Mine tailings storage facilities are dynamic structures designed and managed to store vast quantities of waste material from ore processing, including, in some instances, toxic chemicals and heavy metals. Management of these expansive, water-heavy structures can pose a significant risk to mining operations, as several high-profile disasters have demonstrated. One of the worst was in 2019, when the Brumadinho tailings dam breached at the Córrego do Feijão iron ore mine in Brazil killing 270 people.
This tragic event prompted the development of the Global Industry Standard on Tailings Management (GISTM), published in 2020. Under the standard, convened by the United Nations, the International Council on Mining and Metals (ICMM) and the Principles for Responsible Investment (PRI), among several other requirements, miners must consider alternative technologies to conventional mine tailings management.
Dewatered Tailings
Dewatered tailings is one alternative. In fact, it is one of the most effective tailings strategies for risk reduction available, said Pepe Moreno, a corporate consultant and head of tailings at SRK Consulting, an international mining consultancy.
"The risk associated with tailings is related to the amount of water in the facility: the more there is, the larger the potential consequence could be as the water in the tailings makes waste material flow to mobilise larger volumes and reach further distances," he explained. "In comparison, failure involving a drier mass will not likely have tailings flowing and rather slump, reducing the severity of the consequence of failure."
Dewatered tailings entails extracting water from the mine waste – that would otherwise be pumped into the tailings storage facility (TSF) – to form a paste or bulk cake-like consistency, depending on the dewatering degree. How much water can be removed depends on the type of tailings, but in a classic dewatered setup volumes are expected to be reduced by up to 30-40%, Moreno said.
Dewatering tailings is not new and has been around for a few decades, and only considered by selected miners in areas of water scarcity, such as Chile, to minimise water costs. Only recently has this technology been increasingly incorporated as a risk reduction measure, Moreno explained.
Otherwise, choosing dewatered options has been limited due to cost. "Dewatering solutions require upfront capital for thickening/filtering plants and transporting tailings, it also involves increased energy consumption – the higher the dewatering degree the more energy is required to transport tailings. This means that there should be a strong differentiator to encourage the extra cost of this process," said Moreno, who has over 30 years of experience in mine waste management and specialises in tailings engineering.
Majors are considering dewatered solution for their TSFs
Along with better resource management, today's miners have another reason to invest in technology; increasing pressure from investors who demand less risk in mining operations.
Nowadays, Moreno said, a large proportion of the mining companies and more than 60% of the mining industry's market capital worldwide, are adhering to the GISTM standard.
Many of these are ICMM signatories and so are required to, but other companies are doing so voluntarily.
"These companies want their investors to feel confident their portfolios remain at acceptable risk levels, as endorsed by major pension fund bodies such as the Council on Ethics and Church of England through their convened ‘The Investor Mining and Tailings Initiative'" he explained.
Under GISTM, companies must demonstrate that their final tailings facility choice was not based on cost alone, but also on reducing overall risk, efficient use of resources, and benefits to, and impact on, communities and the environment.
"Trade-off analysis of alternatives in the past typically consisted of modifying the geometry of the same conventional tailings storage facilities or incorporating different embankment construction techniques only, now GISTM indicates alternative technologies need to be explored," Moreno explained.
In this regard, Moreno is working with several major mining companies to assist them in their evaluation of alternative tailings technologies to minimise risk throughout the mining cycle.
"Changing mindsets to adopt new technology is not an easy decision but one that major mining companies are seriously considering despite having their other facilities using conventional methods that have proved efficient through time; it's refreshing and shows industry leadership," he said.
Determining suitability
To determine whether dewatered tailings is suitable for a mine site the operator must conduct a multiple criteria analysis (MCA) at the very early stages of the project. This process creates an auditable trail of how the final solution was decided upon. As Moreno explained, it involves studies of alternatives based on different categories, eg, constructability, risk profile, stability, cost, etc. Fair weighting assigned to each category is crucial to ensure a suitable scoring matrix is obtained. For greenfield projects some critical information is not usually available, therefore expertise is often required to make reasonable assumptions on key design parameters, and the MCA should be revisited and confirmed at later stages of the project.
Different regions will have different suitability for dewatered tailings, however, Moreno said. Most parts of Australia, for example, have ideal conditions due to their semi-arid environments.
The other benefit of dewatered tailings in an arid environment like Australia is recycling the water. Arid environments typically have fewer natural water sources to draw from, supporting the economics of recycling water which is typically more expensive than taking from nearby rivers or other such sources. This provides a cost advantage compared to transporting water from afar. However, water recycling is harder in a tropical environment where there will be surplus water that will need to be treated.
Miners should also consider that some commodities are easier to dewater than others, and the degree of dewatering will vary, Moreno said.
"Therefore, the reduction in risk needs to be enough to justify the extra capital cost. And larger mines will need several modules which increases the operating costs. However, investors are nowadays more open to investing when they know capital is well spent on reducing risk," he added.
Falling costs
But the good news is that dewatered tailings technology is significantly improving, making it easier and more cost-efficient for bigger operations to embrace. Previously, tailings tonnages that could be dewatered were constrained. With improved filter press designs, reduced costs of consumables and maintenance, increased reliability of the process and more energy-efficient systems, overall value through risk reduction can be unlocked, Moreno said. In addition, modules are getting bigger meaning larger mines in particular need fewer of them and can process the same amount of tailings using the same amount of energy, reducing overall operating costs.
"The technology is really evolving. I have been an advocate for dewatered tailings for quite some time and in Australia I see miners shifting to considering and choosing novel tailings management strategies over conventional TSF. It's given confidence to investors that miners are doing the right thing by reducing risk from the start," he said.
Nevertheless, Moreno cautioned the technology is not a silver bullet and needs to be matched with recursive good management.
"Dewatered tailings doesn't completely solve the risk problem – it's a way to set up for success, but also needs appropriate management, governance, monitoring and control."
But its increasing adoption and consideration by the industry shows investors and wider stakeholders that mining companies are serious about managing and mitigating the risks associated with mine tailings facilities, he added before concluding: "This shift is being driven by many factors, and while it's gradual, it's definitely moving in the right direction."
However, a cautionary note, TSFs don't fail because the design was conventional, or because dewatering wasn't considered. TSFs mainly fail due to weak governance, which leads to poor decision-making. Investment in strong governance and leadership is as – or more – important, and GISTM strongly delves into it too.
