Mineral mining is the process of extracting valuable minerals or geological materials from the Earth. The choice of extraction method depends largely on the depth, size, and geological characteristics of the deposit. Factors such as the surrounding rock type, mineral concentration, and economic viability influence the technique selected. These methods range from physical excavation to chemical dissolution, each tailored to specific conditions.
Underground Mining
Underground mining is used when mineral deposits are situated too far below the surface for safe or economic surface excavation. This technique requires the construction of vertical shafts or inclined tunnels, known as declines, which provide access points for personnel, equipment, and ventilation deep into the rock strata. From these main access points, horizontal tunnels called drifts are excavated to reach the ore body itself.
This method is primarily used to recover high-value commodities like gold, copper, zinc, and deep coal seams, where the cost of creating the subsurface infrastructure is justified by the richness of the deposit. The process involves drilling and blasting the ore into manageable pieces, which are then transported to the surface. Because it leaves the overlying rock in place, underground mining results in a smaller surface footprint compared to open-pit operations. Various sub-methods, such as room-and-pillar or longwall mining, are selected based on the ore body’s shape and the surrounding rock stability.
Surface Mining Operations
Surface mining recovers mineral deposits that lie close to the Earth’s surface, making them accessible after removing the overlying waste material. This waste rock and soil, known as “overburden,” must be cleared before the ore can be extracted. Heavy machinery, including earthmovers and large excavators, is used to strip away this overburden.
Open-Pit mining and Strip mining are the two major forms of surface mining, differentiated by the geometry of the ore body. Open-Pit mining is used for large, often low-grade, disseminated deposits. It results in a continuously deepening, cone-shaped excavation with stepped sides. These large pits, sometimes referred to as quarries when mining non-metallic rock like limestone, continue to expand until the mineral grade becomes too low for economic extraction.
Strip mining, conversely, is applied to flat, layered, or tabular deposits, such as seams of coal or phosphate rock. The process involves removing a long, linear strip of overburden to expose the deposit and excavating the mineral. The removed waste material is then deposited into the void created by the previous strip. This technique progresses across a large area, following the horizontal contour of the deposit.
Placer Mining
Placer mining extracts heavy, durable minerals from unconsolidated sediments naturally eroded and deposited by water or wind. These deposits, known as placers, are often found in riverbeds, ancient stream channels, or beach sands, consisting of loose materials like sand and gravel. The minerals sought are those that resist chemical weathering and possess a high specific gravity, allowing them to concentrate naturally.
The methodology relies on gravity separation, exploiting the density difference between the target minerals and the lighter host material. Operations use water to wash the sediment through specialized equipment, such as sluice boxes or jigs. The heavy minerals—like gold, tin, or diamonds—settle out while the lighter materials are carried away. Placer mining is a physical separation process that does not involve the crushing of hard rock or the use of chemical solvents at the extraction point.
In-Situ Leaching
In-Situ Leaching (ISL), also called In-Situ Recovery (ISR) or solution mining, is a chemical extraction method that bypasses the need for physical rock removal. This technique is applied to permeable deposits situated below the water table, preventing the loss of the injected solution. The process begins with drilling a pattern of boreholes into the mineral deposit, creating a network of injection and recovery wells.
A carefully formulated chemical solvent, called a lixiviant, is pumped into the ore body through the injection wells. This lixiviant circulates through the porous rock, chemically dissolving the target mineral into a liquid solution. For uranium recovery, the solvent is often an alkaline solution like sodium bicarbonate, while sulfuric acid is commonly used for certain copper ores. The resulting mineral-rich solution, known as the pregnant leach solution, is then pumped back to the surface via the recovery wells for processing and metal extraction.