Filtered tailings dry stack disposal often requires low cake residual moistures to meet the stacking specifications defined by geotechnical studies. Low residual moisture targets and increasing throughputs require careful definition of filter press process parameters in order to prevent unnecessary extra capital and operating costs. Because of this, alongside the testing phase, a comprehensive characterisation of the material, not limited to standard geotechnical tests, becomes important. Standard physical/chemical characterisation of mining slurries includes both basic (liquor, solid and slurry density, solid concentration, solid specific gravity) and thorough analysis (yield stress, particle size distribution, elements and mineral phase detection). Each of these requires different techniques. Phyllosilicates (i.e. clays) are one of the most common components of mine tailings, together with quartz, feldspar and other aluminosilicates. Clay type and relative content can have a drastic influence on the filtration process and furthermore on filter design. A detailed analysis of clayey phases can give useful information about the filterability of a slurry and, therefore, on the expected filter performance and its variability. A comprehensive study about clay detection and quantification and correlation with dewatering properties is presented here. It involves different analytical techniques (powder X-ray diffraction, Rietveld refinement, qualitative evaluation) and correlation studies based on the lab testing campaigns database. The results show how the filterability can be strongly affected by the phyllosilicate type and small variations in their content, to yield completely different cake properties, showing that smectite group clays have the greatest influence on filterability. The impact of these characteristics on filter sizing, especially for large equipment (like the GHT5000F filter press) and related ancillaries will be discussed.
The diamond industry has been disposing of its coarse tailings using conveyor and stacker systems for many years. The process plant typically generates two tailings products, a grit fraction (sand) and a coarse fraction (gravel), which are often combined on one dump. In some cases, the dump is stable with a single steep slope angle. However, in other situations a composite slope forms with settlement and intermittent slumping behaviour, this impacts on both design and operation. The thermal coal industry has also been disposing of dry ash for many years using either conveyor/stacking systems or haul trucks. Both systems work well, but the costs, deposition plans and stability aspects differ. Management of water and dust are also key factors. There are a number of key design and operational aspects that are similar between these diamond tailings and ash facilities, and would apply equally to filtered and dry stacked tailings. This paper aims to examine these similarities and show how these learnings could be built into new filtered tailings designs and operations to make them more efficient and stable.
At present, most tailings are dewatered by static settling in tailings dams. The environmental impact of these large settlings ponds is very dramatic, not to mention the space requirements and the potential risk of dam failure. This method of disposal also results in the loss of process water due to evaporation. This can be significant in areas where the supply of fresh water is limited. The high centrifugal force in a decanter centrifuge drastically increases dewatering efficiency, as centrifugal separation happens at up to 29,430 m/s² (3,000 × gravitational acceleration or g). Depending on the material, more than 90% of the process water can be recovered using centrifuge technology. It can be reused in the mineral processing operation and will increase the water efficiency of the whole mining community. This paper presents the results of actual projects in Brazil and Peru.
One of the methods that has been studied for the disposal of tailings by the mining industry is dry stacking. Several studies have indicated that the use of filtered tailings is feasible under certain technical and operational conditions. The requirements for deposition, and characterisation of the tailings properties, must be obtained through a wide range of field and laboratory tests. These parameters are key to determining the behaviour of the material during the dewatering process, the optimum moisture content and the maximum density of the tailing cake, as well as the physical and environmental stability of the projected geotechnical structure. The operational aspects related to the handling of the material are also vital to determine the placement of the filtered tailings on the facility. In addition, the advantages with respect to water consumption, reduced and/or flexible stack footprints and geometries, seepage control requirements, closure/reclamation costs and public perception of risk can result in dry stacking being selected as an economically beneficial alternative, particularly if life-of-mine risks and costs are appropriately considered. As with any conventional tailings management approach, careful and diligent planning, engineering and operational controls, quality assurance/quality control (QA/QC), instrumentation and monitoring are required to manage risks and uncertainties. This paper presents the criteria that need to be evaluated in the evaluation of filtered tailings as a business case, based on the experience of the authors developing such projects within the minerals industry.
