Fact sheet: Vitrification—in situ

From: Public Services and Procurement Canada

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Description

In situ vitrification is a thermal rehabilitation technology that uses electricity to heat a matrix of contaminated soils or sludge at high temperatures (1,600 °C to 2,000 °C) to produce an inert glass product. The majority of inorganic or radioactive contaminants present in the matrix can be vitrified, while organic contaminants are destroyed by pyrolysis or volatilized during soil heating. The gases emitted during the vitrification process are captured on the soil surface and must be treated. The glass product is chemically stable, leach-resistant, and has characteristics similar to igneous rocks or basalt.

In situ vitrification allows treatment of inorganic and organic contamination simultaneously. Compared to other soil remediation techniques, in situ vitrification treatment time is usually very short.

Sources:

Implementation of the technology

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.

Recommended analyses for detailed characterization

Chemical analysis

  • pH
  • Organic matter content
  • Metals concentrations
  • Contaminant concentrations present in the following phases:
    • adsorbed

Physical analysis

  • Soil water content
  • Soil granulometry
  • Soil thermal conductivity
  • The mineralogical content of soils (% silica)

Recommended trials for detailed characterization

Physical trials

  • Vapour survey

Hydrogeological trials

  • Permeability test

Other information recommended for detailed characterization

Phase II

  • Contaminant delineation (area and depth)
  • Presence of potential environmental receptors

Phase III

  • Soil stratigraphy
  • Volume of contaminated material to treat
  • Conceptual site model with hydrogeological and geochemical inputs
  • Characterization of the hydrogeological system including:
    • the direction and speed of the groundwater flow
    • the hydraulic conductivity
    • the seasonal fluctuations
    • the hydraulic gradient

Notes:

Some contaminants are incompatible with this technology, and in situ laboratory treatability studies are generally required. Complete characterization of the site and all contaminants present at the site prior to the installation of the vitrification system is essential to identify and measure the glass components already present in the contaminated soil matrix, to determine if the addition of glass stabilizers are required and to plan groundwater management (if necessary). During the vitrification process, analyzes of the soil matrix should be planned to determine the type of vitreous material that is being formed, and to provide for quick solutions (e.g. addition of stabilizing agent) to ensure vitrification success.

Applications

  • The vitrification technique can destroy or remove organic contaminants and immobilize most inorganic contaminants from soils and sludge.
  • A minimum quantity (1.4% of the total mass of the contaminated matrix) of alkaline compounds (sodium, potassium oxides, etc.) is required to ensure the success of the vitrification process.
  • The vitrification process is effective for near-surface contamination. However, new techniques can achieve treatment depths of up to 10 m.
  • Vitrification can treat sites with high clay and moisture content, although treatment costs increase with moisture content.
  • Treatment in a permeable aquifer may require groundwater dewatering, and if the area to be treated contains a high percentage of voids, dynamic compaction is recommended prior to the remediation work.

Applications to sites in northern regions

Remote sites are subject to high mobilization and monitoring costs, limited equipment availability and short work periods. As this rehabilitation technique requires high energy consumption, vitrification is not well suited to northern and remote environments. In addition, cold temperatures can affect vitrification.

Treatment type

Treatment type
Treatment typeApplies or Does not apply
In situ
Applies
Ex situ
Does not apply
Biological
Does not exist
Chemical
Applies
Control
Applies
Dissolved contamination
Does not exist
Free Phase
Does not exist
Physical
Applies
Residual contamination
Applies
Resorption
Applies
Thermal
Applies

State of technology

State of technology
State of technologyExist or Does not exist
Testing
Exist
Commercialization
Exist

Target contaminants

Target contaminantsApplies, Does not apply or With restrictions
Aliphatic chlorinated hydrocarbons
Applies
Chlorobenzenes
Applies
Explosives
Does not apply
Metals
Applies
Monocyclic aromatic hydrocarbons
Applies
Non metalic inorganic compounds
Applies
Pesticides
With restrictions
Petroleum hydrocarbons
With restrictions
Phenolic compounds
With restrictions
Policyclic aromatic hydrocarbons
With restrictions
Polychlorinated biphenyls
Applies

Treatment time

Treatment time
Treatment timeApplies or Does not apply
Less than 1 year
Applies
1 to 3 years
Does not apply
3 to 5 years
Does not apply
More than 5 years
Does not apply

Long-term considerations (following remediation work)

The downstream area of the site must be monitored after the application of the treatment to ensure that no contaminants are released from the vitrified material.

