Mining at our operations in northern Saskatchewan requires the excavation of rock to access the uranium bearing ore.
This waste rock is classified as either mineralized or non-mineralized. Waste rock generated during underground mining is temporarily stored underground before being moved to surface for storage.
At our mill sites in northern Saskatchewan, we also produce tailings comprised of: water, residues (the residual rock left after the uranium is recovered from the ore), mineral precipitates, sewage and minor amounts of other processing reagents. The recovery rates for uranium are very high; however, a small percentage of the uranium remains in the tailings. The primary mineral precipitate in the tailings is gypsum, which is generated in the milling process. Tailings are an inevitable byproduct of milling ore. The annual tonnage of tailings produced is dependent on the ore grade and the production rate.
Tailings are safely stored on site within tailings management facilities that are designed to collect seepage during operations and contain tailings solids permanently. The tailings management facilities are designed and operated to minimize long-term effects. Post-decommissioning scenarios are analyzed to ensure adequacy of containment after operations and decommissioning.
Cameco manages two types of engineered tailings management facilities at its northern Saskatchewan operations: inactive above-ground tailings management facilities and active in-pit tailings management facilities.
Tailings and waste rock management is not required at Cameco’s in situ leach uranium mines in Wyoming, Nebraska and Kazakhstan, or at our refining, conversion and fuel manufacturing facilities in Ontario. This is because tailings and waste rock are not generated at these operations.
More about inactive above-ground tailings management facilities
Cameco’s inactive engineered above-ground tailings management facilities were, historically, used for tailings placement. While we no longer place tailings in these facilities, they remain an important component of the overall waste management system at both the Key Lake and Rabbit Lake operations, as they are designated for disposal of low level radioactive waste or hazardous waste that cannot be recycled.
More about in-pit tailings management facilities
Cameco pioneered the design and use of in-pit tailings management facilities. These engineered storage facilities are designed to protect the environment for thousands of years, and are recognized as a best practice in tailings management around the world. They are inherently more secure than conventional above-ground facilities since waste is stored below grade. Cameco maintains this type of facility at Key Lake and Rabbit Lake.
During operations, dewatering systems are used to draw groundwater toward the in-pit tailings facilities, which ensures contaminants do not move into the surrounding environment. The water collected by these systems is treated to remove contaminants and then released to the environment. After operations have ceased, groundwater moves through a higher permeability zone that surrounds the lower permeability tailings. This simple system ensures that groundwater flow through the tailings is limited, thus minimizing the migration of contaminants in the tailings.
Challenges
Management of waste rock – Waste rock comes in three general types, which must be managed appropriately:
- mineralized waste rock: U3O8 concentration of greater than 0.03%
- non-mineralized waste rock: U3O8 concentration less than 0.03%, and is categorized as either “clean” or potentially acid-generating, based on the likelihood of acidic seepage
More about waste rock generation
The amount of waste rock we generate depends on several factors, including: the depth, shape and rock quality of the ore deposit, the mining method, and the thickness of any mineralized halo around the ore.
Each of these waste rock types presents challenges for storage and decommissioning. Mineralized waste rock exposed to rain and melting snow is known to produce seepage and runoff with elevated levels of uranium and potentially other metals. Potentially acid-generating rock is expected to generate an acidic solution that will result in dissolution of some minerals in the rock, releasing various metals into the seepage. In some cases, clean waste rock can also generate seepage with somewhat elevated levels of metals, but the main challenges are in relation to the sheer mass of material and the difficulty in creating a stable landform and vegetation cover on material that may be erodible or infertile.
Maintaining tailings capacity – The on-going viability of any mining and milling operation is highly dependent on the available capacity for disposal of tailings. The regulatory approval and licensing of tailings facilities is a lengthy process that may require several years to progress from initial planning to final approval. If this process is not started in time, it may not be complete before the available storage space has been filled, resulting in a shut-down of operations.
