JEB PIT SUBAQUEOUS DISPOSAL OF
PASTE TAILINGS STRATEGY
Arnaud de Bourayne
COGEMA Resources Inc.
817 825 - 45th Street West
Saskatoon, SK S7K 3X5
COGEMA Resources Inc.'s operation at McClean Lake will consist of milling ores from the highest grade uranium deposits in the world. These deposits are located in the Athabasca Basin in northern Saskatchewan, Canada and typically include the Cigar Lake, McClean Lake and Midwest Projects.
All the ores from these high grade deposits will be processed at the McClean Lake JEB mill. The ore grades vary up to 30% uranium and in some cases pure massive pitchblende is encountered.
The McClean Lake mill is designed with an initial capacity of 6 million pounds U3O8 annually. Following approval of the Cigar Lake Project by both governments, the capacity of the mill will be expanded to 24 million pounds U3O8 annually, making the JEB mill, the largest in the world.
Although the initial grade of ores to be processed in the JEB mill (2% uranium to 4.75% uranium) are much less than those expected from Cigar Lake (up to 30% uranium), they are still high enough to warrant special radiation protection measures.
The mill therefore has been designed to control radiation, protect worker health and safety, and protect the environment in the short and long term. This design ensures a maintenance free solution after decommissioning to protect future generations and the land.
With respect to the management of tailings, COGEMA Resources with its other partners have adopted an innovative solution that is unique to the McClean Lake site. This solution encompasses three areas.
The McClean Lake deposit is located in the Athabasca Basin in northern Saskatchewan, Canada where the highest grade uranium orebodies in the world are located (Figure 1). These high grade orebodies typically include the Cigar Lake and McArthur River Projects and to a lesser extent, the McClean Lake and Midwest Projects. Consolidated reserves in this area exceed 1 billion lbs. U308, with ore grades ranging from 2% uranium to almost 70% uranium.
Fig. 1. Athabasca Basin
Because the Cigar Lake ores will be processed at the McClean Lake Mill when mining commences, the project design philosophy at McClean Lake took into account several issues including health, safety and environmental protection for the short and long term.
For example, the decommissioning scenario of the tailings management facility will provide a maintenance free solution in the long term to protect future generations and the land; the mill design and tailings management facility incorporate all the elements of a sound business solution, radiation protection, risk management, environmental protection and safety.
The McClean Lake Project consists of four open pits and one underground mine namely; the JEB open pit, Sue A, B and C open pits and the McClean underground (Figure 2). The mill, the construction of which is almost completed, has an annual capacity of 6 million pounds U308 to process the McClean Lake and Midwest projects ores. Processing of these ores will maintain the life of the project for about 14 years. When mining of Cigar Lake ore commences, the McClean Lake mill (renamed the JEB Mill) will be expanded to a capacity of 24 million pounds U308 a year.
Fig. 2. McClean Site Location
Mill feed grades from the McClean / Midwest Projects would range from 2% uranium to 4.75% uranium. The grade from Cigar Lake will vary up to a maximum of 30% uranium during the first decade of production. Although the grades from McClean / Midwest are lower than those expected from Cigar Lake, they are high enough to warrant special radiation protection measures.
The concepts of shielding and containment of contaminants have been incorporated in the innovative methods of mining, milling and tailings management (Figure 3). In the mine, workers will be shielded from high radiation areas by the natural innocuous basement rock; in the mill, engineered structures and suitable design methods will shield and protect workers from radiation and in the tailings impoundment area, a water cover will shield and protect workers from radiation and dust.
Fig. 3. Radiation Protection Concept for High Grade U Deposits
McCLEAN LAKE MILL DESIGN
The mill design considered the chemistry, the mineralogy and the radioactivity of the ores to be processed. Although a conventional milling process is appropriate for processing most of the ores at McClean Lake, the most significant difference for reflection in the design principle is the uranium grade of the ores, especially Cigar Lake's.
The milling process is largely conventional and includes an ore receiving circuit, an atmospheric and pressure leaching circuit, stripping and purification, solvent extraction, precipitation, bulk neutralization and packaging and drying.
The Cigar Lake and Midwest ores will be shipped to McClean Lake as a high density slurry with a high solids content by weight. The slurry will be shipped in steel containers on B train trucks to the McClean Lake ore receiving facility. The full container will be picked up by remote equipment and its contents will be emptied into a dilution sump. The empty container will be washed prior to shipment back to Cigar Lake or Midwest. This measure is designed to prevent the spread of contamination in the facility and on the roadways.
The contents of the dilution sump will be pumped to a re-pulp tank and into storage pachucas ready for leaching.
Due to the high grade of the Cigar Lake ore and the high arsenic and nickel content of the Midwest, Sue A and Sue B ores, these ores will be leached in pressure leaching circuits using steam, oxygen and sulphuric acid as a lixiviant.
