Cyanide Ex-Situ Thermal Remediation (ESTR)
White Paper
1. Introduction
1.1 Cyanide Remediation Summary
Cyanide, a toxic substance commonly found in industrial effluents, is widely used in metal processing due to its strong metal-binding properties. However, accidental cyanide spills can have significant environmental impacts, contaminating both soil and water. To mitigate these effects, several remediation techniques are employed(Young & Jordan, 1995), each offering distinct advantages:
· Physical Methods: Techniques such as dilution, membranes, and electrowinning help concentrate cyanide for recycling but do not effectively destroy cyanide. While simple and cost-effective, these methods can leave behind significant amounts of cyanide, posing long-term risks.
· Adsorption: Utilizing materials like activated carbon or resins, adsorption captures cyanide from solutions efficiently, especially at dilute concentrations. This method offers high adsorption rates but requires further processing to destroy the captured cyanide.
· Complexation: This approach binds cyanide with metals to form less toxic complexes, such as iron-cyanide complexes, which are stable but can potentially re-release cyanide under certain conditions.
· Oxidation Techniques: Including chemical oxidation (e.g., using hydrogen peroxide or ozone) and bio-oxidation, these methods convert cyanide to cyanate or other less harmful compounds. While effective, these techniques can be cost-intensive and may not fully address complex cyanide species or require stringent operational controls.
These conventional methods come with benefits like simplicity, high efficiency in specific cases, and potential recycling capabilities. However, limitations persist, such as incomplete cyanide destruction, dependency on precise conditions, and potential byproducts that require further treatment. These constraints highlight the need for an approach capable of addressing cyanide in various forms comprehensively and effectively(Dong et al., 2021).
1.2 Thermal Remediation and its Feasibility Studies for Cyanide Treatment
Thermal remediation stands out as an effective solution for cyanide-contaminated soils by applying controlled heat to break down stable cyanide compounds, including complex iron-cyanides, significantly reducing environmental impact. This method operates across a broad temperature range (typically 200–400°C), enabling targeted treatment for specific cyanide compounds.
During thermal treatment, iron-cyanide complexes decompose, leading to the release of toxic free cyanides (CN⁻) and their subsequent oxidation to non-toxic products like CO₂ CO, NOx and N₂. Research (Wei et al., 2020)shows that less than 10% of the cyanide destruction results in toxic intermediates such as CN⁻ or HCN, with the majority converting to non-toxic end products. This highlights the efficacy of thermal treatment in minimizing hazardous byproducts while ensuring thorough cyanide breakdown.
Studies demonstrate that for cyanide compounds like potassium ferricyanide (K₃[Fe(CN)₆]) and potassium ferrocyanide (K₄[Fe(CN)₆]), removal efficiencies exceeding 99.9% are achieved at temperatures over 350°C within a 1-hour heating duration. For more resilient compounds, such as ferric ferrocyanide (Fe₄[Fe(CN)₆]₃), temperatures above 450°C are needed to achieve similar results(Wei et al., 2020). However, research also indicates that extending the heating duration from hours to weeks can significantly lower the target temperature required, with 250°C being sufficient for prolonged treatment to meet strict remediation goals.
Field applications have supported these findings, demonstrating that thermal remediation at 200–350°C can effectively reduce cyanide levels to meet regulatory standards. This balance between temperature and treatment duration enhances the feasibility and scalability of the method for different site conditions.
Thermal remediation offers a robust and adaptable solution for managing cyanide pollutants in contaminated soils(Dong et al., 2021). It provides comprehensive cyanide breakdown with minimal environmental risk, efficiently converting even complex compounds into non-toxic products. Its capability to achieve high removal efficiencies, coupled with the flexibility of temperature and duration adjustments, underscores its role as a leading strategy for cyanide remediation and site redevelopment.
2. Gas Thermal Remediation (GTR) heating
Gas Thermal Remediation (GTR™), a form of Thermal Conduction Heating (TCH), is a robust and widely utilized method for thermal remediation, effectively targeting temperatures from 100°C to 400°C. GEO’s patented GTR™ technology allows for the use of natural gas, propane, or liquid fuels to power the system, providing efficient vaporization of VOCs, SVOCs, and cyanide pollutants from soil or groundwater. Contaminants released during heating are captured through soil vapor extraction wells and treated using a vapor treatment system, ensuring safe release into the atmosphere.
Advantages of GTR™ over Electrical Heating:
· No Electrical Upgrades: Many sites can avoid the need for expensive electrical system upgrades.
· Flexibility: Utilizes existing natural gas supplies for small to medium sites and offers easy deployment of mobile fuel sources (e.g., propane or liquid gas tanks) for remote locations.