This paper presents some details of the feasibility study for a proposed dry stacked tailings management system for the Mahenge Graphite Project, in Tanzania. Black Rock Mining Limited is focused on developing its Mahenge Graphite Project, which holds the largest high-grade flake graphite resource reserve in Tanzania the fourth largest graphite resource in the world. Several potential graphite processing operations are being proposed at various locations around the world and tailings management is a unique challenge given the geochemical characteristics of the graphite tailings and the topographical constraints of the site. Dry stacking of the graphite tailings offers a solution which minimises the environmental risks associated with conventional wet tailings and permits water reuse as part of the tailings management process. The project is located in a tropical savannah environment with a distinct difference between wet and dry seasons. Dry stacking offers a unique and innovative approach to minimising potentially large volumes of excess water during periods of high rainfall. At least 80% of the process water can be recycled.
The mining industry is becoming more and more attracted to dry stacking as a method of mine tailings management. The development of bigger-sized plates now allows metal producers to utilize pressure filtration technology to process flow rates of up to 200,000 tonnes per day of dry solids, which was undreamed of just a few years ago. The advantages of this technology include very low cake moisture content, significant savings in water usage and the possibility to recover product (when required) without the need of CCD washers. The filter press technology combined with bigger-sized plates enables filters to be installed in remote sites with high average rainfalls, offering the mining company the possibility of a safer and more sustainable environmental impact, thanks to a stable stack of solid material that requires less room than traditional thickened residue dams. The aim of this presentation is to show the latest studies where this technology has been applied.
In the practice of geotechnical engineering applied to mining, dry stacked tailings heap projects are analysed by limit equilibrium methods and stress-strain analyses. These methods and model assumptions do not consider the influence of the partially drained conditions, strain softening, and transition from overconsolidated to normally consolidated behaviour of the foundation. This situation can lead to calculations of deterministic safety factors that are often overestimated, according to the nature of the foundation. This article proposes a methodology for numerical modelling of the foundation during staged construction of a heap, and considers loading time as a critical variable in calculating the Factor of Safety of the heap dry stacked of elaborating a numerical model considering the foundation and construction. The physical and geotechnical properties of the materials were derived through the model, and are the interpretation of data obtained from a geotechnical investigation and laboratory testing campaign. Stressstrain analyses are performed for different stacking speed scenarios to assess its influence on the safety factor at each loading stage. The analyses are coupled, where the distribution of stresses and deformations within the foundation and the dissipation of porewater pressures over time are simulated. The results show the evolution of the Factor of Safety, the spatial distribution of the regions with excess porewater pressure, allowing the optimisation of both the rate of rise of the dry stack and external batter slope for the project to minimise ground improvement. It is observed that there is a strong dependence of the coefficient of permeability of the foundation soils on the porewater pressure dissipation time. Numerical modelling considering partially drained conditions allows greater understanding of foundation behaviour and performance during the development of the dry stacked heap. The analysis is considered applicable to both soft soil foundation conditions and conversion of existing wet disposal tailings facilities to dry stacking atop, referred to as piggybacking within the mining industry.
This paper presents the geotechnical characterization of tailings generated in Cuiabá Mine Site to support the applicability of FT technique in dry stacking design (compacted FT disposal for buttressing downstream of the dam embankment and across the tailings reservoir). It included seismic piezocone SCPTU tests of tailings disposed in the reservoir and laboratory tests with disturbed tailings samples at different void ratios to evaluate the effect of compaction effort on the geotechnical parameters of FT to support this new TSF method. The main results obtained from the geotechnical tests to support the design have shown: (i) tailings inside the existing TSF have presented contractive behaviour under high strain conditions and undrained strength ratio of 0.21; (ii) the denser the tailings, the higher the undrained strength ratio and the total friction angle. Besides, the contractive or quasi-steady state behaviour obtained to higher void ratios changed to quasi-steady state or strain-hardening behaviour with density increase; and (iii) the tailings critical state friction angle was closed to 32º.