The design used in the leaching circuit area is called the "elevated shielding slab concept" (Figure 4) and is based on a two floor design aimed at protecting operating personnel from significant gamma emitting sources.
Figure 4. Atmospheric Leaching
The two floors are separated by an elevated concrete slab of approximately 30 - 40 cms thick. The thickness of the slab has been calculated to provide the right amount of shielding from the high grade ores being processed. All the operating switches, valves, energizing equipment etc. are located on the upper floor above the slab. Operating personnel will also be located on the second floor, separated from the process and further shielded from potential gamma radiation by the concrete floor.
On the lower level, radioactive process tanks are contained within individual cells to control contamination and to reduce the number of sources of gamma radiation exposure in any one area. These cells are constructed of a combination of steel, concrete and lead where required. Each cell is provided with sumps, appropriate sloping floors and floor surfaces, for ease of clean-up should there be a spill. Sump pumps and process pumps are located outside the concrete cells, which house the tanks where significant gamma emitters are expected. All these measures are designed to protect operators and maintenance personnel from gamma emitting sources. Access to the interior of these cells is strictly limited.
Building Air Design
For the purposes of ventilation and radiation protection, the mill is divided into three areas (Figure 5):
A dual ventilation system has been designed with positive pressure gradients from the "clean" areas to the potentially contaminated areas of the mill. The two speed ventilation rates will control and exhaust, not only radon and radon progeny, but also chemical fumes and steam. The single pass ventilation system will provide fresh outside air to replace the air exhausted from the building and the process circuits. The single pass ventilation ensures that workers will always breathe fresh air while potentially contaminated air is exhausted to the outside.
Fig. 5. Building Air Profile
In addition to the ventilation system, area exhaust fans are mounted on the roof to exhaust any radon and radon progeny that may accumulate in the cells where the vessels are located. Process tanks are exhausted to the outdoors and maintained at a lower pressure than the ambient air in the cells to prevent radon leakage. In each cell, the process vessels and equipment are equipped with ventilation hoods and exhaust fans to control airborne releases.
In case of an upset condition, the dual speed ventilation fans can be adjusted to increase the extraction ventilation capability and control the rate of airborne radiation. In the event of an exhaust fan failure, the positive pressure produced by the general ventilation system is sufficient to maintain a reduced airflow in the proper direction through the cell openings and the process ventilation system.
MILL RISK MANAGEMENT
In addition to the control of radiation, the mill has been zoned according to the different unit operations to control other industrial hazards (Figure 6). Areas where high gamma radiation, radon and radon progeny are potential hazards have been grouped together. Areas where there is a risk of fire or danger of ingesting uranium have also been located differently. Areas representing more conventional industrial hazards, such as chemical irritants, caustic or acidic chemicals are sited around the periphery of the facilities.
Fig. 6. JEB Mill Risk Zoning
WASTE MANAGEMENT SYSTEM
Tailings Neutralization & Effluent Treatment
The waste neutralization circuit will combine the various waste streams (from counter current decantation (CCD) underflow, raffinate, process water, waters from the water treatment plant, solvent extraction, acid plant, crystallization circuit and the molybdenum eluate) neutralize and treat them before any release to the environment. Bulk neutralization was selected as the preferred process for tailings. Bulk neutralization will entail the addition of ferric sulphate in the neutralization circuit to optimize the absorption of arsenic onto ferric oxides followed by the addition of barium chloride to remove radium 226 from solution. This combination of treatment methods will ensure that the long term arsenic concentration in the pore waters will be approximately 1 mg/L and the radium 226 source term will be approximately 5 Bq/L for the combined McClean Lake, Midwest and Cigar Lake tailings. Primary treatment of the liquid effluent will occur immediately after neutralization followed by solid / liquid separation which will remove the majority of residual contaminants. Secondary treatment is then required for the final polishing prior to recycling to the process or release to the environment.
Tailings from the Midwest and Cigar Lake ores require special management due to the potentially high pore water radium levels, arsenic, gamma fields and radon emanation rates from the tailings surface. The design of the tailings preparation process therefore included a two stage thickening process in which treated solutions will be mixed with the tailings being neutralized and reformatted to paste tailings. The subaqueous paste tailings system ensures that personnel are shielded from gamma radiation. There are no airborne dust emissions and radon emanation is significantly reduced due to the shielding effect of the water cover.
