Functions of Geosynthetics in Mining

Mining has been and continues to be critical to human development. Nearly everything we take for granted in modern living uses elements dug up from deep underground. Lithium—essential to our battery-run world—has seen its price double in just two years. But that is expected to change.

Keen on cashing in on the lithium craze, mining operations have probed new territories in search for the white metal. Lithium mining in Chile, which also holds the largest reserve, is expected to reach nearly a third of the world’s market supply by 2025. Analysts believe this increase in supply will help offset the demand for the element (1).

Operations such as the one in Chile are able to scale up in such a short period of time thanks to mining techniques that leverage geosynthetics. In this article, we will explore two mining techniques and the various geosynthetic products involved with five key functions in mining: separation, reinforcement, filtration, drainage, and barrier.

Brine evaporation ponds in Chile

Lithium exists in high concentration as a layer of sediment more than 100 ft below the surface of dried out salt lakes in Chile. To process the precious element, it must first be separated through evaporation. Brine evaporation ponds are constructed near the salt lake source to reduce transportation costs and are relatively shallow to improve evaporation rates. Geosynthetic liners (geomembrane) play a key role in the construction of brine evaporation ponds, serving as a barrier to prevent loss of the precious slurry. The barrier also separates the brine from the surrounding soil. Thus, the geomembrane is among the most important components of a brine evaporation pond.

Geosynthetic liners made with high density polyethylene are lightweight, durable, and very flexible—lowering the cost of transportation and installation. HDPE geomembrane liner has high tensile strength and can be coextruded with a conductive layer to assist with leak detection quality control during the installation process. Additionally, a white layer can be added to the top-facing surface to reduce liner heat absorption to further simplify installation.

Heap leaching in the United States

The heap leaching process has also been used to extract high concentrations of lithium and boron in the United States (2). Heap leaching is among the most cost-effective mining applications with substantially lower operating and capital costs compared with other forms of leach processing. It enables the operation to run at low costs for the long-term.

Heap leaching utilize either a single or double composite liner. For flat pad, on-off pads, or valley pads, a single composite liner is sufficient. The single liner composite comprises multiple layers including the existing foundation (subgrade), a low permeable layer underneath the geomembrane (either a Compacted Clay Liner (CCL) or a Geosynthetic Clay Liner (GCL)), the geomembrane, a protection layer (e.g., geotextile), and the mineral drainage layer (including solution collection/air injection piping) (3).

In this application, the HDPE geomembrane liner serves as a barrier for separation and is also the most important component in heap leaching. The geomembrane must resist chemical attack, point loads from typically high heap loading conditions, site-specific topography, site-specific climate conditions, and site-specific construction conditions.

GCL is a cost-effective alternative for a composite liner system, especially if on-site sources of clay are insufficient. GCL comprises a bentonite (water absorbing mineral) layer between two nonwoven geotextile layers. A 10 mm thick GCL will typically perform similarly or better than a 30 cm CCL and even demonstrate self-healing capabilities. GCL also provides additional filtration and barrier capabilities.

The protection layer is made with a nonwoven geotextile and helps distribute loads and provide reinforcement for the underlying geomembrane. The protection layer also assists with filtration and drainage, by preventing small particles and gravel from reaching the geomembrane.