Tailings
Tailings consist of ground up rock and process effluents that are generated during the processing of ore. It is what is left over after the metals or minerals of economic interest have been extracted from the ore.
But tailings can present a significant risk if improperly stored, as the fine-grained tailings material can be prone to erosion or distributing the tailings over a broad area. For this reason, we treat all our tailings with care.
How we deal with tailings
The process of product extraction is not 100 per cent efficient - the ore may contain other metals or minerals that pose environmental concerns. It is not possible to recover all of the reagents and chemicals used in ore processing, but careful management of tailings is the first step in reducing the potential risk.
We usually store tailings in specially designed and engineered repositories. These structures are designed to be geotechnically stable during operation and after closure. They must also be able to withstand extreme events, such as floods and earthquakes.
Chemically reactive tailings are stored in repositories that allow contact water to be captured and recycled, or treated for discharge at designated outfall points. When a mine is closed, we revegetate tailings impoundments to help control dust, reduce erosion and re-establish ecosystems.
Where possible, we reuse our tailings to backfill open pits and underground workings. This minimises the footprint of land we disturb by mining and, in some cases, reduces the amount of material that we have to buy in or mine specifically for this purposes.
The geotechnical stability and design of tailings storage facilities is checked every two years by an external expert. We also lead an internal Strategic Technical Risk Review Program to confirm that all geotechnical hazards associated with tailings storage facilities are identified and addressed. We also conduct internal acid rock drainage risk reviews at tailings storage facilities that may pose a geochemical risk.
Our results
An estimated 150 million tonnes of processing mineral waste (predominantly tailings) was generated in 2007. This represents an increase of 2.9 percent from 2006 when we produced 146 million tonnes. Four per cent of our processing mineral waste was used in pit or underground backfill, and 78 per cent was placed in specially designed and engineered repositories. About 12 per cent of our inert mineral waste was disposed of safely at the bottom of a lake.
57 percent of the processing mineral waste that we generate is considered to be geochemically reactive. Ninety nine per cent of this chemically reactive waste is either used for in-pit disposal or is stored in engineered repositories.