Design and monitoring considerations for heap leach pad facilities constructed in environments with steep topography and complex hydrogeological regime

Michel R. Julien, Fiona Esford, Gerd Janssens

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Mining Geotechnics

ABSTRACT: The design of heap leach facilities in areas with significant topographic relief is done on a routine basis. In these cases, certain design aspects require detailed analysis (i.e. material testing, stability analysis, hydrogeologic assessment) or at the very least, additional sensitivity analysis should be conducted to understand potential risks. Ore gradation is typically assumed during design based on some preliminary crushing tests of small samples of ore that may or may not include agglomeration. Experience tends to show that conditions vary during operation with different rock types or degree of weathering encountered resulting in varying gradations with time that in turn affects permeability, solution levels, and ultimately ore recovery. Such variability may have an impact on the hydraulic conductivity of the ore placed in the heap leach facility and may also affect stability. Designs should therefore include some allowances to account for the possible variability of these aspects during the operation. Shear strength parameters of the different materials constituting the liner system also need to be properly characterized and accounted for within the design stability analysis. Underdrain system can be impacted by evolving hydrogeological conditions during operation, and is a critical component to the heap leach facility system. Overall, more emphasis should be placed on-going monitoring during ore placement and after to verify design assumptions and to allow the identification of potential issues that may be developing during the course of the operation so that mitigation measures can be implemented, if required. RÉSUMÉ La conception de plate-forme de lixiviation dans des régions avec beaucoup de topographie se fait de façon routinière. Certains aspects du processus de conception dans ces cas requièrent des analyses détaillées ou à tout le moins des analyses de sensibilité additionnelles. La granulométrie est typiquement considérée durant la phase de conception à partir d'essais de concassage de petits échantillons de minerai pouvant ou non inclure de l'agglomération. L'expérience tend à montrer que les conditions d'opération et le type de roche auront tendance à varier durant la vie de la mine et résulte en une granulométrie qui évoluera au fil du temps. Cette variabilité aura un impact sur la conductivité hydraulique des matériaux sur la plate-forme de lixiviation et sur sa stabilité. Le processus de conception doit alors inclure la possibilité de cette variabilité. Les paramètres de résistance des différents matériaux constituant les différents éléments du système d'étanchéité doivent être de plus bien caractérisés et pris en compte dans les analyses de stabilité. Le système de drainage sous la plate-forme peut par ailleurs être impacté par des conditions hydrogéologiques variant au fil du temps. Plus d'emphase doit être placé également sur le monitoring durant et après la mise en place du minerai afin de vérifier continuellement les hypothèses de conception et afin d'identifier des enjeux durant les opérations et développer des mesures de mitigation si requis. 1 INTRODUCTION This paper discusses some key aspects associated with the design and operation of heap leach facilities (HLF) in areas with significant topographical relief and associated complex hydrogeological regime. HLF have been a key method for ore extraction in warm and arid climates Œ at least since the 1980s (Smith, 2008). Now use is expanding in other areas with colder climates and areas with higher precipitation. For example, recent successful applications of the technology have been reported at Kinross Fort Knox mine in Alaska (Bradner, 2012). HLF design is influenced and benefited from materials and techniques used in landfill design. These designs share many similarities but also many differences (Renken et.al., 2005). Typically in the mining industry, extractive processes to recover minerals from the orebody are done fairly close to deposit being mined. HLF can therefore be located in challenging terrains and environments, including in areas with significant topographical relief, complex foundations and hydrogeological conditions. In contrast, landfills tend to be located in more suitable locations. Heap leaching is an attractive technology for mineral extraction offering many advantages when compared to more traditional methods. It allows the exploitation of low grade orebodies, has fairly low operating and capital costs. The mined ore is usually crushed and placed on a low permeability geomembrane and/or clay lined surface where it can be irrigated with a solution that percolates through the ore and gradually dissolves or removes the valuable metals/minerals. The leach solution containing

RÉSUMÉ: n and monitoring considerations for heap

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Michel R. Julien; Fiona Esford; Gerd Janssens (2014) Design and monitoring considerations for heap leach pad facilities constructed in environments with steep topography and complex hydrogeological regime in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper139,author = Michel R. Julien; Fiona Esford; Gerd Janssens,title = Design and monitoring considerations for heap leach pad facilities constructed in environments with steep topography and complex hydrogeological regime ,year = 2014}