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Annual Report FY 2003 - Activities III Projects
Activity III Overview—Pilot-Scale Demonstrations
The objective of this activity is to demonstrate innovative and practical remedial technologies at selected waste sites, a key step in proving value for widespread use and commercialization. Technologies and sites are selected primarily from projects selected by the Technical Integration Committee, the prioritized lists generated in the Volumes from Activity I, or they may be a scale-up from bench-scale experiments conducted under Activity IV.
Activity III, Project 3: Sulfate-Reducing Bacteria Demonstration
Project Overview
This project focuses on an acid mine drainage source-control technology
that can significantly retard or prevent acid generation at affected
mining sites. Biological sulfate reduction is being demonstrated
at an abandoned hard-rock mine site where acid production is occurring
with associated metal mobility.
Technology Description
For aqueous waste, this biological process is generally limited
to the reduction of dissolved sulfate to hydrogen sulfide and
the concomitant oxidation of organic nutrients to bicarbonate.
The particular group of bacteria chosen for this demonstration,
sulfate-reducing bacteria (SRB), require a reducing environment
and cannot tolerate aerobic conditions for extended periods. These
bacteria require a simple organic nutrient.
This technology can reduce the contamination of aqueous waste in three ways. First, dissolved sulfate is reduced to hydrogen sulfide through metabolic action by the SRB. Next, the hydrogen sulfide reacts with dissolved metals forming insoluble metal sulfides. Finally, the bacterial metabolism of the organic substrate produces bicar¬bonate, increasing the pH of the solution and limiting further metal dissolution.
At the acid-generating mine site chosen for the technology demonstration, the Lilly/Orphan Boy Mine near Elliston, Montana, the aqueous waste contained in the shaft is being treated by using the mine as an in situ reactor. A substrate composed of cow manure, wood chips, and alfalfa was added to promote growth of the organisms. This technology will also act as a source control by slowing or reversing acid production. Biological sulfate reduction is an anaerobic process that will reduce the quantity of dissolved oxygen in the mine water and increase the pH, thereby, slowing or stopping acid production.
The shaft of the Lilly/Orphan Boy Mine was developed to a depth of 250 feet and is flooded to the 74-foot level. Acid mine water historically discharged from the portal is associated with this level.
Pilot-scale work performed prior to the field demonstration determined how well bacterial sulfate reduction lowers the concentration of metals in mine water at the shaft temperature (8?ºC) and pH (3).
Status
Fiscal year 2003 was the ninth year of this field demonstration.
Figure 2 shows a cross-section of the underground mine with the
technology installation.
The analytical data generally demonstrates a decrease in dissolved metals concentrations as shown in Figure 3. Manganese, however, is not removed because SRBs are not effective in its removal. The plot indicates that during a spring runoff event there is a significant increase in dissolved metals concentrations; however, the levels decreased when flow rates returned to normal.
Two inlet sampling well points were installed in late FY03. Each was located at a potential feed
Source to the Bioreactor.
These sampling locations will be monitored to more fully assess
the treatment being conducted and to provide greater confidence
for project reporting activities. Sampling at these additional
locations will assist in relating feed water origin to the treatment
and effluent flows and in the characterization of water flow through
the mine and treatment system. Sampling of this project is currently
scheduled to continue through September 2004.
Figure 2. Cross-section of the Lilly/Orphan Boy Mine and the technology installation.
Figure 3. Metal removal efficiency at the Lilly/Orphan Boy Mine.
Activity III, Project 8: Underground Mine Source Control
Project Overview
A significant environmental problem at abandoned underground mines
occurs when the influx of water contacts sulfide ores and forms
acid and metal-laden mine discharge. The Underground Mine Source
Control Project demonstrated that grout materials can be used
to reduce and/or eliminate the influx of water into the underground
mine system by forming an impervious barrier that results in reduced,
long-term environmental impacts of the abandoned mine.
Technology Description
Groundwater flow is the movement of water through fractures, faults,
or intergranular spaces in the earth. Some of the fractures are
naturally occurring; others were the result of blasting during
mining. For this demonstration, a closed-cell, expandable polyurethane
grout was injected into the fracture/fault system that intercepted
the underground mine workings. The demonstration consists of three
phases: 1) extensive site characterization; 2) source control
material identification and testing; and 3) source control material
emplacement.
Phase One, completed in 1999, consisted of site selection and characterization including hydrogeological, geological, geochemical, and geophysical information gathering directly related to the mine and its operational history. The Miller Mine near Townsend, Montana, was selected for the demonstration because the underground workings were accessible, had point-source inflows to the underground workings, the inflow was slightly acidic and laden with heavy metals, and could potentially be controlled using a source control technology.
Phase Two encompassed source control material testing according to ASTM methods for acid resistiveness, shear strength, plasticity, compressive strength, compatibility, and viscosity. The source control grout material selected for injection was Hydro Active Combi Grout, a closed-celled, expandable polyurethane grout manufactured by de neef Construction Chemicals, Inc. This material offered the following advantages: greater retention of plasticity; less deterioration due to the acidic conditions and rock movement; and better rheological characteristics.
Phase Three, the field emplacement was performed during two periods. The first technology emplacement, completed in October 1999, used a core drill to drill and grout greater than 70-foot core holes that extended over the underground mine workings and intercepted the Miller Mine reverse fault system. A second grout injection was completed in April 2001 using short holes drilled using a jackleg drill. Workers drilled approximately 400 feet of holes and grouted using about 68 gallons of grout.
As a result of the October 1999 grout emplacement, the first year monitoring results indicated that flow into the underground mine was reduced from approximately 11 to almost 1 to 1.2 gpm in the W1 drift, the drift with acidic water and only drift grouted, see Figure 4. The average flow from the Miller Mine portal was reduced by approximately 76% after all grout was emplaced, see Figure 5. However, improvements in the metals loading at the Miller Mine portal were recognized; the main metals with reduced loading were iron, nickel, manganese, and zinc. Other metals were near the instrument detection limits.
Status
The final report for the project is being completed in 2004 and
will be available on the MWTP website when it is finalized.
Figure 4. Manual flow measurements for the W1, W2, and W3 are inflow, and W4 measures the total outflow from the mine.
Figure 5. Miller Mine portal heavy metals loading rates before and after the grout emplacement
Activity III, Project 15: Tailings Source Control
Project Overview
Processing metallic ores to extract the valuable minerals leaves
remnant material behind called tailings. In the case of sulfide
mineral-bearing ores, process tailings often contain large quantities
of sulfide minerals that do not meet the economic criteria for
extraction. These remnant sulfide minerals are usually pyrites
and nonextracted ore minerals. The exposure of these minerals
to air and water often leads to detrimental environmental conditions
such as increased sedimentation in surface waters due to runoff
events, increased wind borne particulate transport, generation
of acid mine drainage, and increased metals loading in surface
and groundwaters.
