The recent commissioning of a Dry Stack Tailings (DST) system was supplied by Takraf to a Brazilian mine. While the dry stacking concept is not new, with references as early as 1909 in Australia, technological advances have made DST an increasingly important alternative to conventional methods for handling mine waste, enabling mining companies to significantly decrease their risk profile.
Currently, the most widely utilised method of disposing tailings (waste material from the mineral extraction process) is to pump the tailings slurry to a dedicated impoundment area (pond or dam) to allow sedimentation to occur and solids to settle out. Part of the water is then recovered and re-used in the process. However, the risks and challenges associated with this method, combined with evolving mining conditions, mean that the safe and effective handling and deposit of tailings, while also considering the environment, is of ever-greater importance.
Challenges include the considerable space requirement when storing tailings in impoundments, while the catastrophic tailings dam failures, which have occurred, resulting in significant loss of life and environmental damage, are a stark reminder of the risks.
Exacerbating the situation are current mining conditions, which utilise advanced mineral processing technologies to allow lower grade ores to be processed. This results in a much larger quantity of tailings that needs to be safely stored. This ever-increasing tonnage of generated tailings makes a conventional deposit in dams an increasingly significant burden curtailing the expansion and development of mine operations and further highlights the potential of DST as a viable and beneficial alternative.
Advantages of DST
With DST, the moisture content in the tailings is reduced by a mechanised and controlled process to the point that it can be safely transported, deposited, and stacked as solid cake-form tailings. This yields several advantages over other surface-tailings storage options, including:
- Significant safety improvement with the risk of catastrophic dam failure and tailings run-out being eliminated.
- Transformation of tailings into a stable landform, and facilitation of rehabilitation and eventual closure.
- Smaller tailings footprint and can be employed in undulating or steep terrain.
- Suitability to areas of high seismic activity and areas where there is limited construction material to develop a conventional retention impoundment.
- Reduced requirements for make-up water in plants, principally achieved by recycling process water, and near elimination of water losses through seepage and/or evaporation.
As tailings processing is complex, a detailed understanding of and expertise in the different steps specific to the commodity and project location are required. All equipment needs to be designed and/or adapted to specific project requirements and to be integrated seamlessly into the overall system. As a result, Takraf adopts a comprehensive and holistic approach to the design of its DST systems, combining its proven expertise in dewatering and materials handling with a dedicated team of mining, materials handling, and processing specialists.
This two-fold approach – bottom-up, equipment supply, and top-down, planning a long-term sustainable system – enables the company to develop solutions that best suit a client’s specific requirements, as well as providing the client with a single point of responsibility during implementation. This approach has resulted in clients not only procuring equipment for tailings treatment, but also in contracting Takraf to carry out conceptual studies and economic trade-offs for DST systems. All this culminated in a recent DST system order being placed in January 2019.
The order, placed by Mineração Usiminas, one of the Usiminas companies, and one of the largest steel producers in the Americas, comprised basic and detailed engineering, manufacturing, supply, transportation, and site assembly supervision of equipment to process iron ore tailings.
The system is designed as a fully integrated dewatering process via thickening and filtration and includes the following main equipment:
- 1 x flocculant plant
- 1 x coagulant plant
- 1 x Delkor high-rate thickener: 35m-diameter, 680t/h (nominal); 748t/h (design)
- 1 x 300sqm slurry tank with agitator
- 4 x double stage centrifugal slurry pumps
- 4 x Delkor filter press overhead beam (FP OH): 2m x 2m plates, each 170t/h (nominal) and 187t/h (design), capacity for 215 chambers
- 4 x compressors with tanks (process and instrument air)
- 4 x Takraf belt feeders: 2.0m width, 31m length with transfer chutes
Mineração Usiminas implemented the DST system to enable the filtering and stacking of tailings as the deposit area of the existing tailings dam reached full capacity. The process dewaters the tailings slurry in a single stream – a two-stage process using a Delkor high-rate thickener followed by the filtration of thickener underflow using overhead filter presses. The next process step allows further water recovery through the filter press, designed to achieve a moisture content level of about 14% (dry basis). Particular attention has been given to the redressing of the slurry using reagents to improve maximum recovery. The result is a dry filter cake that can be effectively handled and deposited.
The tailings material was extensively tested prior to the selection of the equipment. For example, the Delkor filter presses, designed with a collaboration partner as an overhead beam type, are best suited for the operational conditions. The design of such machines enables them to process large volumes of material, yet are robust and maintenance-friendly, since they provide easy access to the filter plates and filter cloths.
“Sustainable water management, together with safety, are increasingly important topics for mining operations and all stakeholders around the world; especially in areas where water conservation is critical and/or tailings failure risks are significant as has unfortunately been the case in Brazil. The Mineração Usiminas project is testimony that implementing a full DST solution, in line with the overall mine development plan, is the most environmentally-friendly and beneficial approach for all stakeholders,” concludes Thiago Machado, head of Delkor products at Takraf Brazil.