Safer tailings with better margins
by Matt Pyle MAusIMM, Senior Process Engineer, Ausenco
Improving the storage of fine waste from mineral processing plants is one of the biggest challenges for the mining industry.
The current practice of wet storage in tailings dams is an environmental concern, consumes large amounts of water, impacts communities, and is expensive with long-term management costs and liabilities. And – if wet storage tailings dams fail – they can lead to widespread environmental and social disaster, loss of reputation and massive regulatory fines.
The problem really boils down to the water content. Fine tailings tend to retain water and are difficult to dewater below saturation. Saturated fines are geotechnically unstable. Remove water from tailings, therefore, and they become much more stable from a structural perspective, but sulfides and reactive species are prone to oxidation.
The challenge is that dewatering and stacking of tailings is not friendly to project economics. Traditional approaches to tailings filtration require more project capital to develop and operate. Only those projects with very high grade ore bodies (and therefore higher margins per tonne of ore) can afford this route and hence industry uptake is limited.
However, recent developments in dewatering and preconcentration can enable alternative approaches which, when considered across the whole value chain, can actually generate revenue whilst enabling dry stack tailings economics to compete with traditional wet tailings storage. This allows leading mining companies to increase production rates, reduce capital and unit operating costs, and simultaneously minimise the consumption of energy and water, and the environmental impact of their tailings activity.
Why is it so difficult?
As an industry, we can be short-sighted at times. “Mine-to-mill” concepts and understanding is taught in university, yet “mine-to-tailings” does not exist as an industry philosophy. Our industry is often so focused on the detail that we miss the big picture. And as we develop technically as specialists, many forget the value of being a generalist and working across fields of different expertise. The result? Specialists and experts can be guilty of operating in silos, without sufficient understanding of the big picture issues and thus they overlook opportunities and the benefits of a holistic approach. As organisations grow and the potential to impact increases, hierarchies can form, eroding communication and the exchange of ideas and inhibiting progress.
And yet, the answers are right in front of us. We just need to take a step back.
The solutions
Pre-concentration is a key part of the solution, and there are a number of technologies that can be applied in the pre-concentration process. Examples include shovel sensing, bulk ore assaying and ore tracking sensors – all help ensure that sub-economic material is removed between the mine and the mill, and therefore not milled in the first place, effectively reducing tailings management requirements.
In the process plant, screening, bulk sorting and SAG mill pebble rejection approaches reject coarser waste earlier in the process. These technologies are not new, and in some cases, have been used for decades. For example, at Bougainville Copper, coarse and low grade waste was screened off at high throughputs in the 80s. Ravenswood Gold, operating today, recently upgraded its crushing circuit and employs a similar approach. Mt Isa applied dense media separation for over 40 years to reject low grade lead and zinc material. Bulk sorting is in operation at Anglo American’s El Soldado and Mogalakwena mines, and SAG mill pebbles are rejected at countless sites. At Newcrest’s Telfer operation, pebbles are particle-sorted and rejected.
In the process plant, a key advancement is the use of Coarse Particle Flotation (CPF) technologies. Whereas typical milling processes tend to grind material to a particle size of somewhere between 75 µm to 250 µm, CPF technology can enable grind size to be coarsened to around 300 µm to 400 µm, without compromising recovery. CPF is tested and demonstrated. In fact, back in 2018, we designed the first CPF circuit to be used in a hard rock application at Newcrest Gold’s Cadia mine site. Since then, CPF circuits have been installed at Kennecott, El Soldado, Mogalakwena and two more trains at Cadia, with another under construction at Quellaveco.
How does this lead to safer tailings?
Put all of these technologies and approaches together, and the quantity of fine tailings can be reduced by 65per cent or more. By producing, dewatering and blending coarser wastes, and co-mingling dry coarse material with wetter fine material in the right ratios, the geotechnical stability of the tailings waste can be massively increased. This allows for higher stacking heights and more robust stacking system operation. It also relaxes the moisture requirements for the fine tailings, thereby making more traditional dewatering approaches (like centrifuging, terraflowing or filtering) more effective.
But is it economical?
The objective of the mine-to-tailings value chain is to improve the overall business case by rejecting lower value or waste streams at the right time, to the right extent and with minimum effort. We can improve the economics of the operation by de-bottlenecking the mill constraint and enabling cost-effective dry tailings with reduced financial, reputational and environmental risks (and associated mitigation costs).
Energy savings of up to 30 per cent for every tonne milled are possible. And water consumption per tonne milled can be greatly reduced, providing further savings for some projects in arid regions. For projects with tailings constraints, the ability to stack and store dryer tailings can extend the mine life substantially.
What’s slowing adoption?
The "standard" approach has been the lowest cost. Cost structures (energy, water, GHG emissions, closure) are changing and these changes will lead to revision of the "standard" based on the lowest cost outcome.
Once the cost structures change or improve, enabled by technology and capability, human factors such as awareness, understanding and risk management become the rate-limiting drivers.
Conclusion
There is significant opportunity to improve the way tailings are planned, managed and maintained. I believe it can be done in a way that increases production rates, and reduces capital and unit operating costs, while simultaneously minimizing the energy, water and environmental impact of tailings activity.
Contact the author
If you’re interested in finding out more, please get in touch with Matt Pyle via the contact form on the Ausenco website here