By Felipe Saavedra, Environment Researcher and Project Manager at SMI-ICE-Chile
As commented in Part 1, the challenges of appropriately managing tailings are threefold, considering the tailings of yesterday, the tailings of today and those of tomorrow. Among the primary aspects to address are water recovery and its inherent difficulties, aspects of circular economy for the conversion of tailings into other materials, and the adaptation of processes to move closer a tailings-free mining.
Water value and supply costs are high in most of the copper mining districts and any water recovery from the tailings is a valuable achievement. However, efficient water recovery is constrained by investment costs of conventional equipment and by high energy demands, particularly for filtration processes. Water recovered by conventional processes presents high concentrations of dissolved constituents and can only be returned to the process plant, with high risk of building up impurities that will require additional treatment.
Converting tailings into useful materials means managing the throughput scale and their difficult characteristics, among which are high-water content and low particle size, both which affect physical and chemical stability of tailings. The enormous amounts of tailings produced every year by the copper industry makes their complete conversion to useful commercial products extremely challenging. Numerous studies have proposed to transform tailings into various useful products, such as ceramics or bricks, but unfortunately, currently, there is no market able to absorb these vast amounts of construction materials. Moreover, most of the mining sites are in remote areas far from potential markets and, from both an economic and environmental sustainability perspective, long distance transport may represent an important and almost insuperable limitation for their practical use.
It becomes clear that the challenges linked to mine tailings establish a difficult and up-hill battle against time to come up with viable and long-lasting solutions which may address yesterday, today and tomorrow’s tailings.
THE CASE FOR A SOLAR SOLUTION
The primary copper reserves in the world are found in countries with high rates of solar radiation. The Atacama Desert in the north of Chile has one of the highest rates of solar radiation in the world, which is precisely where the world’s largest copper reserves are located. This scenario inspired the solar tailings transformation concept presented in the BHP Tailings Challenge in 2020 and chosen as one of the 10 projects for the proof-of-concept stage. The Solar Tailings Transformation project (STT) is a novel, circular, and forward-looking approach to tailings management that combines i) sustainably dewatering tailings, and ii) repurposing tailings into marketable final end-products through solar thermal energy. This approach, promoted by the consortium formed by SMI-ICE-Chile, SEENSO, IMDEA Energy and Aiguasol, is a smart and sustainable solution for converting tailings into valuable materials with a low carbon footprint, which seeks to virtually eliminate the need for traditional tailings facilities in the future.
The first stage of the STT approach uses high-performance solar evaporation and subsequent condensation of water applied to the tailings to recover up to 90% of high-quality water and dry the tailings that will be subjected to an agglomeration process in a second stage (explained below). This innovative process for water recovery, integrates two patented technologies already tested by prototypes currently operating in Chile and in Spain. The prototypes produce higher quality water than the water recovered currently, using conventional tailings dewatering technologies (i.e., tailings filtering). The process also involves the repurposing of the salts present in the tailings (e.g., gypsum precipitates) into binders’ additives for the agglomeration process of the STT approach – see the general concept in the figure below:
The Solar Tailings Transformation approach can recover high quality water and transform the tailings into a pelletized inert material. This approach is compatible and complementary to other reprocessing solutions, such as those focussed on recovering metal(loid)s present in the tailings or those attempting to transform part of the tailings content into valuable products.
STT’s second stage is where the dry tailings are subjected to a disruptive high temperature agglomeration process using cutting edge solar thermal energy technologies to transform the tailings into physically and chemically stable eco-friendly pellets suitable for different engineering and materials applications.
The solar high temperature melting-agglomeration-sintering process is inspired on natural processes observed in some volcanic eruptions where lapilli are formed by accretion and agglomeration of dust particles around condensed liquid or melted glassy nuclei.
The chemically and physically stable pellets will be easy to handle, transport and store, so the cost and carbon footprint of managing tailings as pellets would be lower than the current tailings management practices. In addition, the eco-friendly pellets could be used for engineering applications such as road stabilisation, dust control, covers to preclude drainage generation and emissions (dust and odours) from mines and landfills or as footpath base and for landscaping application in parks on rehabilitated areas, including mine waste impoundments. The melted tailings and pellets may also be used to produce other by-products from tailings, such as ceramics, lightweight aggregates, sorbent materials, structure for mirrors used in concentrated solar power applications, etc.
STT technology will have significant and positive environmental and social impacts. The approach is compatible and complementary with other tailings reprocessing solutions, such as those focused on recovering metal(loid)s present in the tailings or those attempting to transform part of the tailings content to valuable products.
THE REQUIRED PARADIGM SHIFT
The Solar Tailings Transformation concept is a sustainable approach that could lead to a paradigm shift in the mining industry’s tailings management. This concept takes a complex and difficult to handle mine waste and transforms it into an opportunity to support a more sustainable industry. For this shift to take place, the industry needs to identify the weaknesses and strengths in their currently knowledge and actions pertaining to tailings, as well as evaluate the risks and opportunities of novel and innovative approaches created by academia for the improvement of processes that impact a diverse and large group of stakeholders.