Inpit Tailings Storage Facilities

In-pit tailings deposition has grown in popularity for mature mining projects, new projects being developed and potential projects being studied. In-pit tailings storage facilities (IPTSFs) maximise the use of resources and offer many economic and environmental benefits. This technique is not suited to all mining sites. For sites with abandoned open pits it requires careful consideration of the geochemical characteristics of the pit, the tailings or residue, the local groundwater quality and the current and future potential end users of that groundwater. For projects with active open pits adjacent to the proposed IPTSF careful consideration of the geotechnical aspects are required given the risks involved. Within the right setting this technique does enable the available resources to be optimised to achieve the desired post closure outcomes of safe, stable structures with minimal environmental impact.

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Integrated Waste Landforms

The development of new tailings storage facilities (TSF) for mining projects, both new and existing projects, requires innovative design approaches to optimise the available resources to achieve the desired outcomes of safe stable structures with minimal environmental impact. The use of integrated waste landforms and in-pit tailings storage facilities maximises the use of resources and offers many economic and environmental benefits in achieving the desired outcomes. From a tailings management perspective the IWL can be simply defined as a tailings storage facility (TSF) that is located inside the waste rock storage. It is formed by placing controlled, compacted, earthworks against the mine waste to form a containment embankment to retain the tailings. Mine waste is placed around the outer edge of this compacted containment embankment such that a void, or storage area, is formed within the waste rock storage. This void allows for further controlled, compacted, earthworks to be placed around the circumference of the void to form a perimeter containment boundary between the tailings and the mine waste.

One of the advantages of this structure is that continued staged construction is in the downstream direction, that is, each lift is laid back against the surrounding mine waste rock storage and is placed over previously placed mine waste. Construction of future embankment lifts therefore does not rely on the strength of the deposited tailings and, as such, the rate of rise and strength of the tailings does not impact on construction considerations.

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Dry Stacked Tailings Studies

Dry stacking is the terminology which describes the placement of tailings which has been thickened to a slurry density of 65% - 70% solids by weight before being filtered/screened to 80% - 90% solids by weight to further reduce the volume of liquor prior to transportation and placement of the tailings. The tailings are transported by conveyor or truck from the filter/screening plant and then placed, spread and compacted to form an unsaturated, dense and stable tailings ‘stack’. Dry stacking of tailings has been adopted where:

• Water conservation and reuse is vital, for example, Jabil Sayid in Saudi Arabia, Skorpion Zinc Mine in Namibia; La Copia Mine in Chile, Alamo Dorado in Mexico.
• Seismicity of the site is such that the tailings need to be dewatered as much as possible to maintain stability of the facility, for example La Copia, El Peñon and Can Can Mines in Chile.
• It is environmentally advantageous to do so, for example in cold climates (arctic and sub-arctic) where freezing of the tailings slurry and supernatant water in the water pond make the components of the slurry (solids and liquids) very difficult to handle. Examples are the Greens Creek and Pogo Mines in Alaska and Raglan Mine in Quebec.
• Improved stability and storage capacity where required, for example Cupias Tailings Dam near Tayoltita in Durango State, Western Mexico; and La Herradura near Caborca Sonora, northwestern Mexico, El Indio in Chile.
• Improved handling is required such that the tailings can be mixed with mine waste.
• There is limited land for storage and the tailings have to be mixed with mine waste.

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