Image: Coppermine Chuquicamata, Chile. Photo courtesy of Reinhard Jahn/Wikipedia.
As mining involves extraction of minerals from the earth’s crust, different techniques are used to dig the deposits. Before commencing the mining, prospecting of deposits is carried in modern mining process, to ascertain the resource potential of the proposed mine.
The exploration and development of mining sites also require heavy machinery that may include bulldozers, drilling equipment, explosives and trucks. Machinery plays a significant role breaking and removing rocks that contain minerals.
The two commonly used techniques in mining are surface mining and underground mining. At present, surface mining is the most common technique used across the world. In mining, the target sites are divided in two categories such as placer deposits and lode deposits.
While placer deposits comprise minerals that are mostly found within river gravels and beach sands, lode deposits constitute minerals that are present in mineral grains. However, both surface and underground methods are followed in the mining operations performed on the two types of ore deposits.
To reach ore deposits buried in the earth, surface mining involves removal of surface vegetation, dirt and in some cases layers of bedrock. The techniques followed in surface mining include open-pit mining, quarrying, strip mining and mountaintop removal.
Surface mining methods are commonly used in recovering minerals present in placer deposits due to their occurrence at shallow depths in the ground. While open-pit mining involves extraction of minerals from an open pit in the ground, strip mining includes stripping surface layers to reach ore deposits. Contour strip mining, area strip mining and auger mining are other techniques followed in surface mining.
However, these techniques are mainly used in coal mining.
Contour strip mining and area strip mining are the two forms of strip mining, which is often used to extract coal and lignite. Strip mining is feasible only when the ore deposits are located near the surface of the ore body. In contour strip mining, bulldozers or power shovels are deployed to take away huge amounts of ground material in terraced strips for extracting coal from a seam on a hill.
Area strip mining is commonly used on flat terrain to extract deposits spread over a large area.
Mountaintop removal technique involves removal of the mountaintop overlying the coal seam. The rock layers present above the seam will be broken up by using explosives. In recent years, the technique has found a wide usage in the Appalachian coal fields of West Virginia, Kentucky, Virginia and Tennessee in the US.
Highwall mining is another form of surface mining, which involves penetration of coal seam by a continuous miner. The miner will be propelled by a hydraulic Pushbeam Transfer Mechanism (PTM). The Pushbeam system is said to have an ability to penetrate deeper into a coal seam.
Underground mining involves digging tunnels or shafts underneath the earth’s surface to access buried ore deposits. Through the tunnels and shafts, ore deposits will be transported to the surface for processing. Tunnels are also used to bring waste rock to the surface. While drift mining involves usage of horizontal access tunnels, diagonally sloping access shafts are used in slope mining.
However, different techniques are deployed in mining in hard and soft rock formations. Underground mining also involves other methods such as shrinkage stope mining, longwall mining, and room and pillar mining. In case of room and pillar mining, ores are taken out from rooms, leaving ore pillars at the site for support. Room and pillar mining is usually employed in flat ore bodies.
Longwall mining is an advanced technique of underground mining to extract coal developed as an alternative for room and pillar mining. It is used to mine a long wall of coal that is a few kilometres long and some hundred meters wide.
Longwall mining technique is found to have better resource recovery rate compared to room and wall mining.
Cut-and-fill stoping is another technique followed in underground mining. It is mainly used to mine irregular ore deposits. The method involves breaking up of the ore into horizontal slices from the stope’s bottom to the surface. The process results in formation of empty space that it filled with a material to offer support for the extraction of the ore.
In-situ leach mining
Most of the rare earth elements are mined by using a method called in-situ leaching that involves no surface or underground digging. Minerals that can dissolve in water such as potash, potassium chloride, and sodium chloride are extracted by this technique. The technique, which is also known as solution mining, is mainly used in the extraction of uranium. The activities that are involved in conventional mining such as extraction and breaking the ore to recover the targeted minerals are not followed in in-situ leaching.
Instead, in this type of mining, ore is left in the ground, but minerals are recovered from it by dissolving them. The solvent solution is brought to the surface to obtain the minerals. This type of mining has some advantages such as causing only slight surface interruptions and generation of no waste rock.