Beyond a careful analysis of ore types and process choices, industry experts emphasize that the geotechnical properties of leach operations play a significant role in a variety of leach-related concerns that range from leach kinetics to environmental and safety risks. For that reason, geotechnical testing and data analysis should be included at all stages of a heap leach project to avoid common problems such as cost overruns, lower than expected metal recovery, leakage, and heap instability.
One example: SRK Consulting’s geotechnical team has worked on numerous projects related to heap leach pads and spent ore dumps since 2007. Carolina Vergara, principal civil engineer with SRK, recently published useful recommendations about leach pad design on the company’s blog page, with insight gained from those years of experience. The article, the full version of which is available at www.srk.com/en/publications, is presented below, edited for length.
Heap leach pads are usually composed of crushed ore deposited on a drainage system designed to remove leach solution after irrigation. In the case of dynamic heap leaching, after the leaching process, spent ore is transported by conveyor belts for disposal at dump sites. It is common to observe physical stability problems in these dumps that could compromise the operation of stacker equipment along with the operational continuity of the mine. These problems are directly associated with excessive water content present in the spent ore, due to geotechnical characteristics of these materials or to drainage problems in the leach pads. Therefore, efforts are often focused on finding different ways to control moisture content in the spent ore.
Considering the common difficulties that we have observed, it is very important to carry out a “dynamic” geotechnical design that is updated with the different phases of ore exploitation in the pit. Thus, it’s necessary to complement the design with permanent operational controls in such a way as to define an action plan against the arrival of spent ore of poor geotechnical quality — e.g., excessive fines with high plasticity or materials with high water content.
SRK’s recommendations to minimize the volume of spent ore with high water content and its attendant problems include:
• In early stages during which metallurgical testing is conducted or during a pilot plant implementation/execution stage, geotechnical testing should be included to identify and implement any necessary changes in the metallurgical process.
• After irrigation begins, it is important to identify zones in leach pads that exhibit water pooling and/or slope sliding.
• Preparation of spreadsheets or databases containing information related to spent ore behavior in the leach pads (hazard maps) and water content in spent ore before it is dumped.
• Operational controls for spent ore after leaching are needed to obtain particle size distribution.
• Periodic maintenance of the drainage system in the heap leach pad is necessary to ensure correct function.
• A temporary dump for material with high water content can provide additional resting time before disposal in the final dump.
• Installation of piezometers in the spent ore dump can alert operators about formation of a phreatic level.