· Modular Design: Eliminates the need for complex central distribution systems.
· Cost-Effective: Lower energy and equipment costs compared to electrical systems.
GTRTM | Electric TCH | |
Energy Source | Natural gas, LNG, propane, or diesel | Electricity |
Energy Supply Method | · Flexible for various site sizes with individual burners. · Easier to meet power demands compared to electric systems. · Capable of operating in remote areas using alternative fuel sources, such as propane. | · Requires costly transformers and high site voltage, regardless of site size. · For larger sites, extreme electric load requirements can be challenging to supply. |
Energy Efficiency | 35–75% | 30–75% |
Energy Cost ($/MMBTU) | $10 | $25 |
Well Spacing | 1.5 -6 m | 1.5 -6 m |
Well Depth | Less than 120 feet | No limit |
Site Safety | Safe (low gas pressure, no open flames) | Safe (high site voltage) |
3. ESTR Technology Summary
Ex Situ Thermal Remediation (ESTR) is a system that is also used for onsite soil treatment, when the soil is treated on site after excavation (no transportation off site). Figure 1 below shows the ESTR works model. Batch Pile Treatment is one of the most commonly applied methods of ESTR for the remediation of soils. When soil is heated, the organic contaminants extracted from the soil pile or are destroyed in place. Concentrations of chlorinated VOCs in soil can be reduced by almost 100% at temperatures much lower than traditional “rotating kiln” arrangements. Diesel and motor oil range petroleum hydrocarbon and most SVOC contaminants can be removed by 99% or greater, and are most reliably remediated from the soil matrix at temperatures between 200°C to 350°C by the following mechanisms:
· Hot water flushing with increasing solution up to 80°C,
· Steam stripping of NAPLs up to 80°C,
· Hydrolysis with hot water or steam below 100°C
· Evaporation (volatilization) from 100°C to 300°C+,
· Oxidation from 100°C to 300°C+, and
· Pyrolysis from 100°C to 300°C+.
Batches ranging in volume from 250 m3 to 10,000 m3 have been successfully implemented in the United States and at several projects in Asia, Europe and Africa. Common applications have treated Pesticides, Di- & Trichlorobenzene, Heavy Oils, PAHs (majorly BaPs), PCBs, Paraffins, Dioxins, Jet Fuel, PCE and TCE.
ESTR offers several advantages for managing cyanide-contaminated tailings through its effective and efficient treatment process:
· High Contaminant Removal Efficiency: ESTR works by heating contaminated materials, causing cyanide compounds and other hazardous substances to volatilize and separate from the soil or waste matrix. This process achieves high removal efficiency, making it suitable for handling severe contamination levels.
· Versatility in Handling Various Contaminant Levels: The ESTR method can effectively manage a wide range of contaminant concentrations, making it adaptable for various cyanide levels and types of VOCs without requiring dilution or pre-treatment adjustments. This versatility supports a wide application scope across different site conditions.
· Faster Treatment Time: ESTR enables accelerated cleanup due to its efficient heating process, significantly shortening the project timeline compared to traditional methods. This reduction in processing time not only speeds up site turnover but also minimizes the time the equipment and workforce need to be on-site, optimizing operational efficiency.
· Cost-Effective Lifecycle Management: Faster remediation with ESTR reduces project duration and associated costs, including labor, equipment maintenance, and energy consumption. By enabling quicker site closure, ESTR minimizes the overall lifecycle costs, making it a financially advantageous solution, particularly for large-scale remediation projects.
· Reduced Environmental Impact: ESTR’s efficiency and shorter operational timelines lower the environmental footprint by reducing prolonged exposure to contaminants and the need for extensive on-site resources. This contributes to more sustainable remediation practices and safer site conditions for surrounding areas.
Overall, ESTR provides a robust, adaptable, and cost-effective approach to cyanide-contaminated site cleanup, making it an optimal choice for projects requiring efficient and reliable remediation outcomes.
4. ESTR Installation Pile Construction Sequencing
Construction of a batch pile for thermal desorption generally follows these steps：
(1) The area the batches will be built upon must be cleared and be somewhat level for the pile to be constructed upon.
(2) A foundational drainage layer will be placed along the base of the batch to insulate the batch from the ground to prevent heat loss from the pile, and to drain liquids from the bottom of the pile. A layer of crushed rock on top of concrete and/or HDPE sheathing is generally used, with slotted steel piping placed as drainage through the crushed rock.
(3) An excavator will pile the dirt onto the batch, one section at a time. Each level must be done one at a time to allow for the heating pipes and SVE pipes to be placed into the pile.
