Vitrification uses electrical energy to create the heat necessary for the melting of soils. There are two methods for producing heat and treating contaminated soils, either the conventional method that uses electrodes, or a more recent method that uses plasma arc technology.
In the conventional method, the electricity required for vitrification is introduced in the soil by electrodes that are inserted into contaminated soils. The electrodes circulate a high voltage electric current between them. The heat generated by the passing electrical current is distributed to surface soils, then as the ground melts, the electrodes sink into the ground which increases the depth of distribution of the heat. When the electric current is turned off, the molten soils cool and vitrify by encapsulating and immobilizing the contaminants in the vitrified material. This process depends on the presence of alkali metal oxides in the soils to be treated, to ensure an equilibrium between electrical conductivity and melting temperature. Excessive alkali content increases the conductivity to a point where heating is insufficient. If the soil's silica content is high enough, contaminated soils can be vitrified.
During the treatment, the installation of a fume hood above the area to be treated (zone treated by section) is necessary, in order to capture the residual gases and direct them to a processing unit. The treatment chain generally consists of a system that cools the gases to a temperature of 100 °C to 400 °C and, depending on the treatment, a scrubber, a demister, particulate filters and oxidizing activated carbon through which the residual gases circulate. In some applications, a thermal oxidizer is used to treat the residual gases before they are released into the atmosphere. The effluent water from the gas scrubber and demister may also require secondary treatment.
In situ vitrification using plasma arc technology has been demonstrated but has not yet been commercialized. The process involves lowering a plasma torch into a cased hole and initiating bottom-up column fusion. The torch can reach temperatures above 7000 °C and theoretically, it can operate at any depth. The residual gases are collected in a fume hood and treated.
Following vitrification, the soil surface in the treated area sinks slightly; clean soil must be imported to backfill and level the treated area.
Implementation of in situ vitrification rehabilitation may include:
- Mobilization, access to the site and setting up temporary installations.
- Groundwater dewatering (lowering of groundwater level) if required.
- The installation of an electricity supply system.
- Installation and insertion of high voltage electrodes into the soil.
- Installation of a vapour extraction pipeline system, a gas treatment system and air emissions control system.
- The restoration of the site following the vitrification work.
Materials and Storage
The implementation of in situ vitrification requires the installation of some specialized equipment. Transportable commercial equipment is available for the treatment process. Chemicals include additives that are mixed with the soil, notably alkali metal oxides.
During installation and processing, a crane and other support equipment are required. The crane is used to mount the fume hood during assembly, to move the fume hood to each soil zone prior to treatment, and to install the electrodes prior to treatment. Other equipment, such as a forklift, may be needed to move the equipment to the site.
The required electricity can be supplied through a trailer containing diesel generators, in cases where the construction of a connection to the power grid would be impracticable. The costs of using electricity generated by diesel, however, are generally higher.
The gas recovery fume hood and treatment system can be built on-site or pre-assembled and transported to the site.
Earth-moving equipment (digger, dump truck, loader, etc.) is required to backfill the subsidence of the treated areas.
Residues and Discharges
Vitrification produces solid and gaseous residues. The residual material resulting from this rehabilitation technique is the mass of vitrified soils that remains in place following the treatment. This mass can take up to 1 or 2 years before cooling completely.
Used adsorbent materials (activated carbon) or other products used in the treatment of residual gases must be collected and disposed off-site in an authorized facility.
The implementation of the system could lead to the management of contaminated soils resulting from drilling or excavation activities. In this case, these soils must be removed off-site.
Composed by : Josée Thibodeau, M.Sc, National Research Council
Updated by : Martin Désilets, B.Sc., National Research Council
Updated Date : November 27, 2013
Latest update provided by : Nathalie Arel, P.Eng., M.Sc., Christian Gosselin, P.Eng., M.Eng. and Sylvain Hains, P.Eng., M.Sc., Golder Associés Ltée
Updated Date : March 22, 2019