Secondary by-products and/or metabolites

  • The heat used in the vitrification process can destroy toxic compounds by producing secondary compounds and releasing toxic gases on the soil surface.
  • Gaseous emissions during the vitrification process must be captured and processed.
  • The vitrification process rapidly volatilizes volatile and semi-volatile organic compounds as well as volatile radionuclides, such as Cesium-137, Strontium-90 and Tritium (Hydrogen-3).

Limitations and Undesirable Effects of the Technology

  • Depth of treatment limits the application of vitrification.
  • Future usage of the site may stress and alter the integrity of the vitrified material and may affect its ability to contain contaminants via immobilization.
  • The presence of vitrified material may reduce the possibility of using the site after restoration.
  • The vitrification technique cannot be used when there is underground infrastructure on the contaminated site.
  • The presence of buried waste, such as barrels, piping, etc., cannot exceed 20% of the total mass of the contaminated material.
  • Heating the contaminated matrix can cause certain contaminants to migrate to an uncontaminated area.
  • The vitrification technique cannot be used in the presence of explosives or flammable contaminants.
  • Application of the technique to contaminated areas below the water table requires control of the groundwater level and aquifer recharge.
  • The technique requires the use of high-voltage lines, which represents a potential danger to health and safety at the site during rehabilitation.
  • The vitrification technique involves high energy costs, which increases the overall remediation costs.

Complementary technologies that improve treatment effectiveness

  • The vitrification technique reduces the volume and mobility of radionuclides, but it doesn’t reduce their radioactivity. Therefore, protective barriers that limit exposure to radioactive emissions may still be required at some sites.
  • Any wet soil must be dried first to prevent steam from forming. The release of steam can splash hot, melted soil above ground.

Required secondary treatments

  • The gaseous emissions must be captured and treated.
  • The downgradient area around the treated area must be monitored to ensure that no migration of contamination from the glass material has occurred.

Application examples

The vitrification technique has been tested in pilot tests and at the following contaminated sites:

  • Geosafe Corporation's test site
  • DOE's Hanford Nuclear Reservation
  • DOE's Oak Ridge National Laboratory
  • DOE's Idaho National Engineering Laboratory

The following websites provide application examples:

Performance

The vitrification process has been pilot tested for volatile and semi-volatile organic compounds, organic compounds such as dioxins and furans and polychlorinated biphenyls, and for several inorganic compounds. The effects of long-term weathering of the vitrified material can significantly affect the stability of the vitrified material. The long-term mobility potential of contaminants cannot be assessed by laboratory testing or pilot testing.

The vitrification technique has been tested on more than 170 contaminated sites of varying importance, containing soils or sludge. However, the technique is provided by very few private companies.

Measures to improve sustainability or promote ecological remediation

  • Optimized device installation that provides energy to reduce energy requirements.
  • Optimization of the time of year when the process is in operation to reduce energy costs (avoid doing work during winter time).
  • Optimization of the calendar to promote the sharing of resources and reduce the number of days of mobilization.
  • Use of energy-efficient equipment.
  • Process optimization to reduce waste and consumables.
  • Use of waste or products derived from industrial processes, if relevant, as additives or reagents.

Potential impacts of the application of the technology on human health

Main Exposure Mechanisms

Applies or Doesn’t Apply

Monitoring and Mitigation

Dust

Doesn’t apply

N/A

Atmospheric/Steam Emissions—Point Sources or Chimneys

Applies

Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Atmospheric/Steam Emissions—Non-point Sources

Applies

Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Air/steam—by-products

Applies

Emissions monitoring (choice of parameters, types of samples and type of intervention [source, risk or local requirements])

Runoff

Doesn’t apply

N/A

Groundwater—displacement

Applies (if soil dewatering is necessary)

Modelling the effects of required pumping and monitoring using pressure sensors

Groundwater—chemical/ geochemical mobilization

Applies

Groundwater quality monitoring

Groundwater—by-product

Doesn’t apply

N/A

Accident/Failure—damage to public services

Applies

File checks and pre-work permits, development of drilling procedures and emergency response

Accident/Failure—leak or spill

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

Accident/Failure—fire or explosion

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

Other—Manipulation of contaminated soils, sludge and / or sediments

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

Other—Exposure to melting material

Applies

Risk review, development of accident and emergency response plans, monitoring and inspection of unsafe conditions

References

Author and update

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

Version:
1.2.1