Successful conversions of former slurry tailings embankments into filtered tailings storage facilities in Durango, Mexico

This case study presents successful conversions of two conventional slurry tailings storage facilities (TSFs) to filtered tailings storage facilities in Durango, Mexico, for two mining operations with production rates ranging from 1,200 to 4,500 t/d (1,320 to 4,960 stpd). In both cases, the decision to make the conversion was driven by: Ability to increase the storage capacity via stacking of filtered tailings. Reduction of the tailings’ impacted footprint. Mitigation of long-term geotechnical stability risks. Alleviation of environmental risks by reducing tailings water as a source of pollutants, and a net reduction in process make-up water demand. The case studies can be divided in two classes: class 1 facilities, where slurry tailings are loaded almost immediately with filtered tailings because of storage area limitations; and class 2 facilities, where the conversion is immediate but the available area allows gradual or eventual loading of the previously stored slurry tailings. Figure 1: Original Guanaceví conventional tailings storage facility. The condition where slurry tailings must be loaded almost immediately represents the greatest engineering challenge because of the geotechnical risks of instability and liquefaction (under both static and dynamic loads). Regardless of whether these facilities are converted immediately or eventually, they all require monitoring of pore water pressure conditions within the slurry tailings being loaded as the depth of placement of the filtered tailings above the slurry tailings increases. Additionally, where loading rate exceeds the pore pressure dissipation rate, tailings dewatering may be required to increase the rate of consolidation and strength and mitigate the risks of foundation failure. Guanaceví tailings storage facility conversion The Guanaceví facility conversion falls within class 1, where land availability limitations required the previously stored slurry tailings to be loaded almost immediately with filtered tailings. Because of local regulations in Mexico, and the use of cyanide leaching in the process at the Guanaceví Mine (Endeavour Silver Corp.), the conventional TSF was geomembrane lined and lacked any drainage system. The conventional TSF prior to the conversion is shown in Fig. 1. The original condition of the conventional TSF allowed a maximum placement of just a few meters of filtered tailings before the geotechnical factor of safety dropped below the minimum required in the design criteria or potentially triggered instability of the facility because of static liquefaction. Further analyses also indicated that piezometric pressure conditions of the slurry tailings had to be consistently lowered as the filtered tailings stack was being raised. Given the importance of drainage for this conversion, various passive and active dewatering options were considered to reduce the in situ tailings pore water pressure and allow for the loading of the slurry tailings deposits. The passive options included vertical wick drains and horizontal drains. Active options included the removal or dredging of the slurry tailings, and the construction of an extraction pumping well network. Because of the need to load the existing slurry tailings as soon as feasible, and because of the opportunity to recover metal-rich solution from the tailings pore water, the decision favored the installation of a network of extraction wells early in the filtered tailings placement phase as soon as a thin cap of drain material or filtered tailings would facilitate equipment access. Figure 2 shows the converted TSF. Cupías tailings storage conversion The Cupías facility belongs to class 2, where the available area allows gradual or eventual loading of the previously stored slurry tailings. This slurry TSF originally consisted of a closed valley embankment built using an upstream raise construction method. Currently, the Cupías TSF is owned and operated by First Majestic Silver Corp. Unlike Guanaceví, the flotation mineral processing allowed for an unlined TSF that could drain through the foundation, as well as through decant towers placed within the tailings impoundment (Fig. 3). To improve the stability of the tailings embankment and achieve the maximum required storage capacity, stabilization berms (or buttresses) were designed at the downstream toe of the TSF. During design, it was determined that at least three stabilization berms would be required. Given the volumes of material necessary for the construction of these berms/buttresses, consideration was given to changing the tailings processing technology from conventional slurry to filtered tailings. Figure 2: Converted Guanaceví filtered tailings storage facility. Once a decision was made to convert the conventional TSF to a filtered TSF, a portion of the planned buttress at the toe of the TSF was constructed with filtered tailings, which resulted in approximately 10 years of storage life before the slurry tailings within the existing impoundment had to be loaded. This period allowed for some drainage and consolidation of the previously deposited slurry tailings to facilitate loading by construction equipment and eventually loading by the filtered tailings stack, reducing risk of liquefaction. Figure 4 shows the current configuration of the Cupías TSF with the buttresses completed and progressively reclaimed and filtered tailings being placed on top of the former slurry tailings impoundment. Figure 3: Original Cupías conventional tailings storage facility. Conclusions Although these case studies apply to production rates below 10 kt/d (11,000 stpd), advancements in filtering technology and co-deposition of various types of waste and dewatered tailings will make conversion of existing conventional TSFs a more common practice. In addition to the technical challenges, the cases presented in this study required the education, understanding and commitment of the mine personnel at various levels, from operations and construction, to be able to implement a filtered tailings stack construction plan that was not prescriptive, but dependent on the evolution of site conditions and the level of dewatering achieved in the underlying slurry tailings impoundments. This level of interaction is key in the successful conversion of a conventional TSF to a filtered TSF. In both cases, storage capacity has more than doubled because of the successful conversion to filtered tailings management. Figure 4: Condition of the Cupías tailings storage facility through the years after conversion to filtered tailings.

Successful conversions of former slurry tailings embankments into filtered tailings storage facilities in Durango, Mexico

Authored by: Humberto F. Preciado, Daniel A. Servigna and Pedro V. Porcayo

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