Even if adequate capacity is approved, other factors can lead to premature filling or shut-down of a tailings facility. For example, if the density of the tailings is lower than expected, the facility will fill up too quickly, or if the geotechnical stability of the structure is called into question, capacity may be reduced or eliminated.
Optimizing tailings properties – Understanding and managing the geotechnical (or physical) and geochemical properties of tailings is a key issue.
Geotechnical properties that can affect tailings management include the percentage of fine particles, particle size distribution, and slurry density. For example, the percentage of fine particles affects things like consolidation rate, hydraulic conductivity and final density of the settled tailings. A high proportion of fine particles will result in lower density and longer consolidation times, as well as a lower hydraulic conductivity.
The geochemical properties of tailings are important in determining the environmental load generated by the facility. The tailings geochemistry is dependent on the ore properties, the milling process and the final processing of the process waste streams. Ideally, the tailings slurry will have low concentrations of constituents of particular concern, which will remain low or even reduce as the system ages.
Maintaining hydraulic containment – One of the consequences of using in-pit style facilities is that groundwater must be managed in order to prevent the facility from filling with water, and to prevent migration of contaminants away from the facility.
The tailings facilities are equipped with dewatering systems that remove enough water from the facility to ensure that groundwater flows toward the facility on all sides, ensuring that no escape of contaminants is possible while the pumping system is operational.
Prevention of freezing – The climate in northern Saskatchewan is classed as sub-arctic, with short, cool summers and long, extremely cold winters. Experience has shown that tailings discharged during winter will freeze in accumulating layers, such that by the end of a winter, there may be several metres of frozen tailings.
These frozen layers may then be buried by subsequent tailings deposition in the summer, insulating the material from the summer’s heat. Once frozen, the tailings do not consolidate, resulting in loss of disposal space. The frozen tailings will thaw naturally over a period of decades, resulting in a slow release of pore water requiring treatment, meaning that final decommissioning is delayed.
Accordingly, one of the primary constraints on the design of tailings facilities in this environment is the prevention of freezing.
Managing tailings and waste rock responsibly is key to the sustainability of our operations in northern Saskatchewan.
Taking Action
Managing tailings and waste rock responsibly is key to the sustainability of our operations in northern Saskatchewan.
Waste rock management – operational management of waste rock focuses on minimizing the short-term interaction of waste rock stockpiles with the local environment, while evaluating and implementing, where appropriate, long-term reclamation and decommissioning plans. Seepage collection and treatment systems are used to collect and control any potentially contaminated runoff water, while extensive monitoring is conducted to ensure the environment is not adversely affected.
Waste rock reuse – the amount of waste rock we can reuse depends on the type and amount of rock available, as well as the projects we have ongoing. We use mineralized waste as blend material for processing high-grade ore, and we use potentially acid generating waste rock as a substitute for sand in the production of concrete for use as backfill in our mines. We also use clean waste rock as aggregate for maintaining roads, and in concrete mixtures. All of these re-use initiatives reduce the size of our waste rock piles on surface, and reduce the consumption of other natural resources for our operations. In particular, the use of potentially acid-generating and mineralized waste rock will ensure that for most locations, none of these waste types is left on surface when operations cease.
Waste rock storage – while most of Cameco’s waste rock is not acid generating, we currently store any mineralized or potentially acid generating material on lined pads. These provide a barrier between the waste rock and the ground, and employ systems to divert or collect runoff water for treatment. We monitor groundwater near the piles regularly to make sure the pads are working as designed. Some legacy piles of mineralized waste (from the 1970s) are stored on surface at Rabbit Lake; however, this material is within the groundwater containment zone of the in-pit tailings facility, so all seepage is captured and treated.
At the conclusion of operations, all potentially acid-generating waste and most mineralized waste will have been consumed. Any remaining mineralized waste will either be buried below grade or isolated with engineered covers in above-grade facilities.
