Tailings are the left-over materials from the processing of mined ore. They consist of ground rock, unrecoverable and uneconomic metals, chemicals, organic matter and effluent from the process used to extract the desired products from the ore.
Tailings generally leave the mine processing plant in a slurry form (diluted with water) and are often but not always stored on the surface in storage facilities called tailings dams, or tailings storage facilities (TSFs).
How tailings dams differ from conventional water dams
The principal difference between tailings dams and conventional water dams is that tailings dams are dynamic structures that grow over the life of the mine to accommodate increased tailings. Tailings dams, and particularly ‘upstream’ dams (see types of dams below), often rely on the strength of hydraulically placed tailings that are variable in strength due to mine processing changes, operational practices on deposition and climate variations.
Tailings dams are usually constructed from earth and rock waste materials from the mine with variable design life (continuously constructed) whereas water dams can be constructed from earth, rock, concrete and other materials and are typically erected in a single phase of works.
Life cycle of tailings dams
Tailings dams evolve over their life cycle, from planning, design, construction, operation, closure and post closure. There may also be periods of care and maintenance between operational phases.
The duration of the closure phase in a tailings dam’s life cycle can often exceed the duration of the operational phase. It may include transition from operations to active care such as ongoing water, geochemical and physical management to maintain integrity.
Over time, inactive TSFs may transition to passive care when the ongoing water, geochemical, and physical management requirements are reduced or eliminated.
Types of tailings storage facilities construction
There are three broad types of dam construction which are depicted below. The choice of design is based on factors such as dam location, geology, seismicity, climatic conditions, construction materials and the nature of the tailings.
Key factors that may influence tailings dam integrity
Maintaining dam integrity requires ongoing focus on appropriate engineering design, quality construction, operating discipline and effective governance with independent reviews. Key internal and external factors that influence dam integrity include: assessment of site conditions (such as seismicity, climate, geology, hydrology, tailings characteristics); the quality of assurance and quality control of dam construction; and ongoing dam operating discipline management including tailings characteristics, water management, monitoring and consideration of changes to any of these factors.
For more information on how BHP approaches management and governance of our TSFs, please see Tailings storage facilities management: Governance, our approach, risk management and key targets.