Being proactive on tailings when deciding how to leach/process

Improved planning at the early stages of mine design is needed for safe, effective tailings disposal, but essential tools to do this are currently lacking. Many environmentally adverse mining legacy issues can be consequences of sub-optimal choices, committing to processing and tailings disposal routes at the “feasibility study” stage that limit tailings management options. For example, the environmental impact and risk associated with acid mine drainage from sulfide minerals in tailings could be substantially reduced by adopting a processing route that removes or eliminates the sulfide minerals. Robust analytical and numerical tools to provide reliable assessment of the physical and chemical risk of tailings at the feasibility stage do not currently exist, so any environmental benefit is overlooked against processing risks or higher initial costs in adopting different or new technologies.

Mineral processing innovation can take ≥15 years for adoption (Breytenbach, 2017). Such innovation is typically assessed in the feasibility stage against current industry practice, but rarely considering downstream and environmental impacts. Tailings management (disposal options, rehabilitation and legacy) is usually only considered later at the “bankable feasibility study” stage, by which time the mineral processing route and tailings disposal methods are already locked in.

Sustainable development frameworks/tools exist to consider risks associated with tailings across the life of a mining project, including closure and beyond, such as SUSOP® (Corder et al., 2012). However, the science and tools required at the feasibility study stage are lacking to evaluate and understand the risks for different processing and tailings disposal options. Its rare such studies properly characterise the short-term thickening response for tailings – complex environmental assessment of tailings disposal options and rehabilitation strategies can take significant time (months to years) and effort, not fitting the short timeframe of feasibility studies.

Be proactive on tailings, not reactive

While there are limited cases where reprocessing of tailings can lead to products, in the majority of applications the tailings are a waste that must be dealt with. Depending on the hydrometallurgical or processing route used, tailings can often have less-than-desirable properties, e.g. high pH, residual levels of toxic lixiviants. There are many options to treat tailings to change such properties, but large volumetric tailings flows can make these options costly. However, what if the properties can be changed at the time the tailings are being generated?

An example is in considering alternative lixiviants to cyanide for gold recovery. Leaching with cyanide requires the pH to be maintained at >10, with a high pH (and residual cyanide) persisting through to tailings. Alternatives such as thiosulfate can be viable (Sitando et al., 2018) and offer the opportunity to leach at a neutral pH, providing a large shift in tailings surface chemistry not readily achieved in cyanide-based systems. The sustainable development implications of alternatives as commercialised in CSIRO’s Going for Gold technology should be significant, but even the SUSOP® approach will struggle to properly capture this without access to appropriate characterisation of how the physical behaviour of the tailings (with and without polymer treatment) would change. Work within CSIRO Mineral Resources is now seeking to take advantage of its leading expertise in quantifying flocculation, thickening and dewatering responses in detail to demonstrate the potential downstream impacts of using the new technology.

Environmental and infrastructure risks are increasing pressure on the mining industry to move away from wet tailings disposal to “dry” paste or stacked tailings (dry in this sense means solids are wet but contain insufficient moisture to flow, such that they can be deposited in a pile or down a slope), or even to filtered tailings. The residual environmental risk and legacy of these different disposal options can be quite different, and can also change substantially in response to upstream implementation of processing innovations. Therefore there is a significant need for reliable tailings management decision-making tools and methodologies and quick access to relevant data for assessing processing innovations, particularly at the feasibility study stage of a mining project.

References

• Breytenbach, M., 2017, “Despite glorious past, mineral processing innovation faces uncertain future”, Mining Weekly, online 26th May 2017;
• Corder, G., McLellan, B., Bangerter, P., Van Beers, D., Green, S.,2012. Engineering-in sustainability through the application of SUSOP®, Chemical Engineering Research and Design 90, 98-109.
• Sitando, O., Senanayake, G., Dai, X., Nikoloski, A., Breuer P.,2018. A review of factors affecting gold leaching in non-ammoniacal thiosulfate solutions including degradation and in-situ generation of thiosulfate. Hydrometallurgy 178, 151-175.