By Jacques Wiertz, Collaborating consultant SMI-ICE-Chile and Adjunct Professor Universidad de Chile
Tailings are the most challenging massive mine waste on the road for environmentally and socially sustainable mining. The tragical failures we have witnessed in recent years in different parts of the world have focused attentions on the need to better guarantee people’s safety. Safety is and must continue to be the central axis of tailings handling and management. This translates into the adoption of an international standards for tailings management (Global Tailings Standard), promoted by a consortium made up by the International Council on Mining and Metals (ICMM), the United Nations Environment Programme (PNUMA – UNEP) and the NGO Principles for Responsible Investment (PRI). The standard, currently being implemented by various mining companies operating in Chile, contemplates a new governance model for tailings dams and series of measures to prevent failures, as well as improve the responsible management of waste. The document specifically emphasizes management transparency and the respect and participation of potentially affected communities.
The primary problems associated with tailings are due to their intrinsic characteristics. Tailings are formed by very fine mineral particles presented as pulp, containing great quantities of water. Their physical instability; the great consumption or loss of water it represents; the air pollution created by fine air-borne fine particles; or the water contamination by the migration of solutions from the deposits, are some of the main issues associated to the characteristics of tailings.
The challenge of sustainable tailings management is triple. Indeed, it’s necessary to manage and handle the tailings of yesterday, the tailings of today and the tailings of tomorrow.
Greater water recovery from tailings complies with 3 complementary objectives. First, allows for greater water recirculation and reduction of freshwater consumption. On the other hand, ensures greater physical and chemical stability of the deposits. However, it also prolongs the life cycle of the deposits by decreasing deposited pulp volumes. This calls for the development of more efficient solid/liquid separation technologies for separation and for energy consumption, which are applicable in a large scale.
Another opportunity is the possible recovery of valuable elements or compounds from tailings, or from the processing plants where these are generated. Using the high degree of mineral particle liberation, the possibility to recuperate currently undervalued elements or compounds of interest must be evaluated. For this we need, on the one hand, to characterise tailings in a very detailed manner and, on the other, to develop efficient, low cost and low impact, recovery technologies.
Through a similar strategy, the possibility of eliminating potentially contaminating elements and compounds from tailings must be studied. Removing minerals such as pyrite reduces the potential generation of acid drainage and allows for the removal of other potential contaminants, like heavy metals. However, this requires greater integration between the plant and the management of tailings, as well as the study of alternatives for the removed materials.
Both the recovery of valuable elements and compounds and the elimination of potential contaminants apply for past tailings dams as well. Under the safety central axis, priority must be set for tailings dams that present the greatest risks. Recovery of valuable elements and compounds would eventually allow to absorb part of the reprocessing costs and reordering tailings in safer environmental conditions. The remediation of tailings dams that are out of commission –or abandoned– should not only consider its possible reprocessing. New alternatives for a sustainable closure and rehabilitation of tailings dams must be studied, for past deposits as well as for those in operation, which sooner or later will have to close.
A CIRCULAR ECONOMY OUTLOOK
The above constitutes a circular economy outlook applied to tailings management. This requires greater integration between processes and the stages of a mining project. It must be addressed starting with process design and should be based on a deep and complete characterisation of minerals, which in turn allows for the identification of associated opportunities and risks of tailings. It is also important to consider mine closure stages and the contribution of a correct and early tailings management process could have in their potential transformation during the restoration of mine sites.
Circular economy outlook applied to tailings management
For the tailings of the future, the scenario is even more challenging. The growing rejection towards tailings deposits will make the approval of new deposits increasingly more difficult, both to obtain environmental permits, but also obtaining the social license. The industry must, therefore, aim towards mining without tailings deposits.
For this, we identify 3 options. The first would be a complete process transformation to have tailings-free mining, replacing current processes for novel processes such as in situ leaching or another alternative. The second option would be to return the tailings to the mine, avoiding mine waste outside of the mined area. This option is already applied, at least partially, with stopes filled with thickened tailings or non-operational pits with filtered tailings. Their generalised application is dependent on the development of new mining and tailings disposal technologies. A third option is to transform tailings to a different material, easy to manage and safer, or which can be used as construction materials or as raw material for other processes and uses. However, the enormous quantity of tailings generated by mining and the non-sustainable transporting process of huge amounts of materials limits this type of applications, which could be deposited safely and close to mining projects.
To confront these challenges, a multidisciplinary approach is required, with professionals capable of facing technological, environmental, and social aspects of these innovative proposals. With this in mind, the SMI-ICE-Chile team and specialists from SMI at The University of Queensland, in collaboration with Instituto IMDEA, Aiguasol and SEENSO from Spain, are developing an ambitious project, chosen as one of the 10 finalists for the Proof of Concepts stage of the BHP Tailings Challenge.