Remediation Technologies: Vitrification

University of Calgary

Department of Civil Engineering

Schulich School of Engineering

653 Soil Remediation Remediation Technologies: Vitrification

Jesica Goya Snchez

April 25th, 2014

Contents31 Introduction

42 Different Vitrification Methods

42.1 In-situ vitrification (ISV)

42.1.1 Static electrodes.

92.1.2 Mobile electrodes

112.1.3 Plasma arc torch

132.2 Ex-situ vitrification

132.2.1 Cold top

153 Conclusion

164 References

1 IntroductionContamination of the land has become a serious issue since we now know the earth has a limited assimilative capacity and not every waste can be "assimilated". There are some contaminated sites in which the contaminants that are found are too dangerous to be removed from site or too difficult or expensive to remediate with the common technologies, that is when vitrification becomes a very good alternative to remediate a site.

Vitrification is the creation of, usually, an oxide solid from the melting of the soil in which the contaminants are found, this decreases the mobility of the hazardous substances. The heating process in this methodology is useful for a lot of soils and organic and inorganic materials. When the soil is heated some organic material may be destroyed or volatilized and has to be properly collected for later disposal. (Evanko, C. And Dzombak, D. 1997)

This technology works with contaminants like Radionuclides, halogenated compounds, arsenic, lead, chromium, mercury, dioxins, PCBs, pesticides and herbicides but is not recommended for soils with high levels of organics ( > 10% by weight) (Evanko, C. and Dzombak, D. 1997). In Figure 1 a vitrified monolith is shown.

Figure 1. Vitrified monolith (http://www.kurion.com/applications/stabilization/contaminated-soil-at-nuclear-weapons-site)2 Different Vitrification Methods

2.1 In-situ vitrification (ISV)

2.1.1 Static electrodes.

This technology is preferred to ex-situ because it eliminates the cost of transporting the material and the hazard the excavation poses to people working on site.

One of the procedures for ISV is reported in an US EPA Bulletin from 1989. The technology is described to use four electrodes that are inserted in the soil. A current flow is established between these electrodes with the help of graphite and glass frit that are spread between the electrodes to help increase soil's conductivity. With this current, the soil reaches a 1600 - 2000 C temperature which is enough to melt silica, one of the main components of every soil. As the soil is being melted and reaches the desired depth, the current is removed and the soil allowed to cool. Figure 2 is a diagram of an ISV array. (USEPA 9200.5-251FS)

Figure 2. Diagram of an ISV array. Taken from US EPA 9200.5-251FS.

A protective cover is placed on top of the system so if there are any volatiles they can be collected and treated before being released into the atmosphere. This cover also helps to bring in air to help with the combustion. Some of the organic materials may produce volatiles that combust when they reach the surface and the oxygen. Table 1 depicts the effectiveness of ISV on different contaminant groups (USEPA 9200.5-251FS). Treatability Groups

Effectiveness

Organics

Halogenated volatiles

Halogenated semi-volatiles

Non-halogenated volatiles

Non-halogenated semi-volatiles

PCBs

Pesticides

Dioxins/Furans

Organic cyanides

Organic corrosives

Inorganics

Volatile metals

Non-volatile metals

Asbestos

Radioactive materials

Inorganic corrosives

Inorganic cyanides

Reactive

Oxidizers

Reducers

Table 1. Effectiveness of ISV on different contaminant groups. Adapted from USEPA 9200.5-251FS.The Geosafe Corporation was a pioneer in the US in the ISV technology, their equipment was designed to be quickly assembled on site, to achieve this it's set in three over-the-road trailers. The off-gas collection hood is an 18 m piece that channels ambient air and the gasses that are produced from the process into the off-gas treatment system (Figure 4). The picture of the assembled equipment is shown in Figure 3 (Edward J. Calabrese, Paul T. Kostecki, 1992).

