Tackling the tailings challenge
Taking risk in a tailings facility is counter intuitive to the industry trend of safety at all costs in their operations. Strategies that build stability into to placement of the waste are inherently better than strategies that rely on containment of wastes and deferment of rehabilitation.
Estimating low probability of failure and using discounted rates for closure can seem like a good financial strategy but this does not account for the resiliency of a company to survive a multibillion dollar cleanup. Risk management of tailings facilities is a complex challenging issue. Plans change and older facilities can face challenges unforeseen by the original designers. Population centres tend to build up around mine sites that were never there when planning began.
Ore bodies are discovered and new technologies substantially extend the mine life which may require substantial raising the walls of tailings facilities dams. The best solutions are stable by design and future proof and tailings strategies should be living documents adapting to realised performance, changing technologies and circumstance.
Recent developments in tailings filtration have substantially increased unit capacities making tailings filtration plants in the 50 to 100kt/d feasible for future plants requiring dry stacking of their tailings.
At Outotec our mission is to take a holistic approach to every project with a design philosophy to provide the best possible solution and service support in terms of technology and system design.
Efficient solid-liquid separation
Sedimentation or thickening is the most cost effective option for recovering and recycling plant water. The performance of any equipment is based on the physical properties of the slurry being thickened but the degree of separation is a choice of the designer.
Innovations in rake design and tank floor slope make it possible to thicken slurries to a high yield stress paste limited only by the ability to pump the material. Developments in paste thickener technology have pushed the yield stress envelope approaching the point where pumping is no longer possible.
Beyond sedimentation filtration is required to recover more water and increase the solid content of tailings streams. This is evident in paste backfill applications where an extreme paste is required and part of the stream is filtered and then recombined with thickener underflow to produce a high yield engineered paste.
Pressure filters have developed in scale and reliability, while being limited by the chamber volume and batch operation pressure filters have one big advantage for tailings filtration and that is size.
No other filtration technology can be built on such a scale. When the limitations of the technology are well understood they and the plants they are installed in can be adapted to the needs of the mining industry delivering the possibility to handle 100kt/d of tailings for dry stack disposal. With tailings the sheer quantity of material to be treated will be the challenge and if filtration is to be used in tailings there will have to be very large filtration units, with high reliability. While large pressure filters are possible they are by no means the solution for all tailings applications.
Choices for tailings disposal
Solutions from conventional tailings disposal through paste thickening and beach disposal to dry stack are all possible and a holistic view of each sites requirement are needed to define the best solution. In areas of high rainfall a dam may still be required to capture runoff and minimal water recovery is required. Paste deposition and the high beach angles that it can provide represents an excellent option when water is scarce and the TSF area should be minimised. The principal driver in any tailings management system is the geography of the site and the area available for safe tailings storage. Only by understanding the tailings material properties and the site requirements from establishment through to closure can the dewatering system requirements be defined.
High Yield stress Paste surface disposal systems (utilizing positive displacement pumps) are in operation. From a point of practicality some sites are shifting from these plastic pastes towards thickening solutions that optimise the total thickening solution incorporating pumping and operability in the system definition to achieve practical systems. These systems limit the sheared yield stress to utilise centrifugal pumping systems at the expense of beach angles.
In terms of disposal area dry disposal represents the smallest footprint and highest soda recovery method. Due to the height of stacking possible the disposal could be reduced to 1/8th of that required for conventional disposal methods. Dry stacked sites can be re-habilitated sequentially.
Total cost of tailings
By shifting to a dry filtered disposal strategy significant cost advantages in closure and rehabilitation can be realised. To find the best holistic Solution Outotec can look at the water recovery and the technologies required for recovering that water. We can simulate the deposition strategies required for the life of mine storage and then map those costs against the operating and capital costs of the different deposition strategies to give the mine owner the information decide which technology is best for their needs.
Conventional tailings disposal is considered in many different disciplines and often each discipline solves the design with the generic information given rather than interacting with all aspect of the tailings disposal to create the best overall solution. By having all of the disciplines working together Outotec can offer significant improvements in the overall tailings system. The underflow density can then be evaluated for the properties that define pumping and thickening. Yield stress in the sheared and unsheared form has significant impacts on slurry pumping and may have huge effect on CAPEX if we step from centrifugal pumping to positive displacement. This step change in CAPEX can feed back into the facility design taking a holistic view of the tailings system can provide significant benefits’ in terms of tailings facility capacity, water quality, pumping costs and indeed the reliability of the pumping system.
In the not too distant future tailings legislation will have a significant impact on the way mining operations plan and operate their tailings storage facilities. To ensure profitable production of metals miners will have to develop strategies for tailings handling that satisfy both the local communities and shareholders. Technologies already exist to achieve these objectives and the best outcomes will be achieved by considering the life of mine costs and project risks to find the optimum tailings solution.