Technology Description
The objective of this demonstration was to identify potential
source control materials and apply one or more of them at a selected
site. The demonstration consists of two phases: 1) site characterization
and materials testing; and 2) materials emplacement and long-term
monitoring and evaluation.
Phase one consisted of the site characterization studies, including hydrogeological, geological, and geochemical information directly related to the tailings impoundment. The materials testing and development involved testing, evaluation, and formulation of source control materials for application at the selected site.
Figure 6. Mammoth Mine Tailings site showing the three applied technologies. Left to right, modified polyurea, ISCRETE, and Krystal Bond cementaceous grouts.
Phase two encompassed the application of three selected source control materials at the demonstration site and an evaluation of the material application and feasibility. Long-term evaluation of the materials included air borne particulate testing, moisture profiles generated from reflectometers, in situ permeability tests (using Guelph Permeameters), ex situ permeability tests, and freeze/thaw testing (flexible wall permeameter).
Status
The Mammoth Tailings site located adjacent to the historic mining
town of Mammoth, Montana (see Figure 6) was the project site selected.
Material testing was completed during the first quarter of 2000.
Three source control materials were applied at the site during
the summer of 2001. These materials included two, polymeric cementitious
grouts that incorporate the tailings material as a filler material
(IESCRETE and Krystal Bond) and a spray-applied, modified polyurea
chemical grout. A year of volumetric soil moisture testing and
material evaluation was completed by the end of calendar year
2002. The final report with the results of the monitoring and
material evaluation will be finalized in the first part of calendar
year 2004.
Activity III, Project 16: Integrated Passive Biological Treatment Process Demonstration
Project Overview
The Integrated Passive Biological Treatment Process project will
demonstrate a technology consisting of a series of biological
processes for the complete mitigation of acid mine drainage (AMD).
As the first part of this project, the technology was tested successfully
at bench-scale. Now, demonstration of the process is being attempted
in the field at a remote, abandoned mine, the Surething Mine,
near Elliston, Montana. At this site, the bacteria live within
a series of reactors constructed in the ground outside an AMD
discharging mine (see Figure 7). Both anaerobic and aerobic bacteria
are being used to mitigate AMD. Toxic dissolved metallic and anionic
constituents are being removed, and the pH of the final process
effluent is near neutral.
Technology Description
The majority of the treatment is conducted in anaerobic, sulfate-reducing
bacteria (SRB) bioreactors. When provided with sulfate (present
in the AMD) and a carbon source (provided in the pit reactor is
a 50% cow manure and 50% walnut shell mixture), SRB produce bicarbonate
and hydrogen sulfide gas. The bicarbonate neutralizes the pH of
the AMD while the hydrogen sulfide gas reacts with the dissolved
metal ions to precipitate them as metal sulfides.
Additional treatment is conducted in an aerobic, manganese-oxidizing bacteria (MOB), bioreactor that was designed to have an indigenous bacteria population self establish as a biofilm on the limestone cobble. Required micronutrients are to be derived from the organic matter in the water carried over from the upstream SRB Reactors.
Figure 7. Integrated Passive Biological Treatment Process Demonstration Site.
Status
The field demonstration has been monitored since September 2001.
The data generally demonstrates a significant decrease in metals
concentrations (see Figure 8), with the exception of manganese.
Shortly after the system started operation, over 95% of the influent manganese was removed. Removal was less over the winter months and rose again in the spring. Thereafter, the manganese removals were drastically lower, indicating that a population of manganese-oxidizing bacteria was having problems developing. It was determined that the original passive aeration configuration was not sufficient to treat the amount of sulfide being carried over from the upstream SRB Reactors. Over the summer, a new aeration system was installed but was not fully effective. Additional process changes will be implemented during the next operational season to make the system favorable to MOB growth.
Figure 8. Metal removal efficiency for the Integrated Passive Biological System.
Activity III, Project 21: Integrated Process for Treatment of Berkeley Pit Water
Project Overview
The objective of this project is to develop integrated, optimized
treatment systems for processing Berkeley Pit water. The Berkeley
Pit is an inactive open-pit copper mine located in Butte, Montana.
Currently containing approximately 37 billion gallons of acidic,
metals-laden water, the Berkeley Pit is filling at a rate of approximately
3 million gallons per day and is a good example of acid rock drainage.
Two optimized flowsheets will be developed. One flowsheet is to be oriented toward minimizing the overall cost of water treatment to meet discharge requirements; this will include not only water treatment equipment but also sludge handling/management. The other flowsheet is to be oriented toward also meeting discharge requirements, but includes the recovery of products from the water (e.g., copper, metal sulfates, etc.) to potentially offset treatment costs and result in overall better economics.
Technology Description
The project evaluated proven technologies (e.g., precipitation,
ion exchange, cementation, solvent extraction, electrolysis, filtration
options, etc.) as well as technologies with credible pilot-scale
supporting data. Technologies with only laboratory testing history
were not included. The goal is to assemble the sequence of unit
operations resulting in the most attractive overall economics.
Status
In FY03, work progressed on the project final report, focused
primarily on the product recovery flowsheet. Results indicate
that discharge of sludge from treatment to the Berkeley Pit, using
a high-density sludge lime treatment process, is the most economically
favorable water treatment approach. Efforts expended on metals
recovery indicate that most of the metals present cannot be profitably
recovered due to their low value and dilute concentrations. Copper
can be recovered profitably using the existing cementation process,
recovery as a sulfide, or electrolytically by the EMEW cell technology.
Upgrading the sulfide to a sulfate (e.g., via sulfation roasting)
or oxide may improve the economics of that option. It appears
that no other metals are economically recoverable. If zinc can
be cheaply upgraded to a form other than sulfide, it may also
be economically recoverable, but probably not at current zinc
prices. Fiscal 2004 plans include completing the project final
report.
Activity III, Project 24 Improvements in Engineered Bioremediation of Acid Mine Drainage
Project Overview
Investigations conducted for this project focused on the improvements
of engineered features and the predictability of a passive technology
that could be used for remediation of thousands of abandoned mine
sites existing in the Western United States. This passive remedial
technology, a sulfate-reducing bacteria (SRB) bioreactor, takes
advantage of the ability of SRB that, if supplied with a source
of organic carbon, can increase pH and alkalinity of the water
and immobilize metals by precipitating them as metal sulfides
or hydroxides.