Figure 3. Conditions for the ISV. Taken from Edward J. Calabrese, Paul T. Kostecki, 1992

Figure 4 Picture of the assembled equipment. http://www.geosafecorporation.com/images/photo_03_big.jpg

This technology can be used in a big area by changing the position of the equipment to an adjacent portion of untreated soil when the previously treated soil has been allowed to cool for four to five days. After removing the equipment, the glass is covered with clean soil. For the soil to reach an ambient temperature, several months may be needed (USEPA 9200.5-251FS).

The US EPA printed a document in which the advantages, disadvantages and limitations of the method are stated and shown here (USEPA 9200.5-251FS): Advantages Immobilize all contaminants.

Reduce the volume of waste. It is done on site.Disadvantages Big off gas collection and treatment system.Limitations There is a limit for organic and inorganic content in soil: this is 5 - 10 weight percent for organics and 5 - 15 weight percent for inorganics. If the contaminated site is saturated with water, it must be first evaporated and this will create additional costs and require more time. There is a limit for the amount of metals (like drums) that can be found in soil, the metal weight can range from 5 to 16 percent of the melt weight, this limit exists because a bigger amount could cause shorting between electrodes. When the soil's permeability is greater than 1x10-5 cm/sec the evaporation doesn't occur as fast and the soil is re-saturated continuously, this is when a ground water diversion system has to be installed. If there are combustible liquids in an amount greater than 5 to 10% by weight a time limit should be set for the off-gas system to contain combustion gas as well as increasing the hood capacity and processing at a slower rate. If there are combustible solids in an amount greater than 4.7% by weight a time limit should be set for the off-gas system to contain combustion gas as well as increasing the hood capacity and processing at a slower rate. Considerations The biggest volume that can be treated with these four electrodes is 9 x 9 x 9 m. The largest amount of contaminated soil by each time that the whole system is set is 800 to 1,000 tons. The maximum depth of 9 m can't be reached when a 9 x 9 area is used because the mass limit will be accomplished first. On the other hand, the smallest area that can be treated and the recommended depth is 1.5 to 2 m. (USEPA 9200.5-251FS).

There are several configurations that can be followed in order to achieve bigger or more specific melting zones, they're shown in Figure 5.

Figure 5. Various configurations for ISV. Taken from EPA/540/R-94/520, March 1995.

Cost and operation time The amount of soil that can be treated with ISV is 4 to 6 tons with a requirement of 0.3 to 0.5 kwh by 0.45 kg of soil. A 1.9 Mw/phase power level is required for this operation. (USEPA 9200.5-251FS)

The cost for an ISV study is estimated at $350 to $450 per ton taking into account direct and indirect costs. The technical implications that would raise the costs are: the rates for electricity, how much water extraction is needed, how deep will the melting go and the chemical analysis required (By Edward J. Calabrese, Paul T. Kostecki, 1992).

Regulatory criteriaThe resulting monoliths from the melting process have been analyzed by the US EPA to check for compliance of the regulatory criteria. The parameters that have been addressed are (US EPA/540/S-94/504):

1. Structure: the glass matter has been tested for structural strength and has been found that is 10 times greater that unreinforced concrete both in tension and compression. 2. Weathering: when exposed to repeated freeze/thaw and wet/dry weathering tests the mass was found to be unaltered.3. Chemical leaching: The product was found to be in compliance with the TCLP (Toxic Characteristics Leaching Procedure). 4. Biotoxicity properties: It has been found that the product of ISV isn't toxic to life forms that are found near the surface.

2.1.2 Mobile electrodesFor this variant of the method the four electrodes, in a square configuration, are first inserted in the ground at a depth of 30 to 60 cm and a conductive material is lain between to help start the melting process, once the soil starts to melt it becomes electrically conductive (US EPA/540/R-94/520). In the first stages of melting the applied current is low, 100 to 200 amps per phase and a high voltage of 4,000 volts per phase. In time the soil becomes more conductive and the voltage decreases and the current increases so the average voltage needed is of 600 volts and a current of 2,900 amps. The total power that can be supplied to the electrodes is 3.5 MW, 1.75 MW per phase (US EPA/540/R-94/520). Once the melting has started and the soil is conductive the lowering of the electrodes can start until reaching the desired depth of melting. With this type of array the area that can be treated is 6 to 12 m with up to 6 m deep. The rate at which the soil can be treated is of four to six tons per hour until the amount of 800 to 1,200 tons has been reached (US EPA/540/R-94/520).