(4) A crane will be used to place the heating wells onto the dirt after the first layer has been formed. The crane will lift the pipes up using a spreader bar as shown.
(5) After the first row of heating pipes has been placed, more contaminated dirt will be placed on top of the heating pipes.
(6) After the dirt has been added, another layer of heating pipes will be placed on top of the dirt.
(7) This process will repeat until the pile has been completely built. The stages of the construction are shown below. Once one area is complete, the excavator will move back and begin the process again until the entire batch has been constructed.
(8) After the construction has been complete, insulation material(s) must be placed along the sides and top of the pile mitigate heat loss to atmosphere. Once the insulation has been placed, supplemental material may be placed on the outside of the pile to keep rain from entering, cooling the soil.
5. Heating Stage
1) Increase to ~90°C: NAPLs (if exist) skimming, and hot water flushing
2) Remain at ~100°C: Steam stripping, and potential hydrolysis
3) Increase to >300°C: Desorption and vaporization, and potential oxidation
4) Remain at target T (>300°C): Complete remediation
6. Vapor and Water Treatment System
The treatment of exhaust gases after extracting contaminants from the soil is a critical step in the remediation process. GEO’s exhaust vapor treatment system and water treatment system are discussed in this section.
6.1 Vapor Treatment (scrubber, cooling condensation, C3 System)
GEO’s exhaust vapor treatment system for Cyanide Remediation shows as below includes: 1. Scrubber, 2. Cooling and Condensation System 3. C3 System 4. GAC
6.1.1 Scrubber
In scrubbers, gases pass through acid or alkaline solutions that neutralize specific contaminants. Acid scrubbing targets basic compounds, while alkaline scrubbing removes acidic gases, ensuring thorough neutralization.
6.1.2 Cooling and Condensation System
This step cools the exhaust gases, allowing condensable substances to separate out, reducing both volume and concentration of contaminants in the gas phase.
6.1.3 C3 System
C3 Technology high-pressure cryogenic refrigeration system for high and low concentration vapour treatment applications. The C3 Technology process and the technical aspects of how it works are described and illustrated for users to incorporate into feasibility evaluations of efficacy or design applications.
The C3 Technology developed by G.E.O. is a combination of high pressure (~150psi) and cryogenic cooling combined with proprietary regenerative adsorption technology that efficiently condenses and recovers contaminants. Applications include soil vapour extraction (SVE) or dual phase extraction (DPE) systems or industrial type off-gas treatment. The chemical is recovered as a non-aqueous phase liquid (NAPL) that is temporarily containerized in appropriate vessels for recycling or proper disposal.
Process Description:
· Contaminated vapour is extracted from the soil by a vacuum blower and delivered to the air compressor
· Entrained liquids from extraction wells are separated at the water knockout tank.
· Separated liquids are securely drummed and transported off-site or treated with GAC before discharged to sewer or storm water in accordance with all regulatory requirements.
· Process air is compressed to approximately 150 pounds per square inch (psi) by the compressor.
· The process stream is then cooled to ambient temperature with an air-to-air heat exchanger.
· The process stream is further cooled through multiple step wise cooling stages to approximately -40° C in the refrigerated heat exchangers, where the chemical constituents are condensed and separated from the vapour stream.
· The process stream is then passed through a regenerative adsorber, which removes any residual contaminant in vapour phase and directs it back to the inlet stream.
6.1.4 GAC
Vapour effluent contaminant concentration is finally polished through granular activated carbon (GAC) prior to discharge to atmosphere. Vapor concentration before releasing into the atmosphere will be monitored to ensure meeting air permitting requirements.
6.2 Water Treatment System
GEO’s exhaust vapor treatment system for Cyanide Remediation shows as below includes: 1. Weir Tank, 2. LGAC 3. Holding Tank
Weir Tank (WT). Aqueous phase liquids separated are first directed to one weir tank. This weir tank will equalize flow through the remaining system and separate solids, preventing downstream clogging. Separation of solids, NAPLs and aqueous phase liquids will occur in this tank. LGAC Tanks. Liquid-phase GAC tanks will be installed in series to treat wastewater after NAPL separation. The effluent water will be stored in the holding tank, prior to discharge. Holding Tanks. Treated water exiting LGAC tank will be directed to a holding tank consisting of two frac (fixed axle storage) tanks each.
7. Soil Confirmation and Site Closure
After ESTR treatment, soil samples are analyzed to confirm that cyanide levels are below regulatory limits. Soil testing verifies that contaminants have been reduced to safe levels, validating the effectiveness of the ESTR process. Once confirmation testing indicates compliance with environmental standards, site closure procedures commence. These include dismantling the heating and extraction infrastructure, restoring the site to its original or designated condition, and documenting the remediation outcomes. Site closure is the final stage in ensuring that treated sites meet safety requirements, allowing them to be repurposed or safely integrated back into the environment.