Technology Description
The remoteness of acid mine drainage (AMD) sites, their abundance,
and related economical aspects require that the design of an SRB
bioreactor is simple and relatively inexpensive and that the bioreactor
is capable of treating any AMD flow rate. Therefore, bioreactors
need to be prefabricated and designed to a size that allows for
transportation using backcountry roads in mountain regions. These
conditions require that the design of the bioreactor be modular
so the treatment system can be assembled at the mine site and
consists of the number of modules as required by the AMD flow
rate and the metals load. This modular configuration of the SRB
treatment system needs to support the prime functional aspects
of a bioreactor such as high permeability, ample supply of organic
carbon, ability to maintain anaerobic conditions, capacity to
accumulate precipitated metals, and means for their periodical
removal, if needed. In addition, the configuration of the bioreactor
should allow for an easy replacement of organic carbon, if needed.
Obviously, the design and sizing of such a treatment system needs
to be streamlined and performance of the system predictable. Therefore,
the two main objectives of the project were:
- developing an SRB bioreactor whose reactive material (organic matter) would not be prone to plugging and, if exhausted, would be easy to replace; and
- quantifying reactivity of organic material and developing a simulator that would facilitate optimization of the design parameters and the size of the SRB treatment system.
These project objectives were achieved through the implementation of the
following work tasks:
- selecting the medium with organic carbon;
- designing an organic carbon replaceable cartridge (RC); and
- developing a computer simulator to design and size an SRB treatment system for the given dissolved metal concentration in AMD and its flow rate.
Status
The project was completed, and the results are reported below.
Selection of the medium with organic carbon was accomplished through a literature search. All information gathered during the literature search is contained in the database assembled using Microsoft (MS) AccessTM. The report of this investigation (MSE Technology Applications, Inc., report MWTP-188) includes the list and references regarding substrate mixture components used in SRB treatment systems and their effectiveness. The report identified 36 organic substrates and lists the main conditions that need to be considered for selecting the most appropriate organic medium. Based on the results of this search, a new organic substrate, a mix of walnut shells and cow manure (W/M), was developed and selected for the project. Some advantages of using this mix are listed below.
- Cow manure is an easily biodegradable organic matter that ensures a quick startup of the bioreactor.
- Cow manure includes nitrogen needed by other microorganisms for the initial decomposition of manure. Moreover, the nitrogen is in the form of ammonium that is easier for microorganisms to use than nitrates.
- Walnut shells are more recalcitrant to biodegradation, thus, supporting good long-term operation of a bioreactor.
- Walnut shells provide a solid matrix structure because individual shells actually rest on each other. This structure prevents time-driven compaction (settling), thus, works toward preservation of the initial permeability of the medium.
- Walnut shells contain a high percentage (56%) of total organic carbon (TOC). The TOC of manure is lower and varies depending on the manure source, from 8% to 20%.
- The sustainable hydraulic conductivity of the mixture is 0.01 centimeter per second (cm/s) or higher, based on the results of experiments conducted for the project. The investigations leading to the recommended design of the RC included testing of the W/M organic medium for its permeability and sulfate reduction rates (SRR).
The permeability tests were conducted in two configurations: 1) an upward vertical flow and 2) a horizontal flow. Results of these tests indicate that the long-term permeability of this medium is significantly higher for flow in a horizontal plane. This phenomenon is attributed to the deformation of the W/M organic medium in which the finest particles are mobilized by the flowing water and migrate downward by gravity to settle at a certain level, usually at the bottom of the container, blocking the flow. In the case of a horizontal configuration, the migrating particles also settle in the bottom of the container; however, they do not block the entry of water that flows above them as it is fed laterally. The experiments conducted showed that for a horizontal flow configuration the sustainable hydraulic conductivity (K) of the mixture is 0.01 cm/s or higher. Generally, the hydraulic conductivity value for the 50% to 50% W/M organic medium mix was 1 order of magnitude smaller than the K value for the 80% to 20% W/M mix.
The laboratory work to determine SRR included six experiments conducted for two synthetic compositions of AMD at three temperatures [i.e., 44 ?F (6.7 ?C), 58 ?F (14.5 ?C), and 77 ?F (25 ?C)]. The two synthetic AMDs, referred to in this document as medium and strong, had pH values of 4.2 and 2.6, respectively. The SRR values ranged from 0.17 moles per day per cubic meter [mol/(d*m3)] to 0.79 mol/(d*m3) with the overall mean value of 0.40 mol/(d*m3). Sulfate reduction rate values in these experiments seemed to be independent of the strength of the influent and the temperature at which the experiments were conducted.
The recommended design of the RC (Figure 9) uses a commercially available cylindrical or cuboidal plastic tank most often constructed of high-density polyethylene (HDPE) or polypropylene. Such a tank needs to be equipped with necessary features to accommodate the W/M organic medium and serve as one SRB RC. These modifications will be made in a machine shop, and the tank will then be transported to a mine site. The tank will be installed either aboveground or belowground at the mine site, as required by the site conditions. An appropriate piping system will convey the AMD into the RC.
The 5-gallon bags with W/M 0.8/0.2 organic medium shown in this figure are made of plastic netting that is commonly used by grocery shops for prepacked fruits. Each bag has a loop in its top portion to facilitate the placement and the removal of the bags from the RC using a rod with a hook. A plastic tarp (not shown in the picture) placed on the top of the bags creates anaerobic conditions. The cost of production and installation (excluding transportation to the site) of such an RC housed in a 2,500-gallon HDPE tank is approximately $8,100. The cost may vary depending on local supply and labor rates applicable at the given location.
The number of RCs and the system configuration is determined through modeling conducted using the bioreactor economics, size and time of operation (BEST) computer simulator developed for this project. This simulator is a spreadsheet-based model that is used in conjunction with a public domain computer software package, PHREEQCI geochemical modeling program. While PHREEQCI calculates geochemical equilibrium for the advective-reactive transport of AMD through the bioreactor, the spreadsheet portion of the simulator handles issues of AMD flow rate, size of the bioreactor, its operational time, and its economics.
In general, the BEST simulation process is based on the chemical composition of the AMD and its flow rate; TOC content in the organic matter; cost of material and production of a typical RC; the SRR of the organic matter used in the treatment system; and the discount rate (DR) and operation and maintenance (O&M) cost for calculation of the net present value (NPV). The BEST simulator was developed and formulated so that it can be operated by a user with minimum modeling experience.
The BEST simulator, saved as a Microsoft ExcelTM workbook, consists of an input/output (I-O) worksheet and 14 additional worksheets, most of them linked together and working in the "background" of the I-O worksheet. The I-O worksheet allows for entering the majority of input data and shows the most important results.