One of the great advantages of this method is that each electrode if introduced into the soil independently so each of them can be at a different depth. This can be very helpful in sites with a great amount of metal since if an electrical short happens, that electrode can be raised above the melted material and be reinserted once the melting has moved on past the zone rich in metal (US EPA/540/S-94/504).

Considerations The space that the application of this technology needs starts with the trailers that are 3 by 36.5 m, the protective hood takes up 18 m in diameter and the treated area 64 m2. A safety zone is also needed between the treated area and non contaminated zones or buildings to ensure there are no secondary effects or damaged infrastructure (US EPA/540/S-94/504).

There are no real climatic limitations for the operation of ISV because the equipment can be protected from the elements and only cold weathers could slow down the first stages of melting (US EPA/540/S-94/504).

2.1.3 Plasma arc torch This technology can help to the process of vitrification in a faster and more precise way. Plasma torches can produce temperatures from 4000 to 7000 C with an electric energy to heat conversion efficiency of 85 to 93% with energy levels from 100 kw to 10 MW.

This technology works using a stream of plasma that is generated by a high energy electric arc or ionized gas. There are two types: transferred arc and non-transferred arc. The transferred arc has a single electrode in the arc and an external one; the non-transferred arc is formed with two electrodes inside the torch. The length of the arc, in both cases, is what determines the amount of heat produced (Patent 5,276,253).

The procedure for vitrification with torches differs from the classic one in that boreholes are done here covering the entire contaminated area in such an array that the vitrified area of each borehole is overlapping the next one to create a perimeter around the contamination. Such boreholes are protected from collapse by inserting a metal casing that can be melted. A diagram is shown in Figure 6 (Patent 5,276,253).

Figure 6. Diagram of the borehole array. Taken from Patent 5,276,253

To create this melting of the materials, the torch is lowered to the desired depth to form a column of remediated material, the gases that are produced during this process are collected in the upper part of the borehole and then treated. A side view of the schematics of the process is shown in Figure 7 (Patent 5,276,253).

Figure 7. Side view of the schematics of the process.

Taken form Patent 5,276,253

Here we can see that the highest temperatures are found right beneath the plasma arc, there the soil melts and accumulate in a pool. The next zone has a lower amount of heat (around 900C), the curing zone, here the soil is hardened into a brick-like material. Next we have a deplasticized zone which is not cured but has been exposed to at least 200 C and all water has been removed, so it becomes rigid and non-absorbent envelope of soil. The distance of this zones to the plasma torch depend on the moisture content of the soil, the power level of the torch and the nature of the soil (Patent 5,276,253).

Considerations

This technology was developed because the original method has certain disadvantages (Patent 5,276,253): The process of melting has to start at the surface of the soil and then proceed downwards. The depth of operation is not so big, and it can't operate at selective depths. Once the melting has occurred below ground level, restarting of the current flow is very difficult. Since the maximum temperature that can be reached is of 2000 C there are times when soil additives have to be added to lower the melting point of some soils. When there is too much water or metal in the soil, the method doesn't work properly. 2.2 Ex-situ vitrification

2.2.1 Cold top Cold top ex-situ vitrification technology was designed to melt soil that has been previously dug from site and transported into the New Jersey facilities belonging to Geotech Development Corporation (Geotech).

