Mining is the process of extracting valuable minerals or geological materials from the earth, such as metals like copper and gold or non-metallic resources like coal. The selection of a mining method is a complex decision, influenced by the deposit’s characteristics, economic factors, and environmental considerations. All mining operations are categorized based on the location of the extraction relative to the earth’s surface.
Surface Mining
Surface mining involves the removal of rock and soil, known as overburden, to access mineral deposits that lie close to the ground’s surface. This approach is favored for shallower deposits, offering a lower cost per ton of material extracted compared to subsurface methods. The large scale of operations allows for the use of massive machinery, such as large excavators and trucks, resulting in high production rates. This method is also considered safer for workers because they avoid the hazards of deep underground environments.
Surface Mining Methods
Specific methods fall under the umbrella of surface mining, each suited to different deposit shapes and materials. Open-pit mining creates a wide, bowl-shaped excavation with stepped sides, often used for large, disseminated deposits of hard-rock commodities like copper and iron. Strip mining involves removing horizontal layers of overburden to expose shallow, bedded seams, most commonly for coal. Quarrying is focused on extracting dimension stone, such as granite or marble, or construction materials like sand and gravel.
Underground Mining
Underground mining is necessary when mineral deposits are located too deep beneath the surface to be accessed economically by surface methods. This technique requires the construction of vertical shafts, angled tunnels (declines or slopes), or horizontal tunnels (adits) to provide access to the deep ore body. Once access is established, the ore is excavated from below the surface through a network of tunnels and rooms. This method has a significantly smaller physical footprint on the surface compared to large open pits.
However, underground operations present greater operational complexity and higher inherent safety risks. Miners work in confined spaces, which necessitates robust ground support to prevent cave-ins and requires sophisticated ventilation systems to provide fresh air and remove hazardous gases. Methods like room-and-pillar mining involve excavating a checkerboard pattern of rooms while leaving columns of rock to support the roof. Longwall mining, commonly used for coal, utilizes a large machine called a shearer to slice off material along a long face.
Why Methods of Extraction Differ
The choice between surface and underground methods is primarily determined by the geological characteristics of the deposit. The most influential factor is the depth of the deposit, as surface mining becomes increasingly impractical below a certain point, often around 60 meters, due to the volume of overburden that must be removed. Engineers use the “stripping ratio” to assess economic viability, which compares the amount of waste rock to the amount of ore.
The geometry and grade of the ore body also heavily influence the decision. Low-grade, dispersed deposits require the high-volume efficiency of surface mining to be profitable. Conversely, high-grade, concentrated veins can justify the high cost of underground mining because the valuable material is more concentrated. Finally, the stability of the surrounding rock and the presence of geological features like faults and fractures dictate the feasibility and safety of creating deep, unsupported tunnels.