Most worksheets are linked together, i.e., any change of input data causes appropriate changes of the results calculated by the respective worksheet. However, the PHREEQCI model and its data input file (one of the "background" worksheets) are not automatically linked with the rest of the worksheets, thus, required changes need to be made manually. Two flow charts explained the navigation between these “background” worksheets.
The time of operation calculated by BEST is based on the available carbon present in the W/M organic medium divided by the safety factor of 4. This safety factor is used because the investigations conducted for the project did not focus on confirmation of whether the organic carbon present in the medium is entirely available for the SRB.
 |
Figure 9. Sulfate-reducing
bacteria replaceable cartridge system |
An example simulation provided in the final report considers AMD flowing at 1 gallon per minute (gpm) and laden with 17.78 milligrams per liter (mg/L), 6.12 mg/L, 0.08 mg/L, and 40.4 mg/L of zinc (Zn), copper (Cu), cadmium (Cd), and aluminum (Al), respectively. An SRB treatment system to remove Zn, Cu, and Cd as sulfides would require three RCs and the capital cost of $24,244. The NPV is $37,768 based on a DR of 3.2%, O&M at $1,000/year, and the operational time of 18 years.
Activity III, Project 26 Prevention of Acid Mine Drainage Generation from Open-Pit Mine Highwalls
Project Overview
Exposed, open-pit mine highwalls contribute significantly to the
production of acid mine drainage (AMD) and can be problematic
upon closure of an operating mine. Four innovative technologies
were evaluated under the Mine Waste Technology Program (MWTP)
Prevention of AMD Generation from Open-Pit Highwalls Demonstration
Project. The objective of the field demonstration was to evaluate
technologies for their ability to decrease or eliminate acid generation
from treated areas of the highwall, compared to untreated (control)
highwall areas.
Technology Description
Generation of AMD from open-pit mine highwalls has been addressed
in a limited manner, and little information is available on the
subject. However, highwall generated AMD will continue to be produced
for indefinite periods of time as weathering occurs and the flushing
action of atmospheric precipitation and/or groundwater infiltration
through the highwall takes place.
The main purpose of this project was to research technologies applicable to controlling or eliminating AMD generated from open-pit mine highwalls and then apply and monitor the potential technologies under actual field conditions. For this demonstration, four technologies having potential to passivate AMD from a highwall were selected for field application. The application methods required for each technology varied along with the application time and the materials.
The demonstration consists of three phases: 1) extensive site characterization and gathering background information; 2) technology identification and field application; and 3) long-term field monitoring and laboratory testing for confirmation of field results.
Site characterization in Phase I included core drilling the highwall to determine geology, hydrogeology, and extent and depth of acid generation (i.e. geochemical analysis), and performing background sampling at all of the sampling ports placed on the highwall. Phase II involved selecting a site [the Golden Sunlight Mine (GSM) near Whitehall, Montana], the highwall technologies, and applying the technologies in the field. The four technologies that were spray-applied to the highwall included: furfuryl alcohol resin sealant (FARS), Eco BondTM, a magnesium oxide (MgO) and a potassium permanganate (KP) treatment.
The third phase of the project involved monitoring the technologies using ASTM D5744-96, Accelerated Weathering of Solid Materials using Modified Humidity Cells, residual wall rinse sampling at the treated highwall plots, and microscopy.
Monitoring results indicated that all treatment technologies reduced the amount of AMD generated on the highwall. Each technology performed differently based on metals removal/reduction. When compared to the background plot during the 41 weeks of humidity cell testing, the results indicated that the overall effectiveness of the technologies from least to most effective was FARS, Eco BondTM, KP, and MgO. For the mine wall rinse sampling, the residual wall rinse results indicated that the overall effectiveness of the technologies from least to most effective was Eco BondTM, MgO, KP, and FARS. Over the course of the project, deterioration of the highwall led to fewer available mine wall rinse sampling stations and less data for evaluation of the plots. The pH of the highwall at each plot initially started at greater than pH 10 except for FARS, which started at 4.5. The pH at the last sampling event shows that the pH at each plot has decreased over time except FARS, and the wall rinseate results for iron indicated how each technology affected the metals loading from the highwall.
Figure 10 shows pH results from the highwall residual wash sampling. The GSM (background) results are compared to the FARS, Eco Bond (MT2), MgO, and KP technologies.
Figure 10. pH results from the highwall residual wash sampling.
Figure 11 shows the iron totals metals loading results from the highwall residual wash sampling. Other metals were observed in the same manner and are included in the final report.
Figure 11. Iron totals metals loading results from the highwall
residual wash sampling.
Status
The field demonstration was performed at GSM, a subsidiary of
Placer Dome, an operating gold mine located near Whitehall, Montana.
The ore body at GSM is sulfidic, and the exposed highwall provides
an AMD source. Phase one, site characterization, was completed
in September 2001.
Phase two, which included technology selection, was completed by May 2001. Placement of the technologies was performed between October and December 2001.
The technologies were evaluated in 2002 using a residual wall rinse sampling method and a modified humidity cell testing method. The humidity cell testing was extended to 41 weeks and was completed in FY03. The final report will be completed in FY04.
Activity III, Project 29 Remediation Technology Evaluation at the Gilt Edge Mine
Project Overview
The objective of this project is to generate performance and cost data
for promising new technologies for preventing the oxidation of sulfide
waste rock, which may be applicable to many mine waste sites. The new
technologies will be compared to the presumptive remedy of lime treatment
as well as to controls in which no treatment is performed. The technology
demonstration was performed at the Gilt Edge Mine, a 270-acre, open-pit
cyanide heap leach gold mine located about 5 miles southeast of Lead,
South Dakota. The immediate area was the site of sporadic mining activity
for over 100?years. The Gilt Edge Mine was operated by
Brohm Mining Corporation, a wholly owned subsidiary of Dakota
Mining Cooperation from February 1986 until July 1999. Brohm’s
activities included developing several open pits, crushing and
placing the ore on a heap leach pad for gold leaching by cyanidation,
and Merrill-Crowe gold recovery in an on-site mill. In July 1999,
the mine’s owners (Dakota Mining Corporation) declared bankruptcy,
resulting in the Gilt Edge site being returned to the State of
South Dakota for management. After incurring significant costs
for water treatment to ensure no discharge of acidic mine water
to the environment occurred, the State of South Dakota requested
that EPA Region VIII take over the site and list it on the National
Priorities List (NPL) as a Superfund site. The Gilt Edge Mine
site presents an opportunity to evaluate emerging acid mine drainage
(AMD)-treatment technologies while gathering data leading to a
Record of Decision (ROD) for the site.
