust could occur.
4. Contamination of the plant devices itself should be noted. High concentrations
of Hg in the surfaces of the devices, for example the oven, could limit the final
concentrations of the processed material.