Planning, operating, monitoring and closing a tailings storage facility (TSF) can present many challenges, especially in dynamic mining environments where site conditions vary spatially and with time. However, big impacts can be made at relatively small cost once the tailings management system, design and performance are well defined and understood. This paper presents various examples of initiatives aimed at achieving the design intent that have been adopted by Rio Tinto Iron Ore, which also reduce risks and improve tailings management performance. Examples presented include development and communication of short-term, long-term and life-of-facility deposition plans, implementation of simple deposition management tools, monitoring and managing slurry density, development and continual oversight of water balance models, and sound investment in water management infrastructure extending to safe performance in emergency situations. Regular governance was also implemented to provide assurance that these controls remain effective.
For the alternative disposal method of dry disposal of tailings, two aspects of the process have to be evaluated, namely the maximum dryness of the separated solids and the maximum liquid recovery. For most mining operation, dry disposal is a new concept. Presenting these companies with examples of operations where this processing option has been successfully applied and proven, will help them to adapt to the changing requirements in the future. Focus has been placed on adapting decanter centrifuges to the mining environment. Modern decanters stand out in terms of their small footprint, low water demand, high availability and their high degree of automation, combined with the excellent cost / performance ratio. Decanter centrifuges have started to play a key role in applications such as tailing dewatering, drilling and tunneling muds processing, hydrometallurgical processes of gold, nickel or zinc and separating SX crud in copper refineries. This paper will present advantages of decanter centrifuges compared to the traditional technology and corroborate these advantages by case studies, especially from South America.
In 2012, SIMEC Mining commenced a detailed investigation into changing the way the magnetite tailings storage facility (Mag TSF) operates at the South Middleback Ranges (SMR) to increase water recovery and provide a sustainable cost-base for tailings management. Changes were also necessary to support the Magnetite Expansion Project (MEP) that was destined to be commissioned in October 2013. A feasibility study was performed with Golder Associates to understand the technical and commercial influences and provide a capital estimate for several options. The selected option from the study was a redesign of the current dual discharge TSF to a perimeter discharge, central decant (PDCD) design. Application of Nalco WaterShed polymer at the Big Baron Pit (Verdoornet al. 2018) revealed the technology would greatly assist in the successful conversion of the TSF to a PDCD configuration. Expectation was high that WaterShed polymer treatment would allow greater beach angle control, improved water recovery, and a reduction in surface water pooling across the TSF with water pooling concentrated around the central decant allowing for efficient removal prior to loss via evaporation or seepage. A conceptual design for the polymer tailings dewatering application was developed in collaboration with Nalco Water and dosing commenced in October 2013. Due to unknown risks associated with dewatering magnetite tailings, the project was split into two stages, namely, phase 1: a proof of concept trial to establish the applicability of Watershed on the magnetite tailings prior to commissioning of MEP; and phase 2: fully operationalise the PDCD configuration. Golder was engaged to develop a life-of-mine plan for the TSF at SMR that could be safely operated to a planned final height of RL 199 m. Throughout 2013 and 2014, design and construction occurred to convert the Mag TSF to a PDCD facility. A master plan was developed to manage tailings storage for five years from March 2014, referred to as the First 5 Year Plan. This involved six wall raises that would eventually fill the three voids near the western embankment and bring the height of the TSF to RL 172 m. The civil concept selected was based on an alternatives assessment that presented three options. SIMEC Mining chose the lowest cost approach of filling the voids with WaterShed polymer treated tailings to provide a base for 3 m wall raises upstream. Strict deposition and water recovery models were followed to ensure sufficient dewatering and the subsequent drying of the tailings layers. There was also extensive test work completed prior to each of the individual embankment raises to ensure that the dewatered tailings had the appropriate density and strength properties to support the raises before commencing with the lifts. During the first five years of operation, water recovery was around 60% and the volume utilisation was in line with the deposition model. The high percentage of water recovered enabled the processing plant to reach its new design capability, reduce significant downtime due to water availability and provide the mining operations with sufficient water for dust suppression. The second five-year plan is currently being finalised and progress is consistent with the tailings deposition and the dewatering model.
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.
The benefit of paste and thickened tailings (P&TT) was used for decades in the trona industry with a highdensity type thickener installed as early as 1994. The extraction process produced supersaturated liquor, which makes liquor recovery paramount. The insoluble material consisting largely of clays and silts produces the common problem of safe containment of the tailings. Both mine backfill and surface stacking of the tailings are being used. This paper provides a commodity review for trona, discussing the benefit of paste-type thickeners in the soda ash circuit. The topics include recovery from counter current decantation (CCD) circuits versus dilution washing; before and after retrofit data establishing improved underflow density and clarity from the latest thickener designs and operation; and the difficulties of surface stacking and drying due to residual soda ash forming a decahydrate crust sealing the stack and preventing drying. The trona industry provides a good review of paste-type thickeners and their benefits.