Mining is one of the world’s oldest industrial activities. It is the extraction of economically important materials, such as gold, coal, and gemstones, from below the surface of the earth. Modern mining technologies have evolved in response to the negative impacts to mining workers and the environment. Safety has been significantly improved by this new technology along with the economic improvements. Mining invariably involves surface and subsurface excavation of soil and rock to access the materials of interest. Surface mining is more common due to simple ease of access compared to subsurface work. It is estimated that 85 percent of minerals in the United States are mined from near the earth’s surface. Placer deposits are removed from river, beach, or oceanic sediments. Lode deposits, are found in geologic intrusions, or veins, generally found within a mass of soil or rock.
Ore is removed and processed to obtain the important materials. Processing of placer ore material is done using gravity to separate the materials. Sluice boxes, or shakers disaggregate the materials. Processing of lode ore requires crushing and pulverizing to allow extraction of the valuable materials. Final processing of both types of ore is done using mechanical or chemical means. Removal of materials that typical occur in small quantities, like rare earth elements, is done by in-situ leaching, using water or acidic and basic solutions to remove the material without excavation.
Surface mining removes the overlying layers of soil and bedrock to access ore deposits. Open-pit and strip-mining, quarrying, and mountaintop removal are the prevalent methods. Sub-surface mining is done by subsurface excavation with a combination of human and mechanical means. Access tunnels and shafts are made to reach buried ore deposits. Sub-surface mining is classified by the type of access or extraction method, which is dependent on the type and hardness of the rock to be excavated. Access can be horizontal, sloping, or vertical. Drift mining decries horizontal tunnels, slope mining uses diagonally sloping shafts. Shaft mining techniques include shrinkage stope, which creates a sloping underground room; room and pillar mining is a technique of excavation that allows safer access to the ore. Room and pillar mining is used with retreat mining,in which pillars are removed, allowing a controlled cave in, which loosens additional ore without labor. Long wall mining creates a long accessible ore surface.