Mining Mirror January 2019 | Page 24

Mining in focus Tailings dams form an important part of a mine’s infrastructure. In picture is the construction of the tailings dam at Sepfluor’s Nokeng fluorspar mine in Gauteng, South Africa. in a classifier. The coarse-grained material is used to build the wall and the finer material is discharged into the interior of the dam. The slimes materials will settle on the inside face and penetrate a short distance, forming an impervious layer that prevents leakage. To raise the wall, the classifier will be mounted on a temporary track on the dam wall. The tailings material is then pumped through the classifier and the coarse material is discharged until the secondary wall reaches the desired height. The classifier is then moved a distance to construct the next portion of the wall on the same level. Another method is to install stationary classifiers at short intervals along the dam wall, under the tailings launder. The overflow is directed to the inner edge of the existing embankment by short lengths of launder, which are shifted to construct a continuous wall of the desired dimensions. The embankments are constructed with the steepest possible slopes to reduce cost. Deposition The design and operation of a tailings storage facility depend on the mode of deposition selected. This could range from conventional deposition of relatively low-density tailings, to paste deposition of high-density tailings. In both cases, however, metallurgical test work needs to be conducted on the slurry prior to deposition in the tailings facility, notes Andrew Vietti, director of Vietti Slurrytec. This would entail, but not be limited to, understanding the properties of the slurry, notably the clay mineral content and process water quality as well as the dewatering and rheological behaviour of the tailings. These test data [22] MINING MIRROR JANUARY 2019 provide the basic input on which the geotechnial engineers are able to provide secure construction and operational designs. Vietti adds that Vietti Slurrytec has developed ClariVie44, a process water conditioning reagent for the improvement of flocculation efficiency and overflow clarity, particularly in high-clay-containing slurries or any other poor settling slurry environment where thickening or dewatering of the tailings is problematic. Commenting on the re-processing of old tailings dumps, Vietti notes that the company has developed the KraVi de-slimer washing device to remove fine slimes, or fine, light particles from larger or heavier particles in any metallurgical slurry. In most instances, these fine particles comprise the clay mineral or fine silicate fraction. The KraVi finds its application in any mineral slurry stream where there is a need to remove a problematic fine clay/silicate particle fraction to improve the metallurgical efficiency of a secondary upgrading or recovery process. Vietti explains that the main benefit of this product is that it is highly efficient at separation, based on size and density and a cost-effective and high-throughput device. It offers an alternative to ultra-fine screens and de-sliming cyclones and to cyclone classification for tailings dam wall building. Legislative requirements South African legislation requires that mine residue deposits (MRDs), tailings storage facilities, tailings deposits, or slimes dams be managed over their entire life cycle by appropriately qualified persons, so that it does not pose an unreasonable risk to the public and the environment. This includes the controlled seepage from the tailings facilities and containing hazardous materials, such as cyanide, from entering the water system. The issue of acid mine drainage (AMD) has been a hot topic over the years. The other environmental and health concerns are related to the fine dust blown from these tailings dams and affecting the surrounding communities. Groundwater monitoring is also conducted from monitoring wells surrounding the tailings dam for the continued life of the installation. Geotechnical perspectives From a geotechnical perspective, tailings facilities must be designed to withstand the force of earthquakes, have adequate drainage to remain stable, and account for ground conditions. If there is excess seepage, liners are often used, particularly in gold mining where cyanide is used to leach the metal from the ore. There is no one-size-fits-all approach that can be used in the design of tailings dams. The design of the tailings dam is dependent on a number of factors; among others, the quantity of tailings being produced and the rate at which it is being produced. The topographic setting needs to be considered, along with climatic factors and the hydrological characteristics. Maintenance on tailings In terms of maintenance, typical work includes keeping tailings pipes and water reclamation systems in repair; monitoring water levels in the embankment’s fill and foundation to make sure it is meeting safety requirements; and monitoring for deformations within the structure itself. Survey pins or instruments inside the structure are used to detect any www.miningmirror.co.za