The JEB tailings management facility will combine an engineered tailings from three uranium projects into a single facility. The subaqeous tailings management system operates on the principle of providing a permeability contrast between the tailings and the surround, to ensure long term containment of contaminants when operations cease (Figure 7). The tailings management system design consists of a dewatering drift or raise connected to a pump chamber. An underdrain system consisting of gradated drainage filter comprising of layers of drainage rock, filter gravel and sand will be placed at the base of the JEB pit. Tailings will be delivered to the JEB tailings management facility as a paste of about 35% solids and will be placed subaqeously from a barge to minimize segregation of the tailings, eliminate concerns of freezing and dusting and to control radiation and radon emanation from the pond.
Fig. 7. Tailings Disposal System for High Grade Ore
The "paste tailings subaqueous" system works by virtue of the contrast in permeability provided by the site specific higher permeability country rock around the JEB open pit and the consolidated tailings. During the mining of the JEB ore, perimeter dewatering wells were installed around the periphery of the JEB pit to maintain the pit in a dewatered state. These wells will be used to intercept clean ground water inflow to the JEB tailings management facility, by maintaining the surrounding acquifer at a slightly higher level than the tailings pond level to ensure consistent hydrodynamic containment throughout tailings placement. A differential head will also be maintained between the pond level and the water level in the underdrain sump to promote the consolidation of the tailings and enable consolidation waters (pore waters) to be collected and either recycled to the mill or treated in the water treatment plant to releasable water quality specifications. The permeability contrast also ensures maximum containment of contaminants in the long term by channeling clean ground water around the clay-like consolidated tailings, via the path of least resistance.
A monitoring program has been developed to provide data for operational control and to measure the long term geochemical/geotechnical performance of the tailings management facility against predictions. Short term data will be obtained from instrumentation installed during construction.
Operational control data will be collected from water level measurements from perimeter wells, tailings pond and raise water system, Eh probes for changes in water chemistry and piezometric head data from the network of piezometers located around the site. Long term monitoring will comprise of in-situ pore water samples for chemical analysis and consolidation measurements.
At decommissioning, the tailings are expected to consolidate to a maximum. Pumping from the perimeter and raise wells and treatment of contaminated water will continue until the tailings are consolidated. At that time, the wells will be shut down and a cover placed on the tailings. Final decommissioning will consist of back filling the pit completely with waste rock and overburden which will consolidate the tailings further and provide proper shielding to recreate pre-development conditions. Re-vegetation will then be implemented.
Long term monitoring will confirm that the tailings disposal is functioning according to design and predictions.
A financial assurance for decommissioning the McClean Lake site has been posted to both governments to cover the projected costs of decommissioning the facility. This amount will be revised when processing of Cigar Lake ores commences. This initiative has been taken to ensure that there will be no burden on future generations as a result of the company's activities and the environment and the people will be safe and well protected.
RADIOLOGICAL AND ENVIRONMENTAL IMPACT
Radiological studies based on time study calculations were conducted for the most exposed group of workers, the mill/tailings operators. These workers will receive radiation exposures of about 5mSv/year on average. This includes both gamma radiation and radon progeny exposures, assuming 1WLM is equivalent to 5mSv.
Potential radiological exposures to critical hypothetical receptors in the vicinity of the McClean Lake Project were estimated for the operating and decommissioned phases. These receptors were assumed to receive part of their subsistence from the project area. The most exposed receptor will receive maximum radiation exposures of 307ÁSv/year, which is about 10% of natural background radiation, during the operating phase and 3.6ÁSv/year (about 0.1% of natural background radiation) during the decommissioned phase.
The McClean Lake project has the potential to affect both surface and groundwaters by discharge of treated effluent from the mill facility and seepage from the disposal of tailings. However, the concern for surface water impact is significantly reduced by virtue of a three stage water treatment system which will treat the waters to remove radionuclides and heavy metals prior to release to the environment. Monitoring programs have been designed to ensure that the effluent meets all applicable government regulations and licenses prior to discharge.
Similarly, potential groundwater contamination is highly unlikely during the operating phase since hydrodynamic containment will be maintained at all times.
Atmospheric radon concentrations have been estimated to determine the potential risks to members of the public during the operating and post-operating phases of the project. In both instances, the incremental radon concentrations decrease rapidly with distance falling below 1Bq/m3 beyond the McClean Lake property. In comparison, the background radon levels range between 10Bq/m3 and 30Bq/m3, based on pre-operational monitoring data.
Technological innovations and engineering design have made it feasible for high grade uranium deposits such as Midwest and Cigar Lake to be mined and their ores processed in a safe and environmentally friendly manner. At the McClean Lake site, high grade uranium ores will be processed in the mill, without dilution with low grade or waste rock materials. All this is being achieved without sacrificing the high standards with respect to radiological and environmental goals. The proposed tailings management facility represents the state of the art in the industry for engineered tailings, deposition techniques and control of releases of contaminants.
This project has been designed in such a way that the risks are reduced and the impacts are minimal in the long term.