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Gold mining involves various methods to extract gold from its raw state in gold ore, including physical force, heat exposure, or chemical means. Gold is found in soil, alluvial deposits, and rocks, and is recovered through methods such as placer mining, dredging, and hard rock mining. New technologies have made it possible to recover gold from hard rock without affecting its quality or quantity, using methods such as crushing, hydrometallurgy, and pyrometallurgy. Cyanidation and landfill leaching are common hydrometallurgical processes, while pyrometallurgy involves smelting, drying, and roasting.
Gold mining refers to the methods used to remove gold from its raw state in gold ore. Numerous processes are employed to achieve the mining, including the separation of gold from its surroundings by physical force, heat exposure, or chemical means. The ore is typically mined first, although this is not always necessary, and then subjected to a mining method appropriate for its quality and location.
One of several precious metals, gold is found in soil, alluvial deposits, non-cohesive sediments, or other rocks. It is found in vein and grain formations in flakes or as whole nuggets and is recovered from the earth by methods such as placer mining, which includes panning, slucing and dredging.
For burial, a large box with ridges is set up at the base of the loose rock. It picks up the gold as the water pushes the less dense components of the ore up the ridges. Dredging is the method by which a team of extractors funnel loose soil from the bottom of a lake or flow into a sluice that floats in a body of water. The less dense metals are pushed away as the gold descends to the base of the box. Most of the world’s gold supply, however, is the product of hard rock mining, conducted in open pits or underground mines, down to 12,800 feet (3,900 meters) deep.
Prior to the mid-1900s, digging for gold from hard rocks was less of a concern, as most people who panned for gold did so using placer mining methods. These miners sought alluvial deposits thought to be rich in gold, usually flakes or grains, and often used water as a means to break up the ground and recover the gold. This recovery method is tedious, time consuming and requires a large number of workers to be successful, especially commercially.
After the mid-20th century, new technologies were developed that made it possible to successfully recover the mineral found in hard rock without seriously affecting its quality or quantity. Various mineral processing methods are used, often in combination with each other, to extract the mineral from its rock coating. Crushing, hydrometallurgy, and pyrometallurgy are used to mine gold.
Shredding is often one of the first steps in gold mining. Miners break up the hard rock containing the ore. This is not done in every mining, but is often used to remove high quality or hard to recover minerals.
Hydrometallurgical processes are generally performed by leaching. For heap leaching, the rock is broken up into smaller pieces and placed on a liner. A cyanide solution is introduced, which liquefies the gold, releases it from the rock, and allows it to drain for processing. This gold mining method, also known as gold cyanidation, takes several months. Landfill leaching is used to extract mostly gold and lower grade metals. It does not require breaking the rock into small pieces and is cheaper and less effective.
In some cases, the gold cannot be removed with cyanidation alone. Pyrometallurgy can also be used for gold mining. This can include smelting, where the ore is reduced to a molten state, as well as drying and roasting, where other materials in the rock are oxidized to prevent them from hindering the mining process.