In this site, the soil is meted in a double-walled, water-cooled steel furnace with submerged-electrode resistance heating. Figure 8 shows a scheme of this furnace, in which the contaminated soil is fed from the top, while the melt is poured from the bottom. The melting temperature that can be achieved in this furnace is of 2870 C (US EPA/540/R-97/506a). The furnace works with a 1,350 kVA resistance, and once the voltage has been adjusted the operation is continuous (US EPA/540/R-97/506). Before the contaminated soil is loaded into the furnace, it is loaded with a mixture of sand and alumina/silica clay. This mixture is heated in order to form a molten pool to which the contaminated soil is added. As the melting is achieved more soil is added to keep a "cold top". The configuration of the system includes an off-gas treatment system which provides an integral treatment of the contaminant (US EPA/540/R-97/506).

Figure 8. Scheme of cold top furnace.

Taken from US EPA/540/R-97/506.Geotech has vitrification facilities in New Jersey, New York. This furnace can process soil and create blocks of glass of up to 135 kg which have an economic value when being used as concrete aggregates, ceramic fiber, etc. This facility converts 50 tons of contaminated soil each day (US EPA/540/R-97/506a).

Cost

The estimated cost for this kind of treatment goes from $77 to $207 per ton, varying because of the transportation costs, electricity costs, disposal costs and potential credits for the vitrified product (US EPA/540/R-97/506)a. Considerations

In this type of vitrification, the advantages of an in-situ operation are lost and shipping costs should be considered as well as the distance to the facilities and since there are not too many, this is not an option for many sites. The mobile version of this type of technology is under development, when it becomes operable, the costs of transportation and disposal will need to be addressed. 3 Conclusion

The different vitrification methods exposed in this paper have some differences in their performance, but overall they're very effective in terms of the confining of a dangerous contaminant that is not easy to move and treat off site. This is a very effective way of treating wastes that can remain in site after vitrified and just be covered with some soil so the site will be in conditions for use.

These methods are safe and are in compliance with the TCLP test which ensures that no leachate will be generated from the remaining glass monolith. The issue with this type of technology is the cost, which can be quite high so it should be considered for a site in which the contaminant is too dangerous to be handled or where too many other technologies would have to be used, and instead if vitrification is used only one method would have to be applied. The amount of space that is needed for the installation of all the equipment is something that needs to be considered since a large space has to be available.

Other limitations are the noise and light that comes from the remediation site, even if the other site characteristics are met, if this were to be implemented near a neighbourhood people would have to agree to this kind of operation, since it is operated continuously. This type of methodology should be considered as a good option for remediation of complicated sites when the site characteristics allow for the installation of the system. 4 References United States Environmental Protection Agency (USEPA). SITE Technology Capsule. Geotech Development Corporation Cold Top Ex-Situ Vitrification Technology. EPA/540/R-97/506, March 1998. United States Environmental Protection Agency (USEPA). Geotech, Ic. Cold Top Ex-Situ Vitrification System. Innovative Technology Report. US EPA/540/R-97/506. December 1999. United States Environmental Protection Agency (USEPA). Innovative Technology. In-Situ Vitrification. Directive 9200.5-251FS. November 1989

United States Environmental Protection Agency (USEPA). Technology Screening Guide for Radioactively Contaminated Sites. EPA 402-R-96-017. November 1996

United States Environmental Protection Agency (USEPA). Engineering Bulletin. In Situ Vitrification Treatment: US EPA/540/S-94/504, October 1994.

Evanko, C. and Dzombak, D. 1997 Remediation of metals-contaminated soils and groundwater

Calabrese, Edward and Kostecki, Paul. Principles and Practices for Petroleum Contaminated Soils. CRC Press, 1992. http://books.google.ca/books/about/ Principles_and_Practices_for_Petroleum_C.html?id=oj26OAVD-G8C&redir_esc=y

Geosafe Corporation In Situ Vitrification. Innovative Technology Evaluation Report. US EPA/540/R-94/520. March 1995

Circeo, Jr. et al. "In-Situ remediation and vitrification of contaminated soils, deposits and buried materials." Patent 5,276,253. 9 September 1992.

Brouns, R. et al. "In-situ Vitrification of soil" Patent 4,376,598. 15 March 1983. 655 Hazardous Waste and Contaminated Site ManagementJesica Goya Snchez

2