At the start of the design process, the design problem and solution have to be conceptualised to allow for the design objectives for a tailings facility to be formulated. A range of techniques can then be used and applied to undertake the design, including appropriate analyses, to ensure water saving at mines, says consulting engineers and scientists SRK Consulting South Africa principal civil geotechnical engineer Adriaan Meintjes.
Meintjes adds that, in the past decade and following a series of significant, recent tailings storage facility (TSF) failures worldwide, mining houses, consulting engineers and TSF contractors have refocused on new best-practice principles for the design of tailings dams.
These incorporate extending and expanding appropriate techniques for the design of these facilities and include laboratory and field testing to obtain relevant suites of parameters for use in stability and seepage analyses, as well as stress- and deformation-related suites of geotechnical parameters where relevant.
Also included are total stress and effective stress slope stability analyses linked to the projected rate of the rise of the TSFs and waste structures, seismic and dynamic stability analyses, static liquefaction analyses and, where relevant, progressive failure analyses. Specific design and analysis considerations for the full life-cycle behaviour of these structures should also be taken into account.
Additional techniques embrace relevant water management, which must also consider extreme rainfall associated with climate change and rising temperatures, as well as design and analysis techniques related to the site’s geological and geographical conditions, and the impacts of local climate.
Meintjes notes that all of these techniques have to be aligned with addressing the relevant environmental and water-related legislation. For example, filtered tailings is among the water-saving approaches that can be considered in relation to tailings dams, which can substantially improve the recovery of water that is stored interstitially.
“The technology has been used for many years in South Africa, although mainly on limited elements of the material stream, such as the final concentrate. There are substantial costs involved, so these need to be well budgeted for in viability costings,” he says.
Meintjes adds that barriers/liners underneath tailings dams have become a worldwide trend, and are generally a legal requirement, unless an alternative, acceptable risk mitigation plan is accepted by the authorities.
An added advantage of filtered tailings may be that it can obviate the need for a liner, as the amount of seepage possible after filtering is insignificant.
Water security and tenure, similar to access to electricity, are essential elements of any current and future mining project. As no new mining projects or expansions can be considered without sufficient water available, there is a growing focus on saving water that is traditionally pumped to tailings dams,” says Meintjes.
“Considering that all people in South Africa have a right to water, the consequence is that mines have started to consider the issue of water security at the very early stages in their new project conceptualisation and design, as well as for their current operations.”
There are three main factors in tailings-related water losses: interstitial storage, evaporation and seepage.
For current tailings operations, as it can be appreciated, it is difficult to materially change interstitial storage and seepage. Decant water on and from the TSF can more optimally be stored to achieve some additional water savings. Under these conditions, a mine is doing well to save an additional 0.05 m3 to 0.1 m3 of water for each tonne of tailings processed.
For new TSFs, bigger water savings can potentially be affected. If a suitable seepage barrier is provided (also considering site geology) potential savings of up to 0.15 m3 of water per ton of tailings processed can be achieved. In the water scarce parts of South Africa, if filtered tailings can be used, it may be possible to save an additional 0.15 m3 of water on improved interstitial water storage and evaporation behaviour.
By addressing each of these, mines can reduce water loss, making it more readily available for recycling and reuse. The final new tailings dam design solution can then comprise either thickened tailings, paste or filtered tailings in combination with a suitable seepage barrier, in water-scarce areas, Meintjes concludes.