This project was a collaboration between EPA Region VIII and the EPA Mine Waste Technology Program (MWTP). The objective of Region VIII was to conduct a treatability study as part of the remedial investigation/feasibility study process for the site—providing data to help make decisions supporting the ROD for the site. The technical and economic information will be summarized in a final report to assist Region 8.
The project involved constructing test cells that were loaded with sulfide-bearing waste rock from the Gilt Edge Mine site. EPA Region VIII (or its contractors), assisted by the U.S. Bureau of Reclamation designed and constructed the test cells, as well as loaded the waste rock into the cells. Three technology providers installed their respective technology for reducing AMD generated by the waste rock. The project occurred west of the Anchor Hill Pit at the Gilt Edge Mine. The test cells received ambient precipitation, and an irrigation system applied additional simulated precipitation to the test cells. A system for managing and sampling leachate quality designed by EPA Region VIII was integrated into the cell design. Twelve test cells were planned. Two cells were dedicated to each of the three technologies to show performance repeatability. Three control cells containing only waste rock (with no additional treatment) and three cells representing the presumptive remedy of blending lime with the waste rock were also constructed. The performance of the installed technologies was judged primarily by comparing leachate water quality from the installed technology cells with that of the control and presumptive remedy (lime treatment) cells. The test cells were constructed and loaded in September 2000.
Technology Description
The three technologies demonstrated were:
- Silica microencapsulation [Klean Earth Environmental Company (KEECO)];
- Envirobond [Metals Treatment Technologies (MTT)]; and
- Passivation technology [Mackay School of Mines, University of Nevada, Reno (UNR)].
KEECO has developed a treatment technology for treating and preventing metals-contaminated waters, soils, and possibly sulfidic waste rock called silica microencapsulation (SME). This technology encapsulates metals in an impervious microscopic silica matrix (essentially locking them up in very small sand-like particles) that prevents the metals from leaching and migrating. Its chemical components react when introduced to water, creating an initial pH adjustment and electrokinetic reaction. The electrokinetic reaction serves to facilitate electrokinetic transport of metal particles toward the reactive components of the SME product, enhancing its efficiency. Metal hydroxyl formation follows; next, silica encapsulation of the metals occurs, forming a dense, stable coating. Contrary to conventional treatment process where sludges typically degrade over time, the SME silica matrix appears to continue to strengthen and tighten, providing for long-term isolation of contaminants from the environment. Silica microencapsulation has been applied to wastewater, sediment, sludge, soil, mine tailings, and other complex media but has never been applied and tested directly on sulfidic mine waste rock materials.
The Envirobond (Metals Treatment Technologies) technology is similar to
the KEECO technology except that it involves phosphate stabilization chemistry
rather than silicates. The technology has been applied at mining sites,
firing ranges, sediment removal
sites, and others to produce a solid treatment material meeting
Toxicity Characteristic Leaching Procedure criteria. The technology
can be adapted for a variety of wastestreams and soil conditions.
Over the past few years, DuPont developed a novel coating method known as a passivation technology. Recently, the technology was donated to UNR for further development and commercialization. The passivation process essentially creates an inert layer on the sulfide phase by contacting the sulfide with a basic permanganate solution to produce an inert manganese-iron oxide layer. This layer prevents contact with atmospheric oxygen during weathering of the sulfide rock, thus, preventing sulfuric acid generation. Another critical element of the process is the addition of trace amounts of magnesium oxide during pH adjustment. Magnesium oxide addition enhances the coating strength.
Status
The treatment cells (Figure 12) were loaded and treated by the
technology vendors in November 2000. Treatment monitoring started
May 2001 and continued through December 2002.
Data analysis from the sampled leachate included comparing parameters of each leachate to the South Dakota Water Quality Criteria (SDWC) applicable to the Gilt Edge Mine. Two indicators that reflect overall effectiveness are the total dissolved solids (TDS) concentrations and the pH trends of each technology. Figure 13 shows the TDS trends. The presumptive remedy cells of 4, 7, and 12 all reached the SDWC of 2500 milligrams per liter (mg/L). The UNR cell 3 trend was very close to the 2500 mg/L limit. Figure 14 shows the pH trend for the technologies. The SDWC pH range is 6.5 to 8.8. The MT2 cells 5 and 11 trends were within this range for the duration of the test, while the UNR cell 3 was in the range for the majority of the test.
MSE Technology Applications, Inc., is currently evaluating sampling and cost data to evaluate the treatment technologies. A final report will be written and should be finalized in FY04.
Figure
12. Treatment cells.
Figure 13. Total dissolved solids trends.
Figure 14. pH trend.
Activity III, Project 30 Acidic/Heavy Metal-Tolerant Plant Cultvars Demonstration, Anaconda Smelter Superfund Site
Project Overview
Presently, grass, forb, and shrub species commercially available
for reclaiming acidic/heavy metals-contaminated (A/M) soils often
come from outside the Northern Rocky Mountain region. These cultivated
varieties may not tolerate the climatic-edaphic stresses (in addition
to A/M stresses) as well as would A/M ecotypes indigenous to the
region. Over the past several years, plant populations exhibiting
A/M tolerance potential have been collected from the Anaconda
Smelter Superfund Site and evaluated in laboratory, greenhouse,
and preliminary field trial studies. The results indicate that
self-sustaining plant communities comprised of native A/M tolerant
ecotypes are possible. Thus, the goal of this project is to formally
compare the performance of local seed mixes against comparable
mixes now commercially available. If the local ecotypes (of the
given grass/forb species) are indeed best performing, they would
be made available for numerous full-scale reclamation of the hardrock
mine/mill/smelter sites in the region.
Technology Description
The team comprised of the Deer Lodge Valley Conservation District
(DLVCD), USDA/Bridger Plant Materials Center (BPMC), and MSE Technology
Applications, Inc., selected and evaluated the most promising
grass/forb accessions at test sites in the Anaconda area over
the 2002?2004 growing seasons. Shrub species were evaluated at
another site that is not formally part of the Mine Waste Technology
Program funded study.
The initial (FY02) study design used four grass/forb mixtures from southwestern Montana and taxonomically similar mixtures of commercially available cultivars. Four replicates of each mixture were planted (in late fall 2001) at both the Stucky Ridge and Mill Creek test sites. However, a hypothesized combination of drought, soil acidity, and plant available metals levels precluded effective seedling growth and establishment of any of the mixtures at both sites during the 2002 field season.
Subsequently, regulatory personnel from the State of Montana and EPA agreed to pretreatment (i.e., liming and fertilizing) of the soils before planting. The study’s goal was also modified to allow comparison of growth performance of species-specific accessions in the field (versus greenhouse), when grown either by themselves or in defined mixtures of various species.
