While South African mines have filtered tailings for many years, this water-saving practice has usually been limited to certain aspects of the material stream (such as concentrate production).
Now, the country’s water scarcity may demand its wider application.
Tailings represents a substantial repository of water on most mines, with the volume of water in conventionally thickened slurry amounting to between 0.75 and 1.10 m³ for every ton processed in a metallurgical plant. Of this amount, between 0.25 and 0.35 m³ of water is ‘locked up’ interstitially – between the particles – per ton of tailings deposited on the tailings storage facility (TSF).
According to Adriaan Meintjes, principal civil geotechnical engineer at SRK Consulting SA, this interstitially locked-up water is not usually an easy source that can be made available for other uses. However, no new mining projects or expansions can be considered if there is insufficient water available.
“Hence the growing focus on saving, by considering the full cycle of losses and gains in all processes after the extraction of the metals,” says Meintjes.
Such a focus aims to reduce the volume of that is traditionally pumped to tailings dams, as well as the losses on the tailings dam through evaporation and seepage.
“Filtered tailings is among the most significant water-saving approaches that can be considered in relation to tailings dams,” Meintjes continues.
“This can not only improve the recovery of water that is stored interstitially but – by reducing water deposition on the tailings dam – also reduces seepage and evaporation losses. Depending on the degree of filtering of the tailings, the total water losses could be decreased to as little as 0.2 to 0.3 m³ per ton of tailings.”
However, he warns that there are substantial costs involved, so these need to be well budgeted for in a mine’s viability costings – considering the full life cycle of the mining venture.
Indeed, he argues that greater investment in saving techniques is likely to be inevitable as water security becomes a strategic issue for mine sustainability.
“South Africa is a water-scarce country, so we have directed our work at mine level increasingly towards water security,” he notes.
“For this reason, our clients tend to engage us at early stages in a project, so that their tailings dam design optimises water usage.”
Dewatering tailings through filtering can also have environmental and financial benefits: it can reduce the need and the extent of the liner barrier system, for instance, as the amount of seepage possible after filtering is significantly decreased.
“Liners underneath tailings dams have become a world-wide trend, and are generally a legal requirement unless an alternative, acceptable risk mitigation plan is accepted by the authorities,” he says.
Meintjes also notes that these concerns about better conservation of water also need to be addressed in the context of new best practice principles relating to safety and the environment.
“In the past decade – and also following a series of recent very significant TSF failures around the world – mining houses, consulting engineers and TSF contractors have aligned new best practice principles for the design of tailings dams.
"The principles involve extending and expanding appropriate techniques for the design of these facilities for closure. If the TSFs are constructed and operated for closure, then reduced life cycle overall costs can be a possible project benefit.”
These techniques include laboratory and field testing – to obtain relevant suites of parameters for use in stability and seepage analyses. There are also total stress and effective stress slope stability analyses required, linked to the projected rate of rise of the TSFs and waste structures.
Specific design and analysis considerations must be applied for the full life-cycle behaviour of these structures.
“Relevant water management must be applied, which must consider the more extreme rainfall events that are related to climate change and increased rises in temperature,” Meintjes notes.
“Design and analysis techniques must also relate to the site’s geological and geographical conditions, as well as the impacts of local climate.”
SRK assists mines to consider two other key factors in tailings-related water loss: evaporation and seepage. By addressing these alongside interstitial loss, mines can considerably reduce water loss – making it more readily available for re-cycling and re-use.
The final tailings dam design solution can then either comprise thickened tailings, paste or filtered tailings in water-scarce areas.