The steps outlined below must be followed for hot soil sampling.
1. Call the thermal remediation operator the day prior to sampling to schedule a partial or complete shutdown of ESTR system components for an appropriate duration prior to sampling.
2. An authorized person (trained and certified in lock-out and tag-out procedures, or equivalent) shall de-energized the applicable TCH wells or areas of the site using the site-specific instructions.
3. If possible, samples should be collected in order from locations having the lowest anticipated concentrations of contaminants of concern to location having the highest concentrations. Regardless, equipment that is not dedicated to each borehole shall be decontaminated between samples.
4. Soils will be obtained by the licensed drilling contractor using sampling tubes and liners for the specific sampling locations. As the soil liners become available, handle carefully due to heat/steam. Protection for handling of hot push rods and soil core barrels/liners is important and is required.
5. The ends of the liners should be capped and the liners placed onto ice to allow cooling. Visually and regularly check liners to avoid allowing any melted ice water to enter the liners. When liner feels cool to the touch, use infrared thermometer to check temperature. Alternatively, remove cap and place digital thermometer into soil liner to determine temperature.
6. When the temperature is between 21°C to 27°C, the soils should be removed as intact as possible, either by cutting the liner longitudinally to expose the soils, or push extracting the soils from the liner tube.
7. The soil liner interval should be screened with a PID. The distinct depth interval aliquots to be sampled shall be obtained for head-space screening by PID, and a split sample placed into sampling containers (with preservative, if appropriate) for the analyses necessary. Care should be taken to select the from near the center of the core barrel where evaporative losses are minimized.
8. All samples are labeled, preserved and shipped per the UFP-QAPP. Document all necessary field and sampling information as per UFP-QAPP in groundwater Chain of Custody form, sampling field form, field notebook, etc.
9. When sampling is complete, contact ESTR system operator to allow re-energizing of well field or portion thereof to resume treatment
 |  |
Ice bath | Testing the temperature of the sampling core |
 |  |
Opening sampling core after cooling | Core; ready to collect soil samples |
8. In-Situ Thermal Remediation (ISTR) Option
In situ thermal remediation (ISTR) is a technology widely used around the world. (ISTR Model show as right figure). Individual GTRTM heating well is used to heat soil and/or groundwater to 100°C or higher. GTRTM significantly accelerates contaminant removal through NAPL skimming, steam stripping volatilization, or thermal oxidation. ISTR enables faster and more complete remediation, even in low-permeability soils like clay and bedrock.
ISTR is an excellent choice for cyanide remediation due to several key advantages. It requires no excavation, minimizing site disruption, and offers fast treatment, typically completed within 3 to 6 months. For smaller sites, no utility upgrades are necessary. The technology provides reliable, repeatable results and ensures permanent contaminant removal. Additionally, guarantees are available, making it a dependable and effective solution.
9. Summary
Ex-situ Thermal Remediation (ESTR) offers several advantages for cyanide remediation:
· High Efficiency and Versatility – ESTR can handle various types of cyanide compounds and contamination levels, providing a flexible solution across different site conditions.
· Rapid Treatment and Cost Savings – The phased heating approach accelerates remediation, reducing project duration and associated costs.
· Environmental Compliance – Vapor treatment and emission control mechanisms ensure compliance with environmental regulations, minimizing the risk of secondary contamination.
· Complete Detoxification – By reaching temperatures > 300°C, ESTR ensures that even the most persistent cyanide compounds are thoroughly broken down, achieving high contaminant removal rates.
· Reduced Lifecycle Costs – Faster project completion and minimized operational time lead to significant long-term savings, making ESTR cost-effective for large-scale remediation projects.
REFERENCES
Dong, K., Xie, F., Wang, W., Chang, Y., Lu, D., Gu, X., & Chen, C. (2021). The detoxification and utilization of cyanide tailings: A critical review. Journal of Cleaner Production, 302, 126946. doi:https://doi.org/10.1016/j.jclepro.2021.126946
Wei, Y., Wang, F., Liu, X., Fu, P., Yao, R., Ren, T., . . . Li, Y. (2020). Thermal remediation of cyanide-contaminated soils：process optimization and mechanistic study. Chemosphere, 239, 124707. doi:https://doi.org/10.1016/j.chemosphere.2019.124707
Young, C., & Jordan, T. S. (1995). CYANIDE REMEDIATION: CURRENT AND PAST TECHNOLOGIES. Paper presented at the 10th Annual Conference on Hazardous Waste Research.
Comments