The laboratory and field data gathered during the remaining seasons will be statistically analyzed to determine whether any of the local seed mixes outperforms their commercial counterparts. Results to date are summarized in Table 2.
Status
The following activities continued in FY03: collection and laboratory
analysis of plant and soil samples from the Anaconda area; field evaluations
of plant performance; and production of seeds (at BPMC) from the most
promising grass/forb accessions. The test site was relocated to the north
side of Stucky Ridge in the fall of 2002. The upper 6 inches of soil were
amended with lime kiln waste (approximately 22 tons per acre) in mid-November;
about 500 pounds per acre of NPK fertilizer was tilled into the soils
in April 2003. The grass, forb, and grass/forb seed treatments were planted
in mid-May 2003; rooting zone soil samples were then collected for acid
extractable metal levels and saturated paste pH. The laboratory results
are shown in Table 2 and indicate that phytoxic levels of copper may exist
within the rooting zone.
Overall, seedling density decreased between late June and late
August for all three treatment types. The record high temperatures
and low precipitation in July and August seem to be the dominant
factors affecting the decline.
None of the local grass accessions had significantly greater densities than for the nonlocal accessions. With the exception of a nonlocal accession of winterfat, the forb/subshrub accessions exhibited poor emergence and consequently poor seedling densities. This effect may be due to an insufficient period of cold-moist stratification (i.e., it would have been better for the seeds to have over-wintered in the soil).
The seed mixtures consisting of commercially available nonlocal varieties had slightly higher seedling densities but were not significantly greater than those exhibited by the local accessions. The Waste Management Area seed mixtures that consisted of both introduced and native species performed as well as the upland seed mixtures that included only native species.
Analytesb |
||||||
| Treatement Type | As |
Cd |
Cu |
Pb |
Zn |
pH |
|---|---|---|---|---|---|---|
| Grass Accessions (only) | 151±22 |
1.2±0.2 |
870±50 |
46±6 |
192±14 |
8.0±0.1 |
| Forb Accessions (only) | 120±8 |
1.4±0.4 |
909±102 |
38±2 |
194±9 |
7.8±0.1 |
| Grass/Forb mixtures | 186±77 |
1.4±0.4 |
801±64 |
55±23 |
186±18 |
7.7±0.1 |
| Notes: aFor 0-6-inch (below ground surface) soils collected at the Moto-Cross site in late June 2003 bAcid-extractable metals/metalloid (As) in mg/kg; 1:1 soilA water pH in standard units cValues exceed the 250 to 500 mg/kg thresold of concern stated in the 2001 Quality Assurance Project Plan. |
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Activity III, Project 33 Microencapsulation to Prevent Acid Mine Drainage
Project Overview
This technology demonstration project was conducted on a cost
share basis with the Minnesota Department of Natural Resources.
The objectives were to evaluate the potential field application
success in preventing acid mine drainage and to estimate requirements
for field applications.
An unoxidized sulfidic rock material was tested with three application levels of two commercially available microencapsulation technologies: Klean Earth Environmental Company (KEECO) and Envirobond. They were evaluated in comparative laboratory studies using modified humidity cell operation.
Technology Description
Microencapsulation is the isolation of sulfide minerals by precipitating
a chemical coating on unoxidized pyrite or where the material
is reacted with an oxidizing agent to produce ferric ions.
The KEECO KB-SEA technology uses a soluble silica to produce an insoluble ferric silicate precipitate that encapsulates solid media particles. The materials become stabilized as this silica coating helps to control future acid generation.
The Envirobond EcoBond-ARD technology uses a soluble phosphate to form a ferric phosphate precipitate that prevents the leaching of metal contaminates by creating an impenetrable chemical bond.
Humidity cells containing three application rates (high, medium, and low) with duplicates for each along with control cells were tested and leached weekly.
Status
The control reactors had acid drainage with a pH >6 after 1
week and a pH of 3.3 at 60 weeks. Over the first 60 weeks of testing,
the KB-SEA treatment was successful in preventing acid drainage;
however, it must be noted that initially very high pHs were generated
in comparison to the controls (see Figure 15).
The EcoBond treatment delayed the onset of acidification but was not successful in preventing acid drainage. Project testing of the EcoBond cells was discontinued at 60 weeks with one set of the EcoBond-ARD duplicate cells being sent to the technology provider for reapplication and continued leaching.
In 2003, the humidity cell phase for the Mine Waste Technology Program was completed, and a microscopic investigation of the test materials was initiated. The purpose of this work is to evaluate the formation of the actual microencapsulation layers along with trying to determine any failure mechanisms of the microencapsulation layer. Both treated, unleached material as well as the spent humidity cell material for each technology will be studied during FY04.
Figure 15. Microencapsulation humidity cell testing.
Activity III, Project 34 Bioremdiation of Pit Lakes (Gilt Edge Mine)
Project Overview
This project is being conducted at the Gilt Edge Mine Superfund
site near Deadwood, South Dakota. The project is a collaboration
between the Mine Waste Technology Program and the U.S. Environmental
Protection Agency’s (EPA) Region VIII Superfund office.
MWTP is taking the prime role in this project with support from
EPA Region VIII. EPA Region VIII=s interest is to conduct a treatability
study as part of the site Remedial Investigation/Feasibility Study
(RI/FS) process, while MWTP’s interest is to develop data
applicable to other similar sites. An in situ treatment of the
Anchor Hill Pit, an open pit at the Gilt Edge site containing
approximately 70 million gallons of acidic water containing high
levels of metals, sulfate, and nitrate, is being performed. The
treatment consisted of an initial neutralization step, followed
by a biological treatment to further improve water quality and
create a long-term, stable system. After the two-step treatment,
the project entered a monitoring mode where the pit lake was physically
and chemically characterized on a quarterly basis for several
years. The purpose was to see how well the treatments work and
how stable the pit lake water becomes, e.g., if metal sulfides
are produced, does the system reoxidize and remobilize those metals
Technology Description
After initial chemical/physical characterization of the pit lake,
the neutralization step was implemented by Shepherd-Miller, Inc.
(SMI) of Fort Collins, Colorado, under subcontract to MSE Technology
Applications, Inc. SMI used a Neutra-Mill fed with lime (CaO).
The Neutra-Mill is simply a floating platform containing an apparatus
to mix a reagent in with the water it is floating on. The Neutra-Mill
was developed by Earth Systems, Pty. of Australia; SMI holds the
U.S. license to apply the technology. Neutralization occurred
between March and May 2001.