Waste rock decommissioning – where possible, clean waste rock material is re-used for road construction, foundations or other purposes. However, most of this material is, or will be, decommissioned on surface. Piles with little concern related to seepage quality are typically re-contoured and revegetated to integrate them into the surrounding environment.
In some cases, clean waste rock piles exhibit unacceptable seepage and surface water quality for re-use. In these cases, a so-called “store and release” cover of local soil is typically installed over a compacted waste rock surface. The compacted waste rock surface serves to limit the rate at which water can enter the pile. The cover absorbs water (during rain and snowmelt periods), which is then released by direct evaporation or transpiration by plants. These covers protect groundwater by reducing the volume of water that percolates through the waste. Surface water is also protected, since any water that runs off the pile does not directly contact the waste, but instead, flows over the clean cover material.
Maintaining tailings capacity – Cameco has acted proactively to ensure that tailings capacity is available at the Key Lake and Rabbit Lake sites.
At Key Lake, Cameco has regulatory approval and licences in place for capacity in excess of that required for McArthur River’s currently known reserves to be milled at this site.
At Rabbit Lake, Cameco is currently pursuing an application to extend the capacity of the Rabbit Lake tailings management facility to accept tailings generated by the known mineable reserves at this site. Approval is expected prior to the need for this space.
Optimizing tailings properties – Cameco has infrastructure, processes and monitoring systems in place to ensure that tailings are optimized in terms of geotechnical and geochemical properties.
Rabbit Lake and Key Lake have slurry thickening machinery to ensure that the density of the slurry is at or above that required to minimize particle segregation. The operational performance of this equipment is monitored through the use of real-time systems and laboratory analysis of periodic samples. The particle size distribution of the as-discharged tailings is measured on a regular basis for comparison against long-term values and samples collected during drilling investigations.
The geochemical stability of tailings is largely dependent on the treatment processes applied after the uranium has been extracted. Cameco has extensive expertise in tailings treatment. Waste solutions are neutralized in a specific series of pH steps to maximize the creation of stable precipitates.
Drilling programs are conducted every five years to investigate and characterize the tailings after deposition. The information gained in these programs has helped to improve our treatment processes and provide source terms for modelling the long-term environmental impacts of the facility.
Maintaining hydraulic containment – Cameco has systems in place to ensure that hydraulic containment of groundwater is maintained at all times for both Key Lake and Rabbit Lake.
At Key Lake, dewatering wells are located around the perimeter of the tailings management facility, which pump water from the sand formation around the facility. Water is also pumped from a raise well connected to a sand drain in the bottom of the facility. These two systems ensure that groundwater is always moving toward the facility and that seepage water from the tailings is continuously collected. All of the collected water is treated or re-used before release to the local environment.
The Rabbit Lake tailings management facility is equipped with a bottom drain and raise well that captures both seepage from the tailings and local groundwater. Dewatering wells are not necessary in this case due to the particular geology of this site. Here too, all collected water is either used in the mill or treated and released.
Prevention of freezing – in northern Saskatchewan’s cold environment, the most effective method for preventing the freezing of tailings is to deposit them under a water cover. The water cover must be of sufficient depth that it will not freeze to the bottom, while also allowing space for all of the tailings to be deposited each winter.
The tailings management facility at Key lake was designed to include a deep water cover (presently about 40 metres), which eliminates any concern for freezing.
The original design of the Rabbit Lake tailings management facility resulted in a well-drained tailings surface, causing the formation of numerous frozen layers. Starting in 1999, a deep injection system was used which thawed portions of the frozen zones and reduced the amount of new tailings frozen each year. However, as the tailings deposit became progressively thicker, technical difficulties with installation of injection wells and maintaining the required pressure made it uneconomic. Beginning in 2012, a modification to the design of the tailings management facility was made, creating a pond on the tailings surface. The pond was designed to maintain a water cover depth of about two metres, enough to prevent freezing of the tailings surface. The case study below describes the process by which this concept was developed, tested and ultimately implemented.