After neutralization, the pit was allowed to sit undisturbed for several weeks to allow precipitated solids to settle and the system to stabilize. After stabilization, the pit lake was once again characterized. Following this, in late May 2001, material consisting of methanol, molasses, and phosphoric acid was added to the pit lake. This process has been patented by Green World Science, Inc., of Boise, Idaho. The purpose of the organic carbon addition is to produce reducing conditions in the water and stimulate the activity of indigenous bacteria. This should have the effect of reducing or eliminating nitrate/nitrite and selenium, and polishing toxic metals concentrations to very low levels by precipitating them as sulfides (produced by reducing some sulfate to sulfide by sulfate-reducing bacteria activity), as well as adding bicarbonate alkalinity to the water to provide buffering capacity.
Status
Project accomplishments in FY03 included continuation of monitoring
the pit water chemistry via obtaining analytical samples regularly
as well as vertical profiles of physical measurements. Results
were very encouraging, with completion of denitrification and
initiation of sulfate reduction during FY03. Late in FY02, the
pit received additional treatments including a pH increase to
neutral (from approximately 5.5 to 6.0) using sodium hydroxide
and additional dosage of molasses and methanol. The purpose of
these treatments was to ensure that the pit water column was in
optimum condition for bacterial activity to occur entering the
winter of 2002 to 2003. These treatments did have the desired
effect. A distinct drop in nitrate/nitrite concentrations was
immediately observed. Nitrate/nitrite reached essentially nondetectable
levels by approximately March 2003. Physical evidence of sulfate
reduction was then observed beginning in April 2003, specifically,
the presence of black precipitates and the smell of hydrogen sulfide
gas whenever the water was stirred up or agitated.
Concentrations of metals that form metal sulfide precipitates (e.g., copper, cadmium, and zinc) immediately decreased dramatically. The metal-sulfide precipitates were very small and slow-to-settle so that dissolved metals values (i.e., those filtered through a 0.45-?m filter) were much lower than total, unfiltered values. Much of FY03 was spent waiting for these suspended materials to settle. Both total and dissolved metals values gradually decreased through the remainder of FY03. An attempt was made in August 2003 to generate an algae bloom; it was thought that an algae bloom would improve metals concentrations by adsorbing dissolved metals and by “flocculating” suspended metals as the algae died and sank. The algae bloom did not noticeably materialize, possibly due to the system being nitrogen-limited. At the end of FY03, the water in the Anchor Hill Pit met South Dakota requirements for discharge from the site, with the exception of specific conductance, total dissolved solids, total suspended solids, biological oxygen demand, and dissolved oxygen. Dissolved metals analyses met South Dakota requirements for dissolved toxic metals. Total metals values were still significantly higher than the dissolved metals values, as well as being above the discharge criteria for dissolved metals.
Figure 16 presents the average concentrations of nitrate/nitrite and dissolved copper, cadmium, and zinc versus time. As seen in the figure, nitrate/nitrite approached zero in the spring of 2003, and dissolved metals values immediately decreased thereafter as a result of subsequent sulfate reduction. This was encouraging since, thermodynamically, sulfate reduction should not occur until denitrification is complete, and this is exactly what occurred.
Figure 17 presents the average total and dissolved concentrations of copper, cadmium, and zinc versus time. This figure illustrates the significant difference between dissolved and total analyses caused by the presence of small, slow-settling metal sulfide precipitates. It can be seen that both total and dissolved values decreased through the year, and further, so that by the end of the year, acceptable discharge values were reached for dissolved analyses but not for total analyses.
Lastly, Figure 18 presents the alkalinity values obtained. It can be seen that the three deepest sample points have had increasing alkalinity throughout FY03, further confirmation of biological activity. Bicarbonate alkalinity is a product of both denitrification and sulfate reduction.
Vertical profiles obtained indicate that the pit lake has become strongly meromictic, i.e., stratified such that the vertical water column does not mix during the year. This meromictic condition may be used as an advantage in future water treatment efforts. For example, partially neutralized acid rock drainage could be added to the lower depths of the pit, which should remain anoxic and might provide conditions for biological nitrate and sulfate reduction in future water treatment. Plans for FY04 include continuation of monitoring the pit water chemistry, as well as investigating means for subjecting the pit water to ex situ filtration followed by aeration in preparation for possible discharge of a portion of the water contained in the pit.
Figure 16. Nitrate/nitrite and dissolved copper, cadmium, and
zinc versus time.
Figure 17. Total and dissolved copper, cadmium, and zinc values
versus time.
Figure 18. Alkalinity values versus time.
Activity III, Project 38: Contaminant Speciation in Riparian Soils Demonstration
Project Overview
This evaluates phosphorus-lead soil interactions with respect
to mineralogical stability. It is an investigation into the reaction
processes that take place when phosphate amendments are added
to riparian soils containing lead and other solid phase material
(i.e., soils, mineral and organic matter).
Technology Description
Phosphorus has shown excellent potential for the remediation of
lead-contaminated soils and reduction of lead bioavailability.
However, no existing information correlates the reaction mechanisms
of lead in field remediated soils with toxicological studies on
waterfowl. This project will serve to fill this gap. In addition,
this project will also serve to monitor how the speciation and
bioavailability of the other contaminants are affected by phosphorus-based
remediation treatments.
This study builds upon and links a previously initiated investigation on the ability of phosphate amendments in lead contaminated riparian soils to reduce the bioavailability of lead to waterfowl. Previous work was performed by the Idaho Department of Environmental Quality (IDEQ) and the U.S. Fish and Wildlife Service (USFWS) and was jointly funded by the Coeur d’Alene Basin Commission and U.S. Environmental Protection Agency (EPA). In previous work, different soil amendment treatment technologies were applied at a field site in the Lower Coeur d’Alene River Basin.
Status
The fate of the lead in these previously remediated soils is being
investigated. Experiments using advanced spectroscopic and microscopic
techniques are being conducted along with an in-vitro test method
for simulation of waterfowl digestive systems to demonstrate the
effectiveness of phosphate soil amendments to reduce lead bioavailability,
solubility, and leachability through the formation of low-solubility
lead compounds. This project is scheduled to conclude in June
2005.
Activity III, Project 39: Long-Term Monitoring of Permeable Treatment Wall Demonstration
Project Overview
The objective of this project was to demonstrate a technology
that uses a fishbone apatite treatment media to passively remove
zinc from the water. In the initial stages of the project, under
U.S. Department of Energy funding, a fully contained subsurface
retention basin and treatment system was designed to capture and
treat a specified volume of water discharging from the Nevada
Stewart Mine. The work scoped under the Mine Waste Technology
Program (MWTP) project funding included monitoring the total system
and the nearby receiving stream. The MWTP was tasked with defining
a baseline metals concentration and then determining the percent
reduction of dissolved metals in the effluent from the apatite
treatment system over a 2-year period. Flow monitored under this
project include the in- and out-flows of the system, and upstream
and downstream of the treatment system effluent location.
Technology Description
The Nevada Stewart Mine site is located in Shoshone County near
the headwaters of the Highland Creek drainage approximately 5
miles south of Pinehurst, Idaho. This site consists of an adit
and several surface waste piles. Approximately 5,200 cubic yards
of mine waste were previously removed by the U.S. Bureau of Land
Management (BLM) from the site and disposed in the Central Impoundment
Area at the nearby Bunker Hill Site. BLM recently contoured the
site to prevent erosion and further contaminant loading to the
receiving stream, Highland Creek. Approximately 40 to 60 gallons
of water discharge continuously from the Nevada Stewart Mine adit
into Highland Creek. Analytical results indicate high levels of
dissolved zinc, manganese, and iron in the soils and adit discharge.
The technology deployed for this project is an apatite-based treatment media (Apatite II) that passively removes zinc from water (either surface water or groundwater). The treatment media was placed into a fully contained subsurface retention basin and treatment system (see Figure 19). By placing the treatment media into a contained subsurface retention system, several advantages over vault and barrier systems are gained, which include:
- significant odor control;
- protection from freezing;
- protection from vandalism and damage from animals;
- ability to change out treatment media;
- ability to accurately monitor inflow/outflow and water quality to determine metals loading; and
- does not detract from the landscape.
Status
The Nevada Stewart Mine was selected for implementation of the
technology in August 2002, and construction of the apatite treatment
system was completed at the end of September 2002. Treatment of
the Nevada Stewart adit discharge began on October 1, 2002, and
the flow through the system was 18 gallons per minute.
The first baseline sample was taken in November 2002 and showed that the system reduced the total metals loading of zinc by 90%, iron by 90%, and manganese by 50%. System monitoring will be performed monthly until September 2004.
After the system operated for a few months, the treatment system plugged. The plugging occurred at the effluent catch basin illustrated in the lower, right corner photo of Figure 19. The geotextile fabric plugged with precipitate suspended in the effluent discharge and bypass mine water. The catch basin was opened and gravel was placed in the system to create a French drain to reduce plugging. Additionally, settling of the apatite media in the treatment cells has reduced the permeability and effectiveness of the treatment system. To reestablish the systems permeability and effectiveness to remove metals, low pressure compressed air was used to agitate the apatite/gravel media. These solutions have worked effectively to reduce plugging and increase the longevity of the treatment system.
After monitoring the apatite system for the initial 9-month period, approximately 162 pounds of zinc, 53 pounds of iron, and 13 pounds of manganese had been removed for the effluent water entering Highland Creek, see Figure 20.
Figure 19. Installation of the fishbone, Apatite treatment system
at the Nevada Stewart Mine site.
Figure 20. Total amount of metals taken out of the Nevada Stewart
Mine water as it flowed through the fishbone apatite system over
9 months.
Activity III, Project 40: Electrochemical Tailings Cover
Project Overview
This project is being conducted at the MSE Technology Applications’
(MSE) test facility in Butte, Montana. The purpose of the demonstration
is to gather performance information for the electrochemical cover
technology developed by Enpar Technologies, Inc., of Guelph, Ontario,
Canada. Fresh, nonoxidized tailings along with soil cover were
transported from the Golden Sunlight Mine (GSM), owned by Placer
Dome, Inc., near Whitehall, Montana. Lined, in-ground test cells,
each equipped with a leachate collection system, along with a
common sprinkler system, were constructed at MSE’s test
facility. Two test cells, loaded with tailings and capped with
soil cover, were constructed as identical control cells, receiving
no electrochemical cover treatment. Two additional test cells,
loaded with tailings and capped with soil cover, were constructed
as identical test cells and were equipped with the electrochemical
enhancement. Equal amounts of water were applied to all four test
cells following installation. Water application will continue
through the summer of 2004. Leachate will be pumped from each
test cell to maintain an artificial water table and to provide
water for analytical purposes. Leachate water quality will be
monitored by regular sampling and analyses. Oxidation of the acid-generating
tailings in all four will be assessed primarily by monitoring
sulfate concentrations along with conductivity measurements. Sulfate
mass produced by the two cells equipped with electrochemical cover
treatment will be compared to that produced by the two control
cells with no electrochemical cover treatment. It is anticipated
that the two control cells will show higher sulfate concentrations
and higher conductivity, resulting from tailings oxidation. The
field installation will be monitored for 1 year, at which time
it will be dismantled and the tailings returned to GSM.
Technology Description
This technology is intended to be an enhancement of a soil cover
to greatly reduce or eliminate the oxidation of sulfide materials,
thereby, reducing or eliminating acid rock drainage produced by
the covered material. The electrochemical cover consists of sacrificial
anodes (e.g., magnesium) overlying the soil cover, which further
overlies a cathode consisting of a steel mesh. The soil cover
essentially is the conductive dielectric between the cathode and
anodes. Oxygen is consumed and alkalinity generated at the cathode
by the reaction O2 + 2H2O + 4e- => 4OH-, with the needed electrons
produced at the cathode by passive, galvanic corrosion of the
anodes. The anodes can be sized so they last for as long as desired;
Enpar has typically sized them so they last for 30 to 35 years.
Status
Originally, the project was intended to be conducted at larger scale Tailings
Impoundment #1 (GSM). Due to concerns over quantifying oxidation of the
dry tailings at Impoundment #1, as well as budgetary limitations, the
project was scaled back and rescoped to be conducted under smaller, more
controlled conditions at MSE’s facility. During FY03, Enpar completed
characterization of the Impoundment #1 tailings and soil cover to support
design of the field system. Enpar also completed laboratory tests focused
on the use of oxidation-reduction potential measurements as an indicator
of tailings oxidation; however, this approach was not used due to concerns
over the accuracy of the technique. Also in FY03, the project work plan
was modified to reflect the new approach at MSE’s facility, and
the project quality assurance project plan and the field installation
were completed. Water was applied to the test cells to “charge”
the cells with water, and then the project was shut down for the winter.
The test cells equipped with the electrochemical cover treatment have
an electrical junction box containing test points that can be used to
measure voltage and current values to assess the health of the
electrochemical system. Measurements taken in the first month
of operation indicated that the system was performing as expected.
Plans are to begin applying water and pumping/analyzing leachate
in the spring of 2004, continuing doing this through the summer
of 2004, then dismantling the system and returning the tailings
to GSM in the fall of 2004.
Figure 21 shows the steel mesh cathode placed above the tailings in one test cell. Figure 22 shows the final field installation.
Figure 21. Steel mesh cathode placed on top of tailings in test
cell.
Figure 22. Final field configuration.